Research On Voltage Balancing Control And Fault Ride-through Of Modular Multilevel DC Transformer

The DC power grid is an important part of the future smart transmission and distribution system,which is of great significance for promoting energy conservation and emission reduction and achieving sustainable energy development.As the core equipment of the DC power grid,the DC transformer can realize the functions of DC voltage conversion,DC power transmission and control,and has a decisive influence on the performance of the DC power grid,which has caused widespread attention and research by scholars at home and abroad.In the case of medium and high voltage DC,the modular multilevel DC transformer solves a series of problems such as low power density and complex insulation design of the Input Series Output Parallel structure.However,the researches on arm voltage balancing control,fault characteristics and fault ride-through for modular multilevel DC transformers are very limited at present.It is difficult to support the dynamic operation and fault ride-through of the transformer.In order to solve this problem,this paper focuses on the capacitor voltage balancing control technology and fault ride-through control technology of modular multi-level DC transformers.Firstly,the mechanism analysis of the modular multilevel DC transformer is carried out,and the power model under quasi-square wave modulation is established.By approximating the stepped wave corresponding to quasi-square wave modulation to a trapezoidal wave,the complexity of calculation can be greatly simplified,and the resulting power model has high accuracy and is suitable for transformers with different numbers of cascaded modules.Furthermore,research on the capacitor voltage balancing control of modular multilevel DC transformer.An improved voltage balancing control algorithm based on sorting is proposed for the voltage balancing control problem of the cascaded modules in the arm of the modular multilevel DC transformer.A voltage balancing control based on duty cycle adjustment is proposed for the problem of voltage balancing control between the upper and lower arms of modular multilevel DC transformers.Optimize the working mode judgment unit in the existing sorting voltage balancing control algorithm by using the power phase shift angle as the criterion of the transformer’s working mode,which can effectively improve the perception speed of the sort-based voltage balancing control algorithm to the working mode change,and avoid the capacitor voltage divergence.As a result,the dynamic performance of the voltage balancing control in the arms will be improved.By establishing common-mode and differential-mode current equivalent circuits,the relationship between the duty ratio and the transmission power between the upper and lower arms in different modes is obtained.By adjusting the duty ratio,the transmission power between the upper and lower arms is directly controlled to achieve the balance of the capacitor voltage between the upper and lower arms.This algorithm eliminates the hysteresis control process in traditional methods and effectively optimize the voltage balance effect.Finally,the fault characteristics of the modular multi-level DC transformer when the highvoltage DC bus fails are analyzed,and a low-voltage ride-through control strategy is proposed.In the topology structure,full-bridge sub-modules are used instead of half-bridge sub-modules,and the blocking characteristics of full-bridge sub-modules can be used to effectively improve the speed of transformer fault clearing.With the ability of the full-bridge sub-module to output negative voltages as help,it can keep the stabilization of the high-voltage side voltage of the high-frequency transformer by real-time adjusting the output voltage of each arm.As result,the DC transformer can achieve fault ride-through during voltage falling.