maximum three phase generator current imbalance rule of thumb? 2

[takoateli]

I have a 110KVA three phase generator which has the following current imbalance
ph1 60A
ph2 80A
ph3 110A

Is this within the acceptable range? The genset was made in Brazil. The generator's nameplate says it's 110/100KVA, 263A max and .8pf max.

Is there a rule of thumb for what's a tolerable amount of imbalance between phases?

This generator is three phase but we have no three phase loads. For a while we had a three phase delta/wye step down transformer bank plus three other independent single phase/split phase step down transformers, but a lightning strike took out our three phase step down bank and now we're running only single phase/split phase step down transformers. I might be able to balance out the load a little better. I'm wondering how far I have to go.

Is an imbalance of 60A, 80A, 110A outside the acceptable range?

Thanks!
Greg

 

[wayne440]

If it were my own, I wouldn't be concerned. Even your "loaded" phase is far from 263A.

 

[takoateli]

That's what I used to think, then I learned that the concern is the condition the unbalance causes in the rotor. You end up smoking the rotor due to high currents. There's no metering on the rotor so you have no idea what you're doing to it. But some pros clued me into that.

Greg

 

[wayne440]

I think you are making too much of the issue in your case, however I have only been dealing with gensets for 30 years. For what it is worth, I have not yet burned up a stator or rotor, but then I'm not the one that has to fix your set if it breaks. Accordingly, I advise you to do whatever makes you comfortable.

 

[takoateli]

Wayne,

Well 30 years is a lot of experience! Your opinion makes me feel more comfortable about running the generator this way.

Thanks!
Greg

 

[ScottyUK]

For such a small set I think Wayne is right - don't worry about it. If this was a big utility-class machine of a few hundred MW output then the continuous maximum negative sequence current is only a few percent.


----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[takoateli]

Thanks! I appreciate that.

Greg

 

[waross]

Scotty, can you comment on the possible rotor effects of this practice:
It is common for sets of this size to be reconnected for single phase use. no matter what connection is used (zig-zag, double delta or Collins (bar-diamond)) The following will apply;
The current in one phase will be in phase and will supply 1/2 of the load.
The current in the second phase will be equal but will be 120 degrees leading and will supply 1/4 of the load.
The current in the third phase will be equal but will be 120 degrees lagging and will supply 1/4 of the load.
Although the currents will be equal in each winding, will the phase angle differences cause rotor heating?
In practice the rotors in small diesel driven machines are well able to self cool, but would one of your large machines be able to withstand this treatment.
Thanks Scotty.

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

 

[ScottyUK]

Hi Bill,


The little 'uns have got me awake early today and I am desperately in need of a second coffee, but I think the following is true:

With a three-phase set connected for single phase the currents in the windings are the same, so there's no current imbalance. The condition is more-or-less the same as that which would exist if the machine was operating with a load of different PF on each phase. The phase angle between the currents mean that they do not form a balanced symmetrical 3-phase set, so using symmetrical component theory to describe this set of currents there must be negative and/or zero sequence components present too. That being the case then yes the rotor will see an additional heating effect.

I don't agree with the 120° angles you mention though - if you draw out the voltages as a three phase group with normal 120° phase displacement and then superimpose the phase currents one is in-phase, and the others are at 60° lag and 60° lead. If you put those values into a free symmetrical components calculator then it spits out some fairly alarming numbers. A big machine would definitely be in trouble with this kind of imbalance.

Hopefully someone will check the above for errors and correct me where necessary.


----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[ScottyUK]

FWIW, here's the results I got:

http://files.engineering.com/getfil...8d01-4d17-aa8c-d401f2d5f586&file=untitled.JPG

----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[ScottyUK]

On reflection I probably haven't played with that calculator enough before posting a result. I'd incorrectly assumed I could enter the phase displacement between current and voltage for each phase when in fact the current is all zero sequence. There will be a large 50Hz (60Hz) component circulating in the rotor forging which will heat it up. It won't be as severe in terms of heating effect as a 100Hz (120Hz) NPS current, but it will be bad enough.

Told you I needed coffee!


----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[takoateli]

Guys,

I really appreciate all the comments. One thing I take away from what you guys say is that apparently there is no current imbalance rule of thumb for generators of this size (110KVA)

Something else I take away is that nobody is saying "AAAAH! YOU CAN'T RUN THAT GENERATOR LIKE THAT! So apparently a current imbalance of 60A-80A-110A isn't alarming to anyone.

So I'm making the assumption that we are not damaging the rotor running like this and we are not shortening the life of our generator.

I do plan on getting the balance better over the next few days.

Greg

 

[takoateli]

Here's what I got on the Symmetrical Components Calculator

I entered the phase magnitudes (currents) and phase angles and it calculated the rest. Is that right?
I'll be googling pos neg and zero sequence. I don't know what they mean.
Data I entered:

Phase 1 magnitude : 60 Phase 1 angle (deg) : 120

Phase 2 magnitude : 80 Phase 2 angle (deg) : 120

Phase 3 magnitude : 120 Phase 3 angle (deg) : 120
__________________________________________________________________________
Data the calculator calculated:

Positive Sequence magnitude : 14.53 Positive Sequence angle (deg) : 336.6

Negative Sequence magnitude : 14.53 Negative Sequence angle (deg): 263.4

Zero Sequence magnitude : 83.33 Zero Sequence angle (deg) : 120

 

[Hoxton]

Sorry, but I cannot remember what ANSI; NEMA; IEC actually say, but please remember that even under unbalance, the generator line current must not exceed the rated current. Unbalance always means a derate from rated kVA (happy to be corrected!)

I think the unbalance for salient pole generators is 10% and cylindrical rotor (turbo) generators is 5%.

 

[waross]

That may be for salient pole machines above 2 or 3 megawatts.
Right on about the KVA.
There used to be a lot of 10 lead generators on the surplus market. The bar-diamond or Collins connection is not well known. These machines gave good service as single phase standbys using two phases and ignoring the third phase. (120:208 Volts)

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

 

[takoateli]

Guys,

Thanks so much! I'm getting quite an education. Since this is a 110KVA generator driven at 1800RPm is it a given it's a salient pole device?

If it is a salient pole device, the I need to stay under 10% current imbalance? I'm very far from that. I think I'd better get to balancing the loads ASAP.

Greg

 

[wayne440]

Greg- I remain convinced that given your relatively small set, you are fine. If both the generator and load were 10 times larger, then maybe time to start worrying a little. But as I noted earlier you will be the one fixing it, not me so...

Onan(once the "big" name in standby sets below 500KW or so, and still a major player), published a tech bulletin #T-009 that states (for their models) any conbination of single and 3 phase loads is acceptable as long as rated current is not exceeeded on any lead. I have used T-009 in the absence of specific literature for sets such as yours for years. A download of T-009 can be found on the web easily, you may find it helpful.

 

[takoateli]

Wayne,

Thanks! I guess this issue keeps eating at me because I *could* balance the loads better, it's just a matter of climbing the pole and switching things around. I think because I *can* do it, I should and will, just to feel better about the system and myself.

I really do appreciate all the information and knowing that it's probably still acceptable as it is now.

I'm trying to make contact with the company in Brazil that made the set and see if they have any guidelines as well. Till I hear back from them I'll just go by T-009 for now.

Thanks!
Greg

 

[waross]

Your machine will be a salient pole machine.
Don't worry about your balance.
If you add loads try to do it in such a way as to improve the balance.
Don't exceed the rated current on any terminal and you will be fine.
Serious unbalance problems don't start until the machine gets 10 or 20 or more times larger than yours.
I had a 1.2 MegaWatt machine with a bad balance. probably worse than yours. After years of service (and after I had left) someone convinced the manager that the rotor should be checked. They removed the rotor and shipped it first by ship and then by truck to the shop. No tests were done onsite. In the shop they claimed that the megger reading was low and rewound the rotor. I am extremely skeptical that there was any real damage.
This machine was over ten times as large as your machine and the generator was not showing any signs of distress.
As I said, I am very skeptical about the whole procedure. I seriously doubt the competence of anyone who would remove and transport a rotor with no testing whatsoever onsite.
This machine was over 10 times as large as yours and there was no real evidence of damage from unbalanced loading.
As machines get larger, they become less able to reject the heat from a rotor. That is what gives rise to overheating issues in larger machines.
For what it's worth. I installed and serviced several dozen residential sets for over 15 years. The majority of these sets were reconnected three phase sets with the inherent unbalance due to leading and lagging currents that Scotty and I discussed. Although the sets were on standby service, several hours of running was common every Sunday. I NEVER HAD A ROTOR FAILURE.

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

 

[takoateli]

Bill,

Thanks so much! Last night I was visiting in the village (I live in a Yanomami Indian village in the Amazon jungle) at a friends house and we were shooting the breeze talking about Yanomami language and culture when there was a really big bang and a ball of light right next to me. I was sure someone had stuck the muzzle of a shotgun under his outside wall pointing into the house and pulled the trigger by the sound and ball of light, but when the lights immediately went out I realized it was a lightning strike. It was raining at the time and there was lightning far off but nothing close. Even if I had had my computer with me (our generator is controlled via the wireless network I installed on the base and we can start and stop it remotely) I wouldn't have put the generator off because it seemed like the storm was not approaching. Anyway, after troubleshooting this morning I found three high voltage switches blown open, two of which had their flexible wire that moves with the switch arm blown out, and worst of all we also lost a 15KVA transformer. With that transformer out more than half of the village's Indian houses will be in darkness till they/we can get another transformer. We also lost a piece of wireless networking gear and I lost the power supply for my MacBook Pro which was plugged in back at my house but happily I had a spare.

The good news is after the rewiring I had to do the current balance is now 48A 70A and 80A so it's even better than it was before.

Thanks so much for your help. If you want to see pics of the transformers and the place here you can go to gregihnen.me, that's my site. I didn't put the link because I don't know if that's allowed on this forum.

Thanks again to all you guys who commented. We all here really appreciate your helping me keep the lights on.

Greg