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CHOOSING CAPACITOR OR VOLTAGE REGULATORS FOR VOLTAGE REGULATION


Which is better for voltage regulation, capacitor ot voltage regulator?

One of the greatest advantages gained by the proper sizing and location of distribution capacitors is voltage improvement. By placing leading volt-amperes reactive (VAR) loads (capacitors) near lagging VAR load centers (motors for example), the lagging VARs on a system basis are cancelled with an associated increase in voltage.

However, care is required not to exceed the lagging VAR requirement at any time. Capacitors that may be sized for peak load requirements may need to be removed from the circuit as the load drops, usually through switched controls. Capacitors draw a specific leading current that generates a voltage rise through the reactive ohms of the system impedance (see Section 2.6 of this bulletin for these calculations). This voltage rise may be unneeded and even undesirable during low load conditions.

Capacitors or Voltage Regulators

Care should be taken in choosing between capacitors and voltage regulators for voltage improvement. Often, both are necessary to have a well-balanced system operating at maximum efficiency. Shunt capacitors provide some voltage rise and can do so at a lower cost than a line regulator. Sample calculations are shown in the following sections. However, for some load conditions, the voltage rise offered by capacitors may be excessive and cause problems for customers’ connected equipment. Higher cost regulators offer a means for maintaining more constant system voltage. The combination of regulators and capacitors provides the best of both worlds.

A $1,500 investment in 300 kilovolt-amperes reactive (kVAR) of fixed capacitors will provide about a 3 volt rise (more or less, depending on where the capacitors are located) when connected on a distribution feeder. That rise is either on or off depending on whether the capacitors are on line or off. This capacitance provides power factor correction by cancelling the effects of 300 kVAR of lagging reactive load.

A single-phase line regulator, costing about $8,500, can provide sixteen, 3/4 volt (5/8 percent) steps up or down (on a 120 volt base), depending on whether the regulator is raising or lowering the voltage. Although this step range approximates a 12-volt boost or buck capacity, the Rural Utilities Service (RUS) suggests that effective voltage analysis has shown that the system operator should allow only an 8-volt variation per regulator. Moreover, from voltage analysis, the application of only two regulators in series along a feeder is recommended as a maximum in addition to the substation regulator or Load Tap Changing (L.T.C.) transformer. If more than two series regulators are boosting and there is a fault near the end of the line when an oil circuit recloser (OCR) opens, the line voltage can go up too high and damage customer owned equipment. This means that if any line regulator needs to raise to step 11 or greater, the incoming voltage, serving the last consumer prior to the regulator is below 118-volts, which is outside the Class A voltage limits that RUS recommends be observed as a design criteria.

Engineers should be wary of the temptation to install three times the needed capacitors instead of three regulators. At $4,500 (3*$1,500), a 12 volt voltage improvement can be gained fairly inexpensively with capacitors, relative to voltage regulators at $25,500 (3*$8,500). This gain may, however, be at the cost of higher losses and power factor penalty charges when the capacitors needed for the 12 volt voltage improvement are far in excess of connected inductive loading and they are allowed to drive the power factor leading. In general, voltage regulators should be used to maintain accurate control of voltage throughout the load cycle (control voltage fluctuation), and shunt capacitors should be used to correct low power factors.

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