Multilevel D-STATCOM for sag and swell mitigation using modulation index control

The D-STATCOM has been utilized to regulate the voltage to its nominal value by exchanging reactive power with the distribution network. For an effective compensation of the sag and swell, two control approaches, namely the phase angle and modulation index schemes have been utilized. For the former, rapid adjustment in the injected or absorbed reactive power cannot be achieved. Thus, this paper proposes a D-STATCOM using a 15-level cascaded inverter based on the modulation index approach. The advantage of this scheme is rapid adjustment of the reactive power, which in turn results in a rapid regulation of the bus voltage. It allows a direct connection to the medium voltage grid (11 kV) without using bulky transformers while maintaining very low total harmonics distortion without using any filter. The performance of the system with its controller is tested using Simulink-MATLAB. The results indicate very good performance in mitigating the voltage sag and swell; the controller is able to recover very rapidly (less than 1 cycle) with THD less than 6% for the whole range of operation.