What is the best approach for reducing harmonic distortion?

Question

I have a power system where on some buses, large converter driven machines result in high harmonic distortion. As far as I know, there are mainly two ways to reduce the harmonics.

1. A filter to remove (reduce) unwanted harmonics within a range of frequencies
2. Reactors to create a smoother wave form.

I'm wondering what are the pros and cons for the two alternatives, and are there any other (better) alternatives? I cannot change any of the existing equipment, but I can add equipment (criteria: smallest size then lowest cost). The converters are 6-pulse rectifiers without filtering, and cannot be substituted (12-pulse would typically give a smoother wave).

The filter will only reduce some harmonics, and I have problem with the 5th, 7th and odd harmonics between 25-37th. I could design a filter to reduce the higher order harmonics, but then I would be stuck with the high 5th and 7th components. (Or have a filter to remove the 5th and 7th, leaving me with the higher order components).

Reactors seems like the best approach, but almost all similar systems I've seen have filters. The size and weight must be as low as possible (even if that might increase the price of the equipment).

The equipment rating is in the range 800-1500 kVA and I have THD above 10%. There are no resonance frequencies in the network (the lowest is around the 180th harmonic).

Is one of the approaches generally better than the other. Will there be any significant difference in size for the two?

I hope someone has any views on this. Thanks!

Answer 1

Robert, Nonlinear loads would introduce odd harmonics in current (mainly 3rd, and next 5th). The Voltage supply THD would be effected by harmonic current due to input impedance drop. My first reaction is to measure the harmonics in current drawn by VSD by a PQ meter; it is likely to be rich in 150Hz and 250Hz. Simulate the power supply and VSD load and fine tune the filter required. By rule of thumb, connect a 30% kVA rating LC shunt filter tuned for 150Hz, on load bus. Pure inductance in series or Capacitor in shunt are not an elegant solution. I have faced this problem on 25kV 50Hz single phase ac electric traction supply and shunt tuned LC filter solution has worked for 10 years, after burning quite a few MVAR pure shunt PFC. I can simulate the system for you for free (no charge) if the source impedance, supply cable and load parameters details are known.You can try MATLAB or PSPICE yourself too. Ofcourse there is an expensive solution of DPFC too. Best of luck.

Answer 2

Capacitors will pass high frequency signals. Inductive coils will block high frequency signals. An inductive coil before the converter will do two things for you: #1 it will block high frequency signals from repeating back into the distribution system #2 it will block high frequency signals (spikes from the utility system) from damaging the converter. You can start searching here: http://www.newark.com/webapp/wcs/stores/servlet/Search?catalogId=15003&langId=-1&storeId=10194&categoryId=800000005762&st=3%20phase%20choke%20coil&pageSize=25&showResults=true&pf=810456104

The above link is for applications up to 150HP (112 KVA). I see you provided your equipment rating at 800 to 1500 KVA (2000HP). In large applications such as this "Isolation Transformers are used. These are essentially the same as the Line Reactors listed in the link but on a larger scale. Unfortunately on equipment this size there will be nothing small both in the ease of installation and size/weight and cost.