Metallized film shunt capacitors are also known as self-healing shunt capacitors due to their self-healing property (i.e., the ability to restore insulation) when insulation breakdown occurs. In the reactive power compensation of low-voltage distribution systems, self-healing low-voltage shunt capacitors have been widely used, and cases of failure or even damage are quite common. There are many reasons for damage, sometimes due to design and manufacturing issues, sometimes due to improper use, and sometimes due to the power grid itself.

To better improve product quality and analyze the causes of damage, here is a brief introduction to the composition of self-healing low-voltage shunt capacitors. Self-healing low-voltage shunt capacitors use single-sided corona-treated polypropylene film as the dielectric and single-sided vacuum-deposited metal layers as the electrodes. They are wound in a non-inductive manner to form a cylinder, with both ends of the cylinder sprayed with metal, leads welded, and then encapsulated to form the capacitor element. Finally, the elements are placed in a metal casing to become the finished capacitor. The quality of the elements directly affects the quality of the capacitor. From the dissection of damaged capacitors, it is almost always due to the breakdown of the elements, and the breakdown often occurs at the end faces of the elements. Therefore, the quality of the end faces is crucial. The quality of the sprayed metal surface is not only related to the equipment but also to the composition of the sprayed metal material, the pressure, the speed of the spray gun, and the misalignment of the capacitor itself. Here, we will briefly analyze the design of the capacitor elements, the control of various parameters in the spraying process, and practical production experience.

1. Selection of Element Line Current

The element of a metallized capacitor is formed by winding two films with single-sided metal layers. The current is drawn through the metal layers sprayed on both end faces of the element. Therefore, the surge current capability of the element end faces is affected by the bonding between the sprayed metal layer and the element end, as well as the line current designed for the element. We use the following formula:

IC=I1×L

Where:

I1 = Line current (mA/m)

IC = Current through the element end face (mA)

L = Length of the metallized film in the element (m)

Therefore, when designing the element, it is essential not to set the line current too high, as this would compromise the quality of the end face. If the line current exceeds the maximum limit that the end face can withstand, the end face will break down. While theoretically, the smaller the line current, the more reliable the end face quality, practical design must consider multiple factors to choose a reasonable value. Typically, a line current design value of around 50 mA/m is suitable.

2. Composition of End Face Loss in Sprayed Metal Layers

The end face of a metallized element is shown in Figure 1, and the equivalent circuit is shown in Figure 2:

C = Capacitance of the capacitor element

R = Resistance of the conductive part, including the resistance of the sprayed metal layer R1, contact resistance R2, and resistance on the film R3.

The loss of the element tanδ=ωRC. Theoretically, the contact resistance R2 should be zero, but in practice, R1 is zero, and R2 is not zero. This is because the connection between the sprayed metal layer and the metal layer on the film surface is in an ohmic contact state between two metal materials, resulting in a certain contact resistance.

R3=2(R□/L)[W3+(m+d)]R3=2(R□/L)[w/3+(m+d)]

Where:

R□ = Sheet resistance of the metal layer on the film

L = Effective length of the film (cm)

W = Effective width of the film (cm)

m = Edge margin (cm)

d = Misalignment (cm)

Thus, the loss of the element is actually determined by R2 and R3, and the size of R2 and R3 reflects the size of the element loss. Therefore, the damage process of the element end face is essentially the change process of R2, which directly reflects the quality of the element end face.

(1) If the quality of the sprayed metal layer is poor, R2 increases.

The size of the sprayed metal particles, the dryness of the compressed air, the distance between the spray gun and the element end face, and the oxidation level of the sprayed zinc wire, if not meeting the process requirements, will lead to an increase in the contact resistance between the sprayed metal layer and the metal layer, causing the element end face to break down.

(2) In practice, if the oxidation of the metal layer on the film increases the sheet resistance, R3 increases.

Therefore, in the process, if the element is wound too loosely, allowing and air between the films and the element end faces, the metal layer will oxidize under the combined effect of heat and electricity during operation, increasing the sheet resistance and thus the element loss, leading to element breakdown.

From the above analysis, it can be understood that the quality of the element end face is not only related to the line current value but also critically depends on the sprayed metal process. Therefore, it is essential to strengthen the setting of various parameters in the sprayed metal process, as controlling the sprayed metal process will directly affect the quality of the metallized capacitor elements.

3. Factors Affecting Sprayed Metal Equipment Parameters

To improve the level of the sprayed metal process, considerations should be made from the setting of sprayed metal process parameters and equipment improvements. Currently, there are two types of sprayed metal processes: oxygen-acetylene flame method and arc method. The arc method's automatic sprayed metal process parameters are easier to control and ensure. Our company uses the arc method. The main parameters of the sprayed metal process include: voltage and current; sprayed wire feed speed and its stability; compressed air pressure; angle between the spray direction and the element end face; thickness of the sprayed metal layer and workpiece movement speed; dust removal intensity of the dust collector and exhaust air flow rate in the room; air composition in the sprayed metal room, etc. These factors interact to affect the sprayed metal quality, and comprehensive consideration of each process parameter is required, with process parameter selection based on extensive testing.

3.1 Influence of Voltage and Current of Sprayed Metal Equipment

Since the sprayed metal equipment uses the electric spray method, the equipment voltage is generally controlled at 26V when idle and around 20V during operation, with a working current of (260-270)A. When the applied voltage is around 20V, the intermittent current peak reaches over 150A, and the metal particles become coarse, splashing in clusters. To better increase adhesion, a method of spraying fine crystals first and then coarse crystals is used, which is beneficial for welding and increases the adhesion of the sprayed metal layer. During equipment operation, it is essential to select the appropriate voltage and current to ensure that the particles sprayed by the equipment meet the process requirements.

3.2 Influence of Sprayed Wire Feed Speed

Generally, the diameter of the sprayed wire is about 2.0mm, with an average feed speed of 10.40m/min. The feed speed should be appropriate; too fast will not allow the sprayed wire to fully melt, and too slow will cause the melting temperature to be too high. The speed should also be stable; otherwise, the uniformity of the sprayed metal layer particles will be poor.

3.3 Adjustment of Compressed Air Pressure

The compressed air pressure for coarse crystals is (4.5-5) bar, and for fine crystals, it is 5.5 bar. This pressure can atomize the melted sprayed metal particles and spray them onto the capacitor end face, firmly combining with the dielectric, and the sprayed metal layer particles are fine. If the pressure is too high, the misaligned polypropylene film will tilt, not only preventing the metal layer from being sprayed but also affecting the firmness of the end face contact, greatly reducing the contact area. Additionally, if the sprayed metal layer contains high impurities, the contact loss will increase. Especially, the oil content in the compressed air, under the action of the arc, decomposes into various harmful components, corroding and oxidizing the sprayed metal layer, significantly increasing the contact loss. Therefore, while adjusting the compressed air pressure, more attention should be paid to the quality of the compressed air to ensure that the compressed air reaching the element end face is clean, free of impurities, oil, water, etc.

3.4 Influence of Spray Gun Direction

Based on years of experience and extensive testing, the spray gun direction should form a 110° angle with the element end face, as shown below:

This allows the melted sprayed metal particles to enter the gaps between layers, firmly attaching to the metallized layer of the film, increasing the contact area and enhancing the firmness of the bonding layer. However, the distance should not be too small, and the relative angle should not be too large; otherwise, the melted particles will become larger, affecting product quality.

3.5 Influence of Sprayed Metal Layer Thickness and Workpiece Movement Speed

Based on experience, the sprayed metal layer thickness is generally (0.5-0.6) mm, with requirements: coarse crystals (0.23+0.02) mm; fine crystals (0.23+0.02) mm. If too thin, the film layer is easily burned during welding; if too thick, the cost is high, and the sprayed metal layer is prone to peeling when the small core is separated after spraying. The control of the sprayed metal thickness can generally be achieved by adjusting the feed speed and workpiece movement speed. However, increasing the feed speed may affect the metal particle size. The workpiece movement speed should be controlled within the range of (0.5-2.3) m/min. To control the capacitor end face at 0.46mm, the movement speed should not be too fast; otherwise, the sprayed metal layer will be too thin, increasing contact loss and being unfavorable for lead welding.

3.6 Influence of Dust Removal Intensity and Exhaust Air Flow Rate

The dust removal intensity of the dust collector should be strong enough to remove suspended particles and impurities in the sprayed metal room, preventing the sprayed metal layer from containing impurities and particles. At the same time, the exhaust air flow rate in the sprayed metal room should be about 20% higher than the compressed air flow rate, to fully exhaust the large amount of ozone generated by the arc, preventing ozone from oxidizing the melted sprayed metal material remaining in the sprayed metal room, increasing contact loss. The air in the sprayed metal room must be clean, free of water and oil content, to increase the purity and bonding force of the sprayed metal layer and reduce loss.

4. Improvement Measures for Equipment to Enhance Sprayed Metal Quality

4.1 Reducing End Face Pollution and Increasing Adhesion of Sprayed Metal Layer to Core End Face

Pollution on the core end face severely reduces the adhesion strength between the sprayed metal layer and the end face, increases the equivalent series resistance, causing the capacitor's tan⁡δtanδ to increase, current-carrying capacity to decrease, and reliability to reduce. From the dissection of failed products, a certain proportion of failures are caused by end face pollution. The pollution on the core end face mainly comes from the reflected metal dust when spraying the first end of the core, and this pollution is not easy to remove. As they adhere to the core surface, when spraying this side, they block the sprayed metal particles from entering between the films, thus reducing the adhesion of the sprayed metal layer to the core end face.

Through observation, it is found that this pollution is mainly caused by airflow disorder. There are two reasons for airflow disorder: first, the suction port of the dust collection device is on one side of the sprayed metal machine, where the airflow is at a right angle to the compressed air jet, causing airflow disorder; second, the hollow area of the core holder is insufficient, and the sprayed airflow hitting the bottom of the holder causes certain reflection, polluting the lower end face of the core.

To address these two issues, we have taken the following measures: (1) Changed the suction port position of the sprayed metal machine from side suction to bottom suction, making the jet airflow and suction airflow in the same direction; (2) Improved the holder to increase the hollow area, reducing the resistance of the jet airflow.

After implementing these two measures, the adhesion of the sprayed metal layer to the core end face has been enhanced. The sprayed metal quality of the improved capacitor core has shown significant improvement through charge-discharge experiments (see Table 1), and later core end faces did not show pollution-induced failures.