JP5520722B2 - Grid-connected inverter system - Google Patents

Description

  The present invention relates to a grid-connected inverter system including a plurality of inverter devices connected in parallel.

  2. Description of the Related Art Conventionally, a grid-connected inverter system has been developed that converts DC power generated by a solar cell or the like into AC power and supplies it to an electric power system.

  Due to recent changes in the world situation (Fukuda Vision at the Toyako Summit, Obama administration's Green New Deal policy, etc.), it is necessary to increase the capacity of distributed power sources such as solar cells. Therefore, in order to increase the capacity of the grid-connected inverter system to the megawatt level, as shown in FIG. 9, a grid-connected inverter system in which a plurality of (three in FIG. 9) inverter devices 200a to 200c are connected in parallel. A100 is manufactured.

  The inverter circuit 202 in the inverter device 200a in FIG. 9 is a circuit that converts a DC voltage input from the DC power supply 10a into an AC voltage by switching of a switching element (not shown). The inverter circuit 202 receives a current signal and a voltage signal from each sensor (not shown) and performs switching of the switching element based on a PWM signal generated by a built-in control circuit (not shown). The filter circuit 203 is a circuit that removes the switching frequency component included in the AC voltage output from the inverter circuit 202. The transformer 208 converts the AC voltage output from the filter circuit 203 into the system voltage of the power system 40. At the same time as boosting (or stepping down), the inverter device 200a and the power system 40 are electrically insulated.

The negative electrode of the output line of the DC power supply 10a is grounded by the ground line _LG1a . Thin film solar cells, which have become popular in recent years, deteriorate unless the negative electrode is grounded. In countries such as the United States, it is obliged to ground one pole of a solar cell. In order to detect a ground fault current, it is necessary to ground one output line (generally, the negative electrode side) of the DC power supply 10a. Although is grounded by the power system 40 also ground line L _G2, a current path is not formed since the inverter device 200a and the electric power system 40 by a transformer 208 provided in the inverter device 200a is insulated, The direct current is prevented from flowing into the power system 400.

  The configurations of the inverter devices 200b and 200c are the same as those of the inverter device 200a. The inverter devices 200a, 200b, and 200c are connected in parallel and connected to the power system 40.

  In the grid-connected inverter system A100 shown in FIG. 9, the inverters 200a, 200b, and 200c each have a built-in transformer 208. There is an advantage that the circulating current between the inverter devices 200a, 200b, and 200c can be prevented. However, since each of the inverter devices 200a, 200b, and 200c is provided with the transformer 208, three transformers 208 are required. Since the transformer 208 is used at a commercial frequency (50 Hz or 60 Hz), the transformer 208 is generally large in size, heavy, and expensive. Therefore, the grid-connected inverter system A100 that requires three transformers 208 has disadvantages such as an increase in the overall size, an increase in weight, and an increase in manufacturing cost. This inconvenience becomes more remarkable as the number of inverter devices connected in parallel increases.

  Further, in the transformer 208, power loss occurs due to winding resistance and iron core eddy current. Since power loss occurs in each of the transformers 208 of the inverter devices 200a, 200b, and 200c, there is a disadvantage that the power conversion efficiency in the entire grid-connected inverter system A100 is lowered.

  Regarding the invention for solving these problems, the applicant of the present application filed a patent application (Japanese Patent Application No. 2009-180457) on August 3, 2009. The invention according to the patent application (hereinafter referred to as “prior application invention”) relates to a grid-connected inverter system capable of suppressing the number of necessary transformers even when a plurality of inverter devices are connected in parallel. is there.

  FIG. 10 is a block diagram for explaining a grid interconnection inverter system according to the invention of the prior application.

  The grid-connected inverter system A200 is configured by connecting inverter devices 20a to 20c in parallel, but the inverter 208 is not provided in each of the inverter devices 20a to 20c, and the connection point on the output side of the inverter devices 20a to 20c The point in which the transformer 30 is provided between the electric power grid | systems 40 differs from the grid connection inverter system A100 shown in FIG.

  Since the grid interconnection inverter system A200 is not provided with the transformer 208 in each of the inverter devices 20a to 20c, the overall size is reduced as compared with the conventional grid interconnection inverter system A100 (see FIG. 9). Weight can be reduced and manufacturing costs can be reduced. Moreover, power loss can be reduced and power conversion efficiency can be improved. Moreover, since the transformer 30 is provided between the connection point on the output side of the inverter devices 20a to 20c and the power system 40, and the inverter devices 20a to 20c are electrically insulated from the power system 40, each inverter device The outflow of direct current from 20a, 20b, 20c to the electric power system 40 can be prevented.

  In the grid-connected inverter system A200, for example, a current path through which current circulates between the inverter device 20a and the inverter device 20b is formed. FIG. 11 is a diagram for explaining a current path formed between the inverter device 20a and the inverter device 20b. As shown in the figure, since the negative electrode a of the DC power supply 10a and the negative electrode b of the DC power supply 10b are grounded and electrically connected to the ground G, the inverter device 20a and the inverter device 20b are connected on the input side. Electrically connected. Further, for example, since the U-phase output line of the inverter device 20a and the U-phase output line of the inverter device 20b are connected at the connection point c, the inverter device 20a and the inverter device 20b are electrically connected on the output side. It is connected. As a result, a current path I (thick line path shown in the figure) of the negative electrode a, the inverter device 20a, the connection point c, the inverter device 20b and the negative electrode b is formed.

  In order to suppress the circulating current flowing through the current path I, the applicant of the present application is the circulating current suppression control (high frequency component suppression control, direct current component suppression control, and tertiary component suppression control shown in the prior invention, Details are omitted.) The circulating current flowing through the current path I is suppressed by the circulating current suppression control. Therefore, it can suppress that the output electric power of each inverter apparatus 20a, 20b, 20c falls by circulating current.

JP 2008-182836 A

  However, when there is a sudden output power fluctuation of the solar cell, a sudden voltage fluctuation of the power system 40 such as a momentary drop, an output voltage fluctuation at the time of starting and stopping the inverter device, the control control of the circulating current cannot be followed. In addition, the circulating current may increase rapidly. Further, in order to actually connect the grid-connected inverter system A200 to the power system 40, each inverter device 20a, 20b, 20c is provided with a configuration for detecting a ground fault or overcurrent and stopping the operation. There is a need. Accordingly, when the circulating current suppression control cannot follow during a transient and the circulating current rapidly increases, it may be determined that the increase in current due to the circulating current is due to a ground fault or may be detected as an overcurrent. In this case, for safety, the inverter device that detects a ground fault or overcurrent is stopped and disconnected from the grid interconnection inverter system A200.

FIG. 12 is a diagram for explaining a configuration for detecting a ground fault, which is provided in the grid interconnection inverter system A200. In the figure, only the inverter device 20a, the DC power source 10a, and the ground line L _G1a are shown, and other configurations (inverter devices 20b, 20c, DC power sources 10b, 10c, ground lines L _G1b , L _{The description of G1c} , transformer 30, and power system 40) is omitted. The configurations of inverter devices 20b and 20c and ground lines L _G1b and L _G1c are the same as inverter device 20a and ground line L _G1a , respectively.

The inverter device 20a is provided with a circuit breaker 24 and a ground fault determination circuit 25, the ground line _LG1a is provided with a circuit breaker Sa, and a current sensor 25a of the ground fault determination circuit 25 is disposed.

The ground fault determination circuit 25 detects a ground fault. The ground fault determination circuit 25 detects the current flowing through the ground line _LG1a by the current sensor 25a, compares the detected current value with a predetermined threshold value, and if the detected current value exceeds the threshold value, a ground fault occurs. Is determined to have occurred. When it is determined that a ground fault has occurred, the ground fault determination circuit 25 outputs a detection signal to the DC / DC converter circuit 21, the inverter circuit 22, the circuit breaker 24, and the circuit breaker Sa. The DC / DC converter circuit 21 and the inverter circuit 22 are stopped when a detection signal (stop signal) is input from the ground fault determination circuit 25. The circuit breaker 24 and the circuit breaker Sa are disconnected from each other when a detection signal (cut-off signal) is input from the ground fault determination circuit 25. Thereby, it is possible to prevent the influence of the ground fault accident from spreading.

When circulating current flows in the grid-connected inverter system A200 (see FIG. 11), the circulating current suppression control (high frequency component suppression control, DC component suppression control, and tertiary component suppression control) shown in the invention of the prior application works. , Circulating current is suppressed. However, if the circulating current flowing through the ground line L _G1a suddenly increases without being able to follow the suppression control during a transient, the current value detected by the current sensor 25a (see FIG. 12) disposed on the ground line L _G1a becomes the ground value. The threshold set in the fault determination circuit 25 may be exceeded, and the ground fault determination circuit 25 may erroneously detect a ground fault. In this case, the inverter device 20a stops and is disconnected from the power system 40.

  FIG. 13 is a diagram for explaining each output waveform of the grid-connected inverter system A200 in a simulation in a case where the output of the solar cell rapidly changes during steady operation. The simulation is for a case where two inverter devices are connected in parallel (that is, when only the inverter device 20a and the inverter device 20b are connected in parallel), and the output power of the DC power supply 10a at time t1. Is suddenly changed to 1/3.

FIG. 5A shows a waveform E ₁ of the output voltage of the DC power supply 10a and a waveform E ₂ of the output voltage of the DC power supply 10b. Both the output voltage of the DC power supply 10a and the output voltage of the DC power supply 10b have decreased from time t1. FIG. 2B shows the waveform of the output current of each phase of the inverter device 20a, and FIG. 4C shows the waveform of the output current of each phase of the inverter device 20b. FIG. 6D shows the waveform of the current detected by the current sensor 25a (see FIG. 12) of the ground fault determination circuit 25 of the inverter device 20a (hereinafter referred to as “ground fault determination current”). FIG. 4E shows the waveform of the ground fault determination current of the inverter device 20b. In addition, in the same figure (d) and (e), the case where an electric current flows into the ground side is shown as plus.

From time t1, the waveform of the output current of each phase of the inverter device 20a changes to the minus side (lower side in the figure), and the waveform of the output current of each phase of the inverter device 20b goes to the plus side (upper side in the figure). (See (b) and (c) in the figure). Further, the magnitude of the ground fault determination current of the inverter device 20a and the magnitude of the ground fault judgment current of the inverter device 20b have increased since time t1 (see FIGS. 4D and 4E). This is because the voltage at both ends of the filter circuit 23 of the inverter device 20a becomes lower than the voltage at both ends of the filter circuit 23 of the inverter device 20b because the output power of the DC power supply 10a has suddenly decreased. This is due to the circulating current flowing in the opposite direction of the arrow. That is, the output current of each phase of the inverter device 20a is superposed on the circulating current flowing from right to left in FIG. 11 so that the output current shifts to the negative side, and the output current of each phase of the inverter device 20b is left The output current is shifted to the positive side by superimposing the circulating current flowing from right to left. In addition, since a circulating current flows to the ground line L _G1a toward the ground side, the current sensor 25a of the ground fault determination circuit 25 of the inverter device 20a detects a positive current, and the ground line L _G1b is detected from the ground side. When the circulating current flows, the current sensor 25a of the ground fault determination circuit 25 of the inverter device 20b detects a negative current.

  Since the suppression control of the circulating current cannot follow at the transient time, the circulating current increases rapidly, and the ground fault accident is erroneously detected because the ground fault determination current exceeds the threshold at time t2. As a result, at time t3, the inverter device 20a and the inverter device 20b are stopped, and the output current and ground fault determination current of each phase are “0”.

  The present invention has been conceived under the circumstances described above, and suppresses circulating current during a transition when a current path for circulating current flows between inverter devices connected in parallel. An object of the present invention is to provide a grid-connected inverter system that can be used.

  In order to solve the above problems, the present invention takes the following technical means.

  A grid-connected inverter system provided by the present invention includes a plurality of DC power supplies, and each DC power supply connected to each DC power supply. The DC power from the connected DC power supplies is converted into AC power, and the AC power is converted into a transformer. A plurality of inverter devices that output without passing through, a transformer provided between a power system and a connection point on the output side where the plurality of inverter devices are connected in parallel to each other, and the DC power source A plurality of ground wires each grounding one of a pair of output terminals of at least two DC power supplies, and each ground wire is provided with an impedance section for increasing the impedance of the ground wire. It is characterized by that.

  In a preferred embodiment of the present invention, the impedance part is a resistance element.

  In a preferred embodiment of the present invention, the impedance part is a reactor.

  In a preferred embodiment of the present invention, a capacitor further connected in parallel to the impedance unit is further provided.

  In preferable embodiment of this invention, the ground fault detection apparatus which detects a ground fault accident is further provided.

  In a preferred embodiment of the present invention, the ground fault detection device detects a ground fault by comparing a current flowing through the ground line with a predetermined threshold value.

  In preferable embodiment of this invention, the overcurrent detection apparatus which detects that the electric current output from the said inverter apparatus is an overcurrent is further provided.

  In a preferred embodiment of the present invention, there are two DC power supplies, and the impedance section is provided only on one of the ground lines of these two DC power supplies.

  In a preferred embodiment of the present invention, the direct current power source includes a solar cell.

  According to the present invention, the current path of the circulating current is formed by the plurality of inverter devices, the ground line, and the connection point. However, since the impedance portion is provided on the ground line, the peak value of the circulating current at the time of transient fluctuation can be suppressed. Therefore, even when a ground fault detection device or an overcurrent detection device is provided, it is possible to suppress erroneous detection of a ground fault or an overcurrent due to the circulating current.

  Other features and advantages of the present invention will become more apparent from the detailed description given below with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS It is a block diagram for demonstrating 1st Embodiment of the grid connection inverter system which concerns on this invention. It is a figure for demonstrating the current pathway formed. It is a figure for demonstrating each output waveform of the grid connection inverter system in simulation. It is a block diagram for demonstrating 2nd Embodiment of the grid connection inverter system which concerns on this invention. It is a block diagram for demonstrating 3rd Embodiment of the grid connection inverter system which concerns on this invention. It is a figure for demonstrating the current pathway formed. It is a block diagram for demonstrating the example which provided another ground fault determination circuit. It is a block diagram for demonstrating the structure which detects an overcurrent. It is a figure for demonstrating the conventional grid connection inverter system. It is a block diagram for demonstrating the grid connection inverter system which concerns on prior invention. It is a figure for demonstrating the current pathway formed. It is a figure for demonstrating the structure for a ground fault detection provided in an inverter apparatus. It is a figure for demonstrating each output waveform of the grid connection inverter system in simulation.

  Embodiments of the present invention will be specifically described below with reference to the drawings.

  FIG. 1 is a block diagram for explaining a first embodiment of a grid-connected inverter system according to the present invention.

As shown in the figure, the grid-connected inverter system A includes three DC power supplies 10a, 10b, and 10c, three inverter devices 20a, 20b, and 20c, and one transformer 30. The DC power supplies 10a, 10b, and 10c are connected to inverter devices 20a, 20b, and 20c, respectively. The output terminals of the inverter devices 20a, 20b, and 20c are connected in parallel to each other. The transformer 30 is connected between the connection point of the inverter devices 20a, 20b, and 20c and the power system 40. Therefore, the grid-connected inverter system A includes a configuration in which three distributed power sources including a DC power source and an inverter device are connected in parallel to the power system 40 through one transformer 30, and the DC power source 10a, The DC power output by 10b and 10c is converted into AC power and supplied to the power system 40. The power system 40 is a three-phase three-wire (or three-phase four-wire) power system, and any one phase (for example, V phase) is grounded by a ground line _LG2 .

  In the first embodiment, the number of distributed power sources is three, but this is an example, and the number of units connected in parallel may be two or four or more, and can be arbitrarily set.

The DC power supplies 10a, 10b, and 10c supply DC power to the grid interconnection inverter system A, and include solar cells. The solar cell is a thin-film solar cell, and one of the output lines of the DC power supplies 10a, 10b, and 10c (in FIG. 1, the negative output line) is grounded by ground lines L _G1a, L _{G1b, and} L _G1c . In addition, a solar cell is not limited to a thin film type solar cell, Other types of solar cells may be sufficient. A solar cell generates direct-current power by converting solar energy into electrical energy. DC voltages output from the DC power supplies 10a, 10b, and 10c are input to the inverter devices 20a, 20b, and 20c, respectively.

The ground line L _G1a is provided with a circuit breaker Sa and a resistor Ra, and a current sensor 25a of a ground fault determination circuit 25 (described later) is disposed. The circuit breaker Sa cuts off the connection between the output line of the DC power supply 10a and the ground line _LG1a when a ground fault occurs. The circuit breaker Sa normally connects the output line of the DC power supply 10a and the ground line _LG1a , but when the detection signal (cutoff signal) is input from the ground fault determination circuit 25, the circuit breaker Sa cuts off the connection. This prevents the influence of the ground fault accident from spreading. The circuit breaker Sb provided on the ground line L _G1b and the circuit breaker Sc provided on the ground line L _G1c have the same configuration as the circuit breaker Sa provided on the ground line L _G1a .

The resistor Ra is for suppressing an increase in circulating current, and a resistance element of several tens of ohms is used in this embodiment. The resistor Ra is not limited to the resistive element, or the ground line L _G1a to high electrical resistivity material, by or reduce the cross-sectional area of the ground line L _G1a, also includes increasing the resistance value. Note that the resistance Rb provided on the ground line L _G1b and the resistance Rc provided on the ground line L _G1c are also for suppressing an increase in circulating current, similarly to the resistor Ra provided on the ground line L _G1a . Is.

  FIG. 2 is a diagram for explaining a current path formed between the inverter device 20a and the inverter device 20b, and corresponds to FIG. 11 described above. As shown in the figure, the inverter device 20a and the inverter device 20b are electrically connected on the input side and the output side, and a current path of a negative electrode a, an inverter device 20a, a connection point c, an inverter device 20b, and a negative electrode b. I (thick line path shown in the figure) is formed. However, since the resistor Ra and the resistor Rb are provided between the negative electrode a and the negative electrode b on the current path I, the circulating current flowing due to the potential difference generated on the current path I is suppressed. Thereby, the operating characteristic at the time of the transient of the grid connection inverter system A improves.

  In addition, since the circulating current is suppressed by the suppression control of the circulating current, deterioration due to electrolytic corrosion which is a problem in the thin film solar cell is suppressed. However, if the resistance value of the resistor Ra is too large, the negative electrode a and the ground G The fluctuation of the potential during the period becomes large, and deterioration due to electrolytic corrosion cannot be ignored. Therefore, the resistance values of the resistors Ra, Rb, and Rc need to be reduced. On the other hand, when the resistance values of the resistors Ra, Rb, and Rc are too small, the circulating current cannot be appropriately suppressed. It is necessary to determine the resistance values of the resistors Ra, Rb, and Rc for each system so that deterioration due to electric corrosion does not occur and the circulating current can be appropriately suppressed.

Returning to FIG. 1, the current sensor 25a detects the current flowing through the ground line _LG1a . In addition, the current sensor 25a of the ground fault determination circuit 25 of the inverter device 20b and the inverter device 20c is also disposed on the ground line L _G1b and the ground wire L _G1c , respectively.

  The inverter devices 20a, 20b, and 20c convert the DC voltage input from the DC power supplies 10a, 10b, and 10c, respectively, into an AC voltage and output the AC voltage to the transformer 30.

  As shown in the figure, the inverter device 20a includes a DC / DC converter circuit 21, an inverter circuit 22, a filter circuit 23, a circuit breaker 24, and a ground fault determination circuit 25.

  The DC / DC converter circuit 21 is a step-up converter that boosts a DC voltage input from the DC power supply 10 a and outputs the boosted voltage to the inverter circuit 22. The DC / DC converter circuit 21 switches a switching element (not shown) on and off based on a PWM signal generated by a built-in control circuit (not shown), thereby generating a DC voltage input from the DC power supply 10a. The voltage is boosted to a predetermined voltage level and output. Note that the configuration of the DC / DC converter circuit 21 is not limited to this, and any known boost converter may be used. Further, when a detection signal (stop signal) is input from the ground fault determination circuit 25 (described later), the DC / DC converter circuit 21 stops the boost operation by stopping the generation of the PWM signal by the control circuit. Thereby, it is possible to prevent the influence of the ground fault accident from spreading.

  The inverter circuit 22 converts the DC voltage input from the DC / DC converter circuit 21 into an AC voltage and outputs the AC voltage. In the first embodiment, since the power system 40 is a three-phase three-wire power system, the inverter circuit 22 includes a three-phase full-bridge inverter. Therefore, the inverter circuit 22, the filter circuit 23, the circuit breaker 24, and the transformer 30 are connected to the power system 40 by three output lines of U-phase, V-phase, and W-phase output voltages. Note that the configuration of the inverter circuit 22 is not limited to the three-phase full-bridge inverter, and may be appropriately determined according to the connected power system 40 and other conditions. That is, the inverter circuit 22 may include a single-phase inverter instead of a three-phase inverter, and may include a half-bridge inverter instead of a full-bridge inverter.

  The inverter circuit 22 has a three-phase bridge circuit (not shown) including three arms in which two switching elements are connected in series, and each switching element is based on a PWM signal generated by a control circuit (not shown). By switching on and off, the DC voltage input from the DC / DC converter circuit 21 is converted into an AC voltage. The AC voltage output from the inverter circuit 22 is input to the filter circuit 23.

  The control circuit of the inverter circuit 22 generates a PWM signal by a triangular wave comparison method that compares the command value signal with a triangular wave carrier signal. The command value signal is a sine wave signal having a frequency close to the frequency (50 Hz or 60 Hz) of the power system 40. The command value signal is a direct current and a direct current voltage input to the DC / DC converter circuit 21, a direct current voltage (hereinafter referred to as "bus voltage") output from the DC / DC converter circuit 21 (input to the inverter circuit 22). ), The AC current output from the inverter circuit 22, the AC current and AC voltage output from the filter circuit 23, and their target values. In the figure, a detection circuit for detecting the input current and input voltage to the DC / DC converter circuit 21, the bus voltage, the output current of the inverter circuit 22, and the output current and output voltage of the filter circuit 23 is shown. Description is omitted. The carrier signal is a triangular wave having a frequency (for example, 4 kHz) that is several times or several tens of times that of the command value signal. The inverter circuit 22 adjusts the output voltage of the inverter circuit 22 by controlling on / off switching of each switching element with a PWM signal.

  Further, when the detection signal (stop signal) is input from the ground fault determination circuit 25 (described later), the inverter circuit 22 stops the power conversion operation by stopping the generation of the PWM signal by the control circuit. Thereby, it is possible to prevent the influence of the ground fault accident from spreading.

  The filter circuit 23 removes a switching frequency component from the AC voltage input from the inverter circuit 22. The filter circuit 23 includes a low-pass filter including a reactor and a capacitor (not shown). The AC voltage from which the switching frequency component has been removed by the filter circuit 23 is output to the transformer 30. The configuration of the filter circuit 23 is not limited to this, and any known filter circuit for removing the switching frequency component may be used.

  The circuit breaker 24 interrupts the connection between the output line of the inverter device 20a and the transformer 30 when a ground fault occurs. The circuit breaker 24 normally connects the output line of the inverter device 20a and the transformer 30, but when the detection signal (breaking signal) is input from the ground fault determination circuit 25 (described later), the circuit breaker 24 is disconnected. . Thereby, the inverter device 20a is disconnected from the grid interconnection inverter system A, and the influence of the ground fault is prevented.

The ground fault determination circuit 25 detects a ground fault. The ground fault determination circuit 25 detects the current flowing through the ground line _LG1a by the current sensor 25a, compares the detected current value with a predetermined threshold value, and if the detected current value exceeds the threshold value, a ground fault occurs. Is determined to have occurred. When it is determined that a ground fault has occurred, the ground fault determination circuit 25 outputs a detection signal to the DC / DC converter circuit 21, the inverter circuit 22, the circuit breaker 24, and the circuit breaker Sa. The DC / DC converter circuit 21 and the inverter circuit 22 are stopped when a detection signal (stop signal) is input from the ground fault determination circuit 25. The circuit breaker 24 and the circuit breaker Sa are disconnected from each other when a detection signal (cut-off signal) is input from the ground fault determination circuit 25. Thereby, it is possible to prevent the influence of the ground fault accident from spreading.

  Although not shown in the figure, the internal configuration of the inverter devices 20b and 20c is the same as that of the inverter device 20a.

  The transformer 30 boosts the AC voltage output from the inverter devices 20a, 20b, and 20c to a voltage for supplying to the power system 40. Moreover, the transformer 30 is connected between the connection point on the output side of the three inverter devices 20a, 20b, and 20c connected to each other and the power system 40, and the inverter devices 20a, 20b, and 20c are supplied with power. It is electrically insulated from the system 40. Accordingly, it is possible to prevent a direct current from flowing out of inverter devices 20a, 20b, and 20c to power system 40.

The three inverter devices 20a, 20b, and 20c are electrically connected to each other at the connection point on the output side, and the input side is also electrically connected by the ground lines L _G1a , L _G1b , L _G1c and the ground (ground). It is connected to the. Therefore, a current path through which a current circulates is formed between the inverter devices 20a, 20b, and 20c. However, the circulating current is suppressed by the suppression control of the circulating current, and at the time of the transient fluctuation in which the suppression control cannot follow, the circulating current is caused by the resistors Ra, Rb, Rc provided on the ground lines L _G1a , L _G1b , L _G1c , respectively. The peak value of is suppressed. Therefore, since it is possible to suppress the circulating current flowing through the ground lines L _G1a , L _G1b , and L _{G1c from exceeding the} threshold set in the ground fault determination circuit 25, the ground fault determination circuit 25 is connected to the ground fault. It is possible to suppress erroneous detection of an accident.

  FIG. 3 is a diagram for explaining each output waveform of the grid interconnection inverter system A when a simulation similar to the simulation in FIG. 13 is performed in the grid interconnection inverter system A. The simulation was performed under the same conditions as the simulation in FIG. 13, and the output power of the DC power supply 10a was reduced to 1/3 at time t1 with only the inverter device 20a and the inverter device 20b connected in parallel. It was a sudden change.

FIG. 5A shows a waveform E ₁ of the output voltage of the DC power supply 10a and a waveform E ₂ of the output voltage of the DC power supply 10b. The output voltage of the DC power supply 10a has decreased since time t1. FIG. 2B shows the waveform of the output current of each phase of the inverter device 20a, and FIG. 4C shows the waveform of the output current of each phase of the inverter device 20b. FIG. 4D shows a waveform of a ground fault determination current (detected current of the current sensor 25a) of the ground fault determination circuit 25 of the inverter device 20a, and FIG. 5E shows a ground fault determination of the inverter device 20b. A current waveform is shown. In addition, in the same figure (d) and (e), the case where an electric current flows into the ground side is shown as plus. Even after time t1, the waveform of the output current of each phase of the inverter device 20a and the waveform of the output current of each phase of the inverter device 20b hardly change, and the ground fault determination current of the inverter device 20a and the ground fault determination of the inverter device 20b. The current has not changed significantly. That is, even when the output power of the DC power supply 10a is suddenly changed to 1/3, the circulating current is suppressed and the ground fault determination current does not change greatly, and the ground fault determination circuit 25 erroneously detects a ground fault. There wasn't.

  FIG. 4 is a block diagram for explaining a second embodiment of the grid-connected inverter system according to the present invention.

In the figure, only the inverter device 20a, the DC power source 10a, and the ground line L _G1a are shown, and other configurations (inverter devices 20b, 20c, DC power sources 10b, 10c, ground lines L _G1b , L _{The description of G1c} , transformer 30, and power system 40) is omitted. The configurations of inverter devices 20b and 20c and ground lines L _G1b and L _G1c are the same as inverter device 20a and ground line L _G1a , respectively. In the figure, the same or similar elements as those in the grid interconnection inverter system A shown in FIG. The grid-connected inverter system A ′ according to the second embodiment is characterized in that a capacitor is provided in parallel with the resistor and the circuit breaker provided on the ground line, and the grid-connected inverter system A according to the first embodiment. Is different.

As shown in FIG. 4, a capacitor Ca is connected in parallel with the resistor Ra and the circuit breaker Sa to the ground line _LG1a of the grid-connected inverter system A ′. Therefore, the high-frequency component of the circulating current (mainly the switching frequency component of the inverter circuit 22) flows to the capacitor Ca and is not detected by the current sensor 25a of the ground fault determination circuit 25, thereby further suppressing the erroneous detection of the ground fault. be able to. Further, the direct current component and the low frequency component of the circulating current are suppressed by the circulating current suppression control, and the high frequency component of the circulating current is passed through the capacitor Ca, so that the direct current between the negative electrode of the direct current power source 10a and the ground (ground). Since the potential difference can be reduced, deterioration due to electrolytic corrosion can be further suppressed.

  In the first and second embodiments, the case where three DC power supplies and three inverter devices are provided has been described. However, the present invention is not limited to this. The present invention can be applied even when two DC power supplies and two inverter devices are provided, or when four or more DC devices are provided. In particular, when two units are provided, a resistor may be provided only on one of the ground wires. Hereinafter, a case where two DC power supplies and two inverter devices are provided and a resistor is provided only on one of the ground lines will be described as a third embodiment.

  FIG. 5 is a block diagram for explaining a third embodiment of the grid interconnection inverter system according to the present invention.

In the figure, the same or similar elements as those in the grid interconnection inverter system A shown in FIG. Interconnection inverter system A "according to the third embodiment includes a DC power supply 10c, and that the inverter 20c, and the ground line L _G1c is not provided, and that it does not resistance Rb is provided to the ground line L _G1b Thus, it is different from the grid interconnection inverter system A according to the first embodiment.

  6 is a diagram for explaining a current path formed in the grid-connected inverter system A ″ (between the inverter device 20a and the inverter device 20b), and corresponds to FIG. 2 described above. 6, since the resistor Ra ′ is provided between the negative electrode a and the negative electrode b on the current path I, the circulating current flowing due to the potential difference generated on the current path I is suppressed. The resistance value of the resistor Ra ′ needs to be a resistance value obtained by adding the resistance values of the resistor Ra and the resistor Rb (see FIGS. 1 and 2) in the first embodiment.

Note that even when four or more DC power supplies and inverter devices are provided, or when three DC devices and three inverter devices are provided (first embodiment), it is not always necessary to provide resistors for all the ground lines. The total resistance value in the current path formed between each inverter device is a range of resistance values (hereinafter referred to as “appropriate”) that does not cause deterioration of the solar cell due to electrolytic corrosion and can appropriately suppress the circulating current. Resistance value range ”). For example, in the case of the first embodiment shown in FIG. 1 (when three DC power supplies and three inverter devices are provided), the resistance value of the resistor Ra, the resistance value of the resistor Rb, and the sum of the resistors Ra and Rb By setting the resistance values of the resistors Ra and Rb so that all the resistance values are within the appropriate resistance value range, the resistor Rc of the ground line _LG1c can be omitted.

  In the first to third embodiments, the case where a resistor is provided on the ground line has been described. However, the present invention is not limited to this. Even when a reactor is provided instead of the resistor, the circulating current flowing through the current path formed between the inverter devices can be appropriately suppressed.

  Although the said 1st thru | or 3rd embodiment demonstrated the case where the ground fault determination circuit 25 detected the electric current which flows through a grounding line, and detected a ground fault, it is not restricted to this. For example, the ground fault determination circuit may detect a zero fault by detecting a zero-phase current.

  FIG. 7 is a block diagram for explaining an example when the ground fault determination circuit detects a zero fault by detecting a zero-phase current.

In the figure, only the inverter device 20a, the DC power source 10a, and the ground line L _G1a are shown, and other configurations (inverter devices 20b, 20c, DC power sources 10b, 10c, ground lines L _G1b , L _{The description of G1c} , transformer 30, and power system 40) is omitted. The configurations of inverter devices 20b and 20c and ground lines L _G1b and L _G1c are the same as inverter device 20a and ground line L _G1a , respectively. In the figure, the same or similar elements as those in the grid interconnection inverter system A shown in FIG.

  In the example illustrated in FIG. 7, the ground fault determination circuit 25 ′ detects a ground fault by detecting a zero-phase current, which is different from the ground fault determination circuit 25 of the grid interconnection inverter system A according to the first embodiment. Different. The zero-phase current sensor 25b of the ground fault determination circuit 25 'is disposed on the two output lines of the DC power supply 10a. Even in this case, since the circulating current is suppressed, the generation of the zero-phase current is suppressed, and the erroneous detection of the ground fault can be suppressed.

  Although the said 1st thru | or 3rd embodiment has demonstrated that the erroneous detection of the ground fault accident by the ground fault determination circuit 25 can be suppressed, this invention can also suppress the erroneous detection of an overcurrent.

  FIG. 8 is a block diagram for explaining a configuration for detecting an overcurrent that has not been described in the first to third embodiments.

In the figure, only the inverter device 20a, the DC power source 10a, and the ground line L _G1a are shown, and other configurations (inverter devices 20b, 20c, DC power sources 10b, 10c, ground lines L _G1b , L _{The description of G1c} , transformer 30, and power system 40) is omitted. The configurations of inverter devices 20b and 20c and ground lines L _G1b and L _G1c are the same as inverter device 20a and ground line L _G1a , respectively. In the figure, the same or similar elements as those in the grid interconnection inverter system A shown in FIG.

  As shown in FIG. 8, the inverter device 20a is provided with an overcurrent determination circuit 26 and an OR circuit 27. In addition, although description and description are omitted in the first to third embodiments, the inverter device 20a in the grid-connected inverter system A, A ′, A ″ (FIGS. 1, 4, 5, and 5). 7) is also provided with a configuration for detecting an overcurrent.

  The overcurrent determination circuit 26 detects that the current output from the inverter device 20a is an overcurrent. The overcurrent determination circuit 26 detects the current flowing through the output line of each phase by the current sensor 26a, compares the detected current value with a predetermined threshold value, and detects the overcurrent when the detected current value exceeds the threshold value. It is determined that The overcurrent determination circuit 26 outputs a detection signal (for example, a high level signal) to an OR circuit 27 (described later) when an overcurrent is detected. The ground fault determination circuit 25 also outputs a detection signal (for example, a high level signal) to the OR circuit 27 when a ground fault is detected.

  The OR circuit 27 outputs a detection signal based on the input signal from the ground fault determination circuit 25 and the input signal from the overcurrent determination circuit 26. That is, when a detection signal is input from the ground fault determination circuit 25 or when a detection signal is input from the overcurrent determination circuit 26, the DC / DC converter circuit 21, the inverter circuit 22, the circuit breaker 24, and the circuit breaker A detection signal (for example, a high level signal) is output to the device Sa. That is, when the detection signal from the ground fault determination circuit 25 and the detection signal from the overcurrent determination circuit 26 are high level signals, the logical sum circuit 27 performs a logical sum of the input signals from the ground fault determination circuit 25 and the overcurrent determination circuit 26. Is output. The DC / DC converter circuit 21 and the inverter circuit 22 are stopped when a detection signal (stop signal) is input from the OR circuit 27. The circuit breaker 24 and the circuit breaker Sa are disconnected from each other when a detection signal (cut-off signal) is input from the OR circuit 27. Thereby, when a ground fault occurs or when an overcurrent is detected, it is possible to prevent these effects from spreading.

  Since the overcurrent determination circuit 26 detects an overcurrent by comparing the current value detected by the current sensor 26a with a predetermined threshold value, for example, when the circulating current flows in the direction of the arrow of the current path I shown in FIG. In some cases, the circulating current is superimposed on the output current to increase the detection value of the current sensor 26a. However, as described above, since the circulating current flowing through the current path formed between the inverter devices 20a, 20b, and 20c is suppressed, the current flowing through the output lines of the inverter devices 20a, 20b, and 20c is overcurrent. It can be prevented that the threshold value set in the determination circuit 26 is exceeded. Therefore, it is possible to suppress the overcurrent determination circuit 26 from erroneously detecting the overcurrent.

  In the said 1st thru | or 3rd embodiment, although the case where DC power supply 10a, 10b, 10c produced | generated DC power with a solar cell was demonstrated, it is not restricted to this. For example, the DC power supplies 10a, 10b, and 10c may be fuel cells, storage batteries, electric double layer capacitors, lithium ion batteries, or generated by diesel engine generators, micro gas turbine generators, wind turbine generators, or the like. The AC power may be converted into DC power and output.

  The grid interconnection inverter system according to the present invention is not limited to the above-described embodiment. The specific configuration of each part of the grid-connected inverter system according to the present invention can be varied in design in various ways.

A, A ′, A ″ Grid-connected inverter system 10a, 10b, 10c DC power supply 20a, 20b, 20c Inverter device 21 DC / DC converter circuit 22 Inverter circuit 23 Filter circuit 24 Circuit breaker 25, 25 ′ Ground fault determination circuit 25a current sensor 25b zero-phase current sensor 26 overcurrent determination circuit 26a a current sensor 30 the transformer 40 power system _{L G1a, L G1b, L G1c} , L G2 ground line Sa, Sb, Sc breaker Ra, Rb, Rc resistance Ca, Cb , Cc capacitor

Claims (9)

Multiple DC power supplies,
A plurality of inverter devices connected to each of the DC power sources, converting DC power from the connected DC power sources into AC power and outputting the AC power without a transformer;
A transformer provided between an output-side connection point and a power system in which the plurality of inverter devices are connected in parallel with each other;
A plurality of ground wires each grounding one of a pair of output terminals of at least two of the DC power sources;
With
Each ground line is provided with an impedance portion for increasing the impedance of the ground line.
This is a grid-connected inverter system.   The grid-connected inverter system according to claim 1, wherein the impedance unit is a resistance element.   The grid interconnection inverter system according to claim 1, wherein the impedance unit is a reactor.   The grid interconnection inverter system according to claim 1, further comprising a capacitor connected in parallel to the impedance unit.   The grid interconnection inverter system according to any one of claims 1 to 4, further comprising a ground fault detection device for detecting a ground fault accident.   The grid-connected inverter system according to claim 5, wherein the ground fault detection device detects a ground fault by comparing a current flowing through the ground line with a predetermined threshold.   The grid interconnection inverter system according to any one of claims 1 to 6, further comprising an overcurrent detection device that detects that the current output from the inverter device is an overcurrent. There are two DC power supplies,
The impedance section is provided only on one of the ground lines of these two DC power supplies.
The grid connection inverter system in any one of Claims 1 thru | or 7.   The grid-connected inverter system according to any one of claims 1 to 8, wherein the DC power source includes a solar battery.