Use 2 phases of a 3 phase generator?

[takoateli]

I'm a technician on a mission base in the Amazon. A few days ago we suffered a lighting strike to our electrical system when the generator was not running. The strike damaged one of our three step up transformers for our electrical distribution system. In a pinch could I run the generator only using two of it's three phases? Can I leave the third phase unloaded? I know people always say you can't but if you be sure to not exceed the rating of any one phase does it matter if the other phases are loaded or not?

The transformers are oil cooled 75KVA pole mountable units. The high voltage sides are wired delta, with the output like this:

trans1---------trans2
\ /
\ /
\ /
\ /
trans3

Also since we lost one transformer could I use just two transformers with the high voltage sides wired like this and just use two phases?:

---------------
trans1 trans2
---------------

Thanks!
Greg

 

[takoateli]

It's me again. On second though, about using just two transformers, I think it would work like this:

o---trans1----o----trans2---o
1 2 3

The three phases of the distribution system would get connected to the "o"'s and only two of the three line to line connections used:

Load on 1 to 2
Load on 2 to 3
Disconnect any loads on 1 to 3

Would that work?
Thanks!
Greg

 

[davidbeach]

No. You might get away with it if you rewire the generator for single phase output, but you'll not have a pleasant experience having it wired for three phase and only connecting two of the three phases.

 

[takoateli]

Thanks for the reply!

 

[waross]

It depends on the size of the generator and the type of AVR.
A large set, up in the megawatt range you will damage the rotor. A smaller set, below about 500 KVA the rotor may be able to cool itself.
Around a megawatt I have seen sets run 5 or more years with unbalanced loads before the rotor needed to be repaired.
The AVR should be on the loaded phases. With a three phase averaging avr you may be able to get by with an adjustment if the load is fairly steady.
WATCH YOUR LOADING. The engine will easily be able to drive overloads that will fry your set.


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

 

[takoateli]

Waross,

Thanks so much! It sounds like you have lots of experience. I really appreciate you taking the time to share that!

This is a 110KVA generator You can see it at my web site. The site is gregihnen.me. I didn't put a link because I don't know if this forum allows that.

I'm starting to get the picture now. A three phase alternator being unbalanced causes overloading of the rotor, so even if you keep the current within the specs on the stator, you'd could be causing excessive currents in the rotor. Since there's no metering on the rotor you'd have no idea what's happening. Very interesting.

Well you answered my question once and for all. I'll have to keep the phases balanced.

The plan is today to separate three step down transformers, steal one and use it to replace the step-up transformer, and wire the remaining two step-down transformers for split phase to feed the houses. The distribution to the houses wasn't supposed to be three phase but the people that installed the system didn't know how to do anything else.

Since I've got a pro on the line I'd like to ask you another question regarding the possibility of rewiring the generator's alternator to change it from 3 phase to single phase. I've read that can be done but you lose a third of the alternator's capacity. Is that correct? If so we'll stay with three phase on the HV distribution system and use split phase coming off.

Thanks so much! I can't tell you how much I appreciate your help!!!

Greg

 

[waross]

Big utility size alternators typically have relatively long slender rotors which do not cool well. Unbalanced loading causes extra heating in the rotor and the rotor tends to overheat under unbalanced conditions.
The smaller diesel sets have rotors that may b e short in relation to the diameter. They also may have salient poles rather than being a smooth cylinder. They will also generate extra heat when the load is unbalanced but they are much better able to shed the extra heat and the temperature does not rise as high. Typically single phase sets in your size range are re-connected three phase sets which cause the same rotor heating issues even when the single phase load is balanced (three wire systems). The sets are typically not de-rated for single phase operation.
If you have a delta distribution system the system will operate with one transformer missing in what we call (in North America) "Open Delta". The capacity of the transformer bank will be 58% of the capacity of the original three transformers.
However moving the other transformer in sounds like a better solution.
Yes single phase will cost you 1/3 of your KVA capacity but only about 17% of your KW capacity. However if your power factor is low you may run up against your KVA limit before your KW limit.
Your 110 KVA will deliver 88 KW.
On single phase it will deliver 73 KVA and 73 KW, whichever comes first.

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

 

[takoateli]

Bill,

Thank you so much! This is all such great information!!! I can't tell you how much I appreciate it!

Today I took the cluster of three step-down transformers which were wired for wye and removed one (to use it as a step-up transformer at the generator) and wired the two remaining transformers for split phase.

The step-down transformers have four connections as there are two low voltage windings. The transformers were originally wired wye with the two low voltage windings in each transformer wired in parallel. Then one side of each transformer's low voltage output went to the common neutral, and the other end of each transformer's low voltage output was a "hot" phase.

I rewired the low voltage side of both of the remaining transformers putting the low voltage windings of each transformer in series yielding three connections, the two "hot" ends and the center connection which is neutral.

One question I have is this. We now have two step-down transformers and each one is independent with a center tap that will be our neutral and the two end taps that will be the "hot" connections. Our low voltage distribution bus has only five wires. Can I combine the two neutral connections, in other words can the two neutral connections from both transformers share a common wire, and then each transformer's hot connection will get it's own wire. My understanding is that the transformers are electrically isolated from each other and ground, so they could safely share one neutral wire, just as they did when they were wired wye. Now that they're wired for split phase they could still share one neutral, correct?

So today we took the one step-down transformer we pulled from what was the three transformer step-down wye configuration and used it to replace the damaged step-up transformer which had a bad short. I rewired the two remaining step down transformers and wired them for split phase. We then powered up the system and with the shorted step-up transformer out of the picture the generator ran fine (when the shorted transformer was still in the circuit the generator would bog down hard), but the village was drawing very little current and we were getting strange voltage readings on the low voltage side of the step-down transformers. One transformer was putting out the normal voltage but the others were not. We did a lot more troubleshooting and it appears that there's a second(!) step-up transformer that's out. It's apparently it's failed open rather than shorted.

So it looks like we're going to have to end up taking yet another one of the step-down transformers and put it in service as a step-up transformer. That will leave one 50KVA transformer to feed 7 houses (which were originally fed from three 50KVA transformers in a wye) but these are jungle houses not North American houses and they present much less of a load so we should be OK. Our load on the generator is way under it's 110KVA rating. Besides this one 50KVA step-down transformer that will remain, there is also a 25 KVA step-down that feeds another house which uses a lot of power, and two 15KVA step-down transformers which feed all the Indians' houses so we should have enough individual loads to juggle around to balance the three phases of the generator.

Thanks so much! I eagerly await your next reply!!!!

Greg

 

[cuky2000]

As you mentioned above, If the load is "way under 110KVA generator rating" and there is well balance load, I believe that should not be a major problem to use the open delta as a temporarily connection.

Beware that one transformer is producing reactive power which the other one is consuming. This reduce the bank capacity in approximately 10% (i.e. 2xTranf KVA*90%). Therefore, made sure that you not overload the derated transformer bank.

See if the enclosed diagram could help.

Good luck and let us know if you see any smoke.

 

[takoateli]

Cuky2000,

Thanks so much! I really appreciate that information!

One big question I still have is where I have two 50KVA step-down transformers which each are wired with their two low voltage windings in series to give 120V from the center tap which will be neutral to the ends and 240V end to end, can I tie the center taps together and let them share one wire on our distribution system? Our low voltage bus only has five wires. If those two transformers' center taps can share one wire that would work nice. I don't see why they couldn't share a neutral. In a wye configuration the transformers share a neutral. With the transformers wired as stated above for split phase and sharing a neutral it would be like four windings sharing a neutral (see attached file) as opposed to 3 windings sharing a neutral in wye configuration.

Thanks!
Greg

 

[waross]

Sharing a neutral is not a good idea when the transformers are on two different phases. The neutral currents don't cancel because of the phase angle difference. Neutral current may be 141% of line currents, or more with different power factors on the two transformers.
Do you want to go single phase? The conversion of the plant is not difficult.
The norm now for new sets is the double delta connection. It is nice and symmetrical looking.
The zig-zag connection is preferred for older sets, particularly when the leads are not well marked.
Some older sets are ten lead sets. For these we use the bar-diamond or Collins connection.
Check your set. It is probably a 12 lead. Let me know and I can walk you through the changes. You can do this change safely with no markings on the generator leads.

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

 

[waross]

By the way, one of the unpleasant experiences Mentioned by David Beach about loading only two phases may be unequal voltages.
Please check your AVR. If you give us a make and model I may be able to check for possible issues with a single phase conversion.

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

 

[takoateli]

Bill,

Thanks!

The generator is running as I write this. We got it going this evening. I moved a transformer down to the step-up bank to replace the one we lost to lightning, so we're back with three working transformers at the generator, wired wye on the low voltage side and delta on the high voltage side. We have only three wires on the high voltage distribution system since the HV side is wired delta.

The transformer I moved to the step-up bank I took from a bank of step-down transformers where it wasn't needed. We had three 50KVA transformers (wired just like the step-up bank: in delta on the high voltage side, wye on the low voltage side) were we only really needed 50KVA capacity total. There was a downside to having the step-down wired for three phase output which is we have no three phase loads. Using two legs of three phase which only yields around 208V to power 240V single phase household appliances is not optimal. So I had a number of incentives to break up the three phase step-down bank and convert to split phase:

- I needed to steal one of the transformers
- we didn't need three phase power
- switching to split phase gives better 240V power
- now the step down transformers are not interdependent. When we had three step down transformers in wye/delta service if one went down it took the whole bank out of service. Of the three 50KVAs that were the three phase bank we're using one in the step-up bank, another one is being used for step down, and the third one is now out of service and is a spare, disconnected from the system so it can't get damaged by lightning while it's in storage.

I could leave the system as it is which is with one 50KVA off line and the following transformers in service: one 50KVA step-down, one 25KVA step down, and two 15 KVA step-down transformers. I can get reasonably good balance that way. But I could get better balance if I had the two 50 KVAs in service.

I trust your knowledge and experience but I still have one burning question. If the 50KVA transformers are in service separately (not sharing any common connections, and their low voltage windings are wired split phase and each transformer has it's center taps grounded to be neutral/ground, then the center taps of both transformers would be a zero volts or very close to it with respect to ground. If both transformers center taps are the same voltage then it seems like they could be connected together with no ill effects. If there's no continuity between the low voltage windings of the transformers other than at the center taps then it seems like there could be no current flow between the two transformers' low voltage circuits. Current from transformer 1 couldn't flow into transformer 2 via the neutral because there would be no return path in the low voltage circuit.

Am I missing the boat?

Thanks!
Greg

 

[waross]

You haven't missed the boat. You may run a common neutral between the single phase transformers. It is a good idea to use a ground rod at each transformer and at each service. Distribution circuits are an exception to the "Only one connection between neutral and ground!" rule. It is called "Multiple Protective earthing".

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

 

[ccjersey]

Using a common neutral (the 5th wire in your case) can result in overloading that conductor since unbalanced current on both transformers Line-neutral loads will have to return to the 2 transformer bank on a single wire. Balanced line-neutral currents on each transformer do not return on the neutral because of the split phase transformer arrangement, only the unbalanced current.

I believe the worst case would be maximum 120 volt load on a single line out of each transformer. I assume that is what Bill was referring to when he posted the 141% figure above. Your neutral may be large enough to carry this with no problem in the unlikely event there was ever maximum load on a single line from each transformer and none on the other lines. I suppose there is some load diversity even in a small village like Cosh.

Interesting project! Makes me appreciate paying my electric bill.

 

[takoateli]

Guys,

Thanks so much! I really appreciate it! I see your point about how the imbalance current from two transformers could add up. The one good thing is they'd be 120 degrees out of phase so that should help a bit.

Our houses are reasonably well balanced and the conductors are large enough that I believe we'd have no problems.

I really do need to bring the other 50KVA online to better balance the three phases of the generator's output. And to do that I need to share that neutral.

Thanks again!!!
Greg

 

[waross]

The 120 degrees out of phase doesn't help. It is the reason that the currents increase instead of cancelling.
But. in agreement with ccjersey, you are probably okay.

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

 

[takoateli]

Bill,

My understanding is that if the currents are zero degrees out of phase they'd add completely, if they are 180 degrees out of phase they'd cancel completely, and if they're anywhere in between there is some cancellation. It might not be all that significant at 120 degrees instead of 180 but at least it's going in the right direction.

Does that sound right?

I really appreciate you guys taking the time to instruct me! These forums are such a great resource because of people like you.

Greg

 

[waross]

In a two phase system at 90 degrees equal currents combine to 141% of one current.
At 120 degrees, the neutral current is equal to either line current. Not good if you are in an area where the neutral conductors are often undersized.

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

 

[takoateli]

Bill,

That's great to have those figures! Thanks!

Here the neutral is not undersized so I think we'll be fine. In the houses the balance between the two legs of the 240 isn't too bad so I think we'll be OK.

What does throw the balance off here is the way the houses are wired. These are houses in the jungle and everyone has a big inverter to carry the 120V loads in the house when the generator is off. When the generator is on the inverter passes through the generator power to it's loads and the inverter also charges it's batteries. The inverter is 120V in and out so it only draws from one leg of the 240V from the generator. Since the inverters are 120V in and out each house has two breaker boxes, a 240V box fed from the generator. That 240V breaker box feeds the 240V loads such as drier, hot water heater, one leg feeds the 120V input to the inverter, and some 120V load not wanted on the inverter such as fridges and freezers and 120V hot water heaters. So the inverter's feed from the 240V breaker box goes to the inverter (120V), through the inverter when the generator is on, out the inverter output to a breaker box that is only 120V with the two hot buses bridged. That 120V breaker box is powered by one leg of the house's 240V when the The balance in the 240V box is good. But the 120V feed out of the 240V box that goes to the inverter the inverter uses to charge it's batteries which is a reasonably heavy load plus the inverter passes through (a big relay) generator power to power to the 120V breaker box which has all the lights and outlets so that's all the small appliances, TVs, computers, fans, etc that the user wants to have inverter power when the generator is off. So that leads to some imbalance on of the legs of the 240V coming into the house. But overall it's not bad. And I can swap the legs in the 240 breaker box on some of the houses to get the neutral current even lower if need be.

I probably could just leave well enough alone, our current balance at the generator on it's three phase output is 60 amps, 80 amps and 110 amps. Before the lightning strike and reconfiguration of the system the currents were all around 100 amps and the difference between legs was normally only around 10 amps between them.

Greg