Research On The Key Technology Of Protected Control Method For MMC-HVDC

By using the fully controlled power semiconductors,the voltage source converter based high voltage direct current(VSC-HVDC)transmission system has the self-commutated capability,which enables independent control of the active and reactive power.In addition,VSC-HVDC has other characteristics,such as fast control of power flow,flexible and reliable transmission,power support to isolated load,small short circuit capacity,etc.It is suitable for renewable energy sources integration,urban power supply,and inter-area ac power grid interconnection.And it can realize multi-power and multi-point by electricity.Nowadays,the MMC is considered as the most popular topology in VSC-HVDC transmission systems.MMC has the ability to transfer mass power due to the scalability of the sub-modules in the converter system.In this paper,the key technologies of the VSC-HVDC based on MMC topology are analyzed under grid-side unbalanced faults and dc-side faults.The main work is as follows:(1)The basic operation principle,the ac and dc decoupled model and the control algorithm of the MMC are presented.Based on the Kirchhoff’s law and energy equations,the mathematical model can be obtained.The control method is presented when the MMC works under the normal operating condition.The module capacitor voltage can be well controlled by the individual capacitor voltage controller,and the double-line frequency oscillation in circulating current can be controlled by the proportional resonant(PR)controller.(2)Under the unbalanced grid fault,the traditional control methods are analyzed in details,and a new multi-variable coordinated control scheme in dual d-q axis is proposed,which can flexibly adjust to satisfy different control requirements.By using this method,not only the grid currents can be controlled,but also the inner unbalanced currents,capacitor voltages,voltage differences between the upper and lower arms,and the power ripples in the MMC can be controlled indirectly.(3)Considering the allowable maximum of three-phase current,the allowable capacitor voltage ripple and the maximum reactive power set-point,the design principle of the multi-variable control method is proposed.And the P-Q diagram of MMC systems and the power transferring capability are proposed in terms of the safe operating area under unbalanced conditions.On the other hand,the suitable injected power can be deduced by using the proposed control scheme in order to achieve the best effect of voltage support.(4)In order to block the inrush current in the VSC-HVDC when a short circuit happens on the dc side,the asymmetrical double commutated cell(ADCC)is proposed.The principle of the dc fault clearance for the ADCC based MMC(ADCC-MMC)system is clarified,and the ADCC-MMC losses are deduced.Considering the higher loss in the ADCC-MMC than that in the half bridge MMC,a hybrid MMC which combines the ADCC and half-bridge cell is proposed.Furthermore,the design principle of the hybrid MMC is also proposed.(5)The control strategies of three-terminal HVDC under ac-side fault and the treatments of the three-terminal HVDC under dc-side fault are studied.For the open and short circuit faults on the dc side,the treatments of the system after the failure happens are proposed.When the ac side fault happens,the operating condition of the power-limiting point is added in the master-slave and the droop control algorithm according to the safe operating range of the system.The effect of the multi-variable coordinated control scheme in the three-terminal radial structure is analyzed and the design of multi-variable coordinated control is proposed according to the double-line frequency oscillation of the dc link voltage and current.