Upgrading Transmission Capacity by Altering HVAC Into Fractional Frequency Transmission System

Transmission adequacy of power system has been challenged by high investment of new line construction and limit line-corridor space. In this paper, a novel solution to increase transmission capacity by altering HVAC lines into fractional frequency transmission system (FFTS) is proposed. FFTS is a transmission that utilizes a relatively low grid frequency, e.g., 50/3 Hz, to extend the transmission capacity of ac lines. The proposed solution can minimize the construction efforts since the existing HVAC transmission lines can be directly employed by FFTS without any change. The structure and principle of FFTS for transmission line conversion (TLC) are introduced firstly. Then, performance of conventional ac equipment in FFTS is evaluated and comparison with respective counterpart in HVDC is carried out. Next, the economic performance of FFTS for transmission capacity upgrade is conducted based on a typical benchmark, and sensitivity analysis results are presented to determine the economic zone of FFTS. The results show that the uniform annual value (UAV) of TLC into FFTS is 24.9% and 31.6% cheaper than TLC into HVDC and constructing new HVAC lines, respectively. Therefore, FFTS is a superior scheme for transmission capacity boost.