Abstract:
A load bypass switch enables continuous power to remote loads in the event of 1) failure of one or more remote loads, or 2) faults within the remote loads, within a dc power system. The bypass switch utilizes the passive components of the dc loads or inverters and therefore reduces overall component count. A black start method for the remote dc system uses the same passives present inside the loads/inverters and simultaneously uses some of the features of the bypass switch. A bypass-module-yard uses multiple bypass switches enabling continuous power to the remote loads in the event of failure of one or more power distribution cables (in-feed to the remote loads) located remotely in the dc system.

Description:
BACKGROUND 
       [0001]    This subject matter of this disclosure relates generally to control systems, and more particularly to a control system and method of control that can be easily integrated with a modular stacked DC (MSDC) topology for sub-sea applications. 
         [0002]    Modular stacked DC converter architectures are well suited for sub-sea 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 a modular stacked DC converter. The MSDC architecture gets its name from the fact that the architecture uses several DC-DC/AC-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. 
         [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. Direct current cables are the most economic choice for long distance power transmission because DC power transmission and distribution can fundamentally overcome the cable capacitance and reactive power issue associated with AC power delivery. 
         [0004]    Direct current 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 busbar voltage constant by way of boosting the voltage at the end of the transmission line. 
         [0005]    The loads at the remote subsea location of a subsea power transmission and distribution (T/D) system  10  that employs a MSDC architecture are connected in series  11  on the distribution side  12 , such as illustrated in  FIG. 1 . Such a topology is valid not only for a MSDC system, but for any system where the transmission line current  14  is controlled to be stiff, such as, for example, a classic line commutated HVDC system. 
         [0006]    A load bypass switch  16 , such as shown in  FIG. 1  may be required for each remote load and/or variable frequency drive (VFD)  18 . Each load bypass switch  16  is connected in parallel to a respective remote load  18 . The bypass switches  16  provide a bypass path to the transmission line current  14  in the event of open-circuit fault VFDs or loads  18  to ensure point-to-point power flow is maintained. 
         [0007]    Bypass switches  16  ensure that continuous point-to-point power flow is maintained. Known systems and methods generally provide switching operations at best within a few milliseconds. Fast operation of the bypass switches  16  is desirable to ensure reliable protection against open-circuit fault transients. 
         [0008]    In view of the foregoing, there is a need to provide a control system and method of control that can bypass a transmission current within a few microseconds. The system and method of control should be applicable to any current source based DC T&amp;D architecture. The system and method of control should, for example, be capable of being easily integrated with a modular stacked DC (MSDC) topology for sub-sea applications. 
       BRIEF DESCRIPTION 
       [0009]    An exemplary embodiment of the disclosure is directed to a remote module bypass system. The exemplary embodiment further comprises a plurality of remote modules connected in series and receiving DC current in response to a DC transmission line current. A plurality of load bypass switches is configured such that each bypass switch is connected in parallel with a distinct and respective remote module selected from the plurality of remote modules and further such that each module is associated with a distinct and respective bypass switch. Each bypass switch provides a bypass path to a corresponding remote module DC current during an open-circuit load fault associated with the respective remote module. Each bypass switch comprises a coupled DC-choke and a thyristor over-voltage protection circuit integrated within and connected to the coupled DC-choke. 
         [0010]    Another embodiment is directed to a remote module bypass switch comprising a DC-choke coupling a DC source current to one or more remote modules. A thyristor over-voltage protection circuit is integrated within and connected to the coupled DC-choke such that the coupled DC-choke and thyristor over-voltage protection circuit form a bypass current path to a remote module subsequent to a remote module open circuit fault. 
         [0011]    Operation of the bypass switch for open circuit faults described above is just one example of protection using a bypass switch due to one type of fault which is open-circuit. However, the bypass switch can also be used in case of other faults that prevent point-to-point power delivery to other series connected loads. For example—a trip of the motor circuit breaker, or a compressor fault. In addition to fault handling, the bypass switch can also be activated from the control in order to intentionally bypass the loads. The intentional bypass can be due to several reasons such as tripping of a subsea inverter or any failure. 
     
    
     
       DRAWINGS 
         [0012]    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: 
           [0013]      FIG. 1  is a simplified diagram illustrating a known sub-sea power transmission/distribution system with load bypass switches on the sub-sea side of the system; 
           [0014]      FIG. 2  illustrates one portion of a DC power transmission/distribution system that employs a load bypass switch according to one embodiment; 
           [0015]      FIG. 3  is a simplified diagram illustrating connection of the bypass switch within the coupled DC choke depicted in  FIG. 2 ; 
           [0016]      FIG. 4  illustrates the switching path resulting during operation of the load bypass switch depicted in  FIGS. 2 and 3 ; 
           [0017]      FIG. 5  illustrates in more detail, a DC power transmission/distribution system that employs a plurality of load bypass switches according to one embodiment; 
           [0018]      FIG. 6  illustrates a DC power transmission/distribution system that employs a power supply integrated with the load bypass switch depicted in  FIGS. 2 and 3  according to one embodiment; 
           [0019]      FIG. 7  illustrates a DC power transmission/distribution system that employs a bypass module yard interconnecting a plurality of load bypass switches according to one embodiment; 
           [0020]      FIG. 8  illustrates a very low frequency, small AC current generated by the sending end stacked converter station (located on power generation side), to flow through the dc transmission cable over the DC transmission current, causing induction at the remote location e.g. inside the subsea inverter or bypass switch module to generate a power supply, according to one embodiment; 
           [0021]      FIG. 9  illustrates a bypass-module-yard and closing of one bypass switch in the event of damage to a DC transmission cable, according to one embodiment; and 
           [0022]      FIG. 10  illustrates a bypass-module-yard topology supporting a star configuration of loads, according to one embodiment. 
       
    
    
       [0023]    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 
       [0024]    Subsea cables or umbilicals are by far the most expensive components in long distance transmission systems. The embodiments described herein with reference to the Figures are directed to power transmission in the range of Megawatts to subsea loads and 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. 
         [0025]      FIG. 2  illustrates one portion of a DC power transmission/distribution system  20  that employs a load bypass switch  22  according to one embodiment. The load bypass switch  22  is a hybrid electronic/mechanical switch comprising an SCR  24  in combination with a normally open (NO) mechanical switch  26  and a normally closed (NC) mechanical switch  28 . The SCR  24  advantageously comprises a switching reaction time in the microsecond range; while the mechanical switches  26 ,  28  comprise switching reaction times in the millisecond range greater than five milliseconds. 
         [0026]    During operation of the DC power T/D system  20 , SCR  24  is triggered instantaneously by a break over diode (BOD)  29  in the event of an open circuit fault on the remote VFD or load  18 . The normally closed switch  28  helps in black start operation, described in further detail herein. 
         [0027]    The electronic bypass switch  22  is realized by integrating a thyristor over-voltage protection circuit connected within a coupled DC choke  30 , more clearly illustrated in  FIG. 3 .  FIG. 3  is a simplified diagram illustrating connection of the bypass switch  22  within the coupled DC choke  30  of the VFD  18  depicted in  FIG. 2 . The resultant topology advantageously eliminates the requirement for additional passive components and ensures benign dv/dt and di/dt for the thyristor  24  during bypass operations. The coupled DC choke is preferable; however, a discrete dc choke can also be used. 
         [0028]    An open-circuit faulted VFD  18  or load may cause overvoltage across the thyristor  24 . The bypass switch  22  functions as an over-voltage protection circuit that is implemented in a different way from a classical over-voltage protection circuit to turn-on the thyristor  24  and hence create a bypass path in just a few microseconds. Subsequent to turn-on of thyristor  24 , the normally open mechanical switch  26 , without any current braking capability, closes within a few milliseconds to create a more permanent bypass path for the transmission line current. 
         [0029]      FIG. 4  illustrates the switching path  40  resulting during operation of the load bypass switch  22  depicted in  FIGS. 2 and 3 . The voltage V_AK rises, and when V_AK&gt;V_BOD, the BOD  29  triggers SCR  24  to provide a continuous path for the transmission line loop current in the event of flashover or any open circuit fault in the VFD or load  18 . This switching path event is completed within a microseconds time period that is substantially less than one millisecond. The NO switch  26  is activated subsequent to the establishment of the bypass switching path  40  as stated herein to provide a more permanent continuous path for the transmission line loop current. 
         [0030]      FIG. 5  illustrates in more detail, a DC power transmission/distribution system  50  that employs a plurality of load bypass switches  22  according to one embodiment. T/D system  50  can be seen to employ a parallel connected output transformer  52 - 56  topology. 
         [0031]      FIG. 6  illustrates a DC power transmission/distribution system  60  that employs an auxiliary power supply  62  integrated with the load bypass switch  22  also depicted in  FIGS. 2 and 3  according to one embodiment. The auxiliary power supply  62  supports startup of the VFD/load  18  during a black start event. 
         [0032]    Black start of a system/load refers to a situation when startup of a load is required while auxiliary power is not available for the load. A small power, referred to as auxiliary power is required for a control system to start the load at a remote location connected to a power distribution grid. 
         [0033]    An uninterruptible power supply (UPS) for energy storage is typically available which provides sufficient auxiliary power for control and accessories to start a remote load connected to a power grid. Some applications where accessing the remote load is very expensive, such as subsea applications where the loads are located up to 3000 meters deep and more than 100 miles away from the shore, may not be serviceable by a UPS due to UPS breakdowns or complete discharge of the UPS. 
         [0034]    With continued reference to  FIG. 6 , the auxiliary power supply  62  provides an inexpensive mechanism to provide auxiliary power to the VFDs or loads  18  in the absence of a UPS or other inexpensive means of supplying the necessary auxiliary power. The auxiliary power supply  62  comprises at least one additional winding  64  that is wound on a predetermined winding of the existing DC coupled choke  30 . The auxiliary power supply  62  operates when a control scheme commands a very low frequency, small AC current to flow over the DC transmission current, causing induction at the remote location, as shown in  FIG. 8 . This induction generates a small voltage for the auxiliary power supply. During a black start event, the NC breaker  28  provides the necessary circulation for the DC current. The coupled winding  64  acts like a very bad transformer, generating enough power to wake up the load or VFD  18 . 
         [0035]    In summary explanation, sending low frequency AC current (small amplitude) over DC transmission current (large amplitude) using a DC transmission cable as a medium, and using this low frequency AC current component (very low frequency as compared to 60 Hz and therefore requiring low reactive power from the sending end during black start) to generate small control voltage by using an existing DC choke of the subsea inverter is a novel technique for supporting startup of the VFD/load  18  during a black start event. 
         [0036]      FIG. 7  illustrates a DC power transmission/distribution system  70  that employs a remote bypass-module-yard  72  interconnecting a plurality of load bypass switches  22  according to one embodiment. The T/D system  70  shown in  FIG. 7  illustrates bypassing a load set  74  from the remote bypass-module-yard  72 . This embodiment advantageously provides a method to more easily locate faults in the system  70  and also enables continuous power flow to the remote loads upon failure of one or more power distribution cables located remotely in the MSDC system  70 . 
         [0037]      FIG. 9  illustrates a bypass-module-yard and closing of one bypass switch  22  in the event one of the transmission cables is operationally damaged. Additional isolators are also shown which may be used to physically connect/isolate when current is not flowing in the cables. Such applications are typically used for maintenance. It should be noted that in the presence of remote bypass-module-yard  72 , the NO switch  26  is redundant since with the help of bypass switches  22  in the bypass-module-yard  72 , the faulty remote loads can be permanently bypassed as well. 
         [0038]    The bypass-module-yard  72  enables star configuration of loads as shown in  FIG. 10 . The subsea system can stay operational even after serious faults, e. g. an anchor of a ship that completely destroys the distribution system. The bypass-module-yard  72  can always be of the same design (standardized and qualified once for subsea use); and it could have multiple ports (more than actually needed for the specific application) and one or two spare cables connected to these ports. In case of a fault with a subsea distribution cable, the affected MSDC module can be reconnected to one of these spare ports. 
         [0039]    In further summary explanation, control methods and system topologies employ a load bypass switch described herein for MSDC applications to enable continuous power flow to viable remote loads, even subsequent to failure of one or more remote loads inside the MSDC system. An inexpensive auxiliary power supply integrated with the load bypass switch enables black start of the MSDC system. A distribution cable layout associated with the load bypass switch enables power flow to the remote loads, even during failure of one or more power distribution cables that feed the remote loads located remotely in the MSDC system. It will be appreciated by those skilled in the relevant art that MSDC is one of many examples of a current-link based DC T/D systems. The principles described herein are applicable to any system where loads are connected in series being supplied through a current source and hence requiring bypassing of loads in the event of faults/intentional load disengagement. 
         [0040]    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.