Direct-current system switch function equivalence method applied to background harmonic analysis

The invention provides a direct-current system switch function equivalence method applied to background harmonic analysis. The direct-current system switch function equivalence method comprises the following steps of calculating direct-current system impedance under the condition that influences of an opposite-side converter substation on the direct-current system impedance are considered; calculating by using a switch function algorithm to obtain current generated by positive sequence background harmonic voltage and current generated by negative sequence background harmonic voltage so as to obtain a mathematic relation between harmonic voltage of an alternating-current side of a local-side converter substation and harmonic current of the alternating-current side of the local-side converter substation; performing equivalence on a two-port network of the local-side converter substation into simple impedance by using a Thevenin theorem; and calculating to obtain equivalent direct-current impedance, which is equivalent from the direct-current system impedance, of an alternating-current system of the local-side converter substation under the condition that influences of the opposite-side converter substation are considered. Influences on impedance characteristics of an alternating-current and direct-current mixed connection system connecting bus can be calculated by using the obtained equivalent direct-current system impedance, the amplification characteristic of background harmonic waves, which are injected to the bus, in the system is obtained, influences on an ultra-high-voltage direct-current power transmission system due to amplification of the harmonic waves are avoided, and efficient and stable operation of the ultra-high-voltage direct-current power transmission system is guaranteed.