This paper proposes a new control scheme for the low frequency AC transmission (LFAC) system aiming at extending the point-to-point configuration to form a multi-terminal electrical energy network. The multi-terminal low frequency ac (MT-LFAC) system configuration is based on the use of modular multilevel matrix converters (M3Cs) and virtual synchronous generator (VSG) control. The M3C is the next ac/ac converter generation, which is used as an interface with the conventional AC network and the LFAC electrical energy system. Application of VSG control is proposed to enable proper power sharing, to provide synchronization of each terminal, and frequency stabilization, thus, to offer multiterminal forming capability. Two different operation modes are applied in the system to damp the frequency deviation after a dynamic perturbation, which provides additional stabilization feature to the VSG. Frequency restoration mode and commanded mode of power sharing are applied as dynamic states to validate the robustness of the VSG control system. Besides, to solve the negative impact of low X/R ratio in the LFAC electrical energy system, we enhance the VSG control by proposing a virtual-impedance-based solution, which increases the output total impedance on the low frequency side and prevents the coupling between P and Q. The operation of the proposed system is examined by simulation results with a precise model of M3Cs in the PSCAD/ EMTDC software environment (version 4.2.1, Winnipeg, MB, Canada).Energies 2020, 13, 747 2 of 19 delivery. However, neither technology is flawless. For example, the existence of the power electronic switches in the dc circuit breakers make them more vulnerable than the HVAC transmission system, especially when a fault takes place in the converter, due to the low dc-side impedances and sensitive semiconductor power converters [1,2]. On the other hand, the HVAC system offers advantages, such as more reliability of the well-known protection schemes and the change capability of voltage levels using transformers [3]. However, the HVAC transmission system's main disadvantage is the existence of the reactive power in comparison to no reactive power in the HVDC electrical energy system. Due to that, the existence of the reactive power can be translated as losses in the grid [4].The aforementioned points have led to an alternative solution which combines both sets of HVACs and HVDC advantages, known as a low frequency ac (LFAC) transmission system, or a fractional frequency transmission system (FFTS) [5][6][7]. This system transmits the power at a lower frequency range of about 1-60/3 Hz. The LFAC has already been proposed for the offshore wind energy and has offered attractive advantages compared to both transmission solutions. For example, in the HVDC electrical energy system, the implementation of a high capacity dc circuit breaker is difficult due to the absence of the zero crossing points of the DC current [8]. On the other hand, the LFAC system has zero-crossing, thus, the protection is more advantag...
This paper proposes a low-frequency ac (LFAC) transmission system capable of providing frequency regulation support for a remote ac grid aiming at enhancing its frequency stability. The LFAC transmission system using modular multilevel matrix converters (M3Cs) for a direct ac-ac conversion is receiving noticeable attention as an alternative solution for a long-distance transmission system. One of its potential applications is the system connection between a large ac grid and a remote ac grid, whereas the latter usually suffers from a fluctuant frequency due to lack of inertia. The core of this work is addressing a novel control scheme of the remote ac grid-side M3C, which makes both interactions with the LFAC system and the remote ac grid behave like a synchronous generator using a dual virtual synchronous generator (VSG) control scheme. This control scheme can enhance the frequency regulation in the remote ac grid, which the existing control schemes cannot provide. Average dc capacitor voltage versus active power (V ave -P) droop control is proposed to coordinate the power flow between the VSG controls applied in both sides of the M3C. To demonstrate the problem of the existing control scheme, as well as the benefits offered by the proposed control scheme, transient performance including load variation and fault events in the remote ac grid is studied and examined in the EMTDC/PSCAD software environment.INDEX TERMS Frequency control, low-frequency ac transmission, modular multilevel matrix converter, power control, power grids, power transmission, remote grid, virtual synchronous generator.
This study presents the results of the investigation of the behaviour of ZhS32-VI single-crystal nickel alloy in hydrogen environment or argon atmosphere at 850°C. The microstructure and chemical composition of corrosion deposits were studied by scanning electron microscopy and X-ray energy-dispersive analysis. It has been established that in argon containing an admixture of oxygen a dense scale rich in cobalt and nickel oxides is formed. At atmosphere containing 65% hydrogen and 35% argon an increased content of aluminum in the surface composition was noted. The influence of 100% hydrogen leads to segregation of tungsten and rhenium with the formation of convex growths. The data of simultaneous thermal analysis revealed that the amount of desorbed hydrogen can be from 0.08 to 0.14%.
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