12-09-2012, 03:55 PM
Control Scheme of Hybrid Active Filter for Power Quality Improvement
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Abstract
A control scheme of hybrid active filter with
integrated series active filter and parallel passive filter
for power quality improvement is presented. The system
is capable of compensating the current and voltage
harmonics and voltage sag in distribution system. The
line current is controlled to be sinusoidal in phase with
mains voltage and load terminal voltage is controlled to
be fixed under a wide range of distorted line voltage. The
power rating of inverter in the series active filter is
reduced by comparing with that of pure active filter to
suppress the harmonics. A series voltage compensator is
used to improve the power quality of load terminal
voltage. The control concept of hybrid active filter is
analyzed with the experimental results in a scaled-down
laboratory prototype to demonstrate the proposed control
strategy.
Introduction
Power quality is becoming important due to a large
of power electronics used in the electric power system.
Nonlinear loads in power system such as diode rectifiers,
adjustable speed drives and arc fumaces have caused
problems of current harmonics and voltage distortion.
These drawbacks cause many problems, such as low
power factor and large losses in the transmission and
distribution lines. For the critical and sensitive loads such
as digital electronics, communication products and
hospital equipment, ideal sinusoidal mains voltage on the
load terminal is required for their proper operation. It is
an important issue for the utility to improve the power
quality and power pollution. Passive filters tuned at the
dominant harmonic frequencies are employed to partially
solve load harmonic currents. However series and
parallel resonance with the mains impedance, strong
dependence on mains impedance and variation of filter
characteristics with the time are the main drawbacks.
Altematively, active filters [I-IO] are the solution to
suppress the harmonics and compensate the reactive
power.
DC Link Voltage Regulation
The dc link of voltage source inverter must be
maintained at the desired level to keep the proper
operation. Series active filter compensates the load
terminal voltage including voltage sag. In this condition
the dc link will supply (or absorb) real power to the
system. In the adopted system a low power rating active
rectifier with bi-directional power flow is employed to
regulate the dc bus. The error between the reference dc
bus voltage and actual dc voltage is input to the voltage
controller to obtain the amplitude of rectifier current
command. Because the active rectifier is operated under
the unity power factor condition,' the amplitude of
rectifier current is multiplied by a sinusoidal fundamental
wave in phase with mains voltage to generate the
instantaneous current command of active rectifier.
Experimental Results
The proposed control scheme of the hybrid active
filter for power quality improvement is verified by the
experimental results in a single-phase scaled-down
laboratory prototype. An experimental IKVA
single-phase voltage source inverter is employed as a
series active filter. The inverter is made of high voltage
IGBT switching devices and is controlled by using the
hysteresis control method. A single-chip microcontroller
(Intel 80196MC) is adopted as the kernel in the
implementation of a digital controller and all tasks such
as current and voltage sampling. The line current, dc-link
voltage and output voltage of converter are measured
with current sensors, optocoupler and potential
transformer. In the first test, we assumed that line voltage
is a pure sine wave, i.e. v , ~ = O . Fig. 6(a) shows the
measured waveforms with only passive filter operation.
In this case the active filter is not operated, i.e. v*,=O.
The line current contains some high frequency harmonic
currents. The passive filters tuned at the dominant
harmonic frequencies are used to filter the low order
harmonics. The current distortion is observed in the
mains current.
Conclusion
A control scheme of hybrid active filter which is a
combination of series active filter, active rectifier and
shunt passive filter is presented to improve the power
quality and reduce the power pollution due to the
nonlinear loads. It can suppress the harmonic currents
and harmonic mains voltage and to regulate the load
voltage due to the utility under-voltage or over-voltage
case. To suppress the harmonics of nonlinear load, the
impedance variation method is used to generate the
equivalent output voltage of active filter. A series voltage
compensator is employed to provide a sinusoidal load
voltage. The active rectifier is employed to perform
constant dc bus voltage for proper operation of series
active filter. A hysteresis voltage comparator is uscd to
track the output voltage command of series active iilter.
The experimental results verify the validity of‘ the
proposed control scheme.