wye vs delta secondary

[PJGafc]

I have an old 3 phase 4160 to 480 volt 2400 KVA transformer that is a wye primary and a ungrounded delta secondary. We just lost one of the cans. We are installing a new set of transformers. the electric company says that I can no longer use a delta secondary without putting in automatic switch to protect their service. Does anyone know why this is? Electric company suggest we use grounded wye secondary and we can avoid putting in switch.
For an industrial application, what are advantages of grounded wye vs. ungrounded delta secondary?

 

[robbietan]

the utility wants a ground reference for their protective devices, which is probably why they want a wye secondary.

in our utility, they would insist a delta primary-wye secondary

 

[dereckbc]

For an industrial application, what are advantages of grounded wye vs. ungrounded delta secondary?

Both have pro's and cons. Delta is the choice for industrial process applications where outages cannot be tolerated from ground faults like extrusion process or petroleum refining where a failure would mean costly shut-down and clean-up. Down side is you have to have trained personnel available to work on and monitor the system.

Wye systems are very economical and use simple Over Current Protection Device schemes, but are prone to ground faults.

 

[jtester]

I have an old 3 phase 4160 to 480 volt 2400 KVA transformer that is a wye primary and a ungrounded delta secondary. We just lost one of the cans. We are installing a new set of transformers. the electric company says that I can no longer use a delta secondary without putting in automatic switch to protect their service. Does anyone know why this is? Electric company suggest we use grounded wye secondary and we can avoid putting in switch.
For an industrial application, what are advantages of grounded wye vs. ungrounded delta secondary?

Are you taking service at 4160 volts from the utility? If so and the transformer is yours, I don't know why they'd care what the connection is, unless you generate lots of harmonics.

You probably do need a switch between you and them to protect their system regardless of the transformer connection.

Jim T

 

[beanland]

Wye-Delta

Wye-Delta

If the utility is connected grounded wye and the customer is delta, the transformer bank becomes a ground fault source for the utility. You do not want that. Some utilities use ungrounded-wye primary and delta secondary to solve this. But, a delta secondary is a problem; you need a ground fault path. Corner grounding works but should be avoided because of the odd voltages in the system. Center-side grounding is common. This is 240/480V, 3-phase, 4-wire. Most rural utilities would prefer a Wye-Wye bank; no ground fault source and good ground reference on service. You could also use a Wye-Delta bank and a zig-zag transformer to create a ground reference on the delta winding, but ground source is still an issue for the POCO. I am not sure what kind of switch the utility is asking for. Can you expand?

 

[coulter]

If the utility is connected grounded wye and the customer is delta, the transformer bank becomes a ground fault source for the utility. ...

beanland/robbietan- -
Can you give me some references on this? I don't disagree. I'm just not knowledgeable in this area.

carl

 

[jtester]

If the utility is connected grounded wye and the customer is delta, the transformer bank becomes a ground fault source for the utility. You do not want that. Some utilities use ungrounded-wye primary and delta secondary to solve this. But, a delta secondary is a problem; you need a ground fault path. Corner grounding works but should be avoided because of the odd voltages in the system. Center-side grounding is common. This is 240/480V, 3-phase, 4-wire. Most rural utilities would prefer a Wye-Wye bank; no ground fault source and good ground reference on service. You could also use a Wye-Delta bank and a zig-zag transformer to create a ground reference on the delta winding, but ground source is still an issue for the POCO. I am not sure what kind of switch the utility is asking for. Can you expand?

If the transformer high side is ungrounded wye, it isn't a ground source. It shouldn't be connected grounded wye under any condition. If the utility distribution system is underground for any distance, then ferroresonance could be a problem.

Delta secondaries aren't a problem, although they are a bit unusual in many locations. I work with delta secondaries all the time, mostly in rural distribution, but never underground.

Jim T

 

[jcormack]

I am not sure I followed beanland & robbietan's logic either. If it is being said that a Wye primary creates a ground current source for the POCO - that is only true if the neutral point of the Wye is connected to ground (not required & in fact discouraged) - & ground currents in that scenario would only flow under imbalances or fault conditions.

Floating Deltas are used quite often in industrial plants - a ground fault simply creates a "Corner grounded Delta" & other than a "spark" caused by capacitive current flow, it does not cause a major problem (this is of course assuming any surge devices were rated L-L not L-N - - because then we might have some fuse blowing / CB tripping) Now, a second ground fault of course is now a L-L fault and devices trip.

On Y-Y transformer connections in my area we NEVER connect the primary side neutral to ground, why would you? the POCO's side is already referenced to ground on their side (typically solidly grounded wye secondary). A typical system here would be Delta-Wye. Often on the higher voltages the POCO uses Autotransformers on say 138 KV to 69KV, in that case a ground on the 69 KV IS a ground on the 138 - but on lower voltages where you have primary and secondary coils you are creating isolated systems.

 

[jtester]

.......On Y-Y transformer connections in my area we NEVER connect the primary side neutral to ground, why would you? the POCO's side is already referenced to ground on their side (typically solidly grounded wye secondary). A typical system here would be Delta-Wye.

An ungrounded wye-grounded wye would be considered very unstable. I've never heard of a wye-wye without grounding both sides. If you install a delta-wye, then no problem.

Jim T

 

[jcormack]

An ungrounded wye-grounded wye would be considered very unstable. I've never heard of a wye-wye without grounding both sides. If you install a delta-wye, then no problem.

Jim T

OOPS Sorry, forgot to say that when we do that we always use a High Impedance ground system on the secondary (1 amp at 480, 4 amp at 4160) - if solidly grounded, then yes ground the primary. All our systems are resistance grounded.....I often forget that when talking to other industries

Also Y-Y passes harmonics straight through, as well as circulating current issues.

As Jim mentioned a wye primary can cause unstable voltage levels on the primary under certain conditions (mainly single-phasing on the primary side - where some ferroresenant conditions may cause voltage levels to spike). Ungrounded wye primaries are not recommended when connecting to the POCO or other others. All of my installations like this are under my control. I jumped in as I don't like Y-Y as a general rule - I use D-Y whenever possible, D-D as a second choice with a zig-zag & use Y-Y only when I have to

 

[robbietan]

the utility in our area would like their system to be protected from disturbances from the customer loads, hence they insist on a delta-wye transformer. the utility tansformers are often grounded wye-delta and they require customers a delta-grounded wye for connection

 

[jtester]

the utility in our area would like their system to be protected from disturbances from the customer loads, hence they insist on a delta-wye transformer. the utility tansformers are often grounded wye-delta and they require customers a delta-grounded wye for connection

If I have primary metering and my own distribution, how would they know my transformer connections? Many utilities, perhaps not yours, use Y-Y in higher voltages, 25kv for example, because of the cost of the higher insulation levels.

Jim T