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Topic Title: Effect of MVARs on a Generator?
Topic Summary: Query Regarding Starting of HV Drives
Created On: 14 March 2014 09:36 AM
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 14 March 2014 09:36 AM
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niallcampbell

Posts: 17
Joined: 25 July 2008

Hi folks,

Does anyone know of a good resource that I can read in order to improve my understanding of the following? (Relationship between Generator standing load and effects of starting HV machines).

My problem is as follows.

I have a 6.6kV Gas Turbine driven Generator (rated at 18 MW, 23.5MVA, 0.8pf) currently running on low load on an offshore platform.

Standing load is approx 5MW at 3MVARs. The generator therefore has approx 13MW spinning reserve.

Currently the platform are reluctant to start a 1890kW HV motor as in the past starting an HV machine against one generator has sufficiently drawn down the system voltage to the point where the emergency generators kick in (typically these don't auto-start until they see less than 300V on the essential switchboard bus bars).

Motor Rating:
Power - 1890kW
RPM - 1759
FLC - 192
Starting - DOL
PF @ FLC - 0.89
Eff - 94.3
Starting PF - 0.2

Can anyone help me to understand why a generator, with 13MW spinning reserve, is struggling to start the above HV motor? I suspect the increase in the VARs is the issue but I'd just like to be able to prove it or have peace of mind!

Thanks for any information.

Niall
 14 March 2014 03:58 PM
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statter

Posts: 126
Joined: 06 February 2013

Presumably its an Induction motor. If so using DOL starting it will draw about 7 x Full Load Current initially and this will fall off as the motor comes up to speed.
The issue is in simple terms that this will exceed the generator rating for a short period, and the generator AVR may struggle to deal with this sort of spike. I suspect that you have both of these effects acting together.

You either need more inertia on the system - to hold up the voltage during starting or more generation capacity or a different starting method. Ideally soft start which would limit current to much closer to FLC.

Hope this helps.
 24 March 2014 07:15 AM
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niallcampbell

Posts: 17
Joined: 25 July 2008

Hi,

Thanks for your reply.

Soft start isn't an option here, we have many HV drives on the installation, all DOL. What we are trying to prove, one way or another, is if the one 18.8MW 6.6kV Gas Turbine-Alternator is sufficient to start the necessary HV drives for production. Rule of thumb indicates that there's plenty spinning reserve to get the motor started, and doing a quick calc based on Generator Capability indicated to me that both the increase in MW and MVARs should be OK for the machine.

So I want to know why the motor rated as above affected the system so badly.

As you say, it could be that the old Generator MAVR isn't responding quickly enough to the drop in system voltage. We have an ongoing project to replace these with Brush A32's.

If anyone knows of any good books on the subject I'd appreciate a link to them.

Thanks,

NC.
 24 March 2014 11:53 AM
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statter

Posts: 126
Joined: 06 February 2013

hello
by my calcs full load =1890/0.89 =2123kVA. Starting load approx. 7 x 2123 = 14,865kVA assuming the volts hold up. you nominally have this available (except your volts are falling) but the power factor will be well under 0.8 -( you say 0.2 above )whilst the motor starts and that's the root of the problem. You're trying to work the generator / AVR outside of its design envelope.

Are you starting the motor off load?
Statistics

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