TRANSFORMER BUSHINGS BASICS AND TUTORIALS

TRANSFORMER BUSHINGS BASIC INFORMATION

What Are Transformer Bushing? Functions Of Transformer Bushing?

Bushings may be classified generally by design as follows:

a) Condenser type

1) Oil-impregnated paper insulation, with interspersed conducting (condenser) layers or oil impregnated paper insulation, continuously wound with interleaved lined paper layers

2) Resin-bonded paper insulation, with interspersed conducting (condenser layers)

b) Noncondenser type

1) Solid core or alternate layers of solid and liquid insulation

2) Solid mass of homogeneous insulating material (e.g., solid porcelain)

3) Gas filled

For outdoor bushings, the primary insulation is contained in a weatherproof housing, usually porcelain. The space between the primary insulation and the weathershed is generally filled with an insulating oil or compound (also, plastic and foam).

Some of the solid homogenous types may use oil to fill the space between the conductor and the inner wall of the weathershed. Bushings may also use gas such as SF6 as an insulating medium between the center conductor and outer weathershed.

Bushings may be further classified generally as being equipped or not equipped with a potential tap or power-factor test tap or electrode. Note Potential taps are sometimes also referred to as capacitance or voltage taps.)

The bushing, without a potential tap or power-factor tap, is a two-terminal device that is generally tested overall (center conductor to range) by the GST method. If the bushing is installed in an apparatus, such as a circuit breaker, the overall GST measurement will include all connected and energized insulating components between the conductor and ground.

A condenser bushing is essentially a series of concentric capacitors between the center conductor and the ground sleeve or mounting range. A conducting layer near the ground sleeve may be tapped and brought out to a tap terminal to provide a three-terminal specimen.

The tapped bushing is essentially a voltage divider and, in higher voltage designs, the tap potential may be utilized to supply a bushing potential device for relay and other purposes. In this design the potential tap also acts as a low-voltage power-factor test terminal for the main bushing insulation, C1.

Modern bushings rated above 69 kV are usually equipped with potential taps. (In some rare instances 69 kV bushings were equipped with potential taps.) Bushings rated 69 kV and below may be equipped with power factor taps.

In the power-factor tap design, the ground layer of the bushing core is tapped and terminated in a miniature bushing on the main bushing mounting range. The tap is connected to the grounded mounting range by a screw cap on the miniature bushing housing.

With the grounding cap removed, the tap terminal is available as a low-voltage terminal for a UST measurement on the main bushing insulation, C1, conductor to tapped layer.

TRANSFORMER LOSSES DEFINITION BASIC AND TUTORIALS

TRANSFORMER LOSSES COMPONENTS TUTORIALS
What Are Transformer Losses Components?

Transformer Losses is a natural occurrence in the Power System Cycle. Below are the different components of the Transformer Losses.

No-Load Loss and Exciting Current
When alternating voltage is applied to a transformer winding, an alternating magnetic flux is induced in the core. The alternating flux produces hysteresis and eddy currents within the electrical steel, causing heat to be generated in the core. Heating of the core due to applied voltage is called no-load loss.

Other names are iron loss or core loss. The term “no-load” is descriptive because the core is heated regardless of the amount of load on the transformer. If the applied voltage is varied, the no-load loss is very roughly proportional to the square of the peak voltage, as long as the core is not taken into saturation.

The current that flows when a winding is energized is called the “exciting current” or “magnetizing current,” consisting of a real component and a reactive component. The real component delivers power for no-load losses in the core.

The reactive current delivers no power but represents energy momentarily stored in the winding inductance. Typically, the exciting current of a distribution transformer is less than 0.5% of the rated current of the winding that is being energized.

Load Loss
A transformer supplying load has current flowing in both the primary and secondary windings that will produce heat in those windings. Load loss is divided into two parts, I2R loss and stray losses.

I2R Loss
Each transformer winding has an electrical resistance that produces heat when load current flows. Resistance of a winding is measured by passing dc current through the winding to eliminate inductive effects.

Stray Losses
When alternating current is used to measure the losses in a winding, the result is always greater than the I2R measured with dc current. The difference between dc and ac losses in a winding is called “stray loss.”

One portion of stray loss is called “eddy loss” and is created by eddy currents circulating in the winding conductors. The other portion is generated outside of the windings, in frame members, tank walls, bushing flanges, etc.

Although these are due to eddy currents also, they are often referred to as “other strays.” The generation of stray losses is sometimes called “skin effect” because induced eddy currents tend to flow close to the surfaces of the conductors.

Stray losses are proportionally greater in larger transformers because their higher currents require larger conductors. Stray losses tend to be proportional to current frequency, so they can increase dramatically when loads with high-harmonic currents are served. The effects can be reduced by subdividing large conductors and by using stainless steel or other nonferrous materials for frame parts and bushing plates.

Harmonics and DC Effects
Rectifier and discharge-lighting loads cause currents to flow in the distribution transformer that are not pure power-frequency sine waves. Using Fourier analysis, distorted load currents can be resolved into components that are integer multiples of the power frequency and thus are referred to as harmonics. Distorted load currents are expected to be high in the 3rd, 5th, 7th, and sometimes the 11th and 13th harmonics, depending on the character of the load.

Odd-Ordered Harmonics
Load currents that contain the odd-numbered harmonics will increase both the eddy losses and other stray losses within a transformer. If the harmonics are substantial, then the transformer must be derated to prevent localized and general overheating.

ANSI standards suggest that any transformer with load current containing more than 5% total harmonic distortion should be loaded according to the appropriate ANSI guide (IEEE, 1998).

Even-Ordered Harmonics
Analysis of most harmonic currents will show very low amounts of even harmonics (2nd, 4th, 6th, etc.) Components that are even multiples of the fundamental frequency generally cause the waveform to be nonsymmetrical about the zero-current axis.

The current therefore has a zeroth harmonic or dc-offset component. The cause of a dc offset is usually found to be half-wave rectification due to a defective rectifier or other component. The effect of a significant dc current offset is to drive the transformer core into saturation on alternate half-cycles.

When the core saturates, exciting current can be extremely high, which can then burn out the primary winding in a very short time. Transformers that are experiencing dc-offset problems are usually noticed because of objectionably loud noise coming from the core structure.

Industry standards are not clear regarding the limits of dc offset on a transformer. A recommended value is a dc current no larger than the normal exciting current, which is usually 1% or less of a winding’s rated current (Galloway, 1993).