ML12305A251

Safety Guide 9, Selection of Diesel Set Capacity for Standby Power Supplies

ML12305A251
Person / Time
Issue date:	08/31/1971
From:	Office of Nuclear Regulatory Research
To:
References
Safety Guide 9 RG 1.9
Download: ML12305A251 (3)

Text

8/10/71 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 sive steps in a loading sequence.

sets of sufficient capacity and margin to imple-Generally 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 mating the loads conservatively and by select-rating, used as a datum for the overload rat- ing the continuous rating of the diesel genera-ings, has been termed variously the "continu- tor set so that it exceeds the sum of the loadsI ous," "guaranteed," or "long term" rating. The needed at any one time. A more accurate esti-definition used throughout this guide for "con- mate of safety loads is possible during the tinuous rating" is "that load for which the sup- operating license stage of review due to the plier guarantees continuous operation at a high completion of the detailed designs and the availability (expected to be about 95%) with availability of preoperational test data. This an annual maintenance interval". The over- permits the consideration of a somewhat less load ratings are similarly defined except that conservative approach, such as operation with the specified maintenance intervals are shorter. safety loads within the 2000 hour0.0231 days <br />0.556 hours <br />0.00331 weeks <br />7.61e-4 months <br /> overload For example, the following are the load ratings rating of the diesel generator set. A conserva-of a typical diesel generator set: tive estimate of safety loads based on design or measurements taken during preoperational Ratings Maintenance Interval testing of engineered safety features does not, Continuous 2500 kW Annual (8760 hr) however, represent with certainty the actual Overload 2850 kW 2000 hr loads experienced under accident conditions.

2950 kW 7 day 3050 kW 30 min Therefore, an adequate margin is still essential.

If the power output is increased into the over- C. Regulatory Position load ratings, wearout is accelerated and the maintenance interval needed to assure high 1. At a time when the characteristics of reliability is reduced. This discussion assumes loads are not accurately known, such as that the diesel generator set is utilized solely during the construction permit stage of as a standby power supply and that it does not design, each diesel generator set on a serve a secondary function such as power gen- standby (onsite) power supply should eration for peak demand periods of a transmis- be selected to have a continuous load sion network. The secondary functions, since rating equal to or greater than the they would affect wearout and availability of sum of the conservatively estimated loads needed to be powered at any one the diesel generator set, will be evaluated on an individual case basis. If found acceptable, time. In the absence of fully substan-the total amount of operation between mainte- tiated performance characteristics for mechanical equipment such as pumps, nance intervals will be limited by the technical specifications. This guide covers diesel gen- the electric motor drive ratings should erator sets used solely as a standby power sup- be calculated using conservative esti-mates of these characteristics. (For ply which is the design most widely adopted.

example, pump run-out conditions and The tabulation illustrates the sensitivity of motor efficiencies of 90% or less.)

the deterioration rate to increases in load above the continuous rating. For example, if the 2. At the operating license stage of re-design basis loading were that corresponding view, the predicted loads should not to the 2000-hour rating, an error of only 8 per- exceed the smaller of the 2000-hour cent in estimating the loads could result in rating, or 90 percent of the 30-minute rating of the set.

operation at the 30-minute rating. Although operation at the 30-minute rating would not 3. During preoperational testing, the pre-be expected to stall the engine, such operation dicted loads should be verified by tests.

could lead to the danger of early failure. 4. Each diesel generator set should be ca-The uncertainties inherent in estimates of pable of starting and accelerating to safety loads at the construction permit stage rated speed, in the required sequence, of design are of such magnitude that it is pru- all the needed engineered safety fea-dent to provide a substantial margin in the ture and emergency shutdown loads. At selection of the diesel generator set load capa- no time during the loading sequence bility. This margin can be provided by esti- should the frequency and voltage de-9.2

crease to less than 95 percent of nomi- lower. Voltage should be restored to nal and 75 percent of nominal, respec- within 10 percent of nominal and fre-tively. During recovery from transients quency should be restored to within 2 caused by step load increases or result- percent of nominal in less than 40 per-ing from the disconnection of the larg- cent of each load sequence time inter-est single load, the speed of the diesel val.

generator set should not exceed 75 per- 5. The suitability of each diesel generator cent of the difference between nominal set of the standby power supply should speed and the overspeed trip set point be confirmed by prototype qualification or 115 percent of nominal, whichever is test data and preoperational tests.

9.3