Transformers for grounded and ungrounded system 2

[arbil]

We need one generator transformer (11/7.2 KV ,Dyn11 ) .The star side will be connected to ungrounded system and hence neutral will be isolated.However the delivery of the transformer is bit delayed .We are having a spare transformer of same rating , the only difference being that the spare transformer was in use in a grounded system with its neutral grounded .
The standard says that the insulation level of the transformer depends on whether the neutral terminal is intended to be directly earthed or not.Therefore we are in a dilemma whether to use the spare transformer or not.
Please help !!

 

[Marmite]

A couple of options I can think of:-
1. Put a surge arrestor on the star point
2. Connect a VT to the star point and potentially use it for neutral displacement protection if you wish.
Without knowledge of the design of transformer its impossible to say with certainty that the suggestions would work. You need to know the specification the transformer was built to, and in particular whether the winding insulation was graded down towards the star point.
Regards
Marmite

 

[Masbibe]

Interesting question.

Marmite,
You cant connect VT between star point and ground because system is isolated.
Only thing that you can do is to use open delta transformer for detecting zero voltage.
Transformer will work in normal situation but it could be problems in case of ground fault.

Milovan Milosevic

 

[waross]

A solidly grounded wye transformer need only be insulated for 58% of line to line voltage. A transformer with a floating neutral or an HRG transformer must be insulated for full line to line voltage.

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

 

[arbil]

Thanks for the replies .

 

[Masbibe]

Also putting surge arrestor is not good idea.
When you have isolated star and when you have ground fault star point will go to potencial 7.2kV/sqrt(3). And probably you should choose surge arrestor to react on this or smaller value of voltage. Beacause if surge arrestor doesnt react on this voltage then it is like you dont have surge arrestor.
So if your surge arrestor react in case of ground fault your system will behave like grounded system (neutral point is grounded in that case) and you dont want that because you need isolated system.

Milovan Milosevic

 

[odlanor]

MilovanSRB
Grounded system refers to operation with rms current industrial frequency (50 or 60Hz).
Surge arrester operates for travelling waves voltages and transient discharge current.
They seal-in automatically , blocking any industrial current.

Arbil,
Surge arrester or VT could be a solution. Nevertheless , BIL of transformer shall be adequated for the
ungrounded system in case of fail of equipment (SA or VT).
Remember: Insurance transformer are applicable only to equipment that obeys standards.

 

[Masbibe]

Odlanor,
So what is the function of SA in this situation when it is connected to star point if it will not react on 50 or 60 hz overvoltage as you said ( I believe you on this).
To me there is no any function of SA connected like this.

Milovan Milosevic

 

[odlanor]

Surge arrester protect equipment against travelling waves voltage originated from lightning strokes. If winding insulation does not support this kind of voltage, will be a break of insulation and consequently a short circuit.Last one is the actual 60Hz frequency voltage that will dammage your equipment.
The function of SA is to guarantee no travelling voltage wave reflected at neutral due to high impedance(isolated system) They are refleted in 2X original value.
If your transformer was sized to work with neutral solid grounding it can not support to be neutral disconnected otherwise it will receive a 2X more overvoltage that it could support.

 

[Masbibe]

Odlanor,
Thanks for explanation. It deserve star from me.
But please answer me then one more thing.
If main problem with transformer on isolated network is reflecting of voltage waves then why standard dont say that isoloation of transformer for isolated network should stand double value of voltage in relation to transformer for grounded network if what warros said is correct:
" A solidly grounded wye transformer need only be insulated for 58% of line to line voltage. A transformer with a floating neutral or an HRG transformer must be insulated for full line to line voltage. "

Milovan Milosevic

 

[prc]

arbil,As I understand you are connecting delta 11 kV to generator and 7.2 kV to some local load.

Normally 7.2 kV windings are fully insulated and you can use your spare transformer with out any problem.Please check the neutral bushing and confirm that this is same as line bushing.Check test report to confirm that 7.2 Kv winding is tested as fully insulated.

Surge arrester works the same way on isolated or earthed circuit.It is connected between line and earth.In isolated neutral circuit,in case of a line fault arrester will see line to line voltage.Hence arrester for such applications are rated for line to line voltage.In earthed circuits for line to earth voltage with margins for system overvoltages

 

[odlanor]

MilovanSRB
I do notknow how explain to you. Perhaps Waross could justify why 58% because I do not remember.
To fill the complexity of Insulation Coordination ,the first thing you should understand is that:
There exists little doubt that in actual system , the standard lightning impulse waveshape ( is 1.2/50us ) never has appeared across a piece of insulation. For example, the actual voltage at a transformer has an oscillatory waveshape.
Therefore it is proper to ask why the 1.2/50us shape was selected. It is true that, in general, lightning surges do have short fronts and relative short tails, so that the times of standard waveshape reflect this observation.
But of importance in the standardization process is that all laboratories can with ease produce this waveshape.

 

[PHovnanian]

Perhaps Waross could justify why 58% because I do not remember.

58% of the system line to line operating voltage is the line to neutral voltage. Which only is useful if the Y is grounded at its neutral. Otherwise, any winding could see continuous operation at up to the system line to line voltage above ground.

A second issue has been raised, which is the BIL rating of the transformer. A transformer intended for grounded operation may have winding close to its neutral point that will not withstand the full impulse voltage (including the effects of wave reflection) that the HV end will see.

arbil, you need to get the design data from the manufacturer. Its possible that your spare may have been designed to operate either grounded or ungrounded. But just because there is a neutral bushing and grounding jumper available to unbolt doesn't mean that its OK to run it that way.

 

[waross]

Well said, PHovnanian.
1[÷]?3 = 0.57735026918962576450914878050196
Which rounds to 58%

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