25-08-2017, 09:32 PM
A Novel SRF Based Cascaded Multilevel Active Filter for Power Line Conditioners
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Abstract
This paper presents a novel synchronous reference
frame (SRF) controlled five-level cascaded multilevel inverter
based shunt active filter for power line conditioners (PLCs) to
improve the power quality in the distribution system. The SRF
based compensation is developed by sensing load currents only,
which require for harmonics and reactive power compensation
due to non-linear loads. The attempt is to come up with a simple
control strategy. Incidentally it is different from conventional
methods and provides superior performance. The cascaded
multilevel voltage source inverter switching signals are generated
from proposed triangular-periodical current controller; that
gives better dynamic performance under transient and steady
state conditions. The proportional integral (PI) controller is used
to maintain the capacitance voltage of the cascaded inverter
almost constant. The extensive simulation results demonstrate
that the cascaded multilevel inverter based active power filter
using the synchronous reference frame controller effetely
compensates the current harmonics and reactive volt amperes
under both steady state and transient conditions.
INTRODUCTION
Nonlinear loads such as diode/thyristor rectifiers, switched
mode power supply (SMPS), welding equipment,
incandescent lighting, and motor drives are degrading power
quality in transmission and distribution grid systems. These
non-linear loads result in harmonic or distortion current and
create reactive power problems [1]. These harmonics induce
malfunctions in sensitive equipment, overvoltage by
resonance, increased heating in the conductors and harmonic
voltage drop across the network impedance that affects power
factor [2]. Traditionally passive filters have been used to
compensate harmonics and reactive power; but passive filters
are large in size; aging and tuning problems exist and can
resonate with the supply impedance. Recently active power
line conditioners (APLC) or active power filters (APF) are
designed for compensating the current-harmonics and reactive
power simultaneously [3].
SRF Control strategy:
The synchronous reference frame theory is developed in
time-domain based reference current generation techniques.
The SRF is performing the operation in steady-state or
transient state as well as for generic voltage and current; it’s
capable of controlling the active power filters in real-time
system. Another important characteristic of this theory is the
simplicity of the calculations, which involves only algebraic
calculation. The block diagram of the synchronous reference
frame controller is shown in Fig 3.
SIMULATION RESULT AND ANALYSIS
The performance of the proposed SRF controller based
cascaded active filter is evaluated through Matlab/Similink
power tools. The system parameters values are; Line to line
source voltage is 440 V; System frequency (f) is 50 Hz;
Source impedance of LS is 1 mH; Filter impedance of Rc, Lc is
0.1 Ω; 1 mH; diode rectifier RL, LL load: 20 Ω; 100 mH; DC
side capacitance (CDC) is 2100 μF; Reference voltage (VDC, ref)
is 150 V; Power devices are IGBTs with freewheeling diodes.
The simulation result of the rectifier load current or source
current before compensation is shown in Fig 5 (a). The
reference fundamental current is extracted from the loads
using the proposed SRF controller that is shown in Fig 5(b).
CONCLUSIONS
This paper indicates the suitability of cascade multilevel
inverter based active filter for power line conditioning of
distribution networks. The cascaded inverter provides lower
cost, higher performance and higher efficiency than the
traditional PWM-inverter for power line conditioning
applications. The cascaded inverter switching signals are
derived from the proposed triangular-periodical current
modulator that provides good dynamic performance under
both transient and steady state operating conditions. SRF is
employed to extract the fundamental component from the nonlinear
load currents. This controller is developed by sensing
load currents only. This approach is fairly simple to
implement and is different from conventional methods. The
PI-controller maintains the capacitance voltage of the
cascaded inverter nearly constant. The extensive simulation
results demonstrate the performance of the APF under
different non-linear load conditions.