ML20202H247
| ML20202H247 | |
| Person / Time | |
|---|---|
| Site: | Maine Yankee |
| Issue date: | 04/03/1986 |
| From: | NRC |
| To: | |
| Shared Package | |
| ML20202H240 | List: |
| References | |
| NUDOCS 8604150180 | |
| Download: ML20202H247 (5) | |
Text
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SAFETY EVALUATION OF DEGRADED GRID VOLTAGE PROCEDURE ,
MAINE YANKEE ATOMIC POWER COMPANY MAINE YANXEE ATOAIC POWER STAT 1DN DOCKET NO. 50-309 -
INTRODUCTION .
During normal plant operation, the safety class IE power distribution t usses !
are supplied electrical power by the.. plant's s.afn turbine generator Through the unit station service transformer. Power can also be supplied to the class IE
. distribution busses from tne off-site transmission grid via the reserve station service transformer. When neither the unit nor une off-site supply is avail-able to the class IE busses, the busses will be supplied power from their respective emergency diesel generators (EOG). Should either the unit or reserve power source voltage be belbw the class IE equipment qualified opera-ting voltage, equipment damage could occur. _.
In a Generic Letter dated June 3,1977, the NRC requested that the licensee address this concern. In this letter, the staff requested the licensee to analyze the class IE system to determine the grid vcitage at which damage could occur. The voltage sensing devices would cause the off-site grid power supply to be disconnected from the class 1E system at voltages where damage could occur. The class 1E busses, upon being disconnected, would then be supplied power from their respective EDG.
The Itcensee was concerned that significant degradation of the grid would result if the above automatic disconnection occurred because of the number .of nuclear plants in the New England area. It was thiir view that automatic -
disconnection from the grid should only be required if the degraded grid voltage condition occurred at the same time as a loss of Coolant Accident (LOCA). They agreed that, should their plant have a LOCA at the same time there was a degraded grid voltage condition, they would automatically disconnect from
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the off site grid. The EDGs would then supply power to their respective class IE distribution busses. However, if there was a degraded grid voltage con-dition without a LOCA, the operator would take manual action to protect- the equipment that was supplied power from the class IE system.
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j Safety Evaluation 2 _.
This proposal has been rvviewea and accepted by the NRC and is documented in the staff's Safety Evaluai. ion Report of September 29, 1982, and Update Safety Evaluation of March 27, 1984. The report of March 27, 1984 identified that the licensee should develop an operating procedure to provide the necessary operator guidances to protect the class 1E electrical equipment during a de-graded grid voltage condition without a LOCA condition. Also, the licensee !
should provide technical specification changes to cover setpoints and toler- <
ances, limiting conditions for operat1on and surve111ance testing of the under- !
. voltage protection system. In addition, the technical specification should include a test that simulates a loss of offsite power condition coincident with a _ an accident signal verifying the diesel start, load shedding and load. sequen-cing. By letter dated January 25, 1984, the licensee _ submitted for staff review a draft abnormal operating procedure. The procedure was subsequently.
revised February 7, 1985. _This safety evaluation is for these procedures.
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DISCUS'S ION The licensee submitted the draf t operating procedure AOP 2-12, Revision 0, by letter dated January 25, 1984. This procedure was designed to assure protec-tion of class IE systems under degraded gfid voltage conditions without a con-current LOCA. The prucedure was reviewed during a site visit discussed in Inspection Report No. 50-309/84-25. _.
The following were identified during the visit:
The initiating low voltage alarm setpoint was 95% of the motor designed -
voltage of 4000 volts. Since the' voltage was sensed at the bus, an additional 20 volts was added to the setpoint for motor lead voltage drop.-
The alarm was set for 3S20 solts with a 10 second time delay. The time i delay blocks the alarm to allow for a temporary low voltage condition due to motors starting. -
Ouring a low voltage condition, an alarm would sound and be indicated on -
the main control board annunciator. The operator would determine if the low voltage condition was caused by in plant conditions or the off-site grid system.
If the low voltage was determined to be an off-site problem, the operator would contact the system dispatch center in Augusta, Maine, (Augusta Dispatch) to raise the system voltage. _
If operational limits were exceeded, the- unit would be _ shutdown. Plant loads would be reduced as much as possible to raise the stil.lon service voltage.
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Safety Evaluation 3 1
If the low vcltage condition was nct cre. ting operational concerns but the dispatcher was not able to raise the system voltage and the condition was '
expected to last for more than one hour, the operator would perform the following actions: -
Start the emergency diesel generators (EDG).
Synchronize the EDG with their busses.
Separate the off-site source from the busses.
EVALUATION
- Ouring this ' review, the staff discussed with the licensee procedural changes that would clarify and aid operator decision making.
- Changes / improvements were
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The operator action to supply power to the class IE buspes from the' EDG and separate from the off-sita source should not only be done if the operator determines from the Augusta Otspatch that the degraded voltage condition is expected to last more than one hour but also, if the condi-
- tiDn continues for one hour.
- The operator should receive .ssu'rance from the Augusta Dispatch of grid stability before establishing normal plant electrical lineup after the underveltage conditioil has been corrected.
t The grid stability condition should be specified to prevent potentiaf equipment damage resulting from An unstable grid. The concern is .that an unstable grid with multiple voltage excursions outside of the-nonnal volt-age range could result in overheating of the safety related equipment.
- This could occur even thougn each excurston is less than one hour in dura-tion. The grid stability should be defined in tems of voltage magnitude ,
and time duration tv provide a stability period of sufficient duration so -
that safety related equipment temperature would return to normal following i each voltage excursion. -
l The itcensee made the following additional changes to the procedure to resolve the scaff's concerns. These changes are included in the licensee's final procedure, Abnormal Operating Procedure No. AOP 2-12, Revision 1, Issued -
February 7,1985.
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Safety Evaluation 4 -
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! 1. The operator would snutdown the anit should sne?e be any 6,900 volt or
4,160 volt motor thermal overload or overcurrent alarm condition resulting r i from a degraded grid voltage condition. Reference to a de' graded grid
. voltage time duration was removed from the procedure. The above operator action addresses both the- time durstion and grid stability concerns. 1 i'
_ 2. Should t'he degraded grid voltage condition create or were it likely to create operational concerns, the operatur would start the EDGs and !
synchronize them with the class IE busses. The off-site source would then , ;
, be disconnected from the class IE busses. _
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' 'The staff finds that the procedure with the above changes is acceptable to prc-vide the necessary operator guidance to protect the class IE electric system under degraded grid voltage conditions without a LOCA condition.
The licensee proposed ano developed procedures in lieu of technical specifica- l tions for the calibration and pertocic surveillance testing of the undervoltage !
protection system. In addition, a test that simulates a loss of offsite power {
condition coincident with an accident signal will be conducted. This test will i
i verify the diesel start, load shedding and load sequencing.
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The staff has re tewed the test surveillance and calibration activities of the )
degraded grid voltage protection system. These activities have been incorpor-I 1 i
ated into the Test Procedures Nos. 3.1.14A and 3.1.148 for Emergency Diesel Generator (EDG)/ Emergency Core Cooling System (ECCS) Cold Shutdown. Both pro-cedures are Revision 15, issue date September 28, 1985. The staff's review of .
the test procedure identified the following: setpoints and tolerances, sur- -l i
veillance tests and system integrated test conditions: ;
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! -. Degrided grid undervoltage relay setpoint was 3820 volts. This agrees with procedure AOP 2-12. The staff finds the setpoint to be acceptable because it exceeds the minimum Toltage that will cause damage to the motors. -
Undervoltage relay setpotrIt tolerance was plus or minus 40 volts. Using.
the minus 40 volts the trip value is 3870 volts. This value is 160 volts
'; higher than the equipment lowest qualified voltage reflected at the bus and is acceptable.
l The time delay relay setpoint was 10 seconds with a tolerance of plus or minus 1 second. This agrees with ADP 2-12. Operation .of the equipment for this short time would not' effect the equipment during a low voltage condition and is acceptable. '
- The above components wil1~ be tested for operability and accuracy dur'ing i each refueling outage. This time between tests is the same as for other j
protective relays used in the Class IE system.
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R Safety Evaluation 5 c.
A test that simalate3 a loss of offsite power coincident with an accident signal will be conducteo during each refueling outage. This frequency of I testing is consistent with .the frequency of similar testing at other
. facilities and is acceptable. _
The low voltage condition for this test is simulated by the manual opening of the bus incoming _ supply breaker.
System action starts at 3255 volts whien occurs with or without an acci- 1 i dent signal.
System action is to start the diesel generator (EDG), load shedding, and load sequencing.
The above system action occurs at'the degraded grid voltage setpoint of 3820 volts after_ a 10 second time delay coincident with an. accident signal; however, it.is not part of the refueling outage integrated test.
After the initial integrated verification test of the degraded grid volt-age protection system, partial testing up to the time delay relay output contact is acceptable and conforms with the Standard Review Plan.
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load shedding is reinitiated"if the EDG breaker is inadvertently This is a requirement for proper function and is acceptable. ,
Load sequencing takes place when the EDG is reclosed after the above ,
condition. This is a requirement for proper function and is acceptable.
The staff finds that these p'rocedures provice the necessary vertftcation of operability and accuracy of the degraded grid voltage system in lieu of technical specificetions. .
~ CONCLUSION ,
1 The staff has conciuded, based on the considerations discussed above, _that:
The revised Abnormal Operating Procedure No. AOP 2-12, Revisien 1 Issue Date 2-7-85, provides technically acceptable procedures covering operator i
actior.s required during degraded grid voltage conditions without concur-rent LOCA conditions. _
l The Test Procedure No>. 31.14A & B, Revision 15, Issue Date September 28, 1985, provides technically acceptable procedures covering . test survett-lance and calibration of the degraded grid voltage system in lieu of technical specifications. '
Principal Contributor: Frederick P. Paulitz, ORS, Region-I e
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