Capacitor trip devices

04/01/2006 - Volume III - Issue I

The most common use of capacitor trip devices (CTDs) is to trip medium-voltage circuit breakers. A secondary application is for actuation of lockout (device 86) relays powered from the ac supply of a control power transformer. A separate CTD is required for each circuit breaker or lockout relay. A CTD must never be connected to parallel (multiple) loads.

 

The principle of a basic capacitor trip device is very simple. A capacitor is connected to a half-wave rectifier or a bridge rectifier and charged from the normal ac control power supply. The charging time of the capacitor is typically in the vicinity of cycles or so. The charging current is limited by a series resistor, both to protect the capacitor from excess current and to protect the bridge rectifier. The capacitor is isolated with no continuous load connected to the capacitor output circuit. When a protective relay or any other trip contact closes, the capacitor output is connected to the circuit breaker trip coil circuit (or to the solenoid circuit of a lockout relay), and the stored capacitive energy is released to trip the circuit breaker or lockout relay.

 

When the ac supply is at rated voltage (240Vac, for example), the capacitor will charge to the peak of the ac voltage or 339Vdc. The capacitor stays at this voltage as long as the incoming supply voltage is maintained. When the ac voltage is lost, the capacitor begins to discharge slowly. If a trip command is received, the charge on the capacitor is released to trip the circuit breaker.

 

The capacitor size is selected so that it has sufficient energy to operate the trip coil of the circuit breaker. Ideally, the capacitor size and charge current magnitude are tuned to the inductance and resistance of the tripping solenoid (an RLC series circuit). To produce a discharge current through the tripping solenoid, which emulates the magnitude of current and current duration which the solenoid would experience if operated from a dc trip coils on the circuit breaker, in line with the objective of matching the coil characteristics to the decaying dc output of the capacitor. CTDs are nearly always furnished with a capacitor size that provides more energy than the ideal minimum.

 

An important consideration in the design of the capacitor trip circuit is that it must have sufficient energy to trip the circuit breaker even when the ac control power supply is at the minimum voltage of the allowable range in ANSI C37.06. For a 240Vac supply, ANSI stipulates that the circuit breaker shall operate properly with a minimum control voltage of 208Vac. Our practice during production tests is to charge the capacitor from a source adjusted to 208Vac, and then disconnect the source. The CTD must be able to trip the circuit breaker if the tripping command is issued 10 seconds after the ac supply is removed. This assures that the CTD has enough energy to perform its design function even when conditions are not optimal. For perspective, the rated (maximum) permissible tripping delay specified for a medium-voltage circuit breaker in ANSI/IEEE C37.04 and ANSI C37.06 is two seconds, so the 10 second value used in our production testing provides a large margin compared to the requirements of the standards.

 

So far, we have discussed the basic concept of a capacitor trip device, as typically installed directly on a circuit breaker. There are also more complex devices, which include an electronic circuit to maintain capacitor charge after the ac supply is lost. The electronic circuit is powered by rechargeable batteries, typically size AA. The Enerpak model A-1 is an example of this type of unit. This device is designed to maintain a voltage on the capacitor sufficient to trip the circuit breaker for 140 hours after the ac supply voltage is disconnected. While the charging system makes these devices more complex, the underlying principle of the device is identical to the basic device described.

 

The CTD uses a charged capacitor, so care must be exercised when performing inspection or maintenance activities. The capacitor self-discharges after removal of the ac source, but the discharge time is relatively long. The capacitor must always be discharged before any work is done in the area of the capacitor or of wiring to which the capacitor is connected (e.g., the trip circuit of the relays or the tripping contact of a control switch).

 

The preferred method of discharging the capacitor is to disconnect the ac control power, then use the circuit breaker control switch to issue a trip command which discharges most of the stored energy through the circuit breaker trip coil, and finally, short-circuit the terminals of the capacitor to remove any remaining residual charge.

 

Alternatively, the capacitor can be discharged directly. This must not be done with a short-circuiting conductor but rather with a circuit having a resistor to limit the current magnitude. A 5-watt, 500 ohm resistor works well for this purpose.

 

Advantages

  • Economic for small installation with only a few circuit breakers, compared to use of a battery.

  • Particularly suited to installations in isolated locations or unattended substations where the user wishes to avoid the initial cost and ongoing maintenance of a station battery.

  • Suitable for use in outdoor installations where battery capacity is reduced at low temperatures.

Disadvantages

  • Capacitor trip devices cannot be used for continuous loads; thus, it cannot be used with a red light in the trip circuit to monitor trip coil integrity nor with a trip coil supervision circuit of microprocessor relays.

  • Use of ac control power precludes use of communications devices (relays, power meters) which require dc control power for communications when the ac power is off (e.g., immediately after a fault).

  • Uneconomic for large installations, compared to use of a battery.

  • An electrolytic capacitor is used, which has limited life, particularly in high temperatures. The periodic maintenance program must include functional testing (annually) of the capacitor trip device.