ML20213E880

Forwards Summary of Dec 1982 Low Voltage Incident & Description of Corrective Actions,Per 860805 Request.Results of Evaluation of Data to Assess Offsite Power Supply Load Changes Will Be Made Available

ML20213E880
Person / Time
Site:	Maine Yankee
Issue date:	11/07/1986
From:	Whittier G Maine Yankee
To:	Thadani A Office of Nuclear Reactor Regulation
References
8097L-SDE, GDW-86-264, MN-86-139, NUDOCS 8611130335
Download: ML20213E880 (5)

Text

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VE MAIRE HARHEE ATOMICPOWERCOMPARUe ,uoug,,K,?n"gTag6 (207) 623-3521

$ November 7, 1986 MN-86-139 GDH-86-264 Director of Nuclear Reactor Regulation United States Nuclear Regulatory Commission Hashington, D. C. 20555 Attention: Mr. Ashok C. Thadani, Director PHR Project Directorate #8 Division of Licensing

References:

(a) License No. DPR-36 (Docket No. 50-309)

(b) MYAPCo Letter to USNRC dated March 24, 1983 (MN-83-54)

Offsite Power Supplies (c) MYAPCo Letter to USNRC dated December 19, 1983 (MN-83-249)  !

Auxiliary Power System Voltage Study for Maine Yankee

Subject:

Maine Yankee Offsite Power Supply Gentle [en:

On August 5, 1986, at the request of the NRC Licensing Project Manager for Maine Yankee, representatives of Maine Yankee and the NRC Staff met to discuss NRCts questions regarding the December. 1982 low voltage incident. This letter provides a summary of the background information regarding the 1982 low voltage incident, Maine Yankee's corrective actions, and our understanding of the results of our discussion.

A copy of the slides used during Maine Yankee's presentation at the August meeting is included as Attachment A.

It is our understanding that a safety evaluation will be prepared to close out this issue.

Very truly yours, f MAINEYANKEEATOM{CPOWERCOMPANY l Jj&nd S

G. D. Whittier, Manager

. Nuclear Engineering and Licensing

. GDH/bjp f

Enclosure cc: Dr. Thomas E. Murley Mr. Pat Sears \

Mr. Cornelius F. Holden cP 8611130335 861107 '

DR ADOCK 0500 9 8097L-SDE s

, , MAINE YANKEE ATOMIC POWER COMPANY December 1982 Low Voltage Incident

Background

The 115 kV System consists of two 115 kV transmission lines entering the Maine Yankee switchyard, one from the Mason Station (Section 207) and one from the Surowiec Substation (Section 69). Either of the two lines are independently capable of supplying the plant auxiliary power requirements (Maine Yankee FSAR Section 8.2.3).

I Descriotion of Event [From Reference (b)]

The Mason 115 kV line (Section 207) was taken out of service at approximately 0800 on December 14, 1982 in order to completely refurbish the line. At the time, Maine Yankee was starting up and required about 29 i MW of power. Also, Mason Station Units 4 and 5 were on line at 0800 on December 14, 1982 carrying a load of 41 MH. At 0900, the units were carrying a load of 55 MH. Shortly after the line was isolated, the 4160 V buses were observed to have dropped to approximately 3750 V and the Section 69 (Surowiec) 115 kV line had dropped to 109 kV.

Prior to and after the incident, the voltage was normal. During the event the voltage was low but was not logged.

The low voltage condition continued from about 0800 until 0840, at which time the Mason line was placed back into service. As a result of the low voltage condition, a high motor current alarm was received on Primary Component Coolant Pump P-9A but the pump did not trip. The only other observable in-plant indications included the tripping of two pieces of equipment, FV-5, main generator seal oil vapor extractor and C1B service air compressor (a second air compressor auto started), tripped on low voltage.

The low voltage occurrence appeared to be related to increased load growth on the Surowiec line. This additional load created a greater than anticipated voltage drop on the line.

Corrective Actions Central Maine Power Company took the Surowiec line out of service for upgrading and refurbishment. The project, completed in September, 1983, increased the line conductor size and thereby improved the voltage regulation at Maine Yankee. Until the project's completion, Maine Yankee's auxiliary power system was supplied by the 115 kV line from Mason Station.

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MAINE YANKEE ATOMIC POWER COMPANY 1

Maine Yankee performed an Auxiliary Power System Voltage Study in 1983, Reference (c), which revealed that the Surowiec line (Section 69) would be ,

capable of independently supplying the plant auxiliary power requirements '

during heavy load conditions, provided the transformer tap on reserve station transformer X-14 was increased by 51,. Also, Procedure 17-300-1 was established, requiring reassessment of the voltage system computer

model by determining, at least once every 18 months, any changes to the capacity of the offsite power supplies. If any significant changes in the offsite or onsite loads have occurred, the auxiliary power system voltage study would be redone to assure the continued capability of either 115 kV line of supplying the plant's safeguards loads. Most in-plant load changes occur during refueling outages and most offsite load changes occur gradually over time. Thus, refueling intervals provide a logical frequency to reassess the voltage loads on the 115 kV lines and plant-load changes to ensure that the capacity of either of the two 115 kV lines is capable of supplying the plant's auxiliary power requirements. >

Yankee Atomic Electric Company is presently evaluating the recently collected data to assess tne offsite power supply load changes. The '

results of this evaluation will be available in the near future.

115 kV Line Monitorina l The quality of power from the 115 kV Switchyard is capable of being

monitored by several methods. A voltage transformer placed in between the low voltage side of Reserve Station Service Transformer X-14 and the incoming breakers to Buses 3 and 4, is connected to an undervoltage relay i; which continuously monitors the voltage of the 115 kV System. Actuation of the relay results in annunciation at SS annunciator 2-2 indicating "115 kV Reserve Station Service Low Voltage". In response to this Panalarm, the operators are required to determine the cause of the low voltage and

! take corrective action. The undervoltage relay is an instantaneous type

! relay set to actuate at 4050 50 V, which corresponds to a voltage of 112

{

i 1.4 kV at the high voltage side of the transformer. A ten second time delay is provided to eliminate nuisance alarms due to transients, etc.

Four independent methods are also provided through voltage transformers j placed on Buses 1, 2, 3, and 4. Each voltage transformer is connected to an undervoltage relay which continuously monitors the voltage of Buses 1, i 2, 3, and 4, respectively. The 115 kV System is monitored through these

! relays whenever the buses are connected to the Reserve Station Service 1 Transformers. Actuation of the relays results in annunciation at SS j annunciators 1-30, 1-3L, 2-30, and 2-3L, indicating Bus 1, 2, 3 or 4 i undervoltage. The undervoltage relays used are induction disc type relays I

which are set to pick up at 5580 V for Buses 1 and 2, and 3255 V for Buses i 3 and 4. This corresponds to a voltage of 93 KV and 90 kV, respectively, j at the high voltage side of the transformers.

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, , MAINE YANKEE ATOMIC POWER COMPANY Four additional methods are provided through voltage transformers placed on emergency Buses 5 and 6. Each voltage transformer is connected to two undervoltage relays which continuously monitor the voltage of Buses 5 and

6. The 115 kV System is monitored through these relays when the buses are connected to the Reserve Station Service Transformers. Actuation of each of the relays results in annunciation at SS annunciators 3-3, 4-3, and 5-9 indicating " Bus 5 Undervoltage", " Bus 6 Undervoltage", and " Bus 5/ Bus 6 Low Voltage". The undervoltage relays used are of two types, each bus has one induction disc type relay set to pick up at 3255 V, and one instantaneous type set to actuate at 3820 1 40 V, a ten second time delay is provided for the signal from the instantaneous relay to eliminate nuisance alarms due to transients, etc. The above setpoints correspond to a voltage of 90 kV for the induction disc relays, and of 105.6 1.1 kV for the instantaneous relays, at the high voltage side of the transformers. The instantaneous relay settings provide each emergency bus with a second level of undervoltage protection and are set above the value required to assure that adequate voltage exists at the terminals of all safety-related equipment fed from these buses.

Voltmeters on the Main Control Board are also another method of monitoring the quality of the 115 kV system. Three voltmeters are provided on the ,

115 kV section of the Main Control Board to display voltages at the Mason '

Line, the Suroweic Line, and the Bus Tie Section, respectively. Six voltmeters are provided on the Station Service section of the Main Control Board to display voltages at Bus 1, 2, 3, 4, 5, and 6, respectively. l These voltmeters reflect 115 kV System voltages when the buses are connected to the Reserve Station Service Transformers.

White indicating lights are provided at switchgear Buses 1, 2, 3, and 4, and at the Main Control Board control switches for reserve source. These eight lights monitor both the availability of power from the 115 kV Switchgear, and continuity of the control circuit established to initiate reserve source auto-throwover. The lights are dual intensity; a bright white light indicates availability of 115 kV voltage, a dim white light indicates 115 kV voltage available but control circuit not set up for auto-throwover. No light indicates that 115 kV voltage is unavailable.

The relays that monitor this voltage are set to pick up at voltage corresponding to 100 kV for Reserve Station service Transformer X-16, and 4

96.8 kV for transformer X-14.

During the May,1985 meeting on this subject, the NRC requested that Maine Yankee conduct a simple test, by transferring plant loads to the Surowiec line during shutdown and verifying that the 115 kV bas voltage read as expected, to verify the adequacy of the Surowiec line. Maine Yankee developed Procedure 1-22-4-1, Surowiec 115 kV Voltage Test, and performed the test during the 1985 refueling outage with acceptable results. It was agreed by all participants at the August 5, 1986 meeting that the test was not necessary given Maine Yankee's program established to ensure the adequacy of the 115 kV system. Accordingly, Maine Yankee will no longer perform the test during each refueling in accordance with Procedure 1-22-4-1.

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, , MAINE YANKEE ATOMIC POWER COMPANY In summary, Maine Yankee has in place a variety of means to ensure that the capacity and capability of the 115 kV system, consisting of two independent 115 kV lines, are sufficient for the system to carry its safeguard loads. The tap on Station Service transformer X-14, which was increased by 5% in 1985 as a result of the December, 1982 low voltage incident, assures that the plant auxiliary power requirements can be provided from the Surowiec 115 KV line. To aid the operators in monitoring the quality of the 115 kV system, one relay, three voltmeters, and eight light indicators monitor the 115 kV system under all operational modes. An additional eight relays and six voltmeters monitor the 115 kV system whenever the buses are connected to the reserve station transformers. Furthermore, Maine Yankee has implemented Procedure 17-300-1, which requires a periodic reassessment of the load growth on the Mason and Surowiec 115 kV lines and at the plant to verify the assumptions used in the voltage system model. The capability of the 115 kV system to perform its intended function as required by General Design Criterion 17 is thus assured through this programatic assessment.

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