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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 =  
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| 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 November 1978 U.S. NUCLEAR REGULATORY
{{#Wiki_filter:"-
COMMISSION
38/10/71 SAFETY GUIDE 9 SELECTION OF DIESEL GENERATOR SET CAPACITY FOR
k REGULATORY
STANDBY POWER SUPPLIES
GUIDE OFFICE OF STANDARDS
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.
DEVELOPMENT
REGULATORY
GUIDE 1.9 SELECTION, DESIGN, AND QUALIFICATION
OF DIESEL-GENERATOR
UNITS USED AS ONSITE ELECTRIC POWER SYSTEMS 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 that the onsite electric power system have such loads and maintain an, Irequency sufficient capacity and capability to ensure within acceptable limit " supply power that (1) specified acceptable fuel design limits continuously to the i V~eded 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 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 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 0 ot .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
ments Genel su ignt Cap ity mrgin t7 rGenerally it has been industry practice to ment General Design Criterion 17.
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 onsi V 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,"'
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
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 es in the margin.'Copies e the Institute of Electrical and Electronics In .. n ited Engineering Center, 345 East 47th Street, ork, New York 10017.A knowledge of the characteristics of each load is essential in establishing the bases for USNRC REGULATORY
(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
GUIDES Regulatney Guides re Issued to descrbe and rnake available to the public methods acceptable to the NRC staff of specific parts of the commlssiora's regultionts, to delinestedvt sdb the staff In avalu-atg spec probles or postulated accidents.
90 percent of nominal in about 1 second.


or to provide guidance to applicants.
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.


Regulatory Guides ae not sutbstitutes for regulations, and corn-plence with them Is not requked. Methods end solutions different from those set out In the guides wil b acceptable If they provide a b"si for the findings re=ui to the issgirce or continuance of a pewrh or Bse e by the Comments and suggestions for Imroernemeis In tees gudes are encouraged at Imes, end guides wil e rwl sd, be aspramopas, to accommodate corn-mene= and to reflect new rmt or Howeve. corment t, this gtuld. If eeed wlfhn about two mond afte Its Issuance, wil be wfttleft useful In awluating the need for an eauly r .evilon.Comments should be sMnt to the Secrety of the Conmi;slon, U.S. Nudcer Regulatory Commurorn , Wastington, D.C. 26. Attention:
cent of nominal speed.
Dockasth and Sevice Branch.The guides are Issued In the following ten broad divisioin:
1. Power React" 2. Reseerch end Teat Reactors 3. Fuels and Materiab Factites 4. Environmental and Sitng rL Matmerilsand Plant protection B. Products


===7. Transportation===
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
& Occupational Health 9. Antitrust and Financial Review 10. General Requests for single copies of Isued guides Iwit-h may be reproducedl or for placerment on en autonlatic:
9.1
distribution list for single copies of fuaum guldes In s divslons should be ,de In wrtltng to the US. NudIsr Reguatory Cornenssion, Washingion, D.C. I, Attentio:
Okector, D,4lon of Technical Informaiwon and Document Control.


the selection of a diesel-generator unit that is able to accept large loads in rapid succession.
nance programs are followed.


The majority of the emergency loads are large induction motors. This type of motor draws, at full voltage, a starting current five to ten times its rated load current. The sudden large increases in current drawn from the diesel generator resulting from the startup of induc-tion motors can result in substantial voltage reductions.
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.


The lower voltage could prevent a motor from starting, i.e. , accelerating its load to rated speed in the required time, or cause a running motor to coast down or stall. Other loads might be lost if their contactors drop out. Recovery from the transient caused by starting large motors or from the loss of a large load could cause diesel engine overspeed which, if excessive, might result in a trip of the engine. These same consequences can also result from the cumulative effect of a sequence of more moderate transients if the system is not permitted to recover sufficiently between suc-cessive steps in a loading sequence.Generally it has been industry practice to specify a maximum voltage reduction of 10 to 15 percent when starting large motors from large-capacity power systems and a voltage reduction of 20 to 30 percent when starting these motors from limited-capacity power sources such as diesel-generator units. Large induction motors can achieve rated speed in less than 5 seconds when powered from adequately sized diesel-generator units that are capable of restoring the voltage to 90 percent of nominal in about 1 second.Protection of the diesel-generator unit from excessive overspeed, which can result from a loss of load, is afforded by the immediate oper-ation of a diesel-generator unit trip, usually set at 115 percent of nominal speed. In addi-tion, the generator differential trip must oper-ate immediately in order to prevent substantial damage to the generator.
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.


There are other pro-tective trips provided to protect the diesel-generator units from possible damage or degra-dation. However, these trips could interfere with the successful functioning of the unit when it is most needed, i.e. , during accident conditions.
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.


Experience has shown that there have been numerous occasions when these trips have needlessly shut down diesel-generator units because of spurious operation of a trip circuit. Consequently, it is important that measures be taken to ensure that spurious ac-tuation of these other protective trips does not prevent the diesel-generator unit from performing its function.The uncertainties inherent in estimates of safety loads at the construction permit stage of design are sometimes of such magnitude that it is prudent to provide a substantial margin in selecting the load capabilities of the diesel-generator unit. This margin can be provided by estimating the loads conservatively and selecting the continuous rating of the diesel-generator unit so that it exceeds the sum of the loads needed at any one time. A more accu-rate estimate of safety loads is possihle duringl the operating license stage of review because detailed designs have been completed and preoperational test data are available.
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.


This permits the consideration of a somewhat less conservative approach, such as operation with safety loads within the short-time rating of the diesel-generator unit.C. REGULATORY
Therefore, an adequate margin is still essential.
POSITION Conformance with the requirements of IEEE Std 387-1977, "IEEE Standard Criteria for Diesel-Generator Units Applied as Standby Power Supplies for Nuclear Power Generating Stations," dated June 17, 1977, is acceptable for meeting the requirements of the principal design criteria and qualification testing of diesel-generator units used as onsite electric.power systems for nuclear power plants subject to the following:
1. When the characteristics of loads are not accurately known, such as during the con-struction permit stage of design, each diesel-generator unit of an onsite power supply system should be selected to have a continuous load rating (as defined in Section 3.7.1 of IEEE Std 387-1977)
equal to or greater than the sum of the conservatively estimated loads needed to be powered by that unit at any one time. In the absence of fully substantiated performance characteristics for mechanical equipment such as pumps, the electric motor drive ratings should be calculated using conservative esti-mates of these characteristics, e.g., pump runout conditions and motor efficiencies of 90%or less.2. At the operating license stage of review, the predicted loads should not exceed the short-time rating (as defined in Section 3.7.2 of IEEE Std 387-1977)
of the diesel-generator unit.3. During preoperational testing, the pre-dicted loads should be verified by tests.4. In Section 5.1.1, "General," of IEEE Std 387-1977, the requirements of IEEE Std 308-1974 should be used subject to the regulatory position of Regulatory Guide 1.32.5. Section 5.1.2, "Mechanical and Electrical Capabilities," of IEEE Std 387-1977 should be supplemented with the following: "Each diesel-generator unit should be capa-ble of starting and 'accelerating to rated speed, in the required sequence, all the needed engineered safety feature and emer-gency shutdown loads. At no time during the loading sequence should the frequency and voltage decrease to less than 95 percent of nominal and 75 percent of nominal, respec-tively"b Frequency should be restored to 1.9-2
* within 2 percent of nominal, and voltage should be restored to within 10 percent of nominal within 60 percent of each load-sequence time interval. (A greater percent-age of the time interval may be used if it can be justified by analysis.


However, the load-sequence time interval should include suffi-cient margin to account for the accuracy and repeatability of the load-sequence timer.)During recovery from transients caused by step load increases or resulting from the disconnection of the largest single load, the speed of the diesel-generator unit should not exceed the nominal speed plus 75 percent of the difference between nominal speed and the overspeed trip setpoint or 115 *percent of nominal, whichever is lowei. Further, the transient following the complete loss of load should not cause the speed of the unit to attain the overspeed trip setpoint." 6. In Section 5.4, "Qualification," of IEEE Std 387-1977, the qualification testing require-ments of IEEE Std 323-1974, "IEEE Standard for Qualifying Class IE Equipment for Nuclear Power Generating Stations,"'
C. Regulatory Position
should be used subject to the regulatory of Regulatory Guide 1.89, "Qualification of C!ass IE Equip-ment for Nuclear Power Plants." 7. Section 5.5, "Design and Application Con-siderations," of IEEE Std 387-1977 should be supplemented with the following: "Diesel-generator units should be designed to be testable during operation of the nuclear power plant as well as while the plant is shut down. The design should include provisions so that the testing of the units will simulate the parameters of operation (outlined in Regulatory Guide 1.108, "Periodic Testing of Diesel-Generator Units Used as Onsite Electric Power Systems at Nuclear Power Plants") that would be expected if actual demand were to be placed on the system."Testability should be considered in the selection and location of instrumentation sen-sors and critical components (e.g., gover-nor, starting system components), and the overall design should include status indica-tion and alarm features.
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.


Instrumentation sensors should be readily accessible and designed so that their inspection and calibra-tion can be verified in place." 8. Section 5.6.2.2, "Automatic Control," of IEEE Std 387-1977 should be supplemented with ,the following:
3. During preoperational testing, the pre dicted loads should be verified by tests.
(3) "With the exception of the engine over-speed trip and the generator differential trip, all diesel-generator protective trips should be either (1) implemented with two or more independent measurements for each trip parameter with coincident logic provisions for trip actuation or (2)automatically bypassed during accident conditions.


The design of the bypass circuitry should satisfy the requirements of IEEE Std 279-1971 at the diesel-generator system level and should include the capability for (I) testing the status and operability of the bypass circuits, (2) alarming in the control room abnormal values of all bypass param-eters, and (3) manually resetting of the trip bypass function. (Capability for automatic reset is not acceptable.)" 9. Section 5.6.3. 1, "Surveillance Systems," or IEEE Std 387-1977 should be supplemented with the following: "In order to facilitate trouble diagnosis, the surveillance system should indicate which of the diesel-generator protective trips is acti-vated first." 10. In Section 6.3, "Type Qualification Test-ing Procedures and Methods," of IEEE Std 387-1977, the requirements of IEEE Std 344-1975,"Recommended Practices for Seismic Qualifica- tion of Class 1E Equipment for Nuclear Power Generating Stations," for seismic analysis or seismic testing by equipment manufacturers should be used subject to the regulatory posi-tion of Regulatory Guide 1.100, "Seismic Quali-fication of Electric Equipment for Nuclear Power Plants." 11. The option indicated by "may" in Sec-tion 6.3.2(5)(c)
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-
of IEEE Std 387-1977 should be treated as a requirement.
9.2


12. Section 6.5, "Site Acceptance Testing," and Section 6.6, "Periodic Testing," of IEEE Std 387-1977 should be supplemented by Regu-latory Guide 1.108.13. Section 4, "Reference Standards," of IEEE Std 387-1977 lists additional applicable IEEE standards.
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.


The specific applicability or acceptability of these referenced standards has been or will be covered separately in other regulatory guides, where appropriate.
5. The suitability of each diesel generator set of the standby power supply should be confirmed by prototype qualification test data and preoperational tests.


==D. IMPLEMENTATION==
9.3
This proposed guide has been released to encourage public participation in its develop-ment. Except in those cases in which an appli-cant proposes an acceptable alternative method for complying with specified portions of the Commission's regulations, the method to be described in the active guide reflecting public comments will be used in the evaluation of applications for construction permits docketed after the implementation date to be specified in the active guide. This implementation date will in no case be earlier than July 1979.If an .applicant wishes to use this draft guide in an application docketed prior to the imple-mentation date, the pertinent portions of the application will be evaluated on the basis of this draft guide.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)
v • d • e


"-

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.

9.3

/


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