Low-Frequency Stability Analysis of Power Electronic Traction Transformer Based Train-Network System

Power electronic traction transformers (PETTs) will be increasingly applied to locomotives in the future for their small volume and light weight. However, similar to conventional trains, PETTs behave as constant power loads and may cause low-frequency oscillation (LFO) to the train-network system. To solve this issue, a mathematical model of the PETT is firstly proposed and verified based on the extended describing function (EDF) method in this paper. In the proposed model, the LLC converter is simplified to an equivalent circuit consisting of a capacitor and a resistor in parallel. It is further demonstrated that the model can apply to various LLC converters with different topologies and controls. Particularly, when the parameter differences between cells are not obvious, the PETT can be simplified to a single-phase rectifier (i.e., conventional train) by equivalent transformation. Based on the model of PETT, the system low-frequency stability and influential factors are analyzed by using the generalized Nyquist criterion. Lastly, the correctness and accuracy of theoretical analyses are validated by off-line and hardware-in-the-loop simulation results.