Patent Publication Number: US-2015070939-A1

Title: Electric power conversion system and method of operating the same

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH &amp; DEVELOPMENT 
     This invention was made with Government support under contract number DE-AR0000224 awarded by the Advanced Research Projects Agency-Energy (ARPA-E). The Government may have certain rights in this invention. 
    
    
     BACKGROUND 
     The field of the disclosure relates generally to high voltage direct current (HVDC) transmission systems and, more particularly, to electric power conversion systems and their methods of operation. 
     At least some known electric power generation facilities are physically positioned in a remote geographic region or in an area where physical access is difficult. One example includes power generation facilities geographically located in rugged and/or remote terrain, for example, mountainous hillsides, extended distances from the customers, and off-shore, e.g., off-shore wind turbine installations. More specifically, these wind turbines may be physically nested together in a common geographic region to form a wind turbine farm and are electrically coupled to a common alternating current (AC) collector system. Many of these known wind turbine farms are coupled to AC transmission systems through a power converter, a power transformer, and an AC circuit breaker. Many of the known power converters are a power conversion assembly, or system, electrically coupled to the AC collector system and power transformer. Such known power conversion assemblies include a rectifier portion that converts the AC generated by the power generation facilities to direct current (DC) and an inverter portion that converts the DC to AC of a predetermined frequency and voltage amplitude. Many of the known AC circuit breakers facilitate uncoupling the power generation facility from the AC transmission system when the power generation facility needs to be isolated, e.g., when the facility is removed from service due to an electrical fault on either side of the circuit breaker. 
     Similarly, some wind turbine farms are coupled to a high voltage (HVDC) transmission system through a separated power conversion assembly, or system, electrically coupled to the AC collector system and the HVDC transmission system. In such configurations, the rectifier portion of the separated power conversion assembly is positioned in close proximity to the associated power generation facilities and the inverter portion of the separated full power conversion assembly is positioned in a remote facility, such as a land-based facility. Such rectifier and inverter portions are typically electrically connected via submerged HVDC electric power cables that at least partially define the HVDC transmission system. Also, at least some known HVDC transmission systems are coupled to DC loads that do not require an inverter portion of AC conversion. 
     Many known HVDC transmission systems include isolation devices, e.g., DC circuit breakers and reclosers, positioned to define isolatable portions of the system. Such isolation devices may be opened to isolate electrical faults and, possibly, closed to restore as much of the system to service as possible once the fault is isolated. However, these devices may be rated for voltages in excess of 100 kilovolts (kV) and DC circuit breakers rated for such high voltages are typically not commercially available other than through custom design and manufacture. Therefore, the costs associated with construction and maintenance of HVDC transmission systems may be significantly increased. Moreover, any alternatives to fast-acting circuit breakers would need to provide similar fast-acting characteristics for timely and effective fault isolation. 
     In addition, many known HVDC transmission systems include a Supervisory Control and Data Acquisition (SCADA) system, or some equivalent, that includes current and voltage sensors positioned therein to facilitate isolation and restoration operations. However, in contrast to AC transmission systems, due to the nature of DC, i.e., no zero-crossing of the amplitudes of DC voltages and currents as a function of time, such opening of the DC circuit breakers requires opening the devices under load, including higher than typical loads due to the fault, thereby increasing the risk of arcing at the contactor portions of the mechanical isolation devices with a potential decrease of service life of the contactor portions. Also, in the event of an upward DC current excursion, with increased loading due to the fault, it is generally considered that an operation of a mechanism has approximately five milliseconds (ms) to isolate the associated fault. 
     BRIEF DESCRIPTION 
     In one aspect, an electric power conversion system is provided. The electric power conversion system is coupled to a high voltage direct current (HVDC) transmission system. The electric power conversion system includes a plurality of power conversion modules. At least one power conversion module of the plurality of power conversion modules includes at least one power converter coupled to at least one DC power terminal. The power conversion module also includes at least one isolation device coupled to the at least one power converter. The at least one power converter and the at least one isolation device at least partially define an isolatable portion of the electric power conversion system. The at least one isolation device is configured to remove the isolatable portion from service. The at least one power converter is configured to decrease electric current transmission through the isolatable portion prior to opening the at least one isolation device. 
     In a further aspect, a control system for an electric power conversion system is provided. The electric power conversion system is coupled to an HVDC transmission system. The electric power conversion system includes a plurality of power conversion modules. The at least one of the power conversion module includes at least one power converter coupled to at least one DC power terminal. The at least one power conversion module further includes at least one isolation device coupled to the at least one power converter. The at least one power converter and the at least one isolation device at least partially define an isolatable portion of the electric power conversion system. The at least one isolation device is configured to remove the isolatable portion from service. The at least one power converter is configured to decrease electric current transmission through the isolatable portion prior to opening of the at least one isolation device. The control system includes at least one sensor configured to transmit at least one signal representative of a value of electric current transmission through at least one of the electric power conversion system and the HVDC transmission system. The control system also includes at least one processor coupled to the at least one sensor, the at least one power converter, and the at least one isolation device. The at least one processor is configured to determine at least one of electric current transmission approaching, attaining, and exceeding at least one parameter of at least one of the electric power conversion system and the HVDC transmission system. The at least one processor is also configured to regulate electric current transmission through the isolatable portion at least partially as a function of the value of electric current transmission through at least one of the electric power conversion system and the HVDC transmission system. The at least one processor is further configured to open the at least one isolation device when said at least one sensor measures electric current transmission at a predetermined value. 
     In another aspect, a method of operating an electric power conversion system is provided. The electric power conversion system is coupled to a high voltage direct current (HVDC) transmission system. The electric power conversion system includes a plurality of power conversion modules. Each power conversion module includes at least one power converter coupled to at least one DC power terminal. The at least one power conversion module further includes at least one isolation device coupled to the at least one power converter. The at least one power converter and the at least one isolation device at least partially define an isolatable portion of the power conversion module. Each power conversion module also includes at least one sensor configured to transmit at least one signal representative of a value of electric current transmission through at least one of the electric power conversion system and the HVDC transmission system. The method includes determining electric current transmission through at least one of the electric power conversion system and the HVDC transmission system is one of approaching, attaining, and exceeding at least one parameter of at least one of the electric power conversion system and the HVDC transmission system. The method also includes regulating electric current transmission through the isolatable portion at least partially as a function of the value of electric current transmission through at least one of the electric power conversion system and the HVDC transmission system. The method further includes opening the at least one isolation device when the at least one sensor measures electric current transmission at a predetermined value. 
    
    
     
       DRAWINGS 
       These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein: 
         FIG. 1  is a block diagram of an exemplary computing device; 
         FIG. 2  is block diagram of a portion of an exemplary monitoring and control system that may include the computing device shown in  FIG. 1 ; 
         FIG. 3  is a schematic diagram of an exemplary electric power conversion system that may be monitored and controlled using the system shown in  FIG. 2 ; 
         FIG. 4  is a schematic diagram of an exemplary power conversion module that may be used with the electric power conversion system shown in  FIG. 3 ; 
         FIG. 5  is another schematic diagram of the power conversion module shown in  FIG. 4 ; 
         FIG. 6  is a schematic diagram of a portion of the electric power conversion system shown in  FIG. 3  with a fault shown; 
         FIG. 7  is a schematic diagram of an alternative power conversion module that may be used with the electric power conversion system shown in  FIG. 3 ; 
         FIG. 8  is a schematic diagram of an alternative electric power conversion system that may be monitored and controlled using the system shown in  FIG. 2 ; 
         FIG. 9  is a schematic diagram of an alternative power conversion module that may be used with the electric power conversion system shown in  FIG. 8 ; 
         FIG. 10  is another schematic diagram of the power conversion module shown in  FIG. 9 ; 
         FIG. 11  is a schematic diagram of another alternative electric power conversion system that may be monitored and controlled using the system shown in  FIG. 2 ; 
         FIG. 12  is a schematic diagram of yet another alternative electric power conversion system that may be monitored and controlled using the system shown in  FIG. 2 ; 
         FIG. 13  is a schematic diagram of an alternative power conversion module that may be used with the electric power conversion system shown in  FIG. 8 ; and 
         FIG. 14  is a schematic diagram of another alternative power conversion module that may be used with the electric power conversion systems shown in  FIGS. 8 ,  11 , and/or  12 . 
     
    
    
     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 
     In the following specification and the claims, reference will be made to a number of terms, which shall be defined to have the following meanings 
     The singular forms “a”, “an”, and “the” include plural references unless the context clearly dictates otherwise. 
     “Optional” or “optionally” means that the subsequently described event or circumstance may or may not occur, and that the description includes instances where the event occurs and instances where it does not. 
     Approximating language, as used herein throughout the specification and claims, may be applied to modify any quantitative representation that could permissibly vary without resulting in a change in the basic function to which it is related. Accordingly, a value modified by a term or terms, such as “about” and “substantially”, are not to be limited to the precise value specified. In at least some instances, the approximating language may correspond to the precision of an instrument for measuring the value. Here and throughout the specification and claims, range limitations may be combined and/or interchanged, such ranges are identified and include all the sub-ranges contained therein unless context or language indicates otherwise. 
     As used herein, the term “computer” and related terms, e.g., “computing device”, are not limited to integrated circuits referred to in the art as a computer, but broadly refers to a microcontroller, a microcomputer, a programmable logic controller (PLC), an application specific integrated circuit, and other programmable circuits (none shown in  FIG. 1 ), and these terms are used interchangeably herein. 
     Further, as used herein, the terms “software” and “firmware” are interchangeable, and include any computer program stored in memory for execution by personal computers, workstations, clients and servers. 
     Moreover, as used herein, the term “non-transitory computer-readable media” includes all tangible, computer-readable media, such as a firmware, floppy disk, CD-ROMs, DVDs and another digital source such as a network or the Internet, as well as yet to be developed digital means, with the sole exception being a transitory, propagating signal. 
     Furthermore, as used herein, the term “real-time” refers to at least one of the time of occurrence of the associated events, the time of measurement and collection of predetermined data, the time to process the data, and the time of a system response to the events and the environment. In the embodiments described herein, these activities and events occur substantially instantaneously. 
     The electric power conversion systems for HVDC transmission systems described herein provide a cost-effective method for transmitting HVDC power. The embodiments described herein facilitate transmitting HVDC power across relatively large distances while facilitating rapid detection and isolation of affected electric power conversion systems in the event of electrical faults on the HVDC transmission system. The embodiments described herein also facilitate rapid restoration of the affected electric power conversion systems after the faulted portions of the system are effectively isolated. Specifically, the devices, systems, and methods described herein include a plurality of bi-directional power converters and mechanical isolation devices that are operatively coupled to the bi-directional power converters. The bi-directional power converters and mechanical isolation devices define isolable portions of the electric power conversion system. The bi-directional power converters facilitate real-time decreasing of electric current through the isolable portions in the event that electric current sensed being transmitted through the conversion system or on the HVDC transmission system exceeds parameters as communicated in real time to a Supervisory Control and Data Acquisition (SCADA) system. Specifically, in the event that sensed electric current exceed parameters, the bi-directional power converters decrease the electric current transmitted therethrough and as the current approaches or attains a value of approximately zero, the SCADA system initiates opening the associated mechanical isolation devices with a significantly reduced load that will approach zero amperes, thereby mitigating a potential for damage to the contactors of the mechanical isolation devices. Fault isolation occurs approximately three orders of magnitude more rapidly than the typical five milliseconds (ms) needed to reduce a partial for a reduction of service life to the affected components. 
     Also, the devices, systems, and methods described herein facilitate system restoration. Once the electrical fault is cleared, the SCADA system will initiate post-fault recovery actions. Specifically, the cleared mechanical isolation devices will reclose under near-zero loads and the associated bi-directional power converters will increase the current transmitted through the restored isolable portions to restore DC power transmission to the affected portions expeditiously. 
       FIG. 1  is a block diagram of an exemplary computing device  105  that may be used to perform monitoring and/or control of a high voltage direct current (HVDC) transmission system and, more specifically, an electric power conversion system (neither shown in  FIG. 1 ). More specifically, computing device  105  monitors and/or controls any piece of equipment, any system, and any process associated with an electric power conversion system and a HVDC transmission system, e.g., without limitation, bi-directional power converters, mechanical isolation devices, and monitoring devices (neither shown in  FIG. 1 ). Computing device  105  includes a memory device  110  and a processor  115  operatively coupled to memory device  110  for executing instructions. In some embodiments, executable instructions are stored in memory device  110 . Computing device  105  is configurable to perform one or more operations described herein by programming processor  115 . For example, processor  115  may be programmed by encoding an operation as one or more executable instructions and providing the executable instructions in memory device  110 . In the exemplary embodiment, memory device  110  is one or more devices that enable storage and retrieval of information such as executable instructions and/or other data. Memory device  110  may include one or more computer readable media. 
     Memory device  110  may be configured to store operational measurements including, without limitation, real-time and historical vibration values, and/or any other type data. Also, memory device  110  includes, without limitation, sufficient data, algorithms, and commands to facilitate monitoring and control of the components within a HVDC transmission system and an associated electric power conversion system. 
     In some embodiments, computing device  105  includes a presentation interface  120  coupled to processor  115 . Presentation interface  120  presents information, such as a user interface and/or an alarm, to a user  125 . In some embodiments, presentation interface  120  includes one or more display devices. In some embodiments, presentation interface  120  presents an alarm associated with the HVDC transmission system and associated electric power conversion system being monitored, such as by using a human machine interface (HMI) (not shown in  FIG. 1 ). Also, in some embodiments, computing device  105  includes a user input interface  130 . In the exemplary embodiment, user input interface  130  is coupled to processor  115  and receives input from user  125 . 
     A communication interface  135  is coupled to processor  115  and is configured to be coupled in communication with one or more other devices, such as a sensor or another computing device  105 , and to perform input and output operations with respect to such devices while performing as an input channel. Communication interface  135  may receive data from and/or transmit data to one or more remote devices. For example, a communication interface  135  of one computing device  105  may transmit an alarm to the communication interface  135  of another computing device  105 . 
       FIG. 2  is block diagram of a portion of a monitoring and control system, i.e., in the exemplary embodiment, a Supervisory Control and Data Acquisition (SCADA) system  200  that may be used to monitor and control at least a portion of an electric power conversion system  300  and an associated HVDC transmission system (not shown in  FIG. 2 ). As used herein, the term “SCADA system” refers to any control and monitoring system that may monitor and control electric power conversion system  300  across multiple sites, remote sites, and large distances. In the exemplary embodiment, SCADA system  200  includes at least one central processing unit (CPU)  215  configured to execute monitoring algorithms and monitoring logic. CPU  215  may be coupled to other devices  220  via a communication network  225 . 
     Referring to  FIGS. 1 and 2 , CPU  215  is a computing device  105 . In the exemplary embodiment, computing device  105  is coupled to network  225  via communication interface  135 . In an alternative embodiment, CPU  215  is integrated with other devices  220 . 
     CPU  215  interacts with a first operator  230 , e.g., without limitation, via user input interface  130  and/or presentation interface  120 . In one embodiment, CPU  215  presents information about electric power conversion system  300 , such as alarms, to operator  230 . Other devices  220  interact with a second operator  235 , e.g., without limitation, via user input interface  130  and/or presentation interface  120 . For example, other devices  220  present alarms and/or other operational information to second operator  235 . As used herein, the term “operator” includes any person in any capacity associated with operating and maintaining electric power conversion system  300 , including, without limitation, shift operations personnel, maintenance technicians, and facility supervisors. 
     In the exemplary embodiment, electric power conversion system  300  includes one or more monitoring sensors  240  coupled to CPU  215  through at least one input channel  245 . Monitoring sensors  240  collect operational measurements including, without limitation, AC and DC voltages and currents generated within and transmitted through electric power conversion system  300 . Monitoring sensors  240  repeatedly, e.g., periodically, continuously, and/or upon request, transmit operational measurement readings at the time of measurement. CPU  215  receives and processes the operational measurement readings. Such data is transmitted across network  225  and may be accessed by any device capable of accessing network  225  including, without limitation, desktop computers, laptop computers, and personal digital assistants (PDAs) (neither shown). In alternative embodiments, CPU  215  includes, without limitation, sufficient data, algorithms, and commands to facilitate control of the DC current transmission through electric power conversion system  300 . 
       FIG. 3  is a schematic diagram of electric power conversion system  300  that may be monitored and controlled using SCADA system  200  (shown in  FIG. 2 ). In the exemplary embodiment, electric power conversion device  300  couples alternating current (AC) electric power generation devices  302  (only one shown) to a HVDC electric power transmission grid, or system  304  that may be positioned hundreds, or thousands, of kilometers from devices  302  across rugged and/or remote terrain, e.g., mountainous hillsides, large bodies of water, and extended distances from users. AC electric power generation devices  302  may be rated for voltages in excess of 100 kilovolts (kV) AC and HVDC electric power transmission system  304  may be rated for voltages in excess of 100 kV DC. In some embodiments, devices  302  define an AC electric power system or grid. 
     In the exemplary embodiment, AC electric power generation devices  302  may be positioned off-shore, i.e., devices  302  may be off-shore wind farms. Alternatively, AC electric power generation devices  302  may include any type of renewable electric power generation system including, for example, and without limitation, solar power generation systems, fuel cells, thermal power generators, geothermal generators, hydropower generators, diesel generators, gasoline generators, and/or any other device that generates power from renewable energy sources. Alternatively, AC electric power generation devices  302  may include any type of non-renewable electric power generation system including, for example, and without limitation, coal- and oil-fired facilities, gas turbine engines, nuclear power generation facilities and/or any other device that generates power from non-renewable energy sources. Moreover, any number of electric power generation devices  302  may be used. Also, AC electric power generation devices  302  may include an AC power grid. 
     In the exemplary embodiment, electric power conversion system  300  includes a plurality of power conversion modules  306  electrically coupled in series for each phase on the AC side through AC terminals  305 , with three parallel AC phases shown, and in series on the DC side through DC terminals  307 . Each of the three phases includes n power conversion modules  306 . In the exemplary embodiment, each power conversion module  306  is configured to generate a unipolar output voltage and a bi-polar current. Alternatively, power conversion modules  306  are configured to generate electric power with any voltage and current characteristics that enable operation of electric power conversion system  300  as described herein. 
       FIG. 4  is a schematic diagram of power conversion module  306  that may be used with electric power conversion system  300 .  FIG. 5  is another schematic diagram of power conversion module  306 . Power conversion module  306  includes a plurality of bi-directional power converters  308 . A first bi-directional power converter  310  is coupled to AC electric power generation devices  302 . First bi-directional power converter  310  is configured to convert AC power to DC power when power flows from AC electric power generation devices  302  toward HVDC electric power transmission system  304 . Also, first bi-directional power converter  310  is configured to convert DC power to AC power when power flows from HVDC electric power transmission system  304  toward AC electric power generation devices  302 . 
     A second bi-directional power converter  312  is coupled to first bi-directional power converter  310  through a first DC link  314  and DC terminals  315 . Second bi-directional power converter  312  is configured to convert DC power to high frequency AC (HFAC) power when power flows from AC electric power generation devices  302  toward HVDC electric power transmission system  304 . Also, second bi-directional power converter  312  is configured to convert HFAC power to DC power when power flows from HVDC electric power transmission system  304  toward AC electric power generation devices  302 . 
     A third bi-directional power converter  316  is coupled to HVDC electric power transmission system  304  through a second DC link  318 . Third bi-directional power converter  316  is configured to convert HFAC power to DC power when power flows from AC electric power generation devices  302  toward HVDC electric power transmission system  304 . Also, third bi-directional power converter  316  is configured to convert DC power to HFAC power when power flows from HVDC electric power transmission system  304  toward AC electric power generation devices  302 . 
     Power conversion module  306  includes an AC bypass switch  320  coupled to first bi-directional power converter  310  and AC electric power generation devices  302 . Power conversion module  306  also includes a DC bypass switch  322  coupled third bi-directional power converter  316  and HVDC electric power transmission system  304 . Power conversion module  306  further includes a pair of two-port passive networks  324  and a voltage matching transformer  325  therebetween to facilitate zero-voltage-switching (ZVS) and/or zero-current-switching (ZCS) of power converters  308 ,  312 , and  316  to increase operational efficiency as well as facilitating reduction of electromagnetic interference (EMI) induced by switching effects of power conversion module  306 . 
     Power conversion module  306  includes at least one isolation device, e.g., without limitation, mechanical isolation device  326  positioned between the DC-side two-port passive network  324  and third bi-directional power converter  316 . In the exemplary embodiment, mechanical isolation device  326  is a switch device with open and closed positions (shown open). Alternatively, mechanical isolation device  326  is any device that enables operation of power conversion module  306  as described herein, including, without limitation, a circuit breaker and a recloser. Mechanical isolation device  326  and second bi-directional power converter  312  are operatively coupled (discussed further below) to define an isolable portion  328  of power conversion module  306 . 
     Each of first, second, and third bi-directional current converters  310 ,  312 , and  316 , respectively, includes semiconductor devices  330 , e.g., insulated gate bipolar transistors (IGBTs) (only shown in  FIG. 5 ), with off-on characteristics, in parallel with an anti-paralleling diodes  332  (only shown in  FIG. 5 ). Alternatively, any semiconductor devices that enable operation of bi-directional current converters  310 ,  312 , and  316  as described herein are used, including, without limitation, insulated gate commutated thyristors (IGCTs), silicon controlled rectifiers (SCRs), gate commutated thyristors (GCTs), symmetrical gate commutated thyristors (SGCTs), and gate turnoff thyristors (GTOs). 
     Each power conversion module  306  in electric power conversion system  300  is coupled to SCADA system  200  (only shown in  FIG. 4 ). SCADA system  200  includes a plurality of monitoring sensors  240  that include, without limitation, voltage monitoring sensors and current monitoring sensors. Power conversion module  306  and SCADA  200  may also include any other sensing devices that enable operation of power conversion module  306  as described herein. Monitoring sensors  240  are configured to transmit real-time current and voltage monitoring information to SCADA system  200 . Any positioning and configuration of monitoring sensors  240  that enables operation of power conversion module  306  as described herein may be used. Moreover, mechanical isolation device  326 , two-port passive networks  324 , and first, second, and third bi-directional current converters  310 ,  312 , and  316 , respectively, are configured to receive commands from SCADA system  200  and transmit status and feedback information to SCADA system  200 . 
     Alternatively, any control system architecture that enables operation of power conversion module  306  and SCADA system  200  is used. For example, and without limitation, each of first, second, and third bi-directional current converters  310 ,  312 , and  316 , respectively, and mechanical isolation device  326  may include at least one controller (not shown) that includes at least one processor (not shown). As used herein, the terms “controller”, “control system”, and “processor” include any programmable system including systems and microcontrollers, reduced instruction set circuits, application specific integrated circuits, programmable logic circuits, and any other circuit capable of executing the functions described herein. The above examples are exemplary only, and thus are not intended to limit in any way the definition and/or meaning of the term processor. Moreover, each controller may include sufficient processing capabilities to execute support applications including, without limitation, those for SCADA system  200 . Each associated controller may be coupled to associated monitoring sensors  240  and may also be coupled to and/or a portion of SCADA system  200 . 
     In operation, AC electric power generation devices  302  generate three-phase, sinusoidal, AC power (only one phase shown in  FIGS. 4 and 5 ). First bi-directional current converter  310  receives and rectifies the sinusoidal, AC power to DC power at predetermined first voltage (typically in excess of 500V) and energizes first DC link  314 . Second bi-directional current converter  312  receives the DC power transmitted from first bi-directional current converter  310  through first DC link  314  and converts the DC power having a first voltage to HFAC with an approximate square wave and a predetermined HFAC voltage. The HFAC power is transmitted through two-port passive networks  324  and voltage matching transformer  325  to facilitate ZVS/ZCS operation and reducing electromagnetic interference (EMI) induced by switching effects of power conversion module  306  and to condition the amplitude of the HFAC power. Mechanical isolation device  326  is typically closed. Therefore, the conditioned HFAC power is transmitted to third bi-directional power converter  316  that converts the HFAC power to HVDC power in a predetermined voltage range (typically in excess of 500V) that energized second DC link  318  and transmits power to HVDC transmission system  304 . 
     In some circumstances, electric power may flow in the reverse direction, i.e., bi-directional power converters  310 ,  312 , and  316  facilitate converting HVDC power from system  304  to AC power that is transmitted to system  302 . Also, in operation, monitoring sensors  240  transmit real-time operational status and feedback information to SCADA system  200 , and in some embodiments, directly to, without limitation, each of first, second, and third bi-directional current converters  310 ,  312 , and  316 , respectively, and mechanical isolation device  326 . 
       FIG. 6  is a schematic diagram of a portion, i.e., one of the three phases of electric power conversion system  300  with a fault  334  shown in HVDC electric power transmission system  304 . Referring to  FIGS. 4 ,  5 , and  6 , fault  334  induces a rapidly increasing electric current flow through modules  306  that may be accompanied by voltage fluctuations, both sensed by monitoring sensors  240 . The information is transmitted to SCADA  200  and/or second bi-directional current converter  312  and mechanical isolation device  326 . SCADA  200  determines if electric current transmission through at least one of electric power conversion system  300  and HVDC transmission system  304  is one of approaching, attaining, and exceeding at least one parameter of at least one of electric power conversion system  300  and HVDC transmission system  304 . If SCADA system  200  determines that protective action is required, electric current transmission through isolatable portion  328  is decreased through regulation of second bi-directional current converter  312  until the current decreases to a predetermined value, e.g., approximately zero amperes. Such protective action is completed in less than approximately 100 microseconds (μs), and may be completed in less than 50 μs. SCADA system  200  then opens mechanical isolation device  326  when current transmitted therethrough is approximately zero amperes, such opening taking approximately 1 millisecond (ms). In the event the fault clears quickly enough, mechanical isolation device  326  may not need to open. Any current value that enables operation of SCADA system  200  and electric power conversion system  300  may be used. While only one power conversion module  306  is shown in  FIGS. 4 and 5 , and  FIG. 6  only shows one of three phases, all of power conversion modules  306  in all phases are similarly, and substantially simultaneously, isolated such that electric power conversion system  300  is fully removed from service until fault  334  is cleared. 
     SCADA system  200  may also facilitate restoration of electric power conversion system  300  to service. Mechanical isolation device  326  is closed in each module  306  substantially simultaneously such that isolatable portion  328  is unisolated and full service is restored by increasing electric current transmission through isolable portion  328  through regulation of second bi-directional power converter  312 . 
       FIG. 7  is a schematic diagram of an alternative power conversion module  340  that may be used with electric power conversion system  300 . Power conversion module  340  includes two sub-modules, i.e., a first sub-module  342  that is similar to module  306  with the exception of connections to two second sub-modules  344 . Power conversion module  340  facilitates greater power transmission through power converter system  300  than module  306  due to the two additional sub-modules  344  in parallel with sub-module  342 . SCADA  200  (shown in  FIG. 2 ) is used to monitor and control operation of power conversion module  340  and operation of each of sub-modules  342  and  344  is similar to operation of modules  306 . Any combination of sub-modules  342  and  344  may be used in any of the phases and any of the modules  340  that enable operation of power converter system  300  as described herein. 
       FIG. 8  is a schematic diagram of an alternative electric power conversion system  400  that may be monitored and controlled using SCADA system  200  (shown in  FIG. 2 ). In this alternative embodiment, electric power conversion system  400  couples a first DC system  402  to a second DC system  404  that may be similar to HVDC electric power transmission system  304  (shown in  FIGS. 3-7 ). Both DC systems  402  and  404  may be rated for voltages in excess of 100 kilovolts (kV). Alternatively, DC systems  402  and  404  may have any voltage rating that enables operation of electric power conversion system  400  as described herein. Also, in this alternative exemplary embodiment, electric power conversion system  400  includes a plurality of power conversion modules  406  electrically coupled in series to first DC system  402  and second DC system  404  through a first set of DC terminals  405  and a second set of DC terminals  407 , respectively. Electric power conversion system  400  includes n power conversion modules  406 . 
       FIG. 9  is a schematic diagram of alternative power conversion module  406  that may be used with electric power conversion system  400 .  FIG. 10  is another schematic diagram of power conversion module  406 . In this alternative embodiment, electric power conversion system  400  couples first DC system  402  to second DC system  404 . Power conversion module  406  includes a first mechanical isolation device  408  and a second mechanical isolation device  410  positioned between a first bi-directional power converter  412  and a second bi-directional power converter  416 . First and second mechanical isolation devices  408  and  410 , respectively, are similar to mechanical isolation device  326  (shown in  FIGS. 4 ,  5 , and  7 ). First bi-directional power converter  412  is similar to second bi-directional power converter  312  (shown in  FIGS. 4 ,  5 , and  7 ) and second bi-directional power converter  416  is similar to third bi-directional power converter  316  (shown in  FIGS. 4 ,  5 , and  7 ). 
     Also, in this alternative embodiment, first bi-directional power converter  412  and second mechanical isolation device  410  are operatively coupled to define a first isolable portion  420  of power conversion module  406 . Similarly, second bi-directional power converter  416  and first mechanical isolation device  408  are operatively coupled to define a second isolable portion  422  of power conversion module  406 . SCADA  200  (shown in  FIG. 2 ) is used to monitor and control operation of power conversion module  406  and operation of module  406  is similar to operation of modules  306  (shown in  FIGS. 3-6 ). 
       FIG. 11  is a schematic diagram of another alternative electric power conversion system  430  that may be monitored and controlled using SCADA system  200  (shown in  FIG. 2 ). In this alternative embodiment, electric power conversion system  430  couples first DC system  402  to second DC system  404 . Also, in this alternative embodiment, electric power conversion system  430  includes a plurality of electric power conversion systems  400  coupled to each other through a plurality of DC rails  432 . Further, in this embodiment, electric power conversion systems  400  are vertically stacked (shown slightly off-center for clarity) to form electric power conversion system  430 . Alternatively, any number of electric power conversion systems  400  may be configured in any arrangement that enables operation of electric power conversion system  430  as described herein. SCADA  200  (shown in  FIG. 2 ) is used to monitor and control operation of power conversion system  430  and operation of system  430  is similar to operation of modules  306  (shown in  FIGS. 3-6 ). 
       FIG. 12  is a schematic diagram of yet another alternative electric power conversion system  450  that may be monitored and controlled using SCADA system  200  (shown in  FIG. 2 ). In this alternative embodiment, electric power conversion system  450  couples first DC system  402  to second DC system  404 . Also, in this alternative embodiment, electric power conversion system  430  includes a plurality of electric power conversion systems  400  coupled to each other through a plurality of first DC conduits  452  configured for a first polarity and second DC conduits  454  configured for a second polarity. Alternatively, any number of electric power conversion systems  400  may be configured in any arrangement that enables operation of electric power conversion system  450  as described herein. SCADA  200  (shown in  FIG. 2 ) is used to monitor and control operation of power conversion system  430  and operation of system  450  is similar to operation of modules  306  (shown in  FIGS. 3-6 ). 
       FIG. 13  is a schematic diagram of an alternative power conversion module  506  that may be used with electric power conversion system  400  (shown in  FIG. 8 ). In this alternative embodiment, power conversion module  506  couples first DC system  402  to second DC system  404 . Power conversion module  506  includes mechanical isolation device  326  and bi-directional power converter  412  that are operatively coupled to define an isolable portion  520  of power conversion module  506 . Power conversion module  506  also includes a diode rectifier device  525  that facilitates one way electric current transmission from first DC system  402  to second DC system  404  and prevents current transmission in the opposite direction. SCADA  200  (shown in  FIG. 2 ) is used to monitor and control operation of power conversion module  506  and operation of module  506  is similar to operation of modules  306  (shown in  FIGS. 3-6 ). Power conversion module  506  may be used with electric power conversion system  400  to form stacked systems similar to system  430  (shown in  FIG. 11 ) and paralleled system similar to system  450  (shown in  FIG. 12 ). 
       FIG. 14  is a schematic diagram of another alternative power conversion module  606  that may be used with electric power conversion system  400  (shown in  FIG. 8 ). Power conversion module  606  is similar to power conversion module  506  with the exception that power conversion module  606  includes a levelizing electric power converter  625  coupled to electric power converter  412  through a DC link  630 . Levelizing electric power converter  625  facilitates equal power sharing with a linear control of levelizing converter  625 . Such linear levelizing facilitates increasing the efficiency of power conversion through an even distribution of power as compared to non-linear control of converters  412  in power conversion module  506 . Power conversion module  606  may be used with electric power conversion system  400  to form stacked systems similar to system  430  (shown in  FIG. 11 ) and paralleled system similar to system  450  (shown in  FIG. 12 ). 
     The above-described electric power conversion systems for HVDC transmission systems provide a cost-effective method for transmitting HVDC power. The embodiments described herein facilitate transmitting HVDC power across relatively large distances while facilitating rapid detection and isolation of affected electric power conversion systems in the event of electrical faults on the HVDC transmission system. The embodiments described herein also facilitate rapid restoration of the affected electric power conversion systems after the faulted portions of the system are effectively isolated. Specifically, the devices, systems, and methods described herein include a plurality of bi-directional power converters and mechanical isolation devices that are operatively coupled to the bi-directional power converters. The bi-directional power converters and mechanical isolation devices define isolable portions of the electric power conversion system. The bi-directional power converters facilitate real-time decreasing of electric current through the isolable portions in the event that electric current sensed being transmitted through the conversion system or on the HVDC transmission system exceeds parameters as communicated in real time to a Supervisory Control and Data Acquisition (SCADA) system. Specifically, in the event that sensed electric current exceed parameters, the bi-directional power converters decrease the electric current transmitted therethrough and as the current approaches or attains a value of approximately zero, the SCADA system initiates opening the associated mechanical isolation devices with a significantly reduced load that will approach zero amperes, thereby mitigating a potential for damage to the contactors of the mechanical isolation devices. Fault isolation occurs approximately three orders of magnitude more rapidly than the typical five milliseconds (ms) needed to reduce a partial for a reduction of service life to the affected components. 
     Also, the devices, systems, and methods described herein facilitate system restoration. Once the electrical fault is cleared, the SCADA system will initiate post-fault recovery actions. Specifically, the cleared mechanical isolation devices will reclose under near-zero loads and the associated bi-directional power converters will increase the current transmitted through the restored isolable portions to restore DC power transmission to the affected portions expeditiously. 
     An exemplary technical effect of the methods, systems, and apparatus described herein includes at least one of: (a) decreasing a time period of fault detection and isolation on a HVDC transmission system and associated electric power conversion systems through a mechanical isolation device to less than 100 μs, and in some cases, less than approximately 50 μs; (b) decreasing exposure of electric power conversion system components to elevated electric current conditions due to a fault through decreasing power transmission through isolable portions of the electric power conversion systems to near-zero values and opening an associated mechanical isolation device in less than 100 μs, and in some cases, approximately 50 μs, i.e., approximately three orders of magnitude less than the time to isolate through a mechanical isolation device; (c) decreasing transmission of electric current through mechanical isolation devices to near-zero values to facilitate rapid opening to clear electrical fault conditions and reclosing to restore power transmission; and (d) substantially reducing a need to use slower acting and costly DC circuit breakers to isolate faults. 
     Exemplary embodiments of HVDC transmission systems and electric power conversion systems for coupling power generation facilities and the grid, and methods for operating the same, are described above in detail. The HVDC transmission systems, bi-directional power converters, and methods of operating such systems and devices are not limited to the specific embodiments described herein, but rather, components of systems and/or steps of the methods may be utilized independently and separately from other components and/or steps described herein. For example, the methods may also be used in combination with other systems requiring HVDC transmission and power conversion and the associated methods, and are not limited to practice with only the HVDC transmission systems, bi-directional power converters, and methods as described herein. Rather, the exemplary embodiment can be implemented and utilized in connection with many other DC transmission applications that are currently configured to receive and accept bi-directional power converters, e.g., and without limitation, DC distribution systems in remote areas and industrial facilities. 
     Although specific features of various embodiments of the invention may be shown in some drawings and not in others, this is for convenience only. In accordance with the principles of the invention, any feature of a drawing may be referenced and/or claimed in combination with any feature of any other drawing. 
     Some embodiments involve the use of one or more electronic or computing devices. Such devices typically include a processor or controller, such as a general purpose central processing unit (CPU), a graphics processing unit (GPU), a microcontroller, a reduced instruction set computer (RISC) processor, an application specific integrated circuit (ASIC), a programmable logic circuit (PLC), and/or any other circuit or processor capable of executing the functions described herein. The methods described herein may be encoded as executable instructions embodied in a computer readable medium, including, without limitation, a storage device and/or a memory device. Such instructions, when executed by a processor, cause the processor to perform at least a portion of the methods described herein. The above examples are exemplary only, and thus are not intended to limit in any way the definition and/or meaning of the term processor. 
     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 language of the claims.