Analysis and Control of Modular Multi-terminal DC Power Flow Controller with Fault Current Limiting Function

In order to overcome the problems of power flow control and fault current limiting in multi-terminal high voltage direct current(MTDC)grids,this paper proposes a modular multi-terminal DC power flow controller(MM-DCPFC)with fault current limiting function.The topology structure,operation principle,and equivalent circuit of MM-DCPFC are introduced,and such a structure has the advantages of modularity and scalability.The power balance mechanism is studied and a hierarchical power balance control strategy is proposed.The results show that MM-DCPFC can achieve internal power exchange,which avoids the use of external power supply.The fault characteristics of MM-DCPFC are analyzed,fault current limiting and self-protection methods are proposed,and the factors affecting the current limiting capability are studied.The simulation models are established in PLECS,and the simulation results verify the effectiveness of MM-DCPFC in power flow control,fault current limiting,and scalability.In addition,a prototype is developed to validate the function and control method of MM-DCPFC.

A Review of Series Voltage Source Converter with Fault Current Limiting Function

The series voltage source converter(SVSC)is widely used in the power electronic equipment,such as series active power filter,dynamic voltage restorer,unified power flow controller and so on.However,while the SVSC is more vulnerable to the impact of fault current,its applications are increasing,bringing huge challenges to the safe operation of the grid.In recent years,the topology and control strategy of the series voltage source converter with fault current limiting(SVSC-FCL)are a research hotspot.In this paper,it suggests classifying SVSC-FCL based SVSC into two groups:the control scheme optimization group and the existing topology improvement group.The research challenges and perspectives of the SVSC-FCL are introduced in detail.This paper aims to illustrate current research progress on SVSC-FCL and enrich the available pool of the multi-functional power electronic equipment.

Optimum Control Scheme of Output Voltage Based on Cascaded H-bridge DVR

This study presents an optimum control scheme to maximize the output voltage level number of the cascaded Hbridge dynamic voltage restorer(CHB–DVR).The relationship between the modulation index and the output voltage level number is analyzed in detail.The compensation reference voltage value is adjusted with the voltage drop depth to obtain high-quality output voltage with an acceptable total harmonic distortion.Thus,the modulation index remains within a certain range and thus meets the requirements of the maximum level number technique(MLNT).In addition,an improvement method based on the MLNT is proposed to achieve minimum active power absorption from a direct current link of the CHB–DVR.The traditional in-phase compensation and optimum control strategies are implemented to analyze the output voltage quality for verifying the feasibility of the proposed approach.Simulation and experimental results show the effectiveness of the proposed control scheme.

A nonlinear repetitive controller

A nonlinear repetitive controller is proposed. The new method is mainly composed of a repetitive control part and a deadband relay. Whenever the input error goes beyond the range of the deadband relay, the control loop is driven dominantly by the deadband relay to obtain fast dynamic response and meanwhile to avoid the saturation of the repetitive control part. After the input error falls within the range of the deadband relay, the deadband relay automatically turns off and the repetitive control alone governs the current control to eliminate the steady state error. A systematic methodology is established and it is linked to the conventional control system design. The proposed scheme is practically applied to the current control of active filter. Experimental results verified the feasibility of the proposed method.

Design-oriented Transient Stability Assessment for Droop-controlled Converter Considering Grid Voltage Phase-angle Jump

Droop-controlled voltage-source converters(VSCs)can provide frequency and voltage support to power grids.However,during a grid fault,VsCs may experience transient instability,which can be significantly affected by both the control parameters and fault conditions.This mechanism has not been fully elucidated in previous studies.In particular,grid-voltage faults are commonly accompanied by a grid voltage phase-angle jump(VPAJ),which is typically ignored in the evaluation of the transient stability of VSCs.To address this issue,this study comprehensively assesses the impact of the VPAJ and key control parameters on the transient characteristics of VSCs.Furthermore,the critical clearing angle and critical clearing time are quantitatively calculated to define the transient stability boundary.In addition,a transient stability-enhancement control method that considers the transient stability constraints is proposed.Finally,simulations and experimental tests are conducted to validate both the theoretical analysis and proposed method.