HOUSTON (Reuters) - Texass largest and oldest electric power cooperative on Monday filed for bankruptcy protection in federal court in Houston, citing a disputed $1.8 billion bill from the states grid operator.

Brazos Electric Power Cooperative Inc is one of dozens of electricity providers facing enormous charges stemming from a severe cold snap last month. The fallout threatens utilities and power marketers who collectively face billions of dollars in blackout-related charges, executives said.

Unusually frigid temperatures knocked out nearly half of the states power plants in mid-February, leaving 4.3 million people without heat or light for days and bursting water pipes that damaged homes and businesses. Brazos and others that committed to provide power to the grid and could not, were required to buy replacement power at high rates and cover other firms unpaid fees.

The states grid operator, Electric Reliability Council of Texas (ERCOT), on Friday said $2.1 billion in initial bills went unpaid, underscoring the financial stress on utilities and power marketers. More providers likely will reject the bills in coming days, executives said.

The municipal power sector is in a real crisis, said Maulin Patani, a founder of Volt Electricity Provider LP, an independent power marketer that is not a member of the Brazos coop. ERCOT should suspend the service charges to halt further defaults, he said in an interview on Sunday.

The city of Denton, in north Texas, last week sued ERCOT in a state court to prevent it from charging it for fees unpaid by other users of the grid. Denton Electric could face tens of millions of dollars for fees that were not collected from others, the suit claimed.

Debt analyst Fitch Ratings last week also warned of potential downgrades to all Texas municipal power firms that use the states grid. Costs from the storm could exceed the liquidity immediately available to these issuers, Fitch said.

ERCOT triggered the squeeze when it pushed up spot-market rates to $9,000 per megawatt hour (mwh) over more than four days and levied huge fees for services. The service fees were 500 times the usual rate, according to industry executives.

Brazos Electric coop executive Clifton Karnei, who sat on ERCOTs board of directors until last week, signed the Brazos coops bankruptcy submissions. Through its 16 utility company members, Brazos provides electricity to more than 660,000 customers across the state of Texas.