25-02-2013, 12:32 PM
Active Filters for Power Quality Improvement
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Abstract
This paper deals with problems related with
harmonics in power system networks. Several international
standards issued to control power quality problems are briefly
described and some important methods to analyse electrical
circuits with non-sinusoidal waveforms are introduced and
evaluated. One of these methods - the p-q theory - was used to
implement the control algorithm of a shunt active filter, which
is also described in this paper as an application example. The
filter can compensate for harmonic currents, power factor and
load unbalance. Both simulation and experimental results are
presented, showing that good dynamic and steady-state
response can be achieved with this approach.
INTRODUCTION
Due to the intensive use of power converters and other
non-linear loads in industry and by consumers in general, it
can be observed an increasing deterioration of the power
systems voltage and current waveforms. Figure 1 presents a
power system with sinusoidal source voltage (vs) operating
with a linear and a non-linear load. The current of the nonlinear
load (iL1) contains harmonics. The harmonics in the
line-current (is) produce a non-linear voltage drop (Δv) in
the line impedance, which distorts the load voltage (vL).
Since load voltage is distorted, even the current at the linear
load (iL1) becomes non-sinusoidal.
POWER QUALITY STANDARDS
To assure the harmonization of legislation within the
European Community, without which the free interchange
of goods and services would be affected, several directives
have been released. One of such directives is the Council
Directive 85/374, related to the liability for defective
products. Its 2nd article defines electricity as product and in
this sense it becomes necessary to establish its
characteristics.
ACTIVE FILTERS
Active filters are special equipments that use power
electronic converters to compensate for current and/or
voltage harmonics originated by non-linear loads, or to
avoid that harmonic voltages might be applied to sensitive
loads.
There are basically two types of active filters: the shunt
type and the series type. It is possible to have active filters
combined with passive filters as well as active filters of both
types acting together [6].
Figure 3 presents the electrical scheme of a shunt active
filter for a three-phase power system with neutral wire,
which, can both compensate for current harmonics and
perform power factor correction. Furthermore, it allows load
balancing, eliminating the current in the neutral wire. The
power stage is, basically, a voltage-source inverter with only
a single capacitor in the DC side (the active filter does not
require any internal power supply), controlled in a way that
it acts like a current-source. From the measured values of
phase voltages (va, vb, vc) and load currents (ia, ib, ic), the
controller calculates the reference currents (ica*, icb*, icc*,
icn*) used by the inverter to produce the compensation
currents (ica, icb, icc, icn). This solution requires 6 current
sensors and 4 voltage sensors, and the inverter has 4 legs
(8 power semiconductor switches). For balanced loads
without 3rd order current harmonics (three-phase motors,
three-phase adjustable speed drives, three-phase controlled
or non-controlled rectifiers, etc) there is no need to
compensate for the current in neutral wire. These allow the
use of a simpler inverter (with only three legs) and only 4
current sensors. It also eases the controller calculations.
ACTIVE FILTER SIMULATION RESULTS
Figure 9, presents simulation results using
Matlab/Simulink [14, 15] for a three-phase power system
with a shunt active filter with control based on the p-q
theory. It includes the following waveforms, corresponding
to two-cycles of steady-state operation: phase voltages
( va, vb, vc ); load phase and neutral currents ( ia, ib, ic, in );
total instantaneous power at load ( p3 ) and source ( p3s ); and
source phase and neutral currents ( isa, isb, isc, isn ).
ACTIVE FILTER EXPERIMENTAL RESULTS
The next figures illustrate the performance for different
operating conditions of a shunt active filter with control
based on the p-q theory. This active filter was developed at
University of Minho, and uses digital control implemented
with a standard microcontroller [16, 17]. Although the
power system and the active filter are three-phase, for the
sake of better understanding, the figures only show
waveforms for phase a: reference compensation current
(ica*), compensation current (ica), phase voltage (va), and
supply current (isa).
CONCLUSIONS
This paper deals with problems related with harmonics in
power system networks. Several international standards
issued to control power quality problems are briefly
described and some important tools to analyse electrical
circuits with non-sinusoidal waveforms are introduced and
evaluated. Among other application, these tools are useful in
the implementation of control algorithms for active filters.
Active filters are an up-to-date solution to power quality
problems. Shunt active filters allow the compensation of
current harmonics and unbalance, together with power
factor correction, and can be a much better solution than the
conventional approach (capacitors for power factor
correction and passive filters to compensate for current
harmonics).