By Luis Marti and Cynthia Yiu USTY RECORDS OF ONTARIO HYDRO'S activities in the study of geomagnetically induced currents (GICs) and their effects on the power system go as far back as the late 1970s. In the early 1980s, a network of four GIC monitors was in place long before the notorious March 1989 geomagnetic disturbance (GMD) event that caused the Hydro Quebec blackout. Through the 1980s and 1990s, the intensity of GMDrelated activities in Ontario Hydro and in Hydro One (its successor transmission and distribution company) appear to have waxed and waned along with the sunspot cycles. However, since sunspot cycle 22, there have been periodic reviews of the effects of GICs on protection systems, resulting in operational measures on the basis of forecasts of GMD events of intensity K7 and above as well as GIC measurements. For cycle 23, operational measures [in addition to safe posture as recommended by the Northeast Power Coordinating Council (NPCC)] included recalling outages on key 500-kV circuits due to the risk of the cascading loss of an entire 500-kV corridor caused by the potential misoperation of electromechanical relays that triggered on-line imbalance. The NPCC's Procedures for Solar Magnetic Disturbances Which Affect Electric Power Systems (Document C-15) provided actions (safe D real-Time management of Geomagnetic disturbances Hydro One's eXtreme Space Weather control room tools. Digital Object Identifier 10.1109/MELE.2015.2480637 Date of publication: 2 December 2015 46 I E E E E l e c t r i f i cati o n M agaz ine / december 2015 2325-5987/15©2015IEEE