DC Traction Power Supply System Based on Modular Multilevel Converter Suitable for Energy Feeding and De-icing

A novel DC traction power supply system suitable for energy feeding and de-icing is proposed in this paper for an urban rail transit catenary on the basis of the full bridge submodule (FBSM) modular multilevel converter (MMC). The FBSM-MMC is a novel type of voltage source converter (VSC) and can directly control the output DC voltage and conduct bipolar currents, thus flexibly controlling the power flow of the urban rail transit catenary. The proposed topology can overcome the inherent disadvantages of the output voltage drop in the diode rectifier units, increase the power supply distance and reduce the number of traction substations. The flexible DC technology can coordinate multiple FBSM-MMCs in a wide area and jointly complete the bidirectional control of catenary power flow during the operation of the electric locomotive, so as to realize the local consumption and optimal utilization of the recovered braking energy of the train. In addition, the FBSM-MMCs can also adjust the output current when the locomotive is out of service to prevent the catenary from icing in winter. The working modes of the proposed topology are illustrated in detail and the control strategy is specially designed for normal locomotive operations and catenary de-icing. Simulation cases conducted by PSCAD/EMTDC validate the proposed topology and its control strategy.

Single-ended Fault Detection Scheme Using Support Vector Machine for Multi-terminal Direct Current Systems Based on Modular Multilevel Converter

This paper proposes a single-ended fault detection scheme for long transmission lines using support vector machine(SVM)for multi-terminal direct current systems based on modular multilevel converter(MMC-MTDC).The scheme overcomes existing detection difficulties in the protection of long transmission lines resulting from high grounding resistance and attenuation,and also avoids the sophisticated process of threshold value selection.The high-frequency components in the measured voltage extracted by a wavelet transform and the amplitude of the zero-mode set of the positive-sequence voltage are the inputs to a trained SVM.The output of the SVM determines the fault type.A model of a four-terminal DC power grid with overhead transmission lines is built in PSCAD/EMTDC.Simulation results of EMTDC confirm that the proposed scheme achieves 100%accuracy in detecting short-circuit faults with high resistance on long transmission lines.The proposed scheme eliminates mal-operation of DC circuit breakers when faced with power order changes or AC-side faults.Its robustness and time delay are also assessed and shown to have no perceptible effect on the speed and accuracy of the detection scheme,thus ensuring its reliability and stability.

High-frequency Resonance Analysis and Impedance Reshaping Control of MMC-HVDC System Based on Frequency Coupling Impedance Model

In recent years, high-frequency resonance (HFR) events occurred in several modular multilevel converter based high-voltage direct current (MMC-HVDC) projects. The time delay of an MMC-HVDC system is the critical factor that induces HFR. The frequency coupling affects the impedance characteristics of an MMC and further deteriorates system stability. Therefore, in this paper, a multi-input multi-output admittance model of an MMC-HVDC system is developed to analyze its frequency characteristics. The effects of current loop, power loop, phase-locked loop, and operating point on the MMC frequency coupling degree are analyzed in detail. Meanwhile, to further suppress HFR in the MMC-HVDC system, an enhanced impedance reshaping control strategy based on the equivalent single-input single-output impedance model is proposed. Finally, the accuracy of the enhanced impedance model and the effectiveness of the impedance reshaping control are verified by electromagnetic transient simulations in PSCAD.

Fault Ride-through Hybrid Controller for MMC-HVDC Transmission System via Switching Control Units Based on Bang-bang Funnel Controller

This paper proposes a fault ride-through hybrid controller(FRTHC)for modular multi-level converter based high-voltage direct current(MMC-HVDC)transmission systems.The FRTHC comprises four loops of cascading switching control units(SCUs).Each SCU switches between a bang-bang funnel controller(BBFC)and proportional-integral(PI)control loop according to a state-dependent switching law.The BBFC can utilize the full control capability of each control loop using three-value control signals with the maximum available magnitude.A state-dependent switching law is designed for each SCU to guarantee its structural stability.Simulation studies are conducted to verify the superior fault ride-through capability of the MMC-HVDC transmission system controlled by FRTHC,in comparison to that controlled by a vector controller(VC)and a VC with DC voltage droop control(VDRC).

Impact of Different AC Voltage Control Modes of Wind-farm-side MMC on Stability of MMC-HVDC with Offshore Wind Farms

Wind-farm-side modular multilevel converters(WFMMCs) used in modular multilevel converter based highvoltage direct current(MMC-HVDC) transmission systems must be able to control the AC grid voltage in offshore wind farms. Different AC voltage control strategies can significantly affect the dynamic characteristics of WFMMCs. However, existing studies have not provided a general methodology of controller parameter design, and few comparative studies have been conducted on control performance under varying operating conditions as well as the effects of different AC voltage control modes(AVCMs) on the stability of MMC-HVDCs with offshore wind farms. This paper provides a controller parameter design method for AVCMs, which is tested in various operating scenarios. Sequence impedance models of offshore wind farms and WFMMCs under different AVCMs are then developed. The effects of AVCMs on the small-signal stability of the interconnected system are then analyzed and compared using the impedance-based method. Finally, case studies are conducted on a practical MMC-HVDC system with offshore wind farms to verify the theoretical analysis.