Low frequency AC transmission systems for offshore wind power plants: Design, optimization and comparison to high voltage AC and high voltage DC

Abstract

Low frequency AC technology has been presented as a configuration that can potentially reduce the total cost of existing transmission systems. Such cost reduction could be achieved by increasing the power transfer capacity of High Voltage AC cables operating at low frequency and removing the offshore converter when compared to High Voltage DC systems. The purpose of this paper is to suggest an optimal design of Low Frequency AC transmission systems. This results in obtaining the optimal Low Frequency AC voltage level, location, number of reactors and the Back-to-Back converter size for an Offshore Wind Power Plants with a defined rated power and distance to shore. Additionally, the Low Frequency AC, High Voltage AC and High Voltage DC equivalent configurations are compared in terms of total cost and the final transmission system is selected. The analysis of a practical case study is presented in detail, followed by an expansion of the methodology application on various configurations. Also, cost sensitivity analyses for two opposed cases are provided to identify the variation of the break-even distance of Low Frequency AC with High Voltage AC and High Voltage DC. Lastly, different non-standard transmission frequencies are analyzed and an optimal frequency is defined for a range of Offshore Wind Power Plants rated powers and distances. The paper presents a methodology that can be applied considering different relevant parameters, e.g. transmission voltages, cable cross sections and cost data. Based on parameters available in the literature, the study results show that Low Frequency AC is not currently viable.