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8/4/2019 SVC Tutorials
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STATIC VARCOMPENSATORS
By
Prof. C. Radhakrishna
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CONTENTS
STATIC VAR COMPENSATORSSVCs are used for
Types of SVC and Controllers
SVC Control CharacteristicsModelling of SVC
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STATIC VAR COMPENSATORS
If the system has to operate close to stability limits insteady state, (with minimum margins), fast control ofreactive power is essential.
In recent years, thyristor controlled static varcompensators are being used for fast reactive powercontrol.
SVCs are used for
1. Increasing power transfer in long lines2. Stability improvement (both steady state and transient)
with fast acting voltage regulation
3. Damping of low frequency oscillations (correspondingto electromechanical modes)
4. Damping of subsynchronous frequency oscillations(due to torsional modes)
5. Control of dynamic over voltages
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Types of SVC and Controllers
Three basic types of SVCs1. Variable impedance type2. Current source type
3. Voltage source type
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SVC Control Characteristics
The voltage is measured at the high voltage side of the
transformer (or network bus) feeding the SVC and thecurrent is the reactive current, considered to be positivewhen SVC is inductive (absorbing reactive power).
In the control range, the SVC voltage is not maintainedconstant. The slope of the control characteristics is
positive (3 to 5%) and helps in (a) stable parallel operationof more than one SVC connected at the same orneighbouring buses and (b) prevent SVC hitting the limitstoo frequently.
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V
ES
Capacitive Inductive ISVC
Figure 1. Steady state control characteristics of variable impedance SVC
The reference voltage of the SVC is chosen such that, undernormal operating conditions, SVC delivers close to zeroreactive power so that the full control range of SVC isavailable for use whenever there is a transient.
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Figure 2: Equivalent circuit for SVC in control range
It is to be noted that when the SVC hits the capacitive limit, itbehaves like a fixed capacitor. Similarly, when it hits theinductive limit, it behaves as a fixed inductor. In the controlrange, the SVC can be modelled as a nonlinear voltage source
Es in series with a fictitious, fixed inductor Xs. The phase angle of the voltage source is same as that of the
bus voltage Vs. The value of the inductor is related to the slope of the
control characteristic.
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Modelling of SVC
For steady state analysis, it is adequate to model the steady state controlcharacteristics of SVC. Even for transient stability studies, where low
frequency phenomena are of interest, and AC network transients areneglected, steady state representation of SVC may be adequate as a firstapproximation. However to model the damping contribution of SVC, it is
necessary to consider the dynamics of SVC controller. The output of SVC is a time-varying susceptance Bsvc. The inclusion ofthis in the network results in a time varying admittance matrix which can beproblematic. This equivalent has to be updated at every time step when SVC current
is to be calculated.
REFERENCES :
[ 1 ] K.R. Padiyar : Power System Dynamics : Stability and Control , 2nd edition, BSPublications, 2002.
Figure 3: Modelof SVC controller
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CONCLUSIONS
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THANK YOU