An Investigation of Excessive Rural Network Harmonic Levels Caused by Particular Irrigation Pumps
High and increasing voltage Total Harmonic Distortion (THD) and 5th harmonic voltage levels have been measured in recent years on Orion NZ Ltd's rural 11 kV distribution network in Canterbury, New Zealand. On occasion, at Points of Common Coupling (PCC), the THD has exceeded the 5% limit and the 5th harmonic has exceeded the 4% limit. It has been suspected that these summer season high harmonic levels are due to the large existing and increasing density of farm irrigation deep well pump loads. It is likely that adverse effects on the network and neighbouring loads would become apparent if the increasing harmonic levels are left unchecked. A practical and theoretical investigation was conducted by the Electric Power Engineering Centre (EPECentre), University of Canterbury, on behalf of Orion to determine what are the primary causes of the high harmonic levels and provide mitigation options. Several farms with installed irrigation pumps were visited and the harmonic current and voltage levels were measured when the pumps were operating. Substation measurements were also simultaneously taken. A computer harmonic model of the local 11 kV network and loads was created that provided an accurate description of actual network conditions. An examination of Orion's system from the view of harmonic management was performed. The investigation revealed that the most significant cause of high harmonics levels is the comparatively large harmonic current injections by local irrigation pumps using Variable Speed Drives. A secondary cause is resonances in the network, often dynamically created by Power Factor (PF) capacitors switching online at various farms. To keep the rural network infrastructure in an effective and sustainable operating condition, mitigation measures are needed. Options researched included simulations of harmonic filters placed at various network points, and the use of alternate vector group transformers. Suggestions are made regarding objectives for harmonic limits at different network voltage levels, so as to determine harmonic limits for load installations.