Cause analysis of dielectric loss exceeding standa

Abstract: This paper introduces the phenomenon that the dielectric loss of LB-220 current transformer rises after it is put into operation and the change of dielectric loss with voltage, and analyzes the causes of the dielectric loss exceeding the standard under the same vacuum degree

key words: the dielectric loss of the current transformer exceeds the standard. The dielectric loss value of the main insulation of the oil paper capacitive current transformer can sensitively reflect the state of the main insulation, which is an important basis for the safe operation of the transformer. In view of the importance of the dielectric loss value of the main insulation of the capacitive current transformer, the 96 edition "preventive test code for power equipment" has increased the allowable dielectric loss value of the main insulation of the oil paper capacitive current transformer in operation from the original 1.5% to 0.8%, which greatly reduces the fault rate of the transformer in operation and puts forward higher quality requirements for the manufacturer. This paper analyzes the reason why the dielectric loss of the main insulation of the capacitive current transformer exceeds the standard combined with typical examples and the most used materials in FDM printing

1 when draining oil, open the oil nozzle at the bottom of the left side of the dynamometer to analyze the test data

a substation 2321 line current transformer (model LB-220, delivered in August 1989 by Xi'an High Voltage Switchgear Factory), which has been in operation for less than 3 years, a and C phases TG δ The value seriously exceeds the standard (Table 1). In order to analyze the reasons for the rise of dielectric loss of this group of current transformers, the dielectric loss TG was carried out in the test hall δ The relationship curve between value and voltage is measured, and the test data is shown in Table 2. Dielectric loss TG according to table 2 δ Based on the test data of the relationship between value and voltage, the dielectric loss Tg of the above three current transformers is plotted δ The characteristic curve of the relationship between value and voltage is shown in Figure 1. 2 cause analysis of dielectric loss exceeding standard

the primary conducting rod of LB-220 current transformer is bent into a "U" shape structure by flat copper wire or aluminum tube. The main insulation is made of multi-layer cable paper and thin aluminum foil or semiconductor paper, each layer of which is alternately made into capacitive insulation, all of which are wound on the "U" shape primary conducting rod, and the winding is tight. In order to make the inner insulation easy to dry, holes with uniform spacing are punched on the aluminum foil. See Figure 2 for the insulation equivalent diagram of capacitor screen. Total capacitance of primary to terminal screen CX = C1 × C2/(C1 + C2) total dielectric loss TG δ X＝（C1tg δ 2＋C2tg δ 1）／（C1＋C2）。

if a new way is opened for the effective development of polylactic acid, the holes on the aluminum foil are blocked by foreign matters or overlapped with each other in the winding process, when the entire capacitive current transformer is dry, the moisture of the outer insulation is easy to be eliminated, while the moisture of the inner insulation is not easy to be eliminated, resulting in incomplete insulation drying

for the samples with series structure, the total dielectric loss mainly reflects the external insulation condition of the smaller part of the capacitance, that is, TG δ X≈tg δ 2. With the extension of operation time, the moisture of the inner insulation gradually spreads to the outer layer, C2, TG δ 2 gradually increase, so that the overall dielectric loss value of insulation increases significantly

it can be seen from table 2 that three LB-220 current transformers pair the end screen TG at one time δ＝ The f (U) test data showed an obvious downward trend, and the maximum decrease was 42.8%. When water and impurities exist in the primary main screen of capacitive current transformer due to incomplete insulation drying, these ionic impurities will produce ionic conductivity loss under the action of alternating voltage. When the voltage increases, the movement speed of ions accelerates, and in the half wave time of alternating current, the operation time of ions in a fixed space distance decreases, and the active power loss decreases accordingly, making TG δ It shows a downward trend with the increase of voltage.

3 conclusion

the insulation resistance and insulating oil tests of the three LB-220 current transformers are all qualified, ruling out the possibility of water ingress and moisture of the three current transformers. If the dielectric loss value in the pre-test exceeds the standard and the dielectric loss value decreases under high voltage, it indicates that the three current transformers have congenital deficiencies, that is, the deep insulation is not completely dry in the process

about the author: Ying Zhiqin (1950 -), female, engineer, engaged in high voltage technology