Parelleling pad mounted transformers

[mbrooke]

How is this typical done in the industry? Presently the plan is to parallel 3 750kva 12.47 Delta-480 wye units to a single collector buss. All 3 will be fed from the same primary feeder under any contingency thus phase angles will be identical. Primary protection is via bayonet fuses integral to the pad but can be supplemented via S&C PME switch-gear protective package add on, however the secondary protection (if at all) is up in the air.

 

[Bugman1400]

The 3 xfmrs will need to have the same or nearly the same impedances for obvious reasons. The highside fuses will take care of the individual xfmr overload or short. The collective secondaries need to be protected as close to the xfrms as possible. I would also check the fault availability to ensure MCC can handle since the parallel xfmrs greatly reduces the impedance.

 

[augie47]

Our auto manufacturing plants do that as standard practice. We had concern as we had a 2000 amp buss duct being fed by multiple 2000 amp breakers but the system was designed under engineeering supervision and each buss supply is monitored by computer to assure the buss ampacity is not exceeded. OCP for any "tie connected" secondary circuits and available fault current will probably necessitate engineering input.
Some of the engineers on the Forum can shed far more (and better) light on this than I. Hopefully they will.

 

[mbrooke]

The 3 xfmrs will need to have the same or nearly the same impedances for obvious reasons. The highside fuses will take care of the individual xfmr overload or short. The collective secondaries need to be protected as close to the xfrms as possible. I would also check the fault availability to ensure MCC can handle since the parallel xfmrs greatly reduces the impedance.

What makes closeness to the secondary beneficial? Im thinking that as long as secondary breaker disconnects from the collector buss any faults between the pad and breaker will be cleared via primary side protection. Vista and other pad mount gear can be ordered with breakers where their relays can be programmed with time current curves which would help "see" the typical blind spot between the pad and service.


Fault current will be looked at in depth, however one fortunate factor are the lengths of both the primary and alternate source 12kv feeders which are calculated to produce L-G fault current in the 2000 to 3000 amps range max.

Our auto manufacturing plants do that as standard practice. We had concern as we had a 2000 amp buss duct being fed by multiple 2000 amp breakers but the system was designed under engineeering supervision and each buss supply is monitored by computer to assure the buss ampacity is not exceeded. OCP for any "tie connected" secondary circuits and available fault current will probably necessitate engineering input.
Some of the engineers on the Forum can shed far more (and better) light on this than I. Hopefully they will.

Thanks for the reassurance I knew this was not off the wall outside of an underground secondary network.The secondary breakers will need to have reverse power flow detection where they will open for energy flow into the padmount, but unsure if specific parameters apply for this application.

 

[mivey]

I would recommend supplemental switchgear or keeping extra fuses. When the HS fuses on one unit goes out, guess what happens next? It would be better to switch out the parallel units to prevent those fuses from blowing on overload, unless the parallel units are for redundancy.

Why not just use a 2500 kVA unit?

 

[mbrooke]

I would recommend supplemental switchgear or keeping extra fuses. When the HS fuses on one unit goes out, guess what happens next?

Ferroresonance or in the case of a 3 legged wye grounded primary core inductive tank heating.

It would be better to switch out the parallel units to prevent those fuses from blowing on overload, unless the parallel units are for redundancy.

What makes parallel units susceptible to fuse blowing?

Why not just use a 2500 kVA unit?

Combination of redundancy and whats normally in stock (750 and 1000kva units are like pole pigs around here). The other option is ordering two 2,500 kva units loaded 1/2 each with a N.O. buss tie or 3 subsurface network transformers which come with fuses and network protector.

 

[Bugman1400]

What makes closeness to the secondary beneficial? Im thinking that as long as secondary breaker disconnects from the collector buss any faults between the pad and breaker will be cleared via primary side protection. Vista and other pad mount gear can be ordered with breakers where their relays can be programmed with time current curves which would help "see" the typical blind spot between the pad and service.

You are correct but, I've seen ground faults at the long end of a secondary before the main breaker that did not clear the highside fuse for some reason. Maybe the fuses were oversized....can't recall. Perhaps this would not be an issue for parallel xfmrs and a collected bus.

 

[mbrooke]

You are correct but, I've seen ground faults at the long end of a secondary before the main breaker that did not clear the highside fuse for some reason. Maybe the fuses were oversized....can't recall. Perhaps this would not be an issue for parallel xfmrs and a collected bus.

Could it be that the sputtering fault itself introduces impedance? I know in underground networks POCOs will sometimes rely on the impedance of a fault to burn the cable clear without actually overheating the cable insulation itself.

 

[mivey]

Ferroresonance or in the case of a 3 legged wye grounded primary core inductive tank heating.

A reduced concern at 12 kV.

What makes parallel units susceptible to fuse blowing?

Overloading of the remaining units, unless they were for redundancy.

Combination of redundancy and whats normally in stock (750 and 1000kva units are like pole pigs around here). The other option is ordering two 2,500 kva units loaded 1/2 each with a N.O. buss tie or 3 subsurface network transformers which come with fuses and network protector.

12kV/480V, 2500 kVA utility padmounts are pretty common. Even if you might be small enough to not have one in the local yard, the rep can find one at a utility yard to have delivered for emergency and get them a replacement on standard delivery.

 

[JoeStillman]

If the purpose of having three-in-parallel instead of one big transformer is redundancy, then you'll want to be able to isolate the primaries and secondaries.

This looks to me like a good application for differential protection. If there is a fault in one of the transformers, you could isolate it from the others without shutting down the whole collector bus.

 

[mbrooke]

A reduced concern at 12 kV.

But still possible with underground cables (high capacitive reactance)

Overloading of the remaining units, unless they were for redundancy.

The load has been calculated such that the reaming 2 units will be loaded to 100% (actually a bit more to take advantage of cyclic loading, but still not anticipated to shorten life).

12kV/480V, 2500 kVA utility padmounts are pretty common. Even if you might be small enough to not have one in the local yard, the rep can find one at a utility yard to have delivered for emergency and get them a replacement on standard delivery.

I might take this approach now that I look at it.


What would the impedance (short circuit increase) be of the combined units if the 3 pad approach is taken?

If the purpose of having three-in-parallel instead of one big transformer is redundancy, then you'll want to be able to isolate the primaries and secondaries.

This looks to me like a good application for differential protection. If there is a fault in one of the transformers, you could isolate it from the others without shutting down the whole collector bus.

I may go that route, but I am seriously considering a main-tie-main.

 

[mivey]

But still possible with underground cables (high capacitive reactance)

Possible but not a high probability.

The load has been calculated such that the reaming 2 units will be loaded to 100% (actually a bit more to take advantage of cyclic loading, but still not anticipated to shorten life).

Then losing one would be ok.

I might take this approach now that I look at it.

IMO it make protection easier in some cases. Paralleling more than two can get complicated if you are also trying to protect the unit.

What would the impedance (short circuit increase) be of the combined units if the 3 pad approach is taken?

The fault current for three paralleled units will be about the same as the single unit. Whether paralleled or a single unit, we usually spec the impedance to keep the maximum fault to around 10,000 amps or less at the 12 kV level as it makes life much simpler for other reasons.