Wye Delta Wye GSU delta winding 1

[Engineer1916]

Hello,
I have a 25MVA GSU for a solar field YDY configuration. There is no load on Delta, it is only there for stability/Harmonic suppression. I have a couple of questions about the buried delta.
1. Under what conditions should we bring out the corner of a burred delta outside the tank through bushings? What sort of testing is performed on delta winding?
2. How to select the MVA and kV rating of tertiary winding if it is not used for any load?
3. Under what conditions is it better to corner ground this delta winding vs not grounding it?

Thank you in advance.

 

[RRaghunath]

Delta winding MVA rating shall be such that it is good enough for carrying the expected harmonics continuously.
Further, the windings also carry circulating current during earth faults in Y-connected systems. This will be short time and the short time rating of delta winding needs to be good enough for the duty.
The magnitudes you need to estimate with the help of inverter and system data.
With regard to grounding one terminal of the delta, it is a mandatory requirement as any metal in the vicinity of current carrying conductors is expected to be earthed.
Whether the terminals of delta winding need to be brought out or not is a matter of your requirement. If the windings are not designed for any loading in future, I don't see any need for bringing out the terminals.

 

[Engineer1916]

RRagunath,
Thank you for your response. Here are few further questions:
1. How to calculate the harmonic load, to specify continuous kVA rating? How to specify kV rating?
2. How to calculate circulating current in delta winding during short circuit to specify short time rating?
3. How will any testing be performed on delta winding in the future if no terminals are brought out?

Any example would be highly appreciated.

 

[prc]

Is it a GSU (Generator station unit=generator transformer) or solar collection transformer? Normally solar inverter transformer is yD and collection transformer dYN, with a grounding transformer on delta bus. If your transformer is YNyn (ie solidly grounded on both sides) delta stabilizing delta tertiary is not required. In case you are not loading the stabilizing tertiary, you can leave the rated voltage to manufacturer. Rating should be 1/3 of main windings. Tertiary may carry unbalanced load in addition to LG line fault currents. In service you have to ground one corner of delta that may be brought out through a bushing. Still better bring out one corner through two bushings and form delta by shorting two bushing terminals. Testing of winding can be done by opening out this delta outside.

 

[sparky_007]

I used the IEEE Guideline "IEEE C57.158-2017 IEEE Guide for the Application of Tertiary and Stabilizing Windings in Power Transformers" to prepare our specification for 25MVA and 50MVA transformers. This guideline covers the delta winding voltage, KVA rating, short circuit withstand, testing, etc.

Similar to your case, we had delta winding for stability/harmonics etc. Per the IEEE guideline, we had two bushings for the delta winding for one of the corner of the delta winding. These two bushing were permanently connected external to tank (on top) with copper bus and grounded. Recommend one CT installed on any leg of the delta and sized based on the maximum rating of the delta.

 

[Engineer1916]

Thank you very much everybody for your valuable comments. I really appreciate it.

Sparky 007,
Can you please tell me what sort of testing can be performed on delta winding through external terminals?

 

[sparky_007]

Per IEEE C57.158, recommended tests on stabilizing winding (when the open delta with two bushings is connected externally and grounded):
1. turns ratio test
2. resistance measurement
3. winding insulation resistance and capacitance measurement
4. applied voltage test

 

[Engineer1916]

sparky_007,
Thank you very much. I promise this is the last question. In your experience, at what size have you seen these terminal brought out? Because I have not seen them in west coast upto 50MVA.

 

[prc]

It is not mandatory to bring the corner of delta outside through two bushings and short to form delta. So size of transformer has no role in it. The only extra test that you can do with two bushings is winding resistance measurement of tertiary winding. Ratio measurement cannot be done with one terminal of winding(see clause 9.1.1 of above standard ) The major advantage,is in future,if the need arise,you can remove the effect of tertiary by opening out the shorting out side and earth one terminal. This may be required in some occasions. Let me give some examples from personal experience. 1) A lot of 10-20 MVA transformers from a specific make were failing in service during LG line faults due to mechanical failure of tertiary winding. By opening out tertiary winding, balance units were saved.2) In an oil refinery a batch of 174 MVA 220/33 kV Units were in service for many years. After some system expansion,they found 33kV tie breaker fault level was below actual bus fault level. Either breakers to be changed or fault level to be reduced. By opening out tertiary transformer zero sequence impedance increased and fault level came down.

 

[Engineer1916]

prc,
thank you for the clarification. I will look into how the zero sequence will increase with opening delta.
But anyway I don't think my application would need delta brought out. The transformer is for a solar field and most likely its gonna spend its whole life in the same substation.