Contributions to the mitigation of voltage imbalances in modern distribution networks with modular multilevel static synchronous compensators: modelling, control and energizing strategies

Abstract

This work presents contributions to the operation and control of three-phase modular multilevel static synchronous compensators for applications in modern electric networks. Firstly, an energization strategy is proposed for the static synchronous compensator. The energizing circuit comprises a controller and two thyristors shunt-connected to the contacts of the mechanical switch used to connect the converter to the electric network. At the first energizing stage the lower and upper arm DC capacitors of two phases of the converter are charged with a current drained from the mains. The maximum value of the energizing current is controlled by the thyristors firing angle. The developed mathematical model allows to obtain an energization characteristic curve that relates the DC terminal voltage of the static converter and the thyristors firing angle. This characteristic curve is represented by a piece-wise linear approximation in order to reduce the computational effort of the energizing current control method. In the following energization stages specific converter’s semiconductors are switched so that the submodule’s DC capacitors of the other converter’s arms receive part of the energy stored in the capacitors already charged during the first energizing stage. Posteriorly, current and voltage control loops, based on mathematical models in the synchronous reference frame, are presented to control the static compensator in order to regulate the positive-sequence voltage and to compensate the negative- and zero-sequence voltages at the point of common coupling. Moreover, control loops used to regulate the DC terminal voltage and to compensate the circulating currents of the modular multilevel converter are also presented. Lastly, a consumer voltage unbalance assessment methodology is presented and adapted to the time domain. Then, the developed dynamic equations are incorporated into the controller of the static compensator in order to compensate for only the negative- and zero-sequence voltage unbalances caused by a consumer connected to the point of common coupling.