Busbar for transformers

What the busbar does in a transformer

A transformer busbar is the conductor between the winding and the outside world. It may connect a low voltage winding to a terminal plate, link a tap changer to the winding, carry a neutral bar to an earth point or bring a high voltage lead out through a bushing. The shape is usually a flat copper or aluminium bar, formed and insulated to fit a tight space inside a tank or an enclosure.

The current is not one value. The busbar carries the rated current of the winding, the current at the tap position in use and, for a short time, the fault current until the protection operates. In a dry type transformer the same bar also has to manage the heat inside an enclosure with limited airflow. In an oil filled unit the bar sits in a dielectric liquid, which changes both the cooling and the insulation rules.

The design therefore starts with the connection map: which winding, which terminal, which tap and which fault path. A bar that is drawn from the terminal backwards without that map often ends up with a joint in the wrong place or a bend that cannot be insulated.

Current, temperature and section

The continuous current sets the conductor section. The bar has to stay below its allowable temperature at the ambient inside the tank or enclosure, and the allowable temperature depends on the insulation class and the surrounding materials. A bar that runs hot can degrade the insulation it touches, even if the conductor itself is within its own limit.

The current is rarely a clean sine wave. Harmonic loads, non-linear loads and the transformer's own magnetising current all add heating. A neutral bar in a distribution transformer can carry a high third harmonic current, so sizing it as a fraction of the phase bar can be wrong. The load profile and the harmonic content should be part of the sizing input.

The section also has to suit the fault. A fault current that lasts for one second heats the bar far more than the continuous rating suggests, because the heating is proportional to the square of the current and the time. The bar must survive that without annealing, softening at a joint or damaging the insulation it supports.

• Continuous winding current at the hottest tap position
• Harmonic and neutral current where the load is non-linear
• Fault current and clearing time from the protection study
• Ambient in the tank or enclosure and the cooling method
• Allowable temperature for the conductor and the adjacent insulation

Bracing against fault force

When a fault current flows, the magnetic field around each conductor produces a force. Two parallel bars carrying current in the same direction attract, and bars carrying current in opposite directions repel. The force is proportional to the square of the current and inversely related to the distance between the conductors, so a close phase spacing and a high fault level together produce a large load.

The supports and the bar have to take that load without the bar moving into a clearance or the joint opening. The bending moment between supports rises with the square of the span, so halving the support spacing reduces the moment by a factor of four. That is usually the cheapest way to raise the fault withstand: add a support rather than add copper.

The support itself has to be strong enough and insulated enough. A moulded insulator can crack if the force is applied at a sharp edge, and a metal support must not form a loop or a path. The support should locate the bar in both directions, because the force can push the bar sideways or pull it along its length depending on the layout.

Insulation in oil, air and sealed enclosures

The insulation rules change with the medium. In oil, the dielectric strength is high, but the oil has to circulate and the bar has to avoid sharp points that concentrate the field. In air, the clearance and creepage depend on the voltage, the pollution degree and the altitude. In a sealed enclosure the gas or the dry air may be cleaner, but the heat has nowhere to go.

Creepage is the path along the surface of the insulation, and it is usually the limiting dimension. A bar that is close to a grounded tank wall at its edge has a short surface path, even if the air gap looks generous. Barriers, sleeving or a coated bar can increase the creepage, but each adds thickness and a qualification question.

The insulation also has to survive the manufacturing process. A bar that is bent after insulation is applied can crack the coating at the bend. A bar that is bolted through an insulated support can crush the insulation if the washer is too small. The drawing should show the insulation, the masked areas and the order of operations.

Joints, terminals and tap links

A transformer busbar usually has at least two joints: one at the winding and one at the terminal or the tap link. Each joint adds resistance and a place where the fault force can open a gap. The joint faces should be flat, clean and plated for the medium. In oil, a bare copper joint can oxidise before the tank is filled, so the surface preparation and the time before filling matter.

The terminal connection has to suit the external conductor. A flat palm needs a matching lug and a washer that covers the contact area. A threaded stud needs a torque value and a thread that will not strip in service. A tap link needs a joint that can be opened for maintenance without disturbing the winding connection.

The joint should be accessible for a torque check. A joint buried under a winding or behind an insulating barrier cannot be checked after assembly, so the design should either make it accessible or make it a permanent connection with a demonstrated resistance.

• Plate the joint faces for the medium and the environment.
• Specify the fastener, washer and torque on the drawing.
• Keep the joint face flat and free of burrs before assembly.
• Record the contact resistance on the first article.
• Leave access for a torque check or a millivolt drop check.

Tests that prove the connection

The first article of a transformer busbar should include a dimensional check, a plating check and a resistance measurement across each joint. A millivolt drop test at a stated current gives a number that can be compared with a limit and repeated in production. A visual check of the insulation, the masked areas and the bend radii catches the defects that a dimensional report misses.

For the insulation, a power frequency withstand test and an insulation resistance test are the usual checks. If the bar is oil immersed, the test may be done on the assembled transformer rather than on the bar alone. A partial discharge measurement is useful where the voltage class and the insulation system require it.

A fault withstand test is not practical on every bar, so the design is usually justified by calculation and by a type test on a representative assembly. The calculation should show the force, the support reaction and the stress in the bar at the specified fault current and clearing time.

What to send for a quote

Send the transformer drawing, the connection diagram, the voltage class, the rated current at each tap and the fault level. State the insulation medium, the ambient, the cooling method and the standard the installation must meet. Add the terminal type, the external conductor and the access available for assembly.

If the bar fits inside an existing tank, send the tank drawing and the winding outline. The space around the bar is often the hardest constraint, and a supplier cannot price a bend or an insulation barrier without it.

• Transformer and connection drawings
• Voltage class, rated current and tap range
• Fault current and clearing time
• Insulation medium, ambient and cooling
• Terminal type and external conductor
• Tank or enclosure outline and access
• Annual volume and inspection level