POWER TRANSFORMERS CORE IMPROVEMENT BASIC AND TUTORIALS

POWER TRANSFORMERS CORE IMPROVEMENT BASIC INFORMATION
What Are The Transformer Core Improvements?


The major improvement in core materials was the introduction of silicon steel in 1932. Over the years, the performance of electrical steels has been improved by grain orientation (1933) and continued improvement in the steel chemistry and insulating properties of surface coatings.

The thinner and more effective the insulating coatings are, the more efficient a particular core material will be. The thinner the laminations of electrical steel, the lower the losses in the core due to circulating currents. Mass production of distribution transformers has made it feasible to replace stacked cores with wound cores.

C-cores were first used in distribution transformers around 1940. A C-core is made from a continuous strip of steel, wrapped and formed into a rectangular shape, then annealed and bonded together.

The core is then sawn in half to form two C-shaped sections that are machine-faced and reassembled around the coil.

In the mid 1950s, various manufacturers developed wound cores that were die-formed into a rectangular shape and then annealed to relieve their mechanical stresses. The cores of most distribution transformers made today are made with wound cores.

Typically, the individual layers are cut, with each turn slightly lapping over itself. This allows the core to be disassembled and put back together around the coil structures while allowing a minimum of energy loss in the completed core.

Electrical steel manufacturers now produce stock for wound cores that is from 0.35 to 0.18 mm thick in various grades.

In the early 1980s, rapid increases in the cost of energy prompted the introduction of amorphous core steel. Amorphous metal is cooled down from the liquid state so rapidly that there is no time to organize into a crystalline structure.

Thus it forms the metal equivalent of glass and is often referred to as metal glass or “met-glass.” Amorphous core steel is usually 0.025 mm thick and offers another choice in the marketplace for transformer users that have very high energy costs.

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.