Investigation On Control Strategy Of DFIG With Inertia Characteristics Under Nonideal Power Grid

Due to the continuous consumption of traditional fossil energy and the environmental pollution caused by its combustion,wind energy as a green renewable energy has attracted wide attention.The doubly fed induction generator(DFIG)system has attracted more and more attention due to its small converter capacity,safety and reliability,high efficiency and practicability,and has become a research hotspot in the field of wind power generation.In order to optimize the frequency response characteristics of DFIG,virtual synchronous generator control(VSG)technology is applied to the control of DFIG,by simulating the rotor motion swing equation of synchronous generator,the rotor rotation kinetic energy of DFIG can be used to provide frequency support for the power grid,so that the doubly fed wind power generation system has certain inertia characteristics.Because most wind turbines requiring virtual synchronous control technology are located in remote and weak power grid areas,under such conditions,not only the frequency of power grid may fluctuate because of the sudden increase or decrease of the load,but also the phenomenon of the sudden drop or imbalance of the grid voltage often occurs.Because of its insufficient control bandwidth and slow dynamic response speed,the original virtual synchronous generator control can not adapt to the change of grid voltage,which will lead to the decline of the output power quality and deterioration of operation performance of DFIG,and even threaten the operation safety of DFIG control system.In view of the two non-ideal grid conditions which are the sudden drop and imbalance of the grid voltage,the improved measures this paper proposed on the basis of the original virtual synchronous generator control are as follows:i)In view of the problem of the large rotor impulse current of DFIG and long time oscillation of electromagnetic torque with the original virtual synchronous control system under the condition of symmetrical drop of the three-phase grid voltage,this paper proposed that by adding positive sequence compensating voltage and transient voltage control instruction to limit the amplitude of rotor impulse current,by adding the fast demagnetization control outer loop to accelerate the attenuation speed of stator transient flux and then to reduce the oscillation time of electromagnetic torque,at the same time,by introducing the rotor current detection module,the improved control strategy which coordinates the rotor current limit and the stator transient flux fast demagnetization is realized;In view of the problem of the large rotor impulse current of DFIG with the original virtual synchronous control system under the condition of unsymmetrical drop of the three-phase grid voltage,this paper proposed that by adding rotor transient current restraint outer loop,rotor negative sequence current restraint outer loop and unified compensation voltage,the maximum restraint of rotor impulse current is achieved,and the safety of the control system is ensured.ii)In view of the problem of the stator and rotor's currents distortion of DFIG,active/reactive power fluctuation and electromagnetic torque oscillation with the original virtual synchronous control system under the condition of imbalance of the three-phase grid voltage,this paper makes full use of the second order generalized integrator(resonator)in the forward synchronous rotating coordinate system to control the negative sequence component or fluctuation component of DFIG systems and six improved control strategies are proposed,which is improved strategy based on direct resonance control outer loop,improved strategy based on stator current resonance control outer loop,improved strategy based on rotor current resonance control outer loop,improved strategy based on torque-reactive power resonance control outer loop,improved strategy based on active power-reactive power resonance control outer loop and improved strategy based on extended power resonance control outer loop.Through the above six improved control strategies,the DFIG virtual synchronous control system can flexibly select control objectives according to the real-time requirements of the grid under unbalanced grid voltage conditions,thereby improving the output power quality and operation performance of the system.Through the research of this paper,the doubly fed wind power generation system based on virtual synchronization generator control not only has a certain adjustment ability for the change of the grid frequency,but also has a certain adaptability for the change of the grid voltage.