Capacitor Bank

Capacitor Bank Protection

•          Over current and earth fault protection

•          Under voltage protection

•          Over voltage protection

•          Unbalance protection

Over current and earth fault protection

•          To guard against the faults on capacitor leads between the circuit breaker and the units, it is normal to provide overcurrent and earth fault protection using Inverse Minimum Definite Time (IDMT) Relay

•          A small time delay is provided to ensure that these relays do not maloperate on capacitor “switching-in (inrush)” currents

•          When the source is earthed, two overcurrent and one earth fault relays are used so as to cover both phase and earth faults

•          Instantaneous overcurrent units are not used as they are liable to operate during normal switching condition (because inrush current of very short duration flows during switching-in).

•          To limit the inrush current during ‘switching-in’ as well as overload current due to harmonics sometimes series reactors are used with capacitor bank

Under-voltage Protection

•          During under-voltage conditions, the capacitor will discharge into the system.

•          It is desired to trip the capacitor with a time delay (time delay to prevent transient operation).

•          under-voltage relay for this purpose have a setting of 30-90 % with built-in variable time delay and high reset ratio.

•          The incoming AC voltage on restoration will get superimposed on to the remanent voltage and hence the over-voltage. To avoid this a time delay between successive switching is provided so that the capacitor will discharge to a safe value.

•          The time delay between successive switching is a function of electrical system and capacitor voltage rating : say 1 minute for 415 V capacitor and 5 minutes for 11 kV capacitors

•          Hence timer is recommended for closing interlock in each capacitor panel.

Over-voltage Protection

•          The capacitors are overstressed when subjected to prolonged over-voltage condition.

•          This increases fatigue and shortens the life of the capacitor.

•          The over-voltage is caused due to various conditions: -

–        Increased bus voltage during lightly loaded conditions due to Ferranti effect.

–        Harmonic over-voltage due to neighborhood of arc furnace, thyristor controlled devices like convertors.

–        Switching surges

–        Transient over-voltages due to CB operating mechanism including lightning

Unbalance Protection

•          Unbalance protection is provided either by voltage or current measuring relays.

•          Voltage relays are generally applied to single star with isolated neutral and delta banks

•          Current relays are applied to double star, delta and single phase capacitor banks

Unbalance Protection for Delta Banks

•          In delta capacitor bank installations, the unbalance protection can be provided by either current or voltage operated relays

•          Unbalance voltage detection

–        The VTs are connected across any one of identical series groups of each phase.

–        The secondaries of the VTs are connected in open delta to feed the voltage relay.

               Unbalance voltage detection

•          Under normal healthy condition distribution of voltage across each series group of the 3 phases are identical in magnitude, but 120⁰ displaced in phase. Hence the open delta output is zero.

When the failure occurs in any one of the series groups of any phase, the voltage distribution among the series groups of any phase is distributed, thereby providing an open delta voltage to operate the relay.

Unbalance current detection

•          A CT is installed on the tie connection between two parallel halves of each delta leg.

•          The CT from each leg is connected to a relay

•          A tunned relay along with a timer is recommended.

•          The relay have a setting of 5-20 % or 10-40% of rated current and time delay to prevent transient operation of the relay

Double star capacitor bank

•          For a double star capacitor bank with floating neutral connection, a current relay operated off a CT in the floating neutral connection is used.

•          Either definite time or inverse time relays can be used for this application

•          The time delay must be provided to co-ordinate with the fuse blowing time.

Neutral current detection

•          In case of grounded wye capacitor bank, CT is installed in capacitor bank neutral to earth connection.

•          An unbalance in the capacitor bank say due to failure of capacitor unit (s), causes current to flow in the neutral.

•          IDMTL relay connected across the CT secondary operates and trips the bank.

Open delta voltage detection

•          An unbalance relay will be connected to secondary of VTs (in open delta), the primary of which are connected across tap points of the capacitor bank.

•          Under balanced conditions, the output of open delta secondary is the addition of 3-phase voltages, which is zero.

•          When unbalance occurs due to failure of capacitors, the voltage distribution across the series group varies in the faulted phase. Hence the variation in the voltage of VT primary gives rise to open delta secondary output to actuate the relay.

Single Phase Capacitor Banks

•          Single phase capacitor banks are generally used for Railways applications

•          The protection for single phase banks are: -

–        Earth fault protection is provided by differentially connecting the CTs on the phase and neutral side of the capacitor bank

–        Over-current protection

–        Over-voltage protection

–        Under-voltage protection alongwith timer for closing interlock

–        Unbalance protection