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B4-67 Harmonic aspects of VSC HVDC, and appropriate harmonic limits

The rapid proliferation, increasing size and technical advances of VSC HVDC in recent years has revolutionized the industry. In the sphere of harmonics and filtering, the impact has been highly significant, with the harmonic footprint of these converters being much less than equivalent LCC schemes, but also radically different in terms of frequency ranges of interest, and the significant deliberate generation of inter-harmonics.

The existing national and international regulations and recommendations governing harmonics were originally formulated considering the types of converters and associated harmonics then prevalent. In many senses they are proving inadequate to deal with the new technology and its consequences. Individual regulatory bodies are hastening to catch up, but sometimes lack a firm basis of appropriate technical knowledge.

Furthermore, the implications of VSC transmission for harmonic generation are not widely enough understood – in terms of range, magnitude and the necessity or otherwise of having dedicated filters. The modelling of a VSC converter as a harmonic voltage source is also not fully appreciated in its implications for regulatory methodologies. The implications of the generation of non-integer harmonics by the latest generations of converters also has profound implications.

A further topic is the effect of VSC installations on pre-existing (background) harmonics. Some designs of VSC converter now produce a quasi-sinusoidal waveform so clean that in some cases harmonic filters may not be required to mitigate the harmonics generated by the converter. However, the converter will still have a harmonic impedance as seen from the network, and it is important to be able to assess this harmonic impedance and calculate its impact in terms of possible amplification (or damping) of the pre-existing network harmonics.

It is therefore an appropriate task for CIGRÉ SC B4 to assess this area and provide expert technical advice which can be referred to where necessary.

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