TYPES OF FAILURES IN POWER TRANSFORMERS BASIC AND TUTORIALS

The electrical windings and the magnetic core in a transformer are subject to a number of different forces

during operation, for example

a) Expansion and contraction due to thermal cycling

b) Vibration

c) Local heating due to magnetic flux

d) Forces due to the flow of through-fault currents

e) Excessive heating due to overloading or inadequate cooling

These forces can cause deterioration and failure of the electrical insulation of the transformer windings. Statistics for the causes of transformer failures experienced in U.S. utilities are not readily available.

The detection systems that monitor other transformer parameters can be used to indicate an incipient electrical fault. Prompt response to these indicators may help avoid a serious fault.

Some examples of actions taken to detect undesirable operating conditions are as follows:

1) Temperature monitors for winding or oil temperature are typically used to initiate an alarm requiring investigation by maintenance staffs. At this stage, the operators may start to reduce the load on the transformer to avoid reaching a condition where tripping the transformer would be required.

2) Gas detection relays can detect the evolution of gases within the transformer oil. Analysis of the gas composition indicates the mechanism that caused the formation of the gas, e.g., acetylene can be caused by electrical arcing; other gases are caused by partial discharge and thermal degradation of the cellulose insulation.

The gas detection relays may be used to trip or to generate an alarm depending on the utility practice. Generally, gas analysis is performed on samples of the oil that are collected periodically. A continuous gas analyzer is available to allow online detection of insulation system degradation.

3) Sudden-pressure relays under oil respond to the pressure waves in the transformer oil caused by the evolution of gas associated with arcing.

4) Sudden-pressure relays in the gas space respond to sudden changes in the gas pressure due to evolving gases from an arc under oil.

5) Oil-level detectors sense the oil level in the tank and are used to generate an alarm indicating minor reductions in oil level and trip for severe reductions.

6) Online devices monitor bushings of the transformers, CTs installed in those bushings, and surge arresters installed on the transformers and generate an alarm indicating that repair is needed urgently so that major damage is avoided.

Details of the modern techniques for monitoring these components are given by Coffeen et al.

EFFECTS OF SHORT CIRCUITS ON TRANSFORMERS BASICS AND TUTORIALS

EFFECTS OF SHORT CIRCUITS ON TRANSFORMERS BASIC
What Are The Effects Of Short Circuits On Transformers?


Transformers are susceptible to damage by secondary short-circuit currents having magnitudes that can be many times rated load current. The damage results from the following effects:

• The I 2R losses in the winding conductors are increased by the square of the current. This increases the temperature rise of the windings.

Because protective devices limit the duration of short circuits (as opposed to overloads), the temperature rise of the winding can be calculated by dividing the total energy released by the I 2R losses by the thermal capacity of the conductor.

• The short-circuit currents exclude flux in the core and increase stray flux around the core. This stray flux induces currents in metallic parts other than the winding conductors, which can be damaged thermally.

• A short circuit applied to the secondary circuit of an autotransformer can substantially increase the voltage across the series winding and across the common winding through induction.

This not only presents the possibility of damaging the winding insulation by overvoltage, but will also drive the core into saturation and significantly increase core losses with potential damaging effects from temperature.


• Bushings and tap changers have current ratings that are usually only marginally greater than the rated load of the transformer.

Since fault currents are many times rated currents and these components have short thermal time constants, they can be seriously overloaded and thermally damaged.

• Stray flux in the vicinity of current-carrying conductors produces mechanical forces on the conductors. When a short circuit is applied to a transformer, there is a significant increase in stray flux, resulting in greater mechanical forces on the windings, leads, bushings, and all other current-carrying components.

These components, especially the windings, must be braced to withstand these forces.

A good transformer design must take all of the above effects into account to minimize the risk of damage and assure a long service life.

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.