ML20151K249
| ML20151K249 | |
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|---|---|
| Site: | Oconee  |
| Issue date: | 03/29/1984 |
| From: | Battle R OAK RIDGE NATIONAL LABORATORY |
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- INSTRUMENTATION AND CONTROLS DIVISION i
l OCONEE-1 AC ELECTRICAL DISTRIBUTION CONTROL J
AND PROTECTION DESIGN FEATURES R. E. BATTLE MARCH 29, 1984 Internal Use Only NOTICE:
This document has not been given final patent clearance, and the dissemination of its information is only for official use. If this document is to be given public release, it must be reviewed in accordance with internal release procedures (SPP D-8-5).
DRAFT l
Prepared by the OAK RIDGE NATIONAL LABORATORY l
Oak Ridge, Tennessee 37831 operated by Martin Marietta Energy Systems, Inc.
for the U. S. DEPARTMENT OF ENERGY under Contract No. DE-AC05-840R21400 8406270394 940424 i
t PDR ADOCK 05000269 P
PDR c.
HIGHLIGHTS Operatior of the Oconee 1 ac distribution system was described and evaluated for failure modes that could cause the failure of more than one Motor Control Center (MCC).
The methods of system control and protection were described for the 6900 V, 4160 V, 600 V, and 208/120 V distribution buses. Most of the plant ac loads have normal and alternate power sources with automatic or manual transfer.
The 600 V MCCs have two breakers to protect against faults and overloads.
There are lockout relays to prevent automatic transfer of a MCC if it trips because of overload or fault current.
There are no single failures that would cause multiple MCC failures.
DC control power to the circuit breakers is arranged such that failure of a 4160 V bus and failure of a de panelboard would result in a temporary loss of ac power to more than one MCC.
Local, manual transfer to an alternate source would remain functional. Protection of the 600 V MCCs is independent of de control power.
Administrative procedures are used to assure separation of the three Class IE divisions. A common-cause failure potential exists because of the interconnection feature, but common-cause failure can be kept small by strong administrative procedures.
f iii i
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s TABLE OF CONTENTS Page iii HIGHLIGHTS 1
1.
INTRCDUCTION 1
2.
PLANT ELECTRICAL DISTRIBUTION SYSTEM CONTROL 1
2.1 AC DISTRIBUTION SYSTEM CONFIGURATION.
2 2.2 6900 V.
4 2.3 4160 V....
4 2.4.1 Non-Class 1E Electrical System Control 8
2.4.2 Class 1E Electrical System Control 8
2.5 208/120 V 9
3.
PLANT ELECTRICAL DISTRIBUTION SYSTEM PROTECTION.
9 3.1 4160 V PROTECTION 9
3.2 600 V PROTECTION.
9 3.2.1 600 V LC 10 3.2.2 600 V MCC.
10 3.3 208/120 V MCC PROIECTION.
10 4.
ELECTRICAL DISTRIBUTION SYSTEM RELIABILITY 11 4.1 RELIABILITY CHARACTERISTICS AT OCONEE 11 4.1.1 Faul Protection Design Features..
12 4.1.2 Electrical System Control-Power Design Features 12 5.
SUMMARY
REFERENCES 15 i
r.
OCONEE 1 AC ELECTRICAL DISTRIBUTION CONTROL AND PROTECTION DESIGN FEATURES R. E. Battle 1.
INTRODUCTION The Oconee concaruction permit (CP) was issued by the Nuclear Regulatory Commission in 1967, prior to the issuance of IEEE Standard 308-1971. Therefore, Oconce would not be expected to comply with this standard, unlike most other plants now operating which received their cps after the issuance of IEEE Standard 308. With a few exceptions such as connections between class IE systems, the Oconee electrical system has features similar to many other plants now operating.
The purpose of this review, however, is to describe the Oconee electrical distribution system and to identify features of the electrical distribution system that could cause or contribute to control system failures that may have safety implications. The probabilities of failure and the consequences that result are not included in this report. The description that follows applies to Oconee 1; Oconee Units 2 and 3 are similar.
i 2.
PLANT ELECTRICAL DISTRIBUTION SYSTEM CONTROL 2.1 AC DISTRIBUTION SYSTEM CONFIGURATION The Oconee Nucicar Station has three 1038 MVA nuclear units, two 87.5 MVA Emergency Safety Features (ESF) hydro units (instead of diesel engines), and three gas turbines 8.5 mi from Oconee that can be isolated from the remainder of the grid to serve Oconee. Normally, the electrical power to the Oconee 1 electrical distribution system is supplied by the Oconee 1 main generator through a unit transformer IT.
The bulk of the power from Unit 1 is transmitted through stepup transformer No. I to a 230 kV switchyard. Eight transmission lines connect this switchyard to the electrical grid.
An autotransformer bank connects the 230 kV switchyard to a 500 kV switchyard.
The gas turbines at Lee Steam Station i
2 l
are connected to Oconee by a 100 kV transmission line through a trans-former CT5. When the Oconce 1 main generatcr is not operating, the plant f
electrical distribution system is powered from a startup transformer CTl which receives power from the 230 kV switchyard. When all power from the electrical grid to Oconee is lost the Keowee hydro units start automati-cally. The Keowee units can supply power through the 230 kV switchyard to transformer CTl or through a 13.8 kV underground feeder to transformer CT4. Transformers CT4 and CT5 do not have the capacity to supply all of the plant equipment, but were designed to supply the ESF loads of all three units. The electrical distribution system including the 4160 V Main Feeder Buses and typical Load Centers and Motor Control Centers are shown in Fig. 1.
The ac distribution voltages are 6900 V, 4160 V, 600 V, and 208/120 V; the de distribution voltages are 125 V and 250 V.
The 6900 V switchgear buses, ITA and ITB, supply the four reactor coolant pumps.
There are no ESF lodds on these two bases.
Other than the two 6900 V buses all of the buses at Oconee are powered through two 4160 V Main Feeder buses No. I and No. 2 (other exceptions are load center IX8 and 1X9 and 600 V MCC XOD1 that can be powered from the Oconee 2 electrical system). The Main Feeder buses connect to three 4160 V buses ITC, ITD, and ITE. These buses supply the ten 600 V Load Centers (LC). These LCs supply the 600 V Motor Control Centers (MCC); the 600 V MCCs supply the 208/120 V MCCs and the battery chargers. A more detailed description of the operation of the distribution system at each voltage level follows.
2.2 6900 V The o900 V switchgear buses ITA and ITB are shown in Fig. 1.
Switchgear ITA supplies reactor coolant pumps (RCP) 1A1 and 1B1, and ITB supplies RCPs lA2 and 1B2. During normal operation the 6900 V buses are supplied from transformer IT.
When Unit 1 is not supplying power to transformer IT, the 6900 V buses are normally supplied from transformer CT1. Bus transfers may be manual, but there is an automatic transfer on undervoltage when the normal power source is lost.
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1 2.3 4160 V Normal power to all but the 6900 V switchgear is through the Main Feeder Buses No. 1 and No. 2 which are connected in a double bus, double circuit breaker arrangement as shown in Fig. 1.
These two buses supply three 4160 V buses ITC, 1TD, and ITE.
All the normal and ESF loads receive power through these three buses. During normal operation the Main Feeder Buses are supplied from transformer IT, but during Unit i shutdown, transformer CT1 supplies the power. Main Feeder bus transfers may be manual, but after a trip of the main generator, the transfer is automatic. For a loss of all offsite power, the Keowee hydro units pro-vide emergency power, or if necessary, the 100 kV line to the Lee Steam Station can be isolated from the rest of the system, and the gas turbines at the Lee Steam Station can provide emergency power.
2.4 600 V LCs and MCCs The 600 V distribution system has ten LCs - 1X1 through 1X10 - each of which is fed by one of three 4160 V switchgear bus sections ITC, ITD, or ITE. LCs IX8, 1X9, and 1X10, are Class 1E and supply power to the three ESF MCCs. LCs IX8 and 1X9 have alternate connections to LC 2X11 in Unit 2, but the transfer is made by removing the breaker from its normal position and placing it in the feeder from LC 2X11. The 600 V LCs feed twenty-seven 600 V MCCs. Twenty of the MCCs have alternate power sources with an automatic transfer, the three ESF MCCs have a manual transfer to an alternate source, MCC X0D1 has a manual transfer to a source from Unit 2, and three buses do not have alternate sources.
The MCCs with automatic transfer to an alternate LC have lockouts that prevent auto-matic transfer on overload or fault currents.
2.4.1 Non-Class 1E Electrical System Control Automatic transfer of the MCCs is accomplished by circuit breakers located in the LC transition sections.
There is another breaker in the feeder, but it serves for protection only and prc71 des no control function. A typical circuit breaker configuration is shown in Fig. 2.
4160/600 V h4160/600 V A A LC 1X6 LC IX5 I&C DC l CBS-3 CB6-4 1DID -
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< IDIC CB6-4 CBS-1 NC CB6-1 NC NC CBL-1 CBL-2 MCC 1XL l
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- Contactor Thermal protection NC - Normally closed 600 V NO - Normally load open Fig. 2. Typical MCC sources and breaker configuration.
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6 Switches for AUTO-MANUAL selection and breaker control are located at each LC.
Normally the AUTO-MANUAL switches will be in the AUTO position.
Loss of voltage on a feeder to a MCC will automatically trip the normal feeder breaker and close the alternate feeder breaker, but there is not an automatic transfer back to the normal feeder when its voltage is 4
restored. Simultaneous loss of voltage to the normal and emergency source will cause the normal feeder breaker to open and the alternate (or emergency) feeder breaker to close regardless of which breaker was closed prior to loss of voltage.
An automatic transfer will not occur if the alternate source is not energized. All of the feed. circuit breakers for a given LC are dependent on I&C de 1DIC or 1DID for automatic transfer control power.
An operator can make a manual transfer by placing t'a selector switch in MANUAL and closing the alternate feeder.
Closing of this breaker will automatically trip the feeder breaker that was originally in use.
When MANUAL is selected an automatic transfer is blocked.
The 600 V MCCs XOD1, XOD2A, and XOD2C, which are not shown in Fig. 1, have an interlocked, manual transfer from a Unit 1 LC to either Unit 2 or Unit 3 LCs. MCC XOD2B is normally powered from Unit 2 with a transfer to Unit 3.
Automatic control of the circuit breakers supplying the non-Class 1E 600 V MCCs is dependent on I&C de power. The I&C de control power is distributed to LC feeder circuit breakers for automatic transfer as follows:
(a) 1DIC - LC 1X2, 1X4, and 1X6, (b) 1DID - LC 1X1, 1X3, 1X5, and 1X7.
Based on this distribution of de control power, the 600 V MCC automatic transfers are dependent on the I&C de sources as shown in Table 1.
If a 600 V MCC is dependent on two de sources for a successful automatic transfer, then failure of one de source would cause an automatic transfer.
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7 to be nonfunctional either because one breaker could not open or the other could not close. Remote operation would not function for the breakers that have lost control power, but local, manual control would be functional. Therefore, restoration of power would depend on dispatching an operator to the Load Center switchgear to open or close the failed circuit breakers.
Table 1.
600 V MCC normal and emergency circuit breaker control power sources I&C de power source i
Normal Emergency 600 V breaker de breaker de MCC source source 1XA 1DID 1DIC 1XB 1DIC 1DIC IXC 1DIC 1DIC IXD 1DIC 1DID 1XE IDID 1DIC 1XF IDID 1DID 1XGA 1DIC 1DID 1XCB IDID 1DIC 1XH 1DID 1DID 1XI 1DIC 1DID IXJ IDID 1DIC 1XK 1DID 1DIC 1XL IDID 1DIC XM No transfer IXN 1DIC 1DID 1XO IDID 1DID 1XP 1DIC 1DID 1XQ IDIC 1DID 1XR 1DID IDID 1XT 1DID 1DIC XWT 1DID 1DID X0D1 Manual transfer Admin.
building No transfer Security building No trcnsfer
l 8
2.4.2 Class 1E Electrical System Control The Class 1E 600 V MCCs have normal and emergency ac power sources.
To reduce the probability of a common failure of these MCCs, their l
alternate ac source breakers are padlocked open. A common-cause failure of two NCCs could result if both MCCs are connected to one LC.
- However, i
at Oconee the shared connections are padlocked open and must be closed manually. The common-cause failure potential at Oconee is that an opera-tor will make an error and close a breaker onto a faulted Load Center or i
inadvertently leave a breaker closed so that two Class IE MCCs remain 4
connected to one LC.
Two Class 1E LCs,1 X 8 and 1 X 9, have alternate sources from Unit 2, but to make this transfer the normal source breaker must be removed and installed in the alternate source breaker position.
A potentially significant common-cause failure mode exists for the Class 1E 600 V MCCs in that all three MCCs can be connected to LC IX9, or two can be connected to LC IX8. The alternate sources are to be used during maintenance as required, but administrative procedure is the method used to ensure that the normal connections are restored after maintenance is performed.
Strong administrative procedures must be enforced to reduce the probability of a common-cause failure of two or three Class 1E 600 V MCCs.
I 2.5 208/120 V I
The 208/120 V system provides power to instrumentation, control, and small power loads. Each 208 V MCC has a single feeder except ESF buses IXS1, 1XS2, and IXS3, which have mechanically-interlocked, manually transferable, redundant feeders from 600 V McCs IXS2, IXS1, and 1XS2, respectively. The 600 V ESF bus IXS3 is not used as an alternate supply-source, but 600 V MCC 1XS2 is an alternate source for two 208 V MCCs.
The 120 V unregulated power loads, which consist mostly of plant light-I ing, are powered from transformers connected directly to 600 V MCCs. All of the 208/120 V feeder breakers are controlled locally at the switchgear but not from the control room.
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9 3.
PLANT ELECTRICAL DISTRIBUTION SYSTEM PROTECTION This description of the plant electrical distribution protection system will begin with the 4160 V Main Feeder Buses, and it will include buses at 4160 V and lower. The switchyard and generator will not be included.
T 3.1 4160 V PROTECTION The 4160 V electrical distribution system is so designed that at least two failures must occur to cause both Main Feeder Buses to fail.
All of the 4160 V circuit breakers have two trip coils powered from separate de sources. The two Main Feeder Buses No. 1 and No. 2, shown in Fig. 1, are protected by differential current relays (bus fault protection) from the source side of each incoming source breaker to the load side of the outgoing feeder breakers. All of the 4160 V breakers have phase overcurrent, ground fault, and breaker failure protective relaying.
3.2 600 V PROTECTION 3.2.1 600 V LC Each of the 600 V LCs is fed from a 4160 V switchgear bus section ITC,1TD, or ITE. Each feeder has two breakers which are in series.
One of the breakers is a 4160 V breaker which has the protection scheme described in Sect. 3.1.
The other breaker is a 600 V breaker which has a direct release (series trip) device that protects against phase over-The direct release breakers-will protect against overcurrent current.
independently of de control power. If the de. control power fails, the circuit breaker trip coil will not function, but the direct trip protec-tion features will remain available. The feeder circuits from the 600 V LCs to the 600 V MCCs contain two breakers protecting each feeder, but only one of these two circuit breakers performs a control function.
Because there are two breakers in series providing overcurrent protection is unlikely that a fault would cause a cascading failure of the elec-it trical distribution system. There are no loads other than MCCs connected-
-directly to the LCs.
10 3.2.2 600 V MCC The 600 V MCCs sources from the 600 V LCs contain two direct-release breakers in series for fault current protection as described in the pre-vious section. The circuit breakers closer to the LCs function on bus The undervoltage, but both series breakers protect against overcurrent.
600 V MCC load breakers protect against fault currents, and the load starters have built-in elements to protect against overload current.
The feeder breakers to the 208/120 V MCCs have fault current protection.
3.3 208/120 V MCC PROTECTION There is only one circuit breaker in each feeder from the 600 V MCCs to the 208/120 V MCCs.
This breaker protects against fault current but it has no automatic control function. The load breakers on the 208/120 V MCCs provide fault current protection, and the load starters have built-in elements to protect against overload current.
4.
ELECTRICAL DISTRIBUTION SYSTEM RELIABILITY The results of a bus or cable fault can be destruction of cables, buses, and other equipment because of the large amount of electrical energy available for conversion to heat or mechanical energy. Electrical systems contain energy sufficient to cause explosions that can destroy switchgear and adjacent equipment. Cables and busways can be melted by heat released by the cable resistance and fault current, or the cables and busways can be bent by the magnetic forces resulting from fault cur-rents. Hot ionized gases created by an electrical are can melt cabinets and cause extensive damage. Therefore, cables, buses, and switchgear are physically protected against faults, and breakers are operated by high-speed protective relays to limit fault duration. There are protective devices for phase overcurrent and ground fault current, and there are differential current protective devices for some buses, generators, and transformers.
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11 Breakers and fuses are coordinated to ensure that wherever possible, unfaulted loads are not interrupted when a protective device Protective devices are coordinated such operates to isolate a fault.
that the devices nearest the faulted equipment operate first, but if those devices nearest the fault fail, the next level of protective There have devices are coordinated to isolate the faulted equipment.
been several reported incidences where electrical failures have caused extensive equipment damage,1 but these failures are not frequent at nuclear power plants.
4.1 RELIABILITY CHARACTERISTICS AT OCONEE The 4160 V Main Feeder Buses at Oconee are critical because all but the 6900 V buses receive power through them even for emergency operation These buses are when the Keowce hydro units are supplying the ac power.
two bus faults, two breaker failures, or two breaker designed such that Because of the control circuit failures must occur to fail both buses.
the breakers are designed with two trip importance of the 4160 V system, coils each such that the buses will retain protection and control even if a single control circuit or de power source fail.
4.1.1 Fault Protection Design Features Failure to clear a fault is unlikely because there are two breakers in series protecting the 4160- and 600-V distribution feeders as shown The IEEE Standard 500 recommended estimate of failure of an in Fig. 2.
ac breaker to open and interrupt fault current on demand is 3 x 10-4/
demand,2 and WASH-1400 estimate is 1 x 10-3/ demand.3 Based on these estimates the probability of two breakers failing independently on demand becomes vanishingly small. Only the 4160 V breakers are dependent on de control power to isolate a fault, but each 4160 V breaker has two trip The 600-V circuit breakers are coils supplied from separate de sources.
Instead these not dependent on station de power to trip on a fault.
breakers have a direct release trip device independent of de power.
Therefore, at least two breaker failures must occur to fail to isolate a fault on a 4160-V or 600-V distribution center and based on the estimated failure of two breakers probability of failure of a breaker, independent is not significant.
12 4.1.2 Electrical System Control-Power Design Features The electrical distribution system at Oconee I was designed such that all but a few non-critical MCCs would have an ac power source even if one of the switchgear bus sections ITC, ITD, or ITE failed to zero voltage. All but three of the MCCs would have operable normal or emer-gency power sources.
(Three of the non-critical MCCs have only one in several source.), However, a combination of two failures could result MCCs being without power. The two failures are loss of one of two de control power sources, IDIC or 1DID, and loss of voltage to one of the three switchgear bus sections ITC, ITD, or ITE.
For this loss of ac power an MCC would not transfer to its alternate source either because a normally closed circuit breaker would not open or a normally open circuit <
breaker would not close.
The circuit breakers would fail to operate because of loss of de control power. The MCCs that would be without power for a failure of a switchgear bus section and a de control power
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source are shown in, Table 2.
The Class lE LCs and MCCs were not included in these tables because they are not dependent on de power sources 1DIC or 1DID.
A 600 V MCC would fail to transfer automatically on loss of voltage if the AUTO-MANUAL selector switch were left in MANUAL. There is an AUTO-MANUAL selector switch for each circuit breaker housed in an LC.
When a selector switch is lef t in MANUAL an automatic transfer to an alternate power source will not occur if the normal source fails, but a manual transfer could be made. It would take several human errors leaving transfer switches in MANUAL to cause the loss of more than one 600 V MCC.
5.
SUMMARY
f We have found no single failures in the Oconee electrical distri-bution system that would lead to a cascading failure of all or part of the ac distribution system. There are some multiple hardware or admin-istrative - failures that could result in the loss of electrical power to i
several MCCs.
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Table 2.
MCCs without power for the loss of a de source and a 4160 V switchyear section I&C 4160 V switchgear ITC 4160 V switchgear ITD 4160 switchgear ITE MCC breaker c:ntrol power Normally closed Normally open Normally closed Normally open Normally closed Normally open source breaker fails breaker fails breaker fails breaker fails breaker fails breaker fails to open to close to open to close to open to close 1DIC Security MCC 1XR PCC XODi*
MCC IXC MCC IXI None Buildingt MCC IXJ MCC IXB MCC 1XK MCC IXN MCC 1XC MCC IXL MCC 1XQ MCC 1XD MCC IXP MCC XOD2A*
MCC 1XE MCC XM Administrgtion g,u Building 1DID MCC IX0 None MCC IXT MCC IXD MCC IXE MCC 1XI MCC 1XR MCC 1XH MCC XM MCC 1XN MCC 1XJ MCC 1XK Administ{ation MCC 1XQ Building i
MCC XOD2C*
MCC 1XGA MCDC IXCB MCC 1XA Security Buildingt
- These MCCs have alternate (emergency) power sources from units 2 or 3.
These MCCs lose power at least tocporarily whenever the associated distribution center ITC, ITD, or ITE fails.
tThese MCCs do not have an alternate source.
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14 Because of the extensiva equipment damage and personnel injuries that could result from electrical faults, electrical equipment is physi-cally protected against faults.to reduce the probability of their occur-rence, and circuit breakers are installed to clear faults rapidly and limit the effect on the' remainder of the system.
In the Load Center feeder circuits, the' Oconee' nualear power plant has installed two circuit breakers in' series to protect against fault currents. The additional breaker provides backup protection that improves the reliability of the system to isolate a fault without a cascading failure. Therefore, a cable, bus, or load fault is not likely to cause multiple bus failures.
Failure of several 600 V MCCs to transfer to alternate ac power sources could occur because of the design of the de control power distri-2 bution system. There are several 600 V MCCs that depend on two de power sources to transfer from normal LC sources to alternate LC sources.
The initiating events' for this failure would be loss of power on a de panel-s board IDIC or'lDID and loss of voltage on a switchgear bus section ITC, 1TD, or ITE. 'Such an occurrence involves multiple failures such as failure of a de panelboard as an initiating event followed by loss of ac power from one of the switchgear buses.
Power could be restored by loc.1, manual operation of the failed breakers.
The effect on the recctor because of the resulting loss of load has not been evaluated in this report, such studies being the responsibility of the companion controlsystemsprkgram,'but the buses that fail do not include the Class 1E 600 V MCCs IXS1', 1XS2, and 1Xs3.
An administrative error that would' permit all three class-lE, 600 V MCCs to be connected to one LC"has potential for a common-cause failure of redundant safety, equipment. The purpose of the alternate source to the class IE 600 V.MCCs'is to keep the MCCs energized during maintenance, but administrative procedures must assure that the normal source is reconnectcd after maintenance. Otherwise a significant common-cause
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15 References 1.
H. C. Iloy, "Potentially Damaging Failure Modes of High-and Medium-Voltage Electrical Equipment," NUREG/CR-3122, August 1983.
2.
"IEEE Guide to the Collection and Presentation of Electrical, Electronic, and Sensing Component Reliability Data for Nuclear-Power i'
Generating Stations," IEEE Standard 500-1977, p. 148, June 30, 1977.
3.
" Nuclear Reliability Assurance Data Source Guide," ORNL/ENG/TM-2, i
- p. 39, November 1976.
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