Protection CT Location for Small Generators 1

[mgtrp]

I'm looking for opinions on the location of protection CTs for small medium-voltage generators (800-1200 kW) that do not use differential (just overcurrent).

Some people prefer them to be mounted on the neutral end of the generator winding, so that they can measure the generator contribution to internal faults. This seems reasonable—a set of grounds left on after maintenance would be quite the fireball otherwise, especially with a PMG-fed unit.

Others prefer them to be mounted on the phase end of the generator winding, so that they measure the system contribution to internal faults. This also seems reasonable—the fault can be cleared much faster (trip instantaneously), and coordination with other units on the same bus is possible. It also allows for current-based ground fault protection of high-impedance or ungrounded generators based on utility contribution. I also note that this seems to be the more typical arrangement of small diesel genset manufacturers.

Any thoughts as to which is preferable? Would operating regime make a difference (units that are run for extended periods vs started and stopped more often)?

Thanks in advance,
mgtrp

 

[davidbeach]

Neutral end CTs can detect winding faults prior to closing the breaker, terminal end CTs won’t see fault current until the breaker is closed.

 

[RRaghunath]

Generator internal faults are most likely earth faults. A CT located in the generator neutral to earth circuit (including in the circuit of NGR in case of medium resistance or high resistance grounded generators) wired to earth fault protection should be able to detect internal faults effectively.
In case of phase faults, it is better to use phase end CTs as the contribution from grid is likely to be greater than that from the generator itself.
If the generator is not intended to be paralleled with grid, there may be slight advantage in locating the phase CTs on neutral side of the generator windings. Generally neutral end terminal boxes are small, so practicability of CT installation could be an issue.

Rompicherla Raghunath

 

[mgtrp]

Most of our existing fleet of diesel generators have either CTs on both ends (for full differential) on the larger units, or phase end only for the smaller units. A lot of the smaller units don't appear to have the neutral ends of the windings brought out to the terminal box, so changing the CT location would be a bit of a project.

I suspect that we might go for something similar to what Mr. Raghunath has suggested for the smaller units, and rely on the AVR to trip on "failure to build voltage" in the event that somebody leaves the ground leads on after maintenance, hoping that this will operate prior to catastrophic failure. This doesn't seem ideal to me, but apparently must be considered an "acceptable" way to protect a small generator since provisions for fitting neutral CTs are so often omitted from the generator design. The loss of a single generator would be a costly, but not the end of the world; a fire causing wide-spread damage to the facility would be very problematic.

Cheers,
mgtrp