POWER DISTRIBUTION--Shunt Capacitors in Distribution Systems

Loads in electric power systems consume real power (MW) and reactive power (M_var). At power plants, many of which are located at long distances from load centers, real power is generated and reactive power may either be generated, such as during heavy load periods, or absorbed as during light load periods. Unlike real power (MW), the generation of reactive power (Mvar) at power plants and transmission of the reactive power over long distances to loads is not economically feasible. Shunt capacitors, however, are widely used in primary distribution to supply reactive power to loads. They draw leading currents that offset the lagging component of currents in inductive loads. Shunt capacitors provide an economical supply of reactive power to meet reactive power requirements of loads as well as transmission and distribution lines operating at lagging power factor. They can also reduce line losses and improve voltage regulation.

EX. 3 Shunt Capacitor Bank at End of Primary Feeder

ILL. 21 Single-line diagram of a primary feeder

Ill. 21 shows a single-line diagram of a 13.8-kV primary feeder supplying power to a load at the end of the feeder. A shunt capacitor bank is located at the load bus. Assume that the voltage at the sending end of the feeder is 5% above rated and that the load is Y-connected with R_Load = 20_/phase in parallel with load jX Load = j40 _/phase. (a) With the shunt capacitor bank out of service, calculate the following: (1) line current; (2) voltage drop across the line; (3) load voltage; (4) real and reactive power delivered to the load; (5) load power factor; (6) real and reactive line losses; and (7) real power, reactive power, and apparent power delivered by the distribution substation. (b) The capacitor bank is Y connected with a reactance XC ¼ 40_/phase. With the shunt capacitor bank in service, redo the calculations. Also calculate the reactive power supplied by the capacitor bank. (c) Compare the results of (a) and (b).

SOLUTION

a. Without the capacitor bank, the total impedance seen by the source is:

c. Comparing the results of (a) and (b), with the shunt capacitor bank in ser vice, the real power delivered to the load increases by 23% (from 6.033 to 7.430 MW) while at the same time:

_ The line current decreases

_ The real and reactive line losses decrease

_ The voltage drop across the line decreases

_ The reactive power delivered by the source decreases

_ The load voltage increases

The above benefits are achieved by having the shunt capacitor bank (instead of the distribution substation) deliver reactive power to the load. 9 The location of a shunt capacitor bank along a primary feeder is important. If there were only one load on the feeder, the best location for the capacitor bank would be directly at the load, so as to minimize I^2 R losses and voltage drops on the feeder. Note that if shunt capacitors were placed at the distribution substation, I^2 R feeder losses and feeder voltage drops would not be reduced, because the total power including MW and Mvar would still have to be sent from the substation all the way to the load. Shunt capacitors at distribution substations, however, can be effective in reducing I^2 R losses and voltage drops on the transmission or subtransmission lines that feed the distribution substations.

For a primary feeder that has a uniformly distributed load along the feeder, a common application is the ''two-thirds'' rule; that's , place 2/3 of the required reactive power 2/3 of the way down the feeder. Locating shunt capacitors 2/3 of the way down the feeder allows for good coordination between LTC distribution substation transformers or voltage regulators at the distribution substation. For other load distributions, computer software is available for optimal placement of shunt capacitor banks. We note that capacitors are rarely applied to secondary distribution systems due to their small economic advantage.

During the daily load cycle, reactive power requirements change as a function of time. To meet the changing reactive power requirements, many utilities use a combination of fixed and switched capacitor banks. Fixed capacitor banks can be used to compensate for reactive power requirements at light loads, and switched capacitor banks can be added during heavy load conditions. The goal is to obtain a close-to-unity power factor throughout the day by switching capacitor banks on when needed and o¤ when not needed.

Methods of controlling switched capacitor banks include the following:

1. Voltage control

2. Current control

3. Var control

4. Time control

5. Temperature control

6. Radio dispatch/SCADA control