Zigzag earthig transformer+ NGR on the secondary side of a 132/33 kV Yd transformer.

[Robert2025]

I am currently working on a 33 kV distribution system, and i must design a Zigzag earthig transformer+ NGR on the secondary side of a 132/33 kV Yd transformer. I need a sample calculation file. If you have any sample files available, I would greatly appreciate it if you could share one with me.

 

[waross]

To what current do wish to limit ground faults?
Use that to calculate your grounding impedance.

 

[Robert2025]

@waross: Thank you for your quick feedback.
Customer has chosen a NGR with 220ohm 105A-10s, To what current should i limit the ground fault?

 

[Kiribanda]

1) How much earth fault current that you are going to limit on the 33kV system?
2) Can't you use only a NGR to achieve that current without a ZIGZAG?
3) What is your main reason for specifying a ZIGZAG?
4)What is your SLD of your 33kV system?

 

[Robert2025]

@Kiribanda : Thanks for the inputs.
We need LV supply and therefore we would like to have a mix of earning and auxiliary transformer at least 50 KVA 33/0,4 ZNyn.
From an old zigzag earthing transformer, i found ghe short circuit impedance Zk% is around 5% and zero sequence is 110/phase.
Without NGR and direct grounding of Zigzag Neutral at 33kv, do I need a NGR? I don’t know even how much must single phase must be limited.
IF(L-G) without NGR=3*33/sqr(3)/(2Zk+ Zo) is 35A.

 

[Masius]

Hi Robert,
Is your system like this ?

 

[Robert2025]

@Masius : It is not exactly what I wanted to post here. Thank you!
At delta side I have a 33 kv switchgear and one of the outgoing feeder is connected to a Zigzag - Aux transformer.
The Zigzag-Aux transformer in my project is 50kVA. How much have you limited L-G fault by use of Zigzag Tr and NGR?

 

[waross]

Your first mistake is a delta secondary in a distribution network.
That configuration was common in the first part of the last century. By the 1950s many delta distribution systems had been converted to wye secondary systems
Eg: 19,000 delta to 33kV/19kV wye.
A common voltage and conversion was from 2400 Volts to 4160/2400 Volts.
The 220 Ohm resistor will limit the ground current to 19,000 Volts / 220 Ohms = 86 Amps
If you must connect 19,000 volt loads on the 33,000 Volt delta, the best solution is to install an artificial neutral locally to derive 19,000 Volts..
Consider this a source of 19,000 Volts AND NOT A NEUTRAL.
The connection to the artificial neutral may rise to line to ground voltage (19,000 Volts) in the event of a ground fault on the 33,000 Volt distribution system.
Grounding the artificial neutral will interfere with the operation of the NGR.
Both conductors of the 19,000 Volts derived with the artificial neutral must be considered "HOT" conductors and must be insulated for at least 19,000 Volts. (Good practice is to use 33,00 Volt rated conductors)
Disconnects must open both conductors,
Even though the derived 19,000 Volts will be near zero Volts to ground under normal conditions, you must consider that ground faults on a distribution system must be considered as WHEN a fault will happen, not IF a fault will happen.

 

[Robert2025]

SLD attached. @Masius , @waross , @Kiribanda please comment.

 

[Robert2025]

Your first mistake is a delta secondary in a distribution network.
That configuration was common in the first part of the last century. By the 1950s many delta distribution systems had been converted to wye secondary systems
Eg: 19,000 delta to 33kV/19kV wye.
A common voltage and conversion was from 2400 Volts to 4160/2400 Volts.
The 220 Ohm resistor will limit the ground current to 19,000 Volts / 220 Ohms = 86 Amps
If you must connect 19,000 volt loads on the 33,000 Volt delta, the best solution is to install an artificial neutral locally to derive 19,000 Volts..
Consider this a source of 19,000 Volts AND NOT A NEUTRAL.
The connection to the artificial neutral may rise to line to ground voltage (19,000 Volts) in the event of a ground fault on the 33,000 Volt distribution system.
Grounding the artificial neutral will interfere with the operation of the NGR.
Both conductors of the 19,000 Volts derived with the artificial neutral must be considered "HOT" conductors and must be insulated for at least 19,000 Volts. (Good practice is to use 33,00 Volt rated conductors)
Disconnects must open both conductors,
Even though the derived 19,000 Volts will be near zero Volts to ground under normal conditions, you must consider that ground faults on a distribution system must be considered as WHEN a fault will happen, not IF a fault will happen.

Could you please have a look into SLD I sent.