Table-mounted transformers are essential components of modern underground distribution systems, especially in residential, commercial, and light industrial areas where aesthetics, safety, and space considerations are critical. These transformers rely on a specialized component called a bushing to ensure a reliable electrical connection between their internal windings and the external circuit.
Understanding the different types of table-mounted transformer bushings, especially the differences between primary (high voltage) and secondary (low voltage) applications, is critical for utilities, contractors, and engineers involved with distribution infrastructure.
What Are Transformer Bushings?
Transformer bushings serve as insulated passageways that allow energized conductors to exit or enter the transformer tank without compromising electrical integrity or safety. They insulate the internal electrical components from the transformer’s grounded metal casing and provide a mechanical connection point for power cables. Without bushings, it would be impossible to safely and effectively route power between a transformer and the external network.
In padmount transformers, bushings are generally categorized into two groups: primary or high-voltage (HV) bushings and secondary or low-voltage (LV) bushings. These categories are determined not just by their electrical ratings but by their position within the transformer’s circuit.
Primary (High-Voltage) vs. Secondary (Low-Voltage) Bushings
In a floor-standing transformer, the primary and secondary bushings are strategically placed in separate compartments within the transformer cabinet to ensure safety and simplify maintenance operations.
The primary bushing is typically located in the left compartment, which houses the high-voltage connection from the utility distribution network. The secondary bushing, on the other hand, is located in the right compartment, where the transformer’s low-voltage output is delivered to the consumer-side power system.
The two sections are separated by a solid metal partition, a key safety feature that provides physical and electrical isolation between the high-voltage and low-voltage areas. This layout not only reduces the risk of accidental contact but also facilitates orderly wiring and complies with utility distribution safety regulations.
Primary (high-voltage) bushings
Primary bushings, also known as high-voltage (HV) bushings, are responsible for receiving incoming power from the underground distribution system. They typically handle voltage levels ranging from 2.4 kilovolts (kV) to 35 kV, depending on the transformer’s rating and its location in the wider distribution network.
Because these bushings operate at high voltages, they are almost universally designed with a dead front interface, meaning that no live metal parts are exposed. This design significantly reduces the risk of arc flash incidents and improves safety for power personnel during installation, maintenance, and switching operations.
High voltage bushings typically support either on-load or off-load disconnect connectors, depending on the current rating and application, and are built to withstand significant electrical stress, environmental exposure, and mechanical loads.
Secondary (Low Voltage) Bushings
Secondary bushings, also known as low voltage (LV) bushings, are located on the output side of the transformer and are used to distribute power to downstream circuits and user loads. They handle voltages typically between 120 volts and 600 volts, which is much lower than the primary voltage, but corresponds to much higher current levels due to the power conversion principle.
These bushings must be designed for high current carrying capacity and mechanical strength. They are typically configured with spade or stud terminations to allow for the connection of large conductors or multiple cables in parallel.
Spade bushings, in particular, offer a bolting surface with multiple holes to accommodate a variety of conductor sizes and configurations. These features make low-voltage bushings ideal for applications that require reliability, ease of installation, and scalability to cope with future load increases.
Functional and structural differences
The unique design and location of primary and secondary bushings reflect their different operating roles in the transformer. The primary bushing is optimized for insulation, arc protection, and voltage resistance, while the secondary bushing focuses on mechanical strength and current handling capabilities.
These functional requirements determine not only the differences in materials and structures, but also the differences in their connection interfaces and safety features. These bushings work together to enable the transformer to act as a safe and efficient bridge between the high-voltage power supply network and the needs of low-voltage users, ensuring its reliable operation in a variety of electrical environments.
200 Amp Loadbreak Bushing Interface
The 200-amp load-break bushing interface is a prevalent type of dead front bushing used in padmount transformers. This setup allows for the safe disconnection of cable connections under load using a hot stick, facilitating maintenance without de-energising the transformer.
Each bushing comprises a well, a clamp, and a threaded insert. The central well houses a ⅜”-16 threaded copper stud, providing a termination point for the transformer’s winding leads on one end and a threaded insert on the other. This design ensures a secure and reliable connection, with the stud capable of withstanding significant torque without damage.
For applications requiring surge protection, a reducing tap plug can be utilized to install standard 200 amp loadbreak arresters. This flexibility allows for enhanced protection against voltage surges while maintaining the compact design of the transformer cabinet.
600 Amp Deadbreak Bushing Interface
For higher load applications, the 600 amp deadbreak bushing interface is employed. Unlike the loadbreak design, deadbreak bushings must be de-energized before disconnection, ensuring safety during maintenance.
These bushings are designed for robust performance, accommodating larger conductors and higher current loads. They are typically used in three-phase transformers rated at 300kVA and above, serving commercial or industrial facilities with substantial power demands.
The 600 amp bushings often feature bolted T-body connectors, providing a secure and low-resistance electrical connection capable of withstanding significant mechanical and thermal stress.
Conclusion
Transformer bushings in padmount transformers are crucial for ensuring safe and efficient operation in underground power distribution systems. They facilitate the safe transition of electric current into and out of the transformer while protecting the system from electrical faults and mechanical stresses.
Understanding the differences between primary and secondary bushings, as well as the various interface types like 200 amp loadbreak and 600 amp deadbreak, is essential for selecting the appropriate transformer configuration for specific applications. Making informed choices ensures operational reliability, safety, and maintainability of the entire power distribution system.