This paper presents a method for rapid detection of faults on VSC multi-terminal HVDC transmission networks using multi-point optical current sensing. The proposed method uses differential protection as a guiding principle, and is implemented using current measurements obtained from optical current sensors distributed along the transmission line. Performance is assessed through detailed transient simulation using Matlab/Simulink R models, integrating inductive DC-line terminations, detailed DC circuit breaker models and a network of fiber-optic current sensors. Moreover, the feasibility and required performance of optical-based measurements is validated through laboratory testing. Simulation results demonstrate that the proposed protection algorithm can effectively, and within very short period of time, discriminate between faults on the protected line (internal faults), and those occurring on adjacent lines or busbars (external faults). Hardware tests prove that the scheme can be achieved with the existing, available sensing technology.
This paper presents a comparison of the steady-state behaviour of four state-of-the-art HVDC converters with DC fault-blocking capability, based on the modular multilevel and alternate arm converter topologies.AC and DC power quality, and semiconductor losses are compared, whilst considering different operating conditions and design parameters, such as the number of cells and component sizing. Such comparative studies have been performed using high-fidelity converter models which include detailed representation of the control systems, and of the converter thermal circuit. The main findings of this comprehensive comparison reveal that, the mixed cell modular converter offers the best design trade-off in terms of power losses and quality, and control range. Moreover, it has been established that the modular converter with a reduced number of cells per arm and with each cell rated at high voltage (i.e. 10-20 kV), tends to exhibit higher switching losses and relatively poor power quality at the DC side.
This paper presents an alternative implementation of a modular multilevel converter (MMC) that generates a large number of voltage levels per phase with high resolution voltage steps from a reduced number of cells per arm. The presented MMC employs a half-bridge chain-link of medium-voltage cells and a full-bridge chain-link of low-voltage cells in each of its arms. The total blocking voltage of the full-bridge chain-link is equivalent to half that of the medium-voltage half-bridge cell. The use of half and full-bridge cells with two distinct rated voltages in each arm permits full exploitation of the full-bridge cells to generate high resolution multilevel voltage waveforms with fine stepped transitions between major voltage steps of the medium-voltage half-bridge cells. In this manner, errors in the synthesis of the common-mode voltages of the three phase legs due to switching of the cell capacitors in and out the power path are reduced. The nested multilevel operation of the proposed MMC results in a number of voltage levels which is related to the product, rather than the sum, of the numbers of half and full-bridge cells. Detailed comparisons with existing MMC implementations show that the proposed MMC implementation offers the best design trade-offs (superior AC and DC waveforms with reduced control and power circuit complexity). The validity of the proposed MMC implementation is confirmed using simulations and experimentally.
This paper presents a small-signal analysis of different grid side controllers for full power converter wind turbines with inertia response capability. In real wind turbines, the DC link controller, the drivetrain damping controller and the inertial response might present contradictory control actions in a close bandwidth range. This situation might lead to reduced control performance, increased component stress and non-compliance of connection agreements. The paper presents an analysis of the internal wind turbine dynamics by considering different grid-side converter control topologies: standard current control used in the wind industry, standard current control with inertia emulation capabilities and virtual synchronous machines. Comments are made on the similarities between each topology and the negative effects and limits, and possible remedies are discussed. Finally, the conclusion poses that the inclusion of a DC link voltage controller reduces the ability of a converter to respond to external frequency events without energy storage. The degradation increases with the DC link voltage control speed.
This work presents the outcome of a comprehensive study that assesses the transient behaviour of two high voltage direct current (HVDC) networks with similar structures but using different converter topologies, termed two-level and half-bridge (HB) modular multilevel converter (MMC). To quantify the impact of converter topology on DC current characteristics a detailed comparative study is undertaken in which the responses of the two HVDC network transients during dc side faults are evaluated. The behaviour of the HVDC systems during a permanent pole-to-pole and poleto-ground faults are analysed considering a range of fault resistances, fault positions along the line, and operational conditions as a prerequisite. Fast Fourier Transform (FFT) has been conducted analysing di/dt for both converter architecture and fault types taking into consideration sampling frequency of 96 kHz in compliance with IEC-61869 and IEC-61850:9-2 for DC-side voltages and currents.
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