MV Soft Starters and Voltage Dip

[ElecDar]

I have four chillers 1.5 MVA each; I’m looking for 11 kV soft starters to limit the voltage dip during starting to be within 3% max. I found 6.6 and 3.3 kV soft starters but I’m trying to save the required transformers and switchgear to step down the voltage from 11 kV to 6.6 or 3.3 kV.

Also what is the accurate method for calculating the voltage dip during the 11 kV chiller starting?

Thanks in advance.

 

[electricuwe]

Have you already found the 1,5 MW motors for 11 kV ? To my mind such a high voltage is not economical for a motor in that power range.

 

[electrageek]

Sometimes it is possible to create a softstart by connecting the motor windings in a Y for the first portion of the start and then switch to a Delta for full voltage. This would give you a reduced voltage starter without the need for transformers. It would require having six wires from the motor to the starter and the have three AC contactors to provide the switching needed. I have seen this approach used on a 13.8Kv 12000HP motor with the \ac contactor being GE Magnablast 15KV breakers. Its a thought anyway.

The voltage dip for starting one motor is calculated by using the locked rotor current of the motor and then calculating the drop across each component that is in series with the motor (i.e. motor wires, feeder wires, step down transformers, back to the main service. Consideration also needs to be given to worst case conditions (i.e. how much load is already on the feeders probably the worst case would be having three of the four chillers already running and then staring the fourth. You will need the impedance of the various components.

I am unaware of any 11KV softstarts. I will look around and see if any of our vendors have anything.

 

[bigamp]

I do not know of any 11kV soft starters, only 3.3kV. You have done well to find 6.6kV. If you need to do a reduced voltage start, looks like your best bet will be a closed Korndorffer autotransformer type. Expensive I know. An autotransformer can be superior to a soft start because it gives you the most motor torque per amp of line current.

Another possibility might be, if you have not committed to a motor type yet and if one is available in your speed/power, to use a slipring type wound rotor induction motor with an automatically controlled stepping resistor for the rotor winding. Start with maximum rotor resistance to give high torque/low starting current and progressively remove resistance to provide torque as the motor accelerates.

Both I guess are "old technology" and expensive. The solution proposed by electrageek would be OK if the motor was Delta but most MV motors are star.

Obtain (from the power or lines company) the source fault level and X/R ratio to the point of common coupling (i.e. the point to which others connect to the power system) then calculate the source resistance and reactance. Work out what your starting current will be and then calculate the voltage dip as described by electrogeek. Don't forget that the motor starting current is at a low power factor (about 0.2) and you MUST take this into consideration when you calculate the voltage dip. The motor vendor will be able to provide all the necessary motor data such as full load amps, locked rotor current and the starting power factor. This voltage dip is the one that is important as it affects other users, not just your plant. It is limited by what the power company or lines company will let you do. Typically this might be up to 3% say 9 times per hour, depends on country. It is a "relative" dip i.e. voltage falls by x during the impact load.

The allowable dip you can cause in the plant itself is up to you. What can you live with? If you take in supply at HV, then you will have an HV/11kV transformer, cables etc. Work out the voltage dip (as described by electrageek). Most of the dip will be across your HV/11kV transformer and you can work out what this will be (as a check) using the transformer regulation formula given in the J&P transformer book.

You should also think about doing a motor starting study. To do this you will need motor and driven equipment torque-speed curves and inertias and also the motor current-speed curve.

Regards

 

[jbartos]

Suggestion: It may be a good idea to model your power distribution system by using electrical power system analysis software, first. Then, it would be prudent to select the hardware optimally for your chillers while adhering to the electrical software results. To do calculations by pencil and paper is not so accurate, and it is more timeconsuming.

 

[gordonl]

Try Benshaw, I beleive they can do it.

 

[redtrumpet]

Benshaw, phone 519-291-5112, ask for Ron Vines. Don't know if they can do 11 kV, but it's worth asking.

 

[gordonl]

Their web site says they do 13.8kV, and they would probably customize. I have some 11kV, and I have it in mind that they said they could do it.

http://www.benshaw.com/literature/specifications/890007-03-xx.pdf

 

[jraef]

If you are still looking for 11kV starters, call Motortronics in Clearwater FL.

http://www.motortronics.com,

or 800-767-7792 That which does not kill me, makes me stronger... and pissed off!

 

[infomaniac]

You can also contact ABB in Wichita Falls, TX.
(888) 385-1221 x 7054

or visit

http://www.abb-control.com

for additional information