Term of office for membership of Labour's National Policy Forum (NPF) extended from two years to four, or from one general election to the next (whichever is longer).
Mr Imchen - a candidate of Nagaland People's Front (NPF) party - was initially detained on Monday morning while he was travelling to his constituency, Koridanga, from the state capital, Kohima.
The site was designated for a new coal-fired power station in the Scottish government's National Planning Framework (NPF), which deems the site nationally important and limits the powers of local people to object.