Paralleling ZigZag Earthing Transformers

[MattMag]

I am looking to determine if there are any potential problems with operating two Zn Earthing transformers in Parallel ?

They are connected to an 11kV system that is energised from 3 ungrounded 11kV generators. The earthing transformers are to the same design, but have slightly different zero sequence impedances (3% different). Operating them in parallel reduces the risk of site black out if one transformer develops a fault (gens are tripped if Eathing Tx tripped) and simplifies interlocking.

The system is fault rated to withstand the additional earth fault current, and the protection grades still. I don't know whether having two Earthing Tx's in parallel will cause any 'circulating current' third harmonic problems.

Does anyone know of any potential problems with doing this ?

Thanks

 

[dpc]

Are the transformers solidy grounded?

There might be some circulating third harmonic current, but it shouldn't be terribly high.

Off the top of my head, I don't see a big problem.

I'm not sure I see the necessity of tripping the generators if you lose the grounding transformers. The generators would be OK ungrounded for a short time period while you switch to the backup grounding transformer.

But it's early Monday morning for me, so I could be overlooking any number of things.

 

[busbar]

Given that one has 97% of the impedance of the other, that’s probably within usual manufacturing tolerances—even if they were built side-by-side, and should be acceptable. [From the description, the “3%” variation in impedance is not, for example, one rated 2% and one rated 5%, right?]

If the zigzag transformers are independently switchable, it would be well worth double-checking neutral currents for each energized independently, and then in the neutrals of each with both on line.

 

[MattMag]

Yes - the Transformers are solidly earthed, and the 3% difference was between the impedances. I'll give some consideration to not tripping immediately - the VT's should give the system a coupling to earth, though without earthing no earth faults would be detected, and if one were to happen (double contingency I know !) the cable insulation would be stressed as some cables do not have phase to phase voltage rating for the phase to earth insulation.

Thanks for the replies.

 

[dpc]

But you're talking about a short period of time when the generators would be ungrounded.

You could add ground fault detection by means of a broken delta PT connection and a voltage relay to detect zero sequence voltage.

 

[cuky2000]

Paralleling two ZN transformers should not be a problem assuming the connections met the following conditions:

o The transformers should have approximately the same relative short-circuit impedance with maximum permissible discrepancy + /- 10%. For 3% differences, the transformers should be capable to handle around 98% of the continuous rated capacity without overload.
o The voltage ratio between the primary and secondary sides must are assumed to be as similar.
o The transformers connection is identical in the LV & HV side.
o Rated output taps ratio if any are assumed to be smaller than 3:1 for each unit.

 

[busbar]

 
With the two banks on line, a check of neutral currents would verify that there is no significant circulating-current problems.