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Grid-connected/Off-grid And Power Control Of Double-mode Direct-drive Wind Energy Conversion System Based On Energy Storage

Posted on:2017-01-21Degree:DoctorType:Dissertation
Country:ChinaCandidate:X H ChaiFull Text:PDF
GTID:1222330503482422Subject:Power electronics and electric drive
Abstract/Summary:PDF Full Text Request
The double-mode direct-drive wind energy conversion system(DDWECS) based on energy storage has two operation modes: stand-alone operation and grid-connected operation. Due to the machine-side converter and load-side converter is decoupling operation in DDWECS, therefore, the grid-connected and off-grid double-mode operation of DDWECS is realized through the load-side converter. In order to make the wind turbine unit capture maximum wind energy, machine-side converter adopts maximum power point tracking(MPPT) control below rated speed, so as to improve the efficiency of the unit and to increase the annualpower generating capacity of the unit. In order to ensure the stable operation of the full-power converter, it is necessary to provide a stable DC voltage technology through the energy storage system(ESS). ESS guarantees the power balance of system, which is in grid-connected operation and stand-alone operation, at the same time smoothes the inverter output power and effectively restrains DC voltage fluctuations. For double-mode DDWECS based on the energy storage, this paper from the machine-side MPPT control implementation, the load-side double-mode operation control, the three-level neutral-point potential(NPP) control and ESS power control and other aspects carries out research, the concrete research content is as follows:(1) At present, the research focus of MPPT control is the improvement of the active power reference algorithm, but for MPPT control based on power signal feedback power(PSF), how to obtain the power feedback signal is also one of the key technologies to achieve MPPT control. For MPPT control based on PSF, the rotor position detection precision, the error of power feedback signal and the performance of the loop controller parameters are greatly influenced on the performance of wind energy tracking. Firstly, it can ensure that the system has good dynamic and static performance through the controller parameters design of the current inner-loop and powerouter outer-loop. Secondly, for the MPPT control based on PSF, the MPPT implementation method based on virtual PSF(VPSF) is adopted, this method improves the system reliability and performance, at the same time reduces the hardware cost and maintenance cost of the system. Finally, for rotor position estimation method based on back electromotive force sliding mode observer(SMO), a sensorless control method based on adaptive SMO(ASMO) is proposed in this paper. To improve the method from two aspects, on the one hand, the adaptive second order generalized intergrator(ASOGI) is introduced to instead of the commonly used low pass filter, it can make the phase error quickly converge to zero and improve the tracking performance of the system. On the other hand, sigmoid function is introduced, it can consider the rapidity and anti-chattering of system.(2) With the NPC three-level converter widely applied to direct-drive wind energy conversion system, the ability to ensure the DC bus neutral-point potential,(NPP) balance has a crucial impact on the system. Research on grid-side NPC three-level converter, it proposes a NPP control method based on zero sequence voltage superposition. The zero sequence voltage of equalizing loop output is added to modulation voltage, and controller parameter is designed ccording to the largest open-loop cutoff frequency. This NPP control method has the following advantages: simple calculation method, effectively suppressing of NPP AC/DC fluctuations, positive and negative DC bus voltage fast dynamic response, high utilization rate of dc voltage.(3) Power electronic interface inverter is the key to achieve stand-alone operation and grid-connected operation for double-mode DDWECS based on the energy storage. For double-mode inverter with grid-connected and off-grid operation mode, a unified control strategy based on improved droop control was proposed in this paper. When operating in stand-alone mode, the improved droop control ensures that the operating frequency of the inverter is fixed on rated frequency and at the same time could reduce effect on the inverter reactive power control performance caused by line impedance. When operating in grid-connected mode, it ensures that inverter output active and reactive powers are in a controllable state and the power is not affected by grid frequency fluctuation. Secondly, the key to achieve smooth switching between grid-connected and off-grid is the synchronization of the voltage of point of common coupling(PCC) and grid voltage. The synchronization contains frequency synchronization, phase synchronization and amplitude synchronization. For double-mode inverter adopting traditional droop control, how to achieve the simultaneous synchronization of frequency and phase is the difficulty of active synchronization control. In order to solve the problem, this paper presents a method of active synchronous control based on frequency recovery control, which enables the working frequency of the inverter under stand-alone operation is fixed on rated frequency, so as to realize the simultaneous control of the frequency and phase by adjusting the phase deviation. This synchronization control method is simple, while avoiding the problem that the system adopting V/f-PQ control is easy to cause the oscillation during operation model switching, namly this method synthesizes the advantages of V/f-PQ control and traditional droop control. The method lays the foundation for realization of direct-drive wind power generation double-mode operation and smooth switching.(4) In order to ensure the steady state power balance of double-mode DDWECS, it must be equipped with ESS which has high energy density. In order to guarantee the system dynamic performance, it must be equipped with ESS which has high power density. In order to optimize the comprehensive performance of ESS, battery-supercapacitor hybrid ESS(HESS) is adopted in this paper. In view of the operation characteristic of double-mode direct-drive wind energy conversion system based on HESS, this paper researches a feasible comprehensive control strategy, which can ensure power balance when system is in grid-connected operation and stand-alone operation, it can smooth the output power and suppress the fluctuation of DC voltage effectively. The uncertainty of wind speed and the inverter load mutationn of stand-alone operation, will cause the mutation of difference power, which leads to the fluctuation of the DC voltage, therefore, ESS has a good anti-jamming performance. In this paper, the effects of DC voltage outer-loop shear frequency cvf and high-pass filter cutoff frequency 0? on HESS anti-jamming performance index and the supercapacitor capacity is detailed analysis by establishing the control model of HESS, and the constraints of supercapacitor power loop design and selection method of cvf is obtained.(5) The simulation model and experimental platform of double-mode DDWECS based on the energy storage are established, the correctness and feasibility of the proposed method is verified by the simulation and experiment in this paper.
Keywords/Search Tags:direct-drive wind energy conversion system(DDWECS), hybrid energy storage system(HESS), double-mode operation, improved droop control, active synchronizing control, sensorless technique
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