CIGRE PowerSystemStabilityNewIndustry KundurPowerSols CAN PP1

7/29/2019 CIGRE PowerSystemStabilityNewIndustry KundurPowerSols CAN PP1

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Power System Stability in the New IndustryEnvironment: Challenges and Solutionspresented by:

Dr. Prabha S. Kundur

Kundur Power Systems Solutions, Inc.

Toronto, Ontario

Canada

Tutorial

Copyright P. Kundur

This material should not be used without the author's consent


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1529pk - 2Copyright P. Kundur

Power System Stability and Control

Tutorial Outline

1. Brief Introduction to Power System Stability

Basic concepts

Classification

2. Examples of Major System Blackouts Caused by Different Forms

of Instability

3. Challenges to Secure Operation of today's Power Systems

4. Major System Blackouts in 2003 and 2004

5. Comprehensive Approach to Enhancing Power System Stability


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Power System Stability

Refers to continuance of intact operation of power system,

following a disturbance

Recognized as an important problem for secure system operation

since the 1920s

Major concern since the infamous November 9, 1965 blackout of

Northeast US and Ontario

criteria and analytical tools used till now largely based on the

developments that followed

Presents many new challenges for today's power systems


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Power System Stability: Basic Concepts

Power System Stability denotes the ability of an electric power

system, for a given initial operating condition, to regain a state of

operating equilibrium after being subjected to a physical

disturbance, with all system variables bounded so that the system

integrity is preserved integrity of the system is preserved when practically the entire power

system remains intact with no undue tripping of generators or loads

Stability is a condition of equilibrium between opposing forces:

instability results when a disturbance leads to a sustained imbalancebetween the opposing forces

Ref: IEEE/CIGRE TF Report, "Definition and Classification of Power System Stability",

IEEE Trans. on Power Systems, Vol. 19, pp. 1387-1401, August 2004


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Basic Concepts (cont'd)

Following a transient disturbance, if the power system is stable it will reach a

new equilibrium state with practically the entire system intact:

faulted element and any connected load are disconnected

actions of automatic controls and possibly operator action will eventually

restore system to normal state

On the other hand, if the system is unstable, it will result in a run-away or

run-down situation; for example:

a progressive increase in angular separation of generator rotors, or

a progressive decrease in bus voltages

An unstable system condition could lead to cascading outages, and a shut-

down of a major portion of the power system


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Classification of Power System Stability

Classification into various categories greatly facilitates:

analysis of stability problems

identification of essential factors which contribute to instability

devising methods of improving stable operation

Classification is based on the following considerations:

physical nature of the resulting instability

size of the disturbance considered

devices, processes, and the time span involved

We should always keep in mind the overall stability !

solutions to problems of one category should not be at the

expense of another


7/1501529pk - 7Copyright P. Kundur

Power System Stability

Frequency

Stability

Small-Signal

Stability

Transient

Stability

Short Term Long Term

Large-Disturbance

Voltage Stability

Small-Disturbance

Voltage Stability

Voltage

Stability

Rotor Angle

Stability

Considerationfor

Classification

Physical

Nature/ Main

System

Parameter

Size of

Disturbance

Time Span

Short Term

Short Term Long Term


8/1501529pk - 8Copyright P. Kundur

Rotor Angle Stability

Ability of interconnected synchronous machines to remain insynchronism after being subjected to a disturbance

Depends on the ability to restore equilibrium between electromagnetic

torque and mechanical torque of each synchronous machine

If the generators become unstable when perturbed, it is as a result of

a run-away situation due to torque imbalance

A fundamental factor is the manner in which power outputs of

synchronous machines vary as their rotor angles swing Instability that may result occurs in the form of increasing angular

swings of some generators leading to loss of synchronism with other

generators


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Copyright P. Kundur

Transient Stability

Term traditionally used to denote large-disturbance angle stability

Ability of a power system to maintain synchronism when

subjected to a severe transient disturbance:

influenced by the nonlinear power-angle relationship

stability depends on the initial operating condition and severity of

the disturbance

A wide variety of disturbances can occur on the system:

The system is, however, designed and operated so as to be stablefor a selected set of contingencies

usually, transmission faults: L-G, L-L-G, three phase


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Copyright P. Kundur

Small-Signal (Angle) Stability

Small-Signal (or Small-Disturbance) Stability is the ability of a powersystem to maintain synchronism under small disturbances

disturbance considered sufficiently small if linearization of system

equations is permissible for analysis

Instability that may result can be of two forms:

aperidic increase in rotor angle due to lack of sufficient synchronizing

torque

rotor oscillations of increasing amplitude due to lack of sufficient

damping torque

In today's practical power systems, SSS problems are usuallyassociated with oscillatory modes

local plant mode oscillations: 0.8 to 2.0 Hz

interarea oscillations: 0.1 to 0.8 Hz


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Copyright P. Kundur

Voltage Stability

Ability of power system to maintain steady voltages at all buses in the

system after being subjected to a disturbance

A system experiences voltage instability when a disturbance, increase in

load demand, or change in system condition causes:

a progressive and uncontrollable fall or rise in voltage of buses

in a small area or a relatively large area

Main factor causing voltage instability is the inability of power system to

maintain a proper balance of reactive power and voltage control actions

The driving force for voltage instability is usually the load characteristics


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Copyright P. Kundur

Short-Term and Long-Term Voltage Stability

Short-term voltage stability involves dynamics of fast acting load

components such as induction motors, electronically controlled

loads and HVDC converters

study period of interest is in the order of several seconds

dynamic modeling of loads often essential; analysis requiressolution of differential equations using time-domain simulations

faults/short-circuits near loads could be important

Long-term voltage stability involves slower acting equipment such as

tap-changing transformers, thermostatically controlled loads, andgenerator field current limiters

study period may extend to several minutes


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Frequency Stability (cont'd)

In a large interconnected system, frequency stability could be of

concern only following a severe system upset resulting in the

system splitting into islands

Depends on the ability to restore balance between generation andload of island systems with minimum loss of load and generation

Generally, frequency stability problems are associated with

inadequacies in equipment responses, poor coordination ofcontrol and protection systems


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Examples of Major System Blackouts Caused by

Different Forms of Instability

1. November 9, 1965 blackout of Northeast U.S. and Ontario

2. April 19, 1972, blackout of Eastern Ontario

3. July 2, 1996 disturbance of WSCC (Western North

American Interconnected) System

4. August 10, 1996 disturbance of WSCC system

5. March 11, 1999 Brazil blackout


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November 9, 1965 Blackout of NE U.S. and Ontario

Clear day with mild weather; load levels in the region normal

Problem began at 5:16 p.m.

Within a few minutes, there was a complete shut down of electric

service to: virtually all of the states of New York, Connecticut, Rhode Island,

Massachusetts, Vermont

parts of New Hampshire, New Jersey and Pennsylvania

most of Ontario

Nearly 30 million people were without power for about 13 hours


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North American Eastern Interconnected System


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Events that Caused the 1965 Blackout

The initial event was the operation of a backup relay (zone 3)

at Beck GS in Ontario near Niagara Falls

opened circuit Q29BD, one of five 230 kV circuits connecting

Beck GS to load centers in Toronto and Hamilton

Prior to opening of Q29BD, the five circuits were carrying

1200 MW of Beck generation, and

500 MW import from Western NY State on Niagara ties

Loading on Q29BD was 361 MW at 248 kV;

The relay setting corresponded to 375 MW


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Events that Caused the 1965 Blackout (cont'd)


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Events that Caused the 1965 Blackout (contd)

Opening of circuit Q29BD resulted in sequential tripping of the

remaining four parallel circuits

Power flow reversed to New York: total change of 1700 MW

Generators in Western New York and Beck GS lost synchronism,

followed by cascading outages: Transient (Angle) Instability !

After about 7 seconds from the initial disturbance

system split into several separate islands Eventually most generation and load lost due to the inability of

islanded systems to stabilize:Frequency Instability !


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Formation of Reliability Councils

Northeast Power Coordinating Council (NPCC) formed in January 1966 to improve coordination in planning and operation among utilities

first Regional Reliability Council (RRC) in North America

Other eight RRCs formed in the following months

National/North American Electric Reliability Council (NERC)

established in 1968

Detailed reliability criteria were developed;

Procedures for exchange of data and conducting stability studies were

established

Many of these developments have had an influence on utility practices

worldwide; still largely used !


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April 19, 1972 Blackout ofEastern Ontario

Copyright P. Kundur


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Transient Response of Nuclear Units with Auxiliary

Governor


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July 2, 1996 WSCC / WECC

(Western North AmericanInterconnected System)

Disturbance

Copyright P. Kundur


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WSCC July 2, 1996 Disturbance (cont'd)

LG fault on 345 kV line from Jim Bridger 2000 MW plant in

Wyoming to Idaho due to flashover to a tree

tripping of parallel line due to relay misoperation

Tripping of two (of four) Jim Bridger units as stability control; this

should have stabilized the system

Faulty relay tripped 230 kV line in Eastern Oregon

Voltage decay in southern Idaho and slow decay in central Oregon


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WSCC July 2, 1996 Disturbance (contd)

About 24 seconds later, a long 230 kV line (Amps line) from western

Montana to Southern Idaho tripped, due to zone 3 relay operation parallel 161 kV line subsequently tripped

Rapid voltage decay in Idaho and Oregon

Three seconds later, four 230 kV lines from Hells Canyon generation to

Boise tripped

Two seconds later, Pacific intertie lines separated

Cascading to five islands 35 seconds after initial fault

2.2 million customers experienced outages; total load lost 11,900 MW

Voltage Instability!!!


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WSCC July 2, 1996 Disturbance (cont'd)


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August 10th, 1996 WSCC Event


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As a result of the undamped

oscillations, the system split

into four large islands

Over 7.5 million customers

experienced outages ranging

from a few minutes to nine

hours! Total load loss 30,500MW

WSCC August 10, 1996 Disturbance (cont'd)


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Sites Selected for PSS Modifications

San Onofre

(Addition) Palo Verde

(Tune existing)


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Power System Stabilizers

With existing controls

Eigenvalue = 0.0597 + j 1.771

Frequency = 0.2818 Hz

Damping = -0.0337

With PSS modifications

Eigenvalue = -0.0717 + j 1.673

Frequency = 0.2664Damping = 0.0429


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Design of HVDC Modulation

HVDC intertie shown (as expected) to have low participation in themode of interest (0.23 Hz interarea oscillations)

Often however, HVDC can be modulated to improve damping, provided

adequate input signal is found and proper compensator is designed

SSAT used to examine frequency response for several potential inputsignals

Frequency response magnitude identified local bus frequency as

having good operability/controllability of the mode of interest

Frequency response phase used to design compensator which

provides proper modulation signal to HVDC controls

TSAT and SSAT used to verify modulation performance


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March 11, 1999 BrazilBlackout

Copyright P. Kundur


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March 11, 1999 Brazil Blackout

Time: 22:16:00h, System Load: 34,200 MW

Description of the event:

L-G fault at Bauru Substation as a result of lightning causing a bus

insulator flashover

the bus arrangement at Bauru such that the fault is cleared by opening

five 440 kV lines

the power system survived the initial event, but resulted in instability

when a short heavily loaded 440 kV line was tripped by zone 3 relay

cascading outages of several power plants in Sao Paulo area, followed

by loss of HVDC and 750 kV AC links from Itaipu

complete system break up: 24,700 MW load loss; several islands

remained in operation with a total load of about 10,000 MW

Transient instability followed by voltage problems


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1529pk - 51

Challenges to Secure Operation of

Today's Power Systems

Copyright P. Kundur

Li it ti f T diti l A h t P


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Limitations of Traditional Approach to Power

System Stability

Focus largely on one aspect of stability: "transient stability"

Deterministic approach for system security assessment

System designed and operated to withstand

loss of any single element preceded by a fault

referred to as N-1 criterion Analysis by time-domain simulation of selected operating

conditions

scenarios based on judgment/experience

Operating limits based on off-line studies system operated conservatively within pre-established limits

"Adhoc Approach" to application of power system stability

controls


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Ch ll t S O ti f T d ' P


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Challenges to Secure Operation of Today's Power

Systems (cont'd)

Complex modes of instability

global problems

different forms of instability: rotor angle, voltage, frequency

"Deregulated" market environment

many entities with diverse business interests

system expansion and operation driven largely by economic drivers

lack of coordinated planning


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Rotor Angles in B.C. and Alberta


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BC Hydro and Alberta Bus Voltages


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Rotor Angles in B.C. and Alberta


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Major Power System Blackouts in 2003

and 2004


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Blackouts in 2003 and 2004

We had several wake up calls since 2003:

August 14, 2003 blackout of North East USA and Ontario

63,000 MW load loss affecting 50 million people

September 23, 2003 blackout of South Sweden and East Denmark

6,500 MW load loss affecting 4 million people

September 28, 2003 blackout of Italy

50,000 MW load unsupplied affecting 60 million people

August 12, 2004 blackout of three Australian States: Queensland,NSW and Victoria

load loss 1,000 MW


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August 14, 2003 Blackout of Northeast US

and Canada


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14 August 2003 Blackout of Northeast US - Canada

Approximately 50 million people in 8 states in the US and2 Canadian provinces affected

63 GW of load interrupted (11% of total load supplied by Eastern

North American Interconnected System)

During this disturbance, over400 transmission linesand 531

generatingunits at 261 power plants tripped

For details refer to: "Final Report of Aug 14, 2003 Blackout in the

US and Canada: Causes and Recommendations", US-Canada

Power System Outage Task Force, April 5, 2004.www.NERC.com


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Sequence of Events

The Midwest ISO (MISO) state estimator and real-time contingency

analysis (RTCA) software not functioning properly from 12:15 to 16:04

prevented MISO from performing proper "early warning" assessments

as the events were unfolding

At the First Energy (FE) Control Center, a number of computer

software problems occurred on the Energy Management System(EMS) starting at 14:14

contributed to inadequate situation awareness at FE until 15:45

The first significant event was the outage of East Lake generating unit

#5 in the FE system at 13:31:34

producing high reactive power output

voltage regulator tripped to manual on overexcitation

unit tripped when operator tried to restore AVR


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East Lake 5 Trip: 1:31:34 pm

ONTARIO

2

1

ONTARIO


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Initial line trips in Ohio, all due to tree contact: Chamberlin-Harding 345 kV line at 15:05:41

Hanna-Juniper 345 kV line at 15:32:03

Star-South Canton 345 kV line at 15:41:35

Due to EMS failures at FE and MISO control centers, no properactions (such as load shedding) taken

Critical event leading to widespread cascading outages in Ohio

and beyond was tripping of Sammis-Star 345 kV line at 16:05:57

Zone 3 relay operation due to low voltage and high power flow

Load shedding in northeast Ohio at this stage could have

prevented cascading outages that followed

Sequence of Events cont'd


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Tripping of many additional 345 kV lines in Ohio and Michigan by

Zone 3 (or Zone 2 set similar to Zone 3) relays

Tripping of several generators in Ohio and Michigan

At 16:10:38, due to cascading loss of major lines in Ohio and

Michigan, power transfer from Canada (Ontario) to the US on the

Michigan border shifted power started flowing counter clockwise from Pennsylvania through

New York and Ontario into Michigan

3700 MW of reverse power flow to serve loads in Michigan and Ohio,

which were severed from rest of interconnected system except Ontario

Voltage collapsed due to extremely heavy loadings on transmission

lines

Cascading outages of several hundred lines and generators leading

to blackout of the region

Sequence of Events


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Primary Causes of Blackout


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Primary Causes of Blackout(as identified by the US-Canada Outage Task Force)

1. Inadequate understanding of the power system requirements:

First Energy (FE) failed to conduct rigorous long-term planning

studies and sufficient voltage stability analyses of Ohio control area

FE used operational criteria that did not reflect actual system

behaviour and needs

ECAR (East Central Area Reliability Council) did not conduct an

independent review or analysis of FE's voltage criteria and operating

needs

Some NERC planning standards were sufficiently ambiguous that FE

could interpret them in a way that resulted in inadequate reliability forsystem operation

cont'd


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Causes of Blackout cont'd

2. Inadequate level of situation awareness: FE failed to ensure security of its system after significant

unforeseen contingencies

FE lacked procedures to ensure that its operators were

continually aware of the functional state of their critical

monitoring tools FE did not have adequate backup tools for system monitoring

3. Inadequate level of vegetation management (tree trimming)

FE failed to adequately manage tree growth into transmission

rights-of-way

resulted in the outage of three 345 kV lines and one 138 kV

line

cont'd


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Causes of Blackout cont'd

4. Inadequate level of support from the Reliability Coordinator

due to failure of state estimator, MISO did not become aware of

FE's system problems early enough

did not provide assistance to FE

MISO and PJM (Regional Transmission operator) did not have in

place an adequate level of procedures and guidelines for dealing

with security limit violations due to a contingency near their

common boundary


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September 23, 2003 Blackout of Southern

Sweden and Eastern Denmark

Copyright P. Kundur


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Blackout of 23 September 2003 in Southern Sweden


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p

and Eastern Denmark

Pre-disturbance conditions:

system moderately loaded

facilities out of services for maintenance:

400 kV lines in South Sweden

4 nuclear units in South Sweden

3 HVDC links to Germany and Poland

The first contingency was loss of a 1200 MW nuclear unit in

South Sweden at 12:30 due to problems with steam valves

increase of power transfer from the north

system security still acceptable

cont'd



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p

and Eastern Denmark (cont'd)

Five minutes later (at 12:35) a disconnector damage caused a

double busbar fault at a location 300 km away from the first

contingency

resulted in loss of a number of lines in the southwestern grid and

two 900 MW nuclear units

At 12:37, voltage collapse in the eastern grid section south of

Stockholm area

isolated southern Sweden and eastern Denmark system from

northern and central grid

The Blackout in Southern Sweden and Eastern


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Denmark, September 23, 2003

TenhultStrmma

Horred

Sdersen

Barsebck

Hemsj

Simpevarp

Nybro

Kimstad

Glan

Kolstad

Hallsberg

Breared Alvesta

The voltage collapse

Maintenance work

The fault in Horred

Line outages due to:

TenhultStrmma

Horred

Sdersen

Barsebck

Hemsj

Simpevarp

Nybro

Kimstad

Glan

Kolstad

Hallsberg

Breared Alvesta


Maintenance work

The fault in Horred



Maintenance work

The fault in Horred


Voltage Collapse

IsolatedSubsystem

The Blackout in Southern Sweden and Eastern


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Denmark, September 23, 2003

The blacked-out area after the grid separation at 12.37



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The isolated system had enough generation to cover only about 30%of its demand

voltage and frequency collapsed within a few seconds, blacking out the

area

Impact of the blackout:

loss of 4700 MW load in south Sweden

1.6 million people affected

City of Malmo and regional airports and rail transportation without

power

loss of 1850 MW in eastern Denmark

2.4 million people affected

City of Copenhagen, airport and rail transportation without power

Result of an (n-3) contingency, well beyond "design contingencies"

p

and Eastern Denmark cont'd


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September 28, 2003 Blackout of Italy

Copyright P. Kundur


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Italian System Blackout of 28 September 2003

Predisturbance conditions (Sunday, 3:00 am):

total load in Italy was 27,700 MW, with 3638 MW pump load

total import from rest of Europe was 6651 MW

Sequence of events:

a tree flashover caused tripping of a major tie-line between Italy and

Switzerland (Mettlen-Lavorgo 380 kV line) at 03:01:22 Sychro-check relay prevented automatic and manual reclosure of line due

to the large angle (42) across the breaker

resulted in an overload on a parallel path

attempts to reduce the overload by Swiss transmission operators by

network change was not successful

at 03:21 import by Italy was reduced by 300 MW but was not sufficient to

mitigate the overload of a second 380 kV line (Sils-Soazza), which tripped

at 03:25:22 due to sag and tree contact


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What Can We Do To Prevent

Blackouts?

Copyright P. Kundur

F t I ti S t S it


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Factors Impacting on System Security

Regulatory Framework

Governments, Reliability Councils

Business Structure

Owning and operating entities; Financial

and contractual arrangements

Physical System

Integrated Generation,

Transmission, Distribution

System

Comprehensive Approach to Enhancing System

St bilit


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Stability

Impractical to achieve 100% reliability of power systems

Good design and operating practices could significantly minimize

the occurrence and impact of widespread outages

Reliability criteria: risk-based security criteria

Improved protective relaying

Robust stability controls

Coordinated emergency controls

Comprehensive stability assessment: analytical tools and models

Real-time system system monitoring and control

Wide-spread use of distributed generation

Reliability Management System

Good vegetation management

R li bilit C it i


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Reliability Criteria

At present, systems designed and operated to withstand loss of anysingle element preceded by single-, double-, or three-phase fault

referred to as "N-1 criterion"

formulated nearly 40 years ago after the 1965 blackout

Need for using risk-based security assessment criteria

consider multiple outages

account for probability and consequences of instability

Built-in overall strength or robustness best defense againstcatastrophic failures !

I d P t ti R l i


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Improved Protective Relaying

State-of-the-art protective relaying for generating units andtransmission lines

adaptive relaying

Replacement of zone 3 and other backup relaying on important

lines with improved relaying

Improved protection and control at power plants to minimize unit

tripping for voltage and frequency excursions

Protective relay improvements to prevent tripping of critical

elements on overload

control actions to relieve overload

R b t St bilit C t l


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Robust Stability Controls

Greater use of stability controls excitation control (PSS), FACTS, HVDC, secondary voltage control

multi-purpose controls

multiple controllers

Coordination, integration and robustness present challenges good control design procedures and tools have evolved

Hardware design should provide

high degree of functional reliability

flexibility for maintenance and testing

Industry should make better use of controls !

E C t l f E t C ti i


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Emergency Controls for Extreme Contingencies

Contingencies more severe than normal design contingencies

multiple contingencies

can occur anywhere on the system in any form

Currently, emergency controls used to protect against some

generator tripping, load shedding, dynamic breaking, controlled

system separation, transfer tap-changer blocking

Need for a systematic approach to cover against all likely extreme

contingencies


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Examples of Response-Based Emergency Control

S h


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Schemes

1. Scheme for prevention of voltage collapse in Eastern Ontario

fully automated and coordinated emergency controls for voltage

stability

2. Transient Excitation Boosting

for enhancing transient (angle) stability of systems with dominant

interarea swing

Example 1: Prevention of Voltage Collapse in

E t O t i


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Eastern Ontario

Implemented in early 1980s to cope with delays in building 500 kV

line

Under high load conditions, loss of a major 230 kV line leads to

voltage collapse of Ottawa area

A coordinated scheme consisting of fast line reclosure, load

rejection, shunt capacitor switching, and transformer ULTC

blocking


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Example 1: (cont'd)


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Example 1: (cont'd)

Coordination provided by appropriate selection of voltage and

time settings

triggered by voltage drop magnitude and duration

Following a contingency, depending on the severity (power flow,

line outage), only the required level of control action provided


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1300 MW

1374 MW


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Response-Based Emergency Controls

Example 2: Transient Excitation Boosting


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Example 2: Transient Excitation Boosting

In situations with dominant interarea swing, PSS reduces

excitation after the first local mode swing

Improvements in TS achieved by keeping excitation at ceiling

until highest composite swing

increase in internal voltage

increase in voltage also increases power consumed by area load

Block Diagram of TSEC Scheme


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Block Diagram of TSEC Scheme

Effect of TSEC on Transient Stability


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Effect of TSEC on Transient Stability

Example 2: (cont'd)


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Example 2: (cont d)

Transient Excitation Boosting, TSEC, applied to four major plants

in Ontario:

Nanticoke (4000 MW), Bruce A and B (6000 MW), Lennox (2000 MW)

signal proportional to angle swing

integrated with PSS and coordinated with terminal voltage limiter

In effect, a nonlinear adaptive closed loop control

may use local or remote signals

imposes little duty on equipment


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State-of-the-Art On-Line Dynamic Security

Assessment (DSA)


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Assessment (DSA)

Practical tools have been developed with the required accuracy, speedand robustness

a variety of analytical techniques integrated

distributed hardware architecture using low cost PCs

integrated with energy management system

Capable of assessing rotor angle stability and voltage stability

determine critical contingencies automatically

security limits/margins for all desired energy transactions

identify remedial measures

The industry has yet to take full advantage of these developments !

Dynamic Security Assessment Tools Developed and

Used by Powertech for System Design and Operation


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Used by Powertech for System Design and Operation

Powerful set of complementary programs:

flexible and detailed models

alternative and efficient solution techniques

Transient (Angle) Stability Assessment: TSAT

Small-Signal (Angle) Stability Assessment: SSAT

Voltage Stability Assessment: VSAT

Frequency Stability Analysis: LTSP*cont'd

LTSP currently not maintained/supported*


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On-Line Voltage Stability Assessment Tool

(VSAT)

Copyright P. Kundur

Key Elements of VSAT


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Key Elements of VSAT

Interface with EMS; Model Initialization

Contingency screening and selection

Determination of secure operating region

using static analysis

Determination of remedial actions

Fast time-domain simulation

validation and checking


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Security Computation Module


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y p

Engine for voltage stability analysis

static analysis with detailed models

Secure region is defined by a number of Coordinates (SRCs)

key system parameters: MW generation, area load, interface

transfers, etc.

Voltage security determined by voltage stability margin

MVAr reserves of key reactive sources

post-contingency voltage decline

Modal analysis of powerflow Jacobian matrix identifies areas prone

to instability

Specialized powerflow dispatcher and solver to quickly search for

stability limit


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Secure Operating Region


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p g g

Remedial Measures Module


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Determines necessary remedial measures to ensure sufficient stability margins

expand the secure region

Preventative control actions:

taken prior to a contingency

caps/reactor switching, generation redispatch, voltage rescheduling

Corrective (emergency) control actions:

applied following a contingency

load shedding, generator runback, transformer tap changer blocking

Ranking of each remedial measure using:

sensitivity analysis

user-defined priorities

Ranking and Applying Remedial Measures


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g pp y g

Objective is to identify the most effective remedial measures to givethe desired stability margin

Obtain solved power flow case for the most severe contingency

gradually introduce the effect of the contingency

bus injection compensation technique

Compute the sensitivities of reactive power (or bus voltage) to

different control measures

rank the remedial measures

Apply controls one at a time in order of ranking until power flowsolves for the most severe contingency


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Fast Time-Domain Simulation Module


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Determines the essential dynamic phenomena without step-by-stepnumerical integration

when chronology of events significant

for validating the effect of remedial measures

Focuses on the evolution of system dynamic response driven by

slow dynamics

transformer tap changers, field current limiters, switched caps

Captures the effects of fast dynamics by solving associated steadystate equations

Mathematical Formulation


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The complete set of differential/algebraic equations of a power

system has the following general form:

Where:

X = state vector

V = bus voltage vector

I = current injector vector

Y = network admittance matrix

Z = variables associated with the slow

control devices including ULTCs, loads, switchable reactors

and capacitors, and field current limiters

Z,V,XfX

Z,V,XIYV

Mathematical Formulation


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At each equilibrium point, Z=Zi and the system operating conditionis obtained by solving the following set of nonlinear algebraic

equations:

As time progresses, the slow control devices operate and the

values of Z change. The above set of nonlinear algebraic equations

is solved every time the values of Z change.

i

i

ZV,X,IYV

ZV,X,f0


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A Practical Tool for TSA


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Overall architecture similar to that of VSA

Time-domain program, with detailed models and efficient solution

techniques, forms simulation engine

EEAC used for screening contingencies, computing stability margin,

stability limit search, and early termination of simulation

Prony analysis for calculation of damping of critical modes of

oscillation

A powerflow dispatcher and solver for finding the stability limit

a fully automated process

No modeling compromises;

can handle multi-swing instability


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EEAC


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Loss of transient stability in a power system always starts in a

binary splitting of generators:

Critical cluster of generators

Rest of the system

At any given point in the

time-domain trajectory of

the system, the system

can be visualized as a

one-machine-infinite-bus

(OMIB) system

EEAC


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The classical equal area criterion can be extended to the visual OMIBsystem

Stability margin of the system is defined as

da

a

ad

ad

d

ad

AAA

AAx

AA

A

AAx

unstableissystemtheif100

stableissystemtheif100

Thus, -100 , and

if the system is stable if the system is unstable

can be used as a stability index

Use of EEAC Theory


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Contingency screening

stability margin gives an indication of the relative severity

Corrective measures for maintaining secure system operation

critical cluster of generators (CCG) provides valuable information

Power transfer limit search

stability limit can be determined in four iterations using stability margin

each iteration involves a detailed simulation and computation of stability

index


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Limit Search Results


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Speed Enhancement: Parallel Processing


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Code parallelization differential equations easily parallelized, but not network equations

speed-ups limited by serial slowdown effect

up to 7 times speed-up can be achieved with 20-30 processors

not an effective way

Conventional serial computers offer much faster computational

per-CPU

For multiple contingencies

perform initialization only once

run contingencies on multiple processors - one processor per

contingency


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Computational Speed of DSA (cont'd)


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Voltage Stability Assessment:

- screening 300 contingencies 20.0 secs

- detailed security analysis 1.2 secs

with 20 critical contingencies- one transfer limit search 12.0 secs

Transient Stability Assessment:

- screening 100 contingencies 75.0 secs

- 10 second simulations with 75.0 secs

10 critical contingencies- one transfer limit search 120.0 secs

- total time for complete assessment < 5 mins

Power System model with 4655 buses, 156 generators, using 1.7 GHz,Pentium 4 PC with 256 MB memory

Future Trends in DSA: Intelligent Systems


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Knowledge base created using simulation of a large number cases and

system measurements

Automatic learning, data mining, and decision trees to build intelligent

systems

Fast analysis using a broad knowledge base and automatic decision making

Provides new insight into factors and system parameters affecting stability

More effective in dealing with uncertainties and large dimensioned problems

We just completed a PRECARN project: "POSSIT"


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Distributed Generation (DG)


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Offer significant economic, environmental and security benefits

Microturbines

small, high speed power plants

operate on natural gas or gas from landfills

Fuel Cells combines hydrogen with oxygen from air to generate electricity

hydrogen may be supplied from an external source or generated inside

fuel cell by reforming a hydrocarbon fuel

Not vulnerable to power grid failure due to system instability or

natural calamities

protection and controls should be designed so that units continue to

operate when isolated from the grid


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Summary


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1. The new electricity supply industry presents increasing challenges forstable and secure operation of power systems

2. State-of-the-art methods have advanced our capabilities significantly

comprehensive stability analysis tools

automated tools for system planning/design

on-line Dynamic Security Assessment (DSA)

coordinated design of robust stability controls

3. Industry is yet to take full advantage of these developments

cont'd

Summary (cont'd)


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4. Future directions will be to explore new techniques which canbetter deal with growing uncertainties and increasing

complexities of the problem

risk-based security assessment

intelligent systems for DSA

"self-healing" power systems

real-time monitoring and control

5. Wide-spread use of distributed generation could be a costeffective means of minimizing the impact of power grid failures

Documents

CIGRE PowerSystemStabilityNewIndustry KundurPowerSols CAN PP1