25-06-2014, 02:18 PM
CASCADED H-BRIDGE MULTILEVEL INVERTER BASED DSTATCOM
CASCADED H-BRIDGE MULTILEVEL INVERTER BASED DSTATCOM.pdf (Size: 504.81 KB / Downloads: 26)
ABSTRACT
The Power Quality (PQ) in distribution system is
affected by the pollution introduced by the customers. Voltage
Sag is one of the Power Quality problem created by the
nonlinear loads. When nonlinear load is connected to a
source, there is a dip in the voltage which could be critical for
the entire system. The power electronics based equipment
known as Custom Power Devices (CPD) is used in
distribution system to solve PQ problems. DSTATCOM
(Distribution Static Compensator) which is connected in
shunt at point of common coupling is one effective solution
for system facing such problems. This work proposes a
Cascaded H-bridge (CHB) Inverter based DSTATCOM to
compensate voltage sag in power distribution network. CHB
converters are being considered for the increasing number of
applications due to their high power capability associated
with low output harmonics and low commutation losses. The
proposed design uses a standard three-leg inverter (one leg
for each phase) and an H-bridge in series with each inverter
leg which uses a capacitor as the dc power source. The
performance of the proposed DSTATCOM is validated
through simulation using MATLAB software with its
Simulink and Power system block set tools and also the
performance of the system without DSTATCOM and with
DSTATCOM is evaluated
INTRODUCTION
Modern power systems are of complex
networks, where hundreds of generating stations and
thousands of load centers are interconnected through
long power transmission and distribution networks. Even
though the power generation is fairly reliable, the quality
of power is not always so reliable. Power distribution
system should provide with an uninterrupted flow of
energy at smooth sinusoidal voltage at the contracted
magnitude level and frequency to their customers. PS
especially distribution systems, have numerous nonlinear
loads, which significantly affect the quality of power.
Voltage sags are one of the most dominating power
quality assets, which dragged the attention of many
researchers as the sensitivity of loads are increasing due
extensive usage of power electronic devices. Fault at
distribution level,
DSTATCOM
Distribution Static Synchronous Compensator
(DSTATCOM) is a shunt connected device. This can
perform load compensation, i.e. power factor correction,
harmonic filtering, load balancing etc. when connected at
the load terminals. It can also perform voltage regulation
when connected to a distribution bus. In this mode it can
hold the bus voltage constant against any unbalance or
distortion in the distribution system. The DSTATCOM
must be able to inject an unbalanced and harmonically
distorted current to eliminate unbalance or distortions in
the load current or the supply voltage. The structure of
DSTATCOM is similar to STATCOM but its control is
different from that of a STATCOM
MODULATION STRATEGY
The modulation methods used in multilevel
inverters can be classified according to switching
frequency. Methods that work with high switching
frequencies have many commutations for the power
semiconductors in one period of the fundamental output
voltage. A very popular method in industrial applications
is the classic carrier-based sinusoidal PWM (SPWM)
that uses the phase-shifting technique to reduce the
harmonics in the load voltage. Another interesting
alternative is the SVM strategy, which has been used to
reduce the harmonics. There are several kinds of
modulation control methods such as traditional
sinusoidal pulse width modulation (SPWM), space
vector PWM, harmonic optimization or selective
SIMULINK MODELLING
Figure-4.1 and 4.2 shows the Matab/Simulink
power circuit model of DSTATCOM. It consists of four
blocks named as source block, nonlinear load block,
control block, measurements block. The system
parameters for simulation study are source voltage of 3.3
kV, 50 Hz ACsupply, Inverter series inductance 10 mH,
Source resistance of 0.001 ohm and inductance of 30mH
Load resistance and inductance are chosen as 100ohm
and 500mH respectively
CONCLUSION
A model of system feeding nonlinear loads has
been developed using MATLAB Simulink.
DSTATCOM with Cascaded H-bridge Inverter is
investigated. The cascaded H-bridge multilevel boost
inverter without inductors uses a standard three-leg
inverter (one leg for each phase) and an H-bridge in
series with each inverter leg. The load voltage, RMS
voltage, current, real power, reactive power under
nonlinear loads is simulated. From the results we can see
that there is an improvement in power factor. Finally,
the performance of the system without DSTATCOM and
with DSTATCOM using CHB is evaluated.