Kundur Power Systems Solutions, Inc.

Kundur Power Systems Solutions, Inc.
Tutorial Power System Stability in the New Industry Environment: Challenges and Solutions presented by: Dr. Prabha S. Kundur Kundur Power Systems Solutions, Inc. Toronto, Ontario Canada Copyright © P. Kundur This material should not be used without the author's consent

Power System Stability and Control
Tutorial Outline Brief Introduction to Power System Stability Basic concepts Classification Examples of Major System Blackouts Caused by Different Forms of Instability Challenges to Secure Operation of today's Power Systems Major System Blackouts in 2003 and 2004 Comprehensive Approach to Enhancing Power System Stability

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

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 imbalance between the opposing forces Ref: IEEE/CIGRE TF Report, "Definition and Classification of Power System Stability", IEEE Trans. on Power Systems, Vol. 19, pp , August 2004

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

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

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 Consideration for Classification Physical Nature/ Main System Parameter Size of Disturbance Time Span

Rotor Angle Stability Ability of interconnected synchronous machines to remain in synchronism 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

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 stable for a selected set of contingencies usually, transmission faults: L-G, L-L-G, three phase

Small-Signal (Angle) Stability
Small-Signal (or Small-Disturbance) Stability is the ability of a power system 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 usually associated with oscillatory modes local plant mode oscillations: 0.8 to 2.0 Hz interarea oscillations: 0.1 to 0.8 Hz

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

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 requires solution 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, and generator field current limiters study period may extend to several minutes

Frequency Stability Ability to maintain steady frequency within a nominal range following a disturbance resulting in a significant imbalance between generation and load Instability that may result occurs in the form of sustained frequency swings leading to tripping of generating units and/or loads In a small "island" system, frequency stability could be of concern for any disturbance causing a significant loss of load or generation

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 and load of island systems with minimum loss of load and generation Generally, frequency stability problems are associated with inadequacies in equipment responses, poor coordination of control and protection systems

Examples of Major System Blackouts Caused by Different Forms of Instability
November 9, 1965 blackout of Northeast U.S. and Ontario April 19, 1972, blackout of Eastern Ontario July 2, 1996 disturbance of WSCC (Western North American Interconnected) System August 10, 1996 disturbance of WSCC system March 11, 1999 Brazil blackout

November 9, 1965 Blackout of Northeast U.S. and Ontario
Copyright © P. Kundur

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

North American Eastern Interconnected System

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

Events that Caused the 1965 Blackout (cont'd)

Events that Caused the 1965 Blackout (cont’d)
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 !

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 !

Special Protections Implemented after the 1965 Blackout
P Relays on Niagara Ties trip Niagara ties to NY when P exceeds set value; cross-trip St. Lawrence ties to NY in place until mid 1980s Underfrequency load shedding (UFLS) throughout the interconnected system beginning of the use of UFLS by the industry

April 19, 1972 Blackout of Eastern Ontario

April 19, 1972 Disturbance: Eastern Ontario
Incident: 230 kV lines east of Toronto tripped due to communication malfunction; ties to New York at St. Lawrence tripped generation rich island formed in eastern Ontario (G=3900 MW, L=3000 MW) frequency rose to 62.5 Hz and then dropped to 59.0 Hz due to speed governor underfrequency load shedding ! frequency rose to 62.6 Hz and dropped to 58.7 Hz stabilized at 60.8 Hz with 1875 MW generation Frequency Instability ! Source of problem: overspeed controls associated with prime-mover governors of Pickering “A:” NGS

MHC Turbine Governing System with Auxiliary Governor

Transient Response of Nuclear Units with Auxiliary Governor

Transient Response of Nuclear Units with Auxiliary Governor Out-of-Service

July 2, 1996 WSCC / WECC (Western North American Interconnected System) Disturbance

WSCC July 2, 1996 Disturbance Started in Wyoming and Idaho area at 14:24:37 Loads were high in Southern Idaho and Utah; High temperature around 38°C Heavy power transfers from Pacific NW to California Pacific AC interties MW (4800 rating) Pacific HVDC intertie MW (3100 capacity)

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

WSCC July 2, 1996 Disturbance (cont’d)
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!!!

Loss of voltage control following the tripping of the Amps line Time in Seconds

TSAT was Used to Replicate Disturbance in Time Domain
MEASURED RESPONSE SIMULATED RESPONSE

August 10, 1996 WSCC (WECC) Disturbance

WSCC August 10, 1996 Disturbance
High ambient temperatures in Northwest; high power transfer from Canada to California Prior to main outage, three 500 kV line sections from lower Columbia River to load centres in Oregon were out of service due to tree faults California-Oregon Interties loaded to 4330 MW north to south Pacific DC Intertie loaded at 2680 MW north to south 2300 MW flow from British Columbia Main outage: Ross-Lexington 230 kV line at 15:47:36 Growing 0.23 Hz interarea oscillations caused tripping of lines resulting in formation of four islands Small-Signal Oscillatory Instability !

August 10th, 1996 WSCC Event

WSCC August 10, 1996 Disturbance (cont'd)
Malin - Round Mountain MW Flow 2300 2400 2500 2600 2700 2800 2900 3000 3 6 9 12 16 19 22 25 28 31 34 37 40 43 47 50 53 56 59 62 65 68 71 74 Time in Seconds

WSCC August 10, 1996 Disturbance (cont'd)
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 30,500 MW

TSAT was Used to Replicate Disturbance in Time Domain
MEASURED RESPONSE SIMULATED RESPONSE

Sites Selected for PSS Modifications
San Onofre (Addition) Palo Verde (Tune existing)

Power System Stabilizers
With existing controls Eigenvalue = j 1.771 Frequency = Hz Damping = With PSS modifications Eigenvalue = j 1.673 Frequency = Damping =

Design of HVDC Modulation
HVDC intertie shown (as expected) to have low participation in the mode 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 input signals 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

TSAT Verification of Effectiveness of HVDC Modulation
Without HVDC Modulation Eigenvalue = j 1.771 Frequency = Hz Damping = With HVDC Modulation Eigenvalue = j 1.797 Frequency = Damping =

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

March 11, 1999 Brazil Blackout (cont'd)
Measures to improve system security: Joint Working Group comprising ELECTROBRAS, CEPEL and ONS staff formed organized activities into 8 Task Forces Four international experts as advisors Remedial Actions: power system divided into 5 security zones: regions with major generation and transmission system; emergency controls added for enhancing stability improved layout and protection of major EHV substations improved maintenance of substation equipment and protection/control equipment improved restoration plans

Challenges to Secure Operation of Today's Power Systems

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

Challenges to Secure Operation of Today's Power Systems
Power Systems are large complex systems covering vast geographic areas national/continental grids highly nonlinear higher order system Many processes whose operations need to be coordinated millions of devices requiring harmonious interplay Increasing use of Wind Power for generation of electricity requires careful consideration in integration with power grids cont'd

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

Example of a Complex Mode of Instability
A transmission line fault causes transient instability of a remote area: Sensitive to conditions in the faulted area Nature of the stability problem is not readily apparent

North American Western Interconnected System

Case A 4-cycle fault on Palo Verde - Devers line (Arizona-California)
Alberta to B.C. transfer 500 MW East of River interface flow 7300 MW Note: power flow conditions considered for this study as unusual, and do not represent present operating conditions

BPA Bus Voltage

BC Hydro Bus Voltage Near Alberta Tie

Rotor Angles in B.C. and Alberta

Case B East of River interface flow reduced to 7000 MW (from 7300 MW for Case A)

BC Hydro and Alberta Bus Voltages

Rotor Angles in B.C. and Alberta

North American Western Interconnected System

Major Power System Blackouts in 2003 and 2004

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

August 14, 2003 Blackout of Northeast US and Canada

14 August 2003 Blackout of Northeast US - Canada
Approximately 50 million people in 8 states in the US and 2 Canadian provinces affected 63 GW of load interrupted (11% of total load supplied by Eastern North American Interconnected System) During this disturbance, over 400 transmission lines and 531 generating units 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,

NERC Regions Affected: MAAC, ECAR, NPCC

Conditions Prior to Blackout
Electricity demand high but not unusually high Power transfer levels high, but within established limits and previous operating conditions Planned outages of generating units in the affected area: Cook 2, Davis Bess plant, East Lake 4, Sammis 3 and Monroe 1 Reactive power supply problems in the regions of Indiana and Ohio prior to noon Operators took actions to boost voltages voltages within limits System operating in compliance with NERC operating policies prior to 15:05 Eastern Daylight Time

Blackout Started in Midwest

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

East Lake 5 Trip: 1:31:34 pm ONTARIO 2 1

Sequence of Events cont'd
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 proper actions (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 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

Areas Affected by the Blackout Some Local Load Interrupted
End of Cascade Areas Affected by the Blackout Service maintained in some area Some Local Load Interrupted

Primary Causes of Blackout (as identified by the US-Canada Outage Task Force)
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 for system operation cont'd

Causes of Blackout cont'd
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 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

Causes of Blackout cont'd
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

September 23, 2003 Blackout of Southern Sweden and Eastern Denmark

The Transmission Grid in the Nordic Countries

Blackout of 23 September 2003 in Southern Sweden 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

Blackout of 23 September 2003 in Southern Sweden 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 Denmark, September 23, 2003
Voltage Collapse Isolated Subsystem

The Blackout in Southern Sweden and Eastern Denmark, September 23, 2003
The blacked-out area after the grid separation at 12.37

Blackout of 23 September 2003 in Southern Sweden and Eastern Denmark cont'd
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"

September 28, 2003 Blackout of Italy

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

Italian System Blackout of 28 September 2003 cont'd
the cascading trend continued and the power deficit in Italy was such that the ties to France, Austria and Slovania were tripped the outages left the Italian system with a power shortage of 6400 MW the frequency decay could not be controlled adequately by under-frequency load shedding over the course of several minutes, the entire Italian System collapsed at 3:28:00 The blackout affected about 60 million people total energy not delivered 180 GWh worst blackout in the history of Italy power was restored after 3 hours in the northern area and during the same day for most of Italy

What Can We Do To Prevent Blackouts?

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 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

Reliability Criteria At present, systems designed and operated to withstand loss of any single 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 against catastrophic failures !

Improved Protective Relaying
State-of-the-art protective relaying for generating units and transmission 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

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 !

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

Need for advancing this technology !
"Defense Plan" Against Extreme Contingencies: Coordinated Emergency Controls Judicious choice of emergency controls protection against different scenarios identification of scenarios based on past experience, knowledge of unique characteristics of system, probabilistic approach Coordination of different emergency control schemes complement each other act properly in complex situations Response-based emergency controls should generally be preferred "self-healing" power systems Need for advancing this technology !

Examples of Response-Based Emergency Control Schemes
Scheme for prevention of voltage collapse in Eastern Ontario fully automated and coordinated emergency controls for voltage stability Transient Excitation Boosting for enhancing transient (angle) stability of systems with dominant interarea swing

Example 1: Prevention of Voltage Collapse in 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

Example 1: (cont'd) The coordinated scheme:
Fast reclosure of major lines (1.3s) first line of defense Load rejection (1.5s) 9 blocks, 750 MW; armed by operator voltage/time dependent Shunt capacitors switching (1.8 to 8.0s) 36 banks in 17 TSs Transformer ULTC blocking at 14 TSs unblocked when voltage recovers

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

1300 MW 1374 MW

Response-Based Emergency Controls 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

Effect of TSEC on Transient Stability

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

Comprehensive Stability Analysis Tools
Powerful analytical tools have been developed capable of comprehensive analysis for system design and operation: all forms of stability large systems with detailed models complementary use of time-domain and modal analysis automated procedures for considering large number of scenarios Industry gradually shifting to the use of new tools Lack of widespread understanding and appreciation for the use of eigenvalue based modal analyses techniques

State-of-the-Art On-Line Dynamic Security Assessment (DSA)
Practical tools have been developed with the required accuracy, speed and 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
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 *

Powertech DSA Tools (cont'd)
Automated procedures for: contingency screening and ranking consideration of a large number of scenarios stability limit search power flow dispatch determination of stability margins identifying remedial measures for maintaining stability and increasing stability margins Significant savings in computation and engineering times

On-Line Voltage Stability Assessment Tool (VSAT)

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

Contingency Selection Module
Impractical to consider every conceivable contingency A limited number (typically 20) critical contingencies determined for detailed studies Performance Indices based on a few power flow solutions and reactive reserve not reliable A fast screening method used: based on exact margin to voltage collapse and full power flow solutions number of power flow solutions 1.2 to 2.0 times number of contingencies Supplemented with user-specified contingencies

Fig. 4 Automatic Critical Contingency Selection

Security Computation Module
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

Modelling: Inputs and Outputs: Inputs Output
generator capability curves governor response, economic dispatch, AGC nonlinear loads control of ULTCs, switched shunts, etc. Inputs and Outputs: Inputs list of contingencies produced by screening and ranking (+user defined) base case powerflow from state estimator definition of SCRs voltage security criteria and definition of parameter of stress Output secure region in secure region space

Secure Operating Region

Remedial Measures Module
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
Objective is to identify the most effective remedial measures to give the 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 flow solves for the most severe contingency

Expanding the Secure Region: Remedial Measures

Fast Time-Domain Simulation Module
Determines the essential dynamic phenomena without step-by-step numerical 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 steady state equations

Mathematical Formulation
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

Mathematical Formulation
At each equilibrium point, Z=Zi and the system operating condition is 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.

VSAT Structure Powerflow Solver

Transient Stability Assessment Tool (TSAT)

Transient Stability Assessment (TSA)
Time-domain simulations essential modeling detail and accuracy Sole dependence on time-domain simulations has severe limitations high computational burden no stability margin/sensitivity information requires considerable human interaction Supplementary techniques for speeding up and automating overall process Methods available for deriving useful indices Transient Energy Function (TEF) Signal Energy Analysis Extended Equal Area Criterion (EEAC)

A Practical Tool for TSA
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

EEAC Integrates the dynamic response in the multimachine space, and maps the resultant trajectory into a set of one-machine-infinite-bus planes By applying complementary cluster center of inertia (CCCI) transformations Keeps all dynamic information in the multimachine space Stability analysis can be quantitatively performed for the image OMIB systems Has the same accuracy and modeling flexibility Fast, quantitative

EEAC 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 The classical equal area criterion can be extended to the visual OMIB system Stability margin of the system is defined as Thus, £ h £ 100, and h> 0 if the system is stable h £ 0 if the system is unstable h can be used as a stability index

Use of EEAC Theory 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

Results - Test System System description Interface Contingency
BC Hydro system 1430 buses 186 generators 4 HVDC links Interface GMS and PCN output Base case transfer = 3158 MW Contingency Three phase fault at GMS 500 kV bus Tripping of one of two 500 kV lines from GMS to WSN

Limit Search Results

Speed Enhancement: Parallel Processing
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 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

Contingency Screening & Ranking (EEAC)
TSAT Structure Powerflow Dispatcher Time-Domain Simulation Stability Indices Increase Transfer Remedial Measures Must Run Contingencies Transaction Definitions Security Limit? Sufficient Margin? STOP Yes No Full Contingency List Solved Powerflow + Dynamic Data Contingency Screening & Ranking (EEAC)

Computational Performance of DSA
Target cycle time from capture of state estimation to completion of security assessment for all specified transactions: 20 minutes TSA and VSA functions performed in parallel distributed processing on separate CPUs This can be readily achieved with low cost PCs

Computational Speed of DSA (cont'd)
Power System model with 4655 buses, 156 generators, using 1.7 GHz, Pentium 4 PC with 256 MB memory Voltage Stability Assessment: screening 300 contingencies secs detailed security analysis 1.2 secs with 20 critical contingencies one transfer limit search secs Transient Stability Assessment: screening 100 contingencies secs 10 second simulations with secs 10 critical contingencies one transfer limit search secs total time for complete assessment < 5 mins

Future Trends in DSA: Intelligent Systems
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"

DSA Using Intelligent Systems

Real-Time Monitoring and Control: An Emerging Technology
Advances in communications technology have made it possible to monitor power systems over a wide area remotely control many functions Research on use of multisensor data fusion technology process data from different monitors, integrate and process information identify phenomenon associated with impending emergency make intelligent control decisions A fast and effective way to predict onset of emergency conditions and take remedial actions The ultimate "self-healing" power system !

Distributed Generation (DG)
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

Reliability Management System
Roles and responsibilities of individual entities well chosen, clearly defined and properly enforced Coordination of reliability management Need for a single entity with overall responsibility for security of entire interconnected system real-time decisions System operators with high level of expertise in system stability phenomena, tools

Summary The new electricity supply industry presents increasing challenges for stable and secure operation of power systems 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 Industry is yet to take full advantage of these developments cont'd

Summary (cont'd) Future directions will be to explore new techniques which can better 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 Wide-spread use of distributed generation could be a cost effective means of minimizing the impact of power grid failures

Kundur Power Systems Solutions, Inc.

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