Relay Tolerance and Setting Errors
Relay Tolerance and Setting Errors
(OP)
In Ieee 242-2001, "Recommended practice for protection and Coordination of Industrial and Commercial Power Systems", Table 15-2, a value of 0.12 sec. is given for relay tolerance and setting errors, static relays, coordination time interval, field calibrated relays.
I'm wondering what the true basis of that number is and what it includes. The relay I'm looking at has a timer accuracy of +/-0.25 cycles for Inst/DT elements, which is not even close to 0.12 Sec.
Does someone have some helpful pointers? I asked the oracle Google, but not much out there. Thanks.
I'm wondering what the true basis of that number is and what it includes. The relay I'm looking at has a timer accuracy of +/-0.25 cycles for Inst/DT elements, which is not even close to 0.12 Sec.
Does someone have some helpful pointers? I asked the oracle Google, but not much out there. Thanks.






RE: Relay Tolerance and Setting Errors
RE: Relay Tolerance and Setting Errors
When you factor in all of the delays and error bands, I suspect 0.12 sec will be about right. When the breaker clearing time is added, 0.12 sec may not be enough.
The last time I went through a similar evaluation, we were trying to coordinate an upstream digital relay with a downstream electromechanical instantaneous trip. With a 5-cycle breaker, I ended up with a time delay of 0.2 sec on the upstream relay.
RE: Relay Tolerance and Setting Errors
However, for the Instantaneous Definite Time region, if all such errors still keep us in that region, then all we have is relay timer accuracy, in my case +/- 0.25 cycle, for 0.50 cycle margin, or 0.01 sec at 50 Hz, or 0.0083 sec at 60 hz.
I don't intend to go below 0.20 sec, but I would like to understand where the numbers come from. I think the O/C region and the DT regions must have separate numbers and the 0.20 CTI is a generalization that needs to be fine-tuned in each app.
RE: Relay Tolerance and Setting Errors
But you have to read the relay specs closely and make sure you are adding in all sources of error and delay, not just timer accuracy.
RE: Relay Tolerance and Setting Errors
RE: Relay Tolerance and Setting Errors
But if you're dealing with metal-clad switchgear, those CTs are usually low accuracy (C100) and can easily saturate on close-in faults. Assumption has always been that the instantaneous unit should operate before saturation gets severe, but who really knows.
I don't think the IEEE guidelines are too useful when dealing with instantaneous and definite-time delays.
RE: Relay Tolerance and Setting Errors
David and Dpc covered all are well.
I would like only add some example of calculation, what I use ( BTW, this calculation are not important, in all cases we put time grading min 0.2sec)
Time grading = dT
Te = tolerance of the relay operation time
Tr = overshoot time (needed time for canceled trip ).
Tcb = CB operate time.
Tm = marginal time (delay on aux relay, by CT saturation, etc, actually about 20ms, in practice it's enough because all settings are delayed).
for digital relay.
dT= 2xTe ( 2x25ms)+ Tr(30ms) + Tcb ( 50ms )+Tm ( 20 ms )=
150ms
Regards.
Slava
RE: Relay Tolerance and Setting Errors
RE: Relay Tolerance and Setting Errors
RE: Relay Tolerance and Setting Errors
There is also the issue of how fast an element resets when current falls below the pickup, and how far below pickup it needs to go for the element to pickup.