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o NRR/SELB INPUT FOR AIT REPORT ON PILGRIM LOOP EVENT OF 11/12/87 Author:
J. Knox 4.1.1.2 Operations and Ecuipment Failures During The Event At 2:05:26 A.M. on November 12, 1987, one of two 345 kV transmission lines (No. 342) connecting the Pilgrim Station switchyard to the Boston Edison transmission net-I work, experienced an "A" phase to "C" phase to ground fault (See Figure No. 3).
As a result of this fault on the No. 342 line, protective relaying operated cau-sing breakers No.103 and No.104 to trip open to isolate the fault. Also, be-cause No. 104 breaker was slow to open, backup or stuck breaker protective relay-ing operated causing a transfer trip signal to be sent to breaker Nos. 103, 105, 2130, 412, and 312. The transfer trip signal caused breaker Nos. 105, 2130, and 312 to trip open and caused the autematic reclose feature on breaker Nos. 103, 105, 2130, and 312 to be removed or locked out.
For a short time following isolation of the initial 'ault on the 342 line described above, the Pilgrim Station was energized from the renaining 345 kV transmission line (No. 355) through breaker 102 and the startup transfomer. At 2:05:34 A.M.
(7 seconds following the initial fault on the 142 line) the 355 line experienced a 'B" phase to "C" phase fault. As a result of this fault en the No. 355 line, protective relaying operated causing breaker 1670 to trip open and to reclose j
0.5 seconds later.
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I During the 0.5 second time required for reclosing, the licensee hypothesized that motors, normally energized from the 345 kV transmission lines through the l
startup transfonner and the Pilgrim distribution system, generated a volt 6ge l
backfeed through the startup transformer to the fault on the No. 355 line.
This backfeed caused the motors to quick.iy slow down with a resultant frequency decay. The frequency decay resultad in an increased voltage-frequency relation-ship to the startup transfortner which caused the HV-1 differential protectivt i
ttleying on the startup transformer to activate. Actuation of differential l
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- relaying caused the trip opening of breaker Nos. 102, 152-104, 152-204, 152-304, 152-404, 152-504, and 152-604 and the loss of offsite power to the Pilgrim sta-tion from the Boston Edison transmission network.
4.1.1.2.3 Swinging / Oscillating itansmission Lines At 2:06 A.M. on November 12, 1987, the No. 342 line experienced an "A" phase to "C" phase to ground fault.
Following this initial fault on the No. 342 line, "the No. 355 line experienced a "B" to "C" phase fault. These faults on the Nos. 342 and 355 lines were believed to be caused by snow and wind. With snow buildup and dropping of snow at different times, the various phase lines and the l
static line would be swinging witt the wind and oscillating at different rates causing them to come into proximity with each other. When the lines came near enough, a fault was detected by protective relaying which isolated the faulted i
lines.
l 4.1.1.2.4 Insulators l
At 2:06 A.M. on November 12, 1987, the Nos. 342 and 355 lines were isolated by protective relaying due to faults on the lines.
Following isolation of the Nos. 342 and 355 lines, the insulators located in the Pilgrim Station switchyard were observed to be packed with snow from top to bottom on the sides facing the northeast wind. Also, the insulator skirts were imbedded in snow. A;; this time with their snow covering, the insulators would haw been unable to perfonn their design function of insulating the 345 kV lines from ground.
The snow would have allowed, if energized, the 345 kV lines to fault to ground.
The insulators required to re-energize the startup transfonner were washed, cleared of snow, and returned to operable status at 1:30 P.M., approximately 11.5 hours5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br /> following the initial isolation of the fault on the Nos. 342 and 355 4
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lines. The insulators required to re-energize the main and unit auxiliary trans-formers for backfeed were washed, cleared of snow, and returned to operable status at 8:00 P.M., approximately 18 hours2.083333e-4 days <br />0.005 hours <br />2.97619e-5 weeks <br />6.849e-6 months <br /> following the initial isolation of the fault on the Nos. 342 and 355 lines.
4.1.1.4 Failure History Since the Pilgrim Station went into operation in June 1972, the station's 345 kV i
offsite system has experienced twenty (20) events, including the November 12, 1987 event, which have caused loss of 345 kV offsite power to the station's onsite i
electric distribution system. Of these twenty events, five (5) were caused by a single line outage occurring during the same time frame that equipmer,t was out of service for maintenance or modification, iive (5) were caused by lighti'g, four (4) by snow and wind, three (3) by insulator salt contamination during ocet stonns,
t and three (3; by other miscellaneous causes.
In addition to the 345 kV offsite system, the Pilgrim Station maintains a secondary i
23 kV offsite power line from a neighboring utility to the insite safety related i
distribution system. This 23 kV offsite line is designed to automatically carry i
the station's safe shutdown loads given loss of the offsite 345 kV and onsite diesel generator systems. The unavailability of this 23 kV line and the 345 kV system will cause a total loss of offsite power (LOOP) event at the station.
t The 23 kV line was unavailable at the Pilgrim Station during four of the twenty events described above. Three (3) unavailabilities were caused by snow and wind and the remaining unavailability (caused by its being tagged out of service for modification during the November 12, 1987 snowandwindstorm)mayalsohavebeen caused by snow and wind. A total of four LOOP events have, thus, occurred over f
the past sixteen years at the Pilgrim Station. The frequency of total LOOP's l
to date at the Pilgrim Sta*, ion, is 4/16 or 0.25 per reactor year (counting the f
November 12,1987 event).
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4.1.1.5 Changes / Improvements Since the Pilgrim Station went into operation in 1972, the licensee has initiated and completed a number of changes for the purpose of improving the overall reli-ability of the 345 kV offsite system.
On March 15, 1979, the licensee initiated a design modification to the 345 kV transmission system in order to reduce the number of double line outages (with consequentreactortrip)beingcausedbylightning. This modification was com-pleted on March 30, 1980.
Prior to modification there had been three double line outages due to lightning.
Following modification there have been no double line outages due to ligt:tning, i
i In 1980, the insulators on the Nos. 342 and 355 lines subject to salt contamina-i l
tion, were replaced with RG units. This type of insulator has a resistance glaze which produces a leakage current of about 0.5m Amperes / unit when connected to an i
energized line. The design principle it that the leakage current will keep the insulator warm resulting in more unifonn and rapid drying during fog or mist con-dition. Hence, the high voltage gradient across the insulator's surface, normally associated with the uneven drying on contaminated conventional insulators, is l
eliminated and in turn the possibility of flashover. The licensee believes that 4
this modification has significantly reduced transmission line flashovers caused
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j by surface contamination since 1980.
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j A program for routine inspection and periodic washing of the switchyard insula-
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tors was initiated to reduce flashover incidents caused by salt contamination
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buildup. The licensee found that this technique has been effective in reducing i
l flashovers attributed to slow buildup of contamination, but has not been as I
effective for a severe buildup over a short time. Thus, the licensee investigated the use of a new silicone elastomeric coating for application on switchyard in-sulators.
Industry experience with this new coating has shown that it will give l
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years of flashover protection on transmission lines even in locations with heavy industrial contamination, dust, pollution from highways, and salt fog. The licensee completed installation of this coating on all but two switchyard in-sulators in April 1987. Operating experience, limited to the November 12, 1987 l
snow storm, gave a preliminary indication, based on an observed reduction of i
insulator noise, prior to the 2:06 A.M. loss of power, that the new coating will j
be tffective in reducing flashover due to salt contamination.
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Other changes which have been complete or will be completed before plant restart F
j include modification of the startup transformer fast transfer logic, modifica.lon l
to protective relaying, and installation of monitoring equipment for better analysis of 345 kV system faults and breaker operation, i
i 4.1.1.6 Conclusions and Recommendations
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L Changes implemented by the licensee for improvement of reliability of the 345 kV l
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offsite system, reduce, to the extent practical, the likelihood of 345 kV system l
failure due to weather conditions.
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i NRR/SELB INPUT l
1 FOR AIT REPORT ON P!LGRIM l
LOOP EVENT OF 11/12/87 i
j Author:
J. Knox j
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4.1.1.2 Operations and Equipment Failures During The Event
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At 2:05:26 A.M. on November 12, 1987, one of two 345 kV transmission lines (No. 342) 1 connecting the Pilgrim Station switchyard to the Boston Edison transmission net-work, experienced an "A" phase to "C" phase to ground fault (See Figure No. 3).
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As a result of this fault on the No. 342 line, protective relaying operated cau-f sing breakers No.103 and No.104 to trip open to isolate the fault. Also, be-
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j cause No. 104 breaker was slow to open, backup or stuck breaker protective relay-l 4
l ing operated causing a transfer trip signal to be sent to breaker Nos. 103, 105, l
2130, 412, and 312.
The transfer trip signal caused breaker Nos. 105, 2130, and I
I 312 to trip open and caused the automatic reclose feature on breaker Nos. 103, 105, f
l 2130, and 312 to be removed or locked out.
4 For a short time following isolation of the initial fault on the 342 line described above, the Pilgrim Station was energized from the remaining 345 kV traasmission I
line (No. 355) through breaker 102 and the startup transformer. At 2:05:34 A.M.
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(7 seconds following the initial fault on the 342 line) the 355 line experienced a "B" phase to "C" phase fault. As a result of this fault on the No. 355 line, I
protective relaying operated causing breaker 1670 to trip open and to reclose I
0.5 seconds later.
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Curing the 0.5 second time required for reclosing, the licensee hypothesized l
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that motors, normally energized from the 345 kV transmission lines through the j
startup transformer and the Pilgria distribution system, generated a voltage
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backfeed through the startup transformer to the
. ult on the No. 355 line.
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This backfeed caused the motors to quickly slow down with a resultant frequency
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decay. The frequency decay resulted in an increased voltage-frequency relation-l ship to the startup transformer which caused the HU-1 differential orotective relaying on the startup transformer to activate. Actuation of differential i
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l relaying caused the trip opening of breaker Nos. 102, 152-104, 152-204, 152-304, j
I 152-404, 152-504, and 152-604 and the loss of offsite power to the Pilgrim sta-tion from the Boston Edison transmission network.
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4.1.1.2.3 Swinging / Oscillating Transmission Lines
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l At 2:06 A.M. on November 12, 1987, the No. 342 line experienced an "A" phase to l
"C" phase to ground fault.
Following this initial fault on the No. 342 line, l
l the No. 355 line experienced a "B" to "C" phase fault.
These faults on the l
Nos. 342 and 355 lines were believed to be caused by snow and wind. With snow buildup and dropping of snow at different times, the various phase lines and the f
static line would be swinging with the wind and oscillating at different rates causing them to come into proxhity with each other. When the lines came near l
j enough, a fault was detected by protective relaying which isolated the faulted i
j lines.
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l 4.1.1.2.4 Insulators l
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I At 2:06 A.M. on November 12, 1987, the Nos. 342 and 355 lines were isolated by 3
j protective relaying due to faults on the lines.
Following isolation of the j
Nos. 342 and 355 lines, the insulators located in the Pilgrim Station switchyard l
1 were observed to be packed with snow from top to bottom on the sides facing the i
northeast wind. Also, the insulator skirts were imbedded in snow. At this time f
i with their snow covering, the insulator would have been unable to perfonn their l
design function of insulating the 345 kV lines from ground. The sn:,w would have allowed, if energized, the 345 kV lines to fault to ground.
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j The irsulators required to re-energize the startup transformer were washed.
i cleared of snow, and returned to operable status at 1:30 P.M., approximately 11.5 hours5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br /> folicwing the initial isolation of the fault on the Nos. 342 and 355 I
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3-I lines. The insulators required to re-energize the main and unit auxiliary trans-formers for backfeed were washed, cleered of snow, and returned to cperable status f
at 8:00 P.M.. approximately 18 hours2.083333e-4 days <br />0.005 hours <br />2.97619e-5 weeks <br />6.849e-6 months <br /> following the initial isolation of the fault l
on the Nos. 342 and 355 lines, j
i 4.1.1.4 Failure History
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i Since the Pilgrim Station went into operation in June 1972, the station's 345 kV offsitesystemhasexperiencedtwenty(20) events,includingtheNovember 12, 1987 event, which have caused loss of 345 kV offsite power to the station's onsite electric distribution system. Of these twenty events, five (5) were caused by l
a single line outage occurring during the same time frame that equipment was out j
of service for maliitenance or modification, five (5) were caused by lighting, four
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(4) by snow and wind, three (3) by insulator salt contamination during ocean stonns.
l and three (3) by other miscellaneous causes.
In addition to the 345 kV offsite system the Pilgrim Station Sintains a secondary l
23 kV offsite puer line from a neighboring utility to the et ste safety related j
distribution system. This 23 kV offsite line is designed to automatically carry the station's safe shutdown loads given loss of the offsite a45 kV and on.ite j
diesel generator systems. The unavailability of this 23 kV line and the 345 kV l
system will cause a total loss of offsite power (LOOP) event at the station.
The 23 kV line was unavailable at the Pilgrim Station during four of the twenty
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events described above. Three(3)unavailabilitieswerecausedbysnowandwind t
and the remaining unavailability (caused by its being tagged out of service for
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mcdification during the November 12, 1987 snow and wind stonn) may also have been j
caused by snow and wind. A total of four LOOP events have, thus, occurred over I
the past sixteen years at the Pilgrim Station. The frequency of total LOOP's to date at the Pilgrim Station, is 4/16 or 0.25 per reactor year (counting the November 12,1987 event).
. 4.1.1.5 Changes / Improvements Since the Pilgrim Station went into operation in 1972, the licensee has initiated and completed a number of changes for the purpose of improving the overall reli-i ability of the 345 kV offsite system.
Or. March 15, 1979, the licensee initiated a design modiffcation to the 345 kV transmission system in order to reduce the number of double line outages (with consequent reactor trip) being caused by lightning.
This modification was com-l pleted on March 30, 1980.
Prior to modification there had been three double line outages due to lightning.
Following modification there have been no double line outages due to lightning.
In 1980, the insulators on the Nos. 342 and 355 lines subject to salt contamina-tion, were replaced with RG units. This type of insulator has a resistance glaza which produces a leakage cu' ;at of about 0.5m Amperes /urit when connected to an energized line.
The design principle is that the leakage current will keeo the insulater warm resulting in more uniform and rapid drying during fog or mist con-dition. Hence, the high voltage gradient across the insulator's surface, normally l
associated with the uneven drying on contaminated conventional insulators, is eliminated and in turn the possibility of flashover.
The licensee believes that this modification has significantly reduced transmission line flashovers caused i
by surface contamination since 1980.
A program for routine inspection and periodic washing of the saitchyard insula-tors was initiated to redrae flashover incidents caused by salt contamination buildup. The licensee found that this technique hos been effective in reducing flashovers attributed to slow buildup of contamination, but has not been as l
effective for a severe buildup over a short time. Thus, the licensee investigated the use of a new silicone elastoreric corting for application on switchyard in-sulators.
Industry experience with this new coating has shown that it will give J
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' i years of flashover protection on transmission lines even in locations with heavy industrial contamination, dust, pollution from highways, and salt fog. The j
licensee completed installation of this coating on all but two switchyard in-sulators in April 1987. Operating experience, limited to the November 12, 1987 l
snow stom, gave a preliminary indication, based on an observed reduction of insulator noise, prior to the 2:06 A.M. loss of power, that the new coe*ing will i
be effective in reducing flashover due to salt contamination.
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Other changes which have been complete or will be completed before plant restart l
include modification of the startuu transformer fast transfer logic, modification to protective relaying, and installation of monitoring equipment for better l
r analysis of 345 kV system faults and breaker operatien.
4.1.1.6 Conclusions and Recomendations Changes implemented by the licensee for improvement of reliability of the 34; kV offsite system, reduce, to the extent practical, the likelihood of 345 kV system j
failure due to weather condition
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