Application of Multilevel Voltage Inverters in Distributed Series Compensation Devices

Abstract

The operation principles of multilevel inverter circuits and methods for controlling them are considered. The operation of a multilevel circuit as part of a distributed transformer-based series compensation device is analyzed. The specific feature pertinent to the operation of a self-excited inverter within a distributed series compensation device is that it is loaded on the transformer connected in series into the power transmission line. In this case, the load is in fact a current transformer. Analytical expressions for calculating the circuit parameters are presented. The results of spectral modeling and simulation of the multilevel circuit in different modes of its operation are given. By using the spectral modeling, the currents and voltages of all power circuit components and their harmonic composition were estimated in the steady-state operation mode, and it has been shown that inductive injection is the heaviest mode of operation, because the inverter current reaches its maximum value in this mode. In using the multilevel circuit, additional requirements are posed to the control system, which, in addition to ensuring stable control of the output voltage, must also balance the voltages across the capacitors in the inverter arm. In selecting a storage capacitor and filter capacitor, it should be borne in mind that the maximum possible voltage across these components is determined not only by the necessary boost voltage, but also by the line current in the rectifier mode and can be significantly higher than the originally evaluated value. The obtained results can be used in designing distributed series compensation devices on the basis of an NPC inverter.