Luis M. Castro

A New VSC-HVDC Model for Power Flows Using the Newton-Raphson Method

The paper presents a new model of the VSC-HVDC aimed at power flow solutions using the Newton-Raphson method. Each converter station is made up of the series connection of a voltage source converter (VSC) and its connecting transformer which is assumed to be a tap-changing (LTC) transformer. The new model represents a paradigm shift in the way the fundamental frequency, positive sequence modeling of VSC-HVDC links are represented, where the VSCs are not treated as idealized, controllable voltage sources but rather as compound transformer devices to which certain control properties of PWM-based inverters may be linked - just as DC-to-DC converters have been linked, conceptually speaking, to step-up and step-down transformers. The VSC model, and by extension that of the VSC-HVDC, takes into account, in aggregated form, the phase-shifting and scaling nature of the PWM control. It also takes into account the VSC inductive and capacitive reactive power design limits, switching losses and ohmic losses.

On the Provision of Frequency Regulation in Low Inertia AC Grids Using HVDC Systems

This paper proposes an improved model of a high voltage dc (HVDC) link capable of providing frequency support to networks with no inertia or near-zero inertia; for instance, ac grids with small synchronous generators. The model is useful to carry out steady-state and dynamic simulations of ac grids described by their positive-sequence representation using phasorial information. The core of the frequency control scheme put forward in this paper uses the angular aperture that exists between the internal phase-shifting angle of the voltage source converter (VSC)-HVDC rectifier and the voltage angle at its ac terminal. This is amenable to power flow regulation in the dc link and, hence, to frequency control in the low-inertia grid. A feature of this model is that the developed VSC-HVDC link model may also be used to feed an island system; for instance, a system with dead load, where the inverter station provides the electrical angular reference. Hence, the inverter acts as a virtual synchronous generator with frequency regulation capabilities as seen from the low-inertia ac grid. The dynamic control scheme that enables the VSC-HVDC to provide frequency control in such operating environments has been comprehensively investigated under a wide range of credible scenarios. Overall, the dynamic system of equations describing the VSC-HVDC and the synchronous generators are discretized using the trapezoidal rule, and the ensuing equations are combined together with the algebraic equations of the ac and dc grids in a linearized reference frame amenable to iterative solutions using the Newton-Raphson method.

A Unified Modeling Approach of Multi-Terminal VSC-HVDC Links for Dynamic Simulations of Large-Scale Power Systems

This paper introduces a new and general frame-of-reference for dynamic solutions of multi-terminal VSC-HVDC systems using the Newton-Raphson method. Three VSC dynamic models are derived to conform to each pairing AC sub-network-the slack converter whose aim is to control its DC voltage, the scheduled-power converter which injects a scheduled amount of power and the passive converter which is connected to an AC network with no frequency control equipment. Each VSC unit makes provisions for the phase reactor, AC filter, DC capacitor, DC smoothing inductor and LTC transformer. The VSC itself is a positive-sequence lumped-type model whose core elements are a phase-shifting transformer and an equivalent shunt susceptance which account for the phase-shifting and scaling nature of the PWM control. In turn, the DC side of each pairing VSC unit is linked to a DC system of an arbitrary configuration. All this enables the assembly of any number of VSCs, giving rise to a comprehensive formulation of multi-terminal VSC-HVDC systems. The prowess of the proposed multi-terminal dynamic model is demonstrated by carrying out a comparison against the widely-used EMT-type package Simulink, using a three terminal VSC-HVDC system, with very good results. Furthermore, a six-terminal VSC-HVDC system forming a DC ring is used to show the applicability of the proposed unified approach when solving multi-terminal VSC-HVDC links for system-wide dynamic studies.

Power flow solutions of AC/DC micro-grid structures

This paper presents a new and general frame-of-reference for the unified, power flow solution of AC and DC micro-grids using the Newton-Raphson method, where the quadratic convergence towards the solution is preserved. The cornerstone of this modeling development in power flow theory is the so-called multi-terminal VSC-HVDC system. In this frame-of-reference, an AC micro-grid of arbitrary configuration is connected to the high-voltage side of the LTC transformer of a VSC station. In turn, the DC side of each VSC is linked to a DC system of arbitrary configuration. Any number of AC micro-grids may exist and the DC system may contain single load or generation points such as a PV installation. Each VSC model takes into account, in aggregated form, the phase-shifting and scaling nature of the PWM control. It also accounts for the VSC current design limits, PWM limits within the linear range, switching losses and ohmic losses.

Tensile and Flexural Strength of Untreated Woven Henequen-Glass Fabric Reinforced Epoxy Hybrid Composites

The use of eco-friendly composites has gained attraction due to its lightweight and moderate strength in recent years. The aim of this paper was to study the influence of the stacking sequence of glass and henequen fabrics on the mechanical properties of epoxy composites. Fiber/Matrix interface adhesion was examined using SEM. It was observed how the tensile and flexural properties of the hybrid reinforced epoxy laminates with henequen and glass fabrics, increase as the number of layers of henequen woven fabric decrease while stacking sequence does not have a great effect on the tensile properties. However, when ten layers of henequen fabric were used, a eco-friendly composite material with good mechanical strength was obtained due to the mechanical anchoring of the henequen fabric with the epoxy resin. Hence, it is clearly shown how by tailoring the geometry of the fabric, improvements in the mechanical properties of eco-friendly polymer composites can be achieved.