For power transmission over distance it is usual to transmit power over transmission lines at high voltage to reduce power losses. Typically power is transferred using high voltage AC power transmission. Three phase AC transmission is the most common, although for supplying some remote areas fewer phases, such as a single phase supply may be used.
On land, outside of urban areas it is typical to transmit power using high voltage AC power distribution using overhead transmission lines. The various conducting lines are suspended so as to be sufficiently far above the ground and away from one another to provide electrical isolation, i.e. the surrounding air acts as an insulator.
In some instances however such overhead lines may not be practical or appropriate. For instance in urban environments it may be desired to transmit and distribute power using buried cables rather than overhead lines. For transmission of power across bodies of water it may also be better to use submarine power cables. This is becoming increasingly of interest with the growing popularity of offshore power generation, e.g. offshore wind farms and the like.
In a power distribution cable it is necessary to insulate the conductors that are used for power transfer from one another and also from the environment. FIG. 1 illustrates the general principles of such power cables and shows a cross-section of one example of a power cable 100 such as may be used, for example, for medium or high voltage AC power transmission, i.e. voltages of tens of possible hundreds of kilovolts.
FIG. 1 illustrates that the cable may have three conductors 101, one for each AC phase, each of which may, for example, be formed from copper or a similar material. The conductors are each sheathed by at least one semi-conductive layer 102 which is surrounded by a respective insulator 103 such as XLPE (cross-linked polyethylene). Each insulator 103 may be surrounded by one or more sheathing layers 104 and 105, which may, for example include at least one semi-conductive sheathing layer 104. For a high voltage cable, layer 105 could be a lead sheath for example whereas for a medium voltage application layer the layer 105 may be a copper screen. It will be understood by one skilled in the art there may be additional or alternative sheathing layers such as semi-conductive polyethylene, aluminium tape, a conductive counter-helix or the like and/or other layers such as swelling tape surrounding the insulator. The sheathing layers, i.e. 102, 104 and/or 105, are arranged to provide electric shielding to shield the other conductors and the environment from any electric field generated by the current flowing in the conductors in use.
The three conductors, with their associated sheathing and insulation layers, are all contained within an armour layer 106 which may for instance comprise a braiding of galvanised steel wires to provide protection for the power cable. The power cable may also have an outer jacket layer 107 such as a polypropylene yarn cladding.
There may be filler material 108 within the cable, which may comprise a plurality of elongate filler elements disposed inward of the armour layer 106. This can give the overall power cable a desired form and ensure the sheathed conductors are held in place within the power cable, as well as providing additional padding/protection.
Additionally it is common to embed at least one optical fibre within the cable, or at least provide the ability for optical fibres to be located within the power cable, for instance to allow for data communication between the various power stations linked by the cable. Thus there may be at least one fibre optic conduit 109 for carrying one or more optical fibres 110, and typically a bundle of optical fibres.
It will be appreciated of course that whilst FIG. 1 shows a single cable 100 comprising three conductors for the respective AC phases, it is possible for each AC phase to be supplied through a separate, single-phase, power cable, which therefore typically has the same general structure as that shown in FIG. 1 but with only one conductor and its associated sheathing and insulation layers (and correspondingly less filler). Such single-phase power cables may sometimes be bound together before deployment. It will also be appreciated that whilst high voltage AC remains the most common power transmission, high voltage DC transmission is also being increasing considered for certain applications, such as power transfer from offshore power generation. High voltage DC may also be transmitted over power cables similar to those illustrated in FIG. 1, with insulation designed to withstand the constant fields generated by the constant DC voltage. Such DC cables may have a single conductor (for an earth ground return) or two conductors, e.g. for a bipolar scheme.
As mentioned above such power cables may be used in various applications and one example is for power transmission from an offshore power generation site, such as a wind farm, as illustrated in FIG. 2. FIG. 2 illustrates that a first power station 201, which in this example may be located on an off shore platform, may be connected to a local source of power, such as a plurality of wind turbines 202. The first power station 201 may receive electrical power from the wind turbines and in some instances transform the voltage to a high voltage for transmission to an on-shore power station 203 via power cable 100. Power cable 100 may be deployed to run along the sea-bed to shore. In examples such as this wind farm example the first power station 201 may be several kilometres or several tens of kilometres from shore.
In use the conductors of the power cable 100 may carry very high voltages, of the order of hundreds of kilovolts for example. Thus good quality insulation is required. In some instances defects resulting from the manufacturing processes and/or degradation of the properties of the insulation over time can result in the insulator failing with a resultant current discharge.
In some instances the insulation may fail such that there is a discharge between conductors of the cable, or between a conductor and the environment, i.e. earth. This can result in a high voltage discharge with a significant fault current, e.g. arcing. Typically such a fault may result in a catastrophic failure of the relevant part of the cable, with potentially explosive failure of the cable. FIG. 2 illustrates a cable insulation failure and catastrophic fault at location 204. This will generally result in power transmission through the cable having to be stopped until the cable can be repaired, for instance by opening one or more high voltage circuit breakers 205.
Power cables of the type described above can be very costly and thus simply replacing the entire cable or providing redundant power cables that can be used in the event of failure of one cable may not be practical. Thus, in the event of a fault, it is typically necessary to repair the cable by removing the damaged section and splicing in a new section of cable. This requires locating the damaged section. However detecting which part of a say 30 km length of subsea cable is damaged is not a trivial endeavour.
For example to undertake visual inspection would require raising progressive sections of cable to the surface for inspection and possible repair, e.g. starting from shore, until the damaged section 204 was found or employing divers or a submersible vehicle to scan the length of the cable until the fault is found. This may take significant time and, as mentioned above, power transmission may be halted until the cable is repaired. Usually it is desired to have power outage for as short a time as possible. Often the cable may be buried 1 or 2 m below the sediment on the sea floor, therefore difficult to examine.
In some instances, prior to a complete failure of the insulation the cable may experience partial discharge. Partial discharge, as will be well understood by one skilled in the art of power cables, is a discharge, such as a gas discharge that does not complete a bridge between the conductors. For power cable insulation partial discharge may occur in cavities or other defects in the insulation. Thus partial discharge occurs, and may regularly occur, at the location of a fault in the power cable. A fault that leads to partial discharge is (not yet) catastrophic, but it is understood that some faults that initially lead to partial discharge can worsen over time and eventually lead to catastrophic failure.
Detection of partial discharges may therefore be an indication that there is a fault with the insulation of a power cable and/or the insulation is in the process of failing. Monitoring for partial discharge is thus sometimes used to determine the health of a power cable system and identify any points of possible failure. A section of power cable where significant partial discharge is occurring could therefore be replaced as part of a planned maintenance before it fails. Planned maintenance is far less disruptive than catastrophic failure and may allow a smaller section of the power cable to be replaced.
Partial discharge can be identified by looking for high frequency effects in the electrical properties of the cable when at voltage. However locating where the partial discharge is occurring is more difficult. One method is based on Time Domain Reflectometry (TDR) of pulses in the cable conductor. However the TDR techniques can't usually be performed whilst the cable is being used for transfer of power and such techniques typically only work on relatively short lengths of cable and the presence of multiple partial discharge sites can confuse the measurements as the various pulses from the partial discharge can superimpose.
Partial discharge can be detected by the use of an external inductively coupled sensor but typically it is not economical to permanently embed such sensors with a power cable. Thus upon manufacture a power cable may be subject to a factory partial discharge test to identify any manufacturing defects prior to installation but detecting the location of partial discharge that develops in a cable that has been deployed remains challenging.
There is therefore a desire to be able to rapidly identify the location of a fault resulting in a current discharge along a power cable, whether a fault leading to partial discharge or a catastrophic fault.
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