Research On The Internal Oscillation Suppression Method Of Modular Multilevel DC Transformer

More and more countries have been focusing on the energy innovation to cope with the high carbon emission and the green house effect caused by the excessive exploitation of fossil fuels.However,the high proportion of new energy will bring a series of severe challenges to the stable operation of AC power grid owing to its inherent intermittency.On the contrary,the DC power grid is free from power compensation and frequency synchronization,it is stable and easily controllable with good prospects.As the key device,DC transformer plays a crucial role on guaranteeing the stable operation of DC distribution network.Up to date,the commonly used modular multilevel DC transformer(MMDCT)has many outstanding merits such as modularity,scalability and redundancy.What's more,it has practical application value to coordinate the prominent contradiction between the low voltage stress of power electronic devices and the high voltage level of the power grid.Unfortunately,the series connected submodule capacitors,the arm inductor and its parasitic resistance may lead to the insufficient damping,which may trigger the underdamped oscillations especially under power/load sudden change and the system stability will be reduced accordingly.To solve this issue,this thesis deeply investigates the oscillation mechanism of MMDCT and proposes the corresponding oscillation suppression method.The main work is organized as follows:1)A detailed illustration of the topology and operation principle of MMDCT is given,the leakage current and power transmission characteristic under quasi-two level modulation is analyzed and the state-space averaged model is established as well.2)A transient oscillation suppression method with adjustable duty ratio is proposed based on the in-depth analysis of the oscillation generation mechanism in MMDCT,and the oscillation suppression angle(OSA)is introduced to modify the submodule pulse edges in upper and lower arms,through which the change tendency of the circulating current is adjusted.Furthermore,the effect of OSA on the system damping ratio is analyzed and the corresponding circulating current closed loop controller is designed based on the second-order small signal state space averaged model at the primary side of MMDCT,aiming at achieving the optimum compensation of the system damping ratio.The effectiveness of the proposed method is verified through PSIM simulation platform,which clarifies that the modulation method and the design of the corresponding closed loop controller can sufficiently suppress the transient oscillation in MMDCT and improve the system stability.3)To solve the large voltage stress on the arm inductor caused by the simultaneous conduction of submodules in upper and lower arms when the circulating current is changed by the OSA,a modified oscillation suppression method is proposed with only two submodules in upper/lower arm changing their turn on or off time in each half switching cycle.Then the second-order small signal model of MMDCT is reestablished with the effect of the modified oscillation suppression angle(MOSA)on the system damping ratio illustrated and the closed loop controller is designed as well.Besides,the capacitor charge difference between arms and within each arm with the cooperation of the inner phase shift angle among submodules and the oscillation suppression angle is analyzed,and the sorting algorithm based on the capacitor voltage and voltage difference is used to realize the voltage balance within each arm.Finally,the modified oscillation suppression method is verified via simulation,which proves the effectiveness of the new method.