Optimal placement of fault current limiters in microgrids in order to improve service continuity

Master's Thesis in Electrical Engineering (Power)

Abstract

Optimum placement of fault current limiters in microgrids in order to improve service continuity

Given the increase in consumption demand and the increasing influence of resources Distributed generation to the power grid, production and distribution systems are becoming more extensive and complex. Connecting these sources to the system increases the level of short-circuit current and causes problems such as disrupting the coordination of the existing protection system in the network. In order to solve this problem, methods such as disconnecting the distributed generation source from the distribution network when a fault occurs, resetting the parameters of the relays and using an adaptive plan are suggested. A suitable method to solve the problems caused by the excess fault current is the use of fault current limiters. but it does not have the difficulty and complexity mentioned in the above methods.

In this thesis, in order to solve the problems caused by the connection of the distributed generation source in the microgrid and to improve the voltage quality in the microgrid buses, it is proposed to use a one-way fault current limiter in the feeders connecting the microgrid network to the main network in order to improve service continuity. For this purpose, this idea has been investigated in two 8-bus networks and the IEEE30-bus distribution network, which is circular. In this research, in order to find suitable impedance values ??of unidirectional fault current limiters in order to maintain protection coordination and improve voltage quality in the IEEE30 bus distribution network, the teacher and student optimization algorithm has been used.

Key words: distributed generation source, fault current limiter, protection, power quality

1-1 Introduction

Concerns Due to the reduction of fossil fuels, the increase in global temperature and environmental problems, the use of scattered production sources [1] based on renewable energies [2] has been the research field of many researchers. Due to the increase in consumption demand and the ever-increasing penetration of distributed production sources and the connection of microgrids [3] to the power grid, the power grids are becoming larger and more complex day by day. Distributed generation sources or independent power plants are connected to the distribution network to increase the system capacity as a backup for uninterrupted supply of local sensitive loads [1]. From the point of view of the consumer, it is very important to produce and transmit electric energy permanently and without interruption. are exposed to errors and cause disruption in the network and lowering the quality of voltage and system power. For this purpose, the existence of a well-coordinated protection system is necessary. This system is adjusted with automatic operation to isolate faults from the network in the shortest time to minimize damage. In the distribution network, an overcurrent relay is usually used to isolate the fault location from the network. With the addition of distributed generation resources to the distribution network, the size and direction of the short-circuit current in the network lines changes, and as a result, the protection system does not function properly when a fault occurs. In addition, the presence of these power plants leads to an increase in the short-circuit current level of the network, which is higher than the maximum tolerable current of the breakers in the network. The complete replacement of existing breakers with higher capacity breakers is not practical because in addition to the high price of breakers, replacement of spare parts is difficult and has a relatively high cost, and also the level of short-circuit current of the network may be higher than the maximum tolerable current of the breakers available in the market. [2, 3]

According to the problems caused by scattered production sources, to maintain the performance of the power system at the highest level of security and reliability [4], several methods have been proposed. The best and cheapest method is to use the fault current limiter [5], which has the ability to limit the first short circuit current peak. This equipment has the potential to minimize the need to add or replace or readjust the equipment if it is used in the right places.

1-2 Importance of the topic

Due to economic and political reasons, the demand for electric power is increasing day by day. The connection of distributed production to the distribution system is expanding rapidly. These scattered production sources, along with their advantages, may have negative effects on the distribution system. [4] One of these negative effects is the connection of scattered production sources on the protection system of distribution networks. [5] In general, circuit breakers [6], protective relays, reclosers [7] and fuses [8] designed for a distribution system without the presence of distributed generation sources will not function properly when the distributed generation sources are present due to the change of the short circuit current level [6, 7] and this issue reduces the safety level of the system. On the other hand, the protection system includes many components, which must be coordinated to eliminate the error. Coordination of these components is done during the system design process based on short circuit calculations. When installing distributed generation sources, the fault current increases in the system, so after installing distributed generation sources, some components of the protection system must be replaced and coordinated again. [3]

Many researches and studies have been done to solve the problems caused by the connection of distributed products in the network. One of the most effective ways to solve problems is to use a fault flow limiter in the network. Researchers have done a lot of research on the types of fault current limiters, the size, location of this equipment in the network, the effects of fault current limiters on the generators in the network and so on. have done Therefore, placing the fault current limiter in the network is to achieve the following goals.

Increasing the capacity of scattered production sources

Increasing the capacity of transferring energy to longer distances

Reducing the voltage drop [9] due to the fault

Improving the stability of the system

Improving the security and reliability of the network

Maintaining the protection system

1-3 review of the studies conducted to reduce the effects of distributed generation source

Until now, various methods have been proposed to reduce the negative effect of distributed generation, some of these methods are reviewed here.

The method presented in reference [8] has investigated the negative effects created after connecting distributed generation with the computational toolbox SiGDist [10]. Based on the obtained results, the limitations of distributed production connection are determined. According to the limitations obtained, the amount of necessary changes in protection system equipment and protection coordination is estimated based on the installation location of distributed generation and the maximum production power of these resources. In [9], the capacity of a wind turbine is determined by considering the voltage regulation and the coordination of overcurrent relays with the help of the proposed formulas during an iterative algorithm. In [10], the maximum allowed capacity of the distributed generation source is obtained with the three constraints of the maximum and minimum allowed size of the busbars [11] of the network after the installation of the distributed generation source, the loss of the network after the installation of the distributed generation source is not greater than the base state and the protection coordination of the fuse and recloser [12] is obtained with a method similar to [9].

Abstract

Optimal allocation of Fault Current Limiters in Microgrids for improving the continuity of Service

By

Seyed Sadegh Mousavi Shoushtari

Due to the rise of power demand and penetration of Distributed Generation (DG), the electric power networks gradually get bigger and more complicated. Adding new DG units in the system increases the level of short circuit that causes lack of coordination of protection system. Using an adaptive protection scheme, readjustment of relay settings, and disconnection of DG form the distribution network during the fault are proposed to resolve the problem.