Patent Document

BACKGROUND 
       [0001]    The subject matter of this disclosure relates generally to subsea control systems, and more particularly to a distributed type direct current (DC) energy storage system that can be easily integrated with a modular stacked DC (MSDC) topology for subsea applications. 
         [0002]    Modular stacked DC converter architectures are well suited for subsea applications requiring transmission and distribution over long distances. Unlike other DC transmission options, wherein the DC transmission (link) voltage is controlled, i.e. maintained nearly constant, the DC transmission (link) current is controlled in the relevant modular stacked DC converter design. One MSDC architecture  10  is depicted in  FIG. 1 . The MSDC architecture gets its name from the fact that the architecture uses several modular DC-DC/AC converter modules stacked and connected in series on the DC side, both at the sending end and at the receiving end of the transmission link such as depicted in  FIG. 1 . The converter modules at the receiving (subsea) end can also be arranged in a distributed way, each module enclosed in a pressure vessel of its own, rather than really stacking them within one vessel. 
         [0003]    All subsea installations require control systems. Subsea control systems may consist of dozens or hundreds of low power consumers, e.g. electrically driven sensors for the physical displacements of valves. Transmitting power for subsea control systems over long distances is challenging because these loads typically require a constant subsea busbar voltage. Constant busbar voltages are known to be difficult to achieve when the loads are supplied by a long transmission cable, e.g. more than 100 km cable length, and some of the loads are intermittent (i.e. actuators for opening or closing valves). Maintaining a constant output voltage on the receiving end despite fluctuating load levels requires a feedback control of the system voltage by power electronics. DC power transmission requires a subsea inverter, e.g. an inverter based on MSDC technology. An MSDC inverter, in addition to converting DC to AC, may keep a subsea DC-link voltage constant by way of boosting the voltage at the end of the transmission line. Because of the intermittent operation requirements described herein, the power consumption of subsea control systems is typically characterized by a continuous rating, e.g. 30 kW for large systems, and an additional short-time power rating that may be, for example, 3-4 times higher than the continuous power rating, e.g. 100 kW for 60 seconds. 
         [0004]    Long distance DC transmission cables must have a sufficiently large cross section to maintain the voltage drop along the cable with acceptable limits. If the voltage arriving at the end of the cable is too low, it cannot be boosted up to the constant busbar voltage required for the subsea loads. Generally, techniques for avoiding subsea cables with a large cable cross-section to achieve a constant busbar voltage when supplying high, short-time power, e.g. 100 kW, employ a centralized subsea energy storage system, e.g. rated to supply 100 kW for 60 seconds (=6 MJ). Centralized subsea energy storage systems are disadvantageous in that they require a significant subsea volume/large and heavy subsea containment. Centralized subsea energy storage systems are further disadvantageous in that a defect in a single storage element may adversely impact the entire energy storage capability. Centralized subsea energy storage systems are further disadvantageous in that significant control scheme changes may be required to implement the requisite energy storage capability. 
         [0005]    In view of the foregoing, there is a need to add an energy storage system to a subsea system in which the energy storage system overcomes the herein described disadvantages of centralized subsea energy storage systems. 
       BRIEF DESCRIPTION 
       [0006]    An exemplary embodiment of the present invention comprises a power distribution system comprising: 
         [0007]    a power source side; 
         [0008]    a load side; and 
         [0009]    a plurality of power converters on each of the power source side and the load side, wherein the power source side converters and the load side converters are each configured to provide a modular stacked dc converter architecture, and further wherein the load side converters are each configured with a respective energy storage system such that together the plurality of energy storage systems deliver energy to a common control system busbar. 
         [0010]    According to another embodiment, a power distribution system comprises a plurality of load side power converters configured in a modular stacked DC (MSDC) converter architecture, wherein the load side power converters each comprise a respective energy storage device such that together the plurality of energy storage devices provides a distributed energy storage system configured to maintain a common control system busbar voltage substantially constant during intermittent load voltage excursions. 
     
    
     
       DRAWINGS 
         [0011]    The foregoing and other features, aspects and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which like characters represent like parts throughout the drawings, wherein: 
           [0012]      FIG. 1  is a simplified diagram illustrating a subsea power transmission/distribution system with modular stacked power converter building blocks configured with distributed energy storage elements on the subsea side of the system according to one embodiment of the invention; 
           [0013]      FIGS. 2-5  illustrate operation of the subsea power transmission/distribution system depicted in  FIG. 1 ; 
           [0014]      FIGS. 6 and 7  illustrate the reaction of the AC-bus voltage and output current of a single converter for an applied load step from ˜38 kW to 100 kW for the subsea power transmission/distribution system depicted in  FIG. 1 ; 
           [0015]      FIGS. 8-11  illustrate a load profile specification that provides 100 kW peak operation for a time period of 60 s for one embodiment of the subsea power transmission/distribution system depicted in  FIG. 1 ; 
           [0016]      FIG. 12  illustrates in more detail, a MSDC converter configured with distributed storage elements according to one embodiment that is suitable to implement the modular stacked power converter building blocks configured with distributed energy storage elements on the subsea side of the system depicted in  FIG. 1 . 
       
    
    
       [0017]    While the above-identified drawing figures set forth alternative embodiments, other embodiments of the present invention are also contemplated, as noted in the discussion. In all cases, this disclosure presents illustrated embodiments of the present invention by way of representation and not limitation. Numerous other modifications and embodiments can be devised by those skilled in the art which fall within the scope and spirit of the principles of this invention. 
       DETAILED DESCRIPTION 
       [0018]    Subsea cables or umbilicals are by far the most expensive components in long distance transmission systems, e.g. for distances larger than 100 km. The embodiments described herein with reference to the Figures are directed to subsea energy storage in combination with long distance power transmission in a topology that alleviates the necessity for subsea cables with an excessively large cable cross-section to achieve a constant bus bar voltage when supplying high, short-time subsea control system power. 
         [0019]      FIG. 1  is a simplified diagram illustrating a subsea power transmission/distribution system  10  with a plurality of modular stacked power converter building blocks  12 , each load side converter configured with one or more distributed energy storage elements  14  on the load side of the system according to one embodiment of the invention. The subsea power transmission/distribution system  10  provides one option for fulfilling the peak power requirement. The distributed storage topology depicted in power transmission/distribution system  10  advantageously provides reliability benefits compared to centralized bulk storage solutions, because a defect in a single storage element  14  will not impact the remaining system storage capabilities. Further, there are no significant modifications required when using a MSDC control scheme due to the simplicity of the storage control scheme. 
         [0020]    With continued reference to  FIG. 1 , each load side DC-AC inverter  12  employed by power transmission/distribution system  10  comprises one or more distributed storage offshore (DSO) elements  14  integrated therein. The plurality of DC-AC inverters  12  and respective energy storage elements  14  are distributed in an offshore facility  13  such as a watercraft or a topside platform that may be fixed or floating according to different aspects of the embodiments described herein. Each DSO element  14  may comprise, without limitation, one or more capacitors such as ultracapacitors or energy storage cells such as rechargeable batteries. An ultracapacitor as used herein means a capacitor that has much greater energy density and power per pound than electrostatic and electrolytic capacitors. Ultracapacitors are also called “supercapacitors.” According to another aspect, the plurality of DC-AC inverters  12  and respective energy storage elements  14  are distributed subsea in close proximity to the subsea loads to form a subsea electric power distribution system. 
         [0021]      FIGS. 2-5  illustrate simulated operation of the subsea power transmission/distribution system  10  depicted in  FIG. 1 , including distributed storage capabilities implemented in the load side converters  12 , according to one embodiment. With reference now to  FIG. 2 , a load increase  16  after t=2 seconds cannot be covered by the transmission capability of the cable  18  and results in a discharge of corresponding link capacitors/DSO elements  14  such as illustrated in  FIG. 3 , thus providing the requisite power to the subsea loads. The peak power is required for only 1 sec, followed immediately by a charge period of the distributed storage which is completed at t=7 sec as depicted in  FIG. 3 . 
         [0022]      FIG. 4  illustrates the onshore transmitted and subsea load DC voltage levels during the same time period depicted in  FIGS. 2 and 3 . The voltage level on the receiving end of the cable (subsea) is almost constant between 3 s≦t≦7 s indicating a constant, but increased (as compared to t&gt;8 s, normal load in steady state) power transmission during that time (as power is proportional to voltage for constant current operation). This additional power transmitted from the shore, used for charging the distributed storage elements  14  can also be determined as the difference between the received power  18  from the transmission system and the power  16  consumed by the subsea loads for 3 s≦t≦7 s ( FIG. 2 ), which is about 10 kW. 
         [0023]    It can be appreciated the minimum voltage level for the storage is not a fixed value since it depends upon the power demand subsequent to the peak period. The maximum power which can be received by the converters  12  is defined by P rec =V sub ·I Ring , where V sub  is the subsea voltage and is linked to the DC link voltage by the duty cycle occurring during the energy storage operation at its limits. The maximum subsea voltage V sub  is therefore equal to the sum of the nominal DC link voltages of the converters  12  as exemplified herein according to one embodiment. 
         [0024]    If for example, the DC link voltage of the distributed storage is discharged to 500V per module  12 , and the ring current such as depicted in  FIG. 5  is 10 A, the maximum power to be transmitted post fault with respect to five operational modules  12  is 5·500V·10 A=25 kW. The converter DC link voltage recovers, and accepts higher power levels to be transmitted from the shore. 
         [0025]      FIGS. 6 and 7  illustrate the reaction of the AC-bus voltage and output current of a single converter  12  for an applied load step from ˜38 kW to 100 kW for the subsea power transmission/distribution system  10  depicted in  FIG. 1 . The voltage level depicted in the center plots of  FIGS. 6 and 7  at the distribution bus is decreased during the high power period  30  because the output voltage of the converters  12  was not controlled during the simulation, power factor was kept to unity, although it can be appreciated the output voltage of the converters  12  would be controlled in a real system. The current levels depicted in the bottom plots of  FIGS. 6 and 7  correspond to a single converter  12 . A voltage spike  32  can be observed in the center plot of  FIG. 7  during the power sag from peak power to nominal power due to the very fast current change in corresponding line and transformer inductors. An appropriate MOV device, for example, could protect the connected loads by limiting the over-voltage to acceptable values. 
         [0026]      FIGS. 8-11  illustrate a load profile specification that provides 100 kW peak operation for a time period of 60 s for one embodiment of the subsea power transmission/distribution system  10  depicted in  FIG. 1 . Although the subsea power transmission/distribution system  10  can survive the 100 kW peak period, it will not however be able to continue operation for an infinite amount of time at the low load level (˜38 kW), as the maximum load to be fed with the post peak period DC link voltage of ˜600V is at most 5·600V·10 A=30 kW, which is below the requested power demand.  FIG. 8  illustrates the DC link voltage is still decreasing after the peak period in which the storage is still in discharge operation. 
         [0027]    Two potential solutions can be realized to prevent power outages subsequent to significant utilization of the energy storage with given limitations. One embodiment comprises increasing the transmission current reference to increase the maximum transferable power by increasing the onshore voltage/nominal voltage limit. Another embodiment comprises reconfiguring a standard converter topology to provide a converter structure such as illustrated in  FIG. 12  that illustrates in more detail a power converter  40  configured with distributed storage elements  14 . Converter  40  is suitable to implement the modular stacked power converter building blocks configured with distributed energy storage elements on the load side of the system  10  depicted in  FIG. 1 . More specifically, converter  40  utilizes one leg from a DC/DC stage  42  as a bidirectional buck-boost converter that decouples the storage State of Charge (SoC) from a DC link voltage  44 . 
         [0028]    The required energy for the peak load period under the assumption of a maximum transferable power Ptrans=40 kW can be calculated as E storage =(P peak −P trans )·60 s=60 kW·60 s=3.6 MJ, which would only be sufficient with a structure fully decoupling the storage voltage level from the converter DC link voltage  44 , as depicted in  FIG. 12 . The effectively transferrable power is dependent upon the DC link voltage; a storage coupled directly to the DC link voltage would require a higher capacity. According to one embodiment, discharging the storage to 50% of the nominal voltage results in a 75% usage of the storage SoC (E mod =(1/2)CU 2 . According to one embodiment based on the 3.6 MJ energy demand, and using predetermined commercially available ultracaps with predetermined commercially available modules, the energy per module can be determined as: 
         [0000]      Energy per module( E   mod )=(1/2) CU   2 =(1/2)(63) F· 125V 2 =0.49 MJ. 
         [0029]    In summary explanation, embodiments of a distributed type direct current (DC) energy storage system that can be easily integrated with a modular stacked DC (MSDC) topology for subsea applications have been described herein. The embodied energy storage in combination with long distance power transmission results in a topology that alleviates the necessity for subsea cables with an excessively large cable cross-section to achieve a constant bus bar voltage when supplying high, short-time subsea control system power. The distributed storage embodiments described herein provide advantages compared to a centralized storage in terms of controllability and reliability. It can be appreciated that particular distributed storage embodiments formulated according to the principles described herein may require a rating of converter modules that is equal to the specified maximum short-time power, divided by the number of converter modules configured in a series topology. 
         [0030]    While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.

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