Protection of a micro-grid in connected and independent state

Dissertation for Master's Degree in Power Electricity

System Orientation

Abstract

Decreasing fossil fuel resources, adverse environmental effects and low efficiency of traditional power grids have increased the desire to produce electricity near the load and distribution grid level using renewable resources. is One of the basic solutions to solve the problems raised is the use of microgrids. A set of small sources of energy production at the level of distribution voltage is called a microgrid. The microgrid is operated in two modes, connected to the network and disconnected from the network. In this research, a differential protection scheme for microgrid protection is presented using time-frequency domain transformation such as S transformation. At first, the current of successive buses is measured and processed using S transformation and their time-frequency contours are obtained. The energy spectrum content of the time-frequency contours of the fault current signals is calculated, then the differential energy is calculated to register the fault patterns in the microgrid in the state connected to the grid or islands. The efficiency of the proposed method has been evaluated in different types of fault (symmetric or asymmetric) and high impedance fault in the microgrid in radial or ring structures. that a specific threshold value for differential energy can be very suitable for sending the excitation signal at the right time in about 2 to 3 cycles from the time of the fault occurrence. The obtained results have shown that the protection scheme based on differential energy can effectively protect the microgrid against various fault conditions. Therefore, the proposed method is a suitable choice for wide area protection.

PSCAD software is used for microgrid simulation and MATLAB software is used to analyze the simulation results. Key:

Microgrid, S conversion, conservation, differential energy

Chapter 1: Introduction

1-1- Preface

Power systems around the world are facing the problem of gradual depletion of fossil resources. On the other hand, the use of fossil resources will cause environmental pollution. These problems have led to the production of power at the level of distribution voltage by renewable energy sources such as: photovoltaic cells, wind farms, fuel cells, cogeneration systems of power and heat, etc. The development of microgrids in order to supply energy in the industry has drawn a bright future, some of the benefits of which are: lower environmental impacts of microgrids compared to large thermal power plants due to the reduction of greenhouse gas emissions, modification of the voltage profile and reduction Losses due to closer electrical and physical distance between production and consumption, increase in power quality due to the decentralization of production and minimizing interruption times and blackouts in the network, as well as due to the utilization of heat losses in CHP systems [1] and reduction of production costs, the microgrid will also bring great benefits in economic matters. are exploited, so the design of the protection system for these networks is not so complicated. However, due to the acceleration of the development of microgrid technology in distribution networks and due to the change in the amount and direction of power distribution as well as the change in the levels of short circuit in different points of the network when a fault occurs, problems have arisen in the coordination between the protective devices in traditional networks. A microgrid is a local network that includes distributed generation units, energy storage systems, and distributed loads that are connected or independent from the grid [[i]]. These sources are located in local areas and have advantages such as low cost for consumers and producers, low voltage, high reliability, increased redundancy and system strength, and high flexibility [[ii]].

    There are two main categories of micro-resources. One is DC sources, such as fuel and solar cells, and the other is high frequency AC sources, such as microturbines, which require rectification. In both cases, the obtained DC voltage must be converted to an acceptable AC voltage.

Microgrid has two working modes. In the mode of connecting to the network to provide peripheral services, reducing the network peak and economic exchange of power benefits the network, and when there is a disturbance and blackout in the main network, it can be separated from the network and provide power to its loads independently. Despite all the advantages of the microgrid, protection is considered one of its most important challenges. The protection philosophy of microgrids is completely different from traditional distribution networks which are radial. The reasons for this difference are:

Since microgrids, unlike traditional networks, contain resources in addition to loads, the two-way flow of power in microgrid feeders affects the performance of microgrid protection equipment. The presence of microgrids transforms passive networks into active networks.

By changing the microgrid from the state connected to the network to the island state, the short circuit capacity of the network also changes. This makes it impossible to use traditional overcurrent relays that are only sensitive to a regulatory short circuit capacity in microgrids.

    In passive distribution networks, the fault flow direction is only in one direction and from the source side to the fault point. In this condition, fault detection is done only through the range of current passing through the detected fault feeder. However, in microgrid feeders, there are scattered production sources for two-way fault currents, so that the fault currents enter the fault point from both sides. If such a fault is not resolved, the scattered production sources are separated from the feeder by the respective controllers, which leads to a significant drop in microgrid production [[iii]].

Therefore, providing a solution to protect a microgrid that has the ability to detect the location of the fault and isolate it is imperative. It is inevitable. Therefore, fault detection in a microgrid should be effective in independent mode and connected to the network and for annular and radial structures, and all loads, lines and sources should be protected in an independent mode from the network.

Today, power networks have changed from stable passive (passive) distribution networks with one-way transmission of electric power to active distribution networks with two-way transmission of electricity. Since electrical energy is produced by the main network for consumers, distribution networks without DG units are passive [2]. When DG units are placed in the circuit, they lead to two-way power flow and turn passive distribution networks into active distribution networks. Microgrids require extensive control to ensure network security, optimal performance, reduce the emission of pollutants, and also change the microgrid from one mode to another. This control is performed by the central controller [3] (CC) and also the controller of small power generation sources (MCs) which are connected to energy storage equipment and distributed generation sources. As the names of these two controllers are clear, MCs are responsible for controlling distributed generation sources. CCs also monitor the operation and overall protection of the microgrid through MCs. The main task of CC is to maintain power quality and reliability through power-frequency control (P-F) voltage control and protection coordination. CC is also for generating Economically, it plans distributed generation resources and also helps to exchange power between the microgrid and the main grid. Therefore, CC not only creates protection coordination in the whole microgrid, but it is also responsible for the control of all MCs. Thus, CC provides energy optimization for the microgrid and keeps the frequency and voltage of the subscribers at an optimal level. This controller also has the ability to operate MCs continuously It monitors through two main modules Energy Management (EMM) and Protection Coordination (PCM) [[iv]].