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Thanks for the answer. Why do you think there would be a lot of false trips? If I set the relay to, let's say, 5Hz/s this is some serious ROCOF now and shouldn't happen under healthy conditions. Unless what the relay is measuring and what is actually happening on the system doesn't align...

I'm thinking about using it as a faster alternative to frequency based load shedding. In case of small inertia of the system load shedding based on frequency may come too late to make any difference (as it takes time for the frequency to fall below setpoint). With ROCOF I was hoping to gain some...

Hi All, Can somebody give me any guidelines on what settings to use for rate of change of frequency? Would you normally do some simulations before choosing settings or just use a 'generic' recommended one. Also, would you tend to use normal ROCOF or frequency supervised one?

Thanks for the interesting paper! Although the phenomena they're describing there is caused by CT ringdown (energy trapped in the CT after CB has opened) so it's different to the transients caused by charging of the capacitance.

Thanks a lot smallgreek. Since I don't have access to any layout drawings my question was more of an academic nature ;) I was just curious of how much of an impact (if any) it may have on CBFail. A quick calculation for 3m high, 2m to the breaker gives around 0.2nF so not much. For the case of...

Thanks for the information. Sorry if my previous response sounded a bit cheeky - I meant I don't go out to substations so have very little idea about substation design. Perhaps I was looking at a metering CT...

Nope, not that far if CT is on the bushing. However for HV CTs are not always mounted on the bushing (at least that's what I heared). I don't know about the distance - I was hopeing to get some input from people who actually visit a substation every now and then (unlike myself).

Thanks davidbeach. Does this apply to CB fail protection as well? I've done some simulations and it seems that under some conditions the charging current can be in the range of 0.05In and this can last 50-100ms (in those simulations I used capacitance of 5nF per phase which might be a bit...

Dear All, In a substation we have protection CTs in one place and the circuit breakers in another. What I'm interested in is what can be the capacitance of the wire connecting both, or at least the distance between CTs and CB. What I'm trying to understand is how much current (and for how long)...

Not quite sure I understand - with multifunctional IEDs you normally have all three voltages connected and the phase selection algorithm for frequency measurement is internal to the IED, so you never know which phase is the relay actually looking at. Am I missing something?

One more question :) When talking about undervoltage - do you mean undervoltage on any phase or all three phases?

Thanks for all your responses.

Hi All, Any ideas why to use undervoltage blocking for frequency functions (except the obvious one that below approx. 10% of Vnominal the accuracy may not be satisfactory)? I heared some people wanting to set undervoltage blocking to 30% Vnom - are there any reasons to do that? Will these...

Does everything said above apply to thermostats (internal to the transformer) as well?

OK, thank you all for your replies.

Hi All, Can you tell me how do you connect Buchholz/SPR to trip CB - do you connect it directly to the CB or do you connect it via IED's binary inputs (or both - in parallel)? By the way - is there a difference between Buchholz and SPR? Regards, Peter

Thanks. [embarrassed]

Hi All, Is there any 'magic equation' that allows to estimate the physical size of CT (i.e. length, width, height) based on the turns ratio, VA rating and type of core? How can I actually make sure that the CT will fit where I want to put it (in a cubical for example). I mean protection CTs...

By BFP do you mean Breaker Failure protection?

Thanks for all your replays. @m3ntosan: Yes, by distribution I mean networks up to 132kV. I'm surprised you mention cables as well - I was under impression that for cables it's mostly current diff.