GENERATOR EARTH FAULT PROTECTION

[genhead]

I have 2 x 1MW 6.6kv diesel generators (paralleled), connected to the delta winding of a 6.6/11kv step-up transformer. A grounding (zig-zag) transformer is also connected here, with grounding via a resistor. There is no earth-fault ct at this neutral-point, and access is very difficult.
The generator neutrals are not connected.
Regarding earth-fault protection, can I use core-balance type toroids (ct) around the 3 phases at each generator, as I already have earth-fault relays fitted on each generator ? Would this give adequate earth-fault protection ?

 

[230842]

There are several ways to detect earth faults (for protection). All of them are based on current fault detection performed by two different procedures:
a) Meassuring current (or voltage)on grounding resistor path.
b) Meassuring zero sequence current either by the sum of the three line currents at secondary side of meassurement CT's or by a core-balance type toroids (ct) around the 3 phases at each generator as you propose.
Proceure a) is the most common and simple way to do that, since grounding resistor path current is exactly the fault current.
Procedure b) is also eassy to improve, since the 3 meassurement CT perform, generally, their secondary windings arranged in star connection whith one common wire (return path) and three active wires. The return wire current amounts three times the current fault value(referred to CT's secondary, of course). Regarding your proposal, core-balance type toroids (ct) around the 3 phases at each generator is quite valid but normally used for low power installations or, as in your case, when no other alternative does exist.
Hoping help you. Julian

 

[peterb]

CTs installed on each generator circuit, as you describe, will give sensitive protection for the individual generators. There needs to be earth fault protection for the earthing transformer and earthing resistor as well, unless both are continuosly rated for full earth fault current.
This would protect the earthing transformer/resistor combination against earth faults on the 6.6 kv busbar or uncleared faults on the generator circuits. If you can't access the earthing lead of the resistor or the connection between the resistor and the transformer to install a CT, then you would probably need to install a toroidal CT on the earthing transformer phase leads in the switchgear.