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With the emergence of the hybrid multi-infeed high voltage direct current (HMIDC) system, there is complex dynamic interaction between the AC/DC system and converter stations, while the interaction mechanism of the HMIDC system is unclear. For this purpose, this paper establishes the HMIDC linearized model based on the motion equation concept. Then, after decoupling and reconstructing the interaction path of the HMIDC system, the damping coefficients are derived. The AC system strength and voltage source converter (VSC) terminal voltage controller parameters are selected to analyze the impacts on damping coefficients, and the interaction mechanism of the HMIDC system is analyzed. In order to solve the potential instability issue, an additional damping controller in the VSC terminal voltage loop is proposed to enhance the voltage stability of HMIDC system, which is realized by adjusting the reactive power/amplitude coupling damping coefficient. Finally, the RT-LAB-based hardware-in-the-loop experiments verify the accuracy of the interaction mechanism analysis of the HMIDC system and the effectiveness of the proposed controller. [ABSTRACT FROM AUTHOR]

DC technology has gained considerable interest in the medium voltage applications due to the benefits over the AC counterpart. However, to utilize the full capacity of this development, the selection of a suitable power electronic converter topology is a key aspect. From the pool of voltage source converters (VSC's), it is unclear which topology is suitable for multi-megawatt applications at medium voltage dc (MVdc) levels. To address this, the paper proposes a selection guideline based on reliability and optimum redundancy levels of VSCs for MVdc applications. This will be combined with other functional factors such as operational efficiency and return-on-investment. Three candidate multi-level topologies namely three-level neutral point clamped converter (3L-NPC), modular multi-level converter (MMC) and cascaded 3L-NPC (which is being used for the first MVdc link in the U.K.) have been evaluated over two-level-VSC from $\pm$ 10 kV to $\pm$ 50 kV. Results show that with the increase of MVdc voltage level MMC shows better performance whereas at low MVdc voltage levels 3L-NPC is the prominent topology. [ABSTRACT FROM AUTHOR]

The increasing use of voltage-source converters (VSCs) in modern power systems has led to extensive studies on their frequency response and stability, which are characterized by power-frequency oscillations and overshoots following a disturbance. However, the existing small-signal analysis models do not adequately describe the impact of the types of disturbance on the torque-frequency dynamics of the multi-VSC parallel system. In this paper, based on the principle of electromechanical analogy, a unified equivalent admittance modeling method is proposed by analogizing the rotor dynamic model of a synchronous generator (SG) to the circuit network, which has the advantages of simple modeling, clear physical meaning and easy expansion. Firstly, the equivalent admittance model of a single SG/VSC is established, and then easily extended to the multi-VSC system. The proposed modelling method not only applicable to a variety of VSCs, but also can handle different types of disturbance. More importantly, the circuit theory can be applied to analyze the torque-frequency dynamic characteristics. Subsequently, the equivalence between the proposed model and the state-space model under different damping terms is compared. In addition, the response characteristics of two different damping terms are compared. Next, the dynamic response characteristics of single VSC and multi-VSC parallel systems under different disturbances are analyzed. Finally, related simulations and experiments are executed to verify the accuracy and effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

This letter proposes a method to model the power converter availability for power system adequacy assessment. The proposed method considers wear-out as well as chance failures and calculates an equivalent failure rate accordingly. In this regard, it is shown that neglecting the wear-out failures and the dynamic behavior of adequacy indices can lead to considerable errors. Moreover, the performance of the proposed availability modeling method is compared to that of the existing solutions through calculating the Expected Energy Not Supplied (EENS) in a case study. The comparison confirms the superior performance of the proposed method in terms of high accuracy and low computational burden. [ABSTRACT FROM AUTHOR]

Local-measurement-based protection elements are essential for the safety and stability of the VSC-MTDC. However, when a DC line fault occurs, the resistive coverage for each side of a protection element is not always identical. As a result, a certain side of the protection element may not be sufficiently sensitive to trip the fault, especially in scenarios of communication failure. To solve this problem, a mutual-speed-up protection strategy is proposed to identify dead-zone grounding faults. First, the time domain characteristics and frequency domain characteristics of the fault-induced primary traveling wave and those of the second traveling wave caused by the opening operation of the DC CB are analyzed. Then, based on the above-mentioned difference, a time difference criterion (TDC) and zero-crossing criterion (ZCC) are proposed, working in conjunction to discriminate the remote-end dead-zone fault and the external fault at the head of the adjacent DC line. Simulation results indicate that the proposed strategy works without communication and is sufficiently fast to operate before the blocking of the converter, thus making it a powerful supplemental strategy for existing main protection elements. [ABSTRACT FROM AUTHOR]