POWER TRANSFORMER TEMPERATURE RISE TEST AT LOAD BEYOND NAMEPLATE RATING BASICS AND TUTORIALS

POWER TRANSFORMER TEMPERATURE RISE TEST AT LOAD BEYOND NAMEPLATE RATING BASIC INFORMATION

How To Conduct Temperature Rise Test For Power Transformer Beyond Nameplate Rating?

After completing the hot resistance tests data recorded during tests may be evaluated to determine preliminary exponents. The preliminary exponents may be used to evaluate whether an excessive top oil temperature or winding hottest spot temperature may occur during this test.

It is suggested that the winding hottest spot temperature be limited to 140 ˚C and top oil be limited to 110 ˚C, unless other values are agreed upon by the manufacturer and user. The top oil temperature and the measured rate of change of the oil level with temperature may be used to evaluate whether excessive oil levels may occur during this test.

If it becomes apparent that excessive values may be obtained, the load may be reduced from the 125% value, so the top oil temperature, winding hottest spot temperature, and oil level are limited to acceptable values.

After the evaluation of risk and the load beyond nameplate to be applied has been determined, proceed with the test as follows:

a) Short-circuit one or more windings, and circulate a constant current , at rated frequency, equal to

125% of rated current (1.25 x IR), plus additional current to produce losses equal to the rated no-load loss.

The current to be circulated may be determined using Equation (3). Continue applying this current until the top oil temperature does not vary by more than 2.5% or 1 ˚C, whichever is greater, in a time period of three consecutive hours.

b) Record all data listed in Clause 6 and Clause 7 after the top oil temperature rise has stabilized and

while is being applied:

c) Reduce the current to 125% of rated current ( ) and hold for a minimum time period of one hour. Calculate and record as measured current/ for later use in 9.8.5.

d) At the end of the one-hour period, while the current equal to 125% of rated ( ) is being applied, record all data.

e) Remove the load current, and measure a series of hot resistances of the windings at appropriate time intervals to determine the average winding temperatures using the cooling curve method in IEEE Std C57.12.90-1999. Only those windings found to be the hottest windings in item e) of 9.5 need be measured.

TRANSFORMER BUSHINGS BASICS AND TUTORIALS

TRANSFORMER BUSHINGS BASIC INFORMATION

What Are Transformer Bushing? Functions Of Transformer Bushing?

Bushings may be classified generally by design as follows:

a) Condenser type

1) Oil-impregnated paper insulation, with interspersed conducting (condenser) layers or oil impregnated paper insulation, continuously wound with interleaved lined paper layers

2) Resin-bonded paper insulation, with interspersed conducting (condenser layers)

b) Noncondenser type

1) Solid core or alternate layers of solid and liquid insulation

2) Solid mass of homogeneous insulating material (e.g., solid porcelain)

3) Gas filled

For outdoor bushings, the primary insulation is contained in a weatherproof housing, usually porcelain. The space between the primary insulation and the weathershed is generally filled with an insulating oil or compound (also, plastic and foam).

Some of the solid homogenous types may use oil to fill the space between the conductor and the inner wall of the weathershed. Bushings may also use gas such as SF6 as an insulating medium between the center conductor and outer weathershed.

Bushings may be further classified generally as being equipped or not equipped with a potential tap or power-factor test tap or electrode. Note Potential taps are sometimes also referred to as capacitance or voltage taps.)

The bushing, without a potential tap or power-factor tap, is a two-terminal device that is generally tested overall (center conductor to range) by the GST method. If the bushing is installed in an apparatus, such as a circuit breaker, the overall GST measurement will include all connected and energized insulating components between the conductor and ground.

A condenser bushing is essentially a series of concentric capacitors between the center conductor and the ground sleeve or mounting range. A conducting layer near the ground sleeve may be tapped and brought out to a tap terminal to provide a three-terminal specimen.

The tapped bushing is essentially a voltage divider and, in higher voltage designs, the tap potential may be utilized to supply a bushing potential device for relay and other purposes. In this design the potential tap also acts as a low-voltage power-factor test terminal for the main bushing insulation, C1.

Modern bushings rated above 69 kV are usually equipped with potential taps. (In some rare instances 69 kV bushings were equipped with potential taps.) Bushings rated 69 kV and below may be equipped with power factor taps.

In the power-factor tap design, the ground layer of the bushing core is tapped and terminated in a miniature bushing on the main bushing mounting range. The tap is connected to the grounded mounting range by a screw cap on the miniature bushing housing.

With the grounding cap removed, the tap terminal is available as a low-voltage terminal for a UST measurement on the main bushing insulation, C1, conductor to tapped layer.