Capacity uplift in low voltage distribution networks using a novel AC/AC power electronic converter

In order to achieve net zero by 2050 in the UK, “Future Energy Scenarios” by the National Grid ESO predict an excessive shift from fossil fuels to more sustainable and green energy resources like wind, solar and hydrogen. This shift makes electricity the dominant energy source in the residential and transport sectors, which includes the adaption of Low Carbon Technologies (LCTs) like electric heating and electric vehicles as well as high penetration of decentralised and distributed generation in Low Voltage (LV) networks. In the UK, distribution network operators will face significant challenges due to this transition to de-carbonised electricity generation and consumption. Therefore, future LV networks should have more capacity and resilience to respond to the demand increase and adaption of these LCTs. Conventional solutions, such as network upgrades and reinforcement, are expensive and disruptive to the public and business and do not address these challenges effectively. One proposed innovative solution is to increase the voltage along the existing distribution feeders and step it back down at a customer’s premises. Utilising the existing LV feeder lines, this solution can potentially increase the network capacity by up to 50% and improve power quality issues such as overvoltage caused by distributed generation units. The existing feeder cables have plenty of spare voltage rating capacity as they are currently rated at the phase voltage of 600 V. This project has identified an AC/AC Power Electronic Converter (PEC) as the most suitable device to carry out the step-down function.

This thesis will focus on two major aspects of this solution: (1) identifying and extracting the mechanical and electrical specifications and requirements of such an AC/AC PEC and (2) investigating and selecting an appropriate circuit to meet these requirements. The Meter Box of each house is found to be the optimal position for the AC/AC PEC, which drives the mechanical specifications and imposes strict constraints on the size and efficiency of any prospective circuit. Additionally, the ideal solution should have minimum disruption to the consumer, including full compliance with existing protection and safety requirements mandated by the law through the standards and regulations. Among these protection and safety requirements, a continuous Neutral point connection and protection against electric shock and overcurrent faults were found to significantly impact the selection and rating of the AC/AC PEC. A solution to this issue was proposed, which includes the operation of the AC/AC PEC in current limit mode during the overcurrent faults and replacing high-current overcurrent protective devices with more sensitive alternatives. This solution is partially validated via a Monte-Carlo statistical study. Based on the extracted and devised specifications, a novel AC/AC Chopper circuit is proposed with a robust operation against the probable errors in the voltage polarity detection circuit. The operation principle of the circuit was fully investigated via analytical and simulation studies. A GaN and SiC based 1.5 kVA prototype was designed and constructed to evaluate the performance of the new circuit and validate the theoretical and simulation studies.