In the early 1990s, Hydro-Québec undertook a joint project with GE to develop and commission a DC current-blocking series capacitor for the transmission lines forming part of its main power system in the area surrounding the grounding electrode of the Radisson HVDC substation. The aim of this project was to minimize the harmful effects of DC current flowing through the main network during the ground-return operating mode of the Radisson-Sandy Pond multi-terminal HVDC link. With a view to having available technology applicable anywhere in the network, TransÉnergie and IREQ, two divisions of Hydro-Québec, later took part in the development of a DC current blocking device that could be installed in transformer neutrals. This work was undertaken not only with immediate needs in mind but also to solve specific problems that could appear sporadically at different network locations. Tests on a prototype of the new blocking device were performed in the High Power Laboratory at IREQ, followed by tests on the preproduction unit, in accordance with the specified requirements. The Neutral DC Current Blocking Device (NCBD) was then installed at a regional substation for a one-year operating period to verify its immunity under normal network events and switching conditions. During this period, no disruption to the smooth operation of the substation or protection of the AC system occurred. The NCBD operated correctly and suffered no fault or unsolicited operation.

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During the spring of 1991, several severe geomagnetic disturbances were recorded in North America, and particularly in Northem Quebec, where Hydro-Quebec's La Grande generation complex and transmission system are located. As a result of these disturbances, the Quebec-New England Phase I1 HVDC system was affected by transformer saturation and harmonic voltage distortion due to geomagnetic induced current (GIC). This paper presents an analysis of the phenomenon observed at Radisson, the northemmost terminal in the system.A central issue which is discussed in the paper is the reflection of the dc side impedance to the ac side for operating converters, and the effect of this reflection on the harmonic impedance as viewed from the ac side.

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