If a fault occurs on a generator stator or its associated cables, the tripping of the generator circuit breaker alone is not sufficient Although the machine will be isolated from the system, the fault will continue to burn, probably causing extensive damage. This can only be stopped by removing the generator excitation. It only two machines are being operated in parallel at the time of the fault there will be the additional hazard of a complete loss of electrical supplies as almost certainly the circuit breakers of both generators will trip. If differential protection is employed this situation will be avoided.
This form of protection is also known as Merz Price or unit protection. Its advantage is that it will respond to faults only within a particular zone, and in this case the generator windings and the cable to the circuit breaker are covered.
Instantaneous operation can be permitted because the relay will be insensitive to faults outside the zone and therefore discrimination need not be considered.
When a fault is detected the excitation is removed, the fuel or steam valve closed, and the generator circuit breaker tripped. To provide both phase-to-phase and earth fault protection the scheme requires six current transformers, three at the generator neutral and three on the circuit breaker together with three relays. The current transformers must be carefully selected to ensure that faults outside the protected zone do not cause spill currents on the secondaries of sufficient magnitude to operate the relay.
It would seem that convincing reasons exist for the use of differential protection, yet its application on marine systems is rare. There are a number of reasons for this, but the most important probably concerns the earthing of the system neutral. Internal generator faults are rare, but when they do occur it is usually between one phase and earth.
On a typical marine system with an insulated neutral this fault can be tolerated until appropriate action can be taken. Cost and complexity also weigh against additional protection equipment
When the system voltage exceeds 1000 V and the generator neutral is earthed via a resistor, generator internal protection is mandatory.
However, il should be noted that if the earth fault current is limited to a very low value the sensitivity of the differential scheme may not be sufficient to ensure operation.
Typically, sensitivities down to 10% of the circuit rating arc possible. Occasionally, designers prefer to specify an unearthed neutral high-voltage system and to increase the insulation to earth accordingly.
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