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

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, W_h =K_h f B_m^1.6 w/m³
K_h – 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, W_e = K_e f ² B_m² t ² w/m³
K_e – 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 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, P_cu = I₁²R1 + I₂²R_{2 ;}

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.

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	Cable AC Hipot Testing	JYCX	Details
	Cable Insulation Resistance Testing	JYM KYORITSU	Details
Transformer	Insulation Resistance of Winding	JYM KYORITSU	Details
	DC Leakage Testing in 1min	JYDHV	Details
	Winding Resistance Testing	JYR50S	Details
	Winding Deformation Testing	JYP	Details
	Turns Ratio Testing	JYT-A	Details
	AC Hipot Testing	JYCX	Details
	Short Circuit Impedance Testing	JYW6300	Details
	Dan Delta Testing	JYC	Details
	Oil Breakdown Voltage (BDV)Testing	JY6611	Details
	Oil Tan Delta Testing	GTD-61A	Details
Switchgear	Circuit Breaker/Disconnector Resistance	JYL-200B	Details
Relay	Relay Performance	JY7003G	Details
On-load	On-load Switchgear Testing	JYK-I	Details
	Tap Changer Dynamic Testing	JYK-I	Details
Instrument   Transformer	Current Transformer (CT)Testing	JYH-C	Details
	Potential Transformer (PT)Testing	JYH-C	Details
Surge Arrestor	Zinc Oxide Characteristics Testing	JY6800	Details
	1mA DC Reference Voltage	JYDHV	Details
Capacitor	Capacitor Current	JY6700	Details
	Capacitor Inductance	JY6700	Details
Ground Grid	Grounding Down Lead Earth	JYD	Details

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

What Tests Must be Done before the Transformer Leaves the Factory?

Main Factors Affecting the Operating Life of Transformers

What is the Purpose of Measuring the DC Resistance of the Transformer Winding?

Points to note in transformer oil breakdown voltage (BDV) test

DC winding resistance tester with 500 times tests on a single charge -JYR9310