No-load Cutting-in And Low Voltage Ride Through Method Based On Flux Linkage Tracking For Brushless Doubly-fed Induction Generator

Nowadays,with the increasing shortage of fossil energy such as petroleum and coal,wind power is increasingly favored.Recent years,wind power generation has got rapidly developed.As the main model in the field of wind power generation,the research of doubly-fed induction generator has been relatively mature,and the machine has the advantages of better control performance and low manufacturing cost.Whereas,due to the presence of brushes and slip rings,the reliability of the system is reduced.At the same time,the brushes are more likely to wear and need to be replaced regularly,so the maintenance cost is greatly increased.Brushless doubly-fed machine,compared with DFIG,has the similar advantages,but it has no brush and slip,which can lower the maintenance cost and enhance the reliability of the whole system,so the BDFM is more suitable for the field of wind power generator generation with less maintenance or even no maintenance.As a result,the BDFM has aroused great research interest and achieved plenty of scientific payoffs.Recent years,a large number of achievements have been obtained in the design methods,structural optimization,control strategies of normal and faulty operation of BDFM.This paper focuses on the short-circuit current calculation,no-load grid-connected technology and low-voltage ride-through control strategy of grid-connected brushless doubly-fed machine(BDFIG).The main research contents are as follows:1.A no-load cutting-in method for BDFIG based on the controlling of control winding flux linkage is proposed.According to the static reference frame mathematic model of BDFM,the relationship between control winding flux linkage and power winding voltage has been deduced,the amplitude and frequency of PW voltage can be controlled by the amplitude and phase of CW flux linkage.Then on this basis,the control system of no-load cutting-in has been established.The experimental results show that the method can adjust the PW voltage according to the speed of the system to realize variable-speed constantfrequency power generation.And the method has the advantages of stable regulation process,short adjustment time,and can effectively avoid the overcurrent during the cutting-in process.2.Based on the PW static reference frame mathematic model of BDFM,the short circuit current for BDFIG terminal is deduced and analyzed when the PW and CW are shorted simultaneously in the case of considering the control winding resistance.The derivation results can be used as a reference for the calculation of the crowbar resistance when using crowbar protection or as a reference for the protection configuration of the distribution network.Simulation results show that the error of PW and CW current are 0.32%and 0.40%,respectively.At the same time,the analysis of this part lays a theoretical foundation for the study of the transient process of BDFIG.3.Based on the original laboratory research results,further theoretical analysis of the performance for the proposed LVRT method has been carried out.The analysis results show that,suitable flux linkage tracking coefficient T1 K can suppress the CW rush current,and the appropriate T2 K can adjust the reactive current inject to the grid during the LVRT process.The experimental results verify the correctness of the theoretical analysis.Then the transient and steady-state characteristics of asymmetric LVRT for Y-connected BDFIG have been analyzed,including voltage,flux linkage,active power,reactive power and electromagnetic torque,etc.,based on the newly developed 15 k W BDFIG.In order to verify the correctness of the theoretical analysis,the simulation model has been rebuilt,and the simulation results show that the proposed method can effectively control the CW current during the fault,meanwhile the amplitude and the phase angle can well track,which lay a theoretical foundation for the further experiment.4.Based on the initial experimental hathpace in the laboratory,a new 15 k W brushless doubly-fed machine hardware platform was established in company with the fellows of research group.The main work is to design the inductor-based voltage sag generator based on PLC control,including the parameter calculation,device selection and the preparation of PLC programs.Experimental results show that the VSG can simulate symmetrical voltage drop with different dip depth and can simulate the asymmetrical fault such as P-N,P-P,P-P-N,meeting the requirements of the national standard.The successful development of this device provides the basis for the subsequent experiments of BDFIG.5.Related procedure has been perfected and debugged.Based on the newly built experimental hathpace,the LVRT of symmetrical fault with a drop depth of81.7% was completed,and the influence of flux linkage coefficient on the CW rush current and the reactive power output was verified.Then,the LVRT under the most severe grid symmetrical fault conditions(ie,the voltage drop to zero,the BDFIG operate at 100% rated speed with rated load before the fault)was completed firstly.The Experimental results show that the CW current can be limited to the allowable range,its standard value is 2.01,which is 16.9% smaller than the experimental results under the 96% rated rotation speed(the same conditions)which proved that the BDFIG traverse the most severe grid symmetrical fault successfully without any extra hardware.And the active power and electromagnetic torque during the low wear process are basically consistent with the theoretical analysis.6.The LVRT procedure for asymmetrical fault has been perfected and debugged.Then the asymmetrical LVRT experiments(including P-N,P-P,P-P-N)have been carried out based on the newly built BDFIG experimental hathpace.Experimental results show that the CW current can be effectively limited to the allowable range,the electromagnetic torque,active power and reactive power are consistent with the theoretical analysis during the fault.