CHAPTER I

INTRODUCTION

The utilization of electric power is increasing day by day as the industrial growth has attained its height. The population explosion and new cities are contributing towards the steady increase in the demand of electricity around the world. To satisfy the different socio-economic demands, different type of industries e.g. floor mills, shoe firms, automobile industry, cold storages, hospitals, air port, street lighting, cement industry, glass industry, etc all have to be established and supplied by the reliable electric power. Many consumer electronics products need good quality power for their reliable and safe operation. To satisfy these demands, the power distribution network is expanding at a steady growth rate of around 8%. The electric utilities are trying their utmost to keep the network efficient and reliable. However, the reliability and good quality of power are difficult to guarantee and outages/disturbances are a reality in most of the electric utility operations. Utility distribution networks, sensitive industrial loads, and critical commercial operations suffer from various types of outages and service interruptions which can cost significant financial loss per incident in terms of process downtime, lost production and idle work force. The types of interruptions which are experienced can generally be classified as power quality problems caused by voltage fluctuation and outages [1]. Recently, the importance of power quality issues has increased due to various reasons [2-3]. Most important, the consumer utilization pattern has a complete shift from the conventional electricity utilization. The characteristics of load have become more complex due to increased use of power electronic equipment, which results in a deviation of voltage and current waveform from its sinusoidal characteristics. On another hand, equipments have become more sensitive to power quality due to its critical nature. Deregulation of the electrical power market is another factor that has increased the importance of power quality in electric utility system. The most frequent disturbance in electric utility system is the Voltage Sag (about 70% of the registered disturbances [4]. It is defined as a momentary decrease in the root mean square voltage from 10% to 90%, with a duration ranging from half cycle up to 1 min [5, 6].

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[5, 6]. Different reasons lead to voltage sags. They can be a result of fault conditions on source or load side due to lightning, wind, storms, contamination of insulators, animals and other accidents [7]. Sags due to these reasons last until the fault ends or the fault is cleared by a fuse or breaker. Large motors startup or connecting large loads to the utility bus in an area close to the adjustable speed drives (ASD) or even at the same plant are also potential reasons for voltage sags.

Fig. 1.1 Power quality events outside the CBEMA curve [8]

According to survey reports, voltage of 10% - 30% below nominal for 3-30 cycle durations account for the majority of power system disturbances, and are the major cause of industry process disruptions [2]. The Computer and Business Equipment Manufacturers Association (CBEMA) curve shown in Fig. 1.1 may give a clear idea about the problem based on detailed surveys. On the average, there are about 289 disturbances per site per year outside the safe operating area as indicated in the Fig.1.1. 90 out of the 289 events are voltage sags and under voltages and 16 interruptions. In the worst locations, it might reach up to 7121 sags and 146 interruptions [6]. Keeping in view the above mentioned problems it becomes necessary that the problem of voltage sags and power interruption should be solved. It is common in many utilities that different feeders exist in the same service area or run side by side feeding different types of loads. Under such a situation these feeders can share their loads during the time-of-day or on seasonal basis. This load sharing between

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two feeders has been the main theme of this dissertation. Load sharing or load balancing phenomenon are of the same meaning i.e. transfer of load from heavily loaded feeder to the lightly loaded feeder. Briefly this can also be termed as feeder reconfiguration for load balancing. There are different techniques for feeder reconfiguration but the main device which is used to implement feeder reconfiguration is ‘switch’. In the distribution system, two types of switches namely sectionalizing switch and tie switch are used for feeder reconfiguration. The main problem with these switches is the speed of operation whether they are manually operated or motor operated. With the use of these switches, power interruption cannot be avoided. To overcome this problem, a fast solid state transfer switch has been developed and implemented for transfer of load from one feeder (preferred feeder) to the other feeder (alternate feeder) when voltage sag or disturbance (abnormality-conditions) occur on the preferred feeder. The STS has the capability to transfer the load back on the preferred feeder when the fault is removed. Different issues of the STS have been addressed in this dissertation. One of the issues is the difference of impedance at the point of connection of both the feeders. Due to impedance difference, the speed of the STS is affected. To match the impedance of both the feeders at the point of common connection, a state estimator has been designed and implemented to verify its performance to match the impedance (chapter 2). Similarly, to overcome the problem of voltage sag STS-system be utilized effectively. The quality of power to the sensitive load can be improved using STS. Voltage sag problem arises when a feeder become overloaded then due to the voltage drop along the feeder, the tail end consumer may face voltage sag problem and the sensitive load and different process may be interrupted. The reliability of the supply is affected when fault occur on the feeder and consequently power to the load is interrupted. To improve the reliability and power quality, solid state transfer switches may be used in the distribution system to implement different feeder reconfiguration schemes. Chapter 2 presents the implementation of state estimation technique for the impedance matching of the distribution feeder. An overview of the state estimation techniques have been demonstrated from the point of view of application to the power delivery system. The problem of state estimation is usually formulated as a weighted least squares (WLS) problem, which is solved by efficient numerical techniques. The problem is addressed by an over-determined system of nonlinear equations i.e. when number of measurements (Nm) is greater than the number of unknown parameters being estimated (Ns). The problem of state estimation may also be either completely determined when Nm = Ns or under-determined when Nm is less than Ns. Most state estimation algorithms use the formulation of over-determined systems of nonlinear equations and solved as WLS problem [9]. The equations have been formulated using the weighted least square method to obtain the solution of the problem. The feeder impedance matching process has been highlighted to reconfigure the feeder when abnormal condition exists on the preferred feeder or to transfer the load of a heavily loaded feeder to a lightly loaded feeder using static transfer switch (STS). The simulation results show the impact of feeder impedance matching on the performance of the STS in terms of its transfer time. It has been observed that the transfer time has been considerably reduced by matching the impedance of both the feeders under study.

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Chapter 3 presents the description of feeder reconfiguration using static transfer switch (STS) system. Different feeder reconfiguration techniques in the literature have been overviewed. Two case studies are included in this chapter to implement the feeder reconfiguration using STS. Chapter 4 deals with the brief overview of the classical switches operated manually and used in the power distribution system. The literature review of the different techniques about the development of the control logic of the static transfer switch (STS) and its application is discussed. The structure of STS-system has been described. The control logic of the STS using delay logic technique and forced commutation technique is developed and different control logic blocks are discussed in detail. This chapter also includes a comprehensive review of the recent research work on performance of static transfer switch (STS) as applied to the utility distribution network. The impact of load power factor on the transfer time of the STS and a comparison of delay logic and forced commutation technique for thyristor based three phase medium voltage STS has been discussed. The proposed forced commutation technique for thyristor based STS has broadens the scope of applications in utility distribution network for implementation of different demand side management options. Most importantly, the technique has shown significant performance for variable loads having variable power factor. PSCAD/EMTDC software is used for different case studies. Chapter 5 deals with the load balancing problem of the overloaded feeder and describes different techniques for the load balancing. The most important technique for load balancing using STS through feeder reconfiguration has been explained in details. A particular distribution feeder named New-Exchange feeder of the Islamabad Electric Supply COmpany (IESCO), Pakistan, has been considered for case studies. The feeder data including the total load connected, rating of the distribution transfer, single line diagram, etc. has been given in detail. The application of the STS-system has been investigated in detail. Simulation studies show that voltage sag occurs during peak hours and the feeder is overloaded. Sensitive bus voltage sag becomes more than the permissible limits as described in IEEE std. 466. Then it becomes necessary that the preferred feeder load, to some extent, must be transferred to another feeder through feeder reconfiguration. To remove the overloading of the feeder under study, some of the load has been shared by the alternate feeder through static transfer switch (STS). It has been shown that STS-system can share the load efficiently when the New-Exchange feeder becomes overloaded. Chapter 6 gives conclusions and summarizes the accomplishments of this research. The contributions includes; implementation of state-estimation technique to match the impedance of two complementary feeders for reconfiguration by implementing the proposed STS-system, comparison of delay logic technique as well as forced commutation technique of the proposed control logic of the STS-system, and simulation of different case studies. The outlines of the possible future work are also given.

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