The decision by one of Colorado’s largest electric cooperatives to pull out of the Comanche Unit 3 plant is the latest twist in what is emerging as the troubled history of the state’s youngest coal-fired generating unit.

A slew of public records detail poor maintenance practices, costly equipment breakdowns and lengthy outages that long preceded CORE Electric’s decision to relinquish its 25% stake in Comanche 3 on Sept. 6. In announcing its decision, CORE blamed Public Service Company of Colorado (PSCo), an Xcel Energy subsidiary, for “ongoing mismanagement” at the 750 MW facility.

A review of documents undertaken by Power Engineering tells a story of a host of problems that have dogged the power unit at the power plant near Pueblo, Colorado.

Comanche 3 reached commercial operation on July 6, 2010. The supercritical plant was scheduled to come online in 2009, but the date was pushed back to make repairs needed for leaking steam tubes. The Colorado Public Utilities Commission (CPUC) indicated the leaks were due to inadequate post weld stress relieving treatments. Repairs were also necessary to install baffles to quiet a high-pitched noise coming from the plant.

Those early problems lasted through the decade: CPUC documents reveal stack noise issues; reduced capacity factors due to outages needed to correct improperly welded components in the boiler; unplanned outages because of slagging due to malfunctioning water cannons; the replacement of the finishing superheater, which was the subject of a recommendation to disallow recovery of $11.7 million in investments; and finally, the extended outage that began in January 2020 to repair and replace steam turbine blades.

CPUC documents indicate poor maintenance likely contributed to the January 2020 outage, when major turbine repair and renovations were required to return the plant to service.

“Inspection of the turbine revealed rubbing on eight of the high pressure rotating blade shrouds with work hardening on three of the shrouds and a section of the shroud was missing,” said a CPUC report. In addition, the inspection revealed “significant seal damage in the lower portion of the casing.”

The cost of repairs from the incident included re-blading the turbine (at a $4.8 million capital cost) and incremental replacement power costs for the outage (estimated at $1.7 million).

Then, at the completion of the 2020 outage, a loss of lubrication to the main turbine shaft while in the process of returning the unit to service resulted in still more damage to the turbine, generator and ancillary equipment. That outage extended after the end of 2020.

The regulatory report attributed this to “unidentified equipment defects, inadequate equipment marking, insufficient communications protocols, lack of thoroughness in procedures and training, and human error.”

CPUC records show costs stemming from the June 2020 incident were even larger and included repair activities totaling $20.4 million in capital and O&M costs. PSCo expected all but the deductible and overhead (about $1.5 million) to be reimbursed by insurance. Regulators said ratepayers incurred about $14 million in incremental power replacement costs, according to PSCo’s simulations with the lengthy outage necessitating expensive short-term market purchases during the summer peak period.

Lowest availability

Despite being the youngest PSCo-owned unit to operate with either a single steam cycle or combined cycle, Comanche 3 had the lowest availability of all units from 2010 through October 2020. State regulators found the plant was offline an average of more than 91 days per year during this time; roughly 27% of the outages were planned, 24% were associated with boiler tube leaks, and the remainder were associated with other unplanned non-routine outages.

Years of challenges led to CPUC to launch an investigation into the history and operations at Comanche 3. Utility regulators found in their investigation indications that PSCo failed to meet basic industry standards throughout much of the plant’s history.

CORE Electric has an open lawsuit against PSCo for breach of contract related to the unit.

“Because of the numerous and lengthy outages at Comanche 3 since it began commercial operation, CORE has suffered millions of dollars in damages,” reads the co-op’s lawsuit. “CORE has spent millions of dollars in additional repair and maintenance costs that were incurred only because of PSCo’s imprudent utility practices and other breaches of the project agreements.”

CORE said because of the outages, it was required to buy replacement electricity from Xcel for $38.5 million, an estimated $20 million above what was expected from Comanche 3.

Regarding the lawsuit and CORE’s plan to pull out of Comanche 3, an Xcel Energy spokesperson said, “We disagree with CORE’s claims and expect to address them through the legal process. Generation at Comanche 3 is very important for all customers and we value all our partners who help provide safe, reliable and affordable energy.”

Problems continued into 2022

Comanche 3 has been out of service for roughly half of 2022, records indicate. According to an Xcel Energy inspection report from an incident on Jan. 28, 2022, one pole of Comanche 3’s 345 kV generator breaker was closed during breaker troubleshooting. The electrical protection circuits for the generator breaker were isolated and the disconnect switches that isolate the generator breaker from the electric system were closed.

Xcel inspectors said “as a result, the generator was significantly damaged, several generating units in the region experienced momentary outages and multiple transmission lines were opened.”

Inspectors attributed the incident to a lack of coordination between substation and plant operations. Their incident report also said that work was conducted on the generator breaker without establishing a robust clearance to protect the generator in case of an unexpected system response.

Both CPUC’s investigation on the 2022 incident and the commission’s larger investigation are still listed as active.

The Comanche plant in Pueblo, Colorado, first entered service in 1973 with its 325 MW unit 1. It added a 335 MW Unit 2 in 1975 and brought Unit 3 into service in 2010.

Comanche 3 has a supercritical Mitsubishi TCRF36, N-61 steam turbine generator set.

The unit’s turbine generator uses a combination of three large rotors coupled together: a shaft for the combined nine-stage high pressure turbine/six-stage intermediate pressure turbine coupled to two shafts operated in tandem that serve two six-stage dual flow low pressure turbines.

Comanche Unit 3 is expected to retire by Jan. 1, 2031, ending Xcel’s use of coal in Colorado. The date comes from a recently revised agreement and is nine years earlier than the retirement date in the originally submitted plan.

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