59N vs 50G/51G

[JLuc]

Hi all,

when protecting a 600V alternator against ground faults (high resistance grounded, 5A); is there an advantage of using a 59N protection over using a 50G/51G with a sensitive CT?

I see common practice is 59N, but basic motor relays don't have this function. I'd use 50G/51G with sensitive CT (0.25 to 25 A primary).


Thanks for your comments.

 

[dpc]

The 59N is the normal approach for high resistance grounding due to the very low level of ground current. With newer digital relays, current detection is feasible, but requires a very low pickup setting.

 

[slavag]

Hi.
What Dave said.
Regards.
Slava

 

[dpc]

If this generator is going to be connected to an existing power system, the system grounding must also be considered. Connecting a generator with high-resistance ground to a power system that has solidly-grounded transformer windings will largely defeat the benefits of the high-res grounding.

You're probably aware of this, but just thought I'd mention this since I've seen this done in quite a few installations.

 

[davidbeach]

Dave, at least it would leave the generator high resistance grounded after the generator breaker opens and that will limit damage during the time between when the generator breaker opens and the generator ceases to produce voltage.

 

[dpc]

Agreed - it does provide some advantages. But there can certainly be much more than 5A flowing into the stator for a ground fault in the stator winding until the generator is isolated from the system.

Thanks for the clarification.

Dave

 

[JLuc]

Thank you all for the comments.

The system is delta on low voltage side, so high resistance grounding will protect the stator of the generator.

JL

 

[NKE007]

The 59N relay is used primarily on ungrounded systems. You need to install 3 PT's connected in grounded wye:broken delta configuration with a loading resistor across the broken delta to avoid ferroresonance. You could use the scheme on a high resistance grounded system, but the settings on the relay would need to be higher.
A 51N relay connected to a CT with a primary rating of 50% of the expected ground fault should provide sensitive operation. So if you expect 100A of ground fault select a CT ratio of 50/5

 

[Kiribanda]

JLuc,

A generator neutral grounded with a properly sized distribution transformer can be incorporated with a 59N relay to detect ground faults. Since the ground fault currents of such a grounding system are very low you cannot use 50G or 51G.

But a motor does not have its star point grounded. So you cannot have 59N. If the motor is connected to a high resistance grounded system, then you can use 51G (actually 51GS) sensitive ground fault protection to detect low level ground fault currents.

Therefore motor relays donot have 59N.

Two different issues.

Kiri

 

[JLuc]

You're right kiribanda,

in my case, it's a 1MW induction generator (almost same thing than induction motor)...

we will use Multilin 369 (motor relay) with 51GS function, with 50:0.025 sensitive CT. This GE CT can read currents as low as 0.25A, which will be OK with a 5A grounding resistor...I guess.

Thank you