Under the deal, Voith will supply two motor-generators, two 25 MW vertical pump turbines, the automation systems as well as the electrical and mechanical balance-of-plant systems.
Voith Hydro will produce the equipment at its manufacturing and engineering facility in Shanghai, China.
The project is situated on the Lam Ta Khong River, a tributary of the Mun River, 200km northeast of the capital city of Bangkok.
The extension project will increase the capacity of the pumped storage plant to 1000MW.
The 500MW phase one of the project was started in 1995 and completed in 2001.
Voith Hydro Shanghai president Martin Andrä said: "As a fully localized company in China, Voith Hydro Shanghai is powering most of the large hydropower projects in China with industry-leading equipment, technology and service.
"Our proven track record in China is now also helping drive our expansion into the Southeast Asia, home to some of the world’s largest untapped hydroelectric resources."
Thailand is planning to expand renewable energies and reduce its reliance on energy imports.
Currently, more than 60% of the country’s primary commercial energy demand is met by imports.
The Thailand Energy Ministry is working on a Renewable and Alternative Energy Development Plan to increase the renewable energy production to 25% in 10 years.