Approximate Circuit Representation of Voltage Unbalance Emission Due to Load Asymmetry in Three-Phase Power Systems

Abstract

This work investigates from a circuit viewpoint the phenomenon of voltage unbalance emission due to load asymmetry in three-phase power systems. Voltage unbalance emission occurs whenever a power system shows a lack of structural symmetry between the three phases. In such a case, in fact, the transformation methods, such as the symmetrical component transformation, provide coupled modal circuits. As a result, in a practical three-phase three-wire power system, the positive-sequence circuit injects current into the negative-sequence circuit and therefore the load voltage shows a negative-sequence component due to modal circuits interaction. Since in many cases of practical interest the asymmetry level in a three-phase load can be regarded as a small perturbation with respect to the ideal symmetrical condition, in this paper a weak-coupling approach between sequence circuits is proposed. More specifically, load asymmetry is investigated, and analytical conditions for circuit weak-coupling are derived. In this case an approximate equivalent circuit for the estimation of voltage unbalance emission is derived. Analytical results are validated by means of numerical simulation of a radial power system with asymmetrical three-phase load.