Abstract:
A high-voltage DC (HVDC) power system and a method of controlling and protecting the HVDC power system includes a plurality of sending-end (SE) modules coupled in electrical series and a plurality of receiving-end (RE) power converter modules electrically coupled to said plurality of SE modules, the RE modules coupled in a switchyard configuration, the switchyard configuration including a plurality of load branches coupled together in electrical series, each load branch including a branch bypass switch configured to bypass load current around an associated load branch, and a branch protection system.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH &amp; DEVELOPMENT 
       [0001]    The U.S. Government has certain rights in this invention as provided for by the terms of Contract No. DE-AC26-07NT42677—RPSEA Sub Contract 08121-2901-01. 
     
    
     BACKGROUND OF THE DISCLOSURE 
       [0002]    This description relates to power distribution systems, and, more particularly, to systems and methods for a high-voltage direct current (HVDC) transmission and distribution system control and protection. 
         [0003]    As oil and gas fields in shallow waters go dry, producers are tapping offshore fields in deeper waters with oil installations that operate far below the surface of the sea. The typical equipment for such subsea oil recovery and production include gas compressors and various pumps for multiple functions. Electric variable speed drive (VSD) and motor systems are one way to power such equipment directly under the deep water. Therefore, the delivery of electric power from a remote onshore utility grid or power generation is important to secure a reliable production and processing of oil and gas in subsea locations. Typically, the transmission power requirement is approximately one hundred megawatts for medium to large oil/gas fields. 
         [0004]    For applications wherein bulk power is transmitted over long distances to offshore locations, alternating current (AC) transmission faces technical challenges, which becomes more significant when transmission distance is in excess of one-hundred kilometers. The significant reactive power drawn from the distributed subsea cable capacitors restrains the power delivery capability as well as increases the system cost. 
         [0005]    Direct current (DC) transmission is more efficient over longer distances than AC transmission. Medium voltage (MV) or high voltage (HV) DC transmission typically requires power electronic converters which are capable of converting between HV AC and HV DC. In conventional converter topologies, each switch of the converter is designed to handle high voltages which may range from tens of kilovolts to hundreds of kilovolts depending upon the application needs. Such switches are typically arranged with series connection of several semiconductor devices such as insulated gate bipolar transistors (IGBTs) and thyristors. Another method is to use switches within modules of lower voltage rating and achieving the high voltages required by connecting as many modules in series as the application requires. Due to the special application in subsea, receiving-end converters need to be designed on a modular-basis which is easy to transport, marinize, install, and retrieve. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0006]    In one embodiment, a high-voltage DC (HVDC) power system includes one or more sending-end (SE) modules coupled in electrical series and one or more receiving-end (RE) power converter modules electrically coupled to said plurality of SE modules, the RE modules coupled in a switchyard configuration, the switchyard configuration including one or more load branches coupled together in electrical series, each load branch including a branch bypass switch configured to bypass load current around an associated load branch, and a branch protection system. 
         [0007]    In another embodiment, a method of protecting and controlling a high voltage DC (HVDC) power system includes coupling a plurality of load distribution branch circuits to a receiving end (RE) power distribution system configured in a switchyard structure, each of the plurality of load distribution branch circuits includes a branch protection system, and an electrical load supplied with electrical power through an RE converter module and respective distribution cable, during a fault in one of the plurality of load distribution branch circuits, at least one of bypassing current around an open circuited load using a plurality of series-connected thyristors coupled in electrical parallel with the load, reducing a reverse voltage spike across the load in event of a ground fault using a diode connected anti-parallel across the load, and absorbing a transient current spike caused by a load-shedding event associated with another load connected to the switchyard structure using a resistor-capacitor-diode (RCD) circuit coupled across. 
         [0008]    In yet another embodiment, a subsea receiving end (RE) assembly of a high-voltage DC (HVDC) power system includes one or more receiving-end (RE) power converter modules coupled in a switchyard configuration comprising a distribution load cable supplying electrical power to a distribution branch for each load and having respective distribution branch protective devices, each RE power converter module supplying a respective load with three-phase alternating current (AC) power for each branch of loads, each distribution branch including a bypass protection device, a current resonance damping circuit, and a diode connected anti-parallel with terminals of load distribution cables. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIGS. 1-13  show exemplary embodiments of the method and system described herein. 
           [0010]      FIG. 1  is a schematic block diagram of a bipolar current-source based modular stack direct current (MSDC) high voltage direct current (HVDC) system  100 . 
           [0011]      FIG. 2  is a schematic block diagram of a receiving-end distribution switchyard structure  200  used to supply power to subsea loads  202 . 
           [0012]      FIG. 3  is a schematic block diagram of a portion of receiving-end distribution switchyard structure  200  including a plurality of branches  206 . 
           [0013]      FIG. 4  is a graph  400  of voltage across branch No. 2 (shown in  FIG. 3 ) during a transient induced by the load-shedding of branch No. 1 illustrated in  FIG. 3 . 
           [0014]      FIG. 5  is a schematic block diagram of a portion of the receiving-end distribution switchyard structure (shown in  FIG. 2 ) including a plurality of branches  206 . 
           [0015]      FIG. 6  is a graph of voltage across the bypass switch of No. 1 load branch during a transient induced by a ground fault in branch No. 1 illustrated in  FIG. 5 . 
           [0016]      FIG. 7  is a graph of the insulation voltage stresses of two wet-met connectors shown in  FIG. 5  of No. 1 load branch during a transient induced by a ground fault in branch No. 1 illustrated in  FIG. 5 . 
           [0017]      FIG. 8  is a graph of the insulation voltage stresses of two wet-met connectors shown in  FIG. 5  of No. 1 load branch during a transient induced by a ground fault in branch No. 1 illustrated in  FIG. 5 . 
           [0018]      FIG. 9  is a schematic diagram of the bypass protection circuit shown in  FIG. 2  illustrating an example using two thyristors in series as a high-voltage bypass device. 
           [0019]      FIG. 10  is a schematic diagram of the bypass protection circuit shown in  FIG. 2  illustrating a single BOD turning on all series connected thyristors. 
           [0020]      FIG. 11  is a schematic diagram of the bypass protection circuit shown in  FIG. 2  in accordance with other example embodiments of the present disclosure. 
           [0021]      FIG. 12  is a schematic diagram of the bypass protection circuit shown in  FIG. 2  in accordance with another example embodiment of the present disclosure. 
           [0022]      FIG. 13  is a schematic diagram of the bypass protection circuit shown in  FIG. 2  in accordance with another example embodiment of the present disclosure. 
       
    
    
       [0023]    Although specific features of various embodiments may be shown in some drawings and not in others, this is for convenience only. Any feature of any drawing may be referenced and/or claimed in combination with any feature of any other drawing. 
         [0024]    Unless otherwise indicated, the drawings provided herein are meant to illustrate features of embodiments of the disclosure. These features are believed to be applicable in a wide variety of systems comprising one or more embodiments of the disclosure. As such, the drawings are not meant to include all conventional features known by those of ordinary skill in the art to be required for the practice of the embodiments disclosed herein. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0025]    The following detailed description illustrates embodiments of the invention by way of example and not by way of limitation. It is contemplated that the invention has general application to protection and control of electric power distribution systems in industrial, commercial, and residential applications. 
         [0026]    Embodiments of the present disclosure relate to a distribution switchyard structure and its bypass device to protect system against open-circuit failure. Due to the operational principle of a modular stacked direct current (MSDC) subsea power system, open-circuit failure mode is regarded as the most critical failure event, which may lead to a catastrophic cascading system failure. The distribution switchyard structure reduces the effects of an open-circuit failures that may occur on any of the distribution cables as well as ride-through the ground-faults and fast load-shedding events. As a key component in the distribution switchyard, a high-voltage bypass device with high reliability includes a passive-component-triggering circuit capable of turning on a plurality of series-connected thyristors in a fast and uniform manner as protection against over-voltage. Without a requirement of using any control power, the proposed passive solution improves the system reliability and ensures turning on multiple-thyristors at the same time. 
         [0027]    For the high voltage bypass with a high surge current capability and a fast turn-on behavior, a thyristor is one of the best options. The typical voltage of each receiving-end module is more than 10 kV, up to 30 kV. For such high-voltage applications, two or more thyristors in series are used. As such, the triggering unit to turn on thyristors at the same time is important. Active triggering using control sensing, pulse power supply, and fiber optics may be used. However, for subsea MSDC system applications, the reliability of the high voltage bypass device is important as a protection circuit. In the subsea harsh environment, such as high-temperature and high pressure, solely relying on electronics/control to turn on the series-connected thyristors may not be able to meet a long life-time target. Any of power supply loss, control unit malfunction, or electronic components failure can results in a failure to turn on the bypass thyristors. Therefore, using a passive scheme without any power supply or control to fulfill all the detection and protection function is desired. 
         [0028]    Breakover diode (BOD) based triggering performs well to protect a single thyristor against over-voltage. It utilizes the energy from a snubber capacitor to fire the thyristor when the BOD breaks over. No active device is needed, which is robust and reliable. Although putting thyristors embedded with a BOD in series can help protect the individual thyristor against over-voltage, it cannot ensure that all thyristors are turned on at the same time due to part-to-part variations of the BODs. Moreover, such a system is a series-tied system. Any BOD failure results in overall failure of the bypass system. For example, if each BOD circuit reliability is a, the total reliability of N series-connected thyristors based on passive triggering is shown in (1), as being significantly smaller than an individual piece where N is the number of thyristors. 
         [0000]        R   total =α N   (1)
 
         [0029]    The following description refers to the accompanying drawings, in which, in the absence of a contrary representation, the same numbers in different drawings represent similar elements. 
         [0030]      FIG. 1  is a schematic block diagram of a bipolar current-source based modular stack direct current (MSDC) high voltage direct current (HVDC) system  100 . In the example embodiment, all subsea receiving-end (RE) modules  102  are connected in electrical series. Any open-circuit fault in system  100  could result in a whole system shut-down, which greatly affects system reliability. To reduce the probability of such an event, an ultra-reliable bypass protection device is used to provide a bypass current path in the event that any RE module fails in an open-circuit mode. Bypassing the open-circuit quickly facilitates MSDC  100  surviving the open-circuit fault. 
         [0031]      FIG. 2  is a schematic block diagram of a receiving-end distribution switchyard structure  200  used to supply power to subsea loads  202 . In the example embodiment, a protection system  204  is implemented in the distribution switchyard structure  200  for each branch  206  of loads. Each branch  206  includes a bypass switch  208  in parallel with protection system  204 , one or more branch distribution cables  210 , RE module  212  and load  202 . Each branch  206  may also include one or more common mode current sensors  214 ,  215  to facilitate detecting, localizing, and isolating a ground fault in branch  206 . In various embodiments, bypass switch  208  may be a manually actuated switch or may be controlled automatically by control devices positioned locally with branch  206  or remotely, for example, at the sea surface proximate the sending end units. 
         [0032]    Protection system  204  includes a resistor-capacitor-diode (RCD) snubber circuit  216 , a bypass protection circuit  218 , and an antiparallel connected diode  220 . RCD snubber circuit  216  is used to absorb a transient current spike as load-shedding of a neighbor branch. RCD snubber circuit  216  is configured to reduce an over-voltage across bypass protection circuit  218  to prevent false-tripping due to such a load-shedding event. The transient current spike is caused by the energy damping from the transmission cables. Bypass protection circuit  218  uses fast solid-state thyristors (shown in  FIG. 10 ) across inlet terminals  222  of distribution cables  210  to provide a bypass current path in case of an open-circuit in branch  206 . Anti-parallel diode  220  is configured to reduce a high reversed voltage spike across bypass protection circuit  218  and reduce a sudden insulation stress polarity reverse on inlet terminals  222  when a ground fault occurs. In addition, ground fault detector devices, for example, current sensors  214 ,  215  are implemented in each terminal of switchyard structure  200  to quickly identify the fault location. 
         [0033]      FIG. 3  is a schematic block diagram of a portion of receiving-end distribution switchyard structure  200  including a plurality of branches  206 . In the example embodiment, branch No. 1 supplies power, for example, to a 12 MW compressor load that has suddenly shut-down or bypassed. In such an event, without RCD snubber circuit  216  employed, the transient is shown in  FIG. 4 . The transient with RCD snubber circuit  216  employed in protection system  204  is also shown in  FIG. 4 . 
         [0034]      FIG. 4  is a graph  400  of voltage across branch No. 2 (shown in  FIG. 3 ) during a transient induced by the load-shedding of branch No. 1 illustrated in  FIG. 3 . Graph  400  includes an x-axis  402  graduated in units of time (seconds) and a y-axis  404  graduated in units of voltage (kV). A trace  406  illustrates the voltage across branch No. 2 when the load is shed on branch No. 1 without RCD snubber circuit  216  employed on branch No. 2. A trace  408  illustrates the voltage across branch No. 2 when the load is shed on branch No. 1 with RCD snubber circuit  216  employed on branch No. 2. In the example embodiment, the over-voltage spike is reduced from approximately 10 kV to approximately 3 kV, indicating how significant RCD snubber circuit  216  is in reducing the voltage spike. 
         [0035]      FIG. 5  is a schematic block diagram of a portion of receiving-end distribution switchyard structure  200  including a plurality of branches  206 . In the example embodiment, branch No. 1 supplies power, for example, to a 12 MW compressor load that has suddenly suffered a ground fault  500 . Anti-parallel diode  220  facilitates avoiding a pronounced reversed voltage spike across bypass switch  208  that is produced during a ground fault. An amplitude of the voltage spike depends on, among other things, where the ground fault is located and an amount of total power consumption at the time of the fault. This reversed voltage occurs because the capacitances of local distribution cables  210  for each load  202  are discharged at different rates. In one embodiment, the voltage is measured at wet-met connectors  502  and  504 . 
         [0036]      FIG. 6  is a graph  600  of voltage across bypass switch  208  of No. 1 load branch  206  during a transient induced by ground fault  500  in branch No. 1 illustrated in  FIG. 5 . Graph  600  includes an x-axis  602  graduated in units of time (seconds) and a y-axis  604  graduated in units of voltage (kV). A trace  606  illustrates the voltage across branch No. 1 when the ground fault occurs on branch No. 1 without anti-parallel diode  220  employed on branch No. 1. A trace  608  illustrates the voltage across branch No. 1 when the ground fault occurs on branch No. 1 with anti-parallel diode  220  employed on branch No. 2. In the example embodiment, the reverse-voltage spikes to approximately −80 kV which can damage bypass device  208 , although the normal operation voltage is only about 27 kV. 
         [0037]      FIG. 7  is a graph  700  of the insulation voltage stresses of two wet-met connectors  502  and  504  (shown in  FIG. 5 ) of No. 1 load branch  206  during a transient induced by ground fault  500  in branch No. 1 illustrated in  FIG. 5 . Graph  700  includes an x-axis  702  graduated in units of time (seconds) and a y-axis  704  graduated in units of voltage (kV). A trace  706  illustrates the voltage stress at connector  502  (shown in  FIG. 5 ) that suddenly reverses polarity when the ground fault occurs on branch No. 1 without anti-parallel diode  220  employed on branch No. 1. Connector  502  requires a greater dielectric strength to sustain this sudden polarity change. A trace  708  illustrates the voltage stress at connector  504  (shown in  FIG. 5 ) when the ground fault occurs on branch No. 1 without anti-parallel diode  220  employed on branch No. 1. 
         [0038]      FIG. 8  is a graph  700  of the insulation voltage stresses of two wet-met connectors  502  and  504  (shown in  FIG. 5 ) of No. 1 load branch  206  during a transient induced by ground fault  500  in branch No. 1 illustrated in  FIG. 5 . Graph  800  includes an x-axis  802  graduated in units of time (seconds) and a y-axis  804  graduated in units of voltage (kV). A trace  806  illustrates the voltage stress at connector  502  (shown in  FIG. 5 ) when the ground fault occurs on branch No. 1 with anti-parallel diode  220  employed on branch No. 1. Anti-parallel diode  220  effectively eliminates the high insulation voltage stress issue. A trace  808  illustrates the voltage stress at connector  504  (shown in  FIG. 5 ) when the ground fault occurs on branch No. 1 with anti-parallel diode  220  employed on branch No. 1. 
         [0039]      FIG. 9  is a schematic diagram of bypass protection circuit  218  (shown in  FIG. 2 ) illustrating an example using two thyristors  902  and  904  in series as a high-voltage bypass device. Each of thyristors  902  and  904  include an anode  906 , a cathode,  908  and a gate  910 . Thyristors  902  and  904  are triggered by respective triggering circuits  912  and  914 . Triggering circuits  912  and  914  each includes a breakover diode (BOD)  916  and a pulse transformer  918  coupled in electrical series across an anode/gate circuit of thyristors  902  and  904 . Respective balancing resistors and snubber circuit  920  and  922  cause thyristors  902  and  904  to share the branch voltage approximately equally. Balancing resistors and snubber circuit  920  and  922  each include a snubber capacitor  924 . With pulse transformer  918  inserted in BOD loop  916 , as soon as BOD  916  breaks over, the BOD current flows through pulse transformer  918 . Then a pulse current is injected into triggering circuit  914  of thyristor  904  by pulse transformer  918 . Therefore, any BOD&#39;s breakover can trigger all thyristors at the same time. Even if the self-embedded BOD  916  fails, thyristor  904  can be still triggered by other BODs, in a case of multiple thyristor circuits connected in series. 
         [0040]      FIG. 10  is a schematic diagram of bypass protection circuit  218  (shown in  FIG. 2 ) illustrating a single BOD  916  turning on all series connected thyristors. Although shown using just two thyristors for clarity, any number of thyristors may be turned on from a single BOD  916  in other embodiments. Assuming, for example, that BOD  902  breaks over first, its voltage suddenly drops to almost zero, snubber capacitor  924  will discharge energy through a BOD loop that includes BOD  916  and pulse transformer  918  (shown by line  1002 ). 
         [0041]    The BOD current, i BoD  is determined as shown in (2). As the primary winding voltage of the pulse transformer (turn ratio is 1) v pri  is the same as the gate voltage of the other thyristor, which is very much lower than the breakover voltage level of the BOD, v scr , v pri  can be ignored in (2). 
         [0000]    
       
         
           
             
               
                 
                   
                     i 
                     BOD 
                   
                   = 
                   
                     
                       
                         
                           v 
                           scr 
                         
                         - 
                         
                           v 
                           pri 
                         
                       
                       
                         
                           R 
                           1 
                         
                         + 
                         
                           R 
                           2 
                         
                       
                     
                     ≈ 
                     
                       
                         v 
                         scr 
                       
                       
                         
                           R 
                           1 
                         
                         + 
                         
                           R 
                           2 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
         [0042]    According to  FIG. 10 , the induced current on the secondary side of transformer i sec  will be almost the same as the BOD current i BOD . 
         [0000]        i   sec   ≈i   BOD   (3)
 
         [0043]    Therefore, gate current appears on both thyristors and force them to turn on. Because the gate energy needed to turn on a thyristor is not large, the stored energy in single snubber capacitor  924  is enough to fire all thyristors  904  plus others if there are more than two thyristors connected in series. 
         [0044]    Since any BOD&#39;s breakover in the system can trigger all thyristors, the reliability can be dramatically increased making the bypass circuit  218  suitable for applications like subsea power system  100 . Assuming the each BOD trigger circuit reliability is a, the total reliability of the bypass system is shown in (4), where N is the number of thyristors. It can be seen that the triggering circuit essentially becomes as a parallel-system, though thyristors are tied in series. The parallel system reliability can be improved dramatically. 
         [0000]        R   total =1−(1−α) N  
 
         [0045]      FIGS. 11 ,  12 , and  13  are schematic diagrams of bypass protection circuit  218  (shown in  FIG. 2 ) in accordance with other example embodiments of the present disclosure. 
         [0046]    This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.