Types of Bushings and Voltage ratings

Bushings in Transmission Systems: Types, Classification & Applications

Bushings are one of the most critical components in transmission and distribution systems. Their primary role is to provide insulation between energized conductors and grounded parts, ensuring safe current transfer through transformer tanks, circuit breakers, switchgear, and other high-voltage equipment.

A failure in bushings can lead to catastrophic equipment damage, fire hazards, or extended outages. Hence, understanding their classification, design, and application is vital for electrical engineers and maintenance teams.

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Classification of Bushings

Bushings can be classified into three main categories:

1.      Based on Insulating Material Used

2.      Based on Construction Type

3.      Based on Internal Insulation

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1. Classification by Insulating Material

Bushings are often categorized based on the type of medium used for insulation:

·         Air-to-Oil Bushing – Air is used at one end, and oil at the other. Since oil has ~2x dielectric strength of air, the oil-side is shorter in length. Common in transformers.

·         Air-to-Air Bushing – Both ends insulated with air. Used in outdoor-to-indoor applications.

·         Air-to-SF₆ Bushing – Applied in SF₆ gas-insulated circuit breakers.

·         SF₆-to-Oil Bushing – Connects SF₆ bus ducts to oil-filled equipment.

·         Oil-to-Oil Bushing – Transition bushing between two oil-filled systems such as transformer-to-bus duct connections.

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2. Classification by Construction

There are two construction types:

(a) Solid Type (Bulk Type) Bushings

·         Conductor with porcelain/epoxy insulation.

·         Used up to 25 kV, typical for distribution transformers, switchgear, and generator step-up transformers.

·         Limitation: Cannot withstand >90 kV at 60 Hz.

(b) Capacitance Graded (Condenser Type) Bushings

·         Used above 25 kV up to 1500 kV.

·         Multiple conductive layers embedded in oil-impregnated paper or epoxy.

·         Better electric field distribution and compact design.

·         More expensive due to complexity but essential for high-voltage equipment.

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3. Classification by Internal Insulation

Depending on the insulating medium inside the bushing, they are further classified as:

1.      Air-Insulated Bushings – Used with air-insulated equipment; simple but limited in application.

2.      Oil-Insulated (Oil-Filled) Bushings – Mineral oil between conductor and insulation. Used in both solid and condenser bushings.

3.      Oil-Impregnated Paper (OIP) Bushings – Combination of oil and kraft paper, highly reliable for HV/EHV equipment.

4.      Resin-Bonded/Resin-Impregnated Paper (RBP/RIP) Bushings – Use resin instead of oil. RIP bushings are preferred in modern designs for reduced fire risk.

5.      Cast-Insulation Bushings – Solid cast epoxy material, robust and widely used in medium voltage systems.

6.      Gas-Insulated Bushings (SF₆) – Use pressurized SF₆ for insulation. Common in GIS and modern HV substations.

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Bushing Interfaces as per IS 2099

High-voltage bushings (>1 kV) are designed as per IS 2099 standards. The standard defines five types of interfaces, based on mounting and contact type:

Interface	Contact Type	Current Rating (A)	1 Sec RMS (kA)	2 Sec RMS (kA)	3 Sec RMS (kA)	Peak Asym. (kA)
A1	Pin & Socket	250	12.5	9.0	7.5	31
B1	Pin & Socket	250	12.5	9.0	7.5	31
B2	Pin & Socket	400	16.0	11.3	9.2	40
C1	Bolted	630	28.0	19.7	16.1	70
C2	Bolted	1250	75.0	53.0	43.3	>150
D1	Bolted	800	50.0	35.3	28.8	125
D2	Bolted	1250	75.0	53.0	43.3	>150
F1	Bolted	2500	-	-	-	-
F2	Bolted	630	28.0	19.7	16.1	70
F3	Bolted	1250	75.0	53.0	43.3	>150

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Applications of Bushing Types

A1 Interface Bushings

·         Used in oil-filled equipment like transformers, switchgear, capacitors.

·         Example types: 180AR-1 (12 kV, 250A), K180AR-1 (24 kV, 250A).

·         Also available as In-Air bushings for dry-type transformers, motors, and capacitors.

B Interface Bushings

·         Common for transformers and SF₆ circuit breakers.

·         Ratings: 12 kV / 400A to 36 kV / 400A.

Higher Interface Types (C, D, F)

·         Designed for high current, high voltage applications.

·         Bolted connections ensure mechanical robustness.

·         Widely used in EHV transformers, generator transformers, and high-capacity switchgear.

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Conclusion

Bushings may appear as a simple connection device, but they are a critical link in high-voltage power systems. Their classification—by insulation medium, construction, or internal insulation—directly influences their application in transformers, switchgear, GIS, and other HV equipment.

For reliable operation, selection must consider dielectric strength, thermal performance, short-circuit withstand, and mounting interface as per IS 2099 standards. With the advent of RIP and SF₆ bushings, the industry is moving towards safer, maintenance-free, and more compact designs.