Interesting IT System Protection and 59N 1

[Gonenc]

Hi Everyone,

I have isolated system which has 154kv/10.5kv YN/D transformer. I have to do protection for delta side of this system. In case of earth fault, I know that I can not protect my system with current. I should use 59N protection. In case of earth fault in any bus in our system, unfaulted phases voltage will be increase for all buses. But in this case how can I localize the faulted part of system.

Best Regards,

 

[sushilksk]

By using zigzag transformer, current based protection can detect faulted part.

 

[rcw retired EE]

Add a zig-zag transformer with a neutral grounding resistor sized to limit fault current to 5-10 amps, making this a high resistance grounded system. The first ground fault will only alarm, not trip. Use a standard pulsing current fault location system to locate & correct the fault while the rest of the system remains in service. .

If this is a utility distribution system that can't handle the extended voltage stress on the unfaulted phases, use a resistor with a higher current rating and standard ground fault current relaying to clear the fault.

 

[Gonenc]

Hi Guys,

Thank you very much for your reply.

Actually, at the beginning of project this site has 20MVA generator (Yn) and 20MVA (YND) step up transformer which is produce electricity for the grid. Then customer informed us, he will shutdown his generator and he will use this transformer just for pump the hot water for the another power plant which is located 1 km away from existing power plant.That is why, without his knowledge their system become IT system.

But pump motors are 2 km far away from 20MVA transformer. That is why if I put earthing transformer with hing impedance which is limit 5-10 Amp, I will not able to sense ground fault again. I am planing to limit short circuit current around 1ka-2ka, after discussion with out transformer factory. Do you have any opinion about impedance of earthing transformer ? or is there any missing part that I do not understand ?

Thanks for you patience and support.
Best Regards,

 

[sushilksk]

I have seen Zigzag transformer with ~ 250A (for 10 sec) setting at 33kV level. For higher currents transformer size and cost will be higher.

 

[prc]

One Indian utility is buying NGT of 1000-2000 A for 33kV system.For your special case please refer to Donald Beeman’s Industrial power system handbook page 386-387 for selecting the rating of NGT.

 

[waross]

Three 37.5 KVA or 50 KVA "Off the shelf" 10.5 kV distribution transformers will make a good grounding network.
These may be cheaper than zig-zag and should not be a special order.
If you go for a 10 second rating you will have to disconnect quickly and will lose the delta advantage of remaining in service with one phase grounded.
On the loss of one supply phase, the delta will back-feed any loads on the missing phase.
Some systems it won't matter.
On some systems you may want to disconnect the wye/delta bank on the loss of a supply phase.
It depends.
It really depends.

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

 

[Gonenc]

Hi;

Thanks again for your reply.

Please also let you explain about below Etap screenshot. PS: I am trying to limit grounding current around 1kA and I will use zigzag transformer. Should I add also, neutral resistor for this application ? If it is possible could you explain me how can I calculate those vaules ?

Continuous Amp= What is it ? As much as I understand this value should determine according to unbalance condition..
Fault Amp = This value represent the current which is passing through artificial neutral point or one phase winding of zigzag transformer during fault condition ?
Ohm= I think, this one is reactance of per phase. As much as I have searched from google, reactance of grounding transformer is really high if I compare with normal power or dist. transformer reactance. I am not expert but as I know reactance of those transformer around mohm. So, if I mention transformer manufacturer, can they adjust the reactance of winding, what I want to limit faulty current ??
X/R = How can I determine it or tell the transformer manufacturer ?

May be those question a bit silly but I would like to learn it..

 

[argotier]

Gonenc,

First of all check out the book prc has recommended you, it will guide you to select the limit ground current in order to minimize transient overvoltages in case of faults to ground. In the book there are pretty straightforward formulas to obtain this rating.

But suppose the 1 kA it's the right choice, let's move on to your questions:

Should I add also, neutral resistor for this application ?

The choice also depends on the transient overvoltages and the maximum ground reactance you can put in your system (again refer to the recommended book).
From my experience, 1 kA may be unusual for a grounding resistor (are most common for < 500 A where I live).

Continuous Amp= What is it ? As much as I understand this value should determine according to unbalance condition

It does account for unbalance conditions. Typical value: 10% of the transient current (but this is conservative, standards may recommend lower values).

Fault Amp = This value represent the current which is passing through artificial neutral point or one phase winding of zigzag transformer during fault condition ?

Typically the rating is for the neutral current but here it depends on ETAP (I don't use it, so I can't help with this).

Ohm= I think, this one is reactance of per phase. As much as I have searched from google, reactance of grounding transformer is really high if I compare with normal power or dist. transformer reactance. I am not expert but as I know reactance of those transformer around mohm. So, if I mention transformer manufacturer, can they adjust the reactance of winding, what I want to limit faulty current ??
X/R = How can I determine it or tell the transformer manufacturer ?

Once you specified the ground current limit, the impedance of the earthing transformer it's already defined (if there is no additional resistance, at least):

z0 = ( 1.73 x VL-L) / I ground-fault = 57.16 ohm (for the 1kA current, if I did it right). And yes, is a per phase impedance.

Hope it helps.

 

[Gonenc]

Dear Argotier,

Thanks for your reply. I am appreciated.