Testing Phase Angle Relationship for Transformer Differential Relay 4

[tmaly1]

Dear folks,

What is the most effective method to confirm and check the phase angle relationship for the load and source side CT's for a transformer differential relay?

The transformer is 30 MVA, 115/12.47 kV, Delta/Wye with 30 degree phase shift. The CT's are connected Wye (on TX delta side) and Delta (on the TX Wye Side).

Thanks for your feedback and help.

 

[stevenal]

Use a dual channel oscilloscope with current probes. Connect at relay inputs on the polarity side. Align probes so they read current entering the relay. Add load slowly, keeping it below relay pickup in case polarity is wrong. Traces should be 180 degrees out.

 

[RalphChristie]

If it is a new transformer I would do a kind of a copper-test on the transformer. Short the LV-winding behind the diff CTs (shorting wires must be able to handle current) and apply a low voltage on the HV-side of the transformer. Gradually increase the voltage till full load current flows. Current will now circulate through the HV and LV CTs and you can measure the values on the relay.

With old relays you can measure the phase angle with a scope.
With newer electronic relays you can actually see the different values on the screen (if programmed correctly)

 

[NormGA]

If you are talking about verifying the current input to your protective relays I suggest that a better way would be to use a phase angle meter in conjunction with an ammeter. That's the way I've been doing it for the last 30 years or so. The high side current connected in wye with low side in delta gives you another 30 degree shift in the opposite direction so that the high side ct current is in phase with the low side ct current. The phase angle of the current going into the relay is 180 degrees out of phase with the current going out of the relay. When I read currents I turn my test plug over so that the angles are the same.

I try to calculate high and low side currents based on transformer watts and vars and ct ratios and compare my calculation with values that I read. Make sure that the three phases are about 120 degrees apart and that the ratio of high side ct current to low side ct current is about the same as the ratio of the tap settings of the relays.

What kind of relays are you using? As someone suggested the new microprocessor relays pretty much do everything for you as long as you give the relay all the correct information. And you can connect the ct's just about any way you wish. Verifying the inputs is a breeze with this type relay.

HTH,
NormGA

 

[ScottyUK]

This sounds like a generator step-up transformer from the rating and large ratio. From the transformer data you've given us, and with the assumption that the impedance is in the region of 15% or so, you should be able to directly connect a mobile generator to the LV winding, short the HV winding, and circulate current through the transformer as Ralph suggests.

You won't be able to circulate rated current in the windings without using either an HV generator or interposing a distribution transformer between the generator and the transformer-under-test, but an LV generator should circulate enough current to allow you to check your CTs and relay without the additional complexity. Circulating full rated current in the windings will require a very powerful test source.

Guessing a couple of values and using your supplied data:

I've assumed
Transformer Z=15%
Mobile generator supply 415V


Rated LV current = 30E6/(sqrt3*12.47E3) = 1389A
LV volts for rated current into HV S/Cct = 12.47E3/0.15 = 1.87kV

LV Current with 415V on LV winding and HV S/Cct = 1389* (415/1.87E3) = 309A.

309A is just within the capability of a 250kVA set, but the genset will probably object to a step load change of 90% of rating. I would probably go up to a 300 or 350kVA unit. The rental cost is not significantly different.


Interposing a typical 11/3.3kV distribution transformer, with the 415V source to the 3.3kV winding and the transformer-under-test's LV winding to the 11kV winding, the current in the LV winding of the transformer-under-test rises to about 1030A, which is about 75% of rated current, at the expense of increasing the load on the test generator to 3500A @ 415V. Sets of this size certainly exist, but arranging such a test is a fairly major undertaking. The directly connected LV generator will show up any CT polarity problems in a sensitive relay such as transformer differential anyway, so you would be making a lot of work for little gain.



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If we learn from our mistakes,
I'm getting a great education!

 

[tmaly1]

Dear folks,

Thank you all for your help and feedback, I really appreciate the time and efforts you took to share your jknowledge and experience with myself.

ScottyUK, actually, the TX is a substation TX at one of our dist. subs. We're replacing the sub. breakers and want to make sure that all CT's and relay connections will remain intact and in the same configurations they were before we replace the breakers and function in the same manner.

NormGA, we're using GEH-1788 GE OC and Westinghouse HU-1 TX Differential type relays, they are all electromechanical.

Thanks again for your valuable posts.

 

[jray480]

tmaly1
Not sure if this helps, I think I understand your question. If you are just replacing the xfmr and not any of the CT wiring, only determing and reterming when the new xfmr arrives consider this. Your 3 line will give the correct terminations at the relay end, label the other end correctly i.e. field or xfmr connections. When you reland all your current circuits and control wiring at the xfmr match them to the name plate and CT drawings. Replace your connection jumpers to retain your delta or wye configuration. Once everything is landed and ready, push your currents back into the relay. The method I use requires 3 lightbulbs @ different wattages i.e. 40w 60w 75w; wire the lights in parrallel. Wire the light bubl supplies together respectively A/B/C with jumpers pigtailed out to connect to the ct circuits respectively. Wire the nuetrals together and connect to return. This setup will be powerd by a suicide cord fused if you desire.
Make sure that the relays will handle the voltage and to my knowledge mostly all will. What were making is a load box thats all. when you make your connections measure your currents at the relay and at the xfmr or breaker ct blocks. make sure your currents matchup A to A / B to B / C to C. Just calculate the current based on your bulb wattage.