1. Coupling Transformer Ignored
2. Coupling transformer considered

**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 V_{SVC} as a function of change in the SVC current I_{SVC}.
Thus for constant equivalent source voltage V_{s},^{}

^{Ø} ∆V_{SVC}
= -X ∆I_{SVC} And also ∆V_{SVC} = I_{SVC} B_{SVC}

^{Ø} For incremental changes the equation is
linearized to give

∆I_{SVC}
= B_{SVC}o ∆V_{SVC} + ∆B_{SVC} V_{SVC0}

On
substitution

∆V_{SVC}
/∆B_{SVC} = - V_{SVC0} / (ESCR + B_{SVCo})

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 X_{T} is separated
from X_{s}.^{}

^{Ø }The high
voltage V_{H} is related to low voltage side V_{SVC} as^{}

V_{SVC} / V_{H} = 1 / 1+ X_{T}
B_{SVC}

^{Ø }Linearizing
the above equation gives^{}

∆V_{SVC}
(1+ XT B_{SVC0}) + V_{SVC0} XT ∆B_{SVC} = ∆VH

On
substitution

∆V_{H}
/ ∆B_{SVC} = - V_{H0} / (ESCR + B_{SVC0}) . (1 - X_{T}ESCR
/ 1 + B_{SVC0})

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