Generator sizing for high voltage motor loads 2

[sohaila7]

Hello everyone
I have to estimate a generator size for motors whichv are of two differnent operating voltages - 380V and 6.6kV.

380V motors range from 1HP to 332 HP. 6.6kV motors range from 350 HP to 6800 HP.

The name plates of most motors have gone vague. Moreover, these motors are according to IEC standards (therefore, the nameplate does not contain any NEMA code). I have assumed starting PF as 0.25 for all motors and motor efficiency as 0.89.

1) Please guide me if my assumptions are OK.
2) Does the formula for calculating running kW and starting kW same for motors of all the voltages?

Sohail

 

[SilverArc]

Speak to a manufacturer and ask for their generator sizing progams and plug in the information you have.

These programs only ask for running power factor. You have to make sure, you also enter the starting method in the proogram used for motors. In the result, if the allowed freq. drop is 8% and voltage drop is approx. 25% during starting, you are good to go.

Now to your questions:
Motor voltages will not make a difference, I hope you know you will have to use a transformer for feeding two different type motors.A generator will generate only one voltage.

I am not familiar with the effect of connecting a generator to a transformer, the inrush might have an effect... I am sure, some body will be able to tip you for that in this forum.

People like "WAROSS.".. literally master these areas.

Cheers !

 

[waross]

I did not comment because I have not sized generators for motors at voltages higher than 600 (575) volts. We don't consider motor voltage in the range of 120 to 600 volts.
Re frequency drop and voltage drop;
The older voltage regulators will keep the voltage at 100% as the frequency drops. (Within reason.)
Virtually all of the voltage regulators supplied for smaller sets (up to 1000 KVA) in the last 15 or more years have Under Frequency Roll Off. (UFRO)
This holds the voltage at 100% for frequency drops of about 2 Hz. If the frequency drops further, the voltage is reduced proportionally. 2 Hz. is about 3 1/3%. A frequency drop of 8% would result in a voltage drop of *8 - 3 1/3% = 4 2/3%. The reduced voltage sheds some of the load and the engine is better able to recover its speed.
respectfully

 

[SilverArc]

Waross,
Just a quick comment on your post, I have never done a hand calculation and have always used Cummins software or CAT package and the results are fine, if the freq. drop is 8% and Vol. drop 25% during starting.
Do you think, I have been doing it wrong ?
You mentioned , the gen. might have to shed some load to recover if the freq. drops more than 2 hz, but if there is no load shedding scheme, than this load will be always there during starting and generator should technically shut down.
Please advise me, about this freq. issue.
Regards,

 

[waross]

Load shedding is done automatically by the Automatic Voltage Regulator. If your frequency drops 8% then the AVR will reduce the voltage setting to reduce current. I am sorry if I have cause confusion with my use of the phrase "load shedding" There is no switchgear to disconnect loads. The load is reduced across the board by the reduced voltage and the resulting lower current.
The action of the AVR is called Under Frequency Roll Off, UFRO.
It typically starts at -2 Hz, or 58 hz. on a 60 Hz. system.
That is 3 1/3% so with an 8% drop in frequency you can expect the AVR to drop the voltage by 8% - 3 1/3% = 4 2/3%.
Re the Cummins software. I have been doing it by hand since before the software was available and I have never bothered to aquire the software.
Given the interest that Cummins has in supplying good installations and at the same time remaining competitive I would expect their sizing software to be excellent.
By the way UFRO also protects the AVR from low frequency burn-out. Before the days of UFRO on AVRs, there was a switch to turn off the AVR. If you had to run the engine slow for some reason and forgot to turn off the AVR, then AVR burnout was almost automatic. Slow running is not a problem with UFRO equiped AVRs. As the frequency drops, the voltage drops until the engine can carry the load. Then, like a VFD, frequency and voltage rise together until the load has accelerated and the frequency is back to normal. If you push it too far, the drop in lighting levels and the beeping of UPSs on under voltage is a concern. Part of my sizing routine is to consider the effect on other loads of too much frequency drop when starting a large load.
respectfully

 

[sohaila7]

Thankyou for this informative discussion

Coming back to my basic question, can Waross (or somebody!!) illustrates how to compute the starting kVA requirement for a 6800 HP motor with the motor data above?

What should be the starting kVA if there two motors, one 6800HP and the other 3800HP? What size of generator should I select?

 

[waross]

Are you able to ALWAYS start the 6800 HP motor first?
Do the motors start in a set sequence or do they start and stop at random?
respectfully

 

[sohaila7]

No, this is not the first motor to start.

Actually I am to calculate a generator required for one Power Station's black start.

This is the biggest motor that is started, in the sequence, in the middle. Rest are much smaller motors compared to this one.

The motors will not be started and stopped randomly.

The next highest rated motor is 2574 HP at 6600V.

What I have done is so far is that I have calclated the running kVA (coming out to be around 12,240 kVA) that will be required. When it came to caluclate the starting kVA, the requirement for 6800HP is comming out to be very very large, 197,721 kW, at that point I got confused and wished to have an expert opinion.

Hope that the picture is clear now.

 

[waross]

It looks like your starting KW is 13 times your running KW.
I would do some more research myself before sizing this set.
However, my rule of thumb is to add all the motor loads at 100% except for the largest, that I add at 300%. On a small installation I will often use nameplate current instead of HP or KW as nameplate current includes the adjustment for power factor and efficiency.
Then I step back and look at the whole picture. What are the loads, a couple of large motors or many small motors and one or two large motors? What lighting and voltage and frequency sensitive loads are included?
I sometimes adjust my figure up or down.
I think that the scope of your project justifies a call to Cat, cummins and Detroit diesel to request a copy of their sizing software. That's what I would be doing.
And don't forget; If the plant is 10% undersized it will never be forgotten. If it is 100% oversized it will be forgotten much sooner.
And consider oversizing the engine 25%. This is often done on Prime power sets so that the engine is still capable of pulling the load when it is old and worn out. The larger engine is also a great help in starting large engines. The alternator windings will tolerate a momentary overload better than the engine. (The engine slows down). With the extra power of the larger engine you can overload your alternator windings for a few seconds of starting duty and hold your frequency and voltage higher.

 

[ScottyUK]

You can not possibly be considering a 200MW set to start and run a 5MW motor and a few smaller drives. Something is desperately wrong with the calculation. I think at more likely power level of 50 - 60MW that Wartsila are still in the frame with a big low speed reciprocating engine but you are realistically looking at a gas turbine running on either gas or distillate for a black start facility. Distillate (diesel / naphtha / kerosene) is easier to store in bulk than gas. Likely OEMs are Rolls-Royce, GE, Solar, Alstom, ABB, Siemens, maybe MHI too. The gas turbine would start up drawing its own auxiliary power from a smaller diesel set of maybe 1000kW or so for a 50MW turbine.

You need a motor starting study to determine your worst case conditions. Rules of thumb are too vague for reliable application on big HV motors which are usually custom built to optimise certain parameters (e.g. to reduce LRC for example or to maximise starting torque).


----------------------------------

Sometimes I only open my mouth to swap feet...

 

[Bahram7]

Large induction motors in the range of several thousand HPs draw between 8-10 pu starting currents. You need to use a motor starting software to evaluate the situation correctly.

Bahram7

http://www.simtech-intl.com