Undervoltage blocking of frequency relay
Undervoltage blocking of frequency relay
(OP)
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 settings be different between different makes of PTs?
Thanks.
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 settings be different between different makes of PTs?
Thanks.






RE: Undervoltage blocking of frequency relay
If you actually have 30% voltage on your system, the frequency is not going to be your main problem.
RE: Undervoltage blocking of frequency relay
The relay uses a clock to count the time between zero-crossings. During a fault, the DC offset of the voltage will shift the zero crossings, which is why you want a delay. After the breaker opens there can be ringing of the remaining voltage, which is why you want a high enough voltage cutoff.
If you want to look at each type of distribution PT, and capactance of each possible remaining system configuration, you can. We just use a level high enough that we don't need to do all the extra analysis.
RE: Undervoltage blocking of frequency relay
1. Load Shedding
2. Generator protection.
3. Loss of main
usually between 0.3-0.6Un
RE: Undervoltage blocking of frequency relay
http://www.eng-tips.com/viewthread.cfm?qid=216805
RE: Undervoltage blocking of frequency relay
RE: Undervoltage blocking of frequency relay
When talking about undervoltage - do you mean undervoltage on any phase or all three phases?
RE: Undervoltage blocking of frequency relay
RE: Undervoltage blocking of frequency relay
Am I missing something?
RE: Undervoltage blocking of frequency relay
RE: Undervoltage blocking of frequency relay
I would consider implementing under-voltage blocking during under-frequency events on islanded systems, small diesel systems and distributed generation plants that may become islanded.
Bill
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"Why not the best?"
Jimmy Carter
RE: Undervoltage blocking of frequency relay
In SEL the under voltage is part of the frequency settings, and dosen't matter.
RE: Undervoltage blocking of frequency relay
RE: Undervoltage blocking of frequency relay
RE: Undervoltage blocking of frequency relay