Undervoltage blocking of frequency relay

[PeterS84]

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.

 

[dpc]

It's just to avoid nuisance tripping on frequency when the voltage or voltage signal is not reliable, such as during faults, or issues with the PTs circuits. 30% is pretty typical, but the exact setting is not critical.

If you actually have 30% voltage on your system, the frequency is not going to be your main problem.

 

[cranky108]

If you look at how the relay determines frequency it will better help you understand why.

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.

 

[slavag]

Depend on the frequency protection functionality.
1. Load Shedding
2. Generator protection.
3. Loss of main

usually between 0.3-0.6Un

 

[slavag]

please see attached thread

http://www.eng-tips.com/viewthread.cfm?qid=216805

 

[PeterS84]

Thanks for all your responses.

 

[PeterS84]

One more question

When talking about undervoltage - do you mean undervoltage on any phase or all three phases?

 

[cranky108]

The phase the under frequency is relay is on. It should be built into the under frequecny relay.

 

[PeterS84]

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?

 

[slavag]

I think, a last 25 years, under voltage release is part of under frequency relay. That means, it's operated on the same measurement circuit. In the digital relays, under voltage release is part of under frequency setting.

 

[waross]

Under-voltage may be a good thing if the frequency draws down as a result of loading rather than a fault. Allowing or causing the voltage to drop with the frequency aids in recovery. Underfrequency may cause saturation in transformers and motors. Dropping the voltage to maintain the Volts per Hertz ratio may allow a recovery rather than experiencing an outage.
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
--------------------
"Why not the best?"
Jimmy Carter

 

[cranky108]

If the manual dosen't say which phase frequency measurement is on then you should pester the manufacture for an answer.

In SEL the under voltage is part of the frequency settings, and dosen't matter.

 

[artvandelayb4u]

You usually do not need to worry about selecting a specific phase to protect with frequency settings and an undervoltage block. Most often all three phases will see a freq change and the under voltage block is usually set as a phase to ground value. The undervoltage block will be active on whichever phase needs it, even if that is all three phases at once.

 

[living2learn]

artvandelayb4u - (Side comment) Would a phase to ground measurment require wye-wye VT's?

 

[cranky108]

You can measure frequency phase to phase.