ML17305B066
| ML17305B066 | |
| Person / Time | |
|---|---|
| Site: | Palo Verde  |
| Issue date: | 06/30/1990 |
| From: | Mazumdar S NRC OFFICE FOR ANALYSIS & EVALUATION OF OPERATIONAL DATA (AEOD) |
| To: | |
| Shared Package | |
| ML17305B065 | List: |
| References | |
| TASK-AE, TASK-E90-05, TASK-E90-5 AEOD-E90-05, AEOD-E90-5, NUDOCS 9009270170 | |
| Download: ML17305B066 (30) | |
Text
AEOD/E90<<05 ENGINEERING EVAL'UATION REPORT OPERATIONAL EXPERIENCE ON BUS TRANSFER June 1990 P.epared.
by:, Subinoy Nazumdar Office for Analysis and Evaluation of Operation Data U.S. Nuclear Regulatory Commission
,9009270170 900912
, 'PDR ADOCK 05000528l p
pwu I,'~,
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iI t
'P
INDEX Summary l.
Introduction 2.
Description of Hillstone 3 Event 3.
Search for Similar Events and Industry Actions 4.
Basic Transfer Schemes Page.No.
5.
Classification of Auxiliary Distribution Systems in Nuclear Plants 14 6.
Consequences of Bus Transfer Fai1ures 7.
Findings 8.
Conclusion 9.
References 15 18 19 20 10.
Diagrams
,22
4l ji P4 P
SURiHARY Analysis by Northeast Utilities established on November 18, 1988, that at Millstone Unit 3, under certain scenarios, the existing 4160V bus transfer practice can result in a common mode failure of Class 1E loads of both trains.
On November 27, 1989, another analysis by the licensee concluded that repeated operation of 'the existing 4160V bus transfer could potentially damage safety-related motors.
8oth analyses were reported in licensee event reports (LERs) for Hillstone 3; On further study of bus transfer failures at other utilities, we have found that though there is no evidence of equipment failures that could be directly attributed to bus transfer, there have been at least 56 LERs issued between 1985 and 1989, reporting failures of bus transfer to, take place.
An unsuc-cessful bus transfer following a nuclear unit trip, has the potential to lead to either a full or a partial loss of offsite power to the station auxiliary electric. system.
A reliable bus transfer scheme is required to meet the intent of general design criteria (GOC) 17 (10 CFR 50, Appendix A) to "...minimize the probability, of losing electric power from any of the;remaining supplies as a
result of, or coincident with, the 'loss of power generated by the nuclear unit, the loss of power from the transmission network,, or the loss of power from the onsite power supplies."
Our detailed study on bus transfer schemes indicates that:
I)
Though the industry standard organizations, American National Standard Institute (ANSI), National Electrical Manufacturers Asso-ciation (NEMA), Institute of Electrical and Electronic Engineers
( IEEE). and Electric Power Reserach Institute (EPRI), have been working for over 7 years to establish a guideline on safe bus
- transfer, they have not come to any decision as yet, and i't may be a few years be'fore appropriate standards are issued.
'i 2)
Host of the licensees have not updated their bus transfer schemes with the state-of-the-art development.
In most of these plants, with suitable improvements in the existing bus transfer schemes, it may be possible to improve their reliability significantly.
3)
It is desirable that relevant parts of this study and subsequent tindings be communicated to the utility companies and the organi-zations trying to establish the guidelines for safe bus transfer.
1.
INTRODUCTION.
At a nuclear plant most electrical buses are provided with power feed fr'om multiple sources with provision for manual or automatic power transfer from one power source to another power source to provide maximum availability of power.
This study covers power transfer in nuclear. plant medium voltage auxiliary distribution systems between 2kV and 15kV.
This study was initiated b
.our r vi y.
eview -of.,tw'q repOrts Submitted by Hor'thgast 1'nd 2) relating to,bu~ tlraiisfer,problems at M
- ni
. owing that review;,we identif'ied s'imilar problems at other operating, nuclear plarits.
Me noted wide vIariations in the medium volta e~ bus transfer'cheme Irised iin.difte'rent plants. 'his rom t d, i'd th' d
. o th,'tat of-th "
s a
e o t e-art of bus transfer-schemes.,
Me have 'also various indu. try organizat e sa ient featyres of our investigation is recorded in the report.
2.
DESCRIPTION. OF MILLSTONE 3 EVENT The attached Figure 1-taken from Mi,llstone,3 LEft 50-422/88-.026-03 is a siiii-lified diagram:of the, ielectrical distr'ibution system at Millstone 3.
The Unit 3 main'enerator is provided with a paiin generatpr breaker between th generator'nd its two,step-up main transformers connected in parallel;.,
Ttie'igh voltage side of'hese ii~aih trainsformers:pre, connected:to the 345kY switchyard in a breaker-and;half-scheme, through breakers
.156-13T-2 and 15G-14T-2.
The main generator'rovides normal power,to the 4160V plant auxi liary buses 34A'nd 348 through the normal lstqti'on service transformer (NSST)
A, and to the 6900Y plant auxiliary..buse's.35A35B, 35C',
and 350 throu h
HSST B.
The 4160V safety related (vital)'uses 34C and 34L~ ar
' ni,rmally fed uses 34C and.34D are provided with 'al(ernai'.e power from the 345kY switchyard through the reserve station service transformer (RSST')
A', and the 6900Y
'buses 3SA,,35B 35C,,
and 350 through
$1SST E).
On November 18, 1988, an engineering analysis by the licensee disci>vera~>
ti>a with the existing
- scheme, certain faults in thee 345kY system could trip o n
the 345kY breakers 156-13T-2 and 156-14T-Z-,
and thereby isolate the main, b
k I
thi generator from the 345kV switchyard without 'tripping th n
his.scenario, witlh the main.generator discc~nnected fr'om the rid but still connecteci to the HSSTs, the turbine could trip due.to a -power, mismatch or turbines'overspee'd.
-Following the turbine trip, the turbine-,
generator would.co'ast-down and the. generator voltage and,f 1
-'ontinue to.,decrea'se.
an, requency woul d-'
licensee compute'r s.imulation indicated that. the voltage of th'e 4160Y bruises would'decay to 3220 volts, and the "frequincy iso 40 hertz, ppproximatel 86 ie un ervo t:age relays on seconds after the turbine trip.
At '3220 volts tt d
uses, 34A and 348 would iiperate wh'itch in turn would trip all motor loads off buses 34A.and 348.
After a time delay of 2 seconds,,
the. su ly breakers from HSST A to bus 34A and 34B wo'uld trip open.
Th t tl normal. supply breakers from HSST A to buse~
34A a~d 74B would lniptpiatge auto matic fast transfer,, of,buses 34C and 340 to RSSf A.
This-fast tran f t'a about 6 cycles.
Thus, prior to the transfer, the voltage, at vital: buses 34C and 340 would be be'low 3220Y/40Hz.
A fast tran<fe~r
$o RSST A at 4160Y/60Hi would cause a trans, tent that cou"id damage the lqadS Corinected to vital,bu es, 34C and 34D.
This Iis a potential connion. mode failure. mechanism aff ti b.th y'-
equipment (see'ection 6'Consequences of Bus Transfer
's ures'-'n damages caused.by bus transfer transients).
3 Another licensee
- analysis, LER 50-422/89-030-00, indicates under the worst-case condition such a fast bus transfer can produce per unit resultant voltage as high as 1.85 per unit volts per hertz, while the ANSI standard C50.41-1982,
" olyphase Induction Motors for Power Generating Stations" recommends that to avoid damages to connected loads, the resultant voltage should be limited to 1.33 per unit volts per hertz.
As a solution to the issue, on, June 22, 1989, the licensee modified their bus fas transfer control circuit to eliminate fast bus transfer on undervolt Th t bus transfer would stroll function as originally designed whenever the
~
~
supply breakers from NSST to buses 34A and 34B open automatically for reasons other than undervoltage (i.e. overcurrent, current differential, etc.).
After this modification, an undervoltage on the non-vital buses 34A and 34B will result in a slow transfer in which the vital to non-vital bus tie breakers will open and the supply breakers from RSST A to 34C and 34D will close on bus undervoltage of 27 percent.
In addition, the control circuits of the 345kV switchyard breakers 15G-13T-2 and 15G-14T-2 have been modified to ensure that whenever both these breakers are open, the main generator output breaker and the supply breakers from the NSST to the 4160V and 6900V auxiliary systems will RSST.
trip open resulting in the fast transfer of the 4160V and 6900V bus t
th On October 26, 1989, we visited Millstone 3 to discuss with Northeast Utility engineers the studies they have made to arrive at the adequacy of their proposed changes.
As a result of this visit, the licensee has initiated an in-depth investigation of the following:
I)
Stuay of the Unit I and Unit 2 bus transfer schemes to ensure they do not have any similar deficiency.
2)
Study of the suitability of adding sync-check relays to prevent, fast bus transfer on out-of-phase switching.
3) 3.
Stuay of alternative schemes in lieu of automatic bus transfer.
SEARCH FOR SIMILAR EVENTS AND INDUSTRY ACTIONS In July 1989, we requested Oak Ridge National Lab (URNL) to search for LERs on failure of medium voltage bus transfer.
A direct search did not identify any LERs.
ORNL then tried indirect methods which identified 183 LERs.
Me also obtained all LERs related to bus transfer that were screened by Reactor Operations Analysis Branch (ROAB) in 1989.
After reviewing all these
- LERs, we identified 56 LERs on medium voltage bus transfer failures in the U.S. nuclear plants between 1985 and 1989.
These LERs, with a brief description of the root causes for failure of the bus transfers, are listed in Table 1.
Because of the limitations of the search procedure, this list does not cover all bus transfer fai lures that have occurred in U.S. nuclear plants during this period.
It can be only used as a broad guide to identify the nature of bus transfer fai1ures and cannot be used to establish the total number of bus transfer failures.
The search did not, identify any event that directly caused equipm'ent
- damage, either. from.failure of the bus 'tr'ansfe'r-"to take )lac'e'or from stresses, caused
,by transients'uei to. the bus transfer.
These 56 LERs.record the -events in which successful bus transfer,was iriihilbitedi. About'alf, of tlhese f'ailures cain be:, attributed to the;desicjn. deficiency of the ibu4 t'ransfer schemes,, -with,15
'failure's-due to'bsence, of a.fast,"bus, transf'er'idhe~,
On 12'c'casions tlhe
'bus tr'ansfer fai*l'ed because of technicia~ ebrolrs'or'oor maintenance.
On fou'r occasions, defective circuits.,'prevenited bus'tr'an~fe'r a'irid'n five occasions, the sync-check relay settings w'ere inadequate.
I'n December 1989,. we visited Arizona Public'Seirvice'.(APS),. Phoenix,, Arizona to study.their bus transfer schiemia.
Their scheme is similar to the basiic scheme 2 explained in Section 4, "Basic. Bus Transfer. Schemes",.
In this scheme, the four 13.8 kY balance of-the. plant buses are-provided with automatic and residual voltage bus tran'sfer;superv'ised by. static syriic-icheck relapses.
'Fhe two 4;16 kY-Class 1E buses:are fied from the 525'V switichyard through two
,525:kV-.13.8 kY startup trainsformers and.13.8 kV-4.16kV ESF service transfo>rmers with provision. for'anual bus tra'nsfer only..
Arizona Power Services.has conducted. detailed computer simulation study.to airriye at optimum settiriig df
'yn'c-,ch'ec k..r e,l ays.
On January 28, 198i6, Robinson 2 experienced a failure of bus transfer du'e.to DC'aturation of cur'rent tiransforme'rs.
This. subject,has been covered i'nI Information Notice No. 86-87,,
".Loss of Offsite Paiwer Upoi> An Automatic.Bus, Transfer",
and Engineering Eiialuation Report. E703, "Loss of Offsite Power l)ue to Unneeded. Actuation of Startup-'Transformer Pr'otectian Differehtia1 Relay'.
To keep abreast. of the current knowledge and efforts. by the electric pow'er'ndustry on the issue of bus transfer;, we esItabli'sh&d corrmunicat'ion withl IIEE, EPRI, Southern Comj>any Services, Generail Electric, and Beckwith Electric'.
EPRI is;actively studying various. problems a'ssbciathd With bus transfer for'h'e power industry,.
In 1986 EPRI pulblished a report, EL428i6, on Bus Transfer" Stuaies.(Reference 8).
EPRI is currient;ly p'ursliing 6 s'tudy (PR2626-1). to establish the, safe prec'losure voltage f'r buls transfer sch'emes.
On Nay, 16
- 1990, EPRI made a Ipresentation on thie pirogress 'they',haVe'made
.thus'- far.
However;- work under this project may. be delayed beca'use cif shortage. of funds and EPRI,would. welcome other'rganizations to Chare th'e 0ost.
GE engineersare also active on this issue. 'hey'4ve conducted an in-depth study of prevalent bus transfer,pr'actices in tijie U.0. under EPRI-sponsored project EL4286 and are working on r'eport PR2626-1.
They have produced a static sync-check relay, SLJ12A, which is presently bkinf:,used by some utilities.
Beckwith Electric has a1Iso developed-a.static sync-check relay and a bus, transfer.
system which are Ibeing used at oper'a'tin'g'tations..On Decemberi12,
'989, Beckwith made a presentation to the. NRC staff at our Mhite Flint offi'ce on different aspects of bus.transfer sch'emes'.
Me visited Southern Company Services, Birmingham, Alabama, in November, 1989.
They have done substantial work on computer modeling and testing of fast bus transfer
- schemes, and have published their results in four technical papers.
On February 7 and 8, 1990, the author attended the 1990 IEEE Power Engineering Society winter meeting to participate in the IEEE working group meeting. on power plant auxi liary systems.
'At this meeting, the four technical papers by Southern Company Services (Reference 3 through 6) were also presented.
Reference 7 reports on the tests and computer simulation study that Consumer Power Company, Jackson, Michigan has conducted on fast bus transfer on a 400 megawatt fossil-fired plant.
Their study demonstrates that in this particular plant both with normal load and shutdown load, the actual bus transfer time was well within the time permitted -by the ANSI C50.41-1982 resultant voltage requirement of 1.33 per unit volts per hertz.
We have discussed the ANSI 1.33 per unit resultant voltage issue with Dr. R. H.
Daugherty of Reliance Electric.
In 1982, Dr., Daugherty first questioned (Reference
- 11) the resultant voltage of 1.33 per unit volts per hertz specified in ANSI C50.41 (Reference 12).
- However, he indicated he had not zeroed in on any particular solution to the problem.
We have also requested nuclear agencies in Mest Germany,
- France, England,
- Sweden, and Canada to exchange their experience with us.
Me have received responses from the Central Electricity Generating Board (CEGB), U.K. and the Swedish.Nuclear Power Inspectorate (Reference 16, 17).
Me are awaiting responses from other countries.
Our in-depth review of these LERs, the single line diagrams of the plants, the ous transfer
- schemes, and'iscussions with some of these licensees indicated:
1)
Wide variation in the medium voltage distribution, system and the bus transfer schemes used in different plants.
A generalized description of these different schemes has been presented in Section 5, "Classification of Auxiliary Distribution System in Nuclear Plants".
2)
While some of the licensees have done in'-depth studies on.the problems associated with bus transfer
- schemes, many of the licensees are not fully knowledgeable of the problems and the state-of-the-art know-how of bus transfer schemes.
3)
Though the industry standard, organizations, (ANSI, NEHA, IEEE, and EPRI),
are aware of the consequences of bus transfer.failures and have conducted significant studies on it, they have not reached any final decision on the subject.
TABLE 1.
FAILURES OF. MEDIUM VOLTAGE BUS TRANSFERS BETWEIEN 1985 and 1989 IN U. S.
NUCLEAR PLANTS PLANT Arkansas 1
Arkansas 1
Arkansas 1
SCf!EMIE *'
LER NO.
87-005-00 89-002-00 89-004-00 ROOT CAUSE Loads dropped out clue.to slow bus. tran. fer.
Bus transfer f:ailecI because of system perturbation and wrong setting of sync-check relay.
Bus transfer inhibi'tel by wrong setting of sync-.check relay.
10 Heaver Ya1'1ey 2
2 Brunswick;2 Ca 1 laway 1
Callaway 1,
Ca 1laway 1
Catawba 1
- Cooper, 87-036-00 89-009-00 85-'005-00 85-038-00 88-015-00.
8'9-001-02 87-013-00.
B'us 'transfer inhibited by defective control circuit.
Technician error caused loss o'f offsite power.
Sync-chec k relay inhibited bus transfer following generator fleld failure.
Sync-check relay inhibited bus transfer fo11lowfng generator fleld f'ai lure.
Operator error caused failur'e of power supply to Class 1E bus.
Train A blackout due to defective relay installation.
Cause of failure unlI:no'wn.,
Crystal River 3 2
89-013-'01 Ur>planned load growth caused excessive vo.',Itage drop after bus tran.fer.
See Section 5, Classification of Auxiliary Di. tribution Systems in Nuclear Plants," for descr'iption of the schemes.
12 13 14 Crystal River 3 2
Davis-Besse 1
Diablo Canyon 2
2-Dresden 2
89-023-00 87-'011-00 88-008-00 85-034-00 Technician error and defective relay. caused failure o'f offsite power supply.
Bus transfer failed due to improper breaker trip lever settin'g.
Bus transfer failed due.to poor maintenance and cable layout.
Bus transfer inhibited by defective control circuit.
16 Dresden 3
89-001-01 Bus transfer inhibited by bad breaker auxiliary contacts.
17 18 19 20 21 22 23 Duane Arnold Farley 2
Hatch 1
LaSal le.2 Maine Yankee 1
2
'Millstone 2 Millstone 3 85-010-00 85-010-00 88-018-00 89-007-00 88-006-00 88-011-01 88-026-03 Bus transfer occured 30 seconds after reactor trip causing dropout of
Loads dropped out because of slow bus transfer.
Loss of offsite power after successful bus transfer due to defective relay.
Loads dropped out because of slow bus transfer.
A fault on main transformer caused sufficient dip in grid voltage to inhibit bus transfer.
Bus transfer inhibited by ground fault on vital 4160V bus caused by personnel error.
Study indicates possibility of loss of redundant trains of. safety related equipment on bus transfer.
-- 24 26 Millstone 3 Nine Mil,e Point 2 2 Oconee 1
89-030-00
'88-012-50 89'010-01 Engineering evaluation established repeated bus transfer operation could damage safety related loaids.
Fast bus, transfer inhibited by defective relay circuits.
Bus transfer considerIed inadeiquate because of'ow HV grid voltage.
27'conee 2
2 88-002-00 Susceptibi'lity of bus transfer failure due to HV Ibrea ker ferroreson anc e.
28 Palisades 87-024-00 Inadvertent operation of deluge system caused fault on startup transforme!r which inhibited bu,s tra,nsfer.
29 Palisades 89-015-00 IBus transfer eliiminated by connecting Class 1IE buses to startup transformer during norraal operation to avoid failiire of bus transfer from stuck lbreaker.
30 31 32 33 34 35 Palo Verde 1
Palo Verde 2
Peach, Bottom 2 2
Peach Bottom,2 2
Point Beac:h 2
2 Point Beach 2
2 89-.004-00 89-001-01 86-010-00 B7-012-00
'B5-005-00 B7-002-00 Bus transfer inhibited by
.stuck breakers.
Simultaneous failuI!e of both transformers caused loss of offsite power.
l.oads dropped out lurin$
bus transfer.
I oads dropped out Hurin) bus transfer.
Sync-check relay i!I!hibiiIed bus transfer.
=
Non-Class 1E bus fhiled to'ransf'er due to transient caused b'y lightning strike.
36 37 38 39 Point Beach 2
2 squad Cities 2
2 Rancho Seco River Bend 89-002-00 87-009-00 88-015-00 88-018-04 Loss of offsite power due to low bus voltage after bus transfer.
Slow bus transfer caused load drop-off.
Loads dropped, out during bus transfer.
'Relay logic inhibited bus transfer.
40 H. B. Robinson 2
1 86-005-00 Bus transfer inhibited by CT DC saturation.
41 42 Salem '2 Seabrook 3'6-007-00 89-004-00
,Bus transfer inhibited by low 'bus voltage.
Loads dropped out during bus transfer.
44 45 46 47 48 49 Seabrook Shoreham Shoreham South Texas 2
3 South Texas 2
3 Susquehanna 1
2 Susquehanna 1
2 89-014-00 87-003-00 87-026 89-009-00 89-014-00 85-035 87-007-00 Bus transfer inhibited by low DC battery voltage.
Differential relays for both normal and reserve station service transformers tripped because maintenance personnel left jumper across these relay CTs.
Loads dropped out during slow bus transfer.
A generator trip caused momentary loss of offsite power.
Trip of the 345 kV breaker feeding the main transformer caused loss of offsite. power.
Loads dropped out during bus transfer Loads dropped out during bus transfer.
- 50 Susquehanna 1
2 87-015-00 Loads, dropped out kecau~e of slow bus transfer.
51 52 Susquehanna 1
Susquehan'na 1
2 87-020-00
.Loa'ds dropped out -Hur'in) bus transfer.
88-014-00 Loads dropped,out Hurin) bus transfer:
53 54 Turkey Point 4 2
Vogtle 2-MNP 2 85-004-00 89-023 85-007-00 A flash
~o'ver on thk 240 kV systeII> caused failure of both sources of power supp ly~
Improper CT connection inhibited bus transfe'r.
Relay failure inhibited bus transfer..
56 Yankee Rowe
<18-008-01 Fast bus transfer failed because of defective ciricuit.
4.
BASIC BUS TRANSFER SCHEMES Bus transfer primarily involves transfer of power.supply to a. bus from one power source to another, normally during nuclear. plant startup or shutdown.
The intent of bus transfer schemes is to provide, power to a bus and its con-nected loads, with minimum or no interruption.
In a majority of the nuclear
- plants, during a unit start up,.the unit auxiliary power is manually trans-ferred from the station offsite power, through station service transformer (SST), to the unit main generator, through the unit auxiliary transformer (UAT.
In this transfer, the two sources are first brought to synchronism (i.e., the magnitude and phase angle of the two source voltages are nearly equal) and then connected in parallel for a short duration (few seconds) before disconnecting the offsite power source.
Because of this, the auxiliary equipment connected to the bus is subjected to minimum electrical and mechanical transients during this transfer.
- However, as the two power sources are connected in parallel, the connected equipment are susceptible to excessive fault current interruption beyond the equipment, rating if a fault were to occur during the bus transfer.,
The bus transfer during the unit shutdown can be either in the normal mode or in the emergency mode.
In the. normal shutdown bus transfer operation, the operator manually transfers the auxiliary power from the UAT to the SST usually in the reverse order of. the startup bus transfer.
The emergency mode bus transfer is usually an automatic bus transfer initiated by a loss of the power supply from the UAT.
The UAT power loss normally occurs after a generator trip, a turbine trip, a reactor trip, or a main/unit aux-iliary transformer trip.
The automatic bus transfer can be in the residual
- mode, the in-phase mode, the fast bus transfer mode, or a combination of the three.
In the residual mode,.also called dead bus transfer, the bus.transfer takes place only after the bus,voltage decays below 25 percent of the rated bus voltage.
This is achieved either by sensing bus undervoltage or by using an adequate time. delay in the bus transfer circuit.
The. main draw-back of this scheme is that it is the slowest of the three automatic bus transfer schemes.
Residual transfers are susceptible to load dropoff and may require sequential start of the loads to avoid block start which can cause excessive voltage drop or excessive stress on the station service transformer (Reference 3).
The in-phase bus transfer scheme uses special relays that, ensure that the transfer wi 11 take place when the incoming source is in phase with the residual voltage of the bus.
This scheme has the same merits as the manual parallel transfer during normal shutdown.
However, this transfer scheme is usually slower than the fast transfer scheme (30 cycles as opposed to 6 cycles) and it may not have any practical advantage over the residual transfer scheme.
~ -12
-'he fast bus transfer sch'erne tries to kee
'll, th' c
1 s.
Th
~yc es.
The two main versions of this scheme are t>>e "break-before-make-sche'me" the incomin c e'me, e incoming-source*breaker closes after.the early "b" contact of the out oiri b
c o
e outgoirig breaker.
Ln the simultaneous'ype,'the g and o~tgoi~g ~re~k~rs. are i~itiat~d at. the same source breaker is signalled 'to clo e simultaneousl
).
No e ou going. sougrce b~eaker is signalIled to o en and taneous ty'pe. has. a" shorter:dead band (the tine er
'1'
,d'so tdfo bth o
o o:
o , u'sually between one or two o t'o b
k 1'e
',,o sv'e to 'teri, cycles in s'ue would be connected in para'l.lel'.
This a
is.s ow n opening, or fa<ls t o e t
r e
.~. nuc ear p
nts,- the seque t al type. of. fas Mashington Nuclear Plant,.Unit 2, Donald C. 'Cook nits h
n
)
5'u p
ypo s
t ar none o t ese units have, re oirted an the two s'ources a'e nearly equal (Vp70X de'lta fC
)'
n 11 1
d f th 0
1 d
e an
- second, tihen bi>th closed breakers still. be There are two wo concerns when-the two sources. are conriected,in parallel.
Th first is that su'ch an. operation can cause iristabilit'n'he,:
stresses in;the conne'cted compone'nts.
?n most, cases b
step-up t a sfomers-and'th t
-the same switchyar'd o'r'from switchyards tha a "e s ii n
s at on reserve tran's'fqrmers are su 1
Fur'ther th'e p'rerequisii'iti f
'.e con pn..or. successfuil bus transfer usua
.1
'onitored by sync-:,check 'relays, en~sures that tie tw s
phas a
d q
a1 in'agnit d
(Th
'he outgoing break'er 'is sldw in.opening, or f i.'s o o en);-
Thus vie..
e, two sources can be in ar'allel onl disturbance arid eq'uuipment:stres
'es ex'p i
d d es exper ence during this transfer is minimal.
The second concern; is the occurrence of elec)rica'), faults beyond the designed u y o t e e'quipment and buses when. the two so s
in para e.
e
. we s
esign has allowed onl a 0.1-ce period in the-.transfer scheme during which b th b o
reakers are permitted to The EPRI Project Report EL-.4286- (Reference
- 8) op bus,transfer studies sridicates, that for,,a typical nuclear plait, th
-c i+ical, comparison between,Ithe merits of si lt fo1 lows:
o s mu'aneous and sequential schemes ar'e as TABLE II.
COMPARISON OF CRITICAL PARAMETERS BETWEEN SIMULTANEOUS AND SEQUENTIAL FAST BUS TRANSFER SCHEMES.
PARAMETER (Maximum Value)
Angle in electrical degrees between motor internal voltage and system Resultant volts per hertz between motor internal voltage and system Motor transient torque per unit on motor basis SIMULTANEOUS SCHEME (2
C cle Deadband 32.9
.575 3.46 SEQUENTIAL SCHEME (6
C cle Deadband) 15'.828 9.36 Motor transient current per unit on motor basis 3.22 9.67 This table shows that in sequential fast bus transfer, the connected electrical equipment is subjected to substantial electro-mechanical stresses and can exceed the 1.33 volts per hertz requirement of ANSI C50.41-1982.
On the other
- hand, these stresses are lower in the simultaneous fast bus transfer scheme (about one-third of sequential transfer) and there is less possibility of any equipment damage.
The worst case resultant volts per hertz between the motor internal voltage and the system is only 43 percent of the ANSI C50.41-1982 requirement.
The fast bus transfer schemes can be either supervised type using a sync-check device or unsupervised type.
In the past, the sync-check relays were of the induction disc type which are much slower than the static relays developed by Beckwith Electric and GE in the 1980s.
Before mid-1950, most fossil-fired plants used a version of the dead bus transfer in which during a loss of the normal power source, all loads were first tripped and finally the auxiliary bus was connected to the incoming source and then the auxiliary loads were re-energized in sequence.
In late 1950, with the introduction of stored energy breakers, fast transfer schemes with about six-cycle dead band gained popularity.
In 1977,. ANSI C50.41, "Polyphase Induction Motors for Power Generating Stations," first introduced the requirement for limiting "pre-closure voltage" to 1.33 per unit volts per hertz
, as it was realized that with the six-cycle or longer dead band bus
- transfer, the connected equipment could be damaged from switching transients.
This issue is discussed further in Section 6 and references 3 and 8.
5.
CLASSIFICATION OF AUXILIARYDISTRIBUTION,YSTEMS IN NUCLEAR'LANTS Electric power systems in US nuclear. power. plants are designed and operaiteH to.meet the requirements aif 10 CFR 50, Appendix A, GDC 17.
This
-criteri'on'n'art states:
"Electric power from the transmission network.to the onsite
'lectric distribution system shal i be.uppli'ed by two physica1lly iridependent circuits....
Each of these circuits shall be designed, to be available.in sufficient time following a 'loss of.all onsite alternating-cur'rent power supplies and'he other offsite power circuit, to assure that specified a'cc<'rp-'able fuel design limits a'nd'design conditions of'he reactor coiolant,pres.ure boundary are not exceeded.
One of these circuits, shall be designed to be
.available within a.few secorids following a loss-of-coolant accidient to a'ssirre'hat core cooling, caintainment integrity, and.other vital safety functions arie maintained.
Provisions. shall be included 'to minimize the. probability of'6si'ng
.electric power froim any of the remaining supiplies as
- a. resvlt of, or coi'ncide'nt with, the loss of, power generated by thee"nuclear
'power unit, the lass of'ower from the transmission network, or the 1,'oss of powe'r from'.the onsite electric power supplies."
,To meet.the intent of the 'crite'rion 'oavai,labi lity.wi'thin
,'a few seconds and -minimizing.the: probabil,ity aif 'losing electric 'power on,loss of.
the'gene::ating unit, power system desigins use the birs transfer'cheme however, there is Iwide variation in the medium voltage (between l?kV and 15kV) auxiliary power distribution systems used iii ni>cleai'plants.
Broadly speaking,,
'these diffe'rent schemes can be 'categiorized i'nt6 f'ov'r'asic schemes from 'the bus
'ransfer'nalysis Ipoint of v1iew.
The first scheme shown in Figur'e 2 uses the iautom'atic bud transfer schemIe tjo
'eet the intent of,,
F'lants that, use 'this 'scheme normally supply all plant auxiliaries from the main generator thIro<irghi t4e unit auxiliary-trans-former.
Upon loss,. of the nuclear, power. unit (i.e'., 'the main geherat'or),
electric power i~:supplied to'he auxiliaries frown the'fts$ te preferred,prIweiI'"
source by,the start-up transformer'.
This tran.fer of.power fram the vnit auxiliary to the'tart-up transformer is-accomplished by. the automatic fast bus transfer scheme.,
For,:plants with.this scheme, a failure of'he bus transfer to take p'lace. leads to '1'aiss,-of the preferred offsite power to -the station auxiliarie's.
The second scheme;shown in Figure 3,(reproduce8
'fI'orrI Figure 2.3 of reference: 8)
's the most popular scheme.
The requirements of a minimum of two offs'ite power sources are provided.
In,soke of the plants'<ing t'.his scheme, each Class
~1E~
'bus is normally,fedI from one of the.two offsite sIources with provisiorI for nianual,transfer
.to the ailterrIiate offsite souI'c6; In other plants, all Class 1E loads are fed fr'om one of'he two-off'site power sources, with provision for'utomatic transfer. to the s'econd offsite soukck., IThe non-Class 1E (balaiIrce of the plant) buses are 'usvally fed from the unit gener'ator thr'ough the unit auxiliary transforIner with prov'ision for automatic fast, bus transfer to rirffisite.
~
power source.
'In 'some aithers,.all loads are, fqd by,the unit avxiliary t0an's-
.former during normal ioperati'on, with provision for autI)matic transfer to one of
.the offsite sources upon
- a. unit trip.,
The third scheme shown in Figure 4 (reproduced from Figure 2.4 of reference,8) uses a generator circuit breaker or load break switch.
Browns Ferry 1, 2 8 3, Catawba 1 5 2, McGuire 1 h 2, Millstone 3, North Anna 1, Seabrook, South Texas 1 5 2 and Summer have generator breakers.
In the event of a generator fault or unit trip, the generator breaker trips open and the auxiliary loads are fed without interruption from the offsite source through the main transformer and the unit auxiliary transformer.
In the event of problems with the main or, unit auxiliary transformer, the non-Class 1E buses
- and, and in some designs, the Class lE buses are usually connected to the offsite source through the station service or start-up transformer using automatic bus transfer scheme.
The Swedish nuclear plants also use such a scheme (Reference 16).
The basic advantages of this scheme are:,
1)
Elimination of a second station service transformer with its associated switchgear, and 2)
Elimination of bus transfer operation on unit trip, The fourth scheme does not use any unit auxiliary transformer and relies on two station service transformers for supplying power to auxiliary loads at all
- times, except for emergency operation of Class lE sources from emergency diesel generators when both or one offsite source fails.
This scheme is used at Calvert Cliff 1 5 2, Fermi 2, Grand Gulf, Hope Creek,
- Shoreham, and TMI I.
In this scheme, bus transfer is not required for any unit trip or fault in the unit generator and the main, transformer.
Our evaluation of the 56 events reported in Table I, indicates that Scheme 3
nad five failures, Scheme 4 had two failures and Scheme 2 had the rest of the ta ilures.
Some licensees (e.g., Palisades, Davis-Besse, Millstone 3) have eliminated or plan to eliminate the fast bus transfer of safety-related buses because of the unreliability of the transfer scheme.
- However, such eliminations. should consider the design bases of the original design, especially as they pertain:
to the intent of GDC 17 on availability and reliability of available sources of power to safety-related loads.
The Standard'eview
- Plan, NUREG 0800, Section 8.2,. deals with the review of offsite power system and the interface between the offsite and onsite power systems.
The medium voltage bus transfer scheme used by nuclear plants in the interface between offsite and onsite power
- systems, would rightly fall into this section.
However, the section contains little or no guidance for the review of the various types of bus transfer schemes used by nuclear plant designs
~
6.
CONSEQUENCES OF BUS TRANSFER FAILURES Bus transfer failures can be divided into two classes:
1)
Failures that inhibit bus transfers.
2)
Fai 1ures that can damage the loads connected to the auxiliary buses.
16-The, first failure, category covers cases where thi transfer does, not take place, resulting in loss'; of power to the bus that 'is being transferr'ed from one, source to the other.
'In several nuclear plants. such a'a'ilure leads to. the loess of a preferred'power source to plant auxiliariesi,:Ihidh 'in some cases, include:hd.
Class 1E loads.
HIenice, with this type of bus transfer failure, the Class
.E loads will have t'o rely on the emergency,diiesel'eneratorsand the reatto'r
'coolant system on natural'irculaticin.
Thu;s such failures defeat the main purpose of providing the, bus transfer facility' that of. meeting the inten't 6f
'GDC 17 on availability arid reliability of the Iprkferred 'source.
Although,riuclear 'plants are designeci for safe shutdown o'n
.failure,of offsite power, it, is desirab'le that such fai lu:res ai e kept to a minimum to reduce stress on the nuclear units and to improve the avai'labil'ity of offsite pow0r
'ources.
Every imiprovement in availability of 'offs'ite power source. is ain improvement on station blackout.
Several factors c'a n inhibit a bus transfer.
Our revidw 'of the 56 l ERs docu-menting the failures.of bus transfers has identif'ie'd defective system design, slow. sync-check relay speed (generally the slow electro-mechanical relays),
improper relay'-settings, slow operat:ion of the outgoir>g brc'.aker, bad:auxiliary
- contacts, system undervoltage, and'hiuman errori a0 the main causes.for thesle
'kind of failures.
.The second. class of'ailures are caused by the e)<cessive voltage d'ifference (in magnitude and phase angle) bietween the auxilia'ry'lo'ad.'bus aind the incoming power source.
Mhen the normal power sourceitoi, an auxiliary bus is interrupted, the trapped magnetic flux and t,he induced voltage of the motors start to decay and. the motor starts to s1low down causing voltage magnitude and phaseangle decay.'hus, when the auxiliary loads are connected to the 'incoming 'source, there.can be substantial voltagie difference between the auxiliary bus and 'the
.incoming, source.
Thiis exc.essive resultant voltage wi111 cause. tran. ient current flows. iri the system'which can dlamage the trhnsf'ohneds tlhe buses, and,the connected loads due to stres;ses fromi. electromagnetic,f'orices,.overheating, and
'transient to'rques; 1'hus far', there has. not been any reported equipment failure in,:nuclear plants'.Chait can'bie d'irect:ly attributeci tio this cause, a1lthiougih potentially, some equipiment is stressed in this process.
It 'is to be noted that such stresses experienced by connected equipment are cumulativ'e in 'natur'e
.and uriless specifica1lly moni'tored for, can remain uindetected until failur'e
'occur's.
ANS'I'50.41'"Polyphase Induction Motors '.for Poiwer Gienerating Stations" fir4t addressed this.issue -i'n'1977,.
AN.iI -C50.41-1982, states that. a motor is inherently capable of'ieveloping, transient torque (and'urrent) consideriab'ly in
'xcess of rated'orque when exposed to ari out-~of-phase bus. transfer or momentary voltage interruption.
The magnitude of'his transient torque can range, from.approximatel~p 2 to 20 times rated torque and is a function ofi.the machin'e, operating conditions, switching, times, system inertia, etc; To limit the possibility of damaging the motor or driven equipment, or both, it is recommended that the power supply system be designed so that the resultant vectorial volts per hertz between the motor residual vo,its per hertz and the incoming source volts per hertz at the instant the transfer or reclosing is completed does not exceed 1.33 per unit volts per hertz on the motor rated voltage and frequency bases (see Fig.
5 reproduced from Figure 1 of Reference 12).
NEMA accepted this as a safe criteria in NEMA MG-1-1978.
In 1982, R. H.
Uaugherty challenged this criterion (Reference
- 11) and J. S.
C. Htsui further substantiated the challenge in 1985 and 1986 (Reference 9 and 10).
They established that ANSI C50.41-1982 criterion on resultant voltage limit of 1.33 per unit volts per hertz is unrelated to the motor shaft torque.
As a result of these
- concerns, NEMA MG-1, 1987 has rejected the ANSI C50.41-1982 criterion.
Recently EPRI has initiated a study on the whole issue (RP2626-1).
GE is conducting this study for EPRI, and the results are expected to be published by the middle of 1991.
At present, at most of the nuclear plants, the bus transfer schemes generally meet the criterion of resultant voltage of 1.33 per unit volts per hertz Mith no equipment failure reported, this appears to be a 'safe figure that can be used in bus transfer design and test verification.
However, there is no strong technical basis for selecting this 1.33 value (see Reference 11, discussion section).
An increase in the value of the resultant voltage will permit more successful bus transfers, but increases the susceptibility of connected equipment to damage, while a decrease of this voltage may unduly increase failure of successful bus transfer.
In this respect it is desirable for the NRC to keep close liaison with the EPRI and IEEE working groups active on these issues.
From the review of the LERs and referenced documents, the following findings relate to bus transfer criteria:
1)
A malfunction of the generator excitation system can cause excessive voltage deviation between the generator terminal and the offsite power.
2)
As established in the Millstone LERs 88-026-03 and 89-030-00 under special operating condition, the generator voltage and frequency can be substan-tially different from the offsite source.
3)
In certain plants there may be significant phase shift between the generator voltage and offsite power source, especially when they are not from the same switchyard.
4)
A fault or lightning can cause transient system disturbance in which
- event, a supervised scheme is the best way to ensure optimum safe transfer.
l8-
)
Loads with-.high:moment of inertia are, suitable pot f t b loads with '
as us 'anszer,
- and, (see Refer'e'nce, 3 and I5).
s ua i.ans er with 'low moment of inertia are, Nore, suitable for resid 1 t f'.
These findin s,l'ead to th g
e.conclusion that,a wag to ensure safe transfer under a
operational conditions is.to use the sync-check relay.
.One way, to ensure adequate setting of 'the sync-check relay is to.carry out field tests under wtirst-case loading conditions.
However, it is not always possible to conduct tests under worst-'case loading coiiditions and some
'icensees have= developed computer simulatipn programs to cover such
'con'dikio~ns.~
Even in computer simulation programs, information qn,,equipment parameters, are assumed and can produci~ err'oneous results sf bropel. c'are is not used in'electing
'these parameters;
,7-.
FINDIHGS
.The salient features of this study are:
A.
There is no information on any equipment, failure. that can be attrihuted to ous transfer'ransient.;:.
From this field ekperience, the present ANSI C50.41-1982 -criterioii for safe bus transfer - 1'.33 per unit volts er
- hertz, appears adequate.
'B.
Between 1985 andri 1989, on at least 56 occasions, bus transfer, failed to take place
'on demand'.. It is pos'sible to improve updn th'is bus transfer tailure rate'.by suitab1y modifying the bus transfer scheme.
Thus far, he:issue has 'not ~eceived the.industry.attention it deserves..
C.,
There: is, a: wide,variation in bus transfer,'chemes used by different icensees.
.While some licensees have performed, in-depth studies on', the
'rob'lems,.associated with bus transfe~
schemes,,many are not fully
- knowledgeable of the;problems and state'-of'-t'he'art developments in lbu) transfer schemes.
D.
Although the industry standard brganizations
.(ANSI, NEN, IEEE and p'PRI)'
are no re:working to establish a guideline for safe bus transfer
.th h
)
y decision yet, and it may be a,feg y'ears ',before. appropriate standards are; issued.
It 'is desirable that the NRC maintains, neces."iary communication'; wi th 'these organiiations.'.
Information regarding the different aspects, kf -bu~ transfer,
'such as types of 'designs an'd their limitations and advantages, conformance-to the requirements of GOC 17, safe bus transfer criteria per ANSI C50.41 and the ongoing industry efforts in this area, the use of state-of-the-art as acting.sync-chieck relay,, etc.
need to.be 'fed back to licenseek ot'perating-nuc1ear plants
-'19-F.:. The schemes that eliminate bus transfer on unit trip, for example the schemes with generator breaker (Figure 3), or the schemes in which the auxiliary loads are normally fed from offsite source have much lower p-obabiiity of failure than the conventional scheme Ipigure 2) ~ popular in the USA in which a bus tr'ansfer is initiated at every unit trip.
G.
Though the sequential fast bus transfer scheme is popular in the USA, the simultaneous bus transfer scheme in conjunction with a main-generator circuit breaker is used in. all Swedish nuclear plants and.appears to be a more reliable scheme.
H.
The Standard Review Plan, NUREG 0800, Section 8.2, contains little guidance for the review of medium voltage bus transfer schemes that are used in nuclear plant designs to meet the requirements of GDC 17.
8.
'CONCLUSION Our review of operational experience and of available documents lile the FSARs, could not identify the design details of existing bus transfer
- schemes, or the modifications done by licensees of operating plants in the area of bus transfer.
Me are aware that several licensees have modified their bus transfer schemes following start-up experience.
The details of the as-installed, as-operating bus transfer
- schemes, can be obtained by means of a 10'FR 50.54(3)(f) request from the licensees of operating nuclear plants.
- However, based on our review and lack of reported equipment failures, it is difficult to make a cost-beneficial argument to justify such a request.
Nevertheless, based on the potential. consequences of failures in the bus transfer
- schemes, it would be prudent for licensees to consider improvements to existing schemes.
Though EPRI is currently active in developing a suitable criteria for reliable bus transfer, our survey indicates many of the licensees are not adequately informed on the state-of-the-art in bus transfer.
By suitably improving their existing bus transfer
- schemes, they can reduce the current rate of bus transfer failures and thereby improve the availability of offsite power significantly and reduce challenges to emergency diesel generators.
The NRC should consider coordinating with industry groups that are active in development of suitable criteria and standards for reliable bus transfers.
Revision of the Standard Review Plan should be considered to include adequate guidance for the review of bus transfer schemes in the design of nuclear plant auxiliary electric systems.
The experiences and problems relating to medium voltage bus transfer schemes at operating nuclear plants should be comnunicated to the industry.
.-;:20-9.
1.
2.
3.
4.
5.
-6.
8.
9'.
10".
12.
REFERENCES:
LER 50-423/88-026-03,
"'Potential'amage to 'Safety Related Equipment t)ue To'esign: Inadequacy."',
Report dated tictober 10, 1989.
LER 50-423/89-030-00, "Potential Damage to Safety Related Motors Due to Fast: -Bus'ransfer Design Inadequacy.",
Rapport dated December 26, 1989.
T.- Higgins; M; Snider,
'P.. Young,:H. Holley,, "Bus Transfer
.Assessment and Application.:"-, Paper 9G MH 220'-4EC-,,
IEEE/PES.Minter'eeting; 1990.
T:., Higgins,iP.,Young,,M.
Sni'der,.H. Holley, '"Computer, Mode1ing for, Bus Transfer, Studies.,","Paper 90 MM'221=2;EC.; IEEE/PES Minter-Me'eting,i.1990'.
- Young, W; Snider, T. Higgins, H. Holley,, "-Bus Transfer Testing
@nd Evaluation.",
Paper 90 MH 220.-0EC, IEEE/PES'iintier Heeting, 1990.
H. Holley., T.. Higgins,:P.
- Young, W. Snider, "A.Compiaris'on:of'Induction Hotor Models for Bus Tr'ansfer Studies.'", 'Papei'0 MM 063-8 EC, IEEE/PES Winter Meeting, 1990.
Y; E. Yeager;",Bus Transfer of 'Yiultiple. 1nductioo Motor 'Loads in.a 400 Megawatt Fossi.l Powei P'lant " IEEE Transaction on Energy.,Conversioh,
'o,lume 3;
No., 3,pp 4!B1.-457, September 1988.
J;.C. Appiarius, E. L. Owen, R.,M. McCoy, A..Mur'dock,. "Improved Motors for Utility,Appl'ications, Volume '2::
Bus Tra'nsfer Studies.-"
EPRI EL-.4286
'olume 2, Pr'object 17b3'-2, Finall'Ieport; October 1986.
J.
S.
C. Htsuii, "Nonsimultaneoi~s Reclotsing 'Air-Gap Transie~t Torque, of Induction Motor'. Part -II; Sample. Studies and Discussion on.ReclosirIg of ANSI C5Q.41;"
IEEE Transactions of Pow'i~r'Apparati>s and',Systems, WM 214-1.
'J., S..'C;. Htsui', *"Maghitude, Ampilitude':and'req'uencies: of Induction - l'totor
'Air-Gap, Tr'ansient Torque 1'hrough'imultaneoiIs Reclosing Mith <ir Without Capacitors.",
IEEE'Transactions on,Power Appar'atus and Systems,
'Volum'e PAS'-,104; No. 6, June 1985.
R; M..Daugherty"Analysis of Transient,.-:Electrical Torques, and Slhaft Torque -in'Induction Motor's as a Resu'lt'of Power Supply Disturbance."
'LEEE Transaction on Power Apparatus and.Systems, Volume"PAS-108, No. 8; 'Augus't 1982.
American National Standard fo'r "Polyphase Induction Motors for Power Generating-Stati'ons.",
C50.41-1982.
- 13.
National Electrical Manufacturers Association, "Motors and Generators",
MG-1-20, 1985.
14.
L. E. Goff, J. B. Williams, J.
C. Appiarius, S. L. Smith, "Application of a
New Synchronism Check Relay.", Georgia Tech Relay Conference, 1979.
15.
R. D. Pettigrew,,E.
L. Johnson, Automated Motor Bus Transfer Theory and Application.", Paper, Thirty Seventh Annual Conference for Protective Relay Engineers, April 16, 1984, Sponsored by Texas A & M University.
16.
Letter dated March 1, 1990 from Swedish Nuclear Power Inspector.
17.
Letter dated April 3, 1990 from the Central Electricity Generating
- Board, U.K..
345 KV North'Bus 15G-'13T-2 15G-14T-2
~ Southington
.(Central-Ct)-
348 Line Manchester (Central Ct) l.
310 Line Unit 2 (J
To Haddem Neck Via Montville
)~ (Eastern Ct) 371 Line Unit 1 Transmission Line
)g~ Card Station
)
(Easterri Ct) 383 Line 345 KV South Bus Main Tra esfoimef ASST "8" RSST "Ara Main NSST "8" NSST "Ara BOP.6.9 KV System Bkr
( Urlit3j Bus 34A (BOP)
BtJS 35A Emergency Qiesei Generator tEDGt I~
,)
Bus Tie Breaker Bus 358 Bus 34C (Class 1E}
Bus 34Q (Class 1E)
I EGG 4.16 KY System Bus Tle -Breaker
)
Bus 35C Bus 35D Bus 348 (BOP)
'Figure 1. Simplified One-Line Dianram of Mi!lstoenee t}~tt 3.
t'
~
r I
I I
I I
i Generator Bus l
I l
l I
l I
I Disc Device Unit Generator 1
I I
I l
I I
I Start-up I
Transformer (Preferred, Power Supply)
I I
1 Generator Step Up Transformer Auxiliary Transformer r
J
)
Switchyard and Transmission Systems Standby Generator LIA LIB Standby Generator LIA QLIA
'attery Charger Battery LIB Charger LIB Battery Supply Battery Supply
)
)
LIA LIA LIB LIB Figure 2. Simplified One-Line Diagram for Nuclear Unit with Single Offsite Power Supply.
Tiansmission Switchyard, Generator
- Step-Up Transformer Station Sewice Transfoneer
'i 2nd Offsite G
Generatoi Unit Aux n
ransformer Source 4L Class 1E Class 1E I
[.
3 3l 3
I G
M.'M M
M
'1 t
Standby Diesel Generators Figure 3; Simplified Oine Line Diagram foi'a ku6le6r. Uni( v'ith the, Conventional Uriit Connected Sicheme.
,Generator
'Step-.Up Transformer Station Seririce
'Transformer Generator Breaker
)
J~
Unit.Aux..
rv i
c.
w 1ransformer Genera'tor
'Auxiliaries Busses
'r M M
M M
M'lass.1E',;,
Class,1E Standby
.Diesel Generator Mi M)
G)
IM M
Standby Diesel Generator Figure 4. Simplifiied'Orie;.Line Diagram'fair a Nuclear-Unit iwith Generator Ci~cuit Breaker, or. Load. Bieak Switch.
Eg d
ER tn this diagram, ER ES
+ EM2 2EgENi cos d where Eg = System equivalent volts.per hertz
= System voltage in per unit of motor rated voltage divided by system frequency'in per unit of rated frequency E~ = Motor residual volts,per hertz
= Motor terminal voltage in per unit of motor rated voltage divided by motor speed-in per unit of synchronous speed E~ = Resultant vectorial voltage. in.per unit volts per hertz on the motor rated voltage and frequency base Figure 5. Determination of Resultant Volts per Hertz on Bus Transfer or Reclosing.
':,~'G S.p
~c! S "gO.'l:"
~l":;r).- >no 4
I A' Q
Q'f~'a ~
)63"
~
~
.i 1 AS'.; ~
pCle -'
-.2PiQh
~,
bc.'-'.
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-AIR 4'~ 6 scf.'e.'l Cfi9 2 ~ y ftl <4~
r:~i'l Stl6d~
cl