Poor power quality, same time every nite

[KentAT]

At another plant like mine, they have a new recurring issue each evening between that lasts up to 20 minutes, and always 10 pm and 11 pm.

The background is that there are normally two directions by which the POCO supplies their plant @ 12600V. One had a problem and was taken out of service for repairs. The POCO was contacted about a possible problem on their side and POCO put a recorder on their primary side for one day and said there was no problem. The plant feels there are loads being brought onto the grid nearby that are causing this problem (there is a nuke plant 5 miles away...).

My co-worker's plant owns transformers to use 480V 3-phase. Every single evening since the one supply was taken out of service, the utility power causes the plant's switchgear to trip out on device 47, a phase protective relay, and start our emergency generator. This relay watches for phase unbalance, phase loss, and phase reversal on the incoming power, and based on his waveforms from last week, the phase unbalance is tripping.

Now, they have PF capacitors, and the tech at that plant said you can hear the power changing and can tell when it is occurring as they really go crazy when the event occurs. The plant electrician captured the same ugly waveforms on the utility power last night but this time, he was running on generator and the utility was not in service at the time.

I'm taking my Power Quality Analyzer over there on Monday to capture the waveforms and check for harmonics, so he can show them to the POCO, and maybe they will do something because they are not sure when the other feed line will be back in service.

One question I have is : if harmonics on the utility are the problem, will they still show up if the plant is not using the utility power as I capture the data (will be running on generator)?

kent

 

[catchtwentytwo]

Kent,

Has the phase protective relay been tested and calibrated? Also you might want try to send those ugly waveforms to Electrotek waveform@electrotek.com and see if they they have an opinion.

John

 

[KentAT]

No, it has not been tested or calibrated. BTW, their unbalance setpoint is at 7%, whereas mine is set for 12%. Both setpoints are from the original commissioning of this switchgear around 1996.

I will be able to post the waveforms if anyone wants to see them.

kent

 

[wirenut1980]

What is the protective relay trip setpoint? Could be that either the POCO or the plant has a bad capacitor switch, and when they switch in, it causes the voltage unbalance to increase.

 

[cadpoint]

Seems I'd check the wire on device 47.

 

[charlietuna]

you need a certified data logger readout of the problem(if there is one) on the incoming service. sounds like other building's equipment shutdowns are effecting the primary feeder which is probably an old backup feeder????

 

[KentAT]

What is the protective relay trip setpoint?

7% unbalance.

POCO is due in today to analyze the power supply. Problem is, the disruption is between 10 and 11PM.

My analyzer is an AEMC PowerPad Model 3945, http://www.aemc.com/products/html/moreinfo.asp?id=110102&dbname=products.

With the plant load on a backup generator and the utility offline tonite, I hope to see some power quality issues on the utility side, like the tech saw Friday nite.

 

[ELA]

I would expect the harmonics to be similar on backup generator, could be better or worse.

It may be possible that the harmonic distortion you experience varies with the source impedance of your supply. Perhaps your source supply was originally more rigid (low impedance) before the change was made.

With a higher source impedance you may be getting a larger distortion on the waveform.

Is the phase monitor relay rated for "true RMS"?


We once had an issue where a phase monitor would report a phase loss when the harmonic distortion was too great. We replaced it with a better quality "true RMS" monitor and the problem went away.

We also found that the original phase monitor did not have problems as often when running on a rigid source. It would actually fail more often when we were running on generator. Our generator source impedance was higher.

 

[charlietuna]

i would set the unbalance to match your's just to see if it holds? this might be your problem?? and is your logger certification up to date? we had a problem in a large building and were there was major liabilities involved ?? the power company kept denying anything on their side until we provided a certified printout of incoming power ! then they had a private meeting and decided to admit they were operating on an old backup feeder that was undersized due to an accident that blew up a transmission yard ! they hate to admit guilt !

 

[catchtwentytwo]

And don't rule out a wiring glitch relating to the wiring to the relay.

I've twice seen "mysterious" reverse KVAR relay tripping that turned out to be caused by the factory mislabeling of internal interconnect conductors. They were terminated at the correct terminals according to the wire labels. Only problem is they were on the wrong wires, at least at one end. In my case, the problem didn't become apparent until the system crossed a load and impedance threshold.

 

[KentAT]

We once had an issue where a phase monitor would report a phase loss when the harmonic distortion was too great. We replaced it with a better quality "true RMS" monitor and the problem went away.

Just gathered data last night. Got 7th, 9th, 11th, and 13th harmonics at the expected time of night. Phase unbalance was only 1.9%, not enough to trip the relay on its own, but the harmonics seem to really jazz it up. THD was up to between 7-9%THD during the bad periods.

POCO due in today to see our data.

Will keep you guys posted - and thanks for the help as always...

Kent

 

[wirenut1980]

Just gathered data last night. Got 7th, 9th, 11th, and 13th harmonics at the expected time of night. Phase unbalance was only 1.9%, not enough to trip the relay on its own, but the harmonics seem to really jazz it up. THD was up to between 7-9%THD during the bad periods.

POCO due in today to see our data.

Will keep you guys posted - and thanks for the help as always...

Kent

So the phase unbalance relay tripped when the plant was shut down?

Current of voltage THD? If you are talking voltage THD and there was no load in the plant, then the POCO has some other customer that is way out of IEEE 519 specs and is distorting the supply voltage. If you are talking current THD, that is a load in the plant possibly causing the problem.

 

[KentAT]

Plant was online. Unbalance relay did not trip because the plant raised the setpoint to keep plant online during data collection. THD was voltage, current probes were not hooked up last night.

Confirmed the protective relay is not a "true RMS" one.

BTW, primary side is 34000, not 12600 like mine.

Today we showed the harmonics and waveforms to POCO from last night's analysis.

We pressed the POCO to examine their grid, find the problem, and solve it. Don't look for us to tell them what is causing the harmonics on the supply, that's not our place. And yes, we expect them to acknowledge the problem and deal with it.

Next step is PUC complaint. It's been 3 weeks, every nite, at the same hour of the day. It almost seems like a shift change - every nite between 10 and 11 pm. The analyzer showed that we're having 4 separate events, timed 8-9 minutes apart. Like I said, there's the nuke plant nearby, and another water plant that has some big loads.

POCO seems more interested today, since we have data to show bad power quality. Looks promising at this point...

kent

 

[wirenut1980]

...current probes were not hooked up last night...
kent

Why not? Unless the plant does not have hardly any harmonic loads, how can you be sure the voltage distortion is coming from the supply and is not caused by the plant's own loads without seeing what the current distortion is?

What harmonic loads does the plant have? Sorry, I must be missing something here.

 

[charlietuna]

harmonics have been around for years and are now decreasing due to methods of manufacturing. the power company doesn't concern themselves with harmonics--thats caused by customer loads --your looking in the wrong place !

 

[KentAT]

Why not? Unless the plant does not have hardly any harmonic loads, how can you be sure the voltage distortion is coming from the supply and is not caused by the plant's own loads without seeing what the current distortion is?

What harmonic loads does the plant have? Sorry, I must be missing something here.

short answer - it wasn't convienient from where we were set up, and the plant tech saw the events occurring last week when the plant load was on their generator instead of the utility.

The plant electrician was going to get both voltage and current this week (Tuesday and later) - don't know if he did yet.

As for loads at the plant - fully loaded the plant might pull only 1200A @ 480 - 3-ph, typical is only about 400-500A. There are no VFDs, no adjustable speed drives.

will check back when I get recent info from the plant guy...

kent

 

[catchtwentytwo]

harmonics have been around for years and are now decreasing due to methods of manufacturing. the power company doesn't concern themselves with harmonics--thats caused by customer loads --your looking in the wrong place !

If there's a dedicated utility stepdown transformer that solely serves the plant that would help block "grid-based" harmonics if they were actually the problem.

I agree with charlietuna, the problem is within the plant. What type of processes/machinery is there and what is the end product? Are there any UPS systems, rectifiers or inverters?

 

[KentAT]

If there's a dedicated utility stepdown transformer that solely serves the plant that would help block "grid-based" harmonics if they were actually the problem.

I agree with charlietuna, the problem is within the plant. What type of processes/machinery is there and what is the end product? Are there any UPS systems, rectifiers or inverters?

Yep, they have a 480V UPS to keep critical 480V motors running while the backup generator starts up, and yes, there is a dedicated stepdown transformer bank that our plant owns 33,000V > 480V.

As it turns out, the POCO confirmed the next day that the nearby nuke plant starts up some monster water pumps every night around 10:30 pm (our guys have seen the impellers, about 15 feet diameter, I'm told). They have 3 to start each night, and they are typically started 8 minutes apart. The POCO switched our plant's feed to another line until the original one is back in service (burned up transformers) in a couple of weeks. All appears OK since they did that on Tuesday.

Learning from you guys, I tend to believe the harmonics were coming from our power factor caps and/or the UPS, but the harmonics appear fine until the utility supply gets jazzed up as those pumps are started. Could that be because of voltage fluctuations as seen in the first picture, and our caps are responding to it? We don't have the harmonics anymore. Does this make sense??

This is a graph of the phase voltages during the monitoring period.

This is the voltage THD during the same period. You can see the THD spikes are 8-9 minutes apart.

Here are the waveforms at the worst time. Remember, I did not capture the current (my bad - still learning):

And the voltage THD info:

 

[catchtwentytwo]

What is going on with the UPS system when you see those THD "spikes"? Are there any logs you can access? Is it a rotary UPS system (like Piller)?

Two things might be happening, one being the UPS goes to battery, or if it is normally runs in a static switch path parallel with the rectifier/battery/inverter path, most likely it turns-off the static switch when it see an disturbance to run solely in rectifier/battery/inverter path.

 

[KentAT]

Thanks for the interest -- The units are Unity 3-phase, they are in a nearby building so I don't know what's going on during the spikes. What we do know, is that there has been only clean power since the utility changed took our plant off the feed we were on.

The plant tech said he didn't see anything on the control/display that indicated the units were disconnecting from supply.

Here is a product sheet:http://www.bomara.com/Best/unity1threephase_spec1_eu.htm

The product overview states "Unlike a double-conversion system, a UNITY/I Three-Phase system draws a clean sinusoidal current. With no rectifier to produce harmful input current distortion, this system requires no costly filtering.

The specs mention input harmonic distortion @ 5% or less.