Small-signal Stability Analysis And Improvement Method For Multi-converter Systems

Power electronic converters have become the core equipment for power conversion in modern power grids due to their strong controllability,fast dynamic response,flexibility and high efficiency.Thus,the power electronic converters play an extremely important role in power generation,transmission and consumption.The grid with multiple power electronic converters will become the inevitable trend of future power systems.However,the interaction among the complex control loops of converters,the passive components of the system,and other converters in the system can easily lead to system instability problems,threatening the safe and stable operation of the system.With the the integration of large-scale and multiple type converters and the increasingly complex operating environment and working conditions of the systems,the instability problems of the converter systems will become more and more prominent.Therefore,it is urgent to carry out the stability modeling and assessment as well as investigation of the influence factors of stability for the multi-converter systems,and then,studying the systems' stability improvement methods,which can be beneficial to ensure the safe and stable operation of the multi-converter systems.Taking the multi-converter system with the new energy power generation converters as the research object,and focusing on the small-signal stability of the system,this paper studies the impedance modeling and stability analysis method for the multi-converter system.The instability mechanisms and influencing factors of the multi-converter system have been explored,and then relevant measures have been put forward to improve the stability of the system.The specific research contents of this paper mainly include the following aspects:(1)The instability mechanism of bidirectional AC-DC converter in full operation condition is studied.By establishing the general small-signal impedance model of bidirectional AC-DC converter that works in different operation modes,the instability characteristics of its multiple control loops and the interaction between control loops and grid impedance are comprehensively studied.The concept of "positive feedback" is used to reveal the mechanism of the difference of stability margin,which is caused by the power flow direction,of bidirectional AC-DC converter under different operation modes.On this basis,the stability improvement measures of bidirectional AC-DC converter are discussed,where the band-pass filter for the sampling signal of the voltage at the point of common coupling(PCC)is proposed to to improve the low stability margin caused by the positive feedback effect introduced by phase-locked loop.(2)Based on the network node admittance matrix,the impedance-based stability modeling and analysis method for multi-converter systems is proposed.The small-signal impedance expression of the converter system is established,and the influence of election of the common coordinate system on the system's stability is explored.On this basis,the stability margins of a multi-converter system affected by the connnecting positions of converters have been studied,and then the appropriate connnecting positions of converters could be selected to maximize the stability margin.For the unstable multi-converter systems,the participation factor of the critical eigenvalues of the loop gain matrix is used to identify the dominant converter that causes the system's instability.(3)A method based on the impedance sensitivity is proposed to intuitively and quantitatively analyze the influences of control parameters and component parameters on the system's stability.The equivalent impedance criteria of typical converter systems,including single input single output Y+Z type,single input single output Z+Z type,multiple input multiple output Y+Z type,and multiple input multiple output Z+Z type,are derived.Based on the equivalent criterion,the dominant control parameters and component parameters that affect the stability of the system can be quantitatively investigated by using the proposed impedance sensitivity.Then according to the sensitivity of the dominant parameters,the relevant parameters can be purposefully adjusted to efficiently improve the stability of the system.(4)Combining with the stability analysis theory of multi-converter and the impedance measurement technology,a stability verification and analysis method for multi-inverter systems based on impedance measurement principle is propo sed,which is suitable for stability verification of multi-inverter systems before inverters are connected to power grid.An active impedance sensitivity analysis method is proposed,which can effectively identify the dominant inverter that causes system instability by using sensitivity calculation based on the impedance measurement data of inverters.The coupling mechanism of multi-inverter system is explored,and the influences of grid impedance on the coupling characteristics of inverter s are analyzed.The effectiveness of the proposed stability modeling and assessment method and stability improvement measures for multi-converter system can be verified by simulations in PSCAD/EMTDC and experiments in RT-LAB hardware in the loop platform.To sum up,aiming at the deficiency of the existing researches,this paper has done some related researches on the small-signal stability modeling method,assessment method as well as the investigation of coupling instability mechanism of the multi-converter systems,which can provide a theoretical basis for improving the stability of the system,helping to ensure the safe and stable operation of multi-converter systems.