Power Factor Correction

Yes. Can be used.

The proper answer requires much more information about the equipment and the nature of the loads. This is not a trivial subject.

"...you can't indiscriminately add power factor correction capacitors to a system without understanding how their presence will affect the system. Variable frequency drives (VFDs) for motors,DC power supplies for computers, electronic ballasts for fluorescent lighting,and other non-linear loads can create harmonics. If you don't consider the existence of these harmonic sources and the inductive reactance of the existing system,you'll create the potential for harmonic resonance..."

From this paper:

http://www.eaton.com/ecm/groups/public/@pub/@electrical/documents/content/ia02607001e.pdf

You only need to consider the issues of resonance if the total of the VFD loads in the system becomes "significant" when compared to the total load...in my experience in industrial plants with 2MW loads, we used a figure of about 30% which we never reached.

I used to go running screaming to the consultants with the power dataloggers to help us with the dreaded harmonics when we added a 5 hp drive somewhere, until an old hand EE quietly suggested it was a load of BS until the harmonics load was high enough compared to the total load. After all, did you notice who publishes all the breathless information on harmonics? The guys who sell the stuff you need to control harmonics!

After that, we simply installed simple line reactors in front of a VFD to reduce the garbage they put onto the plant 3 phase bus. And gave all the BS peddlers the boot.

Jon.

RAMConsult #2 is correct for nonlinear load. But in a most basic form, the load normally is resistive, inductive. The resistive, real part is performing real work and billed accordingly. The inductive part generates circulating currents, that does not perform real work, but needed to be compensated for. Hence a capacitor bank on the user's premises, or extra billing, if the utility have to perform the services.

The explanation is similar for VFD's, in principle only. In practice, it is an engineering speciality to maintain acceptably low harmonics on the grid back from the drives. Given the frequently high powers involved, it is not a lighthearted exercise.

I know low power filtering, but there it is, where it stops, for me in practice.

Yes, you can use them together so long as you also include harmonic filtering.

It just so happens that I am working on the very same problem now.

I found this document from ABB to be very good and went a long way to reinforcing my understanding, eliminating my concerns, alerting me to risks and filling in some knowledge voids.

If you are familiar with PFC, just start reading from page 20....or just read the whole document. It's all good.

Sawmilleng has a valid point regarding how big of an issue it is, but there is risk of damage to the PFC caps if they are not dimensioned with adequate power margins to cope with the harmonic loads.

Filtering the harmonics is not hard to do and will give you peace of mind and economic savings once installed. That's if and only if they are really required.

Which way you decide to go will be heavily influenced by your plant's inventory and your primary power source. Then there's acceptable risk consideration and availability of funds.....

I don't know whether this is a new installation or a retrofit, but either way, if this has become something that you need to care about, you would do well to procur some power monitoring apparatus and become familiar with its correct usage and proper interpretation of the data it will provide you with.

In my installation I will be budgeting for ABB's PQF system (or similar) and make allowance for its accommodation but put off the final purchase decision until the plant has been built and real measurements made under typical loading scenarios. It is too hard to calculate accurately without the risk of over engineering based on worst case.....

At present my installation consists of a 400kVA transformer on a pole and crate loads of refurbished machinery. There aren't alot of VFDs but there are a lot of motors.

No legacy issues to deal with. I have the luxury of a blank piece of paper to work with.

What's your situation?