Grid-Forming Control for Train Drives

With the problems of climate change, the demand for sustainable transportation increases, where trains in particular are a competitive option. However, more trains in the network place higher demands on the system, where one part is the electrical stability. One type of instability that can occur when many trains are parked close to each other or when the distance to the feeding station is large is low frequency oscillations (LFOs), which means that the power starts oscillating at a low frequency. If these oscillations grow, circuit breakers may trip, potentially causing a complete loss of power at a station.

One way to deal with this problem is to make the control system used at trains more robust. One control structure that shows promising results that has been developed for renewable power generation is grid-forming (GFM) control.

The purpose of this study is to investigate if grid-forming control can be used to mitigate the problem with LFOs by answering the question ”how far can the line length be increased before LFOs occur if GFM control is used in trains’ control system?”. That is done by developing a linear admittance model of the line converter module, which is the system on trains used to control the electric power consumed by the train. Furthermore, a GFM controller has also been added to the model. The conventional controller is then compared with the GFM controller concept by analysing poles of the admittance model and with time-domain simulations of a reference model. The results show that using GFM control, the line length the train can operate until unstable LFOs occur increases with approximately 260%.