chillers operate with genset but not with power from utility company 1

[voltesv]

i need help re our chillers.
we have 3 chillers with two compressors each. funny thing is that starting last week we cant operate more than two compressors at a time with electricity coming from the utility company. we can only operate all three chillers with the standby genset power.
the chillers report 'severe phase unbalance'.
we have asked the utility company to check all the wire resistances from the transformer to the chillers and they turned out ok. we have tightened all terminal connections but still the problem exists.

the voltages using utility company are 216/221/216
while the voltages when using the genset are 214/215/214

the genset has a floating ground.
the chiller people say the equipment are ok and the electricity supplied is bad.
the power company says the electricity is ok and the equipment is probably grounded.

i dont know what other info to include here but so far this is what i have.

 

[DanDel]

You have a 2.3% voltage imbalance from the utility. This is not acceptable. Present your findings to the utility, and have them show you where their voltage is accpetable. If the voltages are less than 1% imbalance at the utility connection while that much worse at your chillers, then there is a problem in your system. If not, then it is the utility's problem and must be corrected.

 

[sngpl]

Wee the voltages measured when the chilers were on load... I know you can't run all the chillers by the Utility's supply but did you measure the voltages when at least two of them were in operation?

Apparently the problem lies in the Utility's supply.
What is the kVA rating of the distribution transformer?
Is it overloaded?

Its a tricky situation!!!

 

[Shortstub]

1) Had anything changed between the time when the chillers were OK with utility operation, and now? Or, did you always have a problem?

2) Presuming the noted measurements were phase-to-phase (220V, nominal), what are phase-to-ground measurements of the utility supply w/o the chillers?

3) Ditto for genset operation?

 

[voltesv]

thanks for the inputs guys.
we found out that the ground from the transformer is not connected. the genset also has a floating ground.
is this relevant?
the ground wire from the transformer is only up to the chillers' distribution panel. the chillers are grounded to each of the individual breakers but there there is no direct ground connection from the chiller breakers to the main distribution panel.

to answer some of the questions above, the transformer is rated 2000 kva. we did not have this problem before but the people say that this was also an issue during the time the chillers were installed back in '97. but no one knows what caused it then.

the voltages given above were all done with load, i.e. for the utility co supply with 2 compressors and 4 compressors for the genset.

our electrical engineer is pointing to the improper grounding as the culprit but there is still some doubt in my mind because our grounding system has been like this since the start and it was only now that we encountered this kind of situation.

the utility co. says that phase unbalance is acceptable at the source and that the problem is at the load side.

thanks.

 

[alehman]

Improper grounding would not account for the voltage imbalance seen if those are line-to-line voltages as they appear to be.

When you say "the ground from the transformer is not connected. the genset also has a floating ground.", do you mean the neutrals are not connected to ground? Is there a neutral to ground connection at the distribution panel fed by the transformer?

When you say "the ground wire from the transformer is only up to the chillers' distribution panel.", do you mean the neutral is only connected up to the distribution panel? This would be common. You wouldn't normally need a neutral wire from the distribution panel to the chiller.

Unless your system is designed from the outset to operate ungrounded, the transformer neutral should be grounded. If your system was intended to operate ungrounded, you should consult your chiller supplier as to whether or not their equipment can tolerate an ungrounded system. This will probably require discussion with the chiller engineering folks.

You need to retain the services of a professional engineer to evaluate your grounding situation. Operating a system that is intended to be grounded without a ground is dangerous.

 

[busbar]

ANSI Std C84.1-1995 Annex D seems to imply 3% as an acceptable maximum voltage imbalance. The standard is a consensus of electric-utility engineers and electrical-equipment manufacturers that has been created and revised over many years of discussion.

If I figured right, “216/221/216” works out to be ~1.5%. 1.5% voltage imbalance corresponds to an Annex D motor-horsepower derating of around 3%.

A typical US electric-utility spec is: 2½ percent maximum deviation from the average voltage between the three phases. Nota Bene, this is generally at the building service entrance and not at utilization-equipment terminals. It may be time to independently test the device that’s inhibiting chiller operation, compared to characteristics and numbers listed in its published specs. Additionally, determine the published versus actual time interval needed for an out-of-bounds condition must exist before tripping the compressors.

It is in your best interest to assure that known-accurate instruments are used for the evaluation. Finger pointing based solely on a simple go/no-go indication from an internal appliance-grade protective device can prove embarrassing.

 

[jbartos]

Suggestion: It is better to have system grounding. The transformer neutral without any system grounding can become unstable (rotate) that could add to the voltage imbalance. If the Utility cannot provide the balanced power supply, there may be a need for the line power conditioner.

 

[Shortstub]

Repeat, what are phase-to-ground measurements with genset? With utility?

Note: There is no discussion regarding starting, nor running two machines. Only the operation with a third.

Does the sequence of chillers matter. I had a similar problem with operation of 4-480V gensets in an ungrounded system. Three operated normally, but a 4th tripped the system. Root cause was a an unintentional ground in the control circuit of the generator being brought online! One of its control wires was caught under a wire-duct cover. In addition, one of the first three gensets had an inadvertent ground which went unnoticed because of the ungrounded system. Phase-to-phase short tripped the control circuits of both units, overloading the 3rd and 4th, resulting in a total blackout.

Thus, my question relating to phase-ground measurements. Also, was your system worked on prior to the problem?

 

[DanDel]

voltesv, your problem is the voltage imbalance with the utility connection. Any imbalance above 1% is not acceptable for 3-phase motors. Talk to any motor shop or manufacturer.

Poor or unknown grounding is a very important concern, and must be corrected, but it is not causing your chillers to report 'severe phase unbalance'. Ask the manufacturer why it has this function and what the maximum setting should be.

"the utility co. says that phase unbalance is acceptable at the source and that the problem is at the load side"
What is the voltage imbalance at the source? The cause of the voltage imbalance may very well be in your system, but start with the actual numbers from the utility. If the values at the source stay within 1% when the chiller attempts a start, begin looking at your system for load imbalances or poor connections.