Help decoding strange V,I behavior in facility - Page 1
Forum section: electronics / general

author: Houseman
author_rank: Regular Contributor
timestamp: July 11, 2019, 08:42:57 pm
content: Hi forum. I really need your help decoding a strange Voltage and Current waveform.I am working in a facility as power quality engineer.We have several vacuum pumps that sometimes rattle, lights flickers and PC reboots suddenly.After putting everywhere a power quality meter we have discovered that several times a day also by night, when the pumps are shut down the mains AC 400V power get this typical waveform (typical because it's the same in every moment) as shown in the picture.Voltage fist goes slightly up then it drops freefall and needs some time to recover to normal value.Every load connected to it has this peaks in current absorption, as shown in the picture.We are getting mad trying to understand the cause of this.Any help would be very appreciated.Please feel free to ask for more informationsLSP

author: fourtytwo42
author_rank: Super Contributor
timestamp: July 12, 2019, 09:34:02 am
content: Looking at the last two in particular this would indicate a large load applied external to your facility as the voltage has changed substantially without a change in current, it could also be a sub-station tap changer so I think you should be talking to your utility to improve your supply quality.I don't see anything that should cause a PC to re-boot as any SMPS will simply ride over these very short interruptions so that is likely something else, have you tried putting the power quality monitor directly at the PC outlet as that may have another local problem that needs sorting out.I assume these vacuum pumps are large 3 phase motors, are they driven by inverters or line direct (you don't say) if the latter and they are grumbling there is something much more serious than you have logged on your power monitor such as periodic loss of an entire phase.

author: capt bullshot
author_rank: Super Contributor
timestamp: July 12, 2019, 10:39:43 am
content: The last two ones might be sub-station transformer tap changes, as fourtytwo said.The first one looks like a heavy load being started outside your facility.The second might be similar, but worse since there's a major imbalance over a short period - three phase motors won't like that.The third one, heavy load started outside, and something in turn stopped within your facility (current is reduced). As the voltage rises visibly when your current draw falls, your mains feed looks quite high impedance. You should talk to your utility.Your power quality recorder might be able to record the U and I waveforms, triggering at the voltage dip, maybe these could give some more enlightenment.

author: Houseman
author_rank: Regular Contributor
timestamp: July 15, 2019, 11:43:52 pm
content: I firstly would like to thank very much forty-two and capt bullshot for their reply.Yes you are both right. The last two pictures are the voltage/current captured directly at the vacuum-pump electrical terminal.The pumps are small three phase asynchronous motors 2kW/5kW each but apart from grumbling the main problem here is they are spitting drops of lubricating oil through the Anti-Suck Back Isolation Valve on a random time inside the vacuum chamber. This ruins the whole process and all the chamber needs to be cleaned with IPA (three days work) since I learned that oil tends to flow towards the vacuum.The pumps has been several times retired for revision and the producer has firmly said they has no problems.Apart from changing the pump with a dry one, my job is to find the cause related to the effect and if really the voltage dip is the real issue.I honestly have no clue how to move forward..The strange thing I have noticed is that these dips mostly occurs on Friday morning and on Monday morning on a regular basis (also 10 times in 2/3hours). (not on a regular basis unluckily happens the oil spit).What could cause on a regular basis external to our facility?? We are near (500mt.) a famous national TV broadcast antenna and we are crossed by 15kV electrical rod transmission lines. (not really a safe place to live...)@capt bullshot;- my power quality recorder has a trigger and has recorded U and I waveforms (apart from phase, harmonics and a lot of other parameters.What should I have to search for?- do you think the problem is related more to the voltage drop (does 25V dip cause such a issue) or due to other variations?Thank You again all for your help.Regards from Italy.

author: Dundarave
author_rank: Regular Contributor
timestamp: July 16, 2019, 12:23:17 am
content: I’d immediately contact the power provider and try to make it their problem, or if they are not under any contractual obligation to provide clean power, appeal to their knowledge and need to assist you and their other customers who might be similarly affected.This situation is costing your company significant money, and if it’s caused by the quality of power your provider is sourcing, they likely have some kind of obligation to at least assist in getting to the bottom of it. Thinking on this further, if the power anomalies can't be identified and eliminated, perhaps the use of some kind of motor-generator assembly to electrically isolate the production motors from the incoming power would be called for. Notionally, the flywheel effect of the motor-generator would then eliminate the power feed anomalies and provide a "buffered" 3 phase power source.

author: sibeen
author_rank: Frequent Contributor
timestamp: July 16, 2019, 02:34:14 am
content: I suspect appealing to the good nature of the supply authority may not get you any joy in this instance, they are probably complying with the local electricity distribution code, and that any fix will need to be undertaken within your facility. Are the motors connected through a VFD or are they direct connected?

author: tautech
author_rank: Super Contributor
timestamp: July 16, 2019, 03:13:38 am
content: Could PFC help bridge these mains supply glitches ?

author: Tom45
author_rank: Frequent Contributor
timestamp: July 16, 2019, 03:36:03 am
content: A friend of mine told about a similar problem with a customer of his. The problem turned out to be flaky wiring somewhere at the service entrance to the plant.So it would be worthwhile to keep that possibility in mind.

author: sibeen
author_rank: Frequent Contributor
timestamp: July 16, 2019, 04:32:41 am
content: Quote from: tautech on July 16, 2019, 03:13:38 amShort answer - no.It appears to be a load related voltage dip caused by a step load external to the factory. PFC cannot do anything about that. The current rise seen in the factory as the voltage dips is 'probably' caused by slip on the induction motors, indicating that the motors are 'probably' directly connected to the grid rather than through a VFD.

author: Houseman
author_rank: Regular Contributor
timestamp: July 16, 2019, 07:36:27 am
content: @sibeenYes, you are correct. No VFD is present: Vacuum pumps are directly connected straight to the mains, apart from a protection switch.The main problem here appears that our electrical contractor is saying that these voltage dips are in their 10% tolerances interval so they are not going to do anything, neither let one technician of them come out and see.We have in our nearby a galvanic processing facility but they swear that they are connected to a different sub-station and they start on Monday at 6 and work through the Week without these strange Monday and Friday overlapping dips.What is your opinion about connecting a three phase UPS to the vacuum pumps?Thank You again all for your contribution.Quote from: Tom45 on July 16, 2019, 03:36:03 amThank You but in such a case why these Monday&Friday morning dips?

author: capt bullshot
author_rank: Super Contributor
timestamp: July 16, 2019, 07:58:14 am
content: Regarding the U and I waveforms:It's a guessing game and rather vague, but looking at the voltage and current waveforms that were recorded can reveal hints pointing to the root cause.The RMS recordings already showed that the voltage sag isn't caused by your facility, looking at the waveform itself might show if the voltage drop is caused by a short circuit somewhere else, a heavy load starting, a substation tap switch, ...So you'd look at the recorded voltage waveforms at the moment the sag occurs and maybe you see one phase dropping some time before the others, or all three at the same time, do they drop to near zero and recover, is the drop edge steep or slow and then try to figure out what kind of event might cause that, and then look around if you can find such a thing.Regarding events on a regular base:There's a lot of tell-tales around what can cause such, including the cleaning lady pulling the server plug every friday nightAnyway, it's a good point to start and look around who might be starting e.g. a large motor or other large loads at these times. A TV transmitter station might have a backup diesel generator that is tested on a regular schedule. I don't think the crossing of 15kV line is related to your issue, anyway the local substation might be to weak, so it's kind of related to this line, but not to the fact it's running across your facility.Running the vacuum pump motors through VFDs instead of direct connection to the mains line most probably will stop the motors grumbling and pumps spitting oil (in case the oil spitting is related to the grumbling).Induction (asynchronous) motors don't like voltage sags, this will cause electrical phase shift in the first place, leading to jerk on the mechanical side. A VFD cannot run through the sag either, but should limit the jerking (by slowly decelerating and accelerating the motor). But I can't guarantee that this will work on your place.

author: sibeen
author_rank: Frequent Contributor
timestamp: July 16, 2019, 12:35:49 pm
content: Quote from: Houseman on July 16, 2019, 07:36:27 amHere you need to be cautious. There are a few things that need to be considered.In your present configuration, with the motors connected direct on-line (DOL) the start up (inrush) current to the motor is going to be high at around 5x to 10x In depending upon the quality of the motor. UPSs are generally current limited to around 2x In when operating on inverter. For a motor start when mains is available the UPS would transfer to its bypass (mains) supply to get the motor up and running and would then transfer back to inverter. If the motor attempts to start when there is no mains avaivable, and therefore the UPS is on inverter, the inrush current will exceed the inverter's capability, unless the UPS is well over-sized, and the UPS would shut down on an over-current. So sizing of the UPS and operating scheme needs to be considered.The next, and more problematic, issue is what happens when the motor decides to brake or slow down. In this case it then acts as a generator and will attempt to regenerate current back into the mains. A UPS is normally only designed for two quadrant operation, feed the load forward only, and a backfed current may cause issues or even destroy the UPS. A VFD overcomes this by having included braking resistors which bleed off any power returning to the VFD or they may be an active front end style which allows for 4 quadrant operation and power fed back from the motor can then be fed back into the grid.You may be better off first trying the motors on a VFD, one that includes a braking resistor sized correctly for the motor. See how that goes and if lucky that may ride through the few cycles of low voltage and fix your issue. If that doesn't work you may then have to put a UPS to feed the VFD - and then we run into harmonic issues as the front end of a VFD down at that small size will normally be a 6 pulse rectifier and mitigation of the harmonics via chokes may be required.It's not just a simple fix and a bit of experimentation and stuffing around, with the associated expense, may be involved before you can resolve the issue. That is what you need to pass up the chain to the bean counters - just because I'm spending some money as a 'first try' this does not mean that the issue will be resolved and additional costs will more than likely be incurred before the issue is fixed.<fixed up spelling from 'break' to 'brake' to avoid some confusion.

author: IanB
author_rank: Super Contributor
timestamp: July 16, 2019, 01:42:41 pm
content: Quote from: Houseman on July 15, 2019, 11:43:52 pmAs far as the oil is concerned, maybe you could modify the inlet piping to the vacuum pump to include an oil trap and so prevent the oil reaching the vacuum chamber?

author: GeorgeOfTheJungle
author_rank: Super Contributor
timestamp: July 16, 2019, 02:30:44 pm
content: Quote from: Houseman on July 16, 2019, 07:36:27 amThose big baths of melted zinc draw lots of power.

author: StillTrying
author_rank: Super Contributor
timestamp: July 16, 2019, 03:13:45 pm
content: Quote from: Houseman on July 11, 2019, 08:42:57 pmDo any or all of those screenshots correspond to the same instant of pump, PC or lights flicker, on the last 2 the volt drop is less than 2%.I think some of the screenshots maybe different events.

author: Houseman
author_rank: Regular Contributor
timestamp: July 17, 2019, 11:04:39 pm
content: Quote from: IanB on July 16, 2019, 01:42:41 pmUnluckily not possible. The small drops of vaporized oil would pass through the filter. We have tried if already without effort.Also using special vacuum oil type.These little drops will then re-accumulate inside the chamber and rejoin together and we find them in the slots, o-ring and tubes...

author: Houseman
author_rank: Regular Contributor
timestamp: July 17, 2019, 11:40:39 pm
content: Quote from: StillTrying on July 16, 2019, 03:13:45 pmFirst three screenshot are at the mains entrance on the mains switchboard. You can see a 80-90A total current absorption.The last two are tapped with a second tester with the probes directly connected to one vacuum pump - the three-phase asynchronous motor who is getting the oil problem.The strange thing is that not at every mains dip-event I have seen the same dip-event at the pump at the same time.Thank You

author: Houseman
author_rank: Regular Contributor
timestamp: July 23, 2019, 12:33:38 am
content: Quote from: sibeen on July 16, 2019, 12:35:49 pmSorry Sibeen, I cannot follow You.When should the motor brake or slowdown? It is an asynchronous 3-phase motor that is switched ON/OFF by cutting the main power.By putting it OFF we simply cut power and disconnect it from the UPS.Please explain: In which case should this event happen (brake/slowdown)?Thank you so much for Your very useful support.Regards

author: sibeen
author_rank: Frequent Contributor
timestamp: July 24, 2019, 02:57:58 am
content: Houseman, this is a case where I don't really know your application so was erring on the side of caution.An example is probably the best way to go about it. If a motor is driving an elevator to drive the elevator up the shaft the motor need to provide enough power to overcome gravity, friction etc. Once the elevator has reached the top it now contains a lot of potential energy. When the elevator comes down the motor must act as a brake to the elevator and this energy needs to be bled off. In some variable frequency drives this will be accomplished by the use of braking resistors. In more upmarket VFDs the converters can operate in four quadrant mode and feed the excess energy back into the grid. As an aside, this latter manner can cause other issues when a site is being run on a generator - but that's another storyAnother example is a conveyor belt which is moving stuff along. This has created inertia and if you try to brake or stop the conveyor this energy needs to be dissipated.So it really depends upon your application.*For the elevator aficionados and pedants I've purposely ignored the use of counterweights etc to keep it simple.

author: Houseman
author_rank: Regular Contributor
timestamp: July 26, 2019, 03:03:52 pm
content: Thanks Sibeen.I understand now your example.As far as it appears your explanation brought me to the conclusion that the UPS choice is not the best solution except from supersizing it features letting the cost raise.As you can see from my other post (VFD settings) the pumps has 6x nominal value of current inrush, and there are several maybe starting all at the same time.So I should size the UPS at least 4 times more the nominal power (100kVA) and I think that would be unacceptable also in terms of costs.and the manovre solution to step-in the UPS after poweron should also be unpracticable.I think maybe an active power conditioner should be the closer solution...Thank You again.

author: StillTrying
author_rank: Super Contributor
timestamp: July 27, 2019, 02:15:05 pm
content: The volt droops and current spikes caused bya pumpitself, look very similar to what's apparently coming from elsewhere to me. If not the initial droop, the following deep spikes definitely looks like the pumps themselves, rather than a supply tap change.But maybe that's just because I know next to nothing about 3kW 3PH asynchronous vacuum pumps.https://www.eevblog.com/forum/renewable-energy/help-setting-rpm-and-frequency-on-a-vfd/msg2572122/#msg2572122

author: Houseman
author_rank: Regular Contributor
timestamp: August 03, 2019, 05:13:24 pm
content: Quote from: sibeen on July 16, 2019, 12:35:49 pmHi Sibeen.Luck was NOT at our side,Those Fxxking voltage sags appears already. A little bit "stretched" but present at the output of the Omron V1000 inverter...I attach both at the input and the output of the Inverter.We really do not know what to do...Any help would be very appreciated.Thank You all