Patent Publication Number: US-9853558-B2

Title: High voltage direct current (HVDC) transmission system to compensate for voltage values output from a plurality of potential transformers

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Pursuant to 35 U.S.C. §119(a), this application claims the benefit of earlier filing date and right of priority to Korean Patent Application No. 10-2014-0057357 filed on May 13, 2014, the contents of which are all hereby incorporated by reference in its entirety. 
     BACKGROUND 
     The present disclosure relates a high voltage direct current (HVDC) transmission system, and more particularly, to an HVDC system that may compensate for the deviation of voltage values measured through a plurality of potential transformers. 
     High voltage direct current (HVDC) transmission indicates converting alternating current (AC) power produced at a power station into DC power by a transmission site to transmit the DC power, and then re-converting the DC power into the AC power by a reception site to supplying the AC power. 
     An HVDC transmission system is applied to submarine cable power transmission, large-amount long-distance power transmission, interconnection between AC systems, etc. Also, the HVDC transmission system enables different frequency systems interconnection and asynchronism interconnection. 
     The transmission site converts the AC power into the DC power. That is, since transmitting the AC power by using a submarine cable is significantly dangerous, the transmission site converts the AC power into the DC power to transmit the DC power to the reception site. 
     There are many types of voltage source inverters used in the HVDC transmission system, and a modular multi-level voltage source converter is recently receiving attention most remarkably. 
     A modular multi-level converter (MMC) is a device converting DC power into AC power by using a plurality of sub modules and operates by allowing each of the sub modules to have a charging, discharging and bypass state. 
     The potential transformers are installed at many locations in the HVDC transmission system for the purpose of system control and system protection. 
     However, the potential transformers may have an error of measurement because they measure different measurement values according to a voltage measurement range even when they are the same product, which is recognized as a system fault and thus there may be a need to stop system operation in a serious case. 
     That is, the voltage measurement error of a single potential transformer generally needs to have a range of about 0.2% to about 0.5%. However, a potential transformer applied to the HVDC transmission system experiences a significant increase in an error range when measuring a high voltage, and when a plurality of potential transformers are installed at many locations, the error range caused by the error of measurement that each potential transformer has, has no choice but to increase. 
     To this end, it is also possible to use a more precise sensing device, but there are limitations in installation cost and technology in order to use the more precise sensing device. 
     SUMMARY 
     Embodiments provide an high voltage direct current (HVDC) transmission system that may compensate for the deviation of voltage values measured at a plurality of potential transformers. 
     Technical tasks to be achieved by proposed embodiments are not limited to the above-mentioned technical tasks and other technical tasks not mentioned will be able to be clearly understood by a person skilled in the art from the following descriptions. 
     In one embodiment, a high voltage direct current (HVDC) transmission system includes: an integrated measurement panel configured to detect a voltage measurement deviation between first to Nth potential transformers to be installed at different locations in the HVDC transmission system and determine a compensation value for compensating for the detected voltage measurement deviation; and an HVDC transmission device configured to measure actual voltage values for respective locations at which the first to Nth potential transformers are installed, by using voltage values output through the first to Nth potential transformers, when the first to Nth potential transformers are installed at different locations in the HVDC transmission system, wherein the HVDC transmission device is configured to: receive, from the integrated measurement panel, compensation values for compensating for the voltage values output through the first to Nth potential transformers, compensate for the voltage values output through the first to Nth potential transformers by using the received compensation values, and measure the actual voltage values by using the compensated voltage values. 
     The integrated measurement panel may be configured to: provide the first to Nth potential transformers to be installed at different locations in the HVDC transmission system, supply a first voltage to the first to Nth potential transformers provided, measure the voltage values output through the first to Nth transformers by the first voltage supplied, and determine compensation values for correcting the measured voltage values to the same voltage value when there is a deviation between the measured voltage values. 
     The first to Nth potential transformers provided may be configured to be connected to the integrated measurement panel in common, and the voltage values output through the first to Nth potential transformers provided may be measured through the integrated measurement panel. 
     The integrated measurement panel may be configured to calculate the average value of the measured voltage values, and determine compensation values for compensating for the deviation of the voltage values based on the calculated average value. 
     The integrated measurement panel may be configured to determine compensation values for standardizing the measured voltage values to a first voltage supplied to the first to Nth potential transformers. 
     The integrated measurement panel may be configured to sequentially supply, to the first to Nth potential transformers provided, voltages belonging to a voltage range divided into a plurality of sections, and 
     the compensation values may be determined with respect to the voltage range of each section sequentially supplied. 
     The HVDC transmission device may include: a deviation compensator configured to measure, by using the compensation values, the actual voltage values for respective locations at which the first to Nth potential transformers are installed, and a central controller configured to control the operation of the HVDC transmission device by using the actual voltage values measured through the deviation compensator. 
     The deviation compensator may be configured to transmit information on respective voltage sections of the output voltage values of the first to Nth potential transformers to the integrated measurement panel, and the integrated measurement panel may be configured to extract compensation values for respective voltage sections among the compensation values for the first to Nth potential transformers to provide extracted compensation values to the deviation compensator. 
     The compensation values may be at least one of a transformation ratio, offset and gain values to be applied to the first to Nth potential transformers. 
     According to embodiments, before potential transformers are installed at the HVDC transmission system, a compensation value for compensating for the measurement deviation between the potential transformers installed at the HVDC transmission system and when the potential transformers are installed at the HVDC transmission system, the determined compensation value is applied to measure a voltage value. Thus, it is possible to enhance the reliability of the HVDC transmission system. 
     Also, according to embodiments, it is possible to compensate for the deviation of a plurality of potential transformers by using the same measurement panel and thus, since the deviation of voltage values measured through the plurality of potential transformers decreases, it is possible to perform accurate operations, such as voltage deviation protection as well as it is easy to control the HVDC transmission system. 
     Also, according to embodiments, the HVDC transmission system transmitting power may minimize a loss caused by a measurement deviation and thus it is possible to decrease a system loss. 
     Also, according to embodiments, when a specific potential transformer has a fault and thus needs to be replaced, it is possible to easily replace the specific potential transformer because information used previously for compensating for the deviation of the plurality of potential transformers has been stored. 
     The details of one or more embodiments are set forth in the accompanying drawings and the description below. Other features will be apparent from the description and drawings, and from the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram for explaining the configuration of a high voltage direct current (HVDC) transmission system according to an embodiment. 
         FIG. 2  is a diagram for explaining the configuration of a mono-polar HVDC transmission system according to an embodiment. 
         FIG. 3  is a diagram for explaining the configuration of a bipolar HVDC transmission system according to an embodiment. 
         FIG. 4  is a diagram for explaining the connection of a transformer and a three-phase valve bridge according to an embodiment. 
         FIG. 5  is a schematic block diagram of a modular multi-level converter according to an embodiment. 
         FIG. 6  is a detailed block diagram of a modular multi-level converter according to an embodiment. 
         FIG. 7  represents the connection of a plurality of sub modules according to an embodiment. 
         FIG. 8  illustrates the configuration of a sub module according to an embodiment. 
         FIG. 9  represents the deviation compensation device of a potential transformer according to an embodiment. 
         FIG. 10  shows the configuration of an HVDC transmission system according to an embodiment. 
         FIG. 11  is a flowchart for explaining, step by step, the deviation compensation method of a potential transformer according to a first embodiment. 
         FIG. 12  is a flowchart for explaining, step by step, the deviation compensation method of a potential transformer according to a second embodiment. 
         FIG. 13  is a flowchart for explaining, in detail, the compensation value determination process in  FIGS. 11 and 12 . 
         FIG. 14  is a flow chart for explaining, step by step, a method of measuring a voltage value in an HVDC transmission system according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Some embodiments are described below in more detail with reference to the accompanying drawings. In the following description, since the suffixes “module” and “unit” for components are given and interchanged for easiness in making the present disclosure, they do not have distinct meanings or functions. 
     The effects and features of the inventive concept, and implementation methods thereof will be clarified through following embodiments described with reference to the accompanying drawings. An embodiment may, however, be embodied in different forms and the inventive concept should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided to make this disclosure thorough and complete and fully convey the scope of the inventive concept to a person skilled in the art. Further, the inventive concept is only defined by scopes of claims. The same reference numerals throughout the disclosure refer to the same components. 
     When describing embodiments, detailed descriptions related to known functions or configurations will be ruled out in order not to unnecessarily obscure subject matters of the embodiments. In addition, since the terms used herein are defined in consideration of functions in the embodiments, they may vary depending on a user&#39;s, operator&#39;s intention or practice. Therefore, the definitions need to be made based on details throughout the inventive concept. 
     Combinations of each block of the accompanying drawings and each step of a flowchart may also be performed by computer program instructions. Since the computer program instructions may be loaded on the processor of a general-purpose computer, a special-purpose computer or other programmable data processing equipment, the instructions performed by the processor of the computer or other programmable data processing equipment create a means that performs functions described on each block of a drawing or each step of a flowchart. Since the computer program instructions may also be stored in a computer usable or computer readable memory that may aim at the computer or other programmable data processing equipment in order to implement functions in a specific manner, the instructions stored in the computer usable or computer readable memory may also produce an item that includes an instruction means performing functions described on each block of a drawing or each step of a flowchart. The computer program instructions may also be loaded on the computer or other programmable data processing equipment. Thus, since a series of operation steps are performed on the computer or other programmable data processing equipment to create processes executed by a computer, instructions operating the computer or other programmable data processing equipment may also provide steps for performing functions described on each block of a drawing and each step of a flowchart. 
     Also, each block or each step may represent a portion of a module, a segment or a code that includes one or more executable instructions for performing specific logical function(s). Also, it should be noted that some alternatives may be performed in such a way that functions mentioned at blocks or steps are performed in a different order. For example, two blocks or steps shown one after another may also be performed substantially at the same time or the blocks or steps may also be sometimes performed in a reverse order according to a corresponding function. 
       FIG. 1  shows a high voltage direct current (HVDC) transmission system according to an embodiment. 
     As shown in  FIG. 1 , an HVDC transmission system  100  according to an embodiment includes a power generation part  101 , a transmission-side alternating current (AC) part  110 , a transmission-side transformation part  103 , a DC transmission part  140 , a reception-side transformation part  105 , a reception-side AC part  170 , a reception part  180 , and a control part  190 . The transmission-side transformation part  103  includes a transmission-side transformer part  120 , and a transmission-side AC/DC converter part  130 . The reception-side transformation part  105  includes a reception-side DC/AC converter part  150 , and a reception-side transformer part  160 . 
     The power generation part  101  generates three-phase AC power. The power generation part  101  may include a plurality of power stations. 
     The transmission-side AC part  110  transmits the three-phase AC power generated by the power generation part  101  to a DC substation that includes the transmission-side transformer part  120  and the transmission-side AC/DC converter part  130 . 
     The transmission-side transformer part  120  isolates the transmission-side AC part  110  from the transmission-side AC/DC converter part  130  and the DC transmission part  140 . 
     The transmission-side AC/DC converter part  130  converts, into AC power, three-phase AC power corresponding to the output of the transmission-side transformer part  120 . 
     The DC transmission part  140  transmits transmission-side DC power to a reception side. 
     The reception-side DC/AC converter part  150  converts DC power transmitted by the DC transmission part  140 , into three-phase AC power. 
     The transmission-side AC/DC converter part  130  includes a converter that converts AC power to DC power and the reception-side DC/AC converter part  150  includes a converter that converts DC power to AC power. The converter of the transmission-side AC/DC converter part  130  and the converter of the reception-side DC/AC converter part  150  each includes potential transformers. The potential transformers may be implemented in an arm sensor  231  included in an arm controller  230  or implemented in a sub module sensor  211  included in a sub module  210 . The arm sensor  231  or the sub module sensor  211  is described with reference to  FIG. 6 . 
     The reception-side transformer part  160  isolates the reception-side AC part  170  from the reception-side DC/AC converter part  150  and the DC transmission part  140 . 
     The reception-side AC part  170  provides, to the reception part  180 , three-phase AC power corresponding to the output of the reception-side transformer part  160 . 
     The control part  190  controls at least one of the power generation part  101 , the transmission-side AC part  110 , the transmission-side transformation part  103 , the DC transmission part  140 , the reception-side transformation part  105 , the reception-side AC part  170 , the reception part  180 , the control part  190 , the transmission-side AC/DC converter part  130 , and the reception-side DC/AC converter part  150 . In particular, the control part  190  may control the turn-on and turn-off timings of a plurality of valves in the transmission-side AC/DC converter part  130  and the reception-side AC/DC converter part  150 . In this case, the valve may correspond to a thyristor or insulated gate bipolar transistor (IGBT). 
       FIG. 2  shows a mono-polar HVDC transmission system according to an embodiment. 
     In particular,  FIG. 2  shows a system transmitting single pole DC power. Although it is assumed in the following description that the single pole is a positive pole, there is no need to be limited thereto. 
     The transmission-side AC part  110  includes an AC transmission line  111  and an AC filter  113 . 
     The AC transmission line  111  transmits three-phase AC power generated by the power generation part  101 , to the transmission-side transformation part  103 . 
     The AC filter  113  removes other frequency components excluding frequency components used by the transformation part  103 , from the transmitted three-phase AC power. 
     The transmission-side transformer part  120  includes one or more transformers  121  for the positive pole. For the positive pole, the transmission-side AC/DC converter part  130  includes an AC/positive-pole DC converter  131  generating positive-pole DC power, and the AC/positive-pole DC converter  131  includes one or more three-phase valve bridges  131   a  corresponding to the one or more transformers  121 , respectively. 
     When one three-phase valve bridge  131   a  is used, the AC/positive-pole DC converter  131  may use AC power to generate positive-pole DC power having six pulses. In this case, the primary and secondary coils of the transformer  121  of the valve bridge may have a Y-Y connection or Y-Δ connection. 
     When two three-phase valve bridges  131   a  are used, the AC/positive-pole DC converter  131  may use AC power to generate positive-pole DC power having 12 pulses. In this case, the primary and secondary coils of the transformer  121  of one of the two valve bridges may have a Y-Y connection, and the primary and secondary coils of the transformer  121  of the other of the two valve bridges may have a Y-Δ connection. 
     When three three-phase valve bridges  131   a  are used, the AC/positive-pole DC converter  131  may use AC power to generate positive-pole DC power having 18 pulses. The more the number of pulses of the positive-pole DC power, the price of the filter may decrease. 
     The DC transmission part  140  includes a transmission-side positive-pole DC filter  141 , a positive-pole DC transmission line  143 , and a reception-side positive-pole DC filter  145 . The transmission-side positive-pole DC filter  141  includes an inductor L 1  and a capacitor C 1  and DC-filters positive-pole DC power output by the AC/positive-pole DC converter  131 . 
     The positive-pole DC transmission line  143  may have a DC line for transmission of positive-pole DC power, and earth may be used as a current feedback path. One or more switches may be disposed on the DC line. 
     The reception-side positive-pole DC filter  145  includes an inductor L 2  and a capacitor C 2  and DC-filters positive-pole DC power transmitted through the positive-pole DC transmission line  143 . 
     The reception-side DC/AC converter part  150  includes a positive DC/AC converter  151 , which includes one or more three-phase valve bridges  151   a.    
     The reception-side transformer part  160  includes one or more transformers  161  corresponding respectively to one or more three-phase valve bridges  151   a  for the positive pole. 
     When one three-phase valve bridge  151   a  is used, the positive-pole DC/AC converter  151  may use positive-pole DC power to generate AC power having six pulses. In this case, the primary and secondary coils of the transformer  161  of the valve bridge may have a Y-Y connection or Y-Δ connection. 
     When two three-phase valve bridges  151   a  are used, the positive-pole DC/AC converter  151  may use positive-pole DC power to generate AC power having 12 pulses. In this case, the primary and secondary coils of the transformer  161  of one of the two valve bridges may have a Y-Y connection, and the primary and secondary coils of the transformer  161  of the other of the two valve bridges may have a Y-Δ connection. 
     When three three-phase valve bridges  151   a  are used, the positive-pole DC/AC converter  151  may use positive-pole DC power to generate AC power having 18 pulses. The more the number of pulses of the AC power, the price of the filter may decrease. 
     The reception-side AC part  170  includes an AC filter  171  and an AC transmission line  173 . 
     The AC filter  171  removes other frequency components excluding the frequency component (e.g., about 60 Hz) used by the reception part  180 , from the AC power generated by the reception-side transformation part  105 . 
     The AC transmission line  173  transmits filtered AC power to the reception part  180 . 
       FIG. 3  shows a bipolar HVDC transmission system according to an embodiment. 
     In particular,  FIG. 3  shows a system transmitting two-pole DC power. Although it is assumed in the following description that the two poles are a positive pole and a negative pole, there is no need to be limited thereto. 
     A transmission-side AC part  110  includes an AC transmission line  111  and an AC filter  113 . 
     The AC transmission line  111  transmits three-phase AC power generated by a power generation part  101 , to a transmission-side transformation part  103 . 
     An AC filter  113  removes other frequency components excluding frequency components used by the transformation part  103 , from the transmitted three-phase AC power. 
     A transmission-side transformer part  120  includes one or more transformers  121  for the positive pole and one or more transformers  122  for the negative pole. A transmission-side AC/DC converter part  130  includes an AC/positive-pole DC converter  131  generating positive-pole DC power and an AC/negative-pole DC converter  132  generating negative-pole DC power, the AC/positive-pole DC converter  131  includes one or more three-phase valve bridges  131   a  corresponding respectively to one or more transformers  121  for the positive-pole, and the AC/negative-pole DC converter  132  includes one or more three-phase valve bridges  132   a  corresponding respectively to one or more transformers  122  for the negative-pole. 
     When one three-phase valve bridge  131   a  is used for the positive pole, the AC/positive-pole DC converter  131  may use AC power to generate positive-pole DC power having six pulses. In this case, the primary and secondary coils of the transformer  121  of the valve bridge may have a Y-Y connection or Y-Δ connection. 
     When two three-phase valve bridges  131   a  are used for the positive pole, the AC/positive-pole DC converter  131  may use AC power to generate positive-pole DC power having 12 pulses. In this case, the primary and secondary coils of the transformer  121  of one of the two valve bridges may have a Y-Y connection, and the primary and secondary coils of the transformer  121  of the other of the two valve bridges may have a Y-Δ connection. 
     When three three-phase valve bridges  131   a  are used for the positive pole, the AC/positive-pole DC converter  131  may use AC power to generate positive-pole DC power having 18 pulses. The more the number of pulses of the positive-pole DC power, the price of the filter may decrease. 
     When one three-phase valve bridge  132   a  is used for the negative pole, the DC/negative-pole DC converter  132  may generate negative-pole DC power having six pulses. In this case, the primary and secondary coils of the transformer  122  of the valve bridge may have a Y-Y connection or Y-Δ connection. 
     When two three-phase valve bridges  132   a  are used for the negative pole, the AC/negative-pole DC converter  132  may generate negative-pole DC power having 12 pulses. In this case, the primary and secondary coils of the transformer  122  of one of the two valve bridges may have a Y-Y connection, and the primary and secondary coils of the transformer  122  of the other of the two valve bridges may have a Y-Δ connection. 
     When three three-phase valve bridges  132   a  are used for the negative pole, the AC/negative-pole DC converter  132  may generate negative-pole DC power having 18 pulses. The more the number of pulses of the negative-pole DC power, the price of the filter may decrease. 
     The DC transmission part  140  includes a transmission-side positive-pole DC filter  141 , a transmission-side negative-pole DC filter  142 , a positive-pole DC transmission line  143 , a negative-pole DC transmission line  144 , a reception-side positive-pole DC filter  145 , and a reception-side negative-pole DC filter  146 . 
     The transmission-side positive-pole DC filter  141  includes an inductor L 1  and a capacitor C 1  and DC-filters positive-pole DC power output by the AC/positive-pole DC converter  131 . 
     The transmission-side negative-pole DC filter  142  includes an inductor L 3  and a capacitor C 3  and DC-filters negative-pole DC power output by the AC/negative-pole DC converter  132 . 
     The positive-pole DC transmission line  143  may have a DC line for transmission of positive-pole DC power, and earth may be used as a current feedback path. One or more switches may be disposed on the DC line. 
     The negative-pole DC transmission line  144  may have a DC line for transmission of negative-pole DC power, and earth may be used as a current feedback path. One or more switches may be disposed on the DC line. 
     The reception-side positive-pole DC filter  145  includes an inductor L 2  and a capacitor C 2  and DC-filters positive-pole DC power transmitted through the positive-pole DC transmission line  143 . 
     The reception-side negative-pole DC filter  146  includes an inductor L 4  and a capacitor C 4  and DC-filters negative-pole DC power transmitted through the negative-pole DC transmission line  144 . 
     The reception-side DC-AC converter part  150  includes a positive-pole DC/AC converter  151  and a negative-pole DC-AC converter  152 , the positive-pole DC/AC converter  151  includes one or more three-phase valve bridges  11   a , and the negative-pole DC-AC converter  152  includes one or more three-phase valve bridges  152   a.    
     The reception-side transformer part  160  includes one or more transformers  161  corresponding respectively to one or more three-phase valve bridges  151   a  for the positive pole and one or more transformers  162  corresponding respectively to one or more three-phase valve bridges  152   a  for the negative pole. 
     When one three-phase valve bridge  151   a  is used for the positive pole, the positive-pole DC/AC converter  151  may use positive-pole DC power to generate AC power having six pulses. In this case, the primary and secondary coils of the transformer  161  of the valve bridge may have a Y-Y connection or Y-Δ connection. 
     When two three-phase valve bridges  151   a  are used for the positive pole, the positive-pole DC/AC converter  151  may use positive-pole DC power to generate AC power having 12 pulses. In this case, the primary and secondary coils of the transformer  161  of one of the two valve bridges may have a Y-Y connection, and the primary and secondary coils of the transformer  161  of the other of the two valve bridges may have a Y-Δ connection. 
     When three three-phase valve bridges  151   a  are used for the positive pole, the positive-pole DC/AC converter  151  may use positive-pole DC power to generate AC power having 18 pulses. The more the number of pulses of the AC power, the price of the filter may decrease. 
     When one three-phase valve bridge  152   a  is used for the negative pole, the negative-pole DC/AC converter  152  may use negative-pole DC power to generate AC power having six pulses. In this case, the primary and secondary coils of the transformer  162  of the valve bridge may have a Y-Y connection or Y-Δ connection. 
     When two three-phase valve bridges  152   a  are used for the negative pole, the negative-pole DC/AC converter  152  may use negative-pole DC power to generate AC power having 12 pulses. In this case, the primary and secondary coils of the transformer  162  of one of the two valve bridges may have a Y-Y connection, and the primary and secondary coils of the transformer  162  of the other of the two valve bridges may have a Y-Δ connection. 
     When three three-phase valve bridges  152   a  are used for the negative pole, the negative-pole DC/AC converter  152  may use negative-pole DC power to generate AC power having 18 pulses. The more the number of pulses of the AC power, the price of the filter may decrease. 
     A reception-side AC part  170  includes an AC filter  171  and an AC transmission line  173 . 
     The AC filter  171  removes other frequency components excluding the frequency component (e.g., about 60 Hz) used by the reception part  180 , from the AC power generated by the reception-side transformation part  105 . 
     The AC transmission line  173  transmits filtered AC power to the reception part  180 . 
       FIG. 4  shows the connection of a transformer and a three-phase valve bridge according to an embodiment. 
     In particular,  FIG. 4  shows the connection of two transformers  121  for a positive pole and two three-phase valve bridges  131   a  for the positive pole. Since the connection of two transformers  122  for a negative pole and two three-phase valve bridges  132   a  for the negative pole, the connection of two transformers  161  for the positive pole and two three-phase valve bridges  151   a  for the positive pole, the connection of two transformers  162  for the negative pole and two three-phase valve bridges  152   a  for the negative pole, the connection of one transformer  121  for the positive pole and one three-phase valve bridge  131   a  for the positive pole, and the connection of one transformer  161  for the positive pole and one three-phase valve bridge  151   a  for the positive pole may be easily derived from the embodiment of  FIG. 4 , related drawings and descriptions are omitted. 
     In  FIG. 4 , the transformer  121  having a Y-Y connection is referred to as an upper transformer, the transformer  121  having a Y-Δ connection is referred to as a lower transformer, the three-phase valve bridge  131   a  connected to the upper transformer is referred to as an upper three-phase valve bridge, and the three-phase valve bridge  131   a  connected to the lower transformer is referred to as a lower three-phase valve bridge. 
     The upper three-phase valve bridge and the lower three-phase valve bridge have a first output OUT 1  and a second output OUT 2  that are two outputs outputting DC power. 
     The upper three-phase valve bridge includes six valves D 1  to D 6  and the lower three-phase valve bridge includes six valves D 7  to D 12 . 
     The valve D 1  has a cathode connected to the first output OUT 1  and an anode connected to the first terminal of the secondary coil of the upper transformer. 
     The valve D 2  has a cathode connected to the anode of the valve D 5  and an anode connected to the anode of the valve D 6 . 
     The valve D 3  has a cathode connected to the first output OUT 1  and an anode connected to the second terminal of the secondary coil of the upper transformer. 
     The valve D 4  has a cathode connected to the anode of the valve D 1  and an anode connected to the anode of the valve D 6 . 
     The valve D 5  has a cathode connected to the first output OUT 1  and an anode connected to the third terminal of the secondary coil of the upper transformer. 
     The valve D 6  has a cathode connected to the anode of the valve D 3 . 
     The valve D 7  has a cathode connected to the anode of the valve D 6  and an anode connected to the first terminal of the secondary coil of the lower transformer. 
     The valve D 8  has a cathode connected to the anode of the valve D 11  and an anode connected to the second output OUT 2 . 
     The valve D 9  has a cathode connected to the anode of the valve D 6  and an anode connected to the second terminal of the secondary coil of the lower transformer. 
     The valve D 10  has a cathode connected to the anode of the valve D 7  and an anode connected to the second output OUT 2 . 
     The valve D 11  has a cathode connected to the anode of the valve D 6  and an anode connected to the third terminal of the secondary coil of the lower transformer. 
     The valve D 12  has a cathode connected to the anode of the valve D 9  and an anode connected to the second output OUT 2 . 
     The reception-side DC/AC converter part  150  may include a modular multi-level converter  200 . 
     The modular multi-level converter  200  may use a plurality of sub modules  210  to convert DC power into AC power. 
     The configuration of the modular multi-level converter  200  is described with reference to  FIG. 5 . 
       FIG. 5  is a block diagram of the modular multi-level converter  200 . 
     The modular multi-level converter  200  includes a controller  250 , a plurality of arm controllers  230 , and a plurality of sub modules  210 . 
     The controller  250  may control the plurality of arm controllers  230  and each of the arm controllers  230  may control the plurality of sub modules  210 . 
     The configuration of the modular multi-level converter  200  is described in detail with reference to  FIG. 6 . 
       FIG. 6  is a block diagram of the modular multi-level converter  200 . 
     The modular multi-level converter  200  includes the sub module  210 , the arm controller  230 , and the controller  250   
     The sub module  210  may receive DC power to perform charging, discharging and bypass operations, and includes a sub module sensor  211 , a sub module control unit  213 , a switching unit  217 , and a storage unit  219 . 
     The sub module sensor  211  may measure one or more of the current and voltage of the sub module  210 . 
     The sub module control unit  213  may control the overall operations of the sub module  210 . 
     In particular, the sub module control unit  213  may control the current and voltage measurement operations of the sub module sensor  211  and the switching operation of the switching unit  217 . 
     The switching unit  217  may switch a current input and output from the sub module  210 . 
     The switching unit  217  includes at least one switch and may perform a switching operation according to the control signal of the sub module control unit  213 . 
     Also, the switching unit  217  may include a diode and perform the switching operation and the charging, discharging and bypass operations of the sub module  210  as the rectification operations of the diode. 
     The storage unit  219  may perform a charging operation in which energy is charged based on a current input to the sub module  210 . 
     Also, the storage unit  219  may operate a discharging operation in which a current is output based on the charged energy. 
     The connection of the plurality of sub modules  210  included in the modular multi-level converter  200  is described with reference to  FIG. 7 . 
       FIG. 7  represents the connection of the plurality of sub modules  210  included in the modular multi-level converter  200 . 
     Referring to  FIG. 7 , the plurality of sub modules  210  may be connected in series and the plurality of sub modules  210  connected to a one-phase positive-pole or negative-pole may form an arm.  2   
     The three-phase modular multi-level converter  200  may generally include six arms, because each of three phases A to C may have a positive pole and a negative pole. 
     Thus, the three-phase modular multi-level converter  200  may include a first arm  221  including the plurality of sub modules  210  for an A-phase positive-pole, a second arm  222  including the plurality of sub modules  210  for an A-phase negative-pole, a third arm  223  including the plurality of sub modules  210  for an B-phase positive-pole, a fourth arm  224  including the plurality of sub modules  210  for an B-phase negative-pole, a fifth arm  225  including the plurality of sub modules  210  for an C-phase positive-pole, and a sixth arm  226  including the plurality of sub modules  210  for an C-phase negative-pole. 
     In addition, the plurality of sub modules  210  for a phase may form a leg. 
     Thus, the three-phase modular multi-level converter  200  may include an A-phase leg  227  including the plurality of sub modules  210  for the A-phase, a B-phase leg  228  including the plurality of sub modules  210  for the B-phase, and a C-phase leg  229  including the plurality of sub modules  210  for the C-phase. 
     Thus, the first arm  221  to the sixth arm  226  are included in the A- to C-phase legs  227  to  229 . 
     In particular, the A-phase leg  227  includes the first arm  221  being an A-phase positive-pole arm and the second arm  222  being an A-phase negative-pole arm, and the B-phase leg  228  includes the third arm  223  being a B-phase positive arm and the fourth arm  224  being the B-phase negative arm. In addition, the C-phase leg  229  includes the fifth arm  225  being a C-phase positive-pole arm and the sixth arm  226  being a C-phase negative arm. 
     Also, the plurality of sub modules  210  may form a positive-pole arm  227  and a negative-pole arm  228  according to their polarity. 
     In particular, referring to  FIG. 7 , the plurality of sub modules  210  included in the modular multi-level converter  200  may be divided into a plurality of sub modules  210  corresponding to the positive pole and a plurality of sub modules  210  corresponding to the negative pole around a neutral line n. 
     Thus, the modular multi-level converter  200  may include the positive-pole arm  227  including the plurality of sub modules  210  corresponding to the positive pole and the negative pole  228  including the plurality of sub modules  210  corresponding to the negative pole. 
     Thus, the positive-pole arm  227  may include the first to third arm  221 , the third arm  223 , and the fifth arm  225 , and the negative arm  228  may include the second arm  222 , the fourth arm  224 , and the sixth arm  226 . 
     Subsequently, the configuration of the sub module  210  is described with reference to  FIG. 8 . 
       FIG. 8  illustrates the configuration of the sub module  210 . 
     Referring to  FIG. 8 , the sub module  210  includes two switches, two diodes and a capacitor. The sub module  210  of this type is referred to as a half-bridge type or half bridge inverter. 
     In addition, the switch included in the switching unit  217  may include a power semiconductor. 
     In this case, the power semiconductor indicates a semiconductor device for a power device and may be optimized for the conversion or control of power. In addition, the power semiconductor is also referred to as a valve device. 
     Thus, the switch included in the switching unit  217  may include the power semiconductor, e.g., an insulated gate bipolar transistor (IGBT), gate turn-off thyristor (GTO), integrated gate commutated thyristor (IGCT). 
     The storage unit  219  includes the capacitor and may thus charge and discharge energy. 
     A plurality of sensors is installed at the HVDC transmission system, such as a modular multi-level converter as described above. 
     That is, an arm sensor  231  or the sub module sensor  211  is installed at the HVDC transmission system as described above.  FIG. 7  depicts that only one arm sensor  231  and only one sub module sensor  211  are installed, which is for the convenience of description, and the arm sensor or the sub module sensor  211  is actually installed in plurality. 
     In this case, the arm sensor  231  or the sub module sensor  211  may be implemented in a potential transformer. However, since each of potential transformers has different error ranges, different voltage values may be output according to the potential transformer measuring a voltage even when the same voltage is measured. 
     Thus, before the HVDC system is installed, the inventive concept compensates for the deviation of the potential transformers to be installed at the HVDC transmission system, as described above. 
       FIG. 9  represents the deviation compensation device of a potential transformer according to an embodiment. 
     Referring to  FIG. 9 , the deviation compensation device of the potential transformer includes a plurality of potential transformers  310  to be installed at the HVDC transmission system, and an integrated measurement panel  320  that measures a voltage value output through the plurality of potential transformers  310 , checks the measurement deviation between the plurality of potential transformers  310  by using the measured voltage value, and determines a compensation value for compensating the measurement deviation based on the checked measurement deviation. 
     The potential transformer  310  may include first to Nth potential transformers. 
     The first to Nth potential transformers may be manufactured by the same manufacturer or by different manufacturers. 
     The potential transformers have a deviation in measured voltage value even when the same product is installed at the same location. 
     The integrated measurement panel  320  measures the voltage values output through the plurality of potential transformers  310 , and when there is a deviation in the measured voltage values, the integrated measurement panel  320  sets a compensation value for compensating for it. 
     In this case, the integrated measurement panel  320  includes a memory  330  for storing the set compensation value when the compensation value for each of the plurality of potential transformers  310  is set. 
       FIG. 10  shows the configuration of an HVDC transmission system according to an embodiment. 
     Referring to  FIG. 10 , the HVDC transmission system includes first to Nth potential transformers  310 , an integrated measurement panel  320 , a deviation compensator  410 , and a central controller  420 . 
     Since the first to Nth potential transformers  310  and the integrated measurement panel  320  have been already described, their detailed descriptions are omitted. 
     When the first to nth potential transformers  310  are respectively installed at different locations in the HVDC system, the deviation compensator  410  detects voltage values output through the first to Nth potential transformers  310  and measures, by using the detected voltage values, actual voltage values for each location at which the first to Nth potential transformers  310  are installed. 
     In this case, the first to Nth potential transformers  310  have a deviation as described above. 
     Thus, the deviation compensator  410  transmits compensation value request signals for the first to Nth potential transformers  310  to the integrated measurement panel  321 , when the actual voltage value is measured. 
     The integrated measurement panel  320  transmits the compensation value for the potential transformer requested by the deviation compensator  410  according to the request signal transmitted through the deviation compensator  410 . 
     For example, the deviation compensator  410  may request a compensation value for a first potential transformer and accordingly, the integrated measurement panel  320  transmits the compensation value of the first potential transformer to the deviation compensator  410  in response to the request. 
     In this case, the deviation compensator  410  checks the voltage section of the voltage value output through the first potential transformer and requests a compensation value corresponding to the checked voltage section, and the integrated measurement panel  320  provides, to the deviation compensator  410 , a compensation value for a specific potential transformer in a specific voltage section requested by the deviation compensator  410 . 
     Also, the deviation compensator  410  uses the compensation value provided through the integrated measurement panel  320  to compensate for the voltage value output through the first potential transformer, and sues the compensated voltage value to measure an actual voltage value for the location at which the first potential transformer is installed. 
     In addition, the deviation compensator  410  transmits the measured actual voltage value to the central controller  420 , when the actual voltage value for the location at which the first potential transformer is installed is measured. 
     The central controller  420  uses the actual voltage value transmitted through the deviation compensator  410  to check the voltage value of the location at which the first potential transformer is installed, and controls the overall operations of the HVDC transmission system based on the checked voltage value. 
     In the following, the deviation compensation operation of the potential transformer is described in more detail with reference to the accompanying drawings. 
       FIG. 11  is a flowchart for explaining, step by step, the deviation compensation method of a potential transformer according to a first embodiment,  FIG. 12  is a flowchart for explaining, step by step, the deviation compensation method of a potential transformer according to a second embodiment, and  FIG. 13  is a flowchart for explaining, in detail, the compensation value determination process in  FIGS. 11 and 12 . 
     Firstly, referring to  FIG. 11 , the integrated measurement panel  320  measures voltage values generating through the first to Nth potential transformers in step S 100 . In this case, the same voltage is supplied to the first to Nth potential transformers and accordingly, the first to Nth potential transformers perform a voltage measurement operation on the same voltage supplied. 
     Then, the integrated measurement panel  320  determines whether there is a deviation between the voltage values generating through the first to Nth potential transformers in step S 110 . That is, the integrated measurement panel  320  determines whether the voltage values generating through the first to Nth potential transformers all are the same voltage value or different voltage values. 
     Subsequently, the integrated measurement panel  320  calculates the average value of the voltage values generating through the first to Nth potential transformers in step S 120  when there is the deviation between the voltage values generating through the first to Nth transformers. In the first embodiment, the average value becomes a reference value and the deviation between the first to Nth potential transformers is compensated for by the reference value. 
     When the average value is calculated, the integrated measurement panel  320  determines a compensation value for standardizing, all of the voltage values generating through the first to Nth potential transformers, to the average value in step S 130 . 
     In this case, the compensation value is determined to depend on the voltage values generating through the first to Nth potential transformers. For example, when the average value is 4 and the voltage value generating at the first potential transformer is 5, the compensation value may be set to a value, such as ‘−1’. 
     Also, since the voltage values generating through the first to Nth potential transformers all have different values, the compensation value of each of the first to Nth potential transformers determined by the average value is determined to be different values. 
     In this case, the average value may be the average value of the voltage values generating through only specific potential transformers, not the average value of all of the voltage values generating through the first to Nth potential transformers. 
     For example, when there is big difference between a specific one the voltage values generating through the first to Nth potential transformers and other voltage values, the accuracy of the compensation value may decrease when the average value is calculated by including the voltage value having the big difference. 
     Accordingly, in an embodiment, the average value is calculated by using only specific ones of the voltage values generating through the first to Nth potential transformers and a compensation value for each of the first to Nth potential transformers is determined by using the calculated average value. 
     Subsequently, the integrated measurement panel  320  stores a compensation value for each of the first to Nth potential transformers determined in step S 140 . 
     The compensation value is a value for standardizing the voltage measurement characteristic of the first to Nth potential transformers and accordingly, the compensation value may include at least any one of a transformation ratio, offset and gain values. 
     For example, when a primary voltage value of about 100 V needs to be detected as a secondary voltage value of about 10 V but the secondary voltage value is about 15 V, the compensation value is used to correct the secondary voltage value of about 15 V to the secondary voltage value of about 10 V. In this case, the original voltage change ratio is 100:10 but it is appreciated as 100:15 above and accordingly, when a voltage of about 15 V is detected within a range where a voltage of about 100 V is applied, it is possible to appreciate the primary voltage value as about 100 V. Likewise, it is possible to form the compensation value by using the offset value or gain value, not the voltage change ratio. 
     Next, referring to  FIG. 12 , the integrated measurement panel  320  generates a reference voltage value in step S 200 . 
     In addition, the integrated measurement panel  320  measures the output voltage values of the first to Nth potential transformers measuring the generated reference voltage value in step S 210 . 
     Then, the integrated measurement panel  320  determines whether there is a deviation between the voltage values generating through the first to Nth potential transformers in step S 220 . That is, the integrated measurement panel  320  determines whether the voltage values generating through the first to Nth potential transformers all are the same voltage value or different voltage values. 
     Subsequently, the integrated measurement panel  320  determines a compensation value for correcting the voltage values generating through the first to Nth potential transformers to the reference voltage value in step S 230 , when there is the deviation between the voltage values generating through the first to Nth transformers. 
     In this case, the compensation value is determined to depend on the voltage values generating through the first to Nth potential transformers. For example, when the reference voltage value is 4 and the voltage value generating at the first potential transformer is 5, the compensation value may be set to a value, such as ‘−1’. 
     Also, since the voltage values generating through the first to Nth potential transformers all have different values, the compensation value of each of the first to Nth potential transformers determined by the reference voltage value is determined to be different values. 
     Subsequently, the integrated measurement panel  320  stores a compensation value for each of the first to Nth potential transformers determined in step S 240 . 
     The compensation value is a value for standardizing the voltage measurement characteristic of the first to Nth potential transformers and accordingly, the compensation value may include at least any one of a transformation ratio, offset and gain values. 
     The compensation value needs to be set depending on a measured voltage section. 
     For example, the deviation when a voltage of about 100 V is measured may be deferent from the deviation when a voltage of about 200 V is measured. 
     Thus, the embodiment enables different compensation values to be determined on each of the first to Nth potential transformers according to a measured voltage section. 
     To this end, referring to  FIG. 13 , any one of voltage values belonging to a first voltage range (e.g., about 1 V to about 20 V) is firstly generated in step S 300 . 
     In addition, the output voltage values of the first to Nth potential transformers by the generated voltage values are measured and compensation values for the first to N potential transformers are respectively determined by using the measured voltage values in step S 310 . In this case, the determined compensation value is a compensation value for the first voltage range set. 
     In this case, the compensation value may be determined based on the average value of the voltage values generating through the first to Nth potential transformers or may be otherwise determined based on the generated voltage value belonging to the first voltage range. 
     Then, the integrated measurement panel  320  sets a voltage range to the next voltage range in step S 320 . For example, the integrated measurement panel  320  may set the voltage range to a second voltage range of about 21 V to about 40 V. 
     In addition, the integrated measurement panel  320  generates any one of the voltage values belonging to the next voltage range set in step S 330 . 
     Subsequently, the integrated measurement panel  320  determines a compensation value on the set voltage range based on the generated voltage values in step S 340 . 
     Then, the integrated measurement panel  320  checks whether compensation values for all voltage ranges have been determined, ends if positive, and returns to step S 320  to determine compensation values on the next voltage range in step S 350  if negative. 
     The integrated measurement panel  320  stores a reference value being the basis of determination of the compensation values. 
     In addition, when any one of the first to Nth potential transformers on which compensation values have been determined has a fault and thus needs to replace it with a new potential transformer, a compensation value for the new potential transformer is determined based on the stored reference value. 
       FIG. 14  is a flow chart for explaining, step by step, a method of measuring a voltage value in an HVDC transmission system according to an embodiment. 
     Referring to  FIG. 14 , when the first to Nth potential transformers  310  are respectively installed at different locations in the HVDC system, the deviation compensator  410  measures voltage values output through the first to Nth potential transformers  310  in step S 400 . 
     When the voltage values are measured, the deviation compensator  410  checks the voltage section of each voltage value output through the first to Nth potential transformers  310  in step S 410 . 
     Subsequently, when the voltage section of each voltage value is checked, the deviation compensator  410  transmits information on the voltage sections of the first to the first to Nth potential transformers  310  to the integrated measurement panel  320  and receives the compensation values of the first to Nth potential transformers  310  corresponding to each of the voltage sections from the integrated measurement panel  320  ins step S 420 . 
     In addition, the deviation compensator  410  compensates for the voltage values output through the first to Nth potential transformers  310  by applying the received compensation value of each of the first to Nth potential transformers  310  in step S 430 . 
     Subsequently, the deviation compensator  410  measures, based on the compensated voltage value of each of the first to Nth potential transformers  310 , the actual voltage value at respective locations at which the first to Nth potential transformers  310  are installed in step S 440 . 
     In addition, the deviation compensator  410  transmits the actual voltage value measured for the installation locations of the first to Nth potential transformers  310  to the central controller in step S 450 . 
     According to embodiments, before potential transformers are installed at the HVDC transmission system, a compensation value for compensating for the measurement deviation between the potential transformers installed at the HVDC transmission system and when the potential transformers are installed at the HVDC transmission system, the determined compensation value is applied to measure a voltage value. Thus, it is possible to enhance the reliability of the HVDC transmission system. 
     Also, according to embodiments, it is possible to compensate for the deviation of a plurality of potential transformers by using the same measurement panel and thus, since the deviation of voltage values measured through the plurality of potential transformers decreases, it is possible to perform accurate operations, such as voltage deviation protection as well as it is easy to control the HVDC transmission system. 
     Also, according to embodiments, the HVDC transmission system transmitting power may minimize a loss caused by a measurement deviation and thus it is possible to decrease a system loss. 
     Also, according to embodiments, when a specific potential transformer has a fault and thus needs to replace, it is possible to easily replace the specific potential transformer because information used previously for compensating for the deviation of the plurality of potential transformers has been stored. 
     Although embodiments have been described with reference to a number of illustrative embodiments thereof, it should be understood that numerous other modifications and embodiments can be devised by those skilled in the art that will fall within the spirit and scope of the principles of this disclosure. More particularly, various variations and modifications are possible in the component parts and/or arrangements of the subject combination arrangement within the scope of the disclosure, the drawings and the appended claims. In addition to variations and modifications in the component parts and/or arrangements, alternative uses will also be apparent to those skilled in the art.