Technology Literature

What is the dielectric loss (Tan Delta) of a transformer and How is it tested?

Tan Delta testing is a crucial method for assessing the condition of a transformer's insulation system. Tan Delta, also known as the loss angle tangent, is the ratio of the active current component to the reactive current component in insulating materials, reflecting the dielectric loss of the insulation. Simply put, the smaller the tan δ value, the lower the dielectric loss of the insulating material, and the better its insulation performance. By measuring the tan δ value, the aging degree and moisture condition of the transformer's insulation materials can be evaluated, allowing for the early detection of potential insulation failures. Below, we will analyze the classifications of tan delta and its measurement principles in detail.

Dielectric loss: Under the action of an electric field, the energy loss inside the insulating material due to the hysteresis effect of dielectric conductance and dielectric polarization. Also called dielectric loss, referred to as dielectric loss. Under the action of the alternating electric field, the complementary angle δ of the included angle (power factor angle Φ) between the current phaser and the voltage phaser flowing in the dielectric is called the dielectric loss angle.Under the action of the alternating electric field, the charge accumulated in the dielectric has two components:

(1) Active power. One is the power consumed for heating, also known as the in-phase component;

(2) Reactive power, also known as out-of-phase component. The ratio of the out-of-phase component to the in-phase component is called the dielectric loss tangent tanδ. tanδ=1/WCR (where W is the angular frequency of the alternating electric field; C is the dielectric capacitance; R is the loss resistance). The dielectric loss tangent is a dimensionless physical quantity.

1. The dielectric loss of the transformer generally refers to the magnetic dielectric loss. It mainly includes two parts: one part is hysteresis loss, and the other part is eddy current loss.

Hysteresis loss

The hysteresis loss is due to the existence of a "hysteresis loop" in the iron core, so that the phase difference between the induced electromotive force and the magnetizing current is not equal to 90 degrees. We know that if it is 90 degrees, the current is "reactive". Now it is not equal to 90 degrees, which is equivalent to connecting an active current component in parallel.

By Steinmetz formula hysteresis loss, Wh =Kh f Bm1.6 w/m3
Kh – hysteresis constant
Eddy Current Loss
It is the result of induced current in the core due to the electromagnetic induction in ferromagnetic material, which flows as loops of electrical current within the conductor.
Eddy Current loss, We = Ke f 2 Bm2 t 2 w/m3
Ke – co-efficient of eddy current
t – Thickness of the sheet

Eddy current loss can be reduced by using an iron core made of thin laminated sheets and material with high electrical resistivity. It reduces the induced EMF and the amount of current flow.

2. Specific explanation:

When the transformer is working normally, the magnetic field generated by the secondary side current and the main part of the primary current are offset. The magnetic field left after cancellation should be roughly equal to the magnetic field at no load (when the secondary current is zero) (assuming resistance and leakage flux are ignored). Therefore, the "magnetizing current" I mentioned above should be equal to the no-load current of the transformer.
Ideally, the no-load current should be 90 degrees behind the voltage, which is "reactive". But with the above losses, this lag is not 90 degrees enough. The complementary angle δ of this lag angle can also represent the size of the loss. The mathematical relationship is very similar to dielectric loss.

When the transformer is no-load, if there is no loss, the input characteristics of the primary side should be equivalent to a pure inductance. With loss, the input current is equivalent to adding a resistor in parallel to the inductor. tanδ should be equal to the ratio of the current on this equivalent resistor to the current on the inductor.

Other transformers  loss types:

1. Iron Losses or Core Loss
Iron loss occurs in the transformer core due to the alternating magnetic flux. It consists of eddy current loss and hysteresis loss.

Iron loss = Hysteresis loss + Eddy Current loss

Referring the below equations of hysteresis and eddy current loss,

Both the Eddy current loss and hysteresis loss depend upon the magnetic properties of the core material, f-frequency of the AC supply and the Bm– maximum flux density.
2. Copper Loss
Copper loss occurs due to the ohmic resistance in both primary and secondary winding.
Calculating the copper loss in the primary and secondary winding,

Total Copper Loss, Pcu = I12R1 + I22R2 ;

The values of Resistance R1 of the primary winding and R2 of the secondary winding are constant.

I1 – primary current, I2 – secondary current

Then it is clear from the above equation that the copper loss varies with the amount of current (square of the current) through the windings.
3. Dielectric Loss

It occurs in the insulating material and oil of the transformer. The transformer oils are subject to change its parameters such as dielectric strength, tan delta, moisture, chemical parameters (dissolved impurities – dissolution of copper), physical parameters, etc…

Additional points
1.Iron loss (Hysteresis loss, eddy current loss) and dielectric loss are no-load losses which are independent of the transformer load.
2.Whereas the copper loss and stray loss varies with the load current.
3.Iron loss is a constant loss and copper loss is a variable loss.
4.Copper loss is determined by the short circuit test and Iron loss is determined by the open circuit test.

Measuring tan Delta is of significant importance for transformers. Firstly, it helps identify the aging and deterioration of the internal insulation. Over time, the insulation materials in a transformer degrade due to electrical stress, thermal stress, and environmental influences, resulting in decreased dielectric performance and increased tan δ values. Regular measurement of the tan δ value allows for maintenance and repair before problems escalate into serious faults, preventing sudden power outages and equipment damage. Secondly, tan Delta measurement can detect moisture conditions in the transformer windings. Moisture in insulation materials significantly increases the tan δ value, potentially leading to insulation breakdowns and short-circuit faults. Through regular measurements, moisture issues can be identified and addressed early, ensuring the safe operation of the transformer. Additionally, tan Delta testing can be used for quality control and acceptance of new equipment, ensuring compliance with technical specifications and performance standards.

In summary, tan Delta measurement is a critical step in transformer maintenance. Regular monitoring of insulation status can effectively extend the transformer's service life, reduce the risk of failures, and enhance the reliability and safety of the power system.

Tan Delta Testing Process:

Measuring the dielectric loss (tan Delta) of a transformer typically requires specialized testing equipment. Here are the detailed steps:


  1. Select Appropriate Testing Equipment:

    • Ensure the use of suitable tan Delta testing instruments, such as a Schering Bridge or dielectric loss tester.
  2. Safety Checks:

    • Ensure the transformer is powered off and grounded, and that all connection points and the work area are safe.
    • Wear appropriate personal protective equipment (PPE).
  3. Environmental Conditions:

    • Verify that the testing environment meets the equipment's operational conditions, such as temperature and humidity.

Testing Steps

  1. Connect the Testing Equipment:

    • Connect the high-voltage lead of the testing instrument to the high-voltage terminal of the transformer, and the low-voltage lead to the ground terminal.
    • Ensure all connections are secure and tight to avoid testing errors or safety incidents.
  2. Calibrate the Equipment:

    • Turn on the testing instrument and follow the device manual to perform initial calibration. Ensure the device is zeroed and perform a self-check.
  3. Apply Voltage:

    • Gradually apply the appropriate test voltage based on the transformer's rated voltage and the testing instrument's requirements. Testing is usually conducted at one-third, half, and full rated voltage.
  4. Read Data:

    • Once the voltage is stable, record the tan Delta and capacitance values displayed by the testing instrument.
    • Repeat the above steps to ensure data consistency at different voltages.
  5. Analyze Data:

    • Compare the test values with the standard values provided by the transformer manufacturer to assess the insulation condition.
    • If the tan Delta value is abnormally high, further analyze the causes, such as aging, moisture, or contamination.


  1. Disconnect Equipment:

    • Turn off the test voltage and disconnect the testing instrument leads.
    • Ensure all equipment is safely powered off and grounded.
  2. Record and Report:

    • Organize and record the test data, generating a test report.
    • Based on the test results, decide whether further inspection or maintenance is required.

Important Considerations

  • Safety First: Throughout the testing process, always prioritize safety to ensure the safety of personnel and equipment.
  • Accuracy: Ensure secure connections and proper equipment calibration to guarantee the accuracy of the test results.
  • Regular Testing: To maintain the health of the transformer, regular tan Delta testing is recommended to detect potential issues early.

By following these steps, you can effectively measure the dielectric loss of a transformer, evaluate its insulation condition, and ensure the safe operation of the transformer.

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Relative Tester---Transformer Oil Tan Delta/Dielectric Dissipation Factor (DDF)/Loss Angle Test Kit (GTD-61A)

1. The GTD-61A Automatic Oil Tan Delta & Oil dielectric loss tester is highly automated, allowing for the measurement of temperature rise, dielectric loss, and resistance in one go.

2. The test kit adopts medium-frequency induction heating and a PID temperature control algorithm. This heating method has several advantages, including non-contact heating between the oil cup and the heating body, uniform heating, high speed, and convenient control. As a result, the temperature is strictly controlled within the preset temperature error range.

3. The GTD-61A test kit uses advanced DSP and FFT technology to ensure data stability, accuracy, and reliability.

4. The internal standard capacitor is an SF6 inflatable three-point capacitor. The dielectric loss of this capacitor is not affected by ambient temperature and humidity, so the accuracy of the instrument is still guaranteed after long-term use.

5. The large color touch screen and English operation menu make the GTD-61A test kit easy to operate.

6. The GTD-61A test kit provides tips on opening the cover after shutting off the high voltage, short-circuiting the high and low voltage electrodes of the oil cup to eliminate any hidden safety hazards, and ensuring the safety of the operator and normal operation of the equipment.

7. The test kit has a real-time clock, allowing the test date and time to be saved, displayed, and printed with the test results. The device can also display real-time monitoring of the environment.

8. The GTD-61A can automatically store and store up to 100 sets of test data.

9. The test kit has the function of calibrating the empty electrode cup. The capacitance and dielectric loss factor of the empty electrode cup are measured to judge the conditions of the empty electrode cup. The calibration data is automatically saved to facilitate accurate calculation of relative permittivity and DC resistivity.

10. The GTD-61A has a wireless transmission function, making it easy to connect with a computer for data transmission and counter charge functions.

11. The oil can be drained off automatically without being taken out, making it convenient to clean.

The test items are required for 110kV/220kV substation acceptance and maintenance testing(with corresponding tester types):

Test Object

Testing Item

Tester Type


Cable AC Hipot Testing



Cable Insulation Resistance Testing




Insulation Resistance of Winding



DC Leakage Testing in 1min



Winding Resistance Testing



Winding Deformation Testing



Turns Ratio Testing



AC Hipot Testing



Short Circuit Impedance Testing



Dan Delta Testing



Oil Breakdown Voltage (BDV)Testing



Oil Tan Delta Testing




Circuit Breaker/Disconnector Resistance




Relay Performance




On-load Switchgear Testing



Tap Changer Dynamic Testing



Instrument   Transformer

Current Transformer (CT)Testing



Potential Transformer (PT)Testing



Surge Arrestor

Zinc Oxide Characteristics Testing



1mA DC Reference Voltage




Capacitor Current



Capacitor Inductance



Ground Grid

Grounding Down Lead Earth



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What Test Items are Required for 110kV/220kV Substation Acceptance and Maintenance Testing?

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