Comparison of Capacitor Voltages Balancing Methods for Cascaded H-bridge STATCOM

The object of the research work is the multilevel high-voltage converters with Cascaded H-Bridges (CHB) controlled by two different methods. They are the sorting algorithm and the fundamental harmonic displacement method. The comparison was carried out on the basis of mathematical modeling for the case of cascade connection of 10 cells with the rated voltage on the capacitors equal to 1000 V. Such circuits can be implemented, in particular, in high power STATCOMs with the voltage of 10 kV. Such converters are widely applied to compensate reactive power of electrical facilities at ironworks, for example at arc furnaces or continuous hot or cold rolling mills. In the process of control system design, it is necessary to solve a dual problem, particularly, to provide balancing of capacitor voltages for the minimum time and within a definite range. It is also necessary to avoid the overloading of the slave switching devices caused by the increase in the number of commutations. The modeling results showed that the suggested methods of balancing can provide the acceptable voltage quality at the point of connection, THD of output voltage is 5.23% and 7% for the first and the second method respectively. When the sorting algorithm is used, balancing takes less time as compared with the displacement method. However, this is achieved at the cost of higher load imbalance of semiconductor switches, which is undesirable in terms of thermal conditions. When balancing was carried out using the displacement method, the load of semiconductor switches was more uniform. This method can be recommended for practical application as the main one.