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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