Timing of transmission line repairs considering power system vulnerability

Master's Thesis of Electrical Engineering - Power

Abstract

The discussion of repairs in any system, including the power system, is of special importance. In the traditional structure of the electricity industry, repairs related to the generation and transmission sectors are both carried out by the network operator. With the restructuring of the electricity industry, the proposed repair time for different parts of the system is left to the owners of the parts, and the independent operator of the system is responsible for monitoring and coordinating the repair time. In the models presented for scheduling the transmission system repairs, it is generally tried to choose the best repair time in order to maintain the reliability of the system in a safe area, and the reliability of the system is considered as the most important condition of this issue. After 2001, the study of the effect of deliberate attacks on the power network has gained special importance; Because the application of classical standards to ensure the reliability of the system cannot sufficiently include the existing reality, that is, the issue of deliberate attack on the power grid. In this thesis, in the first step, a new model is presented that can examine the vulnerability of the power system in a time horizon. The "time dimension" of intentional attacks has not been considered in previous researches. The output of this step is a temporal model formulated as a two-level problem. This two-level model becomes a one-level programming problem using dual theory. In the second step, this model is used to present a new formulation for the scheduling of transmission line repairs. In the new formulation, the scheduling of transmission line repairs is considered as a multi-level planning problem, in which the vulnerability of the power system is considered along with the reliability of the system.

The proposed models are implemented on Garver 6-Bus and IEEE-RTS 24-Bus standard networks and the ability of these methods is shown.

Key words: power system vulnerability, multi-level planning, dual theory, scheduling Repairs of transmission lines

The power network is one of the most important infrastructures of a country, in such a way that almost all other infrastructures depend on the correct operation of this network. In any country, there is a close relationship between the economy and its electricity industry, and if the performance of the power grid is disturbed, there will be great economic losses for that country. For example, the damage caused by the blackout[1] that occurred in the United States of America in August 2003 was estimated between four and ten billion dollars. The blackout affected a population of about 50 million people, and in some areas, consumers were left without power for up to four days [1].  The largest blackout in history is the 2012 blackout in India, during which more than 600 million people lost access to electricity. Sometimes the consecutive outages of the transmission lines can prepare the ground for the occurrence of such destructive blackouts. For example, in the 2003 blackout in the United States of America, when a fault occurred on three transmission lines at the same time, these three lines went out of the circuit, and the outage of these three lines caused the rest of the network lines to be overloaded and quickly go out of the circuit, one after the other, and as a result, a load of about 61.8 gigawatts was lost. It is obvious that the importance of such a system makes it very necessary to ensure the correct operation of this system.

The power network is generally composed of four parts: production, transmission, distribution and consumers, and in order to maintain the efficiency of this system, all four mentioned parts need maintenance and repairs. Increasing the reliability of the system and increasing the energy efficiency are among the most important results obtained from performing maintenance.

In various books and standards, multiple definitions and meanings for "repairs" are mentioned; For example, IEEE Std 902-1998 considers maintenance and maintenance of conditions that are necessary and necessary for the correct operation of the equipment, with the same purpose for which the equipment was used [2]. Anyway, what is important is the significant dependence of the correct operation of the power system on the correct and timely repairs of its various parts..

Since the repair period of various power system equipment varies from a few days to a few weeks, for this reason, the repair schedule is also done in several short-term (several weeks), medium-term (about one year) and long-term (about three to four years) time horizons, and these repairs fall into two categories: preventive repairs [2] and corrective repairs [3]. As it is clear from the names of these two categories, the first category of repairs is carried out in order to maintain the system in a suitable state that is favorable in terms of energy efficiency and reliability, and the second category is carried out to return the system to a normal and acceptable state as quickly as possible after an error or malfunction [3]. In addition to the length of time related to the timing of repairs, another issue that is raised in repairs is the coordinated repair of different parts, especially the production and transmission parts. A large number of papers have presented various methods for coordinated maintenance scheduling [4] of the production and transmission sector [4]–[6]. However, the repairs related to each part can be done separately. Among these, the repairs related to the transmission network are of special importance, and short-term, medium-term and long-term repairs can be performed for this part of the system.

In the traditional environment of the electricity industry, the network operator centrally, with the aim of maintaining the reliability of the network, schedules the repairs of the production and transmission parts of the system and assigns the schedule to the production units and transmission lines. does With the restructuring of the electricity industry, the repair time proposal for different parts of the system is left to the owners of the parts, and the independent operator of the system is responsible for monitoring and coordinating the repair time.

Regarding the many researches that have been done in the field of power system repairs, we can refer to an article by Pai Konejo [5] [7], which by presenting a repetitive process [6] tries to present a program that in an environment Restructured, production units can adjust their schedules in a round-trip process in such a way as to maximize their profits and maintain system reliability constraints with ISO supervision. Pandzik [7] [8] also presents a MILP model (which is actually a linearization of a two-level problem) to determine the best scheduling of transmission line repairs in a one-year horizon. In this model, the transmission system operator (TSO) [8] is placed in the high-level problem and sets his objective function to maximize the available transmission capacity during a year. The low-level problem also performs market settlement with the aim of maximizing social welfare [9]. Wu [10] [9], taking into account the uncertainties in the power system, determines the repair program of the production and transmission sector in a coordinated and security-bound manner [11]. Latifi [12] [10] also by presenting an iterative process, adds the limitations and uncertainties in the gas network to the discussion of repairs of production units in a restructured environment, and by establishing a relationship between the gas network operator (GNO) [13], the independent market operator (IMO) [14] and the independent system operator (ISO) [15], the mid-term planning of electricity and gas networks is carried out in a coordinated manner.

In all the models that schedule power system repairs, system reliability is either the objective function itself or is added as a constraint to the problem. In the discussion of system reliability, more attention is paid to events that normally occur in the system itself without the intervention of external factors. Short circuit errors, load interruption, failure of a generator and sudden exit of transmission lines are examples of such incidents.

Another part of errors that are not considered in reliability studies are intentional errors[16] that are made by a person or a specific group with the intention of damaging the power grid. According to the statistics provided by MIPT [17], during a 10-year period, from 1994 to 2004, there have been more than 300 hostile attacks on the power grid all over the world, among which, most of the attacks targeted transmission lines and power transmission towers [11]. To provide statistics in this regard, in the United States of America more than 90% and in other countries about 60% of the attacks have targeted the transmission lines [12]. Statistics like this show that the power system is vulnerable to intentional errors in addition to common errors. Many studies have investigated the vulnerability[18] of the power system against intentional attacks.