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Triple frequency injection adds 30% to the power rating of a transmission line.

New initiative.

In 1984, Duncan Grant and John Houldsworth published a paper in which they proposed the addition of cophasal, triple-frequency harmonics (of 1/6 of the fundamental amplitude) to the output waveforms of a three-phase inverter in order increase the line-line voltage by approximately 15% (for a given dc link voltage). ("The use of harmonic distortion to increase the output of a three-phase PWM inverter". Houldsworth J.A. and Grant D.A. IEEE Transactions of the Industry Applications Society, Sept 1984, pp 1224-1228). This approach has been widely adopted. In 2017, Invro filed a patent application based on the idea of adding 1/6 triple harmonic to the three-phase waveforms on a power transmission line to achieve the same effect. This would permit the power line to carry approximately 30% more power without any increase in the line-ground voltage . Hence there would be no increase in insulator string lengths or insulation thickness in the case of cables. The triple harmonic is stripped out of the phase waveforms on passing through a three-phase transformer with a floating star point.

The patent application was subsequently withdrawn because of prior art (including our own!) and it is quite easy to identify a number of difficulties with the concept (putting impedance between the star point of a transformer and ground is not generally welcomed, handling lightning strikes could become more difficult, large capacitive currents will flow at the triple frequency, etc. ). However, with effort solutions may be found to all the problems. And the benefits of the concept may make the cost of implementation worthwhile. While the basic idea lacks novelty in the eyes of the patent examiner, solutions to the problems may be capable of being protected by a patent. We would like to identify all the problems, create solutions and where possible turn the solutions into intellectual property.

Below is the proposed line-to-ground (or line to neutral) waveform consisting of a sinusoid with 1/6 third harmonic added - before increasing the overall magnitude by 15% to restore the peak value to its original magnitude.

Waveform

If you would be interested in partnering with Invro Ltd in this enterprise, please contact us at duncan.grant@invro.com. Appropriate partners could be major power companies, grid equipment makers, and research organisations with an interest in power transmission and distribution.