DC harmonic filter design and mitigation of induced fundamental frequency currents for the NEA 800 kV HVDC multi-terminal project

This paper describes the DC side filtering for the North-East to Agra 800 kV HVDC transmission project. The multi-terminal nature of this transmission, with numerous possible configurations of the DC side, posed an exceptional challenge for the DC side harmonic filter design.

The design of the DC filters took into account a number of factors including: the optimum balance between shunt DC filter size and smoothing reactor size, keeping the DC filter solution as simple, economical and robust as possible, and the avoidance of amplification of low order harmonics. The design study considered the many combinations of DC side circuit possible with the multi-terminal configurations, including monopolar and hybrid as well as bipolar modes. The chosen design solution uses the same filter configuration at all stations, although with some different details and component ratings.

An important aspect relates to the possible induction, from parallel AC power lines, of fundamental frequency currents in the DC circuit. These currents may pass through the HVDC converter, undergoing a frequency-shifting modulation which produces direct current in the converter transformers, thus risking unacceptable levels of core saturation. The calculated level of induced fundamental frequency was such that mitigation measures in the HVDC system were required. The chosen solution was to install fundamental frequency series blocking filters in the neutrals of the inverter station at Agra.

DC filter component rating was critical, especially in the case of the DC side filter high-voltage capacitors at Agra, which are located indoors due to pollution. As these have to be rated for the harmonic voltage plus the 800 kV direct voltage requirement, their height becomes a crucial factor for the DC hall design.

The entire design study was the most complex since the last true multi-terminal HVDC project some 25 years previously, and provided many valuable experiences which are recounted in the paper.