ML20210F148
| ML20210F148 | |
| Person / Time | |
|---|---|
| Site: | Maine Yankee |
| Issue date: | 02/02/1987 |
| From: | Whittier G Maine Yankee |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| 8386L-GDW, GDW-87-25, MN-87-02, MN-87-2, NUDOCS 8702110008 | |
| Download: ML20210F148 (4) | |
Text
F MAIRE HARHEE 'ATOMICPOWERCOMPARUe avaugray,?,7eCg"al (207) 623-3521 e
February 2, 1987 MN-87-02 GDH-87-25 United States Nuclear legulatory Commission Document Control Desk Hashington, D. C.
20555
References:
(a) License No. DPR-36 (Docket No. 50-309)
(b) MYAPCo. Letter to USNRC dated November 7, 1986 (MN-86'139).
Maine Yankee Offsite Power Supply.
(c) MYC-430, Analysis / Calculation for the Auxiliary Power Supply Voltage Study for Maine Yankee, dated October 25, 1983.
Subject:
Offsite Power Supply Gentlemen:
In August 1986, representatives of NRC and Maine Yankee met to discuss the staff's questions on the 1982 low voltage incident. A synops;s of the information on the 1982 low voltage incident at the Maine Yankee plant was provided in Reference (b).
Information was also requested on the July 18, 1986 low voltage incident.
l Attachment A describes the results of our investigation. The voltage alarm of l
July 18, 1986 was determined to be a low voltage prealarm requiring no follow-up action by the operator.
It has also been determined that the Surowiec 115 kV line was capable of supplying the plant auxiliary power I
requirements, including worst case accident load.
i He plan to reevaluate the adequacy of the Surowiec 115 kV line to supply power to Maine Yankee's safeguard equipment each refueling cycle.
l Please contact me if you have any questions.
Very truly yours, MAINE YANKEE ATOMIC POWER COMPANY
$D00ko$0$0$o9 8k 4
PDR G. D. Whittier, Manager Nuclear Engineering and Licensing GDH/hbg Enclosure A d k @
- W " g,, @ rd cc: Mr. Ashok C. Thadani f
ar Mr. Cornelius Holden 8386L-GDH
MAINE YANKEE ATOMIC POWER COMPANY Attachment A
^ MN-87-02
+
JULY 18. 1986 LOH VOLTAGE INCIDENT AT MAINE YANKEE
. DESCRIPTION OF THE INCIDENT On July 18, 1986, the plant was preparing to come on line with about 22 MH station service load already running. The 115 kV line from Mason (Section 207), was.taken out of service for maintenance by Central-Maine Power Company (CMP) at 0800 hours0.00926 days <br />0.222 hours <br />0.00132 weeks <br />3.044e-4 months <br />. At 0915 hours0.0106 days <br />0.254 hours <br />0.00151 weeks <br />3.481575e-4 months <br />, one hour before starting RCP 1, the Shift Operating Supervisor (SOS) informed CMP's dispatcher of plans to start the pump.
The dispatcher indicated that there would be no problem with that action; however, upon starting RCP 1 at 1016 hours0.0118 days <br />0.282 hours <br />0.00168 weeks <br />3.86588e-4 months <br />, an alarm was received on annunciator window SS 2-2 in the Control Room indicating "115 kV Reserve Station Service Low Voltage". Voltage readings at the Maine Yankee main control board confirmed that the 115 kV line from Surowiec was between 112-113 kV.
The alarm cleared at 1047 hours0.0121 days <br />0.291 hours <br />0.00173 weeks <br />3.983835e-4 months <br /> when Section 207 was returned to service. No other alarms were received, or problems encountered with the 4160 volt buses or other plant auxiliaries.
CAUSES OF THE LOW VOLTAGE CONDITION In order to determine the causes of the low voltage condition at the Maine Yankee 115 kV bus,.the Section 69 configuration, the voltage, and the loading at the time of the incident was investigated.
Suroweic Line Confiauration Figure 1 shows a one-line diagram of the Surowiec to Maine Yankee 115 kV line (Section 69), the Surowiec to Mason 115 kV line (Section'81), the substations at Topsham and Bath, and various 34 kV lines interconnecting these substations.
It can be seen that since Maine Yankee is at the end of the line, the quality of power supplied is influenced by both the distance from Surowiec, and the loads along the route.
SUROHEIC 115 KV VOLTAGE The 345/115 kV transformer at Surowiec is the power source for the Section 69 line; its voltage is crucial in determining the performance and voltage at different points of the line. The transformer at Surowiec has a 33-step automatic load tapchanger which is set to hold the voltage at 121 kV.
Each tap represents a 0.625% change in voltage.
In trying to confirm that the Surowiec substation was being maintained at 121 kV past records were reviewed. The Section 69 voltage at Surowiec recorded by different sources at different times ranged between 117 kV and 118 kV. This disparity was resolved during a visit to Surowiec, when the voltages of five other lines emanating from Surowiec, were all found to be between 121 and 123 kV. He simultaneously asked the Augusta dispatcher to read the voltage at these same points on his remote meters in Augusta; he confirmed that all voltages were in the 120 to 123 kV range.
These two tests confirmed that 121 kV was being maintained by the transformer at Surowiec as designed. CMP personnel believe that the Section 69 values of 117-118 kV are in error because of incorrect calibration, or the age of the equipment being used at Surowiec to transmit the data to Augusta. 8386L-GDH
MAINE YANKEE ATOMIC POWER COMPONV Attachment A
- MN-87-02 Section 69 Loads To get an idea of the Maine-Yankee station service loads at the time, we have reviewed the Maine Yankee station computer hourly recording on both Sections
-207 and 69.
These showed the time at which Section 207 was disconnected; however, the real and reactive power values from these recordings do not add up before and after switching Section 207 in or out and are therefore not considered reliable. A recent test reading of the 115 kV watt and varmeters and simultaneous computer printout confirmed the discrepancies noted above and indicates that the transducers or the station computer input are not calibrated properly. He have therefore used a CMP magnetic tape record showing a demand metering load of 22.8 MH.
The load growth at the various Section 69 substations will adversely affect the quality of power at Maine Yankee and could be the cause of an undervoltage alarm. Data collected in October 1986 indicates that both Topsham and Bath maximum load growth has increased substantially over the previous 1984 values. Data for 1986 is shown with'the 1984 values in parenthesis.
Topsham maximum: 26.76 MVA (18.9 MVA), and Bath maximum:
33.57 MVA (21.5 MVA).
This constitutes a 421 and 56% increase, respectively.
The 1986-87 winter load forecasts obtained from NEPEX predict similar growth.
A preliminary voltage drop calculation using 22.8 MH at 0.75 power factor load for Maine Yankee, 22.8 MH at 0.93 power factor load for Bath, and 22.2 MH at 0.93 power factor load for Topsham, shows that if 121 kV voltage exists at the Surowiec substation, the Maine Yankee voltage would be 113 kV. This agrees with the voltage recorded during the July 18, 1986 incident at Maine Yankee, as well as with the setpoint of 112 + 1.4 kV for Low Voltage Alarm SS 2-2.
It appears that the low voltage at Maine Yankee was mainly a result of a combined heavy load condition at Topsham, Bath, and Maine Yankee; this caused Section 69 voltage to drop to 112-113 kV at Maine Yankee.
THE ABILITY OF SECTION 69 TO CARRY WORST-CASE ACCIDENT LOADS DURING THE TRANSIENT The minimum voltage necessary to assure operation of worst-case accident loads was determined in EDCR 80-50. This EDCR installed two undervoltage relays, one on 4160 V Bus 5 and one on 4160 V Bus 6.
The purpose of these relays was to provide each bus with a second level of undervoltage protection.
These instantaneous undervoltage relays are set to actuate at 3820140 V and correspond to a voltage above that necessary to assure operation of all safety-related electric equipment fed from these buses.
Actuation of the relays results in annunciation at window SS 5-9, indicating
" Bus 5/ Bus 6 Low Voltage". Since neither of these relays actuated during the July 18, 1986 incident, we conclude that voltage was satisfactory to power the worst-case accident loads.
The relay setting of 3820 1 40 V would correspond to a voltage of 105.6 1 1.1 kV on the 115 kV system. 8386L-GDH
MAINE YANKEE ATOMIC POWER COMPANY s
Attachment A MN-87-02 A preliminary calculation was made using the same data and assumptions described above except that the 30 MVA load at Maine Yankee was replaced with an emergency load of 43.85 MVA obtained from Reference (c). The results r
indicate that the voltage would drop to 110 kV at Maine Yankee; this voltage is approximately the same as that predicted in Reference (c) and shows that sufficient margin was available at the time of this low voltage condition to power the worst case accident loads.
UNDERVOLTAGE CONDITION AT PLANT BUSES DURING INCIDENT An undervoltage condition did not exist at the plant buses during the incident.
In addition, it should be noted that Alarm SS 2-2, which actuated on July 18, 1986, was installed to alert the operators to a lon of 115 kV reserve station service; this was its primary function.
It was not intended to be a low voltage alarm, even though it does fulfill this duty by being actuated by an undervoltage relay. Accurate low voltage annunciation is provided to the operators through SS 5-9 " Bus 5/ Bus 6 Low Voltage," which has been described above, hence, Alarm SS 2-2 should only be considered as a secondary prealarm with regard to low voltage. The setpoint of Alarm SS 2-2 corresponds to a voltage of 112 1 1.4 kV; the voltage recorded at Maine Yankee during the incident was 112-113 kV; therefore, Alarm SS 2-2 actuated as designed.
CONCLUSIONS Our investigation has determined that:
The cause of the July 18, 1986 low voltage incident at Maine Yankee was the result of combined heavy load conditions at Topsham, Bath, and Maine Yankee.
The Surowiec line (Section 69) was capable of providing the worst-case accident loads during the low voltage incident.
The computer models used to predict voltage at Maine Yankee need updating to reflect present load values and parallel paths between Topsham and Bath.
1.
The model will be updated by July 1, 1987.
An undervoltage condition did not exist at the plant during the low voltage condition. 8386L-GDH