Effect on measured frequency with PTs at different locations

[GRAEE]

Hi All,

May I ask your experience/explanation of the effect on measured frequency by two separate meters at different locations. We have two frequency meters connected at our 138 kV and 13.8 kV substations respectively...and are supplied by one source...Is there any possibility that they have difference on readings? If so, what are the possible factors that may contribute in different readings? Meters used are the same type but are installed at different locations & PTs.

thank you///

 

[davidbeach]

No, same frequency in both locations. Problem is, with one meter you know the exact frequency, with two meters you'll never know what the frequency is.

 

[GRAEE]

davidbeach,

are different PT types & accuracies used for two voltage levels affect the frequency readings?

 

[Skogsgurra]

The most likely reason for the difference is that you have interference on one signal. Interference usually (always) makes the frequency look higher than the true value.

The easy solution is to trust the lower reading. It is usually correct.

A slightly more elaborated solution is to put a low pass filter in front of the meter to reduce the interference. Make f0 a few hundred Hz. That usually does it.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[electricpete]

are different PT types & accuracies used for two voltage levels affect the frequency readings?

no. pt's can create voltage magnitude errors and phase errors, but not frequency errors.

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(2B)+(2B)' ?

 

[scottf]

What are the 2 readings?

 

[GRAEE]

scottf,

The reading at the 138 kV side is 58.794 Hz while at the 13.8 kV side is 58.83 Hz. A 0.036 Hz difference is observed in both readings.

 

[waross]

I have seen a lot of cheap frequency meters that would show those readings on 60 Hz. I have had electronic frequency meters that would show a reading of 3 times the actual frequency. I have seen cheap meters that would show lower frequencies as they aged.
Can you get a good meter and measure the actual frequency accurately?
An old synchronous motor type electric clock makes an accurate frequency checker. It will lock on to the frequency exactly and your accuracy will be determined by the accuracy of your wrist watch which you will use to compare times and compute the frequency.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[scottf]

What is the stated accuracy of the meters. I don't have a lot of experience with frequency meter readings, but a 0.06% difference doesn't sound that high to me.

What I can say, is the VTs shouldn't be contributing to any difference in the output frequency of each point.

Is the 138 kV unit an inductive VT or a capacitive voltage transformer (CVT)? Have you put a scope on the output of each to make sure there isn't any distortion?

 

[jghrist]

The reading at the 138 kV side is 58.794 Hz while at the 13.8 kV side is 58.83 Hz. A 0.036 Hz difference is observed in both readings.

Is this during a system disturbance? I wouldn't expect the frequency to deviate from 59.3 - 60.7 Hz for more than a minute without tripping generators.