Ferroresonance in distribution systems

[veman]

We have a 1200' run of 13.8 kV underground cable feeding a 3 phase padmounted 500 kVA Yg-Yg transformer from 3 single phase fuses. I have found some new information on ferroresonance that suggests that there still may be ferroresonace occuring in the YY transformer because of the newer low loss 5 legged core designs of recent years.

Is anyone aware of ferroresonance avoidance steps or guidelines that I might use to determine if there is an issue in the above or other cases??

Thank you,
Veman....

 

[jghrist]

See

http://www.ece.mtu.edu/faculty/bamork/FR_WG/Panel/Paper 03GM0829 - Walling-rev1.pdf

Also IEEE C57.105-1978

Title: IEEE Guide for Application of Transformer Connections in Three-Phase Distribution Systems

Abstract: The characteristics of the various transformer connections and possible operating problems under normal or abnormal conditions are treated for three-phase distribution systems. These systems are characterized by primary voltages up to and including 34.5 kV, usually have a preponderance of connected transformers with low-voltage windings below 1000 V, and furnish electric service to consumers. All combinations of Delta and Y, grounded and ungrounded, T connected, zigzag, and certain special connections are considered. Only two-winding transformers are included. Phasing procedures and loading practices are not covered.

 

[Marmite]

See

http://www.schneider-electric.com/d...straints/general-knowledge/ect190.pdf#page=18

for a fairly comprehensive paper on ferroresonance.
Regards
Marmite

 

[veman]

Thanks for the replies... (still looking though)

 

[lansford]

We have seen ferroresonace under the conditions you describe. The optimum solution is to do three pole switching at the riser pole or if you are lucky to have a LBOR swith in the transformer primary.


Jim

 

[dpc]

lansford: You've seen this in a wye-wye transformer?

 

[veman]

Thanks for the replies - I have read that it does occur in Y-Y transformers because of lower loss cores that have been used in recent years. The reason for my question is there is no 3 phase switch in the supply right now and I am debating spending the extra $ on installing one so it looks like this is what I'll end up doing.

 

[magoo2]

Going with a three-phase switch is probably the simplest way to deal with it.

As another option, you could add some load on the transformer, but you typically need about 2% of the transformer rating to suppress the ferroresonance. That would be 10 kW in your case. Sounds like a lot of heat to deal with, so I don't think it's practical!

 

[lansford]

Re DPC,
Yes, seen this in grnd wye/grnd wye. Some literature in the past did say that it was minimized with this type of connection implying that you would not get ferroresonaces. But we did see it.
Jim

 

[magoo2]

It was originally thought that the grounded-wye grounded-wye connection would get you out of ferroresonance. However, this was based on banks of single-phase units and it works well in that case because when you open a primary phase4, it simply gets de-energized.

However, with a three-phase 5-legged core, you get a coupled voltage from the other phase(s) that leads to ferroresonance.

It is more likely with 25 and 35 kV systems, but, with sufficient cable, it does happen on 15 kV systems (12.5, 13.2, 13.8, etc.).