MV Soft Starters and Voltage Dip
MV Soft Starters and Voltage Dip
(OP)
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.
Also what is the accurate method for calculating the voltage dip during the 11 kV chiller starting?
Thanks in advance.






RE: MV Soft Starters and Voltage Dip
RE: MV Soft Starters and Voltage Dip
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.
RE: MV Soft Starters and Voltage Dip
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
RE: MV Soft Starters and Voltage Dip
RE: MV Soft Starters and Voltage Dip
RE: MV Soft Starters and Voltage Dip
RE: MV Soft Starters and Voltage Dip
http://www.benshaw.com/literature/specifications/890007-03-xx.pdf
RE: MV Soft Starters and Voltage Dip
www.motortronics.com, or 800-767-7792
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RE: MV Soft Starters and Voltage Dip
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