Electrical Bushings – Types, Purpose, and Construction with Diagrams
- nishantjoshi27
- 6 days ago
- 5 min read
Updated: 22 hours ago
Electrical bushings are essential components in transformers, serving the critical function of enabling electricity to pass safely through ground barriers such as transformer tanks. By providing strong electrical insulation, bushings ensure the integrity and reliability of high-voltage equipment.
This article explains the purpose of electrical bushings, the main bushing types used in transformers, and the construction principles that allow them to manage electric-field behaviour effectively.
Understanding Electrical Bushings
Electrical bushings guide an energised conductor through a grounded enclosure while maintaining insulation strength and stable operating behaviour. They create a controlled environment around the conductor so the electric field remains contained within the insulation body. This helps equipment operate steadily under routine load and environmental conditions.
Bushings support essential functions that are critical to overall transformer reliability. Their insulation materials are selected to deliver consistent dielectric performance under electrical, thermal, and mechanical stresses. Over long service periods, bushings withstand these stresses under very high voltages and currents, which is why consistent performance is important.
What is Their Purpose
The purpose of electrical bushings extends far beyond creating a simple insulated opening. They create a stable pathway for the electricity as it enters or exits an enclosed system and ensure that the electric field is distributed safely around the insulation body. Their structure also supports the conductor physically, and this helps the equipment withstand temperature-based expansion as well as environmental stresses. Together, these functions support predictable operating conditions and reduce stress on the surrounding components.
Bushings also help maintain efficiency in power equipment by preventing leakage currents and managing electric-field gradients. This ensures the conductor remains isolated from the grounded enclosure. These combined structural and dielectric roles make bushings a critical interface element in transformers.
Primary Transformer Bushing Types
The design and material selection for bushings vary depending on voltage, environmental conditions, and equipment configuration. Understanding the main transformer bushing types helps identify which designs best support specific operating requirements.
Types Based on Construction
Solid-Type Bushings
Solid-type bushings use a central conductor surrounded by a single, homogeneous insulation material such as epoxy resin. This construction without capacitance grading is used in transformers operating at lower and medium voltage levels, where the insulation relies on the bulk dielectric strength of the solid material to control electric-field distribution. Their straightforward structure supports predictable performance and simplified installation on distribution transformers.
Capacitance-Graded Bushings
Capacitance-graded bushings are built with insulation layers that contain embedded conductive foils. These foils create a controlled voltage distribution along the insulation, supporting stable electric field behavior. This design approach helps the bushing maintain uniform insulation characteristics as voltage levels increase. The internal grading arrangement supports high-voltage operation by distributing electrical stress evenly throughout the insulation body, forming a key construction method used in many electrical bushings.
Types of Electrical Bushing Based on Application
Oil-to-Air Bushings
Oil-to-air bushings connect live electrical conductors between two insulation environments. On the transformer side, the conductor is surrounded by insulating oil within the grounded tank, while on the external side it is exposed to air and insulated by the bushing’s internal insulation system and outer housing, supporting safe passage to electricity across differing media.
Air-to-Air Bushings
Air-to-Air Bushings are used where both ends of the bushing terminate in air, this type supports equipment mounted across walls or enclosures that require external-to-internal isolation.
Air-to-SF₆ Bushings
Air-to-SF₆ bushings provide a controlled interface between air-insulated external conductors and SF₆-insulated compartments within gas-insulated environment. The bushing design ensures a continuous dielectric path as the conductor transitions from ambient air into the sealed SF₆ gas environment, maintaining electrical integrity and safe operation under rated voltage conditions.
SF₆-to-Oil Bushings
SF₆-to-oil bushings are used to connect SF₆-insulated environment to oil-filled transformers. They enable the conductor to pass safely between the gas-insulated compartment and the transformer’s oil insulation system, while managing electric field distribution and preserving dielectric coordination across both media.
Oil-to-Oil Bushings
Oil to oil bushings are used where the conductor passes between two oil insulated compartments. Their construction maintains consistent dielectric behaviour across oil-based insulation systems, allowing a secure and controlled transition between adjacent oil filled environments.
Types Based on Internal Insulation Material
Oil-Impregnated Paper (OIP)
OIP bushings use paper insulation saturated with mineral oil, these bushings provide strong dielectric behaviour and are widely applied in condenser-type constructions. Their design supports predictable insulation performance across various applications especially where long-term stability is required.
Resin-Impregnated Paper (RIP)
RIP bushings use epoxy resin to impregnate paper insulation, forming a solid insulation system without oil. Their construction supports consistent dielectric performance and stable operation across varied environmental conditions.
Resin-Impregnated Synthetic (RIS)
RIS bushings use synthetic insulating materials instead of paper to build a solid insulation core. This structure supports low moisture absorption and stable dielectric behaviour under wide temperature variations.
Gas-Insulated Bushings
Gas-insulated bushings use SF₆ as the primary dielectric medium, applied in GIS installations where compact insulation paths with controlled gas-filled environments are required for current conduction.
Cast-Insulation Bushings
Cast-insulation bushings are formed using solid epoxy or porcelain materials. Their construction supports medium-voltage equipment like distribution transformer where a simple internal layout and solid insulation system meet the operational requirements.
Inside the Structure – Electrical Bushing Construction
The structure of a bushing determines how well it manages electric-field distribution and insulation strength. A typical electrical bushing construction includes several components working together to support stable operation.
At the centre is the conductor, surrounded by an insulation system that may be solid, oil-impregnated, resin-impregnated, or gas-based depending on the bushing type. In condenser-type bushings, thin conductive foils are embedded within the insulation to shape the electric field evenly across the layers.
The outer housing protects the insulation from weather, pollution, and moisture, in addition to increasing the creepage distance. Materials may include porcelain or composite silicone, with composite designs offering improved hydrophobic behaviour. The mounting flange secures the bushing to the equipment body, creating a sealed interface. Many bushings incorporate test taps for monitoring parameters such as capacitance and tan-delta. Each component is designed to maintain insulation continuity and long-term operational stability.
Frequently Asked Questions
1. How do grading foils support insulation performance?
Grading foils help control electric field behaviour by distributing voltage across the insulation layers. This supports uniform dielectric response within the bushing core and strengthens the internal structure used in many transformer bushing types.
2. How do composite housings support outdoor performance?
Composite housings exhibit hydrophobic surface behaviour that helps in moisture resistance and maintain stable insulation performance in polluted or humid outdoor environments.
3. What determines the required creepage distance for a bushing?
Creepage distance is influenced by equipment voltage, site pollution level, humidity, and installation environment. These factors guide how the external insulation profile is shaped to support reliable performance in different layouts of electrical bushings.
Conclusion
Electrical bushings enable the safe transfer of electricity between grounded equipment and the grid, by combining controlled dielectric behaviour with robust structural design. Understanding the purpose, construction, and insulation variations of electrical bushings helps engineers make informed decisions during installation, inspection, and maintenance. At Yash Highvoltage®, we engineer the solutions that has proven on-ground track-record across varied operating environments.
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