Integration of renewable energy generations requires the transmission of bulky power over long distance, and high-voltage direct current (HVDC) transmission systems become a more preferable choice compared to conventional HVAC systems. For HVDC systems, one of the important concerns is the DC protection strategy which can significantly impact on the connected AC system performance, e.g. system frequency. The maximum loss-of-infeed for a AC network is highly dependent on the duration of the power outage, and the impacts of DC fault protection arrangements which result in different speed of power restoration on the connected AC system, on the system frequency, have not been properly understood. Different DC protection arrangements using DC disconnectors, fast and slow DC circuit breakers on frequency response of the connected AC networks are investigated. A three-terminal meshed HVDC system is studied to demonstrate system behaviour during DC faults.

This paper presents a detailed comparison of different modelling methods of the half-bridge modular multilevel converter (HB-MMC), namely, switching function, Thevenin equivalent, and averaged, considering both MMC implementations (large and reduced number of cells). The theoretical basis that underpins each modelling method are discussed. Offline PSCAD simulations are used to validate user-defined switching function and averaged MMC models against the Thevenin equivalent model provided in PSCAD library for accuracy, considering steady-state and DC fault conditions. Furthermore, the RTDS based real-time simulation results of the user-defined HB-MMC switching function model are validated against the above mentioned offline models, considering steady-state and DC short circuit fault operations. Simulation speed and efficiency of different offline HB-MMC models being studied here are compared. From comprehensive corroboration of different HB-MMC models presented here, it has been found that the averaged, switching function and Thevenin equivalent models produce practically identical results during steady-state and DC faults. In detailed offline and real-time simulation studies where fundamental and harmonic dynamics are of interest, switching function model is found to be faster and computational efficient compared to the Thevenin equivalent model.

This study extends a recently proposed dc auto-transformer to a true multi-port converter that can function simultaneously as dc auto-transformer and a dc/ac converter for dc voltage matching and tapping, and power control in highly meshed multi-terminal dc grids. The presented multi-port converter can generate multiple adjustable dc voltages from a fixed or variable input dc voltage, or from an active ac grid. Theoretical discussions and simulations indicate that when the proposed multi-port converter is used as a hybrid dc and ac hub to manage congestions and resolve loop flow issues in a highly meshed multi-terminal dc network, large dc power can be fed to the lower dc terminal, without exposing the switching devices of the lower subconverter to excessive current stress. Its increased control flexibility and response to dc faults at high and low dc terminals are validated using simulations and experiments.

DC line faults on high-voltage direct current (HVDC) systems utilising voltage source converters (VSCs) are a major issue for multi-terminal HVDC systems in which complete isolation of the faulted system is not a viable option. Of these faults, single line-to-earth faults are the most common fault scenario. To better understand the system under such faults, this study analyses the behaviour of HVDC systems based on both conventional two-level converter and multilevel modular converter technology, experiencing a permanent line-to-earth fault. Operation of the proposed system under two different earthing configurations of converter side AC transformer earthed with converter unearthed, and both converter and AC transformer unearthed, was analysed and simulated, with particular attention paid to the converter operation. It was observed that the development of potential earth loops within the system as a result of DC line-to-earth faults leads to substantial overcurrent and results in oscillations depending on the earthing configuration.

Voltage source converter interfaced wind turbines connected to weak grids can induce system instability. A state space model is used in this paper to study the stability of large wind farms. Dynamics of the phase locked loop is integrated to the state space model. The advantage of this model is that it allows study the stability of systems with parallel wind turbines. Studies on the dynamic responses of the system show the importance of including phase locked loop as its existence can induce system instability especially under weak network conditions. This model can be used to study the stability of converter interfaced wind farms and helps to design the converter controller to ensure wind farm system stability when connected to weak grids.

Control algorithms for the rotor- and grid-side power converters of a double-fed induction generator (DFIG)-based wind turbine under non-ideal grid voltage conditions are proposed, and guidelines for tuning the controller parameters are presented. The control schemes are based on sliding-mode control (SMC) theory. Apart from directly controlling the DFIG's average active and reactive powers, the proposed methods also fulfil two additional control targets during voltage unbalance and harmonic distortion, that is, the rotor-side converter (RSC) eliminating electromagnetic torque fluctuations and the grid-side converter (GSC) compensating for the stator current harmonics to ensure a sinusoidal total current from the overall generating unit. The described control strategies are proved to be robust against parameter deviations and of fast dynamic response. In spite of the discontinuous nature of the standard SMC, constant converter switching frequency is achieved. Besides, the RSC control algorithm does not require a phase-locked loop and, furthermore, there is no need for decomposing the grid voltage and different currents into symmetrical sequences or harmonic components in any of the converters’ control systems. Finally, the excellent performance of the system, as well as its robustness, is verified by means of simulation results under different grid voltage conditions.

This paper analyzes the behaviour of a modular multilevel converter (MMC) based HVDC system under DC cable fault conditions. The behaviour of the HVDC system during a permanent line-to-line fault is analyzed in details at each stage of the fault timeline. Operation of the proposed system under a specific earthing configuration i.e. converter unearthed /AC transformer secondary side and DC cable solidly earthed, is also analyzed. Simulation studies are provided to assist the analytical analysis. It is observed that both faults leads to substantial AC and DC over-current and result in DC side oscillations.

This paper investigates virtual inertia control of doubly fed induction generator (DFIG)-based wind turbines to provide dynamic frequency support in the event of abrupt power change. The model and control scheme of the DFIG is analysed. The relationships among the virtual inertia, the rotor speed and the network frequency variation are then investigated. The "hidden" kinetic energy that can be released to contribute to the grid inertia by means of shifting the operating point from the maximum power tracking curve to a virtual inertia control curve is investigated. The virtual inertia control strategy based on shifting power tracking curves of the DFIG is proposed and the calculation method for determining these virtual inertia control curves is presented. A three-machine system with 20 percent of wind penetration is used to validate the proposed control strategy. Simulation results show that by the proposed control strategy, DFIG based wind farms have the capability of providing dynamic frequency support to frequency deviation, and thus improving the dynamic frequency performance of the grid with high wind power penetration. (6 pages)

A fast power restoration operational scheme and relevant stabilising control is proposed for active distribution power systems with multi-terminal DC network in replacement of the conventional normal open switches. A nine-feeder benchmark distribution power system is established with a four-terminal medium power DC system injected. The proposed power restoration scheme is based on the coordination among distributed control among relays, load switches, voltage source converters and autonomous operation of multi-terminal DC system. A DC stabiliser is proposed with virtual impedance method to damp out potential oscillation caused by constant power load terminals. The proposed system and controls are validated by frequency domain state-space model and time-domain case study with Matlab/Simulink.

A high-voltage direct current (HVDC) electric power system (EPS) has become an attractive power distribution architecture for the more electric aircraft. The structure of a typical HVDC parallel EPS based on the permanent magnet synchronous machine (PMSM) starter/generators (S/Gs) is proposed in this study. On the basis of this structure, the control strategy for a single PMSM S/G system is presented, which is suitable for wide-speed operation and high-power application. It can not only achieve both starter and generator functions but also achieve the stability of DC bus voltage. A master–slave current sharing method is derived and can be applied in the HVDC parallel system, compared with the droop-control strategy. The validity of the proposed strategies is verified by simulation studies in a Matlab/Simulation environment.