Local Controls
Several options for controls are available for capacitor banks:
• Time clock — The simplest scheme: the controller switches capacitors on and off based on the time of day. The on time and the off time are programmable. Modern controllers allow settings for weekends and holidays. This control is the cheapest but also the most suscep- tible to energizing the capacitor at the wrong time (due to loads being different from those expected, to holidays or other unexpected light periods, and especially to mistakenly set or inaccurate clocks). Time clock control is predictable; capacitors switch on and off at known times and the controller limits the number of switching operations (one energization and one deenergization per day).
• Temperature — Another simple control; the controller switches the capacitor bank on or off depending on temperature. Normally these might be set to turn the capacitors on in the range of 85 and 90°F and turn them off at temperatures somewhere between 75 and 80°F.
• Voltage — The capacitor switches on and off, based on voltage. The user provides the threshold minimum and maximum voltages as well as time delays and bandwidths to prevent excessive operations. Voltage control is most appropriate when the primary role of a capacitor is voltage support and regulation.
• Vars — The capacitor uses var measurements to determine switch- ing. This is the most accurate method of ensuring that the capacitor is on at the appropriate times for maximum reduction of losses.
• Power factor — Similar to var control, the controller switches capac- itors on and off based on the measured power factor. This is rarely used by utilities.
• Current — The capacitor switches on and off based on the line current (as measured downstream of the capacitor). While not as effective as var control, current control does engage the capacitor during heavy loads, which usually corresponds to the highest needs for vars.
Many controllers offer many or all of these possibilities. Many are usable in combination; turn capacitors on for low voltage or for high temperature. Var, power factor, voltage, or current controllers require voltage or current sensing or both. To minimize cost and complexity, controllers often switch all three phases using sensors on just one phase. A control power transformer is often also used to sense voltage. While unusual, Alabama Power switches each phase independently depending on the var requirements of each phase (Clark, 2001); this optimizes loss reduction and helps reduce unbalance. Because capacitor structures are rather busy, some utilities like to use voltage and/or current-sensing insulators. Meter-grade accuracy is not needed for controlling capacitors.
To coordinate more than one capacitor with switched var controls, set the most-distant unit to have the shortest time delay. Increase the time delay on successive units progressing back to the substation. This leaves the unit closest to the substation with the longest time delay. The most distant unit switches first. Upstream units see the change and do not need to respond. This strategy is the opposite of that used for coordinating multiple line voltage regulators.
For var-controlled banks, locate the current sensor on the source (substation) side of the bank. Then, the controller can detect the reactive power change when the capacitor switches. To properly calculate vars, the wiring for the CT and PT must provide correct polarities to the controller.
One manufacturer provides the following rules of thumb for setting var control trip and close settings (Fisher Pierce, 2000):
• Close setpoint: 2/3 × capacitor bank size (in kvar), lagging.
• Trip setpoint: Close set point – 1.25 × bank size, will be leading. (This assumes that the CT is on the source side of the bank.)
For a 600-kvar bank application, this yields Close setpoint: 2/3 × 600 = +400 kvar (lagging)
Trip setpoint: 400 – 1.25 × 600 = –350 kvar (leading)
For this example, the unit trips when the load kvar drops below +250 kvar (lagging). This effectively gives a bandwidth wide enough (+400 to +250 kvar) to prevent excessive switching operations in most cases.
Voltage-controlled capacitor banks have bandwidths. Normally, we want the bandwidth to be at least 1.5 times the expected voltage change due to the capacitor bank. Ensure that the bandwidth is at least 3 or 4 V (on a 120- V scale). Set the trip setting below the normal light-load voltage (or the bank will never switch off).
If a switched capacitor is located on a circuit that can be operated from either direction, make sure the controller mode can handle operation with power flow in either direction. Time-of-day, temperature, and voltage control are not affected by reverse power flow; var, current, and power factor control are affected. Some controllers can sense reverse power and shift control modes. One model provides several options if it detects reverse power: switch to voltage mode, calculate var control while accounting for the effect of the capacitor bank, inhibit switching, trip and lock out the bank, or close and hold the bank in. If a circuit has distributed generation, we do not want to shift modes based on reverse power flow; the controller should shift modes only for a change in direction to the system source.
Capacitor controllers normally have counters to record the number of operations. The counters help identify when to perform maintenance and can identify control-setting problems. For installations that are excessively switching, modify control settings, time delays, or bandwidths to reduce switching. Some controllers can limit the number of switch operations within a given time period to reduce wear on capacitor switches.
Voltage control provides extra safety to prevent capacitors from causing overvoltages. Some controllers offer types of voltage override control; the primary control may be current, vars, temperature, or time of day, but the controller trips the bank if it detects excessive voltage. A controller may also restrain from switching in if the extra voltage rise from the bank would push the voltage above a given limit.