UNDERSTANDING SERIES CAPACITORS

Series Capacitors are generally applied to compensate the excessive inductance of long transmission lines, in order to reduce the line voltage drop, improve its voltage regulation, minimize losses by optimizing load distribution between parallel transmission lines, and to increase the power transfer capability. Similarly, series capacitors are also installed in electrical power systems to improve its voltage stability.

In contrast to a shunt capacitor, series compensation results in a voltage rise at the end of the feeder that varies directly with load current. In other words, voltage rise is zero at no load and maximum at full load. Consequently, series capacitors do not need to be switched in response to changes in power demand. In addition, a series capacitor will require relatively smaller kVAR and kV ratings than a shunt capacitor providing an equivalent voltage regulation.

Series Capacitors

How do they work?

Series capacitors positively affect the voltage and reactive power balance. When the load current passes through the capacitor, the voltage drop over the capacitor varies in proportion to the current. The voltage drop is capacitive, such that it offsets the inductive voltage drop, which also varies with the load current. The result is an automatic stabilizing effect on the voltage in the network.

Voltage Rise Due to Series Capacitors

At the same time, series capacitors generate reactive capacitive power, which improves the system power factor resulting to reduced line current and line losses and the increase of load capacity. The generated reactive capacitive power varies proportionally to the square of the load current. Thus, the reactive power is automatically regulated.

Applications

Series capacitors can correspondingly find purpose at all high-impedance and high-current loads such as, but not limited to the following:

·      Transmission lines of 400 km and more for a radial line, and 800 km and more for a symmetrical line.

·   High voltage distribution network that has a high series inductive reactance to improve its receiving-end voltage.

·        Electric arc and resistance welding transformers as for spot, seam and butt welding.

·        Induction furnace, where the heating is due to eddy current losses induced by the magnetic field.

·        Large scale electrolysis of aluminum, copper or zinc.

·     Electric arc furnace, where heating is caused by arc plasma between the two electrodes. The arcing makes the circuit highly inductive, besides generating unbalanced currents (third harmonics), due to different touchdown arc distances in the three electrodes which make it a non-linear impedance load.

·        Mitigation of voltage fluctuations (flicker).

Disadvantages and Cautions

There are several concerns that must be evaluated in the application of series capacitors. The following disadvantages limit the application of series capacitors on the electric power systems:

·    They cannot provide reactive compensation for feeder loads and do not significantly reduce system losses.

·      Series capacitors can only release additional system capacity if it is limited by excessive line voltage drop. On the other hand, shunt capacitors are effective when system capacity is limited by high line current as well.

·        They cannot tolerate fault current. This would result to an overvoltage and should be prevented by bypassing the capacitor through an automatic switch. In addition, an arrester must be connected across the capacitor to divert current until the switch closes.

·         Resonance and/or hunting with synchronous and induction motors

·         Ferroresonance

References:

Alexander, G.E. (n.d.). Series Compensated Line Protection – A Practical Evaluation

Dugan, R., McGranaghan, M., Santoso, S., and Beaty, H.W. (2004). Electrical Power Systems Quality (2^nd ed.). New York: McGraw-Hill