Jan 22 (Reuters) - National Grid Plc said on Monday it filed a proposal to update rates for one of its U.S. electric and gas distribution utilities in a bid to boost its performance in the country.

The three-year rate proposal for the Niagara Mohawk Power Corporation electric and gas distribution utility would increase revenue, before the impact of deferred credits, by $206 million in 2018-2019, compared to the company's initial request of $317 million, National Grid said.

For 2019-2020 and 2020-2021 incremental year-on-year revenue increases would be $36 million and $60 million, respectively.

The company also expects to invest a total of $2.5 billion over the three-year period, down from the initial proposal of $2.7 billion.

The proposal, filed jointly with the New York Public Service Commission, also includes an allowed 9 percent return on equity for the utilities and estimates customer savings of $76 million from the prospective impact of U.S. tax reform.

The settlement will contribute to the improving financial performance of U.S. operations, National Grid said.

Niagara Mohawk gas and electricity utility serves 2.2 million customers in the upstate New York region and represents about 30 percent of the company's U.S. rate base.

The final decision of the Commission is expected in the spring of 2018 with new rates effective as of April 2018.

(Reporting by Arathy S Nair in Bengaluru; Editing by Sunil Nair)