Uo measured on the Delta side Dy11n Xfmr

[obut4]

Hello,

We have a unit connected generator.
The generator is 11kV, 9MVA and is medium impedance grounded (25 ohms).(high impdedance would have probably been better)
The generator is connected to 2 step up transformers of 5 MVA connected in paralell.
Both transformers are exactly the same and the manufacturer has guaranteed that they can operate in paralell.
This configuration is unusual but the delivery time for 2 transformers of 5MVA was shorter than for one of 10 MVA.

The WYE side are 11kV and connected to the generator.
The DELTAS side are 15.4kv and ungrounded.

There is one voltage transformer (15.4kv/V3 / 110/V3) on each phase on the 15.4kV side connected between phase and ground.

When we started the generator, one protection relay connected on the 15.4kV gave an alarm indicating that the zero sequence voltage is too high.

The voltage measured were (see attached document)
- VAN = 9.375 kV at 0°
- VBN = 9.224 kv at 248.1°
- VCN = 8.154 kv at 123.0°

The calculated zero sequence voltage was 2.272 kV.

We thought there was an insulation failure causing a phase to ground fault on the 15.4kV.

But the isolation test with a megger showed a good insulation (2.8Gohms) between phase to ground.

The transformer manufacturer told us that the relay was set too sensitive (3V0 = 10V secondary = 1400 Vprimary) and that the relay readings should not be taken into account as the DELTA is ungrounded.
According to them the problem will disappear once the unit is synchronized.

Have you ever seen this problem?
Could the problem come from the fact that the transformers are in paralell?

We have a smaller generator with the same configuration except that there is only one 5MVA step up transformer and there is no problem with it.

Thanks for your help,

 

[waross]

Too bad that the transformer wasn't delta on the generator side.
Is the wye point floating, grounded, or connected to the generator neutral?

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

 

[obut4]

Hi Waross,

Usually i also see that the generator is connected on the DELTA side of the transformer, but here it is not the case.

What difference does it make? what is the advantage? is it linked to 3rd harmonics?

The WYE points of the 5MVA transformers are floating.
The generator is connected to ground through a 25 ohms resistor.

I have attached a photo of the single line (T19,T20)

obut4

 

[davidbeach]

Until the unit is synchronized, there is no ground reference on the delta side and the individual phase-ground voltages can be most anything.

 

[obut4]

Thanks for your answer David.

I conclude that the settings of the relay were to sensitive.

If a broken delta transformer with a resistor is used to measure the zero sequence voltage would it change something?


What is the mimimum 3V0 setting recommended for a zero sequence overvoltage relay with PT 15.4kv/V3 / 110/V3 on a ungrounded delta side of transformer?

We use the phase to phase voltage to perform synchronization on the 15.4kV side. So there should not be any problem as the phase to phase voltage is good even if the phase to ground voltage can be almost anything. Is it correct?


Thanks for your help

obut4

 

[davidbeach]

Correct setting (if such exists) depends on what you are trying to protect against. If you just want to know if one phase is grounded, calculate what you'd see for that condition and set at about 80% of that. Other than a grounded phase, the phase-ground voltages of an ungrounded system don't mean much.

 

[jghrist]

If the delta side will be synchronized to a grounded system, then I think the transformer manufacturer is correct. The problem will go away when the system is synchronized. One purpose of the zero-sequence voltage relay may be to detect if the 15.4 kV side loses utility source. You may have to synchronize at 11 kV to avoid the problem.

 

[slavag]

Hi.
As saied before, its volatge shifting in the Ph-N PT/VT connection, difference capacitance in the phases, standard problem of ungrounded sides w/o any load, topology of power cables is influence on the capacitance too.
Your case is very unusial situation.Problem is: not possible decide if its shifting ot ground fault on the delta side.

possible provide two settings for 59N relay:
1.before synchronazing about: 30-40V
2. one after: about 15-25V

 

[waross]

When a generator is feeding a grounded wye system, unbalanced loads are better shared by the generator windings if there is a wye:delta transformer with the delta facing the generator, as opposed to a wye:wye transformer. It gets to be a habit to see deltas facing the generator even when it may not matter that much. The delta:wye will also distribute unbalanced loads on to the three generator windings. Not perfectly but a lot better than the entire unbalance dropping on one phase as with a wye:wye arrangement.

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

 

[obut4]

Hi,

This is just to let you know that the generator has been synchronized.

As foreseen, the zero sequence voltage on the ungrounded Delta side of the transformer has disappeared once the unit has been synchronized (referenced to ground).

Thanks again for your help