Question

With reference to time lag in the operation of over current protection devices

Describe 2 methods of obtaining time lag

Explain the purpose of time lag.

Explain discrimination with respect to electrical distribution system.

Explain inverse current time characteristic

Methods of obtaining time lag

Thermal

Electronic

All methods have an inverse current-time characteristic, i.e. the bigger the current the faster it will operate.

A thermal relay utilised the bending action of a bimetallic bar to trip the circuit-breaker. The time taken to heat the bimetal gives the necessary time lag.

An electronic over current relay usually converts the current into a proportional voltage. This is then compared with a set voltage level within the transistorised monitoring unit. The time delay is obtained by the time taken to charge up a capacitor. This type of relay usually has separate adjustments for current trip level and for trip time. The amplifiers within the electronic relay can be set to give an almost instantaneous trip (typically 0l05 sec) to clear a short-circuit fault.

Purpose of time delay

To delay tripping

To allow current surges for motor starting and generator synchronizing

Also time delay are required with short circuit devices to give discrimination.

The over current relay must be provided with a short time delay even when responding to a short-circuit condition, to maintain discrimination with the feeder circuits. Because of this delay the relay will usually respond only to the steady-state short-circuit current. If the relay operating current is too high, or if the fault current if too low, the circuit breaker may be tripped only after an excessively long time. Therefore, to ensure satisfactory operation of the protective equipment, the generator specification often calls for a minimum steady-state fault current capability of three or four times the full-load current when the machine is warm.

The time delay under short-circuit conditions is usually between 0.1 and 3.0 seconds, the actual setting depending on discrimination requirements.

Discrimination.

The ability of a protection system to disconnect only the faulted circuits and to maintain the electrical supplies to healthy circuits is called protective discrimination.

Discrimination is achieved by coordinating the current ratings and time settings of the fuses and overcurrent relays used between the generator and the load as shown in Fig. The protective devices nearest the load having the lowest current rating and shortest operating time. Those nearest the generator having the highest current rating and longest operating time.

If a short-circuit fault occurs in the lampholder in Fig. the fault current will be large enough to operate all protection devices from the generators to the fault. However, the 5 A fuse protecting the lamp circuit has the lowest current rating and shortest operating time in the system so will be the quickest to operate. This action will clear the fault and leave all other healthy circuits still connected.

Inverse current time characteristic.

All relay types have an inverse current-time characteristic called OCIT (over-current inverse time), i.e. the bigger the current the faster it will operate. See Fig. The basic inverse I/t curve would tend towards zero time for the highest currents. To make the relay action more precise at very high fault currents the action is arranged to operate at a definite minimum time which is fixed by the design. This type is called an OCIDMT (over current inverse and definite minimum time) relay action.