Replacing 1.5MVA Transformer

[Elec-Eng]

Hello All,
I am trying to replace a 1.5MVA 13.8KV – 575V/332V transformer with a same capacity 1.5MVA 13.8KV -575V transformer.
Problem is the existing one is Delta/Wye connected and replacement one is Delta/Delta connected.
They are both three phase and have about the same impedance.
The secondary of the existing one is connected to three phase loads.
Do you see any problem with this change out?
Thanks in advance.

 

[GoldDigger]

Will you be corner grounding the new secondary? It taking advantage of one of the exceptions allowing it to operate ungrounded?

 

[Bugman1400]

You'll lose any use of your ground protection, if you had any. You'll also need to ensure that your wiring and any devices downstream are rated for L-L voltage instead of just L-N voltage. You cannot have any alternate 575V source since it will be out of phase.

 

[zog]

Why not just order a delta/wye as replacement? Save you a lot of headache.

 

[templdl]

Hello All,
I am trying to replace a 1.5MVA 13.8KV – 575V/332V transformer with a same capacity 1.5MVA 13.8KV -575V transformer.
Problem is the existing one is Delta/Wye connected and replacement one is Delta/Delta connected.
They are both three phase and have about the same impedance.
The secondary of the existing one is connected to three phase loads.
Do you see any problem with this change out?
Thanks in advance.

It appears as though the 332v L-N voltage is not required. I'm sure that those who authorized it knew what they were doing and had a good reason for doing so.
With a Delta secondary you will have to consider a corner grounded Delta where you select one of the phases a point to ground at which point your EGC
As such you can still have A-B-C phases only one of the phases is grounded. You will still have an EGC. You will only be lacking the neutral. There will be no 332v linre to neutral.
Regarding phasing, yes the phases FRTOM a wye to a Delta will change but that is of no concern unless you have to coordinate the new Delta try i an existing wye.

 

[Tony S]

Hello All,
I am trying to replace a 1.5MVA 13.8KV – 575V/332V transformer with a same capacity 1.5MVA 13.8KV -575V transformer.
Problem is the existing one is Delta/Wye connected and replacement one is Delta/Delta connected.
They are both three phase and have about the same impedance.
The secondary of the existing one is connected to three phase loads.
Do you see any problem with this change out?
Thanks in advance.

Are you an engineer?

From your question I would recommend hiring a consultant that knows what they are dealing with. Because you clearly don’t!

 

[Haji]

Hello All,
I am trying to replace a 1.5MVA 13.8KV – 575V/332V transformer with a same capacity 1.5MVA 13.8KV -575V transformer.
Problem is the existing one is Delta/Wye connected and replacement one is Delta/Delta connected.
They are both three phase and have about the same impedance.
The secondary of the existing one is connected to three phase loads.
Do you see any problem with this change out?
Thanks in advance.

It appears that you are changing from grounded to ungrounded power system for more uninterrupted service. It requires ground fault detection.
But if you use HRG system instead, it has the same reliability as the ungrounded system minus its liability to insulation damage due to any arcing ground.

 

[paulengr]

Why not just order a delta/wye as replacement? Save you a lot of headache.

If you go corner grounded, it usually requires some training, and you've got to check all of the wiring can handle it. Cost is immaterial but some 480/277 delta-wyes I've encountered used 300 V insulation instead of 600 V. Reliability on ungrounded systems decreases dramatically due to arcing faults where system capacitance becomes a voltage multiplier. HRG gives the best reliability but I'm no fan of zig-zags and delta-wyes as ground protection. Given that the restocking fee is not great and a 1500 kva transformer should have been under $40k US new and even less used, I'd swap it for the right one. By the way, 575 V is a UTILIZATION voltage giving you a utilization voltage of around 540 V. This puts you into custom motor territory where everything has obscene lead times and double or triple the normal price. Check closely if this is even right. Canadian standards are usually 600 V transformers and 575 V motors. US is usually 480 with 460 V motors. European is usually 440 V with ~400 V motors. 575 V transformer just doesn't sound right and sounds a lot like somebody that didn't know what they are doing used tribal knowledge to buy a transformer. Utilization voltage at low voltage is typically 3% below transformer voltage which accounts for a 3% voltage drop in lines due to impedance.

 

[Elec-Eng]

Thanks for your comments. The existing transformer is old and there is no spare for it. But we have a spare with same capacity but delta/delta connected. Transformer secondary is connected to 3 phase 600V feeder breakers, and they mostly feed the utilities through 575v - 120/208V step down transformers. There is no load at transformer secondary connected between phase and ground (at 332V).
Does ungrounded delta/delta transformer secondary cause a big issue?
I am just trying to see if this change out is a possibility.
Thanks.

Note: I am an engineer with lots of experience in Automation, but still at the early stage of power distribution field.

 

[topgone]

Thanks for your comments. The existing transformer is old and there is no spare for it. But we have a spare with same capacity but delta/delta connected. Transformer secondary is connected to 3 phase 600V feeder breakers, and they mostly feed the utilities through 575v - 120/208V step down transformers. There is no load at transformer secondary connected between phase and ground (at 332V).
Does ungrounded delta/delta transformer secondary cause a big issue?
I am just trying to see if this change out is a possibility.
Thanks.

Note: I am an engineer with lots of experience in Automation, but still at the early stage of power distribution field.

Big issue,IMHO!

Since you don't have a neutral to connect to ground, the danger of higher touch potential (should one of your lines gets accidentally grounded) comes to fore as well as losing a simple way of protecting the system from ground faults. You have to "derive" a neutral using a separate transformer. You also have to add ground fault protection relaying unit.

But that's possible, I have done that many times. You will have to bear with having to foot the bill for additional grounding transformer plus relaying.