Protection of Generators
The protections of generator are the most complex and elaborate due to the following reasons:
Generator is a large machine, connected to bus-bars. It is accompanied by unit transformers, auxiliary transformers and a bus system.
It is having excitation system, prime mover, voltage regulator, cooling system etc., and so it is not single equipment. The protection of generator should be co-ordinate with associated equipment's.
It is costly and important equipment. It should not be shut-off as far as possible since that would result in power shortage and emergency.
In the unit system of generator connection, generator is connected to L.V side of the main step-up transformer (GT) and H.V of unit auxiliary transformer (UAT). The HV side of the main transformer is connected to bus via switch gear from where power is transmitted into the grid. The UAT feeds the power to the auxiliaries directly connected with the unit. The generator and the generator main transformer (GT) form a unit and each unit has a boiler, turbine, condenser and other auxiliary systems.
While selecting the scheme for protection of generators, the protection of complete unit and the stability of the system due to disturbance in the generator should be considered in addition to the protection of generator itself.
So, the generator is high-cost equipment and requires protection to cover a wide range of faults which may occur.
Some of the important generator faults are:
Abnormal Running Conditions
Failure of Prime-Mover
When input to the prime mover fails, the generator runs as a synchronous motor and draws current from the supply system. This motoring condition is known as “inverted running”.
In case of turbo alternator set, failure of steam supply may cause inverted running. If the steam supply is gradually restored, the generator will pick up the load, without disturbing the system. If steam supply failure prolongs, the generator is safety shut off and this condition is relatively harmless. Therefore, automatic protection is not required.
In case of hydro sets, protection against inverted running is achieved by providing mechanical devices on the water wheel. When the water flow drops to an insufficient rate to maintain the electrical output, the generator is disconnected from the system. Therefore, in this case also, electrical protection is not needed.
Failure of Field
The chances of field failure in generator are very rare. For modern machines, a field failure relay is employed.
Field failure or loss of field can be caused by opening of field switch or field breaker. In such cases, the generator loses its field excitation, speeds up slightly and acts as an induction generator, deriving excitation from the system and supplying power at a leading power factor. In this condition, stator currents may increase above the normal rating of the machine, resulting in stator winding overheating.
Over current occurs mainly due to partial breakdown of winding insulation or due to overload on the supply system.
Over Speed Protection
Over speed conditions do happen, in the event of sudden loss of all or major portion of load. In such cases, the valve controlling the inlet steam or water as the case may be, is shut off by mechanical devices, thereby dangerous over speeding is avoided.
Over Voltage protection
Over voltage in a generator can occur, when the speed of the prime mover increases due to sudden loss of load on the generator or due to defective very sensitive to speed variations. The exercise continuous checks on over speed and thus prevents the occurrence of over voltage on the generating unit.
In case of hydro generators, when the generated voltage rises 20% above the rated / normal value, the over voltage relay operates to trip the main generator breaker as well as the generator field breaker.
Unbalanced Loading Protection
Unbalanced loading means that there are different phase currents in the generator. This phenomenon occurs from the earth faults or phase faults on the circuit external to the generator. If unbalanced currents, allowed to persist, they may either severely burns the fixing of the rotor core or damage the field winding.
Unbalance conditions give rise to negative sequence currents which produce an armature reaction field which rotates in a direction opposite to that of the rotor and hence produces a flux which sweeps through the rotor with twice the rotational speed. Hence spurious currents of twice the machine frequency are included in the rotor body, in the field winding and the damper windings. These currents cause overheating of stator.
Vibration is caused by overheating of the rotor or some mechanical failure or abnormality. The overheating of rotor is caused due to unbalanced stator currents or rotor ground faults. Over heating distorts the rotor, thereby causing eccentricity. Eccentric running produces vibration. Protection provided for unbalanced stator currents and rotor ground faults minimizes vibration. A vibration measuring device is also used to detect the vibration which is caused either by electrical or mechanical causes. An alarm is actuated if excess vibration takes place.
Bearing Over Heating
The temperature of the bearing is detected by inserting a temperature sensing device in a hole in the bearing. For large machines where lubrication oil is circulated through the bearing, an oil flow device is used to detect the failure of oil cooling equipment. An alarm is actuated when the bearing is over heated or when the circulation of lubricating oil fails.
Voltage Regulator Failure
Modern quick response automatic voltage regulators are very complex. They are subject to component failures. Suitable protective devices are provided against their failure. A definite time DC over current relay is provided which operates when there is over current in the rotor circuit for a period longer than a prescribed limit. In such a situation, the excitation is switched to a pre-determined value for manual control.
The supply for the voltage regulator voltage is given from a separate voltage transformer. Protection against failure of the regulator reference voltage is provided by using a voltage balance relay which compares the voltage derived from the instrument transformer with the voltage of the voltage regulator transformer. If there is manual operation of the voltage regulator due to the reference voltage, the relay operates and switches the excitation to a pre-determined value for manual control.
Stator Winding Faults
- Phase to earth faults
- Phase to phase faults
- Inter-turn faults
- Filed ground faults
- Loss of excitation
- Rotor over heating due to unbalanced 3-phase stator currents
A modern generator is normally provided with the following protective schemes.
1. Stator Protection:
- Percentage differential protection (for phase to phase faults, 3-phase faults and phase to earth faults up to about 85% of windings.
- phase to earth fault protection
- protection against stator inter-turn fault
- Stator over heating protection
- Field ground fault protection
- Loss of excitation protection
- Protection against rotor over heating due to unbalanced three phase stator currents.