Application of Tan Delta Test in Power Transformers

Transformer Dissipation Factor Test Guidelines

The transformer dissipation factor test serves as 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)

Determining Test Scope

Specifically, the transformer dissipation factor test determines the capacitance (insulation) between individual windings, as well as between windings and ground.

Technical Test Preparation

To eliminate the effects of winding inductance on insulation measurements, technicians must short-circuit all terminals of each winding, including neutrals.

Tap Changer Considerations

Furthermore, always check for any arrester elements in the tap changer. It is highly recommended to avoid testing at the rated tap, because some tap changers include a tie-in resistor that remains in the circuit at the rated tap. Ultimately, measuring the dissipation factor at the rated tap may artificially result in a higher value.

International References and Standards

Primary reference documents for insulation diagnostic 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.”

Selecting the Correct Test Voltage

For equipment rated above 12 kV, selecting the test voltage remains relatively straightforward. Conversely, for equipment rated 12 kV or lower, engineers should consider testing at 10–25% above the operating line-to-ground voltage.

IEEE Safety Limits

Additionally, 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. Because the lowest low-frequency test voltage in IEEE C57.12.00 is 10 kV (corresponding to a nominal system voltage of 1.2 kV), technicians can safely apply a 5 kV test voltage to a 1.2 kV transformer in accordance with IEEE standards.

The figures and matrices below detail the specific wiring paths for each type of transformer asset.

Two-Winding Transformer Testing

 

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

Table 1: Test Connections for a Two windings transformer

Sequence	Target Capacitance	High Voltage	INPUT A	MEAS GND	Test Mode
1	CHL	HV	LV	Tank GND	UST A
2	CHG	HV	LV	Tank GND	GSTg-A
3	CHL + CHG	HV	LV	Tank GND	GST
4	CLG	LV	HV	Tank GND	GSTg-A
5	CLH	LV	HV	Tank GND	UST-A
6	CLG + CLH	LV	HV	Tank GND	GST

 

• Autotransformer Testing Procedures

  Unlike a two-winding transformer, you cannot separate the windings of an autotransformer. Consequently, the winding combines the high-voltage (HV) and low-voltage (LV) sections into a single asset.

  Overall Test to Ground (CHG)

  For a conventional autotransformer without a tertiary winding, you can only perform an overall test to ground. First, connect all seven bushings (or three bushings for a single-phase unit) together: HV1+HV2+HV3+LV1+LV2+LV3+N (neutral bushing). Then, follow the same core test procedure as described in the “Shunt Reactor” section.

  Autotransformer with Tertiary Winding

  If the autotransformer possesses an accessible tertiary winding, follow the exact test steps described in the “Two-Winding Transformer” section.

Table 2: Test Connections for an Autotransformer with Tertiary Winding

Sequence	Target Capacitance	High Voltage	INPUT A	MEAS GND	Test Mode
1	CHT	HV+LV+N	T	Tank GND	UST-A
2	CHG	HV+LV+N	T	Tank GND	GSTg-A
3	CHG + CHT	HV+LV+N	T	Tank GND	GST
3	CTG	T	HV+LV+0	Tank GND	UST-A
4	CTG + CHT	T	HV+LV+0	Tank GND	GST

• Three-Winding Transformer Testing

  The test sequence for a three-winding transformer acts as a direct extension of the method used for two-winding assets.

 

Figure 3: Connecting CAPTAN 12 to a three-phase and three-winding transformer for measuring

 

Table 3: Test Connections for three-Phase and three-Winding Transformer

Sequence	Target Capacitance	High Voltage	INPUT A	INPUT B	MEAS GND	Test Mode
1	CHL	HV	LV	TV	Tank GND	UST-A
2	CHT	HV	LV	TV	Tank GND	UST-B
3	CHG	HV	LV	TV	Tank GND	GSTg-A+B
4	CHL+CHT+CHG	HV	LV	TV	Tank GND	GST
5	CLT	LV	HV	TV	Tank GND	UST-B
6	CLG	LV	HV	TV	Tank GND	GSTg-A+B
7	CTG	TV	HV	LV	Tank GND	GSTg-A+B

 

 

• Shunt Reactor Diagnosis

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

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

Reactor construction features only one primary internal capacitance, which is CHG. Technicians measure this capacitance using GST mode as shown below.

Table 4: Test Connections for Shunt Reactors

Sequence	Target Capacitance	High Voltage	MEAS GND	Test Mode
1	CHG	HV	Tank GND	GST

 

 

Note: For a single-phase shunt reactor, operators only perform the overall measurement. You complete this by short-circuiting the winding and conducting a standard GST measurement.

Diagnostic Test Results Analysis

When interpreting your asset readings, always compare the final dissipation factor and capacitance values against:

• Factory test sheets,
• Previous maintenance records,
• Data from similar units in the substation.

Evaluating Capacitance Shifts

Capacitance depends strictly on winding geometry. Therefore, it should remain highly stable over time and temperature shifts. A sudden change in capacitance indicates physical winding movement or distortion, which typically happens during through-faults. These electrical faults primarily warp the CLG and CHL capacitances. Ultimately, the target capacitance should not deviate by more than 5% from the original factory baseline.

Evaluating transformer dissipation factor test Deviations

An increased dissipation factor typically points to general contamination issues, such as degraded or dirty oil. Conversely, a simultaneous increase in both the dissipation factor and capacitance strongly signals water contamination inside the tank.

Modern oil-filled power transformers must maintain an insulation power factor of 0.5% or less at 20°C. If values exceed this limit, the manufacturer must provide a clear justification assuring that incomplete drying did not cause the reading. However, older power and distribution transformers may naturally present power factors exceeding 0.5%.

Investigating Abnormal Power Factors

Technicians sometimes encounter abnormal power factor readings. These anomalies typically stem from:

• High-resistance grounding of the main transformer tank, or
• The deployment of a grounded electrostatic shield between the windings.

When a transformer uses grounded electrostatic shielding, the inter-winding capacitance becomes practically negligible. This design configuration leaves only stray capacitances between the bushing leads to be measured.

Understanding Bushing Impact on Overall Measurements

Although bushings are included in the overall CLG and CHG measurements, their individual effect on the total value may appear quite small. This behavior depends on the relative capacitance of the bushing compared to the huge volume of the winding.

Consequently, a highly defective bushing might go completely undetected during an overall test because the winding capacitance masks the fault. Therefore, you must perform separate individual tests on all transformer bushings to ensure precise defect identification. Running a regular transformer dissipation factor test on the main tank elements guarantees proper coverage of your primary insulation assets.