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Thermal Considerations when Condenser Bushings Are

Applied in Bus Duct

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The Thermal Issues

• Heat generated in the conductor tube due to flow of current and the electrical resistance of conductor tube

• Steady state temperature rise at the hot spot is determined by the temperature rise of the hot spot due to the flow of current and by the heat transfer characteristics of the bushing.

• The heat that is generated within the condenser can only leave the bushing by transfer into the oil or into the air at the terminal ends of the bushing.

• The heat dissipated at the terminal ends is determined by the temperature of the connecting bus and the surrounding medium (air side and oil side)

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Anatomy of a Condenser Bushing

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l

r1r2

• Field strength between co-axial cylinders:

• Capacitance of the cylinder

configuration:

• Capacitances in series:

)/(ln

1)(

12 rr

U

rrE

)/(ln

2

12 rr

lC

n

i iCC 1

11

Condenser Bushing Fundamentals

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Anatomy of a Condenser Bushing

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Anatomy of a Condenser Bushing

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Test Tap

Anatomy of a Condenser Bushing

Test Tap

Test Tap 450 kV BIL Voltage Tap 450 kV BIL

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C11

C12

C13

C14

C1

C2

Anatomy of aCondenser Bushing

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• Defined in IEEE C57.19.00, Requirements and Test Procedure for Outdoor Power Apparatus Bushings

• Section 4.1: “Usual Service Conditions”• Ambient air temperature = 40°C, max; -30°C, min.• 24 hour average transformer oil temperature = 95°C• External bus connection temperature rise = 30°C max, at

rated current.

• Section 5.4.1: ‘Thermal Basis of Rating• Bushing hot spot temperature rise at rated current =

65°C, max, over ambient air temperature.

• Hottest spot bushing temperature at rated current105°C = 40°C + 65°C

Standard Thermal Conditions for Condenser Bushings

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• Defined in IEEE C57.19.00, Requirements and Test Procedure for Outdoor Power Apparatus Bushings

• Section 4.1: “Usual Service Conditions”• Ambient air temperature = 40°C, max; -30°C, min.• 24 hour average transformer oil temperature = 95°C• External bus connection temperature rise = 30°C max, at

rated current.

• Section 5.4.1: ‘Thermal Basis of Rating• Bushing hot spot temperature rise at rated current =

65°C, max, over ambient air temperature.

• Hottest spot bushing temperature at rated current105°C = 40°C + 65°C

Standard Thermal Conditions for Condenser Bushings

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t = 30°C max T = 70 °C

40°C

95°C

t = 65°C T = 105°C

I = Rated CurrentStandard Thermal Conditions for Condenser Bushings

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70°C

95°C

40°C

Standard Thermal Conditions

105°C

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Thermal Conditions Inside ofBus Duct

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• Air side terminal temperature rise can be as much as 65°C according to IEEE C37.23

• Ambient air temperature inside of bus duct will, most likely, be greater than the 40°C specified as standard for condenser bushings.

• 24 hour average transformer oil temperature = 95°C

• The amount of heat generated within the bushing remains the same.

Potential Bus Duct Thermal Conditions

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• Air side terminal temperature rise can be as much as 65°C according to IEEE C37.23

• Ambient air temperature inside of bus duct will, most likely, be greater than the 40°C specified as standard for condenser bushings.

• 24 hour average transformer oil temperature = 95°C

• The amount of heat generated within the bushing remains the same.

Potential Bus Duct Thermal Conditions

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t = 65°C, T = 115°C >>40°C

50°C

Hot Spot = ??

I = Rated Current

Bus DuctThermal Conditions

95°C

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95°C

»» 40°C 50°C

not 70°C but t = 65°CT = 115°C

Bus Duct vs Free Air

t = 65°CT not = 105°CT »» 105°C

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High Temperature Condenser BushingDeveloped for Bus Duct Applications

• Condenser = Nomex™ (Aramid) insulation + aluminum foils

• All Seals = high temperature Viton™ (Fluorocarbon rubber)

• These materials are suitable for continuous operation at 200 °C

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