Phase to Earth Fault Protection
Phase to earth faults normal occur in the armature slots. The ground fault current is usually limited by the resistance in the neutral of the generator. A sensitive earth fault protection relay, as shown in figure is used, to protect stator winding against such faults.
The relays used in this shield (protection) scheme are;
- Inverse time relay – if the generator is directly connected to the station bus.
The earth fault current is usually limited by resistance (for fault current to be limited to between 200 A and 250A) or by a distribution transformer (if the fault current are to be limited to between 4A and 10A) connected in the neutral of the generator.
When the neutral is earthed through a resistor, a CT is mounted in the generator neutral and connected to a relay of type (1) and (2).
For (1), the inverse time relay is requires grading with other earth fault relays in the system. For (2) no discrimination with other earth fault relay is required, since the ground fault is restricted to the stator and transformer primary winding. With resistor earthing, it is impossible to protect 100% of the stator winding. The percentage of the winding protected is dependent on the value of neutral earthing of resistor and relay setting.
When there is an earth fault, the neutral current increases and hence the relay will operate to close the trip circuit.
However, the distributed capacitance of the stator to ground fixes a limit to the value of the resistance (Rn) in the neutral.
i.e., Rn = 106 / (6πfC) ohms,
Where C is the capacitance of stator circuit to earth / phase in μF.
If neutral is earthed through the primary winding of a transformer, earth fault protection is provided by connecting an over voltage relay across the secondary of the distribution transformer. Then the maximum value of resistance is given by Rn = 106 / 6πfN2C ohms.
- Read : Classification of Relays
Percentage of un-protected winding (phase to Earth fault of a generator or transformer)
The value of R and x% determines the relay setting.
The usual practice is to protect 80-85% of alternator or transformer winding against earth fault. The remaining 15-20% from neutral end is left unprotected. It can be seen in the figure for phase to earth fault that the setting required from differential protection is determined by the value of the neutral earthing resistor and also by the amount of stator winding to be protected.
The earth fault If is given by:
IF = (I-X)V/R
This must be equal to the primary fault setting of differential protection, Is for maximum operating current.
IS = (1-X)V/R and IR = V/R
IS = IR –XIR
X = (IR –IS) / IS
Hence % X = ( 1 – IS/IR ) x 100
IO or IS = CT primary fault setting current
IR = Earth resistor current setting (Full load current)
R = Earthing Resistance
∴ % of Stator winding unprotected (1-X) = (IS/IR)x100
i.e., %(1-X) = Is.R / V x 100