High-Voltage Film Capacitors: History
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Subjects: Polymer Science | Engineering, Electrical & Electronic | Engineering, Industrial
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Olatoundji Georges GNONHOUE

High-voltage capacitors are key components for circuit breakers and monitoring and protection devices, and are important elements used to improve the efficiency and reliability of the grid. Different technologies are used in high-voltage capacitor manufacturing process, and at all stages of this process polymeric films must be used, along with an encapsulating material, which can be either liquid, solid or gaseous.

  • high-voltage capacitors
  • resin
  • dielectric film

1. Introduction

High-voltage films capacitors are important components for networks and various electrical devices. They are used to transport and distribute high-voltage electrical energy either for voltage distribution, coupling or capacitive voltage dividers; in electrical substations, circuit breakers, monitoring and protection devices; as well as to improve grid efficiency and reliability. Impregnated either with gas or oil, they can be categorized into six different classes, namely high-power capacitors, high-voltage capacitors, energy storage capacitors, starting capacitors, filter capacitors and discharge capacitors [1]. Table 1 shows the history of the development of capacitors.

One particular category, namely capacitors used in high-voltage applications, is of particular interest in the present study, given the key role these devices play in improving grid efficiency by stabilizing voltage levels. The dielectric materials used for these capacitors play a key role in their performance and long-term reliability. Prior to the 1970s, impregnated kraft paper was the main capacitor dielectric, usually used in combination with mineral oil or polychlorinated biphenyl (PCB) as an impregnating liquid, however today these components are often manufactured using polymeric films. Thanks to their low dissipation factor, high dielectric strength, good stability and high availability, polymer films have gradually replaced the kraft paper used in capacitors. The switch from paper to polymer film has also shortened the capacitor production process by reducing the drying time required before impregnation [2][3]. Furthermore, as with polymer technology, capacitor manufacturing technologies have evolved over time. Film capacitors have, thus, been fabricated using polyethylene (PE), polystyrene (PS), polytetrafluoroethylene (PTFE), polyethylene terephthalate (PET) and polycarbonate (PC) films, and most recently, biaxially-oriented polypropylene (BOPP), which is the current choice for capacitors used in high-voltage applications [4][5]. Additionally, mixed capacitors associating paper layers with polymeric layers have been used with different impregnating components, as shown in Table 2.

Table 1. History of capacitor development [6].

Capacitors Years
Water in Leyden Jar 1746
Franklin’s glass–metal foil 1750
Paper 1876
Electrolytic capacitor 1887
Wax paper–metal foil 1876
Self-clearing capacitor 1900
Mica 1909
Wound electrolytic capacitor 1927
Lacquer on paper WWII
Polymeric films Starting from 1954

Table 2. Comparison of the electrical and dielectric characteristics of power capacitors from 1950 to 1996.

Years Manufacturing Technologies Electric Field of Operation (V/μm) Dissipation Factor (%)
1950–1959 All-paper capacitor using mineral oil 12 3.5
1960–1968 All-paper capacitor using PCB 16 2
1969–1974 Mixed capacitor using PCB 38 0.6
1975–1983 Mixed capacitor using
non-chlorinated liquids		 45 0.45
1984–1987 Mixed capacitor 45 0.45
1988–1996 All-film capacitor 60–75 0.1

Table 2 shows the evolution over time of the different power capacitors, as well as their electrical and dielectric characteristics.

2. Encapsulant

The main role of the encapsulant is to consolidate and improve the dielectric strength of the capacitor assembly by filling any possible voids or air gaps, and thus improving the resistance to partial discharges [7]. In order to prevent faults inside capacitors using a liquid impregnant, the latter should always be in an airtight structure to prevent the loss of the impregnator. Most of the encapsulants used in high-voltage capacitors are polymers, which are initially in liquid form, oils or thermoset materials.

The oils used as an impregnation media for solid dielectrics have evolved over time, going from mineral oils to mono-di-benzyl-toluene (MDBT) and synthetic oils such as polychlorinated biphenyl (PCB), benzyl neocrapate (BNC) and mono-isopropyl-biphenyl (MIPB).

The most common liquid impregnants are mineral oils; vegetable oils such as castor, rapeseed and cottonseed oils; certain waxes; and biodegradable synthetic oils such as phenylxylylethane (PXE), monobenzyltoluene (MBT) and dibenzyltoluene (DBT) (Jarylec). These have completely replaced chlorinated biphenyls, which are currently prohibited due to environmental and health concerns.

Several mineral or synthetic oils are used to impregnate the dielectric. For example, trichlorodiphenyl (TCD) has been used as an impregnator on paper or mixed paper layer dielectrics. The PCB is replaced by synthetic aromatic hydrocarbon (AHC) oils, such as alkylnaphthalene (AN) and alkyldiphenylethane (ADE). Other non-flammable and low-permittivity oils such as silicone oils and alkylphenyl phosphate have also been used. Aromatic hydrocarbon (AHC) oils such AN and ADE both have excellent gas absorption properties, good compatibility and are environmentally safe. However, since AHC has low permittivity and is flammable, other insulating oils such as silicone phenylmethyl oil and mixtures of AHC and TCP are now partially used for power capacitors [8].

Liquid impregnants give excellent dielectric performance to capacitors, but ecological and safety requirements are directing research towards solutions using other impregnants, such as the use of thermosetting epoxy resins, plant-based oils and gases. If a solid impregnator is required, epoxy resins are available and represent a good alternative due to their excellent mechanical, electrical and chemical properties, as well as their advantageous processing possibilities in many applications [9]. Finally, dielectric gases can act as impregnators. The main gaseous impregnator used today is nitrogen (N2), which for certain applications is mixed with other gases such as sulfur hexafluoride (SF6) [10]. Other gases such as atmospheric air, CO2, fluoronitriles ((CF3)2CFCN) and fluoroketones CF3C(O)CF(CF3)2 (C5K ketone) can also be used. The dielectric strength of atmospheric air is 3 kV/mm and that of SF6 is three times higher than that of air, while the dielectric strength for fluoronitriles ((CF3)2CFCN) is twice that of SF6 and that of fluoroketones CF3C (O) CF (CF3)2 (C5K ketone) is 85% of the value of the dielectric strength of SF6 at −15 °C. The dielectric strength of the SF6 mixture (with a proportion of 20% SF6) and N2 is 70% of the value of the dielectric strength of SF6, while that of N2 is 44% of the value of the dielectric strength of SF6 [11][12][13][14][15].

To improve the properties of capacitors, composite polymers can be used. Polymer composites have several advantages, as they have the potential to improve the thermal conductivity and mechanical strength of materials. For the manufacture of polymer composites, polymeric materials such as epoxy resin are used as a matrix for advanced composites due to their ease of processing and low cost.

Nanodielectrics are a particular type of composite material, which as the name suggests, are dielectric materials that once dispersed in a polymer matrix influence the movement of polymer chains, leading to significant changes in the overall properties of the material [5]. Nanoparticles include SiO2, ZnO, Si3N4, BN and other oxides and nitrides, whose size range varies from a few nanometers to about 100 nm. To improve the properties of resins, low molecular weight organic and inorganic materials are often added [16].

It should be noted that the reasons why resins such as epoxies are used is mostly because they have high specific strength, efficient rigidity, chemical resistance and good dimensional stability. However, epoxy resin has certain drawbacks, namely poor thermal and electrical conductivity. To improve the unwanted properties of epoxies, the aforementioned nanocomposites are used to strengthen them and form epoxy composites. The materials used to this end are carbon nanotubes (CNT), silicon carbide nanowires, magnesium oxide, titanium dioxide and calcium carbonate. CNT are the most widely used due to their desirable properties, such as their high aspect ratios and good mechanical, electrical and thermal properties. Certain fillers, such as metallic particles, organic and inorganic particles, carbon, ceramics and fibers can also be added to polymers in order to improve their optical, thermal, electrical, mechanical and magnetic properties. The degree of improvement depends on how the charges disperse in the matrix and on the interfacial interactions between the different materials [17][18][19].

Tables 3 and 4 give a list of the main encapsulating or impregnating materials used in manufacturing high-voltage capacitors, along with their dielectric properties.

Table 3. Overview of thermosetting and elastomer insulating materials [11][20][21][22].

Thermosetting Relative Permittivity tan δ Conductivity (S/m) Dielectric Strength (MV/m)
Epoxy Resins (EP) 3.5 to 4 <10−2 10−12 to 10−15 20
Polyurethanes (PU)
(thermosetting)		 4 2 × 10−2 (1 MHz) 10−11 -
Polyurethanes (PU) (elastomer) 7.4 >5 × 10−2 (1 MHz) 10−10 to 10−12 88.6
Phenolic Resin and Resin bonded Paper (RBP) 5 0.1 (1 MHz) 10−11 -
Silicone 2.8 to 3 0.005 to 0.01 10−13 26–36

Table 4. Overview of insulating liquids [11][23][24][25][26][27][28][29]. 

Liquids

Relative Permittivity

tan δ

Dielectric Breakdown,

(kV)

Fire Point (°C)

Flash Point (°C)

Mineral oil

2.2 at 20 °C

<0.05

(25 °C)

30–85

170

145

Silicone oils

2.7 at 20 °C

2.3 at 200 °C

1 to 2.10−4

35–60

>335

>300

Ester liquids

3.3

≥10−3

45–70

257

310

Vegetable oils

3.1

0.25 (25 °C)

82–97

354–360

310–325

Natural Ester Liquids

3.1 at 20 °C

≤0.2 at 25 °C

33.8

300

275

Natural Ester Liquids

3.1 at 20 °C

≤0.2 at 25 °C

33.8

300

275

This entry is adapted from the peer-reviewed paper 10.3390/polym13050766

References

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