Patent Publication Number: US-2021175711-A1

Title: Power supply system and control method

Description:
TECHNICAL FIELD 
     The present invention relates to a power supply system, and a control method used in the power supply system. 
     BACKGROUND ART 
     Conventionally, in power distribution to a consumer (power use facility) by a power system including a synchronous generator, unbalanced current occurs in the case where a single-phase load and a three-phase load coexist as consumer loads. Unbalanced current decreases the output efficiency of the synchronous generator and, as negative-phase-sequence current of the synchronous generator, causes damage to the synchronous generator due to heating and the like. In view of the problem of unbalanced current, for example, a device that provides a power storage battery and a power conditioner in a consumer and compensates, by the power conditioner, for an unbalance of line current of low-voltage power distribution lines and the like has been proposed (see PTL 1). Moreover, a dispersed power source linked with an inverter, such as a solar power generation device, has been widely used in recent years. 
     CITATION LIST 
     Patent Literature 
     PTL 1: Japanese Unexamined Patent Application Publication No. 2001-231169 
     SUMMARY OF THE INVENTION 
     Technical Problem 
     The present invention has an object of providing a power supply system in which an inverter linked with a dispersed power source is connected to a power generation device such as a synchronous generator, and the inverter is controlled to supply power to a consumer and compensate for negative-phase-sequence current of the power generation device. The present invention also has an object of providing a control method used to control the inverter in the power supply system. 
     Solution to Problem 
     To achieve the stated object, a power supply system according to an aspect of the present invention includes: a power distribution system that is connected to a consumer load, the power distribution system being a three-phase power distribution system; a power generation device that is connected to the power distribution system; an inverter that is connected to the power distribution system, supplies power to the consumer load, and compensates for an unbalanced current of the power generation device; an inverter output component calculator that calculates a voltage positive-phase-sequence component, a current positive-phase-sequence component, a voltage angular velocity, and a voltage value relating to the inverter, based on a three-phase voltage and a three-phase current output from the inverter to the power distribution system; a power generation device output component calculator that calculates a current negative-phase-sequence component and a voltage phase relating to the power generation device, based on a three-phase voltage and a three-phase current output from the power generation device to the power distribution system; an active and reactive power calculator that calculates an active power and a reactive power relating to the inverter, from the voltage positive-phase-sequence component and the current positive-phase-sequence component relating to the inverter; a target output voltage generator that generates a target output voltage which is an amplitude of a vector in a complex plane combining voltages of three phases, based on the voltage value and the reactive power relating to the inverter; a target output phase generator that generates a target output phase which is a phase of the vector in the complex plane combining the voltages of the three phases, based on the voltage angular velocity and the active power relating to the inverter; a negative-phase-sequence compensation voltage generator that generates a first compensation voltage, based on the current negative-phase-sequence component and the voltage phase relating to the power generation device; and a gate command value calculator that generates a gate command to the inverter, according to the first compensation voltage, the target output voltage, and the target output phase. 
     To achieve the stated object, a control method according to an aspect of the present invention is a control method in a power supply system that includes: a power distribution system that is connected to a consumer load, the power distribution system being a three-phase power distribution system; a power generation device that is connected to the power distribution system; and an inverter that is connected to the power distribution system, supplies power to the consumer load, and compensates for an unbalanced current of the power generation device, the control method including: calculating a voltage positive-phase-sequence component, a current positive-phase-sequence component, a voltage angular velocity, and a voltage value relating to the inverter, based on a three-phase voltage and a three-phase current output from the inverter to the power distribution system; calculating a current negative-phase-sequence component and a voltage phase relating to the power generation device, based on a three-phase voltage and a three-phase current output from the power generation device to the power distribution system; calculating an active power and a reactive power relating to the inverter, from the voltage positive-phase-sequence component and the current positive-phase-sequence component relating to the inverter; generating a target output voltage which is an amplitude of a vector in a complex plane combining voltages of three phases, based on the voltage value and the reactive power relating to the inverter; generating a target output phase which is a phase of the vector in the complex plane combining the voltages of the three phases, based on the voltage angular velocity and the active power relating to the inverter; generating a first compensation voltage, based on the current negative-phase-sequence component and the voltage phase relating to the power generation device; generating a gate command to the inverter, according to the first compensation voltage, the target output voltage, and the target output phase; and controlling the inverter by the gate command. 
     Advantageous Effect of Invention 
     According to the present invention, a power generation device such as a synchronous generator and an inverter can supply power to a consumer, and the inverter can compensate for negative-phase-sequence current of the power generation device. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a diagram illustrating the schematic structure of a power supply system according to Embodiment 1. 
         FIG. 2  is a block diagram illustrating an example of the structure of an inverter output component calculator according to Embodiment 1. 
         FIG. 3  is a block diagram illustrating an example of the structure of a power generation device output component calculator according to Embodiment 1. 
         FIG. 4  is a block diagram illustrating an example of the structure of a target output voltage generator according to Embodiment 1. 
         FIG. 5  is a block diagram illustrating an example of the structure of a target output phase generator according to Embodiment 1. 
         FIG. 6  is a block diagram illustrating an example of the structure of a negative-phase-sequence compensation voltage generator according to Embodiment 1. 
         FIG. 7  is a diagram illustrating the schematic structure of a power supply system according to Embodiment 2. 
     
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     Embodiment 1 
     Embodiments are described below, with reference to drawings. The embodiments described below each show a specific example of the present invention. The numerical values, shapes, materials, structural elements, the arrangement and connection of the structural elements, steps, the order of steps, etc. shown in the following embodiments are mere examples, and do not limit the scope of the present invention. Of the structural elements in the embodiments described below, the structural elements not recited in any one of the independent claims are structural elements that may be added optionally. Each drawing is a schematic, and does not necessarily provide precise depiction. 
     A power supply system according to an embodiment of the present invention is described below. 
     (Overall Structure of Power Supply System  1 ) 
       FIG. 1  is a diagram illustrating the schematic structure of power supply system  1  according to this embodiment. 
     Power supply system  1  is a system for supplying power to a consumer (power use facility) by synchronous generator  11  and inverter  21  that is linked with dispersed power source  23 . Power supply system  1  includes synchronous generator  11 , inverter  21 , energy storage  22 , dispersed power source  23 , control device  100 , and power distribution system  40 , as illustrated in  FIG. 1 . 
     Power distribution system  40  includes power distribution lines forming a three-phase transmission path. Power distribution system  40  is connected to synchronous generator  11  and inverter  21  at junction  41 , and transmits power to the consumer. Power distribution system  40  is connected to consumer load  30  (e.g. power use appliance) in the consumer. Consumer load  30  includes three-phase load  32  connected to each of the power distribution lines of three phases, and single-phase load  31  connected to part of the power distribution lines of three phases, as illustrated in  FIG. 1 . 
     Synchronous generator  11  is a power generation device that generates AC power synchronous with the rotational speed of a magnetic field, such as a diesel generator or a gas engine generator. Synchronous generator  11  is connected to power distribution system  40  that transmits generated power. 
     Dispersed power source (dispersion type power source)  23  is a power source that generates power by any of various power generation methods such as solar power generation, wind power generation, and fuel cell. 
     Energy storage  22  is a medium that stores power (energy) generated by dispersed power source  23 , such as a storage battery. Energy storage  22  is connected to dispersed power source  23  and inverter  21 . 
     Inverter  21  is a power conversion device that converts DC power from energy storage  22 , into AC power. Inverter  21  is connected to power distribution system  40  that transmits AC power. Inverter  21  can supply power to consumer load  30  via power distribution system  40 . Inverter  21  also has a function of compensating for negative-phase-sequence current of synchronous generator  11  as unbalanced current, under control of control device  100 . The unbalanced current can occur in power distribution system  40 , due to the influence of consumer load  30  or the like. 
     Control device  100  is a device that is composed of, for example, electronic circuitry including an integrated circuit of a microprocessor, memory, and the like, and controls inverter  21  by performing a predetermined control method. For example, control device  100  may be a device integrated with inverter  21 . Control device  100  determines a control (gate command) parameter (gate command value) for inverter  21 , and controls inverter  21 . At each instant, control device  100  determines the gate command value, based on a result of detecting (measuring) voltage and current output from synchronous generator  11  to power distribution system  40 , and information (own terminal information) of voltage and current output from inverter  21  to power distribution system  40 , which is obtained by measurement and the like. Control device  100  includes inverter output component calculator  110 , active and reactive power calculator  120 , target output voltage generator  130 , target output phase generator  140 , power generation device output component calculator  150 , negative-phase-sequence compensation voltage generator  160 , and gate command value calculator  190  as functional structural elements, as illustrated in  FIG. 1 . 
     Inverter output component calculator  110  has a function of calculating and outputting voltage positive-phase-sequence component V + out_dg(dq), current positive-phase-sequence component I + out_dg(dq), voltage value Vout_dg, and voltage angular velocity ωg_dg, based on input own terminal information. The input own terminal information includes voltage Vout_dg(abc) and current Iout_dg(abc) output from inverter  21  to power distribution system  40 . Voltage Vout_dg(abc) represents voltages Vout_dg_a, Vout_dg_b, and Vout_dg_c for respective axis components of an abc coordinate system, i.e. three phases. Current Iout_dg(abc) represents currents Iout_dg_a, Iout_dg_b, and Iout_dg_c for three phases. Voltage positive-phase-sequence component V + out_dg(dq) represents voltage positive-phase-sequence component V + out_dg_d as a d-axis component and voltage positive-phase-sequence component V + out_dg_q as a q-axis component in a dq coordinate system having orthogonal d axis and q axis. Current positive-phase-sequence component I + out_dg(dq) represents current positive-phase-sequence component I + out_dg_d as a d-axis component and current positive-phase-sequence component I + out_dg_q as a q-axis component in the dq coordinate system having orthogonal d axis and q axis. 
     Active and reactive power calculator  120  has a function of calculating and outputting active power Pout_dg and reactive power Qout_dg, based on voltage positive-phase-sequence component V + out_dg(dq) and current positive-phase-sequence component I + out_dg(dq) received from inverter output component calculator  110 . 
     Target output voltage generator  130  has a function of generating and outputting target output voltage E_dg, based on voltage value Vout_dg received from inverter output component calculator  110  and reactive power Qout_dg received from active and reactive power calculator  120 . Target output voltage E_dg is the amplitude of a vector in a complex plane combining voltages of three phases. 
     Target output phase generator  140  has a function of generating and outputting target output phase θm_dg, based on voltage angular velocity ωg_dg received from inverter output component calculator  110  and active power Pout_dg received from active and reactive power calculator  120 . Target output phase θm_dg is the phase of the vector in the complex plane combining voltages of three phases. 
     Power generation device output component calculator  150  has a function of calculating and outputting current negative-phase-sequence component I − out_sg(dq) and voltage phase θg_sg, based on voltage Vout_sg(abc) and current Iout_sg(abc) output from synchronous generator  11  to power distribution system  40 . Voltage Vout_sg(abc) represents voltages Vout_sg_a, Vout_sg_b, and Vout_sg_c for respective axis components of the abc coordinate system, i.e. three phases. Current Iout_sg(abc) represents currents Iout_sg_a, Iout_sg_b, and Iout_sg_c for three phases. Current negative-phase-sequence component I − out_sg(dq) represents current negative-phase-sequence component I − out_sg_d as a d-axis component and current negative-phase-sequence component I + out_sg_q as a q-axis component in the dq coordinate system having orthogonal d axis and q axis. 
     Negative-phase-sequence compensation voltage generator  160  has a function of generating and outputting first compensation voltage ΔV(αß), based on current negative-phase-sequence component I − out_sg(dq) and voltage phase θg_sg received from power generation device output component calculator  150 . 
     Gate command value calculator  190  has a function of determining a gate command value by a value obtained by coordinate-converting (γδ conversion) target output voltage E_dg output from target output voltage generator  130 , target output phase θm_dg output from target output phase generator  140 , and first compensation voltage ΔV(αß) output from negative-phase-sequence compensation voltage generator  160 . Gate command value calculator  190  issues a gate command based on the determined gate command value, thus controlling inverter  21 . 
     (Structure of Inverter Output Component Calculator  110 ) 
     The structure of inverter output component calculator  110  is described below, with reference to  FIG. 2 . 
       FIG. 2  illustrates an example of the structure of inverter output component calculator  110 . Inverter output component calculator  110  includes three-phase to two-phase converters  111   a  and  111   b , converters (T + )  112   a  and  112   b , converters (T − )  113   a  and  113   b , converters (T +2 )  114   a  and  114   b , converters (T −2 )  115   a  and  115   b , low-pass filters (LPF)  116   a  to  116   d , phase synchronization circuit (PLL)  117 , and calculator  118 , as illustrated in the drawing. 
     Three-phase to two-phase converter  111   a  converts symmetrical three-phase AC voltage Vout_dg(abc) represented in the abc coordinate system into two-phase AC voltage Vout_dg(αß) represented in the αß coordinate system, i.e. two-phase coordinate (α, ß), and outputs voltage Vout_dg(αß). Voltage Vout_dg(αß) represents voltage Vout_dg_α as an α-axis component and voltage Vout_dg_ß as a β-axis component. 
     Converter (T + )  112   a , converter (T − )  113   a , converter (T +2 )  114   a , converter (T −2 )  115   a , LPFs  116   a  and  116   b , and, PLL  117  for dq conversion and the like calculate voltage phase θg_dg according to voltage Vout_dg(αß) output from three-phase to two-phase converter  111   a , and calculate voltage positive-phase-sequence component V + out_dg(dq) and voltage angular velocity ωg_dg. Calculated voltage positive-phase-sequence component V + out_dg(dq) and voltage angular velocity ωg_dg are output from inverter output component calculator  110 . Calculated voltage phase θg_dg is recursively used in converter (T + )  112   a , converter (T − )  113   a , converter (T +2 )  114   a , and converter (T −2 )  115   a.    
     Calculator  118  calculates the magnitude of voltage positive-phase-sequence component V + out_dg(dq), to calculate voltage value Vout_dg. Calculated voltage value Vout_dg is output from inverter output component calculator  110 . 
     Three-phase to two-phase converter  111   b  converts symmetrical three-phase AC current Iout_dg(abc) represented in the abc coordinate system into two-phase AC current Iout_dg(αß) represented in the αß coordinate system, i.e. two-phase coordinate (α, ß), and outputs current Iout_dg(αß). Current Iout_dg(αß) represents current Iout_dg_α as an α-axis component and current Iout_dg_ß as a β-axis component. 
     Converter (T + )  112   b , converter (T − )  113   b , converter (T +2 )  114   b , converter (T −2 )  115   b , and LPFs  116   c  and  116   d  for dq conversion and the like calculate current positive-phase-sequence component I + out_dg(dq), according to current Iout_dg(αß) output from three-phase to two-phase converter  111   b  and voltage phase θg_dg. Calculated current positive-phase-sequence component I + out_dg(dq) is output from inverter output component calculator  110 . 
     (Structure of Power Generation Device Output Component Calculator  150 ) 
     The structure of power generation device output component calculator  150  is described below, with reference to  FIG. 3 . 
       FIG. 3  illustrates an example of the structure of power generation device output component calculator  150 . Power generation device output component calculator  150  includes three-phase to two-phase converters  121   a  and  121   b , converters (T + )  122   a  and  122   b , converters (T − )  123   a  and  123   b , converters (T +2 )  124   a  and  124   b , converters (T −2 )  125   a  and  125   b , LPFs  126   a  to  126   d , and PLL  127 , as illustrated in the drawing. 
     Three-phase to two-phase converter  121   a  converts symmetrical three-phase AC voltage Vout_sg(abc) represented in the abc coordinate system into two-phase AC voltage Vout_sg(αβ) represented in the αß coordinate system, and outputs voltage Vout_sg(αß). Voltage Vout_sg(αß) represents voltage Vout_sg_α as an α-axis component and voltage Vout_sg_ß as a β-axis component. 
     Converter (T + )  122   a , converter (T − )  123   a , converter (T +2 )  124   a , converter (T −2 )  125   a , LPFs  126   a  and  126   b , and PLL  127  for dq conversion and the like calculate voltage phase θg_sg according to voltage Vout_sg(αß) output from three-phase to two-phase converter  121   a , and calculate voltage positive-phase-sequence component V + out_sg(dq) and voltage angular velocity ωg_sg. Calculated voltage phase θg_sg is output from power generation device output component calculator  150 . Calculated voltage phase θg_sg is further recursively used in converter (T + )  122   a , converter (T − )  123   a , converter (T +2 )  124   a , and converter (T −2 )  125   a.    
     Three-phase to two-phase converter  121   b  converts symmetrical three-phase AC current Iout_sg(abc) represented in the abc coordinate system into two-phase AC current Iout_sg(αß) represented in the αß coordinate system, and outputs current Iout_sg(αß). Current Iout_sg(αß) represents current Iout_sg_α as an α-axis component and current Iout_sg_ß as a ß-axis component. 
     Converter (T + )  122   b , converter (T − )  123   b , converter (T +2 )  124   b , converter (T −2 )  125   b , and LPFs  126   c  and  126   d  for dq conversion and the like calculate current negative-phase-sequence component I − out_sg(dq), according to current Iout_sg(αß) output from three-phase to two-phase converter  121   b  and voltage phase θg_sg. Calculated current negative-phase-sequence component I − out_sg(dq) is output from power generation device output component calculator  150 . 
     (Structure of Target Output Voltage Generator  130 ) 
     The structure of target output voltage generator  130  is described below, with reference to  FIG. 4 . 
       FIG. 4  illustrates an example of the structure of target output voltage generator  130 . Target output voltage generator  130  includes Q drooper  131  and PI controller  132 , as illustrated in the drawing. 
     Q drooper  131  has a function of performing droop control to provide droop property between a deviation between voltage value Vout_dg received from inverter output component calculator  110  and predetermined command output voltage value E0 and a deviation between predetermined command reactive power Q0 and target output reactive power, and calculating and outputting the target output reactive power. Predetermined command output voltage value E0 is set beforehand, and is, for example, 200 V. Predetermined command reactive power Q0 is set beforehand, and is, for example, 0 var. Predetermined command output voltage value E0 and predetermined command reactive power Q0 may be stored inside beforehand, or acquired from outside. 
     PI controller  132  has a function of calculating target output voltage E_dg. In detail, PI controller  132  has a function of performing PI control to eliminate a deviation between the target output reactive power output from Q drooper  131  and reactive power Qout_dg received from active and reactive power calculator  120  to thereby follow predetermined command output voltage value E0, thus causing output of target output voltage E_dg from target output voltage generator  130 . Target output voltage E_dg is the amplitude of the vector in the complex plane combining voltages of three phases. 
     (Structure of Target Output Phase Generator  140 ) 
     The structure of target output phase generator  140  is described below, with reference to  FIG. 5 . 
       FIG. 5  illustrates an example of the structure of target output phase generator  140 . Target output phase generator  140  includes governor model unit  141 , calculator  142 , and integrator  143 , as illustrated in the drawing. 
     Governor model unit  141  has a function of performing droop control (speed control) to provide droop property between a deviation between target output angular velocity ωm_dg, which is a time derivative of target output phase θm_dg, and predetermined command angular velocity ω 0  and a deviation between predetermined command active power PO and target output active power, and calculating and outputting target output active power Pin_dg. Predetermined command angular velocity ω 0  is set beforehand, and is, for example, 314 rad/s or 376.8 rad/s. Predetermined command active power PO is set beforehand, and is, for example, 1000 W. Predetermined command angular velocity ω 0  and predetermined command active power PO may be stored inside beforehand, or acquired from outside. 
     Calculator  142  has a function of calculating target output angular velocity ωm_dg, based on voltage angular velocity g_dg received from inverter output component calculator  110 , active power Pout_dg received from active and reactive power calculator  120 , and target output active power Pin_dg output from governor model unit  141 . In  FIG. 5 , virtual inertia constant Jdg used for the calculation of target output angular velocity ωm_dg in calculator  142  represents the magnitude of rotational inertia with which a rotating object to be simulated by the inverter tries to maintain the same rotational motion, and virtual damping constant Ddg represents the magnitude of energy acting in a direction of attenuating the rotation. 
     Integrator  143  has a function of calculating and outputting target output phase θm_dg by time-integrating target output angular velocity ωm_dg calculated by calculator  142 . Target output phase θm_dg is the phase of the vector in the complex plane combining voltages of three phases. 
     (Structure of Negative-Phase-Sequence Compensation Voltage Generator  160 ) 
     The structure of negative-phase-sequence compensation voltage generator  160  is described below, with reference to  FIG. 6 . 
       FIG. 6  illustrates an example of the structure of negative-phase-sequence compensation voltage generator  160 . Negative-phase-sequence compensation voltage generator  160  includes PI controllers  161   a  and  161   b  and converter ((T − ) −1 )  162 , as illustrated in the drawing. 
     PI controllers  161   a  and  161   b  have a function of performing PI control for causing current negative-phase-sequence component I − out_sg(dq) received from power generation device output component calculator  150 , i.e. current negative-phase-sequence component I − out_sg_d and current negative-phase-sequence component I − out_sg_q, to be zero (0). This PI control is control for compensating for negative-phase-sequence current of synchronous generator  11 . Converter ((T − ) −1 )  162  converts the input, by the inverse of matrix T −  (see  FIG. 3 ) set according to voltage phase θg_sg received from power generation device output component calculator  150 . Converter ((T − ) −1 )  162  outputs first compensation voltage ΔV(αß) for such compensation that causes current negative-phase-sequence component I − out_sg(dq) to be zero, based on the relationship with PI controllers  161   a  and  161   b.    
     (Effects) 
     In power supply system  1  described above, inverter  21  operates in response to the gate command (command based on the value obtained by coordinate-converting target output voltage E_dg, target output phase θm_dg, and first compensation voltage ΔV(αß)) received from control device  100  with the foregoing structure. Inverter  21  can thus supply power to consumer load  30  and compensate for negative-phase-sequence current of synchronous generator  11 . 
     Moreover, in power supply system  1 , inverter  21  linked with dispersed power source  23  as an example operates as a voltage source as with synchronous generator  11 , so that individual operation is possible. 
     Embodiment 2 
     Power supply system  1   a  which is a modification to part of power supply system  1  so as to perform control for inserting virtual impedance into inverter  21  is described below. 
       FIG. 7  is a diagram illustrating the schematic structure of power supply system  1   a  according to this embodiment. Power supply system  1   a  includes synchronous generator  11 , inverter  21 , energy storage  22 , dispersed power source  23 , control device  100   a , and power distribution system  40 , as illustrated in  FIG. 7 . Of the structural elements in power supply system  1   a  illustrated in  FIG. 7 , structural elements having the same functions as those of power supply system  1  described above are given the same reference marks as in  FIG. 1 , and their description is omitted. Control device  100   a  includes virtual impedance unit  170  in addition to the structure of control device  100  in Embodiment 1. Features of control device  100   a  not particularly described here are the same as those of control device  100 . 
     Virtual impedance unit  170  has a function of calculating, based on own terminal information (current Iout_dg(abc) output from inverter  21 ), virtual impedance inserted into inverter  21 , and outputting second compensation voltage ΔV2(αß) according to the virtual impedance. 
     In such power supply system  1   a , inverter  21  operates in response to the gate command (command based on the value obtained by coordinate-converting target output voltage E_dg, target output phase θm_dg, and first compensation voltage ΔV(αß) and the value obtained by coordinate-converting second compensation voltage ΔV2(αß)) received from control device  100   a  with the foregoing structure. Thus, inverter  21  can supply power to consumer load  30  and compensate for negative-phase-sequence current of synchronous generator  11 , and also unwanted output power vibration from the inverter can be suppressed. 
     Other Embodiments, Etc 
     While power supply systems  1  and  1   a  according to Embodiments 1 and 2 have been described above, the foregoing embodiments are merely examples, and various changes, additions, omissions, and the like are possible. 
     The functional structural elements (functional structural elements for performing a control method for controlling inverter  21 ) in each of control devices  100  and  100   a  in the foregoing embodiments may be realized by hardware (such as electronic circuitry) without using software, or realized by software. A process by software is realized by a microprocessor in each of control devices  100  and  100   a  executing a control program stored in memory. The control program may be recorded in a recording medium and distributed or circulated. For example, by installing the distributed control program in a device such as a computer and causing a microprocessor in the device to execute the program, the functions of each of control devices  100  and  100   a  can be realized. 
     Any embodiment obtained by combining the structural elements and functions in the foregoing embodiments and the like is also included in the scope of the present invention. 
     The following describes the structures, variations, advantageous effects, etc. of a power supply system according to an aspect of the present invention and a control method used in the power supply system. 
     (1) A power supply system according to an aspect of the present invention includes: three-phase power distribution system  40  that is connected to consumer load  30 ; a power generation device (synchronous generator  11 ) that is connected to power distribution system  40 ; inverter  21  that is connected to power distribution system  40 , supplies power to consumer load  30 , and compensates for unbalanced current of the power generation device; inverter output component calculator  110  that calculates voltage positive-phase-sequence component V + out_dg(dq), current positive-phase-sequence component I + out_dg(dq), voltage angular velocity ωg_dg, and voltage value Vout_dg relating to inverter  21 , based on three-phase voltage Vout_dg(abc) and current Iout_dg(abc) output from inverter  21  to power distribution system  40 ; power generation device output component calculator  150  that calculates current negative-phase-sequence component I − out_sg(dq) and voltage phase θg_sg relating to the power generation device, based on three-phase voltage Vout_sg(abc) and current Iout_sg(abc) output from the power generation device to power distribution system  40 ; active and reactive power calculator  120  that calculates active power Pout_dg and reactive power Qout_dg relating to inverter  21 , from voltage positive-phase-sequence component V + out_dg(dq) and current positive-phase-sequence component I + out_dg(dq) relating to inverter  21 ; target output voltage generator  130  that generates target output voltage E_dg which is an amplitude of a vector in a complex plane combining voltages of three phases, based on voltage value Vout_dg and reactive power Qout_dg relating to inverter  21 ; target output phase generator  140  that generates target output phase θm_dg which is a phase of the vector in the complex plane combining the voltages of the three phases, based on voltage angular velocity ωg_dg and active power Pout_dg relating to inverter  21 ; negative-phase-sequence compensation voltage generator  160  that generates first compensation voltage ΔV(αß), based on current negative-phase-sequence component I − out_sg(dq) and voltage phase θg_sg relating to the power generation device; and gate command value calculator  190  that generates a gate command to inverter  21 , according to first compensation voltage ΔV(αß), target output voltage E_dg, and target output phase θm_dg. 
     With this structure, inverter  21  operates in response to the gate command, and can thus supply power to consumer load  30  and compensate for negative-phase-sequence current of the power generation device (synchronous generator  11 ). This prevents damage to the power generation device. 
     (2) For example, inverter output component calculator  110  may: convert three-phase voltage Vout_dg(abc) and current Iout_dg(abc) output from inverter  21  to power distribution system  40  into two-phase voltage and current represented by two-phase coordinate (α, ß) in an αß coordinate system, by three-phase to two-phase conversion; calculate voltage positive-phase-sequence component V + out_dg(dq) and current positive-phase-sequence component I + out_dg(dq) relating to inverter  21 , based on the two-phase voltage and current obtained by the conversion; and calculate voltage value Vout_dg from voltage positive-phase-sequence component V + out_dg(dq). 
     With this structure, the voltage positive-phase-sequence component, current positive-phase-sequence component, and voltage value of inverter  21  usable for the generation of the gate command are calculated in correspondence with the three-phase voltage and current output from inverter  21  to power distribution system  40 . 
     (3) For example, power generation device output component calculator  150  may: convert three-phase voltage Vout_sg(abc) and current Iout_sg(abc) output from the power generation device to power distribution system  40  into two-phase voltage and current represented by two-phase coordinate (α, ß) in an αß coordinate system, by three-phase to two-phase conversion; and calculate current negative-phase-sequence component I − out_sg(dq) and voltage phase θg_sg relating to the power generation device, based on the two-phase voltage and current obtained by the conversion. 
     With this structure, the current negative-phase-sequence component and voltage phase of the power generation device usable for the generation of the gate command are calculated in correspondence with the three-phase voltage and current output from the power generation device to power distribution system  40 . 
     (4) For example, target output voltage generator  130  may include: Q drooper  131  that calculates target output reactive power to provide a droop property between a deviation between voltage value Vout_dg relating to inverter  21  and predetermined command output voltage value E0 and a deviation between predetermined command reactive power Q0 and the target output reactive power; and a PI controller that calculates target output voltage E_dg to eliminate a deviation between the target output reactive power and reactive power Qout_dg relating to inverter  21 . 
     With this structure, inverter  21  controlled by the gate command produces appropriate output depending on, for example, the state of consumer load  30  which may vary. 
     (5) For example, target output phase generator  140  may include: governor model unit  141  that calculates target output active power Pin_dg to provide a droop property between a deviation between target output angular velocity ωm_dg and predetermined command angular velocity ω 0  and a deviation between predetermined command active power PO and target output active power Pin_dg, target output angular velocity ωm_dg being a time derivative of target output phase θm_dg; calculator  142  that calculates target output angular velocity ωm_dg, based on target output active power Pin_dg, and active power Pout_dg and voltage angular velocity ωg_dg relating to inverter  21 ; and integrator  143  that integrates calculated target output angular velocity ωm_dg, to calculate target output phase θm_dg. 
     With this structure, inverter  21  controlled by the gate command produces appropriate output depending on, for example, the state of consumer load  30  which may vary. 
     (6) For example, negative-phase-sequence compensation voltage generator  160  may include PI controllers  161   a  and  161   b  that perform control to cause current negative-phase-sequence component I − out_sg(dq) relating to the power generation device to be zero (0). 
     With this structure, inverter  21  controlled by the gate command can compensate for negative-phase-sequence current of the power generation device (synchronous generator  11 ). 
     (7) For example, power supply system  1  or  1   a  may further include: virtual impedance unit  170  that generates second compensation voltage ΔV2(αß), based on three-phase current Iout_dg(abc) output from inverter  21  to power distribution system  40 , wherein gate command value calculator  190  also generates the gate command to inverter  21 , according to second compensation voltage ΔV2(αß). 
     With this structure, unwanted output power vibration from the inverter can be suppressed. 
     (8) A control method according to an aspect of the present invention is a control method in power supply system  1  or  1   a  that includes: three-phase power distribution system  40  that is connected to consumer load  30 ; a power generation device (synchronous generator  11 ) that is connected to power distribution system  40 ; and inverter  21  that is connected to power distribution system  40 , supplies power to consumer load  30 , and compensates for unbalanced current of the power generation device, the control method including: calculating voltage positive-phase-sequence component V + out_dg(dq), current positive-phase-sequence component I + out_dg(dq), voltage angular velocity ωg_dg, and voltage value Vout_dg relating to inverter  21 , based on three-phase voltage Vout_dg(abc) and current Iout_dg(abc) output from inverter  21  to power distribution system  40 ; calculating current negative-phase-sequence component I − out_sg(dq) and voltage phase θg_sg relating to the power generation device, based on three-phase voltage Vout_sg(abc) and current Iout_sg(abc) output from the power generation device to power distribution system  40 ; calculating active power Pout_dg and reactive power Qout_dg relating to inverter  21 , from voltage positive-phase-sequence component V + out_dg(dq) and current positive-phase-sequence component I + out_dg(dq) relating to inverter  21 ; generating target output voltage E_dg which is an amplitude of a vector in a complex plane combining voltages of three phases, based on voltage value Vout_dg and reactive power Qout_dg relating to inverter  21 ; generating target output phase θm_dg which is a phase of the vector in the complex plane combining the voltages of the three phases, based on voltage angular velocity ωg_dg and active power Pout_dg relating to inverter  21 ; generating first compensation voltage ΔV(αß), based on current negative-phase-sequence component I − out_sg(dq) and voltage phase θg_sg relating to the power generation device; generating a gate command to inverter  21 , according to first compensation voltage ΔV(W), target output voltage E_dg, and target output phase θm_dg; and controlling inverter  21  by the gate command. 
     With this structure, inverter  21  can supply power to consumer load  30 , and compensate for negative-phase-sequence current of the power generation device (synchronous generator  11 ). This prevents damage to the power generation device. 
     (9) For example, the control method may include: generating a second compensation voltage, based on the three-phase current output from the inverter to the power distribution system, wherein the gate command is also generated according to the second compensation voltage. 
     With this structure, unwanted output power vibration from the inverter can be suppressed. 
     REFERENCE MARKS IN THE DRAWINGS 
     
         
         
           
               1 ,  1   a  power supply system 
               11  synchronous generator (power generation device) 
               21  inverter 
               30  consumer load 
               40  power distribution system 
               110  inverter output component calculator 
               120  active and reactive power calculator 
               130  target output voltage generator 
               131  Q drooper 
               132 ,  161   a ,  161   b  PI controller 
               140  target output phase generator 
               141  governor model unit 
               142  calculator 
               143  integrator 
               150  power generation device output component calculator 
               160  negative-phase-sequence compensation voltage generator 
               170  virtual impedance unit 
               190  gate command value calculator