Sub-Synchronous Oscillation Mechanism and Characteristics Analysis of Flexible Low-Frequency Transmission System

Flexible low-frequency transmission (FLFT) system is an AC transmission system with its operating frequency less than the power frequency. This system contains power electronic devices such as modular multilevel matrix converters (M3Cs), presenting wide-frequency dynamic characteristics in their control links, which may lead to oscillation risks. In light of this, an <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">RLC</i> equivalent circuit of the system is established according to a low-frequency project in Zhejiang, China with the impedance analysis method, where the sub-synchronous oscillation (SSO) risk is determined. Also, the SSO mechanism is analyzed with damping characteristics analysis of the equivalent circuit. Then, the effect of line parameters, low-frequency side M3C controller parameters, and steady-state operating points on the SSO characteristics is investigated using the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">dq</i> impedance model. The analysis results show that the interaction between the low-frequency AC grid and the low-frequency side of M3C can lead to SSOs. The low-frequency side of M3C exhibits “capacitive impedance with negative resistance” at SSO frequency, and the low-frequency AC grid presents “inductive impedance with positive resistance” at the frequency, thus forming an <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">RLC</i> second-order resonant circuit. The negative resistance effect is the main cause of the SSO in the FLFT system, and the outer-loop proportional and inner-loop integral links of the M3C controller have a great impact on the SSO. The strength of low-frequency AC grid also affects the SSO damping. The analysis results have guiding significance to the design of M3C controller parameters and FLFT system parameters in the subsequent FLFT project.