Rotor Fault Protection of Generators: 5 Schemes | Electrical Engineering

The different protection schemes for rotor protection of generators are described as follows: 1. Rotor Earth Fault Protection 2. Loss of Excitation (or Field Failure) Protection 3. Protection against Rotor Over-Heating Because of Over-Excitation 4. Rotor Temperature Alarm 5. Automatic Field Suppression and Use of Neutral Circuit Breaker.

Scheme # 1. Rotor Earth Fault Protection:

Field circuits are operated unearthed and it is not necessary to trip such a circuit completely if there is only one earth fault but the relay should be provided to give an indication that the fault has occurred so that the generator may be taken out of service at leisure since the incidence of second fault would cause serious damage.

One method of detecting earth fault on rotor circuit is illustrated in Fig. 8.16. In this arrangement a high resistance is connected across the rotor circuit and its mid-point is grounded through a sensitive relay. The relay detects the earth fault for most of the rotor circuit except the rotor centre point.

Other methods of rotor earth fault protection include dc injection method and ac injection method. The arrangement is illustrated in Fig. 8.17. In this arrangement either dc or ac voltage is impressed between the field circuit and ground through a sensitive overvoltage relay and current limiting resistor or capacitor (in case of ac voltage). A single earth fault in the rotor circuit will complete the circuit comprising the voltage source (ac or dc), sensitive overvoltage relay and earth fault and thus earth fault will be sensed by the relay.

DC injection method is simple and has no problems of leakage currents. If direct current is used, the overvoltage relay can be more sensitive than if alternating current is used, with ac, the relay must not pick up on the current that flows normally through the capacitance to ground, and care must be taken to avoid resonance between the capacitance and the relay inductance.

Detection of open-circuit fault in rotor circuit is the same as used for detection of loss of excitation.

Scheme # 2. Loss of Excitation (or Field Failure) Protection:

Loss of excitation on the generator can endanger the generator or the connected system or both.

It is caused by accidental tripping of field breaker, short circuit in the field circuits, poor brush contact or operating errors. In absence of field current alternator runs as an induction generator and thus heavy currents are inducted in the rotor teeth and wedges. Most modern generators can only withstand these currents up to a period of 2 to 3 minutes.

When a generator loses excitation, it draws reactive power from the system amounting as much as 2 to 4 times the generator’s rated load. Before it lost excitation, the generator may have been delivering reactive power to the system. Thus, this large reactive load suddenly thrown on the system, together with the loss of generator’s reactive-power output may cause wide spread voltage reduction, which in turn, may cause extensive instability.

If the system is capable enough to tolerate the difference of reactive power then no automatic protection is required for the purpose as an operator has got 2 to 3 minutes to take care of it. But if there is chance of instability of the system, automatic protection is required.

In case of large generators which operate over a wide range of field excitation such a relay may be an embarrassment. Furthermore, field failure due to failure of exciter may not be detected by it as it may be held in by ac induced from the stator. An under-current relay fast enough to drop out on ac cannot be employed as it would be affected by ac induced during synchronising and during external faults.

The most selective type of loss of excitation relay is a directional-distance type relay operated from alternating current and voltage at the main generator terminals.

Several losses of excitation characteristics and the relay operating characteristic on an R-X diagram are illustrated in Fig. 8.19. As soon on the excitation is lost, the equivalent generator impedance traces a curve from the first quadrant of R-X diagram into a region of the fourth quadrant.

By enclosing this region within the relay operating characteristic, the relay will operate when the generator first starts to slip poles and will trip the field breaker as well as disconnect the generator from the system before either the generator or the system can be harmed. The generator may then be returned to service immediately when the cause of excitation failure is corrected.

Scheme # 3. Protection against Rotor Over-Heating Because of Over-Excitation:

In case of overcurrent due to over-excitation in the rotor circuit, a dc relay is usually provided. This relay senses and initiates alarm. However, application of such relay is very much limited as relaying of dc quantities is relatively uncommon and the rotor windings are designed to tolerate over-currents due to over-excitation.

Scheme # 4. Rotor Temperature Alarm:

Such a protection is provided only in case of large generators. It indicates the level of temperature and not the actual hot spot temperature. In the arrangement shown in Fig. 8.20, resistance is measured by comparing voltage and current by a double actuating quantity moving coil relay, the operating coil being used as the voltage coil and restraining coil as current coil. The relay measures the ratio of voltage and current i.e., a resistance giving a measure of rotor temperature.

Scheme # 5. Automatic Field Suppression and Use of Neutral Circuit Breaker:

In the event of a fault on a generator winding even though the generator circuit breaker is tripped, the fault continues to be fed as long as the excitation will exist because emf is induced in the generator itself. For the quick removal of the fault during emergency, it is necessary to disconnect the field simultaneously with the disconnection of the generator.

Thus it is absolutely necessary to discharge its magnetic field in the shortest possible interval of time. Hence it is to be ensured that all the protection system not only trip the generator circuit breaker but also trip the automatic field discharge switch. The field discharge switch is an automatic control unit designed to remove the voltage from the generator after its isolation from the system.

The schematic diagram for field suppressing and opening of the neutral circuit breaker is illustrated in Fig. 8.21 (a). In the event of fault the circulating relay contact is closed and so the trip coils TC₁, TC₂, and TC₃ are energized. The trip coil TC₁ opens the main circuit breaker while the trip coil TC₃ opens the neutral circuit breaker. The trip coil TC₂ opens the upper contacts, shorts the lower contacts so as to short circuit the field winding through resistor R. Thus the energy of the generator is dissipated in the resistor R.

An alternative arrangement of field suppression and dissipation of energy of the generator is illustrated in Fig. 8.21 (b).

Alternative Arrangement of Field Suppression:

This arrangement is similar to that described above except that alternator field winding is also discharged through resistor R₂ by using trip coil TC₄, as illustrated in Fig. 8.22. This process of discharging consists of the isolation of the exciter from the generator rotor field winding and involves the dissipation of magnetic energy stored in the inductive reactance of the rotor and the main exciter windings. This is achieved by switching in the rotor and the dc exciter field windings across the field discharge resistors, as illustrated in Fig. 8.22. In order to avoid any over-voltages, the switching-over is performed immediately and without a break in the excitation system.

The quality of operation of an automatic field discharge system depends upon the rate of decrease of field, which in turn depends upon the time constant of the field winding. This term is also known as the field discharge time constant T_d. Normally the value of field discharge time constant lies between 3 and 8 seconds.

The discharge time T_d is defined as the time required for the drop in voltage at the generator stator terminals to a minimum value (usually 500 V).

The value of resistance R₂ in the rotor circuit is determined on the basis of a fall in generator voltage to 30% of normal voltage within 3 to 4 seconds. Resistance R₂ comes out to be roughly 4 to 5 times rotor winding hot resistance.

The resistance R₁ should be roughly equal to 10 times the exciter field winding resistance.