Research On Neutral Point Voltage Control And Grid-Connected Strategy Based On Svpwm Simplified Algorithm Of Three-Level Converter

With the global energy crisis and environmental pollution worsening, the development and utilization of new energy is widely concerned around the world. As the core of electric energy conversion, the converter plays an important role for the high efficiency and high performance of new energy power generation. The study of the converter’s structure and the control method has the extremely vital significance to improve the power efficiency and stability of the converter system and to reduce the cost. The three-level converter receives extensive attention and application in renewable energy power generation, especially in the field of photovoltaiv, because of its advantages like small distortion of output voltage and current, high power factor, high pressure rating and low switching loss,In order to avoid the complex calculation and complicated control of traditional SVPWM algorithm, this paper basically researches the SVPWM simplified algorithm and mdulation strategy of three-level converter, and achieves grid-connected control of this system. The main work of this paper is as follows:1. Theoretical analysis is carried out for the topological structure of neutral point clamped three-level converter. Mathematical models for the three-level converter are deducted in three phase abc and dq synchronous rotating coordinate systems, based on the switching function. According to the requirements of the grid-connected control, parameters of inductors and capacitors are designed in main circuit.2. The SVPWM simplified algorithm for three-level converter based on reference vector conversion is studied. Three-level reference vectors are transformed to two-level reference vectors via coordinate translation, and adjacent vector affect times are gained by two-level SVPWM algorithm, determining the state of each bridge arm switch. The simplified algorithm avoided the complicated partition algorithm control and was easily realized.3. The causes of neutral point unbalance of the DC capacitors is researched for the neutral point clamped three-level converter. Theoretical methods for eliminating the midpoint offset are put forward both in ways of hardware and software. As the focal point of researching, the software method adjusts the vector affect time on the base of three-level SVPWM algorithm model, achieving the neutral point balancing control.4. The design of the phase locked loop is in the light of grid-connection goals, using the coordinate transformation to detect the grid angular frequency and phase, and eliminating the error signal by the PI filter. A current closed loop controller is designed according to the mathematical model, implementing the zero steady-state error control of grid-connected current tracking the grid voltage phase. In this case, the grid-connected power factor is close to 1.Grid-connected test is carried out on the base of the theoretical analysis above and simulation verification. Simulation is conducted in two cases:symmetrical grid voltage and asymmetrical grid voltage. The grid-connected control is achieved in both cases, verifying the correctness of the control methods in this paper and the design of system parameters.