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{{Adams
{{Adams
| number = ML13350A381
| number = ML012410194
| issue date = 11/30/1978
| issue date = 11/30/1978
| title = Selection, Design, and Qualification of Diesel-Generator Units Used as Onsite Electric Power Systems at Nuclear Power Plants
| title = Selection of Diesel Generator Set Capacity for Standby Power Supplies
| author name =  
| author name =  
| author affiliation = NRC/OSD
| author affiliation = NRC/RES
| addressee name =  
| addressee name =  
| addressee affiliation =  
| addressee affiliation =  
Line 10: Line 10:
| license number =  
| license number =  
| contact person =  
| contact person =  
| document report number = RG-1.009, Rev. 1
| document report number = RG-1.009
| document type = Regulatory Guide
| document type = Regulatory Guide
| page count = 3
| page count = 3
}}
}}
{{#Wiki_filter:Revision 1 U.S. NUCLEAR REGULATORY COMMISSION                                                                                    November 1978 k REGULATORY GUIDE
{{#Wiki_filter:"-
                            OFFICE OF STANDARDS DEVELOPMENT
38/10/71 SAFETY GUIDE 9 SELECTION OF DIESEL GENERATOR SET CAPACITY FOR
                                                                  REGULATORY GUIDE 1.9 SELECTION, DESIGN, AND QUALIFICATION OF DIESEL-GENERATOR
STANDBY POWER SUPPLIES
                          UNITS USED AS ONSITE ELECTRIC POWER SYSTEMS
A. Introduction large increases in current drawn from the die sel generator resulting from the startup of General Design Criterion 17 requires that induction motors can result in substantial volt the onsite (standby) power supply for a nuclear age reductions. The lower voltage could pre power plant be of sufficient capacity and capa- vent a motor from starting or cause a running bility to assure that (1) specified acceptable motor to coast down. Other loads also might be fuel design limits and design conditions of the lost if their contactors drop out. Recovery reactor coolant pressure boundary are not ex- from the transient caused by starting large ceeded as a result of anticipated operational motors or from the loss of a large load could occurrences and (2) the core is cooled and con- cause diesel engine overspeed which, if exces tainment integrity and other vital functions sive, might result in a trip of the machine.
                                                  AT NUCLEAR POWER PLANTS


==A. INTRODUCTION==
are maintained in the event of postulated acci- These same consequences also can result from dents. Diesel generator sets have been widely the cumulative effect of a sequence of more used as the power source for the standby power moderate transients if the system is not per supplies. This safety guide describes an accept- mitted to recover sufficiently between succes able basis for the selection of diesel generator mire tecover suent e.


==B. DISCUSSION==
W sets of sufficient capacity and margin to imple- sive steps in a loading sequence.
General Design Criterion 17, "Electric Power                                          A diesel-generator unit selected for use in an Systems," of Appendix A,                          "General Design                    onsite electric power system should have the Criteria for Nuclear Power Plants," to 10 CFR                                          capability to (1) start and acceate a number Part 50, "Domestic Licensing of Production and                                        of large motor loads in rapid ar                                sion and be Utilization Facilities," includes a requirement                                        able to sustain the loss ok an o%1._y part of such loads        and    maintain        v0I*ge        an, Irequency that the onsite electric power system have sufficient capacity and capability to ensure                                          within acceptable limit                            "        supply power that (1) specified acceptable fuel design limits                                      continuously to the                            i      V~eded bq*"4 to main- and design conditions of the reactor coolant                                          tain the plant in a .*2e c 4itioA if an extended pressure boundary are not exceeded as a                                                loss of offsite po A~r o urý b'
result of anticipated operational occurrences                                                                                      A
*and (2) the core is cooled and containment in-                                            IEEE Std              -197 ;,,;              E Standard Criteria tegrity and other vital functions are maintained                                      for Diesel-G *              k*tor      mits Applied as Standby in the event of postulated accidents.                                                  Power                      " K Nuclear Power Generating Stations            4ceea'kes principal design criteria Criterion III, "Design Control," of Appen-                                        an ,}*ua            a        testing requirements that, if dix B, "Quality Assurance Criteria for Nuclear                                        foI          ,        help ensure that selected diesel- Power Plants and Fuel Reprocessing Plants," to                                      .genrhrat"            Units meet their performance and
10 CFR Part 50 includes a requirement that                                      '          a h*t'requirements. IEEE Std 387-1977 was measures be provided for verifying or checking                                            'f;      ed by Working Group 4.2C of the the adequacy of design by design reviews, J%$.                                            cl r Power Engineering Committee (NPEC)
the use of alternative or simplified calculati al                                    ot0      .he Institute of Electrical and Electronics methods, or by the performance of a suit".e                                            Engineers, Inc. (IEEE), approved by NPEC,
testing program.                                                ,                    and subsequently approved by the -IEEE Stan-
                                                                              - "      dards Board on September 9, 1976. IEEE Std Diesel-generator units have been wiely ud                                        387-1977 is supplementary to IEEE Std 308- as the power source for the onsiV                                    le      ic        1974, "IEEE Standard. Criteria for Class IE
power systems. This regulatory gfiae                                          -        Power Systems and Nuclear Power Generating scribes a method acceptable to the NRC staff                                          Stations,"' and specifically amplifies paragraph for complying with the                      a mission's require-                      5.2.4, "Standby Power Supplies," of that doc- ments that diesel-gene                          units intended for                    ument with respect to'the application of diesel- use as onsite power                      r j          nuclear power                  generator units. IEEE Std 308-1974 is en- plants be selected with                        ic        capacity and                dorsed, with certain exceptions, by Regulatory be qualified for                      v                                              Guide 1.32,              "Criteria              for      Safety-Related Electric Power Systems for Nuclear Power wThe substantial          her of        ges in this revision has made              Plants. "
it impractical to              the c    ges wit*      es in the margin.


'Copies          e                    the Institute of Electrical and                  A knowledge of the characteristics of each Electronics                In ..n  ited Engineering Center, 345 East
ments Genel su ignt Cap ity mrgin t7 rGenerally it has been industry practice to ment General Design Criterion 17.
47th Street,          ork, New York 10017.                                          load is essential in establishing the bases for USNRC REGULATORY GUIDES                                            Comments should be sMnt to the Secrety of the Conmi;slon, U.S. Nudcer Regulatory Commurorn , Wastington, D.C. 26.              Attention: Dockasth and Regulatney Guides re Issued to descrbe and rnake available to the public              Sevice Branch.


methods acceptable to the NRC staff of                   specific parts of the commlssiora's regultionts, to delinestedvt            sdb the staff In avalu-        The guides are Issued Inthe following ten broad divisioin:
specify a maximum voltage reduction of 15 B. Discussion percent when starting large motors from large capacity power systems and a 25 to 30 percent A diesel generator set selected for use as a voltage reduction when starting these motors standby power supply should have the capabil- from limited capacity power sources such as ity to (1) start and accelerate a number of, diesel generator sets. Large induction motors large motor loads in rapid succession, and be supplied with nominal voltage can achieve able to sustain the loss of any such load, and rated speed in less than 5 seconds when pow
atg spec        probles or postulated accidents. or to provide guidance to applicants. Regulatory Guides ae not sutbstitutes for regulations, and corn-          1. Power React"                              B. Products plence with them Is not requked. Methods end solutions different from those          2. Reseerch end Teat Reactors                  7. Transportation set out Inthe guides wil b acceptable If they provide a b"si for the findings        3. Fuels and Materiab Factites                & Occupational Health re=ui      to the issgirce or continuance of a pewrh or Bse e by the                  4. Environmental and Sitng                    9. Antitrust and Financial Review Cornwitiuo*                                                                            rLMatmerilsand Plant protection              10. General Comments and suggestions for Imroernemeis In tees gudes are encouraged                Requests for single copies of Isued guides Iwit-h may be reproducedl or for at Imes, end guides wil bee rwl sd,       aspramopas, to accommodate corn-            placerment on en autonlatic: distribution list for single copies of fuaum guldes mene= and to reflect new        rmt    or aperen*e. Howeve. corment t,                In s ief*cdivslons should be ,de In wrtltng to the US. NudIsr Reguatory this gtuld. If eeed wlfhn about two mond afte Its Issuance, wil be                     Cornenssion, Washingion, D.C.              I, Attentio: Okector, D,4lon of wfttleft useful In awluating the need for an eauly r      .evilon.                  Technical Informaiwon and Document Control.
(2) supply continuously the sum of the loads ered from adequately sized diesel generator sets needed to be powered at any one time. This which are capable of restoring the voltage to guide provides an acceptable way of assuring
90 percent of nominal in about 1 second.


the selection of a diesel-generator unit that is      generator unit so that it exceeds the sum of able to accept large loads in rapid succession.        the loads needed at any one time. A more accu- The majority of the emergency loads are large          rate estimate of safety loads is possihle duringl induction motors. This type of motor draws, at          the operating license stage of review because full voltage, a starting current five to ten          detailed designs have been completed and times its rated load current. The sudden large        preoperational test data are available. This increases in current drawn from the diesel            permits the consideration of a somewhat less generator resulting from the startup of induc-        conservative approach, such as operation with tion motors can result in substantial voltage          safety loads within the short-time rating of the reductions. The lower voltage could prevent a         diesel-generator unit.
these objectives are met. The considerations Protection of the diesel generator set from involved in the need for the diesel generator to excessive overspeed, which can result from a start and achieve rated conditions in a short loss of load, is afforded by the provision of a period of time are evaluated on an individual diesel generator set trip, usually set at 115 per case basis.


motor from starting, i.e. , accelerating its load to rated speed in the required time, or cause a                   
cent of nominal speed.


==C. REGULATORY POSITION==
A knowledge of the characteristics of each A problem arises in assessing whether the load is essential in establishing the bases for goal of continuously supplying the sum of the the selection of a diesel generator set that is needed loads is achieved with sufficient capacity able to accelerate large loads in rapid succes- and margin, because of the various interpreta sion. The majority of the emergency loads are tions of load ratings quoted by diesel generator large induction motors. This type of motor suppliers. The load ratings represent the loads draws, at full voltage, a starting current five at which the set can operate continuously with to ten times its rated load current. The sudden, a high availability, if various specified mainte
running motor to coast down or stall. Other loads might be lost if their contactors drop                Conformance with the requirements of IEEE
9.1
out. Recovery from the transient caused by              Std 387-1977, "IEEE Standard Criteria for starting large motors or from the loss of a            Diesel-Generator Units Applied as Standby large load could cause diesel engine overspeed          Power Supplies for Nuclear Power Generating which, if excessive, might result in a trip of          Stations," dated June 17, 1977, is acceptable the engine. These same consequences can also            for meeting the requirements of the principal result from the cumulative effect of a sequence        design criteria and qualification testing of of more moderate transients if the system is not        diesel-generator units used as onsite electric.


permitted to recover sufficiently between suc-          power systems for nuclear power plants subject cessive steps in a loading sequence.                    to the following:
nance programs are followed.
    Generally it has been industry practice to              1. When the characteristics of loads are not specify a maximum voltage reduction of 10 to 15        accurately known, such as during the con- percent when starting large motors from large-          struction permit stage of design, each diesel- capacity power systems and a voltage reduction          generator unit of an onsite power supply of 20 to 30 percent when starting these motors          system should be selected to have a continuous from limited-capacity power sources such as            load rating (as defined in Section 3.7.1 of IEEE
diesel-generator units. Large induction motors          Std 387-1977) equal to or greater than the sum can achieve rated speed in less than 5 seconds          of the conservatively estimated loads needed to when powered from adequately sized diesel-              be powered by that unit at any one time. In generator units that are capable of restoring          the absence of fully substantiated performance the voltage to 90 percent of nominal in about 1        characteristics for mechanical equipment such second.                                                as pumps, the electric motor drive ratings should be calculated using conservative esti- Protection of the diesel-generator unit from        mates of these characteristics, e.g., pump excessive overspeed, which can result from a            runout conditions and motor efficiencies of 90%
loss of load, is afforded by the immediate oper-        or less.


ation of a diesel-generator unit trip, usually set at 115 percent of nominal speed. In addi-              2. At the operating  license stage of review, tion, the generator differential trip must oper-        the predicted loads    should  not exceed the ate immediately in order to prevent substantial          short-time rating (as  defined  in Section 3.7.2 damage to the generator. There are other pro-          of IEEE Std 387-1977)    of the diesel-generator tective trips provided to protect the diesel-          unit.
The nominal rating, used as a datum for the overload rat ings, has been termed variously the "continu ous," "guaranteed," or "long term" rating. The definition used throughout this guide for "con tinuous rating" is "that load for which the sup plier guarantees continuous operation at a high availability (expected to be about 95%) with an annual maintenance interval". The over load ratings are similarly defined except that the specified maintenance intervals are shorter.


generator units from possible damage or degra- dation. However, these trips could interfere              3. During preoperational testing, the pre- with the successful functioning of the unit            dicted loads should be verified by tests.
For example, the following are the load ratings of a typical diesel generator set:
Ratings Continuous 2500 kW
Overload
2850 kW
2950 kW
3050 kW
Maintenance Interval Annual (8760 hr)
2000 hr
7 day
30 min If the power output is increased into the over load ratings, wearout is accelerated and the maintenance interval needed to assure high reliability is reduced. This discussion assumes that the diesel generator set is utilized solely as a standby power supply and that it does not serve a secondary function such as power gen eration for peak demand periods of a transmis sion network. The secondary functions, since they would affect wearout and availability of the diesel generator set, will be evaluated on an individual case basis. If found acceptable, the total amount of operation between mainte nance intervals will be limited by the technical specifications. This guide covers diesel gen erator sets used solely as a standby power sup ply which is the design most widely adopted.


when it is most needed, i.e. , during accident conditions. Experience has shown that there                4. In Section 5.1.1, "General," of IEEE Std have been numerous occasions when these trips          387-1977, the requirements of IEEE Std 308- have needlessly shut down diesel-generator              1974 should be used subject to the regulatory units because of spurious operation of a trip          position of Regulatory Guide 1.32.
The tabulation illustrates the sensitivity of the deterioration rate to increases in load above the continuous rating. For example, if the design basis loading were that corresponding to the 2000-hour rating, an error of only 8 per cent in estimating the loads could result in operation at the 30-minute rating. Although operation at the 30-minute rating would not be expected to stall the engine, such operation could lead to the danger of early failure.


circuit. Consequently, it is important that measures be taken to ensure that spurious ac-              5. Section 5.1.2, "Mechanical and Electrical tuation of these other protective trips does not        Capabilities," of IEEE Std 387-1977 should be prevent      the   diesel-generator   unit  from        supplemented with the following:
The uncertainties inherent in estimates of safety loads at the construction permit stage of design are of such magnitude that it is pru dent to provide a substantial margin in the selection of the diesel generator set load capa bility. This margin can be provided by esti- mating the loads conservatively and by select ing the continuous rating of the diesel genera tor set so that it exceeds the sum of the loads needed at any one time. A more accurate esti mate of safety loads is possible during the operating license stage of review due to the completion of the detailed designs and the availability of preoperational test data. This permits the consideration of a somewhat less conservative approach, such as operation with safety loads within the 2000 hour overload rating of the diesel generator set. A conserva tive estimate of safety loads based on design or measurements taken during preoperational testing of engineered safety features does not, however, represent with certainty the actual loads experienced under accident conditions.
performing its function.


"Each diesel-generator unit should be capa- The uncertainties inherent in estimates of              ble of starting and 'accelerating to rated safety loads at the construction permit stage of            speed, in the required sequence, all the design are sometimes of such magnitude that it              needed engineered safety feature and emer- is prudent to provide a substantial margin in              gency shutdown loads. At no time during the selecting the load capabilities of the diesel-              loading sequence should the frequency and generator unit. This margin can be provided                voltage decrease to less than 95 percent of by estimating the loads conservatively and                  nominal and 75 percent of nominal, respec- selecting the continuous rating of the diesel-              tively"b Frequency should be restored to
Therefore, an adequate margin is still essential.
                                                  1.9-2


* within 2 percent of nominal, and voltage                      conditions. The design of the bypass should be restored to within 10 percent of                   circuitry should satisfy the requirements nominal within 60 percent of each load-                      of IEEE Std 279-1971 at the diesel- sequence time interval. (A greater percent-                  generator    system  level  and  should age of the time interval may be used if it can                include the capability for (I) testing the be justified by analysis. However, the load-                  status and operability of the bypass sequence time interval should include suffi-                  circuits, (2) alarming in the control room cient margin to account for the accuracy and                  abnormal values of all bypass param- repeatability of the load-sequence timer.)                    eters, and (3) manually resetting of the During recovery from transients caused by                    trip bypass function. (Capability for step load increases or resulting from the                    automatic reset is not acceptable.)"
C. Regulatory Position
  disconnection of the largest single load, the speed of the diesel-generator unit should not            9. Section 5.6.3. 1, "Surveillance Systems," or exceed the nominal speed plus 75 percent of            IEEE Std 387-1977 should be supplemented with the difference between nominal speed and the          the following:
1. At a time when the characteristics of loads are not accurately known, such as during the construction permit stage of design, each diesel generator set on a standby (onsite) power supply should be selected to have a continuous load rating equal to or greater than the sum of the conservatively estimated loads needed to be powered at any one time. In the absence of fully substan tiated performance characteristics for mechanical equipment such as pumps, the electric motor drive ratings should be calculated using conservative esti mates of these characteristics. (For example, pump run-out conditions and motor efficiencies of 90% or less.)  
  overspeed trip setpoint or 115 *percent of               "In order to facilitate trouble diagnosis, the nominal, whichever is lowei. Further, the                 surveillance system should indicate which of transient following the complete loss of load            the diesel-generator protective trips is acti- should not cause the speed of the unit to                vated first."
2. At the operating license stage of re view, the predicted loads should not exceed the smaller of the 2000-hour rating, or 90 percent of the 30-minute rating of the set.
  attain the overspeed trip setpoint."
                                                            10. In Section 6.3, "Type Qualification Test-
  6. In Section 5.4, "Qualification," of IEEE            ing Procedures and Methods," of IEEE Std 387- Std 387-1977, the qualification testing require-          1977, the requirements of IEEE Std 344-1975, ments of IEEE Std 323-1974, "IEEE Standard for            "Recommended Practices for Seismic Qualifica- Qualifying Class IE Equipment for Nuclear                tion of Class 1E Equipment for Nuclear Power Power Generating Stations,"' should be used              Generating Stations," for seismic analysis or subject to the regulatory p*osition of Regulatory        seismic testing by equipment manufacturers Guide 1.89, "Qualification of C!ass IE Equip-            should be used subject to the regulatory posi- ment for Nuclear Power Plants."                          tion of Regulatory Guide 1.100, "Seismic Quali- fication of Electric Equipment for Nuclear
  7. Section 5.5, "Design and Application Con-           Power Plants."
siderations," of IEEE Std 387-1977 should be supplemented with the following:                            11. The option indicated by "may" in Sec- tion 6.3.2(5)(c) of IEEE Std 387-1977 should
  "Diesel-generator units should be designed to          be treated as a requirement.


be testable during operation of the nuclear power plant as well as while the plant is shut            12. Section 6.5, "Site Acceptance Testing,"
3. During preoperational testing, the pre dicted loads should be verified by tests.
  down. The design should include provisions            and Section 6.6, "Periodic Testing," of IEEE
  so that the testing of the units will simulate        Std 387-1977 should be supplemented by Regu- the parameters of operation (outlined in              latory Guide 1.108.


Regulatory Guide 1.108, "Periodic Testing of Diesel-Generator      Units  Used    as  Onsite          13.  Section 4, "Reference Standards," of Electric Power Systems at Nuclear Power                IEEE Std 387-1977 lists additional applicable Plants") that would be expected if actual              IEEE standards. The specific applicability or demand were to be placed on the system.                acceptability of these referenced standards has been or will be covered separately in other
4. Each diesel generator set should be ca pable of starting and accelerating to rated speed, in the required sequence, all the needed engineered safety fea ture and emergency shutdown loads. At no time during the loading sequence should the frequency and voltage de-
  "Testability should be considered in the               regulatory guides, where appropriate.
9.2


selection and location of instrumentation sen- sors and critical components (e.g., gover-                           
crease to less than 95 percent of nomi nal and 75 percent of nominal, respec tively. During recovery from transients caused by step load increases or result ing from the disconnection of the larg est single load, the speed of the diesel generator set should not exceed 75 per cent of the difference between nominal speed and the overspeed trip set point or 115 percent of nominal, whichever is lower. Voltage should be restored to within 10 percent of nominal and fre quency should be restored to within 2 percent of nominal in less than 40 per cent of each load sequence time inter val.


==D. IMPLEMENTATION==
5. The suitability of each diesel generator set of the standby power supply should be confirmed by prototype qualification test data and preoperational tests.
nor, starting system components), and the overall design should include status indica-              This proposed guide has been released to tion and alarm features.        Instrumentation        encourage public participation in its develop- sensors should be readily accessible and              ment. Except in those cases in which an appli- designed so that their inspection and calibra-        cant proposes an acceptable alternative method tion can be verified in place."                        for complying with specified portions of the Commission's regulations, the method to be
  8. Section 5.6.2.2, "Automatic Control," of           described in the active guide reflecting public IEEE Std 387-1977 should be supplemented with            comments will be used in the evaluation of
,the following:                                          applications for construction permits docketed after the implementation date to be specified in
    (3) "With the exception of the engine over-          the active guide. This implementation date will speed trip and the generator differential        in no case be earlier than July 1979.


trip, all diesel-generator protective trips should be either (1) implemented with                If an .applicant wishes to use this draft guide two or more independent measurements              in an application docketed prior to the imple- for each trip parameter with coincident          mentation date, the pertinent portions of the logic provisions for trip actuation or (2)        application will be evaluated on the basis of automatically bypassed during accident            this draft guide.
9.3
 
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1.9-3}}


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Latest revision as of 01:50, 17 January 2025

Selection of Diesel Generator Set Capacity for Standby Power Supplies
ML012410194
Person / Time
Issue date: 11/30/1978
From:
Office of Nuclear Regulatory Research
To:
References
RG-1.009
Download: ML012410194 (3)
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"-

38/10/71 SAFETY GUIDE 9 SELECTION OF DIESEL GENERATOR SET CAPACITY FOR

STANDBY POWER SUPPLIES

A. Introduction large increases in current drawn from the die sel generator resulting from the startup of General Design Criterion 17 requires that induction motors can result in substantial volt the onsite (standby) power supply for a nuclear age reductions. The lower voltage could pre power plant be of sufficient capacity and capa- vent a motor from starting or cause a running bility to assure that (1) specified acceptable motor to coast down. Other loads also might be fuel design limits and design conditions of the lost if their contactors drop out. Recovery reactor coolant pressure boundary are not ex- from the transient caused by starting large ceeded as a result of anticipated operational motors or from the loss of a large load could occurrences and (2) the core is cooled and con- cause diesel engine overspeed which, if exces tainment integrity and other vital functions sive, might result in a trip of the machine.

are maintained in the event of postulated acci- These same consequences also can result from dents. Diesel generator sets have been widely the cumulative effect of a sequence of more used as the power source for the standby power moderate transients if the system is not per supplies. This safety guide describes an accept- mitted to recover sufficiently between succes able basis for the selection of diesel generator mire tecover suent e.

W sets of sufficient capacity and margin to imple- sive steps in a loading sequence.

ments Genel su ignt Cap ity mrgin t7 rGenerally it has been industry practice to ment General Design Criterion 17.

specify a maximum voltage reduction of 15 B. Discussion percent when starting large motors from large capacity power systems and a 25 to 30 percent A diesel generator set selected for use as a voltage reduction when starting these motors standby power supply should have the capabil- from limited capacity power sources such as ity to (1) start and accelerate a number of, diesel generator sets. Large induction motors large motor loads in rapid succession, and be supplied with nominal voltage can achieve able to sustain the loss of any such load, and rated speed in less than 5 seconds when pow

(2) supply continuously the sum of the loads ered from adequately sized diesel generator sets needed to be powered at any one time. This which are capable of restoring the voltage to guide provides an acceptable way of assuring

90 percent of nominal in about 1 second.

these objectives are met. The considerations Protection of the diesel generator set from involved in the need for the diesel generator to excessive overspeed, which can result from a start and achieve rated conditions in a short loss of load, is afforded by the provision of a period of time are evaluated on an individual diesel generator set trip, usually set at 115 per case basis.

cent of nominal speed.

A knowledge of the characteristics of each A problem arises in assessing whether the load is essential in establishing the bases for goal of continuously supplying the sum of the the selection of a diesel generator set that is needed loads is achieved with sufficient capacity able to accelerate large loads in rapid succes- and margin, because of the various interpreta sion. The majority of the emergency loads are tions of load ratings quoted by diesel generator large induction motors. This type of motor suppliers. The load ratings represent the loads draws, at full voltage, a starting current five at which the set can operate continuously with to ten times its rated load current. The sudden, a high availability, if various specified mainte

9.1

nance programs are followed.

The nominal rating, used as a datum for the overload rat ings, has been termed variously the "continu ous," "guaranteed," or "long term" rating. The definition used throughout this guide for "con tinuous rating" is "that load for which the sup plier guarantees continuous operation at a high availability (expected to be about 95%) with an annual maintenance interval". The over load ratings are similarly defined except that the specified maintenance intervals are shorter.

For example, the following are the load ratings of a typical diesel generator set:

Ratings Continuous 2500 kW

Overload

2850 kW

2950 kW

3050 kW

Maintenance Interval Annual (8760 hr)

2000 hr

7 day

30 min If the power output is increased into the over load ratings, wearout is accelerated and the maintenance interval needed to assure high reliability is reduced. This discussion assumes that the diesel generator set is utilized solely as a standby power supply and that it does not serve a secondary function such as power gen eration for peak demand periods of a transmis sion network. The secondary functions, since they would affect wearout and availability of the diesel generator set, will be evaluated on an individual case basis. If found acceptable, the total amount of operation between mainte nance intervals will be limited by the technical specifications. This guide covers diesel gen erator sets used solely as a standby power sup ply which is the design most widely adopted.

The tabulation illustrates the sensitivity of the deterioration rate to increases in load above the continuous rating. For example, if the design basis loading were that corresponding to the 2000-hour rating, an error of only 8 per cent in estimating the loads could result in operation at the 30-minute rating. Although operation at the 30-minute rating would not be expected to stall the engine, such operation could lead to the danger of early failure.

The uncertainties inherent in estimates of safety loads at the construction permit stage of design are of such magnitude that it is pru dent to provide a substantial margin in the selection of the diesel generator set load capa bility. This margin can be provided by esti- mating the loads conservatively and by select ing the continuous rating of the diesel genera tor set so that it exceeds the sum of the loads needed at any one time. A more accurate esti mate of safety loads is possible during the operating license stage of review due to the completion of the detailed designs and the availability of preoperational test data. This permits the consideration of a somewhat less conservative approach, such as operation with safety loads within the 2000 hour0.0231 days <br />0.556 hours <br />0.00331 weeks <br />7.61e-4 months <br /> overload rating of the diesel generator set. A conserva tive estimate of safety loads based on design or measurements taken during preoperational testing of engineered safety features does not, however, represent with certainty the actual loads experienced under accident conditions.

Therefore, an adequate margin is still essential.

C. Regulatory Position

1. At a time when the characteristics of loads are not accurately known, such as during the construction permit stage of design, each diesel generator set on a standby (onsite) power supply should be selected to have a continuous load rating equal to or greater than the sum of the conservatively estimated loads needed to be powered at any one time. In the absence of fully substan tiated performance characteristics for mechanical equipment such as pumps, the electric motor drive ratings should be calculated using conservative esti mates of these characteristics. (For example, pump run-out conditions and motor efficiencies of 90% or less.)

2. At the operating license stage of re view, the predicted loads should not exceed the smaller of the 2000-hour rating, or 90 percent of the 30-minute rating of the set.

3. During preoperational testing, the pre dicted loads should be verified by tests.

4. Each diesel generator set should be ca pable of starting and accelerating to rated speed, in the required sequence, all the needed engineered safety fea ture and emergency shutdown loads. At no time during the loading sequence should the frequency and voltage de-

9.2

crease to less than 95 percent of nomi nal and 75 percent of nominal, respec tively. During recovery from transients caused by step load increases or result ing from the disconnection of the larg est single load, the speed of the diesel generator set should not exceed 75 per cent of the difference between nominal speed and the overspeed trip set point or 115 percent of nominal, whichever is lower. Voltage should be restored to within 10 percent of nominal and fre quency should be restored to within 2 percent of nominal in less than 40 per cent of each load sequence time inter val.

5. The suitability of each diesel generator set of the standby power supply should be confirmed by prototype qualification test data and preoperational tests.

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