PROTECTIVE RELAY CURRENTS EXPERIENCED BY POWER TRANSFORMERS DURING FAULT CONDITIONS

Two characteristics of power transformers combine to complicate detection of internal faults with current operated relays

a)The change in magnitude of current at the transformer terminals may be very small when a limited number of turns are shorted within the transformer.

b)When a transformer is energized, magnetizing inrush current that flows in one set of terminals may equal many times the transformer rating. These and other considerations require careful thought to obtain relay characteristics best-suited to the particular application.

Minimum internal faults

The most difficult transformer winding fault for which to provide protection is the fault that initially involves one turn. A turn-to-turn fault will result in a terminal current of much less than rated full-load current.

For example, as much as 10% of the winding may have to be shorted to cause full-load terminal current to flow.  Therefore, a single turn-to-turn fault will result in an undetectable amount of current.

Maximum internal faults

There is no limit to the maximum internal fault current that can flow, other than the system capability, when the fault is a terminal fault or a fault external to the transformer but in the relay zone. The relay system should be capable of withstanding the secondary current of the CT on a short-time basis.

This may be a factor if the transformer is small relative to the system fault and if the CT ratio is chosen to match the transformer rating.

Through-faults

Fault current through a transformer is limited by the transformer and source impedance. While current through a transformer thus limited by its impedance can still cause incorrect relay operations or even transformer failure, CT saturation is less likely to occur than with unlimited currents.

The above favorable aspect may disappear if the transformer protective zone includes a bus area with two or more breakers on the same side of the transformer through which external fault current can flow with no relationship to the transformer rating. An example is a transformer connected to a section of a ring bus with the transformer protection including the ring bus section.

TYPES OF POWER TRANSFORMER FAULTS BASIC INFORMATION

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)Impact forces due to through fault current

e)Excessive heating to to overloading or inadequate cooling

These forces can cause deterioration and failure of the winding electrical insulation. Table below summarizes failure statistics for a broad range of transformer failure causes reported by a group of U.S. utilities over a period of years.

This guide deals primarily with the application of electrical relays to detect the fault current that results from an insulation failure. The current a relay can expect to see as a result of various types of winding insulation failures.

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. For example

a)Temperature monitors for winding or oil temperature are typically used to initiate an alarm requiring investigation by maintenance staff.

b)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 corona and thermal degradation of the cellulose insulation.

The gas detection relays may be used to trip or alarm depending on utility practice. Generally, gas analysis is performed on samples of the oil, which are collected periodically. Alternatively, a continuous gas analyzer is available to allow on-line detection of insulation system degradation.

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

d)Oil level detectors sense the oil level in the tank and are used to alarm for minor reductions in oil level and trip for severe reductions.

PHASE SHIFTING TRANSFORMER SHORT CIRCUIT CHARACTERISTIC

Short Circuit Requirements 

General
PSTs shall comply with the short circuit requirements of IEEE Std C57.12.00-2000, unless otherwise agreed upon by the purchaser and manufacturer.

Transformer categories
The kVA rating to be considered for determining the category should be the equivalent to the rating according to IEEE Std C57.12.00-2000.


Short-circuit current magnitude
The manufacturer shall determine the most onerous conditions for short circuit on every winding or active part in accordance with IEEE Std C57.12.00-2000.

These conditions should take into account the large impedance swings that can occur as the tap position is changed from the extreme positions to the mid position.

Since the system short-circuit levels are critical to the design of PSTs, the user shall specify the maximum system short-circuit fault levels expected throughout the life of the unit.

If a short-circuit test is performed, it shall be done in accordance with IEEE Std C57.12.90-1993.

The test shall be carried out on the tap position that produces the most severe stresses in each winding. This may require more than a single test depending on the type of construction.

For two-core PSTs this usually requires a test on the zero phase-shift position, as this position involves only the series transformer, and a second test on a position to be agreed upon between customer and manufacturer.

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.

POWER TRANSFORMER SHORT CIRCUIT FORCES BASICS AND TUTORIALS

SHORT CIRCUIT FORCES ON POWER TRANSFORMERS BASIC INFORMATION
What Are The Short Circuit Forces Acting On Power Transformers?


Forces exist between current-carrying conductors when they are in an alternating-current field. These forces are determined using :

F = B I sin x
where

F = force on conductor
B = local leakage flux density
x = angle between the leakage flux and the load current. In transformers, sin x is almost
always equal to 1.


Thus
B = uI
and therefore
F directly proportional to I^2

Since the leakage flux field is between windings and has a rather high density, the forces under shor tcircuit conditions can be quite high. This is a special area of transformer design. Complex computer programs are needed to obtain a reasonable representation of the field in different parts of the windings.

Considerable research activity has been directed toward the study of mechanical stresses in the windings and the withstand criteria for different types of conductors and support systems.

Between any two windings in a transformer, there are three possible sets of forces:

• Radial repulsion forces due to currents flowing in opposition in the two windings

• Axial repulsion forces due to currents in opposition when the electromagnetic centers of the two windings are not aligned

• Axial compression forces in each winding due to currents flowing in the same direction in adjacent
conductors

The most onerous forces are usually radial between windings. Outer windings rarely fail from hoop stress, but inner windings can suffer from one or the other of two failure modes:

• Forced buckling, where the conductor between support sticks collapses due to inward bending into the oil-duct space

• Free buckling, where the conductors bulge outwards as well as inwards at a few specific points on the circumference of the winding

Forced buckling can be prevented by ensuring that the winding is tightly wound and is adequately supported by packing it back to the core. Free buckling can be prevented by ensuring that the winding is of sufficient mechanical strength to be self-supporting, without relying on packing back to the core.