power
DC
drives
as
well as
DC
transmission systems. The
effect of harmonics generated at the low-power side would
be minimised, either naturally or by the installation of
several medium- and low-power active filters downstream
to contribute to the compensation in such cases. The static-
VAR compensation is then the major concern and is
usually compensated for by using traditional static power
conditionersifilters
[I,
2,
51 as well
as
several sets of
synchronous condensers connected in parallel
[
11
and
cascaded multilevel-inverter VAR compensators [61-65].
The required response time for such cases is in the range of
tens
of
seconds, which is sufficient for contactors and
circuit breakers to operate after taking the optimal-switch-
ing decision
[l,
111.
Power fluctuations in the range of
a
few seconds are, on the other hand, treated by the generat-
ing stations' anciliary devices.
One of the few applications of active filters in high-power
systems is the Japanese bullet train (Sinkansen)
[2,
5,
661,
which uses a parallel combination
of
several active filters.
The control and co-ordination requirements of these filters
are, however, complicated [2].
standard lattice voltage
switched
inverter capacitor structure regulator
4
configuration and connections
Classification according to the power-circuit
parallel AF
series AF parallel AF
+ + +
series AF parallel PF
parallel PF
Power-circuit configurations play an important role in the
selection of the applications,
as
some circuits are suitable
only for certain aspects
of
control and power ranges,
as
dis-
cussed in this Section and illustrated in the block diagram
shown in Fig.
3.
4.7
Parallel active filters
This class of filter configurations constitutes those most
important and most widely used in industrial processes
[2-91.
It is connected to the main power circuit, as shown in
the single-line diagram of Fig. 4. The purpose is to cancel
the load current harmonics fed to the supply. It can also
contribute to reactive-power compensation and balancing
of
three-phase currents, as mentioned above. Parallel fdters
have the advantage of carrying only the compensation cur-
rent plus a small amount of active fundamental current
supplied to compensate for system losses. It is also possible
to connect several filters in parallel to cater for higher cur-
rents, which makes this type of circuit suitable for a wide
range of power ratings. This configuration consists of four
distinct categories of circuit, namely inverter configurations,
switched-capacitor circuits, lattice-structured filters and
voltage-regulator-type filters. Information on these circuits
is given in Table 1.
supply
impedance
filter
Fig.
4
paralkd active filter configuration
nonlinear
load
DC-link
71
DC-link
inductor
smi%
and
,
jcapacitor
active
filter
a
Fig.
5
Invcrter
bud
activejilter,s
LI
Current
fed
inverter
h
Voltage
fed
inverter
coupling active
inductors filter
b
/'$7
a b
Fig.
6
Switched ccpacitor
filters
supply
nonlinear
m
b
nonlinear
C
Fig.
7
Lattice structure
configurations
Fig.
3
AF
Active filter
PF:
Passive
filter
Subdiviion
of power
systtmjZters
accordmg
to
power
cirmit configurations
and
connections
IEE
Proc-Electr Power Appl..
Vol.
147,
No.
5,
September.
2000
in
series
with
parallel PF
405