Neutral voltage in 35MVA transformer 3

[Emadshaaban1987]

A new 30MvA 11kv generator connected to a new abb 35MVA 11/20kv ynd11 transformer.to be synchronized with other genset in 20kv busbar.

My questions are:
1-We started the set with no load ,all phases voltages in transformer output are identical 20kv but residual voltage (read from broken delta by) is 350v(CTs polarity tested )Neutral resistor in TR start point IS OPEN and in generator star point is closed what are proper causes?

2-at no load what is the range of normal THD of voltage?

 

[Kiribanda]

1) Pl. confirm whether it is OPEN delta or BROKEN delta connection. Because they are used for two different purposes.
2) What do you mean by NER is open? Does it mean your NER is not used? If yes why?

 

[davidbeach]

Assuming, from various clues that the OP is from the IEC realm where they say open delta to mean broken delta, and that “open NER” means that the generator is ungrounded then the phase-ground voltages have no meaning. Phase-ground voltage will be determined by the balance (or lack there of) of the parasitic capacitances.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[Emadshaaban1987]

Thank you all ,yes I meat broke delta connection from the residual VT connection.
NER :neutral earthing resistor is opened in TR star point and closed in generator star point

 

[RRaghunath]

11kV system is earthed at Generator and 20kV system is isolated. That explains why 20kV system shows residual voltage of 350V as measured on VT secondary (connected in broken delta).
This has no meaning as the 20kV system is floating and the voltage will be determined by the balance (or lack there of) of the parasitic capacitances as mentioned by davidbeach Sir above.

 

[Kiribanda]

That means your 20kV system is ungrounded. There should be a "high impedance ground fault" on the 20kV
system. That should be the reason why the voltage relay connected to BROKEN DELTA protection scheme shows a zero
seq voltage of 350V. Knowing the VT ratio & VT rating you can estimate whether it is a solid ground fault or
a high resistance one. Anyhow, in my opinion you have to clear this fault.
1) What is your reasoning or logic behind operating a 20kV system ungrounded?
2) Are the SA (adequate MCOV) & VTs (adequate voltage factor) connected to your 20kV system are adequately rated for an ungrounded system?
(Note: if they are not adequately rated, they might create an high impedance earth fault on the 20kV system)
2) Could you pl. upload the SLD with SA & VT name plate data?

 

[cranky108]

I believe the reasoning on having a ungrounded generator bus, is that a ground fault will have no current to do damage to the equipment. That with iso-bus ensures that almost any fault will only be a ground fault. It is an equipment protection scheme. It is used mainly on generator buses, because a fault can not be cleared until the generator field dies, which can take enough time to do extensive damage.
This is not an IEC thing, we use it here also.

 

[7anoter4]

I have some questions still:
The measurements were done at Delta connection and here is no neutral.
2) In order to produce voltages at 20 kV part you have to energize the 11 kV star connection. In this case a no-load current will flow.
3) How the residual voltage was then measured?
If the residual voltage is measured between generator neutral and transformer Y [primary] neutral if the voltages at generator terminals are balanced then if the phase cables connecting the generator to transformer impedances are only 92% balanced, you'll get 350 V residual voltage.

 

[Kiribanda]

Hi Zanoter4,
I saw your sketch. May I explain something. The 20kV side is DELTA ungrounded. Therefore, to detect/ alarm/ trip
during a GF on this ungrounded system, three single phase VTs connected in BROKEN DELTA method are to be used.
But your sketch shows two PTs connected in series. I donot know what it is?
During a GF, zero seq voltage appears between the two terminals of the BROKEN DELTA connection for alarm/ tripping.

 

[7anoter4]

Thank you Kiribanda for your remark. However, there are not 2 PT series since 3 potentials [R,S,T] are involved as in broken Delta.

 

[7anoter4]

I am sorry! My confusion between V connection and broken delta.
We usually use a Zig-Zag transformer for ground fault detection.
Thank you Kiribanda again.