Insulation Coordination Linhas Aereas

8/10/2019 Insulation Coordination Linhas Aereas

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AbstractInsulation coordination methodologies are analyzed

and a program built by the author is presented, which executes

the insulation coordination algorithm proposed by the standards

of the International Electrotechnical Comission (IEC). Also

resulting from this analysis, a methodology for compacting

transmission lines is presented, where alternative tower

configurations are proposed and the benefits of this action are

analyzed.

The general analysis of the insulation coordination

methodology is carried out by comparison of the methodologies

presented in the international standards (IEC and IEEE).In what concerns the specific case of line insulation, a more

detailed analysis is made of the insulation coordination

methodology presented in the IEC standards and the Insulation

Coordination Guide provided by REN (national electric grid

operator, former EDP). This analysis begins to focus on the

dielectric stresses that the line insulation must stand, then the

strength provided by the insulation as regards the previously

presented stresses, and finishes with the methods used in the

conjugation of these two factors.

Since this dissertation is concerned about overhead

transmission lines, whose major insulation component is air, an

analysis of the required distances between the variety of

components that constitute a transmission line tower is also made.

For that purpose, beyond the attention given to the evolution of

the models that characterize the dielectric breakdown, the real

distances implemented in the transmission lines owned by REN

are also analyzed.

I ndex TermsInsulation coordination, Overhead transmission

lines, Line compaction, Dielectric breakdown of long air gaps,

Overvoltages, Insulation Standards.

I. INTRODUCTION

NE of the most important requirements of any electrical

equipment or system refers to its capacity of insulating its

conductive parts against the exterior environment. Beyond the

risks that insulation failures may cause on humans and

animals, a correct insulation of an electrical equipment is

essential for an acceptable performance.

However, the insulation of an electric system is a complex

task, which requires the interoperability of several types of

insulator types - solid, liquid or gaseous. To this necessity of

using several insulator types with the objective of containing

electric current, is called insulation coordination.

Due to the non-ideality of all dielectric materials, this

practice is particularly important in high voltage environments,

where the consequently high electric fields may cause

modifications in the properties of the insulation material.

The electric system, mostly its transport and distribution

components, are forced by their function to use high working

voltages. Therefore, because of the expanse of their use, air is

the most used type of insulation, complemented in the towers

by solid insulators. However, the dimensioning of the air

clearances are crucial, since air may become conductive when

exposed to extreme electric fields, causing line faults that

affect negatively their normal operation.In this context, this dissertation proposes to establish the

insulation coordination methodologies used on overhead

transmission line design.

II.

METHODOLOGIES AND STANDARDS OF INSULATION

COORDINATION

This chapters main objective is to analyze the generalmethodology of insulation coordination, by comparing the two

most widespread standards of this matter: the IEC 60071-1 [1]

and IEEE Standard 1313.1 [2].

In this analysis, it was concluded that despite of thedisagreement in the nomenclature of several variables with

similar or even equivalent definitions, we can identify a

common line of action between those two. In both standards,

the first step consists in the analysis of the system and in the

identification of the representative overvoltages that best

characterize it. Then, the second step evaluates the strength

that the dielectric material presents to the overvoltage stresses

and how the relation between those two contribute to the line

performance. The last step takes the result from the previous

one, and returns the withstand voltage of the equipment as the

standardized value that best fits it.

Next, a more detailed analysis of these three steps will bepresented.

A. Determination of the representative overvoltages of thesystem

This step analyzes the more relevant dielectric stresses that

occur in the system, as well as the limitation and protection

devices necessary for the desired line performance.

Therefore, the systems voltage stresses are separated infour main classes.

Insulation Coordination of Overhead

Transmission Lines

Miguel Carlos Valentim do Rosrio, n 56766

O


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Temporary overvoltages;

Slow front overvoltages (switching overvoltages);

Fast front overvoltages (lightning overvoltages);

Longitudinal overvoltages.

B. Comparison of overvoltages and insulation strength

Considering the previous results from the voltage stresses,

this chapter takes in consideration several factors that

influence the choice of the insulation. These factors aresimilarly defined in both standards, and represent the several

uncertainties that characterize the system:

Performance (acceptable number of insulation

failures);

Statistical nature of test results;

Deviation of the characteristics of the insulators

production and installation;

Ageing;

Different shapes of overvoltages, different from the

standardized ones;

Real atmospheric conditions, different from the

standardized ones; Analysis precision.

C. Standardized results

Both standards present the standard insulation level as the

main output of the insulation coordination process. This

standard insulation level is the combination of:

The maximum system voltage;

Set of standard rated values of withstand voltages

that best fit and characterize the insulation, for

each of the overvoltage class that stresses the

system.

III. INSULATION COORDINATION ON OVERHEAD TRANSMISSION

LINES

In this chapter, a more detailed analysis is made of the

insulation coordination methodology, when applied to

overhead transmission line design. With this objective in mind,

another two standards will be compared and analyzed: the IEC

60071-2 [3] and the Insulation Coordination Guide from REN

[4].

This analysis will firstly focus on the dielectric stresses, then

on the dielectric strength of the insulation, followed by the

application of insulation coordination methods that relate both

of these factors, concluding with the air clearances proposed in

both standards.

A. Standard dielectric stresses

Both standards divide the main representative dielectric

stresses in equivalent classes, already analyzed in chapter II.

However, the time lag between these two guides results in

different designations of the classes, as well as different

expected values for the statistical overvoltages. With the

natural technological evolution, since the last version of

RENsguide, theres been an intensification in the use of SF6

in substations, which led to the development of the very fastfront overvoltagesclass, and consequently to a reorganizationof the overvoltage classes.

B. Standard dielectric strength

In what concerns the dielectric strength, the two guides

characterize the accumulated probabilistic distribution of the

dielectric breakdown with very different expressions. IEC uses

a modified Weibull distribution, while REN uses a Gaussian.The differences are negligible, and never higher than 5%.

However, the standard deviation used by these standards is

different. IEC uses a range of 3%-6%, while REN uses 6%-

8%, for disruptions caused by atmospheric and switching

overvoltages, respectively.

The expressions used to relate the U50voltage with the airclearance are also different, but both produce similar results,

always with differences below 4%. In figures 1 and 2, a

comparison is presented between the expressions used for each

standard.

Fig. 1Comparison between expressions of U50(d)for switchingovervoltages

Fig. 2 - Comparison between expressions of U50(d)for lightningovervoltages

C. Insulation coordination method application

Both guides suggest the same insulation coordination

methods, although each one applies a different insulation

coordination factor. The differences found in the deterministic

method are exposed in table 1.


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Table 1Comparison of deterministic insulation coordination factors

IEC REN

Switching 1 1,2: 72,5 kV220kV1,15: 420 kV

Lightning 1 1,2: 72,5 kV220kV1,25: 420 kV

From table 1 its possible to verify that the values proposedby IEC are clearly less conservative than the ones from REN.

Although there are no differences in the probabilistic

method in the two guides, the simplified probabilistic method

is suggested in each standard with different values for the

statistical insulation coordination factor. These differences are

presented in figure 3.

Fig. 3Comparison between statistical insulation coordination factors

D. Standard air clearances

Both guides present mainly different types of air clearances,

as the IEC suggests only insulation coordination air clearances,

while REN presents mainly security related air clearances, due

to conductor movement. However, REN also presents a range

of distances used in protection rods, designed by insulation

coordination. The differences between these two are shown in

figure 4.

From figure 4 its possible do conclude that even themaximum values are similar, the minimum ones are very

different. It can be also seen that from the range of 170kV to

420kV, the minimum values of IEC are consistently smaller.

Fig. 4Comparison between air clearances of protection rods

IV. INSULATION DISTANCES ON OVERHEAD TRANSMISSION

LINES

In this chapter, the minimum air clearances used for

insulation in overhead transmission lines are presented, as well

as the methodology and critical analysis to achieve them.

A.

Characterization of air clearances

In this section, an extensive analysis is made of the

expressions used to calculate the U50 voltage (voltage wherethe air gap has a 50% probability of breakdown) of an air gap

with length d. This analysis is extended to the breakdowncaused by both types of overvoltages, and the several

expressions are compared, with their validity range evaluated.

B. Phase-to-ground air clearances

The phase-to-ground air clearances are divided in two types

of clearances: the air clearances between protection rods, and

between live parts and structures.

The first one is given by insulation coordination, and the

standard values are given by IEC. However, some deeper

research [5] revealed that the standard rated air clearances

used by IEC do not use the most adequate gap factor for

overhead transmission lines. With the proper gap factor, we

could reach a reduction of 6% to 15%, as shown in figure 5.

Fig. 5Comparison between minimum air clearances

On the other hand, the air clearances between live parts and

structures are given by security reasons, due to wind deviation

of the insulator set.

C. Phase-to-phase air clearances

The phase-to-phase air clearances have two types as well.

The values suggested by IEC are concerned with insulation

coordination. In addition, the values proposed by REN and

DGEG are related with security, due to mid-air cable

oscillations caused by wind and snow. These values are

compared in figure 6, where we can conclude that the security

air clearances dominate the dimensioning for the phase-to-

phase air clearances. In phase-to-earth distances on the other

hand, its the insulation coordination that defines this type ofair clearances.


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Fig. 6Comparison between phase-to-phase minimum air clearances

D.

RENstransmission line analysis

This section verifies the air clearances and gap factors

effectively used by REN, in the overhead transmission lines of

the Portuguese national electric grid.

It was concluded that for any class of voltage and air

clearance, the tower dimensions are always at least 23%

greater than any air clearance proposed by standards.

With the use of [6], it was also possible to calculate in a

more accurate way, the gap factor of each of the tower

configurations. Considering this, figure 7 presents the

comparison between the Conductor-Crossarm gap factor

suggested by IEC and the real gap factor calculated.

Fig. 7Comparison between real and IEC Conductor-Crossarm gapfactors

From figure 7, we can assess that the mean value of the

Conductor-Crossarm gap factor (1,447) is very similar to the

one proposed by IEC (1,45). However, this is not true for the

Conductor-Window configuration, as can be seen in figure 8.From figure 8 we can conclude that the mean value of the

calculated gap factor is 1,2, against the 1,25 suggested by IEC.

Once again, we conclude that the value proposed by REN for

this specific gap factor configuration is overestimated,

following the propensity already verified in the methodology

of this guide. For example, considering a switching withstand

voltage of 950kV (average for 400kV class), the air clearance

according to IEC would be 2,49m. However, with the real gap

factor of 1,2, we would obtain 2,64m. Consequently, an IEC

structure with a 2,49m air clearance and a 10% probability of

disruption for that specific switching withstand voltage, would

result in a decrease of breakdown probability to 2,8%, with the

REN air clearance.

Figure 8 - Comparison between real and IEC Conductor-Window gap

factors

V.

CI-LINEPROGRAM

Due to the complexity of the algorithm proposed by IEC

60071-2, a program was developed to execute the

methodology presented by this standard.

This program receives the values of the representative

overvoltages, and returns the air clearances determined by the

standards for a line with specified characteristics.

The general architecture is presented in figure 9.

Inicial data Urp Ucw

UrwVoltage rangeUw

Range 1

Range 1

Range 2

Range 1 air

clearances

Uw

Range 2

Voltage type

Phase-to-

phase air

clearances

Phase-to-

earth air

clearances Phase-to-earth

Phase-to-

phase

Figure 9General architecture of the CI-LINE program

This program can be downloaded from:

https://fenix.ist.utl.pt/homepage/ist156766/dissertacao/progr

ama-ci-line

VI.

OVERHEAD TRANSMISSION LINE COMPACTION

In this chapter, a methodology that describes overhead

transmission line compaction is presented. Subsequently, some

alternative compact tower configurations will be offered, and

their benefits and impacts analyzed.
https://fenix.ist.utl.pt/homepage/ist156766/dissertacao/programa-ci-linehttps://fenix.ist.utl.pt/homepage/ist156766/dissertacao/programa-ci-linehttps://fenix.ist.utl.pt/homepage/ist156766/dissertacao/programa-ci-linehttps://fenix.ist.utl.pt/homepage/ist156766/dissertacao/programa-ci-linehttps://fenix.ist.utl.pt/homepage/ist156766/dissertacao/programa-ci-line


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A. Base configuration

The base configuration for this compaction will be the

400kV YS tower from the Portuguese national electric grid, as

shown in figure 10 [7].

Figure 10YS tower configuration

The only objective of this compaction is to reduce the top

width of 24m, considered the minimum for this class of

voltage. The height of the tower will remain unchanged, as this

compaction would imply the consideration of local installation

factors that are clearly out of scope.

B.

Proposed configuration

From the data obtained previously in chapter IV, we can

collect the minimum air clearances that the standards propose

for the various types of air clearances. This data is shown in

figure 11, along with the actual distances measured in the base

configuration.

Figure 11Comparison between real and normative distances

From figure 11, we can assess that both air clearance types

used in the base configuration can be reduced. The Conductor

Structure air clearance is limited by the IEC value of 3,4 m,calculated by insulation coordination. However, the minimum

distance between phases is limited by the REN guide, which

declares 6 m as the minimum safety distance.

Respecting these distances, an alternative compact

configuration is proposed in figure 12.

10,0

0

2,

23

R 3,40R 3,40

5,10

R 3,40

15,58

7,75

3,

10

3,

10

Figure 12Proposed configuration

C. Live line works

The proposed configuration in figure 12 has been

dimensioned considering only insulation coordination.

However, due to the necessity of service continuity in the

transmission lines, many maintenance operations are made

with the line energized. In these operations, called live lineworks, it is necessary to ensure the safety of the workers, witha minimum air clearance determined by (1) [8].

rpawd UCCCkU 5050 (1)

where

U50rpis the voltage where a rod-plane air gap configuration hasa 50% probability of breakdown;

Cdis a factor that compensates damaged cap&pininsulators;Cw is a factor that compensates the existence of metallicfloating objects and other tools used by workers that weakens

the dielectric strength of the air gap;

Cais a factor that compensates atmospheric variations.However, since 70% of the Portuguese territory is under

400 m of altitude [9], there is no reason to design a tower

configuration for the worst case. Therefore, two configurations

will be proposed, one that can only be used till 400 m of

altitude, and another till 1500 m.

Applying this minimum security air clearance to theproposed configuration, we obtain the configurations in figure

13 and 14.

Considering this, it is possible to reduce the width of the

tower in 24% for the 400m and 18,6% for the 1500m in

relation to the base configuration.


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10,0

0

2,

23

5,10

R 4,20

R 3,83R 3,83

3,1

0

3,

10

18,32

9,21

Figure 13Proposed configuration for altitudes under 400 m

10,

00

2,

23

5,10

R 4,43

R 4,04R 4,04

3,

10

3,

10

19,52

9,79

Figure 14Proposed configuration for altitudes under 1500 m

D. Lightning performance

To conclude the design of the proposed configurations, it is

necessary to calculate the position of the shield wires, which

will be based in two main conditions. First, the minimum

clearance between the shield wires and the phase conductorswill have to be the same as the phase-to-phase clearance.

Second, it will be used the shielding failure current of the base

configuration, and therefore the same striking distance.

The conjugation of these two conditions results in the

configuration proposed in figure 15.

10,0

0

2,

23

5,10

3,1

0

3,1

0

18,32

R 6,00R 6,00R 6,00

1,8

1

3,44

1,

81

3,44

Figure 15Shield wire position for the proposed configuration till 400m

With this shield wire configuration, both the proposed

configurations allow an increase of 27% in lightning

performance (0,55 to 0,4 faults/100km of line/year obtained

with the programIEEE Flash v1,9).In conclusion, the use of these compact configurations

provides not only a reduction in costs of production and

installation, but also an increase in service continuity due to

the enhanced lightning performance.

VII.

CONCLUSION

Pursuing the main objective of this dissertation, an analysis

was made of the general methodology of insulation

coordination presented in the international standards. It was

concluded that both provided a similar path to achieve it.

Then, a more detailed analysis was made, comparing the

insulation coordination methodologies when applied to

overhead transmission lines. Some conclusions were drawn

from the comparison of IEC and REN guides, more

specifically, the consequences of the temporal lag existing

between them and the differences in the application of the

insulation coordination factors.

Subsequently, an analysis was made to the air clearances

needed to specify the insulation of an overhead transmissionline. The phase-to-earth and phase-to-phase air clearances

were specified based on the several existing standards for this

matter. Also, an analysis was made of the real air clearances

and gap factors implemented in REN transmission lines, which

were compared with the values proposed by the standards.

The architecture of a program built by the author was also

presented. The program executes the insulation coordination

algorithm suggested by IEC, and generates all types of air

clearances for the specified overhead transmission line.

Finally, a methodology for overhead line compaction is

presented, and applied to a specific REN line configuration.

Some alternative configurations were proposed, with apossible reduction of 24% in width. The impacts that this

modifications had on the lightning performance were studied,

leading to an enhancement of 27%. This chapter is then

concluded with a brief evaluation of the benefits that can be

drawn from compaction of overhead transmission lines.

REFERENCES

[1] IEC, Standard 60071-1, Insulation coordination - Part 1: Definitions,principles and rules", 2006.

[2]

IEEE, Standard 1313.1 for Insulation Coordination - Definitions,Principles and Rules, 1996.

[3]

IEC, Standard 60071-2, Insulation coordination - Part 2: ApplicationGuide", 1996

[4]

REN (former EDP), Guia de coordenao de isolamento para a redePTI - EDP, 1985.

[5]

L. Thione, Evaluation of the switching impulse strength of externalinsulation,Electra n 94, 1976.

[6] L. Paris et al., Phase-to-ground and phase-to-phase air clearances in

substations,Electra n 29.[7]

Companhia Portuguesa de Electricidade, Esquemas Sumrios dosPostes da C.P.E., 1975.

[8]

CIGR, Dielectric Strength of External Insulation Systems Under LiveWorking, Sesses CIGR de 1994, 33-306.

[9]

A. M. P. Jorge Ferreira, Dados Geoqumicos de Base de Sedimentos

Fluviais de Amostragem de Baixa Densidade de Portugal Continental,Banco de dados do LNEG - Tese de Doutoramento, 2000.

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Insulation Coordination Linhas Aereas