JOHANNESBURG (Reuters) - Standard & Poor's on Tuesday affirmed its BBB- rating with a negative outlook on South African power utility Eskom, citing risks involved with a government financial support package. The government in September announced a support package for the troubled state-run utility, which will include the company raising over $4 billion in debt and receiving an equity injection from the state. Eskom, which produces virtually all of South Africa's power, is operating on thin margins as it scrambles to complete two large new power stations. South Africa has suffered from rolling blackouts several times this year.