Impedance Reconstruction Methods And Stability Analysis For Converters In Photovoltaic/Battery Microgrid

The popularization and application of photovoltaic/battery Microgrid is one of the effective ways to ensure the implementation of China’s energy sustainable development strategy.Due to the randomness,volatility and intermittent characteristic of photovoltaic power generation,large-scale photovoltaic power generations connected to the grid will bring challenges to the safe and stable operation of the grid.The output power of photovoltaic power generation is adjusted by configuring the appropriate amount of energy storage system.It can improve the power controllability and schedulability of photovoltaic power generations and enhance the penetration rate of photovoltaic power generations into the grid.This dissertation is funded by the Key Program of National Natural Science Foundation of China ‘Research on Parallel Resonance and Circulation Current Suppression Theory of Multi-inverter in Microgrid’ [51577056] and National Key Research and Development Program of China ‘Stability Analysis and Control of Power System Integrating Renewable Energy Generation Bases Delivered By HVDC Transmissions’ [2017YFB0902000] and Science and Technology Project of State Grid ‘Research on Inertial Control Method and Port Stability of Power Electronic Converter Connecting to DC Power Grid’ [SGZJ0000KXJS1700510],and also supported by National Energy Conversion and Control Engineering Technology Research Center.This dissertation focuses on the research hotspots and difficulties of photovoltaic/battery Microgrid based on AC-DC hybrid bus.When single-phase inverters with AC loads are injected into energy storage system at the DC bus side,instantaneous output power of single-phase inverters ripples at double output voltage frequency,which leads to generate the second ripple current(SRC)in the input sources such as batteries and energy storage converters(ESCs).At the same time,if the equivalent impedances of parallel ESCs are not equal,it will lead to the circulating current among ESCs,which is not conducive to the stable operation of the DC bus.Due to the existence of grid impedance at the AC bus side,there is impedance coupling between the inverters and the grid,which will form the impedance network.The harmonic current in the impedance network may be amplified under the excitation of harmonic source,which will greatly affect the power quality of the AC bus and pose a threat to the stable operation of the AC bus.Therefore,there is an urgent need to ensure the stable operation of photovoltaic/battery Microgrid based on AC-DC hybrid bus.Based on the above analysis,the key problems affecting the stable operation of photovoltaic/battery Microgrid are summarized as follows: the SRC suppression of parallel ESCs,circulating current control of parallel ESCs,small-signal perturbation modeling and stability analysis for photovoltaic grid-connected inverter and harmonic oscillation suppression(HOS)for photovoltaic grid-connected inverters.These four kinds of key problems have attracted the attention of industry and academia and formed research hotspots.Therefore,the effective impedance reconstruction methods are proposed to solve these four kinds of key problems and improve the system stability.The specific works and innovations in this dissertation can be summarized as follows:(1)To suppress the SRC and circulating current of ESCs,the SRC suppression by two bandpass filters(BPFs)and current sharing method is proposed based on impedance reconstruction.Section 2.1 establishes the system model of parallel ESCs,and analyzes the generation mechanism of circulating current and SRC of parallel ESCs.Section 2.2.1 presents an adaptive droop control method by introducing the fine-tuning virtual resistances in series,which reduces the output voltage deviation of parallel ESCs and suppresses the circulating current.Section 2.2.2 proposes the SRC suppression method by two BPFs for the ESCs based on impedance reconstruction,which reduces the SRC and improves the dynamic performance in case of a lower cutoff frequency in the outer voltage loop.Section 2.3 gives the control parameter design and stability analysis of the ESCs.Finally,simulation and experimental results verify the validity of the proposed method.(2)For the small-signal perturbation modeling and stability analysis for photovoltaic grid-connected inverter,the phase compensation impedance reconstruction method is proposed considering the effect of phase-locked loop(PLL).In Section 3.1,variables in the system dq frame are transformed into the control dq frame by the rotating matrix,and the interaction between two dq frames is obtained.The small-signal perturbation admittance model in consideration of the PLL is built for three-phase LCL-type grid-connected inverter.Section 3.2 studies the output admittance characteristics of the grid-connected inverter,and analyzes the effect of PLL,grid impedance and LCL-type filter parameters on system stability in the system dq frame.Section 3.3 proposes the phase compensation impedance reconstruction method for three-phase grid-connected system considering the effect of PLL.This method not only solves the problem of the system instability due to the larger PLL bandwidth,but also ensures the system dynamic response speed.Meanwhile,the feedforward point of common coupling(PCC)voltage is introduced in this method to reshape the output admittance of the grid-connected inverter.The amplitude of the grid-connected inverter output admittance is maintained at the system cutoff frequency,and the system phase margin is increased.And then the realization and parameter design of this method is given.Section 3.4 analyzes the stability of the system before and after the phase compensation.Finally,simulation and experimental results verify the correctness of the proposed method.(3)To solve the harmonic oscillation for photovoltaic grid-connected inverters,the HOS method by two notch filters is proposed based on impedance reconstruction.Section 4.1 analyzes the high-frequency oscillation mechanism of the paralleled multi-inverter system.In Section 4.2,a basic technique from the viewpoint of inverter output impedance is proposed to take into account the HOS,showing that the inverter output impedance should be designed relatively high at the oscillation frequency,while relatively low at other frequencies.On the basis of the proposed technique,two virtual impedances are added to be in parallel and in series with the original inverter output impedance,respectively.And then the high-frequency oscillation suppression method by two notch filters is proposed to increase the whole system damping.The implementation forms of the virtual impedances are presented by the proposed PCC voltage feedforward and grid-side inductor current feedback with two notch filters.The feedforward path of PCC voltage with the notch filter equals to a virtual impedance in parallel with inverter output impedance,which is designed to show low impedance at the harmonic oscillation frequency.By doing so,most high-frequency harmonic current will flow into the parallel virtual impedance branch,and it effectively suppresses the high-frequency oscillation.Meanwhile,the feedback path of the grid-side inductor current with the notch filter is equivalent to a virtual impedance in series with inverter output impedance,which is designed to display low impedance at the fundamental frequency.This way,most of the fundamental frequency current flows into the grid branch with relatively low impedance.In addition,it improves the power quality of the grid-connected current.Section 4.3 compares and analyzes the system stability in two cases.Finally,simulation and experimental results are provided to verify the validity of the proposed control method.(4)The experimental platform of photovoltaic/battery Microgrid based on AC-DC hybrid bus is built.Section 5.1 gives the overall topology of the main circuit of the experimental platform.Section 5.2 designs the experimental devices such as photovoltaic power generation system,energy storage system and DC/AC converter in detail.Section 5.3 carries out the overall performance test of the experimental platform,mainly including the coordinated control of the DC bus side and the power quality of the AC bus side.Finally,the test results show that the converters can coordinate control at the DC bus side,maintain the constant voltage of the DC bus,and ensure the stable operation of the DC bus.And the phase synchronization between grid-connected currents and grid-connected voltages can be achieved at the AC bus side,which ensures that the grid-connected power factor is close to the unit power factor,and also meets the requirement that the total harmonic distortion rate of grid-connected current is less than 5%.In this dissertation,the key scientific and technical problems of photovoltaic/battery Microgrid based on AC-DC hybrid bus are studied.The theory and method of impedance reconfiguration control for converters in photovoltaic/battery Microgrid are deeply studied from the SRC suppression of parallel ESCs,circulating current control of parallel ESCs,small-signal perturbation modeling and stability analysis for photovoltaic grid-connected inverter and HOS for photovoltaic grid-connected inverters.The research scope covers the problem mechanism,control method,parameter design and stability analysis,which forms a relatively perfect theoretical analysis method and impedance reconstruction control technology scheme for converters in photovoltaic/battery Microgrid.