Generator Cannot Charge UPS
Generator Cannot Charge UPS
(OP)
We have a 60kW 120/208Y 3 phase generator connected to a UPS through a 4 pole ATS (see attached simplified one-line). The normal power to the UPS is through a 75kVA delta/wye transformer. The UPS is an Eaton (Cutler-Hammer) Powerware Blade 48kw unit with batteries and inverter. The UPS works fine on utility power and charges the batteries and supplies the critical load. When utility power is interrupted, the UPS powers the load from the batteries, the generator starts, and after about 4 minutes the UPS rejects the standby power source (generator) due to low frequency, which it says is between 48 and 55 Hz.
Measurements on the generator output show that both the voltage (206 to 217) and frequency (59.8 to 60.1 Hz after initial connection of load) are fine.
This is a new installation for the UPS. The generator worked fine with an APC UPS however the UPS was changed in order to increase its capacity.
Is it possible that there is some incompatibility between the generator and the UPS? The UPS manufacturer puts the blame on the generator.
Any other suggestions will be appreciated.
Measurements on the generator output show that both the voltage (206 to 217) and frequency (59.8 to 60.1 Hz after initial connection of load) are fine.
This is a new installation for the UPS. The generator worked fine with an APC UPS however the UPS was changed in order to increase its capacity.
Is it possible that there is some incompatibility between the generator and the UPS? The UPS manufacturer puts the blame on the generator.
Any other suggestions will be appreciated.






RE: Generator Cannot Charge UPS
RE: Generator Cannot Charge UPS
A lot of frequency circuits have issues with this.
I have a DMM that has a frequency display. Often I must select other unexpected voltages before it outputs the correct frequency. I have to know what frequency I am looking for then 'hunt for it'.
Keith Cress
kcress - http://www.flaminsystems.com
RE: Generator Cannot Charge UPS
I consider an analogue frequency meter on a genset to be a rough indication. If I have a frequency issue I get a good frequency meter and check it on the mains before using it.
Your generator should normally run at 61.8Hz at no load and droop to 60Hz at full load.
Bill
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"Why not the best?"
Jimmy Carter
RE: Generator Cannot Charge UPS
I don't know the specific parameters on the blade systems vs the Powerware 9170s, FERRUPS and Unity Systems I'm familiar with, but there can be other parameters other than frequency window and voltage range that must be changed to run reliably on generator power.
For example on some Powerware systems there are settings for slew rate, phase lock count, freq G1 count, Line G1 count, line delta that must be changed for generator operation. These in essence tune how the UPS samples line power purity; even if the frequency is in range, without these changes they frequently won't transfer back to line power.
Other Powerware systems have an interface that needs a contact closure signal from the generator transfer switch to let it know when it is on generator power. When the UPS sees this signal, the unit will change the above related parameters automatically.
RE: Generator Cannot Charge UPS
I haven't worked with the powerblade style of UPS so don't know what sort of circuit topology it uses, but from the published data I could find on it and a reported effeciency of >97% I can only assume it is some form of off-line style, also as it is stated that the input inrush current is load dependent.
The input power factor is stated as <5% in the brochure. Again I would assume that this is best case (a marketing department wouldn't lie, would they?) and would be with the batteries fully charged and an ideal load. The input power factor is stated as >99% (again load dependent). Given this, during battery recharge you should be drawing approximately 55kW if the UPS was fully loaded.
There used to be a general 'rule of thumb' when sizing generators to supply a UPS load. Depending upon the input rectifier configuration of the UPS (6 or 12 pulse) the generator was sized between 1.5 and 1.8 times the size of the UPS. With a 60 kW generator you are only slightly above the maximum draw of the UPS.
Have a look at the input current distortion when the UPS is charging the batteries and being supplied by the generator. A reasonable high THD with a relatively high load would probably indicate that the generator is under-sized for the application.
RE: Generator Cannot Charge UPS
Keith Cress
kcress - http://www.flaminsystems.com
RE: Generator Cannot Charge UPS
Second I would like to thank everyone for their responses.
Third I would like to let everyone know the resolution.
To answer waross, the frequency measurements were made with a Fluke scopemeter.
To answer DanEE I did call Tech Support immediately after this post. I wanted to get the forum's opinion in addition to Tech Support. My experience with Tech Support is highly variable, it depends both on the company and who you get in contact with at the company. This UPS is so efficient because it basically bypasses the rectifier and inverter when the power is clean. When it senses "dirty" power it goes into "double conversion" mode, which uses the rectifier and inverter to clean up the power. Battery charging load is minimal with fully charged batteries. Tech support was very helpful and suggested that the problem could be due to power grounding or a poor neutral. I asked about phase reversal and they said that phase reversal would cause the unit not to be able to measure the power properties (including frequency) properly due to the software used to calculate the them. This is apparently due to the sudden reversal of phases.
To answer sibeen, I have heard of that general rule before. For this system it would depend on the charging rate of the batteries, any starting loads, and maximum demand. When I got to the site they had already connected a rental 220kW generator and had the same problem so generator capacity probably wasn't the issue. The permanent generator is 60kW and 75kVA.
It turns out as you might have surmised that the generator phase sequence was opposite the utility phase sequence. Both the permanent and rental generator had been connected ABC while the "normal" side was connected ACB. This was determined using a phase sequence meter that I purchased for the occasion since Tech Support had told me that phase reversal could cause the symptoms that were occuring.
My only suggestion to Eaton is that the Installation and Operation manual should mention that phase sequence MUST be consistent between normal and standby sources (although I will agree with anyone who says that it should be checked during installation). You would also think that the UPS unit would alarm "Phase Reversal" instead of just not being able to measure the power correctly. The I&OM is very basic and does not give any circuit diagrams so I cannot tell how they are measuring the power and power quality.
This got me to thinking about standard utility phase sequence so I read the post "How to Recognize Phase A" and I made a comment there.
RE: Generator Cannot Charge UPS
RE: Generator Cannot Charge UPS
Bill
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"Why not the best?"
Jimmy Carter
RE: Generator Cannot Charge UPS
edison123, the UPS is for a small data room only but there were two three phase loads, both air conditioners for the data room.
The client's comment was that "this explains why the air conditioners never came on when we were on standby power". Apparently they never had a long power outage, at least long enough for the rooms to heat up.
RE: Generator Cannot Charge UPS
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Generator Cannot Charge UPS
I sure don't understand the logic. May be some UPS expert will clarify that ?
RE: Generator Cannot Charge UPS
The UPS output is usually synchronised to the bypass source. Unless the UPS can reverse the output phase sequence automatically it will require the correct phase rotation at the input in order to synchronise. The rectifier gate drives will likely be designed for a specific phase sequence too.
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If we learn from our mistakes I'm getting a great education!
RE: Generator Cannot Charge UPS
"Unless the UPS can reverse the output phase sequence automatically it will require the correct phase rotation at the input in order to synchronise"
Doesn't the ups invert from DC ? If so, why would its input phase sequence matter ?
I agree that ups output has to match the original utility supply sequence but I still don't get the input sequence requirement.
RE: Generator Cannot Charge UPS
It shouldn't, however, have only one defined phase sequence for the input. Interesting that the UPS's refusal to transfer (along with incorrect errors) is what gave rise to finding the actual problem.
RE: Generator Cannot Charge UPS
Even on single UPS systems (running on 208vm or 2 phase as many call it), the installation process has detailed checks to make sure the phases aren't turned over.
The system did what it should do, i.e. protect itself from damage and it's amazing that the static bypass was not blown up.
btw, Powerware has a written policy that only their employees can install their 3 phase systems, otherwise the warrany is null and void. The Blade system due to it's unique packaging design to make the 3 phase much more safe to install, is the first exception to this policy.
RE: Generator Cannot Charge UPS
Yes, in a double conversion type it does. Most designs I am familiar with are designed to produce an A-B-C phase rotation and have the ability to alter frequency and phase within certain limits to match up with the bypass supply. Outside those limits the UPS rejects the bypass supply. They simply aren't designed to change the rotation because there is no need to do so when installed correctly.
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If we learn from our mistakes I'm getting a great education!
RE: Generator Cannot Charge UPS
In large UPS systems which use a controlled Thyristor bridge as the rectifier, the phase sequence is definitely required to be A-B-C (U-V-W, R-Y-B) as the thyristors need to fire in a defined sequence.
RE: Generator Cannot Charge UPS
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Generator Cannot Charge UPS
Apparently the UPS inverter will maintain the same phase sequence as the initial incoming power. I never have noticed, but is this the same as a VFD inverter, (i.e. does the VFD maintain the same phase sequence as the incoming power or always maintain a clockwise (or counterclockwise) sequence (unless you tell it through the configuration to reverse direction)?
RE: Generator Cannot Charge UPS
But, what happens when the first power-up phase sequence was reversed by mistake ? Then will it reject the correct phase sequence when the mistake is disovered (the client is sure to notice the wrong sequence in the ups output) and rectified ?
Something wrong here with the logic as well as the fault annunciation scheme.
RE: Generator Cannot Charge UPS
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If we learn from our mistakes I'm getting a great education!
RE: Generator Cannot Charge UPS
I was talking about the ups remembering the first power-up sequence (which could be wrong, even a cable misconnection) and then ups rejecting the subsequent rectification of the input sequence.
I am yet to see about the importance of input phase sequence in an ac-dc-ac ups.
RE: Generator Cannot Charge UPS
I would assume that the UPS could invert to either clockwise or counterclockwise phase sequence since I know that VFDs can do so (I use them in reversing situations frequently and it is just a software or hardware bit to make it reverse). See my question in my previous response in this post.
RE: Generator Cannot Charge UPS
The input phase sequence is of no importance to the rectifier (in theory). The output phase sequence is set and will never change as it is driven by firing order. Bypass won't work if input and output phase sequence are reversed. Not clear what "UPS rejects standby power source" exactly means, but I wouldn't expect the UPS to accept a source it can't bypass to.
RE: Generator Cannot Charge UPS
Other rectifier topologies may require that the phase sequence be correct.
If the rectifier consisted of a simple diode bridge then the phase sequence shouldn't make any difference. The newer style PWM rectifiers often require that the phase sequence is correct.
RE: Generator Cannot Charge UPS
RE: Generator Cannot Charge UPS
THe BladeUPS is a relatively new product in North America (on the market for about 18months now)and I can't say there have been many issues with this particular model as of yet.
There is definitely an alarm annunciated on the screen. I have seen it myself and often rely on the UPS's ability to detect a phase rotation error when commissioning the unit (when I am too lazy to use a phase rotation meter).
It would be in the Status menu and "!" sub menu or in the event history log (which retains the last 128 alarms.
I believe the alarm is "Rectifier Phase Rotation error".
Because the alarm message is alpha-numerically longer than the LCD display can allow, you have to wait a few seconds for that alarm message to scroll the whole text. So you may have skipped to the next message down the list without seeing the whole message.
RE: Generator Cannot Charge UPS
I didn't see that alarm and the tech support guy with initials MN (who like I said was helpful and I called him to tell him that the problem was solved) told me that the BladeUPS did not have a way to check phase sequence and that is why the frequency measurement is in error. I definitely saw the low frequency measurement and low frequency error. I have it in a CSV file of measurements at one minute intervals. I will ask the client for an event history log file dump to check if that error ever came up.
RE: Generator Cannot Charge UPS
I just went through the "Event" CSV dump and it does not give a "phase reversal alarm". Typical alarm sequence is:
AC power has been restored
AC Restored Message sent to registered clients
Utility power has failed
UPS bypass unavailable
UPS switched to battery power
AC Fail Message sent to registered clients
A System / Subsystem Alarm is active
AC Fail Message sent to registered clients
AC power has been restored
AC Restored Message sent to registered clients
There is also a "Data" CSV log file where the frequency is observed (along with the input and output phase voltages, output load percent etc. During the time where the "Utility Power has failed" the voltages read correct (although slightly fluctating) but the frequencies are between 48-53Hz. I guess that the "System/Subsytem Alarm" could be the one where it is displayed however the manual doesn't say that there is any other log besides the event log which has never listed the alarm that you mentioned.