Positive Metered Powerflow from Open Breaker? 1

[drchaos]

Hi,

We have a windfarm that has recently energized. It currently has its breaker open, so the grid trafo 110/20kV is energized from the HV side only.
Both metering and SCADA are showing a few kW flowing from the trafo into the grid. It isn't constant, but doesn't look random or noisy either.
Does anyone have any idea where this power could be coming from??! Could it be due to poor earthing / broken CT?
The fact that two independent systems are giving the same results would suggest the power is real, rather than a measurement error, but any suggestion on what to look at would be welcome!

 

[Marmite]

Where are the metering CT's and VT's fitted?
Can you post a single line diagram?

Regards
Marmite

 

[davidbeach]

You're feeding your transformer no load losses. Probably some station service loads too.

 

[waross]

What David said. If your metering is on the grid side, you may be reading your meter backwards. It may not be your mistake.
The default setting on many electronic meters is to sum positive and negative power flow. If that is the case I hope that the utility plans to reprogram the meter to report positive power flow and negative power flow separately.

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

 

[davidbeach]

If it is line metering, positive power is generally off the line and negative is into the line. Nothing backward about it, plants all show up as negative when seen from the transmission system.

 

[drchaos]

There are various people investigating at the moment. If the trafo was on no-load, would its no load power consumption be varying?
I've attached an SLD of the relevant bit - the CB at the bottom is open.
The chap who does the metering insists that something fishy is going on! Will let you know if we find out the problem - hopefully it is something as simple as incorrect wiring.

 

[waross]

Hi David. I meant to say that they may have been reading positive when they meant to read negative. The electronic meters that I have installed, both single phase and industrial report the sum of the positive and negative power as the default.
The utility meter shop has to reprogram the meter defaults before the meter will report positive power separately from negative power.
Once the wind farm goes online it will be obvious whether or not the meter has been programed properly.
These may be customer owned meters rather than utility meters. In that case the customer may need to spend $5000 or $6000 on the cable and software package to reprogram their meter defaults.



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

 

[waross]

Your one-line doesn't include station loads. Is it reasonable to consider the meter reading as transformer losses plus station air conditioning loads.
I suspect that your problem may be confusion as to positive power flow versus negative power flow. Back to what David said.

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

 

[jghrist]

You could be supplying some unbalance current into the 110 kV system from the grounded wye - delta tertiary transformer connection. This may account for the variation in the no load power. Supplying unbalance current would not be supplying real power, but with the metering at a very low percent of full-scale, CT errors may introduce enough phase angle error to have this show up as a small power flow.

 

[waross]

I missed the delta winding, jghrist.
If the 110kV phase voltages are unbalanced the delta winding will transfer real power from the higher voltage phase(s) to the lower voltage phase(s). The amount of power transferred will vary as the primary voltage unbalance varies.


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