tapchanger maintenance - schedules and scope

[shipmate]

All,
Anyone involved with maintaining tapchangers and associated control circuits in both US and other countries: I am looking for info on the scope of maintenance - what is done, how often, potential problems and their solutions. Any details on oil management (other than DGA) including experience with filtration over and above the OEM recommendations. Problems with diverter and selector contacts in both reactor and resistor set-ups, vacuum interrupter systems, timing problems, mechanical faults such as sloppy shafts, dielectric breakdown on barrier plates, internal explosions, gas relay failures, internal wiring failures, loss of comms between circulating current controllers, etc., etc. Also any technical references that may help on this matter.
Your help is much appreciated.
Regards
Shipmate

 

[busbar]

From a practical standpoint, you may want to review NETA 2001 Maintenance Testing Specs §7.12.

Offhand, speculation is that oil in a separated-compartment OLTC would have significantly different aging characteristics than those of a common compartment.

 

[cabezon]

Shipmate,
You covered the whole spectrum of OLTC related problems, however still it is not clear to me if anything in particular is concerning you, apart from the understandable request for maintenace tips.
A couple of things coming to the top of my head I would like to add: blocking diverter switch operation under occurrence of OL current and shaft gasket leaking into the main tank hence confusing DGA results, to mention just a few.
Hope this help and look forward to keeping this thread open to increase my knowledge on the O&M of OLTC.
Regards, GAC

 

[shipmate]

Cabezon, Busbar,
Thanks for the input. I am at a very early stage of understanding tapchangers, and while mechanics is my bent, I am beginning to try to understand what I am up against in the tapchanger area.
I can understand the main problems that are common to all electrical equipt, such as contact burning, oil degradation due to in-oil arcing etc., overheating: I am also reasonably comfortable with the basic aspects of gas generation.
I am trying to get a handle on what is likely to happen if you do not check a changer for say a year longer than the schedule (what is the usual utility schedule? What controls the periodicity of maintenance - load, accessibility, number of operations? squads of guys in the right place? current-time curves etc?). Also, what are the typical inspection tasks, (check contacts, timings etc are fairly self evident, but does the barrier plate have an impact, how much sloppiness can a MR/GE/McGraw/other unit tolerate before it becomes a liability, what about time to change oil, weather conditions etc etc). Are there any empirical 'rules' associated with changer maintenance that are worth knowing?
Thanks, all, for any input given.

 

[busbar]

CYA notation — Looking at your specialty, my comments apply only to land-based/on-shore systems.

It is very likely that the tapchanger counter is a better indicator for determining maintenance intervals.

 

[mar888]

TC in US differ from Europe. In us are on the low side and in Eurpoe in the high. DGA is not usefull becouse is TCG mainly depend on number of operation. We now investigate other techniques like metals, particles.

 

[tarong]

My experience for what it is worth. I have experienced a major tapchanger explosion. The root cause of a mega million dollar failure was a procurement of incorrect gasket material - less than $100 spare part.

1. Take great care with tap changer oil quality at all stages of specification, procurememnt, storage and use. Specify to suit the OEM recommendations, and ensure high quality procurement and handling standards are met by use of written procedures.

2. A follow on from point 1. Corrosive sulfur was the main agent of destruction in our failure - sulfur seems to corrode Cu in transformers only at certain temperatures of Cu - very selective, so minute concentrations of corrosive sulfur can be catastrophic. Sulfur corroded Cu braids on the moving contacts of the diverter switch caused arcing, production of explosive gas, and then major explosion and destruction of the HV winding on one phase, and contamination of all the windings due to circulation of highly contaminated tapchanger oil in the main tank after the destruction of the tapchanger container.

Therefore, beware of any source of corrosive sulfur in the oil - could be in new oil if not specified correctly (suggest incoming QA inspection testing of tapchanger oil to ensure no corrosive sulfur). Use only gasket material that is free of corrosive sulfur - suggest it is best to stick with only OEM spares for tapchangers.

3. Follow OEM recommendations with planned maintenance - I have only ever seen it based on maximum number of operations.

4. Use only suitably trained people to maintain these critical pieces of plant - if you are doing the maintenance in-house, get your people OEM training and get quality maintenance procedures in place.

5. In operation, try to minimise the number of tapchanger operations - VAr control is usually not that critical that PF need be controlled excessively tightly. This may involve increasing the dead-band in an automatic voltage regulator or even having tap-changing as a manual operation requiring a real human to push a button. Certainly look out for and eliminate any couse of "hunting" tap-changer.