29-01-2013, 02:53 PM
Voltage sag/swell compensation using Z-Source inverter based Dynamic Voltage Restorer
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Abstract
This paper presents the modeling and simulation
of a dynamic voltage restorer as a voltage sag/swell mitigation
device in electrical power distribution networks. The dynamic
voltage restorer, with its excellent dynamic capabilities, when
installed between the supply and a critical load feeder, can
compensate for voltage sags/swells, restoring line voltage to its
nominal value within few milliseconds and hence avoiding any
power disruption to the load. A new topology based on Z-source
inverter is presented in order to enhance the voltage restoration
property of dynamic voltage restorer. Z-source inverter would
ensure a constant DC voltage across the DC-link during the
process of voltage compensation. The modeling of Z-source based
dynamic voltage restorer is carried out component wise and their
performances are analyzed using MATLAB software. The
simulation results shows that the control technique is very
effective and yields excellent compensation for voltage sag/swell
mitigation.
INTRODUCTION
Modern power systems are complex networks, where
hundreds of generating stations and thousand of load centers
are interconnected through long power transmission and
distribution networks. The main concern of customer is the
quality and reliability of power supply at various load centers.
Even though power generation in most well-developed
countries is fairly reliable, the quality of supply is not. Power
distribution system should ideally provide their customers an
uninterrupted flow of energy with smooth sinusoidal voltage at
the contracted magnitude and frequency. However, in practice
power system especially the distribution system, have
numerous non linear loads, which are significantly affect the
quality of power supply. As a result, the purity of waveform of
supply lost. This ends up producing many power quality
problems.
DYNAMIC VOLTAGE RESTORER
Dynamic voltage restorer was originally proposed to
compensate for voltage disturbances on distribution systems. A
typical DVR scheme is shown in Fig. 1. The restoration is
based on injecting AC voltages in series with the incoming
three-phase network, the purpose of which is to improve
voltage quality by adjustment in voltage magnitude, waveshape,
and phase shift. These are important voltage attributes as
they can affect the performance of the load equipment. Voltage
restoration involves energy injection into the distribution
systems and this determines the capacity of the energy storage
device required in the restoration scheme.
Z-SOURCE INVERTER
Z-source inverter has X-shaped impedance network on its
DC side, which interfaces the source and inverter H-bridge. It
facilitates both voltage-buck and boost capabilities. The
impedance network composed of split inductors and two
capacitors. The supply can be DC voltage source or DC current
source or AC source. Z-source inverter can be of current source
type or voltage source type. Fig. 3 shows the general block
diagram of Z-Source inverter.
VOLTAGE SAG COMPENSATION IN DVR SYSTEM
In order to meet the requirement of constant voltage
control, closed loop operation is performed for the desired
value of the voltage according to the need. The simulink
model of closed loop control of voltage sag compensation in a
DVR system is shown in the Fig. 7. Initially the system was
subjected to 25% voltage sag at t=300ms and remains up to
t=700ms with the total voltage sag duration of 400ms, in a run
time of 1000ms.
CONCLUSION
In this paper voltage sag/swell compensation using Z-Source
inverter based Dynamic Voltage Restorer is considered. The
control technique is designed using in-phase compensation and
used a closed loop control system to detect the magnitude error
between voltages during pre-sag and sag periods. The modeling
and simulation of closed loop control of voltage sag/swell
mitigation were carried out using MATLAB software.
The simulation results show that the developed control
technique with proposed single phase DVR is simple and
efficient. From the simulation results it was observed that
dynamic voltage restorer compensates 25% of voltage sag and
30% of voltage swell.