For related oil-tester equipment, Dielectric loss refers to the energy loss within an insulating material due to the delayed effects of dielectric conduction and polarization under the influence of an electric field. It is also known as dielectric loss or simply dielectric loss, abbreviated as tan δ.
2、Dielectric Loss Angle (δ)
Under the action of an alternating electric field, the angle between the phase quantity of the current flowing through the dielectric and the phase quantity of the voltage (power factor angle Φ) is the complement of the loss angle (δ), commonly known as the dielectric loss angle.
3、Dielectric Loss Tangent (tan δ)
Also known as the dielectric loss factor, it represents the tangent of the dielectric loss angle, abbreviated as tan δ. The definition of the dielectric loss tangent is as follows:
If the phase quantities of the current and voltage of the test specimen are denoted as i and u, respectively, the following vector diagram can be obtained:
The total current can be decomposed into capacitive current (Ic) and resistive current (IR), thus:
This is precisely the tangent of the loss angle δ = (90° – Φ). Modern digital instruments fundamentally measure tan δ or Φ to obtain the dielectric loss factor.
Measuring dielectric loss is a traditional and highly effective method for assessing the insulation condition of electrical equipment. A decrease in insulation capability is directly reflected in an increase in dielectric loss. Further analysis can reveal the reasons for insulation degradation, such as moisture ingress, contamination of insulating oil, aging, and deterioration.
Simultaneously measuring dielectric loss provides information on the capacitance of the test specimen. If one or more capacitors in a set exhibit a short circuit or open circuit, there will be a noticeable change in capacitance. Therefore, capacitance is also an important parameter.
4、Power Factor (cos Φ)
The power factor is the cosine of the power factor angle (Φ) and signifies the proportion of active power (P) in the total apparent power (S) of the test specimen. The definition of power factor is as follows:
Some dielectric loss testers commonly display power factor (PF: cos Φ) instead of dielectric loss factor (DF: tan δ). In general, cos Φ < tan δ, and these values are very close when the losses are minimal.
(1) Capacitance and Tolerance: The allowable maximum deviation range of actual capacitance from nominal capacitance. Common capacitance tolerances include J class ±5%, K class ±10%, and M class ±20%. Precision capacitors have smaller allowable tolerances, while electrolytic capacitors have larger tolerances, with different tolerance grades represented by letters: D class—±0.5%, F class—±1%, G class—±2%, J class—±5%, K class—±10%, M class—±20%.
(2) Rated Voltage: The maximum DC voltage that a capacitor can withstand for stable and reliable operation in a circuit, also known as the voltage withstand capability. For devices with the same structure, dielectric, and capacitance, higher voltage ratings result in larger physical sizes.
(3) Temperature Coefficient: The relative change in capacitance for a one-degree Celsius change in temperature within a certain temperature range. A smaller temperature coefficient is preferable.
(4) Insulation Resistance: Indicates the magnitude of leakage current. For small-capacity capacitors, insulation resistance is usually in the hundreds of megaohms or even thousands of megaohms. Electrolytic capacitors typically have lower insulation resistance.
(5) Losses: The energy consumed by a capacitor as heat under the influence of an electric field within a unit time. These losses mainly come from dielectric and metallic losses, often represented by the tangent of the loss angle.
(6) Frequency Characteristics: The properties of a capacitor’s electrical parameters as they vary with the frequency of the electric field. Capacitors operating at high frequencies may experience a decrease in capacitance due to a lower dielectric constant at higher frequencies, resulting in increased losses. Additionally, various distributed parameters, such as electrode resistance, resistance between electrodes, self-inductance of electrodes, and lead inductance, can affect capacitor performance at high frequencies, limiting their usage frequency.
5、Standard Channels of High Voltage Capacitance Bridge:
Standard channel inputs standard capacitor current, and the test channel inputs test specimen current. Measurement of tan δ is performed by comparing current phase differences, while measurement of the test specimen’s capacitance is done by comparing current amplitudes. Therefore, using a capacitance bridge for dielectric loss measurement requires carrying standard capacitors, step-up transformers, and voltage regulators, and the wiring is complex.
6、High Voltage Dielectric Loss Measurement Instrument (Dielectric Loss Analyzer):
Also known as a dielectric loss analyzer, it adopts the principle of a capacitance bridge and utilizes digital measurement technology to automatically measure the tangent of the dielectric loss angle and capacitance. It typically comprises a high-voltage bridge, high-voltage test power supply, and high-voltage standard capacitor. PS-JSB1, using frequency anti-interference principles and Fourier transform digital waveform analysis technology, calculates standard current and test specimen current, exhibiting strong interference suppression capability and delivering accurate and stable measurement results.