CALGARY — TransAlta Corp. says it’s cutting its workforce in its Canadian coal generating unit by 20 per cent as it works towards trimming annual costs by about $12 million.The Calgary-based power producer started the cost-saving initiative in January and it says a “large part” of the savings are already begin captured.It incurred about $7 million in restructuring costs during the first three months of 2015.TransAlta’s first-quarter net income fell nearly 45 per cent to $26 million, from $47 million a year earlier.Revenue for the quarter was $593 million, down 18 per cent from $775 million during the same period in 2014.TransAlta Renewables signs $1.78-billion Australian investment deal with TransAlta CorpTransAlta to build US$536M power station in Australia