325 MVA with 2% Flicker

[JamesBarlow]

I'll start off by saying I'm a mechanical engineer so could the electrical guys write slowly.

I am working on a project where I will be using an AC drive to power a large piece of equipment. It's a government project so I can't be more specific. The power available from the facility is given in the contract as 325 MVA with 2% Flicker. That's all it says.

The motor I am using will run at an idle speed most of the time with period spikes requiring 4.5MVA. A less frequent spike will drive the power up to 8.1MVA. The spikes are less than .1 seconds in duration and will only occur a few times a minutes at most.

The issue I am dealing with is what does the 2% Flicker mean. Some say it means we can only use %2 of the 325MVA, but that doesn't make sense to me. I understand from the IEEE how it's defined but I seem to be struggling to make the connection between how the motor runs and what kind of flicker it will produce in other systems.

Does anyone have experience, or insight, into large motors being run with a Flicker requirement attached. I did search for papers and information but have only been finding things that discus welders and arc furnaces. I have a request for help from the motor drive supplier but so far I have not gotten a reply yet.

 

[Skogsgurra]

Flicker is a 'Human nuisance spec' It says that your equipment shall not load the grid with short and frequent pulses, which produces an unpleasant flicker in lamps connected to mains fed from the same grid as you take your power from.

The specs for flicker are most severe in the tenths of Hz to few Hz range. So, your 'few times a minute' is your ticket there.

Also, if your grid can deliver 325 MVA and your motor is 8.1 MVA with load changes around 4.5 MVA, you can relax. No chance that such a small load change can influence a grid capable of 325 MVA.

You will have to read up on the standards that apply in your country. But my overall feeling is that you do not have any problems at all.

It may be that you are confusing things a bit. If the 325 MVA is your short-circuit power, then your load changes are a little more than 1 percent of that. Still no problem. And, as I said, the frequency is outside the 'nuisance' ragion.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[fernandoelec]

An additional comment to the Skogsgurra´s info, you must be care with the power spikes, cause in some cases with the load attached to the motor shaft an over voltage can be present in the AC drive's DC Bus and it can explode, please ensure that the AC drive has a DC Braking resistor unit this device will be avoiding any failure in the DC Bus.

The other point with the flicker is the waveform, the Flicker exist with a frequency under 35 or 30 Hz, then the AC drive provider must ensure to you that the waveform doesn´t affect the driving circuits of the power semiconductors.

Fernandoelec

 

[rasevskii]

for JamesBarlow-

I suppose this is some sort of short-circuit testing facility involving a large Motor-Generator set. That is a 325MVA ac generator driven by a relatively small motor, whereby the MG set has a large flywheel effect built in, either as an actual flywheel or extra heavy rotor in the generator. In the past, such machines were typically driven by a wound rotor motor with a liquid rheostat in the secondary/slip ring side/ of the motor. The rheostat position was regulated to get a current limit on the motor stator that the supply network could accept.

The load spikes that you mention in this case would not even be seen as a flicker on the supply network due the high mechanical inertia.

Perhaps in your application it is done some other way and the load spikes could be seen on the supply network side.
More info needed. Is this a VFD of some sort?We have many VFD experts -not me- on this forum..

regards, rasevskii