Hi folks, I must calculate the cur

[Alex68]

Hi folks,
I must calculate the current flowing through the earthing disconnector of the transformer HV side. This is the discharging current of the capacitances of the tansformer.

Given data:
1- capacitance between HV winding and LV winding
2- capacitance between HV winding and ground
3- capacitance between LV winding and ground

It is a YNd11 transformer.

Should I consider only the HV circuit or also the low voltage one?
Does the current flow also through the HV winding towards the grounded neutral?
Could you suggest a paper describing such a calculation?

Thanks

 

Please describe your connections properly. Capacitors are rarely connected in star mode, from whose node (neutral) you are trying to measure your ground path current. Any minor current imbalance in the three phase circuit will load it, if a path to the ground exists through it's main neutral node. If you have any capacitors on the line side (HV), they better be in a delta mode, other wise in a worse case scenario, they should remain in an ungrounded star-mode. There remains a high risk of damage by over heating, in case of any electrical imbalace, transients and in the presence of any harmonics etc., in capacitor's dielectric.
Rakesh Kapila

 

[Tuas]

I assume you want to know the leakage current when the transformer is energised, you should add the two charging currents separately, HV and LV charging current and ignore the HV/LV capacitance since it's not to ground. But be careful not to take the capacitance as they are, if the capacitance values are taken from factory tests.

 

[Alex68]

I'm sorry for my bad explanation.

We can imagine that there are capacitances among energised transformer windings. I don't mean that I put a capacitor bank in parallel with the transformer windings, but it is a natural phenomena that happens everytime we have two energised conductors, for example: between a cable core and its armature "there is" a capacitor, or better these two enegised conductors form a capacitor.

The problem is:
I have a 370MVA transformer, which is energised. Perhasps it is also working at full load.
For an unknwon reason I must open the HV c.b.
So the transformer stops working but its windings remain energised, that is the "capacitors" are charged.
For safety reason, I must always close the HV earthing switch. In this way, the "capacitors" of the transformer (but also of the bus duct connecting to the c.b.) discharge.

Well... The question is:
What is this discharging current?
I must verify that the earthing switch we bought is correctly sized or we need a fast closing earthing switch which is able to withstand bigger currents.

Now I'm trying to find the equivalent circuit and so:
1-Should I consider only the HV circuit or also the low voltage one?
2- Does the current flow also through the HV winding towards the grounded neutral?
3- Could you suggest a paper or standard describing such a calculation?

 

[stevenal]

So this switch will never carry a steady state current, but only be used to drain a trapped charge, and provide a safety ground? Then load ratings are meaningless, use the fault ratings. Match the switch fault duty to that of your system, and you should be in good shape.

 

[Alex68]

Thank you but...
this is the simplest but not the cheapest solution!!!
The difference between the costs of the two devices is very high and we have already bought the slowest (and cheapest) one.
A fast earthing switch is able to withstand a 3ph short circuit without damage.
A slow earthing switch can simply connect to ground the transformer and carry a max current of 0.5A.

I must demonstrate if the slow swtich is sufficient or not.
I have to construct the equivalent circuit and to calculate the currents.

 

[zub]

you must consider the HV capacitance only. The HV-LV capacitance will still remain and it becomes a LV to ground capacity parallel to LV capacity when you close the earthing disconnector on HV. And it can be neglectible.

Hope it helps.

zub.

 

[Alex68]

Thanks
I'm going on doing some simulations with PSPICE.
I think to consider the series of The HV-LV capacitance and LV-GND capacitance in parallel with the HV-GND capacitance. Moreover This capacitive circuit is in parallel with the series of the resistance and leakage inductance of the HV winding.
I think that this circuit should be enough to simulate the transformer.

 

[stevenal]

This switch simply must be able to carry system fault current until protection operates. Anything less would be illegal by any code I'm aware of, and violate any engineering code of ethics. If can't aford the project, don't do it.

 

[each]

I agree with stevenal, this switch has to be appropriate for fault current. In regular case it will not take much current.You can connect a CT having less ratio 1/1 A or 0.5/1 A after swiching of Transformer suplly and chrge it again, measure current flowing through Neutral, now you can yourself decide the rating of switch for regular case i.e. for energised condition of transformer. One more thing you can see is the no load current of all three phase of transformer at rated voltage, it is the phase current while it is just charged but not loaded.

 

[Alex68]

Thank Stevenal and Each
but the standard EN60517 "Gas-insulated metal-enclosed switchgear for rated voltages of 72,5 kV and above" at point 5.3.101 reports:

"Earthing may be made by:
a) earthing switches with a making capacity equal to the rated peak withstand current, if there is no certainty that the circuit connected is not live;....."

We have the certainty that our GIS is not live, because we are talking about our GIS whose c.b. and switches are interlocked.
===================
Zub
Thank you but I don't understand your equivalent circuit.
the measured values of capacitances are:
HV-LV capacitance: 7180pF
LV-GND capacitance: 24950pF
HV-GND capacitance: 6550pF

 

[zub]

excuse my english first,
you have CHL=7180 pF, CL=24950 pF, CH=6550 pF.

The equivalent circuit after closing the HV Earthing disconnector is;

An inductance (Transformer HV winding) is earthed from the both ends. (Charged the amount of CH when it was energised) And the discharge current will flow from the both ends (because of earhed Wye). And the CHL will become in parallel to CL because you earthed the HV winding. And it is not affect the HV discharge current at all.

But you should consider the closing time of the disconnector. Because it is important for sizing.

i hope i could describe myself.

zub.

 

[Alex68]

Thank you Zub
We arrived at the same equivalent circuit.

Now I trust to have the right solution

 

[stevenal]

Alex,

EN and GIS standards are not my bag, but this application sure bothers me. You say the interlock provides certainty, but the interlocks I've seen can be defeated easily. Do you realy have certainty? Do the interlocks cover all possible sources and not just the normal ones?

 

[Alex68]

Hi Stevenal,
thank you for your interest.
The answer to your questions is: "Yes, we do!"

We are talking about the main transformer of a power plant and it is connected to the GIS on the HV side and to the generator CB on the other side.
The HV earthing switch is interlocked by the position of the GIS cb and its disconnector, and of the generator cb and its disconnector.
Moreover each cb is protected by a breaker failure protection.
this should be sufficient.

 

[stevenal]

From ANSI C37.38:

Inadvertent
closure into an energized bus shall not result in any danger to personnel, but will necessitate immediate examination
of the switch and probably major overhaul before the equipment can be safely placed back into service.

 

[Alex68]

Thanks for the suggestion
I will verify