Complex sentence: The mercury-arc pole was partially decommissioned in 2007, and work is underway to completely replace the mercury-arc valve converters with a new thyristor pole, with work expected to be completed in 2012.
Split into simpler sentences: The mercury-arc pole was partially decommissioned in 2007. Work is underway to completely replace the mercury-arc valve converters with a new thyristor pole, with work expected to be completed in 2012.