Abstract: This paper analyzes the characteristics and losses of circuit breaker capacitors, explains the common phenomenon that the loss tangent tanδ of circuit breaker capacitors tends to be large under low electric field strength and that tanδ increases with time, and proposes solutions. Keywords: Circuit breaker capacitor, loss tangent tanδ, impurity ion loss 1 Introduction Circuit breaker capacitors (also known as voltage equalization capacitors) are mainly connected in parallel to the circuit breaker contacts to make the voltage between the contacts uniform. The arc-extinguishing chamber of a high-voltage circuit breaker is often composed of several contacts connected in series. Since the stray capacitance to ground C0 is often larger than the contact capacitances C1 and C2, contact C1 bears most of the voltage, resulting in a highly uneven voltage distribution. However, after connecting an equal-capacitance voltage equalization capacitor Cs in parallel to each contact, since Cs >> C0 >> C1 and C2, the voltage division ratio is greatly improved, and n is approximately equal to 1 (about 1.02 to 1.05), making the voltage distribution between the contacts uniform to facilitate breaking and avoiding damage caused by uneven voltage distribution after the contacts are broken. The basic structure of circuit breaker capacitors is similar to that of coupling capacitors, typically using a porcelain casing, with all components connected in series, and an internal metal bellows to compensate for the volume change of the impregnating agent as the operating temperature changes. Its materials, manufacturing process, and quality are basically the same as other capacitors. However, circuit breaker capacitors often exhibit a large loss tangent (tanδ), a phenomenon not prominent in other types of capacitors. Therefore, this paper discusses the reasons for the large tanδ value in circuit breaker capacitors and some improvement methods. 2. Generation of Circuit Breaker Capacitor Losses and Reasons for the Prone Excessive Tanδ Value Impurities in the capacitor impregnating agent, especially ionic impurities, affect the magnitude of tanδ. The energy loss of the dielectric under voltage is caused by polarization losses (such as dipole polarization, interlayer dielectric interface polarization, etc.) and conductivity losses. Since the impregnating agents used in capacitors are currently all weakly polar liquid dielectrics (relative permittivity of 2.2–2.6), the polarization loss is extremely small. Therefore, the loss of the impregnating agent is mainly determined by conductivity losses. In a normally functioning liquid medium, there are two main factors contributing to conductivity: one is the dissociation of molecules from the liquid itself and impurities into ions, constituting ionic conductivity; the other is the formation of charged particles (such as small water droplets suspended in the impregnating agent) after adsorbing charges, constituting electrophoretic conductivity. Since capacitors employ a vacuum impregnation process, with simultaneous vacuuming and oil injection, degassing and dehydration are ensured. Therefore, we focus on analyzing the impact of ionic conductivity on the losses of circuit breaker capacitors. The capacitor element's plates are separated into several regions by layers of solid dielectric (film or capacitor paper). Ions from the impregnating agent fill these regions. When there is no external electric field, positive and negative ions are randomly distributed and do not consume energy. When a voltage is applied, along the direction of the electric field, ions in the oil move back and forth in a narrow space, alternating with the direction of the electric field, resulting in losses. Under power frequency, when a lower voltage UA is applied, within one cycle, ions depart from plate E but before reaching the solid dielectric D, return to plate E due to the alternating electric field. When the voltage is increased to UB, ions travel from plate E to the solid medium D and back to E within one cycle. At this point, the loss increases with the voltage. As the voltage continues to rise, charged particles cannot penetrate the dielectric layer and are blocked at E or D, only able to move back and forth within the narrow space between E and D. At this point, the loss no longer increases, while the reactive power Q increases with the voltage. The loss tangent equals the active power divided by the reactive power; therefore, tanδ decreases with the voltage increase. When the voltage exceeds the rated voltage and continues to rise, under the influence of a high electric field, the increased number of charged particles in the liquid due to dissociation or collisional ionization increases the conductivity, leading to increased leakage current and heat generation. The increase in loss far exceeds the increase in reactive power, thus increasing the tanδ value. Under further strong electric fields, in addition to conductivity losses, there are also losses caused by the ionization of gas in the dielectric pores. When oil decomposes and releases gas to form bubbles, the electric field strength Eg in the bubbles is εr times that of the oil's electric field strength Eo. Since the breakdown electric field strength of the gas is much lower than that of the oil, the bubbles ionize first. This causes the bubble temperature to rise, its volume to expand, and the gas channels to widen, eventually leading to breakdown. Consequently, tanδ increases sharply. In the use or testing of circuit breaker capacitors, the voltage will not exceed the rated voltage Un indefinitely; therefore, region C in Figure 3 is not of our concern. The rated voltage of circuit breaker capacitors is generally 40–360 kV. As the breaking capacity of circuit breakers increases, the rated voltage of circuit breaker capacitors continuously increases. However, users rarely have the opportunity to measure losses at the product's rated operating voltage; most measurements are taken at 10 kV. At this voltage, the tanδ value is in region A of Figure 3. The above analysis suggests that the tanδ value at low electric field strength does not accurately reflect the tanδ value at the product's rated voltage; it must be greater than the tanδ value at the rated voltage. Furthermore, the particularly small capacitance of circuit breaker capacitors also determines that their tanδ value is prone to being overestimated. Because circuit breaker capacitors are electrically connected in series (e.g., approximately 100-140 components in a 110kV insulation level product), their capacitance is extremely small. Therefore, the reactive power Q of circuit breaker capacitors is relatively low. When the active power P increases slightly, the loss tangent of the circuit breaker capacitor will increase significantly. Another common phenomenon with circuit breaker capacitors is that the tanδ value gradually increases over time. This is due to the presence of polar impurities. Impurities adhering to the metal bellows, porcelain bushings, and other components of the circuit breaker capacitor, as well as impurities introduced during assembly, slowly dissolve or suspend in the liquid. This dissolution process is particularly lengthy for rubber gaskets. Over time, impurities seep into the dielectric space from the outside of the component. The number of charged particles in this confined space continuously increases, leading to a continuous increase in dielectric losses and thus a larger tanδ value. 3. Solutions Based on the above analysis of the circuit breaker capacitor loss problem, we can see that increasing product cleanliness and minimizing the content of ionic impurities can improve the problem of high tanδ and its increasing over time. Specifically, the following measures can be taken: ① Strengthen the cleaning of components such as metal bellows, preferably using safe organic solvents. Impurities in metal bellows, such as oil stains and other fluxes such as rosin, are easily soluble in organic solvents. Trichloroethylene and other organic solvents were previously used abroad, but due to the toxicity of trichloroethylene, chloroform and other organic agents were later used to clean components, preventing impurities from remaining on the component surface. ② Pre-treatment methods can be used for sealing gaskets. Circuit breaker capacitors use silicone rubber gaskets to seal the caps. Capacitor oil (diarylethane, benzyltoluene, etc.) is itself an organic solvent, which can dissolve impurities on the rubber (fillers, pigments, lubricants, vulcanizing agents, etc.) in the oil, causing an increase in tanδ. Fluoropolymer materials, such as DuPont's Viton rubber, are used for sealing gaskets abroad, with excellent results, but the price is too high. A pretreatment dissolution method can be used. Taking diarylethane as an example, the molded silicone rubber sealing gasket can be pretreated in hot diarylethane at 100°C for 10-12 hours before use, resulting in a near-zero increase in the tanδ of diarylethane. ③ Furthermore, the possibility of impurities contaminating the product should be minimized during assembly. For example, in oily assembly where gloves cannot be worn, the operator's sweat may mix into the solvent. The ionic impurities in sweat have a much greater impact on dielectric loss than the dust (non-ionic substances such as SiO2 and Al2O3) from air conditioning filters. These often overlooked impurities can potentially lead to an increase in the product's tanδ. The tanδ vs. U curve of the JAM180-0.0012H circuit breaker capacitor manufactured by Guilin Power Capacitor Factory verifies the analysis of circuit breaker capacitor losses and also demonstrates that Guilin Power Capacitor Factory, utilizing the advantages of its high-voltage test hall (750kV test transformer, 800kV standard capacitor), can measure capacitor losses at rated voltage. At the same time, by adopting advanced technology and effective measures, the impurity content is reduced. Even the tanδ (0.066%) of the product under low field strength is far lower than the national standard requirement of tanδ≤2% under rated voltage and rated frequency.