10.5 AC generator protection

AC generator operation can be altered by faults occurring both within the machine and disturbances in the network to which it is connected.

An AC generator protection system therefore has a dual purpose: protecting the machine and protecting the network.

The main faults which may affect a generator are:

• overload;

• external phase-to-phase short-circuit;

• internal phase-to-phase short-circuit;

• internal phase-to-frame fault;

• rotor frame fault;

• phase cutting, inversion of two phases and phase unbalance;

• loss in excitation;

• motor operation;

• under or overfrequency;

• voltage too high or too low;

• loss of utility power supply when generator is connected to it.

Protection against overloads

Overcurrents point to the occurrence of overloads which, if they are prolonged, cause excessive heating, leading to accelerated ageing of the generator.

Protection is ensured:

• either by thermal overload protection (see section 7.7);

• or by temperature monitoring (see section 7.23).

Protection against external phase-to-phase short-circuits

External phase short-circuits are faults appearing on the network fed by the generator. This type of protection depends on whether or not there is a system allowing the short-circuit current to be maintained at roughly 3 I _n for several seconds (see section 4.1.2).

Generator fitted with a system maintaining the short-circuit current at roughly 3 I _n for several seconds

Protection is ensured by independent time phase overcurrent protection (see section 7.1).

The current threshold must be coherent with dowstream protection devices. However, it may be set up to 2.5 I _n.

The time delay: as short as...