I have a 75-year old 115/69 kv tran

[SidiropoulosM]

I have a 75-year old 115/69 kv transformer, which serves a load area in Northern California. My studies show that I can improve the voltage profile on the 69 kv network by changing the fixed no-load tap on the 115 kv side. This would be much cheaper than installing capacitors on the 69 kv system. However, I have received feedback from field staff that because of the transformer's age, an attempt to change the fixed tap may damage the transformer. Has anyone encountered a similar situation; is this a legitimate concern?
Michael Sidiropoulos

 

[DanDel]

Has there been any routine shutdown testing and/or oil sampling done?
If the oil DGA and screening are ok, you might shutdown for a full set of tests, then change taps and test again, comparing results.

 

[busbar]

Er, I can understand why staff wants to take a wide path around gear that was built in the 1920s. What are oil tests showing? It may be remotely possible to lower the oil and physically remove and reconnect leads to bypass the tapchanger contacts, or eliminate it altogether. A concurrent internal visual inspection may be sobering.

Is the 69kV-side voltage profile stable? If even seasonal adjustments are needed, as described it’s hard to imagine that any tapchanger adjustment could be any less than disastrous in terms of service continuity.

Seems like the transformer has given phenomenal service, and it may be high time to get in a high-priority order for replacement. The phrase “borrowed time” may apply here.

The transformer MVA is not specified, but locating a trailer-mounted rental substation may be in order, even without the tap change. The voltages are common for that region. Maybe it could be discussed with some TANC or COTP members.

I mean no offense in these comments, but best of luck to you.

 

[chicopee]

Replacement is warranted in light of the fact that you may have PCB in the transformer. With a leaking transformer containing PCB you will be bying a big chunk of territory

 

[DanDel]

Just because the transformer is old doesn't mean it necessarily has PCB fluid or is PCB contaminated. This should be well known to the owner and is definitely the first point to consider when deciding if replacement is warranted.

 

[RRaghunath]

I think the transformer tap can be changed safely. To be sure, a DGA test can be conducted on oil sample drawn from the transformer after restoring it in to service and on load. Raghunath

 

[SidiropoulosM]

On the PCB issue, we have been actively replacing all equipment with PCB's and this is not one of them.

The transformer is 60 MVA, built by Westinghouse in 1925. It probably belongs to the Smithsonian rather than here !

In any event, we replace transformers because of load growth, not age. The cost of a new transformer of this rating is about $1.5 million and we don't spend that kind of money without a real need.

Thank you all for your comments, you've given me some ideas. First, I'll see if they can do those oil checks. I will also see if we can justify a new transformer based on load growth. Michael Sidiropoulos

 

[cabezon]

Unless there have been some operations to or from the relevant tap recently and oil tests do not suggest problems, I would not recommend moving it without performing a detailed visual inspection.

 

[busbar]

Minor aside: There are some established regional oil-test labs that can provide containers and coaching if oil sampling is a new issue.

http://www.actiinc.com/sampling/freestuff.html

http://www.oliveoilsource.com/olivenews4-11.htm

http://www.nttworldwide.com/products.htm

http://www.transformerservices.com/Lab/Gastest.html