two MV system
two MV system
(OP)
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.
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.






RE: two MV system
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.
RE: two MV system
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.
RE: two MV system
I am not aware of any other method to protect delta connected transformers against line-to-ground faults,
any clue?
RE: two MV system
RE: two MV system
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.
RE: two MV system
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.
RE: two MV system
RE: two MV system
Thanks Davidbeach. Sorry, I am wrong. I should have written BROKEN DELTA.
RE: two MV system
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.
RE: two MV system
"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