Effects of multi-cycle symmetrical control (MCSC) loads on the feeding LV distribution transformers

Multi-Cycle Symmetrical Control (MCSC). The aim of our study is to investigate the potential adverse impact of these loads on the distribution MV/LV transformers that supply them. Indeed, asymmetric load currents contain a DC component, and DC current components may shift the magnetic operating point of the transformers and drive them into saturation, which results in increased magnetic flux and magnetizing currents beyond their rated values. These currents, which have an important spectral content, generate increased losses which could lead to loss of life and/or electrical insulation damage. In the upstream network, the increase in magnetizing currents results in increased reactive power demand and harmonic injection.

An EMT simulation study is conducted for a 50 kVA distribution transformer and typical upstream and downstream network characteristics. Moreover, the sensitivity of the results to the following factors is considered: the unipolar sub-pattern duration, the relative amplitude of the unipolar load, the presence of unipolar loads on one phase only or on the three phases, the presence of symmetric load components, the saturation characteristic of the transformer.

The results show that, depending on the number of cycles of same polarity, the load relative amplitude, and its distribution over the three-phases, MCSC loads could drive the transformers into significant saturation. However, if the MCSC load current pattern has a small enough number of consecutive cycles of the same polarity, the saturation does not occur and the adverse consequences are avoided.