Cold Weather Circuit Breaker Performance

Ken Benoit, Minnesota PowerKasey Borboa, Minnkota Power Cooperative

Minnesota Power System Overview

Number of SF6 Gas Breakers by Voltage

in MN

500 kV 1

230 kV 45

115 kV 162

Transmission Substations by Voltage

in MN

500 kV 1

345 kV 1

230 kV 16

138 kV 2

115 kV 60

Wholesale electric generation and transmission cooperative headquartered in Grand Forks, N.D.

11 member-owner cooperatives• 3 in ND, 8 in MN

12 municipals • Northern Municipal Power Agency

Serve 35,000 square miles

Substations• 40 Transmission• 212 Distribution

174 SF6 Breakers

Minnkota Power Cooperative System Overview

7-day weather outlook in International Falls

• Average Hi -8°F• Average Lo -31°F

Three daily records • 27th low -46°F• 30th hi -19°F• 31st low -45°F

Wind chill -60°F

20--

0 --

20--

40-

°F

Wind(MPH)

Jan-25 Jan-26 Jan-27 Jan-28 Jan-29 Jan-30 Jan-31

7-Day Lo/Hi

3.2 5.0 6.2 5.6 12.9 7.1 2.1

Polar Vortex

-50

-45

-40

-35

-30

-25

-20

-15

-10

-5

0Te

mpe

ratu

re

I-FallsDuluthGrand Forks

January 29th

Hourly Temperatures

January 30th January 31th

Pressure vs Temperature of SF6 Gas at Constant Density

6

Alarms during the Polar Vortex Jan 25 to 31, 2018 SnapshotJan 30 00:53 78L FBS BLOCKED, normal at 11:43, then opened

Jan 30 02:05 1H CAN BLOCKED, normal at 09:56

Jan 30 02:34 16L ARD BLOCKED, normal at 11:21, then opened, BO HTR A Ph.

Jan 30 02:37 5T VRG BLOCKED, normal at 11:57

Jan 30 03:00 5L MLS BLOCKED, normal at 09:08

Jan 30 05:04 1L HYT BLOCKED, normal at 09:09

Jan 30 05:24 26L TMS BLOCKED, normal at 10:30, then opened

Jan 30 09:04 98L ARD BLOCKED, normal at 10:44

Jan 30 09:51 20K SQB BLOCKED, normal at 11:19

Jan 30 20:48 79L CAN Breaker opened, normal at 10:59 on Jan 31

Jan 30 23:21 78L FBS BLOCKED, normal at 11:16 on Jan 31

Jan 31 00:10 50R LFK BLOCKED

Jan 31 10:34 16L VRG NORMAL, repaired BO HTR A Ph.

Jan 31 04:32 94L SHA Blocked, normal at 10:45

345 kV Breakers –1 Low Gas Lockout (TRIP) 230 kV Breakers –

5 CB Low Gas Alarm 115 kV Breakers –

1 Low Gas Alarm 69 kV Breakers – 1 Low Gas Alarm,

1 Low Gas Lockout (TRIP) 13.8 kV Reactor Switcher –

1 Low Gas Alarm

Operational Experience

Operational ConcernsUsually open breakers before being blocked from trip or close

while keeping the system intact If can not open breaker, let the breakers ride through a block.

Usually breaker becomes operational next day when sun comes out Replaced failed heater blankets on 20 breakers in 2018When breakers are in blocked condition, trip coil monitors

alarms from relays are activated No alarming on failed breaker heaters, changed specification

on purchasing breakers to included a tank heater alarm

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Annual breaker maintenance• Check tank and cabinet heaters

• Thermostat and contactor operation are verified

• Heater currents are measured and recorded, compared to previous results and nameplate

• Check SF6 gas levels• Inspect gas level and verify correct

pressure• Add gas as needed

MPC Maintenance and Operation

Operating conditions: -50°CTemperature compensated gaugesLocation of temperature probe

Insulated pipingLoss of AC alarmingTank heater failure alarming

Specification Considerations

Station service loading concerns when changing out Oil Circuit Breakers to SF6 Gas Breakers

500 kV 150A 240 VAC single phase

230 kV 33 to 38A 240 VAC single phase

115 kV 18 to 24A 240 VAC single phase

46 kV 13A 240 VAC single phase