SISO Stability Criterion Derived From a Three-Port Admittance Model of Train-Network System and Its Applications in Low-Frequency Oscillation Analysis

Frequency-domain admittance models have been widely utilized to analyze the low-frequency oscillation (LFO) issue in train-network systems. Since existing dq-frame or $\alpha \beta $ -frame admittance models of train-network systems are usually built from ac-port, impacts of dc-side passive impedance on the LFO cannot be explicitly characterized. In addition, admittance models built from ac-port are multiple input and multiple output (MIMO) systems, which is inconvenient for admittance measurement and stability margin estimation such as a single-input and single-output (SISO) system. As a result, this article builds a three-port admittance model of train-network systems. From the perspective of dc-port, a novel SISO stability criterion is derived to analyze the LFO. It is verified that the proposed SISO stability criterion and the traditional MIMO stability criterion in the dq-frame are different reduced-order forms of the three-port admittance model, and all of them are equivalent under some preconditions. Moreover, impacts of dc-side passive impedance on the LFO are explicitly revealed based on the proposed SISO stability criterion, and a virtual resistor control based on the feedback of dc-link capacitor current is then proposed to suppress the LFO. Finally, the experiment results by StarSim hardware in the loop (HIL) are employed to verify the correctness of theoretical analysis results.