Description

Capacitor arranged in a high pressure environment

Oil production and communication applications in subsea environments require electric devices able to withstand high pressure. However, electronic components, such as capacitors, particularly conventional electrolytic capacitors or MP capacitors are not applicable under high pressure conditions. Such capacitors exhibit electrodes stacked and rolled up in a casing partially filled with an electrolytic fluid, which would collapse under high pressure. Electric devices containing such electronic components are often designed with a pressure proof housing in order to keep the interior of the housing at atmospheric pressure (1 atm) . Due to the high pressure in deep sea environments this housing needs to be adequately massive thus causing high costs. Further drawbacks are the high effort for sealing the housing and the feedthroughs for electrical connections to the outside. In other known approaches the housing of such electric devices is filled with an electrically insulating fluid. Although this allows a lightweight design of the device housing, the risk of damage to the capacitors persists because they are exposed to the ambient pressure transmitted by fluid in the device housing.

US 5,876,872 discloses a battery with at least one electrochemical cell for use and recharging underwater, particularly seawater, at a pressure at or greater than atmospheric pres- sure. The battery has an anode, a cathode and an associated electrolyte in a housing. A pressure compensating fluid with a greater density than water separates the electrolyte from the surrounding water.

WO 00/76013 Al discloses a battery for underwater use comprising a plurality of elements. Each element is provided with an aperture communicating with an external liquid environment. Each element is filled with liquid means for sepa-

rating an electrolyte inside the element from the external liquid environment.

It is an object of the invention to provide an improved ca- pacitor arranged in a high pressure environment and an improved electric device containing at least one such capacitor .

The object is achieved by a capacitor according to claim 1 or 2 and by an electric device according to claim 6.

Advantageous embodiments are the object of the dependent claims .

An electronic component according to the invention for application in high pressure environments comprises a casing entirely filled with an electrically insulating first fluid. The casing is equipped with or connected to a volume compensation unit for compensating a volume change of the first fluid. "Entirely filled" means that there are no or only marginal amounts of residual air or gas in the casing in order to avoid compressing them under high pressure which would lead to a collapse of the casing. The volume compensation unit adapts the fluid volume inside the casing to the outside pressure, so the inside pressure and the outside pressure are balanced. Volume fluctuations due to temperature changes can be compensated the same way. Thus a wall of the casing is relieved from mechanical stress so the casing will not collapse regardless of the outside pressure. Particularly, the accord- ingly designed electronic component is a capacitor, such as an electrolytic capacitor or an MP capacitor. Nevertheless the design may be applied to capsulated inductors as well.

The electronic component according to the invention is par- ticularly disposed in an electric device exhibiting a device housing filled with a second fluid. The ambient pressure outside the device housing is thus forwarded to the electronic component. The electric device can comprise more than one

electronic component according to the invention. The device housing can be designed as a light weight canister because it does not have to withstand mechanical stress due to high pressure. Lightweight means thinner walls of the device hous- ing thus reducing costs and providing better cooling to parts inside the device housing. The second fluid should also be electrically insulating. It can serve as a coolant for semiconductors and other parts inside the electric device. As the pressure inside and outside the device casing is essentially the same under all conditions, the risk of leakage of sea- water into the device is tremendously reduced.

In a preferred embodiment the volume compensation unit comprises a valve allowing the first fluid to flow out of the casing or into the casing. The term valve is supposed to mean a constricted passage allowing the first fluid to pass freely or by a light differential pressure. Such an electronic component is preferably used in an electric device filled with a second fluid which is the same as the first fluid. This em- bodiment is particularly simple and cost-saving.

In an advantageous embodiment the valve comprises a filter keeping impurities and pollution from entering the electronic component .

In another preferred embodiment the volume compensation unit comprises a reservoir at least partially filled with the first fluid and connected to the casing by a tube or hose. The first fluid is separated from a second fluid surrounding the reservoir by a flexible impermeable membrane. The reservoir can be arranged in any place inside the device housing or attached directly to the casing of the electronic component. In this embodiment the first fluid flows from the electronic component to the reservoir or vice versa depending on the pressure conditions. The reservoir may exhibit a slight overpressure compared to the ambient pressure in order to make sure the electronic component is always entirely filled with the first fluid. This can be achieved by partially fill-

ing the reservoir with air or gas. The size of the reservoir has to be adequate in order to keep the electronic components filled regardless of the ambient pressure. Two or more electronic components can be connected to a common reservoir by respective tubes or hoses.

In another preferred embodiment the volume compensation unit is formed as an at least partially elastic wall of the casing, thus enabling the casing to adapt to various volumes de- pendent on the pressure. For instance the partially flexible wall can comprise a bellows.

In a preferred embodiment the device housing exhibits a double wall. A double housing yields an improved protection of the electric device from water leakages.

In a particularly preferred embodiment of the invention the electric device is applied in a deep sea environment, e.g. in oil production or communication installations. Compared to conventional devices having a rather heavy device casing keeping the interior at atmospheric pressure in order to keep the capacitors from crushing, the electric device according to the invention exhibits a lightweight device casing. The efforts for sealing the interior in order to keep sea water outside can be kept relatively low because of the non- existing difference between the ambient pressure and the interior pressure. At the same time a risk for damaging the electronic components, particularly the capacitors under high pressure inside the device is virtually zero.

The invention will now be described in more detail with reference to the accompanying drawings, in which:

Figure 1 is a schematic of an electric device for high pres- sure applications with four electronic components and a reservoir,

Figure 2 is a schematic of an electric device for high pressure applications with four electronic components each one equipped with a bellows, and

Figure 3 is a schematic of an electric device for high pressure applications with four electronic components each one equipped with a valve.

Figure 1 shows a schematic of an electric device 1 for high pressure applications with four electronic components 2 (particularly capacitors) and a reservoir 3 serving as part of a volume compensation unit.

Each electronic component 2 comprises a casing 4, electrodes 5 stacked and rolled up and two electric terminals 6. The casing 4 is entirely filled with an electrically insulating first fluid Fl, so no or only marginal residue air or gas is left inside.

All electronic components 2 are connected to the reservoir 3 with a respective tube 7 flowing into a common tube 8, which for its part leads to the common reservoir 3. A part of the reservoir 3 below a flexible impermeable membrane 9 and the tubes 7, 8 are also filled with the first fluid Fl. The elec- trie device 1 exhibits a device housing 10 keeping water outside the electric device 1. The device housing 10 is filled with a second fluid F2 as well as another part of the reservoir 3 above the membrane 9, the reservoir 3 having an opening to the outside. When the device housing 10 is subjected to a change in pressure, e.g. by submerging it, this change is forwarded to the casings 4 of the electric components 2 by the second fluid F2. At the same time the pressure change is forwarded to the membrane 9 thus subjecting the first fluid Fl behind the membrane 9 and consequently inside the housings or casings 4 the same pressure, so the pressure inside and outside the casings 4 is balanced.

Figure 2 shows a schematic of another embodiment of the invention. An electric device 1 for high pressure applications contains four electronic components 2 (particularly capacitors) . Each electronic component 2 comprises a casing 4, electrodes 5 stacked and rolled up and two electric terminals 6.

The casing 4 is entirely filled with an electrically insulating first fluid Fl, so no or only marginal residue air or gas is left inside. The electric device 1 exhibits a device housing 10 keeping water outside the electric device 1.

The device housing 10 is filled with a second fluid F2. When the device housing 10 is subjected to a change in pressure, e.g. by submerging it, this change is forwarded to the casings 4 of the electric components 2 by the second fluid F2. In order to keep the casings 4 from crushing the pressure inside them has to be equal to the pressure outside. Instead of the reservoir 3 from figure 1, serving as part of a volume compensation unit the volume compensation is achieved by a partially elastic casing 4 of each electronic component 2. Therefore each casing 4 comprises a respective bellows 11, which forwards the pressure outside the casing 4 to the first fluid Fl inside, so the pressure inside and outside the cas- ings 4 is balanced.

Figure 3 shows a schematic of yet another embodiment of the invention. An electric device 1 for high pressure applications contains four electronic components 2 (particularly ca- pacitors) . Each electronic component 2 comprises a casing 4, electrodes 5 stacked and rolled up and two electric terminals 6. The casing 4 is entirely filled with an electrically insulating first fluid Fl, so no or only marginal residue air or gas is left inside.

The electric device 1 exhibits a device housing 10 keeping water outside the electric device 1. The device housing 10 is also filled with the first fluid Fl. When the device housing

10 is subjected to a change in pressure, e.g. by submerging it, this change is forwarded to the casings 4 of the electric components 2 by the first fluid F2 outside the casings 4. In order to keep the casings 4 from crushing the pressure inside them has to be equal to the pressure outside. The volume compensation is achieved by a valve 12 in each casing 4 allowing the first fluid Fl to flow freely or by a slight overpressure in or out the casing 4 dependent on the pressure change. Each valve 12 is equipped with a filter (not shown) for keeping pollution outside the electronic components 2.

The second fluid F2 can be the same as the first fluid Fl in all embodiments.

The device housing 10 can exhibit double walls. Each wall of the device housing can be provided with volume compensation units .

The above shown embodiments for electronic components 2 can be applied to other components, such as inductors, as well.

The electric device 1 may be applied in a deep sea environment, particularly in oil production or communication installations. E.g. it can serve as a control unit or an amplifier.