Direct Online Compensation

[SilverArc]

Hi Everybody,

I have made sure, that I am not asking a question that has been answered before.
I had no doubts earlier about this problem before I spoke to few manufacturers.
The application is a 4160 V 1250 HP Dewatering pump operating power factor .86.

It is started using Direct on line starter, using a Vacuum Contractor. The idea is to connect this medium voltage cap. bank.

As per my information, Cap Banks are of two types; Fixed or Switched. Typically Switched will be an automatic cap bank using some kind of controller.

I called a sales guy from Eaton and he advised that In medium voltage, you cannot connect a 600 KVAR cap. bank right away, because that will create issues of over voltage. So, the best way to avoid is using a CT to measure current and inject 150 KVAR cap bank in sets of four. This CT will activate a controller that will insert each set bank.
He also mentioned some Code regulation and some IEEE standard.

The I spoke to a Engineer rather a Sr. Engineer from GE, who has been involved in starting of 20 MW motors using 10-12 MVAR cap banks. He did not outwardly reject the sales guy idea but advised that you might have to worry about some issues like, when you insert the cap bank after the Contactor before motor, It might lead to over excitation when the power supply is off.

I tried to understand this issue using ETAP and did a motor starting analysis but it did not create any over voltage.So, that was not done the right way.

I tried to understand that phenomena theoritically in a transient fashion: Here are my arguments;
When the motor start, the first thing is Inrush for one cycle or so and then the starting current for 2 secs, In the mean time motor gains speed. How do we account for the Inrush current of capacitor banks.

This was my Story.
Basically, I need advise from people who have used medium voltage capacitor banks for medium voltage motors with Direct on line starting.

Thanks

 

[KVuorio]

Hi!

I was once involved in project where was 6kV induction motor with direct start and 6kV capacitor bank connected to same switchgear.

There was special arrengement how to start the motor, i try to explain:
First the phase1 and phase3 of capacitor bank was closed by contactor and after 5ms (milliseconds) the motor contactor and the phase2 of capasitor bank was closed. They used some synchronous relay to make contactors switch in desired delay.

This arrengement eliminated the overvoltage and inrush current issues.

I made the measurements to show to the end customer that the contacts really are closing with 5ms delay.

 

[LionelHutz]

Hmm, outright I wouldn't say switching 600kVAR is going to cause problems. We did an install switching 3600kVAR on-line. You do have to study the complete system to see what effect it'll have though.

Assuming you are trying to compensate for the system voltage sag the typical install is to put the capacitors on the output of the motor contactor and energize it all at once. However, you do have to be careful of over exciting the motor on coast down as you've already been told. Some times, there is another capacitor contactor that is closed first so that the motor and caps can be energized at the same time but the capacitor contactor allows the caps to be disconnected from the motor when turned off or even disconnected once full-speed is reached.

If you're trying to correct the power factor of the motor then you need to figure out what size of cap will correct the power factor without over correcting (keep the combo inductive, not capacitive). Then, you just connect them to the motor leads. The reactive component of the motor remains fairly constant and a fixed capacitor size will work.

The step controlled capacitor bank you describe is more applicable to correcting the power factor for a whole system. The controller measures the system power factor and adds or removes capacitors as necessary to maintain a certain power factor. 600kVAR might work for about 2500kVA of connected load.

Make sure you compensate the overload settings if necessary since the combined motor and cap current will be less than the motor current alone.

 

[bigamp]

The most economical solution to power factor correction is to connect the capacitor downstream of the motor contactor so that the capacitor is energised at the same time as the motor is (and you use only one item of switchgear). You must be careful in sizing such a capacitor. Its capacity should be such that it draws a current that is no greater than about 90% of the motor no load current. This has been explained very well on this forum a number of times by a guy called MarkE (who is great value). If you oversize the capacitor you can get a resonance problem when the motor is deenergised (the capacitor can excite the motor). I don't know that any software will model such things but fortunately there is more to engineering than software.

The restriction on capacitor size as per above may mean that you are unable to correct your motor power factor to the desired value so you will need to introduce some more capacitance that will need to be separately switched.

If you are trying to minimise voltage dip due to inrush current you need to have quite a lot of capacitance to switch in at the same time as the motor is started but you then need to switch most of it out once the motor is up to speed (may even need to progressively switch some out as the motor runs up).

 

[SilverArc]

Thanks All for your comments.

Could you throw some light on the transient phenomena involved.
When a motor starts, most of the starting current, which is six or seven times and most of this is reactive in nature and if my capacitor is connected at the same time, than the motor will suck power from capacitor. That means there will be no inrush from utility.
I guess I am having a hard time understanding this phenomena.

I know, there must be very few people who could put this theoretically.... .I would appreciate a word.

Thanks