Technical comparison between Active and Passive Harmonic filters. Harmonics are pollutants in the electrical distribution network. Generally current harmonics are the cause for generation of voltage harmonics. This is due to source impedance. This phenomenon can be discussed in details. Obviously a stiff source will generate less voltage harmonics. Normally Harmonics are measured in terms of THD% or individual orders in % of fundamental frequency values. Current harmonics in any network are generated due to use of non linear loads. By non linear loads, we mean, the loads which do not draw sinusoidal current from sinusoidal Voltage wave form. We generally find presence of odd order harmonics in present day industries. The main non linear loads responsible for creating those disturbances are;

Variable frequency drives. D.C. Motors. Rectifiers. Inverters and Converters. All electronic loads.

We understand that all the electrical machines and equipments are designed for giving “BEST” performances when supplied with pure sine wave. Presence of harmonics violates this condition, which results in poor efficiency and high unexplained failure rate in various equipments. To take care of this problem all the electricity boards in India have started imposing regulations with respect to injecting harmonics in to the grid by any consumer. There are few standards like IEEE-519 for generation of harmonics, accepted globally and by our country also. It is beyond any dispute that it is our responsibility to keep the electrical distribution network clean. It is accepted that the present day industry in India cannot do away with VFDs, DC motors and such other non linear loads. But it is also necessary to improve the power quality. To achieve this we have few options available.

1) Use compensated electrical equipments which generate current harmonics within limits. 2) Stop using plain capacitors for power factor improvement, instead use detuned

capacitors. De-tuned Capacitors do not generate harmonics if designed properly. 3) Use passive filters to filter out harmonic currents.

4) Use Active filters. But such filters are useful only in meeting the requirements of

electricity boards. In most of the cases active filters do not provide any solution, when those are intended to eliminate break downs in various equipments due to presence of harmonics.

5) Use of a combination of Active and passive harmonic filters is preferred some times, on

account of commercial and technical reasons. Following is a brief comparison between passive and Active filter systems. The argument are supported by explanation in Drawing No. 01 & 02

PASSIVE HARMONIC FILTERS

1) P.H.F are filters in true sense. 2) Those filters actually short

circuit and remove the high frequency currents (distortions or harmonics) from the distribution network.

3) Such filters do not allow

harmonic currents to circulate in the network, which results in elimination of equipment break down due to harmonics.

4) The reliability of operation of

protection relays improves when harmonic currents are filtered from the network.

5) P.H.F. clean the entire system,

whether down the line or up the line, if the locations of connection are selected properly.

6) P.H.F are very rugged

maintenance free and any

ACTIVE HARMONIC FILTERS

1) A.H.F. are not filters but those only inject opposite nature of distorted current in the line. Thus only the power quality towards the source is improved and never down the line.

2) A.H.F. do not short circuit or

remove the harmonic currents. But only try to improve the waveforms towards source.

3) A.H.F. do not stop harmonic

currents circulating in the network. 4) Protection relays keep

misbehaving. 5) A.H.F. do not show this behavior

because as explained those are not actually filters.

6) A.H.F. are complicated technically,

use sophisticated electronics and

break down can be attended easily by the user at site.

7) There is no software or

electronic components. 8) Passive harmonic filters are

more economical than active harmonic filter.

9) Comparatively the running

losses are low. 10) One major disadvantage with

passive harmonic filter is possibility of those getting over loaded due to sudden change in the loading pattern. But this can be taken care of by over designing the system, looking at future expansion.

11) However in case of additional

non linear loads parallel passive harmonic filter can be used effectively.

software, hence the maintenance is difficult and every time the manufacturer has to be contacted. In case of any break down the user cannot attend to even small problems. The manufacturers of Active filters take advantage of this fact and literally take the user for a ride and charge heavily for after sales service.

7) As explained, Active filters have lot

of sophisticated electronics and software.

8) Active harmonic filters are always

costlier than passive harmonic filters.

9) Running losses are higher than

passive harmonic filters. 10) Active harmonic filter do not have

tendency of getting over loaded. Also many such units can be paralleled for additional loading.

For Waveforms j.a.simon 9841040723