Zigzag Grounding Transformer-ETAP

[Gonenc]

Hi;

I am trying to enter related information to zigzag grounding transformer on ETAP. Our transformer information are below. BUT, when I enter 18,2 ohm/phase to Impedance OHM subtitles, phase to ground current is limit to much. As much as I read, zero sequence of zigzag transformer should be very low. Please see below screenshot and please tell me what should I enter these gaps. OR these required ohm and X/R information base on zero sequence ?

System Voltage (L-L)=10,5Kv
Irated/phase=100A
Short Circuit current in neutral=1000A, 10sec
Ohm/per phase = 18,2 ohm/phase

 

[7anoter4]

The zig_zag transformer impedance has to be negligible. So it is 0.01818 ohm[and not 18.2 ohm].It could be 18.18 milliohm=18.18/1000=0.01818 ohm.

 

[tofulover]

Could I please ask if it is acceptable for the zig-zag TX to be solidly grounded? I know we typically connect a NER to the neutral but would it be just as acceptable to be solidly grounded too?

 

[Gonenc]

Hi Dears,

I supposed that at first post above, I took screenshot from Etap page. I do not how to calculate ''Impedance OHM'' blanks. I supposed it should be zero sequence impedance. Because the only current is going to pass from grounding transformer is zero sequence current. Therefore I turned, how to calculate zero sequence impedance.. The results of these calculation are too different.


1- Actually I am in IEC part of world, but I found IEEE C62.92.3-2014 standard. According this standards I calculated my grounding transformer zero sequence.

IEEE C62.92.3-2014;
20MVA YnD 154kV/10,5kV Transformer
%Uk= 9,938
Isc= (20000kva/(1,73*10,5kv))/0,09938=11066A
Ssc= 11,066ka*10,5kv*1,73=201,25MVA

Use 100MVA Base;
Rated Voltage= 10,5kv
Base Ohms= (1000*10,5kv^2)/(100000) (1000)*(kv^2)/kVA
= 1,1025
Base Amps= (100000)/(10,5*1,73) kVA/(kV*1,73)
=5498,57 A
Z(1)=Z(2)= 100/201,25= 0,4969 pu

Ig = 1000/5498,57 = 0,1818 pu
Io= 0,1818/3 = 0,0606 pu
Ztotal = 1.0/0,0606= 16,495 pu Ohm
Ztotal – 2Z(1)= 16,495- (2*0,4969) = 15,5 pu ohm
In ohms Z(0) =15,5* 1,1025=17,09 ohm/phase

2- But also I found one research it mentioned directly how to calculate zero sequence of zigzag grounding transformer. You can copy paste this header to google you can find it --> ZIG-ZAG ''GROUNDING TRANSFORMER MODELING FOR ZERO-SEQUENCE IMPEDANCE CALCULATION USING FINITE ELEMENT METHOD'' (page10). The author calculated zero sequence like below.

According to it my transformer zero sequence calculation will be like below,

Z0= (1,732*10,5kv)/1000A = 0,01818 ohm/phase.

 

[7anoter4]

In the second formula you have to multiply by 1000 in order to get volts instead of kV.
Z0= (1,732*10,5kv*1000)/1000A = 18.18 ohm/phase.
The second system is neglecting transformer Z1 and Z2 [≈1% of Zo in this case].

 

[7anoter4]

By the way, it is solidly grounded system [Rngr=0]

 

[cuky2000]

Here is a recommended procedure to model a Zig-Zag Grounding Transformer in ETAP
.

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Link

I hope this help

 

[tofulover]

Hi 7anoter4, I am interested to know when should we be using the zigzag solidly grounded and when we should be using this with an NER?

 

[7anoter4]

If the system can work indefinitely with a grounded phase then we need a high resistance resistor. If the system can work a limited time [usually an hour] with a grounded phase then we need also a high resistance resistor.
If we intend to disconnect the grounded phase immediately you may use a solidly grounded Zigzag transformer or limited current using a low resistance Rngr[it depends on the protection discrimination also] . However Ig>3*Ico.
See: IEEE Std 242-2001, IEEE Recommended Practice for Protection and Coordination of Industrial and Commercial Power Systems. Chapter 8 Ground-fault protection