Generator starting of autotransformer start 1500 HP motor

[GirlEE]

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?

 

[rbulsara]

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.

 

[rbulsara]

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.

 

[waross]

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

 

[jraef]

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

 

[GirlEE]

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.

 

[GirlEE]

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...

 

[itsmoked]

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

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[rbulsara]

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.