This letter presents a grid-forming control with adaptive grid-synchronization capability for voltage source converters (VSCs). The proposed control has both phase-locked loop (PLL) and power synchronization loop (PSL), which makes it exhibit hybrid synchronization features. In addition, the VSC's synchronization performance can be dynamically optimized by adaptively adjusting the PLL-PSL weighting coefficient. Theoretical and simulation analyses are performed to verify the effectiveness of the proposed control.

Urban power grids are facing many operational and expansion challenges to meet further demand growth and increased reliability requirements. Advanced transmission technologies have been considered by the electric utilities to effectively increase the utilization of existing infrastructure and operational flexibility. The focus of this paper is on VSC-HVDC technologies for urban power grid enhancement and modernization. First, a potential technical scheme is proposed for converting an existing AC circuit to DC operation, which could boost the power transfer capability of the critical transmission corridor and increase network operational flexibility. Second, this paper proposes three operation modes for the VSC-HVDC interties in urban power grids corresponding to normal, emergency and island operating conditions, respectively. An integrated, adaptive emergency control strategy (AEC) is proposed that can enable adaptive power flow responses of the VSC-HVDC intertie under varying system operating conditions and critical contingencies. The flexibility and effectiveness of the proposed operational principles of urban VSC-HVDC intertie and the corresponding control strategies are verified in PSCAD/EMTDC using a realistic urban power grid in China.

Light-duty offshore wind power system has been a research hotspot in recent years. The diode rectifier based HVDC scheme is a promising solution and has attracted more and more attention. In this paper, a self-synchronizing control for offshore wind farms connected to diode rectifier based HVDC system is proposed. The active power of each wind turbine is controlled by regulating the amplitude of the AC voltage, while the reactive power is used to adjust the offshore frequency. The proposed control is a decentralized scheme. Thus, no communication network or GPS is required and only the local information is used in the control system. The synchronization of wind turbines is achieved by the proposed self-synchronizing frequency control instead of using phase-lock loop. Besides, in order to make offshore wind farms provide necessary frequency support to the onshore power system, a communication-less frequency regulation scheme with artificial coupling method is proposed. The feasibility and effectiveness of the proposed self-synchronizing control and frequency regulation scheme are verified through simulation studies in PSCAD/EMTDC platform.

The modular multilevel converter (MMC) is widely used in high and medium voltage applications. To improve the reliability of the MMC, fast and accurate submodule (SM) fault detection, location, and tolerant control are required to avoid the interrupted power transmission. Existing methods do not consider all fault characteristics of SM in different operation modes when MMC adopts nearest level modulation. This paper proposes a robust SM fault management scheme that enables uninterrupted operation of MMC with nearest level modulation under SM fault. The SM fault characteristics in different operation modes are presented. Then, the fault detection method based on measuring the compensation of the circulating current suppression controller is proposed. Three quantities, including accumulated discharge voltage, unbalanced charge voltage and capacitor voltage, are used to locate the faulty SM and identify the fault type. In addition, a fault-tolerant control method is proposed by injecting a symmetry compensation component into the modulation signals. Finally, simulation and experimental results are presented to verify the proposed fault detection, location and tolerant control methods.

In this paper, the DC short-circuit fault and corresponding clearance solutions of modular multilevel converter-based high-voltage direct current (MMC-HVDC) systems are analyzed in detail. Firstly, the analytical expressions of DC fault currents before and after blocking the MMC are derived based on the operation circuits. Before blocking the MMC, the sub-module (SM) capacitor discharge current is the dominant component of the DC fault current. It will reach the blocking threshold value in several milliseconds. After blocking the MMC, the SM capacitor is no longer discharged. Therefore, the fault current from the AC system becomes the dominant component. Meanwhile, three DC fault clearance solutions and the corresponding characteristics are discussed in detail, including tripping AC circuit breaker, adopting the full-bridge MMC and employing the DC circuit breaker. A simulation model of the MMC-HVDC is realized in PSCAD/EMTDC and the results of the proposed analytical expressions are compared with those of the simulation. The results show that the analytical DC fault currents coincide well with the simulation results.

Cross-seam high voltage DC (HVDC) transmission in the United States can provide an efficient and flexible bulk power delivery highway between the interconnections. Recent studies have shown significant economic benefits of cross-seam HVDC transmission systems by taking the advantages of load diversity, frequency response, renewable energy diversity, and energy arbitrage in the different interconnections. This paper introduces a station-hybrid system scheme for cross-seam interconnections. The proposed station-hybrid system combines the advantages of line commutated converter (LCC) and voltage source converter (VSC) technologies, thus enabling reliable and flexible bidirectional power flows across the interconnections. Control strategies of the station-hybrid system are investigated with a focus on power flow reversal control under normal operation conditions and grid support control under emergency conditions. The feasibility and effectiveness of the proposed station-hybrid system and corresponding control strategies are verified by simulations in PSCAD/EMTDC.