During the 1970s, Hydro-quebec (Montreal, Canada) installed nine 250 MVAR (16-kV) synchronous compensators to better control network voltages through dynamic injection of reactive power. These machines were designed with hydrogen (H{sub}2) cooling under a pressure of 30 psig (2 bars relative). Commissioning tests indicated that under design conditions, these machines could be continuously over-loaded up to 325 MVAR without excessive overheating. Over the years, the utility observed several excesive H{sub}2 leakages. For safety reasons, the H{sub}2 pressure and the maximum MVAR loading were reduced on the compensator with the most leakage. Hydro-Quebec made temperature rise calculations of critical parts under different loadings and H{sub}2 pressure conditions, and concluded that the 325 MVAR loading should still be possible with the H{sub}2 pressure reduced to 15 psig (1 bar relative). However, field tests revealed that, while the machines withstood the overloading, the overheating at 325 MVAR damaged the bushings (phases and neutrals). While the base machine was essentially Class Finsulation, the bushings were Class B.
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