TRANSFORMER OILS TESTING OF NEW OIL PROPERTIES BASICS AND TUTORIALS

TRANSFORMER OILS TESTING OF NEW OIL PROPERTIES BASIC INFORMATION

Testing Of Transformer Oil As Recommended In IEEE Std C57.106-2002

When mineral insulating oil specified to conform to ASTM D3487-00 is received, it should be tested to verify conformance with ASTM D3487-00. Testing of the oil for full conformance of all property requirements of ASTM D3487-00 is only justified under circumstances determined by the purchaser.

As a minimum, it is recommended that the purchaser require the supplier to provide a certified set of tests for the oil that demonstrate that the oil, as shipped, met or exceeded the property requirements of ASTM D3487-00.

For those circumstances where a full set of tests according to ASTM D3487-00 are not justified, it is recommended that, at a minimum, the tests shown in Table 1 of this guide be considered. The purchaser of the oil should conduct tests sufficient to satisfy concerns regarding conditions of shipment that might result in non conformance to ASTM D3487-00 property requirements.  

Table 1 lists several of the more important tests with values that should help in the decision regarding acceptance of the new mineral insulating oil.

Insulating oil is ordinarily shipped in three types of containers: drums or totes, tank trailers, and rail cars. Rail cars are usually under the control of the supplier and dedicated to insulating oil shipment, so they tend to be the cleanest.

Highway trailers are used to transport many different chemical products as well as insulating oil; these trailers are therefore subject to chemical contamination. Special cleaning and drying procedures may be necessary.

If problems are encountered, check the history of the shipping containers to see that they have been cared for properly. It is recommended that the purchaser require the delivery of oil in containers that are properly cleaned to guarantee delivery of oil conforming to ASTM D3487-00.

Drums and totes are the least desirable method of insulating oil transport but may be necessary for small purchases. Drums and totes should be stored under cover to prevent contamination by moisture.

Before processing, it is necessary to check the quality of the oil in each drum or tote or after blending the oil in a large tank. Each tank load or each shipping unit of oil as received at the customer’s site should undergo a check test to determine that the electrical characteristics have not been impaired during transit or storage.

Table 1 contains a list of recommended acceptance tests for shipments of mineral insulating oil as received from the supplier. Some users may not wish to perform all these tests; however, as a minimum, dielectric strength and dissipation factor (power factor) as listed in Table 1 should be performed.

It is satisfactory to accept oils that exhibit characteristics other than those described by the values in Table 1, providing that the users and the suppliers are in agreement.

INRUSH CURRENT CONSIDERATION FOR TRANSFORMERS BASIC INFORMATION

POWER TRANSFORMER INRUSH CURRENT CONSIDERATION

What Are The Inrush Current Consideration For Power Transformers?

Two distinctly different definitions for inrush current have been offered because one definition cannot serve all the purposes where inrush current is of interest.

Inrush Current:

is the maximum root-mean-square or average current value, determined for a specified interval, resulting from the excitation of the transformer with no connected load, and with essentially zero source impedance, and using the minimum primary turns tap available and its rated voltage.

Peak Inrush Current:

is the peak instantaneous current value resulting from the excitation of the transformer with no connected load, and with essentially zero source impedance, and using the minimum turns primary tap and rated voltage.

Magnetic and thermal cut-out devices usually are not responsive to one-half cycle of energy regardless of magnitude, hence two or more half cycles are needed to define the trip-out characteristics. Furthermore, these devices are not responsive to peak values, but rather to energy content. (I2 t) becomes the parameter of interest, using root-meansquare current values for fusing characteristics.

Relays and magnetic cut-outs are responsive to the average current value. Therefore, when inrush current is cited it should be made clear which of the two values (root mean square) (average) is indicated.

It should be noted that the inrush current of a transformer is seldom the same value as the steady-state exciting current, but is typically larger and decays to steady state after several cycles, depending on the condition of the core, the instantaneous value of applied voltage, etc.

It is important to consider this asymmetry of inrush current in the design and use of transformers and particularly in the specification of protective devices for the transformer. Maximum inrush current values occur when a transformer core that has an existing maximum residual flux is switched on at zero instantaneous voltage so the residual flux and the instantaneous magnetizing flux are additive.

Circuits are available using silicon controlled rectifier switching to cause this to happen deliberately. Alternately, random switch on twenty or more times will usually produce a near maximum value for a single-phase transformer.

It may take more times for a three-phase transformer unless all three lines are monitored. For the measurement of root-mean-square or average current it is necessary to use an adequate X-axis spread or chart speed so that curve area per cycle can be measured.

Peak inrush current values are of interest in connection with contact welding problems and with devices sensitive to instantaneous current magnitude. The measurement of true inrush current with any degree of accuracy can be very difficult because of the usual nonavailability of zero source impedance power lines for larger systems.

This problem can best be circumvented when the installed source capacity is known and specified in terms of impedance and phase angle, and rated capacity.

These values can then be used in test or computation to determine the installed inrush characteristics of a system which, of course, is the final value of interest. When inrush current values are presented for conditions other than essentially zero source impedance, the actual source impedance values applicable to the data should also be given.