Impact of VSC Converter Topology on Fault Characteristics in HVDC Transmission Systems

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 pole-to-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

What is the Role of International Law in Global Health Governance on the Period of Covid-19

Rapid globalisation challenges many of the traditional assumptions about International law, which is linked to domestic law, especially the ways in which it is formed and the methods of its implementation. This phenomenon led governments to be more focused on international collaboration to achieve national public health purposes and succeed some audit over the cross-border powers that influence their populations. This essay will analyse the position on what is the role of international law in global health governance. Another significant result of this essay is that Global Actors should create a global health cooperation in order to implement the international law effectively on the period of Covid-19.

Non-unit protection for HVDC grids : an analytical approach for wavelet transform-based schemes

Speed and selectivity of DC fault protection are critical for High-Voltage DC (HVDC) grids and present significant technical and economic challenges. Therefore, this paper proposes a non-unit protection solution that detects and discriminates DC faults based on frequency domain analysis of the transient period of DC faults. The representation of a generic HVDC grid section and the corresponding DC-side fault signatures in the frequency domain form the basis of a generalized approach for analytically designing a protection scheme based on Wavelet Transform (WT). The proposed solution is adaptive within its design stage and offers general applicability and immunity to system changes, while the protection settings are configured for optimized performance. The scheme is validated through offline simulations in PSCAD/EMTDC and the technical feasibility of the algorithm in the real world is demonstrated through the use of real-time digital simulation (using RTDS) and Hardware-in-the-Loop (HIL) testing. Both offline and real-time simulations demonstrate that the scheme is able to detect and discriminate between internal and external faults at a significantly high speed, while remaining sensitive to high impedance faults and robust to external disturbances and outside noise

Real time evaluation of wavelet transform for fast and efficient HVDC grid non-unit protection

This paper presents a real-time evaluation of a Wavelet Transform (WT) for HVDC grid non-unit protection. Due to its time and frequency localisation capability, WT can successfully extract the necessary information present in the voltage transients following a DC fault. This capability is exploited to achieve fast and selective HVDC grid protection. A Digital Signal Processor (DSP) is employed to execute real-time Stationary Wavelet Transform (SWT) on voltage signals using discrete convolution to efficiently compute the WT coefficients. Hardware-in-the loop (HIL) simulation is performed to test a WT-based hardware module using a Digital Real-Time Simulator (DRTS), in which a meshed HVDC grid is modelled. The closed-loop interaction enables the hardware device to emulate a protection relay that can generate trip commands for the HVDC breakers integrated within the HVDC grid model. The real-time simulations demonstrate the technical feasibility, speed and robust performance of the SWT implementation

AI and Big Data: A New Paradigm for Decision Making in Healthcare

The latest developments in artificial intelligence (AI) - a general-purpose technology impacting many industries - have been based on advancements in machine learning, which is recast as a quality-adjusted decline in forecasting ratio. The influence of Policy on AI and big data has impacted two key magnitudes which are known as diffusion and consequences. And these must be focused primarily on the context of AI and big data. First, in addition to the policies on subsidies and intellectual property (IP) that will affect the propagation of AI in ways close to their effect on other technologies, three policy categories - privacy, exchange, and liability - may have a specific impact on the diffusion of AI. The first step in the prohibition process is to identify the shortcomings of current hospital procedures, why we need acute care AI, and eventually how the direction of patient decision-making will shift with the introduction of AI-based research. The second step is to establish a plan to shift the direction of medical education in order to enable physicians to retain control of AI. Medical research would need to rely less on threshold decision-making and more on the prediction, interpretation, and pathophysiological context of contextual time cycles. This should be an early part of a medical student's education, and this is what their hospital aid (AI) ought to do. Effective contact between human and artificial intelligence includes a shared pattern of focused knowledge base. Human-to-human contact protection in hospitals should lead this professional transformation process

Comparison of conventional and improved two-level converter during AC faults

An improved two-level converter (I2LC) is a practical compromise between the conventional two-level converter (C2LC) and modular multilevel converter (MMC), recently proposed for dc transmission system for relatively lower dc voltages and rated powers. The I2LC inherent the ac and dc fault behaviors of the MMC and relative simplicity of C2LC. Therefore, this paper presents a detailed quantitative comparison between the ac and dc responses of the C2LC and I2LC to symmetrical and asymmetrical ac faults. It has been showing that unlike the C2LC, the I2LC provides better controllability than the C2L at system level during asymmetrical ac faults, including two operational objectives simultaneously such as balanced output currents and ripple-free dc-link current. Index Terms—ac and dc faults, two-level converter, improved two-level converter, medium and high-voltage direct current (HVDC) transmission systems

Novel enhanced modular multilevel converter for high-voltage direct current transmission systems

This paper proposes an enhanced modular multilevel converter as an alternative to the conventional half-bridge modular multilevel converter that employs a reduced number of medium-voltage cells, with the aim of improving waveforms quality in its AC and DC sides. Each enhanced modular multilevel converter arm consists of high-voltage and low-voltage chain-links. The enhanced modular multilevel converter uses the high-voltage chain-links based on medium-voltage half-bridge cells to synthesize the fundamental voltage using nearest level modulation. Although the low-voltage chain-links filter out the voltage harmonics from the voltage generated by the high-voltage chain-links, which are rough and stepped approximations of the fundamental voltage, the enhanced modular multilevel converter uses the nested multilevel concept to dramatically increase the number of voltage levels per phase compared to half-bridge modular multilevel converter. The aforementioned improvements are achieved at the cost of a small increase in semiconductor losses. Detailed simulations conducted in EMPT-RV and experimental results confirm the validity of the proposed converter

Assessment of grid-connected wind turbines with an inertia response by considering internal dynamics

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

On the importance of cyber-security training for multi-vector energy distribution system operators

Multi-vector Energy Distribution Systems (EDS) are increasingly connected to provide new services to consumers and Distribution Network Operators (DNO). This exponential growth in connectivity, while beneficial, tremendously increases the attack surface of critical infrastructures, demonstrating a clear need for energy operator cyber-security training. This paper highlights the cyber-security challenges faced by EDS operators as well as the impact a successful cyber-attack could have on the grid. Finally, training needs are contextualised through cyber-attack examples

An Improved Alternate Arm Converter for HVDC applications

This paper presents an Improved Alternate Arm Converter where the director switches of the upper and lower arms of the conventional alternate arm converter are rearranged as a conventional two-level converter. The flying capacitor (i.e. the capacitor across the director switches) in each phase leg facilitates seamless current commutation between the upper and lower arms, and eliminates the need for the main dc-link capacitor across the positive and negative dc rails. The modifications introduced to the power circuit necessitate the proposed converter to adopt a new operating regime that ensures simultaneous conduction of the upper and lower arms of each phase leg as in the modular multilevel converter. The operating principle, modulation methods of the proposed converter, and sizing of its main components are described in detail, and substantiated by simulations