Use of Snubbers

[buddy91082]

Podering when yo use snubbers for primary transforemer protection from a 5kV vaccuum breaker that is connected via cable approximately 15' away. Anyone have a good reference guide or article to use snubbers? I have seen design with them and without them. If transient swicthing with vaccum breakers is such a problem I woudl think it would be standard on all protection of transformers/motors.

Thanks.
b

 

[collies99]

Here in the beautiful province of British Columbia, CANADA, we have 25Kv feeders, using both SF6 and Vacuumn breakers, most are of the outdoors station class.

Feeders breakers are installed with series reactor upstream and tapped of the main bus, feeders are radial.

Anecdotally we had ABB Sache breaker that failed when interrupting due to the TRV, effects of using series reactors to limit downstream fault levels are buying lower rated breakers.

All feeder bus now employ a capacitor to ground to provide a shunting path, feeder bus are mainly fed from a wye grounded secondary or on older delta system with zig-zag installed.

This has now worked for over 30 years and is normal design practice.

 

[prc]

Buddy, normally snubbers are not required with 5kV VCB. Normally this may be required above 11kV and esp with dry type transformers. The latest tutorial documents on the subject are the following with a vast bibliography from 70's to the latest.
1)IEEE C57.17-2012 Requirements of Arc Furnace Transformers Appendix B
2)IEEE C57.142 Guide to describe the occurrence and mitigation of switching transients induced by transformer, switching device and system interaction

 

[mls1]

Having recently completed a transient analysis for a similar situation with a dry type transformer in a 13.8kV system I can tell you there are no good "rule of thumb" recommendations for this. The concern is, as you are probably aware, current chopping events but its more of a concern on lightly or inductively loaded transformers when the magnetizing current looks like a giant inductor and there is insufficient cable capacitance to reduce the peak and slope of the transient. The best references I found, which are referenced in IEEE C57.142, are the Allen Greenwood books "Electrical Transients in Power Systems" and "Vacuum Switchgear".

To identify the need for snubbers you'll need to know the parasitic capacitances (from power factor testing) as well as the normal test data. You can then build a terminal model of the transformer and run the terminal transients. This is good for a terminal transient analysis but it wont' give you internal resonance points. That is much more difficult to predict and better done through frequency response measurement at the transformer terminals.

For my part, I used ATP/EMTP for the analysis and I was surprised by how sensitive the analysis was. I took a conservative approach and added capacitor to the unit.

 

[VTer]

At 5kV, we normally install MOV style surge arresters on the transformer and call it a day.

"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

 

[buddy91082]

What if fuses were used? Is current chop a problem with fuses and snubbers still needed?

 

[7anoter4]

See [for instance]:
DESIGN OF SNUBBERS FOR POWER CIRCUITS By Rudy Severns

http://www.ee.bgu.ac.il/~pel/links/DESIGN_OF_SNUBBERS_FOR_POWER_CIRCUITS.pdf

 

[buddy91082]

Did a little bit more digging through some old text books. This is how i understand why snubbers are used on vacuum breakers:

Vacuum breakers act quickly. So quickly that the current sine wave can be terminated before reaching the zero crossing. When this happens, the current trapped within the windings (this is the current that is setting up the magnetic field in the windings which cannot change instantaneous) has no path that the current can flow through because the circuit is broken. This current trapped within the winding can be released through the system capacitance. When the current is released,it can create a large transient voltage across the transformer windings becuaee the system capacitance is in paralle with the transformer windings. This voltage is depended upon the value that the current is choppped and the transformer surge impedance.

I have come to the conclusion that current chop is not a problem with fuses since fuses do not act as quickly as the vacuum breakers.

b

 

[dpc]

since fuses do not act as quickly as the vacuum breakers.

You might want to do some additional research on current-limiting fuses. They can be much faster than a circuit breaker at high values of fault current. The issue with vacuum breakers is, I think, more related to normal switching operations involving load current.

 

[buddy91082]

After doing a bit more reading, I agree that current limiting fuses also prsent the current chopping problem.

What about on low voltage system 600v and below? Wouldn't we also have the same current chop problem with current limting fuses and breakers?