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Generator starting of autotransformer start 1500 HP motor

Generator starting of autotransformer start 1500 HP motor

Generator starting of autotransformer start 1500 HP motor

(OP)
We're unable to start a 1500hp, 4000 volt motor operating a centrifugal pump under generator power.  Pump starts against closed valve that slowly opens in 3 minutes.  But we don't get there.  Gen manuf.-set CB trips on undervoltage.  Generator manufacturer assures us that the gen-set is sized correctly, 1750 KW.  Very little other loads.  Starter is autotransformer type with taps at 65%.  Starter has power factor correction capacitors.  Generator voltage before adding load is 4160.  New installation, generator manufacture will be out to take a look next week.

Any ideas?  

RE: Generator starting of autotransformer start 1500 HP motor

My first idea, unfortunately, is that your generator is too small.  It's going to be really tough to start a 1500 hp motor on a 1750 kW generator, even with autoxfmr start.

You can try the 50% tap - since this is a centrifugal pump it can probably accelerate on the lower torque.  But the generator is really its own reduced voltage starter in this case - have  you tried measuring the actual voltage on either side of the starter during starting?  

Does the generator have a voltage boost or voltage support option on the voltage regulator?

You also need to question the setting of the generator undervoltage relay.  What it is set at, and why?  Undervoltage doesn't really bother the generator too much - if your other loads can tolerate the voltage dip, you may be able to lengthen the time delay on the UV relay.  

 

 

RE: Generator starting of autotransformer start 1500 HP motor

Couldn't agree more. The basic gross rule-of-thumb is that you need the genset kW to be AT LEAST 1.5x the HP rating (twice the kW of the motor) when using a RV starter on a centrifugal pump, more if trying X-Line. I'd venture to say that at 3 minutes your autotransformer has long been out of the circuit anyway. They are typically rated for no more than 30 seconds.

RE: Generator starting of autotransformer start 1500 HP motor

Also agree the generator is likely too small. It will likely require adjustment of the under voltage relay so it does not trip the breaker during the start.

RE: Generator starting of autotransformer start 1500 HP motor

Replacing the starter with a VFD would likely solve the problem, but will be expensive, obviously.  

RE: Generator starting of autotransformer start 1500 HP motor

Rather than a VFD, I would be looking at ways to reduce the excitation during starting so as to let the generator act as a "Poor mans" VFD.
Lose the undervoltage trip.
Connect the motor to the generator.
Start the generator and ramp the voltage up with the speed. There are a number of ways to do this, both with and without the AVR.
You may use one excitation system for starting and then switch to the AVR.
You may use one or more CTs with resistors across them to develop a load dependent voltage that may be used to fool the AVR into reducing the excitation during starting.
respectfully

RE: Generator starting of autotransformer start 1500 HP motor

Wow waross... Gnarly!

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

RE: Generator starting of autotransformer start 1500 HP motor

I would first try what dpc said, set at lowest tap and bypass the UV trip. If this motor is the only load, even a 30-35% voltage dip should not matter for a moment (as long as the contactors/relays, if invovled do not drop out, hopefully they have separate source of back up power). It should recover pretty fast.

How big is the alternator? The alternator at least should be sized for twice the HP rating.

waross's approach would be like last resort, its far too facny for a new installation. At that point adding a generator in parallel would be more attractive or replace the unit with a bigger one.


RE: Generator starting of autotransformer start 1500 HP motor

The kW requirement during starting is very low ( the p.f. is about 0.2). In the worst case just replacing the alternator with bigger one would be required.

RE: Generator starting of autotransformer start 1500 HP motor

waross,

There must be an great story behind that technique.....

Did it happen in Alaska?

RE: Generator starting of autotransformer start 1500 HP motor

It may sound conuterintutive but sometimes trying to start a motor on higher tap may help. The voltage dip may be bigger but it also recovers faster and motor speeds up quicker.

RE: Generator starting of autotransformer start 1500 HP motor

Hello GirlEE

There are two major components to consider when starting a motor on a gen set.
The engine produces KW and the alternator produces Amps or KVA.

If the engine is too small, the frequency will drop, and if the Alternator is too small, the volts will drop.

Both the engine and the alternator, have an overload capacity and the ability of each to start a motor, is dependant on the overload capacities.

At this stage, it sounds as though you are having a problem with voltage drop, rather than frequency droop, so lets concentrate on this in the short term.

There will be an overload in current during start and the minimum current required is a function of the load torque and the motor starting efficiency. You are already starting against a closed valve, so that minimizes the starting torque requirement.
Using an auto transformer starter can reduce the start current provided that it is set up correctly. In many cases that I see, the motor effectively goes DOL because the transformer steps to full voltage at part speed.
I expect that you are going to need at least the 65% tap to get the pump to full speed, and in many cases, the 80% tap is required to get the motor from 60% speed to full speed. i.e. on the 65% tap, the motor runs out of torque at around 60% speed. This is very much a function of the motor starting characteristics and the load torque.

The first question that I would ask is: Is the trip occuring at the same time as the transformer starter switches to full voltage? Try extending the changeover time ans see if the generator runs for longer.

If the generator is tripping while the starter is on the 65% tap, then you have an excitation problem which is limiting the overload characteristics of the alternator. - This may not be a fault, rather a weakness in the system.
If the alternator is self excited, then the chances are that it will have a short term overload capacity of around 120 - 130%. This is very common on lower cost generator sets. If the alternator has external excitation from a PM generator, then it should be able to supply a short term overload in the order of 300% current.
For this type of application, I always specify a three phase averaging AVR and a PM Generator excitation system. These can usually be added if required.

Self excited systems will commonly require an alternator rated at 250 - 300% of the motor rating to start, PMG excited systems commonly require alternators rated at 120 - 150% to start a pump.

see http://www.lmphotonics.com/genset.htm

Best regards,

Mark Empson
http://www.lmphotonics.com

RE: Generator starting of autotransformer start 1500 HP motor

It is also possible that the setpoints on AVR and/or the governor controls are not set at optimum level. Specially the setting for v/hz response during step loads.

RE: Generator starting of autotransformer start 1500 HP motor

For what it is worth refer to: FAQ237-766

RE: Generator starting of autotransformer start 1500 HP motor

I would also caution that your various starting attempts be far apart or you may end up cooking the motor the generator or both!  You won't find specs for either of your machines relating to failed starts, almost starts, partial starts, etc.  So you need to look at both your machine's starting duty cycles (starts/hr) and then add 30-50% as I suspect things get pretty warm during your aborted starting attempts.

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

RE: Generator starting of autotransformer start 1500 HP motor

Quote (itsmoked):

as I suspect things get pretty warm during your aborted starting attempts

As you will not from various other threads related to this topic in this forum, what gets "warm" the most during these aborted starts is the rotor and it can take a long time for that heat to make it out to the housing of the motor.  Never rely on the housing temperature of motor to decide that it is cool enough to try to start it again.  As long as the prime mover continues to run the generator should be getting plenty of cooling air, but watch that motor rotor.

RE: Generator starting of autotransformer start 1500 HP motor

I see the undervoltage trip as a hinderance.
A VFD was suggested. Rather than using a VFD to ramp up the frequency and voltage, it would be very simple and cheap to use the generator to do the same thing.
With further thought, after disabling the undervoltage trip, if the motor is connected directly to the generator and the engine then started, the underfrequency roll-off feature of the Automatic Voltage Regulator will do a pretty good job of ramping the voltage up as the frequency increases. It should bring the motor up to speed efficiently and with a lot of the advantages of a VFD start.
If this technique is used, I would go direct online and not use the autotransformer starter.
Use a UPS to energise the main contactor in the starter.
A variation of this technique is used to energise Generator Step-up Units. Why not motors?
respectfully

RE: Generator starting of autotransformer start 1500 HP motor

waross:

Your approach has merit. The only difference in a step up tranformer and starting a motor is the starting torque requirement and some real power. Real power is not a problem in this case, the reactive power and the torque requirement is. At very low voltage motor may just stall or draw excessive current.

I believe by defeating UV protection during the start should help a great deal. I would also try at higher voltage than lower, if necessary.  All other cautions about motor heating up mentined above are also valid.

RE: Generator starting of autotransformer start 1500 HP motor

Ramping up the motor with generator is a good idea as long the the other loads mentioned in the OP can be shut off while this  is done or can tolerate the low voltage & freq.

There may be a fair amount of protection that would have to be circumvented.  I agree that that UV relay is probably not necessary unless they plan on operating in parallel with utility at some point.  

RE: Generator starting of autotransformer start 1500 HP motor

Other suggestions:

Got a +500HP motor lying around? Hook it up in a manner where you start it with the generator. Once it's running start the large motor. It will return energy to the system in parallel with the generator. It may be enough to do the trick.  You could disconnect it once it has returned all the energy it's going to, based on the current direction to it. For more stored energy add a flywheel to it.

Another alternative would be a pony motor to get the 1500 up to some speed that the generator can then support a start of.

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

RE: Generator starting of autotransformer start 1500 HP motor

More importantly, does the motor have its own protection relay? It must have, I presume.

If so, and that motor protection relay is not tripping, the generator breaker has no business tripping on UV. Delete/defeat that.

RE: Generator starting of autotransformer start 1500 HP motor

Generator can be setup to trip on UV only when accompanied by large current, indicative of a fault. Even that has to be conditioned for not tripping on a motor start.

RE: Generator starting of autotransformer start 1500 HP motor

If your undervoltage device is a shunt trip, it may be disabled but if it is a low voltage release type trip it will probably have to be physically removed from the breaker.
Re  the reduced speed starting technique:
The Electric Machinery Mfd. Company used to publish an information magazine called the "Synchronizer" (E-M Co. invented, and produced the original field frequency relay.)
In issue #200-SYN-59 they discuss starting large motors on generators.
The techniques discussed:

Shunt capacitors to supply the KVA demand.
About 800% of the capacity used for normal power factor correction will provide about 50% of the starting KVAs. The capacitors must be cut out in steps as the motor is accelerated or serious overvoltages will result with the generator.

Initial voltage boost.
A resistor is inserted into the voltage measuring circuit to fool the AVR into over-exciting and driving the voltage higher. The motor is connected as the voltage is increasing. The timing would be less than a second.

Simultaneous start.
The motor is connected to the generator. The generator field is energised and the generator started and brought up to speed. Applicable to Hydro and steam turbines.

Sub frequency start.
The prime mover is started and run at an idle and the motor is connected. The field is energised and the motor is accelerated up to speed.
Applicable to diesel engine sets.
Note; Under Frequency Roll Off equiped  Automatic Voltage Regulators were not common when this was written.
Voltage is dependant on speed as well as field strength. With full excitation on a set running at 50% speed, the set will produce 50% voltage. As a result we have a fairly constant volts-per-hertz ratio supplied to the motor similar to a VFD drive.
Old style AVRs lacking UFRO would try to maintain full voltage at reduced speed. The normal result was the AVR regulator and failing if the prime mover was run at much below rated speed. The old regulators had a manual switch and were not normally turned on until the prime mover was up to speed.
What I would expect to happen with your set (after you get rid of the undervoltage trip) is that the starting energy demanded by the motor will drag the frequency down. This will result in a reduced demand for starting energy. At some frequency/speed below 60Hz the energy output of the diesel will meet the reducing energy demand of the starting motor and the speed/frequency will stabilize. The UFRO feature of the AVR will have reduced the voltage proportionately. The motor magnetic circuit will not saturate because the volts-per-hertz ratio will have been almost constant. The starting current will be less than across the line because of the reduced voltage. Both the motor and the genset will accelerate together until the frequency is up to 60Hz. The starting cycle will probably be a little longer than across the line utility starting but it should still be quite friendlly towards both the motor and  the generator
I understand that you have already installed the equipment. Equipment changes at this time will be expensive and emarrasing.
The first step, lose the undervoltage trip.
Then do a series of starts and observe the action. Start with the autotransformer starter. Start without the autotransformer.
If you want less voltage drop, try adding a resistor and using some voltage boost.
You may consider taking your notes to a quiet place and spending some time adding the information to your documentation beteew starts. It is a good way to force the more impatient team members to allow adequate cooldown time between starts. You may alsow want to let the motor run for 10 or 20 minutes before stopping it. The rotor cools quicker when it is turning, and a 20 minute run time limits you to 3 starts per hour.
dpc;
Never worked in Alaska. I did have some fun with a couple of small  generators in the Yukon teritory just east of Alaska. The highlight of that adventure was a 10 day canoeing and hunting trip down the Big Salmon river and part of the Yukon river. My partner got a nice moose.  It almost sank the canoe. One trip travelling back south I had a few adventures on the road where jraef ran out of gas. I think he mentioned it in Pat's Pub.
A lot more fun than the generators.
respectfully

RE: Generator starting of autotransformer start 1500 HP motor

Lots of solutions, no feedback from GirlEE as to if any of it is still necessary...

RE: Generator starting of autotransformer start 1500 HP motor

(OP)
Well, anyone curious as to how it turned out.  All incorrect relay settings.  First the undervoltage trip occured because the relay timer on that fct was 100 ms.  Fixed that.  Then we had overvoltage trips.  Two things: relay set at 105% instead of 110%, as reported, and we increased function timer from 1 sec to 2 sec.  We got a beautiful view of the waveform off some fancy recording equipment.  It was great; you could see the overvoltage occur and partially recover right before the 65%-100% transition, then comes a voltage dip that sometimes went low enough to clear the overvoltage fct. timer.  Overvoltage got as high as 1.2-1.25 times at the max, as the inrush current dove.  Lasted above 110% appx. 1.7 sec.  We did all the troubleshooting pumping against a closed valve for very small time periods.  After changing relay setting, we pumped water on generator power successfully.

RE: Generator starting of autotransformer start 1500 HP motor

(OP)
Anyway thanks all of you for the great ideas and further resources.  I guess it's a letdown just to be relay settings.  That old saw about hoofbeats seems applicable...

RE: Generator starting of autotransformer start 1500 HP motor

Good to hear! Glad it was all that simple.
Now where's those cool plots??

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

RE: Generator starting of autotransformer start 1500 HP motor

Girlee:

Thank you for the feedback. I am glad all worked out for you. Hopefully you have all protective settings reviewed by some competent professional.

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