Influence of SVC on the System Voltage

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_SVCo ∆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_TESCR / 1 + B_SVC0)