Unit Capability Factor
(UCF)

PURPOSE

The purpose of this indicator is to monitor progress in attaining high unit and industry energy production reliability. This indicator reflects effectiveness of plant programs and practices in maximizing available electrical generation, and provides an overall indication of how well plants are operated and maintained.

DEFINITION

  • Unit capability factor is defined as the ratio of the available energy generation over a given time period to the reference energy generation over the same time period, expressed as a percentage. Both of these energy generation terms are determined relative to reference ambient conditions.
  • Available energy generation is the energy that could have been produced under reference ambient conditions considering only limitations within control of plant management, i.e., plant equipment and personnel performance, and work control.
  • Reference energy generation is the energy that could be produced if the unit were operated continuously at full power under reference ambient conditions.
  • Reference ambient conditions are environmental conditions representative of the annual mean (or typical) ambient conditions for the unit.

    • DATA ELEMENTS

    The following data are required to determine each unit's value for this indicator:
    • Reference energy generation, expressed in units of megawatt-hours (electric).
    • Planned energy loss: the energy that was not produced during the period because of planned shutdowns or load reductions due to causes under plant management control. Energy losses are considered planned if they are scheduled at least four weeks in advance. Energy losses considered to be under plant management control are further defined in the clarifying notes. Planned energy loss is expressed in units of megawatt-hours.
    • Unplanned energy loss: the energy that was not produced during the period because of unplanned shutdowns, outage extensions, or load reductions due to causes under plant management control. Energy losses are considered to be unplanned if they are not scheduled at least four weeks in advance. Energy losses considered to be under plant management control are further defined in the clarifying notes. Unplanned energy loss is expressed in units of megawatt-hours.

    CALCULATIONS

    The unit capability factor is determined for each period as shown below:

    Value of a unit, UCF (%) = (REG - PEL - UEL) x 100 %
    REG
    Where:
      • REG = Reference energy generation for the period
      • PEL = Total planned energy losses for the period
      • UEL = Total unplanned energy losses for the period
    • Planned energy loss: PEL = å(PPL x HRP)
        • Where:
      • PPL = Planned power loss: the power decrease in megawatts due to a planned event
      • HRP = Hours operated at reduced power (or shutdown) due to the planned event

        • Note: The total planned energy loss for the period is the sum of the losses from all planned events.

    • Unplanned energy loss: UEL = å(UPL x HRU)
        • Where:
      • UPL = Unplanned power loss: the power decrease in megawatts due to an unplanned event
      • HRU = Hours operated at reduced power (or shutdown) due to the unplanned event

        • Note: The total unplanned energy loss for the period is the sum of the losses from all unplanned events.

    • Value for the industry = Median of the unit values
      In general, unit capability factors for individual units will be presented for a three-year period to minimize the impact of annual variations due to refueling and planned maintenance outages. An example of the data collected for this performance indicator and a sample calculation are provided in Attachment A.

    DATA QUALIFICATION REQUIREMENTS

    Data for new units is included in the calculation of industry values beginning January 1 of the first calendar year following commercial operation.

    CLARIFYING NOTES

  • The reference energy generation is determined by multiplying the reference unit power by the period hours.
  • The reference unit power is the maximum power capability of the unit under reference ambient conditions. If a maximum power capability has been determined by formal test, the reference unit power is determined by correcting test results to reference ambient conditions. If a formal test has not been performed, the reference power should be based on design values, adjusted for reference ambient conditions. The reference unit power is expected to remain constant unless design changes that affect the capacity are made to the unit.
  • The reference ambient conditions are environmental conditions representative of the annual mean (or typical) conditions for a unit. It is expected that historical heat sink temperatures will be used to determine the reference ambient conditions. The same reference ambient conditions will generally apply for the life of the unit. Periodic review of these reference conditions is not required.
  • Planned energy losses (those scheduled at least four weeks in advance) caused by the following conditions should be included when computing the unit capability factor because they are considered to be under the control of plant management:
    • Refueling or planned maintenance outages
    • Planned outages or load reductions for testing, repair, or other plant equipment or personnel-related causes
  • Energy losses due to tests may be considered as planned if they are identified at least four weeks in advance and are part of a regular program, even if the precise time of the test is not decided four weeks in advance.
  • Unplanned energy losses caused by the following conditions should be included when computing the unit capability factor because they are considered to be under the control of plant management:
    • Unplanned maintenance outages
    • Unplanned outages or load reductions for testing, repair, or other plant equipment or personnel-related causes
    • Unplanned outage extensions
    • Unplanned outages or load reductions that are caused by, or prolonged by, regulatory actions taken as a result of plant equipment or personnel performance, or regulatory actions applied on a generic basis to all like plants
  • Energy losses due to the following causes should not be considered when computing the unit capability factor because these losses are not considered to be under the control of plant management:
    • Grid instability or failure
    • Lack of demand (reserve shutdown, economic shutdown, or load-following)
    • Environmental limitations (such as low cooling pond level, water intake restrictions, earthquake or deluges that could not be prevented by operator action)
    • Labor strikes (see clarifying note below)
    • Fuel coast downs
    • Seasonal variations in gross dependable capacity due to cooling water temperature variations
  • The values of planned or unplanned power losses to be used in computing energy losses due to a particular event are the losses that would have occurred if the unit were operating at the reference power level at the time of the event. The power losses relative to the reference power may be determined by one of the following techniques:

      (1) Subtracting the actual power level during the event from the power level immediately prior to the event when the power was at or near the reference power level,

      (2) Computing the power level reduction that would have occurred with the unit at the reference power level, or

      (3) Using historical data from similar events occurring at the reference power level.

    For example, if a unit experiences a 10 MW power loss due to an equipment problem while operating at 75 % of the reference power, and it is determined from calculations or from similar events that have occurred at the reference power that the same equipment problem would have resulted in a 20 MW power loss at the reference power level, then 20 MW should be used when computing the energy loss.
  • For events involving planned or unplanned outages and start-up following these outages, the reference unit power should be used as the basis for computing power losses.
  • If energy losses during an event occur due to a combination of causes under management control and causes outside of management control, the portion of the total losses caused by factors under management control should be identified and included when computing the unit capability factor.
  • Outages or load reductions caused by labor strikes that occur while the unit is operating are normally not included as planned or unplanned energy losses because these energy losses are not under the direct control of plant management. However, if during the strike the unit becomes incapable of starting or operating because of equipment failures, maintenance, overhauls, or other activities such as refueling, then the energy losses during the time the unit is inoperable are included. If a labor strike occurs during an outage, any outage extensions are included as energy losses as long as the unit is incapable of being restarted because of equipment failures, maintenance, overhauls, or other activities such as refueling.
  • In general, changes in an outage or load reduction start date must be announced at least four weeks in advance to be considered as "planned." However, if the grid dispatcher requests a change in the start date less than four weeks in advance, the outage or load reduction is considered to be planned.

    The same rule may be used if the change in the start date is decided by plant management, assuming this decision is due to all of the following reasons or circumstances:

    The unit is operating in a deregulated environment, and the management decision to modify the planned outage start date is solely to take advantage of economic situations to maximize, on a short term basis, the economic benefit coming from selling the plant electricity output.

    This economic benefit can be applied to the entire production system of the Utility, not only to the specific unit under consideration.

    • The unit is considered as able to run at maximum power during the four-week period prior to the initial planned outage start date.
    • Any forced or unplanned outage occurring during this four-week period (or before the new start date) shall not become the reason for putting forward the planned outage.
  • If a unit begins an outage or load reduction before the scheduled start date, the energy loss from the beginning of the outage or load reduction to the scheduled start date is an unplanned energy loss.
  • If an outage extends beyond the scheduled start-up date, either to complete originally scheduled work or to complete corrective maintenance work on equipment required for start-up, all energy losses associated with the outage extension should be considered as unplanned. However, outage extensions to complete discretionary work (i.e., preventive maintenance or modifications) not originally scheduled for completion during the outage should be considered as planned if the work is scheduled at least four weeks in advance. Extended outages can be reclassified from unplanned to planned once corrective maintenance activities required for start-up are completed if any remaining planned activities were scheduled at least four weeks in advance. This clarification also applies to load reductions.
  • The scheduled start and end dates of planned outages and load reductions are those dates negotiated with and agreed to by the network and/or grid dispatcher. These dates may differ from dates shown on the detailed schedule of activities used at the unit for directing the outage.
  • Energy losses related to load reduction preceding a shutdown and load increases following the shutdown should be categorized as planned or unplanned depending on whether the shutdown is planned or unplanned. For example, energy losses while entering and recovering from a planned outage will be considered as planned losses. If an outage extension (unplanned outage) occurs at the end of a planned outage, the energy loss during recovery from the outage will still be considered as a planned loss because the shutdown was originally caused by a planned outage (see Attachment A, time period 5 - 6 for an example of this situation.) Energy losses due to required tests following refueling are considered planned losses.
  • A unit that is in reserve shutdown will be considered as available if it can be restarted within the normal time required for unit start-up. If work on plant equipment is undertaken that would prevent a restart, the energy that potentially could have been produced while the plant was unavailable should be computed and used when determining the unit capability factor, even if the plant was not actually required to start-up during the period.
  • Either net or gross energy may be used; however, consistency must be maintained for all energy terms. The use of gross energy is more meaningful in certain situations. For example, it is less confusing for multi-unit stations that may power the station electrical loads from one unit.
  • As a point of interest, the sum of unit capability factor, unplanned capability loss factor, and planned capability loss factor equals 100 percent over a specific time period. Planned capability loss factor can be calculated from this relationship.
    • UCF - ATTACHMENT A

    Example Indicator Calculation

    The following examples and the accompanying power history plot are provided to illustrate methods used in calculating the unit capability factor and the unplanned capability loss factor for a plant under a variety of common situations. The time periods referenced in the example refer to points labeled on the power history plot.

    Initial Conditions:

    • Reference unit power: 985 MW(e).

      It is assumed that this unit has a maximum power output of 1 000 MW(e) under optimum ambient conditions (determined by a formal test). Correction of test results to reference ambient conditions resulted in the reference capacity value of 985 MW(e).

    • Time period being considered: one year (8 760 hours)
    • Reference energy generation for the period (REG):

      (985 MW(e)) x (8 760 hours) = 8 628 600 MW(e).h

    Energy Loss Examples:

    Time Period Description
    1 - 2 Power reduction of 100 MW(e) for 12 hours due to circulating water pump failure. The unit was operating at reduced power due to a load following at the time of the pump failure. The power reduction caused by this failure would have been 201 MW(e) if the failure had occurred at the reference power level.
    UEL = 201 x 12 = 2 412 MW(e).h Unplanned
    2 - 3 Reduced power operation due to ambient conditions and fuel coast down. The lost energy generation is not used in calculations.
    3 - 4 Planned refueling outage. Scheduled length was 45 days (1 080 hours). The outage begins on the scheduled date.
    PEL = 985 x 1 080 = 1 063 800 MW(e).h Planned
    4 - 5 Outage extension of 10 days (240 hours) beyond scheduled length to complete all work scheduled for the outage.
    UEL = 985 x 240 = 236 400 MW(e).h Unplanned
    5 - 6 Power ramp-up following outage. Average power level of 495 MW(e) for three days (72 hours).
    PEL = (985 - 495) x 72 = 35 280 MW(e).h Planned
    6 - 7 Operation above reference unit capacity due to very cold cooling water. The additional energy generation is not used in calculations.
    7 - 8 Shutdown for 32 hours due to reactor scram caused by personnel error.
    UEL = 985 x 32 = 31 520 MW(e).h Unplanned
    8 - 9 Power ramp-up following the scram. Average power level of 490 MW(e) for 8 hours.
    9 - 10 Operation below reference unit capacity due to environmental limitations only. The lost energy generation is not used in calculations.

    Calculations for Unit Capability Factor

    Time Period Energy Loss
    MW(e).h
    Total planned energy loss (PEL) 3 - 4 1 063 800
    5 - 6 35 280
    1 099 080
    Total unplanned energy loss (UEL) 1 - 2 2 412
    4 - 5 2236 400
    7 - 8 31 520
    8 - 9 3 960
    274 292

    Unit Capability Factor (UCF) = (REG - PEL - UEL) x 100%
    REG
    = (8 628 600 - 1 099 080 - 274 292) x 100%
    8 628 600
    = 84.1%

    Calculations for Unplanned Capability Loss Factor*

    Unplanned Capability Loss Factor (UCL) = UEL x 100%
    REG
    = 274 292 x 100%
    8 628 600
    = 3.2%

    * This calculation is provided for use with the Unplanned Capability Loss Factor detailed description.

    Point-to-point power level explanations:
    0 - 1 Reduced power due to load following
    1 - 2 Reduced power due to equipment failure
    2 - 3 Reduced power due to ambient conditions and fuel coast-down
    3 - 6 Unit shutdown (outage) and subsequent ramp-up
    6 - 7 Increased power due to very cold water
    7 - 9 Unit shutdown (operator error) and subsequent ramp-up
    9 - 10 Reduced power due to environmental limitations not under management control
    © 2000 International Atomic Energy Agency. Comments to Project Officer
    This page was automatically created on 10 Feb 2012, 04:02:12