Transformer Components – Bushings, the Tip of the Iceberg

Bushings will last the life of the transformer with the usual proviso that they are not over loaded or over stressed (too much) and escape misadventure from projectiles. The Iceberg reference in the title of this newsletter alludes to the fact that much of a bushing is unseen within the transformer tank and also to the reality that many bushing details are unknown to transformer owners.

Bushings come under scrutiny as transformers age, as the need to squeeze more power through the equipment increases and as the regulations for PCB source identification change.

There are Industry standards for bushings including CEMA/EEMAC for Canada, ANSI for the US, IEC for Europe. These standards did/do not cover all voltage classes of bushings and unfortunately the dimensional parameters are different between Standards and are not even consistent within a given Standard over time.

This newsletter will provide some basic information on bushings and point to potential pitfalls when changing out old bushings with modern replacements.


Bushings 101

Bushings can be divided into two basic groups, Bulk Type and Condenser Type.

Historically, Bulk Type are limited to lower voltage classes and are basically a central conductor stud or tube surrounded by an insulator housing, either Porcelain or Epoxy. Sometimes they are dry (i.e. no oil within the bushing), or sometimes oil is used as part of the insulation structure between the conductor and the housing. Variations in oil filled Bulk Type bushings include: oil shared with the transformer main tank, oil that is sealed within the bushing and oil that can be accessed.

It is important to note that bushings are not routinely oil sampled or require topping up but many oil filled bushings can have samples extracted (with care) for analysis.

Condenser Type bushing are available over a large range of voltage classes and are characterized by utilizing an internal condenser structure within a porcelain housing which is used to grade the voltage stress from the outer terminal to the tank and from the tank to the internal connection point. All Condenser Type bushing are oil filled. Many of these bushings have an oil sight glass or an oil gauge to monitor fluid level.

Applicable to both bushing types, if the current rating is 600 Amps or less, a draw lead is often used to connect from the transformer windings to the external bushing terminals. This draw lead or draw rod allows for the removal of the bushing for shipment without entering the transformer. Bushings with current ratings above 600 Amps are typically solidly connected internally to the transformer winding and require draining oil and entering the transformer tank to gain access.

Key Bushing Information

To replace a bushing, the following electrical and dimensional data is required:
  • Voltage Class, BIL, Current rating
  • Above the mounting flange:
    • Total length above the flange
    • Height to the draw lead/pull through lead connection point (if applicable)
      • Draw lead/pull through lead pin dimension and thread specifics
    • Maximum diameter of live part
    • Top terminal dimensions and thread specifics
  • Mounting Flange:
    • Mounting hole pattern diameter
    • Number of mounting holes and diameter
  • Below the mounting flange:
    • Total length below the flange
    • Ground plane length to accommodate CT’s, also often referred to as the minimum oil level point
    • Maximum diameter below flange
    • Internal hole diameter for draw/pull through lead (if applicable)
    • Bottom terminal dimensions and thread specifics (if applicable)
    • Bottom Shield dimensions and mounting (if applicable)

Not all of these dimensions are critical to match for replacement, i.e. if the height above the flange is a few inches longer or shorter than the original bushing, it is relatively easy to overcome in the Field. Any dimensions below the flange are important as are the draw lead parameters. Adapter flanges and special modifications can be made to modern bushings to facilitate a fit but the original dimensions must be known.

This newsletter has briefly touched on the subject of transformer bushings but hope it has increased awareness of the complexity and significant variation between bushings and the potential complications surrounding oil sampling and component replacement.