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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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THANK YOU