Overvoltage relay low pass filter cutoff frequency

[stason]

Hello all,

can anybody tell me what the cutoff frequency of the input low pass filter of a voltage / frequency protection relay, such as e.g. SEG MRN3, would be?

The manual says the relay applies an analog filter to get rid of higher harmonics before it applies digital DFT filtering. I am talking about the former analog stage.

I've tried various frequencies from 50Hz to 300Hz but have no idea would a real relay would do.

Thanks in advance,
stason

 

[electricpete]

I imagine it is an anti-aliasing filter and the cutoff would be somewhere below twice the sample rate and above the frequencies of interest (I would think line frequency is the only frequency of interest?).

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(2B)+(2B)' ?

 

[stason]

with the sampling rate of 1.25 ms that would lead to something like 63 Hz cutoff, which would cause a pretty strong reduction of the 50Hz when using the first-order low pass.
But, the relay manual does not reveal the type of filter used.
Thanks, pete, that's already something to start with.

 

[PHovnanian]

Hmm. For a 1.25 ms sampling rate, I get a Nyquist frequency of 400 Hz. Plenty of room for a 60 Hz (or 50 Hz) fundamental even given a non-ideal filter.

 

[stason]

I am not sure we are talking about the same filter. I was talking about an analog low pass filter sitting behind the voltage transformers. I am simulating its behavior before the A/D converter. And from what I know the cutoff frequency = 1/(2*pi*R*C)

 

[PHovnanian]

the cutoff frequency = 1/(2*pi*R*C)

Yes. But are you trying to simulate known RC values for a filter? Or are you trying to figure out what they should be? A 1.25 ms sampling rate would require a theoretical cutoff (brick wall filter) at 400 Hz. In reality, a first order RC filter would roll off much sooner to give adequate attenuation at 400 Hz. A higher order filter could have a higher cutoff for the same attenuation. But either way, I imagine that the filter cutoff would be somewhat higher than 63 Hz for a unit designed for 60 Hz or 50 Hz operation, as there would be appreciable attenuation of the signal of interest this close. Remember, 3 db power attenuation is about 70% of actual voltage. While communications people might throw a few db around with no concern, a 30% error is pretty significant when you are trying to set a relay. Its possible for the relay mfg to correct for filter attenuation in s/w. But you'd need to know this in order to simulate that behavior properly.

This is all speculation about what the cutoff for your particular relay might be. If you have the analog filter schematic and component values, you can calculate it.