For each generator circuit breaker must be fitted with an overcurrent relay. On medium-voltage systems this is often an integral part of the circuit breaker, in In the form of direct-acting trips, although for high-voltage systems a separate relay is used. A n inverse time-current characteristic is normally employed. In the actual operating curve of the relay should be selected on the basis of the following requirements:
- Proper system discrimination must be provided, the generator breaker only being tripped if a feeder breaker fails to do so or if a busbar fault occurs.
- The minimum operating current should be in excess of the preference trip relay selling. The overcurrent relay must reset fully after nonessential loads have been shed.
For simple direct-acting trips the reset current is often as low as 80% of the setting current and consequently this can determine the minimum ratio setting current to full-load current. If the full-load current is greater than 80% of the minimum operating current it is possible that, after a transient overload condition, the overcurrent device may remain in a partly operated state. From this position it may ‘creep’, even with normal load current flowing, and could eventually trip the breaker. This problem should not arise when a separate relay is employed as this will tend to reset at currents much closer lo the operating value. Static relays are particularly good in this respect and arc now being made available as an integral part of the circuit breaker, but a direct-acting trip may have to be set to 125% of full-load current Sufficient fault current must he available from the generator to ensure (hat the relay will operate in a reasonably short time it a busbar fault occurs. Il must be remembered that, although the generator will initially contribute as much as ten times full-load current to the fault, the generator impedance will rapidly increase. The fault current will decay until the automatic voltage regulator (a.v.r.) begins to increase the excitation and the current rises again to a steady-state value. The overcurrent 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 is loo low. the circuit breaker may be tripped only after an excessively long time therefore, to ensure satisfactory operation of the protective equipment, (he 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 American Bureau of Shipping requires three times full- load current to be sustained without damage for 2 s.)
It should be noted that the steady-state current can be significantly higher when the generator is cold because the field resistance will then be low.
The relay operating characteristic should reflect the thermal withstand capacity of the generator. Lloyd’s regulations slate that the generator should be capable of withstanding 50% excess current for a period of 15 s. Ideally, the protection characteristic should pass through this point or even give a faster clearance, although this could increase the possibility or an undesired generator trip In practice the setting used is often 20 s at l50°o of full-load current. Similarly, under short-circuit conditions, the circuit breaker must trip before the generator suffers thermal damage. The time delay under short-circuit conditions is usually between 0.1 and 3 s. the actual setting depending on discrimination requirements
When a circuit breaker filled with direct-acting overcurrent trips is ordered, it is important that the manufacturer is informed of the particular application. It can then be ensured that the oil dashpot delay mechanism, if used, is calibrated for operation at the ambient temperature envisaged.
Also, for marine generator protection, an oil of higher viscosity will be required than that used for industrial circuit breakers. Similarly, if a clockwork escapement retarder is to be used, a long time delay type must be fitted.
A fully rated air current breaker will normally be required rather than a moulded-ease circuit breaker (m.c.c.b.). The thermal overloads fitted to the latter rarely have a characteristic suitable for generator protection. Also if is usual, the m.c.c.b. employs instantaneous magnetic trips, discrimination with feeder circuit protection will be extremely difficult. However, for small generators it may sometimes be more economical to use an m.c.c.b. without the thermal overloads filled and with the magnetic trips set above the maximum generator fault contribution (say 12-15 times to the full-load current).