phase shifting in transformers 1

[bartb102]

Unusual transfomer phasing question.

34.5KV/2400V Delta/Wye station stepdown transformer. CT's on the primary are connected wye. I need to use an existing multifunction relay connected to these CTs to monitor power consumption in the transformer, and so need primary voltage. I do not have PTs on the transformer primary, only two secondary 2400:120V connected open delta/open delta. Yes, that will result in less than optimum accuracy, not an issue here, we just want to monitor transfomer losses.

The transformer will phase shift the PT output voltages 30deg. I need to shift them back again, or determine how to setup the relay to compensate for the unwanted shift. I can select most possible connections of the PTs- 3-wire, 4-wire, AB, BC, CA, AN, BN, CN, and set their ratio. The protective relay will be set for over-current functions only, no voltage functions.

One thought is to connect 3- 120V metering PTs wye/delta, but I'd like to avoid that if I can.

Thanks,

 

[dpc]

we just want to monitor transfomer losses

Unless you are monitoring power in and power out, determining transformer losses will require a calculation based on power in or power out.

The losses are two orders of magnitude (or more) less than the through power being measured, so even small errors in metering accuracy will put the accuracy of the loss measurements into question.

The best approach would be to simply meter on the secondary side of the transformer (PTs and CTs) then calculate the losses based on the transformer test data. This is commonly done for revenue metering on distribution substations.

Measuring transformer losses via wattmeters is challenging even during factory testing of transformers due to ratio and phase angle errors inherent in the CTs. And if you are using typical transformer bushing CTs, they will generally not even be of standard metering accuracy, compounding the problem.

 

[davidbeach]

But it's so much fun swatting gnats with sledge hammers, don't ya think? ;-)

 

[bartb102]

We are already monitoring the secondary side through the generator protection relays (I didn't mention that this is a hydro plant with two generators). The transformer also has a tertiary 480V Wye winding for station service (does not have CTs). We'd like to know the difference between generation and output to the utility. The 34.5KV CTs are busbar type, not bushing. I also need to bring the secondary line voltage into the control system, so first (low cost) thought is to simply use the voltage inputs on the transformer protection relay (to keep inventory down we're using same type as on the generator). Without transformer phase correction it can be used for magnetude, but not for any phase related parameter.

 

[rcw retired EE]

I tried doing that and it didn't work well enough to read transformer losses. Typically, the transformer losses are within the error bands of the CT/VT/meter combined system. Add in the inaccuracies of the step-up transformer and voltage transformer ratios and phase angles and you will have unreliable data.

I had more MW output than input = negative GSU losses. This obvioulsy was due to metering errors.