Abstract:
A method of performing line insulation monitoring of a pair of conductor lines at least partially located in a cable is provided, comprising the steps of providing a first power switch in a first conductor line of the pair and a second power switch in a second conductor line of the pair, providing a line insulation monitor at a first end of the pair of conductor lines, electrically connected to the pair of conductor lines, at the second end of the pair of conductor lines, electrically connecting the first and second conductor lines, placing the first and second power switches into a monitoring configuration wherein the first power switch is closed while the second power switch is open, and using the line insulation monitor to monitor the insulation of the conductor lines.

Description:
BACKGROUND 
       [0001]    Embodiments of the present invention relate to a method of performing line insulation monitoring of a pair of conductor lines at least partially located in a cable, a power switching arrangement and a hydrocarbon extraction facility. 
         [0002]    The present embodiments relate to an AC power switching system which is primarily intended for deployment within an underwater (e.g. subsea) AC power distribution network (at a power switching node or similar equipment) for an underwater hydrocarbon extraction facility, to switch AC power (under topside control) to down-stream equipments such as a subsea control module (SCM), or similar equipment, comprising a subsea electronics module (SEM) or similar equipment. It should be noted that the terms SEM and SCM are well-known and standard terms in the subsea hydrocarbon extraction industry. 
         [0003]      FIG. 1  schematically shows a general power distribution network topology for an underwater hydrocarbon extraction facility, in this case a subsea oil/gas facility. The system comprises topside components  1  which would typically be located on the surface, for example on land or on a platform, and subsea components  2  located at the sea floor. The topside components include, as shown, an AC power source and line insulation monitor (LIM)  3 . The subsea components comprise, in this case, a power and communications subsea distribution module including a SEM  4  and distribution means for supplying AC power to a number of separate SEMs  5  via subsea jumper umbilicals  6 . AC power switches  8  are provided in each branch of L 1  and L 2 , these are operable in pairs, so that in a closed configuration, current may flow through both L 1  and L 2 , or in an open configuration, where current flow through each of L 1  and L 2  is prevented. The topside components  1  and subsea components  2  are linked by a topside “umbilical” cable  7 , which typically carries many different lines between the surface and subsea, including various electrical and hydraulic lines as is well-known in the art. Of particular interest for the purposes of embodiments of the present invention is an electrical power conductor pair carried by the umbilical, comprising conductors L 1  and L 2 . 
         [0004]    In more details, the key elements of the power distribution network shown in  FIG. 1  are described below. 
         [0005]    A topside equipment  1  comprises (as a minimum) the following system components that interface directly or indirectly with the topside to subsea umbilical. 
         [0006]    A topside AC power switch or Circuit breaker (CB) is an AC power switch used to switch power to the subsea power network. The switch/circuit breaker can be used to power off the power distribution network in response to a user command, or if a fault condition is detected. 
         [0007]    The power switch/CB operation is not necessarily solid-state or AC power phase controlled or synchronised, and as such the AC power waveform (as delivered to the subsea equipment) can be interrupted at any point in the AC power cycle. 
         [0008]    A step up transformer  26  acts to boost ‘mains’ voltages (115V or 230V/240V) to levels more compatible with subsea AC power transmission (300V to 600V AC). 
         [0009]    The transformer also provides galvanic isolation between the topside AC power source (and associated equipment) and the subsea power distribution network. 
         [0010]    A line insulation monitor (LIM) equipment is used to detect breakdown of the insulation between the power conductor pair L 1 , L 2  and earth. This detection is performed by the LIM measuring the leakage current to chassis earth when a DC bias is applied to the umbilical conductors L 1  and L 2  with respect to chassis earth, as is generally known in the art. 
         [0011]    An up stream topside umbilical  7  is a composite construction subsea umbilical cable, typically combining both hydraulic and electrical services, including power conductor pairs for distribution of topside AC power to the subsea control system. 
         [0012]    A subsea power distribution hub switches AC power delivered from the topside umbilical jumper to one or more down-stream power distribution networks. AC power switching is performed under the control of the topside equipment and is supplemented with protection features to interrupt power delivery if a fault condition (overload condition) is detected. 
         [0013]    A down-stream subsea in-field umbilical or subsea jumper  6  may comprise either a composite construction subsea umbilical (typically combining both hydraulic and electrical services including power conductor pairs) or a simple electrical jumper for distribution of switched hub AC power to the subsea control system. 
         [0014]    The subsea down-stream load (e.g. an SEM  5  or SCM or similar equipment) comprises, as a minimum, the following system components that interface directly or indirectly with the switching hub umbilical (or jumper): 
         [0015]    A transformer to ‘step-down’ the AC power transmission voltage to a level more compatible with AC to DC power conversion. The transformer also provides galvanic isolation between the umbilical/jumper AC power jumper (AC power source) and the subsea power distribution network from the subsea electrical equipment (e.g. an SEM or SCM) internal electronics. L 1  and L 2  are connected to a winding of this transformer. 
         [0016]    An AC to DC power convertor is usually a power-factor corrected converter which presents a constant power demand to the power distribution network as the input supply voltage varies. The converter displays true negative impedance characteristics: as input voltage increases, the input current decreases and vice versa. 
         [0017]    It has been found that it may be beneficial to be able to provide downstream umbilical isolation, and to enable LIM surveying prior to supplying AC power, something not possible with known systems as described above. 
         [0018]    It is an aim of the present invention to achieve these beneficial features. 
         [0019]    This aim is achieved through the use of independently operable power switches in each of L 1  and L 2 , and employing a controlled switching sequence prior to powering on. 
         [0020]    In accordance with a first aspect of the present invention there is provided a method of performing line insulation monitoring of a pair of conductor lines at least partially located in a cable, comprising the steps of: providing a first power switch in a first conductor line of the pair and a second power switch in a second conductor line of the pair, providing a line insulation monitor at a first end of the pair of conductor lines, electrically connected to the pair of conductor lines, at the second end of the pair of conductor lines, electrically connecting the first and second conductor lines, placing the first and second power switches into a monitoring configuration wherein the first power switch is closed while the second power switch is open, and using the line insulation monitor to monitor the insulation of the conductor lines. 
         [0021]    In accordance with a second aspect of the present invention there is provided a power switch arrangement for an underwater hydrocarbon extraction facility connected to a surface location by a pair of conductor lines at least partially located within an umbilical cable, comprising a first power switch located on a first conductor line of the pair and a second power switch located on a second conductor line of the pair, each power switch operable between open and closed configurations for respectively preventing or enabling electrical current flow therethrough, and control means for controlling the configuration of the each of the first and second switches, wherein the first and second power switches are independently operable. 
         [0022]    In accordance with a third aspect of the present invention there is provided a hydrocarbon extraction facility comprising a surface location and an underwater location, the surface location and underwater location being electrically connected by a pair of conductor lines at least partially located within an umbilical cable, comprising the power switch arrangement of the second aspect. 
         [0023]    Other aspects of the present invention are set out in the present disclosure. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]    Embodiments of the invention will now be described with reference to the accompanying drawings, in which: 
           [0025]      FIG. 1  schematically shows a known general power distribution network topology for an underwater hydrocarbon extraction facility; 
           [0026]      FIG. 2  schematically shows the topology of an AC power switch architecture in accordance with an embodiment of the present invention; 
           [0027]      FIG. 3A  and  FIG. 3B  schematically show a LIM surveying process in accordance with an embodiment of the present invention; 
           [0028]      FIG. 4  schematically shows a LIM surveying process in accordance with an alternative embodiment of the present invention; and 
           [0029]      FIG. 5  schematically shows a LIM surveying process in accordance with a further embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0030]    An embodiment of the present invention, employed within a subsea AC power distribution network topology generally similar to that of  FIG. 1 , will now be described, with the AC power switch architecture being outlined in  FIG. 2 . 
         [0031]    As in the known system of  FIG. 1 , power switches  101 ,  102  are respectively provided in each of conductors L 1  and L 2 . Each power switch  101 ,  102  comprises a solid state relay (SSR) element or equivalent. Activation of each switch is controlled by an AC power switch control  11 , which, via an integrated phase synchronised power switch control element  15 , provides independent control of the L 1  and L 2  switches  101  and  102 , providing control signals to the switches via respective isolators  12  and  13 . 
         [0032]    The power switch control  11  receives on/off controls, including power switch phase demands, from a processing core and topside communications link  14 , which is capable of receiving operating instructions from the surface. 
         [0033]    Both the switch control  11  and processing core  14  receive operational power from the surface. 
         [0034]    In alternative embodiments, switch operation is controlled by subsea control means, such as an SCM or SEM. Power could also be received by local subsea power storage or generation means. 
         [0035]    It should firstly be noted that within this architecture, each AC power switch&#39;s SSR elements (or equivalent) could be based on either thyristor, i.e. silicon controlled rectifier (SCR), or insulated gate bipolar transistor (IGBT) technology depending upon the application and the function performed by the SSR. These two alternatives are briefly discussed below: 
         [0036]    (i) SCR-based SSR 
         [0037]    A power switch design based on ‘back to back’ thyristors (SCR) configured as an SSR, where switch ON (conduction) is initiated by triggering the SCR device gate drive and switch OFF occurs via SCR natural commutation (i.e. where the load current falls below the SCR holding current) after the SCR gate drive has been removed/disabled. The SCR gate control can be implemented using pulse control via an isolation transformer, however if the SSR design is to provide clean transitions between positive and negative half-power conduction cycles then use of continuously energised SCR gate drives is preferable to provide good performance. 
         [0038]    (ii) IGBT-based SSR 
         [0039]    An SSR design based on ‘tack-to-back’ IGBT devices configured as an SSR, where switch ON (conduction) is initiated by biasing the IGBT device gate drive, and the IGBT must remain biased for the IGBT conduction to be maintained. IGBT switch OFF occurs when the IGBT gate bias is removed or reversed. No natural conduction commutation occurs in the IGBT, so inductive loads can present power-off transient problems if the IGBT is not suitably protected and controlled. The IGBT gate control requires a continuous drive to provide continuous conduction and clean transitions between positive and negative half power conduction cycles. 
         [0040]    The choice of SSR will also depend on the following factors:
       IGBT leakage current performance is superior to SCR leakage current performance when switched off;   The IGBT does not naturally commutate off when gate drive is removed, so power off transients (due to inductive loads) can present significant design issues;   The IGBT control characteristics enable a more responsive over-current protection feature to be implemented, as the IGBT switches off as soon as the gate drive is removed, so fault currents need not be supported until the power phase current naturally commutates to zero current; and   Back-to-back IGBTs have been employed and are very effective at switching 600V AC (to simulate relay and circuit breaker contact bounce) and produce negligible leakage when off.       
 
         [0045]    A further alternative is to use a combination approach, i.e. a power switch implemented using an IGBT and SCR SSR combination, to achieve the optimum power switch design characteristics. As an example, (assuming a series connected switch pair configuration) the L 1  power switch  101  element could be implemented using a SCR SSR and the L 2  power switch  102  element could be implemented using an IGBT SSR. The SCR SSR could be used for the ‘ultimate’ delivery of power to the load (last switch to be closed and first switch to be opened), and with the IGBT SSR used for the ‘making safe’ and isolation of the load (first switch to be closed and last switch to be opened). The SCR SSR could be employed for phase-controlled power delivery and the IGBT SSR employed to provide high integrity load isolation. A combination of SSR technologies may provide improved performance compared with that offered by a single technology. Furthermore, common cause failure mechanisms may be reduced if different SSR technologies are employed. 
         [0046]    Returning to  FIG. 2 , various other features are shown, which, while generally present in a working switch topology, are not of direct consequence to embodiments of the present invention. These include input and output voltage monitoring potentiometer networks  16 ,  17 , operatively connected to, respectively, a phase control zero crossing detector &amp; telemetry element  18  and telemetry element  19  both integrated into the control  11 , via isolation amplifiers  20 ,  21 ; a current sensing element  22  located on L 2 , which provides monitoring signals to an over-current detector, over current trip &amp; telemetry element  23  integrated within control  11 , via an isolation amplifier  24 ; and a temperature sensing element  25  integrated within control  11 . 
         [0047]    Using these features, the control  11  can provide housekeeping telemetry and over-current trip status telemetry to the processing core  14 . 
         [0048]    A method of providing downstream umbilical isolation and LIM surveying prior to power on will now be described, with reference to  FIG. 3 , where as far as possible the numbering system used for  FIG. 1  has been retained for like components. This schematically shows a simplified topology in two different switching states. In the top figure,  FIG. 3A , both switches  101  and  102  are open, blocking current therethrough, while in the lower figure,  FIG. 3B , showing the switches in a monitoring configuration, one switch  102  has been closed independently of switch  101 , allowing current in L 2  to flow through. In both  FIGS. 3A and 3B , for clarity the only topside components shown are the topside AC power transformer  26  and LIM  27 . Subsea, at the distribution module, the conductor pair branches into one additional isolated conductor pair, L 3 , L 4 , including respective switches  103 ,  104 . Pairs L 1 , L 2  and L 3 , L 4  pass via respective subsea jumper umbilicals to respective SEMs, of which only an input transformer  28  is shown. 
         [0049]    As previously described, switches  101 ,  102  are provided in the L 1  and L 2  power lines to facilitate isolation of the downstream power conductors. Initially, all switches are kept open, as shown in  FIG. 3A . To perform a LIM survey of the downstream umbilical, i.e. the subsea jumper umbilical (power pair), only one of the two in-line power switch elements is switched on prior to full ‘power on’. In  FIG. 3B , this is switch  102 , but switch  101  could equally be used. With one of the series switch elements  101 ,  102  closed, the topside LIM  27  can “see through” the subsea power switch to facilitate line insulation monitoring of the downstream umbilical prior to application of AC power. In other words, closing of one switch creates an unbroken conductor line between the topside, down the topside umbilical (L 2 ), through the closed switch  102 , through the downside umbilical (L 2 ), through a winding of subsea transformer  28  and back up the other conductor (L 1 ) as far as open switch  101 . It should be noted that only one switch need be closed in this way, it is unnecessary to then open switch  102  and close switch  101  (or vice versa). With one switch closed, LIM surveying can be performed. 
         [0050]    Following testing of L 1  and L 2 , conductor pair L 3 , L 4  may then be surveyed in a generally similar manner, with switches  103 ,  104  being independently operable as for  101  and  102 . 
         [0051]    For this step, all four switches  101 ,  102 ,  103  and  104  are held open to prevent current flow therethrough, and then one of switches  103  or  104  is independently closed to enable a LIM survey of conductor pair L 3 , L 4  to be performed. 
         [0052]    It should be noted that additional power branches may also be provided, in which case each power branch could be individually LIM surveyed in this manner. 
         [0053]    Once the or each power branch has been LIM surveyed in this way, the system may be powered on. 
         [0054]    Those skilled in the art may recognize that SCR switch element leakage may make this feature/methodology redundant or impractical if the topside LIM can “see through” the SCR AC power switching elements be they configured ON or OFF. However the dual switch power control methodology described above will be required if dual series connected in-line IGBT based SSRs are employed. Any SCR-based SSR will exhibit a degree of leakage which will facilitate “through-sensing” to some extent. However, if both of the switches include IGBT &amp; SCR elements then line isolation can be assured. 
         [0055]    An IGBT (or similar)-based power switch could be considered as an alternative to the SCR-based power switching module design described above or perhaps a supplementary element (extra in-line switching element) just to provide the required isolation. The IGBT SSR in-line switching element would prevent SCR SSR power switch leakage but would require more attention to the switch gate drive (if used as the primary power switching element rather than an SCR based design) particularly when the device was switched off (as the IGBT would switch off as soon as the gate drive was removed as compared with the SCR which will naturally commutate off, as the thyristor current reduces to near zero value, after the thyristor gate drive is removed). 
         [0056]    Alternatively, IGBT and SCR-based combination series connected SSRs could be employed, assuming that is that the voltage drop and power dissipation in each of the series connected switching elements could be tolerated. SCR-based SSRs could be used for power control requiring natural commutation and IGBT-based SSRs for line isolation. 
         [0057]    In accordance with embodiments of the present invention therefore, the integrity of the downstream umbilical power conductor insulation (both L 1  and L 2  conductors with respect to chassis earth) can be appraised using the topside LIM function prior to powering on the downstream umbilical (the powering on being achieved by closing both of the L 1  and L 2  power switches). This appraisal is achieved by closing either one of the AC power switch elements (i.e. connecting one of L 1  or L 2  but not both together). By closing a single AC power switch element, the downstream umbilical is not subjected to the application of an AC voltage (applied differentially between L 1  and L 2 ), but does enable the topside LIM DC bias voltage to be applied to both the L 1  and L 2  downstream power conductors (as the topside LIM DC bias ‘sees through’ both the closed AC power switch and the downstream load input transformer). 
         [0058]    Thus by providing independent control of the L 1  and L 2  power switches it is possible to assess the integrity of the insulation, using the topside LIM  27 , of both the downstream umbilical and the load input power stage, before applying power differentially to the L 1  and L 2  conductors. 
         [0059]    Furthermore, should an insulation breakdown condition develop to the extent that the topside LIM  27  is tripped, the independent control of the L 1  and L 2  power switches enables the user, under topside command, to investigate and isolate any umbilical elements downstream of the AC power switches (if that is where the insulation breakdown fault has developed, as opposed to an insulation breakdown of the main umbilical). 
       Alternative Embodiment for DC Configurations 
       [0060]    While the above-described embodiment is suitable for AC power distribution networks, it will be appreciated by those skilled in the art that embodiments of the present invention could equally be used to provide LIM surveying in DC networks. The apparatus used would be substantially similar to that previously described, with the main difference that no transformer (e.g.  26  in  FIG. 3 ) would be provided. There would also be no transformer provided in the power delivery train subsea for a DC system. An example of this type of system is shown in  FIG. 4 , where components from  FIG. 3A  retain their reference numerals as appropriate. As the system receives DC power, the topside transformer is replaced with a simple electrical connector  29 . The subsea transformers have been removed, and the isolated conductor pairs connect directly to the subsea loads  30 ,  31 . 
         [0061]    Depending on the nature of the DC input power conversion stage(s), it might be necessary to perform two LIM surveys, in case the DC input does not conduct sufficient DC for the LIM loop back measurement technique to work. In this case, after the initial L 1  and L 2  OFF (open) step, one of these switches could be independently closed and a LIM survey performed, followed by a reversal of the switching states for the two switches, and a LIM survey performed again, e.g.: L 1  ON L 2  OFF followed by L 1  OFF L 2  ON. 
       Alternative Embodiment for Subsea LIM 
       [0062]    With some network arrangements, it is not possible to use, at least exclusively, a topside LIM for insulation monitoring of the power network. For example, the network may include an isolation transformer, located subsea. In that case, a topside LIM would not be able to “see past” the isolation transformer to monitor the insulation down-stream of that transformer. In such cases, it is possible to use a subsea-based LIM to effect monitoring, with the output from the subsea LIM being passed upstream for processing in a conventional manner. The methodology of embodiments of the present invention is equally applicable when using such subsea-LIMs, whether AC or DC is used. An example of this type of system is shown in  FIG. 5 , where components from  FIG. 3A  retain their reference numerals as appropriate. An isolation transformer  32  is located in the network at the subsea end of the umbilical power conductor pair. To enable insulation monitoring to take place, the LIM  27  is located subsea, and connected to the conductor pair downstream of the isolation transformer  32  via connection means  33 . 
         [0063]    It is to be understood that even though numerous characteristics and advantages of various embodiments have been set forth in the foregoing description, together with details of the structure and functions of various embodiments, this disclosure is illustrative only, and changes may be made in detail, especially in matters of structure and arrangement of parts within the principles of the embodiments to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed. It will be appreciated by those skilled in the art that the teachings disclosed herein can be applied to other systems without departing from the scope and spirit of the application.