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.

THE NATURE OF TRANSFORMER LOSSES BASIC INFORMATION

Transformer losses are broadly classified as no-load and load losses. No load losses occur when the  transformer is energized with its rated voltage at one set of terminals but the other sets of terminals are open circuited so that no through or load current flows.

In this case, full flux is present in the core and only the necessary exciting current flows in the windings. The losses are predominately core losses due to hysteresis and eddy currents produced by the time varying flux in the core steel.

Load losses occur when the output is connected to a load so that current flows through the transformer from input to output terminals. Although core losses also occur in this case, they are not considered part of the load losses.

When measuring load losses, the output terminals are shorted to ground and only a small impedance related voltage is necessary to produce the desired full load current. In this case, the core losses are small because of the small core flux and do not significantly add to the measured losses.


Load losses are in turn broadly classified as I2R losses due to Joule heating produced by current flow in the coils and as stray losses due to the stray flux as it encounters metal objects such as tank walls, clamps or bracing structures, and the coils themselves. Because the coil conductors are often stranded and transposed, the I2R losses are usually determined by the d.c. resistance of the windings.

The stray losses depend on the conductivity, permeability, and shape of the metal object encountered. These losses are primarily due to induced eddy currents in these objects.

Even though the object may be made of ferromagnetic material, such as the tank walls and clamps, their dimensions are such that hysteresis losses tend to be small relative to eddy current losses.

Although losses are usually a small fraction of the transformed power (<0.5% in large power transformers), they can produce localized heating which can compromise the operation of the transformer. Thus it is important to understand how these losses arise and to calculate them as accurately as possible so that, if necessary steps can be taken at the design stage to reduce them to a level which can be managed by the cooling system.

Other incentives, such as the cost which the customer attaches to the losses, can make it worthwhile to find ways of lowering the losses. Modern methods of analysis, such as finite element or boundary element methods, have facilitated the calculation of stray flux losses in complex geometries.

These methods are not yet routine in design because they require a fair amount of geometric input for each new geometry. They can, however, provide useful insights in cases where analytic methods are not available or are very crude. Occasionally a parametric study using such methods can extend their usefulness beyond a specialized geometry.