Generation of Abnormal Harmonics in High-Voltage AC-DC Power Systems

The possibility of abnormal harmonic generation in an ac system feeding a converter load is examined from the standpoint of automatic control systems used in modern HVDC transmission systems. It is shown that with certain types of automatic controls, the firing angles of converter valves may sustain some errors, which in turn could generate abnormal harmonic currents in the three- phase ac system feeding the converter. A criterion for predetermining the type and extent of abnormal harmonic generation is defined, and numerical examples given. Methods of reducing abnormal harmonic generation are discussed, and some experimental data supporting the theoretical calculations are also included.