How is the frequency measured in PM8000?

[sesa74337_bridg]

Hi, All

We have a data centre customer trying to understand why the PM8000 showing 50.01 Hz frequency on the generator board when there was only the control power to the meter, no 3-phase voltage and current inputs.

How does the PM8000 measures the frequency?

Thanks

James R

[Charles_Murison]

Hello James,

This  is a very abridged version and there are much more fine details but the general summary is:

Meter will measure the period for the zero crossing on Phase A voltage. Inverse of period is frequency.

For you meter, if you do a manual waveform capture, is there any Voltage on Phase A? 

[Hal_Etheridge]

Some ION meters have the hardware capability to measure frequency on one of phase A, B or C.  In that case if phase A is missing/below the allowed threshold then it would switch to phase B and then switch phase C if needed.  If none of the phases have a usable voltage waveform then my guess is that most likely the frequency register would go N/A.  There should be some mention in the meter documentation what exactly the behavior is for these situations though.

[DanL · Accepted solution]

On the PM8000, the hardware is designed to allow measurement (of both voltage and frequency) between any pair of voltage terminals.

For frequency measurement, the meter prioritizes pairs of voltage terminals with high signal levels (V1-VN, V2-VN, V3-VN, V1-V2, V2-V3, V3-V1).

The PQA module has an output indicating which frequency channel is in use "PQA1 Power Frequency Channel".

On the PM8000, the channels are:

V1N = 8,
V2N = 9,
V3N = 10,
V12 = 12,
V23 = 13,
V31 = 14,

If a signal is not detected on any pair, then the meter will revert to a single ended measurement on one terminal with respect to the meter's floating internal ground. Usually the meter's internal circuits have the same frequency as the auxillary supply voltage.

In the customer's situation where the V1, V2, V3 terminals are disconnected, most likely there is detectable voltage frequency between the Neutral connection on the VN measurement terminal and the meter's internal circuits. Of course when all measurement terminals are disconnected at the meter, there will be no measured frequency.