GENERATOR OVERSPEED PROTECTION

It has been noted above that over-speed causes overvoltage, which may be protected against using overvoltage relays. Basically, however, over-speed control is part of the turbine control system. Most large steam turbine controls have two or three separate speed control units, with one of these being strictly a mechanical centrifugal device that will close the turbine control valves even if electrical power is lost to the controls that require electric input.

Over-speed protection must be selective and must not shut the unit down due to a temporary loss of load, even if the cause is serious, for example, a short circuit. Short circuits anywhere near a generator will collapse the voltage and the generator experiences a loss of load. Since the turbine power is unchanged, the turbine-generator unit will over-speed until the governor throttles the turbine input back. Faults are usually temporary, however, and there is no need to shut the unit down unless the fault is on the generator or GSU transformer. Other nearby faults have exactly the same effect on the generator, however, and the protections must be designed to discriminate. Faults in the generator or step-up transformer will trip the unit before over-speed can become a problem. Nearby transmission faults should not trip the unit, however.
Most large steam turbine units have protective devices that are designed to distinguish between a load rejection and a fault. Both cause over-speed and a sudden loss of generated power, but the generator current behavior is quite different. Faults cause an increase in current, while load rejection causes a decrease in current. The generator should not be tripped for the fault condition, assuming that it is not a generator or transformer fault, but the generator will have to be tripped if the load is lost permanently. Turbine controls are designed to make this distinction, to run back in the case of load rejection and to do nothing in the case of a network fault except the usual speed governing reaction.

These turbine controls are outside the scope of this book, but there are cases where there needs to be coordination between the protective actions taken on the electrical side with those taken in the power plant. In the case of over-speed, backup protection can be interlocked with the turbine controls that will monitor bus frequency and order a generator trip if this frequency becomes too high. Obviously, this must be coordinated with the turbine controls