ML19221A840
| ML19221A840 | |
| Person / Time | |
|---|---|
| Issue date: | 12/31/1972 |
| From: | NRC OFFICE OF STANDARDS DEVELOPMENT |
| To: | |
| References | |
| REGGD-01.006, REGGD-1.006, NUDOCS 7907100064 | |
| Download: ML19221A840 (2) | |
Text
SAFEIT GUIDE 6 INDEPENDENCE BETWEEN REDUNDANT STANDBY (ONSITE)
POWER SOURCES AND BETWEEN THElR DISTRIBUT10N SYSTEMS A. Introduction group is never automatically interconnected under accident conditions with the standby General Design Criterion 17 requires that p wer s urce f a redundant counterpart.
onsite electrical power systems have sufficient There can also be compromises of independ-independence to perform their safety functions ence resulting from automatic bus ties (both assuming a single failure. This aafety guide a-c and d-c) which conn (ct the loads of one load desciibes an acceptable degree of independence group to the power source of another m the between redundant standby (onsite) power sources and between their distribution systems' has failed. The slightly improved defense This guide does not address the suitability of against random failures achieved by these bus nearby hydroelectric, nuclear, or fossil units as ties is more than offset by the additional vul-standby power sources at multiple-unit sites.
nerability to common mode failures which they This matter will be evaluated on an individual create.
case basis.
A special case of the foregoing is the bus that is automatically transferred to one or the other H. Defit.itions of two redundant standby power sources; this Prr/rrred Forcer System: The offsite external is e mmonly referred to as a swing bus. This commercial power system.
arrangement also compromises the independ-
'"*'*I Sta ndby
'cer System: Those onsite power groups while addinglittle to the defenst against 9
sources and their distribution equipment random single failures.
provided to energize devices essential to The inclusion of a swing bus in an otherwise safety and capable of operation independ-well designed system often results from an in-ently of the preferred power system.
compatibility between the number of standby Standby Poicer Source: An electrical generat-power sources (whether a-c or d-c) and the ing unit and all necessary auxiliaries, number of redundant load groups. For example.
usually a diesel generator set, which is part an engineered safety feature system design of the standby power system.
which depends on the operation of at least two Lead Grcap: An arr Angement of buses, trans-
- o. tnree elec,ricady drisen pumps and which formers, switching equipment, loads, etc.,
derives powe.- Irom either of two redundant fed from the same power source.
standby power sources must provide for the swinging of one of the three pump motors in C. Discussion crder to meet the single failure criter:on. A c,mpatible design, such as one based on three There is evidence based on operating experi-rower sources, would not utilize the swing fea-ace and analytical considerations that the par-ture.
allel operation of standby power sources renders The necessity for a swing bus can also result them vulnerable to common mode failures. Cur-from an incompatibility between the a.c and rent designs are therefore based on the concept c.c power sources themselves. An example of independent, redundant load groups. In these would be a three diesel ger.erator, three bus desigr.s. the *tandby power source for one load system utilizing d.c control circuits. If only t-0,1 b
125 OM mu cow { '
d-c sou rces are provided, the switching of
- 4. When operating from the st a ndby diesel generator control circuits between the d c sources, redundant lo.ul groups and ihe sources becomes necessary in order to provide redundant standby sources should be the necessary redandancy. Again, a compatible independent of each other at least to the de sign such as one based on three d-c sources, following extent:
one for each generator, would not utilize a
- a. The st: ndby sou rce of one load swing bus.
group should not be automatically A diesel generator that swings twtween the paralleled with the standby. source load groups of ditTerent units at a multiple unit of another load group under acci-site is not an example of the foregoing since dent conditions; such load groups are r redundant to each
- b. No provisions should exist for auto-ther.
matically connecting one load group to another load group; D. Regulatory Position
- c. No provisions should exist for auto-
- 1. The electrically powered safety loads atically transferring loads be-(a-c and d-c) should be separated into tween redundant power sources; redundant load groups such that loss of any one group will not prevent the
- d. If means exist for manually con-minimum safety functions from being necting redundant load groups to-performed.
gether, at least one interlock should 2.
Each a-c load group should have a con-be provided to prevent an operator nection to the preferred (offsite) power error that w ould parallel their source and to a standby (onsite) power standby power aources.
source (usually a single diesel genera-
- 5. A single generator driven by a single tor). The standby power source should prime m ver is acceptable as the have no automatic connection to any standby power source for each a-c load other redundant load group. At mul-gr up of the size and characteristics tiple nuclear unit sites, the standby typical of recent applications. If other power source for one load group may arrangements such as multiple diesel have an automaQ connection to a load group of a different unit. A preferred generators operated in parallel or mul-power source bus, however, may serve tiple prime movers driving a single redundant load groups.
generator are proposed, the applicant
- 3. Each d-c load group should be energized should demonstrate that the proposed by a battery and battery charger. The arrangement has an equivalent reliabil-battery-charger combination should ity. Cor.imon mode failures as well as have no automatic connection to any randor i sing' 'ailures should be con-other redundant d-c load group.
sidered in the analysis.
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