POWER companies are preparing a fresh round of gas and electricity price rises that will pile the pain on to consumers already struggling to cope with soaring living costs.  Prices are expected to go up by 10% to 15% as early as this summer, industry sources said, as providers pass on the soaring cost of gas and oil. The price of wholesale gas has jumped by a third in the past two months thanks to production cuts in Libya and increased demand in Japan, where nuclear plants have been shut down.  Meanwhile, the wholesale power price, which is linked to gas and coal costs, has gone up by one-f