SIZING GENERATORS 2

[Padlock01]

Hi Guys, Sizing a LV transformer for motor starting is fairly straight forward (VD) however, I have attached an excel spread sheet that I have used
for starting a 110kW DOL drive I wondered whether there is a norm for sizing Standby Gensets.

 

[waross]

I have had good luck with the following rule of thumb:
Connected load in amps plus three times the full load current of the motor to be started.
This has always worked well for me for standby rated sets.
With prime power rated sets you have an extra 25% cushion.
(eg: A 100 KVA standby set will be derated to 80 KVA if it is rated as a prime power set. There may be a more robust oil cooler and a larger sump, but the basic engine and generator end will be the same as the 100 KVA standby set.)
This rule should be applied with some judgement and consideration of the types of loads and the size of the motor to be started in relation to the size of the generator.
Notes:
1. A naturally aspirated set will pick up the load of a large motor starting with less frequency dip than a turbo aspirated set. The less existing load on the set when the set is started, the more significant this may be.
2. Spec a Permanent Magnet Generator (PMG option) for the excitation. It will have less voltage dip when starting large motors.
3. If you are in a CYA situation, (Cover Your ASSets), then go on to the Caterpillar web site and download their specsizer soft ware.
4. When using SpecSizer be aware that this software has a default setting for voltage dip that is very conservative. Allowing a greater voltage dip when starting large motors and changing the voltage dip setting in SpecSizer may often be an economical advantage. (smaller set)

Bill
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"Why not the best?"
Jimmy Carter

 

[ScottyUK]

5. When adjusting voltage dip settings, make sure your contactors don't drop out.

 

[Padlock01]

Thanks Guy for your input I tried the Caterpillar SpecSizer software it confirmed my sizing in my excel spreadsheet

 

[cherryg]

Please note that during software sizing, the Generator insulation class needs to be specified correctly. In one of our projects, Clients requirements exceeded the standard Class H and resulted in bumping up the rating. Also, the sequence in which the blocks of motors are started should be selected based on worst case scenario.