* COO Farrell named as new chief
* Snyder led diversification, expansion drives (Adds details, background)
CALGARY, Alberta, July 27 (Reuters) - Steve Snyder, who led a diversification drive at TransAlta Corp (TA.TO) and stared down an investor pushing for major strategic changes at the power generation company, is stepping down as chief executive after 15 years, the company said on Wednesday.
Snyder, 62, will be replaced by Dawn Farrell, currently the company's chief operating officer, said TransAlta, which is known for its coal- and gas-fired power plants in Canada and the United States as well as a large wind farm unit.
Farrell, 51, will take the helm at the start of 2012, TransAlta said.
During his tenure, Snyder honed the company's focus on power generation, exiting regulated utility and transmission businesses.
Early in the last decade, he bought CE Generation for C$365 million to gain operations in several U.S. States. TransAlta's largest units are still in Alberta, where TransAlta runs Canada's largest coal-fired power plant fleet.
However, Snyder made renewable energy a major part of the company's portfolio. Its biggest renewables move was the C$1.6 billion ($1.7 billion) takeover of Canadian Hydro Developers in 2009.
In 2007 and 2008, U.S. private equity firm Luminus Management LLC launched a bid to get TransAlta to adopt a more aggressive, debt-heavy capital structure as a way to lift the stock price. It proposed a slate of its own directors for the company.
Snyder contended such a move ran counter to a tried-and-true strategy of maintaining a conservative structure and investment-grade credit rating and said it showed that Luminus had a poor grasp of the Canadian electricity generation business.
In the end, Luminus backed down, but not before TransAlta sold off its Mexican assets and instituted a dividend increase.
Farrell has held several executive positions at TransAlta and at British Columbia government-owned BC Hydro, where she served in senior generation, engineering and aboriginal-relations roles.
The company is scheduled to release its second-quarter financial results on Thursday.
TransAlta shares closed down 33 Canadian cents at C$20.51 on the Toronto Stock Exchange on Wednesday.
($1=$0.94 Canadian) (Reporting by Jeffrey Jones; editing by Peter Galloway)