462 AC POWER SYSTEMS
( )Figure 10.3.3Single-phase, three-wire residential wiring circuit.Power-System Loads
Figure 10.3.4 represents aone-line (single-line) diagramof a part of a typical three-phase power
system. Notice the symbols that are used for generators, transformers, buses, lines, and loads.
Recall that a bus is a nodal point.
Let us now consider the addition of aload busto the operating power system. Service
classifications assigned by the electric utilities include residential, commercial, light industrial,
and heavy industrial loads, as well as municipal electric company loads. Let us prepare a load-
bus specification, which is a summary of the service requirements that must be provided by the
electric utility. Referring to the simple model of Figure 10.3.5, as anexample,let the total new
load connected to the system be 220 MVA at 0.8 power factor lagging; let the service be provided
at the subtransmission level from a radial 115-kV circuit; let the 115-kV transmission line have
a per-phase series impedance of 3+j 8 .
The load-bus data (specifications), transmission line data, and source-bus data are given in
Tables 10.3.1, 10.3.2, and 10.3.3, respectively. The reader is encouraged to work out the details.
Looking at the load-bus data, the amount of reactive power that is needed to provide 100%
compensation is 120 MVAR. Let us then add a three-phase shunt capacitor bank with a nominal
voltage rating of 115 kV and a reactive power rating of 120 MVAR. The per-phase reactance of
the bank can be computed asX=V^2 /Q= 1152 / 120 = 110. 2 . The load-bus data with power
factor correction are given in Table 10.3.4.
Thetransmission-system efficiencyis defined as the ratio of the real power delivered to the
receiving-end bus to the real power transferred from the sending-end bus. This efficiency, which
is a measure of the real-power loss in the transmission line, comes out as 94.7% without power
factor correction, and 96.5% with power factor correction.
Thetransmission-line voltage regulation(TLVR) is the ratio of the per-phase voltage drop
between the sending-end and receiving-end buses to the receiving-end per-phase voltage (or
nominal system per-phase voltage). It can also be expressed as%TLVR =VRNL−VRFL
VRFL× 100 (10.3.1)