Current chopping - effect on old PILC-SWA cables
Current chopping - effect on old PILC-SWA cables
(OP)
I'm looking at a retrofit or replacement of an old bulk oil switchboard. Either solution will likely result in use of vacuum or SF6 interrupters.
The majority of the outgoing circuits are distribution transformers ranging from a few hundred kVA up to about 7MVA. I'm concerned about current chopping on these circuits because our switching practice will definitely take the secondary side load of the transformer prior to isolation and leave the HV side breaker to open magnetising current. The cable installation is about 35 years old, in good shape, and is paper insulated, lead covered, steel wire armoured (PILC-SWA). I have relatively limited knowledge of the effect of current chopping on this type of cable and am looking for some thoughts on any likely problems. I really don't want to replace the cables unless absolutely necessary, something which doesn't appear to be the case at present.
The majority of the outgoing circuits are distribution transformers ranging from a few hundred kVA up to about 7MVA. I'm concerned about current chopping on these circuits because our switching practice will definitely take the secondary side load of the transformer prior to isolation and leave the HV side breaker to open magnetising current. The cable installation is about 35 years old, in good shape, and is paper insulated, lead covered, steel wire armoured (PILC-SWA). I have relatively limited knowledge of the effect of current chopping on this type of cable and am looking for some thoughts on any likely problems. I really don't want to replace the cables unless absolutely necessary, something which doesn't appear to be the case at present.
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If we learn from our mistakes I'm getting a great education!






RE: Current chopping - effect on old PILC-SWA cables
We're talking about dozens of 15 kV substations, a power generation station and cable lengths of over 8000 feet with spices in multiple manholes, so it was a pretty good sample.
old field guy
RE: Current chopping - effect on old PILC-SWA cables
We have other problems which I intend to address including fault level right at the limit of the breaker capacity. One option is to open the bus section breakers on the three-section 11kV board and implement a fast bus transfer scheme to provide supply continuity on loss of one of the three incomers.
I have a degree of scepticism toward fast bus transfer schemes and am curious what positive and negative experiences others have had. They are relatively uncommon in the UK and there's a lack of operating experience to draw upon which doesn't help to fill me with confidence.
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If we learn from our mistakes I'm getting a great education!
RE: Current chopping - effect on old PILC-SWA cables
We did this regularly at a power station I worked at without any problems using VCB retrofits in conjunction with existing 11kV PILC cables (multicore and single core). I believe that the problem is less severe nowadays with modern contact materials, but can be a problem with large breakers relative to the size of the load, although we had 1600A breakers (11kV) feeding some small (2.5MVA) transformers without problems.
Beware of surge diverters fitted to trucks in an attempt to alleviate switching spikes on TX and motor circuits as most of the retrofits I have seen have no fuses protecting the diverter and protection of the unit then relies on the busbar protection, which is normally set way too high to protect a diverter. I have heard of some failures and we had some problems with diverters holding a charge. We took the divertors off because we deemed it was better to protect people rather than the equipment.
With regard to your fast bus transfer scheme, I would be more worried about that than current chopping in PILC cables. If I understand your idea correctly, you would be trying to pick up load on a standby feeder as soon as the duty feeder tripped and although VCBs are fast, you are still looking at around 100-150ms break-to-make time to transfer and you then have all transformers picking up mag inrush at the same time.
If this post is related to your earlier post regarding retrofit BVP boards, double check the rating of the boards, as I'm with Marmite, in that I have never seen one of these boards rated at anything more than 350MVA (18kA) @ 11kV, but more commonly 250MVA (13.1kA), though I have no experience of Vac retrofits on these boards. That's probably the reason why you can't source a 26kA retrofit.
Before you embark on purchaing retrofits from any company, ask to see the test documentation for the type test and make sure that the type that you are buying is the same as the type tested and also check that it was tested with same interrupters that you are having fitted. Also check that no parts were replaced part way through the test to get it through. It can be a minefield out there with retrofits. Happy hunting.
RE: Current chopping - effect on old PILC-SWA cables
Many thanks for the reply. The question was related to the earlier one about the BVP board although I posted it separately because the problem would potentially occur whether we replaced it with new or retrofitted the existing board. I don't intend to replace the existing cables although I accept that we will likely have to cut back the PILC cable and joint it to a new piece of XLPE if we replace the board. Another headache for me although nothing new to the utilities I suspect.
Outgoing breakers are all BVP17's with a 400A rating although likely the VCB replacements would be rated for 630A. Some of the transformers are pretty small, 350 and 500kVA although there are bigger ones up to about 7MVA on the 3.3kV network. It's the circuits feeding the little transformers where I think current chopping could have the most severe effect. The incomers and bus sections are the BVP22's and I guess there just weren't that many of these bigger breakers installed compared to the BVP17 which is why there's so little interest in retrofitting them.
That's more good experience regarding the PILC cables to add to that from OFG, and it's on British cables which is reassuring. I admit I hadn't really considered arrestors mounted on the truck itself, I was thinking of retrofitting them to the board itself although the amount of re-engineering required to do this was beginning to alarm me. Without any fuse protection for the arrestor the scope for a major incident looks quite real and with a very high fault level it's an issue we need to seriously consider. We don't have a bus diff scheme so any bang would likely be a big one. I absolutely agree about protecting the personnel rather than the equipment: I'd rather see the whole plant burn down than lose one more friend at work. Losing three in one day is more than enough for my lifetime.
I'm 'aware' of fast bus transfer schemes and I think they are more practical at a motor control centre level although they are pretty much the standard spec for source transfer between unit and station auxiliary transformers for the newer generation of power plants. I will maintain a healthy level of scepticism for the same reasons you have picked out. It's all too easy to see the combined inrush tripping a hi-set overcurrent especially if the residual bus voltage was out of phase. Mechanical damage caused by torque transients to the motor or load concerns me too, especially with a long transfer time.
The rating is definitely 500MVA / 26.3kA at 11kV and this was verified from Areva's records. Areva frustrate me from time to time but the inherited records from GEC and Alstom are very complete so a little 'thank you' to them for that service.
A couple of vendors - or possibly just one vendor whose product is being re-badged by others - has indicated that they can get the breaker type tested at KEMA up to 31.5kA which is the highest ABB's VD4 VCB module is capable of. We'd probably end up bearing some of the cost of the type test which is a further concern: KEMA time doesn't come cheap! For what it's worth the vendor concerned is Igranic Engineering who appear to know what they are doing from all sources who have commented. A few companies appear to re-badge their product. Areva aren't interested in further type testing but they will sell us one their horrible tinplate fixed-pattern switchboards instead.
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If we learn from our mistakes I'm getting a great education!
RE: Current chopping - effect on old PILC-SWA cables
Regards
Marmite
RE: Current chopping - effect on old PILC-SWA cables
About fast transfer.
You are right, it's good for motors or some very critical systems. In the "standard" application, for my pinion, not needed, residual voltage or long time transfer is enough, but usually with motor trip before and with re-starting systems
Best Regards.
Slava
RE: Current chopping - effect on old PILC-SWA cables
I'm trying to resolve some of these nagging doubts early on in the project while we're still in the design and tender stage, rather than miss something important and find myself asking how to fix it once the gear is in service.
Your comments are appreciated.
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If we learn from our mistakes I'm getting a great education!
RE: Current chopping - effect on old PILC-SWA cables
Check the rating of the stabs and clusters on the trucks and the board before declaring that the retrofit is rated at 630A. It is more likely that the VCB cassette is rated at 630A, but the rest of the components of the old BVP truck and the cubicle, will still be rated at 400A. We had 2400A interupters on all our retrofits, but the bushings etc. on some of the trucks and cubicles were only 1200A.
I was involved in some fast transfer schemes at LV level some years ago and yes, they were more suited to MCC type installations, although we had to have battery backed DC control circuits so that the contactors would ride through the mains dip during transfer. It worked, but the control scheme for the breakers was quite complicated.
The station I worked at had transfer schemes for switching between station and unit 11kV supplies, but the switching was done as a routine operation rather than on failure of supplies and the transfer actually created a short time paralleling of the boards before one of the breakers was opened. We had very high fault levels during paralleling (around 75kA) but this was only for 10s and the switchrooms were evacuated whilst this was done (remotely). This doesn't help your idea, but that's the only HV "transfer" scheme I have ever come across.
Never heard anything bad about Igranic.
A big factor for us with our retrofits (not Igranic) was getting an engineer in when something went wrong and getting modifications done when we found "inherent" problems with the conversions. Look at the on-site service and response time as well as the product when you consider who to buy the retrofit from.