Calculation of harmonics and interharmonics in HVDC schemes with low DC side impedance

When HVDC schemes are used to join AC systems of different frequencies, frequencies other than the well known characteristic harmonics are produced in the AC systems, and these are often called interharmonics. This paper examines these interharmonics and shows that, for HVDC schemes with low DC side reactance, another set of interharmonics will exist only on the DC side. These are a function of the impedance of the convertor seen from the DC side. It is shown that this impedance is time varying and, by modulation theory, will produce DC side harmonic frequencies some of which may be at low frequency. It is also shown that the amplitudes of characteristic harmonics and interharmonics will be changed by this effect. The method based upon modulation theory is compared to the results of a time domain computer simulation. The results shown will also be applicable to variable frequency motor drives.