ASD on a generator
ASD on a generator
(OP)
Hi, I do not normally visit here, seeking your help
My client has a well pump that has been operating fine for the past year. 150 hp submersible, powered via an pulse width modulated ASD, includes load reactors due to the distance between the drive and the motor, We added a 230 kW minimum generator to the site, with specifications that the primary load would be the pump via the drive. Just did the startup test of the generator, when we added load from the drive, the flouresecnt lights started to flicker, and the piping started to vibrate, the pressure also started vibrate, with reluctance we continued to increase power to the normal operating range, where the vibrations lessened, we proceeded with the generator proof test and at the end of the hour, we allowed an automatic transfer to normal power, the transfer switch has a phase monitor to allow transfer under load. At transfer there was a slight bump and all vibrations went back to normal on normal power. I have asked the generator manufactorer, whats up?
I have put generators on drives before without issue.
Any comments?
Thanks, Hydrae
My client has a well pump that has been operating fine for the past year. 150 hp submersible, powered via an pulse width modulated ASD, includes load reactors due to the distance between the drive and the motor, We added a 230 kW minimum generator to the site, with specifications that the primary load would be the pump via the drive. Just did the startup test of the generator, when we added load from the drive, the flouresecnt lights started to flicker, and the piping started to vibrate, the pressure also started vibrate, with reluctance we continued to increase power to the normal operating range, where the vibrations lessened, we proceeded with the generator proof test and at the end of the hour, we allowed an automatic transfer to normal power, the transfer switch has a phase monitor to allow transfer under load. At transfer there was a slight bump and all vibrations went back to normal on normal power. I have asked the generator manufactorer, whats up?
I have put generators on drives before without issue.
Any comments?
Thanks, Hydrae





RE: ASD on a generator
Without knowing the procurement specs, it’s hard to judge. 230kW and 150hp seem a little close. It’s likely the genset voltage regulator is having trouble with harmonic content produced by the ASD.
Obviously, I wouldn’t sign off any acceptance until this is resolved. Contract verbiage can make or break who has to resolve the matter, even to the point of liquidated damages if non-performance delays purchase-order/project completion.
RE: ASD on a generator
Just a thought...
BobPE
RE: ASD on a generator
The procurement specs were generic, make it work, here are the loads and set a minimum size of 230kW. Engine was not at full throttle. Amps to the motor at 57 hz 180, drive rated at 199, motor nameplate at 215. (480V)
what confuses me is, more vibration at lower loads...
BobPE
The operating ranges and loads are exactly the same as when on normal power. Start at 40 Hz which is about 100 gpm, (gives cooling flow past motor) ramp up over the course of 5 minutes to operating level, 57hz which is 1200 gpm to flow at the water right, reverse on shut down. It is a submersible motor so there are not the harmonics with a lineshaft. There is very little vibration when on normal power.
and if it was from the pump why did the lights flicker?
Hydrae
RE: ASD on a generator
Unfortunately, “make it work” can have several meanings that may{/will} differ for the seller and buyer. There is incompatibility between the load and genset AVR, but the vendor will likely insist that they are able to “make it work” regardless of the [undesirable or at least questionable to the buyer’s representative—you] mechanical noise present during their contractually mandated demonstration/performance test.
Hate to even suggest that the solution may be to “ask ‘em what they would do” and negotiate some sort of compromise where the difference is split between both of you, and chalk it up to “lessons learned.”
Wigged-out genset AVRs serving UPSs/motor drives are a 20+ year-old situation, but still occur where buyers/facility designers with ‘thin’ ‘value-engineered’ specifications. The genset vendor is likely playing naïve to claim that they have not dealt with the matter before.
RE: ASD on a generator
so where can I learn more on the situation?
I have not got a response back from the Generator company yet, just gathering info now.
Hydrae
RE: ASD on a generator
Discuss it with the drive and pump manufacturer. They may be able to advise you on similar cases and potential problems.
RE: ASD on a generator
I work for a company in the oilfield that manufactures both downhole centrifugal pumps and VSDs and I have encountered similar problems now and then.
The first thing to think about is the genset AVR. You absolutely must specify to the genset supplier that the AVR must be capable of handling the loads associated with a VSD. I know this has been addressed in depth by Caterpillar as I have run about 25 systems on their gensets. Generic specifications have their limitations, and this may be one of those instances.
I spent a Christmas once upon a time in the desert stripping down a VSD before I came across someone who understood what I was on about.
dadfap
RE: ASD on a generator
Rectifiers can look capacitive at low loads. That may be causing oscillations with the generator governor, which are really designed to work only with inductive loads.
RE: ASD on a generator
Aside — IEEE Std 446-1987 "Orange Book" ('96 is current) ...Emergency and Standby Power and found no mention of drives served by gensets other than a passing comment in §3.11.4.7 that load-generated harmonics may exist.
RE: ASD on a generator
1. The generator voltage and current waveforms, voltage dips and sags, and frequency might be observed by oscilloscope or similar
2. Whoever did the integration, might have used some simulation software of the generator and downstream load including transients during the generator starting. If no one did, it may be good idea to simulate it since the generator and load ratings are very close, and there appears to be no load blocks applied during the generator start.
3. Mechanical vibration will be dependent on the generator frequency slew rate and amount of power delivered in specific time intervals (essentially, the motor terminal voltage (VSD output) should be as close as to rated voltage, without harmonics, spikes, sags, dips, swells, etc.).
RE: ASD on a generator
1- What is the drive output kW when running at some speed? From this and/or the pump RPM and discharge pressure calculate the actual motor load HP. Is the load in the region which peebee mentions? I do not know where this region would be, or if it exists.
2- Measure the input kVA to the drive. Is it higher than it should be?
3- Measure the harmonic content of the input current. Is it significant enough to cause voltage distortion? What does the voltage waveform look like?
4- Is the harmonic content exceeding the rating of the generator? If harmonic content is higher than acceptable maybe a line reactor on the line side of the drive would help mitigate the harmonic currents.
5- Could the reactor on the drive ouput be robbing the motor of torque? If the generator supply is soft due to harmonic current content on the line side the output reactor would amplify any motor torque problems.
6- Of course, some advisement by the generator manufacturer would be good also.
Interesting problem, good luck.
RE: ASD on a generator
As for the vibration at lower drive output frequencies, the harmonic output of a PWM drives varies quite a bit with load. Things generally get better the closer it gets to 60 Hz.
The root cause of the problem is almost certainly the high levels of harmonics produced by the drive. Harmonic current distortion may be 70 or 80 percent.
You probably need to enlist help from generator supplier and drive supplier in coming up with a solution. Addition of line reactor or a broadband filter will probably help, but it would be best to figure out what is actually reacting poorly to the harmonic distortion before shotgunning a solution.
Overheating of the generator stator is another concern if it is required to run the pump/drive for long periods of time.
RE: ASD on a generator
The generator supplier will be testing using a load bank next monday, they are suggesting replacing the electronic voltage regulator with a less sophisticated (mechanical?) model or installing an isolation transformer on the control circut.
The following is the spec on the regulator
"The automatic voltage regulator shall maintain voltage within the limits of +/- 1.0% from no load to full load including cold to hot variations at any power factor between 0.8 lagging and unity inclusive of a 4.5% speed variation."
I will measure the KVA tommorow during a normal power run.
do not have an O-scope though.
I will also inform the manufactorer of the drive for their input.
The flow and pressure out of the pump/motor during the gen test was normal, measured with 4.5" ashcroft gauge and 10" mag meter (0.5% accuracy) except for the vibration. So, not less than normal average torque.
Expected run times are 4 to 20 hours per day, but generator will only be used during power outages, which can last up to 5 days, once every five years.
A PLC controls the drive and will not load the generator until a warm up is acheived, so gen start up loaded is not an issue.
The pump only uses 135+/- hp at full speed and we do not need full speed (yet)
Appreicate your inputs.
Hydrae
RE: ASD on a generator
I bet the genset passes the load test on the load bank, if it does then you can also bet that the problem is associated with the interaction between the AVR & the drive. Your spec makes no mention of the drive, it covers the diesel performance and the expected load PF but no mention of the VSD or the harmonic levels the AVR is expected to cope with.
dadfap
RE: ASD on a generator
The Damper windings are there to help in damping torque pulsations from the Genset .
There is / was a regulation which I think was JEM 1354 , which said that the I2 Equivalent negative sequence current should not exceeed 15% of the rated generator current .
Sorry I cannot be more specific .
RE: ASD on a generator
The load table in the specs and drawings listed the motor and drive with brand and model number as the primary load.
Hydrae
RE: ASD on a generator
RE: ASD on a generator
Generator size, voltage, and subtransient reactance (get from generator supplier)
From the VFD, does it have any internal (DC inductor) or input (AC line reactor) impedence?
CB2
RE: ASD on a generator
Thanks for the help, so that I do not release info about the client or their vendors, please send me your email to
hydrae2003.at.yahoo.com
note the .at. is the symbol
Hydrae
RE: ASD on a generator
The ungrounded generator might cause overvoltages. A generator delta winding would call for a system grounding transformer or corner delta system grounding.
RE: ASD on a generator
The impedance of the generator is much higher than that of the ASD and the interconnecting distribution system.
Consider the generator and a source impedance resistor and the ASD's and wiring thereto as another impedance resistor, all in a series circuit. Make the generator resistor several orders of magnitude greater than the ASD's & wiring impedance. Then, introduce significant current harmonics and you can see that the significant voltage drop across the generator impedance results. This equivocates to the voltage distortaion present at the output of the generator.
Now, while the current distortion caused by the non-linear load characteristics of the ASD's has a direct relationship ONLY with the generator impedance; the consequent voltage distortion is present at every device connected to the generator output bus.
I see this weekly as hospitals and water treatment facilites and such add ASD's to replace older constant speed driven Air Handlers & pumps.
Virtually all hospitals have emergency generators that are tested on a weekly basis.
While the ASD's may operate without any noticable ill effects on transformer distribution power, surprise-surprise when they switched over to em.gen. power.
A rule of thumb is that the non-linear load on a generator should not exceed 15% of the generator capacity. This doesn't mean that the percent non-linear loading can't be greater... but if it is ... a Harmonic analyis of the system will show that you will need to had some harmonic mitigation equipment to prevent the THD levels from causing problems such as you have experienced and described.
Some months back, a Hospital in Northern California was experiencing short life on the electronic coil cirucits of their motor starters when the em.gen. power was tested (weekly schedule). Also had a lot of noise on the bus bars and vibration.
The resulting voltage harmonics consequent to the current harmonic interacting with the impedance of the generator, was the culprit.
The solution, after a harmonic analyis was performed, was to add a trap filter at the input bus to the ASD's. Some such filters are designed to attenuate the 5th harmonic; other may also attenuate the 7th harmonic as well. MTE Corp.'s Matrix Filter guarantees either a maximum 12% THiD (or 8%....depending upon which filter is selected).
This will surely solve the problem... but... an harmonic analysis of the system should be done.
Recently, some software for performing an harmonic analyis became available from a company called HarmonicPro. The software is called HA Lite. A 30 demo is available for free download. After that, a license needs to be purchased. Very inexpensive for the results that it serves up. The software allows you to analyze both transformer sourced distribution to the ASD's as well as Induction Generator sourced power to the ASD's .
If interested, see http://www.harmonicpro.com and if you do pursue the software, be sure to read all of the HELP files.
HTH - jOmega
RE: ASD on a generator
Quando Omni Flunkus Moritati
RE: ASD on a generator
I agree with most of the opinions here that harmonics are the problem.
The generator should be rated for rectifier duty. This should not be an unusual request for the manufacturer, even though it might be for their agent.
The generator sounds too small, given the size of VSD and the lack of much rotating plant to calm things down. Adding a fairly large (unloaded) delta-connected motor would make life easier for the generator as it provides an energy store on the system and a circulation path for the triplen harmonics.
The transformer will help IF it introduces a delta winding onto the system which will allow the triplen harmonic currents to circulate, thus keeping them off the generator. The VSD will almost certainly have a large third harmonic content which at 150Hz (180Hz?) is a low enough frequency for a fast electronic AVR to respond to.
if you are stuck with the generator, can you change the drive to one with an active power-factor-controlled front end? Or consider adding an active harmonic filter in parallel with the dirty load, which will suppress everything except the fundamental current.
RE: ASD on a generator
We did a second load test today, swapped out the 1% AVR with a 2%, it helped but not enough, The isolation transformer to the AVR helped only marginally. The Generator tech had a harmonic evaulater, the fifth harmonic exceeds the ieee spec as predicted by cb2 (thanks) and the current and voltage plots look simular to a plot by the program jOmega referenced. The drive manufacturer is recomending a 5% filter on the line side of the drive and to see if it will work, and if not then add a harmonic trap on the fifth since that device is expensive.
Hydrae
RE: ASD on a generator
RE: ASD on a generator
Thank you for posting the latest developments and tests results.
What is the magnitude of the 5th and 7th
current harmonic and voltage harmonic ?
From experience, I think you'll save time and money going with either a 5th harmonic trap filter from someone like TCI (www.transcoil.com) which includes a 5% line inductor.
Or ... the Matrix Filter by MTE will do you well.
I do not think that the 5% line inductor alone will do you much good. HA Lite allows you to try the 5% line inductor for harmonic mitigation in the simulation... you'll be able to assess the results before committing any $$$ to it. HA Lite will also allow you to try a 5th harmonic trap filter ... and again, you can assess the benefit therefrom.
jOmega
RE: ASD on a generator
RE: ASD on a generator
I would have thought that the passive filter would have been sufficient on a generator. On a mains system I would go for the active filter as the passive filter becomes a harmonic "black hole" and sucks up all the harmonics and may leave the "target" harmonic untouched.
dadfap
RE: ASD on a generator
http://www.squared.com/us/products/pwrfacto.nsf/9ad7caf97d46699a852566d5006eb88a/da4cc2f04053bc2485256736006bf339/$FILE/D5800BR9801EP_R2.pdf
http://www.squared.com/us/products/pwrfacto.nsf/9ad7caf97d46699a852566d5006eb88a/660a71862a47f69585256a4d0067edd9/$FILE/AccuSine%20PCS%20Brochure.pdf
http://www.schneider-electric.com.sg/selink/sg/Schinternetnew.nsf/($All)/5F5D95D4E8CBB08048256C60000B637D/$file/release-3211002.pdf
http://www.schneider.co.uk/products/dist/line_con.htm
etc. for more info
Passive harmonic filters are dependent on the system impedance, which is big drawback
RE: ASD on a generator
Most applications need to take down the 5th and possibly the 7th .... 5th harmonic traps are generally not tuned exactly to the 5th harmonic... there's some spill over towards the 7th and consequent benefit.
Passive filters, properly applied do a very good job. Most applications do not need the unwarranted expense of an active filter. They should be saved for the most severe situations. Proper harmonic analysis ahead of time can save a ton of money ... (hdwre + Installation costs).
RE: ASD on a generator
Thanks for your inputs
The client purchased and installed a 5% passive line reactor, works great now, it also improved minor vibrations when on normal power also.
I will know better next time...
Hydrae
RE: ASD on a generator
Thank you very much for the feedback.
Glad the problem is resolved.
Kind regards,
jOmega
RE: ASD on a generator
Fellow practicing Engs.
Thanks for your inputs
The client purchased and installed a 5% passive line reactor, works great now, it also improved minor vibrations when on normal power also.
///Was there a power supply quality analysis performed before the motor application and after the motor application?\\\
I will know better next time...
///Perhaps, when the Utility comes to the Client requesting some further harmonic suppression equipment. Then, the reactor solution will have to be amended.\\\
Hydrae
RE: ASD on a generator
Bear in mind that, from a system standpoint, there are a number of control / system elements at work here - the AVR (as you all noted), the generator (speed) governor, mechanical spring time constant of the piping system, the drive characteristics, the ratio of kW load to motor load, presence of harmonics, etc. Adding the reactor may have dampened one critical portion of the system.
It seems intuitive to me that as the load increased on the system, the overall damping increased as well. When I hear about 'pipes shaking' it makes me think more of an overall systems response rather than zeroing in on harmonics & drive problems. (Remember the old 'poles' in the mechanical / electrical system transfer function - a few too many years since college I'm afraid here). An interesting approach might be to try slow down the time constant or response time of the AVR/drive/governor & see what happens.
That being said, when the problem gets solved, it's good for everyone.
RE: ASD on a generator
Have you ever seen a distribution sytem responding to an 'electrical' harmonic resonance ?
If severe enough, can be "Shake, Rattle, and Blow baby"
Not pretty !
RE: ASD on a generator
My point was that mechanical vibrations (the pipes rattling)may arise from sources other than just harmionic resonance, and as EE's it's easy to focus in on just the electrical causes.