By Julia Payne LONDON (Reuters) -The Ukrainian government has instructed power stations to switch to natural gas to preserve strategic coal reserves, the head of the country’s biggest private power producer said on Saturday. DTEK Chief Executive Maxim Timchenko said the country has 15 to 20 days left of coal reserves, or 730,000 tonnes. DTEK is involved across the energy sector, from renewables to coal and oil, and accounts for a quarter the country’s power generating facilities. Most of Ukraine’s coal production, some produced by DTEK, is located in the country’s east, in or near the two sepa…