Grading

[PUNGLU]

How long should the last relay on the attached drawing (At the 33kV source Bus)take to initiate a trip to opening of the breaker?

I am trying to grade and give the customer (11 kV incomer breaker)some appreciable margin. Any standards used, may be in other utilities. Let me know please and thanks in advance.

 

[davidbeach]

I don't recognize SPAJ, so I don't know what type of protection you are working with. With the transformer neutral shown, I'm going to assume phase and ground fault protection using 50/51 elements.

No ground fault current from 11kV ground faults will be seen at 33kV, but there needs to be coordination of ground fault elements from the transformer out (could have a ground in the delta winding of the transformer). CTs need to be added on the line for breaker 3.

The 11kV should be set as fast/low as it can and still coordinate with the rest of the 11kV system. The neutral overcurrent on breaker 3 needs to coordinate with that including a reasonable CTI, 0.25 sec is probably tight but do able.

Breaker 3 needs ground set really low and fast as the only thing it will see is transformer faults. Phase needs to coordinate with the 11kV for phase and the phase-phase associated with a ground fault on the 11kV. Set the 50 element at 115%-125% of maximum through fault current. Coordinate the 51 with the 11kV including your CTI.

Assuming a short run from the 33kV bus through breakers 1, 2, and 3, you won't be able to set a coordinated 50 for breakers 1 or 2, although you could use a definite time element for currents above the pickup of the breaker 3 50. Stack up your settings plus CTI for each breaker and see where you wind up for breaker 1. With that electromechanical relay I'd probably push my CTI out to 0.3 sec or there 'bouts. Impossible to give you exact numbers and even with enough information to do the settings different people would arrive at different settings.

 

[PUNGLU]

I appreciate. The numerical O/C relays are SPAJ 140C for O/C & residual E/F while the SBEF is for SBEF. BKR1 and BKR2 belong to the utility while BKR onwards is for the client.

 

[slavag]

Hi Punglu.
Is very dificult help you on this issue.
SPAJ140 have only two stage of protection 50 ( DT) and 51 ( DT or IDMT) Spaj115 is SBEF and on the 11kV side that means you don't need any grading with 36kV ( see David's post).
For DT grading is as David saied 0.25sec, possible with SPAJ 0.18-0.2 sec ( depend on the CB opening time).
For the 50 on the 36kV side I recommend use doubling function of SPAJ140 in case of inrush trafo current.
for 51 you can used IDMT curves.
Now, one of the option is: used logical blocking between SPAJ's140, min time in this case 0.1sec.
Regards.
Slava

 

[jghrist]

I'd set the Breaker 2 relay just below the transformer damage curve. This will give maximum coordinating space for the customer's 11 kV incomer and feeders. Are there any lines fed from the bus between Breaker 2 and Breaker 3? If not, then the 11 kV incomer does not need to coordinate with Breaker 2. But then, why is there both a Breaker 2 and a Breaker 3? Why no phase protection tripping Breaker 3?

 

[PUNGLU]

Jghrist, I also felt BKR3 and BKR2 is superfluous but requirement for the utility is that we have BKR2 as the metering BKR. BKR3 is the Trafo BKR. No line are fed form any of the buses. All loads are fed from the 11kV bus. The CTs for Trafo protection are all bushing CTs, no extra CTs have been provided except for the metering CTs for BKR2.

 

[AMBMI]

I would suggest:
- no selectivity between the electromechanical realy and the SPAJ 140C
- 50 Trip threshold set at 200% trafo In.
- use Extremely inverse curve set the time dial just to be below the infrequent fault trafo damage curve
- 51 Trip threshold 130% max short circuit current at secondary side (considering a network infinite short circuit power).
- 51 delay : 0.05 seconds

 

[AMBMI]

obviously in my previous answer please exchange 51 with 50...
I apologize!