FAQs

Table of Contents

1. How do I report GADS data on combined cycle units?
2. What is the concern about "overlapping" deratings?
3. We don't enter fuel quality data for our coal units. Should we?
4. How is the steam cycle heat rate of a combined cycle unit calculated?
5. What is the example 3D calculation option?

 

How do I report GADS data on combined cycle units?

Basically, two methods exist for reporting performance and event data on combined cycle units, as shown below. Solomon 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."

Solomon 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 800 and 899 as a miscellaneous 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)

Cogeneration

(Courtesy of Solar Turbines Inc.)

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What is the concern about "overlapping" deratings?

The starting date/time for a derating is normally defined as the time the system, major component, or piece of equipment became unavailable for service, effecting an actual or potential loss of unit capacity.

Deratings often overlap each other in duration. NERC GADS considers all deratings "additive" except those which are masked (shadowed) by an outage or a larger derating for their entire duration. This means the derating that started first is assumed to be the primary cause of the load reduction until it terminates or a full outage begins.

Deratings that are masked (shadowed) for their full duration by outages or larger deratings are considered non-curtailing in nature; that is, they do not affect the available capacity of the unit.

Occasionally, two or more individual components will fail at the same time. According to the NERC Data Reporting Instructions, you can report each component failure as a separate derating. NERC processes the data first sorting by start date/time (which is now, in this circumstance, exactly the same for the two failures) then by event number. This means that when start dates are identical, the derating with the lower event number will be processed first.

However, this same situation of having derating events with the same start date/time can occur for other reasons (i.e., deratings that carry over from the end of one year and continue into the next year). NERC GADS requires that every event that carries over into the new year be "restarted" by entering the same start date/time of "010100XX."

Therefore, overlapping deratings that carry over into the next year all have the same start date/time, similar to having two or more individual components failing at the same time. However, the "knowledge" as to which one started first is lost due to the restarting of the event data records.

Depending on company reporting policy, some companies artificially stop and then restart deratings that overlap full outage events. This was primarily done because older mainframe GADS software was incapable of dealing with deratings that overlapped full outages; so, the solution was to stop deratings at the same time that the outage started and create new derating events at the end of the outage.

The software options now available allow you to set a consistent methodology for calculating overlapping deratings when these situations occur. The options are as follows:

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We do not enter fuel quality data for our coal units. Should we?

That is a tough question. Typically, the reasons most companies do not enter fuel (primarily lignite and coal) quality data in GADS are:

Coal quality affects many aspects of power plant performance, notably, capacity, heat rate, availability, and maintenance. By collecting the fuel quality data, it may be possible to calculate the effects of fuel quality on unit performance from the GADS data. Attempts to derive availability and maintenance impacts have met with limited success throughout the years. Accurate correlations of these impacts will require development of a comprehensive utility database on coal quality, equipment failures, and maintenance costs.

A reasonable cost-benefit analysis should be done to determine if it is cost-effective to collect the data and expend the time and effort to perform an engineering analysis of the impacts before this question can be answered—the answer is probably unit-specific.

It is difficult to answer what is being reported in the "industry," but we suspect that less than 50% of the units (lignite and coal units) collect fuel quality data.

For more information on this subject, a good place to start is a four-volume EPRI document entitled "Effects of Coal Quality on Power Plant Performance and Costs" – EPRI CS-4283 (Project 2256-1).

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How is the steam cycle heat rate of a combined cycle unit calculated?

A combined cycle plant is a highly fuel-efficient process that involves generating electricity in combustion turbines and utilizing the procedure's hot exhaust (waste heat) to create water vapor, which is then used to generate additional electricity in a steam turbine. The heat rate is defined as a measure of energy efficiency that defines how much fuel it takes to generate a kilowatt-hour of electricity.

The combined cycle plant involves the sequential use of the fuel energy in both the gas turbine-generator and the steam turbine-generator. The steam turbine operates in conjunction with the gas turbine, providing extra generation at an effective heat rate of zero to the station, ignoring any duct burners.

Therefore, heat rates are defined for the gas turbines and for the combined cycle, but not the steam cycle alone.

Using the GADS fuel and generation data and the performance reports from GADS NxL, you can determine the approximate overall heat rate of the combined cycle plant and an approximate heat rate for each of the gas turbines. Based on these heat rates, it is possible to infer the approximate heat rates for the steam turbine; however, the computation is somewhat involved.

Normally, the overall heat rate (Btu/kWh) for the combined cycle is calculated by summing the fuel burned in each of the gas turbines and the fuel burned in the duct burners (expressed in Btu) and dividing the summed Btu value by the total generation (kWh) from the gas turbine generator and the steam turbine generator.

So, the answer is that the steam cycle heat rate is not normally calculated by itself. It can be accurately calculated by testing and performing a detailed heat balance for the steam cycle.

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What is the example 3D calculation option?

This setting applies specifically to the calculation of overlapping deratings. This setting causes all overlapping derating to be calculated as shown in Example 3D in the NERC GADS Data Reporting Instructions in Appendix G.

The calculations deal with overlapping deratings when the first derating ends before the second derating, but the capacity of the unit does not change. The assumption is that the derating "C" in Example 3D is not created by the GADS reporter. It is also assumed that the capacity of the unit stayed at the same net available capacity as the derating "B" until the traveling screen (cause code 3260) repair is completed.

In looking at Example 3D, the unit net available capacity is assumed to continue to be 360 MW from March 10 at 7:45 (the start of the "B" derating) through March 10 at 19:00 (the end of the "C" derating).

Therefore, the software, with the option set, calculates the "C" portion as if the unit capacity stayed at the same available capacity as the derating "B" (i.e., a 240 MW derating for 8.50 hours, which results in 3.40 equivalent hours). As a result, the total "B" equivalent hours are now 4.04 equivalent hours.

Before selecting this option, ensure that this is the manner in which your company assumes overlapping deratings are calculated under these circumstances.

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