Increasing the Energy and Reliability Indicators of Multilevel Frequency Converters for Oil- and Gas-Sector Installations

Abstract

This paper presents a method for improving energy indicators and increasing system reliability by adopting multilevel schemes of power circuits of semiconducting frequency converters of a voltage class of up to 1 kV. An optimum number of phases was selected at the first stage based on the criterion of the minimum cost to solve the problem. Then, this criterion was supplemented by the limitation of the electric losses and, at the final stage, the problem was solved based on the condition of securing the specified indicator of the failure- free operation. Modern semiconducting converters for the large specified capacities are of modular configuration and each of the frequency converter assemblies (rectifier, invertor) includes, as a rule, some parallel connected modules. In these conditions, an increase in the number of phases at which the phase winding of the engine is made multiphase and the previously parallel-connected power modules of self-commutated inverters are galvanically decoupled and connected separately to each of the engine phases, which allows increasing the reliability indicators of the system without any additional capital costs. It is theoretically shown and experimentally confirmed that, due to the development of the modern element base, the use of three-level frequency converters for a voltage up to 1 kV is economically justified. It is determined that, in addition to the known advantages of such a circuit (reduction in the switching losses and overvoltages on the phase winding of the engine), it is possible to increase additionally the coefficient of efficiency of a frequency converter due to a larger number of the degrees of freedom during the switch over to multiphase circuits.