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Maximizing motor load lifetime through the control of harmonic distortion level

Excessive levels of the harmonics are undesirable as they can cause additional heat losses in rotating machinery and therefore reduce the machinery useful lifetime. An approach to reduce the harmonics is proposed. The technique requires estimates of harmonic state in the given network by firstly classifying nodes of the network into one of four types. The condition for minimum measurement set is derived to allow the complete determination of the harmonic state of the network. A technique is then developed to assess the degree of dominance of particular harmonic source to harmonic real power flow in motor loads. The method is centered on the construction of a set of sensitivity indices which relates the change in the real harmonic load power of a given node to that of the harmonic current injection from each of the sources. The effectiveness of controlling the harmonic current injection from each harmonic source to reduce harmonic heating effect in loads can therefore be evaluated. The benefit of the reduced harmonic distortion is then expressed in terms of an increase in the motor load useful lifetime. The final harmonic flow control design is achieved through the maximization of an objective function which includes the cost of controlling the harmonic current and the saving gained from an increase in the motor lifetime. A harmonic injection pattern is obtained to maximize the service useful lifetime of the motor loads for the given cost of harmonic injection control.