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Owing to the charging/discharging flexibility, electric storage system (ESS) is widely recognised as a promising technique that can be introduced to enhance transient stability of power systems. Here, a multi-objective ESS allocating and sizing approach is presented to de-risk the loss of stability for the AC/DC power system. The approach contains two major steps. Firstly, transient stability risk (TSR) is assessed based on the severity and probability of contingencies which is simulated according to the good point set sampling considering multiple uncertainties and probabilistic fault. In the second step, an allocating and sizing model is proposed for ESS to minimise the operational cost, while the AC/DC system is subject to the TSR. Additionally, strength Pareto evolutionary algorithm (SPEA2) is employed to solve the model, optimising ESS allocation and size as well as output of regular generators. A modified AC/DC test system is used to validate the presented approach. The results indicate that, with the appropriate planning strategy, ESS is able to significantly contribute to TSR improvement as it can assist in transient power balancing after a severe fault. [ABSTRACT FROM AUTHOR]

In recent years, the low-frequency oscillation (LFO) problem has become increasingly significant with rapid increase in the size of AC/DC interconnected power system containing large-scale wind power. This article aims to conduct the LFO analysis when an offshore wind farm is integrated into the AC power system through module multilevel converter (MMC)-based high-voltage direct current (HVDC). First, the basic mathematical models including the MMC with detailed control strategy and the permanent magnet synchronous generator (PMSG) are described. The eigenvalue analysis is then applied to conduct the LFO study. An interconnected AC/DC test system with a wind farm connected though the MMC-based HVDC transmission line is designed as benchmark. To validate the analysis results, the detailed simulation models built under the PSCAD/EMTDCTM environment together with the Prony analysis are carried out. Different simulation scenarios involving the changes of point of common coupling (PCC), wind speed, length of transmission line, and wind penetration level are elaborately studied. Some meaningful conclusions are drawn to provide basic foundation for the future design of damping controller to mitigate LFO. [ABSTRACT FROM AUTHOR]

This study introduces a bidirectional power sharing scheme between an ac electric utility and a dc microgrid. These two are connected together through an interlinking converter (IC) and a dual active bridge (DAB). The proposed control scheme allows predefined amount of power to be supplied from the ac to dc side or the other way around. The DAB is connected at the dc side of the IC and it controls the bidirectional power flow. The dc microgrid contains dc–dc converters, where boost converters are used with the sources and buck converters are used to supply loads. These dc–dc converters and the DAB are controlled by state feedback with integral control in a linearised discrete domain. The DAB linearisation and control law are presented in detail. The dc microgrid is controlled by conventional V‐P droop where the power from/to the ac side is considered as negative/positive load. Detailed simulation results are presented to validate the proposal. Also some hardware results are included that support the findings. [ABSTRACT FROM AUTHOR]

The introduce of an HVDC system into the trunk power supply system requires transmitting as much dc power as possible for improving the whole ac/dc system stability. The continued operation scheme of an HVDC system which makes it possible to operate even under very low commutation bus voltage is adopted to cope with such system requirement. Further stability improvement can be achieved by adding damping control which modulates converter active and/or reactive power. To take full advantage of these control effects, it is essential that the HVDC system can maintain stable operation during the ac system faults and reclosing. Particular attention should be given to a repetitive commutation failure following the unbalanced ac transmission line faults. This paper discussed first the cause of the repetitive commutation failure when supplied with unbalanced ac system voltage based on an experiment using the CRIEPI AC-DC Power System Simulator. Then a stabilization control scheme with an on-line extinction angle estimation; commutation failure preventive control. [ABSTRACT FROM AUTHOR]