A method of optimizing parameters of single machine power system stabilizers using modified harmony search algorithm

Dissertation for Master in Electrical Engineering (M.Sc)

Abstract

Power system stabilizer is added to the excitation system in order to improve the power system damping during low frequency disturbances. For large-scale power systems that include a large number of connected generators, adjusting the power system stabilizer parameters will be a complex and difficult process due to the presence of multiple oscillatory modes with low damping, and since continuous changes in the load cause changes in the oscillatory modes and their damping levels, this will make parameter adjustment more difficult. During the last few decades, many methods including linear and classical methods and non-linear methods based on smart algorithms and meta-heuristics have been used in the process of adjusting the parameters of the stabilizer, but the harmony search algorithm has not been used in these studies. The dynamic investigation of the single-machine system connected to the infinite bus is important because it is a small example of the behavior of the entire power system and provides the possibility to make decisions and provide comprehensive algorithms for the entire power system. In the dynamic studies of the single machine system, it is tried to adjust the parameters of the power system stabilizer for a dominant point in the system, where the damping of critical modes is maximum for that point and a percentage of error is accepted for other working points. In fact, the power system stabilizer is installed on the generators in order to improve the dynamic situation and stability of the small signal of the system. In order to investigate low frequency disturbances in the power system, there will be a need for an accurate and comprehensive model of the system that shows the dynamic behavior of the system, generator and its components well. Therefore, in this thesis, the dynamic model of the system has been obtained with the help of linearization of differential equations and using the state space expression of the system. Then, a method for designing the power system stabilizer and adjusting its parameters based on the harmony search algorithm is provided. The simulation results show that the modified harmonic search algorithm has almost better performance than other existing methods in setting the parameters of the single-machine power system stabilizer.

Keywords: power system stabilizer, harmony search algorithm, power system, small signal stability, single-machine power system, state space equations.

 In the late 1950s, all new units that were added to the power system were equipped with automatic voltage regulators. With the progress of the power industry day by day, when these power plants included a large share of the production capacity, it was observed that the operation of the voltage regulator has a detrimental effect on the dynamic stability (small signal stability) of the power system. Oscillations with low amplitude and low frequency usually continue for a long period of time and in some cases can cause limitations in the power transmission capacity in the power system. Power system stabilizers[1] were developed to help dampen these oscillations by modulating the excitation system. The art and science of regulating power system stabilizers has gradually evolved over the past 40 years, since the first stabilizers were widely installed on the American grid until today. This improvement includes the use of different tuning methods, different input signals, and learning how to behave and deal with the torsional mode of the turbine-generator shaft [1]. Power system stabilizers are added to the excitation system to improve the power system damping during low frequency oscillations. Several methods have been used to design stabilizers. Setting up a secondary control system for excitation in order to stabilize the oscillatory modes of the system has been the main goal of most studies in recent years. In general, two basic methods have been successfully used to adjust the power system stabilizer parameters, one is the phase compensation method and the other is the root geometric location method. The most common method used for structure studies includes a filter filter and a postphase-prephase block diagram. Among the input signals, we can mention the terminal voltage, rotor speed, incremental power and electric power. Also, the linear combination of these signals has also been used as an input signal in past studies.. Phase compensation includes adjusting the stabilizer in order to compensate for the post-phase created between the generator, the excitation system and the power system, so that the path of the stabilizer causes changes in the torque, which are in phase with the speed changes [2] and [3].  This method is a direct and understandable method that is widely used in practice. The design of such requires the setting of several parameters for each generator. These parameters generally include the constant gain, the time constant of the filter circuit, and the time constants related to the block diagram of the prephase stabilizer. Some sequential or simultaneous methods of adjusting these parameters have been introduced in [4] and [5]. Also, the above methods have been used in practical systems and satisfactory results have been obtained for damping the fluctuations of local modes. However, the obtained results may not be the best possible results. Therefore, in order to achieve the best possible result, limitations and assumptions are considered in the design process of the stabilizer so that the optimal global response can be achieved as much as possible [6]. The combination with the geometric location of the root will include the displacement of the eigenvalues ??related to the oscillatory modes of the power system, by setting the poles and zeros of the stabilizer, on the imaginary plane [7]. This method provides an additional perspective on the system performance due to its direct connection with the closed-loop characteristics of the system versus the open-loop structure of the phase compensation methods. But applying this method will be very complicated, especially applying this method to field equations is difficult. In addition, the performance of these stabilizers is significantly reduced by changing the operating conditions of the system. In the studies, the presented methods are usually efficient and suitable for single machine systems connected to the infinite bus. As mentioned, several methods have been used to adjust the parameters of the stabilizer connected to a generator connected to an infinite bus, and in this study, a new method based on the meta-heuristic algorithm of harmony search is used. exploit (near its borders). On the other hand, due to the fact that the power system is a dynamic and dynamic system, the operating conditions and working points of the system are constantly changing. Also, when a generator is connected to the network, their mutual effects on each other's performance can disrupt the performance of control systems. Therefore, it will be very useful to adjust the control systems of the generators to each other. On the other hand, the mentioned changes cause local and interregional low frequency fluctuations. These oscillations are continuously present in the system, but when the amount of changes in loads and production is such that it changes the location of the oscillatory modes of the system and moves them to the right side of the imaginary axis, the system can become unstable. Therefore, the occurrence of a system that can increase the amount of damping of oscillatory modes seems necessary. Therefore, a control system called a power system stabilizer is used, which adds additional damping to the excitation system. are usually not installed on all generators, but only on those generators that can create the highest amount of damping in the system and on the oscillatory modes. 1-3- Improving small signal stability using control equipment As mentioned, the main control system that is used to improve small signal stability and increase damping of oscillatory modes is the power system stabilizer. is This device adds additional damping to the system through post-phase compensation created between speed input and output torque, or speed variations. Several methods have been introduced to design the parameters of stabilizers. In this thesis, the harmony search algorithm is used to design the stabilizer. Setting the parameters has a great effect on the stability or instability of the small signal of the system, so how to set them is very important and will require accuracy and sufficient knowledge of the structure and dynamic model of the system. Therefore, after the accurate modeling of the power system, the optimal design will be done. 

1-4- Project objectives

Different studies have been done in the field of stabilizer design for power system. In this thesis, an attempt has been made to discuss the design for the power system from another perspective.