ML19309B061
| ML19309B061 | |
| Person / Time | |
|---|---|
| Site: | Rancho Seco |
| Issue date: | 09/17/1976 |
| From: | Walbridge W SACRAMENTO MUNICIPAL UTILITY DISTRICT |
| To: | Reid R Office of Nuclear Reactor Regulation |
| References | |
| TAC-10968, NUDOCS 8004020667 | |
| Download: ML19309B061 (16) | |
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U.C. NUCLEAR TAULATORY COMV 'SION DOCKET NUMIE R
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TOR.W. REID FROM: SACRAMENTO MUNICIPAL UTI,LITY DATE OF DOCUMENT SACRAMENTO, CALIFORNIA 9-17-76 WM.C. WALBRIDGE DATE RECEIVED 9-27-76
@ LETTER ONoTORizED PROP INPUT FORM NU.MBER OF COPIES RECEIVED
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@UNC L ASSIFIE D OCOPY 40 DESCRIPTION ENCLOSU RE LTR. RE. OUR 8-12-76 LTR..... TRANS THE FOLLOWIN3 RESPONSE TO REQUEST FOR INFORMATION REGARDINC EFFECTS OF DEGRADATION OF GRID. VOLTAGE ON PLANT CLASS'IE ELECTRICAL DISTRUBUTION SYSTEF
( 1" SIGNED CY. RECEIVED)~
( 15 PAGES)
DO NOT REMOVE ACHOEmsp" PLANT NAME:
RANCHO SECO SAFETY FOR ACTION /INFORMATION ENVIRO SAB 9-28-76 I.'.SSICMED.^.D :
I ASSIGNED.AD:
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REID W/6 BRANCH CMTEF*
PROJECT MANAGER:
PROJECT MANAGER!
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SACRAMENTO MUNiclPAL UTILITY DISTRICT O 6201 S Street, Box 15830. Sacramento, California 95813; (916) 452-3211 September 17, 1976 Director of Nuclear Reactor Regulation Attention: Mr. Robert W. Reid, Chief Operating Reactors, Branch #4 U. S. Nuclear Regulatory Commission Washington, D. C.
20555 Docket No. 50-312 Rancho Seco Nuclear Generating Station, Unit No. 1
Dear Mr. Reid:
Please refer to your letter of August 12, 1976 wherein you requested that the District investigate the Rancho Seco Unit iio.1 nuclear generating plant regarding the effects of degraded voltage conditions.
l Enclosed with this letter are 40 copies of our response to your Enclosure No. 2 " Request For Information."
Sincerely yours,
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SACRAMENTO MUNICIPAL UTILITY DISTRICT RANCHO SECO NUCLEAR GENERATING STATION, UNIT NO. 1 RESPONSE T0:
NUCLEAR REGULATORY COMMISSION REQUEST FOR INFORMATION REGARDING EFFECTS OF DEGRADATION OF GRID VOLTAGE ON PLANT CLASS IE ELECTRICAL DISTRIBUTION SYSTEMS DATED AUGUST 12, 1976 September 17, 1976
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- 1 NRC REQUEST FOR INFORMATION
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p la.
Request:
Describe the plant conditions under which the plant auxiliary systems (safety related and non-safety related) will be supplied by offsite power.
Include an estimate of the fraction of normal plant operating time in which i
this'is the case.
1
Response
At the Rancho Seco plant the redundant safety related electrical buses are either supplied from the site 220 KV switchyard (offsite power) through the i
startup transformers or by the emergency diesel generators.
There are no 1
provisions available for connecting the safety related buses to feed from 1
the main generator through the unit auxiliary transformers. Therefore, for normal operating conditions the safety related buses will be supplied from the 220 KV switchyard approximately 100% of the time. During abnormal con-ditions, such as loss of offsite 220 KV power lines feeding the switchyard l
or during special tests, the safety ralated buses will be supplied by the emergency diesel generators.
l In accordance with nonnal plant operating procedures, the non-safety related plant auxiliary buses are supplied from the 220 KV switchyard through the startup transformers whenever the reactor power is 25% or less.
Consequently, this. includes all normal plant startup and shutdown operating phases.
For normal operations it is estimated that the plant non-safety related auxiliary buses would be supplied from the 220 KV switchyard through the si;artup trans-formers for approximately 4 to 6 weeks a year.
During nonnal load operation of the plant with reactor power above 25% the non-safety related buses are manually transferred to the unit auxiliary transformers. Automatic transfer features are provided to transfer the non-safety related buses from the auxiliary transformers back to the startup transformers in the event of a trip or loss of voltage of the main generator.
Please refer to Enclosure No. 1, One Line Diagram, for additional details of the Rancho Seco plant auxiliary power supply system.
lb. Request:
I The voltage used to describe the grid distribution system is usually a
" nominal" value. Define the normal operating range of your grid system voltage and the corresponding voltage values at the safety related buses.
Response
The normal operating range of the 220 KV system voltage at the Rancho Seco switchyard is 224 KV to 234 KV.
Listed below are the calculated steady state voltages at the safety related buses for full plant auxiliary load I
and no plant auxiliary load over the normal Rancho Seco switchyard operating voltage range:
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l Case I:
a)i Switchyard voltage at 224 KV.
b Maximum plant safety and non-safety related auxiliary loads.
c) All load supplied by the startup transformers.
Calculated Calculated % of Safety Bus No.
Bus Voltage Rated Motor Voltage 4160 Volt Bus 4A 3888 93%
4 l
4160 Volt Bus 48 3888 93%
m 480 Volt Bus 3A (and MCC) 417 91%
480 Volt Bus 3B (and MCC) 415 90%
Case II:
a) Switchyard voltage at 224 KV.
l b) No plant safety or non-safety related loads.
j 1
Calculated Calculated % Of Safety Bus No.
Bus Voltage Rated Motor Voltage 4160 Volt Switchgear 4A 4250 102%
4160 Volt Switchgear 4B 4250 102%
480 Volt Bus 3A 478 104%
480 Volt Bus 3B 478 104%
Case III: a) Switchyard voltage at 234 KV.
b) Maximum plant safety and non-safety related auxiliary loads.
c) All plant auxiliary loads supplied by the startup transformers.
Calculated Calculated % Of Safety Bus No.
Bus Voltage Rated Motor Voltage 4160 Volt Bus 4A 4061 98%
4160 Volt Bus 4B 4061 98%
480 Volt Bus 3A 439 95%
480 Volt Bus 3B 435 94%
Case IV:
a) Switchyard voltage at 234 KV.
b) No safety or non-safety related auxiliary loads.
Calculated Calculated % Of Safety Bus No.
Bus Voltage Rated Motor Voltage 4160 Volt Bus 4A 4435 106%
4160 Volt Bus 4B 4435 106%
480 Volt Bus 3A 499 108%
480 Volt Bus 3B 499 1 08%
Ic.
Request:
The transformers utilized in power systems for providing the required voltage at the.various system distribution levels are normally provided
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. with tap 3 to allow voltage adjustment.
Provide the results of an analysis of your design to determine if the voltage profiles at the safety related buses are satisfactory for the full load and no load conditions on the system and the range of grid voltage.
Response
The criteria used in selecting the transformer tap settings for the Rancho Seco plant safety related buses was to provide the running motors with a i
voltage level which would be as near as possible to the rated voltage of the motors for normal plant operating conditions and not exceed a voltage level of 10% above or 10% below rated motor voltage at the buses for opera-i tion at the upper or lower limits of the normal 224 KV to 234 KV ' voltage range of the 220 KV ~ switchyard buses.
Included in the response to Item lb above and ' Item ld below are the calculated safety features bus voltages expressed in percent of rated motor voltage.
id.
Request:
Assuming the facility auxiliary loads are being carried by the station generator, provide the voltage profiles at the safety buses for grid vol-tage at the normal maximura value, the normal minimum value, and at the degraded conditions (high or low voltage, current, etc.) which would require generator trip.
Response
With the plant non-safety related auxiliary loads being supplied from the station generator through the unit-auxiliary transformers, the following calculated steady state voltages will exist at the safety related buses for 220 KV switchyard voltage at the normal maximum value and the normal minimum value:
Case I:
a) Switchyard voltage at 224 KV.
b) Maximum safety related load.
Calculated Calculated % Of Safety Bus No.
Bus Voltage Rated Motor Load 4160 Volt Bus 4A 4137 99%
4160 Volt Bus 4B 4137 99%
480 Volt Bus 3A 448' 97%
480 Volt Bus 3B 442 96%
Case II:
a) Switchyard voltage at 234 KV.
b) No safety related load.
i Calculated Calculated % Of Safety Bus No.
Bus Voltage Rated Motor Load i
4160 Volt Bus 4A 4435
.106%
4160 Volt Bus 48 4435-
.106%
480 Volt Bus 3A 499 1108%
480 Volt Bus 38 499 108%
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. The generator is not provided with an automatic trip for conditions of low vol tage.
However, to ensure plant operatian at reduced 220 KV system vol-tages the generator will be manually isolated from the system if the generator voltages drops below 19 KV, for extended periods, which is equivalent to a 220 KV switchyard bus voltage of 209 KV if the unit is assumed operating at rated load and power factor.
This is necessary to prevent tripping of plant motors on overcurrent due to the undervoltage condition. At this condition, the voltage at the safety related buses are calculated to be as follows:.
Case III:
a) Switchyard voltage at 209 KV.
b) Non-safety related loads being supplied from the unit aux-iliary transformers.
c) Maximum safety related load.
Calculated Calculated % Of i
Safety Bus No.
Bus Voltage Rated Motor Voltage 4160 Volt Bus 4A 3860 93%
4160 Volt Bus 4B 3860 93%
480 Volt Bus 3A 418 91%
480 Volt Bus 3B 413 90%
The generator is provided with an automatic trip for high voltage conditions.
The trip setpoint is 24 KV on the generator bus.
The maximum conceivable overvoltage conditions at the 220 KV bus and safety related buses would be the case where all offsite 220 KV transmission lines were disconnected, the generator operating at 24 KV, the high voltage terminals of the main step up transformer connected to the switchyard 220 KV buses, and the startup transformers at no load.
For this case, the 220 KV bus voltage would be at 253 KV.
The calculated safety features bus voltages would be as follows:
Case IV:
a) All offsite power lines disconnected.
b Main generator at 24 KV.
c Switchyard voltage at 253 KV.
d No safety features load.
e) Plant non-safety related auxiliary load connected to the unit auxiliary transformers.
Calculated Calculated % Of Safety Bus No.
Bus Voltage Rated Motor Voltage 4160 Volt Bus 4A 4796 115%
4160 Volt Bus 4B 4796 115%
430 Volt Bus 3A 540 117%
480 Volt Bus 3B 540 117%
le.
Request:
Identify the sensor location and provide the trip set facility's Loss of Offsite Power (undervoltage trip) point for your instrumentation.
include the basis for your trip setpoint selection.
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. Response:
Loss of offsite power is sensed by an undervoltage relay connected to each 4160 volt safety related switchgear bus through potential transformers.
The relay senses bus voltage and is set to isolate the safety related bus from the offsite power source at 3534 (85% of 4160) volts at the 4160 volt bus. The basis for this setting is to allow for continuous operation of the safety related buses down to 90% of rated motor voltage and prevent unnecessary transfers of the buses on short ti; e voltage excursions between 85 and 90 percent.
If.
Request:
Assuming operation on offsite power and degradation of the grid system voltage, provide the voltage values at the safety ralated buses corresponding to the maximum value of grid voltage and the degraded grid voltage corre-sponding to the undervoltage trip setpoint.
Response
The conditions for the conceived maximum degraded value of 220 XV switchyard bus voltage are described in the response to Item ld above, Case IV.
For this same condition, but with all of the safety related and non-safety related loads being supplied by the startup transformers, the calculated voltage values at the safety related buses would be as follows:
l Case I:
a) Generator at 24 KV.
b) Switchyard bus voltage at 253 KV.
c) All plant auxiliary loads supplied by the startup transformers.
d) Maximum plant safety and non-safety related loads on.
Calculated Calculated % Of Safety Bus No.
Bus Voltage Rated Motor Voltage 4160 Volt Bus 4A 4393 106%
4160 Volt Bus 4B 4393 106%
480 Volt Bus 3A 470 102%
480 Volt Bus 3B 475 103%
The calculated voltage conditions at the safety related buses corresponding to the undervoltage relay setpoint which isolates the safety features buses and starts the emergency generators is as follows:
Case II:
a) The safety related 4160 volt buses at the undervoltage trip point.
b) Maximum safety features load on.
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?b
. Calculated Calculated % Of Safety Bus No.
Bus Voltage
. Rated Motor Voltage 4160 Volt Bus 4A 3534 85%
4160 Volt Bus 4B 3534 85%
480 Volt Bus 3A 382 83%
480 Volt Bus 3B 379 82%
For this. condition, the calculated switchyard 220 KV bus voltage would be 203 KV.
Ig. Requesh Utilizing the safety related bus voltage values identified in (f), evaluate the capability of all efety related loads, including related control cir-cuitry and instrumentat:on, to perform their safety functions.
Include a definition of the voltage range over which the safety related components and non-safety components, can operate continuously in the performance of their design function.
Response
The safety related electrical loads can be grouped into the following categories:
1.
Continuous running electric drive motors.
2.
Intermittent duty electric drive motors for motor operated valves.
3.
Battery chargers.
4.
Resistance type electric heat tracing equipment.
5.
AC control systems associated with motor control centers.
Continuous running electric motors for both safety features and non-safety related service were specified to be capable of operacing in accor-dance with NEMA (MG-1) standards which requires continuous operation at a voltage range from 10% below to 10% above rated voltage.
For 460 volt motors this range would be from 414 to 506 volts.
For 4160 volt rated motors this voltage range would be from 3744 to 4576 volts.
Design speci-fications for the safety related motors required that they be capable of starting and accelerating their connected loads at 75% of rated voltage.
This would be 345 volts for 460 volt rated motors and 3120 volts for 4160 volt motors.
The ability of each motor to operate continuously without malfunction outside of its rated voltage range is unknown.
The District's specifications for motor operated safety related valves required that they be capable of operating the valve to the safety features actuation position over a voltage range of from 70 to 110% of
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. rated voltage. This corresponds to a voltage of 322 to 506 volts for the 460 volt rated valve drive motors.
Non-safety related motor operated valves were specified to operate over the voltage of 10% below to 10%
above rated voltage (414 to 506 volts).
In that motor operated valves only operate for short time periods, we consider that they can perform without malfunction within the voltage values identified in Item lf above.
The safety related battery chargers, in addition to charging the batteries, supply DC power to safety related DC control systems, inverters, DC motors, and control room emergency lighting.
The battery chargers for the 125 volt DC battery systems, safety and non-safety related, were specified to main-tain the setpoint output DC voltage level to within 1/2% over an input voltage range of 432 to 528 volts AC.
While the capability of the battery chargers to operate continuously without malfunction outside of this voltage range is unknown, protective features are provided to prevent discharge or damage to the batteries in case of charger malfunction.
Diodes in the charger output prevent discharge of the batteries back into the chargers in the event that the DC output of the chargers drops below the battery voltage.
Each battery charger is provided with an output DC overvoltage protective relay which will trip the charger input breaker if the DC output exceeds 140 volts. An alarm is provided in the Control Room for loss of battery charger output voltage. Also, in that the safety related batteries were designed to actuate and maintain the safety related DC power requirements for a 2-hour period without the chargers in operation we do not consider that the battery chargers are of concern relative to the voltage values stated in the respon,se to Item lf above.
The heat tracing system for the safety related pipes, tanks, and valves containing boric acid consists of strip type resistance heating tape, transformers, thermostats and contactors.
The extreme upper and lower voltage limits at which this equipment can operatt,ithout malfunction is unknown. The controls for the safety and non-safety related heat tracing systems are designed in acccrdance with NEMA Industrial Controls and Systems standards which in general specifies a continuous operating capability of from 10% below to 10% above rated voltage. On a 480 volt base this would range from 432 to 528 volts.
Each safety related heat tracing system is provided with undervoltage and temperature monitors which alarm in the Control Room should loss of voltage occur or the temperature of the heat traced system deviate by more than 10 F above or below the setpoint.
The temperature of the boric acid solution is maintained at a level of approximately 40*F above the point at which crystallization would occur.
Because of these factors and the heat retaining capacity of the system due to insulation we do not consider that the heat tracing system should be of concern relative to operating at the voltage limits stated in the response to Item if above.
Other than the controls for the electric heat tracing systems, the only additional safety related controls directly effected by AC bus voltage deviations are the controls for the safety features motor control center circuits. All remaining safety related control systems are supplied from the DC buses which are buffered from AC bus voltage transients by the battery chargers previously discussed.
p ej l The control voltage for each 480 volt motor control center feeder circuit is obtained from each feeder line tap through a 480 to 120 volt stepdown control power transformer.
The safety and non-safety related motor control centers for the plant were specified to be designed and constructed in accordance with the NEMA Standards for Industrial Controls and Systems.
The NEMA standards state that the continuous voltage rating for this type of control system is from 10% below to 10% above rated voltage. This would be equivalent to a voltage range of from 432 to 528 volts which does not encompass the low voltage range stated for the 480 volt safety related buses in Item lf above. The actual maximum and minimum voltage limits at which these control systems can operate continuously without malfunction is unknown. Tests have been conducted on motor starting contactors and it has been found that the contactors pick up at approximately 71% of rated voltage and drop out at approximately 46% of rated voltage. The auxiliary machine tool control relays used in the motor control centers have also been tested and these relays will pick up and hold in at approxi-mately 63% of rated voltage.
The shunt trip devices for the molded case circuit breakers in the motor control feeder circuits operate at test values around 50% of rated voltage.
Starting tests of safety features motors using the emergency generators has given results which show that the control circuits for the motor control centers will operate without malfunction down to 72% of rated volts under short time undervoltage transient conditions.
All safety related AC instrumentation is supplied from a DC bus through an inverter with automatic voltage regulation and is considered to be insensitive to AC bus voltage fluctuations.
Some non-safety related AC instrumentation is supplied directly from a 480 volt motor control center through a 480 volt to 120 volt stepdown transformer.
The rated operating range for this instrumentation is from 108 to 132 volts.
This would be equivalent to 432 to 528 volts on the 480 volt AC bus.
Ih.
Request:
Describe the bus voltage monitoring and abnormal voltage alarms available in the Control Room.
Response
The following bus voltages are monitored in the Control Room:
1.
230 KV Switchyard Voltage 2.
Both 6.9 KV Buses 3.
All 4160 V Buses 4.
All 480 V Switchgear Buses l
The following low voltage alarms are annunciated in the Control Room:
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% Of Description Alarms At Rated Rating Base Low Generator Voltage 21 KV 95%
22 KV Low 6.9 KV Bus Voltage 5.58 KV 85%
6.6 KV
- Low 4.16 KV Bus Voltage 3.72 KV 89%
- Low 480 Volt Switchgear Voltage 420 91%
460 Volts
- Safety Related and Non-Safety Related 2.
Request:
The functional safety requirements of the undervoltage trip is to detect the loss of offsite (preferred) power system voltage and initiate the necessary actions required to transfer safety related buses to tha onsite power system. Describe the load shedding feature of your design (required prior to transferring to the onsite [ diesel generator] systems) and the capability of the onsite systems to perform their function if the load shedding feature is maintained after the diesel generators are connected to their respective safety buses.
Describe the bases (if any) for reten-tion or reinstatement of the load shedding function after the diesel generators are connected to their respective buses.
Response
The following excerpts from the FSAR for Rancho Seco Unit No. I describes the automatic sequence of events that occur on loss of normal (offsite) voltage supply to the 4160 volt safety related buses:
" Step 1.
All the closed breakers on the bus will be tripped and the diesel engine will start.
Step 2.
After the unit comes up to speed and voltage, the auxiliary diesel generator breaker will close, pro-vided the bus is not faulted.
Step 3.
The station service transformer breaker will close 5 seconds after Step 2.
Step 4.
The diesel room ventilating fans will start 15 seconds after Step 2.
Step 5.
The nuclear service raw water pump, which provides water for cooling the closed cooling water system of the diesel engine, will start 25 seconds after Step 2.
Equipment required for safe plant operation will be started manually.
If there is a requirement for safety features system operation, the automatic sequential starting of safety features equipment will be c::complished accoiding to Table 8.2-2."
,o G
. The undervoltage automatic load shedding feature is not maintained or reinstated after the diesel generators are connected to their respective 4160 volt safety features buses.
The safety features actuation system load shedding scheme is maintained after the diesel generators are connected to their respective buses.
The safety features actuated load shedding scheme is maintained for the purpose of providing for normal sequential safety equipment loading of the diesel generators should the need occur after the diesel generators are connected and operating on the safety features buses.
3.
Request:
Define the facility operating limits (real and reactive power, voltage, frequency, and other) established by the grid stability analyses cited in the FSAR.
Describe the operating procedures or other provisions presently in effect for assuring that your facility is being operated within these limits.
Response
On Page 8.2-1, Paragraph 8.2.1.2(A), of the Rancho Seco FSAR reference is made to a strbility analysis. This analysis pertains to stable operation of the transoission system for a full load trip of Rancho Seco Unit-No.1.
The study determined that a full load trip of Rancho Seco will not disrupt the availability of the offsite power.
This study did not establish any operating 1*mits for the main generator.
However, to prevent out of step operation of the generator and possible unit or system instability problems due to either a decrease in excitation or loss of excitation or underfrequency conditions, the unit is provided with the following:
1.
An automatic minimum excitation limiter which will prevent lowering of the excitation to a point where the machine would pull out of step and become unstable.
2.
A loss of field protective relay that will automatically trip and isolate the generator from the system for excessive reactor power flow'into the machine as would occur on an out of step condition.
3.
The unit will automatically separate from the grid at the l
following frequencies i
Frequency Time Before Trip 58 Hertz 3 Minutes 57 Hertz 1 Minute 56 Hertz 0.50 Saconds
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. The main generator limits for real power and reactive power are shown in.
Written operating procedures are available in the plant Control Room which define the operating limits for the msir. generator with regards to voltage, speed (frequency), and real and reactive power.
Plant prccedures also define operating limits for the various plant buses.
4.
Request:
Provide a description of any proposed actions or modifications to your facility based on the results of the analyses performed in response to Items 1-3 above.
Response
Proposed facility modifications:
1.
We propose to install a new overvoltage trip circuit on the 4160 volt safety related buses with a setpoint to trip at 111% of rated motor voltage.
This circuit would isolate the safety related buses from the system, and provide the same identical diesel generator starting and bus load-shedding sequence as does the existing undervoltage trip sequence (refer to the response for Item No. 2 above).
1 2.
We propose to restudy our transformer tap settings and existing safety features bus undervoltage trip settings to determine if additional optimization of the system can be made in regards to undervoltage operating conditions.
If this study shows that practicable improvements can be made then we will notify the NRC within 30 days of the date of this response.
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