Application of Tan Delta Test in Power Transformers

The dissipation factor test is a comprehensive method for detecting moisture, carbonization, and other forms of contamination in the windings, bushings, and liquid insulation of:

• Distribution transformers (rated ≤ 500 kVA),
• Power transformers (rated > 500 kVA).

Test Scope:

The dissipation factor test determines the capacitance (insulation) between individual windings and windings and ground.

Test Preparation:

• To eliminate the effects of winding inductance on insulation measurements, short-circuit all terminals of each winding, including neutrals.
• Check for any arrester elements in the tap changer. It is recommended to avoid testing at the rated tap, as some tap changers include a tie-in resistor that remains in the circuit at the rated tap. Measuring the dissipation factor at the rated tap may result in a higher value

References and standards for dissipation factor tests include:

• IEC 60076-1 (2000), Clause 10.1.3: “Measurement of the dissipation factor of the insulation.”
• IEEE Std C57.12.90-1999, Clause 10.10: “Insulation power-factor tests.”

Test Voltage:

• For equipment rated above 12 kV, selecting the test voltage is straightforward.
• For equipment rated 12 kV or lower, consider testing at 10–25% above the operating line-to-ground voltage.
• IEEE C57.12.90 recommends that the test voltage for insulation dissipation factor tests should not exceed half the low-frequency test voltage specified in IEEE C57.12.00.
• The lowest low-frequency test voltage in IEEE C57.12.00 is 10 kV, corresponding to a nominal system voltage of 1.2 kV. Therefore, a 5 kV test voltage can be applied to a 1.2 kV transformer, in accordance with IEEE standards.

The wiring for each type of transformer is shown in pictures below.

• Two-Winding Transformer

Figure 1: Connecting CAPTAN 12 to a two-winding transformer for measuring

Table 1: Test Connections for a Two windings transformer

• Autotransformer

Unlike a two-winding transformer, the windings of an autotransformer cannot be separated. The winding is a combination of the high voltage  (HV) and low-voltage (LV) windings.

Testing Procedure:

Overall Test to Ground (CHG):

• For a conventional autotransformer without a tertiary winding, only an overall test to ground can be performed.
• Connect all seven bushings (or three bushings for a single-phase unit) together: HV1 + HV2 + HV3 + LV1 + LV2 + LV3 + N (neutral bushing). Follow the same test procedure as described in the “Reactor” section.

Autotransformer with Tertiary Winding:

• If the autotransformer has an accessible tertiary winding, follow the same test procedure as described in the “Two-Winding Transformer” section.

Table 1: Test Connections for an Autotransformer with Tertiary Winding

• Three-winding Transformer

The test procedure for a three-winding transformer is an extension of the method used for two-winding transformers.

Key Considerations:

In some three-winding transformers, one of the interwinding capacitances may be negligible. This can occur due to:

• A grounded electrostatic shield between two windings, or
• A concentric-winding arrangement that places one winding between two others.

The grounded shield or concentric arrangement effectively eliminates the interwinding capacitance, leaving only stray capacitances between bushing leads.

Figure 1: Connecting CAPTAN 12 to a three-phase and three-winding transformer for measuring

Table 1: Test Connections for three-Phase and three-Winding Transformer

• Shunt Reactor

Oil-filled shunt reactors are used in high voltage  (HV) systems to limit over-voltages caused by long transmission lines. They compensate for capacitive generation on power lines, preventing uncontrolled voltage rises, especially on lightly loaded lines.

There is only one capacitance in reactor construction which is . This capacitance is measued in GST mode as shown in picture below.

Figure 1: Connecting CAPTAN 12 to a three-phase shunt reactor for measuring

Table 1: Test Connections for Shunt Reactors

Note: For a single-phase shunt reactor, only the overall measurement is performed. This is done by short-circuiting the winding and conducting a GST measurement (refer to row #1 in the table above).

• Test Results Analysis

When interpreting test results, compare the dissipation factor and capacitances with:

• Factory data,
• Previous test results,
• Results from similar units, if available.

Capacitance:

• Capacitance is a function of winding geometry and is expected to remain stable over time and temperature changes.
• A change in capacitance may indicate winding movement or distortion, often caused by through-faults. Such faults primarily affect the CLG and CHL capacitances.
• The target capacitance value should not change by more than 5% from the factory or comissioning test results.

Dissipation Factor:

• Increased dissipation factor values typically indicate general issues such as contaminated oil.
• A simultaneous increase in both dissipation factor and capacitance suggests water contamination.
• Modern oil-filled power transformers should have insulation power factors of 0.5% or less at 20°C.
• Higher values should be justified by the manufacturer, with assurance that they are not due to incomplete drying.
• Older power and distribution transformers may have power factors exceeding 0.5%.

Abnormal Power Factor

These anomalies may result from:

• Improper (high-resistance) grounding of the transformer tank, or
• The use of grounded electrostatic shielding between windings.

In the case of grounded electrostatic shielding, the inter-winding capacitance becomes practically negligible, leaving only stray capacitances between bushing leads.

Bushing Impact on Measurements:

Although bushings are included in the CLG and CHG measurements, their effect on the overall value may be small, depending on the relative capacitance of the bushing compared to the total CLG or CHG. A defective bushing might go undetected in an overall test due to the masking effect of the winding capacitance. So, separate tests must be performed on all transformer bushings to ensure accurate detection of defects.