TRANSFORMER BUSHINGS FOR SPECIAL APPLICATIONS

High-Altitude Applications
Bushings intended for application at altitudes higher than 1000 m suffer from lower air density along the outer insulator. Standards specify that, when indicated, the minimum insulation necessary at the required altitude can be determined by dividing the standard insulation length at 1000 m by the correction factor given in Table 3.2.2.

For instance, suppose that the required length of the air insulator on a bushing is 2.5 m at 1000-m altitude. Further, suppose that this bushing is to be applied at 3000 m. Hence, the air insulator must be at least 2.5/0.8 = 3.125 m in length.

The air insulator on the bushing designed for 1000 m must be replaced with a 3.125-m-long insulator, but the remainder of the bushing, i.e., the central core and the oil insulator, will remain the same as the standard bushing because these parts are not affected by air insulation. These rules do not apply to altitudes higher than 4500 m.

Highly Contaminated Environments
Insulators exposed to pollution must have adequate creep distance, measured along the external contour of the insulator, to withstand the detrimental insulating effects of contamination on the insulator surface. Figure 3.2.2 shows the undulations on the weather sheds, and additional creep distance is obtained by adding undulations or increasing their depth. Recommendations for creep distance are shown in

Table 3.2.3 according to four different classifications of contamination. For example, a 345-kV bushing has a maximum line-to-ground voltage of 220 kV, so that the minimum creep is 220 X 28 = 6160 mm for a light contamination level and 220 X 44 = 9680 mm for a heavy contamination level. The term ESDD (equivalent salt-density deposit) used in Table 3.2.3 is

TABLE 3.2.2 Dielectric-Strength Correction Factors for Altitudes
Greater than 1000 m
Altitude, m Altitude Correction Factor for Dielectric Strength
1000 1.00
1200 0.98
1500 0.95
1800 0.92
2100 0.89
2400 0.86
2700 0.83
3000 0.80
3600 0.75
4200 0.70
4500 0.67
Source: ANSI/IEEE, 1997 [1]. With permission.

TABLE 3.2.3 Recommended Creep Distances for Four Contamination Levels
Contamination Level
Equivalent Salt-Deposit
Density (ESDD), mg/cm2
Recommended Minimum Creep
Distance, mm/kV
Light 0.03–0.08 28
Medium 0.08–0.25 35
Heavy 0.25–0.6 44
Extra heavy above 0.6 54
Source: IEEE Std. C57.19.100-1995 (R1997) [8]. With permission.

the conductivity of the water-soluble deposits on the insulator surface. It is expressed in terms of the density of sodium chloride deposited on the insulator surface that will produce the same conductivity.

Following are typical environments for the four contamination levels listed:

Light-contamination areas include areas without industry and with low-density emission-producing residential heating systems, and areas with some industrial areas or residential density but with frequent winds and/or precipitation. These areas are not exposed to sea winds or located near the sea.

Medium-contamination areas include areas with industries not producing highly polluted smoke and/ or with average density of emission-producing residential heating systems, areas with high industrial and/or residential density but subject to frequent winds and/or precipitation, and areas exposed to sea winds but not located near the sea coast.

Heavy-contamination areas include those areas with high industrial density and large city suburbs with high density emission-producing residential heating systems, and areas close to the sea or exposed to strong sea winds.

Extra-heavy-contamination areas include those areas subject to industrial smoke producing thick, conductive deposits and small coastal areas exposed to very strong and polluting sea winds.

TRANSFORMER BUSHING STANDARDS REFERENCE BASIC AND TUTORIALS

TRANSFORMER BUSHING STANDARDS REFERENCE BASIC INFORMATION

What Are The IEEE Reference To Transformer Bushing Standards?

Several bushing standards exist in the various countries around the world. The major standards have been established by the Transformers Committee within the IEEE Power Engineering Society and by IEC

Committee 37. Five important standards established by these committees include the following:

1. ANSI/IEEE Std. C57.19.00, Standard Performance Characteristics and Test Procedure for Outdoor Power Apparatus Bushings.

This is the general standard that is widely used by countries in the Western Hemisphere and contains definitions, service conditions, ratings, general electrical and mechanical requirements, and detailed descriptions of routine and design test procedures for outdoor-power-apparatus bushings.

2. IEEE Std. C57.19.01, Standard Performance Characteristics and Dimensions for Outdoor Power Apparatus Bushings.

This standard lists the electrical-insulation and test-voltage requirements for power-apparatus bushings rated from 15 through 800-kV maximum system voltages.

It also lists dimensions for standard-dimensioned bushings, cantilever-test requirements for bushings rated through 345-kV system voltage, and partial-discharge limits as well as limits for power factor and capacitance change from before to after the standard electrical tests.

3. IEEE Std. C57.19.03, Standard Requirements, Terminology and Test Procedures for Bushings for DC Applications [7]. This standard gives the same type of information as ANSI/IEEE Std.

C57.19.00 for bushings for direct-current equipment, including oil-filled converter transformers and smoothing reactors. It also covers air-to-air dc bushings.

4. IEEE Std. C57.19.100, Guide for Application of Power Apparatus Bushings [8]. This guide recommends practices to be used (1) for thermal loading above nameplate rating for bushings applied on power transformers and circuit breakers and (2) for bushings connected to isolated-phase bus.

It also recommends practices for allowable cantilever loading caused by the pull of the line connected to the bushing, applications for contaminated environments and high altitudes, and maintenance practices.

5. IEC Publication 137 [9], Bushings for Alternating Voltages above 1000 V. This standard is the IEC equivalent to the first standard listed above and is used widely in European and Asian countries.

TRANSFORMER BUSHING INSULATION TYPES BASIC AND TUTORIALS

INSULATION OF TRANSFORMER BUSHING BASIC INFORMATION

What Are The Different Types Of Transformer Bushing Insulation?

Another classification relates to the insulating material used inside the bushing. In general, these materials can be used in either the solid- or capacitance-graded construction, and in several types, more than one of these insulating materials can be used in conjunction.

The following text gives a brief description of these types:

Air-Insulated Bushings

Air-insulated bushings generally are used only with air-insulated apparatus and are of the solid construction that employs air at atmospheric pressure between the conductor and the insulators.

Oil-Insulated or Oil-Filled Bushings

Oil-insulated or oil-filled bushings have electrical-grade mineral oil between the conductor and the insulators in solid-type bushings. This oil can be contained within the bushing, or it can be shared with the apparatus in which the bushing is used.

Capacitance-graded bushings also use mineral oil, usually contained within the bushing, between the insulating material and the insulators for the purposes of impregnating the kraft paper and transferring heat from the conducting lead.

Oil-Impregnated Paper-Insulated Bushings

Oil-impregnated paper-insulated bushings use the dielectric synergy of mineral oil and electric grades of kraft paper to produce a composite material with superior dielectric-withstand characteristics.

This material has been used extensively as the insulating material in capacitance-graded cores for approximately the last 50 years.

Resin-Bonded or -Impregnated Paper-Insulated Bushings

Resin-bonded paper-insulated bushings use a resin-coated kraft paper to fabricate the capacitance graded core, whereas resin-impregnated paper-insulated bushings use papers impregnated with resin, which are then used to fabricate the capacitance-graded core.

The latter type of bushing has superior dielectric characteristics, comparable with oil-impregnated paper-insulated bushings.

Cast-Insulation Bushings

Cast-insulation bushings are constructed of a solid-cast material with or without an inorganic filler. These bushings can be either of the solid or capacitance-graded types, although the former type is more representative of present technology.

Gas-Insulated Bushings

Gas-insulated bushings use pressurized gas, such as SF6 gas, to insulate between the central conductor and the flange. It uses the same pressurized gas as the circuit breaker, has no capacitance grading, and uses the dimensions and placement of the ground shield to control the electric fields.

Other designs use a lower insulator to enclose the bushing, which permits the gas pressure to be different than within the circuit breaker. Still other designs use capacitance-graded cores made of plastic-film material that is compatible with SF6 gas.

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.