This is from the official Network Rail media release headed "Severn Tunnel modernisation":
We’re working to transform the Severn Tunnel. The130-year old structure will close for six weeks from 12th September – 21st October 2016 to allow us to install electrical equipment that will ultimately power the new passenger trains.
The new fleet of electric trains will mean faster, quieter, greener, more reliable journeys between Swansea and London. With reduced journey times and more seats, journeys will be more comfortable for passengers.
The railway is set to be electrified from London to Cardiff by 2019 and to Swansea by 2024.[My emphasis]
Only last December, the then Minister of Transport, Patrick Mcloughlin, asserted that the line through Neath and Port Talbot to Swansea would be electrified by 2020 or 2021. Since then, there has been no statement from government about this further delay to the programme, which under the coalition would have seen Cardiff electrified by 2017 and Swansea soon thereafter.