Three phase pad mount distribution transformers with 240/120v delta secondarys

[jcarter12]

We have been requested to replace overhead facilities with underground in our downtown area. Most of the area requires 120/240 v three phase which do not offer because of what we have been told is ferroresonance and harmonic issues. Has anyone used these type transformers and if so, were there any problems? We are getting quotes for 500 and 750 KVa transformers.

 

[waross]

Use delta primaries.
Wye/delta transformers and transformer banks are a bad idea for distribution. Back feeding, circulating currents,transformer burnouts, blown primary fuses due to ground faults elsewhere on the circuit, customer issues, burned out refrigeration equipment. I spent too many years in an area of the third world where wye/delta connections were state of the art. Most pole mounted three phase wye/delta banks spent most of their time with one fused cutout hanging down and the transformer bank running happily on open delta.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Mbrooke]

Delta secondary? Was this an IT (ungrounded)230v system?


Anyway, in regards to the OP's predicament. Grounded wye delta and wye delta both have issues. wye delta is a ferro-resonance concern on an open cutout or single phase switching. Grounded wye delta is a concern with acting as a grounding (zero sequence) source on faults. A downed line can blow a fuse on the bank.

What is the connection now? What is giving 120/240 3 phase delta?

There are several options.

One is a delta delta bank. Slightly less susceptible to Ferroresonance, and not likely at 12kv and below.

Second :A switched grounded wye delta. Basically the XO or primary wye is closed when switching cutouts open or closed and then opened when the bank is in service.

Third is converting customers to wye wye, giving 208. This may or may not be an issue. Most motors that run on 240 volts are rated 230 volts and can run down to 207 volts while still running within design limits. Of course this assumes the run is short and the nominal voltage at supply is high enough that the voltage at the motor is at least 207 volts.

In some areas spot networks/mesh networks are intentionally run on the high side (125/216Y)with tight upper limits to over come this.

However, not ever piece of equipment may be happy. A single machine or set of small loads needing true 240 can obtain it via buck boost transformer. A reimbursement for the bill might help them agree to it.

Forth: A grounded wye open delta. Doable, but seeing you are looking at 500kva units may represent substantial 3 phase customers with extensive motor needs. In such as an open delta would be a poor choice.

Fifth: A delta delta protected by a recloser or switch. This prevents single phasing and thus Ferroresonance is less of a concern.

Personally I would go with this option as its the most feasible IMO if a 120/208Y service is out of the question.

sixth: Using a lower primary voltage such as 5kv to reduce ferroresonance risk. Chances are this may be the last option in that most pocos do not want to create a new system voltage or continue using the same one that is forcing them to upgrade to begin with.

 

[jraef]

It's not what you think mbrooke. Here in the US, we have a common configuration for light industrial /commercial installations where we use a 240V delta secondary with one winding, typically the A-C winding, center tapped and referenced to ground as a neutral. So you get 240V 3phase for larger loads, 240V single phase from any two legs but then for lighting and outlets, you get 120V single phase from A or C to neutral. Then on the B phase, we mark the conductor orange and call it the "High leg" or "Stinger leg" because if you try to use it for single phase to neutral, you get 208V, which stings when you fry your 120V equipment...

Jcarter12,
The problem you are running into is likely because of the size. 240/120 3phase 4wire High leg delta services are typically limited in scope to I think 150kVA, because the transformer is, by design, going to run unbalanced. A local grid with multiple 500+kVA versions presents a problem for the utility.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[jcarter12]

Thanks for the replies. Good info.