Protection System Design For Power Distribution System In The Presence Of Distributed Generation

ABSTRACT

Due to the increasing demand of energy and the need for nonconventional energy sources, distributed generation (DG) has come into play. The trend of unidirectional power flow has been gradually shifting. With new technology comes new challenges, the introduction of DG into the conventional power system brings various challenges; one of the major challenges is system protection under DG sources. These sources pose a significant challenge due to bidirectional flows from DGs as well as lower fault current contribution from inverter interfaced DGs. This paper reviews existing protection schemes that have been suggested for active distribution networks. Most of these protection strategies apply only to smaller distribution systems implying that they may need to be extended to larger systems with a much higher penetration of distributed generation. In the end, a potential protection scheme has also been recommended as a future work.

COVER PAGE

TITLE PAGE

APPROVAL PAGE

DEDICATION

ACKNOWELDGEMENT

ABSTRACT

CHAPTER ONE

• INTRODUCTION
• BACKGROUND OF THE PROJECT
• PROBLEM STATEMENT
• AIM AND OBJECTIVES OF THE PROJECT
• PURPOSE OF THE STUDY
• SCOPE OF THE PROJECT

CHAPTER TWO

LITERATURE REVIEW

• REVIEW OF THE STUDY
• PROBLEMS DUE TO DG PENETRATION IN MICROGRIDS
• PROBLEMS REGARDING PROTECTION
• VARIOUS SOLUTIONS FOR THE PROTECTION ISSUES

CHAPTER THREE

3.0      RESEARCH METHODOLOGY

• CASE STUDY OF 33 KV TRANS-AMADI RADIAL DISTRIBUTION NETWORK
• DESIGN ANALYSIS

CHAPTER FOUR

4.1      ANALYSIS OF RESULTS

CHAPTER FIVE

• CONCLUSION AND RECOMMENDATION
• RECOMMENDATION
• REFERENCES

CHAPTER ONE

1.0                                          INTRODUCTION

1.1                            BACKGROUND OF THE STUDY

An electric power system is a network of electrical components deployed to supply, transfer, and use electric power. An example of an electric power system is the grid that provides power to an extended area.  There exist three major levels in electric power system which includes generation, transmission and distribution. Electric power is conveyed from a central and remote source location (generation) to distant location for the end users via transmission and distribution networks with the distribution networks being the closest to the users. These distribution networks can be radial or ring but in most case traditional radial network is adopted due to its low cost, simple protection scheme, facilitates network stability and reduction in number of protective device [1, 2]. Radial network is a form of unidirectional power flow from source to load points. The power loss due to voltage drop in transmitting power to the consumer end and the cost incurred in building new transmission lines have led to the growing interest and wide acceptability of distributed generation.

DG requires use of large small size distributed generators within the radial network [3, 4]. DG can be categorized into direct current (d.c) power generators or alternating current (a.c) power generators [3] and distributed generator referred to in this study is the Trans Amadi gas fired alternating current generator.

These distributed generators hen connected to the network, come along with the following benefits: improvement of network reliability and voltage profile, cheaper electric power supply, reduction of load supplied from a central source, increased network capacity, reduction in electric losses and environmental pollution [5 – 8]. But distributed generation penetration in a network is not without a cost as it gives rise to technical challenges whenever they are connected to a network. Studies have shown that the unidirectional flow of power in a traditional radial distribution network changes to bidirectional when distributed generator is connected [9]. Change in power flow impacts on the network in the following areas; reliability, operation, protection and control of existing power source and islanding operations [10, 11]. The impact on the existing protection scheme of network can also be broken down into areas such as protection blinding, decrease or increase in fault current due to  DG removal or connection, sympathetic and abnormal tripping of protective devices [10]. Also, protection scheme can be applied either or on both the feeder side and generator side in the network, requiring protective devices such as fuses, relays or reclosers [12]. This study centres on protection on feeder side of network in the presence of distributed generators through an efficient and reliable protection scheme. The complexity of a protection scheme depends on the nature of distribution network. While a closed loop (ring network) offers lots of benefits such as continuity of power supply and improved security compared open loop (radial network), the protection scheme of a closed loop is more complex than open loop. For instance, the use of pilot wires instantaneous protection for a closed loop network is a complex protection scheme [1] compared to the technique of inverse definite minimum time overcurrent protection (IDMT) and time graded overcurrent protection have been employed in the design of a protection scheme for a radial network with emphasis on the coordination of the protective devices. Moreover, most protection scheme ensures the coordination of protective devices in the distribution network. But most protective device coordination fails as well as its reclosing operation, due to changes in power flow, direction and magnitude of fault current contribution from the inserted generators in the network [6]. But this technical challenge of mis- coordination has been overcome through several proposed approaches as stated in literatures. Most solutions to these technical challenges have been summarized into wide area protection scheme, adaptive protection and communication/interaction between relay protective devices [13]. A combination of communication technology, distribution system automation and multi-agent based protection scheme has been used to solve the problem of coordination of protective devices in a network [14]. The technology of superconducting fault current limiter (SFCL) have been used to solve coordination problems of protective devices which can limit fault current contributions of distributed generators [15]. There is little or no power loss with this proposed method. But SFCL have inherent properties which affects the coordination of devices so that they get out of their initial setting values unless its resistances are carefully selected within accepted ranges. Through the use of fault current limiter (FCL) which is more cost effective than SFCL, incorporated in a protection scheme, fault current contribution from DG have been properly dealt with, leading to a more efficient and effective protection scheme whose devices are well coordinated [16].

This ensures that main protective device operate quickly in the event of a fault, otherwise a back-up device operates. Device coordination can also be referred to as selectivity. In this case, devices provide back-up protection to other zones of protection by delaying operation while at the same time operate as faster as possible within the main/primary zone of protection. This is possible using the technique of inverse time overcurrent relays which functions in such a way that as the operating time increase, then the current magnitude decreases. The whole essence of selectivity or coordination is to ensure maximum power delivery with minimum power system disconnection [17].

Protection schemes are provided for distribution systems for quick disconnection of faulty section from the remaining healthy portion of power system. Main aim of protection schemes is to restrict the fault spread. Normally distribution lines and feeders are protected by over current relays. Over current relays are used as primary as well as in backup protection relays for the distribution networks. However, their slow operating speed is not a desirable feature for their application as primary protection schemes for sub transmission systems. For sub transmission systems, distance relays are ideal choice for primary protection and over current relay are used in back up protection relays.

For an interconnected power networks, for each fault location on a line, relays are installed at near end bus and far end bus. The relay which is supposed to clear the fault first is known as primary relay. In over current relay coordination studies, failure of one over current relay is backed with other over current relays. The relays which are operating when the main relay fails to operate are known as backup relays.

1.2                                   PROBLEM STATEMENT

Power grids have gathered a significant amount of attention within the past decade and becoming an essential asset in the energy industry. The ability to integrate sustainable energy generation methods into the distribution network is one of the main reasons for microgrids popularity. A wide variety of Distributed Generation (DG) including wind and other micro-turbine generation, photovoltaic generation along with energy storage, makes the microgrid viable in both grid-connected and islanded modes while reducing the power losses. There are various technical challenges to be tackled in order to harvest the full potential of microgrids, and protection is one of them. When a distributed generator is not protected failure will become inevitable. Various solutions were introduced, driven by the development of protection techniques which is known as protection scheme.

1.3                   AIM AND OBJECTIVES OF THE PROJECT

The main aim of this to design a protective scheme which provides an understanding of protective device coordination that involves the choosing of relay protective devices in relation to their time-current setting within the feeder length in order to isolate equipment and feeders from faults.

At the end of this study, this study will also includes SIMULINK modeling approaches for protection scheme and the use of a programmable FCL. Also the programmable FCL makes the protection scheme more extensible to allow additions of future DGs with little or no modifications to existing scheme.

1.4                                  PURPOSE OF THE STUDY

The purpose of this work is to provide security to the distributed generation and to ensures that main protective device operate quickly in the event of a fault.

1.5                                                 SCOPE OF THE PROJECT

Generally power distribution systems are protected with the help of dedicated over current based protection schemes. But increasing share of distributed energy resources penetration in electric utilities poses a serious threat to the existing protection coordination schemes of the distribution systems. Distributed energy resources connected distribution networks become interconnected in nature and protection coordination schemes, which are designed for unidirectional flow of fault currents become ineffective/non-functional.

Based on the knowledge revealed in the above literatures and analysis, this study adopts the design approach presented in Abdi et al. [16]. In the work, a radial distribution network was considered and the design approach focused on a radial distribution network and his design approach is on coordination of protective device with inserted FCL that handles increases in fault level.

CHAPTER TWO

2.0                                    LITERATURE REVIEW

2.1                                   REVIEW OF THE STUDY

A microgrid is a form of active small-scale distribution system operated in a modern smart grid structure, which has gained more importance, especially in the manner of flexible operation in grid-connected and islanded modes to enhance reliability and security of electricity grids. The integration of distributed generators (DGs) to the microgrid is an essential part of supplying electricity to the loads when operating in the islanded mode. Distributed generations provide higher system efficiency and power quality with fewer environmental impacts than those of the traditional centralized power generation system