A power component detection (PCD) method and an improved current order limiter control (improved COL) are proposed to mitigate the commutation failure in a line-commutated converter-based high-voltage direct current (LCC-HVDC). Integrating the instantaneous voltage and instantaneous current characteristics under single-phase and three-phase-to-ground faults, a fault detection method based on the power component is proposed, and the corresponding setting principle is presented. Then, an improved dc current order limiter is introduced. To evaluate the effectiveness of the proposed methods, a dual-infeed HVDC system is developed in PSCAD/EMTDC, and the presented methods are also implemented. The transient performances of dual-infeed HVDC, under single-phase and three-phase-to-ground faults at the inverter ac busbar of one LCC-HVDC link, are investigated. Simulation results show that the PCD method, with the voltage and current components as an additional judgment criterion, is more sensitive to detect the single-phase and three phase faults, and even more reliable than the detect method taking only voltage or current components for faults recognition. It can also be concluded that the improved COL that is integrated with the advancing firing angle control has the ability to make the LCC-HVDC less susceptible to commutation failure. Finally, the improvement of commutation process by improved COL is further demonstrated.

High voltage direct current (HVDC) power transmission is becoming increasingly important due to steadily rising need for bulk power delivery and interconnected power transmission and distribution systems. DC grids are vulnerable to dc faults, which lead to a rapid rise in dc fault currents. The dc faults must be cleared within timeframe of milliseconds to avoid collapse of the HVDC system. In the event of primary protection (PP) failure, back-up protection (BP) must be applied to clear the fault. In this paper, a novel algorithm based on a Naïve Bayes classifier is proposed to determine threshold levels and operational time frames for primary and back-up protection in multi-terminal voltage source converter based HVDC (VSCHVDC). Local voltage and currents are measured to detect and identify the kind of fault. A four terminal HVDC transmission system is developed in PSCAD/EMTDC and is subjected to lineline faults at different locations and time, to assess the designed protection schemes. Results show that relaying algorithm effectively detects the fault and expedite the primary protection operation. On malfunctioning of PP, BP is accelerated in a short delay of 0.2ms. Furthermore, the relaying algorithm provides faster protection compared to techniques available in the literature. The resulting reduced fault clearance time truncates the maximum fault current and inevitably, leads to reduced power ratings required for dc grid equipment.

This paper proposes a new droop coefficient design method with the aim of improving the power-sharing accuracy among the converters in a multi-terminal dc (MTDC) system. The proposed droop coefficient design method works by adjusting the droop coefficient and can realize an arbitrary power-sharing ratio among all the converters in an MTDC system. This method does not rely on a communication network and therefore has the potential for higher reliability than the alternative methods. Mitigating the impact of the variation of dc line resistances on the power-sharing is discussed. Simulation of a four-terminal MTDC system is carried out by using PSCAD/EMTDC. The experimental results under a scaled-down four-terminal dc grid platform demonstrate the effectiveness of the proposed method.

This paper deals with the power dispatch and direct voltage control in multiterminal high voltage direct current (MT-HVDC) systems. Generalized voltage droop (GVD) control is adopted for voltage source converters (VSC)s of a MT-HVDC system. A mechanism has been designed based on the power ratio within the GVD controlled stations to achieve flexible autonomous coordination control among VSC-HVDC stations, without need for communication. In this paper, several alternatives are considered to guarantee fault ride through of onshore converter stations. The performance of the proposed control strategy is analyzed with time-domain dynamic simulations, in an EMDTC/PSCAD platform, and experimentally validated. Results demonstrate the robust performance and capabilities of the proposed control strategy during changes in the power demand of the ac grids, unexpected change in wind power generation, and eventual permanent VSC-HVDC station disconnection.

Voltage-source converter-based multiterminal high-voltage direct current (VSC-MTDC) systems/grids are prone to system instability. This critical issue is overlooked in the literature. In order to improve system stability, this paper proposes an effective active damping method as a remedy to suppress voltage and power resonances in the VSC-MTDC grids by injecting damping signals into the inner current loops of VSC-MTDC stations. With dynamic regulation of the damping current, resonance is suppressed by power converter controllers without any additional current and voltage measurement. In this paper, modeling and stability analysis of the VSC-MTDC system/grid is presented considering the dc-side energy storage components, and control with a droop control structure. Then, single-frequency and multifrequency resonance mechanisms of dc-bus voltage and power in the event of transients are analyzed. Later, the stability effect of the MTDC system/grid inductance and capacitance values to the resonance amplitude and frequency droop coefficients is investigated. A PSCAD/EMTDC platform is developed to conduct dynamic simulations, and a scaled-down four-terminal 20-kW experimental prototype is used to validate the effectiveness of the proposed control methodology.

The impedance interactions between weak grid and LCL-filtered converter can bring severe harmonic resonance. Recently, frequency domain passivity theory has been applied to converter damping design. However, it is usually not easy to achieve full frequency passivity. What’s more, traditional passivity based damping design is not enough for system robustness. This paper first proposes a relaxed passivity based active damping design for capacitor current feedback. Then the system stability under grid impedance variation is also analyzed. It has been found that the wide variation of grid impedance can decrease system stability margin. As a result, critical oscillation can even happen in the passive region of converter output admittance, causing power quality problems. In order to further improve system stability, this paper proposes an enhanced passivity based impedance shaping method, which is based on hybrid damping and combined active damping. Effects of damping methods and damping parameters are also investigated. A step by step damping design procedure is formulated, which is easy to implement. Simulation and experimental results validate the effectiveness of proposed method.