The Operation of a Power Transmission Line With Injected Third Harmonic Voltage

An alternative transmission line design method is proposed that reduces the required conductor-to-ground clearance by injecting third harmonic voltage. In this method, third harmonic voltage is injected to the line, such that the peak amplitude of phase voltages is reduced by 13.4% while the fundamental component remains intact. The injected third harmonic voltage does not influence the load or generation sides. The objective is to utilize the insulating strength of the conductor-to-ground clearance almost continuously rather than only during the crest voltage, as opposed to conventional ac lines. With this approach, for the same fundamental voltage level, the proposed line requires lower conductor-to-ground clearance. This could considerably reduce the line construction capital investment by using shorter transmission towers. Furthermore, the existing ac lines could be upgraded with this method to increase their ground clearance, if desired. Injection of third harmonic voltage can also increase the loadability of existing ac lines, if the new phase-to-phase clearance is met. In this paper, the operation of a power transmission line with injected third harmonic voltage and practical consideration for its implementation are studied.