Dielectric Fluids for Capacitor Impregnation
What an impregnant has to do in film and paper-film capacitors, which properties matter, and where synthetic esters fit.
Why capacitors are impregnated
In power capacitors, the dielectric is a wound stack of polypropylene film, sometimes combined with paper. After winding, the elements are dried under vacuum and impregnated with a liquid. The liquid fills the gaps between the layers and at the edges of the electrodes. This removes air pockets in which partial discharges would otherwise start, and it helps carry heat out of the winding.
Key properties of a capacitor impregnant
| Property | Why it matters | Typical test |
|---|---|---|
| Dissipation factor (tan δ) | Dielectric losses and self-heating of the capacitor | IEC 60247 |
| Relative permittivity | Field distribution between film and liquid | IEC 60247 |
| Gassing tendency | Gas-absorbing liquids help suppress partial discharges | IEC 60628 |
| Breakdown voltage | Electrical strength of the liquid | IEC 60156 |
| Water and particle content | Purity directly affects losses and breakdown | IEC 60814, ISO 4406 |
| Compatibility with polypropylene | The liquid must not swell or dissolve the film excessively | Immersion and dimensional tests |
| Viscosity and pour point | Impregnation speed and low-temperature behaviour | ISO 3104, ISO 3016 |
| Fire point | Fire safety of the installation | ISO 2592 |
From PCBs to today’s impregnants
For decades, polychlorinated biphenyls (PCBs) were used as capacitor impregnants until they were banned worldwide under the Stockholm Convention. Today, aromatic hydrocarbons such as benzyltoluene and phenylxylylethane (specified in IEC 60867) are widely used in film capacitors. Vegetable oils and synthetic esters are used in designs where a high fire point and high moisture tolerance are required, or where the impregnant has to match specific materials and processes.
Synthetic esters in capacitors – what to consider
- Fire safety: synthetic esters have a fire point above 300 °C (K-class according to IEC 61039)
- Permittivity: esters have a higher permittivity than aromatic hydrocarbons, which changes the field distribution between film and liquid and has to be considered in the design
- Moisture: esters tolerate more water without losing dielectric strength, but water content must still be low for low losses
- Gassing: the behaviour under electrical stress should be tested with the actual capacitor design
- Purity: capacitor grades require tighter limits for dissipation factor, water and particles than transformer grades
Relevant capacitor standards
- IEC 60871 – shunt capacitors for AC power systems above 1,000 V
- IEC 60831 – shunt power capacitors of the self-healing type up to 1,000 V
- IEC 61071 – capacitors for power electronics
IsoTec SE 300 CG
IsoTec SE 300 CG is our capacitor-grade synthetic ester impregnant. We agree limits with each customer and qualify the fluid with your film, paper and process in our laboratory. More about synthetic esters in general: Synthetic ester guide.
Frequently asked questions
Can a transformer-grade ester be used in capacitors?
Not without qualification. Capacitor grades need lower dissipation factor and tighter limits for water and particles, and the gassing behaviour and compatibility with the film must be tested.
Why is gassing tendency important?
Under electrical stress, impregnants can either evolve or absorb gas. A gas-absorbing liquid helps to suppress partial discharges in the capacitor element.
Which capacitors use liquid impregnation?
Mainly high-voltage power factor correction and filter capacitors, and some power-electronics and special capacitors. Many low-voltage self-healing capacitors are dry or resin-filled.
Qualifying an impregnant for your capacitors?
Tell us about your design and process – we test the fluid with your materials. IsoTec products are supplied to customers outside Germany.