INFLUENCE OF SVC ON THE SYSTEM VOLTAGE
1. Coupling Transformer Ignored
The SVC behaves like a controlled susceptance and its effectives in regulating the system voltage are dependent on the relative strength of the connected ac system. The system strength or equivalent system impedance, primarily determines the magnitude of voltage variation caused by the change in the SVC reactive current.
A simplified block diagram of the power system and SVC control system
Ø The variation in the VSVC as a function of change in the SVC current ISVC. Thus for constant equivalent source voltage Vs,
Ø ∆VSVC = -X ∆ISVC And also ∆VSVC = ISVC BSVC
Ø For incremental changes the equation is linearized to give
∆ISVC = BSVCo ∆VSVC + ∆BSVC VSVC0
∆VSVC /∆BSVC = - VSVC0 / (ESCR + BSVCo)
Where ESCR = Effective short circuit Ratio
2. Coupling Transformer Considered
Representation of power system and the SVC including the coupling transformer
Ø The representation of the SVC coupling transformer creates a low voltage bus connected to the SVC and the transformer reactance XT is separated from Xs.
Ø The high voltage VH is related to low voltage side VSVC as
VSVC / VH = 1 / 1+ XT BSVC
Ø Linearizing the above equation gives
∆VSVC (1+ XT BSVC0) + VSVC0 XT ∆BSVC = ∆VH
∆VH / ∆BSVC = - VH0 / (ESCR + BSVC0) . (1 - XTESCR / 1 + BSVC0)
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