Unbalance current in generator phases 1

[SyedUmairAli]

Hi all,
I encountered a problem that we are running 10 generators of 415volts in parallel. In 2 generators out of 10 generators, we have 100 ampere high in B phase. We also verify this high current by other sources too, so what is the cause behind this??? If we have unbalanced load so every generator should have same problem because all generators run in parallel.So please help in this regard.

 

[waross]

Generator size?
Normal full load current?
Age of mechines/installation?
Has this issue been present since new or has it developed recently?

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

 

[SyedUmairAli]

1064kW
1850Amp
2 years
Not confirmed...i have noted this since last month.

 

[waross]

That's only a little over 5%. You may have uneven air gaps in the two subject generators.

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

 

[SyedUmairAli]

Yesterday our one generstor tripped on short circuit/overload indication...when we checked the trend we observed 125amp high in B phase of that generator...we did complete insulation resistace test of generators and cables but nothing found suspicious...what is the reason behind this????we completely inspected rotor and stator...excitation system but no abnormality found...

 

[RodneyDixon]

What are the pitch angles of the generators, are they the same?

 

[davidbeach]

It could be useful to take one of the generators that exhibits the high B phase current and roll its connection to the system. If that generator winding continues to have high current the problem is likely with the generator, but if the problem remains with B phase it is a system issue. Minute differences in connection resistance can make for very noticeable differences in currents.

 

[SyedUmairAli]

Rodney generator pitch is 5/2 & 3/2...

 

[SyedUmairAli]

David...how can i roll the connections???generators run in parallel...

 

[RodneyDixon]

Have you considered the different pitch angles to be a possible source?

Paralleling dissimilar generators with different pitch angle can give rise to high levels of current flowing on the neutral/harmonic current (imbalance). Have you measured the neutral?

Best results are paralleling generators with same pitch angle.

See the following paper for additional information: [URL unfurl="true"]http://power.cummins.com/sites/default/files/literature/technicalpapers/PT-9016-P2-Dissimilar-en.pdf[/url]

 

[crshears]

David...how can i roll the connections???generators run in parallel...

I don't intend to speak for David, but what I've always understood that term to mean is to [a] take a full set of system and generator phase current readings with the unit in question on load, then remove it from service, [c] establish isolation and work protection, [d] disconnect A phase generator winding and connect it to the B phase of the system, disconnect B generator phase and connect it to C phase of the system, and disconnect C generator phase and connect it to A phase of the system, [e] remove work protection, then [f] place the unit in service; once loaded, [g] take another full set of readings and see which phase now shows high or low current compared to the previous readings. The results will indicate whether the problem is system induced or within the generator itself.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[davidbeach]

Yep, that's pretty much it. May or may not actually be possible, but it would answer a lot of questions. Minor variations in very small impedances, either system or generator, can have significant impact on the numbers.

 

[catserveng]

The winding pitch numbers don't sound right, typical generator winding pitches in that size range are 2/3. 4/5, 5/6 and 3/4 ( 3/4 mainly in oil field applications). A single line would be helpful, along with some details on which generators are the problem, such as are the 8 generators not affected one pitch and the two that are a different pitch?

Who makes the generator end can have an influence, one manufacturer I know of uses a process called "square lam" in how they cut the stator steel, it results in a slight phase to phase variation that can cause the types of problems you are seeing. Before you roll phases (make sure you maintain sequence if you do and recheck rotation before paralleling again) I would take a pretty complete set of measurements with a high quality test instrument. We had a number of generators that had the phase leads improperly marked and had to do as David described to get everything playing nice again.

I recently worked on a system with a similar complaint that ended up being a combination of some damaged CT's (mounted out in the generator) and some wiring problems giving reading errors. A good quality three phase analyzer may be helpful to make sure you don't have something odd going on and to help you see everything at once.

Hope that helps, MikeL

 

[SyedUmairAli]

Rodney,

We have about 40 to 60 amperes on our neutral. Yes rodney i know pitch angle problem in parallel generators but this system is very old. Expansion take place years by years that why here we have different pitch configuration generators.

I will share our SLD in later evening.

I have confirmed from individual three meters that one phase ampere was high.

 

[omidsyt]

Hi Dears

One of our generators output current has unbalanced current.

What is cause of generator unbalanced current?

What is its cause possibly?

Six similar Genset connected to one bus. One of those Gensets have unbalanced current.

Is it possible its because of air gap of the failed phase?

What is solution for this problem?

What is cause of generator unbalanced current?

Regards

 

[oneninefivenine]

Set up some power quality equipment and look at 3rd, 5th, 7th harmonics. Are the two gens running alot higher temp?

 

[omidsyt]

Dear oneninefivenine

I measured frequency with HoldPeak HP870N which shows first harmonic 50 (49.98)Hz for three phase, however S-phase at first was about 234 Hz and after one second it change to 50 Hz.

All gens are work in a place with similar air conditioner design and so on.

What do you think?

 

[omidsyt]

All Dears

Thank you.

Here it is our power plant gens currents:

gen1:

R=1677

S=1596

T=1540

gen2:

R=1524

S=1362

T=1360

gen3:

R=1220

S=1394

T=1351

gen4=

R=1239

S=1188

T=1195

All bus voltages are 400v.

What do you think about this?

 

[waross]

What David said, roll the connections.

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