Cables - Guidelines for the Interpretation of DGA for Paper-Insulated Underground Transmission Cable Systems

Guidelines for the Interpretation ofDissolved Gas Analysis (DGA) for Paper-Insulated Underground TransmissionCable Systems

Technical Report

EPRI Project ManagerW. Zenger

EPRI • 3412 Hillview Avenue, Palo Alto, California 94304 • PO Box 10412, Palo Alto, California 94303 • USA800.313.3774 • 650.855.2121 • [email protected] • www.epri.com

Guidelines for the Interpretation ofDissolved Gas Analysis (DGA) forPaper-Insulated UndergroundTransmission Cable Systems

1000275

Final Report, September 2000

DISCLAIMER OF WARRANTIES AND LIMITATION OF LIABILITIES

THIS DOCUMENT WAS PREPARED BY THE ORGANIZATION(S) NAMED BELOW AS ANACCOUNT OF WORK SPONSORED OR COSPONSORED BY THE ELECTRIC POWER RESEARCHINSTITUTE, INC. (EPRI). NEITHER EPRI, ANY MEMBER OF EPRI, ANY COSPONSOR, THEORGANIZATION(S) BELOW, NOR ANY PERSON ACTING ON BEHALF OF ANY OF THEM:

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ORGANIZATION(S) THAT PREPARED THIS DOCUMENT

Detroit Edison Company

ORDERING INFORMATION

Requests for copies of this report should be directed to the EPRI Distribution Center, 207 CogginsDrive, P.O. Box 23205, Pleasant Hill, CA 94523, (800) 313-3774.

Electric Power Research Institute and EPRI are registered service marks of the Electric PowerResearch Institute, Inc. EPRI. ELECTRIFY THE WORLD is a service mark of the Electric PowerResearch Institute, Inc.

Copyright © 2000 Electric Power Research Institute, Inc. All rights reserved.

iii

CITATIONS

This report was prepared by

Detroit Edison Company2000 Second Ave.Detroit, MI 48226

Principal InvestigatorsN. SinghO. Morel

This report describes research sponsored by EPRI.

The report is a corporate document that should be cited in the literature in the following manner:

Guidelines for the Interpretation of Dissolved Gas Analysis (DGA) for Paper-InsulatedUnderground Transmission Cable Systems, EPRI, Palo Alto, CA: 2000. 1000275.

v

REPORT SUMMARY

Laminar dielectric underground transmission cables represent a utility investment ofapproximately $20 billion. Protection of this investment depends on proper condition monitoringto avoid unscheduled outages, which can amount to hundreds of thousands of dollars perincident. The dissolved gas analysis (DGA) technique—which has been successfully applied totransformers—has now proven itself a cost-effective alternative for condition assessment ofthese paper-insulated underground transmission cable systems. This guide explains the basicelements of successful DGA application to such systems, with emphasis on proper datainterpretation.

BackgroundLaminar dielectric cables include high-pressure fluid-filled (HPFF), self-contained liquid-filled(SCLF), and high-pressure gas-filled (HPGF) cable systems. Insulation for these cables consistsof cellulose paper and a dielectric fluid, both of which govern the condition and life of thesecables (assuming that the cable’s steel pipe or lead sheath maintains integrity and protects theinsulating system from moisture ingress). Under the thermal and electrical stresses experiencedby the cable, the insulating system generates gases such as carbon dioxide, carbon monoxide,hydrogen, and lower and higher hydrocarbon gases. The type, distribution, and concentration ofthese gases in cable fluid samples form the basis of DGA condition assessment. Recognizing thepotential for DGA condition assessment of installed laminar dielectric underground transmissioncable systems, EPRI initiated the first DGA project at Detroit Edison in 1983. Detroit Edison hascontinued work under EPRI sponsorship, resulting in several comprehensive reports based onextensive laboratory, field, and Waltz Mill cable testing studies.

ObjectiveTo develop a guide for applying the DGA technique to condition assessment of paper-insulatedunderground transmission cable systems.

ApproachDevelopers of this guide drew from the extensive laboratory and field work performed by DetroitEdison on more than 6000 samples. This field work focused on condition assessment of a varietyof cable designs, vintages, and accessories using DGA techniques. Detroit Edison performed thework for EPRI, Empire State Electric Energy Research Corporation (ESEERCO), and numerousNorth American, South American, European, and Southeast Asian utilities; however, the vastmajority of data relates to U.S. HPFF cable systems.

vi

ResultsThe DGA technique—which can be applied to energized HPFF and HPGF cables—offers anumber of unique advantages for distinguishing between a satisfactory and a problem cablesystem and assessing the severity of any problems detected. First, the technique is easy to applyin the field, with a sampling time of under 15 minutes. It is reliable, has proven extremely cost-effective, and requires only minimal lightweight field equipment. Moreover, DGA is highlysensitive to electrical and thermal stresses and can detect gases at very low levels, with relativelymodest equipment investment. The DGA process is, in fact, accumulative, meaning it is able todetect activity that has already occurred or is intermittent in nature. Finally, the DGA techniquemay prove viable for use in performing remaining life assessments.

DGA success depends on several factors, including proper fluid sampling and handling, accuratechemical analysis, sound interpretation of the generated DGA data, and a knowledge of cablehistory. This guide addresses such factors, aided by tables, illustrations, and photographs so thatutilities can cost-effectively apply the DGA approach for condition monitoring of installedpaper-insulated transmission cable systems. A flowchart defines use of the DGA technique forthe entire cable system—including terminations, trifurcators, splices, and cable runs. In additionto these features, the guide also provides training material for engineers and technicians to use inmaintaining cable systems via the DGA process.

EPRI PerspectiveUtilities are facing erosion of cable expertise through the loss of experienced personnel, many ofwhom were involved in the development, installation, and maintenance of laminar dielectrichigh-voltage cable systems. The need for diagnostic capabilities becomes all the more imperativedue to the advancing age of such cable systems. It is particularly noteworthy that nearly 15% ofthese cables are reaching or have exceeded their design life of 40 years and 30% are more than30 years old. Rapidly emerging competition in the utility industry dictates minimumexpenditures to prolong the use of existing cable assets in order to defer cable replacements.

As a part of its continuing efforts to ameliorate this situation, EPRI plans to issue a set of guidesthat will readily train new engineers and technicians in the operation and maintenance ofunderground transmission systems. This first training guide describes the application of DGAtechniques to condition monitoring of installed paper-insulated transmission cable systems andthe encouraging results obtained to date. Related EPRI research includes Dissolved-Gas AnalysisMethod for HPFF Paper Cable (EL-7488-L); Dissolved Gas Analysis (DGA) by EPRIDisposable Oil Sampling System (EDOSS) (TR-111322); and Transmission Cable LifeEvaluation and Management (TR-111712).

KeywordsTransmission cablesCondition assessmentDissolved gas analysis

vii

ABSTRACT

Condition monitoring of installed laminar dielectric underground transmission cables is crucial ifutilities are to remain competitive, protect their $20 billion investment in these systems, anddefer additional investments in cable replacement. Proper condition monitoring will also helpprevent unscheduled outages that run into hundreds of thousands of dollars per incident. Utilitiesare increasingly using the dissolved gas analysis (DGA) technique—which has been successfullyapplied to transformers—for condition monitoring of these paper-insulated undergroundtransmission cable systems, with encouraging results. Under the thermal and electrical stressesexperienced by the cable, the insulating system generates gases such as carbon dioxide, carbonmonoxide, hydrogen, and lower and higher hydrocarbon gases. The type, distribution, andconcentration of these gases in cable fluid samples form the basis of DGA condition assessment.

The DGA technique—which can be applied to energized HPFF and HPGF cables—offers anumber of unique advantages for distinguishing between a satisfactory and a problem cablesystem and assessing the severity of any problems detected. First, the technique is easy to applyin the field, with a sampling time of under 15 minutes. It is reliable, has proven extremely cost-effective, and requires only minimal lightweight field equipment. Moreover, DGA is highlysensitive to electrical and thermal stresses and can detect gases at very low levels, with relativelymodest equipment investment. The DGA process is, in fact, accumulative, meaning it is able todetect activity that has already occurred or is intermittent in nature. Finally, the DGA techniquemay prove viable for use in performing remaining life assessments.

Recognizing the potential for DGA condition assessment of paper-insulated undergroundtransmission cable systems, EPRI initiated the first DGA project at Detroit Edison in 1983. Thisguide explains the basic elements of successful DGA application to these cable systems, withemphasis on proper data interpretation. Detroit Edison has continued work under EPRIsponsorship, resulting in several comprehensive reports based on extensive laboratory, field, andWaltz Mill cable testing studies.

ix

PREFACE

Utilities are facing erosion of cable expertise through the loss of experienced personnel, many ofwhom were involved in the development, installation and maintenance of laminar dielectric highvoltage cable systems, namely, High Pressure Fluid-Filled (HPFF), High-Pressure Gas-Filled(HPGF) and Self Contained Liquid-Filled (SCLF) cable systems. As a part of its continuingefforts to ameliorate this situation, EPRI has planned a set of guides that will readily train newengineers and technicians alike in the operation and maintenance of such undergroundtransmission systems. The first training guide issue consists of the interpretation ofDissolved-Gas Analysis (DGA).

The aim of this Guide is to explain through simple language, illustrations, and pictures, the basicelements involved in the successful application of DGA to assess the condition of laminardielectric transmission cable systems. Included in the basic elements are:

• Fluid sampling

• Fluid sample handling

• Fluid sample storage

• Fluid sample transportation

• Chromatographic gas analysis

• Interpretation of results and recommendations

It will be assumed that the reader knows little about DGA but has some basic understanding ofthe various laminar dielectric cables, cable accessories and general cable installation.

xi

ACKNOWLEDGMENTS

The valuable contribution of Dr. Reza Ghafurian of Consolidated Edison Company of NewYork, Mr. Mohammad Khajavi of Los Angeles Department of Water and Power (LADWP)and Mr. Takashi Kojima of BC Hydro toward the preparation of this guide is gratefullyacknowledged. Thanks are in order for many utility engineers, too numerous to mention, inNorth America and overseas who provided opportunities to develop a DGA database. Thecontinued support and guidance of Mr. Walter Zenger of EPRI is deeply appreciated.

xiii

CONTENTS

1 INTRODUCTION.................................................................................................................. 1-1

2 FLUID OR GAS SAMPLING FOR CABLE SYSTEMS, INCLUDING HANDLING,STORAGE AND TRANSPORTATION.................................................................................... 2-1

EDOSS Method .................................................................................................................. 2-1

EDOSS Sampling Procedure ......................................................................................... 2-4

Vial Number Code.......................................................................................................... 2-6

EPOSS Method .................................................................................................................. 2-6

EPOSS Sampling Procedure ......................................................................................... 2-7

Cell Filling ...................................................................................................................... 2-8

Disassembling................................................................................................................ 2-8

Application of DGA to Laminar Dielectric Cables, Splices and Terminations....................... 2-9

Sampling of HPFF & SCLF Cable Accessories................................................................... 2-9

Sampling of HPFF Terminations ....................................................................................... 2-10

Sampling HPGF Cables.................................................................................................... 2-11

3 GAS ANALYSIS OF COMBUSTIBLE AND NON-COMBUSTIBLE GASES:AVAILABLE METHODS, INCLUDING EPRI METHODS........................................................ 3-1

Dynamic Headspace........................................................................................................... 3-1

Static Headspace ............................................................................................................... 3-2

EDOSS System .................................................................................................................. 3-3

4 INFLUENCE OF ORIGINAL FLUID QUALITY WITH EMPHASIS ON ELECTRIC ANDTHERMAL ACTIVITY; EFFECT OF THE TYPE OF FLUID, NATURAL VERSUSSYNTHETIC HYDROCARBONS ............................................................................................ 4-1

5 INTERPRETATION OF DGA DATA .................................................................................... 5-1

6 FLOW-CHART ON SAMPLING PROCEDURE, PROBLEM LOCATION AND DGAINTERPRETATION................................................................................................................. 6-1

xiv

7 CONCLUSIONS................................................................................................................... 7-1

A GLOSSARY ........................................................................................................................A-1

xv

LIST OF FIGURES

Figure 2-1 Glass Sampling Vial .............................................................................................. 2-2

Figure 2-2 Two Views for Quick-Connect Vial Holder ............................................................. 2-3

Figure 2-3 Sampling Tool Showing the Metallic Housing for Needle Support and the3-Way Valve .................................................................................................................... 2-3

Figure 2-4 Quick-Connect Coupler and Vial, Before and After Sampling ................................ 2-4

Figure 2-5 Schematic Diagram for EDOSS Sampling Procedure............................................ 2-5

Figure 2-6 Schematic of Duplicate Fluid Sampling by EPOSS Method Along WithFittings and Valves .......................................................................................................... 2-7

Figure 2-7 DGA Sampling by EPOSS..................................................................................... 2-8

Figure 2-8 Sampling of a 345 kV Cable Splice by EDOSS ................................................... 2-10

Figure 2-9 Sampling Kit for HPGF Cables ............................................................................ 2-12

Figure 2-10 Field Sampling of a HPGF Cable....................................................................... 2-13

Figure 3-1 Headspace Analyzer Showing the Sampling Vials and the 50-SampleCarrousel......................................................................................................................... 3-3

Figure 3-2 Gas Chromatographs Utilized in DGA ................................................................... 3-4

Figure 3-3 Schematic Diagram of a Gas Chromatograph for Dissolved Gas Analysis ............ 3-4

Figure 5-1 Bar Chart Showing Normal Gas Concentrations for HPFF Splices ........................ 5-2

Figure 6-1 Flow-Chart for Fluid Sampling and DGA Interpretation.......................................... 6-2

xvii

LIST OF TABLES

Table 1-1 Estimated Circuit Length and Investment of Laminated UndergroundTransmission Cables in the U. S...................................................................................... 1-2

Table 4-1 Effect of Peroxide Content on Gas Content of New Fluids...................................... 4-2

Table 4-2 Gas Concentration for Fluid Samples From Various Sources ................................. 4-3

Table 5-1 Gas Concentration Limits for Splices and Cable Runs of Static HPFF Cables........ 5-6

Table 5-2 Gas Concentration Limits for Terminations of HPFF Cables................................... 5-7

Table 5-3 Gas Concentration Limits for Forced-Cooled HPFF Cables.................................... 5-8

Table 5-4 Gas Concentration Limits for SCLF Cable Splices.................................................. 5-9

Table 5-5 Gas Concentration Limits for Terminations of SCLF Cables ................................. 5-10

Table 5-6 Gas Concentration Limits for HPGF Cables (200 psi Nitrogen) ............................ 5-11

Table 5-7 DGA Schedule for Splices and Terminations........................................................ 5-12

Table 5-8 Gas Concentrations and Ratios Included in the Six-Digit Code for theInterpretation of Dissolved Gas Analysis of HPFF Cables and Accessories................... 5-13

Table 5-9 Fault Diagnosis of Cables Through Dissolved Gas Analysis................................. 5-16

Table 5-10 Six Digit Code Showing Arcing Pathway to a 345 kV Cable Failure.................... 5-16

Table 5-11 Example of Application of Condition Assessment Code to HPFF CableTermination – DGA Data ............................................................................................... 5-17

Table 5-12 Example of Application of Condition Assessment Code to HPFF CableSplice – DGA Data......................................................................................................... 5-18

1-1

1 INTRODUCTION

Laminar dielectric underground transmission cables, which are an integral part of the electricpower system in the United States, represent a utility investment of approximately 20 billiondollars. It is essential to protect this investment. This can be accomplished by proper conditionmonitoring to avoid unscheduled outages, which can amount to several hundred thousands ofdollars for laminar dielectric underground transmission cable systems. The need of this diagnosisbecomes imperative all the more due to the advancing age of such cable systems. It isnoteworthy that nearly 15% of these cables are reaching or have exceeded their design life of40 years and 30% are over 30 years old. The rapidly emerging competitive utility climate alsodictates that prolonged and trouble-free use of cable assets be made with minimum expenditures,deferring cable replacements.

The laminar dielectric cables include High-Pressure Fluid-Filled (HPFF), Self-Contained Liquid-Filled (SCLF) and High-Pressure Gas-Filled (HPGF) cable systems. The three key componentsof such cables are: conductor, insulation, and a metallic covering, namely, steel pipe for HPFFand HPGF cables and lead or aluminum sheath for SCLF cables. The insulation consists ofcellulosic paper and a dielectric fluid. This combination governs the condition and life of thesecables, assuming that the steel pipe or lead sheath maintains integrity, protecting the insulatingsystem from moisture ingress.

Whereas fluids of high (3500 SUS @ 100 F) and low (125-600 SUS @ 100 F) viscosities serverespectively as the impregnating and pipe filling pressuring (200 psi) medium in a HPFF cable, asingle dielectric liquid of low (60 SUS @ 100 F) viscosity is used in SCLF cable at a pressure inthe 15-50 psi range. The pipe filling pressuring medium for a HPGF cable is nitrogen at 200 psi,the viscosity of the highly viscous impregnating fluid being 1000 to 3000 SUS at 210 F. Theinsulating system generates gases such as carbon dioxide, carbon monoxide, hydrogen, andlower and higher hydrocarbon gases under thermal and electrical stresses experienced by thecable. The type, distribution and concentration of such gases form the basis of conditionassessment of laminar dielectric cables through DGA on fluid samples removed from suchcables.

The year of introduction, voltage class along with the estimated average age, circuit lengths andpresent-day Investment dollars are given in Table 1-1 for the three types of cables. While thepresent market for SCLF is limited, HPFF cables are still being purchased by U. S. utilities fornew, replacement and re-routed circuits. Recently, HPGF cables have been receiving a renewedinterest at 115 through 138 kV as they offer the rugged and reliable pipe-type constructionwithout raising environmental concern because of free dielectric fluid.

Introduction

1-2

Table 1-1Estimated Circuit Length and Investment of Laminated Underground Transmission Cablesin the U. S.

Cable TypeYear of

IntroductionCircuit Length

(in miles)Average Age

(Years)

Present DayInvestment

(in $millions)

HPFF, 69-345 kV 1935 2,900 25 $15,000

HGFF, 69-138 kV 1941 300 35 $750

SCLF, 69-525 kV 1927 450 45 $1,500

The Dissolved-Gas Analysis (DGA) technique, which has been successfully applied totransformers, is being increasingly considered for laminar dielectric transmission cables, withencouraging results. This technique offers the following unique advantages:

• Easy-to-apply in the field, sampling time is under 15 minutes

• Fast and reliable approach

• Highly cost effective, requiring minimal light weight field equipment

• Applicable to energized HPFF and HPGF cables, a particularly desirable feature

• High sensitivity to electrical and thermal stresses, with discerning capability

• Gases detectable at very low levels with relatively modest equipment investment

• Accumulative process able to detect activity that has already occurred or is intermittent innature

• Potential for remaining life assessment

The success of DGA depends on several factors such as proper fluid sampling, handling,accurate chemical analysis and sound interpretation of the generated DGA data. In addition, aknowledge of cable history: failures, repairs, cable additions, re-routing, fluid leaks, make-upfluid or nitrogen as for HPGF cables, initial quality of pipe filling fluid for HPFF orimpregnating fluid for SCFF cables, disposition of dielectric fluid reservoir(s) as well as anycommon reservoirs, presence of hot-spots and loading is helpful. The initial quality of nitrogenfor HPGF cables is important for condition assessment through gas analysis.

Data interpretation is of utmost importance as improper interpretation can lead to wrongdecisions that can be very expensive. While there is some commonality in transformer and cableDGA, marked differences exist as a result of distinct designs, materials and operations of the twoelectrical products. Accordingly, the transformer DGA experience cannot be directly applied tocables. Application of transformer guidelines can shut down otherwise satisfactorily operatingcables. The guide draws from the extensive laboratory as well as field work (over 6,000 samples)performed by Detroit Edison for EPRI, ESEERCO and numerous North American and overseasutilities toward the condition assessment of a variety of cable designs, vintages, and their

Introduction

1-3

accessories through DGA since the early 1980s. It should be stressed that vast majority of thesedata relates to HPFF cable systems.

The purpose of this guide is to address these various factors with the aid of tables, illustrationsand pictures so that it can be conveniently and profitably applied to monitor the condition of in-service laminar dielectric transmission cable systems by means of DGA. It is also meant to serveas a training guide for engineers and technicians for the maintenance of such cable systemsthrough DGA.

The various steps and procedures involved in the sampling, storage, handling and transportationof in-service cable fluids have been discussed, facilitating the role of field technicians. Particularattention has been paid to the different sampling approaches relating to splices, terminations andcable runs so that DGA can be properly performed on these three components. The interpretationaspects and methodology of DGA are addressed along with the frequency of sampling.

Depending on the condition of the cable system, the gas concentration limits are classified intofour categories, namely, normal, acceptable, concern and action level. These levels are arrangedin increasing order of severity. The normal and acceptable levels are self-explanatory. Concernlevel means that the concentration of key gases is high enough to require close monitoringthrough DGA. The action level category requires prompt attention, which includes inspectionand possibly repairs. It is advisable to consult with experts before the expensive decision to openthe cable system is made.

A flow-chart delineating the DGA technique for the entire cable system, including terminations,trifurcators, splices and cable runs has been presented to aid in the application of DGA, includingboth static and forced-cooled lines.

2-1

2 FLUID OR GAS SAMPLING FOR CABLE SYSTEMS,INCLUDING HANDLING, STORAGE ANDTRANSPORTATION

The importance of correct fluid sampling under service conditions cannot be overemphasized;the results are no better than a representation of the fluid through proper chemical analysis. Theremoved sample should faithfully correspond to the fluid within the cable system beingdiagnosed and not the fittings involved. The sampling should cover both the cable and itsaccessories together with the trifurcators and the pressurizing system. Splices and terminationsusually have fluid sampling ports. Unlike static lines, it is enough to sample a circulating line at asingle point. Any point along a cable run can be reached by fluid drainage. A termination alwayshas at least one sampling port (or valve). If a valve is not available at the splice, it can be readilyprovided. The sampling of terminations from the top port implies de-energization. However, thesplices of HPFF cables can be sampled without de-energization but this does not hold for thesplices of SCLF cables due to safety considerations. A recent Japanese development allows thesampling of splices in energized SCLF cables1.

While the same general sampling approach is used for cables, splices and terminations, there arecertain differences with respect to these individual products and the type of laminar cable. Thechoice of sampling system is governed by the DGA method, for example, the conventionalASTM DGA method (D 3612) utilizes both glass syringes and stainless steel cylinders, andShimadzu method employs only the former.

EDOSS Method

The EPRI method named EDOSS (EPRI Disposable Oil Sampling System), which is nowexclusively utilized by Detroit Edison for DGA, uses small evacuated glass vials. The EDOSSmethod along with its sampling vials and chemical instrumentation will be described in detail. Itshould be noted that the EDOSS method is a much simpler and inexpensive version of theprevious EPRI method called EPOSS (EPRI Pressurized Oil Sampling System) which had beenextensively used for laboratory, field and Waltz Mill investigations.

Unlike the EPRI DGA methods, the ASTM D 3612 and Shimadzu DGA methods were primarilydeveloped for power transformers and they continue to serve the transformer industry well.However, the higher gas content in cables coupled with the presence of nitrogen in HPFF and

1 “Development of oil sampling devices for energized oil filled cable system” Okada M. Tenaka, Tamura K. Tsuji,Jicable, pp. 272-277 (1999)

Fluid or Gas Sampling for Cable Systems, Including Handling, Storage and Transportation

2-2

SCLF cables can lead to the formation of a fairly large size bubble after sampling. This bubblepresents some difficulties in the subsequent gas analysis process, impacting the results to someextent. Gases with low solubility such as hydrogen, nitrogen and carbon monoxide tend tolargely collect in the bubble, giving rise to higher or lower concentrations, depending whether ornot the bubble is included.

Contrary to the recommended ASTM practice, this bubble is often pushed out from the glasssyringe in the field while keeping the dielectric fluid. There is no correct way of treating thebubble in the Shimadzu method, as there is no provision to handle the bubble in the samplingloop. The EDOSS vial overcomes the bubble difficulties as the analysis system is designed tooperate on the headspace principle, which inherently accounts for both the gaseous and liquidphases.

The EDOSS vial consists of a disposable evacuated crimp-top 22 mm x 75 mm glass cell with anominal capacity of 20 cm3. However, only about 6 cm3 of fluid sample is needed. The vial issealed with a suitable elastomeric plug, which is secured with a metallic crimp cap. Aphotograph of the vial is shown in Figure 2-1. This vial requires two more important componentsto accomplish the sampling process, namely, the quick-connect coupler modified to hold thedisposable crimp-top vial and a suitable adapter incorporating a hollow needle through which thefluid is admitted into the vial and a 3-way valve. A metallic housing protects the needle. Thequick-connect coupler (or vial holder) and the metallic housing containing the needle and the3-way valve are shown respectively in Figures 2-2 and 2-3. Two vials, supported by thequick-connect coupler, are shown in Figure 2-4, one of which shows the collected fluid sample.A schematic diagram of the key components of the EDOSS system, as they are put together insequence for sampling, is illustrated in Figure 2-5.

Figure 2-1Glass Sampling Vial


2-3

Figure 2-2Two Views for Quick-Connect Vial Holder

Figure 2-3Sampling Tool Showing the Metallic Housing for Needle Support and the 3-Way Valve


2-4

Figure 2-4Quick-Connect Coupler and Vial, Before and After Sampling

EDOSS Sampling Procedure

The sampling procedure is schematically shown in Figure 2-5, and it illustrates in sequence thevarious steps involved in the sampling process. It is recommended to take duplicate samples ateach location. The steps for duplicate sampling are described as follows:

• Attach the sampling tool to the equipment being sampled utilizing proper reduction fittings.With the 3-way valve handle pointing downward, drain about ½ quart of fluid to flush thefittings by opening he main valve of the equipment. After flushing, point the 3-way valvehandle upward to direct fluid through the needle. Let some fluid flush through the needlebefore taking the first sample. Maintain the 3-way valve open during the entire samplingoperation.

• Make sure the rubber seal (gray disc in Figure 2-2) is in place inside the quick-connect vialholder. Secure the empty vial to the quick-connect vial holder. To do so, place the vial in theholder while holding the outer-ring down and then releasing it.

• With fluid running through the needle, gently push the quick-disconnect holder with a vialinto the needle housing. Keep the quick-connect vial holder aligned with the needle toprevent any damage to the needle. Once in place, the upper edge of the vial holder shouldalign with the upper edge of the needle housing, refer to Figure 2-5.

• Admit fluid up to about the center blue line of the vial as shown in Figure 2-5. To retrieve thevial, simply pull the vial straight up from the housing and then remove it from thequick-disconnect.

• Repeat the procedure with a second vial while keeping the initial fluid flow through the needle.

• After the second sample is taken, close the main valve and then turn the 3-way valve handledownward to release fluid pressure before disassembling. Restore the corrosion protectiontape to protect the main valve


2-5

• Zep Brake Wash spray can be used to clean the sampling device. Be careful, Zep BrakeWash is very flammable. Place the vials with the red crimp down in the foam filled boxessupplied by Detroit Edison and ship without delay.

Fluid f lush

Fluid f lush

Fluid level

Figure 2-5Schematic Diagram for EDOSS Sampling Procedure


2-6

Vial Number Code

The disposable sampling vials are under vacuum, with a shelf life of about 2 months. Theprepared vials are coded with a 9-digit identification number before shipment. The first fourdigits indicate the month and day (February 2 as 0202) of shipment. Digits 5 and 6 indicate thefractional part of the empty vial weight and the last three digits a sequential number 001 through999 to identify the vial. There is no need to use the vials in sequential order.

EPOSS Method

In the previous EPOSS method, a small volume of cable fluid (~20 cm3) is taken in a glasscylinder, which had been previously evacuated to a low pressure (<0.1 mtorr). With the carefullychosen components of this sampling system, the internal pressure can be maintained belowdetectability limits of the gas analysis system for about one month. The EPOSS cell has a sturdy3-arm valve fitted with zero clearance o-ring face fittings. Figure 2-6 shows duplicate EPOSScells; two cells are used in case one cell breaks in transit, leaks or there is a sampling error.Through this valve and with an adapter, the cell can be connected to the sampling equipment,permitting trouble-free sampling. As the fluid is admitted into the EPOSS cell, dissolved gasesrapidly evaporate from the fluid until equilibrium is obtained, as is true for its modified version,the EDOSS cell. The evolved gases remain confined in the cell until the analysis is completed.

This prevents accidental contamination and gas losses through bubble formation. The gasanalysis is carried out in the same cell, which becomes the extraction vessel. As a result, thisnovel method provides for a simple sampling process with no liquid handling needed duringsampling and analysis.


2-7

1/4" VCO ConnectorsVCO to P ipe Thread Adapter

Bal l ValveFlush Out let

EPOSS Ce l l s

St i rrers

Magnet ic

Three-Way Va lves

Sample In let

Figure 2-6Schematic of Duplicate Fluid Sampling by EPOSS Method Along With Fittings and Valves

EPOSS Sampling Procedure

The procedure to perform DGA sampling by the EPOSS method is described below. Aphotograph, showing the EPOSS system during fluid sampling from HPFF terminations, is givenin Figure 2-7.

The EPOSS sampling cells are under vacuum. Do not open the cell top valve unless the sample isbeing collected. The T-branch under the valve is always open for drainage and is not affected bythe status of the valve.

• Connect together two EPOSS cells plus the ball valve at one end, as shown in Figures 2-6and 2-7.

• Attach the valves to the sampling port using the supplied VCO to ¼″ male adapter.

• You must supply your own fittings to attach the VCO to ¼″ male NPT to the equipmentsampling valve.


2-8

• Once the cells are attached to the sampling port, open the ball valve fully and then slowlyopen the valve attached to the equipment being sampled.

• Drain sufficient fluid to flush all connecting fittings and manifolds (about a quart should besufficient). At this point, you can take the sample in the amber bottles and syringe.

Cell Filling

• When ready to take the fluid sample in the EPOSS cell, slowly open the cell valve, allowingsufficient fluid in the cell to reach the red line (1″ from the bottom).

Disassembling

• Once the fluid sample has been taken, close the valve connecting to the equipment beingsampled and open the ball valve to release the pressure in the sampling manifold.

• Remove the EPOSS cells, drain some of the fluid in the T-branch and reinstall the plasticseals on either side of the EPOSS cell.

• Identify the location of the sample by writing on the supplied labels.

Figure 2-7DGA Sampling by EPOSS


2-9

Application of DGA to Laminar Dielectric Cables, Splices and Terminations

The sampling procedure for HPFF and SCLF cables and splices is essentially the same.However, sampling of terminations requires a different procedure. The sampling of HPGF cablesrequires steel cylinders because the sample of gas has to be taken at operating pressure. Eachprocedure is addressed below.

Sampling of HPFF & SCLF Cable Accessories

Although there are no general rules for the installation of valves in the splice casing of HPFFcables, most splices have two valves, a 2″ gate valve at the top and a 1″ gate valve at the bottom,Figure 2-8.

Samples for DGA can be taken from either the top or bottom valve. However, a top sample ispreferred for convenience. Before taking the cable fluid sample from these large valves, it isessential to drain sufficient fluid to ensure that the sample reflects the fluid within the splice.

It should be noted that the sample taken from the splice does not represent the condition of thecable away from the splice for a static line. To assess the condition of the cable through DGA,fluid has to be drained. Knowing the fluid volume per foot of cable pipe, any location in thecable within adjoining splices can be reached by removing the proper amount of fluid. Thedirection of fluid movement is governed by the configuration of the pumping plant(s). Whereastwo pumping plants will enable fluid movement in either direction, cable systems with only onepumping plant can allow movement only in the direction away from the pumping plant.However, this limitation can be readily overcome by selecting the next splice away from thepumping plant. It should be noted that DGA is the only available technique that allows conditionassessment at any point away from the splice in HPFF and SCLF cables. Fluid movement hasbeen successfully applied to assess cable condition2.

The sampling of SCLF cables is essentially the same as that of HPFF cables, except that theaccess valves have smaller sizes. Unlike HPFF cables, SCLF cables have to be de-energized forsampling for safety.

2 “Accurate Gas Analysis Reveals Damaged Cable Section” Innovators with EPRI Technology, IN-101579, October(1992)


2-10

Figure 2-8Sampling of a 345 kV Cable Splice by EDOSS

Sampling of HPFF Terminations

There is limited exchange between the cable and termination fluid. The check valves provided atthe bottom of the termination limit this exchange. The long narrow annular space between theinner porcelain wall and the cable and stress assembly does not facilitate convective fluidmovement.

Once dissolved gases are formed, they tend to stay in the same location. The sampling ofterminations should reflect the entire length of the termination. The condition assessment ofHPFF terminations should be taken seriously due to the physical hazards involved in theshattering of HPFF terminations.

Terminations always have a valve at the top. In most cases, this is a small ¼″ NPT valve that isnormally utilized to bleed the termination after installation. Some terminations are fitted with asampling valve in the base spool, below the stress cone region. A valve at the end of the risersection, a few feet from the bottom of the termination, is in provided in some cases. Due to therestricted fluid flow between the terminations and the pipe, samples taken from the riser pipe donot provide a representative sample. The fluid emerging from the raiser valve originates from thecable pipe and might have only a very small amount of gases emanating from the termination.


2-11

The valve at the termination base spool offers a good sample due to its proximity to the stresscone region; this is invariably true for capacitor graded designs employed at 230 and 345 kV. Asecond sample should always be taken from the top termination valve, followed by a thirdsample, as fluid is drained from the top.. These samples allow the assessment of the upper &middle regions of the termination. For terminations without a bottom valve, sampling from thetop with drainage is the only option. This evidently requires de-energization of the line. With theexception of the sampling of a 345 kV termination with at the bottom value, the sampling of any138 kV or 230 kV termination requires de-energization regardless of accessing the bottom or topvalue or safety considerations. The clearances are too small to allow live-line sampling ofterminations. Drainage is needed to cover the entire length of the termination.

Considering all these designs, different procedures must be planned ahead in order to obtaingood samples. Details of the different sampling scenarios are given as follows:

1. Only top valve, 138 kV HPFF termination

• Take sample from the top valve without drainage

• Drain 1 to 2 gallons

• Take a second sample from the top valve

• Drain another 1 to 2 gallons

• Take a third sample, after draining 1 to 2 gallons

2. One top valve and one bottom valve, 138 kV HPFF termination

• Take sample from the bottom valve without drainage

• Take first sample from the top valve without drainage

• Drain 2 to 3 gallons

• Take second sample from the top valve

230 kV and 345 kV terminations will invariably have valves both at the bottom and top.However, the drained volume between samples should be increased to account for the largervolume and length of termination.

Sampling HPGF Cables

The absence of a liquid dielectric renders the sampling of HPGF type cables quite simple toperform. This is also true for the chromatographic analysis. Although the gas sample is taken andmaintained at system pressure, the analysis must be conducted at atmospheric pressure, resultingin a dilution equivalent to the ratio of system pressure to atmospheric pressure. Accordingly, theanalysis results must be multiplied by this ratio before the data can be compared to those fromHPFF type cables. Despite the pressure correction, results from HPGF cables are significantlylower than those of HPFF cables due to the diffusion of the generated gases along the pipe,which does not allow the accumulation of gases at a location in HPGF cables. Moreover, the


2-12

amount of the total insulating fluid for HPGF cables is significantly lesser than that HPFF cables,due to the replacement of he pipe fluid by nitrogen gas.

Stainless steel cylinders are utilized to sample HPGF lines. These cylinders have two valves, oneat each end as shown in Figure 2-9. The set-up for field sampling of HPGF cables is shown inFigure 2-10.

The gas sample is taken by connecting one end of the cylinder to the sampling port at the spliceor pipe. Often, valves are provided at the risers. With both cylinder end-valves open, open thecable valve and allow gas to flush through the cylinder. Let gas purge for 2 to 3 seconds and thenclose the outermost valve followed by the other valve. Then close the cable valve and disconnectthe cylinder. It is recommended to sample a HPGF cable at all the splices along the line. Anenergized HPGF cable can be sampled at its splices, as is true for the case of a HPFF cable.

Figure 2-9Sampling Kit for HPGF Cables


2-13

Figure 2-10Field Sampling of a HPGF Cable

3-1

3 GAS ANALYSIS OF COMBUSTIBLE ANDNON-COMBUSTIBLE GASES: AVAILABLE METHODS,INCLUDING EPRI METHODS

The analysis of gases dissolved in dielectric fluid presents two main challenges:

• Gases must be extracted from the liquid matrix

• Technique must be sufficiently sensitive to detect trace level gas concentrations

Gases separated from the liquid matrix can be separated and quantified utilizing standard gaschromatographic techniques with readily available columns and detectors. However, samplescannot be directly injected into a separation column due to high fluid viscosity and elevatedboiling point (low vapor pressure) of the fluid. Hence, sample preparation techniques must beemployed to allow the separation of the gaseous components from the liquid.

Gases can be separated from the liquid matrix in two ways. Each is addressed below.

Dynamic Headspace

Gases are stripped from the liquid sample by bubbling a carrier gas through a column containingthe sample. The carrier gas with the extracted gases is injected into a gas chromatograph for gasseparation and measurement.

Commercial application of dynamic headspace is represented by Shimadzu TOGAS(Transformer Oil Gas Analysis System). This equipment, supplied by Shimadzu, consists of adedicated gas chromatograph connected in tandem with a gas extraction column for transformerDGA. In this unit, the sample is injected directly into a stripping column. The outlet of thestripping column is connected to a chromatographic column(s) for the separation andmeasurement of gaseous components. Since it is impossible to reach 100% gas extraction forvarious gases, the extraction efficiency must be determined for a characteristic fluid sample. Theextraction efficiency is expected to vary with the viscosity and nature of the fluid. This extractionefficiency is determined by standard gas in oil solutions. Since mineral oil is invariably used inpower transformers, only one determination of extraction efficiency is needed. However, severalfluids with varying viscosities are employed in cables. The determination of the extractionefficiency is needed in each case. The fluid samples must be collected in 50 cm3 glass syringes.This system has good accuracy, repeatability and is fast.

Gas Analysis of Combustible and Non-Combustible Gases: Available Methods, Including EPRI Methods

3-2

Static Headspace

In static headspace, the liquid sample is injected into an evacuated constant temperaturecontainer of known volume. After phase equilibrium, the headspace is analyzed by gaschromatography. The equilibrium concentration of each component in the liquid phase iscalculated from the known concentration of the gas phase and their solubility values (Henry’sLaw). The final concentration of gases in the original sample is then obtained. This is expressedin terms of total gas content (gas in gas phase + gas in liquid phase) in cubic centimeters at STP(standard temperature, 273°C & pressure, 1 atm) or NTP (normal temperature, 298°K &pressure, 1 atm) conditions. A factor of 1x106 is applied to transform the cm3 gas per cm3 oil ratioto volume parts per million. A gas concentration in ppm at STP is about 10% smaller than ppmat NTP.

Two DGA methods based on static headspace are available: the traditional ASTM procedure(ASTM D-3612) and automatic headspace analyzers. Headspace is the volume of gas in contactwith a fluid in a confined vessel. Headspace analyzers are devices that allow the sampling of theheadspace for final analysis by gas chromatography.

The ASTM D-3612 requires a fluid sample of about 50 cm3 , which is injected into an evacuatedglass container of about 500 cm3. With such a fluid volume to headspace ratio, only a smallamount of gas remains in the fluid. The extracted gas is later compressed with mercury into agraduated burette to determine its volume. Due to its hazardous nature, mercury is not favored inutility laboratories.

A portion of the extracted gas is subsequently injected into a gas chromatograph to determine itsgas composition. When fluid samples are taken from high pressure cables with glass syringes,bubbles are formed. According to the ASTM standard, the entire sample including the bubblemust be injected in the extraction equipment.

Commercially available headspace analyzers allow unmanned analysis of a large number ofsamples, Figure 3-1. The control of temperature and pressure, in addition to the unmannedhandling of gases, leads to a significant increase in accuracy and reproducibility; both areimportant in the effective application of gas analysis.

In headspace chromatography, the fluid sample has to be introduced in a glass vial. This isaccomplished by taking the field sample in glass syringes and then transferring to the glass vialin the laboratory. This two step process reduces the accuracy and adds to the cost of the analysis.

A novel-sampling device developed under EPRI sponsorship has reduced the two step process toa one step. The developed method, called EDOSS (EPRI Disposable Oil Sampling System),enables the direct filling of the glass vial in the field. The glass vials are discarded after analysisand hence the 'disposable' designation.

The glass sampling vials are pre-evacuated to under 1 mtorr (one micron of Hg), weighed,labeled and then delivered to the users in foam packed cardboard boxes with 18 vials each. Eachvial weighs about 17 g. Details of the sampling procedure have been provided in Chapter 2.


3-3

Procedures for the calculation of the original gas concentration in the fluid sample consist ofbasic mass balances. This requires the value of the equilibrium pressure after extraction and gassolubility data at equilibrium temperature. Day to day gas chromatograph calibration is achievedwith certified gas blends. Solubility must be determined for each different fluid. Solubilitycorrections are needed for hydrocarbon gases C2 and higher.

Figure 3-1Headspace Analyzer Showing the Sampling Vials and the 50-Sample Carrousel

EDOSS System

The EDOSS system corresponds to a static headspace approach. It uses commercially availableequipment with some modifications. However, the sampling system, an important part of DGA,is custom-made.

In the EDOSS method, the utilization of two gas chromatographs in parallel significantly reducesthe time required to perform the analysis, Figure 3-2. Basically, a gas chromatograph is a devicein which a carrier gas is forced through a separation column(s) connected to a gas detector(s).The gas sample, placed in a sampling loop, is swept by the carrier gas stream before it enters theseparation column(s), Figure 3-3. Analysis of H2, O2, N2, CH4, CO and CO2 is performed by a gaschromatograph as shown in Figure 3-3, while another gas chromatograph is used for theseparation of hydrocarbons C2 through C4 hydrocarbons.


3-4

Figure 3-2Gas Chromatographs Utilized in DGA

Carrier Gas( )

Regulator

Flow

Controller

Pressure

Sample In

Sample Loop

6-WayValve

Flow Restriction

6-WayValve

Thermal

Conductivit

DetectorMethanize

Flame

Ionizatio

Detector

Hydroge Hydroge Oxygen

Column 1

Column 2

Figure 3-3Schematic Diagram of a Gas Chromatograph for Dissolved Gas Analysis

4-1

4 INFLUENCE OF ORIGINAL FLUID QUALITY WITHEMPHASIS ON ELECTRIC AND THERMAL ACTIVITY;EFFECT OF THE TYPE OF FLUID, NATURAL VERSUSSYNTHETIC HYDROCARBONS

The condition of the original pipe fluid has a bearing on the interpretation of the DGA results.Accordingly, it is essential to utilize a gas-free fluid so that DGA can be properly performed.Fluid for bulk use is always delivered in large railways or truck tanks under a 5 psi nitrogenblanket.

It should be noted that the fluid for some of the older installations was delivered under a nitrogenblanket containing appreciable amounts of saturated hydrocarbon gases such as methane, ethane,propane and butanes. As a result, there are many old operating HPFF cables with largeconcentrations of such gases. Although the presence of these gases has no effect on theperformance of the cable, it can interfere with DGA interpretation.

Another source of gas contamination found in new fluids is due to the presence of oxidationproducts, namely, peroxides. Several important diagnostic gases such as hydrogen, carbon oxidesand hydrocarbons are generated by the decomposition of peroxides at temperatures as low as80°C3. Although the yield of these gases is not large, occasionally concentrations of 100 ppm and1000 ppm of hydrogen and carbon oxides, respectively, have been observed in new fluids takendirectly from tank-cars.

Formation of peroxide compounds results from fluid handling operations involving hightemperatures and small amounts of air, for example, during fluid distillation processes. Someperoxide compounds are very unstable and their decomposition can result in gas formation atrelatively low temperatures. Peroxide compounds can be readily eliminated through treatmentwith activated clays (Fuller’s earth) or silica. Portable degassing units can be fitted withabsorption columns to recondition dielectric fluids and reduce dissipation factor and peroxidelevel of used fluids.

Alkylbenzene fluids are characterized by a low peroxide content, below 1 ppm. However,polybutene fluids tend to have a comparatively large peroxide content, 2 to 5 ppm. It isrecommended that new polybutene fluids should have a low peroxide content, not exceeding 2 to3 ppm. A high peroxide content will lead to high content of carbon oxides and hydrogen. Thiseffect is shown in Table 4-13.

3 Singh, N., Morel, O.E., Singh, S.K. and Rodenbaugh, T.J., “Predictive Maintenance of Fluid-Filled Taped Cablesthrough a Novel Dissolved Gas Analysis Method: US Field Experience”, CIGRE 15/21/33-16 (1996)

Influence of Original Fluid Quality With Emphasis on Electric and Thermal Activity; Effect of the Type of Fluid,Natural Versus Synthetic Hydrocarbons

4-2

The dissolved gas analysis data for fluids with high peroxide content and non-standard nitrogenblanket are compared to a well degassed fluid in Table 4-2. The first column shows the gascomposition of a fluid sample taken directly from a degassing unit. This is considered to be ahigh quality, gas-free fluid sample. Column two represents a new fluid with high peroxideoxidation products. This fluid, a polybutene, was sampled directly from a tank-car at a U.S.utility. This situation is not desirable from a DGA standpoint.

Table 4-1Effect of Peroxide Content on Gas Content of New Fluids

FluidCO

(v/v ppm)CO2

(v/v ppm)i-C4H8

(v/v ppm)Actual Peroxide

(w/w ppm)Estimated Peroxide4

(w/w ppm)

DO 1001 0 129 101 0.2 0.2

06CS2 541 1,336 1,325 2.6 3.8

DO 5003 40 186 761 0.3 0.9

1 Low viscosity alkylbenzene2 Medium viscosity polybutene3 Medium viscosity blend, 75% alkybenzene: 25% polybutene4 Peroxide content estimated from the yield of carbon oxides

Influence of Original Fluid Quality With Emphasis on Electric and Thermal Activity; Effect of the Type of Fluid,Natural Versus Synthetic Hydrocarbons

4-3

Table 4-2Gas Concentration for Fluid Samples From Various Sources

Gas

UnusedDegassed Fluid

(ppm)

Fluid From Tank CarShowing High Peroxide

Content (ppm)

In-Service Fluid With Non-Standard Nitrogen Blanket

(ppm)

Methane 0 23 53,850

Ethane 0.3 7 11,840

Ethylene 0 2 2

Acetylene 0 0 0

Propane 0.4 14 4,383

Propylene 0 7 11

Isobutane 0.6 23 238

n-Butane 0.8 28 365

Isobutylene 5.4 85 10

C. Monoxide 0 243 14

C. Dioxide 0 822 424

Hydrogen 0 100 23

Nitrogen 530 179,056 77,462

Oxygen Nil Nil Nil

Column three shows the DGA for a 230 kV cable operating satisfactorily but showing very highmethane and ethane levels. The large concentrations of methane, ethane and propane comparedto ethylene and propylene should be noted. The former gases essentially came with the originalfluid (a mineral oil), which did not have a standard nitrogen blanket during transportation and/orstorage in that this nitrogen blanket contained appreciable concentrations of saturatedhydrocarbons as opposed to the recommended pure nitrogen. The non-saturated hydrocarbons,namely, ethylene and propylene were probably formed in-situ.

5-1

5 INTERPRETATION OF DGA DATA

The insulation system of laminar dielectric cables is composed of paper insulation and adielectric fluid. These two components are subjected to both electrical and thermal stressesduring cable operation. As a result, a large number of combustible and non-combustible gasesare generated. These gases, which include carbon dioxide, carbon monoxide, hydrogen and lowerand higher hydrocarbon gases, remain dissolved in the dielectric fluid in a liquid state, and hencethe name “dissolved gases”.

Once formed, they remain in place, even if, the cause of generation has disappeared. This confersa unique advantage upon DGA over partial discharges that disappear due to many factors such asmodifications of the internal surface of the void, or its pressure or shape.

While carbon oxides along with minute amounts of hydrogen and methane are evolved frompaper, the dielectric fluid yields hydrogen, methane, ethane, ethylene, acetylene, propane,propylene, n-butane, isobutane, isobutylene, t-2-butene and 1-butene. The pipe fluid can alsoyield carbon oxides, depending on its peroxide content as discussed in Chapter 4.

Cables operating in a normal manner have a low dissolved gas content. The presence of high gasconcentration indicates that the cable system is experiencing unusual electrical and/or thermalactivity. This has been demonstrated both by laboratory and field studies under EPRIsponsorship. While the laboratory studies may not establish a quantitative relationship betweenlaboratory and field data, they clearly interrelate the specific types of gases and their relativelevels generated under electrical and /or thermal conditions encountered in field conditions.

The type, concentration, and distribution of gases are governed by the specific nature of theproblem faced by a cable. Certain gases can be associated uniquely with either electrical orthermal activity, thereby, giving clues as to the condition of the cable. Dissolved gases do notonly lend themselves to accurate measurements but their generation is also highly sensitive tothermal and electrical stresses. Moreover, such gases can be detected at extremely low levelswith relative ease and modest chemical instrumentation, unlike partial discharges that requireelaborate and sophisticated field equipment.

Carbon dioxide and carbon monoxide are related to the thermal condition of paper, the rest ofgases evolve from fluid. In normally operating cables, the concentration of carbon dioxide ismuch larger than that of carbon monoxide. However, the carbon dioxide to carbon monoxideratio varies as the paper ages. The rate of carbon dioxide evolution tends to decrease with time,but the rate of carbon monoxide tends to increase. Thus the ratio of carbon dioxide to carbonmonoxide decreases as the paper ages, giving a clue as to the degradation of paper. Whereashydrogen is associated with low level electrical activity in the fluid, acetylene is related to strongelectrical activity involving visible arching. Electron bombardment of hydrocarbon chains leads

Interpretation of DGA Data

5-2

to the removal of hydrogen from the hydrocarbon chain, leading to an increase in hydrocarbonunsaturation. A similar effect is seen from exposure to elevated temperature, wherein thestability of saturated compounds such as ethane and propane decreases, favoring the formation ofethylene and propylene. Hydrogen and methane tend to increase with temperature. Whenhydrocarbons are subjected to pronounced ionization activity, acetylene and some carbonizedfluid along with other unsaturated (double bonds) species such as ethylene, propylene, andbutenes are formed.

Typical normal gas concentrations for HPFF cable splices are shown in bar-chart form,Figure 5-1. Acetylene, the single most important diagnostic gas, should be zero for cablesoperating in a normal fashion. Even a few ppm of can be a cause of concern, requiring attention.It is worth pointing out that in the IEEE transformer DGA guide the level of acetylene is beingsignificantly reduced to less than 5 ppm. The presence of acetylene is always accompanied by anincrease in the concentration of ethylene, frequently making the ratio ethane to ethylene closer toone or smaller.

The generation of isobutylene is connected with the thermal decomposition of the dielectricfluid, particularly polybutenes for which isobutylene is the starting material. It should beemphasized that both the levels and ratios of gases are important. It is essential to consider theentire pattern of gases rather than rely on a few individual gases for proper data interpretation.

500

100

100050

200

0

20

100

70

0 200 400 600 800 1000

Gas Concentration (ppm)

C. Dioxide

C. Monoxide

Hydrogen

Propylene

Propane

Acetylene

Ethylene

Ethane

Methane

Figure 5-1Bar Chart Showing Normal Gas Concentrations for HPFF Splices

While sound DGA experience can readily distinguish between a normal and problem cable,periodic monitoring to establish gas generation trends and rates is essential for cases posingconcern and problem. The gas generation rates should be periodically determined to establish


5-3

whether or not a particular cable is maintaining dielectric integrity or deteriorating. Such periodicmonitoring also confirms, if any previous unusual electric or thermal activity has ceased.

The cable per se, splices and terminations are the sources of gases. It has been observed that inrare circumstances the components of the pumping plant can generate gases that can confuse theDGA picture. However, this invariably involves hydrogen generation as a result of the presenceof galvanized fittings or rusting of associated pipes and/or fittings with the reservoir and/orpiping. Minute amounts of oxygen in water can accelerate the production of hydrogen.

Compared to cables, splices and terminations are the most likely sources of gases. It is alsoimportant that the initial dielectric fluid is basically free of gases. This should be ensured in thebeginning, as discussed in Chapter 4.

For a new fluid, acetylene and hydrogen must be both zero, with the remaining gases generallybelow 10 ppm. Before nitrogen blanket became the normal practice for cable fluid shipment,ethane, propane or methane were sometimes involved as a part of the nitrogen blanket for suchgases are readily available in the refinery. It has been observed that some older cables could havelarger amounts of such gases.

Terminations can yield significantly larger concentrations of some key gases due to the nature ofthe electric field, relatively smaller fluid volume, and possible exposure to external/internalover-voltages. Thus, the limits of key gases for terminations are significantly different fromthose of the splices and cables, with terminations yielding higher limits. It should be noted thatfor SCLF terminations with reservoirs, the fluid is free to move out of the termination into thereservoir, resulting in a dilution effect contrary to the case of HPFF terminations and SCLFterminations without direct connection to the reservoir.

Static Versus Forced-Cooled Cables

The influence of forced-cooling on DGA is discussed for the case of HPFF cables as this modeof operation is confined only to HPFF cables in the U.S. The gas concentration limits for forced-cool lines are significantly lower than those for the static lines. Gases generated at a givenlocation in a forced-cooled cable are rapidly mixed and distributed throughout the large volumeof the free fluid as a result of fluid circulation. The dilution factor depends on the total volume ofthe pipe fluid, which is proportional to the cable and return line length, and the number of gassources, mostly the splices. The individual gas limits for a splice are the same for both types oflines. Therefore, the gas limits for a forced-cooled line must be calculated by the application of adilution factor that incorporates the effect of mixing.

Dissolved gases, which are mostly localized once formed, generally affect the fluid to a distanceof about 150 ft , on either side of the source of generation. This corresponds to a fluid volume ofabout 750 gallons (2.5 gallons/ft.). Assuming a 5-mile long line, the dilution factor becomes(5 [mile] x 5,280 [ft./mile/300 [ft.] = 88). The dilution factor becomes 44 for a 2.5-mile long line.Assuming 2 splices per mile, a 5-mile line will have 10 splices. For a splice giving 10 ppm ofacetylene, the forced-cooled line would have approximately 0.11 (10/88) ppm of acetylene. Iftwo splices are equally involved, each yielding 10 ppm, this line will have .22 ppm of acetylene.


5-4

This illustrates the marked differences between static and forced-cooled lines. It follows that toidentify a problem splice, one has to shutdown the cable for several months.

Forced-cooled lines lend themselves more readily to the determination of the rate of gasgeneration. It is sufficient to sample a forced-cooled line at a single point along the line todetermine the average gas concentration; the inlet or outlet of the pump is a convenient above-ground location. However, for a static case, one has to sample each splice to identify the problemand establish trends. A problem on a cable length can be established by moving fluid as DGAsamples are taken for a static line. It is not possible to distinguish whether a splice or a cable isinvolved in a circulating line; only the stoppage of the fluid circulation can allow gasaccumulation at the problem spot.

The principle of the gas ratios holds for both static and forced-cooled cables. However, thepresence of splices complicates the picture. If one splice is yielding an equal amount of carbondioxide and carbon monoxide, this may not be true for the rest of splices, and this makes theapplication of ratios difficult. The levels of gases can provide significant clues to the condition offorced-cooled cables.

Tables 5-1 through 5-6 provide the normal, acceptable, concern and action levels together withtwo key gas ratios for various types of paper-insulated transmission cables and their accessories.The term normal means that the cable system is operating in a satisfactory manner. Such cablesare characterized by low gas concentrations, requiring long sampling intervals. The gasconcentrations in the acceptable condition are higher than the corresponding values for thenormal condition, as shown in column 3 of Table 5-1. Under these conditions, small amounts ofacetylene are present as well larger concentrations of other gases, however, such levels are lowenough not to pose any concern. The frequency of sampling should be increased to ensure thatthe cable system continues to be acceptable. The concern levels signify that the gasconcentrations have increased enough to pose concern, calling for a further increase in samplingfrequency. The schedule of sampling is discussed later. The action level category requiresprompt attention, ranging from close monitoring of the system to inspection and repairs. It isadvisable to consult with experts before the expensive decision to open the cable system is made.Because of the safety hazards associated with the potential shattering of porcelain, a terminationproblem requires prompt visual examination.

The gas levels and the ratios presented in these tables, according to the status of various types ofcable systems, are based on laboratory and field data. Based on the values of gas concentrationsgiven in tables 5-1 through 5-6, the DGA schedule for splices and terminations is presented withthe recommended action in Table 5-7.

While the laboratory studies sponsored by EPRI at Detroit Edison have been helpful, this guideis primarily based on DGA data predominantly generated on HPFF cable systems in the field.Because of the limited sampling of SCLF circuits, it has not been possible to refine the gasconcentrations for SCLF cable systems to the extent of HPFF cable systems. However, theconcept of carbon dioxide-to-carbon monoxide and ethane-to-ethylene ratio is equally applicableto both types of cable systems. Accordingly, the user is urged a degree of caution in theapplication of individual gas limits for concern and action levels for SCLF cable splices(Table 5-4), particularly the latter. While the gas levels for terminations of both HPFF and SCLF


5-5

are in a better agreement, it is not the case for acetylene concentration in the splices of both typesof systems. The significantly higher level of acetylene observed in SCLF cable splices isattributed to the very small volume of fluid between the lead sheath and the overall body of theinsulated splice. Unlike its HPFF splice counterpart, the fluid being sampled at the splice casingis not diluted, hence the higher concentration of acetylene for SCLF cable splices.

As mentioned earlier in Chapter 5, both the level and the overall pattern of individual gases isimportant for data interpretation. Accordingly, the data presented in each table should beconsidered in its entirety. However, the two key gases in order of importance are acetylene andhydrogen. While the gases are a direct consequence of electrical and thermal activity, DGAcannot identify the location of the gas generation source, particularly the electrical type, acrossthe cable cross-section. Thus, DGA will not readily identify if the source is confined to skid-wires for HPFF cables and the outer regions of the insulation of both HFF and SCLF cablesystems as opposed to the inner parts of the cable. It appears that the higher concentrations ofacetylene and hydrogen are confined not to the inner cable insulation but rather the outer layersand shielding. Every effort should be made to understand such a situation, which is not common,through regular DGA.


5-6

Table 5-1Gas Concentration Limits for Splices and Cable Runs of Static HPFF Cables

GasNormal Range

(ppm)Acceptable

(ppm)Concern Level

(ppm)Action Level

(ppm)

Hydrogen (H2) 0 – 1,000 < 10,000 10,000 – 40,000 40,000 +

Acetylene (C2H2) 0 < 1 1 – 5 5 +

C. Monoxide (CO) 0 – 300 < 500 500 – 1,000 1,000 +

C. Dioxide(CO2) 0 – 1,000 < 5,000 5,000 – 10,000 10,000 +

Methane (CH4) 0 – 400 < 1,000 1,000 –4,000 4,000 +

Ethane (C2H6) 0 – 300 < 500 500 – 1,000 1,000 +

Ethylene (C2H4) 0 – 100 < 200 200 – 500 500 +

Propane (C3H8) 0 – 500

Isobutylene (C2H8)

Polybutene Fluids 0 – 1,500 < 5,000 5,000 – 10,000 10,000 +

Alkylbenzene Fluids 0 – 100 < 500 500 – 1,000 1,000 +

Mineral Oils 0 – 200 < 1,000 1,000 – 2,000 2,000 +

Oxygen (O2) 0

Nitrogen (N2) 0 – 80,000

Gas Ratios 1

CO2/CO 5 – 10 > 1 0.75 – 1 < 0.5

C2H6/C2H48 – 10 > 1 0.75 – 1 < 0.5

(1) Ratios only apply to gas concentration levels larger than 50 ppm for carbon oxides and larger than20 ppm for hydrocarbons


5-7

Table 5-2Gas Concentration Limits for Terminations of HPFF Cables

GasNormal Range

(ppm)Acceptable

(ppm)Concern Level

(ppm)Action Level

(ppm)

Hydrogen (H2) 0 – 1,000 < 1,500 1,500 – 10,000 10,000 +

Acetylene (C2H2) 0 < 30 30 – 150 150 +

C. Monoxide (CO) 0 – 300 < 300 300 – 2,000 2,000 +

C. Dioxide(CO2) 0 – 1,000 < 5,000 5,000 – 10,000 10,000 +

Methane (CH4) 0 – 400 < 1,000 1,000 – 4,000 4,000 +

Ethane (C2H6) 0 – 300 < 500 500 – 1,000 1,000 +

Ethylene (C2H4) 0 – 100 < 200 200 – 500 500 +


Isobutylene (C2H8)



Mineral Oils 0 – 200 < 1,000 1,000 – 2,000 2,000 +

Oxygen (O2) 0

Nitrogen (N2) 0 – 80,000

Gas Ratios 1

CO2/CO 5 – 10 > 1 0.75 – 1 < 0.5

C2H6/C2H48 – 10 > 1 0.75 – 1 < 0.5



5-8

Table 5-3Gas Concentration Limits for Forced-Cooled HPFF Cables

GasNormal Range

(ppm)Acceptable

(ppm)Concern Level

(ppm)Action Level

(ppm)

Hydrogen (H2) 0 – 500 < 1,500 1,500 – 3,000 3,000 +

Acetylene (C2H2) 0 < 2 2 – 10 10 +

C. Monoxide (CO) 0 – 100 < 300 300 – 500 500 +

C. Dioxide(CO2) 0 – 500 < 1,000 1,000- 5,000 5,000 +

Methane (CH4) 0 – 200 < 500 500 – 1,000 1,000 +

Ethane (C2H6) 0 – 200 < 500 500 –1,000 1,000 +

Ethylene (C2H4) 0 – 50 < 100 100 - 500 500 +


Isobutylene (C2H8)

Polybutene Fluids 0 – 2,000 < 5,000

Alkylbenzene Fluids 0 – 200 < 500

Mineral Oil 0 – 200 < 500

Oxygen (O2) 0

Nitrogen (N2) 0 – 80,000

Gas Ratios (1)

CO2/CO 5 – 10 > 1 0.75 – 1 < 0.5

C2H6/C2H48 – 10 > 1 0.75 – 1 < 0.5



5-9

Table 5-4Gas Concentration Limits for SCLF Cable Splices

GasNormal Range

(ppm)Acceptable

(ppm)Concern Level

(ppm)Action Level

(ppm)

Hydrogen (H2) 0 – 600 < 1,000 1,000 – 3,000 3,000 +

Acetylene (C2H2) 0 1- 5 5 – 25 25 +

C. Monoxide (CO) 0 – 300 < 500 500 – 1,000 1,000 +

C. Dioxide(CO2) 0 – 1,000 < 5,000 5,000 – 10,000 10,000 +

Methane (CH4) 0 – 400 < 1,000 1,000 – 4,000 4,000 +

Ethane (C2H6) 0 – 300 < 500 500 – 1,000 1,000 +

Ethylene (C2H4) 0 – 100 < 200 200 – 500 500 +


Isobutylene (C2H8)



Mineral Oils 0 – 200 < 1,000 1,000 – 2,000 2,000 +

Oxygen (O2) 0

Nitrogen (N2) 0 - 80,000

Gas Ratios 1

CO2/CO 5-10 > 1 0.75 – 1 < 0.5

C2H6/C2H48-10 >1 0.75-1 <0.5



5-10

Table 5-5Gas Concentration Limits for Terminations of SCLF Cables

GasNormal Range

(ppm)Acceptable

(ppm)Concern Level

(ppm)Action Level

(ppm)

Hydrogen (H2) 0 – 1,000 < 2,000 2,000 – 5,000 5,000 +

Acetylene (C2H2) 0 – 2 < 5 5 – 50 50 +

C. Monoxide (CO) 0 – 300 < 500 500 – 1,000 1,000 +

C. Dioxide(CO2) 0 – 1,000 < 5,000 5,000 – 10,000 10,000 +

Methane (CH4) 0 – 400 < 1,000 1,000 –4,000 4,000 +

Ethane (C2H6) 0 – 300 < 500 500 – 1,000 1,000 +

Ethylene (C2H4) 0 – 100 < 200 200 - 500 500 +


Isobutylene (C2H8)



Mineral Oils 0 – 200 < 1,000 1,000 – 2,000 2,000 +

Oxygen (O2) 0

Nitrogen (N2) 0 – 80,000

Gas Ratios (1)

CO2/CO 5 – 10 > 1 0.75 – 1 < 0.5

C2H6/C2H48 – 10 > 1 0.75 – 1 < 0.5



5-11

Table 5-6Gas Concentration Limits for HPGF Cables (200 psi Nitrogen)

GasNormal Range

(ppm) (1)Acceptable

(ppm)Concern Level

(ppm) 1Action Level

(ppm) 1

Hydrogen (H2) 0 – 75 < 700 700 – 2,200 2,200 +

Acetylene (C2H2) 0 < 0.07 0.07 - 0.4 0.4 +

C. Monoxide (CO) 0 – 15 < 75 75 – 150 150 +

C. Dioxide(CO2) 0 – 75 < 150 150 – 350 350 +

Methane (CH4) 0 – 30 < 75 75 – 300 300 +

Ethane (C2H6) 0 – 15 < 40 40 – 75 75 +

Ethylene (C2H4) 0 – 80 < 15 15 – 40 40 +


Isobutylene (C2H8)

Polybutene Fluids 0 – 100 < 350 350 - 750 750 +

Alkylbenzene Fluids 0 – 10 < 40 40 - 100 100 +

Mineral Oils 0 – 20 < 80 80 – 200 200 +

Oxygen 0

Gas Ratios 2

CO2/CO 5 – 10 > 1 0.75 – 1 < 0.5

C2H6/C2H48 – 10 > 1 0.75 – 1 < 0.5

(1) Gas concentrations must be multiplied by the ratio of cable operation pressure in psi to atmosphericpressure in psi. The numbers given in this table have already been corrected



5-12

Table 5-7DGA Schedule for Splices and Terminations

Accessory Normal Concern Action

Splice 2 to 4 years,depending on cablevoltage class

6 months to 1 year Establish whether or not the splice is thesource of gas. Drain fluid to obtain samplefrom the cable, both sides if possible. Twopumping plants or one pumping plant anda return line enable inspection of bothsides through fluid movement. Consultwith experts before opening the splice.

Cable Run 1 to 2 years, if aproblem has beenidentified

6 months to 1 year Consider changing cable length inquestion

Termination 2 to 4 years 6 months Open termination for visual inspection/rebuild.

To further enhance the interpretation of DGA data, a code combining gas levels and ratios hasbeen developed based on over 6,000 field samples relating to cables splices, cable runs,trifurcators, reservoirs and terminations. This Code basically holds for static HPFF cablesystems, which form the bulk of U.S. paper cable systems. While the code is helpful for SCLF,HPGF and forced-cool cable systems, it is recommended to refer to the applicable Tables, 5-1through 5-6 for such cable systems.

Similar to the code developed originally by Rogers for transformers and later modified in theC57.104 IEEE Standard for transformer DGA data interpretation, this Guide contains six digits.Compared to transformers, cables operate in an oxygen-free, sealed system from which gasescannot escape. Cables also operate at significantly higher electrical stresses than transformersand the designs, materials, operating conditions and fluid volumes are markedly different in thetwo products. Accordingly, the type, distribution and concentration vary significantly, withcables showing much larger gas concentrations. For instance, hydrogen levels close to100,000 ppm have been occasionally observed for HPFF cables, however, this gas rarely exceeds2,000 ppm in transformers. Likewise, acetylene levels over 150 ppm have been observed forin-service HPFF cable systems, a situation uncommon for transformers.

The code presented in this work has six digits. The definition of each digit, according to theseverity of the problem, is given in Table 5-8 below:


5-13

Table 5-8Gas Concentrations and Ratios Included in the Six-Digit Code for the Interpretation ofDissolved Gas Analysis of HPFF Cables and Accessories

Digit Description Level Magnitude

1 Acetylene Concentration (ppm)Acceptable

123

Splices: < 1 = 01 < 5 = 15 <10 = 2=>10 = 3

Terminations:0 < 30 = 030 < 100 = 1100 < 150 = 2 >= 150 = 3

2 Ethylene/Ethane Ratio Acceptable123

< 0.5 = 00.5 < 1.0 = 11.0 < 2.0 = 2 => 2.0 = 3

3 Hydrogen/Methane Ratio Acceptable1

< 4 = 0=> 4 = 1

4 Hydrogen Concentration (ppm) Acceptable123

< 5,000 = 05,000 < 10,000 = 110,000 < 25,000 = 2=> 25,000 = 3

5 Total concentration of saturatedhydrocarbons and isobutylene. Amongthe saturated HC are: methane, ethane,propane and butanes (ppm)

Acceptable12

< 2,500 = 02,500 < 5,000 = 1=> 5,000 = 2

6 Carbon Dioxide/Carbon Monoxide Ratio Acceptable1

> 1.4 = 01 <= 1.4 = 1< 1 = 2

The utilization of gas concentrations and gas ratios given in Table 5-8 offers the assessment ofthe condition of cable system along with the severity of the problem involved, as follows:

Digit One

Acetylene concentration forms the first digit of this code. Acetylene concentrations under 1 ppmin splices and cable runs or under 30 ppm in terminations are deemed acceptable. However,acetylene should not normally be present in splices and cable runs, and it is often the case. Anincrease in acetylene concentration with time signifies a potential problem. Compared to splices,the higher acetylene concentration limits for terminations results from the nature of the electricalfield, over-voltage conditions experienced by terminations, limited fluid volume associated withterminations, and the restricted fluid movement.

Acetylene is produced in discharges involving arcing. Acetylene can also be generated bythermal exposure alone, provided the temperature reaches over 600°C, which is not practical forcables. Discharges can take place external to the cable insulation such as skid-wires, looseground connections, arcing over insulated flanges, and circulating currents. In these cases, the


5-14

presence of acetylene is not associated with the insulation condition, but can eventually lead tofailure.

Digit Two

Digit two represents the magnitude of the ratio between ethylene and ethane. A ratio rangingfrom 0.1 to 0.125 means a normal condition. The increase of this ratio up to 0.5 represents anacceptable condition, the assigned code being 0. As this ratio exceeds 0.5, codes of 1, 2, and 3with the corresponding ratio ranges are provided, Table 5-8.

Ethylene is produced by the thermal decomposition of the dielectric fluid. This gas can begenerated in fairly large concentrations at temperatures above 150°C. However, suchtemperatures are not likely in cable systems. Ethylene observed in cable systems resultsprimarily from electrical discharges that expose minute fluid volumes to elevated temperatures.As the intensity of the discharge increases i.e., as more current is involved, both the temperatureand volume of the affected fluid increases and so does the yield of ethylene.

Ethylene-to-ethane ratios close to, and over 1, are a cause of concern, except for cases where theconcentrations of both ethane and ethylene are so small (under 10 ppm) that a high ratio canresult from an error in analysis. To overcome this situation, digit 5 involving the total amount ofsaturated hydrocarbon gases is utilized.

Digit Three

Digit three represents the hydrogen to methane ratio. This ratio has been introduced todistinguish between electrical and thermal origin of gases. A high ratio with more hydrogen thanmethane generally indicates the presence of partial discharges, where a highly limited generationof heat is involved. The opposite is expected from purely thermal effects.

Digit Four

Digit four represents the total concentration of hydrogen. Compared to transformers wherehydrogen seldom exceeds 2000 ppm, HPFF cable systems yield large concentrations ofhydrogen. Hydrogen concentration of 10,000 ppm is acceptable for splices and cables. Startingwith zero code corresponding to 5000 ppm of hydrogen, codes 1 though 3 have been presented tocover hydrogen levels up to 25,000 ppm and above. It should be noted that extraordinarily largehydrogen concentrations of the order of 100,000 ppm have been observed for cable runs andsplices of HPFF cable systems in some cases.

Although the generation of hydrogen is attributed to low level partial discharges in fluid resultingin large hydrogen concentrations, such an activity apparently does not impact the cable life.Nevertheless, the presence of large amounts of hydrogen should be carefully addressed, withperiodic monitoring. The appearance of acetylene, together with large concentration of hydrogen,is an indication of increased discharge activity. It is noteworthy that hydrogen is also evolved asa corrosion product of iron. Both moisture and oxygen are required to produce hydrogen; theformer should be measured when large concentrations of hydrogen are involved utilizing a glassbottle as opposed a glass syringe so that the settling of water can be observed. This situation has


5-15

been encountered for a pumping plant reservoir with rusted fittings, and a HPFF cable withmoisture content above 110 ppm.

Digit Five

Code five indicates the level of total saturated hydrocarbon gases. The hydrocarbons included inthis code are methane, ethane, propane, iso and normal butane and isobutylene. Isobutylene isnot a saturated hydrocarbon but this gas can be found in large amounts in polybutene fluids forwhich it forms the starting material.

It is common to find large amounts of saturated hydrocarbons in fluid samples taken from earlyvintage cables. These hydrocarbons were used in varying distributions by the supplier as part ofthe blanket gas during storage and/or handling of these fluids. It is also likely that older cablesdid not utilize properly degassed fluids, resulting in high concentration of such saturatedhydrocarbons. Thus digit five has been introduced to distinguish such cases.

Digit Six

The last digit, namely Code 6, represents the ratio between the concentration of carbon dioxideand carbon monoxide. Thermal decomposition of paper evolves large amounts of carbon oxides,however, the concentration of carbon dioxide is always larger than that of carbon monoxide.Although age and temperature can decrease this ratio, it is always greater than one based solelyon thermal considerations.

Compared to carbon dioxide, the presence of electrical activity can increase the yield of carbonmonoxide, resulting in a ratio smaller than one. This has been observed in cases where thermalmechanical bending (TMB) is suspected in cable splices. In addition, laboratory investigationsunder EPRI sponsorship have shown that the yield of carbon monoxide is always higher than thatof carbon dioxide when paper is exposed to electrical discharges. A CO2/CO ratio below onecombined with the presence of acetylene indicates that both paper and fluid are being affected byelectrical discharges.

The six digits along with the corresponding codes with the increasing order of severity of theproblem faced by the cable are given in Table 5-9. The propagation of a failure has beenfollowed through this coding system for a 345 kV HPFF cable that failed at Waltz Mill,Table 5-10. The pathway shown in Table 5-10 indicates a continuous increase in digits 1 and 2.Digit 3 remains at 1, indicating that the problem is of electrical nature and no overheating isinvolved. The level of hydrogen in this cable did not show a significant increase. The level oftotal saturated hydrocarbons was low as this fluid was thoroughly degassed before filling thepipe. A significant decrease in the CO2/CO ratio was observed as the cable approached failure.However, due to the unusually high background of carbon dioxide, this ratio could not becomeless than the first cutoff point of 1.4 as shown in Table 5-8. The large concentration of carbondioxide originated from the high temperature utilized to accelerate the aging of the test cable.Tables 5-11 and 5-12 demonstrate the application of the interpretation code system to splices andterminations for several HPFF systems.


5-16

Table 5-9Fault Diagnosis of Cables Through Dissolved Gas Analysis

Digit-1 Digit-2 Digit-3 Digit-4 Digit-5 Digit -6

C2H2

C2H4

C2H6

H2

CH4 H2 TSHCCO2

CO Comments

0 to 1 0 0 to 1 0 0 0 Normal

0 0 0 0 1 to 3 0 Normal, hydrocarbon contamination from originaloil

1 1 1 0 0 1 Acceptable low intensity discharge activity

0 2 to 3 1 0 0 0 to 1 Acceptable, reversed ethane/ethylene ratio,repeat analysis

1 0 0 0 0 0 Acceptable, low level acetylene

1 1 0 0 2 1 Low level discharge activity with saturatedhydrocarbon contamination

1 1 1 0 0 0 Low intensity discharge activity

1 2 to 3 0 to 1 0 0 0 Discharges in oil

2 1 to 3 0 to 1 0 0 0 High acetylene, strong discharge activity

3 2 to 3 0 0 0 0 Arcing

3 2 0 2 to 3 2 1 Sustained strong arcing activity

3 3 1 0 0 0 Discharges in oil

4 2 1 0 0 0 Arcing in oil (skid wires)

4 3 0 to 1 0 2 0 Strong or sustained arcing activity

Table 5-10Six Digit Code Showing Arcing Pathway to a 345 kV Cable Failure

Digit-1 Digit-2 Digit-3 Digit-4 Digit-5 Digit-6

Sample C2H2

C2H4

C2H6

H2

CH4 H2 TSHCCO2

CO

13 month 0 0 1 0 0 0

15 month 1 0 1 0 0 0

17 month 1 1 1 0 0 0

18 month 1 2 1 0 0 0

18 month (failure) 4 3 1 0 0 0*

* Excessive CO2 background to reach cutoff point


5-17

Table 5-11Example of Application of Condition Assessment Code to HPFF Cable Termination – DGA Data

Voltage Class 345 kV 138 kV 138 kV 345 kV 345 kV 138 kV 138 kV 345 kVCooling Type Static Static Static FC Static Static Static StaticSample date Dec-89 Apr-93 Sep-95 Sep-96 Nov-89 Jan-95 Apr-93 Sep-92

Gases (ppm)Methane 102 156,906 18,561 673 777 81,848 159,898 3,530Ethane 134 57,752 3,608 283 172 13,691 60,626 596Ethylene 54 384 2,597 305 201 14,494 407 1,620Acetylene 45 10 20 65 118 113.8 146 182Propane 155 22,687 3,887 427 201 14,027 22,236 463Propylene 46 33 7,022 326 110 31,351 35 1,442Isobutane 68 6,411 1,953 469 93 7,895 6,755 606Nbutane 99 3,023 551 225 46 1,455 3,174 296Isobutylene 98 28,030 22,519 1,425 149 81,704 29,588 2,106Hydrogen 783 858 2,769 919 2,018 22,321 975 3,499C. Monoxide 159 21 335 291 321 820 26 488C. Dioxide 828 115 903 413 857 1,009 116 403

Ratios:C2H4/C2H6 0 0 1 1 1 1.06 0 3H2/CH4 8 0 0 1 3 0.27 0 1Sat. + Isobutylene 557 271,787 50,529 3,277 1,392 199,166 279,103 7,301CO2/CO 5.21 5.58 2.69 1.42 2.67 1 4.44 0.83

Codes:Digit-1 (C2H2) 0 0 0 1 2 2 2 3Digit-2 (C2H4/C2H6) 0 0 1 2 2 2 0 2Digit-3 (H2/CH4) 1 0 0 1 1 0 0 0Digit-4 (H2) 0 0 0 0 0 2 0 1Digit-5 (Tsat) 0 2 2 1 0 2 2 2Digit-6 (CO2/CO) 0 0 0 0 0 1 0 2Condition Normal

terminationNormal

terminationwith original oilcontamination

Normal terminationwith original oil

contamination andhigh

ethylene/ethaneratio

Discharges inoil

Strongdischarge

activity

Sustainedarcing activity

Discharges inoil

Sustainedarcing activity


5-18

Table 5-12Example of Application of Condition Assessment Code to HPFF Cable Splice – DGA Data

Voltage Class 138 kV 138 kV 230 kV 138 kV 138 kV 220 kV 138 kV 345 kVCooling Type Static Static Static Static Static Static Static StaticSample Date Sep-91 Mar-95 Nov-98 Dec-94 Mar-92 May-99 Dec-96 Jun-88

Gases (ppm)Methane 46 76 253 4,204 44,731 0 12 20,776Ethane 6 65 65 1,329 16,524 4 23 1,074Ethylene 3 84 225 12 0 3 47 1,387Acetylene 1 0 0 0 0 0 14 147Propane 9 132 45 588 4,809 11 43 309Propylene 4 67 126 101 34 3 83 820Isobutane 1 4 4 425 919 1 7 196Nbutane 2 159 91 140 1,140 3 17 104Isobutylene 7 18 66 6,130 41,554 15 44 2,335Hydrogen 75 1,786 108 2,929 1,198 24,355 0 14,278C. Monoxide 27 474 18 156 167 32 7 1,068C. Dioxide 156 8,402 119 706 242 180 38 1,042

Ratios:C2H4/C2H6 0 1 3 0 0 1 2 1H2/CH4 2 23 0 1 0 0 1Sat. + Isobutylene 69 296 432 12,676 108,537 32 130 24,690CO2/CO 5.78 17.73 6.72 4.51 1.45 5.70 5.80 0.98

Code:Digit-1 (C2H2) 0 0 0 0 0 0 3 3Digit-2 (C2H4/C2H6) 0 2 2 0 0 1 2 2Digit-3 (H2/CH4) 1 1 0 0 0 0 0 0Digit-4 (H2) 0 0 0 0 0 2 0 2Digit-5 (Tsat) 0 0 0 2 2 0 0 2Digit-6 (CO2/CO) 0 0 0 0 0 0 0 2Condition Normal Acceptable,

reversed ratioethane/ethylene

Acceptable,reversed ratio

ethane/ethylene

Acceptable,possible

contaminationof original oil

Acceptable,possible

contaminationof original oil

H2 evolutionfrom low levelPD activity or

metal corrosionby free water

Discharges inoil

Arcing

6-1

6 FLOW-CHART ON SAMPLING PROCEDURE,PROBLEM LOCATION AND DGA INTERPRETATION

The application of DGA depends on three inter-dependent steps, namely, fluid sampling,chemical analysis and interpretation of the generated data. Accordingly, the success of DGA isgoverned by the proper execuation of each step. A knowledge of cable operating history relatingto repairs, fluid leaks, make-up fluid, initial pipe fluid quality and presence of any hotspots isimportant for data interpretation. Improper interpretation can lead to wrong decisions that can beexpensive.

The various aspects on these three steps have been addressed in a comprehensive fashion inprevious chapters; tables, illustrations and photographs are an integral part of this coverage. Theentire process of DGA is summarized in a flow-chart, which covers the sampling procedures onsplices, terminations, static versus forced-cooled cables, and frequency of sampling, Figure 6-1The approaches to locate the source of problem in splices, terminations and cable runs throughfluid drainage have been included in the flow-chart. While this flow-chart contains all theimportant details in a sequential fashion, the user should refer to the report for backupinformation to make sound decisions on cable system condition and follow-up action(s).

The flow-chart is fully applicable to HPFF cable systems, both static and forced-cooled. It holdsequally for splices and cable runs of SCLF cables, with the exception of SCLF terminations.Unlike HPFF terminations, the design of SCLF terminations complicates the coverage of theentire termination length through fluid drainage from the top as the termination fluid and thehollow core conductor fluid are in contact at this location. However, if a bottom valve isprovided for a SCLF termination, fluid drainage is possible. Because of the limited fluid supplyin SCLF cables, it is recommended to perform only two fluid drainages for SCLF terminations, ifa bottom valve is available. It should be ensured that the connector needle valve is open whenfluid is drained from the bottom valve. The flowchart applies to the splices and terminations ofHPGF cables. However, the process of gas drainage to locate the problem source at a distance isnot applicable to HPGF cables due to the high diffusion rate of the generated gases along thepipe in such cables, as opposed to HPFF cable systems.

Flow-Chart on Sampling Procedure, Problem Location and DGA Interpretation

6-2

Start

Ident i fy cable system(stat ic or forced-cooled, no. of spl ices & pumping

plant(s); cable history, etc)

Valve on al l spl ices?

Instal l valvesNo

Yes

Sample sp l ices(wi thout cable shutdown)

Take two samples a t each locat ion

Deenergize cable for terminat ionsampl ing

Bot tom va lveavai lable ?

No

Yes

Forced-coo led?

Sample at a s ingle locat ion(c i rculat ing pump)

No

Yes

1st sample f rom bot tom va lve wi thoutdra inage, 2nd and 3rd samples f rom top

valve wi th 2 - 4 ga. dra inage

1st sample f rom top va lve wi thoutdra inage, 2nd and 3rd samples f rom

top valve wi th 2 - 4 ga. dra inage

Cont inued on next page

Figure 6-1Flow-Chart for Fluid Sampling and DGA Interpretation

Flow-Chart on Sampling Procedure, Problem Location and DGA Interpretation

6-3

Results for spl ices

Gases Acceptable?

Yes

Repeat sampl ing in 2 to 4 years

N o

Locate gas source byf lu id movement

Results for terminat ions

Gases Acceptable?

Yes

Repeat sampl ing in 2to 4 years

Sample tr i furcator

N o

Gases Acceptable?

Problem in terminat ion Yes

N o

Problem is the in raiser orfurther away in the pipe

Move f luid to local izesource

Consult with experts

Consult with experts

Concentrat iondecreases wi th

drainage

Yes

Gas source in spl ice

N o

Gas source in cable run

Per form DGA

From previous page

Figure 6-1Flow-Chart for Fluid Sampling and DGA Interpretation (cont.)

7-1

7 CONCLUSIONS

This guide is based on laboratory and field data generated by Detroit Edison since 1986. Whilethe former is important to understand the role of the electrical and thermal stresses involved ingas generation from fluid and cellulosic paper, it is the field data that serves as the foundation forthe guidelines to interpret DGA data. Since the vast majority of the field data has been generatedin the U.S., it involves predominantly HPFF cable systems. Accordingly, the guidelines providedfor SCLF cable systems, particularly the splices, should be used with care. As more field databecomes available, it will be prudent to update this document, as is regularly done fortransformers. Nevertheless the present document covers its intended function, providing the userwith sampling and analysis procedures along with guidelines to interpret the field data.

The potential of DGA is being recognized and U.S. utilities are increasingly applying DGA toassess the condition of paper-insulated transmission cable systems in service. DGA can readilydistinguish between a satisfactory and a problem cable system. While it can assess the severity ofthe problem, DGA data cannot, like any other diagnostic technique, assess the immediateconsequences of the problem, particularly problems associated with cable runs, whichfortunately are not common.

A-1

A GLOSSARY

Alkylbenzene dielectric fluids: Synthetic fluids containing a paraffinic hydrocarbon (alkyl)group attached to a benzene ring. The alkyl group can be either linear or branched. Linear groupscan more easily be degraded by micro-organisms and, therefore, are preferred over branchedgroups. Examples of alkylbenzenes used in cables are dodecylbenzenes and tridecylbenzenes.Alkylbenzene fluids are characterized by low viscosity.

DGA: Dissolved gas analysis, this term refers to the identification and quantification of gaseouscomponents in a dielectric fluid sample, usually through gas chromatography.

Degassing: Removal of gases from a fluid by exposure to vacuum and heat. Elevatedtemperatures are applied to reduce the fluid viscosity and moisture content. A lower fluidviscosity facilitates the degassing process.

Dissolved gas: This term indicates that the gas forms a liquid solution in the complete absence ofa gas phase (bubbles). A gas will form a liquid solution only if the pressure applied to thesolution is greater than its vapor pressure. The total amount of gas that can dissolve in a fluiddepends on the pressure and temperature. For most gases, solubility increases linearly withpressure. According to the nature of the gas, solubility can increase, decrease or remainunaffected by temperature.

EDOSS: Abbreviation for EPRI Disposable Oil Sampling System.

EDOSS Method: DGA performed with EDOSS vials by headspace chromatography.

EPOSS: abbreviation for EPRI Pressurized Oil Sampling System.

EPOSS Method: DGA performed with EPOSS cells by headspace chromatography.

Gas chromatograph (GC): Device utilized to separate mixtures of gases or vapors. Briefly, thisconsists of a controlled temperature oven to support the separation column(s), a carrier gas & itsflow control system, a sample injection system and a detection system. In the case of gassamples, the sample is introduced into a fixed volume loop contained in a remotely actuatedmulti-port valve. Upon the start of the analysis, the sample is forced through the separationcolumn(s) with an inert carrier gas. The retention time of a gas in the separation column isgoverned by the nature of the gas and its affinity to the column-packing material. Thus gases areidentified by their retention times. The separated gaseous components exit the column and enterthe detection system. Calibration of the system is made with certified gas blends containing thecomponents of interest.

Glossary

A-2

Gas in Liquid Solubility: Ability or tendency of a gas to dissolve uniformly in a liquid. Thedissolved gas does not behave as a gas but as a liquid. It has, therefore, no buoyancy and will notaccumulate in terminations or other elevated locations along a cable system. Bubble formationcan only occur when the solution becomes over-saturated as the system pressure drops below thesaturated solution pressure.

Headspace Chromatography: In this technique, the gas phase components are related to thegases and their concentrations in the liquid phase. This enables the determination of the levels ofgases in the original liquid.

HPFF: Abbreviation for High-Pressure Fluid-Filled Cables. It is a cable system having threeimpregnated paper-insulated conductors, corresponding to the three phases, placed in a commonsteel pipe. A high pressure (200 psi) fluid is utilized to fill the pipe. Because of the large volumeof fluid involved, large storage tank and pressure control units with pumps and relief valves areneeded to accommodate fluid expansion and contraction.

HPGF: Abbreviation for High-Pressure Gas-Filled cables. This is a variation of HPFF cableswhere high-pressure nitrogen (200 psi), instead of a fluid (a liquid dielectric), is utilized to fillthe steel pipe. These cables do not require permanently connected gas reservoirs or pumpingplants; nitrogen is supplied to maintain the pressure as needed.

Hydrocarbons: Organic compounds consisting exclusively of the elements of carbon andhydrogen. These can be organized into three broad classes: aliphatics (paraffins, olefins,acetylenes), alicyclic (naphthenes) and aromatics.

Over-saturated gas solution: If the vapor pressure is larger than the pressure applied over thesolution, the solution becomes over-saturated and a gas phase will separate in the form ofbubbles.

Peroxides: Peroxides are organic compounds containing an -O-O- group in their structure. Inhydrocarbon fluids, the formation of peroxides starts with the break up of the hydrocarbonbackbone (R - R' ! R• + •R'), followed by radical combination of the broken piece withO2 (R• + O2 ! ROO•). Subtraction of H from a neighboring molecule can lead to the formationof a hydroperoxide (ROO• + •H ! ROOH), or reaction with another R' fraction can lead to aperoxide (ROO• + •R' ! ROOR'). The stability of these compounds strongly depends on thestructure of R• and/or •R'.

Polybutenes dielectric fluids: Synthetic polyolefin resulting from the polymerization ofisobutylene gas. The polybutene structure contains a straight chain with alternating (CH3)2Cgroups and a double bond at the end of the chain. Also known as polyisobutylene orpolyisobutene fluids.

ppm: Abbreviation for parts per million. This expression is the ratio of the volume of a solute(gas in this case) to the total volume of the solution or the weight of the solute to the total weightof solution, multiplied by 1x106.

Glossary

A-3

Saturated gas solution: If the pressure over the solution equals the vapor pressure of the gas,the solution is said to be saturated. A saturated solution will not uptake more gas withoutforming bubbles.

Saturated hydrocarbons: Hydrocarbons without double bonds or cycles in their structure.Straight-chain paraffins are typical saturated compounds

SCLF: Abbreviation for Self-Contained Liquid-Filled cables. It represents the earliestimpregnated paper-insulated transmission cable, which was developed in the early 1920s. SCLFcable is comprised of a single hollow conductor impregnated paper-insulated cable with a lead oraluminum sheath serving as moisture barrier and mechanical protection. In present day practice,the sheath is covered with a polymeric jacket. SCLF cables are generally installed in ducts butcan be directly buried. Fluid pressure is maintained through the hollow conductor by means ofpressurized tanks, which also accommodate the expansion and contraction of the fluid.Submarine SCLF cables frequently require pumping plants at both ends.

Unsaturated hydrocarbons: Hydrocarbons that contain cycles and/or double or triple bondsbetween their carbon atoms, for example, acetylene, ethylene and propylene.

Mineral Oils: Liquid petroleum derivatives containing complex mixtures of paraffinic,naphthenic and aromatic hydrocarbons with a specified range of boiling points and viscosities.

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Target:

Underground Transmission

EPRI • 3412 Hillview Avenue, Palo Alto, California 94304 • PO Box 10412, Palo Alto, California 94303 • USA800.313.3774 • 650.855.2121 • [email protected] • www.epri.com

About EPRI

EPRI creates science and technology solutions for

the global energy and energy services industry. U.S.

electric utilities established the Electric Power

Research Institute in 1973 as a nonprofit research

consortium for the benefit of utility members, their

customers, and society. Now known simply as EPRI,

the company provides a wide range of innovative

products and services to more than 1000 energy-

related organizations in 40 countries. EPRI’s

multidisciplinary team of scientists and engineers

draws on a worldwide network of technical and

business expertise to help solve today’s toughest

energy and environmental problems.

EPRI. Electrify the World

Documents

Cables - Guidelines for the Interpretation of DGA for Paper-Insulated Underground Transmission Cable Systems