presentation on "short-term voltage instability: effectson synchronous and induction machines "

7/27/2019 presentation on "Short-Term Voltage Instability: Effectson Synchronous and Induction Machines "

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Short-Term Voltage Instability: Effects

on Synchronous and Induction Machines

By : Under the guidance of :

SUDHAKAR .C .J Prof.T.S.PRASANNA


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INTRODUCTION

Several factors point out at an increasing risk of

voltage instability in the short term. increasing proportion of general induction motor load

increased use of various types of electronically controlled loads dispersed generation consuming reactive power without voltage control

Voltage instability on the short term is driven by fast recovering

load components that tend to restore power consumption in the

time scale of a second after a voltage drop caused by a

contingency

A typical such component is the induction motor


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Induction generators produce active power, but similarly to

motors, they consume reactive power.

In new wind farm installations, the trend is to use variablespeedwind generators

Induction machines are usually shunt-capacitor

compensated to improve their power factor. The reactivesupport provided by shunt capacitors varies with the square of

the voltage

Thus, in order to avoid induction machine instability,

dynamic and fast reactive compensation, such as provided byan SVC or a STATCOM, may become necessary


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Voltage instability in the cases discussed in this paper is driven by

induction machine loss of equilibrium, the latter is not likely to happenunless the voltage support provided by local synchronous generators is

lost or reduced.

This is usually the result of rotor current limitation brought about by the

over excitation limiter (OEL) of the synchronous machine

Usually the OEL acts as a slow device

In this case, other control mechanisms, such as load tap changers (LTCs)

have time to act, and the voltage stability problem becomes a long-term

one.

However, even transient over excitation is not allowed above an

instantaneous limit that must be enforced in the short term


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OEL MODELING ASPECTS

A.Transient OEL Modeling

The modeling of OEL is detailed in and has a significanteffect on voltage stability analysis

The field current fed back into the OEL is given in the per

unit system of the synchronous machine by the following

equation:


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Therefore, the OEL output signal VOEL isgiven by the following function:


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Evolution of Synchronous generator rotor current under

limitation


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CASE STUDY I: INSTABILITY OF INDUCTIONWIND GENERATORS IN A REALISTIC SYSTEM

The system studied here corresponds roughly to theSouth Evia region of the Hellenic InterconnectedSystem

The examined network consists of a localconventional steam power plant with twosynchronous generators of 176.5 MVA each and 19wind farms of total nominal capacity of 200 MW

connected to the distribution network It was seen in that with proper capacitor

compensation, the system is voltage stable in the longterm, even for very severe contingencies


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These results led to the conclusion that normal shunt

capacitor banks were sufficient to maintain stability by

allowing the synchronous machines a wider margin of

reactive support, and no dynamic reactivecompensation was deemed necessary

The system is unstable after the following double

contingency.

At t=10s , there is an outage of one local generator.

At t=50s , there is loss of one interconnection line


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Simulation results for South Evia network


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CASE STUDY II: VOLTAGE INSTABILITY

AFFECTING SYNCHRONOUS MACHINE

A. Test System Description

The test system analyzed is the 11-bus network presented in andcommonly used in voltage stability studies.

The one-line equivalent diagram is shown in Fig


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Simulation results (a) , (b) Induction motors . (c) , (d) Static loads


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(a) Synchronous generator rotor angle. (b) Induction

motor speed.


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UNDERVOLTAGE MOTOR SHEDDINGMachines terminal voltage (IM2 undervoltage shedding)


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Local generator rotor current (IM2 under voltage

shedding).


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(a) ,(b) Synchronous generator rotor angle. (c) ,(d) Induction

motor speed (IM2 undervoltage shedding).

CONCLUSION


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CONCLUSION

In this paper, we presented short-term voltage stability resultson two test systems. In both cases, the instability was initiated

after a contingency that forced a local synchronous generatorto its transient over excitation limit, which was taken to belower than is usual in practice. The driving force of theinstability was identified in both cases to be the inductionmachines, either wind generators or equivalent motorsrepresenting industrial and residential components of load.

Of particular interest in the second case was that the instabilityof induction motors was also affecting the local synchronousgenerator that was losing synchronism, thus leading to a localblackout.

Finally, an induction under voltage motor shedding wasproposed, in order to prevent the detected instability fromleading to a local blackout.

REFERENCES


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REFERENCES [1] C. W. Taylor, Power System Voltage Stability. New York:

EPRI/Mc- Graw-Hill, 1994.

[2] P. Kundur, Power System Stability. New York: McGraw-Hill,1994, EPRI Power System Engineering Series.

[3] B. M. Nomikos, E. G. Potamianakis, and C. D. Vournas,

Oscillatory stability and limit cycle in an autonomous system

with wind generation, in Proc. IEEE St. Petersburg Power Tech

Conf., St. Petersburg, Russia, Jun. 2730, 2005.

[4]Short-Term Voltage Instability: Effects On Synchronous

and Induction Machines Emmanuel G. Potamianakis andCostas D. Vournas, Fellow, IEEE, IEEE transactions on

power systems, vol. 21, no. 2, may 2006.


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presentation on "short-term voltage instability: effectson synchronous and induction machines "

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