18-01-2013, 02:32 PM
Cascaded Multilevel Inverter Based Shunt Active
Power Filter in Four-wire Distribution System
[attachment=47522]
harmonics and the reactive power in a four-wire distribution
system, using five-level cascaded inverter based shunt active
power filter (SAPF). The distribution network which supplies
mixed nonlinear loads and employing SAPF is simulated by
MATLAB/SIMULINK software. The Simulation results indicate
that with the proposed control method, the SAPF is capable of
compensating the source current harmonics and the reactive
power. Simultaneously the THD is minimized and the neutral
current is maintained close to zero.
Keywords- Harmonics and reactive power compensation,
multilevel voltage source inverter, shunt active power filter, total
harmonics distortion.
INTRODUCTION
Nowadays the power electronic equipments are widely used
in distribution networks which act as nonlinear loads. Many
power quality disturbances such as harmonics pollution,
unbalanced load currents, and reactive power problems are
caused by the nonlinear loads. As a result poor power factor,
weakening efficiency, over heating of motors and
transformers, malfunction of sensitive devices etc. [1]-[4] are
encountered.
Active power filters (APFs) are previously not implemented
in power net works, because of unavailability of high speed
power switching devices. Recently the power electronic
development spurred the interest in IGBTs, MOS-FETs etc
[5], [6] and then two-level APFs are developed incorporating
power electronics technology to support the needs of industry.
APFs conventionally used to mitigate harmonics, and to
compensate reactive power in industrial networks, were
already established in around 1970s [5], [7]-[9]. Recently
multilevel converters have gained importance in medium and
high-voltage applications. Multilevel inverters were first
introduced in 1981 by Akira Nabae as three-level neutralpoint-
clamped pulse width modulation (PWM). Now there are
three major types of the multilevel inverters: diode-clamped,
flying-capacitor clamped and cascaded multilevel inverter.
The general concept of multilevel inverters involves producing
an AC waveform from small voltage steps by utilizing a bank
of series capacitors or separate DC sources [10].
This paper presents a compensating system for harmonics
and reactive power in a four-wire distribution network by a
five-level cascaded H-bridge voltage source inverter
(CHB-VSI) based SAPF using phase-shifted sinusoidal PWM
(PS-SPWM) method which is considered as an effective
control method. Among the multilevel topologies, the
cascaded multilevel inverter presents better advantages. The
multilevel inverter unlike the two-level inverter, do not need a
transformer for connection to the medium or high voltage
networks.
II. MULTILEVEL INVERTER AS SAPF TOPOLOGIES
The two-level VSI using IGBTs is clearly the dominating
converter topology in low, medium, and high voltage power
systems. Nowadays, there has been an increasing interest in
using multilevel inverters for medium and high power energy
conversion, especially for drives and reactive power
compensation. Besides, multilevel PWM inverters can be
connected to the medium or high voltage networks without a
coupling transformer. The three-level neutral point clamped
VSI (3L-NPC VSI) is mostly applied in industrial medium
voltage converters. The hard-switching transients of the power
semiconductors at high commutation voltage cause high
switching losses and a poor harmonic spectrum. Subsequently,
problems are created by over-voltages in cables and machines
due to the steep-switching transients [11], [12]. For these
reasons, multilevel inverters (MLIs) have been receiving more
attention in the recent years and have been proposed as the best
choice in a wide variety of medium and high voltage
applications [13]. Fig. 1 show the power circuit of a five-level
cascaded inverter employed as SAPF.
An MLI structure enables to reduce voltage stress and
improve harmonic spectrum without a series connection of
devices, which can be considered as a major merit. Other
several advantages of the multilevel inverters are
• Very low distortion in the voltage output waveform.
• Minimum input current distortion.
• Utilization of low voltage devices in medium voltage
application.
• Low switching frequency.
, ksramarao[at]petronas.com.my, perumal@ petronas.com.my
(a)
(b)
Five-level cascaded converter (a) Power circuit (b) Gating pulses
• Reduced electromagnetic interference (EMI) problems
• Lower overall losses in some cases.
BASIC CONFIGURATION OF THE SHUNT ACTIVE POWER
FILTER
SAPF can suppress the current harmonics in the network by
generating and injecting current harmonics at the PCC which
have the same magnitude but opposite in phase of the current
drawn by the nonlinear loads. The instantaneous power
supplied to the load force the source current as close as
sinusoidal, and the source power factor PF close to unity. The
PWM switching frequency for the VSI is chosen to be high.
The reference compensating currents required to be injected at
the PCC are generated by a p-q theory based controller.
Power Filter in Four-wire Distribution System
[attachment=47522]
harmonics and the reactive power in a four-wire distribution
system, using five-level cascaded inverter based shunt active
power filter (SAPF). The distribution network which supplies
mixed nonlinear loads and employing SAPF is simulated by
MATLAB/SIMULINK software. The Simulation results indicate
that with the proposed control method, the SAPF is capable of
compensating the source current harmonics and the reactive
power. Simultaneously the THD is minimized and the neutral
current is maintained close to zero.
Keywords- Harmonics and reactive power compensation,
multilevel voltage source inverter, shunt active power filter, total
harmonics distortion.
INTRODUCTION
Nowadays the power electronic equipments are widely used
in distribution networks which act as nonlinear loads. Many
power quality disturbances such as harmonics pollution,
unbalanced load currents, and reactive power problems are
caused by the nonlinear loads. As a result poor power factor,
weakening efficiency, over heating of motors and
transformers, malfunction of sensitive devices etc. [1]-[4] are
encountered.
Active power filters (APFs) are previously not implemented
in power net works, because of unavailability of high speed
power switching devices. Recently the power electronic
development spurred the interest in IGBTs, MOS-FETs etc
[5], [6] and then two-level APFs are developed incorporating
power electronics technology to support the needs of industry.
APFs conventionally used to mitigate harmonics, and to
compensate reactive power in industrial networks, were
already established in around 1970s [5], [7]-[9]. Recently
multilevel converters have gained importance in medium and
high-voltage applications. Multilevel inverters were first
introduced in 1981 by Akira Nabae as three-level neutralpoint-
clamped pulse width modulation (PWM). Now there are
three major types of the multilevel inverters: diode-clamped,
flying-capacitor clamped and cascaded multilevel inverter.
The general concept of multilevel inverters involves producing
an AC waveform from small voltage steps by utilizing a bank
of series capacitors or separate DC sources [10].
This paper presents a compensating system for harmonics
and reactive power in a four-wire distribution network by a
five-level cascaded H-bridge voltage source inverter
(CHB-VSI) based SAPF using phase-shifted sinusoidal PWM
(PS-SPWM) method which is considered as an effective
control method. Among the multilevel topologies, the
cascaded multilevel inverter presents better advantages. The
multilevel inverter unlike the two-level inverter, do not need a
transformer for connection to the medium or high voltage
networks.
II. MULTILEVEL INVERTER AS SAPF TOPOLOGIES
The two-level VSI using IGBTs is clearly the dominating
converter topology in low, medium, and high voltage power
systems. Nowadays, there has been an increasing interest in
using multilevel inverters for medium and high power energy
conversion, especially for drives and reactive power
compensation. Besides, multilevel PWM inverters can be
connected to the medium or high voltage networks without a
coupling transformer. The three-level neutral point clamped
VSI (3L-NPC VSI) is mostly applied in industrial medium
voltage converters. The hard-switching transients of the power
semiconductors at high commutation voltage cause high
switching losses and a poor harmonic spectrum. Subsequently,
problems are created by over-voltages in cables and machines
due to the steep-switching transients [11], [12]. For these
reasons, multilevel inverters (MLIs) have been receiving more
attention in the recent years and have been proposed as the best
choice in a wide variety of medium and high voltage
applications [13]. Fig. 1 show the power circuit of a five-level
cascaded inverter employed as SAPF.
An MLI structure enables to reduce voltage stress and
improve harmonic spectrum without a series connection of
devices, which can be considered as a major merit. Other
several advantages of the multilevel inverters are
• Very low distortion in the voltage output waveform.
• Minimum input current distortion.
• Utilization of low voltage devices in medium voltage
application.
• Low switching frequency.
, ksramarao[at]petronas.com.my, perumal@ petronas.com.my
(a)
(b)
Five-level cascaded converter (a) Power circuit (b) Gating pulses
• Reduced electromagnetic interference (EMI) problems
• Lower overall losses in some cases.
BASIC CONFIGURATION OF THE SHUNT ACTIVE POWER
FILTER
SAPF can suppress the current harmonics in the network by
generating and injecting current harmonics at the PCC which
have the same magnitude but opposite in phase of the current
drawn by the nonlinear loads. The instantaneous power
supplied to the load force the source current as close as
sinusoidal, and the source power factor PF close to unity. The
PWM switching frequency for the VSI is chosen to be high.
The reference compensating currents required to be injected at
the PCC are generated by a p-q theory based controller.