FACTORS AFFECTING DRY TYPE TRANSFORMER'S LIFE BASICS AND TUTORIALS

DRY TYPE TRANSFOMER FACTOR THAT AFFECT ITS LIFE BASIC INFORMATION

What Are The Factors That Affects The Life OF Dry Type Transformers?

Dry-type transformers may be loaded above rated kilovolt-amperes under conditions other than those specified in the preceding clauses, with a sacrifice of life expectancy dependent on the load capability of the transformer and on the actual operating conditions.

The overload capability of dry-type transformers varies widely and is affected by the following characteristics:

a) Hottest-spot winding conductor rise over ambient;

b) Ratio of load losses to no-load losses;

c) Time constant;

d) Ambient temperature.

Operating conditions for dry-type transformers are so variable that no single set of practical loading data can be presented for all possible combinations of conditions and loading.

However, methods are outlined in this clause whereby the user can estimate allowable loads for the user’s own conditions by taking into account the probable number and nature of such loads during the life of the transformer, and the approximate percentage of life expectancy that the user is willing to sacrifice.

In general, permissible temperatures and loads calculated by the means outlined here will be higher than those loading for life expectancy which are necessarily conservative in order to cover the wide variation in sizes and makes of dry-type transformers.

The necessary curves, tables, equations, and definitions used as a basis for the methods given here are presented in Clause 6 of IEEE STD-C57.56. Information given is considered to be the best that can be produced from the present knowledge of the subject.

In spite of its approximate nature, it is believed that it will be of value as a guide to the user.

ROGOWSKI COILS BASICS AND TUTORIALS

ROGOWSKI COILS BASIC INFORMATION
What Are Rogowski Coils? How Rogowski Coils Work?


Rogowski coil, is an air-core current transformer that is especially well suited to measuring ripple currents in the presence of a DC component or measuring pulsed currents. The raw output is proportional to the derivative of the current, and the current can be recovered by an integrator or a low-pass filter.

The output voltage is given by:

Rogowski Coil

where
n
is the number of turns,
A
is the cross sectional area of the toroid,
and
s
is the centerline circumference.

The coil is wound on an air-core form of suitable size for the current conductor. The winding should be applied in evenly spaced turns in one direction only—not back and forth—so that capacitive effects are minimized.

The far end of the winding should be brought back around the circumference of the coil to eliminate the turn formed by the winding itself. The winding must generally be shielded, since the output voltage is relatively low.

The shield should be applied so that it does not form a shorted turn through the opening, and the coil should be equipped with an integral shielded output lead with the ground side connected to the coil shield.

Output from the Rogowski coil can either be integrated with a passive network as an R/C low-pass filter or with an operational amplifier. The advantage of the R/C network is that no power is required for operation.

The disadvantages are that it cannot be gated and that the output voltage becomes very low if low-frequency response is required. Although a toroidal form is shown in the sketch, Rogowski coils are commercially available that are wound in the form of a very long, flexible solenoid that can be wrapped around a conductor and then secured mechanically.

Rogowski coils are largely unaffected by stray fields that have a constant amplitude across the coil. A field gradient across the coil, however, will introduce a spurious output if the field is time varying. It is good practice to make the coil as small as possible within the electrical and physical constraints of the equipment.

The Rogowski coil can be calibrated from a 50/60-Hz current assuming, of course, that the bandpass of the filter or integrator extends down to those frequencies.