Stability Research On Back-to-back Converter Based On Impedance Analysis

With the target of realizing "carbon peak" in 2030 and "carbon neutral" in 2060 in China,a lot of measures have been proposed to strengthen the environmental protection.Among them,the vigorous development of the renewable energy resources,such as wind power and solar energy,has become one of the important means to realize the "dual carbon" goal.Therefore,how to improve the power system’s ability to accept distributed energy has become a hot research topic.Converters bring about a variety of new oscillation problems in the power system frequently after being connected to the power grid,because of strong nonlinearity,short time scale,small damping inertia and other shortcomings.At present,it is considered that the main reason of oscillation problems is the coupling between converters,or the strong interactive coupling between power electronic equipment and weak power grid,lines,etc.For the instability of renewable energy grid-connected system,there are many studies considering the grid-connected stability of single energy transmission.However,it is necessary to research the bidirectional interactive stability between converters when power flows in bidirectional directions for systems that require bidirectional operation,such as the system with energy storage elements.Meanwhile,the equivalent topology of back-to-back converters is widely used in the flexible direct current transmission systems.Therefore,the back-to-back interconnected converter is chosen as the research object,and the coupling interactive problems between interconnected converters is analyzed from the three aspects respectively:stable operation range,bidirectional flexible interconnection and grid-connected stability.The impedance characteristics are analyzed and the corresponding control strategies are proposed to improve the operating range of the interconnected system,improve the stability differences of bidirectional operation and enhance the grid-connected stability.The main work and achievements are as follows:Research on the DC-link bilateral impedance characteristics of back-to-back converter.At present,the research on the impedance characteristics of converters mostly focuses on the input impedance or output impedance,and the analysis on the bilateral stability characteristics of the bidirectional interconnected converters is less.The output and input dual-port impedance models of the DC interconnect interface are established respectively for the converter with dual-loop voltage regulation control and the converter with single-loop power control.The reasons for interconnection instability problem caused by the resonant peak of the output impedance gain and the negative resistance of the input impedance under bidirectional power flowing are revealed.The influence of controller parameters in control loops on the impedance characteristics and the stability of the interconnected converters is analyzed,and the method of improving system’s stability by adjusting proportional coefficient and bandwidth of voltage loop is proposed.To improve the stable operation range of back-to-back converters,an active control strategy based on current feedback is proposed.On the basis of the input and output impedances established,the influence of active power output increased on the amplitude-frequency characteristics and the phase-frequency characteristics of the impedance are analyzed respectively.It is revealed that the increase in the active power can cause the decrease of the input impedance gain in the low frequency band,which is easy to intersect with the output impedance and produce an unstable region.The equivalent DC-link current is introduced into the front-stage converter to reduce the output impedance gain at the low frequency band,and the frequency domain range intersecting with the output impedance gain is reduced,so as to expand the stable region.The control strategy is verified in Matlab/Simulink simulation software and 2 kW experimental prototype is set up in the laboratory.The proposed scheme can effectively improve the stability margin of the interconnected converters,and also increase the stable operation range of the system.The stability characteristics of back-to-back converters in bidirectional operation are studied,and the bidirectional energy coordinated stability control scheme is proposed.The stability margin of the interconnected converter system is significantly different when the power is transmitted in bidirectional directions.The stability margin of the back-to-back converter is decreased when the voltage source converter is used as the source converter.The state variables in the system have the fast response characteristics and can reflect the control model better.In this paper,the capacitance current observed in the front and rear converters is introduced into the control loop,which improves the impedance gain of the power load converter in the full frequency range and reduces the resonance value of the voltage source converter.The scheme proposed,avoids the intersection of impedance gain as much as possible.The stability difference of the interconnected system is reduced when the power flows in bidirectional directions and the feasibility of the proposed control scheme is verified by simulation and experiment.Aiming at the problem that the position region of equivalent eigenvalue in backto-back converter model is limited,an optimal linear quadratic optimal regulation strategy is proposed.The state space model with phase-locked loop is established,and the characteristics of output variables and state variables are taken into account in the conventional linear quadratic optimal control.The feedback matrix K is modified,so that the equivalent eigenvalue of the system can be set in a wider area to significantly improve the stability and dynamic characteristics of the back-to-back converter.The proposed control strategy increases the damping of the interconnected converters and accelerate the dynamic response,which ensure that the back-to-back converter works stably with better performance.At the same time,simulation and experiments are also implemented to verify the effectiveness of the proposed scheme.