Analysis And Suprress On The Overcurrent Occruing In Converter Station Of VSC-HVDC System

The converter station of VSC-HVDC?Voltage Source Convert-High Voltage Direct Current?system is the hub of energy convertion and transmission,which is expensive and important.Its interior mainly consists of converter elements,including converter transformers,filters,IGBTs,diodes and DC capacitors.Actually,with the limitation of manufacturing technology and material level,the current tolerance level of those converter elements is very limitated.Therefore,the survivability after an overcurrent occurring is critical to the stable operation of VSC-HVDC system.In practical engineering,when the start-up control strategy is unreasonable or a DC fault occurs,the system will face the challenge of the overcurrent.To this end,the overcurrent based on a two-level VSC system is researched from two aspects,viz.?i?limiting the start-up overcurrent,?ii?limiting the DC fault overcurrent.The main research work can be described as follows:?1?Analysis and suppression of the start-up overcurrentAccording to whther IGBTs are blocked or not,the start-up process is divided into uncontrolled rectifier stage and voltage control stage.Meanwhile,the start-up overcurrent characteristics of two stages are analyzed in depth.Then,a simple but effective starting-up control strategy is proposed.It consists of a current limiting resistance(R_in)and three improved controllers.R_in is supplemented into AC side of converter to suppress the overcurrent during uncontrolled rectifier stage,and the design method of R_in is performed in detail.Then,three improved controllers,including DC voltage slope controller,active power adaptive droop controller and reactive power deviation correction controller,are designed to restrain the overcurrent during controlled voltage stage.By introducing this starting-up control strategy,the overcurrent in the VSC-HVDC system is restrained effectively and the system can start up steadily.?2?Analysis and suppression of DC fault overcurrentThe single pole ground fault and the pole-to-pole short-circuit falut that located in DC bus are researched.Firstly,this paper considers IGBTs are immediately blocked after DC fault occurring.Secondly,by establishing equivalent circuits in different fault stages,the fault process is divided into capacitance discharge stage and grid-side current feedback stage,and the fault mechanism of the whole fault stage from occurrence to steady state is qualitatively analyzed.Furthermore,the existing conditions,occurrence time and the effect of grid-side feedback stage are revealed.Then,according to the magnitude of the fault resistance,the transient process is divided into three states,viz.,under-damped,critical-damped and over-damped,the analytical expressions of fault voltage and fault current under three states are derived and are verified by simulation.Based on this,the influence on the fault characteristic is analyzed when adding adding resistance and inductance in the fault circuit.The results show that increasing the impedance of the fault circuit can effectively suppress the overcurrent.As a design idea,a hybrid current limiter with resistance-inductance capacitor commutation is proposed.Its working principle,component parameters,and control signal are introduced in detail.Additionly,the relationship between the component parameters and the current limiting effect is analyzed as well.Finally,substantial simulation results show that the proposed current limiter can effectively restrain the rise rate of DC fault current,save time for DC breakers,and reduce the energy dissipation of DC breakers.