Research On Influence Of Flexible DC Grid-connected On Short-circuit Current Of AC System

Flexible HVDC has a wide application scenario and is considered to be the future development direction of the power grid.It is particularly suitable for solving the problem of insufficient power and reactive power support capability of the power supply zoning under the "hollowing" form of the local power supply of the urban power grid.The network structure of urban power grids is dense and the level of short-circuit current is high.The influence of flexible DC on the short-circuit current of AC systems must be studied in depth.The existing short-circuit current calculation standards and methods do not involve the influence of the flexible DC on the short-circuit current of the AC system in the short circuit fault of the AC system.In this paper,based on PSCAD,an AC/DC hybrid system with two levels of flexible DC at both ends is built.The system adopts PWM pulse width modulation method.The transient characteristics of the short circuit current provided by the flexible DC system are studied.The working principle and types of the inverter switching device IGBT,the classification of the flexible DC and the control strategy are explained.The mechanism of injecting short-circuit current into flexible DC-to-AC system is analyzed.When a short-circuit fault occurs on the receiving AC line,the voltage on the AC side rapidly decreases and the current rapidly increases.The same pair of bridge arms of a conventional DC converter can be turned on at the same time,causing a short-circuit path to cause a commutation failure,and it is not possible to continue to provide a short-circuit current to the AC side.Flexible DC adopts a voltage source-based converter,and its voltage value will not change drastically during a fault.The inverter does not have the condition of commutation failure,and can continuously change phases,and the current flowing through the half bridges can be measured through simulation.It is found that the current is large and one or more paths can be formed between the upper and lower arms,and the flexible DC continues to provide current to the short-circuit point.Through simulation analysis,the mechanism of injecting short-circuit current into the flexible DC-to-AC system is verified.The effects of short circuit type,short circuit occurrence time,flexible DC transfer power,short-circuit point location,control system parameters,and output current limit on the magnitude of the short-circuit current on the AC side of the flexible DC injection were analyzed,and the following conclusions were drawn:Compared with other short-circuit faults,In the case of a three-phase permanent short-circuit fault,the short-circuit current provided by the flexible DC is the largest,and the permanent fault is greater than the short-circuit current provided by the soft fault during transient faults;the short-circuit fault occurs when the current crosses in the positive or negative direction,and the short-circuit current provided by the flexible DC is the largest;The short-circuit current provided by the flexible DC is positively correlated with the transmission power;affected by the line reactance and capacitance to the ground,the farther the short-circuit point is located,the short-circuit current provided by the flexible DC first decreases and then increases;the constant-voltage control proportional control coefficient kp for the flexible DC The effect of the short circuit current provided is small;the output current limiting circuit can significantly reduce the short circuit current provided by the flexible DC,and the advantages and disadvantages of different current limiters are compared.The solid state current limiter has better economical efficiency and can significantly suppress the short circuit.Current.When considering the access to the flexible HVDC transmission system in the power network planning stage,the current provided by the flexible DC to the short circuit point should be taken into consideration when the AC system on the inverter side is short-circuited.During the simulation,the three-phase permanent short-circuit fault at the outlet of the commutation transformer can be analyzed according to the most serious situation,and the magnitude of the short-circuit current provided by the flexible DC to the AC system is analyzed,and the appropriate current-limiting reactors and circuit breakers are selected to ensure the flexible DC.After the access,the level of short-circuit current is controllable and the power grid operates safely and stably.