Basically, two methods exist for reporting performance and event data on combined cycle units, as shown below. The Project recommends reporting them as individual components since this method allows for more detailed analysis of the combustion turbines (CTs), as well as the steam cycle.
1. As a single combined unit
(all of the CTs and steam cycle are considered one large combined unit)
Historically, NERC has recommended this method. In this scenario, most component outages (e.g., a CT forced outage) result in a derating for the combined unit. Attempted and actual starts are not counted unless the entire combined unit "starts"; the combined unit is not considered offline until the last component's breaker opens and is considered to be online when the first component's breaker closes. The combined unit is generally considered to be "available" when at least one of the components is available. Normally, the combined unit is assigned a unit code between 800 and 899, and the unit type is considered to be "miscellaneous".
2. As individual components
Under this reporting method, each CT is treated as if it was a stand-alone unit; the steam cycle is also considered a stand-alone unit. In this scenario, most component outages (e.g., a CT forced outage) result in reporting two events: an outage (e.g., U1) on the affected CT and a comparable derating (e.g., D1) on the steam cycle. All data such as attempted and actual starts are determined on a unit-basis. When determining the performance statistics for the "combined cycle," the individual unit data is combined using data "pooling" methods similar to calculating the statistics for a small "fleet."
The Project recommends that the boundary between the CTs and the steam cycle be established in the exhaust duct of each CT downstream of the diversion dampers (if installed) and upstream of the HRSG/duct burners. While some consultants recommend including the HRSGs as a part of the CT because of the physical location of the HRSG relative to the CT, we find that it makes more engineering sense to include the HRSG and duct burners with the steam cycle. Our reasoning is based on the fact that the HRSG is a part of the water/steam circuit piping of the steam cycle and functions in the same capacity as a boiler for the traditional fossil-fueled steam unit. The heat source for the steam cycle is "waste heat" and any installed duct burners.
For these arrangements, the CTs are assigned unit codes in either the 300-399 or the 700-799 range, and the steam cycle is assigned a unit code between 100 and 199 as a fossil-steam unit type.
Examples of various typical arrangements of combined cycle and cogeneration components are shown below:
CT x 1
CT x 2
CT x 3
(Courtesy of Kawasaki)
(Courtesy of Solar Turbines Inc.)