Research On Control Strategy Of Permanent Magnet Direct-drive Waste Heat Power Generation Grid-connected System

With the continuous improvement of the level of science and technology,the demand for energy is also increasing,the role of new energy generation technologies is becoming increasingl y prominent.The potential of waste heat energy development is great,and the development prospects in the field of new energy power generation are promising.The permanent magnet direct drive waste heat power generation grid-connected s ystem eliminates the gear box between the expander and the generator,the structure is compact,the cost is low,the efficiency is high,and becomes the important development direction of the waste heat power generation field.In this paper,the permanent magnet direct-drive waste heat power generation grid-connected s ystem is taken as the research object.The main contents of study are as follows:Firstly,a permanent magnet direct drive waste heat power generation grid-connected s ystem based on dual PWM converter is proposed and its topological structure is given,which is the structure of expander + permanent magnet s ynchronous generator(PMSG)+ dual PWM converter + power grid.The working principle of each component of the system is analyzed,mathematical modeling of each part is performed,and the relationship between various import parameters under different working conditions of the expander is analyz ed,which provides a theoretical basis for follow-up research.Secondly,in the traditional waste heat power generation system,there is a problem that the expander does not work at the highest efficienc y point according to the flow rate change of the imported working fluid under changing conditions,and a maximum efficienc y tracking control strategy for the waste heat power generation g rid-connected s ystem has been proposed.Based on the anal ysis of the mathematical model of the expander and its operating characteristics,the expander optimal speed calculation method for the optimal efficiency point under variable conditions was obtained.When the reference power of the expander changes,the speed control was used in generator side rectifier to track the optimal speed to achieve the optimal efficienc y point tracking of the expander.The voltage and current double-loop control was adoped in grid side inverter to stabilize the bus voltage and complete power transmission.Matlab simulation model has been established.The simulation results show that the proposed control strategy can achieve power transmission and flexible control of system sp eed and power,the effectiveness of the strategy was verified.Finally,because the expander operates at a wide speed under variable conditions,and high conversion efficiency can usuall y be achieved in a high speed region.To match the expander operating characteristics,a composite vector control strategy for the s ystems has been proposed.Q-axis current reference obtained from speed-loop output.Below the base speed,the MTPA control was adopted in generator side converter to make full use of generator stator current to increase s ystem efficiency,d-axis current reference calculated based on MTPA operating conditions.When the speed rised to the generator output voltage exceeding its maximum value,the flux weakening control was switched automaticall y b y detecting the speed and generator output voltage.The d-axis current reference value was recalculated according to the field weakening operating condition to reduce the generator terminal voltage.A current vector control algorithm based on the current decoupling control and voltage command compensation has been proposed to improve the dynamic performance.Simulation model was built in MATLAB,the results showed that the range of waste heat power gene ration speed can be widened by the proposed composite vector control strategy,the generator output voltage can be reduced to prevent regulator saturation,with this method,the fast response and small overshoot of the system were also guaranteed.