Ferroresonance

[kitha]

Hi
Does anybody know where theres a good reference or a document that suggests limitng length of cables terminating to a 1500KVA 27.6/600 delta-wye transformer? In my application total length of cables is approximately 438m (375m underground and the rest is above ground on a roof) before it terminates to a 1500KVA transformer. In order to convince the owner to extend the aerial network closer to the transformer, need a credible document.

 

[ScottyUK]

Wow. I would have thought the cost of the LV cable would be sufficient to convince most people. You would need to consider what protection you have. Some sort of unit scheme covering the transformer and LV cables would be worthwhile. I don't know of any standard precluding the arrangement you describe though.

 

[kitha]

ScottyUK- it's not LV cables. Its 27.6K primary cable that's 438m underground run. They do not want to build up overhead in that area as is a flight manufacturing plant which requires flights to move around the area.

 

[PHovnanian]

If the customer places a high value on keeping overhead clear, perhaps a three phase switching scheme could be incorporated to prevent the single phasing that would result in ferroresonance.

A three phase pole mounted vacuum switch and set of PTs just beyond the fuses for the underground run could be set up with loss of phase protection.

 

[Mbrooke]

Does your transformer ever experience loads below 20% of the KVA rating or feed a building with a large back up generator? If not ferroresonance may not be a concern to begin with.



Would it be possible to mount a 34.5kv recloser with a 3 phase transformer that will open the cable with a loss of a phase at the point where the line goes underground? It may not be the best solution but its the only solution I can think of at the moment.

 

[kitha]

Mbrooke- Transfomer is 1500KVA and when the production is halted after midnight work shift and the offices are closed, only available load is the plant lights which is approximately 120KVA. So it means its around 8% and with some minor building ventillation fans coming on/off it may be around 10%. The standby genset is only taking care of emergency life safety loads and it is around 225kVA.

What I am wondering is even if Ferroresonace phenominon is a singnificant issue in the power system distribution/protection, we are not able to find a published document by any of the utilities that outliens sugested cable lenghts for various voltage and transformer sizes?

 

[VTer]

Use a 3-phase gang-operated switching/protection to avoid ferroresonance. There are no simple guidelines for this besides some some typical scenarios and likelihoods where it may occur. Some utilities have even adopted standards that smaller size transformers (smaller than 3MVA in my area) that are connected to their system must be wye-wye configuration for this very reason.

"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

 

[daviandgo]

This organization published a study on ferroresonance and cable lengths.

http://www.dstar.org/research/project-desc/ferroresonance/.

 

[magoo2]

daviandgo
The DSTAR results look about right for 7.2 kV phase to ground. Couldn't resist!

With a delta high side connection and 27.6 kV, it is likely that you've gone beyond the limits of cable lengths to avoid ferroresonance. Without looking at the DSTAR results, I think they were all based on grounded wye grounded wye transformers since our company and two others participated in the field tests.

The 8% minimum load will likely reduce the voltage magnitudes when single phase switching occurs but going with a gang operated three phase switch would be a wiser idea.

 

[prc]

This paper gives the information required- R H Hopkinson " Ferroresonant overvoltgaes due to open conductors"Distribution ,Fourth quarter,1967.
For a 1000 KVA delta-wye transformer critical length of conductor at 13.8 kV is 69 ft cable and at 34.5 kV , 11 ft cable.But this may not be an issue, if only three phase switching is used and switches are located at the transformer terminals.