A power capacitor is an actual capacitor, not an ideal capacitor. Under the action of an external AC voltage, in addition to outputting a certain capacity of reactive power Q, in the internal medium of the capacitor, in the plate (aluminum foil) of the capacitor, A certain amount of active power loss P will be generated in conductors such as lead wires and leakage current between porcelain bottles.
Usually, the ratio of the active power P of a capacitor to the reactive power Q is called the loss tangent of the capacitor, and it is expressed by the following formula:
In the formula: tanδ—the loss tangent of the capacitor (%);
P—the active power of the capacitor (W);
Q—the reactive power of the capacitor (var).
Dielectric loss angle (dielectric loss angle) is an important index reflecting the insulation performance of high-voltage electrical equipment. The change of dielectric loss angle can reflect insulation defects such as damp, deterioration or gas discharge in the insulation. Therefore, measuring the dielectric loss angle is an important part of studying the characteristics of insulation aging and on-line monitoring of insulation conditions.
The insulating structure of capacitive equipment is usually composed of a variety of insulating materials. The internal insulation medium of electrical equipment is divided into three categories, among which liquid medium includes transformer oil, capacitor oil, etc.; solid medium includes mica, electric porcelain, glass, silicone rubber, etc.; gas materials include sulfur hexafluoride, etc. Under the action of an electric field, physical phenomena such as polarization, conductance, and loss will occur in insulating media. Dielectric loss is the energy loss caused by the insulation equipment under the action of the electric field. The consumed energy is converted into heat energy, causing the temperature of the insulation material to rise. When the temperature is too high, it will cause the insulation material to age and deteriorate. Insulation dielectric loss is mainly divided into three types: polarization loss, ion loss and conductance loss.
Tanδ is called the tangent of the dielectric loss angle. It is the ratio of the active component to the reactive component in the dielectric under AC voltage. It is a dimensionless number that reflects the energy loss per unit volume in the dielectric.
The dielectric loss is proportional to the applied voltage, power frequency, dielectric capacitance C and dielectric loss factor tanδ. However, it is inconvenient to use the dielectric loss P to indicate the quality of the medium, because the P value is related to factors such as the test voltage and the medium size (shape, size, thickness, etc.), and it is difficult to compare different equipment, so it cannot be accurately determined. Reflects the insulation condition of the dielectric. When the applied voltage and frequency are constant, the dielectric loss is only related to the equivalent capacitance and dielectric loss factor of the medium. For a certain structure and formed dielectric, the equivalent capacitance is a fixed value, so tanδ fully reflects the dielectric loss situation, which can be Used to evaluate the insulation level of high-voltage power equipment, it is a physical quantity that only depends on the characteristics of the material and has nothing to do with the size of the material. Therefore, in engineering, the value of the tangent tanδ of the dielectric loss angle is used to judge the quality of the medium and characterize the loss of the dielectric.
Significance of detection
By measuring the tangent value of the dielectric loss angle, a series of insulation defects can be reflected, such as insulation damp, poor quality change or tile gap discharge. Measuring the tanδ value of the dielectric is convenient for quantitative analysis of the loss characteristics of insulating materials, which is beneficial to the analysis, research and structural design of insulating materials. The tanδ of the material is large, indicating that the dielectric has a large loss during operation, and is prone to heat and aging. For example, the adhesive paper capacitive bushing has high mechanical strength, and the lower dimension can be made very short, but often due to the unsatisfactory quality of the glue, the tanδ is relatively large, so it is difficult to use in the ultra-high voltage system. Castor oil can be used in DC or pulse capacitors, but it cannot be used in AC capacitors because of its large tanδ. The connecting cable used for shock measurement requires that its tanδ must be very small, otherwise, when the shock wave propagates in the cable, the waveform will be severely distorted and affect the measurement accuracy.
But for concentrated defects, if the volume they occupy is small, then the dielectric loss at the concentrated defect will account for a small proportion of the total dielectric loss of the insulation, and the effect of the tanδ method will be poor at this time. Therefore, it is essential to test the dielectric loss factor of the bushing or transformer, but it is not necessary for equipment such as motor cables. When judging the insulation condition by the tanδ value, it must be compared with the tanδ value of the equipment over the years and compared with other equipment under the same operating conditions. Even if tanδ does not exceed the standard value, when tanδ suddenly increases significantly compared with the past and other equipment under the same operating conditions, it must be dealt with.
In the insulation design, attention must be paid to the tanδ value of the insulating material. If the tanδ value is too large, it will cause severe heating, accelerate the aging of the insulation, and may even cause thermal breakdown. And under DC voltage, tanδ is small and can be used to make DC or pulse capacitors.
The value reflects the condition of the insulation, and the process of transformation from a good state to a deteriorated state can be judged by measuring the relationship curve of tanδ=f(ф), so the measurement of tanδ is a basic item in the insulation test of electrical equipment.
By studying the influence of temperature on the value of tanδ, the value of tanδ at the working temperature should be the minimum value and the maximum value should be avoided.
The polarization loss increases with the increase of frequency, especially when the capacitor adopts polar dielectric, the polarization loss increases rapidly with the increase of frequency. When high-order (such as 3rd and 5th) harmonics appear in the power supply, the It is easy to cause breakdown of capacitor insulating material due to overheating.
The tanδ of the insulation of the connecting cable used for impact measurement must be very small, otherwise the measured impulse voltage will undergo serious waveform distortion after passing through the cable, which will affect the accuracy of the measurement.