TYPES OF SWITCHED CAPACITOR CONTROLS PART 1

TYPES OF SWITCHED CAPACITOR CONTROLS PART 1
What are the different types of switched capacitor control?

There are many methods of automatic control of switched capacitors. The On and Off switching controls can be achieved by a time clock, an ambient temperature sensor, a line voltage-sensing device, a line current-sensing device, a power factor sensor, line kVAR flow sensing or other means such as a remote control. Different types of capacitor controls can be used for various installations on the same feeder, and combination control schemes can be used even on a single installation. The following is a summary of the advantages and disadvantages of various capacitor control methods.

Time-Clock Control

A time-clock control switches a capacitor on for those hours of the day when load is expected to be the highest, and switches the capacitor off for expected light-load hours. Controls are available that automatically change the on-off schedule during weekends or even seasons and holidays. The advantages of a time-clock control include: (1) It is simple to install, set, test, and maintain; (2) It does not require that a current sensor be installed on the primary line conductor; and (3) It is not susceptible to unstable behaviour leading to frequent random operations.

The main disadvantage of a time-clock control is that it responds to expected loads instead of true loads. For example, a time-clock controlled capacitor will switch on during an unexpected cool summer day when air conditioning load does not develop and the capacitor is not needed. During such a day, several time-clock controlled capacitors could produce a leading power factor on the feeder or excessive voltage, or both.

Temperature Control

A temperature control switches a capacitor on during high or low outdoor temperatures and switches the capacitor off when the outdoor temperature is mild. The advantages of a temperature control include: (1) The load typical of most feeders is temperature sensitive, so the control responds to actual conditions rather than expected conditions; (2) It is simple to install, set, and maintain; (3) It does not require that a current sensor be installed on the line; and (4) If properly set, it is not susceptible to unstable behaviour.

The disadvantages of a temperature control include; (1) Temperature sensors are often inaccurate and difficult to calibrate, so the capacitor may not switch when expected; (2) Several inspections under a variety of weather conditions are necessary to truly test whether or not the capacitor is switching properly; (3) The control does not responds to loads that are insensitive to outdoor temperature, such as some industrial loads; and (4) The control does not distinguish between high power factor loads and low power factor loads. Inappropriate switching resulting from any of the above shortcomings can lead to out-of-range feeder power factor, or voltage, or both.

Remote Control

A remove control switches a capacitor on or off in response to a signal from a distributor’s SCADA control center. The advantages of a remote control include: (1) A large number of remote controlled capacitors gives the distributor a means to take immediate action to correct delivery point and feeder power factor problems, support voltage during peak loads, and supress overvoltage during light loads; and (2) For a distributor that has a SCADA system and personnel familiar with it, the hardware and technical support to control capacitors by remote control is readily available.

The disadvantages of a remote control include; (1) It does not automatically respond to conditions in its own local area, possibly leading to localized voltage and line loss problems; and (2) since the control comes from human or central computer action, human or computer errors at the SCADA control center could have widespread adverse consequences such as improper voltage at services, large line loss, or delivery point power factors that increase wholesale charges.

GO TO PART 2