@bstephens4801

1 year ago

Any 230KV transformer requires about 5-7 days period to bowser (multiple vacuum pulldowns, vacuum fill the oil from the bottom up, heat the oil, circulate the oil through water separation filtration). There is careful testing done to assure moisture levels are safe for energization. Once the envelope is penetrated by moist ambient air, there is no recourse. You have to perform this process. This time doesn't include draw down of the remaining oil, repair of the radiators, etc. Typically, the radiators can be repaired by welding. Also, utilities used to keep wooden pegs to drive into the holes to 1) stop leaking and 2) possibly prevent tripping, 3)refill oil if the envelope still has a nitrogen blanket. The low oil tripping schemes are pretty high up in the tank. The float will go to the alarm level first and then trip afterwards. The goal is to trip before any sensitive components are above the oil. I have first hand experience with this type failure (low tank leaks) and generally there is a small vacuum formed by the escaping oil and it does leak but not at a tremendous rate. Probably the worst of the leak is due to the flow rate of the nitrogen regulator trying to maintain the blanket. So it takes a while for the transformer to go into alarm and then trip. Luckily all of this happens before serious damage is done. If the radiators are penetrated in a way that damages multiple layers, those have to be removed, cut apart, repaired and rewelded/layered to a full assembly. That takes a while. Many utilities have a full repair facility for power equipment and that would be done there and returned to the substation. In some cases, it might be possible to get spare radiators from other transformers. For instance, the 230/115KV Autobank(s) in a substation could have all good radiators put on one transformer if they are in fact the same dimensions and mounts. Also, radiators could be removed, the flanges blocked off and operate the transformer at reduced load until scheduled repairs can occur. In a situation like this, you do whatever you have to to get the lights on. That location is unusual with a looped 115 system and no alternate feeds. Generally there is a large 115 network and 230 network with multiple dispersed 230/115 locations where those networks tie. A side note these 230/115 autotransformers also play an important role as a ground current source. A ground fault on the 115kv will be seen as a 3 phase event on the 230kv network due to the closed delta tertiary windings. This allows the generators and 230kv system & up to function as a three phase source for any 115kv problems. A useful feature, it works the same way in either direction. Another note, the low oil tripping systems are usually hardwired. No operator intervention required. The nature of these gauges, microswitches and the fact that the transformer is a grounding source plays into the design of the system. Most use a blocking method where the tripping coil is shorted by one contact which must open before the contact that makes causes the coil to operate. So a contact must part and one must make for the trip to occur. There is usually a current limiting resistor in there in case both contact are made at the same time. It is pretty hard to have a misop of that system unless you are doing maintenance on the indicator and forget to block it. Yep, it happens. It's not hard for everyone to 'know your name'.

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