Dissertation
Master's degree
Department: Electrical Engineering - Power trend
Abstract
Today, despite the wide use of sensitive loads such as power electronics, computers and non-linear loads in distribution networks, the issue of power quality has received more attention. Most of these loads are sensitive to voltage changes, such as undervoltage and overvoltage, and need a sinusoidal voltage source for proper operation. Therefore, it seems necessary to use power quality improvers to reduce the adverse effect of these disturbances on the performance of sensitive loads. In recent years, various solutions have been proposed to deal with this problem, and one of the best and most effective methods is the use of DVR[1]. The purpose of this thesis is to improve the quality of power in power distribution networks despite disturbances such as voltage shortages and excesses using the proposed DVR. Also, comparing the performance of four different types of DVR in compensating for voltage deficiency and excess can be counted among the other goals of this thesis. Various voltage source converters for use in DVR have been presented in previous researches. In this thesis, in order to achieve the above objectives, a multi-level voltage source converter with modularized structure and cascade connection ([2]MMCC) is presented to improve the performance of DVR in compensating voltage disturbances.
In order to observe the performance of the proposed DVR in improving power quality, it is simulated on the test system in MATLAB/SIMULINK environment. In order to evaluate the voltage quality and observe the performance of the DVR using the proposed converter, the THD of the two load voltages and the injected voltage have been calculated by the proposed three, five and seven level DVR and compared with the normal DVR (based on the two level PWM inverter [4]). The results of the simulation confirm the performance speed and accuracy of the proposed DVR in restoring double-ended voltage.
Keywords: dynamic voltage restorer, overvoltage, undervoltage, power quality, multilevel converter with modular structure and cascade connection.
[1] Dynamic voltage Restorer
[2] Modular Multilevel Cascade Converter
[3] Total Harmonic Distortion
[4] Pulse Width Modulation
Table 2?1 Recommended value for maximum oil moisture for 69 kV voltage. 16
Table 2-2 criteria for estimating the end of life of transformer insulation. 21
Table 3-1 Comparison of oil circulation method combined with vacuum and low frequency heating method. 37
Table 5-1 nominal specifications of low frequency heating device. 41
Table 5-2 Comparison of TMS320F243 and TMS320F2812 processors. 43
Table 5-3 approximation of the square with exponential transformation functions of different degrees. 53
Table 5-4 Constraints applied to the optimization of the current compensator with quadratic approximation. 59
Table 5-5 Parameter values ??for the soft start test. 67
Table 6-1 Resistances measured by the old method in the delta connection (simulation results) 88
Table 6-2 Resistances measured by the old method in the star connection (simulation results) 88
Table 6-3 Resistances measured by the new method in the triangle connection (simulation results) 88
Table 6-4 Resistances measured by the new method in the star connection (simulation results) 88
Table 6-5 Resistances measured by the old method in the delta connection (laboratory results) 91
Table 6-6 Resistances measured by the old method in the star connection (laboratory results) 91
Table 6-7 Resistances measured by the new method in the delta connection (laboratory results) 91
Table 6-8 New Barosh's measured resistances in the star connection (laboratory results) 91
Table 6-9 Relative error of the test results of resistance estimation in unbalanced load. 93
Table 6-10 simulation results of resistance estimation (percentage) in different states of imbalance. 94
Table 6-11 relative error of estimated resistances in percentage. 95
Table 8-1 Test conditions and parameters for initial resistance measurement. 109
Table 8-2 test conditions and parameters for evaluating the performance of the current compensator. 112
Table 8-3 Test conditions and parameters for soft start. 114
Table 8-4 Test conditions for measuring resistances in balanced and unbalanced load.. 117
1-1- Preface
Ignorance of the concept of power quality, poor power network design, the presence of loads sensitive to voltage changes and the increase of non-linear loads in distribution networks have made the investigation and analysis of power quality an important matter. In order to maintain the desired power quality in distribution networks within the standard range, parameters related to power quality should be identified, evaluated and measured and then by finding and finally applying the necessary solutions to improve and control it, effective steps should be taken. Due to the existence of problems caused by the unfavorable quality of electricity, it seems necessary to use appropriate methods to improve it, which requires appropriate and new solutions. The rapid growth of nonlinear loads and power electronics leads to a decrease in power quality. Weakening of power quality in the network causes damage to sensitive loads connected to the network, for example, lack and excess voltage [1] in the network can destroy sensitive loads and cause them to malfunction and damage them. Therefore, the power industry has moved towards the use of power quality improvements. On the other hand, economic issues, increasing the awareness of subscribers regarding power quality issues, the high sensitivity of new electrical equipment to changes in power quality, the existence of an integrated and interconnected network, and the improvement of the overall efficiency of the power network, prompt the managers of the electricity industry to pay more attention to the issue of power quality and its improvement. All the above-mentioned reasons have led to extensive research aimed at improving power quality. In order to maintain power quality within the specified range defined by power quality standards, compensation methods should be used to reduce the impact of destructive loads on the network. In recent years, paying attention to the power improvement system has created a suitable solution for compensating power quality problems. Using the technology of the power improvement system to improve the power quality is one of the ways that is now suggested to compensate for power quality disturbances. According to the definition, CUPS devices [2] are the use of electronic power controllers in distribution systems to increase power quality and network reliability. Various types of CUPS devices such as D-STATCOM, DVR, UPQC, UPFC, SVC and . have In fact, the idea of ??using CUPS tools is based on the old switches and connectors from the late 19th century. But now, after nearly a century of human exploitation of electrical energy, the designers and planners of the electrical industry around the world have turned to the new idea of ??compensators and are working to improve their performance. Financial losses caused by low power quality due to disturbances such as voltage shortage and excess are one of the main incentives for the development of CUPS devices. This technology based on power electronics is installed near the location of the sensitive load, the result of which is the reduction and elimination of power quality disturbances and the protection of sensitive loads in the power distribution system. In general, improving power quality, freeing up the capacity of distribution systems, reducing the financial losses of network consumers, improving productivity and increasing security for sensitive and important loads of distribution networks are among the positive results of using CUPS devices for consumers in electricity distribution networks. Among the CUPS devices, the best option for controlling and compensating voltage deficiency and excess is Dynamic Voltage Recovery (DVR). DVR is basically a controlled voltage source that is installed between the power bus and sensitive loads, and by injecting a controlled dynamic voltage, it controls the range and phase of the network voltage so that despite the disturbance in the source voltage, a symmetrical three-phase voltage with a certain range is generated at both ends of the sensitive load. One of the suggested methods to increase the compensation capability of this equipment is to use multi-level voltage source converters instead of normal (two-level) converters. Multi-level converters have the ability to produce voltage with total harmonic distortion (THD%) and less, so the need for large filters in the DVR is eliminated, which reduces the volume, size and cost of the DVR. Also, multi-level converters have the ability to be used at high voltage levels. 1-2- The motivation for using multi-level voltage source converters in dynamic voltage recovery and the main objectives of the thesis. Due to disturbance in distribution networks, sensitive loads suffer from problems. Therefore, to prevent sensitive loads from being damaged, it is suggested to use CUPS compensation devices, which is the best option for compensating the lack and excess of voltage, DVR.