7/29/2019 Voltage Instability Collapse an Overview

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Voltage InstabiIity/Collapse - An OverviewRodolfo J. KoesslerPower Technologies, Inc.Schenectady, New York

AbstractVoltage instability and voltage collapse are a frequentconcern on heavily stressed power systems. This papercontains an overview of what, in the author's view, arethe main aspects to be considered when addressingsuch problems. Also included in the paper is a reviewof some of the analytical tools that are available forvoltage stability analysis, and a discussion of thefundamental physics behind the phenomena.The ProblemThe cause behind most voltage stability problems is alack of sufficient reactive power resources to satisfy thereactive component of loads and losses.

Figure1Insufficient Reactive Resources

It is clear, for example, that in an isolated systemdeficient in reactive power sources, such as that on theleft-hand side 0 1 Figure 1, there is no steady-statesolution, whatever the system operating voltage.Most modern power systems, however, areinterconnected (riight-hand side of Figure 1) ,and servetheir loads by a combination of local resources(generators, shurit capacitors,SVCs,etc.) and importedreactive power. There is a widespread conception,however, that there is alwavs a low enough voltage, atwhich the necessary reactive power will flow into sucha system. This i:j not true. Consider for example, thecase in Figure 2, where a radial system is fed over along transmisssion line. The figure shows net reactivepower flowing from the line to the load, as a function ofload voltage, ancl for different MW flow levels.

It is clear from Figure 2 that although it is true thatvoltage reductions will indeed lead to an initial increasein reactive imports, a point is always reached wherefurther reductions in voltage will not only fail to increasereactive imports but, instead, will lower the reactivesupport to the load.This is because, whereas reactive power imports arelinearly proportional to voltage gradient, the relationbetween reactive losses and current is quadratic and,consequently, so is their relationship with voltage. Apoint is therefore reached where losses overwhelmvoltage gradient induced imports, thus setting a limit onthe reactive power support that transmission canprovide a system, at whatever voltage the receiving endoperates, and independently of the resources that areavailable at the sending-end of the system.This limit decreases as the MW loading of thetransmission increases, potentially leading totransmission becoming a consumer, rather than asupplier, of reactive power.

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7/29/2019 Voltage Instability Collapse an Overview

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ComplexitiesAlthough the above description addresses thefundamentals of most voltage stability problems, theireffective analysis and solution constitutes a significantchallenge to many planners and operators. This isbecause of, among other reasons:

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0 Dimensionality. Whereas anguladfrequencydynamics are normally limited to those of powerplants, any bus in the system, from bulktransmission to distribution, is a potentialcandidate for voltage instability.

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0 Local Nature. Voltage instability can occur in themidst of an otherwise perfectly healthy system.Contingencies that are critical to voltage stabilitymay not be the same than those critical toangular stability.0 Data Requirements. In-depth analysis of voltage

instability may require information not normallyavailable to the system planner. This includes:overexcitation limiter characteristics, compositionof the load (motors, thermostat heating, etc.),on-load tap changer (OLTC) characteristics,location of shunt compensation relative toOLTCs, etc.0 Understanding and Application of AnalyticalTools. The myriad techniques available forvoltage stability analysis (VP and QV Curves,Optimal Power Flow, Modal Analysis, and Long-Term Dynamic Simulation, to name a few) can

become quite overwhelming to the analyst, whomay lose sight of the underlying nature of theproblemSteady State Analytical ToolsThe best way to start a voltage stability investigation iswith a conventional power flow, with a constant powerload model, and reactive power limits on generators.Base cases with loads and/or transfers of 5 to 10%above peak levels should be employed to guaranteeadequate margins to voltage stability.If, after comprehensive contingency analyses, nodivergent cases are detected, it is likely that voltagestability is not a problem. On the other hand,contingencies that fail to converge are candidates forfurther analyses.Two of the most popular conventional power flow-basedtools that can be used for such analyses are Voltage-Power ( VP) and Reactive-Voltage (QV) curves. Theirapplication is illustrated in the curves of Figures 3and4, respectively, which were derived from a highly-stressed 345 kV system, before and after a criticalstuck-breaker contingency. These are two alternative

ways of analyzing the same phenomena. Operatingconditions in the VP plane may be "transformed" intothe QV plane, and vice versa. See, for example, pointsA, B and C in both figures.The VP curves (Figure 3) are the result of a series ofload flow solutions with gradually increasing powertransfers. V is the voltage at a critical bus, P is thepower transfer across a specified interface. There is atransfer level beyond which the power solution fails,shown in Figure 3 by an increase in solutionmismatches. This is the "knee" of the VP Curves. Mostutilities plan the network operating point to the left ofthis knee following disturbances. A stable operatingpoint isusually determined by requiring a large enough"power margin" between the operating point and thepoint of instability.

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- TapRatioI . 4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . II+/- 115 kV Voltage II - istribution Vollage 11.1 -

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0.8 _ _ _ - - ~ ~ - +---J 0.80 1 2 3 4Time (Minutes)

Figure10Separation from System"B"Figure 10 shows voltage traces and transformer tapperformance for a case with a 100% constant currentreal part and 100% constant admittance reactive partload model. The traces indicate a steeper voltagedecay than in the reactive-deficiency example of Figure6.(Note: the time scale of Figure 10is one tenth of thatin Figure 6). This is the result of a weaker system andthe lack of local generation. Without the temporaryoverload capability of generators, the rate of decay insystem voltages is dictated by transformer tap delaysettings.Figure 11 corresponds to the same disturbance as inFigure 10, except that 60 % of loads are assumed to

consist of small motors. System performance is nowgoverned by the fast dynamics of motors stalling, whichleads to an almost immediate voltage collapse..

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Figure11Separation from System "B" with 60% Motor Load

ConclusionsThis paper summarizes the concepts and techniquesthat, in the author's opinion, are necessary for athorough review of voltage instability and collapse. Thetechniques range from the simpler contingencyanalyses to the more comprehensive dynamicsimulation.System performance is shown to be affected by boththe fast dynamics common in normal stabilityinvestigations and the slow dynamics of tap changersand maximum excitation limiters. Other effects such asload self-restoration, automatic capacitor switching andAGC action may also play a significant role.The simulations show a variety of possible outcomes tovoltage instability. Some of them are slow indevelopment and thus provide an opportunity foroperator action. Others may lead to system collapsewithin minutes or seconds following a disturbance.BiographyRodofo J . Koessler (M'86) received the degree ofEngineer of Electromechanics from the University ofBuenos Aires, Argentina and the M.E. degree in PowerSystems from the Rensselaer Polytechnic Institute,Troy, NY in 1979 and 1982, respectively.From 1978 to 1981, he was with Servicios Electricosdel Gran Buenos Aires, working with the PowerSystems Planning Department. He joined PowerTechnologies, Inc. in 1985 and is presently a SeniorEngineer in the Utility System Performance unit. Mostof his work at PTI has been in the area of dynamicperformance and model development. He is a SeniorMember of the IEEE-and its Power EngineeringSociety.

0 1997The Institution of Electrical Engineers.P rinted and published by the IEE, Savoy Place, London WC2R OBL, UK .

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