TRANSFORMER FAILURE MODE BASICS AND TUTORIALS

TRANSFORMER FAILURE MODE BASIC INFORMATION
What Is Transformer Failure Mode?


Transformer Failure Modes
The failure of a power transformer is almost always a catastrophic event that will cause the system to fail, and the result will be a messy cleanup job. The two primary enemies of power transformers are transient overvoltages and heat.

Power input to a transformer is not all delivered to the secondary load. Some is expended as copper losses in the primary and secondary windings. These I2R losses are practically independent of voltage; the controlling factor is current flow.

To keep the losses as small as possible, the coils of a power transformer are wound with wire of the largest cross section that space will permit. A medium-power, 3-phase power transformer is shown in Figure 4.29.

A practical transformer also will experience core-related losses, also known as iron losses. Repeated magnetizing and demagnetizing of the core (which occurs naturally in an ac waveform) results in power loss because of the repeated realignment of the magnetic domains.

This factor (hysteresis loss) is proportional to frequency and flux density. Silicon steel alloy is used for the magnetic circuit to minimize hysteresis loss.

The changing magnetic flux also induces circulating currents (eddy currents) in the core material. Eddy current loss is proportional to the square of the frequency and the square of the flux density.

To minimize eddy currents, the core is constructed of laminations or layers of steel that are clamped or bonded together to form a single magnetic mass.

TRANSFORMER CORE DESIGN AND CONSTRUCTION BASICS AND TUTORIALS

TRANSFORMER CORE DESIGN AND CONSTRUCTION BASIC INFORMATION
Transformer Core Design and Construction: A Tutorial 


Air gaps in a magnetic core will add considerable reluctance to the magnetic circuit. Remembering that the inductance of a coil and the magnetic reluctance are inversely proportional, air gaps reduce the inductance of the coil and increase the magnitude of magnetizing currents. In practical transformers, we want to reduce magnetizing currents to almost negligible levels; it is therefore important to eliminate all air gaps if possible.

One approach would be to make the core from a solid block of material. This is impractical from the standpoint of fabricating the transformer, since the coils would have to be wound through the core window.


Also, since metallic core materials conduct electric current as well as magnetic flux, the induced voltages would produce large circulating currents in a solid core. The circulating currents would oppose the changing flux and effectively ‘‘short out’’ the transformer.


A practical solution is to fabricate the core from thin laminated steel sheets that are stacked together and to coat the surfaces of the laminations with a thin film that electrically insulates the sheets from each other. Steel not only has excellent magnetic properties but is also relatively inexpensive and easy to fabricate into thin sheets.


In a modern transformer plant, steel ribbon is cut into sections by a cutting/punching machine commonly called a Georg machine. The sizes and shapes of the sections are determined by the core design of the individual transformer.

The thickness of the sheets varies somewhat; core laminations operating at 60 Hz are between 0.010 and 0.020 in. thick, with 0.012 in. being the most common thickness in use today.


Different methods of stacking core steel have been used in the past. One such method is called the butt lap method using rectangular core sections and is illustrated in Figure 1.11

Even if the edges of the segments do not butt together perfectly, as shown in the exaggerated edge view at the bottom of the figure, the alternating even and odd layers assure that the magnetic flux has a continuous path across the surfaces of the adjacent layers.