Generator Protection - Rotor Earth Fault Protection (64R1/ 64R2)

Rotor Earth Fault Protection (64R1/64R2)

The field circuit of generator (i.e. rotor winding) is an isolated D.C. circuit and not earthed anywhere. The field can be exposed to abnormal mechanical or thermal stresses due to vibration, excessive currents or choked cooling medium flow. This may result in a breakdown of the insulation between the field winding and the rotor iron at one point where the stress has been too high.

A single earth fault in the field winding or its associated circuits, therefore, gives rise to a negligible fault current and does not represent any immediate danger. If however a second ground fault should occur, heavy fault current and severe mechanical unbalance may quickly arise and lead to serious damage. It is essential therefore that any occurrence of insulation failure is discover and that the machine is taken out of service as soon as possible. Normally the machine is tripped instantly on occurrence of second rotor earth fault. Three method are available to detect this type of fault.

First Rotor Earth Fault Protection 64 R1

Ø  Potentiometer method

Ø  A.C. injection method

Ø  D.C. injection method

POTENTIOMETER METHOD

In this scheme, a centre tapped resistor is connected in parallel with the main field winding as shown in Fig. C. The centre point of the resistor is connected to earth through a voltage relay. An earth fault on the field winding will produce a voltage across the relay. The maximum voltage occurring for faults at the ends of the winding.

A ‘blind spot’ exists at the centre of the field winding, this point being at a potential equal to that of the tapping point on the potentiometer. To avoid a fault at this location remaining undetected, the tapping point on the potentiometer is varied by a push button or switch. It is essential that station instructions be issued to make certain that the blind spot is checked at least once per shift. The scheme is simple in that no auxiliary supply is needed. A relay with a setting 5% of the exciter voltage is adequate. The potentiometer will dissipate about 60 volts.

Fig C: Potentiometer Method

A.C. INJECTION METHOD

This scheme is shown in Fig. D, it comprises of an auxiliary supply transformer, the secondary of which is connected between earth and one side of the field circuit through an interposed capacitor and a relay coil.

The field circuit is subjected to an alternating potential at the same level throughout, so that an earth fault anywhere in the field system will give rise to a current which is detected by the relay. The capacitor limits the magnitude of the current and blocks the normal field voltage, preventing the discharge of a large direct current through the transformer.

This scheme has an advantage over the potentiometer method in that there is no blind spot in the supervision of the field system. It has the disadvantage that some current will flow to earth continuously through the capacitance of the field winding. This current may flow through the machine bearings, causing erosion of the bearing surface. It is a common practise to insulate the bearings and to provide an earthing brush for the shaft, and if this is done the capacitance current would be harmless.

Fig D: A.C Injection Method

D.C. INJECTION METHOD

The capacitance current objection to the a.c. injection scheme is overcome by rectifying the injection voltage as shown in Fig. E. The d.c. output of a transformer rectifier power unit is arranged to bias the positive side of the field circuit to a negative voltage relative to earth. The negative side of the field system is at a greater negative voltage to earth, so an earth fault at any point in the field winding will cause current to flow through the power unit. The current is limited by including a high resistance in the circuit and a sensitive relay is used to detect the current.

The fault current varies with fault position, but this is not detrimental

Provide the relay can detect the minimum fault current and withstand the maximum.

The relay must have enough resistance to limit the fault current to a harmless value and be sufficiently sensitive to respond to a fault which at the low injection voltage may have a fairly high resistance.

Insulation leakage current, taking into account of the high voltage to earth at the negative end of the winding and any over voltage due to field forcing and so on.

              

Fig E: D.C. Injection Method

2ND ROTOR EARTH FAULT PROTECTION (64R2)

In this test system is replaced by a replica field system in the form of potential divider , two work 1 K potentiometer in parallel with station D.C. is used so in fig.(F) SW1 at 1st rotor E / F position. Closed switch S1 check that 1st rotor E/F relay VAEM (64R1) operated. Shift S1 to balance. Obtain balance on the mA meter (Galvanometer) by coarse / fine adjustment of potentiometer. Shift SW1 on test position check operation of relay 64R2 by closing switch S2 thus creating an unbalance which simulates second E/F.

Fig F: Second Rotor Earth Fault Protection

ROTOR EARTH FAULT

The scheme to detect rotor earth in case of brushless excitation system is show in fig.(G) in this case, rotor earth fault relay forms the three arms of a bridge whose forth arm is the field winding capacitance to rotor body. During rotor earth fault, this capacitance gets shorted and the bridge becomes unbalanced operating the relay. Main exciter winding, rotating diodes and generator field winding is protected by this relay.

Fig G: Rotor Earth Fault