ML20063F926
| ML20063F926 | |
| Person / Time | |
|---|---|
| Site: | San Onofre  |
| Issue date: | 02/08/1994 |
| From: | Marsh W SOUTHERN CALIFORNIA EDISON CO. |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| NUDOCS 9402150247 | |
| Download: ML20063F926 (16) | |
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Southem Califomia Edison Company 23 PAHKER S TREET IRVINE, CALIFORNIA 92718 February 8, 1994 WAM ER C. MARSH namme MAWit M (W NLK44AP9 MEG 'LAICNtY A8'F#dHS (714)4S4 4403 U.S. Nuclear Regulatory Commission Attention: Document Control Desk Washington, D.C. 20555 Gentlemen:
Subject:
Docket Hos. 50-361 and 50-362 Additional Information for Amendment Applications 136 and 120 Offsite Power Supply Compliance with General Design Criterion 17 San Onofre Nuclear Generating Station Units 2 and 3
Reference:
September 29, 1993 letter from R. M. Rosenblum (SCE) to Document Control Desk (NRC)..
Subject:
Amendment Applications 136 and 120, Proposed Change to Technical Specification 3/4.3.2 Provided as an enclosure is additional infonnation regarding the planned enhancement to the undervoltage protection scheme at San Onofre Units 2 and 3 in support of the proposed Technical Specification change provided in the above Reference. The NRC requested an evaluation of how the existing and proposed offsite power systems comply with 10CFR50 Appendix A, General Design Criterion (GDC) 17.
The evaluation concludes that the San Onofre Units 2 and 3 offsite power system meets the design requirements of GDC-17.
This is true for both the existing (as-licensed) configuration of San Onofre Units 2 and 3 and the configuration following installation of the plannea enhancement to the degraded voltage protection scheme.
In addition, Southern California Edison is currently evaluating the need to implement administrative controls for certain scenarios which are beyond the design requirements of GDC-17.
If you have any questions regarding this matter, please let me know.
Sincerely, Y
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Enclosure:
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K. E. Perkins, Jr., Acting Regional Administrator, NRC Region V J. A. Sloan, NRC Senior Resident Inspector, San Onofre Units 2 and 3 H. B. Fields, NRC Project Manager, San Onofre Units 2 and 3 l
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's ENCLOSURE 1 0FFSITE POWER SUPPLY j
COMPLIANCE WITH GENERAL DESIGN CRITERION 17 3
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'OFFStrE POWER SUPPLY COMPLIANCE WITH GENERAL DESIGN CRITERION 17 SAN ONOFRE UNITS 2 AND 3 1.0 INTRODU(TION i
1.1 IIistory at San Onofre
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i Following the permanent shutdown of Unit 1 in 1992, stability and reliability.
of the Southern Cali'.ornia Edison (SCE) and San Diego Gas and Electric l
(SDGo.L, listribution systems were reevaluated. Studies were performed to evaluate the loss of major transmission lines which are remote to the San Onofre Nuclear Generating Station (SONGS) switchyard. Multiple line losses 1
were considered if they shared common equipment, such as a transmission tower. These studies also considered a subsequent loss of all generation capacity at SONGS.
l These studies identified six scenarios in which a switchyard voltage below the j
minimum analyzed value of 218 KV could occur. Each of these six scenarios requires all the following and a subsequent trip of the operating SONGS unit:
- 1) one SONGS unit offline; and
- 2) loss of a major transmission line or tower; and
- 3) system loads in excess of specified thresholds.
1 Should a subsequent trip of the operating SONGS unit occur under these conditions, voltage levels at the 230 KV switchyard would drop below the minimum analyzed value of 218 KV.
j As a prompt compensatory action, SCE has implemented procedure changes L
to ensure that adequate voltage is maintained to support accident loads under the conditions noted above. These procedures require the _SCE Energy Control Center Operator to notify the SONGS Control Room whenever the y
three conditions stated above exist. The SONGS operator would then transfer j
the Class 1E buses in the operating SONGS unit from their Reserve Auxiliary Transformer to the Unit Auxiliary Transformer, and disable ~ the Resetve Auxiliary Transformer auto-transfer circuit. This ensures that the Class 1E buses shed their loads following a Loss of Voltage Signal.(LOVS), and automatically transfer to. their respective Emergency Diesel Generators
.(EDGs). Currently, the operator would also declare the offsite power source ~
j INOPERABLE and enter the appropriate Technical Specification Action.
1 Statement.
As a long term measure, an enhancement of the existing degraded voltage i
protection scheme (second level of undervoltage - protection) will be
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i implemented to provide more reliable protection against degraded grid conditions. This enhancement will also transfer the Class 1E buses directly to their respective EDGs when a loss of voltage or degraded voltage condition exists concurrent with_ a Safety Injection Actuation Signal (SIAS). This eliminates the time delay inherent in the existing scheme wherein the logic first seeks the alternate offsite power source before transferring the buses to l
the EDGs.
The results of the new grid studies have also prompted a review of SONGS 2&3 compliance with General Design Criterion (GDC) 17, Electrical Power Systems.
GDC-17 establishes the fundamental design requirements for th, esite and offsite power systems at all nuclear plants. Therefore, the results of tht~ tudies and the implementation of the planned enhancement to the undervoltage protection scheme have the potential to impact compliance with GDC-17. This report documents the results of the GDC-17 review.-
1.2 Scope of Review The scope of GDC-17 is broad in that it addresses both the offsite pawer.
supply and the onsite supplies such as the EDGs and the Class 1E batteries.
The scope of this review is limited to those portions of GDC-17 which adoress the design requirements of the offsite power supply. This includes:
e independence of the offsite supply from the onsite supplies interactions which may exist between the offsite supply and the onsite supplies
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t the permanent shutdown of SONGS 1 and the recently issued grid stability l
e studies.
i These issues are evaluated with respect to the compliance of SONGS 2&3 with the design requirements of GDC-17.
Standard Review Plan (SRP) 8.2 is the principal source of guidance for -
determining compliance with GDC-17. The approach used in this review is to isolate the components of GDC-17 which address design requirements of the offsite power supply, and then evaluate those components using guidance provided in SRP 8.2. Other regulatory documents and standards (referenced in Section 2.2 below) are used if SRP 8.2 lacks sufficient detail or is otherwise unclear.
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2.0 BACKGROUND
2.1 General Design Criterion (GDC) 17 (of 10CFR50 Appemlix A). Electric Power Systems i
General Design Criterion 17 states:
I An onsite electric power system and an offsite electric power system shall be provided to permit functioning of structures, systems, and components important to safety. The safety function for each system (assuming the other system is not functioning) shall be to provide sufficient capacity and capability -
to assure that (1) specified acceptable fuel design limits and design conditions of the reactor coolant pressure boundary are not exceeded as a result of anticipated. operational occurrences and (2) the core is cooled and-l containment integrity and other vital functions are maintained in the event of postulated accidents.
The onsite electric power supplies, including the batteries, and the onsite -
electric distribution system, shall have sufficient independence, redundancy, and testability M perform their safety functions assuming a singlefailure.
i Electric power from the transmission network to the 'onsite electric distribution system shall be supplied by two physically independent circuits (not necessarily on separate rights of way) designed and located so as to minimize to the extent practical the likelihood of their simultaneous-failure under operating and postulated accident and environmental conditions. - A switchyard common to both circuits is acceptable. Each of these circuits shall be designed to be available in sufficient time following a loss of all onsite alternating current power supplies and the other offsite electric power circuit, j
to assure that speified acceptable fuel design limits and design conditions of -
the reactor coolant pressure boundary are not exceeded. One of these circuits l
shall be designed to be avaihble within a few seconds following a !oss-of-coolant accident to assure that core cooling, containment integrity, and other -
i vital safety functions are maintained.
Provisions shall be included 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 power unit, the loss of power from the transmission network, or the loss of power from the onsite electric power supplies.
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2.2 Regulatory Documents and Industry Standards The following documents and standards provide guidance on the interpretation l
of those portions of GDC-17 that address design requirements of the offsite power supply:
Standard Review Plan (SRP) 8.2, Revision 2, July 1981, Offsite e
Power System Standard Review Plan (SRP) 8.3.1, Revision 2, July 1981, A-C o
Power Systems (Onsite)
Branch Technical Position ICSB 11 (PSB), Stability of Offsite i
e Power Systems IEEE 765-1983: IEEE Standard for Preferred Power Sunnly for Nuclear Power Generating Stations e
NUREG 0712: Safety Evaluation Report for SONGS 2 and 3
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L 2.3 Definition of Preferred Power Supply (PPS)
The ofpite power system called out in GDC-17 is often referred to as the preferred power supply. Tbe term preferred power supply is defined in IEEE 765-1983:
preferred powersupply. 'nat power supply from the transmission system to the Class lE distribution system which is preferred to furnish electric power under accident and past accident conditions.
For SONGS 2 & 3 the preferred power supply consists of the following:
SCE and SDG&E 230 KV grid Transmission lines between the grid and the SONGS switchyard o
230 KV switchyard Circuits from the switchyard up to and including the auxiliary e
transformers, and Circuits from the auxiliary transformers to the 4.16 kV Class 1E e
switchgear The boundary between the preferred power supply and the Class 1E system is at the incoming terminals of the supply breakers to the Class 1E buses.
For the purpose of this review, the terms offsite power system and preferred powersupply are considered equivalent and are used interchangeably.
3.0 ASSESSMENT
OF GDC-17 COMPLIANCE I
3.1 ONSITE AND OFFSITE POWER SYSTEMS GDC-17 Requirement An onsite r!raricpower system and an opite electricpower system shall be provided to permit functioning of structures, systems, and components i
important to safety.
Evaluation The offsite power system is the preferred power supply defined in Section 2.3 above. From SRP 8.3.1, A-C Power Systems (Onsite), the onsitepowersystem includes "those standby power sources, distribution systems, and vital supporting systems provided to supply power to safety-related equipment."
i For SONGS 2&3 the standby power sources are Emergency Diesel Generators G002 and G003 and their support systems and equipment._ The distribution system is the 4.16 kV Class 1E switchgear and their associated buses, breakers, cables, transformers, and other equipment.
The vital.
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supporting systems include the Class 1E batteries and inverters and their associated distribution equipment.
3.2 SAFETY FUNCTION
.I GDC-17 Requirement
.. He safety function for each system (assuming the other system is not -
functioning) shall be to provide sufficient capacity and capability to assure that (1) specified acceptable fuel design limits and design condbne or the reactor coolant pressure boundary are not exceeded as a result of anticipated operational occurrences and (2) the core ia cooled and containment integrity and other vital functions are maintained in the event of postulated accidents...
Evaluation The phrase "cach system" refers to the onske power system and offsile power system called out in the previous sentence. The basic requirement is that each system must.be capable of providing adequate power under all normal, abnormal, and post accident operating conditions, given that the other system is unavailable.
As to the requirement that the offsite system provide
" sufficient capacity and capability" for design basis accident mitigation, SRP 8.2.1.3 states the following:
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Design information and analyses demonstrating the suitability of the power sources from the grid, including transmission lines, breakers, and transformers used for supp!ying preferred power from distant sources are reviewed to assure that each path has sufficient capacity and capability to perform its i
intended function. This will require examination of loads required to be.
powered for each plant operating condition; continuous and fault ratings of breakers, transformers and transmission lines; loading, unloading, and transfer -
effects on equipment; and power capacity available from each source.
To demonstrate compliance with this part of GDC-17 it is necessary. to perform an integrated review of the capability of the preferred power supply based upon the worst case loading of the in-plant equipment for all normal, abnormal, and post-accident operating conditions. This requires, in part,.
system studies which evaluate the staM.ty and capacity of the preferred power supply under various operating cor.ditions. GDC-17 does not provide any '
specific requirements in this area.
However, guidance is provided in SRP 8.2, IEEE 765-1983, Branch Technical Position j
ICSB 11, and the NRC Safety Evaluation Report (SER) for SONGS' 2 and 3 1
(NUREG 0712). SRP 8.2.III.1(f) states:
ne results of the gridstabilityanalysis must show that the loss of the largest singlesupply to the grid does not result in the complete loss of preferred power. He analysis should consider the loss, thrtmgh a single ewnt, of the largest capacity being supplied from the grid, removal of the largest load from the grid, or loss of the most critical transmission. His could be the total output of the station, the largest station on the grid,or possibly several large I
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'E stations if these use a common transmission tower, transformer, or breaker in a remote switchyard or substation.
A grid stability analysis which meets the above requirements should identify and analyze the worst case single contingencies which could impair the ability of the preferred power supply to support the maximum load demand at the nuclear plant. Note that an analysis which~ addresses multiple contingencies is not required. The need to perform a worst cas_e single contingency analysis is also stated in IEEE 765-1983, Section 5.1.4, System Studies:
1 System studies shall be performed to demonstrate that the preferred power supply in not degraded below a level consistent with the availability goals of the plant as a result of contingencies such as:
(1)
Loss of the nuclear power generating unit or (2)
Loss of the largest generating unit or (3) less of the largest transmission circuit or intertie or (4) less of the largest load The requirement for single contingency studies is also stated in Branch Technical Position ICSB 11 (PSB), Stability of Offsite Power Systems, Section -
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Background:
'Ihe staff has traditionally required each applicant to perform stability studies for the electrical transmission grid which would be used to provide offsite power to the plant. The basic requirement is that the loss of the largest operating unit on the grid willnot result in loss of grid stability and availability of offsite power to the plant under consideration; The requirement for single contingency studies was also referred to in Section 8.2 of the SER for SONGS 2&3 (NUREG 0712). In the discussion regarding grid stability studies, the staff notes that in one case a double line outage was.
j marginally stable (three phase fault affecting the Chino and Villa Park 230 KV lines). The staff concluded that the result was-acceptable "because we consider the limiting case described to be a double contingency event, and our requirements for transient stability are satisfied by analyzing single contingency-events."
i SCE has performed grid stability studies which identify and analyze worst case single and multiple contingencies on the SCE and SDG&E grids in accordance with the SCE and SDG&E Bulk Power Transmission System Reliability j
Criteria. (These criteria documents are provided as Appendices 8.2A and 8.2B in the SONGS 2&3 UFSAR.) Additional studies were performed to evaluate the effects of line outages for those trans' mission lines which are ~
directly connected to'the SONGS switchyard.. In all cases, the worst case single contingency' analyses. demonstrate that the stability, capacity, and.
j capability of the offsite power supply is adequate to support all normal, abnormal, and post-accident plant operating conditions. This is true of both the previous grid studies which supported the licensing of SONGS 2&3 and of the recent grid studies which were performed as a result of the permanent 7
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shutdown of SONGS 1.
The GDC-17 requirements regarding the safety-
~I function of the preferred power source are therefore met both prior to and following the permanent shutdown of SONGS 1.
The recent grid studies also analyzed the effects of certain multiplc contingency events on the offsite power system. The2e studies identified six multiple contingency scenarios in which the switchyard voltage could dropL i
below the minimum analyzed value of 218 KV. All other multiple contingency -
scenarios that are within the reliability criteria used in the study.are.
acceptable.
Multiple contingency scenarios are beyond the design requirements of GDC-17.
Based on the acceptability of all worst case single contingency scenarios, SONGS 2 and 3 and the offsite power system comply with the design requirements of GDC-17.
3.3 INDEPENDENCE OF ONSITE POWER SUPPLIES 1
GDC-17 Requirement
. The onsite electric power supplies, including the batteries, and the onsite electric distribution system, shall have sufficient independence, redundancy, and testability to perform their safety functions assuming a single fallare..
Evaluation This statement refers to the EDGs and their support systems, the Class 1E' batteries and vital buses, and the Class 1E distribution system. These onsite power supplies are outside the scope of this review. The adequacy of the onsite system has been previously evaluated, and is documented in the San -
Onofre Updated Final Safety Analysis Report.
3.4 CONNECTIONS TO Tile TRANSMISSION NETWORK GDC-17 Requirement Electric power from the transmission network to the onsite electric.
distribution' system shall be supplied by two physically independent circuits (not necessarily on separate rights of way) designed and located so as to minimize to the extent practical the likelihood of their simultaneous failure i
under operating and postulated accident and environmental conditions. A switchyard common to both circuits is acceptable..
Evaluation li Electric power is supplied from the transmission network to the
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SONGS switchyard by eight transmission lines on two rights of way (four lines.
on each right of way). SONGS 2&3 share a common switchyard. Any two of
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P the eight transmission lines into the switchyard satisfy the requirement for two physicallyindepemient circuits. Note that the physicalindependence of the two circuits ceases within the common switchyard. At that point the two circuits are no longer distinct. Upon exiting the switchyard, the independence of the l
two circuits is reestablished.
i Two physically independent circuits provide preferred power from the switchyard to the onsite power system of each unit. One of these circuits is connected to the reserve auxiliary transformers of a given unit. This circuit provides an immediately available source of preferred power to the Class 1E buses. This source is referred to as the nonnalpreferred power supply. An attemate preferred power supply is provided to the Class 1E buses from the Class 1E buses of the companion unit via a 4.16 kV cross-tie. These two circuits exit the switchyard at.two widely separated locations and are routed independently, one to each of the two units. These two circuits meet the separation and independence requirements of GDC-17.
i A third source of preferred power can be made available to each Class 1E bus in approximately eight hours by disconnecting the main generator links and backfeeding power through the main and the unit auxiliary transformers. This-power supply is available only when a unit is shutdown and is therefore not credited for accident mitigation. However, with one unit shut down, this l
source may be credited as a source of attemate preferred power to the companion unit's Class 1E buses.
i 3.5 AVAll ABIIIIY OF OFFSITE POWER CIRCUITS GDC-17 Requirement Each of these circuits shall be designed to be available in sufficient time followinga loss of all onsite alternating current power supplies and the other offsite electric power circuit, to assure that specified acceptable fuel design limits and design conditions of the reactor coolant pressure boundary are not exceeded. One of these circuits shall be designed to be available within a few seconds following a loss-of-coolant accident to assure that core cooling, containment integrity, and other vital safety functions are maintained...
Evaluation The basic requirement is that both circuits be available for shutdown of the unit and that at least one of the two preferred power supply circuits should be available almost immediately (within a few seconds) following a loss-of-coolant accident. GDC-17 does not explicitly state the availability requirement I
for the other preferred power supply circuit. Additional guidance in this area is provided in IEEE 765-1983:
1 Section 4.5 Availability. A minimum of two circuits from the transmission system to the Class lE power system shall be designed to be available during'
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startup and_ normal operation to meet accident, - post-accident, and safe shutdown requirements in accordance with the following guidelines.
(4)
A minimum of one PPS circuit shall be designed to be available automatically to provide power to the Class IE buses within a few seconds following a design basis accident.
(5)
A second circuit shall be designed to be available' within a time period demonstrated to be adequate. by the safety analysis of the station.
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An improved design should have two circuits automatically available to provide preferred power to the Class IE buses within a few seconds following an accident.
A circuit meeting the requirements described in item (4) above is referred to as an immediate access circuit. A circuit meeting the requirements described in item (5) above is referred to as a delayed acccvs circuit. The original design of SONGS 2&3 conforms to item (6) above where the normal nnd alternate-preferred power supplies are both immediate access circuits. The planned i
enhancement to the degraded voltage protection scheme converts the attemate preferred power supply from an immediate access circuit to a delayed accces 3
circuit during accident conditions where a SIAS is present. Because only one immediate access circuit is required, the design 'arter implementation of the planned enhancement will still meet the requirements of GDC-17.
3.6 POWER SOURCE INRRACTIONS GDC-17 Requirement Provisions shall be included to minimize the probability oflosing electric power from any of the remaining supplies as a result of.or coincident with the loss of power generated by the nuclear power unit, the loss of power from the i
transmission network, or the loss of power from the onsite electric power supplies.
Evaluation S R P 8.2.111.1 provides additional guidance in this area:
(g)
Dunng the review of the electrical schematics, it should be detennined that loss of standby power willnot result in loss I
of preferred power, loss of one preferred power circuit will not result in the loss of the other circuit, and loss of the main generator will not result in loss of either preferred
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power circuit.
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The' preferred power system must be independent of the
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onsite power system. The basis for acceptance in that no singteevent, including a single protective relay, interlock, or switchgear failure, in the event of loss of all standby power 10
4 sources, will prevent the separation of the preferred power system from the onsite distribution system or prevent the l
preferred power system from accomplishing its intended functions. In addition, the preferred and standby power supplies should not have any common failure modes...
'j Regarding item (g), the design of the standby power supplies (EDGs) has j
been previously reviewed, and there are no credible failures of these supplies
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that can result in the loss of any preferred power circuit. The SCE and l
SDG&E grid stability studies demonstrate that there is no single contingency, i
including a trip of a SONGS unit, that will result in loss of a preferred power
.j circuit. The grid stability studies did identify six multiple contingency scenarios which could result in a degraded preferred power supply. These multiple
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contingency scenarios are beyond the design requirements of GDC-17. The requirements of item (g) are therefore met.
Regarding item (h), there is no single failure that can prevent the separation of the preferred power system from the onsite distribution, and the preferred l
and standby power supplies have no common failure mode. The requirements i
of item (h) are therefore met.
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4.0 CONCLUSION
S In Section 3.0, the GDC-17 requirements for onsite and offsite power sources are divided into six components:
e Onsite and Offsite Power Systems Safety Function e
Independence of Onsite Power Supplies e
e Connections to the Transmission Network e
Availability of Offsite Power Circuits Power Source Interactions Section 3.1 describes the systems which serve as the onsite and offsite power systems required by GDC-17.
Section 3.2 evaluates the safety function requirements of GDC-17. Grid studies have identified and evaluated all worst case single contingency scenarios. In all cases, the worst case single contingency analyses demonstrate that the stability, capacity, and capability of the offsite power system are adequate to support all normal, abnormal, and post-accident plant operating conditions. This is true of both the previous grid studies which supported the licensing of SONGS 2&3, and of the recently issued grid studies which were performed as a result of the permanent shutdown of SONGS 1.
The recently issued grid studies also identify six multiple contingency scenarios in which the switchyard voltage could drop below the minimum analyzed value of 218 kV. All other multiple contingency scenarios that are within the reliability criteria used in the studies are acceptable.
These multiple contingency scenarios are beyond the design requirements of GDC-17. Based on the acceptability of all worst case single contingency scenarios, SONGS 2 and 3 and the offsite power system comply with the design requirements of G DC-17.
Section 3.3 describes the independence of the onsite power supplies. The recent shutdown of SONGS 1 and the planned enhancement to the degraded voltage protection scheme have no effect on the independence of the onsite power supplies.
Section 3.4 evaluates the connections between the transmission network and the onsite distribution system. The GDC-17 design requirements for two independent circuits are met.
Today, preferred power is supplied to each unit by two immediate access circuits. Following installation of the planned enhancement to the degraded i
voltage protection scheme, one of these circuits (4.16 kV cross tie) will be converted to a delayed access Qcuit (manual connection from the control room) when a SIAS is presem.
Section 3.5 demonstrates that both 12
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. configurations (two immediate access circuits, or one immediate access circuit and one delayed access circuit) meet the design requirements of GDC-17.
Section 3.6 evaluates interactions between the SONGS main generators, the offsite power source, and the onsite power supplies. There are no interactions within the design requirements of GDC-17 in which the loss of one supply (including a unit / main generator trip) ctm cause the loss of any other supply.
The GDC-17 design requirements for power source interactions are therefore met. There are, however, six multiple contingency scenarios in which a unit / main generator trip could cause degradation or loss of the offsite power system. These multiple contingency scenarios are, however, beyond the design -
requirements of GDC-17.
Summary The SONGS onsite power system and offsite power system meet the design requirements of GDC-17. Grid studies have shown that there are no single contingency scenarios which can render the offsite power supply inadequate to support all normal, abnormal, and post-accident plant operating conditions.
The SONGS units therefore satisfy the design requirements of GDC-17. This is true for both the existing (as-licensed) configuration of SONGS.2&3 and the :
configuration following installation of the planned enhancement to the degraded voltage protection scheme.
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