OPTIMAL POWER FLOW WITH FACTS DEVICES

Abstract

An electric power system consists of three principle divisions, the generating stations, the transmission systems, and the distribution systems. Electric power is produced by generators, consumed by loads, and transmitted from generators to loads by the transmission system. The transmission systems are the connecting links between the generating stations and the distribution systems and lead to other power systems over interconnections. In the present day scenario, transmission systems are becoming increasingly stressed, more difficult to operate, and more insecure with unscheduled power flows and higher losses because of growing demand and tight restrictions on the construction of new lines. However, many highvoltage transmission systems are operating below their thermal ratings due to constraints such as voltage and stability limits. In addition, existing traditional transmission facilities, in most cases, are not designed to handle the control requirements of complex, highly interconnected power systems. This overall situation requires the review of traditional transmission methods and practices and the creation of new concepts which would allow the use of existing generation and transmission lines up to their full capabilities without reduction in system stability and security. Another reason that is forcing the review of traditional transmission methods is the tendency of modern power systems to follow the changes in today's global economy that are leading to deregulation of electrical power markets in order to transfer desired power and stimulate competition between utilities. In the past, most control of power systems was aided by mechanical devices and actions. This came at the expense of providing greater operating margins and redundancies. The rapid development of power electronics has made it possible to design power electronic equipment of high rating for high voltage systems. The voltage stability problem resulting from transmission system and cheap power transfer may be, at least partly, improved by use of the equipment wellknown as Flexible AC Transmission Systems (FACTS) controllers. This concept was introduced by the Electric Power Research Institute (EPRI) in the late 1980. The objective of FACTS devices mainly Thyristor Controlled Series Compensators (TCSC), Unified Power Flow Controller (UPFC), Abstract Generalized Unified Power Flow Controller (GUPFC), and Interline Power Flow Controller (IPFC) etc., technology is to bring a system under control and to transmit power as ordered by control center economically. It also allows increasing the usable transmission capacity to its maximum thermal limits. With the progress of installing FACTS devices, the latest generation of FACTS devices, named, the Convertible Static Compensators (CSC) was recently installed at the Marcy 345 kV substation. Several innovative operating concepts have been introduced to the historic development and application of FACTS. There are several possibilities of operating configurations by combining two or more converter blocks with flexibility. Among them there are two novel operating configurations, namely GUPFC and IPFC, which are significantly extended to control power flows of multi-lines rather than control power flow of single line by a TCSC and UPFC. In the present day scenario private power producers are increasing rapidly to meet the increase demand due to heavily loaded customers. New transmission lines or FACTS devices on the existing transmission system can eliminate transmission over loading, but FACTS devices are preferred in the modern power systems based onits overall performance. Moreover, a limited amount of FACTS-based transmission capacity is seen as an environmentally and economically sound alternative to transmission line-based transmission capacity. It has been reported that in the United States new transmission lines, assuming they can be built at all, can take between ten and twelve years to be completed and that FACTS systems, on the other hand, can be installed in less than two years. In order to fulfill the demands posed by the deregulation process and the power system, it is necessary to operate the transmission systems in a reliable and secure way. For this purpose, the existing utilities, independent system operators (ISOs), Transmission System Operators (TSOs) and private or Independent Power Producers (IPPs) all need to be committed and dispatched optimally. Thereafter, power flows as well as optimal power flow (OPF) studies are required to be performed, in order to analyze and improve the operation ofthe transmission system with the incorporation of FACTS devices. Therefore, the new power system modeling required to be modified by including FACTS devices accordingly. So in this thesis, research work has been carried out with an objective to Abstract develop efficient algorithms for both power flow and optimal power flow incorporating FACTS devices after determining their optimal locations. By using FACTS devices, new control variables and control objective equations are usually added in conventional power flow and optimal power flow equations. In this work, benefits, commercial status, cost, control attributes, and future developments of the basic models of the FACTS devices are described, investigated and presented. Two port current injection steady state models have been discussed and developed for the FACTS devices mainly for TCSC, UPFC, GUPFC, and IPFC such that they can be easily inserted in the power flow and optimal power flow algorithms. Power flow is a function of transmission line impedance, the magnitude of the sending and receiving end voltages, and the phase angle between the voltages. By controlling one or a combination of the power flow arguments, it is possible to control the active, as well as the reactive power flow in the transmission line. In this work, a new current injection model of the modified power system using Newton Raphson method for desired power transfer has been proposed. So that the FACTS devices can be easily incorporated in the proposed algorithm and therefore whole system with these devices can be easily translated to power injection models without change of original admittance and the Jacobian matrices. The proposed algorithm can easily be extended to multiple FACTS devices by adding a new Jacobian corresponding to the devices. However, to achieve the good performance of FACTS devices, proper placement of the controlling devices in the grid is an important and an effective control strategy. Hence, it is important that proper placement strategy must be followed before the installation of any such devices. In many cases, a single transmission line is determined for the placement of a FACTS device simply because the transmission line is very long. However, if FACTS devices are to be used for the control of power flows, reduction in generation cost and the enhancement of the power system stability, the placement problem is more difficult. In addition, the placement of a FACTS device depends primarily on the objective. Optimal locations based on sensitivity analysis have been presented in many literatures. They used different sensitivity indices to show optimal placement options to reduce either real Mi Abstract power flow over a particular line or total system power losses. This research work, utilizes static considerations based on the following objectives such as reduction in the real power loss of a particular line-/ (Pu), reduction in the total system real power loss (Pu), reduction in the real power flow performance index (PI) andminimum system generation cost and /or minimum of total real power transmission losses. Suitable methods to determine the optimal locations of TCSC and UPFC have been suggested in this work. The approach is based on the sensitivity of first three objectives mentioned above. If the objective of FACTS device placement is to provide minimum the total generation cost and/or minimum ofthe total system real power losses, the devices may beplaced randomly by trialand- error. So the approach based on the genetic algorithm (GA) has been suggested for the fourth objective. Arguably, the load flow and dynamic stability areas of study have received the most attention. Comprehensive models have been developed that very efficiently simulate the operation ofFACTS equipment in large power networks. More involved power systems applications, such as Optimal Power Flows (OPF), have received relatively little attention. Optimal Power Flow (OPF) is a static nonlinear programming problem aimed at scheduling the controls of the power system in a manner that optimizes a certain objective function while satisfying a set of physical and operational constraints imposed by equipment limitations and security requirements. An OPF program determines the settings of the selected control variables to achieve an optimal steady-state operation ofa power system. After obtaining the OPF solution, the implementation ofthe optimal control variables will bring the system to the "optimum" state. OPF problem is in general non-linear and non-convex as a result, many local minima may exist. Under the present state ofmodern power sector, this non-convexity is further increased when FACTS devices are included in the network. So it is necessary to develop efficient algorithms those are independent ofinitial conditions ofthe FACTS control variables and converges easily. Aiming at advancing the understanding of how FACTS equipment impacts on the power transfer and economy ofthe wider power network, a fully-fledged FACTS-OPF algorithm has been developed and validated. It uses Newton's method, leading to a very efficient algorithm that can solve largepower networks very reliably. IV Abstract Newton's method provides a suitable vehicle for incorporating the newly developed non linear, linearized FACTS models. A firing angle-based Thyristor Controlled Series Capacitor (TCSC) model, a coordinated two-voltage source Unified Power Flow Controller (UPFC) model, Genralized Unified Power Flow Controller (GUPFC) model and an Interline Power Flow Controller (IPFC) model are presented in this work. The ability of the FACTS-OPF algorithm to solve networks with a mix of power flow controllers is also shown for IEEE test systems. In this Work, mainly a generalized new multi-objective optimal power flow algorithm for minimization of generation cost and/or the minimization of total real system losses, suitable for modern power systems has been developed and analyzed with all types of modern FACTS devices. Optimal placement of single and multiple FACTS devices are also considered in this work using both sensitivity analysis and genetic algorithms. The main objective of the proposed algorithm is to optimize the location of the FACTS devices and then minimize the generation cost and/or minimize the total real system transmission losses. To validate the performance of the proposed algorithms, 6-bus, IEEE-14 bus, IEEE-30 bus, IEEE-57 bus and IEEE-118 bus test case systems have been used in different stages of the work. Later the algorithms have also been applied to a practical Uttar Pradesh State Electricity Board (UPSEB) Indian utility 75-bus test case system. Although this work has been exercised for multiple FACTS devices, but it is also applicable for multiple and multi type FACTS devices commonly seen in modern deregulated power systems. So this research work will be a very useful contribution in the field of optimal power flow (OPF) with FACTS devices for modern power systems. The new developed algorithm will be a great help to present power industry.