Low Frequency AC Transmission Upgrades with Optimal Frequency Selection

TL;DR

This paper explores low frequency AC transmission upgrades, proposing models to select optimal frequencies, and demonstrates that LFAC can outperform HVDC in certain scenarios by improving power flow and system efficiency.

Contribution

It introduces a novel optimal power flow model incorporating frequency as a variable and analyzes LFAC as an effective upgrade alternative to HVDC.

Findings

LFAC can match or surpass HVDC in system performance.

Optimal frequency depends on system topology and operating conditions.

LFAC offers improved power flow control and infrastructure utilization.

Abstract

The advantages of operating selected transmission lines at frequencies other than the standard 50 or 60 Hz are numerous, encompassing increased power transfer capacity and better utilization of existing infrastructure. While high voltage DC (HVDC) is by far the most well-established example, there has been an emerging interest low frequency AC (LFAC) transmission in applications ranging from offshore wind to railway systems and mining. In this paper, we investigate the use of LFAC as a transmission upgrade and propose models and analysis methods to determine the optimal choice of frequency. The paper first presents an optimal power flow model with frequency as a variable, assuming modular multilevel converters for frequency conversion. Using this model, we analyze LFAC as an embedded upgrade in a transmission system using existing lines. We quantify the system-wide advantages from improved power flow control and frequency reduction and find that an LFAC upgrade achieves similar and sometimes better results compared with HVDC upgrades. Finally, we analyze the factors which determine the optimal frequency for these upgraded transmission lines, and we demonstrate the benefits of changing the frequency in response to different system topologies and operating conditions.