Harmonic assessment of the impact of rural irrigation pumps

by Dr S. Hardie and Assoc. Prof. Neville Watson

The high returns to dairy farmers has resulted in many sheep and crop farmers converting to dairy farming. In order to maintain adequate grass growth for the cows, a large existing and increasing density of farm irrigation deep well pumps are extensively used during the summer. High levels of harmonic voltages and Total Harmonic Distortion (THD) are being experienced due to this equipment, and in some cases, is having adverse consequences.

In order to better understand the harmonic problem, an investigation was performed using computer simulation. A harmonic model was developed that modelled part of the South Island of New Zealand and modelled in detail one local area substation and one 11 kV feeder with dairy farms connected. Two field trips were made to gather data. The effect of harmonic currents injected by irrigation pumps on the harmonic voltages at the farms and substation were measured simultaneously. These were used to help develop and verify the harmonic model.

The investigation revealed that the most significant cause of high harmonic voltage levels is the comparatively large harmonic currents injected by irrigation pumps that use Variable Speed Drives. A secondary cause is resonances in the network, often caused by the Power Factor (PF) capacitors connected to the pumps to improve their Power Factor. Various mitigation options were then investigated using the developed computer model.