240/480 Transformer for 277/480 Generator

[machodelmonte]

Hi all, thanks in advance for any help!

I'm running a microgrid in rural Haiti with approximately 850 metered customers connected to a diesel microgrid. I am in the process of upgrading the grid from it's current service, which is lines of 120/240 triplex distribution running through town and to the customers. Our lines got so long that we had lots of voltage drop at the ends of the lines. So we're now in the process of installing a 13.2/22.8kV medium voltage line with 120/240 split phase distribution across 4 transformers located throughout the town.

The new generators are 2 - Caterpiller 100kVA DE110 diesel units, connected with a double throw (i.e. only one can power the grid with one genset at a time, and switching requires a short power outage for our customers). We requested the generators be wired 277/480 wye. Then we plan to run that through our 3 - 75kVA step-up transformers to boost it to our 13.2/22.8kV 3-phase medium voltage, and then will drop the voltage down to the local 120/240 at each of the distribution transformers.

I know electrical basics but have limited knowledge of medium-voltage, so I worked with a local engineer to design and procure materials for the system. However, the transformers delivered to the site yesterday have me worried: they are rated for 240/480, rather than 277/480, although the step up voltage is correct at 13.2/22.8kV. Since we plan on connecting each line to the transformer, I feared that the 277 voltage would mean our line would be running at around 15.2/26.3kV or so, meaning our service would be higher than the 120/240 it was supposed to be.

I've attached the nameplate of the transformer.

Is there something I'm missing?! Hoping there is a way to make these work as we are in quite a remote part of Haiti; although we need to make sure we have the correct stuff.

 

[winnie]

At a quick glance, the transformers are single phase, and rated to take _either_ 240 or 480V up to 13200V

If this is the case, then you will bank the transformers in a 'delta to wye' configuration, where you connect each transformer 'line to line' to feed it 480V on the input, and 'line to neutral' to get 13200/22860 wye on the output.

This seems correct to me, but I am not actually a distribution guy

-Jon

 

[GoldDigger]

You should be able to use those transformers (three of them!), connecting the low voltage windings from line to line (480) without using the center tap. The voltage on the high side will be correct.

But there is potentially a different problem: It is not clear from the label that these transformers were designed as step up transformers or designed for use as either step up or step down.
If they were designed as step down, you may have a problem with excessive startup surge current when you energize the 480V winding. That may cause the generator(s) to trip out.

You are probably OK, but you need to confirm that with more detailed transformer specs.

A 277/480 transformer would have to be three phase transformer (three sets of windings on one core.)

 

[topgone]

You should be able to use those transformers (three of them!), connecting the low voltage windings from line to line (480) without using the center tap. The voltage on the high side will be correct.

But there is potentially a different problem: It is not clear from the label that these transformers were designed as step up transformers or designed for use as either step up or step down.
If they were designed as step down, you may have a problem with excessive startup surge current when you energize the 480V winding. That may cause the generator(s) to trip out.

You are probably OK, but you need to confirm that with more detailed transformer specs.

A 277/480 transformer would have to be three phase transformer (three sets of windings on one core.)

His generators max at 240V. He can't possibly get a 480 supply!
machodelmonte,
Connect your new transformers into a three-phase bank, 3 X 75kVA; 240V-13.2/22.8kV. Since your transformer capacity is 225kVA, it will work fine with your existing load.

 

[GoldDigger]

His generators max at 240V. He can't possibly get a 480 supply!

You are thinking about his existing generators. The transformers are for the new generators:

We requested the generators be wired 277/480 wye

 

[topgone]

You are thinking about his existing generators. The transformers are for the new generators:

I guess they will have to forego reconfiguring their generator into 277/480 with the new development re arrival of 3 X 75kVA with 240/13.2-22.86kV bank!

 

[GoldDigger]

I guess they will have to forego reconfiguring their generator into 277/480 with the new development re arrival of 3 X 75kVA with 240/13.2-22.86kV bank!

And just where in the transformer nameplate wiring diagram do you see 240 only?
Note that it shows both series and parallel configurations for the two low voltage windings, with no need to connect the center tap to anything in the series setup.

BTW, does "distribution transformer" in the name normally imply only the ones doing the step down or does it apply to both ends of the MV when step up is used?
The way the LV winding is described and the use of the center tap of the LV winding makes me think that this transformer might be designed for step down only.

 

[310 BLAZE IT]

If the generator is wye there must be something done with the neutral. I'm going to say it can't just be grounded. Anyone have a code section on this?

Sent from my SM-N920V using Tapatalk

 

[topgone]

And just where in the transformer nameplate wiring diagram do you see 240 only?
Note that it shows both series and parallel configurations for the two low voltage windings, with no need to connect the center tap to anything in the series setup.

BTW, does "distribution transformer" in the name normally imply only the ones doing the step down or does it apply to both ends of the MV when step up is used?
The way the LV winding is described and the use of the center tap of the LV winding makes me think that this transformer might be designed for step down only.

Please take a second look at the attached nameplate where you can see the two possible voltage connections on the low side.

 

[GoldDigger]

Please take a second look at the attached nameplate where you can see the two possible voltage connections on the low side.

Yes, I see two possible connections on the LV side.
One of them is center tapped 480 and the other is parallel 240. Is there any reason not to use the first configuration with line to line 480, leaving the center tap isolated?
It all depends on the interpretation of the dashed line from one set of terminals to case ground. If that represents removable internal or external jumper all is well.
If B is solidly grounded internal to the transformer, then there is indeed a problem.

And, raising the issue again, the presence of the LV neutral connection on the series winding configuration seems to me to be only consistent with the LV winding being used as an output.
That is also consistent with the tap changer being on the HV (primary) side. To keep the transformer operating in the correct magnetic property region the tap changes to compensate for voltage drop are normally made on the primary side. In this case though since the LV voltage is fixed, then as long as it is kept very close to 240/480 the transformer should be OK.
But given the OP's uncertainty about whether the order was placed properly I would also want him to address that with the transformer manufacturer.

 

[electrofelon]

If the generator is wye there must be something done with the neutral. I'm going to say it can't just be grounded. Anyone have a code section on this?

Sent from my SM-N920V using Tapatalk

I dont see anything wrong with just grounding and bonding the neutral and not using it if its not needed.

 

[GoldDigger]

I dont see anything wrong with just grounding and bonding the neutral and not using it if its not needed.

If you connect the LV line-to-line on a 480Y/277 system (generator output), it is not possible to ground the center tap (whether you use it or not!)
It would not be a neutral.
Now the wye point of the generator will probably be grounded just because operating the generator as an ungrounded system will just add complexity.
(Although I suppose you could see it as a reliability measure?? But the length of the generator to transformer connections will be short and not particularly exposed to damage.)

 

[electrofelon]

And, raising the issue again, the presence of the LV neutral connection on the series winding configuration seems to me to be only consistent with the LV winding being used as an output.
That is also consistent with the tap changer being on the HV (primary) side. To keep the transformer operating in the correct magnetic property region the tap changes to compensate for voltage drop are normally made on the primary side. In this case though since the LV voltage is fixed, then as long as it is kept very close to 240/480 the transformer should be OK.
But given the OP's uncertainty about whether the order was placed properly I would also want him to address that with the transformer manufacturer.

This is an interesting issue. I have been working on some step up to MV, step down to LV setups lately and there appear to be few, if any transformers intended for stepping up LV to MV. Pole transformers are, obviously, pretty much for the utility market so definitely step down, and I am nearly certain that they dont have the NEC required marking as being suitable to be used in reverse. The most "NEC oriented" MV to LV units are dry types and I just looked at hammond's offerings, and didnt see any mention of reverse feed suitability/labeling, but a call would be in order to know for sure. That said, I have reverse fed MV to LV step downs and never had any trouble starting them, although of course if a generator is involved, perhaps we should be more concerned with inrush....

 

[electrofelon]

If you connect the LV line-to-line on a 480Y/277 system (generator output), it is not possible to ground the center tap (whether you use it or not!)
It would not be a neutral.
Now the wye point of the generator will probably be grounded just because operating the generator as an ungrounded system will just add complexity.
(Although I suppose you could see it as a reliability measure?? But the length of the generator to transformer connections will be short and not particularly exposed to damage.)

I agree that if the transformers have a non removable center tap bond, that would be an issue. I was speaking to the wye point of the generator.

 

[Tony S]

 

[GoldDigger]

Looks very nice, and the unconnected wye point of the three LV winding sets is just the configuration to use if you have a 480Y source.
But the generators are 480Y/277, with a line to neutral voltage of only 277V. 480 is the line to line voltage here, and the LV windings of the transformer need to see a full 480.

 

[Saturn_Europa]

Very interesting project. Did you end up using the transformers they shipped?