DLA Testing

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Dielectric loss angle tests, also called dissipation factor, power factor or tan delta tests, determine the insulation dielectric power loss by measuring the power angle between an applied AC voltage and the resultant current. In the ideal insulator / dielectric, the power angle would be 90°C as it is purely capacitive and non-conducting. However in real insulators, there is some leakage current and resistive losses through the dielectric.

Relative increases in dielectric power losses are indicative of insulation deterioration and may further accelerate degradation due to increased heating. Note that dielectric power loss does not translate to dielectric strength, though there are often common causes for increases in power loss and decreases in dielectric strength.

Relationship between Power Factor and Dissipation Factor

Tan Delta.png

The cosine of the power angle ([math]\theta[/math]) is called the power factor. The complement of [math]\theta[/math] is called the loss angle and is denoted by [math]\delta[/math] in the diagram above. The power factor can also be approximated by taking the tangent of [math]\delta[/math] (hence the name tan delta). This approximation is called the dissipation factor (or loss tangent) and is roughly equal to the power factor between values of 0 and 0.08 pu, which covers the majority of tests (in insulating oils of good condition, the dissipation factor / power factor is in the order 0.005). Therefore, dissipation factor and power factor can be considered interchangeable.

The exact relationship between dissipation factor (DF) and power factor (PF) is as follows [2]:

[math]DF = \frac{PF}{\sqrt{1 - PF^{2}}} [/math]

The dissipation factor is essentially the ratio between the resistive and capacitive components of the insulation and can be measured directly (e.g. with a capacitance bridge circuit - more on this later). The lower the quality of the insulation condition, the more resistive it will appear and the more power loss will be dissipated through it (in the form of heat), and thus the dissipation factor will be higher.

The increase in the dissipation factor values as the test voltage is increased is called the "tip-up" (dissipation factor or power factor tip-up).

Measurement

Schering Bridge

Schering capacitance bridge circuit

As mentioned earlier, the dissipation factor can be measured directly using a Schering capacitance bridge circuit (named after Harald Schering). The basic circuit is shown in the figure right where [math]C_{x}[/math] is the unknown capacitance under test. Like other bridge circuits (such as the Wheatstone bridge), the circuit is tuned until the current through the middle of the bridge is zero and the circuit is balanced. For a capacitance [math]C_{n}[/math] with low losses, the unknown capacitance and dissipation factor can be calculated as follows:

[math]C_{x} = C_{N} \frac{R_{4}}{R_{3}} [/math]
[math]DF = \omega C_{4} R_{4} \, [/math]

The Schering bridge is typically accurate for measuring dissipation factors down to 0.001 (i.e. good for liquid insulating materials). For other solid insulating materials such as XLPE and polypropylene that have dissipation factors <0.001, the Schering bridge may not be able to provide the accuracy without careful screening and good earthing.

Transformer Ratio-Arm Bridge

TBA

Interpretation of Results

Dissipation factor or power factor is either expressed as a decimal per-unit value (e.g. 0.005) or as a percentage (e.g. 0.5%).

TBA

The technical literature on this subject has noted that this test is useful for detecting moisture ingress in bushings and windings. About 90% of bushing failures may be attributed to moisture ingress evidenced by an increasing power factor from dielectric loss angle testing on a scheduled basis.

References