Mutual Interaction between a Harmonic Load and the Feeding Transformer

Stefan Fassbinder

Deutsches Kupferinstitut, Germany

Summary : Transformers in public electricity grids do more than just provide galvanic isolation and alter the ratio of voltage by current for a given amplitude of apparent power. They also introduce a substantial portion of inductive reactance into the system, which is important for both the feeding grid’s and the fed mains’ behaviour in cases of short circuits and earth faults. They are also capable of creating a system with a star point out of a system – of the same or a different voltage level – which does not provide a star point connection.     

Now the influences of all of these properties upon the system can today be calculated reasonably well by means of adequate programmes, but the calculations always relate to the rated mains frequency of 50 Hz or 60 Hz. If the transformer is feeding substantial shares of non-linear loads, then the total load current includes quite a number of frequencies rather than just one, and each of these is influenced by the transformer in a different manner. Vice versa, load currents of different frequencies take different effects upon a transformer. How can these influences be quantified, and how do they have to be taken into account when designing an electricity supply system today? This paper wants to offer a practical approach for those cases where theory gets stuck.

Although the mutual effects are basically quite evident, they are a bit difficult to understand and to explain consecutively, because they occur simultaneously and are interdependent. It shall be tried below to explain separately what actually happens jointly.