Patent Publication Number: US-2019190403-A1

Title: System-connected inverter device and method for operating same

Description:
FIELD 
     Embodiments described herein relate generally to a system-connected inverter device and method for operating same. 
     BACKGROUND ART 
     There is a system-connected inverter device that converts direct current power into alternating current power and supplies the alternating current power after the conversion to an alternating current power system. In the system-connected inverter device, a three-level inverter is used; and a three-level voltage is output. The three-level inverter includes multiple switching elements. For example, the ON/OFF of each of the switching elements of the three-level inverter is controlled by a three-level PWM modulation method. Thereby, the three-level voltage is output. Compared to a two-level inverter, the output voltage waveform can approach a sine wave better in the three-level inverter. For example, the harmonic components can be suppressed; and downsizing of the filter on the output side can be realized. 
     As three-level PWM modulation methods, for example, a unipolar modulation method in which a positive pulse-form voltage or a negative pulse-form voltage is output continuously, a dipolar modulation method in which positive and negative pulse-form voltages are output alternately via a zero voltage, etc., are known (e.g., Non-Patent Literature 1). Compared to the unipolar modulation method, the waveform of the output voltage can approach a sine wave better in the dipolar modulation method. On the other hand, compared to the dipolar modulation method, the switching loss that accompanies the ON/OFF of each of the switching elements can be suppressed in the unipolar modulation method in the case where the direct current voltage of the steady-state operation is high. 
     In system-connected inverter devices in recent years, a FRT (Fault Ride Through) function is desirable in which the operation is continued without an abnormal stop even in the case where a temporary alternating current power system fault such as an instantaneous voltage drop or the like occurs. 
     In the case where the unipolar modulation method is used in a system-connected inverter device having a FRT function, low-order harmonics undesirably occur easily in the case where an instantaneous voltage drop occurs and the modulation factor decreases. In other words, the output voltage waveform undesirably distorts in the FRT operation interval. In the case where the dipolar modulation method is used, the occurrence of harmonics in the FRT operation interval can be suppressed; but, on the other hand, the switching loss of the steady-state operation undesirably increases. 
     Therefore, in the system-connected inverter device, it is desirable to obtain a more stable operation while suppressing the switching loss. 
     PRIOR ART DOCUMENTS 
     Non-Patent Literature 
     [Non-Patent Literature 1] 
     
         
         Fukuda, Shoji and Suzuki, Kunio “Harmonic Evaluation of Carrier-Based Multi-Level PWM Methods,” Institute of Electrical Engineers of Japan Transactions on Industry Applications, Vol. 119-D, No. 6, 1999, p. 769-775 
       
    
     SUMMARY OF INVENTION 
     Problem to be Solved by the Invention 
     Embodiments of the invention provide a system-connected inverter device and a method for operating the system-connected inverter device in which the switching loss is suppressed and the operation is stable. 
     Means for Solving the Problem 
     According to an embodiment of the invention, a system-connected inverter device that includes a three-level inverter, a voltage sensor, and a controller is provided. The three-level inverter includes multiple switching elements, is connected to an electric power system of alternating current and a direct current power supply, converts direct current power supplied from the direct current power supply from direct current power into alternating current power by an ON/OFF of the multiple switching elements, and supplies the alternating current power to the electric power system. The voltage sensor detects an alternating current voltage of the electric power system. The controller detects an instantaneous voltage drop of the electric power system based on a detection result of the voltage sensor, and controls the converting by the three-level inverter from the direct current power into the alternating current power by controlling operations of the multiple switching elements using a unipolar modulation method in a state in which the instantaneous voltage drop is not detected and by controlling the operations of the multiple switching elements using a dipolar modulation method in a state in which the instantaneous voltage drop is detected. 
     Effects of the Invention 
     According to embodiments of the invention, a system-connected inverter device and a method for operating the system-connected inverter device are provided in which the switching loss is suppressed and the operation is stable. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram schematically illustrating a system-connected inverter device according to an embodiment. 
         FIG. 2A  and  FIG. 2B  are graphs schematically illustrating an example of the operation of the PWM controller according to the embodiment. 
         FIG. 3  is a flowchart schematically illustrating an example of the method for operating the system-connected inverter device according to the embodiment. 
         FIG. 4  is a block diagram schematically illustrating an example of the three-level inverter according to the embodiment. 
         FIG. 5  is a block diagram schematically illustrating another example of the three-level inverter according to the embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Embodiments will now be described with reference to the drawings. 
     The drawings are schematic or conceptual; and the relationships between the thicknesses and widths of portions, the proportions of sizes between portions, etc., are not necessarily the same as the actual values thereof. Further, there are also cases where the dimensions and/or the proportions are illustrated differently between the drawings, even in the case where the same portion is illustrated. 
     In this specification and each drawing, components similar to ones described in reference to an antecedent drawing are marked with the same reference numerals; and a detailed description is omitted as appropriate. 
       FIG. 1  is a block diagram schematically illustrating a system-connected inverter device according to an embodiment. 
     As illustrated in  FIG. 1 , the system-connected inverter device  10  includes a major circuit portion  12  and a controller  14 . The major circuit portion  12  includes a three-level inverter  20 , circuit breakers  21  and  22 , filter capacitors  24  and  25 , a filter reactor  26 , voltage sensors  31  to  33 , and current sensors  36  to  38 . 
     The three-level inverter  20  is connected to a direct current power supply  2  via the circuit breaker  21 . Also, the three-level inverter  20  is connected to an alternating current electric power system  4  via the circuit breaker  22 . The three-level inverter  20  converts direct current power supplied from the direct current power supply  2  into alternating current power and supplies the alternating current power after the conversion to the electric power system  4 . 
     The direct current power supply  2  is, for example, a solar power generator. In such a case, the system-connected inverter device  10  also may be called a power conditioner. The direct current power supply  2  is not limited to a solar power generator and may be any generator or power supply that can supply direct current power to the system-connected inverter device  10 . 
     The alternating current power of the electric power system  4  may be single-phase alternating current power or may be three-phase alternating current power, etc. The three-level inverter  20  may convert the direct current power into single-phase alternating current power or may convert the direct current power into three-phase alternating current power. 
     The three-level inverter  20  converts the direct current power supplied from the direct current power supply  2  into the alternating current power by an ON/OFF of each of switching elements  40 . 
     For example, based on a direct current voltage V DC  supplied from the direct current power supply  2 , the three-level inverter  20  outputs a three-level voltage of 0 V, ½V DC , and V DC . More specifically, the voltages of −V DC , −½V DC , 0 V, ½V DC , and V DC  are output. Thereby, the three-level inverter  20  converts the direct current power into the alternating current power. The circuit configuration of the three-level inverter  20  may be any circuit configuration that can output a three-level voltage. 
     Each of the switching elements  40  includes, for example, a self arc-extinguishing type semiconductor element such as a GTO (Gate Turn-Off thyristor), an IGBT (Insulated Gate Bipolar Transistor), etc. Each of the switching elements  40  includes a control terminal and a pair of major terminals. The control terminal is, for example, a gate terminal. Each of the switching elements  40  is switched between an ON state and an OFF state according to the voltage of the control terminal. The control terminal of each of the switching elements  40  is connected to the controller  14 . The controller  14  controls the conversion from the direct current power into the alternating current power of the three-level inverter  20  by switching the ON/OFF of each of the switching elements  40 . 
     The circuit breaker  21  is provided between the direct current power supply  2  and the three-level inverter  20 . The circuit breaker  21  switches between a state in which the three-level inverter  20  is connected to the direct current power supply  2  and a state in which the three-level inverter  20  is cut off from the direct current power supply  2 . The circuit breaker  22  is provided between the electric power system  4  and the three-level inverter  20 . The circuit breaker  22  switches between a state in which the three-level inverter  20  is connected to the electric power system  4  and a state in which the three-level inverter  20  is cut off from the electric power system  4 . The switching of the states of the circuit breakers  21  and  22  are controlled by, for example, the controller  14 . For example, the circuit breakers  21  and  22  may automatically perform the switching of the states according to the current value, the voltage value, etc. The circuit breakers  21  and  22  each are provided as necessary and are omissible. 
     The filter capacitor  24  is provided between the direct current power supply  2  and the three-level inverter  20 . In the example, the filter capacitor  24  is provided between the circuit breaker  21  and the three-level inverter  20 . For example, the filter capacitor  24  suppresses noise included in the direct current power from the direct current power supply  2 . In other words, the filter capacitor  24  smoothes the direct current voltage. 
     The filter capacitor  25  and the filter reactor  26  are provided between the electric power system  4  and the three-level inverter  20 . In the example, the filter capacitor  25  and the filter reactor  26  are provided between the circuit breaker  22  and the three-level inverter  20 . 
     One end of the filter reactor  26  is connected to the alternating current output terminal of the three-level inverter  20 . The filter capacitor  25  and the filter reactor  26  suppress the harmonic components of an output voltage V OUT  and an output current I OUT  output from the three-level inverter  20  and cause the output voltage waveform and the output current waveform to better approach sine waves. 
     In the example, one filter capacitor  25  and one filter reactor  26  are illustrated for convenience. For example, in the case where the alternating current power of the electric power system  4  is three-phase alternating current power, the filter capacitor  25  and the filter reactor  26  are provided to correspond to each phase of the alternating current power. In other words, in the case of three-phase alternating current power, three of the filter capacitors  25  and three of the filter reactors  26  are provided. 
     The voltage sensor  31  is provided between the direct current power supply  2  and the circuit breaker  21 . The voltage sensor  31  is connected to the controller  14 . The voltage sensor  31  detects the direct current voltage V DC  of the direct current power supply  2  and inputs the detection result to the controller  14 . 
     The voltage sensor  32  is provided between the filter reactor  26  and the circuit breaker  22 . The voltage sensor  32  is connected to the controller  14 . The voltage sensor  32  detects the output voltage V OUT  of the three-level inverter  20  and inputs the detection result to the controller  14 . 
     The voltage sensor  33  is provided between the circuit breaker  22  and the electric power system  4 . The voltage sensor  33  is connected to the controller  14 . The voltage sensor  33  detects an alternating current voltage V AC  of the electric power system  4  and inputs the detection result to the controller  14 . 
     Also, in the case where the output voltage V OUT  of the three-level inverter  20  and the alternating current voltage V AC  of the electric power system  4  are three-phase alternating current voltages, the voltage sensor  32  and the voltage sensor  33  detect the voltage value of each phase of the three-phase alternating current voltage and input the detection result to the controller  14 . 
     The current sensor  36  is provided between the circuit breaker  21  and the three-level inverter  20 . The current sensor  36  is connected to the controller  14 . The current sensor  36  detects a direct current I DC  of the direct current power supply  2  and inputs the detection result to the controller  14 . 
     The current sensor  37  is provided between the three-level inverter  20  and the filter reactor  26 . The current sensor  37  is connected to the controller  14 . The current sensor  37  detects the output current I OUT  of the three-level inverter  20  and inputs the detection result to the controller  14 . 
     The current sensor  38  is provided between the filter reactor  26  and the circuit breaker  22 . The current sensor  38  is connected to the controller  14 . The current sensor  38  detects an alternating current I AC  of the electric power system  4  and inputs the detection result to the controller  14 . 
     In the case where the output current I OUT  of the three-level inverter  20  and the alternating current I AC  of the electric power system  4  are three-phase alternating current, the current sensor  37  and the current sensor  38  detect the current value of each phase of the three-phase alternating current and input the detection result to the controller  14 . 
     The controller  14  includes a control board  60 , a PWM (Pulse Width Modulation) controller  62 , a gate board  64 , and an instantaneous drop detector  66 . The detection results of the voltage sensors  31  to  33  and the current sensors  36  to  38  each are input to the control board  60 . Also, the current command value of the output current I OUT  of the three-level inverter  20  is input to the control board  60 . In other words, the current command value of the output current I OUT  is the current command value of the alternating current I AC  of the electric power system  4 . The current command value is, for example, the effective converted value after d-q conversion. The current command value may be, for example, a signal having a sinusoidal waveform. The current command value may be a predetermined constant value or may be changed by an input from a higher-level controller, etc. 
     Based on the detection results of the voltage sensors  31  to  33 , the detection results of the current sensors  36  to  38 , and the current command value that are input, the control board  60  generates a voltage reference VR for causing the output current I OUT  to approach the current command value (referring to  FIG. 2 ). Then, the control board  60  inputs the generated voltage reference VR to the PWM controller  62 . The voltage reference VR is, for example, a signal having a sinusoidal waveform. In the case where the output of the three-level inverter  20  is three-phase alternating current power, the control board  60  generates the voltage reference VR for each phase. 
     Based on the voltage reference VR that is input, the PWM controller  62  generates a PWM signal for switching the ON/OFF of each of the switching elements  40  of the three-level inverter  20 . The PWM controller  62  generates the PWM signals by comparing the voltage reference VR to carrier signals CS 1  and CS 2  (referring to  FIG. 2 ). The carrier signals CS 1  and CS 2  are, for example, signals having triangular waveforms. For example, the PWM controller  62  generates multiple PWM signals corresponding respectively to the switching elements  40 . The PWM controller  62  inputs, to the gate board  64 , the PWM signals that are generated. 
     The gate board  64  is connected to the PWM controller  62  and connected to the control terminal of each of the switching elements  40 . The gate board  64  generates multiple gate signals (drive signals) for each of the switching elements  40  from the input PWM signals and inputs the generated gate signals respectively to the control terminals of the switching elements  40 . Thereby, the controller  14  controls the ON/OFF of each of the switching elements  40 . 
     The detection result of the alternating current voltage V AC  of the electric power system  4  from the voltage sensor  33  is input to the instantaneous drop detector  66 . Based on the detection result of the alternating current voltage V AC  that is input, the instantaneous drop detector  66  detects the instantaneous voltage drop of the electric power system  4  and inputs the detection result to the PWM controller  62 . For example, in the case where the residual voltage of the alternating current voltage V AC  has become less than a first threshold, the instantaneous drop detector  66  detects the occurrence of the instantaneous voltage drop of the electric power system  4 . For example, after detecting the instantaneous voltage drop, in the case where the residual voltage of the alternating current voltage V AC  becomes a second threshold or more, the instantaneous drop detector  66  detects the restoration from the instantaneous voltage drop of the electric power system  4 . 
     The residual voltage is the proportion of the voltage after the drop to the voltage before the drop. The first threshold is, for example, 80%. The second threshold is, for example, 90%. For example, in the case where the residual voltage of the alternating current voltage V AC  has become less than 80%, the instantaneous drop detector  66  detects the occurrence of the instantaneous voltage drop of the electric power system  4 ; and in the case where the residual voltage becomes 90% or more, the instantaneous drop detector  66  detects the restoration from the instantaneous voltage drop of the electric power system  4 . Thus, the second threshold is greater than the first threshold. In other words, the determination of the residual voltage of the alternating current voltage V AC  has hysteresis. Thereby, the undesirable switching of the output of the instantaneous drop detector  66  alternately between the detection state and the nondetection state of the instantaneous voltage drop can be suppressed. It is not always necessary for the determination of the residual voltage of the alternating current voltage V AC  to have hysteresis. The second threshold may be the same as the first threshold. It is sufficient for the second threshold to be the first threshold or more. 
       FIG. 2A  and  FIG. 2B  are graphs schematically illustrating an example of the operation of the PWM controller according to the embodiment. 
       FIG. 2A  schematically illustrates an example of the operation of the unipolar modulation method of the PWM controller  62 .  FIG. 2B  schematically illustrates an example of the operation of the dipolar modulation method of the PWM controller  62 . 
     The PWM controller  62  generates the PWM signals by using the unipolar modulation method and the dipolar modulation method and by switching between the methods. 
     As illustrated in  FIG. 2A  and  FIG. 2B , the PWM controller  62  uses one voltage reference VR and two carrier signals CS 1  and CS 2  in each of the unipolar modulation method and the dipolar modulation method. The direct current bias component of the carrier signal CS 2  is different from the direct current bias component of the carrier signal CS 1 . In other words, in the example, the unipolar modulation method is a double-carrier unipolar PWM method; and in other words, the dipolar modulation method is a double-carrier dipolar PWM method. 
     In the unipolar modulation method, the amplitudes of the carrier signals CS 1  and CS 2  are 0.5. Also, in the unipolar modulation method, the direct current bias component of the carrier signal CS 1  is 0.5; and the direct current bias component of the carrier signal CS 2  is −0.5. 
     In the dipolar modulation method, the amplitudes of the carrier signals CS 1  and CS 2  are 1.0. Also, in the dipolar modulation method, the direct current bias component of the carrier signal CS 1  is 0.5; and the direct current bias component of the carrier signal CS 2  is −0.5. 
     The amplitudes and the direct current bias components of the carrier signals CS 1  and CS 2  of the methods are not limited to those recited above and are settable arbitrarily in ranges in which the operation of the three-level inverter  20  is controllable. For example, the generation method of the PWM signals and the method for controlling the switching elements  40  of the three-level inverter  20  for the methods are described in more detail in Non-Patent Literature 1 recited above, etc. 
     The PWM controller  62  switches between the unipolar modulation method and the dipolar modulation method according to the detection result of the instantaneous drop detector  66 . In the case where the instantaneous drop detector  66  does not detect the instantaneous voltage drop, the PWM controller  62  generates the PWM signals using the unipolar modulation method. Also, in the case where the instantaneous drop detector  66  detects the instantaneous voltage drop, the PWM controller  62  generates the PWM signals using the dipolar modulation method. 
     The PWM controller  62  switches from the unipolar modulation method to the dipolar modulation method according to the detection of the instantaneous voltage drop by the instantaneous drop detector  66  and switches from the dipolar modulation method to the unipolar modulation method according to the detection of the restoration from the instantaneous voltage drop. 
     For example, the PWM controller  62  switches between the unipolar modulation method and the dipolar modulation method by changing the amplitudes of the carrier signals CS 1  and CS 2 . For example, the PWM controller  62  switches from the unipolar modulation method to the dipolar modulation method by changing the amplitudes of the carrier signals CS 1  and CS 2  from 0.5 to 1.0. At this time, for example, the PWM controller  62  gradually changes from the unipolar modulation method to the dipolar modulation method by monotonously increasing the amplitudes of the carrier signals CS 1  and CS 2  from 0.5 to 1.0 over a prescribed amount of time. Thereby, an abrupt change of the modulation method can be suppressed. For example, the generation of noise accompanying the abrupt change of the modulation method, etc., can be suppressed. 
     Similarly, for example, the PWM controller  62  gradually changes from the dipolar modulation method to the unipolar modulation method by monotonously reducing the amplitudes of the carrier signals CS 1  and CS 2  from 1.0 to 0.5 over a prescribed amount of time. 
     The switching between the unipolar modulation method and the dipolar modulation method is not limited to the amplitudes of the carrier signals CS 1  and CS 2  and may be performed using the direct current bias components of the carrier signals CS 1  and CS 2 . For example, the PWM controller  62  switches between the unipolar modulation method and the dipolar modulation method using at least one of the amplitude or the direct current bias component for each of the carrier signals CS 1  and CS 2 . For example, the amplitude of the voltage reference VR also may be changed in the switching between the unipolar modulation method and the dipolar modulation method. 
     For example, in the switching between the unipolar modulation method and the dipolar modulation method, the frequencies (the carrier frequencies) of the carrier signals CS 1  and CS 2  may be changed. For example, the frequencies of the carrier signals CS 1  and CS 2  in the dipolar modulation method are set to half of the frequencies of the carrier signals CS 1  and CS 2  in the unipolar modulation method. Thereby, for example, the switching frequencies of the switching elements  40  of the three-level inverter  20  can be substantially the same between the methods. 
     In the switching between the unipolar modulation method and the dipolar modulation method, at least one of the amplitude or the direct current bias component for each of the carrier signals CS 1  and CS 2  may be changed gradually as recited above or may be selectively switched between the value of the unipolar modulation method and the value of the dipolar modulation method. In the case where the at least one of the amplitude or the direct current bias component is changed gradually, the value of the at least one of the amplitude or the direct current bias component may be changed continuously or may be changed in stages. 
     Also, in the case where the at least one of the amplitude or the direct current bias component is changed gradually, it is favorable for the prescribed amount of time necessary to change each of the methods to be less than 0.1 seconds. For example, it is favorable for the prescribed amount of time to be not less than about 0.01 seconds but less than about 0.1 seconds. 
       FIG. 3  is a flowchart schematically illustrating an example of the method for operating the system-connected inverter device according to the embodiment. 
     As illustrated in  FIG. 3 , when starting the operation, the controller  14  of the system-connected inverter device  10  causes the instantaneous drop detector  66  to detect the instantaneous voltage drop (step S 1  of  FIG. 3 ). Based on the detection result of the alternating current voltage V AC  input from the voltage sensor  33 , the instantaneous drop detector  66  detects the instantaneous voltage drop of the electric power system  4  and inputs the detection result to the PWM controller  62 . 
     Also, when starting the operation, the controller  14  causes the control board  60  to start generating the voltage reference VR. Based on the detection results of the voltage sensors  31  to  33 , the detection results of the current sensors  36  to  38 , the current command value, etc., the control board  60  generates the voltage reference VR and inputs the voltage reference VR to the PWM controller  62 . 
     In the case where the instantaneous voltage drop is not detected, the PWM controller  62  generates the PWM signals using the unipolar modulation method and inputs the PWM signals to the gate board  64  (step S 2  of  FIG. 3 ). 
     Based on the PWM signals that are input, the gate board  64  controls the ON/OFF of each of the switching elements  40  by generating the gate signals of the switching elements  40  of the three-level inverter  20  and by inputting the gate signals to the control terminals of the switching elements  40 . In other words, the conversion from the direct current power to the alternating current power by the three-level inverter  20  is controlled (step S 3  of  FIG. 3 ). 
     In the case where the instantaneous voltage drop is not detected by the instantaneous drop detector  66 , the controller  14  repeatedly executes the processing of step S 1  to step S 3  recited above. Thereby, the direct current power of the direct current power supply  2  is converted into alternating current power; and the alternating current power after the conversion is supplied to the electric power system  4 . 
     On the other hand, in the case where the instantaneous voltage drop is detected by the instantaneous drop detector  66 , the PWM controller  62  switches the modulation method from the unipolar modulation method to the dipolar modulation method. At this time, the PWM controller  62  gradually changes from the unipolar modulation method to the dipolar modulation method over a prescribed amount of time. Then, the PWM controller  62  generates the PWM signals using the dipolar modulation method and inputs the PWM signals to the gate board  64  (step S 4  of  FIG. 3 ). 
     Similarly to step S 3 , the gate board  64  controls the ON/OFF of each of the switching elements  40  by generating the gate signals of the switching elements  40  from the PWM signals (step S 5  of  FIG. 3 ). Thereby, the controller  14  provides the FRT function of continuing the operation even when the instantaneous voltage drop occurs. More specifically, a LVRT (Low Voltage Ride Through) function is provided. 
     In the case where the occurrence of the instantaneous voltage drop is detected by the instantaneous drop detector  66 , the controller  14  starts timing from the timing of the detection of the instantaneous voltage drop and determines whether or not the prescribed amount of time has elapsed (step S 6  of  FIG. 3 ). The prescribed amount of time is, for example, 1 second. 
     In the case where the prescribed amount of time has not elapsed, the controller  14  returns to step S 1 . In the case where the instantaneous voltage drop continues, the processing of step S 4  to step S 6  is repeated; and the operation continuation for when the instantaneous voltage drop has occurred is executed. On the other hand, in the case where the restoration from the instantaneous voltage drop is detected before the prescribed amount of time has elapsed, the PWM controller  62  switches the modulation method from the dipolar modulation method to the unipolar modulation method and returns to the steady-state operation of step S 1  to step S 3 . 
     In the case where it is determined that the prescribed amount of time has elapsed from the occurrence of the instantaneous voltage drop, the controller  14  stops the control of the switching elements  40  of the three-level inverter  20 . In other words, in the case where the prescribed amount of time has elapsed from the detection of the voltage drop, the controller  14  determines that there is a system fault of the electric power system  4  and performs an error stop of the operation of the three-level inverter  20 . 
     Thus, in the system-connected inverter device  10  according to the embodiment, the operations of the switching elements  40  of the three-level inverter  20  are controlled using the unipolar modulation method in the state in which the occurrence of the instantaneous voltage drop is not detected; and the operations of the switching elements  40  of the three-level inverter  20  are controlled using the dipolar modulation method in the state in which the occurrence of the instantaneous voltage drop is detected. In other words, in the state having the high modulation factor, the system-connected inverter device  10  uses the unipolar modulation method; and in the state having the low modulation factor, the system-connected inverter device  10  uses the dipolar modulation method. The modulation factor is the proportion of the direct current voltage and the alternating current voltage represented by V AC  (effective value)/V DC . 
     In the case where the modulation factor is high (e.g., 0.5 or more) in the dipolar modulation method, the switching loss that accompanies the ON/OFF of each of the switching elements  40  undesirably increases compared to that of the unipolar modulation method. The system-connected inverter device  10  uses the unipolar modulation method in the state having the high modulation factor in which the occurrence of the instantaneous voltage drop is not detected. Thereby, in the system-connected inverter device  10 , the increase of the switching loss of the steady-state operation can be suppressed. 
     Also, in the case where the modulation factor is low (e.g., less than 0.5) in the unipolar modulation method, low-order harmonics undesirably occur easily compared to the dipolar modulation method. The system-connected inverter device  10  uses the dipolar modulation method in the state having the low modulation factor in which the occurrence of the instantaneous voltage drop is detected. Thereby, in the system-connected inverter device  10 , the occurrence of the harmonics in the FRT operation interval can be suppressed. For example, in the FRT operation interval as well, a waveform that is near a sine wave can be output; and a stable operation can be obtained. 
     Thus, in the system-connected inverter device  10  according to the embodiment, the unipolar modulation method and the dipolar modulation method are switched according to the detection result of the instantaneous voltage drop. Thereby, the switching loss can be suppressed; and a stable operation can be obtained. 
     For example, in the FRT function of solar power generation, it is desirable for the operation continuation to be performed without a gate block in the case of an instantaneous voltage drop in which the residual voltage is 20% or more and the duration is within 1 second, and for the output after the restoration of the voltage to be restored within 0.1 seconds to 80% or more of that before the voltage drop. 
     Conversely, in the system-connected inverter device  10 , for example, the occurrence of the instantaneous voltage drop is detected in the case where the residual voltage has become less than 80%; the operation is continued by switching from the unipolar modulation method to the dipolar modulation method within 0.1 seconds from the detection of the instantaneous voltage drop; and in the case where the residual voltage becomes 90% or more, the restoration from the instantaneous voltage drop is detected; the switching from the dipolar modulation method to the unipolar modulation method is performed within 0.1 seconds from the detection of the restoration; and an alternating current voltage that is 80% or more of that before the voltage drop is output. Thereby, in the system-connected inverter device  10 , the FRT function of the solar power generation can be satisfied. 
       FIG. 4  is a block diagram schematically illustrating an example of the three-level inverter according to the embodiment. As illustrated in  FIG. 4 , a three-level inverter INV 1  ( 20 ) includes the multiple switching elements  40 , multiple rectifying elements  41  and  42 , and multiple charge storage elements  43  and  44 . In the example, the three-level inverter INV 1  is a three-phase bridge-type. In the example, the alternating current power of the electric power system  4  and the alternating current power converted by the three-level inverter INV 1  are three-phase alternating current power. 
     The three-level inverter INV 1  includes direct current terminals  20   p  and  20   n , alternating current terminals  20   u ,  20   v , and  20   w , and six arms AU, AV, AW, AX, AY, and AZ. The three-level inverter INV 1  is connected to the direct current power supply  2  via the direct current terminals  20   p  and  20   n . Also, the three-level inverter INV 1  is connected to the electric power system  4  via the alternating current terminals  20   u ,  20   v , and  20   w.    
     The arms AU, AV, AW, AX, AY, and AZ each are provided between the direct current terminals  20   p  and  20   n . In the three-level inverter INV 1 , the connection point between the arm AU and the arm AX, the connection point between the arm AV and the arm AY, and the connection point between the arm AW and the arm AZ respectively are the alternating current terminals  20   u ,  20   v , and  20   w.    
     In the example, the three-level inverter INV 1  includes twelve switching elements  40 , twelve rectifying elements  41 , six rectifying elements  42 , and two charge storage elements  43  and  44 . The switching elements  40  have a three-phase bridge connection. The rectifying elements  41  respectively are connected in anti-parallel with the switching elements  40 . The charge storage elements  43  and  44  are connected in series between the direct current terminals  20   p  and  20   n . The charge storage elements  43  and  44  are, for example, condensers. Thereby, the connection point of the charge storage elements  43  and  44  is a neutral point  20   c.    
     The configurations are substantially the same between the arms AU, AV, AW, AX, AY, and AZ of each phase connected to the alternating current terminals  20   u ,  20   v , and  20   w . Accordingly, here, the two arms AU and AX that are connected to the alternating current terminal  20   u  (the U-phase) are described as an illustration. 
     The arm AU that is on the positive side includes two switching elements Q 1  and Q 2  connected in series, rectifying elements DF 1  and DF 2  connected in anti-parallel respectively with the switching elements Q 1  and Q 2 , and a rectifying element DC 1  connected between the neutral point  20   c  and the series connection point of the switching elements Q 1  and Q 2 . 
     Similarly, the arm AX that is on the negative side includes two switching elements Q 3  and Q 4  connected in series, rectifying elements DF 3  and DF 4  connected in anti-parallel respectively with the switching elements Q 3  and Q 4 , and a rectifying element DC 2  connected between the neutral point  20   c  and the series connection point of the switching elements Q 3  and Q 4 . 
     The two arms AU and AX are connected in series between the direct current terminals  20   p  and  20   n ; and the series connection point of the two arms AU and AX is connected to the alternating current terminal  20   u  of the U-phase. The potential of the series connection point of the switching elements Q 1  and Q 2  is clamped to the neutral point potential via the rectifying element DC 1 . Similarly, the potential of the series connection point of the switching elements Q 3  and Q 4  is clamped to the neutral point potential via the rectifying element DC 2 . The rectifying elements DF 1  to DF 4  (the rectifying elements  41 ) are so-called reflux diodes. The rectifying elements DC 1  and DC 2  (the rectifying elements  42 ) are so-called clamp diodes. 
     The configurations of the arms AV and AW are substantially the same as the configuration of the arm AU. The configurations of the arms AY and AZ are substantially the same as the configuration of the arm AX. Thereby, the potentials of the alternating current terminals  20   u ,  20   v , and  20   w  are clamped to the potential of one of the three levels of the direct current terminal  20   p , the direct current terminal  20   n , and the neutral point  20   c  according to the switching of the switching elements  40 . The three-level inverter INV 1  is a so-called neutral-point-clamped converter. The three-level inverter INV 1  is a so-called NPC (NPC: Neutral-Point-Clamped) inverter. 
     In such a NPC-type three-level inverter INV 1  as described above, the control is performed by switching between the unipolar modulation method and the dipolar modulation method according to the detection result of the instantaneous voltage drop. Thereby, the switching loss can be suppressed; and a stable operation can be obtained. 
       FIG. 5  is a block diagram schematically illustrating another example of the three-level inverter according to the embodiment. 
     As illustrated in  FIG. 5 , the three-level inverter INV 2  ( 20 ) includes the multiple switching elements  40 , the multiple rectifying elements  41 , and the multiple charge storage elements  43  and  44 . Compared to the three-level inverter INV 1  described in reference to  FIG. 4 , the rectifying element  42  that functions as the clamp diode is omitted from the three-level inverter INV 2 . Components that are substantially the same functionally and configurationally as those of the three-level inverter INV 1  described in reference to  FIG. 4  are marked with the same reference numerals; and a detailed description is omitted. 
     In the example, one of the switching element Q 1  or Q 4  is provided in each of the arms AU and AX. Also, the two switching elements Q 2  and Q 3  that are connected in series are provided between the alternating current terminal  20   u  and the neutral point  20   c.    
     The orientation of the current flowing in the switching element Q 2  is the reverse of the orientation of the current flowing in the switching element Q 3 . When the switching element Q 2  is set to the ON state, the orientation of the current flowing in the switching element Q 2  is the direction from the neutral point  20   c  toward the alternating current terminal  20   u . When the switching element Q 3  is set to the ON state, the orientation of the current flowing in the switching element Q 3  is the direction from the alternating current terminal  20   u  toward the neutral point  20   c . In other words, the three-level inverter INV 2  of the example is a so-called T-type NPC inverter. 
     In the three-level inverter INV 2  as well, similarly to the three-level inverter INV 1 , the switching loss can be suppressed and a stable operation can be obtained by switching between the unipolar modulation method and the dipolar modulation method according to the detection result of the instantaneous voltage drop. 
     Thus, the three-level inverter  20  may have any circuit configuration that can output a three-level voltage and is applicable to a control using a unipolar modulation method and a control using a dipolar modulation method. The circuit configuration of the three-level inverter  20  is not limited to the three-level inverters INV 1  and INV 2  recited above and is modifiable as appropriate. 
     Hereinabove, embodiments of the invention are described with reference to specific examples. However, the embodiments of the invention are not limited to these specific examples. For example, one skilled in the art may similarly practice the invention by appropriately selecting specific configurations of components included in the system-connected inverter device such as the three-level inverter, the voltage sensor, the controller, etc., from known art; and such practice is within the scope of the invention to the extent that similar effects can be obtained. 
     Also, any two or more components of the specific examples may be combined within the extent of technical feasibility and are within the scope of the invention to the extent that the spirit of the invention is included. 
     Further, all system-connected inverter devices and methods for operating system-connected inverter devices practicable by an appropriate design modification by one skilled in the art based on the system-connected inverter devices and the methods for operating the system-connected inverter devices described above as the embodiments of the invention also are within the scope of the invention to the extent that the spirit of the invention is included. 
     Further, various modifications and alterations within the spirit of the invention will be readily apparent to those skilled in the art; and all such modifications and alterations should be seen as being within the scope of the invention. 
     While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the invention.