TYPES OF SWITCHED CAPACITOR CONTROLS PART 2

TYPES OF SWITCHED CAPACITOR CONTROLS PART 2
What are the different types of switched capacitor control?

In continuation of the types of switched capacitors control, we have include some intrinsic characteristics of the power system as the basis for capacitor switching which includes;  voltage, current, kvar, power factor, and combination of some of mentioned capacitor controls.

Voltage Control

A voltage control switches a capacitor on when line primary voltage is low, and switches the capacitor off when the voltage is high. The advantages of a voltage control include: (1) Primary line voltage is often closely related with line load and power factor, so the control responds to actual conditions; (2) Line voltage is an important index of the overall quality and efficiency of the feeder, so the control responds to a critical line condition; (3) It is simple to install, set, test and maintain; and (4) Although it requires a voltage signal, it does not require a line current signal, which is much more difficult to provide.

The disadvantages of voltage control include: (1) Primary line voltage in many cases does not correlate with line load and power factor. (2) A voltage control is prone to unstable operation if not set properly.

Current Control

A current control switches a capacitor on when a primary line current is high and switches the capacitor off when the current is low. The main advantage of a current control is that is sensitive to actual line loading in place of sensitivity to some often inaccurate estimate of line loading.

The disadvantages of a current control include: (1)It is insensitive to the power factor of the load, so it does not follow the true feeder kVAR requirements; (2)It does not act directly to help correct line voltage; and (3)It requires that a current sensor be installed on the primary line. The current sensor can either be a current transformer (CT) or a less expensive specialty current sensor designed for switched capacitor use. A current transformer is a relatively expensive device and it is troublesome to install on the main line conductor. A specialty current sensor is less expensive and relatively easy to install, but these sensors can be extremely difficult to calibrate for acceptable accuracy because they are sensitive to the type of line conductor, to insulator tie wires, and to nearby conductors of other phases.

Power Factor Control

A power factor control switches a capacitor on when lagging power factor is low and switches the capacitor off when lagging power factor is very high or when power factor is leading. The main advantage of a power factor control is that it is sensitive to a parameter related to kVAR load for which the capacitor is intended to compensate.

The disadvantages of a power factor control include: (1) It is sensitive to load magnitude, making sensitivity to power factor of little value in detecting actual kVAR load; (2) It requires a current sensor (as well as a voltage sensor), leading to the problems stated above concerning a current control; and (3) It does not act directly to help correct line voltage. The disadvantages of a power factor control far outweigh the advantages, so this type of control is not recommended.

kVAR Control

A kVAR control switches a capacitor on when load kVARs are high and switches the capacitor off when kVARs are low. The main advantage of a kVAR control is that it is directly sensitive to the correct load component, kVARs, rather than sensitive to an often inaccurate estimate of kVAR flow. Other advantages include a lesser likelihood of coordination problems with other capacitors or voltage regulators on the same feeder and better direct response for correcting power factor at wholesale delivery points.

The disadvantages of a kVAR control include: (1) It requires a current sensor, leading to the problems stated above concerning a current control; (2) It does not act directly to help correct line voltage; (3) Relatively sophisticated technical expertise is required to install, calibrate, test, and maintain this type of control. For many distributors, technical personnel man-hours are not available to adequately attend to a large number of complex capacitor controls. Improper calibration of a kVAR control immediately negates its main advantage (accurate response to the proper load component) and may cause the capacitor to become part of the problem rather than the controls are installed, calibrated, and maintained diligently.

Combination Control

A combination control switches a capacitor on or off by means of a control circuit that responds to more than one input signal. Combination controls are used to combine the advantages of two or more simpler controls and eliminate some of the disadvantages. The most frequently used type of combination control is a voltage override installed with a time-clock, temperature, current, or kVAR control. The voltage override eliminates one of the disadvantages of the listed simpler controls – lack of direct voltage control – by switching the capacitor to help correct voltage regardless of how the simpler control would act.

The main disadvantage of combination controls is their complexity. For combination controls to work properly, they must be correctly installed, calibrated, tested, and maintained. The distributor using such controls must have personnel available who are trained in control circuit wiring and who have time to devote to capacitor controls.

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