25-01-2013, 02:28 PM
Power Quality Improvement using Repetitive Controlled Dynamic Voltage Restorer for various faults
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Abstract
In this work a repetitive controller has been used in the control system of the DVR [1] to compensate for key power quality problems namely voltage sags, harmonic voltages and voltage imbalances. This control system contains a feed forward term to improve the transient response and a feedback term to enable zero error in steady state. This paper proposes to verify results given for a line to line fault in the reference [1] and extends to conduct three phase fault and single line to ground fault. All results are presented and analyzed. MATLAB/SIMULINK software is used for simulation. The Simulation results shows that the repetitive control approach performs very effectively and yields excellent improvement in voltage quality problems.
INTRODUCTION
HE importance of power quality (PQ) has risen very considerably over the last two decades due to a marked increase in the number of equipment which is sensitive to adverse PQ environments, the disturbances introduced by nonlinear loads, and the proliferation of renewable energy sources, among others. At least 50% of all PQ disturbances are of the voltage quality type, where the interest is the study of any deviation of the voltage waveform from its ideal form . The best well-known disturbances are voltage sags and swells, harmonic and inter harmonic voltages, and, for three-phase systems, voltage imbalances. A voltage sag is normally caused by short-circuit faults in the power network [2], [3] or by the starting up of induction motors of large rating [4]. The ensuing adverse consequences are a reduction in the energy transfers of electric motors and the disconnection of sensitive equipment and industrial processes brought to a standstill. A comprehensive description of voltage sag can be found in [5].
DESIGN OF THE CONTROL SYSTEM
The aim of the control system is to regulate the load voltage in the presence of various kinds of disturbances. The control structure proposed in this paper is based on the use of a feed forward term of the voltage at the PCC to obtain a fast transient response, and a feedback term of the load voltage to ensure zero error in steady state.
CASE STUDY
A Simulink Model
The power system and the control system have been implemented in MATLAB .The test system is comprised of a 400 V 50 Hz source which feeds three different loads: 1) A squirrel –cage induction machine 2) A non-linear load which consists of an uncontrolled three phase rectifier with an inductive-resistive load and 3)A three phase sensitive load which consists of a star made up of a resistance connected in series with an inductance in each phase. A two level DVR is connected between the PCC and the sensitive load by means of a 20-KVA coupling transformer with a unity turns ratio and a star connected secondary winding. The voltage of the DC storage device is 650 V.
CONCLUSION
The DVR with the repetitive controller presented in this thesis is able to improve the Power Quality problems like the voltage sags, harmonic voltages and voltage imbalances for different types of faults .The results showed that irrespective of the magnitude and duration of the fault the DVR is able to improve the voltage sags, harmonic voltages and voltage imbalances across the sensitive load.