Controls and applications of electric springs

Abstract

The over-reliance in past decades on fossil fuels to generate electricity has adversely affected eco systems, leading to environmental issues such as global warming and climate change. Renewable energy sources, such as solar and wind, are increasingly adopted for electricity generation due to their environmentally friendly and sustainable natures. Grid voltage fluctuation and utility frequency deviation are the two major challenges that can be attributed to the high penetration of renewable energy sources into power grids. Different technologies have been introduced to resolve these two main issues. However, these techniques have drawbacks and limitations, such as the difficulty of adopting flexible AC transmission technologies into the distribution networks, security issues on the information and communication technology using smart metering, and the high cost and environmental issues related to the overuse of batteries. Recently, a technology for automatic demand-side management, called electric springs (ES), has been proposed to offer a new approach to perform power quality and reliability improvement of emerging power systems. This power-electronic technology involves the participation of non-critical loads at the demand side of the power network to resolve the problem of fluctuating power generations, while also serving as a distributed energy storage system to provide a power buffer for the network. Conceptually, ES involves a small-scale device that can be embedded into industrial and commercial loads, and can be installed on buildings at the demand-side level on a massive and widely distributed scale. Hence, ES can provide local grid voltage regulation and also the supply-demand power balancing function. However, this new technology is at the development stage, and its optimization and potential are yet to be fully explored. Extensive analysis on the suitable control schemes and the possible functionality of ES is therefore needed. In this research work, a new control method for ES, called radial chordal decomposition (RCD), has been proposed to resolve the issues and the inability of existing control methods of ES for concurrent multi-function controls. This control method can resolve the conflicts on controlling the active and reactive power of the ES-integrated smart load. It is found that the power-factor and grid-voltage control loops can be implemented simultaneously and instantaneously on a single ES without mutual effect on one another. In addition, a unified power-flow analysis has been derived for single-phase, three-phase power-balanced, and three-phase power-unbalanced AC power systems. With the control strategy derived from the unified power-flow analysis, ES can be used for three-phase power balancing, with energy-storage utilization, in an unbalanced three-phase power system. Moreover, the implementation of the ES in a DC power system has been developed in this research work. The properties of the ES-integrated DC system, under different types of non-critical loads, has been identified. These theories provide a foundation on the use of the ES concept in DC power systems. The findings from this research have verified that the concept of ES can be effectively applied to DC, single-phase AC, and three-phase AC power systems, where it can provide various power-quality and power-security improvement functions.