Computer Simulation And Analysis Of High-voltage Frequency Converter

There are abundant energy resources in our country and the total energy storage is very large, but the average storage is fairlysmall because of our overlarge population. Over years, there has been a lack of power energy in more and more regions at home due to the lagging construction of power sources. The wanting of coal and oil is occasionally occurring. These exacerbate the urgent energy situation of our country. On the other hand, the structure of energy consumption is unreasonable, the energy efficiency is low and the waste of energy is prevalent.There are many schemes to solve the situation of energy lack and new resources exploitation and power conservation are emphasized. On one hand, exploration of conventional energy and new energy must be strengthened; on the other hand, power conservation is advocated and old equipments and conventional technics should be improved with new technology in order to increase energy efficient rate and to reduce energy dissipation. Thus the industry structure of our country should be converted to low dissipation and high output to accelerate our economic development.Nowadays, there are over 0.4 billion kW motors accounting for 60 percent upon all power loads in our country. The motor capacity chosen is larger than needed, hence, the power conservation potential of motor system is considerable. In many power plants in our country fans and pumps are driven by high-voltage motors and the flow is regulated respectively by baffles and valves. The motors always run at rating speed, as gives rise to additional power dissipations and high pressure on pipes. It is proved that approximate 20-30 percent power energy can be saved at the same loads if the fans or pumps are regulated by ac adjustable speed equipment instead of baffles or valves.In addition, some statistical documents show that there are about 10% motor faults caused by overlarge starting current and electromagnetic force. Ac adjustable speed techniques can improve motor's starting performance, therefore, motor's longevity can be prolonged and the product cost can be reduced.Based on the reasons mentioned above, the simulation and analysis of high-voltage frequency converter are selected to be studied in this thesis. The PSpice software package and Simulink package in the MATLAB are used to model and simulate the 2000kV/6kV frequency converter being developed by a power electronic company. Themain contents are following:(1) The power electronic device IGCT is modeled and simulated and the simulation curves of IGCT during turn-off are similar to the tested curves , which indicates that a turn-off snubber circuit should be installed in some situations.(2) One inverter unit is simulated by using the sub-circuit model of IGCT, the influences of stray inductances on the anode-cathode voltage are analyzed and the importance of the layout of equipment is emphasized.(3) The 36-pulse rectifier is simulated by employing Simulink of MATLAB.(4) In PSpice, the macro models of frequency converter under SPWM(Sinusoidal Pulse Width Modulation) and SHE-PWM(Selected Harmonics Elimination PWM ) are established. Based on these models, a converter system with resistant load, inductance load and asynchronous motor are respectively simulated. The output voltages of converter are analyzed by Fourier Transform and all voltage harmonic ratios and the voltage total harmonic distortion are calculated. The simulation results are consistent with the tested data, so the precise of models is proved.Based on the study of this thesis, some conclusions can be drawn:(1) Through the applications of simulation tools, it can be seen that the dominant device models in PSpice can deal with the dynamic characteristics of power electronic device well and the numerous modules of power system in Simulink are fit for the simulation of power electronic system when the static input-output characteristic is the main goal.(2) When simulating of complicated power electronic system it is necessary for different study aims to make different layer models. In power electronic simulation, modeling can be divided into three layers, namely, device, circuit and system. In one power electronic system, the simulation results of device can be used as the base of the system simulation. That is to say, the different layer models are relative.(3) A fault and an arbitrary presumed fault can be simulated. During the process of design and developing, some corresponding data can be figured out, thereby, the period of design can be shortened and the cost can be reduced.(4) Simulation doesn't replace experiments in laboratory and it is only an aid for electronic design because there are some unavoidable differences between virtual data and simulation results due to uncertain stray parameters.(5) A turn-off snubber circuit is needed installing unless the configuration is compact and the parasitic inductance is small.(6) Modulation scheme and LC filter design are critical for frequency converterbecause they determine the output voltage quality, and the layout is also a key step. The structure of power electronic equipment should be compact on the basis of satisfying electrical and insulated requirement.(7) Control system plays a key role in power electronic equipment and is subject to electromagnetic interference, and the uncertain stray parameter factor is added, the experiment environment and connection should simulate the conditions where the equipment will run in the future so that the virtual problems may be reduced .Over years, a delightful developing trend is that modern control theory is applied to ac adjustable speed system, e.g. applications of optimization control theory and double-bit simulation adjuster that can improve the dynamic characteristics of system, and self-adapting control that can enhance the overall performance of the system. However, these applications are still in the stage of study and experiment. We believe that the corresponding products will come out in the future. The developing potential of adjustable speed technique is considerable, and the market prospect is promising.