Impacts of grounding and winding configurations on voltage harmonics in 3φ power transformers

Three phase (3φ) power transformers are available in various configurations, which offer the advantage of creating local neutral and grounding points. The different configurations of the primary and secondary sides of 3φ transformers can alter the paths, through which harmonic components of exciting currents flow. Such an alteration can cause the induction of undesired harmonic components in the voltages on both sides of a 3φ power transformer. Among these undesired harmonic components is the third harmonic, which can exceed the standardized limits, and can adversely impact the transformer, transmission system, grounding system, protective devices, and/or fed loads. The magnitude of the voltage third harmonic component is highly dependent on the core type, configuration of primary and secondary windings, and ground return paths. This paper experimentally examines the presence of the third harmonic component in primary and secondary side voltages for a 3φ transformer for different primary and secondary winding configurations, separately derived point-of-supply, and isolated and directly connected grounding. The different configurations will be examined to determine what may or may not be considered a separately derived source.