Stabilisation of asymmetrically structured back-to-back static synchronous compensator system with non-linear damping control

This study describes a new back-to-back static synchronous compensator (BTB STATCOM) system with asymmetrically structured converters and develops its mathematical model. BTB STATCOM uses pulse-width modulation technology for a rectifier station along with multipulse technology for an inverter station. The authors propose a control strategy for the following three control variables: DC voltage and both active and reactive rectifier powers. In the rectifier station, input-output linearisation via the feedback control law is applied for independent control of the active and reactive powers. Conversely, the controller is not applicable for maintaining the DC voltage because of its irregularity. They design an alternative indirect damping controller for the reactive current within the inverter station. The proposed controller improves the stability margin and decreases the oscillation of DC voltage with a degraded reactive current tracking performance of the inverter. Using simulation, they validate the proposed method, comparing its effectiveness to that of a proportional-plus-integral-plus-derivative controller. They investigate the semi-globally exponential stability of the damped internal dynamics using a parameter-dependent Lyapunov (PDL) function. Using a composite Lyapunov function based on the PDL function, the uniform boundedness of the output regulation error as well as the closed-loop stability is established.