72 kV Station Service VT Connection Concerns

[ThePunisher]

The design proposed to use 3 x ABB SSVT-350 (IPC) Voltage Station Transformers. A wye-neutral solidly grounded secondary winding is made by interconnecting the X2 terminals to ground and using the X1 terminals for the line conductors (see attached image). Each primary H1 terminals are connected to the 72 KV phases and H0 are connected to earth via ground rods.

My concerns are as follows:

1. There is no primary fuse protection. At 318 kVA capacity, the equivalent primary fault for a 3 phase or line-line fault at the secondary will be too small to be sensed by the power line remote overcurrent protection. Therefore I would insist for a primary fuse protection. Is my concern valid?

2. Upstream 72kV is a resistance grounded system. Since the SSVT primary H0 is solidly grounded, will this pose any risk to the upstream resistance grounded system? With the connection shown in the attachment, a ground fault at the secondary will be reflected at the primary. Will it be possible to re-connect the primary as a DELTA instead?

I was informed that there are already old installations in the facility that have the same configuration but I am still not that convinced they did the right thing.

Can anyone help me get the guidance I need?

I attached the SSVT Installation and Maintenance Guide on the second post

Thank you in Advance,

 

[ThePunisher]

Installation and Maintenance Manual SSVT (IPC)

 

[RRaghunath]

"Therefore I would insist for a primary fuse protection. Is my concern valid?"
It is a VT by design and thus it is standard not to provide fuses on primary. On the other hand your concern is valid and I suppose the VT tank mounted secondary terminal box has fuses and should take care of any fault in the secondary system. Faults internal to VT will have to be cleared by the remote line protection.
"Since the SSVT primary H0 is solidly grounded, will this pose any risk to the upstream resistance grounded system?"
No. VT is not a source. So, solid grounding cannot increase the overall ground fault current magnitudes in 72kV system. None of the secondary windings will be connected in Delta anyway.
"Will it be possible to re-connect the primary as a DELTA instead?"
No. If you want that, you need to order a conventional transformer. VT neutral side bushing is designed for solid grounding. For connecting the windings in Delta, the neutral side bushing has to be of same rating as the Line side bushing.
Just make sure the secondary terminal box has fuses (and a disconnecting switch additionally if you like for maintenance isolation). Otherwise, the arrangement is standard and is fine.

 

[Kiribanda]

ThePunisher,
My main question to you is how one can connect any single phase device(s)
on a high resistance grounded system (HRG)?
Yes, you can connect a DELTA connected transformer to the HRG since a DELTA connected
transformer is fed by line-line. But you cannot use this given transformer for DELTA connection
because per the nameplate each primary winding is rated for 72kV/1.732=41569kV only.
My other question is are you 100% sure that the 72kV network is HRG because above 35kV, HRG systems
are not practical due to capacitance.

 

[ThePunisher]

Kiribanda, its not HRG, its a resistance grounded system with an NGR rating of 200A

 

[Kiribanda]

If 72kV system is grounded with a 200A NGR it means that 72kV system is effectively grounded.
That means you can solidly ground all H0 terminals as shown in the dwg. If you really need you can
put a fuse to the primary side. But a 100kVA/ 41569V rated transformer needs at least
3A/ 42kV rated dropout fuse on the primary.Is it available? Therefore, if you could put a
correctly sized fuse or a circuit breaker to the 600V secondary side, it provides O/L protection
for the primary too of 100kVA SST.

 

[jghrist]

I question whether a system with a 200A NGR can be considered effectively grounded. A ground fault in the secondary will probably result in excessive H1-H0 voltages in the primary on the unfaulted phases. There is a MMOV rating of 1.73 pu for 60 sec. Ensure that any ground fault will clear in less than 60 sec. Also make sure that you don't have any relay protection on the 72 kV that would trip on overvoltage.

 

[Kiribanda]

"I question whether a system with a 200A NGR can be considered effectively grounded"
Thanks jghrist for raising the question.
Assuming approximately 5A/km of charging curent for a 72kV cable, 200A rating is on the high side
because at least 13km length of 72kV cables are required to decide a 200A NGR (200/3=67A).
Therefore, I donot think any utlity will allow to maintain their 72kV system non-effectively grounded.
The only way that we can confirm this is to get calculated COG from ThePunisher.