(Reuters) - The largest city-owned utility in Texas on Friday sued the state’s grid operator alleging it levied “excessive” power prices during a February deep freeze, and seeking to bar the grid from issuing a default that could affect its credit rating. High prices for emergency fuel and power during a severe cold spell left Texas utilities facing about $47 billion in one-time costs. Those costs have led to two bankruptcies and knocked two other electric providers off the state’s power grid because of payment defaults. San Antonio’s municipal utility, CPS Energy, faces about $1 billion in extraordinary charges for natural gas and electricity during a five day deep freeze last month. CPS Energy has about 820,000 electricity customers. “We are fighting to protect our customers from the financial impacts of the systemic failure” of the state’s grid operator,” said Paula Gold-Williams, chief executive of the city-owned utility. The lawsuit, filed in a Bexar County court, alleges grid operator Electric Reliability Council of Texas (ERCOT) mismanaged the cold weather crisis, and overcharged for power. It asked the court to prevent ERCOT from declaring it in default and to prevent ERCOT from charging CPS Energy for other grid defaults. An ERCOT spokeswoman did not immediately reply to a request for comment. Nearly $3.1 billion in power charges to grid users have not been paid so far, ERCOT said on Thursday. ERCOT officials hiked power prices by about 400 times the usual rate to $9,000 per megawatt for five days last month in a vain effort to bring in more power. State officials this week called on ERCOT and the utility regulator to cut those charges for high-price power for the 32-hour period after the grid emergency passed. CPS Energy’s lawsuit called ERCOT’s handling of the crisis “one of the largest illegal wealth transfers in the history of Texas.” It brought the complaint “to protect its customers from excessive and illegitimate power and natural gas costs,” according to the lawsuit. Credit rating firms have warned CPS and other Texas municipal and rural cooperative power provider could have their ratings lowered. The state’s largest and old cooperative, Brazos Electric Power Cooperative Inc, filed for bankruptcy earlier this month owing $1.8 billion to ERCOT.