JAKARTA, Jan 4 (Reuters) - Indonesia state utility Perusahaan Listrik Negara said as of Tuesday it had secured 7.5 million tonnes of additional coal for January but aims to continue to raise supply to maintain a minimum 20-day stock, it said in a statement. The world"s top thermal coal exporter on Saturday banned exports of coal this month to make sure domestic power plants have enough to avoid widespread outages that drove up prices in major buyer China on Tuesday. read more