Delta Transformers on Wye Distribution System

[Josh111]

Hopefully this has not already been asked. It would be hard to do a forum search with the right words to find it if it has been already.

I am wondering if it is ever possible or common place to generate power in a Wye generation scheme and then connect it directly to a transformer with a Delta primary.

If this is done, what would be the major setbacks.

Also, how would one MCOV ratings of elbow arrestors at the high side of a transformer?

 

[zoki85]

I am wondering if it is ever possible or common place to generate power in a Wye generation scheme and then connect it directly to a transformer with a Delta primary.

Possible of course.

 

[jim hardy]

That's how our central station main generator is connected to the main stepup transformer. Generator neutral was resistance grounded .

I don't know anything about the surge arrestors.

 

[dlgoff]

That's how our central station main generator is connected to the main stepup transformer. Generator neutral was resistance grounded .

I don't know anything about the surge arrestors.

I found this Addressing Ground Faults on MV Generators from our old friend, http://ecmweb.com/. They address advantages/disadvantages of Low & high impedance grounding and single & multiple point grounding. Then they show an optimal Hybrid grounding solution.

I Love their site.

 

[zoki85]

Big sync generators (S_n>50 MVA) I know of, are directly grounded (at least that's the practice in my country). However, I know case where a relatively small generator (S_n< 10 MVA) is grounded via quite a large Petersen coil.

 

[jim hardy]

Big sync generators (Sn>50 MVA) I know of, are directly grounded (at least that's the practice in my country). However, I know case where a relatively small generator (Sn< 10 MVA) is grounded via quite a large Petersen coil.

I find that surprising. The reason for the resistor is to limit current to just a few amps should a ground fault develop, so there's not an electrical explosion.
Our machine was 894MVA at 22kv. Ten amps at 22kv is of course 220KW so the grounding resistor was big as a pickup truck. A relay monitors the voltage across it and trips the unit in case of a ground fault.
That design pays off - at another plant somebody left a wrench inside the generator. When it fell shorting one terminal to frame the unit tripped on "ground fault" with no damage, just had to go in and retrieve the wrench.

 

[zoki85]

I find that surprising. The reason for the resistor is to limit current to just a few amps should a ground fault develop, so there's not an electrical explosion.
Our machine was 894MVA at 22kv. Ten amps at 22kv is of course 220KW so the grounding resistor was big as a pickup truck. A relay monitors the voltage across it and trips the unit in case of a ground fault.
That design pays off - at another plant somebody left a wrench inside the generator. When it fell shorting one terminal to frame the unit tripped on "ground fault" with no damage, just had to go in and retrieve the wrench.

Jim, you are right! I rechecked my data and big generators in my country are also impedance-grounded on regular basis. I don't know why but seems I confused generators data with HV power transformers...