Selection of NGT and NGR in Industrial Power System. 2

[prc]

I have some queries with regard to selection of NGR( Neutral Grounding Resistor) and NGT( Neutral grounding transformer) in ungrounded grids of modern industrial power systems.

1) This type of delta connected systems will be at 6.6,11,22,33 or 66 kV voltages and the neutral current of NGT (fault current) selected is 1320 (6.6 kV) to 750A (33&66 KV) J&P Transformer Hand book (ed13.0,2007) . At any voltage level, how this current is selected? As I understand,these MV systems are usually low resistance grounded. Currents of 300-1500 A are seen selected. Is it based on safety angle( arc flash over) or damage to core of rotating,machine?
In Beeman's Handbook on Industrial Power system (1955) it is recommended as 25 % of the main transformer full load current. What is the current practice?

2) Earlier manuals suggest a rating of 30 sec for NGT &NGR, bur recently the ratings are based on 10 sec ratings. Is it due to the developments in protection relays? The equivalent rating of NGT based from short time rating was used to be selected from the Westinghouse T& D handbook.( 1940 vintage) Any latest guidelines?(Apart from IEEE 32-1972)Any equivalent IEC to IEEE 32?


3) How to select the "R" of NGT &NGR? Let us take an example. In case of a 6.6 kV system, let us select a L-G fault current limit of 300 A. The main Transformer 20 MVA with 6.6 kV delta secondary with full load current of 1760 A and impedance of 12.5 %. ie X1=0.125 pu. To limit current to 300 A nearly 38 ohms per phase will be required in NGT alone is used. We have several options . Select a NGR of 9 Ohms & NGT of 9 Ohms so as to get a net R of 13 ohms(9+j9) I felt this will be best as even with a damage /flashover in NGR ,the over current will be limited in NGT.Or from cost angle, one should go for say 2 ohms for NGT &12 Ohms for NGR?

4) Beeman ( also IEEE) recommends X0/X1 less than 10 and R0/X0 more than 2 (R0=3 times R of NGR).What is the significance of these parameters?
Inviting guidance from experts.

 

[odlanor]

query 4-

Electrical Transmission and Distribution Reference book -1964
1- chapter 19 page 660 item 20.2
2- chapter 18 page 626 fig 29 a b c

 

[odlanor]

query 4-

Electrical Transmission and Distribution Reference book -1964
1- chapter 19 page 660 item 20.2
2- chapter 18 page 626 fig 29 a b c

I figured out zigzag transformer for grounding 69kV-delta of 230kV/69kV-Ynd1 transformers of subtransmission utilities.
with parameters Ro/X0> 3 and 3 <Xo/X1 <10.
zig-zag (no resistor) we adopted X0/X1=3 and lightning arresters we adopted X0/X1 = 10.
The aim was to ensure fault-to-ground was less than three-phase fault. (I guess was 20-30% less than the three-phase fault).
After years of evolution of the 69kV system, X0/X1> 10 then LA were damaged (good phases) due to ground fault on 69kV system.
So,we had to resize the zig-zag transformer.

 

[dpc]

I would suggest you get a copy of "Industrial Power System Grounding Design Handbook" by Dunki-Jacobs, et al.

The levels of neutral current you are proposing are quite high by today's standards, IMO. The main benefit of low-resistance grounding over high-resistance grounding at MV is the ability to selectively coordinate using standard overcurrent relaying. With new digital relays, the fault current can be substantially reduced while still allowing good coordination. The lower the current, the less damage to equipment. The fewer levels of overcurrent devices that you are trying to coordinate the less current is required. 100 A should be adequate in most cases. To further reduce damage, use high-resistance grounding.

A 10 sec rating is standard for low-resistance grounded system. Longer ratings are fine, but the resistor can get rather large. You also have to factor in continuous current due to third harmonic current. Unfortunately, the resistors rated for 10 seconds do not have a defined continuous rating in the standard, IIRC.

 

[prc]

Thank you all !

 

[prc]

Here is a good tutorial on Neutral Earthing:

http://www.nzccpts.co.nz/publications/Neutral Earthing Resistors or Reactors Application Guide.pdf