Modelling and control of a series static synchronous compensator using low-frequency, fixed- modulation index techniques

This paper presents the modelling and control design of a converter used in a Static Synchronous Series Compensator (SSSC) application, employing low frequency, fixed modulation index strategies. In those, the magnitude of the output waveform is directly related to the dc-bus level by a fixed relation. Consequently, the dc-bus has to be varied to obtain the desired output voltage. A large signal model for the converter is derived in the paper, subsequently linearized to obtain a small signal model, which was used to propose a control strategy. The theory is then validated experimentally on a novel Voltage Sourced Converter (VSC) configuration. A 200 kVA small-scale SSSC test bench, connected to an emulated transmission line was built for this purpose. This configuration has been used for the first time in a 47 MVAR demonstrator built for the Spanish transmission grid.