Importance of transformer bushing/polarity

1. Transformer Bushings

In electric power, a bushing is a hollow electrical insulator that allows an electrical conductor to pass safely through a conducting barrier such as the case of a transformer or circuit breaker without making electrical contact with it.

The two most typical kinds of bushings used on transformers as main lead entrances square measure solid ceramic ware bushings on smaller transformers and oil-filled condenser bushings on larger transformers.

Solid ceramic ware bushings encompass best ceramic ware cylinders that conductors go through. Outside surfaces have a series of skirts to extend the leak path distance to the grounded metal case. High voltage bushings square measure usually oil-filled condenser sort. Condenser sorts have a central conductor wound with alternating layers of paper insulation and tin foil and crammed with insulating oil. This leads to a path from the conductor to the grounded tank, consisting of a series of condensers.

The layers square measure designed to supply some equal voltage drops between every condenser layer. Acceptance and routine maintenance tests most frequently used for checking the condition of bushings square measure Double power issue tests.

The ability issue of a bushing in physical fitness can stay comparatively stable throughout the service life. A decent indication of insulation deterioration could be a slowly rising power issue. The foremost common reason for failure is wet entrance through the highest bushing seal. These conditions are discovered before failure by routine double testing.

If double testing isn’t performed frequently, explosive failure is that the ultimate results of a unseaworthy bushing. This, many times, leads to a harmful and expensive failure of the Transformer furthermore.

2.Transformer Polarity

Polarity is extremely necessary for the operation of transformers and protection instrumentation. a transparent understanding of polarity is helpful in understanding and analyzing transformer connections and operations also as testing protection relays and systems.It’s also essential understanding facility performance throughout each traditional and abnormal operations.

The two elementary rules of transformer polarity, following:

  1. Current flowing in at the polarity mark of 1 winding flows out of the polarity mark of the opposite winding. each currents square measure well in-phase
  2. The dip from polarity to non-polarity across one winding is basically in section with the dip from polarity to non-polarity across the opposite winding(s).


With power or distribution transformers, polarity is very important as long as the necessity arises to parallel transformers to achieve extra capability or to attach 3 single-phase transformers to create a three-phase bank. The means the connections square measure created affects angular displacement, section rotation, and direction of rotation of connected motors. Polarity is additionally vital once golf shot up current transformers for relay protection and metering.

Transformer polarity depends on that direction coils square measure wound round the core (clockwise or counter clockwise) and the way the leads square measure brought out. Transformers square measure generally marked at their terminals with polarity marks.

Often, polarity marks square measure shown as white paint dots (for plus) or plus-minus marks on the Transformer and symbols on the plate. These marks show the connections wherever the input and output voltages (and currents) have identical instant polarity.

More often, Transformer polarity is shown just by the American National Standards Institute (ANSI) designations of the winding leads as H1, H2 and X1, X2. By ANSI standards, if you face the low-tension aspect of a single-phase Transformer (the aspect marked X1, X2), the H1 affiliation can forever get on your left. See the single-phase diagrams in figure fourteen.

If the terminal marked X1 is additionally on your left, it’s ablative polarity. If the X1 terminal is on your right, it’s additive polarity. Additive polarity is common for little distribution transformers. Giant transformers, like GSUs at Reclamation power plants, square measure usually ablative polarity.

It is additionally useful to consider polarity marks in terms of current direction. At any instant once this direction is into a polarity marked terminal of the first winding, this direction is out of the terminal with identical polarity mark within the secondary. it’s identical as if there have been an eternal circuit across the 2 windings. Polarity could be a convenient means of stating however leads square measure brought out. If you wish to check for polarity, connect the Transformer 

 A Transformer is alleged to own additive polarity if, once adjacent high- and low-tension terminals square measure connected and a meter placed across the opposite high- and low-tension terminals, the meter reads the total (additive) of the high- and low-tension windings. It’s ablative polarity if the meter reads the distinction (subtractive) between the voltages of the 2 windings. If this check is conducted, use the bottom AC voltage offered to scale back potential hazards. AN adjustable ac voltage supply, like a variac, is usually recommended to stay the check voltage low.

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