Direct Connected Generator Protection:

Direct connected generators are normally of smaller ratings and a typical scheme of Direct Connected Generator Protection for a 30 MW generator is shown in Fig. (6.21).

It consists of the following protections:

  • Unbiased differential protection.

  • Backup overcurrent protection.

  • Negative phase sequence protection.

  • Standby earth fault.

In addition to these protections the following may also be provided:

  • Field failure protection.

  • Rotor earth-fault protection.

  • Reverse power protection (to be provided on back pressure turbine sets and engine driven sets).

Direct Connected Generator Protection

Relay Tripping Functions:

The various systems have been described simply as means of operating the protective relays whose function is to energize a multi contact tripping relay, which in turn operates the following:

  • The main circuit breaker to isolate the generator from the system.
  • The neutral earthing switch, if any, to interrupt earth-fault current.
  • The field switch to begin the decay of the generated voltage feeding the fault, in some cases by switching in a suppression resistor.
  • An alarm relay.

Relays on the unit transformer LV side trip only the unit transformer, leaving the generator in service. The auxiliary protections energize the alarm relay.