Key Issues Research On Network Controlled Inverter And Inverter Parallel System

In recent years, energy crisis attracts great attention worldwide. The control of inverter, along with inverter parallel system, which plays an important role in the development of new energy technology, receives more attention from experts. Meanwhile, networked control is the future trend of technological revolution. The introduction of networked control into inverter and inverter parallel system would bring about several advantages. First, data produced either online or offline could be transferred to upper computer, which with its high calculation ability, would make correct judgment regarding system operation condition, system performance, or send control signal to local controller directly. The satisfactory computing capability of upper computer allows advanced control method with huge amount of data, leading to high quality operation performance. Compared to the traditional analogue circuit or low speed field bus, the high speed, real time network facilitates information exchange between inverters in a parallel system efficiently and speedily, which is a target electrical engineers have been striving for. With the concept of networked control, the main contribution of this dissertation is as follows.Firstly, based on Ethercat protocol, methodology for establishing a network controlled inverter and its parallel system is proposed. This parallel system allows complicated network topology. After the introduction of principles of distributed clock, the parameter range for a stable control system with a proportional-integeral controller is determined by D partition method.Secondly, by the way of networked identification, an inverse model based repetitive controller for inverter is proposed. In this method, the least-square-method is employed, along with M sequence as the input data. By processing input data and output data, parameters for inverter model are obtained. Next, the inverse model is modified in advance of being used as a compensator in the repetitive controller. Compared to results from traditional method, in certain conditions, a better control performance based on inverse damping model under nonlinear load is shown.Thirdly, in order to improve voltage performance under nonlinear load, in the context of networked control, an inverter parallel system based on repetitive control method is proposed. In this way, method to obtain stable control parameters of current sharing controller with two modified repetitive controller is discussed in detail. It is stated in theory that the performance of current sharing controller with traditional repetitive method is poorer than that of two modified repetitive controller. After theoretical analysis and experimental results, the difference of dynamic performance of two modified repetitive controller is discussed. According to automatic control theory, the parameter selection method of those modified repetitive controller is provided with the consideration of switch characteristic and load condition.Fourthly, a proportional plus multi-resonant controller with inverse damping model is proposed for inverter parallel system to achieve satisfactory steady-state and dynamic performance of output voltage. The system stability, anti-interference ability and steady-state performance are investigated in detail. Methods for converting continuous model to discrete model for a resonant controller is discussed, which shows several conversion methods are not suitable for a resonant controller, since the magnitude of resonant frequency would decrease by those conversion methods. After the discussion of single inverter design, design procedure of the current sharing controller for a parallel system is shown in detail. Also, the robustness of current sharing controller is proven by theoretical analysis when the value of inductor and capacitor-is changed within a certain range.