Harmonic measurements on transmission systems

Harmonic measurements by the National Grid Company (NGC) on its 400 kV and 275 kV system confirm an upward trend in harmonic levels at transmission level voltages. It is believed that this is due largely to the changing nature of the loads connected at lower voltage, particularly the proliferation of consumer electronic equipment containing components and power supplies with nonlinear characteristics. This paper indicates some of the results from a harmonic monitoring exercise across the transmission system and discusses the critical influence of harmonic impedance on the levels observed