1.  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.

2. 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

3. 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