Subsea power grids have been developed for supplying power to devices that are located on the seabed. For example, a compressor located on the seabed may be used to compress gas from an offshore gas field, so that the compressed gas can be pumped through a pipeline to an onshore destination, and the compressor can be supplied with power from a subsea power grid.
Generally, a subsea power supply such as a variable speed drive (VSD) will receive power over a fixed frequency AC transmission line. The variable speed drive converts the voltage, current and frequency into a level suitable for operating the machinery located on the seabed. Variable-speed drives (also called adjustable speed drives) can be preferred since these allow an efficient use of power in varying load situations. For example, a variable speed drive can be used to drive a compressor according to a fluctuating gas inflow level.
A variable speed drive usually contains several power cells. A power cell generally consists of a rectifier (comprising a number of diodes), a DC-link capacitor or re-chargeable battery, and an inverter (comprising several insulated gate bipolar transistors or IGBTs). It may thus also be termed converter cell. The inverter, whose outputs connect to a load, may typically be realized using an H-bridge configuration, so that a voltage can be applied across the load in either direction. In the VSD the supply voltage or input voltage is transformed down to a voltage suitable for the semiconductors, i.e. the diodes and IGBTs. The voltage is rectified to a DC voltage by the diode rectifier, which in turn is connected to the DC-link capacitor. The inverter module converts the power cell's DC output into an AC output voltage (usually single phase AC) for the load. Several of these power cells can be connected in series to increase the total available output voltage (and thus output power) of the VSD. One series-connected ‘branch’ of power cells is generally used for each output phase. For example, in a three-phase system, three such branches of series-connected power cells could be deployed.
An AC motor may be supplied with variable frequency AC voltage from a VSD. Power to the variable speed drive is supplied from the AC power supply, as described above. The components of a subsea power cell arrangement must be contained to avoid any contact with the underwater environment, and must also be able to withstand the pressure at the point of installation, usually the seabed. For example, the pressure at a depth of 3000 m is about 3×107 Pa. Therefore, to avoid material damage, the pressure inside a container placed at that depth should equal the pressure exerted on the container.
In prior art approaches, the interior volume of a variable speed drive is filled with dielectric fluid to achieve a pressure-compensated system. This main or shared fluid volume fills any spaces between elements of the power cells arranged in a housing of the variable speed drive and can enter into the housings of the power cells mounted inside the drive housing. The fluid volume is pressure-compensated to a pressure very close to the outside sea water pressure so that the housing is not exposed to a pressure difference or differential pressure and thus is not exposed to high forces exerted on it from outside.
The power cells of existing subsea arrangements are housed in polymer containers. While such a polymer container is relatively easy to form, it has a number of disadvantages. The polymer material must be compatible with the dielectric fluid (for example an ester-based dielectric fluid) used to fill the interior, but suitable polymers may be expensive and/or difficult to obtain. For example, polyisocyanurate can be difficult to produce with the necessary material quality and is therefore expensive, while polyurethane is incompatible with a favoured ester-based dielectric fluid.
Furthermore, a power cell enclosure made of such a polymer material lacks material strength and may deform over time, thus increasing the likelihood that the power cell will become damaged and fail. This leads to the additional problem that particles from a damaged power cell can spread through the variable speed drive and can enter into other power cells arranged in the shared fluid volume, thus increasing the likelihood that these will also fail.
Another problem associated with a polymer power cell enclosure is that such an enclosure may not be able to withstand shock and vibration tests. An enclosure must fulfil certain minimum test requirements in order to ensure that the power cell does not suffer damage during transportation and installation at the subsea site.
A power cell of a subsea power supply arrangement can fail in many ways. For example the semiconductors, capacitors or electrical conductors can short circuit. The cause of many of these failures is very often contamination in the power cell environment. Contaminations can originate from other components and solid materials in the VSD, for instance if a polymer material is not compatible with the surrounding fluid. These contaminations can easily spread throughout the shared fluid volume and can reduce the expected lifetime of other power cells and/or components. The reason for this is because a VSD is usually realized to have a built-in redundancy, by providing a number of additional or extra power cells for each output phase of a multi-phase system. For example, if four power cells are required in each phase to obtain the rated output power, the VSD can be realized to include five, six or more power cells so that, if one cell should fail, it can be bypassed or disconnected from the circuit, and one of the ‘extra’ power cells can take over its function. However, in the known designs, contamination from a failed power cell can be transported by the main fluid volume of the VSD and can enter the interior fluid volume of a functional power cell, and can thereby lead to the failure of that power cell also.