| Due to the clean and renewable characteristics,the proportion of renewable energy generation in the power system has increased rapidly.Compared with the traditional generator,the renewable energy converter has the advantages of fast response and flexible power control.However,the converter does not have the ability of spontaneous frequency response in the phase-locked control mode and is difficult to realize fault ride through under the limitation of insulation and over-current withstand level,which seriously weakens the frequency and voltage stability of the power system.Most of the existing methods of improved control strategies and additional hardware devices are to simulate the frequency and voltage response of synchronous generator,but do not really have its dynamic characteristics.The renewable energy grid-connection via motor-generator pair(MGP)inherits the excellent properties of the synchronous machine and retains the fast and accurate control characteristics of the converter.Based on this novel grid-connection mode,this dissertation analyzes the operation mode and power transmission characteristics of grid-connected renewable energy via MGP from the perspective of physical structure and electrical characteristics,and proposes corresponding operation control methods,and then studies the inertial response,primary frequency regulation,fault ride through and reactive power regulation characteristics of grid-connected renewable energy via MGP in different scenarios.The main work of this dissertation are as follows:(1)Based on the operation characteristics of renewable energy,the connection structures of single/multi inverters and MGP and three kinds of MGP grid-connection operation modes,i.e.single/multi inverters drive and condenser mode,are proposed.Based on the theory of electrical machine,the phase sequence of stator winding and the mechanical structure of rotor of two synchronous machines in MGP are deeply analyzed,and the coupling mechanism between MGP and power grid is revealed.On this basis,the electromagnetic mechanical coupling model of MGP in the form of unified phasor is established.(2)Based on the power angle characteristics of synchronous machine,the change law of power angles of two synchronous machines is revealed when the transmission power of MGP changes.Through the reasonable simplification of the MGP motion equation,it is concluded that the relationship between the transmitted active power of MGP and the power angle of synchronous motor is approximately linear.Then.a coordinated control strategy of source side and generator side converters is proposed.On this basis,the q-axis current control method and no speed feedback control method are proposed,respectively.Both of the two methods are the adaptive transformation of the existing variable frequency speed-regulating control methods of synchronous machine,which can realize the fast connection between theory and practical application.The simulation and experimental results verify the feasibility of the two control methods.(3)The mathematical model of inverter driving MGP for grid-connection system based on LCL filter is established,and a double loop control method of generator side and source side currents is proposed.Then it is applied to the grid-connection control of multiparallel inverters via MGP,which can control the q-axis current of each inverter on motor side accurately and independently.The expressions of system impedance and transfer function are derived by impedance analysis and superposition theorem.The frequency domain analysis results show that the LCL filters have parallel coupling and resonance characteristics because they are parallel to the stator side of the synchronous motor,and the coupling strength and resonance peak value is strongly relevent to the number of parallel inverters.The software control system of dual parallel inverters driving MGP is designed by using MBD method,and the experimental bench is built to verify the effectiveness of the proposed current double loop control method.(4)The promotion effect of MGP on inertial response of renewable energy is studied,and then the scheme of improving inertial response capability of MGP is given.By comparing with the structure and frequency regulation principle of thermal power units,it is revealed that the electromagnetic mechanical coupling characteristics of MGP can enhance the frequency response of renewable energy generation units.And then an active power control strategy based on de-loading control and rotor speed feedback is proposed to realize the participation of renewable energy driving MGP in the primary frequency regulation of power grid.A small signal stability analysis mathematical model of the system is established and the stability analysis is carried out.Three scenarios of different parameters,different proportions of renewable energy and the change of source/load power are set in the simulation model,and compared with the virtual inertia control and virtual synchronous generator control.The simulation results verify the effectiveness of the proposed control strategy.A multi-machine experimental bench is built to verify the improvement of MGP on the frequency response capability of renewable energy generation units.(5)For the transient process of MGP during power grid fault,the isolation and attenuation effects of rotor shafting and damping on transient fault components are studied by phasor analysis method.The promotion effect of MGP in terms of power grid fault isolation and reactive power regulation on the fault ride through capability of renewable energy is revealed.The simulation results illustrate that the renewable energy grid-connection via MGP can achieve the low voltage ride through standards specified in the operation regulations,and have good fault ride through capability for different fault durations,asymmetric fault,overvoltage fault,multiple low voltage faults,etc.A sing machine grid-connection experimental bench is built to verify the promotion effects of MGP on the fault ride through capability of PV under different grid faults. |
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