Research On Resistance To Commutation Failure Of HVDC Transmission System With Semi-controlled Switches And Fully-controlled Switches In Series

The line commutated converter high voltage direct current(LCC-HVDC)technology based on grid commutated converters has a large number of engineering applications in long-distance and large-capacity transmission occasions due to its technical and economic advantages.Driven by the goal of "emission peaking and carbon neutrality",the participation of LCC-HVDC in power grid construction will be further expanded to promote the development of new infrastructure.However,LCCHVDC uses thyristors,and its semi-controlled characteristics determine that the commutation process depends on the AC system.Therefore,disturbances in the AC system can cause the system commutation failure,resulting in a short circuit on the DC side,resulting in voltage fluctuations,insufficient power,and system instability.The existing commutation failure defense measures are mainly divided into four categories:coordinating DC power,optimizing control and protection,adding compensation equipment,and constructing new topologies.Among them,constructing new topologies tries to fundamentally solve the problem of commutation failure from the perspective of devices,and it is universal.The new topology scheme based on the series connection of semi-controlled switches and fully-controlled switches improves the flexibility and controllability of the commutation process.It provides a new idea for converter topology research,and has become a hot research direction at present.This paper focuses on the HVDC transmission system with semi-controlled switches and fully-controlled switches in series.The related research work is mainly carried out from the following aspects:(1)Explore the basic mechanism of the series topology of semi-controlled switches and fully-controlled switches.This paper starts from the macroscopic and microscopic levels of commutation failure,and analyzes the external system influencing factors and the intrinsic reasons of internal devices that lead to commutation failure in detail.Aiming at the key factors of commutation failure,this paper forms the basic idea of the topology of semi-controlled switches and fully-controlled switches in series,and then designs the basic structure of the hybrid series converter valve comprehensively.The traditional thyristor commutator valve completely relies on the reverse voltage provided by the AC grid during the turn-off process,and the commutation process is uncontrollable and has poor reliability.The turn-off process of the designed hybrid series converter valve has the participation of the fully-controlled switches,does not completely depend on the AC reverse voltage,and has the ability to turn off autonomously.The commutation process is controllable under fault conditions,and the reliability is strong.Furthermore,this paper proposes a design method for the measurement of the arc extinguishing angle of the hybrid series converter valve,which provides a technical basis for the system simulation test.(2)The coupled turn-off characteristic analysis of the series topology of the semicontrolled switches and fully-controlled switches.The current research on the series topology of semi-controlled switches and fully-controlled switches ignores the switching characteristics,resulting in unclear turn-off models and inability to determine whether the blocking capability of the thyristor is restored.In this paper,a thyristor turn-off model is established based on the current segmental characteristics and charge continuity of the thyristor.Combined with the charge continuity equation,the carrier recombination expression for the thyristor turn-off process is deduced.After analyzing the thyristor current and carrier recombination changes process,the mechanism of AC voltage affecting the reliable turn-off of the thyristor of the converter valve is revealed.Further,this paper analyzes the flow path of the turn-off current of the hybrid series converter valve.Combined with the typical IGBT turn-off process model and the thyristor turn-off model,the coupled turn-off characteristic model of the series topology of semi-controlled switches and fully-controlled switches is established.The established model can accurately judge the recovery degree of thyristor blocking ability and determine the commutation result.(3)Research on the coordination strategy of the series topology of semi-controlled switches and fully-controlled switches.The influence of the hybrid series converter valve on the commutation results depends on the reasonable turn-off timing.Based on this,this paper proposes a series coordination strategy of semi-controlled switches and fully-controlled switches.The proposed strategy considers the blocking characteristics of the semi-controlled switches,and combines the advantages of different simulation platforms.It achieves the effect that the fully-controlled switches participate in the commutation process flexibly and controllably,and the semi-controlled switches’ turnoff process is safe and reliable under fault conditions.Furthermore,this paper proposes a novel coordinated control strategy for the enhanced line commutated converter.Combined with the characteristics of the controllable voltage sub-module in the original topology,the proposed strategy achieves two active control effects of auxiliary commutation and reliable shutdown,which changes the status quo of passive commutation of line commutation converters.The system becomes more resistant to commutation failure.(4)Resistor-capacitance buffer design for series topology of semi-controlled switches and fully-controlled switches.In the series topology of semi-controlled switches and fully-controlled switches,the speed and timing of switch action are different,so the pressure is inconsistent.Analyzing the electrical stress of the switches and solving the overvoltage problem during the turn-off process is a necessary condition for engineering applications.Based on this,this paper puts forward the design and calculation principles of the dynamic pressure equalization resistance-capacitance parameters of the hybrid series converter valve.The proposed principle can realize the optimal calculation of the damping capacitance and damping resistance of the fullycontrolled switches,and the turn-off overvoltage of the fully-controlled switches can be reduced to 3.34kV,which meets the requirements of the blocking characteristics of the switch device.Furthermore,this paper proposes a resistance-capacitance coordination scheme for the enhanced line commutated converter.In this scheme,controllable capacitors and damping resistors are reasonably designed to achieve commutation voltage compensation and DC current suppression effects.The proposed scheme reduces the risk of capacitors being bypassed,and prolongs the action time of the auxiliary commutation voltage,thereby enhancing the system’s reliability.(5)Multi-infeed system analysis of the series topology of semi-controlled switches and fully-controlled switches.Chinese power grid has presented a pattern of largecapacity DC and multiple feeds.This feature makes the commutation failure,especially the cascade commutation failure of multiple DCs,a serious threat to the safe and stable operation of the power grid.Based on this,this paper takes the traditional double-infeed system as an example,uses the hybrid series converter valve and the enhanced grid commutation converter to transform the inverter at the receiving end respectively.The new DC transmission systems verify the series topology of semi-controlled switches and fully-controlled switches against commutation failure.Furthermore,the calculation and analysis are carried out from the aspect of equipment loss.The engineering potential of the series topology of semi-controlled switches and fully-controlled switches in multi-infeed DC transmission systems is demonstrated.