OLTC with diverter switch in the vacuum chamber

[krisys]

We are in the process of procuring 33/11.5 kV, 40/50 MVA, Delta/wye power transformers for an oil & gas processing plant. The 11 kV side neutral is resistively earthed though 400 A Neutral Grounding Resistor (NGR).

The transformers are specified to have On Load Tap Changer (OLTC). One of the vendors is offering the OLTC with diverter switch in the vacuum. Conventionally, the OLTCs are with oil filled diverter switch. If the OLTC operation is frequent, the OLTCs with oil filled diverter switch are prone to give trouble.

I am accustomed with the conventional OLTC having oil filled diverter switch. I would like to know whether anyone has experience in the OLTC having the diverter switch in the vacuum. I would appreciate your view and share your experience on this.

 

[prc]

Please check with the vendor. He may be offering OLTC with vacuum interrupters(contactors) similar to the ones used in MV switchgear. Instead of arcing contacts in oil,arcing will be inside the contactor so that the oil in the diveterswitch will not get deteriortaed quickly.Also the contact life will be high. This technology is available for the past 10 years and in US even earlier.

 

[krisys]

Thanks prc.
Is the vacuum type OLTC so common in use?
Is it more reliable than oil type?
Appreciate your inputs.

 

[stevenal]

Vacuum is the only way to go in my opinion. With no arcing under oil, no oil filtering is needed. Add a self regenerating dehydrating breather, and you have as close to a maintenance free LTC that you can get. [URL unfurl="true"]http://www.reinhausen.com/en/desktopdefault.aspx/tabid-310/94_read-65/[/url]
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[krisys]

Is there any explosion hazard due to the vacuum bottle rupture?

 

[prc]

As I said earlier,in US, such vacuum OLTC is available with reactance type OLTC since late 1980's. Later MR started vaccum tap changers with in tank, resistor type OLTC too. The trend is definitely for vacuum technology and there is no explosion hazard reported.

 

[stevenal]

An explosion hazard could occur should a vacuum bottle fail. There is circuitry to inhibit motor operation and alarm for that condition. Hand cranking the LTC on an energized transformer might be one's last mistake. I recall a report of an incident involving an LRT (GE) tap-changer.

 

[ScottyUK]

Manually operating an OLTC on an energised GSU transformer was a nerve-racking undertaking. Local operation of the motor drive mechanism was bad enough. Local tapping operations activities were permanently suspended following a tapchanger-related accident at our site: if auto control wasn't available then we did our best to make the reactive despatch using the AVR and made up the shortfall on other generating units.

 

[low1]

My experience with OLTCs with vacuum bottles has been disabling them and permanently bypassing the tap gear. These were station service, air type transfomers, 13.8kv/4kv. The failures we were experiencing were related to the bridging reactors. They had a tendency of exploding catastrophically. There were 3 failures I was aware of, after that the tap changers were run to neutral and blocked, then as outages permitted, permanently bypassed. I'm not sure they were related to vacuum bottle failures, but I know we found many of them cracked and damaged.