Fault Control And Symmetrical Short-circuit Current Calculation For AC/DC Hybrid Power System

High voltage direct current(HVDC)is widely used due to its significant advantages in long-distance and large-capacity power transmission and asynchronous power systems interconnected.Compared with the traditional DC power transmission technology,HVDC-flexible transmission technology has the advantages of no commutation failure,rapid decoupling control of active and reactive power,and convenient and fast power flow reversal,and has become an important direction in the field of DC power transmission.Now,China has built the world’s largest AC/DC hybrid D systems with the largest scale and most complex operating conditions.Ensuring the safe and stable operation of large AC/DC hybrid power systems has become an important and pressing demand.Short-circuit calculation is the basis of the protection of AC/DC hybrid power systems,so taking the fault analysis and short-circuit calculation can provide a complete theoretical method for the protection scheme of AC/DC hybrid power systems,and ensure the security and stability of the power grid.This paper focuses on the fault control strategy of AC/DC hybrid nodes and the method of symmetrical short-circuit current calculation in AC/DC hybrid power systems.The main contents of this paper are as follows:(1)The fault control strategies of the converters of AC/DC hybrid nodes and their corresponding relationships with converters states under AC and DC fault conditions were analyzed.Aiming at the topological structure of full-controlled converters,the fault control strategies of the DC side of converters based on improved modular multilevel converters were summarized.Aiming at the double-loop control model of full-control converters,a fault control strategy on the AC side of hybrid nodes based on active fault current management was proposed,and the expression of reference phase angle for fault currents management was derived.On this basis,the coupling relationships between AC/DC sides of converters under different fault control strategies were analyzed,and the equivalent modesl of AC/DC side were established.Further,the corresponding relationships between different fault control strategies and converters states were obtained.(2)Considering the constraint of transient electrical quantities of AC and DC side,The transfer matrix of port transient of AC/DC converters was established.When a short-circuit fault occured in the power system,the fault-ride modes of the fully-controlled and phase-controlled converters were analyzed.Considering the equivalent circuit constraints of the port transient voltage and current of short-circuit calculation of converters,the transfer matrix of port transient of AC/DC converters was established according to the average voltage and current of dual-port converters of different fault-ride modes.(3)A symmetrical short-circuit current calculation method for AC/DC hybrid power systems based on the transfer matrix of port transient was proposed.The nodes of AC/DC hybrid power systems were classified according to whether they were hybrid nodes or not,and the nodal voltage equation of AC and DC power systems were obtained separately according to the equivalent network of short-circuit current calculation.Combining the transfer matrix of port transient of AC/DC converters and the nodal voltage equation of AC and DC power systems,the nodal voltage equation of AC/DC hybrid power systems based on hybrid nodes eliminated was derived.By using the superposition principle,the short-circuit currents of AC/DC hybrid power system were calculated when AC three-phase short circuit of grid or DC symmetrical short-circuit happened.Then,a symmetrical short-circuit current calculation method for AC/DC hybrid power systems based on the transfer matrix of port transient was proposed,and the correctness and effectiveness of the proposed method was verified by simulation.