Grading
Grading
(OP)
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.
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.






RE: Grading
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.
RE: Grading
RE: Grading
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
RE: Grading
RE: Grading
RE: Grading
- 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
RE: Grading
I apologize!