Shunt Capacitors have several uses in the electric power systems. They are utilized as sources of reactive power by connecting them in line-to-neutral. Electric utilities have also connected capacitors in series with long lines in order to reduce its impedance. This is particularly common in the transmission level, where the lines have length in several hundreds of kilometers. However, this post will generally discuss shunt capacitors.
Shunt capacitors are usually called “power factor correction capacitors,” although they also serve other functions and provide multiple benefits, which will be discussed in the succeeding paragraphs. Also, they are used at all voltage levels from end-user utilization to extra high voltages.
Shunt capacitors, either at the customer location for power factor correction or on the distribution system for voltage control, dramatically alter the system impedance variation with frequency. Capacitors do not create harmonics, but severe harmonic distortion can sometimes be attributed to their presence.
A shunt capacitor at the end of a feeder results in a gradual change in voltage along the feeder. Ideally, the percent voltage rise at the capacitor would be zero at no load and rise to maximum at full load. However, with shunt capacitors, percent voltage rise is essentially independent of load. Thus, automatic switching is often employed in order to deliver the desired regulation at high loads, but prevent excessive voltage at low loads. Moreover, capacitor switching may result in transient overvoltages inside customer facilities.
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Applications
Utilities use shunt capacitors at distribution and utilization voltages to provide reactive power near the inductive loads that require it. This reduces the total current flowing on the distribution feeder, which improves the voltage profile along the feeder, frees additional feeder capacity, and reduces losses. In fact, substation transformers experience lower loadings when utilities install sufficient capacitors on the distribution system. The reduced loadings not only improve contingency switching options on the distribution system, but also extend equipment life and defer expensive additions to the system.
At the transmission and subtransmission levels (69 kV and above), shunt capacitors increase the power transfer capability of a transmission system without requiring new lines or larger conductors. The long lead-time, problems associated with transmission line construction and high cost have driven most utilities to use high voltage capacitors more frequently than ever.
High voltage shunt capacitors also support the transmission system voltage, which is often necessary when the transmission grid is pushed to and perhaps beyond its design limits as a result of open access to the grid and decreased capital spending on network upgrades. Since the capacitors produce reactive power (VARs), generators no longer need to produce as much, enabling them to operate at higher power factors and produce more real power (watts). Also, fewer VARs transported through the transmission system not only release additional capacity on the lines, but also reduces system losses by reducing the total current flowing on the lines.
Shunt capacitors also slightly increase transmission bus operating voltages. As the transmission voltage increases, less current is necessary to supply a typical load, so transmission losses decrease again.
References:
Dugan, R., McGranaghan, M., Santoso, S., and Beaty, H.W. (2004). Electrical Power Systems Quality (2nd ed.). New York: McGraw-Hill
Fehr, R. (2003). The Trouble with Capacitors 1
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