Electric Power Generation, Transmission, and Distribution

The per-unit value of the susceptance can also be determined
by first computing the actual value [Eq. (21.184)] and then
dividing by the base admittance of the system.
With the susceptance computed, the line currents serving
the capacitor bank are given by

ICa¼jBaVan

ICb¼jBbVbn

ICc¼jBcVcn (21:188)

21.1.7.2 Delta Connected Capacitor Bank

A delta connected capacitor bank is shown in Fig. 21.28.
Equations (21.184) through(21.187)can be used to determine the value of the susceptance in actual
Siemans or per unit. It should be pointed out that in this case, the kV will be a line-to-line value of the
voltage. Also, it should be noted that in Eq. (21.187), the base line-to-neutral voltage is used to compute
the per-unit line-to-line voltage. This is a variation from the usual application of the per-unit system
where the actual line-to-line voltage would be divided by a base line-to-line voltage in order to get the
per-unit line-to-line voltage. That is not done here so that under normal conditions, the per-unit line-
to-line voltage will have a magnitude of

ffiffiffi

3

p
rather than 1.0. This is done so that Kirchhoff’s current law
(KCL) at each node of the delta connection will apply for either the actual or per-unit delta currents.
The currents flowing in the delta connected capacitors are given by

ICab¼jBabVab

ICbc¼jBbcVbc

ICca¼jBcaVca (21:189)

The line currents feeding the delta connected capacitor bank are given by

ICa

ICb

ICc

2

4

3

(^5) ¼
10  1
 110
0  11
2
4
3
5
ICab
ICbc
ICca
2
4
3
(^5) (21:190)
21.2 Analysis
21.2.1 Power-Flow Analysis
The power-flow analysis of a distribution feeder is similar to that of an interconnected transmission
system. Typically what will be known prior to the analysis will be the three-phase voltages at the
substation and the complex power of all the loads and the load model (constant complex power,
constant impedance, constant current, or a combination). Sometimes, the input complex power
supplied to the feeder from the substation is also known.
In Sections 21.1.3, 21.1.4, and 21.1.5, phase frame models were presented for the series components of
a distribution feeder. In Sections 21.1.6 and 21.1.7, models were presented for the shunt components
(loads and capacitor banks). These models are used in the ‘‘power-flow’’ analysis of a distribution feeder.
A power-flow analysis of a feeder can determine the following by phase and total three-phase:
.Voltage magnitudes and angles at all nodes of the feeder
.Line flow in each line section specified in kW and kVAr, amps and degrees, or amps and power
factor
ICab
Bab
Bbc
Bca
ICca ICbc
ICa
ICb
ICc
FIGURE 21.28 Delta connected capaci-
tor bank.