Research On MMC-RPC Modeling And Control Strategy

With the fast development of electrified railway,a variety of energy quality problems have been exposed in the traction power supply system,such as negative sequence current,harmonic,reactive power,inertia difference and so on.They not only seriously affect the normal operation of the traction power supply system,but also have a huge threat to the upstream power grid.V/v transformers are widely used in China's traction substation,Railway static power conditioner(RPC)with the modular multi-level converter(MMC)back-to-back connections,it has the advantages of high pressure resistance,large compensation capacity,good dynamic and static characteristics and no need to connect the step-down transformer to the traction network.It stand out in a number of power and electronics power compensation devices.In this paper,studied the MMC-RPC connected to the traction power supply system with V/v transformer,and did the following works:1)The modeling of MMC-RPC is studied,the mathematical model of the conventional time domain ? ?? coordinate ? dq rotating coordinates is discussed,constructing the orthogonal virtual component of ? axis by using the second order generalized integrator method to complete the transformation process.Secondly,the small model of MMC-RPC is established,and the design of the controller is provided by the precise small signal model.2)By analysising the harmonic in the traction power supply system connected by V/v transformer a proportional complex integral control method(PCI)is proposed for MMC-RPC.The analysis of its control system shows that it can produce infinitely great gain at the target frequency,by this control method can control specific harmonic,and it can take account of the problems of negative order and reactive power quality.3)Considering the power loss on the connection line between MMC-RPC and traction network,and in order to make the MMC-RPC accurate and effective to control the power quality in the traction power supply system,the relationship between P-Q and f-U on the MMC-RPC AC side is deduced on the basis of the system model,set up the droop control.On this basis,a novel droop control strategy combining power coupling and resistance ratio is constructed by combining power coupling and line resistance ratio.It makes the MMC-RPC more flexible and effective to output the desired power.4)As an excellent nonlinear control theory,the Differential Flatness theory is often used in the trajectory planning of the aircraft and the design of the robot control system,through the model analysis,it can be find that the MMC-RPC system satisfies the differential flat condition,a series of cascade PI controllers for MMC-RPC based on the Differential Flatness theory is constructed.The inner ring of this controller produces a flat output reference trajectory,and the outer ring produces the desired output voltage d and q components.The accurate and stable output of MMC-RPC is guaranteed by feed forward control and error feedback control.Analysising the power on both sides of V/v transformer,from the angle of power,the active and reactive power of the power supply arm is selected as the output vector of the differential flat power controller,construct the controller in the power angle of MMC-RPC,in the same way,the main control amount is generated by the feedforward link and the correction amount is generated by the feedback control link.Compared to the differential and flat cascaded PI control,the new power control has the advantages of stable output and rapid reaction.5)The short circuit ratio of the traction power supply system shows the nature of the weak AC system,there are some defects in it,such as low inertia,poor disturbance resistance and so on.The inertia in the rotor of a synchronous motor is adopted in the MMC-RPC control system,thus the MMC-RPC can provide a certain inertia support for the traction power supply system and improve the anti disturbance capability of the system.Moreover,the output stability of MMC-RPC is improved by using the controller structure without phase locked loop.The corresponding control system theoretical analysis and simulation experiments verify that this virtual inertia based power synchronization mechanism has a very good theoretical and practical value.6)It is also aimed at the weak AC characteristics of the traction power supply system,MMC-RPC directly be adopted the control mode of virtual synchronous machine,which enables MMC-RPC to have internal and external characteristics as synchronous motor,and provide necessary inertia and damping support for traction power supply system,so as to improve the whole system's anti disturbance ability and operation stability.