Hi to all,

I like to install one or more, 90kw inverter with a built-in AC choke but still can contribute as much as 30% to THD, in my power room. The room consist of some soft-starter (110kw x 10 units) and a capacitor bank for power sag and power correction.

I would like to install the VSD to replace some of the old soft-starter and to have speed control. I was advised by experts that it is not advisable to install in the room.

Question:

1. Why is this so? And what is the complications?

2. If it cannot be done what kind of devices can be added and what other ways to install to overcome the problem?

3. Do VSD have such limitations in installations as advise by inverter supplier?

Regards

K C

 

If your equipment is in steel enclosures RF should not be a problem. Harmonics on the line may be a problem and an iso transformer may be needed.

 

If you install VSD in that room you can expect to lose some fuses in your capacitor bank. You can expect to have some resonancy between VSD and your capacitor bank. This can blow your capacitor fuses.

We had the same problem and we just made home made coil on the capacitor bank. We made 4 turns coil at each phase at the capacitor end. This solved all our blow fuses problemes.

 

Dear KC,

You can remove the capacitor banks and use just the VFD at unity power factor.
OR
You will need to install active or passive harmonic filters before every VFD you will install and should be able to reduce Harmonics to less than 6-8%.These filters are also should be capable to reduce all higher harmonics.
Also you will need filters for your capacitor bank to by pass all higher harmonics.
OR
For the cost of 10 harmonic filters you can as well locate the VFD else where. Most of the VFD manufaturers Do not give sufficient data for you to work. They do not even tell you whether the data is for voltage or current harmonic.
The best one I have seen is ABB followed by Danfoss. The worst one is telemecanique.
Talk to your VFD supplier if you can install harmonic filters or remove the capacitor banks since you can also run VFD at maximum power factor.

regards,
Sekar