Generator Cannot Charge UPS

[gepman]

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.

 

[gepman]

My understanding in talking the Powerware UPS tech support is that the UPS does NOT care what the phase sequence is at the input, it just doesn't want it to change. Apparently at the application of power to the UPS it notes the phase sequence and expects it to remain that way. I don't think that is a problem since you would want and expect the phase sequence to be consistent. The real issue to me is that the UPS does not give a phase reversal alarm but does give an incorrect low frequency alarm.

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)?

 

[edison123]

So, the ups remembers the first power-up phase sequence and maintains that the same sequence in the output. And it demands the phase sequence from the genny too. Fair enough.

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.

 

[ScottyUK]

No, it is just designed for a specific input sequence. How else would you design the firing circuit for the input rectifier? Your logic is like saying a motor should always rotate in the 'correct' direction regardless of the input phasing. The output cables can always swapped to reverse direction but the electronics can not provide the reversal: they are designed for a specific sequence.


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

 

[edison123]

No, That's not what I said, Scotty.

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.

 

[gepman]

If a mistake was made on first power-up sequence then you would have to go through the power up sequence again with the correct phase sequence. I don't know if this requires complete removal of the batteries or if it can be initiated through software. Remember this is a UPS so after initial configuration it won't loose power unless the batteries are completely discharged.

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.

 

[davidbeach]

Frequency went from 60Hz to -60Hz; doesn't that qualify as low frequency? ;-)

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.

 

[sibeen]

davidbeach, I’d have to disagree that the input phase sequence has no importance to the rectifier. It does depend upon the style of rectifier that is used. In larger UPS systems the rectifier is normally a controlled thyristor rectifier, 6 or 12 pulse. The rectifier outputs a controlled DC voltage for input to the inverter and for proper battery charging. If the phase sequence is incorrect the firing of the thyristors in the rectifier won’t be correct progression and the rectifier won’t operate correctly.

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.

 

[davidbeach]

Right, I was thinking diode based rectifier.

 

[Jasonarc]

I've been servicing Powerware equipment since 1988. Startups, repairs you name it...

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.

 

[gepman]

Jasonarc

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.

 

[gepman]

Jasonarac

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.