We have three gensets operated in parallel; one of them 11 KV, has a neutral earthed through resistance. The other two are 400 v connected to delta transformer 0.4KV delta / 11KV star, this star is earthed direct. The 11 KV of those transformers and the first genset are connected to one bus bar, which is connected also to a zigzag transformer with neutral earthed. when the genset 11KV is the only one on the bus, without connecting the Z transformer and the 400 v genset, the neutral ct equal zero.
In case of connecting either the Z transformer or the 400 v genset to the bus with 11 KV genset the neutral CT equal 27AMP measured on 11kv genset, and 12 AMP measured on Z transformer neutral wire, and 15 AMP measured on 400 volt transformer neutral wire. All currents have 148 HZ. please advise is this problem is harmonics or earthing problem?

 

I have seen something similar on a power plant in Lake Lure, NC. They have a 2 generator hydro station of 1200kw and 2400kw respectively. To prevent a ground fault trip of the ground fault ct[s, the ground bus is opened on one or the other based on which unit is running. If only one unit is running it is the grounded unit. If both are running only the larger of the two is grounded. I did not design the system but I do know it works. However, it is the only system I have ever seen designed this way.

Hope this helps,

David

 

This is in response to Anonymous' Fri, Mar 5, 6:19pm query:

I investigated a problem similar to this about 2 years ago, except there were two MV gensets, and one LV genset. In order to provide you with meaningful answers, please comment on my interpretation of the information you provided:

1) The 11kV generator (I'll call it G1) is connected to an 11kV bus.

2) G1 is wye-connected and its neutral point is connected to earth via a resistor.

3) An 11kV to 0.4kV transformer (I'll call it T1) is connected between the 11kV bus and a 0.4kV bus.

4) T1's 11kV winding is wye-connected and its 0.4kV winding is delta-connected.

5) T1's neutral point is solidly connected to earth.

6) A zig-zag "earthing" transformer (I'll call it Tz) is connected to the 11kV bus.

7) Tz's neutral point is solidly connected to earth!

8) Two 0.4kV generators (I'll call them G2 and G3) are connected to the 0.4kV bus.

9) G2 and G3 windings are wye-connected.

10) G2 and G3 have their neutral points solidly connected to earth.

Please answer the following:

Q1) Is this a new system being commissined? Or, was G1 added to an existing 0.4kV system? Or, were G2 and G3 added to an existing 11kV system?

Q2) Is there load connected to the 11kV bus? The 0.4kV bus? Or both?

Q3) Is there just one earth system? Or, are separate earths provided... one for the 11kV system and one for the 0.4kV system?

Q4) What test instrumentation is available to you?

Regards,
Phil Corso, PE {Boca Raton, FL, USA}
[[email protected]] ([email protected]) {[email protected]}

 

Any generator creates 3rd harmonics which through the neutral. In fact the absence of these indicates a genset fault, however, you have to have a return path ( in your case the zig zag earths on the 11/400V Txs.
When you connect the genset to the Z transformer then the 3rd harmonics circulate around the transformer. In general your system should have only one earth point. As a zig zag tx is installed then this is the designers ground system and I would have expected the neutrals on the 400V/11kV TXs to be normally lifted. Often zig zag neutrals have resitors to limit the earth fault as well. The reason for the neutral resistor in the 11kV genset is to limit the ground fault current