two MV system

[ciy]

Dear All,

Please find the attached, there are two (2) one line dia. Case A and Case B separately.

Case A, this is a scheme from someone project that I study recently. The secondary side of transformer is ungrounded of Wye
connection and equipped with Lightning arrester (LA) in the system.
I wonder this system how to finish the ground fault protection? It shall have a higher voltage when SLG fault happen.
LA can low down the voltage, but it is still a ungrounding system.

Case B, as show this method is I am used to adopt in MV system for motors in general cases. I always install ZCT to protect
from ground fault.

As above, please advise what are the pros and cons between Case A and Case B.

Thanks.

 

[Kiribanda]

ciy,
CASE A: Operating and ungrounded (isolated) 6.6kV network is not recommended due to inherent zero seq capacitance. The transient overvoltages that will
occur during a ground fault are excessive and therefore might damage the insulation. Specially the 6.6kV motors. All VTs, surge arresters installed on the
ungrounded 6.6kV network have to be rated for full line to line voltage. In addition you should have a good insulation monitoring system on the 6.6kV network.

CASE B: Operating a solidly grounded 6.6kV network is also not recommended due to high ground fault currents. It will be very difficult to have a coordinated
ground fault protection scheme for a solidly grounded network. Therefore, it is always preferred and very typical to have either a low resistance (max allowable 400A)
or a high resistance grounded system. The main criterion for having a HR grounded network is the requirement for continuation of power supply during a ground fault.
Out of the two my preference is always a low resistance grounded network.

 

[ciy]

Kiribanda,

Thanks for your great reply.

In case of CASE A already existing, how to perform ground fault protection?

All we can do just only phase fault protection, that is true?

Please more advise, thanks.

 

[Power0020]

for Case A, to protect against ground faults, an earthing transformer (Zig-zag) should be provided.

I am not aware of any other method to protect delta connected transformers against line-to-ground faults,

any clue?

 

[davidbeach]

It is quite straight forward to detect ground faults on an "ungrounded" (aka capacitively grounded) system by looking at the 3V0 value. Today 3V0 is calculated from the phase voltages by the relay, in the past it would have come from a broken delta VT secondary. Tripping for a ground fault is then easy enough.

 

[Kiribanda]

ciy,
There are two methods as mentioned by Power0020 and Davidbeach.
1) Use a zig zag grounding transformer to derive an artificial ground on 6.6kV network. You have to perform system
studies in order to size the zig-zag transformer which depends on zero seq capacitance on the 6.6kV network.
2) Use measuring 3V0 across an open delta VT connection. Connect three single terminal VTs (voltage factor =1.9xUN for 8Hours)
as GROUNDED WYE connection for the primary and open DELTA (not broken delta) for the secondary and then connect a voltage relay
(BASLER/BE1-59N relay) or any other multifunction relay having ANSI#59N function. You have to calculate the maximum zero seq
voltage at full neutral offset condition and then set alarm & trip settings accordingly.

 

[Power0020]

I suggest the zero sequence detection method as you can utilize the existing bus VTs (I assume there are some!) instead of getting a heavy zig-zag transformer with MV connections.

I doubt there will be some L-G arcing faults on 6.6 kV side? in case of a zig-zag, the capacitance will be discharged continuously to ground. Check whether the installation is in a fire hazard application or not.

 

[davidbeach]

Actually it is broken delta. Open delta is a connection of two of the three transformers that would be used to form a closed delta. Broken delta is three windings with one corner broken. The two open delta transformers are connected phase-phase on both sides, while a broken delta has a grounded wye on the system side and the delta on the instrument side. Typically there will be a loading (ferroresonance suppression) resistor in the broken corner.

 

[Kiribanda]

ciy,
Thanks Davidbeach. Sorry, I am wrong. I should have written BROKEN DELTA.

 

[ciy]

Dear All,

Thanks for your nice advice.

In my understanding, whether grounding system or ungrounding system, a good power system shall take the grounding protection into account.

There is a V-V connection PT in the said CASE A scheme, but without any Grounding Potential Transformer (GPT).

Obviously, CASE A cannot cover the grounding protection. That is why I think CASE A is not a good power system plan.

 

[VTer]

CIY, Just curious but why is the neutral on the wye transformer floating in case A?

"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic — and this we know it is, for certain — then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". – Nikola Tesla