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They were often able to maintain generation and load balance simply through slight adjustments to their baseload resource output. In Graph 3, dots represent hourly energy output from wind as a percent of total load. The horizontal bars indicate the annualized energy penetration for the wind resources. As the average energy supplied by wind increases, the highest percent hourly penetrations occur during the off-peak hours when loads are lower and there is less down regulation capability. During these hours, the Balancing Area must still maintain compliance with the CPS standards. The necessary flexible dispatch comes at the expense of increased Ramp Cycling of baseload resources, as well as increased State Cycling.
This operating impact is most clearly demonstrated on the PSCo system, where in some off-peak hours the company must now operate a very flexible gas-fired power plant to provide the necessary balancing flexibility. Note that PSCo does not yet have the ability to participate in a coordinated regional economic dispatch market, which would help mitigate these off-peak
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PSCo is working to mitigate stand-alone challenges to balancing flexibility. PSCo is seeking partnerships with other Western Interconnection utilities that are willing to participate in the development of full-scale regional energy markets. It also is participating in an Area Control Error Diversity Initiative (ADI). The project began operation in June 2009 and is an attempt to achieve economic compliance with the balancing reliability standards. Xcel Energy also supports initiatives underway at NERC to establish new balancing standards that are improvements to the current Control Performance Standard requirements, given the increasing penetration of wind.
Other initiatives are needed in order to manage increasing wind penetration. Xcel Energy has several initiatives underway. They include Smart Grid projects to increase demand response capability, and one benefit of the project might be to provide increased flexibility to the Balancing Area operators in matching generation and load. Xcel Energy also has recently increased its natural gas storage capability. It turns out that with the peaker generators used to provide much of the flexibility in response to wind, gas pipeline delivery schedules and flexibility of the gas fuel supply also become critical. Lastly, Xcel Energy is exploring energy storage technology. This might be useful in integrating large amounts of system variability. The company is experimenting with a large battery test installation, as well as collaborating with the Electric Power Research Institute on additional initiatives.
Generation Issues
For Xcel Energy’s dispatchable generating resources, we are experiencing both increased State Cycling and Ramp Cycling as wind penetration increases. Generation units that were originally operated as baseload facilities are now running in State Cycling and/or increased Ramp Cycling. This change in unit mission is having an operating impact to the dispatchable fleet in several ways.
- Increased number of state cycles accelerating wear and tear, causing advancement in major overhaul schedules to minimize reduced reliability issues.
- Increased up/down ramp rates causing more component damage per thermal cycle.
- Increased load follow range or turn-down (lower minimum load) causing more component damage per thermal cycle.
- Increased number of ramp cycles accelerating creep fatigue failure mechanisms.
- Increased fuel cost due to impacts of increased state and ramp cycling, and accelerated wear and tear.
- Reduction in emission control efficiency due to load variability.
- Reduction in the life of key plant components and overall unit life.
- Decrease in overall unit reliability.
Each time a conventional steam power plant is State or Ramp Cycled, the boiler, steam lines, steam turbine and auxiliary components go through unavoidable thermal and pressure stresses, which cause long-term non-reversible structural damage due to thermal, corrosion and mechanical cycling.
