Electric Power Generation, Transmission, and Distribution

where P 2 ¼Power transmitted through the

transmission system

V 1 ¼Voltage at sending end of the line

V 2 ¼Voltage at receiving end of trans-

mission line

XL¼Reactance of the transmission line

d¼Phase angle between V 1 and V 2

Equation (18.1)shows that if the total react-

ance of a transmission system is reduced by

installing capacitance in series with the line, the

power transmitted through the line can be in-

creased.

With a series capacitor installed in the line, Eq.

(18.1) can be written as

P 2 ¼

V 1 V 2 sind

XLXC

(18:2)

¼

V 1 V 2 sind

XL(1K)

(18:3)

where K¼

XC

XL

is degree of the compensation,

usually expressed in percent. A 70% series com-

pensation means the value of the series capacitor

in ohms is 70% of the line reactance.

18.5 Series Capacitor Bank

A series capacitor bank consists of a capacitor
bank, overvoltage protection system, and a bypass breaker, all elevated on a platform, which is insulated
for the line voltage. See Fig. 18.6. The overvoltage protection is comprised of a zinc oxide varistor and a
triggered spark gap, which are connected in parallel to the capacitor bank, and a damping reactor. Prior
to the development of the high-energy zinc oxide varistor in the 1970s, a silicon carbide nonlinear
resistor was used for overvoltage protection. Silicon carbide resistors require a spark gap in series
because the nonlinearity of the resistors is not high enough. The zinc oxide varistor has better nonlinear
resistive characteristics, provides better protection, and has become the standard protection system for
series capacitor banks.

V

I

(a)

V

I

(b)

V

I

(c)

FIGURE 18.3 TCR voltage (V) and current (I) wave-
forms for three conduction levels. Thyristor gating
angle¼a; conduction angle¼s. (a)a¼ 908 ands¼
1808 ; (b)a¼ 1208 ands¼ 1208 ; and (c)a¼ 1508 and
s¼ 608.

MVAR

THYRISTOR

CONDUCTION

ANGLE

CAPACITOR BANKS

SWITCHED

0 1 23

0

180 °

FIGURE 18.4 Reactive power variation of TCR with switched capacitor banks.