Designing a robust controller in power electronic cutters and comparing it with linear control methods

Dissertation for obtaining a master's degree (. M.Sc)

Electrical-Power Engineering

Abstract

In this thesis, the design of a resistant controller in power electronic circuit breakers and its comparison with linear control methods are examined. The main goal of research and development in this field is always to find the most suitable control method in order to implement closed loop control on different topologies of power electronic cutters. In other words, the purpose of choosing a control method is to improve the efficiency of the converter, reduce the impact of disturbances (line and load changes), reduce the effects of electromagnetic interference, and also be less affected by changes in the elements of the converter. In this thesis, the study of different control methods implemented on switching power supplies such as linear control and sliding mode is given. Advantages and disadvantages of each control method are also given. In this regard, linear and sliding mode control methods have been selected and their responses have been tested on step-down DC to DC converters. Matlab software is used to simulate the linear control method as well as the sliding mode method in power electronic circuit breakers. In the end, the comparison of the effects of linear control and sliding mode on the steady state response of buck converter under linear fluctuations, load fluctuations and changes of different parts has been done.  In this thesis, it is shown that, compared to linear control, the sliding mode method provides better steady state response, better dynamic response and more resistance to possible system disturbances. Keywords: DC to DC power converter, variable structure system, sliding mode control, sliding plate, dynamic response and steady state. Introduction Switching power supplies (SMPS[1]) in order to convert Electrical energy is needed from one type to another. SMPS are widely used in DC-to-DC converters, where the input source is a voltage (for example, in a rectified linear voltage, the output voltage of a power factor correction circuit (PFC [2]), a battery, or the voltage of a fuel cell).

In some power converters, DC-to-DC converters operate at relatively high switching frequencies, and this property enables them to use small inductive components that improve the behavior.

Contrary to the advantages mentioned above for SMPS, there are parameters that are not very favorable and have a great impact on the behavior of converters, which mainly include:

Nonlinear components in the converter structure;

Line and load fluctuations

Electromagnetic interference ([3]EMI).

The DC to DC converter has nonlinear components. (diode, transistor, etc.) The factor that changes the nonlinearity is the disturbance of the converter and the other is the change in time. The effects of these changes in the parameters of the converter are given in [6].

To design the DC to DC converter, nominal input voltage and load values ??are predicted. In practice, these nominal values ??may have a slight deviation. For example, 20% change (oscillation) of the line is expected, or the nominal load may deviate towards no load or full load. These cases are studied in [1], [2], [3], [4] and [6].

The purpose of studying electromagnetic interference (EMI) is to make sure that the electronic system can operate in an electromagnetic environment, without responding to electrical noise or unwanted electrical interference. For example, in a DC-to-DC power supply, EMI affects the converter components. The effects of EMI on the DC to DC converter have been studied in reference [7]. 

The above may cause the converter to deviate from the optimal performance conditions. If the deviation of the parameters increases, it is possible that the converter does not operate in steady state (becomes unstable). A number of control methods have been used to control SMPS and have solved the mentioned problem. Since a specific control method may be the most suitable method (under certain conditions) compared to other methods, each control method has its own advantages and disadvantages. Usually, it is desirable to achieve a control method that has the best performance under certain conditions. 1-2-Research Objectives This dissertation has introduced the reasons that lead to the selection of a specific control method, namely the sliding mode control method ([4]SMC) compared to other control methods.A detailed analysis of SMC implemented on DC-to-DC converter topologies has been done. In order to capture the advantages of SMC, another control method has been chosen. The response of the converter in the steady state and the dynamic regions controlled by two different control methods (PID [5] and SMC) have been compared. The research work has been carried out in the following logical sequence: The study of DC to DC converter topologies; It has been done on them;

Choosing conventional PID control as a control method;

Choosing SMC as the second control method;

Study the behavior of DC-DC converters in steady state and under dynamic conditions when two control methods are implemented. Under dynamic conditions, it is better than the PID control method.

The research process is described as follows:

In the second chapter, the linear power supply is examined and its advantages and disadvantages are discussed.   In the next section, SMPSs are studied in detail and their advantages and disadvantages compared to linear regulators are shown. The classification of SMPS in isolated and non-isolated DC to DC converters is given. The resonant DC-DC converter has been studied and its advantages over DC-DC converters with hard switching have been shown. DC-DC converters operate in both continuous conduction mode (CCM) and discontinuous conduction mode (DCM). Analyzes are given to select the initial value of the inductor for three basic DC-DC converters (buck, boost and buck-boost) that require the converter to operate in both CCM and DCM modes. A table is given to select the critical value of the inductor. At the end of this chapter, a classification of familiar DC-DC power supplies is shown. In the third chapter, a control method used in DC-DC converters including PID control, hysteresis control, adaptive control, current planning control, variable structure control ([6]VSCS) and sliding mode control (SMC) is studied and each mode is analyzed separately. The control analysis of each of the control methods is given and the advantages and disadvantages of each are shown. The reasons for choosing SMC as the main control method for SMPS are given in the research work. The basic principles of SMC are shown by mathematical equations.

The fourth chapter explains SMC in detail and briefly reviews its history. The introductory part of the chapter introduces the reasons. Utkin, who is one of the first scientists who dealt with the subject [1], briefly introduces his most important research in this field. In the next section, an overview of the SMC theory is given, especially the existence conditions, the obtaining conditions, the description of the system in sliding mode and Kettering vibrations[7]. These mathematical equations are used to prove the analysis. The research and applications of SMC in electrical and mechanical systems are shown in a diagram.

In the next part of the fourth chapter, SMC is implemented on the step-down converter and the theories of SMC mentioned above are proved. The obtained results have been confirmed using the simulation results. The research results and SMC applications for DC-DC converters are given in more detail.

In the fifth chapter, the results obtained from the research work are given and the published works are explained. Finally, the main parts of this thesis are summarized and suggestions for further research work are given.

1-3 Research Background

1-3-1-Introduction

In the formulation of control problems, there are usually differences between the actual performance of the machine and the mathematical model developed for the controller design. This inconsistency can be due to changes in system parameters or approximation of the complex behavior of the machine with a simple model. It should be noted that the resulting controller, despite the inconsistency in the mathematical model and the machine, must guarantee the ability to create the required performance levels in practice. This issue motivates research in the field of developing robust control methods to solve this problem. The sliding model control method provides a special approach for robust controller design.