Types of pad mounted transformer bushing

Definition of a Bushing

A bushing is an insulating component installed on the enclosure of a transformer, switchgear, or other electrical equipment. It serves to route high-voltage or low-voltage conductors from the sealed interior to the external environment while providing electrical insulation and mechanical support. In pad-mounted transformers, bushings are critical interfaces that connect internal windings to external cables or overhead lines. They must meet requirements for waterproofing, pollution resistance, weatherproofing, and electrical insulation.

Differences Between High-Voltage and Low-Voltage Bushings

Pad-mounted transformers are widely used in distribution systems, especially in urban and residential areas where safety and aesthetics are important. The high-voltage bushings and low-voltage bushings on a pad-mounted transformer differ significantly in terms of voltage levels, current capacity, insulation requirements, structural design, and connection methods.

Below are the key differences:

2.1 Electrical Parameters

2.2 Structural Design and Insulation

High-Voltage Bushings:

• Require higher dielectric strength.
• Designed with longer creepage distances and greater flashover resistance.
• Often use interface-type connectors (e.g., 200A loadbreak 600A deadbreak or 900A deadbreak) for connection with high-voltage cables.

Low-Voltage Bushings:

• Lower insulation requirements due to lower voltage.
• Emphasize current-carrying capacity and heat dissipation due to high current loads.
• Usually feature bolt-type or stud-type terminals, compatible with large cross-section copper or aluminum conductors.

2.3. Heat Dissipation and Electrical Connection

• The high-voltage side connects to overhead lines or underground cables, carries less current, and generates less heat.
• The low-voltage side supplies end-user loads with higher current, so bushings are designed to be thicker with larger contact surfaces to reduce resistance and manage heat.

2.4. Maintenance and Inspection

• High-voltage bushings focus on partial discharge (PD), dielectric loss, and overall insulation performance.
• Low-voltage bushings are mainly inspected for contact resistance, overheating, and connection integrity.

Separable insulated connector classification

3.1 According to the operation mode, they can be classified as loadbreak connectors and deadbreak connectors.

a) loadbreak connector

• The Load Break Connector is a type of connector designed to safely disconnect or connect under load. It features an arc suppression device inside, enabling safe operation under load conditions.
• With an arc suppression device inside, it allows for opening and closing operations under load.
• It offers greater operational flexibility, allowing for maintenance and switching under normal operating conditions.
• It is suitable for applications that require frequent disconnection and connection, such as load management and maintenance operations in distribution networks.

b) deadbreak connector

• The Dead Break Connector is a type of connector that cannot be disconnected or connected under load. It can only be operated when the system power is off (i.e., in a no-load state).
• Its structure is relatively simple and it is usually suitable for applications that do not require frequent disconnection and connection.
• It has high operational safety because it is operated without load, reducing the risk of arc generation.
• Its service life is relatively long, but its flexibility is relatively low.

3.2 It can be classified by voltage level as 15kV, 25kV, 28kV and 35kV.

The voltage level of the loadbreak connector

The voltage level of the deadbreak connector

3.3 It can be classified by current rating as 200A, 600A, and 900A.

The current rating of the loadbreak connector

The current rating of the deadbreak connector

3.4 According to their shape and function, they can be classified as elbow connectors, straight connectors, Tee connectors and insert connectors.

Elbow Connector:

• It is designed in an L-shape or T-shape and is widely used in power distribution equipment, especially in areas with limited space.
• Application scenarios: It is commonly used to connect high-voltage cables with transformers or switch cabinets.

Straight Connector:

• It has a straight design and is usually used in situations requiring a straight connection.
• Application scenarios: It is used for connections between cables and equipment.

Tee Connector:

• It allows for multi-directional connections in cable lines and is often used to branch out a branch circuit from the main line.
• Application scenarios: In power distribution networks, it is suitable for distributing current from the main cable line.

Insert Connector:

• It is designed to be inserted into a sleeve well and is used in conjunction with other connector components (such as elbows).
• Application scenarios: It is widely used in sleeve wells of high-voltage equipment.

3.5 According to the protection function classification, there is the arrester connector.

• Arrester Connector: This connector integrates the function of a lightning arrester and is used to protect electrical equipment from lightning strikes or overvoltage damage.
• For example: Elbow Arrester and Tee Arrester

200 Amp Pad mounted Transformer Bushing Inserts

4.1 Bushing well

Definition:

• A housing for an electrical device, featuring small cavities for inserting connector components (such as housing inserts)

Explanation:

• Bushing Well (also known as a “bushing”) is a component used in transformers or switchgear to accommodate and secure cables or other conductors.
• It is typically an interface with a cavity that can accommodate various connector components (such as “bushing insert”), ensuring a reliable connection with external cables.

Application scenarios:

• Bushing Well plays a crucial role in the power system, providing a safe and reliable interface for the connection between high-voltage cables and equipment.

Summary:

• Bushing well (a well with a casing) plays a crucial role in the power system, providing a safe and reliable interface for the connection between high-voltage cables and equipment.

4.2 Bushing insert

Definition:

• A connector component, designed for insertion into a bushing well and used in conjunction with another connector component (such as an elbow head).

Explanation:

• Bushing Insert is a component that is inserted into the bushing well.
• It can be used in conjunction with other connectors (such as elbow connectors) to ensure the integrity and safety of the electrical connection.
• The design of the bushing insert is compatible with various types of connector components, ensuring flexible connection and disconnection in the power system.

Application scenarios:

• The Bushing Insert is typically used in high-voltage power equipment, such as transformers and switch cabinets, to provide a stable connection point and facilitate installation, maintenance, and replacement of cable components.
• It helps achieve a secure connection between the equipment and the cables, while providing electrical insulation and mechanical strength to prevent electrical failures and accidental disconnections.

Summary:

• The bushing insert is an important component in the power system, used to ensure a safe and reliable connection between electrical devices.

4.3 Insulated Cap

Definition:

• An accessory device designed to provide electrical insulation, electrical shielding, and mechanical sealing for casing inserts or complete casings.

Explanation:

• Insulated Cap is a protective device used for the components of the power system’s bushings.
• Its main functions include:

1.       Electrical insulation: Prevents current leakage and ensures the safe operation of the electrical system.
2.   Electrical shielding: Provides electromagnetic shielding to prevent electrical interference.
3.   Mechanical sealing: Prevents moisture, dust, or other contaminants from entering the inner part of the bushing, maintaining the integrity and performance of the connector.

Application scenarios:

• The Insulated Cap is typically used on the ports of unused connectors or bushing inserts to provide protection and insulation, especially in outdoor and harsh environments, to ensure the safety and long-term use of the equipment.

Summary:

• The insulated cap (Insulating Cap) plays multiple protective roles in power equipment and is an important accessory for ensuring the safe and stable operation of the electrical system.

4.4 Integral bushing

Definition:

• A type of equipment sleeve, designed to be used in conjunction with other connector components (such as elbow connectors).

Explanation:

• Integral Bushing (a type of connection component used in electrical equipment) is typically employed to connect external cables or other electrical components (such as elbows).
• Its function is to provide an insulated electrical connection point for electrical equipment, while maintaining the safety and reliability of the electrical system.

Applications:

• Integral Bushing is mainly used in high-voltage equipment, such as transformers and switch cabinets, to help connect the equipment to external cables.
• The design of this bushing ensures a reliable electrical connection between electrical equipment, while providing necessary insulation and mechanical protection.

Summary:

• Integral bushing (a type of connection component) is an important component in electrical equipment used for connecting external cables and electrical components, ensuring the safety and effective operation of the equipment.

4.5 Elbow connector

Definition:

• A device with two interconnected ports, used for electrical insulation, electrical shielding, and sealing of these two ports. These two ports are arranged at right angles, with one port being a cable port and the other port being a bushing port. Typical devices include 200A load-breaking elbow terminals and 200A power-off-breaking elbow terminals.

Explanation:

• The Elbow Separable Insulated Connector is a common electrical connector with an elbow-shaped structure (90-degree angle), used for connecting high-voltage cables to equipment such as transformers in electrical systems.
• This connector has two interconnected interfaces:

Cable interface: for connecting high-voltage cables.

Capillary interface: for connecting to the caps of transformers or switchgear.

• This connector provides electrical insulation and shielding, while ensuring mechanical sealing at the connection point to prevent the influence of the external environment.

Application scenarios:

• The 200A load-breaking elbow connector and the 200A power-off-breaking elbow connector are commonly used in distribution transformers and switchgear, allowing for safe connection and disconnection operations, ensuring the reliability and maintenance convenience of the electrical system.

Summary:

• The elbow separable insulated connector is a key component for safe and reliable connections between high-voltage power equipment and cables, providing flexible connection methods while ensuring electrical insulation.

4.6 Dead Front Arrester Elbow

Definition:

• An insulated and shielded electrical surge protection device. These devices consist of a surge arrester and one or more interfaces. Typical devices include bushing arrester, bushing well arrester, Tee arrester, parking stand arrester

Explanation:

• Dead-front: Refers to the situation where there are no exposed live components on the outside of the equipment, and all live components are covered by insulating materials or enclosures to ensure that contact with high voltage is avoided during operation.
• Separable Insulated Connector: It refers to an insulated connector that can be separated, used for flexible connection and disconnection operations.
• Arrester: An arrester is an electrical surge protection device used to prevent overvoltage or lightning from damaging electrical equipment.

Application scenarios:

• bushing arrester, elbow arrester, Tee arrester, etc. are common protective devices, installed in power equipment to protect the system from the impact of lightning or other transient voltages.

Summary:

• Dead-front separable insulated connector arrester is a protective device used in power systems, providing overvoltage protection and ensuring the safety and flexibility of operation, and is widely used in distribution networks and transformer equipment, etc.

4.7 200A loadbreak separable insulated connector configuration

4.8 200A deadbreak separable insulated connector configuration

600/900 Amp Pad mounted Transformer Bushing Inserts

5.1 Tee tap connector

Definition:

• A Tee Tap Connector is a “T”-shaped connector, which is commonly used to branch out an additional cable connection from the main cable line.

Purpose:

• Provide branch connections from the main line to the load or other equipment.
• The design purpose of this connector is to provide a branch connection point without interrupting the power supply of the main line.
• It is usually used in power distribution networks, where it is necessary to connect or disconnect a certain branch without interrupting the main power supply.

5.2 Tee connector

Definition:

• A Tee Connector is a standard “T”-shaped connector used to connect three cables or devices together to form an “T”-shaped electrical connection point.

Usage:

• It connects three cables together, allowing current to flow in three directions.
• It is mainly used to connect different parts of the power system to ensure the continuity between electrical equipment.
• It is applicable in situations where power needs to be diverted from a main line to two other directions.

5.3 Reducing tap plug

Definition:

• An accessory device designed to convert a 600-ampere or 900-ampere dead-break system into a 200-ampere dead-break or load-break system.

Explanation:

• The Reducing Tap Plug is an adapter-type device that enables the transition or connection between systems with different rated currents. In practical applications:
• It can connect a system with a higher current rating (such as a 600A or 900A dead-break system) to a system with a lower current rating (such as a 200A dead-break or load-break system).
• This device is typically used in power systems’ transformers or switchgear to ensure compatibility between different devices and components.

Application scenarios:

• Used when connecting devices with different current ratings are required, for example, transitioning from a high-current-rated system to a low-current-rated system.
• It provides flexibility, allowing components in the power system to be interconnected without being limited by their respective current ratings.
• By using the reducing tap plug, a smooth transition and compatibility between different components in the power system can be achieved, ensuring the safe and reliable operation of the system.

5.4 Connecting plug

Definition:

• A connector that provides two parallel tube interfaces.

Explanation:

• The Connecting Plug is an electrical connection device designed to connect two tube interfaces, typically used in electrical systems to directly connect two devices (such as transformers or switchgear) together.
• Its design enables a reliable electrical connection between the parallel tube interfaces, ensuring the safe transmission of current between the devices.

Application scenarios:

• The Connecting Plug is commonly used in high-voltage power equipment, especially in situations where two devices need to be connected through tube interfaces, such as connecting two cable ends or the tube interfaces of different electrical devices.

Summary:

• The connecting plug is a key component for connecting two tube interfaces, ensuring safe and effective transmission in the electrical system.

5.5 Insulated plug

Definition:

• An accessory device designed for electrical insulation, electrical shielding, and mechanical sealing of elbow-type interfaces.

Explanation:

• The Insulated Plug is a device used to enclose and protect elbow-type interfaces, ensuring the electrical insulation and shielding at the connection point.
• It also has a mechanical sealing function to prevent the influence of external environments (such as moisture or dust) on the elbow-type interface.

Application scenarios:

• The Insulated Plug is commonly used in elbow-type connectors in the power system. When the elbow-type connector is not in use temporarily, it provides a safe insulation and sealing, ensuring the integrity and safety of the system.

Summary:

• The Insulated Plug is a protective device for elbow-type heads, providing multiple protection functions, including electrical insulation, shielding, and mechanical sealing.

Common configuration

6.1 Typical Configuration 1

6.2 Typical Configuration 2

6.3 Typical Configuration 3

6.4 Typical Configuration 4

6.5 Typical Configuration 5

6.6 Typical Configuration 6

6.7 Typical Configuration 7

6.8 others

Types of pad mounted transformer Secondary/LV Bushings

Definition

Low-voltage transformer bushings are available in a wide variety of formats to accommodate different system voltages and current demands across various installations. Common configurations include 4-hole up to 24-hole designs. Regardless of the total number of holes, the layout typically follows a standardized pattern: each hole is 9/16″ in diameter, arranged on a square grid with 1 ¾” spacing both vertically and horizontally. This ensures full compatibility with standard 2-hole lugs, whether mechanical or compression type. For higher-capacity bushings like the 10-hole variant, each terminal is reinforced with dedicated support brackets to ensure structural stability. The current-carrying elements are made of tin-plated copper for excellent conductivity and corrosion resistance.

Pad mounted transformer Spade Bushings

CSA C227.4:21 standard

IEEE Std C57.12.34-2022

Low-voltage bushing drawing example

4-holes spade Secondary/LV bushing

6-holes spade Secondary/LV bushing

8-holes spade Secondary/LV bushing

10-holes spade Secondary/LV bushing

12-holes spade Secondary/LV bushing

16-holes spade Secondary/LV bushing

Nylon bushing

Material:

• Made of high-strength nylon, typically a flame-retardant and reinforced type, offering excellent insulation performance.

Advantages:

• Lightweight and easy to install;
• Cost-effective;
• Corrosion-resistant, suitable for humid or coastal environments;
• Good thermal endurance.

Disadvantages:

• Lower mechanical strength compared to metal bushings;
• Limited thermal aging resistance under long-term high-load conditions.
• Nylon bushings are generally used in pad mounted transformers with smaller capacities (e.g., ≤300 kVA), or in applications where cost efficiency and corrosion resistance are prioritized.