INSULATION STRUCTURES DESIGN OF POWER TRANSFORMER BASIC INFORMATION

Three factors must be considered in the evaluation of the dielectric capability of an insulation structures —the voltage distribution must be calculated between different parts of the winding, the dielectric stresses are then calculated knowing the voltages and the geometry, and finally the actual stresses can be compared with breakdown or design stresses to determine the design margin.

Voltage distributions are linear when the flux in the core is established. This occurs during all power frequency test and operating conditions and to a great extent under switching impulse conditions (Switching impulse waves have front times in the order of tens to hundreds of microseconds and tails in excess of 1000 μs.)

These conditions tend to stress the major insulation and not inside of the winding. For shorter-duration impulses, such as full-wave, chopped-wave, or front-wave, the voltage does not divide linearly within the winding and must be determined by calculation or low voltage measurement. The initial distribution is determined by the capacitative network of the winding.

For disk and helical windings, the capacitance to ground is usually much greater than the series capacitance through the winding. Under impulse conditions, most of the capacitive current flows through the capacitance to ground near the end of the winding, creating a large voltage drop across the line end portion of the coil.

The capacitance network for shell form and layer-wound core form results in a more uniform initial distribution because they use electrostatic shields on both terminals of the coil to increase the ratio between the series and to ground capacitances.

Static shields are commonly used in disk windings to prevent excessive concentrations of voltages on the line-end turns by increasing the effective series capacitance within the coil, especially in the line end sections.

Interleaving turns and introducing floating metal shields are two other techniques that are commonly used to increase the series capacitance of the coil.

Following the initial period, electrical oscillations occur within the windings. These oscillations impose greater stresses from the middle parts of the windings to ground for long-duration waves than for short-duration waves.

Very fast impulses, such as steep chopped waves, impose the greatest stresses between turns and coil portions. Note that switching impulse transient voltages are two types— asperiodic and oscillatory. Unlike the asperiodic waves discussed earlier, the oscillatory waves can excite winding natural frequencies and produce stresses of concern in the internal winding insulation.

Transformer windings that have low natural frequencies are the most vulnerable because internal damping is more effective at high frequencies. Dielectric stresses existing within the insulation structure are determined using direct calculation (for basic geometries), analog modeling, or most recently, sophisticated finite-element computer programs.

Allowable stresses are determined from experience, model tests, or published data. For liquidinsulated transformers, insulation strength is greatly affected by contamination and moisture. The relatively porous and hygroscopic paper-based insulation must be carefully dried and vacuum impregnated with oil to remove moisture and gas to obtain the required high dielectric strength and to resist deterioration at operating temperatures.

Gas pockets or bubbles in the insulation are particularly destructive to the insulation because the gas (usually air) not only has a low dielectric constant (about 1.0), which means that it will be stressed more highly than the other insulation, but also air has a low dielectric strength.

High-voltage dc stresses may be imposed on certain transformers used in terminal equipment for dc transmission lines. Direct-current voltage applied to a composite insulation structure divides between individual components in proportion to the resistivities of the material.

In general the resistivity of an insulating material is not a constant but varies over a range of 100:1 or more, depending on temperature, dryness, contamination, and stress. Insulation design of high-voltage dc transformers in particular require extreme care.

INSULATION CLASSES OF POWER TRANSFORMER BASIC INFORMATION AND TUTORIALS

The type of insulation used in dry-type–transformer design and construction has a definite bearing on the size and operating temperature of the unit. Currently four classes of insulation, each having a separate NEMA specification and temperature limit, are being used.

A look at these will facilitate selection of the proper unit to meet prescribed installation and operating conditions.

1. Class 130 insulation-system transformers. When properly applied and loaded in an ambient not over 40 C, these transformers will operate at not more than a 60 C temperature rise on the winding.

These units can be used as control-type transformers when higher temperatures might affect other temperature-sensitive devices in the enclosure or as distribution transformers in locations (textile mills, sawmills, etc.) where combustible flyings might be present in the surrounding atmosphere.

2. Class 150 insulation-system transformers. These units have a higher-temperature insulating system and are physically smaller and about half the weight of Class A units of corresponding rated capacities. When properly loaded to rated kilovolt-amperes and installed in an ambient not over 40 C,

Class 150 units will operate at a maximum 80 C rise on the winding. For years dry-type distribution transformers have been of the Class 150 type.

3. Class 200 insulation-system transformers. These units also have a high-temperature insulating system and, when properly loaded and applied in an ambient not over 40 C, will operate at no more than 130 C rise on the winding.

The units are smaller in size than similarly rated Class 150 units and currently are available from a number of manufacturers in ratings of 25 kVA and lower, both single- and three-phase design. One manufacturer designs in-wall, flush-mounted dry-type transformers as Class 200 units.

4. Class 220 insulation-system transformers. These units are insulated with a high temperature system of glass, silicone, and asbestos components and are probably the most compact ones available.

When properly loaded and applied in an ambient not over 40 C, Class 220 transformers will operate at a maximum 150 C rise on the winding. This class of insulation is used primarily in designs in which the core and coil are completely enclosed in a ventilated housing.

Generally, this stipulation covers units with ratings of 30 kVA and larger. Some experts recommend that the hottest spot on the metal enclosure be limited to a maximum rise of 40 C above a 40 C ambient. It should be noted that Class 150 insulation is being replaced with Class 200 or 220 insulation in transformers of recent design.

Another significant factor which concerns all dry-type transformers is that they should never be overloaded. The way to avoid this is to size the primary or secondary overcurrent device as close as possible to the full-load primary or secondary current for other than motor loads.

If close overcurrent protection has not been provided, loads should be checked periodically. Overloading a transformer causes excessive temperature, which, in turn, produces overheating. This results in rapid deterioration of the insulation and will cause complete failure of the transformer coils.

POWER TRANSFORMER INSULATION CLASS SYSTEM BASIC AND TUTORIALS

The type of insulation used in dry-type–transformer design and construction has a definite bearing on the size and operating temperature of the unit. Currently four classes of insulation, each having a separate NEMA specification and temperature limit, are being used.

A look at these will facilitate selection of the proper unit to meet prescribed installation and operating conditions.

1. Class 130 insulation-system transformers. When properly applied and loaded in an ambient not over 40 C, these transformers will operate at not more than a 60 C temperature rise on the winding.

These units can be used as control-type transformers when higher temperatures might affect other temperature-sensitive devices in the enclosure or as distribution transformers in locations (textile mills, sawmills, etc.) where combustible flyings might be present in the surrounding atmosphere.

2. Class 150 insulation-system transformers. These units have a higher-temperature insulating system and are physically smaller and about half the weight of Class A units of corresponding rated capacities.

When properly loaded to rated kilovolt-amperes and installed in an ambient not over 40 C, Class 150 units will operate at a maximum 80 C rise on the winding. For years dry-type distribution transformers have been of the Class 150 type.

3. Class 200 insulation-system transformers. These units also have a high-temperature insulating system and, when properly loaded and applied in an ambient not over 40 C, will operate at no more than 130 C rise on the winding.

The units are smaller in size than similarly rated Class 150 units and currently are available from a number of manufacturers in ratings of 25 kVA and lower, both single- and three-phase design. One manufacturer designs in-wall, flush-mounted dry-type transformers as Class 200 units.

4. Class 220 insulation-system transformers. These units are insulated with a high temperature system of glass, silicone, and asbestos components and are probably the most compact ones available. When properly loaded and applied in an ambient not over 40 C, Class 220 transformers will operate at a maximum 150 C rise on the winding.

This class of insulation is used primarily in designs in which the core and coil are completely enclosed in a ventilated housing. Generally, this stipulation covers units with ratings of 30 kVA and larger. Some experts recommend that the hottest spot on the metal enclosure be limited to a maximum rise of 40 C above a 40 C ambient.

It should be noted that Class 150 insulation is being replaced with Class 200 or 220 insulation in transformers of recent design.

Another significant factor which concerns all dry-type transformers is that they should never be overloaded. The way to avoid this is to size the primary or secondary overcurrent device as close as possible to the full load primary or secondary current for other than motor loads.

If close overcurrent protection has not been provided, loads should be checked periodically. Overloading a transformer causes excessive temperature, which, in turn, produces overheating. This results in rapid deterioration of the insulation and will cause complete failure of the transformer coils.

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.

DESIGN OF POWER TRANSFORMERS INSULATION STRUCTURES BASICS AND TUTORIALS

DESIGN OF POWER TRANSFORMERS INSULATION STRUCTURES BASIC INFORMATION
How To Design Power Transformer Insulation Structures?


Three factors must be considered in the evaluation of the dielectric capability of an insulation structure—the voltage distribution must be calculated between different parts of the winding, the dielectric stresses are then calculated knowing the voltages and the geometry, and finally the actual stresses can be compared with breakdown or design stresses to determine the design margin.

Voltage distributions are linear when the flux in the core is established. This occurs during all power frequency test and operating conditions and to a great extent under switching impulse conditions.

(Switching impulse waves have front times in the order of tens to hundreds of microseconds and tails in excess of 1000 μs.) These conditions tend to stress the major insulation and not inside of the winding.

For shorter-duration impulses, such as full-wave, chopped-wave, or front-wave, the voltage does not divide linearly within the winding and must be determined by calculation or low voltage measurement.

The initial distribution is determined by the capacitative network of the winding. For disk and helical windings, the capacitance to ground is usually much greater than the series capacitance through the winding.

Under impulse conditions, most of the capacitive current flows through the capacitance to ground near the end of the winding, creating a large voltage drop across the line end portion of the coil.

The capacitance network for shell form and layer-wound core form results in a more uniform initial distribution because they use electrostatic shields on both terminals of the coil to increase the ratio between the series and to ground capacitances.

Static shields are commonly used in disk windings to prevent excessive concentrations of voltages on the line-end turns by increasing the effective series capacitance within the coil, especially in the line end sections.

Interleaving turns and introducing floating metal shields are two other techniques that are commonly used to increase the series capacitance of the coil.

Following the initial period, electrical oscillations occur within the windings. These oscillations impose greater stresses from the middle parts of the windings to ground for long-duration waves than for short-duration waves.

Very fast impulses, such as steep chopped waves, impose the greatest stresses between turns and coil portions. Note that switching impulse transient voltages are two types— asperiodic and oscillatory.  Unlike the asperiodic waves discussed earlier, the oscillatory waves can excite winding natural frequencies and produce stresses of concern in the internal winding insulation.

Transformer windings that have low natural frequencies are the most vulnerable because internal damping is more effective at high frequencies.

Dielectric stresses existing within the insulation structure are determined using direct calculation (for basic geometries), analog modeling, or most recently, sophisticated finite-element computer programs.

Allowable stresses are determined from experience, model tests, or published data. For liquidinsulated transformers, insulation strength is greatly affected by contamination and moisture. The relatively porous and hygroscopic paper-based insulation must be carefully dried and vacuum impregnated with oil to remove moisture and gas to obtain the required high dielectric strength and to resist deterioration at operating temperatures.

Gas pockets or bubbles in the insulation are particularly destructive to the insulation because the gas (usually air) not only has a low dielectric constant (about 1.0), which means that it will be stressed more highly than the other insulation, but also air has a low dielectric strength.

High-voltage dc stresses may be imposed on certain transformers used in terminal equipment for dc transmission lines. Direct-current voltage applied to a composite insulation structure divides between individual components in proportion to the resistivities of the material.

In general the resistivity of an insulating material is not a constant but varies over a range of 100:1 or more, depending on temperature, dryness, contamination, and stress. Insulation design of high-voltage dc transformers in particular require extreme care.

POWER TRANSFORMERS INSULATING LIQUIDS BASICS AND TUTORIALS

POWER TRANSFORMERS INSULATING LIQUIDS BASIC INFORMATION
What Are The Insulating Liquids Of Power Transformers?


Insulating Liquids
Dielectric liquids of various types are used as an insulating medium as well as a means of cooling liquid-filled transformers. Common insulating liquids include the following:

• Mineral oil. A mineral oil-filled transformer is generally the smallest, lightest, and most economical transformer available. Mineral oil has excellent properties for use in transformers, but it has the inherent weakness of being flammable. Its use, therefore, is restricted to outdoor installations or when the transformer is installed within a vault if used indoors.

• Silicone. A wide variety of synthetic polymer chemicals are referred to by the generic term silicone. Silicone transformer liquids are actually known chemically as polydimethylsiloxane (PDMS). PDMS is a water-clear, odorless, chemically stable, nontoxic liquid.

• High-molecular-weight hydrocarbon (HMWH). HMWH is another high-firepoint dielectric that is widely used as a transformer liquid. It has similar values for dielectric strength and dielectric constant, power factor, and thermal conductivity as mineral oil.

There are no established standards for testing the fire safety of transformers. Factory Mutual Research (FM) and Underwriters Laboratories (UL) both have different criteria for listing transformer liquids. Fire properties of dielectric fluids are typically classified by the following characteristics.

• Flash point: the temperature at which vapors from a liquid surface will ignite in the presence of a flame.
• Fire point: the temperature at the surface of a liquid that will sustain a fire.
• Flame spread: a series of consecutive ignitions.
• Ease of ignition: how readily the liquid will generate and maintain a flammable fuel/vapor mixture at the surface.
• Heat release rate: the product of vaporization rate and the heat of combustion of the fluid. The higher this rate in a large-scale fire, the higher the degree of fire hazard.

Selection of the dielectric liquid depends on the transformer application. Normally, the choice is mineral oil if the device is to be located outdoors.

The National Electrical Code (NEC) does, however, specify certain limitations regarding the use of oil filled transformers in particular outdoor locations. The selection of less-flammable liquids (PDMS and HMWH) often depends upon personal preference, the liquid used in other transformers on the site, or the transformer manufacturer's recommendation.

THE BH CURVE APPLICATION IN POWER TRANSFORMER TUTORIALS

THE BH CURVE APPLICATION IN POWER TRANSFORMER BASIC INFORMATION
What Is BH Curve? How It Is Applied In Transformers?


For actual transformer core materials, the relationship between B and H is much more complicated. For a flux that periodically changes, the B-H curve depends on the magnitude of the flux density and the periodic
frequency.

Figure 1.7 plots the B-H curve for a ferromagnetic core with a 60 Hz sinusoidal flux density having a moderate peak value.

The B-H curve is a closed loop with the path over time moving in a counterclockwise direction over each full cycle. Note that when the magnetizing current is zero (H 0) there is still a considerable positive or negative residual flux in the core.

This residual flux is from crystalline structures in ferromagnetic materials that remain magnetically aligned even after the MMF is removed.

For a given peak amplitude of flux density, the B-H loop becomes narrower at frequencies below 60 Hz, although the width of the loop is not directly proportional to frequency. Even at very low frequencies approaching DC, the B-H curve has a finite area contained in the loop.

As seen in Figure 1.7, magnetic materials are highly nonlinear, so treating m as a constant is clearly an oversimplification. Nevertheless, assuming that materials are linear, at least over some range of flux density, is required in order to do quantitative analysis.

As the peak amplitude of the flux increases, the core goes into saturation; i.e., B increases at a much smaller rate with respect to increasing H. This means that μ gets effectively smaller as B increases. In saturation, the slope dB/dH is approximately equal to μ0. Figure 1.8 plots a typical B-H curve for a ferromagnetic core with a 60 Hz sinusoidal flux density having a large peak value.

This core material saturates at approximately +/- 1.5 Wb/m2 ( +/- 1.5 T), which is a typical saturation value for materials used in power transformers.

The magnitude of H increases greatly when the core goes into saturation, meaning that the peak magnetizing current increases dramatically. Again, the width of the B-H loop becomes narrower at frequencies below 60 Hz for a given peak amplitude of flux.

DISTRIBUTION TRANSFORMER COOLANTS BASIC AND TUTORIALS

DISTRIBUTION TRANSFORMER COOLANTS BASIC INFORMATION
What Are The Different Distribution Transformer Coolants?

Mineral Oil

Mineral oil surrounding a transformer core-coil assembly enhances the dielectric strength of the winding and prevents oxidation of the core. Dielectric improvement occurs because oil has a greater electrical withstand than air and because the dielectric constant of oil is closer to that of the insulation.

As a result, the stress on the insulation is lessened when oil replaces air in a dielectric system. Oil also picks up heat while it is in contact with the conductors and carries the heat out to the tank surface by self convection. Thus a transformer immersed in oil can have smaller electrical clearances and smaller conductors for the same voltage and kVA ratings.

Askarels

Beginning about 1932, a class of liquids called askarels or polychlorinated biphenyls (PCB) was used as a substitute for mineral oil where flammability was a major concern. Askarel-filled transformers could be placed inside or next to a building where only dry types were used previously.

Although these coolants were considered nonflammable, as used in electrical equipment they could decompose when exposed to electric arcs or fires to form hydrochloric acid and toxic furans and dioxins.

The compounds were further undesirable because of their persistence in the environment and their ability to accumulate in higher animals, including humans. Testing by the U.S. Environmental Protection

Agency has shown that PCBs can cause cancer in animals and cause other noncancer health effects. Studies in humans provide supportive evidence for potential carcinogenic and noncarcinogenic effects of PCBs (http://www.epa.gov). The use of askarels in new transformers was outlawed in 1977 (Claiborne, 1999).

Work still continues to retire and properly dispose of transformers containing askarels or askarel-contaminated mineral oil. Current ANSI/IEEE standards require transformer manufacturers to state on the nameplate that new equipment left the factory with less than 2 ppm PCBs in the oil (IEEE, 2000).

High-Temperature Hydrocarbons

Among the coolants used to take the place of askarels in distribution transformers are high-temperature hydrocarbons (HTHC), also called high-molecular-weight hydrocarbons.

These coolants are classified by the National Electric Code as “less flammable” if they have a fire point above 300˚C.

The disadvantages of HTHCs include increased cost and a diminished cooling capacity from the higher viscosity that accompanies the higher molecular weight.