GB2172432A - Self-healing metallized film capacitor - Google Patents

Abstract

A self-healing metallized film capacitor has a dielectric sheet which is metallized with a high resistivity alloy that is chosen so as to provide a desired sheet resistivity for a given metallization thickness. The invention is applicable to any metallized film capacitor in which electrode metallization thickness and clearability must be controlled.

Description

SPECIFICATION Self-healing metallized film capacitor This invention is concerned with self-healing metallized film capacitors, that is, capacitors in which electrode thickness and clearability (self-healing characteristics) are important considerations, for example capacitors subjected to high electrical stress.

It is known that improved self-healing of metallized film capacitors can be obtained by decreasing the thickness of the metallized layer. However, decreasing the electrode thickness decreases the quality of any end connection, increases the edge field, and increases the difficulty in controlling electrode thickness, and, in the case of an AC capacitor, increases the rate of electrode loss due to corrosion.

It has been proposed to use an aluminium-copper alloy metallization to reduce capacitance loss in metallized film capacitors. However, with this alloy, it was deemed necessary to reduce metallization thickness in order to improve the self-healing characteristics of the capacitor.

Aluminium-copper alloy metallization in normal thicknesses has been used in the prior art for self-healing capacitors, but at a high copper content.

We have now developed a self-healing metallized film capacitor in which the electrode thickness and sheet resistivities are chosen independently of each other.

According to the present invention, there is provided a self-healing film capacitor which comprises a capacitor section having a dielectric film which is metallized with an alloy, the alloy being chosen by independently selecting electrode thickness and sheet resistivity, and then determining the composition of the alloy by a desired bulk resistivity value computed from said electrode thickness and said sheet resistivity.

In general, the metallization thickness for a given capacitor application is determined from the properties needed for that application, including the thickness necessary for good quality reliable end connections. The desired sheet resistivity is determined from the electrical stress to which the capacitor will be subjected in any particular application. Then, the metaliization alloy is selected based on its total resistivity that will provide the desired sheet resistivity and self-healing characteristics (clearability) at the specified electrode thickness.

As a first approximation, the alloy is selected by determining the bulk or total resistivity needed by multiplying the desired electrode thickness in centimeters times the desired sheet resistivity in ohms/sq.

After a trial metallization, the sheet resistivity measurements indicate what adjustments must be made, for example more or less alloying metal or a different alloy. As experience is gained with a particular alloying metal or group of alloys, it is possible to select the particular alloy with greater certainty from the first approximation results.

Metallized capacitors according to the invention find use in a variety of applications, such as DC capacitors, energy storage capacitors, and Ac capacitors. When the self-healing alloy metallized film capacitor of this invention is to be used for AC application, it is necessary to incorporate into the capacitor a material which will prevent capacitance loss and prolong AC life. This is contrary to prior art disclosure which teach that the use of alloy metallization alone will reduce or prevent capacitance loss.

The preferred materials for AC use are urethanes containing unreacted isocyanate groups as described in U.S. Patents 4,317,158 and 4,317,159 and in U.S. Application Serial No. 710897; British Application 8603724 filed concurrently herewith. The urethane material may be used alone as a liquid or potting compound, or may be incorporated into the dielectric field.

For a better understanding of the invention, preferred embodiments thereof will now be described, by way of example, with reference to the accompanying drawing, in which: Figure 1 shows a partly unwound capacitor section using alloy metallization of selected thickness and resistivity, Figure 2 is an elevation, partly broken away, of a capacitor as shown in Figure 1 with a fluid urethane additive, and Figure 3 is an elevation, partly broken away, of a capacitor as shown in Figure 1 in which a urethane is used as a potting compound.

Figure 1 shows a partly unrolled metallized film capacitor section 10 having two metallized film electrodes 12 and 14 which have a high resistivity alloy metallization. Electrodes 12 and 14 are provided with unmetallized margins 13 and 15, respectively, which are oppositely positioned in the wound section 10, so that the metallized portion of each electrode is available for terminal contact at only one edge of the winding. The ends of section 10 are covered with metallic spray or solder 20, and terminal leads or tabs 16 and 18 are connected thereby to electrodes 12 and 14, respectively.

Figures 2 and 3 show AC capacitors having a section 10 located within a housing 30. Electrode tab 16 (not shown) and tab 18 are connected to terminals 34 and 36, respectively, located in cover 37. In Figure 2, a urethane containing unreacted isocyanate groups is present in fluid form 31 by itself or dissolved in a dielectric fluid. In Figure 3, the urethane is used as a potting compound 32.

Suitable urethanes are those described in the patents and patent application noted above, the preferred urethane being a diphenyl methane diisocyanate having 33.4% unreacted isocyanate groups. This material reduces the capacitance loss of an AC capacitor during its operating life.

It is well known that thinner, and consequently high resistance, electrode metallization provides better self-healing (clearability) characteristics in high stress AC film capacitors. It was not well understood whether this improved self-healing was a function of electrode metal thickness or of the increased resistivity of the electrode.

It has now been determined that the dominant factor in self-healing is the sheet resistivity of the electrode, rather than the thickness of the electrode. Since it is well known that the addition of a second metal (creating an alloy) will normally result in a resistivity which is greater than that of the base metal, it is possible by proper choice of metals, to tailor a system in which both the electrode sheet resistance and thickness are selected independently. This result has applications not only for DC application, but for other uses also, for example, energy storage, AC etc. The alloy system must, of course, be chosen so that it does not have any deleterious effect on the performance of the capacitor.

The base metal may be any metal which is useful in capacitors, such as aluminium or zinc, and is preferably aluminium. Suitable aluminium alloys are those containing chromium, copper, iron, lithium, magnesium, manganese, nickel, silicon, titanium, vanadium, zinc, and zirconium. Their bulk resistivities at 1% and 5.5% are (in micro ohm-cm) respectively: Cu 2.99,4.54; Fe 2.84, 3.10; Li 5.96; 16.91; Mg 3.19 5.62; Mn 5.59, 9.25; Ni 2.75, 3.02; Si 3.67 4.67; Ti 5.33, 6.01; Va 4.58, 5.66; Zn 2.74, 3.17; and Zr 3.17, 3.37.

In order that the invention may be more fully understood, the following examples are given by way of illustration only.

Example 1 This example shows the metallization of a film dielectric, specifically polypropylene of 8 pwm thickness, with aluminium and with an aluminium alloy containing 4 wt% copper in three nominal sheet resistivities of 4, 6 and 8 ohm/sq. (Sheet resistivity is measured on a square piece, and the result obtained is independent of the size of the square, hence ohm/sq.) Since the chosen alloy has a resistivity of about twice that of aluminium, some metallized films were obtained having different resistivities at the same metallization thickness and some with different thicknesses.

The resistivity and metal thickness of the metallized films used to prepare the test capacitors as set out in Table 1a. Each result is an average of two lots metallized to the 4, 6 and 8 ohmlsq. nominal resistivity; both nominal and measured resistivities are given in ohms/sq., and the approximate aluminium surface density is given in wg/cmz of the surface area.

TABLE la Resistivity Sample Metal Nominal Measured Density 1-2 Al-Cu 4 3.45 7.2 3-4 Al-Cu 6 4.45 5.2 5-6 Al-Cu 8 9.25 3.3 7-8 Al 4 3.74 4.3 9-10 Al 6 6.50 3.3 11-12 Al 8 9.95 2.4 Three capacitors made from each of the above samples were pulse-tested to determine the effect of electrode thickness on end connection quality. The ratio of the number of failures to total units are given for pulses of 0.6, 0.8 and 1 ampere per inch of end connection.

TABLE ib Failures Sample Resistivity Density O.6Alin 0.8Alin lAlin 1 3.45 7.7 0/3 0/3 0/3 2 3.45 6.8 0/3 0/3 0/3 3 4.25 5.5 1/3 1/3 1/3 4 4.25 4,6 0/3 0/3 0/3 5 9.25 3.2 1/3 1/3 1/3 6 9.25 3.4 2/3 3/3 3/3 7 3.60 4.2 0/3 0/3 0/3 8 3.60 4.4 0/3 0/3 0/3 9 5.51 3.1 0/3 1/3 3/3 10 5.51 3.5 0/3 1/3 2/3 11 8.99 2.2 1/3 3/3 3/3 12 8.99 2.6 1/3 3/3 3/3 These results show that good end connections can be made using electrode thicknesses greater than about 5 yg/cm2, and particularly in the range of 7-8 Fg/cm2. There is no indication that resistivity has any effect on end connection quality.

For a self-healing capacitor, the DC breakdown is a measure of the self-healing qualities of the system.

When the DC breakdowns were plotted as a function of resistivity and of thickness, it was found that breakdown, and hence ability to self-clear, is a unique function of resistivity rather than electrode thickness (in the thickness ranges considered here).

The surface density, surface resistivity, and breakdowns in volts DC are presented below: TABLE 1c Density Resistivity Breakdowns Al-Cu 7.2 3.45 1700,1700,1800,1600,1500,1600 5.0 4.25 2200,2300,2100,2300,2000,2200 3.3 9.25 2700,2800,2700,2400,2400,2400 Al 4.3 3.6 1600,1700,1700,1900,1700,1800 3.3 5.51 2500,2600,2400,2400,2600,2700 2.4 8.99 3000,3100,3100,3100,3000,3000 Comparing DC breakdown for 3.45 and 3.6 ohm/sq resistivities and for both 3.3 g/cm2 densities, it can be seen that DC breakdown is determined by resistivity rather than electrode thickness.

Example 2 This example shows the use of alloy metallization in AC capacitor applications. As has been shown in the U.S. Patents noted above, the loss of capacitance of a metaliized polypropylene capacitor on AC voltage due to corrosion can be controlled by the addition of unreacted isocyanate to the dielectric fluid.

Normally, 8 Fm thick polypropylene is used for AC capacitors to be operated at 370 VAC, and 10 iim polypropylene for those to be operated in the 440 to 480 VAC range. Since such limitation is predominately determined by the loss of capacitance due to corrosion, it is to be expected that the addition of isocyanate to the dielectric fluid would allow 6 Fm polypropylene to be operated at 370 VAC, and 8 Am polypropylene to be operated in the 440 to 480 VAC range. However, when aluminium metallized capacitors manufactured in this manner were tested under accelerated conditions using standard industry requirements, the results showed a high and unacceptable number of failures due to poor self-healing.

Capacitors were constructed from polypropylene dielectric with electrodes of aluminium containing from 5 to 6% copper and with a surface resistivity from 6.5 to 8 ohms/sq. Diphenyl methane diisocyanate (89) was added to the dielectric fluid. The units were tested according to industry standards with the following results.

TABLE 2 Cap. Dielectric Test No. of Test No. of Thickness Conditions Units Hrs. Failures 40sLf 8Am 600VAC/25 C 60 120 0 40;f 8lim 585VAC/80"C 12 500 0 45 > f 6pwm 500VAC/25"C 60 120 1 45pf 6m 466VAC/80 C 12 500 0 These results are well within industry requirements, are unattainable using standard aluminium metallizing, but are attainable with alloy metallization and show that a thinner dielectric film may be used with alloy metallization.

Although the specific example cited here is for metallized polypropylene, it should be recognized that this invention can be used with any other dielectric suitable for use in a metallized capacitor, such as polyester, polyvinylfluoride, polycarbonate, or kraft paper.

The particular dielectric film thickness used will depend on the capacitor application. For example, for energy storage capacitors, the thickness may be 10 to 12clam; while for DC applications, the thickness will most likely be 6 to 8 Fm or less.

Claims (7)

1. A self-healing film capacitor which comprises a capacitor section having a dielectric film which is metallized with an alloy, the alloy being chosen by independently selecting electrode thickness and sheet resistivity, and then determining the composition of the alloy by a desired bulk resistivity value computed from said electrode thickness and said sheet resistivity.

2. A capacitor according to claim 1, in which the sheet resistivity is 7 to 8 ohms/sq.

3. A capacitor according to claim 1 or 2, in which the alloy is an aluminium alloy or an aluminiumcopper alloy.

4. A capacitor according to claim 3, in which the aluminium-copper alloy contains 5 to 6 wt% copper.

5. A capacitor according to any of claims 1 to 4, in which the capacitor section is an AC metallized film capacitor section and is in contact with a urethane containing unreacted isocyanate groups.

6. A capacitor according to claim 5, in which the capacitor is impregnated with a dielectric fluid and the urethane is present as an additive in the fluid.

7. A self-healing film capacitor according to claim 1 substantially as herein described in either of the Examples.