Technical Field

The aim of the invention is to provide an improved capacitor housing for use in subsea power supply modules.

Background

Subsea power supply arrangements are required for supplying power to equipment which operates on or near the seabed. This is particularly relevant for applications in the oil and gas industry. The equipment used in these environments can include motors, pumps, compressors and power grids for subsea processing. Components of subsea installations may be installed at water depths in the range of 1 ,000 to 7,000 metres. Subsea installations may also be towed at these depths. Any equipment which is to be used at subsea levels therefore needs to be able to withstand very high hydrostatic pressures, typically in excess of 100bar, and usually in the range of 200- 500 bar, although the pressures can be much higher, for example, up to 700 bar.

Most motors for subsea processing require a variable speed drive (VSD) due to the expected fluctuations in load throughout the motor’s lifetime. A variable speed drive converts the voltage, current and frequency into a level suitable for operating machinery. A variable speed drive usually contains several power cells, each consisting of a rectifier, a DC-link capacitor and an inverter.

Conventional capacitors and/or electronic components of subsea power supply modules are either designed to be exposed to the high-pressure environments during subsea operation, or they are fitted within larger, pressure-resistant housings. There are advantages and disadvantages associated with each arrangement.

The most common approach is to place the electronics within a larger pressure housing. This has the advantage that standard components with known reliability can be used. These standard components are designed to be operated at pressures of about 1 bar (e.g. during normal use on land /topside). When used in subsea applications with this alternative approach, they are located within a module housing which is designed to protect the contents of the vessel from the surrounding pressure. This reduces and limits the amount of pressure-limit testing and qualification work required for the electrical components themselves, such as the capacitors. Pressure limit testing and qualification procedures are time consuming and cost intensive processes, therefore the removal of this requirement is desirable. The disadvantage is that a larger external housing is required for the module, and this large external housing must be capable of withstanding pressure differentials of up to 700 bar. A large housing which is suitable for withstanding such high pressures requires an increased wall thickness, and these housings tend to be expensive, heavy and hard to handle.

A relatively new solution is to expose the electronic components to the high pressures of the subsea environment. The pressure inside the module is approximately equal to that of the surrounding (ambient) sea-water, and therefore the module experiences only a small pressure-differential, if any. The advantage of this approach is that the weight and footprint of the module is reduced as the external housing does not need to withstand a high pressure-differential. The disadvantage of this approach is that each component needs to be qualified for operation in a high-pressure environment. There have been issues with regards to reliability, especially for capacitors.

WO2015/154916 (Siemens) discloses a power cell arrangement positioned within a metal container or housing. The power cell comprises capacitor banks arranged inside a metal capacitor housing. This document discloses an example of the first approach, where each of the components of the capacitor are designed to be exposed to the high-pressures. The metal capacitor housing is filled with capacitor fluid and the device comprises internal busbars within the capacitor housing. In addition, external busbars (13, 15) are attached to the outside of the capacitor housing. The capacitor housing is located inside a fluid-filled container. The capacitor housing has means to compensate for pressure differences between the interior and exterior of the capacitor housing. These means can be openings in the capacitor housing permitting fluid exchange between the capacitor housing and the surrounding container.

EP2846343 (ABB Technology) describes an example of subsea DC switchgear which uses controllable semiconductor switches (preferably IGBTs) to break a circuit, for example to a power converter. This document provides an example of the second approach, i.e. where the majority of the electrical components are included in a single pressure housing which protects the internal components from the external pressure. The electronics for controlling IGBTs (the controllers, 4) are placed inside the pressure chambers, together with capacitors.

Summary

According to the present invention there is provided a power supply module having an outer container further comprising at least one inner pressure vessel or pressure housing, whereby each inner pressure housing contains at least one capacitor. The inner pressure housing is a pressure-isolated container adapted to maintain a pressure inside the inner pressure housing that is independent from the pressure in the outer container. The outer housing can be fluid-filled. The power supply module further comprises a low inductance external busbar which is connected to the outer side of the inner pressure housing.

The present system utilises a plurality of capacitors mounted in individual pressure housings. Each of the standard capacitors are placed in an individual container, canister or housing which is designed to withstand high pressure differentials. The gate driver circuitry associated with the capacitor is arranged such that it is external to the pressure housing, but within the outer container. The high pressure differentials are in excess of 50 bar, typically in the range of 100 bar to 500 bar, but may be as high as 700 bar). Conventional electrolytic capacitors can be used, without requiring pressure-limit testing and qualification work, as the capacitors are protected by the inner pressure-resistant housing. The capacitors within the pressure housings or containers are then electrically connected to the rest of the system within a larger pressure-compensated oil-filled volume (or housing).

The power supply module is designed such that the inner pressure housing can withstand a high pressure-differential, whilst the oil-filled container is designed to maintain an internal pressure which is roughly equivalent to that of the surrounding subsea environment. Usually, the inductance between the capacitors and the switching devices becomes too large if both the capacitors and the connections are housed in a pressure housing such as a single container. This problem is solved by using smaller individual pressure housings which are located within a pressure- compensated oil volume.

As the capacitors are not exposed to high ambient pressures at sub-sea level it is possible to use standard electrolytic capacitors. Although the capacitors do not have to withstand high pressures (and can operate at pressures of about 1 bar), the other components in the system, including the busbars, are exposed to the high pressures of the subsea environment. Previously, the use of standard capacitors would have necessitated large external housings capable of withstanding high pressures. The claimed power supply module overcomes this disadvantage.

In high-power inverter systems it is crucial that the inductance between the switching devices and the DC-link capacitors are as low as possible. This can be achieved by connecting the capacitors directly to low inductance busbars. The low inductance busbars are formed from sheets of conductive material with an insulating material positioned in between the layers, and are designed to be exposed to a high-pressure environment.

The capacitors and low-inductance busbars are connected to the outside of the pressure housings, where the busbars are exposed to ambient pressures at subsea level (which may be high, for example between 100 to 500 bar). In electromagnetic surveying there is a need to control the current output to the electromagnetic antenna. The subsea power supply module of the system described herein can be used to vary the output to the antenna.

Within a closed and sealed vessel, such as the inner housing which contains the capacitor, there is no air flow. The capacitors may generate heat during use. The capacitor may expand and contract to a certain degree with temperature fluctuations. It may be desirable to ensure that there is effective heat transfer from the capacitors to the inner pressure housings.

Brief Description of the drawings

The invention will now be described by way of example, with reference to the accompanying drawings:

Figure 1 is a schematic of a power supply module comprising a capacitor,

Figure 2 depicts a cross-sectional view of an embodiment of the capacitor in an inner housing 2,

Figure 3 illustrates a perspective view of an embodiment of the exterior of the inner housing,

Figure 4 provides a perspective view of an embodiment of multiple capacitor units 2 connected to an external busbar,

Figure 5 is a graph of the measured voltage over an IGBT during a trial of the power supply module.

A simplified schematic diagram of an embodiment of a subsea power source module is provided in Figure 1. In this schematic, container 1 is an outer housing which is exposed to the surrounding or ambient pressure at subsea level. Throughout the description the terms housing and container are used interchangeably.

The container or outer housing 1 can be constructed of aluminium or stainless steel. The volume of the outer container can be up to several cubic meters. The outer housing 1 or container may be filled with a fluid, such as an oil. The oil is preferably a synthetic ester such as Midel 7131 , silicone oil or mineral oil.

Inner container 2 is a pressure-resistant housing, the exterior of which is exposed to ambient pressures. The interior of the inner container 2 is isolated from the surrounding pressure. The inner container 2 houses a capacitor, which is connected to the source and drain of one or more insulated gate bipolar transistors (IGBTs) 7. The IGBTs together with the inverter control 6 form a chopper module 3. Preferably, more than one IGBT is used for each inverter control 6. There may be one or more inverter control 6 and there may be multiple chopper modules within the system. The IGBTs 7 and the inverter control 6 are arranged adjacent to the inner container 2 within the outer container 1. The IGBTs 7 and inverter control 6 are exposed to ambient pressures.

The main high voltage to low voltage transformer 4 is connected to a diode rectifier bridge 5. The chopper modules 3 are connected to filter inductors 8 before connection to a load 9. The load 9 may be a resistor arranged in series with an inductive component. Preferably, multiple capacitors are used, each of which is located within an inner container 2. Each capacitor can be connected to one or more external busbars to reduce the inductance. Multiple capacitors are preferred in high power inverter modules.

Figure 2 illustrates an embodiment of a capacitor 16 arranged in an inner pressure housing 2. The inner pressure housing 2 is positioned within the outer housing 1. The outer housing 1 may contain a fluid, such as an oil, and the inner housing 2 is sealed against the ingress of any external fluid. The capacitor 16 itself is positioned within a canister or inner housing 2, which is formed from a vessel 19 having a lid 18. The exterior of the housing 2 is exposed to ambient pressure during subsea operation (which can be in the range of 100-500 bar). The housing 2 is designed to isolate the capacitor from the surrounding pressure, thus maintaining the internal pressure within the housing at around 1 bar. By placing the capacitor 16 within the pressure-resistant inner housing 2, a standard capacitor 16 can be used. Preferably, the capacitor 16 is designed to have a very low inductance; with the capability of handling a rapid rate of change in the current flowing through the capacitor. Preferably, the capacitor is a DC-link capacitor. DC-link capacitors typically operate at voltages in the range of 300-400V. The DC-link capacitor is connected to the DC-link of a power converter. The purpose of the capacitor is to provide charge to the DC-link and to act as a power reserve during switching and dynamic load changes. Due to the higher currents which pass through DC-link capacitors, an increase in heat and hydrogen production can be seen in comparison with other types of capacitors.

The vessel 19 and the lid 18 may be formed of metal or any material which is capable of withstanding high differentials in pressure (in excess of 100 bar). The vessel is typically constructed out of aluminium or titanium. The vessel 19 and lid 18 may have to maintain an isolated pressure within the inner housing 2 when the external pressures acting upon the inner housing 2 are over 100bar. The inner housing 2 is designed to withstand pressure differentials (differences in pressure between the interior and exterior of the inner housing 2) of at least 10Obar, typically 200 - 500 bar, possibly as high as 700 bar. The vessel 19 and lid 18 may be formed from metal, the thickness of which should be selected according to the required pressure rating. The housing 2 may have a length in the range of 5cm - 50cm, preferably 10cm - 30cm, most preferably 20 cm. If the housing 2 is cylindrical, the diameter of the housing 2 may be in the range of 2.5cm to 30cm, preferably 5cm to 15cm, most preferably 10 cm. The diameter will be dependent on the capacitor size. The wall thickness of the housing 2 can typically be in the range of 5mm to 20 mm.

A conductor 10 extends through the lid 18 to one or more internal busbars 14. The conductor will provide an electrical connection to an external busbar (which is not illustrated in this figure). The conductor 10 is fitted within an insulator 1 1 , as shown in the illustrated embodiment. Preferably, the conductor 10 is selected to be as short as possible in order to reduce inductance. The conductor may form part of a penetrator which can be connected to the lid 18 by means of a fluid-tight seal. Further, in the illustrated example an optional thermal transfer pad 17 can be located at the base of the capacitor 16 and within the housing 19. Often, capacitors tend to generate heat during operation. A heat/ thermal transfer pad 17 can be utilised to improve the dissipation of heat from the capacitor.

The lid 18 is arranged to form a fluid-tight seal with the vessel 19, in order to form a housing 2 which is capable of withstanding pressures of over 100 bar. The fluid- resistant seal can be achieved by any suitable means. In the illustrated embodiment, a high-pressure O-ring seal 12 is utilised. Alternative means of providing a pressure resistant seal include the use of an X-ring, Q-ring, Quad Ring™, rings with square profiles, seals with knife edges, and flat gaskets.

The capacitor 16 may be kept in place within the container 2 by a flexible or elastic element 15. The elastic element 15 may be a ring or clamp or alternative fixing device formed from a flexible material. The flexible material may be a silicon, rubber (including nitrile rubber, and natural rubber), thermoplastic elastomer (including fluorocarbon elastomer, such as Viton ^® rubber), flexible PVC (polyvinyl chloride), or a fluorosilicone, to provide a few examples. An elastic element 15 can be used to assist with keeping the capacitor in a desired position within the vessel 19. The flexible elastic element 15 could compensate for slight changes in capacitor dimensions; allowing for thermal contraction/expansion. An elastic element 15 can be used to ensure that the capacitor 16 is kept in thermal contact with the vessel 19. By maintaining the application of a force onto the capacitor 16, the capacitor may be forced into contact with the base of the vessel 19. Optionally, a thermal transfer pad 17 or an alternative device for improving thermal transfer can be included in the vessel 19. An elastic element 15 may keep the capacitor in thermal contact with the thermal transfer pad 17 or thermal transfer means. Optionally, the elastic element can be situated between the capacitor 16 and the lid 18.

A further rigid clamping device 13 may be fitted within the housing. The clamping device may be a ring, clamp or fixing device formed from a material that may be less flexible than the material used for elastic element 15. Examples of materials which may be used for the clamping device 13 include a metal ring, ceramic ring, and a hard plastic ring. The clamping element may be positioned between the capacitor 16 and the lid 18.

The rigid clamping device 13 can assist with maintaining the internal spacing for the internal busbar 14. Preferably, the internal busbar 14 is selected such that the material has low inductance. There may be one or more internal busbars 14, or alternatively, the housing 2 can act as one of the electrical connections. Preferably there are two internal busbars. The internal busbars 14 can be arranged within the vessel 19 such that the internal busbars are in electrical contact with the capacitor 16. Preferably, each of the low inductance internal busbars 14 are formed from a flexible conductor. Preferably, each of the internal busbars 14 are bent towards each other to reduce the distance between the conductor 10 and the capacitor 16. Reducing the distance between the internal busbars can assist with the reduction of the inductance. The conductor 10 is an external electrical terminal. The busbars can be constructed from aluminium or copper, with a thickness of less than 5mm. Preferably, the thickness of the busbars is in the range of 1 -3 mm. The distance between the internal busbars can be 1-10 mm. If the separation distance between internal busbars is small, the busbars need to be electrically insulated. This can be achieved using sheets of insulating material or an insulating coating.

The exterior of the capacitor pressure housing 19 and the various connections protruding from the lid 18 according to a first embodiment are illustrated in Figure 3. The positive and negative electrical terminals 25 may be positioned on the exterior of the lid 18 for connection to the exterior busbar 20. The electrical terminals 25 are exposed to high (ambient) pressures. The external electrical terminal 25 is formed from a conductor.

Pressure and temperature monitoring can be conducted within the inner housing 2. The signals from the sensor as well as the power wires for the monitoring system can be connected to inner housing 2 via a penetrator 26, which can be attached to the lid 18 of the inner housing 2. When the penetrator 26 is connected to the lid 18, the seal between the penetrator 26 and the lid 18 is designed to be resistant to fluid ingress.

Figure 4 illustrates one embodiment of a plurality of inner housing 2 modules connected in parallel to a low inductance busbar 20 which may be positioned within a pressure-compensated oil-volume in an outer housing 1. The outer housing 1 is designed to be exposed to ambient pressures, such that the pressure inside the outer housing 1 is adjusted to be similar to the subsea pressure external to the outer housing 1. The busbar 20 has positive 21 and negative 22 connection points for electrical connection to the capacitor 16 (located inside the capacitor pressure housing 19). There may be a plurality of external busbars 20, each of which are connected to several capacitors. The number of capacitors connected to each busbar is preferably between 4 and 20, more preferably between 5 and 20. The number of busbars within each subsea module is preferably between 2 and 10, more preferably between 4 and 10. For example, there may be 14 capacitors connected to each busbar, and there may be 4 busbars within the module.

There are several types of capacitors which could be suitable for use in a power supply module. If electrolytic capacitors are used, these can release hydrogen during operation, which may accumulate in the capacitor. In addition, the capacitor may expand or contract depending on temperature conditions, leading to volume fluctuations inside the capacitor housing. Low pressure within the inner housing 2 can be maintained (i.e. overpressure can be avoided) by one of the following three options:

(a) venting when the inner housing 2 is in a low-pressure environment (e.g. topside),

(b) absorbing the hydrogen onto hydrogen absorbents positioned inside the inner container (2),

(c) providing a fluid outlet from each inner housing 2, which can be connected to a larger gas accumulating chamber which may be located inside the oil-filled volume/ outer housing 1 via a fluid connection. Figure 4 includes an optional pressure release valve 24 for venting excess gas or pressure when the container is in a low-pressure (e.g. around 1 bar) environment, for example when the power module has been retrieved from the subsea environment. The preferred approach for the avoidance of overpressure within the inner container/ housing 2 is to include hydrogen absorbents within the inner housing 2. The absorbents are selected such that they are capable of absorbing any hydrogen which is produced by the capacitor. The hydrogen absorbents may be palladium. The hydrogen absorbents can be in the form of metallic sheets covering the outside surface of the capacitor, for example.

The third optional solution is to provide a fluid connection to a separate container, or accumulator, to collect any gas that has been produced by the capacitor. This gas accumulator may be located inside the oil-filled volume/ housing 1 , and may be capable of isolating the interior of the accumulator from the external ambient subsea pressures.

Example 1

A power supply module was assembled comprising four half-bridge inverter modules with fourteen capacitors connected to a busbar in each of the four modules. In each half-bridge module, the busbar was connected to eight insulated gate bipolar transistors (IGBTs) which were exposed to the ambient pressure together with control electronics and inductive components. The power supply module was used in a towed subsea installation for an experimental trial. During the experiment, the inner housing was exposed to ambient pressures in the range of approximately 130 bar. The module was used for more than 300 hours, during which the total output current was 7000A. The capacitor solution has been tested down to 440 bar in laboratory tests. The other components in the system are also qualified for exposures to similar pressures.

A measure of the inductance was conducted in the laboratory, and the resulting graph is displayed in Figure 5. The voltage rating of the IGBT was 1200V. The maximum voltage experienced by the IGBT was 460V. The trial conducted in subsea conditions also demonstrated that the inductance was sufficiently low based on flawless switching during the entire trial.

During a controlled laboratory test, measurements were taken of the temperature within the inner container 2. The temperature inside the inner container 2 was measured every 10 minutes, at 35A RMS (root mean square) current at 50Hz in an oil bath of 50°C. It was found that the cooling of the capacitors was more than sufficient to operate the system at full power.

The present invention has been described with reference to some preferred embodiments and some drawings for the sake of understanding only and it should be clear to persons skilled in the art that the present invention includes all legitimate modifications within the scope of the appended claims.