Research On Line Protection Of Flexible DC Distribution Network

Flexible DC distribution networks coordinate renewable energy sources and flexible loads to enhance the consumption rate of renewable energy for the energy transition.Due to the low current tolerance of power electronics,short-circuit currents generated by DC-side line faults will bring serious safety hazards to power electronic devices.This paper combines the development trend of DC systems and the characteristics of urban distribution networks,a multi-terminal flexible DC distribution network with multiple branch overhead lines is constructed.The protection scheme is proposed for the line fault between two converters,which can be divided into a multiple branch line and a branch busbar.An accelerated protection strategy is proposed for multiple branch line faults,which can be split into two steps.The first step emphasizes speeding and proposes a line fast pilot protection scheme,which uses the polarity of the line mode current mutations on both sides to identify the fault area.The Direct Current Circuit Breaker(DCCB)removes the entire line between the two converters,which marks the completion of the first step.The second step is to enter the line reclosing phase,and it is necessary to quickly and accurately select the faulty branch line in the case of a permanent fault.The second step emphasizes selectivity and proposes a double-ended line reclosing scheme through communication and a single-ended line reclosing scheme using local information.From the perspective of waveforms,there is a difference in symmetry in the time axis between the fault current waveforms on both sides of the line.This feature is used to construct the protection criterion for the double-ended line reclosing scheme.Using the time sequence of the DCCBs on both sides to close,split the power supply lines at both ends into two independent radiation networks.The single-ended line reclosing scheme considers that the transient energy of the fault current in a single radiation network is used to identify the fault area.Branch busbar fault can also be equated to special line faults,and non-differential busbar protection is proposed based on the symmetry and trend polarity of the current changes flowing into and out of the busbar.The proposed busbar protection scheme has good performance in various high resistance DC busbar faults.So,it can be used as primary protection in conjunction with differential backup protection to improve the reliability of the system.To better verify the feasibility of the relevant line protection scheme proposed in this paper,a four-terminal flexible DC distribution network model containing multiple branch overhead lines was built in electromagnetic transient simulation software.A large number of simulations verify that the protection schemes are correct.And considering a variety of different harsh working conditions,the results prove that the protection scheme has good performance.