ASD on a generator 3

[hydrae]

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

 

[busbar]

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.

 

[BobPE]

this is from the pump side of the problem....when you brought the pump up to speed, was it in a range that they operated the pump before? From your post it seems that maybe you used a very low speed to start to load the generator....vertical turbines have very querky operating ranges that a VFD can find when you over and underspeed them....

Just a thought...

BobPE

 

[hydrae]

Busbar

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

 

[busbar]

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.

 

[hydrae]

OK
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

 

[busbar]

Discuss it with the drive and pump manufacturer. They may be able to advise you on similar cases and potential problems.

 

[dadfap]

hydrae

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

 

[peebee]

I'd start by checking out the possibilities that busbar mentioned.

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.

 

[busbar]

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.

 

[jbartos]

Suggestions:
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.).

 

[gcaudill]

How about a couple troubleshooting of ideas:

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.

 

[dpc]

Operation of rich harmonic loads on a small local generator can often cause problems. As dadfap mentions, voltage regulators used to be common problem in these applications. Older Basler regulators had problems with the harmonic voltages present. Newer regulators have fewer problems.

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.

 

[hydrae]

Fellow practicing Engs.

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

 

[dadfap]

hydrae

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

 

[HarryDampers]

If more than approx 30% of the VA rating of a generator goes into 6 pulse drives then there is a distinct possibilty that the Damper windings will overheat and damage .

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 .

 

[hydrae]

dadfap
The load table in the specs and drawings listed the motor and drive with brand and model number as the primary load.
Hydrae

 

[peebee]

Your spec says the generator has to work from .8PF lagging to unity. Note that as I mentioned before, the ASD may well be capacitive (leading PF) at low loads.

 

[CB2]

If you provide the following information, I will run a hrmonic analysis. That will help in dealing with the generator supplier.

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

 

[hydrae]

CB2

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