Practical tips for the protection of generators and power transformers

Protection overview

There are usually several ways of protecting HV/MV generators and transformers. It should be noted that one protection relay is often used for several functions, for example, an overcurrent relay can be used for both overcurrent and earth fault protection.

The following combinations are commonly found in practice:

1. Inverse time overcurrent and earth fault;
2. Inverse time with instantaneous high-setting overcurrent, with or without inverse time earth fault; and
3. Thermal overcurrent, with instantaneous overcurrent and earth fault.

The power system neutral is considered to be earthed when the neutral point is connected to earth directly or through a resistor. Earth fault protection is applicable in all such cases.

The neutral point is considered to be insulated when the neutral point is not connected to earth, or is earthed through a continuously rated arc suppression coil or through a voltage transformer.

The short-rated coil is short-circuited, either directly or through a resistor after a short time, so that persistent earth faults can be cleared by normal discriminative earth fault protection.

Interesting reading about arc suppression reactors.

How Arc suppression reactors (Earth fault neutralisers or Petersen-coils) work?

Generators

The core of an electrical power system is the generator, requiring a prime mover to develop mechanical power from steam, gas, water or diesel engines. The range of size extends from a few hundred kilovolt-amperes up to turbine driven sets exceeding 600 MV-A in rating.

Small sets may be directly connected to the distribution system, while the larger units are associated with an individual step-up transformer through which the set is coupled to the transmission system.

Faults of many kinds can occur in such a system, for which diverse protective means are needed.

The amount of protection applied will be governed by economic considerations. In general, the following faults need to be considered:

1. Stator insulation earth faults;
2. Overload;
3. Overvoltage;
4. Unbalanced loading;
5. Rotor fault;
6. Loss of excitation;
7. Loss of synchronism;
8. Failure of prime mover;
9. Low vacuum in condenser;
10. Lubrication oil failure;
11. Overspeeding;
12. Rotor distortion;
13. Differential expansion; and
14. Excessive vibration

The neutral point of a generator is normally earthed, with some impedance inserted in the earthing lead to limit the magnitude of earth fault current to values from a few amperes to about rated full load current.