59N Trip Vs. 59N Alarm?

[ACCOTTER]

Can anyone differentiate the philosophy behind tripping on 59N vs. alarming on a 59N?

Factors for consideration include:
This is in a Combustion Turbine Generating Unit.
VT on the LV side of the Generator Step Up Tx (after the Generator circuit breaker) is providing the open delta potential.

59N Element will be included in the GSU Transformer protective relay (generator protection is being covered by other relays).

There is a tap from the isolated phase bus between GCB and GSU to a Unit Auxiliary transformer (UAT protection similar to GSU).

It appears to be a standard to alarm only on 59N given this configuration. If so, why exactly? Thanks.

 

[slavag]

Hi Clos.
First rule ( ) -not open delta---broken delta.
As I know, it's should be ( must be) trip, not alarm.
Isn't distribution application.
With some time delay ,of course, more than stator earth/ground 95% or 100% fault protection of generator.
Regards.
Slava

 

[dpc]

With high resistance grounding (or ungrounded systems), it is possible to just alarm a ground fault and not trip. With any other type of grounding, you have to trip, or start getting out of the way.

In the US, I don't know of any large generator where only alarm is used. Maybe they are around, but I've never seen or heard of one. They generally always trip, in my experience.

The problem with alarm only, is that the ground generally does not go away and often escalates when another phase goes to ground (causing a phase-phase fault). The purpose of high-resistance grounding is to minimize damage to the generator stator from ground faults. If you don't trip right away, you are greatly increasing the risk of doing some major damage to generator.

 

[RRaghunath]

clos,

If I understand correctly, the relay you mention is meant for earth faults when generator CB is open.

The amount of time that the GT/UAT remain connected to grid, with GCB open is very limited. If an earth fault takes place in the GT-LV/UAT-HV/connected busbar system, the said earth fault relay operates. During normal working (with Generator on bars), other protection on generator side of GCB will cover the system on GY/UAT side of GCB as well and the above protection is not required.

Thus, to me, it appears it is acceptable/desirable to wire the said protection only for alarm.

 

[ACCOTTER]

raghun,

I think you’re right on with your assumptions. I was leaning towards your explanation but I wanted some confirmation. The area (zone) of protection changes as soon as the GCB is open and the 59N element from the generator protection (i.e. Beckwith M-3425A) is lost and the unit is being backfed from the GSU-UAT. At that point (although a short and unlikely amount of time) the transformer protection scheme should pick up that 59N element. In the end…the transformer protection should keep this as an alarm only pickup and the generator protection should be doing the tripping on this element.

 

[slavag]

Dear Clos and Rughun.
I'm sorry, but I'm comletly desagree with you.
Isn't important what is a protection zone: with GCB or without.
You have only one generator protection and you must ( OK for me) some duplicate or overlapping of protection functions.
Yes, you have some generator earth fault protection in one relay, but you haven't any back-up of this.
Lot ( most ) of generator faults is stator earth-faults, trafo earth faults. In case of some problem with one relay--what protect on your stub ?
59N is very important protection function.
But it's my opinion.
Regards.
Slava

 

[ACCOTTER]

Slavag,

Understood. I did not mention that we are also implementing secondary generator protection (Siemens 7UM621). This would be the back-up that you are referring to. Both the Beckwith M-3425A and the Siemens 7UM621 will cover the same protective elements i.e. 59N, 27TN. Given this redundancy and the fact that the transformer protection scheme will also cover 50/51G (with GCB open or GCB closed), in my opinion the GSU transformer protective relays (Siemens 7UT613) can cover the 59N as an alarm only.

Mode of operation-

Unit On-Line:
Gen Protection- 59N, 27TN covered by Siemens (7UM621) and Beckwith M-3425A [Trips]
GSU Protection- 59N covered by Siemens (7UT613) [Alarms]

Back-Feeding:
GSU/UAT Protection- 50/51G (7UT613) [Trip]
GSU/UAT Protection- 59N covered by Siemens (7UT613) [Alarm]

Of course, the usual additional protective elements will also be implemented for both the Gen protection and the Tx protection. I was just giving an overview of the earth ground protection and their respective TRIP or Alarm relationship.

 

[slavag]

Hi Clos.
What is a size of your Unit, I assume > 100MVA.
O.K., we all agree, no standards on this issue, only recommendations and opinions.
We always try for large size generators build redundancy on all levels: protection functions, DC, protective relays, CT/VT's.
1. You have redundancy now on the protective relays level.
2. I assume on the DC level too.
3. I assupe you also split CT/VT between protective relays.
But you have only one VT on the generator star point ( something like to 15kV/240V with resistor). And maybe connect this VT in parallel to both protective relays.
Please take in account, you haven't any indication about this wiring ( normal situation is "0" voltage). Where now your back-up, you can add next numerical relay, dosen't help. For this you have additional broken delta connection are connected to the trafo protective relay, reason, I assume only one, number of analog inputs in the generator protective relays ( if I remember right, are 12 and 12 in both of relays).
BTW, for curious only , what is protection functions split between those relays.
And, why you have problem with 59N trip, why only alarm, what is a reason, what is a advantage?
59N generator protection delay for example 0.5s, put 59N additional on the 2sec it's all.
Regards.
Slava