MV capbank & MV softstarters

[colonelbogart]

Need some advice...

We have 4160V MCC serving 1-650kW, 2-310kW and 5-460kW motors driving screw compressors for refrigeration system. All motors are dol

1. There are some suggestions to install softstarters for these though we did operate for almost 2 years now w/o problem.

2. The overall powerfactor is 80-82 only and another suggestion is to install MV capacitor bank to improve pf and save money

We are normally served by Utility company but we have our own standy generators in case of power failure...Compressors operates automatic depending on the load (modulates) and in "auto sequence mode" computer will start compressors as needed or programmed. We did not tried to operate this system (100%) in standby power ever since.

Considering huge initial investment for those suggestions are they justified enough?

Need your views guys...

 

[Marke]

Hello colonelbogart

1. The use of soft starters depends on your requirements.
The soft starter will eliminate current and torque transients and steps. This will reduce supply voltage transients and surges, plus reduce the shock loading on the mechanical systems. The soft starter can reduce the starting current which can be beneficial if there are voltage regulation issues with the full start current. The minimum start current is a function of the speed torque curve of the compressor and also the speed/torque and speed/current curves of the motor.
A correctly selected and commissioned soft starter will give a "gentle" start which will reduce start related problems and extend equipment life.

2. Improving the power factor will not alter the KW drawn by the motor. It can reduce the supply losses and if you are paying a penalty or surcharge for either poor power factor, or KVA maximum demand, there can be a payback. If you pay on KWHr only, there will not be any saving worth considering unless you are close to overloading your own supply transformers. The reduction in KVA could enable you to increase the KW loading without increaseing transformer sizing.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[jraef]

Excellent advice from Marke. I know that several PFC capacitor salespeople will attempt to convince you that capacitors will save energy, but Marke made it clear that the money savings come in the form of avoided penalties only if you are being assessed power factor penalties to begin with.

Regarding soft starters, in general they will save on long term mantenance costs of your motors and power train components, but the amount of savings may be difficult to quantify without detaled mantenance records. The only other consideration is that because of sizing of major components, the smallest 4160V soft starter that you can buy is capable of 1000kW (although most can be turned own to control anything smaller). This translates to more of a cost difference when compared to DOL starters for those motors. If, however, you cannot start the motors when under standby power, that must be the overriding consideration because money saved on cheaper starting methods is a waste if the system capacity is lost on standby power.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[colonelbogart]

Thanks for your valuable suggestions...Probably we will just live with what we have now. Budget cannot be justified...