گزیده ای از یکی از کتب مرجع در ارتباط با تست تلفات عایقی یا tanδ در زیر آمده است.
The PF (Power Factor) insulation tests were used in the laboratory since the early 1900s by cable manufactures, and in the field for testing bushing since 1929. The DF (Dissipation Factor or tan δ) is based on the Schering bridge which was developed also in the early 1900s to evaluate insulation by separating the capacitive and real components of the charging current. Today, the PF and DF tests are considered to be synonymous because they both refer to the AC dielectric loss test. PF and DF are but two of the several measurable characteristics that can be obtained from an AC dielectric loss test used for evaluating the condition of the insulation system. Both of these tests are effective in locating weaknesses in the electrical insulation and hazard in the power apparatus before impending failure.
PF and DF tests are not go-no-go tests, and can measure dielectric loss, capacitance, and AC resistance of the insulation of the electrical apparatus. These tests can measure the presence of bad insulation even when there may be a layer of good insulation in series with the bad insulation. These tests provide information on the overall condition of the insulation in terms of a ratio (i.e., PF or DF value of insulation) that is independent of the volume of the insulation being tested. Moreover, they provide assessment of the insulation under normal frequency (60 Hz) operating conditions which is not time dependent like the direct current (DC) voltage tests. The PF and DF tests do not overstress the insulation and can determine if the insulation is slowly degrading by comparison with previous year’s test results, or with test results of similar equipment.
The PF/DF tests measure insulation capacitance, AC dielectric losses, and the ratio of the measured quantities. When insulation is energized with an AC voltage, the insulation draws a charging current. This charging current comprises of two components called capacitive current and resistive current. The capacitive current leads the applied test voltage by 90°, whereas the resistive current is in phase with the voltage as shown in Figure above. The capacitive current is directly proportional to the dielectric constant, area, and voltage and inversely proportional to the thickness of the insulation under test. The capacitive current may calculated by the following formula:
Changes in the capacitive current indicate degradation in the insulation, such as wetness or shorted layers, or change in the geometry of the insulation.
The resistive current supplies the energy lost due to dielectric losses such as carbon tracking, volumetric leakage, surface conduction, and corona.
Dielectric losses due to water contamination or carbon tracking or other forms of deterioration increase by the square of the voltage, where as dielectric losses due to corona increase exponentially as the voltage increases. PF/DF testing is sensitive enough to detect a deteriorated moisture problem in the insulation compared to an insulation resistance test.
It should be noted that in the electrical industry, especially in the utility transmission and distribution (T&D) arena, the PF test may be referred to as Doble test* because of the use of the Doble test sets when performing PF tests. The terms PF test and Doble test are one and the same.
There are several manufacturers of the PF/DF test equipment. However, there are two main suppliers of PF/DF instruments in the United States, and they are Megger Incorporated, Valley Forge, Pennsylvania and Doble Engineering, Watertown, Massachusetts.
Here briefly describes the Megger DF test equipment, its theory, and its operation for performing PF tests. The description of theory and operation for all types listed are essentially the same. A transformer ratio-arm bridge circuit is used as shown in the simplified diagram of Figure below.
The circuit consists of a standard reference capacitor (CS) and the insulation under test (CX). A special multiwinding transformer is the characteristic feature of the circuit. A voltage is applied to both CS and CX. The ratio arms, NS and NX are adjusted to balance capacitive current, and the variable resistor (RS) is adjusted to balance resistive current. The null indicator is used to determine when the bridge circuit is balanced. The values of NS and NX are used to determine capacitance and the value of RS correlates to power ( dissipation) factor of the test insulation. Model CB-100 is a low voltage (LV) bridge which is manually balanced for both capacitance and DF, but it is direct reading. The semiautomatic models require manual balancing for capacitance, but they provide a direct digital readout of DF. Model Delta-3000 is automatic balancing for both capacitance and PF, and can also display DF directly (below Figure). Some of the major features of the Delta-3000 test set are listed below:
Completely self-contained test set including 0 to 12 kV power supply, standard capacitor, instrumentation, test leads, and printer
It is simple to operate, providing automatic balancing and digital display for voltage, current, dielectric loss (in watts), capacitance, and PF.
Readings are adjusted to equivalent values of 10 or 2.5 kV
Readings may be recorded on a thermal printer for hardcopy, and/or on a removable data key for download at a later time to a standard PC
It achieves high accuracy under severe electrostatic and electromagnetic interference conditions, such as encountered in HV substations
Safety features include two hand-operated interlock switches, open ground detection circuitry and zero voltage initiation of tests
Built-in diagnostic and calibration self-check
The formulas used for calculating the PF or DF are illustrated by using an example (below Figure) with an insulation of PF = 1.0%. The value of PF is given by the cosine of the q angle, or the equation can be written as