Design Of Observer,Controller Of MMC Based On Passivity Theory And Research On Improved Topology For Variable Frequency Applications

For the complexity of the control system caused by the increase of the number of the cascaded modules of MMC in the high voltage applications,and the difficulties in designing a stable controller for the nonlinear MMC system,a full-dimensional state space model considering the SM capacitor voltage dynamics is established.From the perspective of stability,and ultilizing the passivity analysis method,a persistently and sufficiently exciting condition is given.And a uniformly asymptotically stable discrete-time reduced-order observer of the SM capacitor voltage is designed.Based on the balancing control of capacitor voltage,an average model of MMC is established and then a uniformly asymptotically stable closed-loop controller is designed.Finally,two improved MMC topologies are proposed to solve the problem of the capacitor voltage fluctuation and common mode voltage existing in the field of medium voltage motor dirves.The specific contents of this paper are as follows:1)Based on the topology and operating principle of MMC system,a full-dimensional state space model considering the dynamic characteristics of SM capacitor voltage is derived.From the point of view of guaranteeing the stability of the system,the persistently and sufficiently exciting condition of the MMC system is given.On the one hand,the stability problem of the MMC open-loop system is solved in theory.On the other hand,a theoretical support is also presented for the stability design of the MMC observer and controller.2)With the increasing of the voltage and power levels of MMC system,the number of the cascade SMs and capacitor voltage sensors is larger,which increases the hardware cost and reduces the reliability of the control system.In this paper,a design method of SM capacitor voltage discrete-time reduced-order observer is proposed.Only one arm current sensor and one arm voltage sensor are needed for each arm.At the same time,from the view of stability,the selection method of the observer gain matrix to ensure the uniformly asymptotic convergence of the observation errors is given.The designed observer considers the charging and discharging process of the SM capacitor voltage,which is simple and can obtain a relative small observation error.3)To solve the control problem of the strong coupling and nonlinear MMC system,based on the single-phase model of MMC system,a feedback linearization controller from the input to state is designed,which can guarantee the global asymptotic stability of the closed-loop system.However,the control performance would be affected for the medium voltage application because the coupling of different phase units is not considered.Therefore,based on the three-phase average model considering the coupling of different phase units,a state-error feedback controller of MMC system is designed.Ultilizing the passive method and loop transformation method,a selection method of the controller gain matrix to ensure the uniformly asymptotic stability of the MMC closed-loop system is given.Besides,the designed controllers need only one control loop and the parameter regulation is simple.4)When MMC with motor load operates at low speed,there exists the large capacitor voltage fluctuation and the high common-mode voltage problems.In this paper,two improved MMC topologies are proposed to solve the two problems.One kind is HMMC-CFB topology,that is,the cascaded full-bridge SMs are connected in series at the AC side of MMC,which are used to absorb the high frequency zero-sequence voltage component injected by the high frequency injection algorithm.Another is HMMC-CHBD topology,that is,the cascaded half-bridge SMs are symmetrically connected in series at the DC side of MMC.The DC bus voltage is reduced at the low speed,reducing the charging and discharging power of SM capacitor voltage.The two improved MMC topologies presented in this paper have strong scalability and need simple control.