• Energy Research

Recent developments in micro-grids have led to increased interest in DC distribution due to its high efficiency in distributing energy from renewable energy sources to DC loads. This paper seeks to analyse the performance of AC and DC systems in a relatively large-sized 6 kW PV installation to determine the level of improvement in efficiency provided by DC distribution and to identify methods for further improvement. Baseline annual data for the AC system were collected from a live installation on a national school in Inis Oirr, an island off the west coast of Ireland. The results indicate that usage of a DC distribution system has the potential to reduce system losses by up to 50% as well as the ability for an annual saving in grid energy of 5% compared to the existing AC system. Moreover, the analysis reveals that DC outperforms AC distribution more in spring and autumn, when power consumption is comparable to the system production, but there is less impact in summer, when PV production is significantly higher than demand. These findings provide insights into the benefits of future DC distribution systems in individual buildings and in larger-scale micro-grids.

Recent developments in renewable energy installations in buildings have highlighted the potential improvement in energy efficiency provided by direct current (DC) distribution over traditional alternating current (AC) distribution. This is explained by the increase in DC load types and energy storage systems such as batteries, while renewable energy sources such as photovoltaics (PVs) produce electricity in DC form. In order to connect a DC distribution system to the alternating current grid (e.g., for backup, delivering energy storage to the grid) there is a need for a bidirectional inverter, which needs to operate over a wide range of source and load conditions and is therefore critical to the overall system performance. However, DC distribution in buildings is relatively new, with much of the research focused on the control of the DC bus connection between sources and loads, rather than on the grid connection. Therefore, this review aims to explore recent developments in bidirectional inverter technologies and the associated challenges imposed on grid-connected DC distribution systems. The focus is on small-scale building applications powered by photovoltaic (PV) installations, which may include energy storage in the form of batteries. An evaluation of existing inverter topologies is presented, focusing on semiconductor technologies, control techniques, and efficiency under variable source and load conditions. Challenges are identified, as are optimal solutions based on available technologies. The work provides a basis for future developments to address current shortcomings so that the full benefits of DC distribution can be achieved.

The high voltage direct current (HVDC) transmission lines represent the prospective way for long-distance transmission between countries, remote areas, and offshore wind farms to decrease power loss. However, the HVDC protection systems have many challenges against any system issue, such as maintenance and short circuits. Thus, the vital role played by high voltage direct current circuit breaker CB has made it the center of attention in HVDC protection systems. The main challenge in HVDC CB is the lack of naturally exiting current zero that allows the breaker to extinguish the arc during the opening process. Thus, a commutation L-C circuit is required to inject an oscillating current and enforce a zero-crossing. Nevertheless, the L-C branch is affected directly by the arcing time, the transient recovery voltage (TRV), and the rate of rise of recovery voltage (RRRV). This paper investigates the parametric uncertainties of L-C and SF6 mechanical interrupters, including cooling power and arc time constant upon TRV and RRRV, based on Mayr’s black-box model. Furthermore, a part of 3000 MVA, 500 kV HVDC transmission line between Egypt and the Kingdom of Saudi Arabia is used as a testing system employing ATP/EMTP software package to demonstrate the effect of CB’s parameters variations. The results indicate that the capacitance represents the major parameter having a direct impact on TRV and RRRV. In essence, the proper parameterization of the CB is highly required in the design process of HVDC CB to enhance the decision-making and ensure that the L-C is capable of reducing arcing time and TRV simultaneously.