Fault Current Interruption Using Dynamic Voltage Restorer (DVR) Assignment Solution

INTRODUCTION

1.1 Overview
Modern electric power systems are complex networks with hundreds of generating stations and thousands of load centers are interconnected through long power transmission and distribution networks. Power quality is major concern in industries today because of enormous losses in energy and money. With the advent of myriad sophisticated electrical and electronic equipment, such as computers, programmable logic controllers and variable speed drives which are very sensitive to disturbances and non-linear loads at distribution systems produces many power quality problems like voltage sags, swells and harmonics and the purity of sine waveform is lost. Voltage sags are considered to be one of the most severe disturbances to the industrial equipments.

Power quality problems are associated with an extensive number of electromagnetic phenomena in power systems with broad ranges of time frames such as long duration variations, short duration variations and other disturbances. Short duration variations are mainly caused by either fault conditions or energization of large loads that require high starting currents. Depending on the electrical distance related to impedance type of grounding and connection o f transformers between the faulted/load location and the node, there can be a temporary loss of voltage or temporary voltage reduction (sag) or voltage rise (swell) at different nodes of the system.

Power distribution systems, ideally, should provide their customer with an uninterrupted power flow at smooth sinusoidal voltage at the contracted magnitude level and frequency .A momentary disturbance for sensitive electronic devices causes voltage reduction at load end leading to frequency deviations which results in interrupted power flow, scrambled data, unexpected plant shutdowns and equipment failure. Voltage lift up at a load can be achieved by reactive power injection at the load point of common coupling (PCC). The common method for this is to install mechanically switched shunt capacitors in the primary terminal of the distribution transformer. The mechanical switching may be on a schedule, via signals from a supervisory control and data acquisition (SCADA) system, with some timing schedule, or with no switching at all. The disadvantage is that, high speed transients cannot be compensated. Some sag is not corrected within the limited time frame of mechanical switching devices. Transformer taps may be used, but tap changing under load is costly.

Another power electronic solution to the voltage regulation is the use of a dynamic voltage restorer (DVR). DVR’s are a class of custom power devices for providing reliable distribution power quality. They employ a series of voltage boost technology using solid state switches for compensating voltage sags/swells. The DVR applications are mainly for sensitive loads that may be drastically affected by fluctuations in system voltage.

DVR is the one of the best FACTS device compared to all other FACTS control devices. DVR is well suited to protect sensitive loads from balanced/unbalanced voltage sag and swell. DVR is basically a controlled voltage source installed between the supply and a sensitive load. The DVR injects voltage to the system in order to compensate any disturbance occur due to supply.

The operation of DVR depends up on control strategy. In this paper SRF Theory and Proportional Integral (PI) controller technique is used for mitigation of balanced/Unbalanced voltage sag and swell. The proposed method is evaluated through MATLAB/Simulink software. In the present chapter literature survey, proposed circuit and organization of the thesis are presented.

1.2 Problem Formulation

Power quality is an issue that is become increasingly important to electricity consumers at all levels of usage. Sensitive equipment and non-linear loads are now more common place in both industrial sectors and domestic environment. Power quality problems encompass a wide range of disturbances such as voltage sag, voltage swell, flickers, harmonic distortions, impulse transients Land interruptions. Faults at either the transmission or distribution level may cause transient voltage sag or swell in the entire system or swell in the entire system or large part of it. Voltage drop will also occur in the heavy load conditions. Short circuits, starting large motors, sudden changes of load, and energization of transformers are the main causes of voltage sags.

Without good power quality, commercial buildings and industrial facilities can suffer from repeated equipment failures, safety hazards, process interruptions and shutdowns. Even two cycles of a 25% voltage dip can cause unprotected microprocessors to malfunction. Electronic controllers on variable speed motors are even more vulnerable to voltage sags than computers.

Many power quality improving techniques have been proposed amongst which DVR is recognized as one of the most effective one. A Dynamic voltage restorer compensates both the transient functions like voltage sag and swell.

The control method of DVR divides in to two parts
a) Derivation of reference signal obtained from feedback signals
b) Generation of gate signals by using PWM controller comparing the reference signal and sinusoidal signal.

In this proposed control strategy reference signals obtained from by using SRF control technique and Proportional Integral (PI) controller technique.

1.3 Conventional DVR Circuit Topology
The new configuration of the proposed DVR block diagram prototype unit connected between the supply and the load. The supply of a three phase voltage is fed into the programming AC power source. The output of the AC power source unit is connected to the Delta–Wye isolation transformers. The active power for injection is obtained from the super capacitor as energy storage. Single-line diagram of a DVR connected distribution system is shown in Fig.1.1.