Across-the-line motor starts vs VFD starts seen at Power Co.

[fastline12]

We have an issue with power and see no way around it but to bend the rules if we can. Our power company has provided us with plenty of "ampacity" but then goes on to say "nothing larger than 10HP on the line". This is a rural area. I have asked the question about VFDs as I assume their only real issue with bigger motors has to do with the inrush but not quite sure. I would think I could bring a 100HP motor up carefully on the line and apply a nice constant load and they would never know the difference? We maybe power factor but I seem to remember VFDs running darn good PF as well?

What I am proposing is that if 10HP across the line starts are fine, I would think a 20-25HP would be fine with a 1-2sec accel rate?

Anyone have any ideas or thoughts we have not considered that might throw a red flag with the power co? Only thing I can think of is the constant load/unload of the motor loads from industrial equipment. That might wiggle down to 100A hits on the secondary. This is a 7.2kv primary so about 3A delta on the primary.

I think this all comes down to customer calls from lights flickering or maybe upsetting their own regulators or something.


I think we have discussed maybe even beefing up the DC buss capacitor bank in the VFDs as a reserve or motor starts but not yet sure if we will need to install a delay device so they don't all try to charge at once. I would really like to figure out how much capacitance it might take to make things more stable.

 

[GroovyGuy]

Can your Utility provide the system impedance, or fault levels, at the PoCC? With that number in mind it is easy to calculate the voltage drop caused by motor starting.
What is the nature of your load, constant, or variable torque? If constant torque, your only option may be a VFD. Over the last several years I have been working in rural areas as well. The utility there generally will insist on a either a soft-starter or VFD on anything larger than 100hp. In most of these instances we have installed VFDs since the load is constant-torque, plus the Operators like to have the turn-down.
Oh, per your request above, the peak inrush current to a VFD can be quite high. This is due to the charging current of the DC caps. The impact of this is reduced significantly since the OEM will normally insert a reactor on the AC input. However I have seen the impact of not installing a reactor on the AC input on MV VFDs. The AC bus voltage dropped to 0.7puV for one cycle, this was sufficient to extinguish most of the HPS lights in the plant.


"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)

 

[HCBFlash]

Rural and other small power companies treat single phase customers quite differently than 3 phase customers. They act like,and apparently believe that they know everything about 1ph., and don't want the rest of the world in on their knowledge, but they use consultants or contracted engineers for 3ph issues, which is what precipitates reason. I recently had my own B.S. with my local power monopoly, and just did it their way for the sake of time and hassle.

I'd say you can stay below their radar by going with that slow start on your unmodified equipment. If they come around sniffing, tell 'em you went with different equipment after speaking with them, and SHUT UP. I've seen it done plenty. If you want to fight them you'll need lots of time, and a pretty good legal and engineering budget

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Me wrong? I'm just fine-tuning my sarcasm!

 

[waross]

It doesn't matter what the Engineer says. The tariffs are the rules.

I've asked a couple of times for some information to allow us to look for the tariffs online.
You don't know what the rules are until you check the tariffs.


Bill
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"Why not the best?"
Jimmy Carter