Providing an algorithm for islanding power systems while maintaining security criteria

Doctoral Dissertation in Electrical Engineering - Power

Abstract

The islanding of interconnected power systems, which is also known as the isolation and breaking of power systems, is the last line of defense to deal with the collapse of the system and prevent catastrophic incidents in the power network.

Islanding of interconnected power systems as an extensive control method is proposed as a comprehensive decision-making problem with many details and as an important part of corrective control strategies. After the occurrence of a major disturbance in a power system, if there is no suitable solution plan and model, this disturbance may lead to the total collapse of the system.

According to the definition of islanding of power systems, it means determining the correct points of separating the integrated system into a number of smaller islands, if it is not possible to maintain the integrity of the system.

In this treatise, a new method and optimized for islanding interconnected power systems. The presented algorithm is designed in such a way that it can overcome many of the existing limitations in the discussion of islanding and provide acceptable results and achievements. In the proposed method of this treatise, the static and dynamic characteristics of interconnected power networks are used to determine the number of islands and the correct points of their breaking. In this thesis, first by using the theories of dynamic clustering and alignment, the approximate boundaries of possible islands are determined according to the grouping of aligned machines, and then by applying a strong search algorithm based on graph theory, the exact boundary of the primary islands is determined. In the first part of the algorithm, the goal is to quickly and generally determine the number and approximate border of the islands according to the dynamic limitations and network topology and the clustering of machines in aligned groups. In the next step, the exact border of the areas is determined so that after separation, there is minimal unloading between the islands. Due to the fact that the islanding algorithm should ideally be in real time and on the other hand due to the high complexity and the wide dimensions of its search space, a fundamental effort is necessary to be able to provide an accurate algorithm and increase the speed of its calculations and overcome the time problem. with the possibility of providing more stability. Since the stability of designated islands is one of the major issues in island building and needs a lot of attention, it is very important. In this research, it has been tried to be able to predict the stability of the islands and examine them before applying the separation algorithm. An accurate and correct islanding means determining islands that are stable after isolation and have minimal unloading. Another part of the efforts of this research has been to select the boundaries of the proposed islands with higher accuracy. The exact demarcation of the islands is determined using strong algorithms in graph theory. These algorithms are direct and non-repetitive search algorithms and provide definitive answers, which allow accurate decision making for islanding.

In general, three basic questions are raised in the discussion of power system islanding, which are stated as follows. Does it need islanding?

The answer to this question reveals the necessity of islanding.

b - If the answer to the above question is positive, where should the desired power system be broken? And where are the borders of separating the islands located?

Answering this question means determining the exact points of separation of the connected system.

C- How should islanding be done and what are the order and timing of opening the lines?

Answering this question means determining the correct moments and order of opening the lines for the purpose of the island. It is a construction.

The purpose of this research is to answer the above questions with an emphasis on questions number (b) and (c).

According to the results of the latest research, there is still no general strategy that can adequately answer all three of the above questions in a short time, and reaching this comprehensive goal requires extensive research. Unfortunately, comprehensive answers to questions (b) and (c) have not been provided so far, and the research on these questions faces serious challenges.

In a word, a comprehensive and integrated solution to the correct islanding problem means answering all the above questions in the shortest possible time.

Key words: stability of power systems, islanding, protection patterns Khas, graph theory, multi-objective optimization, intelligent algorithms

Introduction

The power network is the largest and most complex interconnected network that has been designed by humans so far, so it is very difficult to control it. With the emergence of privatization and restructuring of the power network, the operation of the power system has imposed increasing tensions on the power system due to the pressures of network restructuring, which pursues new technical and economic goals in the operation of the power system. When the power system works near the operating limits, weak connections, unexpected accidents, hidden faults in protection systems, human errors and a set of other factors may cause system instability or even lead the system to catastrophic failures. Therefore, the systematic study of the power network and the design of a comprehensive strategy for its control have received increasing attention [1-3]. In general, the ability of a power system to deal with the occurrence of a certain disturbance depends on the operating conditions of the system at the moment of its occurrence, and any form of adaptive control must be designed in such a way that it is activated only in suitable operating conditions of the system. On the other hand, it is important to pay attention to the fact that when severe disturbances occur in the power grid, to check its transient stability, the system is generally nonlinear, and to predict its stability or instability, only the theory of nonlinear systems should be used, which will make its analysis more difficult in these conditions. In general, two types of control methods can be applied on the power grid, the first is known as Preventive Control and the second is known as Corrective Control [4-10]. Corrective control strategies contribute to solving security-related problems in many aspects, such as line overload, voltage problems, and power system transients [11]. When the system is in the alarm state, a relatively large disturbance may cause it to enter an emergency state in which the voltages of many buses are below their normal limits and one or more system elements may be overloaded. In this case, the network is still in operation and it is possible to return it to the warning mode by using corrective controls such as power system reconfiguration (TSR: Transmission System Reconfiguration), changing the production schedule (GR: Generators Scheduling), load shedding (LS), etc. will be In this case, corrective control methods include load shedding and power system islanding (CSI: Controlled System Islanding) [12]. This type of control aims to maintain the network as much as possible and prevent its total collapse. In general, such control systems are called Special Protection Scheme (SPS), Special Protection System (SPS), or Remedial Action Scheme (RAS). Therefore, SPS is a protection scheme that is designed to detect specific conditions of the power system that have caused unusual stresses in the system, to perform a series of predetermined control actions to deal with the conditions created in a controlled manner. In some cases, SPS is used to detect specific system conditions such as overload, instability and network collapse in the system.