Transformer connection 2

[surineman]

We have a utility supply of 24.9/14.4 kv.
The project is a new water treatment plant and the MCC would be 600v.

What is the best selection for primary and secondary connections?(delta-wye-...)
What are the parameters to be considered?
Any reference would be appreciated.

Regards,
Surineman

 

[RalphChristie]

surineman

Check

http://www.beckwithelectric.com/powerlines/powerlines-37.html

for the pros and cons of the different transformer connections.

 

[mc5w]

347Y600 volts solidly grounded or resistance grounded would be the superior systems. Since circuit breaker and fuse cost is equal except for the main breaker the choice is up to you. On a resistance grounded system the main breaker should be 4 pole that disconnects the neutral so that you grounding resistors can be automatically deenergized. However, a resistance grounded system should be metered with a form 9s meter with isolating voltage transformers that have a solidly grounded 115Y200 volt secondary. Same goes for your own power monitoring equipment. For revenue metering and power monitoring of the service you want the metering to agree with the power source, not the load. For loads you can just as easily meter 3-wire 3-phase for loads that do not use the neutral.

Ungrounded 3-wire 3-phase is banned by law in New York State unless it is a separately derived system that is confined to a single building.

You still need both service and feeder ground fault protection for either system that is automatic. For branch circuit ground faults a current transformer and cheap ammeter with manual tracing will work on a resistor grounded system that has say a 1 hour ground fault delay at the service and 1/2 hour at the feeders. However, getting integrated ground fault protection that is that slow is hard to obtain.

A resistance grounded system is also less painful in a certain body part if you have a double ended switchboard, cogeneration, or both. Solidly grounded is less painful in that all ground faults will clear automatically.

 

[mc5w]

I forgot to mention that 277Y480 volts solidly grounded might be a less expensive system depending on circuit breaker cost. You might want to consider small 277Y480 volt and 120Y208 volt dry distribution transformers with 600 volt delta primaries for your smaller loads.

 

[davidbeach]

Just a note on voltage designations - Per the IEEE, the voltages listed above should be 480Y/277V and 600Y/347V. I don't have the full reference at hand at this point, but for three phase systems, the line-to-line voltage always comes before any line-to-neutral voltage. Single phase voltages are listed in the opposite order; so minus other designations it is possible to distinguish between 240/120 and 120/240. 240/120V (without the standard at hand, I can't remember exactly what should be in with the slash) is a three phase delta with the mid-point of one winding grounded to provide the 120V. 120/240 is a single phase voltage, the most common US residential service voltage, with two lines, phased 180 degrees apart, with a neutral at the mid-point.

There are about a dozen specific examples, and I believe they are in the Red Book.

 

[mc5w]

First Energy's documents list the voltage as 277/480 volts. Same for the UL voltage rating of circuit breakers and pnaelboards.

Somewhat strangely, SquareD lists their type NF panelboards and circuit breakers in the Digest as rated 277/480 volts but the panelboard nameplate will usually read 480Y/277 volts.

It does not really matter what the order is, just as long as we the lower number is the phase to neutral voltage. The only possible confusion is that there is such as thing as 120/240 volts single phase 3-wire and 240/120 4-wire delta 3-phase. However, the latter system should always say, $-wre delta 3-phase" to avoid confusion especially if you have the bogus 240/480 4-wire delta 3-phase system where the high leg is 416 volts to neutral.