JP2022055035A - Power conversion device, power conversion system, solar power generation system, and control method - Google Patents

Abstract

To provide a power conversion device, a power conversion system, a solar power generation system, and a control method that can reduce power loss and achieve a simplified system.SOLUTION: A power conversion system includes a plurality of power conversion devices, each of which converts direct current power output from a direct current generation device into AC power and outputs the same, a connection node that collects the AC power output from each of the plurality of power conversion devices and outputs the same as combined AC power to a first trunk line, and a step-down transformer that steps down the combined AC power transmitted to the first trunk line and outputs the step-down AC power. The effective value of the voltage of the AC power is greater than or equal to the voltage value of the DC power.SELECTED DRAWING: Figure 1

Description

The present disclosure relates to a power conversion device, a power conversion system, a photovoltaic power generation system and a control method.

Solar power generation systems are widespread as renewable energy. Patent Document 1 below discloses a photovoltaic power generation system including a photovoltaic module, a power conditioner, an AC current collector box, and a cubicle. In this photovoltaic power generation system, the power conditioner converts the DC power output from the solar modules connected in series and parallel into AC power. The AC current collector box collects AC power output from a plurality of power conditioners and converts the collected AC power into high-voltage power. The cubicle receives high-voltage AC power output from a plurality of AC current collector boxes in a state of being collected. With this configuration, in this photovoltaic power generation system, a thick copper wire cable is not often used between the current collector box and the cubicle, so that work efficiency at the time of wiring or the like can be improved.

The following Patent Document 2 discloses a control method of a system having an AC bus in which the outputs of photovoltaic inverters connected to each of a plurality of PV (Photo Voltage) arrays are connected to each other. Conventionally, in such a system, each photovoltaic inverter independently performs maximum power point tracking MPPT (Maximum Power Point Tracking) with respect to the corresponding PV array. On the other hand, in this control method, MPPT control is performed on the premise that the difference between the DC voltages input to the corresponding photovoltaic inverters from each of the plurality of PV arrays is within a predetermined range. This reduces the common mode circulating current.

Japanese Unexamined Patent Publication No. 2018-74636 Japanese Unexamined Patent Publication No. 2016-127996

In the solar system disclosed in Patent Document 1, the supply voltage from the solar module is 600 to 1000 V, which is a relatively low voltage in the voltage range classified as high voltage (DC 750 V or more and 7 kV or less). Therefore, there is a problem that the number of wirings between the solar module and the power conditioner and the number of power conditioners are relatively large, which is a complicated system. In addition, the high-voltage DC voltage is once stepped down to the low-voltage AC voltage (200 to 480V) by the power conditioner, transmitted to the AC current collector box, and boosted to the high voltage of 6600V by the AC current collector box, resulting in power loss. However, there is also a relatively large problem.

In the system control method disclosed in Patent Document 2, it is necessary to measure the output voltage of each PV array, and there is a problem that wiring for transmitting the measured voltage value is separately required.

Therefore, it is an object of the present disclosure to provide a power conversion device, a power conversion system, a photovoltaic power generation system, and a control method capable of reducing power loss and realizing a simple system. It is preferable that the number of wirings for supplying DC power from a DC power generation device such as a solar module can be reduced, and it is not necessary to provide communication wiring for transmitting power information separately from the power wiring. Further, it is more preferable if a power conversion device capable of high-speed operation and compact can be provided.

The power conversion system according to a certain aspect of the present disclosure includes a plurality of conversion devices that convert the DC power output from the DC power generation device into AC power and output the AC power, and an AC output from each of the plurality of conversion devices. The effective value of the AC power voltage includes a connection node that collects power and outputs it as synthetic AC power to the first trunk line, and a step-down substation that lowers and outputs the combined AC power transmitted to the first trunk line. Is greater than or equal to the voltage value of DC power.

The photovoltaic power generation system according to another aspect of the present disclosure includes the above-mentioned power conversion system and a solar panel, and the power conversion system uses the solar panel as a DC power generation device and the main line system as the first main line. The DC power generated by the solar panel is converted into AC power and output to the main line system.

The power conversion device according to still another aspect of the present disclosure includes a conversion unit that converts input DC power into AC power and outputs the AC power to the trunk system, and wiring for transmitting the AC power output from the conversion unit. The communication unit receives phase information from the outside via wiring, and the conversion unit adjusts the voltage phase of AC power using the phase information.

The control method according to still another aspect of the present disclosure is a control method of a power conversion system including a plurality of conversion devices, connection nodes connected to the output units of the plurality of conversion devices, and a step-down substation. A step of converting the input DC power into AC power in each of the plurality of converters, and AC power output from each of the plurality of converters is collected and generated by the connection node in the step-down substation. The effective value of the voltage of the AC power is equal to or higher than the voltage value of the DC power, including the step of lowering the combined AC power.

According to the present disclosure, it is possible to provide a power conversion device, a power conversion system, a photovoltaic power generation system, and a control method capable of reducing power loss and realizing a simple system.

FIG. 1 is a block diagram showing a configuration of a power conversion system according to an embodiment of the present disclosure. FIG. 2 is a block diagram showing an internal configuration of the first power conversion device shown in FIG. FIG. 3 is a block diagram showing an internal configuration of the second power conversion device shown in FIG. FIG. 4 is a flowchart showing the operation of the first power conversion device shown in FIG. FIG. 5 is a flowchart showing the operation of the second power conversion device shown in FIG.

[Explanation of Embodiments of the present disclosure]
The contents of the embodiments of the present disclosure will be listed and described. At least a part of the embodiments described below may be arbitrarily combined.

(1) The power conversion system according to the first aspect of the present disclosure is a plurality of conversion devices, each of which converts the DC power output from the DC power generation device into an AC power and outputs the AC power, and each of the plurality of conversion devices. AC power includes a connection node that collects AC power output from The effective value of the voltage of is equal to or higher than the voltage value of the DC power.

As a result, it is possible to reduce the power loss when supplying the generated power of the DC power generation device (for example, a photovoltaic power generation module) to the first trunk line, and it is possible to realize a simple system. Further, the AC power stepped down by the step-down substation can be used by itself as the power required for the equipment for maintaining the power conversion system.

(2) It is preferable to further include a step-up substation device that boosts the combined AC power transmitted to the first trunk line and outputs it to the second trunk line. Thereby, for example, the extra high voltage generated from the high voltage supplied to the first trunk line can be transmitted to the second trunk line, and the electric power generated by the DC power generation device can be supplied to a long-distance consumer.

(3) More preferably, the power conversion system further includes a phase detection device that acquires the phase information of the AC voltage of the first trunk line, and each of the plurality of conversion devices connects the outputs of the plurality of conversion devices to each other. The first conversion device, which includes a communication device that communicates via wiring and is one of a plurality of conversion devices, adjusts the phase of the output voltage of the first conversion device using the phase information, and the first conversion device has a phase information. The conversion device 1 further transmits the phase information to the communication device included in the second conversion device other than the first conversion device among the plurality of conversion devices via the communication device included in the first conversion device. Then, the second conversion device adjusts the phase of the output voltage of the second conversion device by using the phase information received from the first conversion device. As a result, the phase of the AC voltage output from each conversion device can be aligned with the phase of the AC voltage of the first trunk line, and the power output from each of the plurality of conversion devices can be supplied to the first trunk line without any trouble. .. Further, by using the wiring for transmitting electric power for communication, it is not necessary to separately provide the wiring for communication.

(4) More preferably, the power conversion system further includes a power detection device that detects power information representing power fluctuations in the first trunk line, and the first conversion device is a first conversion device according to the power information. The output power is adjusted, and the first conversion device transmits the power information to the communication device included in the second conversion device via the communication device included in the first conversion device, and the second conversion device. Adjusts the output power of the second conversion device according to the power information received from the first conversion device. As a result, the generated power can be appropriately supplied to the first trunk line (reverse power flow).

(5) Preferably, each of the plurality of conversion devices further includes a monitoring unit for monitoring the power generation state of the DC power generation device that supplies power to the conversion device, and each of the plurality of conversion devices is included in the conversion device. The AC power output by the converter is adjusted according to the power generation status monitored by the monitoring unit. As a result, the generated power of the DC power generation device can be efficiently used in the maximum power state.

(6) More preferably, the DC power generation device includes a plurality of photovoltaic power generation modules connected in series and parallel to each other, and the voltage value of the DC power is 1000 V or more and 6600 V or less. As a result, the number of wires for supplying DC power from the solar module can be reduced.

(7) More preferably, the voltage value of the DC power is 1500 V or more and 3300 V or less. As a result, the current density at the same power can be reduced as compared with the case of lower voltage, and the withstand voltage design between directly connected PV modules becomes easy.

(8) Preferably, the effective value of the voltage of the AC power is 6600V. As a result, the power loss when the generated power of the DC power generation device is supplied to the first trunk line can be further reduced. Moreover, the pressure resistance design is easier than in the case of higher pressure.

(9) More preferably, each of the plurality of converters includes a SiC element used for converting DC power into AC power. As a result, each of the plurality of conversion devices can be operated at high speed, and power saving and compactness can be achieved.

(10) The photovoltaic power generation system according to the second aspect of the present disclosure includes the above-mentioned power conversion system and a solar panel, and the power conversion system uses the solar panel as a DC power generation device and the main line system as the first. As one trunk line, the DC power generated by the solar panel is converted into AC power and output to the trunk line system. As a result, it is possible to reduce the power loss when the DC power output from the solar panel is supplied to the main line system.

(11) The power conversion device according to the third aspect of the present disclosure converts the input DC power into AC power and outputs the AC power to the trunk system, and the AC power output from the conversion unit. The communication unit includes a communication unit that communicates via wiring to be transmitted, the communication unit receives phase information from the outside via wiring, and the conversion unit adjusts the voltage phase of AC power using the phase information. As a result, the generated AC power can be appropriately supplied to the first trunk line (reverse power flow).

(12) The control method according to the fourth aspect of the present disclosure is a control method of a power conversion system including a plurality of conversion devices, a connection node connected to each output unit of the plurality of conversion devices, and a step-down substation device. Therefore, the step of converting the input DC power into AC power in each of the plurality of conversion devices and the AC power output from each of the plurality of conversion devices are collected in the step-down substation by the connection node. The effective value of the voltage of the AC power is equal to or higher than the voltage value of the DC power, including the step of stepping down the combined AC power generated in the above process. As a result, it is possible to reduce the power loss when the DC power output from the DC power generation device (for example, a photovoltaic power generation module) is output to the first trunk line.

[Details of Embodiments of the present disclosure]
In the following embodiments, the same parts are given the same reference numbers. Their names and functions are also the same. Therefore, detailed explanations about them will not be repeated.

(Embodiment)
(System configuration)
The power conversion system 100 according to the embodiment of the present disclosure with reference to FIG. 1 includes a first power conversion device 110, a second power conversion device 112, an operation device 140, a detection device 142, and wirings 176 and 186. , The connection node 178 and the power receiving cubicle 162. The first power conversion device 110 and the second power conversion device 112 are supplied with power from the PV strings 120 to 126 via the junction boxes 130 and 132. That is, the power conversion system 100 constitutes a photovoltaic power generation system together with the junction boxes 130 and 132 and the PV strings 120 to 126. The power conversion system 100 supplies power to the medium-range trunk line 190. The power conversion device included in the power conversion system 100 is not limited to two, the first power conversion device 100 and the second power conversion device 110, and may be three or more. Here, the power conversion system 100 does not include the medium-distance trunk line 190, but the power conversion system 100 may include the medium-distance trunk line 190.

The medium-distance trunk line 190 is also connected to the grid interconnection transformer 160 and the power receiving cubicle 162. Therefore, the power supplied from the power conversion system 100 to the medium-distance trunk line 190 is supplied to the long-distance trunk line 192 via the grid interconnection transformer 160 and to the short-distance wiring 194 via the power receiving cubicle 162. To. The medium-distance trunk line 190 and the long-distance trunk line 192 constitute the trunk line system 196. The medium-distance trunk line 190 transmits high-voltage power (DC 750 V to 7,000 V or less, AC 600 V to 7,000 V or less). The long-distance trunk line 192 transmits power of extra high voltage (both DC and AC exceeding 7,000 V). The short-distance wiring 194 transmits low-voltage (DC 750V or less, AC 600V or less) power.

The PV string 120 includes a plurality of PV modules 114 (solar panels). These plurality of PV modules 114 are connected in series with each other. The PV module 114 is, for example, a module in which a plurality of PV cells (minimum units capable of receiving light (sunlight, etc.) and generating electricity) are connected in series and arranged on a flat surface, and sealed with tempered glass or the like. Each of the PV strings 122 to 126 has the same configuration as the PV string 120. That is, each of the PV strings 122 to 126 includes a plurality of PV modules connected in series with each other. Each of the PV strings 120 to 126 is preferably configured to output the same DC voltage (eg 1500V).

The output power of the PV strings 120 and 122 is transmitted to the junction box 130 via the wirings 170 and 172, respectively. The junction box 130 collects the input electric power (hereinafter referred to as current collector) and transmits it to the first power conversion device 110 via the wiring 174. The wirings 170 and 172 are connected in parallel to the input portion of the junction box 130. That is, a plurality of PV modules constituting the PV strings 120 and 122 are connected in series and parallel. The output voltage of the junction box 130 is the same as that of each of the wiring 170 and 172 (for example, 1500V). Similarly, the output power of the PV strings 124 and 126 is transmitted to the junction box 132 via the wires 180 and 182, respectively. The junction box 132 collects the input electric power and transmits it to the second power conversion device 112 via the wiring 184. The wirings 180 and 182 are connected in parallel to the input portion of the junction box 132. That is, a plurality of PV modules constituting the PV strings 124 and 126 are connected in series and parallel. The output voltage of the junction box 132 is the same as that of the wiring 180 and 182 respectively (for example, 1500V).

The junction box 130 includes the communication unit 154, and the junction box 132 includes the communication unit 156. The communication units 154 and 156 communicate by the PLC method. Specifically, the communication unit 154 is connected to the wiring 174, and signals are transmitted / received (transmitted and received) by the PLC method via the wiring 174. Similarly, the communication unit 156 is connected to the wiring 184, and signals are transmitted and received by the PLC method via the wiring 184. As the PLC communication protocol, for example, TCP / IP, UDP / IP, or the like can be used. As the modulation method of the PLC method, for example, an Orthogonal Frequency Division Multiplexing (OFDM) method, a Spread Spectrum method, or the like can be used. As the communication frequency of the PLC system, a frequency in the range of 10 kHz to 450 kHz or 2 MHz to 30 MHz is used. However, the communication protocol, communication frequency, and modulation method of the PLC method are not limited thereto.

As will be described later, the first power conversion device 110 converts the DC power input via the wiring 174 into AC power and outputs the DC power to the wiring 176. As will be described later, the second power conversion device 112 converts the DC power input via the wiring 184 into AC power and outputs it to the wiring 186. The first power conversion device 110 includes a communication unit 150, and the second power conversion device 112 includes a communication unit 152. The communication units 150 and 152 communicate by the PLC method in the same manner as the communication units 154 and 156. The communication unit 150 is connected to the wirings 174 and 176. The communication unit 150 communicates with the communication unit 154 in the junction box 130 via the wiring 174, and communicates with the communication unit 158 in the detection device 142 described later via the wiring 176. The communication unit 152 is connected to the wirings 184 and 186. The communication unit 152 communicates with the communication unit 156 via the wiring 184. Further, the communication units 150 and 152 communicate with each other via the wirings 176 and 186.

The wiring 176 connected to the output unit of the first power conversion device 110 and the wiring 186 connected to the output unit of the second power conversion device 112 are connected at the connection node 178, and both are connected to the medium-distance trunk line 190. ing. Therefore, the output power of the first power conversion device 110 and the second power conversion device 112 is output to the medium-distance trunk line 190.

The operating device 140 includes an input device for inputting an instruction to the first power conversion device 110. The input device is, for example, a keyboard or mouse for a computer. However, an input device other than these may be used. The user (administrator or the like) operates the operation device 140 to operate the first power conversion device 110 with the data necessary for the first power conversion device 110 and the second power conversion device 112 to operate, and the communication unit 152. Enter the address (for PLC communication) of ~ 158.

The detection device 142 constantly monitors the power fluctuation of the medium-distance trunk line 190 on the trunk line system side of the connection node 178. Specifically, the detection device 142 detects the voltage and current (hereinafter referred to as power information) of the medium-distance trunk line 190. Further, the detection device 142 calculates the phase information (for example, the timing (time) of zero crossing of the AC voltage) from the observed waveform of the AC voltage. That is, the detection device 142 functions as a power detection device and a phase detection device. The detection device 142 includes a communication unit 158. The communication unit 158 communicates by the PLC method in the same manner as the communication unit 150 and the like. The communication unit 158 is connected to the wiring 176, communicates with the communication unit 150 via the wiring 176, and transmits the power information and the phase information detected by the detection device 142 to the communication unit 150.

The grid interconnection transformer 160 boosts the voltage (high voltage) of the medium-distance trunk line 190 to an extra high voltage (for example, 22 kV) in order to transmit electric power over a long distance, and outputs the voltage to the long-distance trunk line 192. That is, the grid interconnection transformer 160 is a step-up transformer. The power receiving cubicle 162 steps down the voltage (high voltage) of the medium-distance trunk line 190 to a low voltage (for example, 200V) and outputs it to the short-distance wiring 194 in order to supply power to a short-distance consumer. That is, the power receiving cubicle 162 is a step-down substation device.

(Configuration of first power converter)
With reference to FIG. 2, the first power conversion device 110 includes a control unit 200, a first filter unit 202, a booster unit 204, an inverter 206, a second filter unit 208, a circuit breaker 210, a storage unit 212, and a communication unit 150. include. The control unit 200 includes a CPU (Central Processing Unit), a microcomputer, and the like, and outputs a control signal for operating the booster unit 204 and the inverter 206, which will be described later. The storage unit 212 is, for example, a rewritable non-volatile semiconductor memory, and stores a program executed by the control unit 200. The control unit 200 appropriately temporarily or continuously stores the data acquired from the outside in the storage unit 212.

The first filter unit 202 is connected to the wiring 174 and the boosting unit 204, and outputs the electric power input from the wiring 174 to the boosting unit 204. As will be described later, in the first filter unit 202, noise such as a ripple current generated by the on / off switching operation (hereinafter, also referred to as switching) of the switching element constituting the booster unit 204 and the inverter 206 causes the wiring 174. It is prevented from flowing to the PV strings 120 and 122 through the PV strings 120 and 122. This suppresses high frequency noise radiation and the like in the PV strings 120 and 122. The first filter unit 202 is, for example, a bidirectional low-pass filter.

The booster section 204 is connected to the first filter section 202 and the inverter 206, and includes, for example, a booster chopper circuit including a semiconductor switching element. The booster unit 204 switches the switching element according to the control signal from the control unit 200. As a result, the booster unit 204 boosts the DC voltage (for example, 1500 V) input from the wiring 174 via the first filter unit 202, and outputs the boosted DC voltage (hereinafter referred to as boosted voltage) to the inverter 206. do. For example, a MOSFET (Metal-Oxide-Semiconductor Ductor Field-Effective Transistor) can be used as the switching element constituting the booster unit 204. The switching element is more preferably a SiC (Silicon-Carbide) MOSFET.

The inverter 206 is connected to the booster section 204 and the second filter section 208, and includes a semiconductor switching element. The inverter 206 switches the switching element (for example, PWM control) according to the control signal from the control unit 200. As a result, the inverter 206 converts the boost voltage (DC voltage) input from the boost unit 204 into a predetermined AC voltage and outputs it to the second filter unit 208. The inverter 206 includes a DC / AC conversion circuit. The DC / AC conversion circuit is realized by, for example, a full bridge circuit. The switching element constituting the inverter 206 is, for example, a SiC MOSFET. The voltage value (eg, effective value 6600V), frequency (eg, 60Hz or 50Hz) and phase of the generated AC voltage can be used to determine the operation of the first power converter 110. Of these, the voltage value and frequency of the generated AC voltage are input in advance from the operating device 140 as described above. The input voltage value and frequency are stored in the storage unit 212 and used by the control unit 200 to generate the control signal of the inverter 206. As described above, the phase information is transmitted from the detection device 142 via the communication unit 158 and received by the communication unit 150. The control unit 200 also uses the phase information received by the communication unit 150 to generate a control signal for the inverter 206. Further, each time the control unit 200 receives the phase information from the detection device 142, the control unit 200 transmits the received phase information to the second power conversion device 112 via the communication unit 150.

When the output power of the first power conversion device 110 and the second power conversion device 112 is supplied to the main line system 196 (reverse power flow), the voltage of the main line system 196 fluctuates under the influence of the supply (reverse power flow). According to the "Guidelines for grid interconnection technology requirements for ensuring power quality" (October 7, 1st year of Ordinance) prepared by the Agency for Natural Resources and Energy, when connecting power generation equipment to grid wiring, the voltage in the grid wiring should be within the appropriate range. It is required to maintain. The electric power company maintains the wiring voltage within a predetermined range in consideration of load fluctuations during on-peak and off-peak times, voltage drop of high-voltage wiring, and the like. For the voltage of the high voltage system (for example, 6.6 kV), the fluctuation range is operated and controlled so as to be within about 5 to 10%. Therefore, it is preferable that the power supplied from the first power conversion device 110 and the second power conversion device 112 to the main line system 196 via the connection node 178 is similarly adjusted.

Therefore, in the present embodiment, the first power conversion device 110 executes grid interconnection control (voltage / current control), power factor control (voltage rise suppression), and the like. That is, the control unit 200 controls these using the power information (voltage and current) of the medium-distance trunk line 190 acquired from the detection device 142 via the communication unit 158 and the communication unit 150. Regarding grid interconnection control, for example, the control unit 200 has a first power conversion device for the voltage of the medium-distance trunk line 190 so that power is supplied from the first power conversion device 110 to the medium-distance trunk line 190. The output voltage of 110 is changed (the control signal of the inverter 206 is changed so that the voltage becomes a little higher). Further, regarding the power factor control, the control unit 200 changes the phase of the output current in order to suppress an increase in the voltage of the medium-distance trunk line 190 due to the power supply from the first power conversion device 110 ( The power factor of the output power of the first power conversion device 110 is changed by changing the control signal of the inverter 206). As a result, reverse power flow can be appropriately performed.

The power generation state of the PV strings 120 and 122 changes depending on the irradiation state of sunlight on the PV strings 120 and 122 (hereinafter referred to as a sunshine state). Therefore, it is preferable that the first power conversion device 110 is configured to perform maximum power point tracking (MPPT) control. To realize MPPT control, the first power conversion device 110 includes a monitoring unit (not shown in FIG. 2) that monitors the power generation state of the PV strings 120 and 122. The monitoring unit is included in, for example, the booster unit 204, and monitors (detects) the input voltage to the booster unit 204 and the current flowing through the booster unit 204. The control unit 200 calculates electric power (= voltage × electric power) from the voltage and current detected by the monitoring unit, and controls signals (PWM control signals) of the booster unit 204 and the inverter 206 so that the calculated electric power becomes maximum. ) Is adjusted. As a result, the generated power of the PV strings 120 and 122 can be efficiently used in the state of maximum power. At this time, grid interconnection control and power factor control are also taken into consideration.

The second filter unit 208 is connected to the inverter 206 and the circuit breaker 210. The second filter unit 208 removes high-frequency components such as ripples generated by switching of the switching elements constituting the inverter 206 from the output of the inverter 206, shapes the output of the inverter 206 into a sine wave, and turns the circuit breaker 210 into a sine wave. Output (harmonic suppression function). The second filter unit 208 is a filter circuit including a reactor and a capacitor. The second filter unit 208 may have an EMC (Electromagnetic Compatibility) filter function.

The circuit breaker 210 is connected to the second filter unit 208 and the wiring 176. The circuit breaker 210 includes a switching element and has a function of short-circuiting and opening between the second filter unit 208 and the wiring 176 under the control of the control unit 200. A short circuit (energization) of the second filter unit 208 and the wiring 176 is realized by turning on the switching element, and opening (cutting off) is realized by turning off the switching element. The switching element is, for example, a SiC MOSFET. The circuit breaker 210 may be a mechanical circuit breaker.

As described above, the communication unit 150 is connected to the wirings 174 and 176, and communicates via the wirings 174 and 176 by the PLC method.

(Configuration of second power converter)
The second power conversion device 112 is configured in the same manner as the first power conversion device 110. With reference to FIG. 3, the second power conversion device 112 includes a control unit 220, a first filter unit 222, a booster unit 224, an inverter 226, a second filter unit 228, a circuit breaker 230, a storage unit 232, and a communication unit 152. include. The control unit 220, the first filter unit 222, the booster unit 224, the inverter 226, the second filter unit 228, the circuit breaker 230, the storage unit 232, and the communication unit 152 are the control unit 200 and the first power conversion device 110, respectively. It corresponds to the filter unit 202, the booster unit 204, the inverter 206, the second filter unit 208, the circuit breaker 210, the storage unit 212, and the communication unit 150.

The control unit 220 includes a CPU, a microcomputer, and the like, and outputs a control signal for operating the booster unit 224 and the inverter 226, which will be described later. The storage unit 232 is, for example, a rewritable non-volatile semiconductor memory, and stores a program executed by the control unit 220. The control unit 220 stores the data acquired from the outside in the storage unit 232 as appropriate, temporarily or continuously.

The first filter unit 222 is connected to the wiring 184 and the boosting unit 224, and outputs the power input from the wiring 184 to the boosting unit 224. As will be described later, the first filter unit 222 prevents noise such as ripple current generated by switching of the switching elements constituting the booster unit 224 and the inverter 226 from flowing to the PV strings 124 and 126 via the wiring 184. do. This suppresses high frequency noise radiation and the like in the PV strings 124 and 126. The first filter unit 222 is, for example, a bidirectional low-pass filter.

The booster unit 224 is connected to the first filter unit 222 and the inverter 226, and includes, for example, a booster chopper circuit including a semiconductor switching element. The booster unit 224 switches the switching element according to the control signal from the control unit 220. As a result, the booster unit 224 boosts the DC voltage (for example, 1500 V) input from the wiring 184 via the first filter unit 222, and outputs the boosted voltage to the inverter 226. For example, a MOSFET can be used as the switching element constituting the booster unit 224. The switching element is more preferably a SiC MOSFET.

The inverter 226 is connected to the booster section 224 and the second filter section 228, and includes a semiconductor switching element. The inverter 226 switches (PWM control) the switching element according to the control signal from the control unit 220. As a result, the inverter 226 converts the boost voltage (DC voltage) input from the boost unit 224 into a predetermined AC voltage (for example, an effective value of 6600 V) and outputs it to the second filter unit 228. The inverter 226 includes a DC / AC conversion circuit. The DC / AC conversion circuit is realized by, for example, a full bridge circuit. The switching element constituting the inverter 226 is, for example, a SiC MOSFET. The voltage value and frequency of the generated AC voltage are stored in the storage unit 232 in advance. That is, the voltage value and frequency are data necessary for the second power conversion device 112 to operate as described above, are input from the operation device 140 to the first power conversion device 110, and communicate via the communication unit 150. It is transmitted to the unit 152 and stored in the storage unit 232. The control unit 220 reads out the voltage and frequency from the storage unit 232 and uses them to generate the control signal of the inverter 226. As described above, the phase information is the phase information received from the detection device 142 by the first power conversion device 110 (control unit 200), is transmitted via the communication unit 150, and is received by the communication unit 152. The control unit 220 also uses the phase information received by the communication unit 152 to generate a control signal for the inverter 226.

Since the AC power generated by the inverter 226 is supplied to the main line system 196 (reverse power flow), the second power conversion device 112 executes system interconnection control, power factor control, and the like in the same manner as the first power conversion device 110. do. Therefore, the control unit 220 acquires the power information (voltage and current) of the medium-distance trunk line 190 acquired from the detection device 142 by the first power conversion device 110 via the communication unit 150 and the communication unit 152, and obtains the power information (voltage and current) thereof. The control signal of the inverter 226 is changed accordingly. As a result, reverse power flow can be appropriately performed.

The power information (voltage and current) of the medium-distance trunk line 190 observed by the detection device 142 is directly transmitted from the communication unit 158 to the second power conversion device 112 (communication unit 152) without going through the first power conversion device 110. ) May be sent. That is, the detection device 142 may transmit the detected power information from the communication unit 158 to the communication unit 152.

The power generation state of the PV strings 124 and 126 changes depending on the sunshine state of the PV strings 124 and 126. Therefore, it is preferable that the second power conversion device 112 is configured to perform MPPT control. To realize MPPT control, the second power conversion device 112 includes a monitoring unit (not shown in FIG. 3) that monitors the power generation state of the PV strings 124 and 126. The monitoring unit is included in, for example, the booster unit 224, and monitors (detects) the input voltage to the booster unit 224 and the current flowing through the booster unit 224. The control unit 220 calculates electric power from the voltage and current detected by the monitoring unit, and adjusts the control signals (PWM control signals) of the booster unit 224 and the inverter 226 so that the calculated electric power becomes maximum. As a result, the generated power of the PV strings 124 and 126 can be efficiently used in the state of maximum power. At this time, grid interconnection control and power factor control are also taken into consideration.

The second filter unit 228 is connected to the inverter 226 and the circuit breaker 230. The second filter unit 228 removes a high frequency component from the output of the inverter 226, shapes the output of the inverter 226 into a sine wave, and outputs the output to the circuit breaker 230 (harmonic suppression function). The second filter unit 228 is a filter circuit including a reactor and a capacitor. The second filter unit 228 may have an EMC filter function.

The circuit breaker 230 is connected to the second filter unit 228 and the wiring 186. The circuit breaker 230 includes a switching element and has a function of short-circuiting and opening between the second filter unit 228 and the wiring 186 under the control of the control unit 220. A short circuit (energization) of the second filter unit 228 and the wiring 186 is realized by turning on the switching element, and opening (cutting off) is realized by turning off the switching element. The switching element is, for example, a SiC MOSFET. The circuit breaker 230 may be a mechanical circuit breaker.

As described above, the communication unit 152 is connected to the wirings 184 and 186, and communicates via the wirings 184 and 186 by the PLC method.

With the above configuration, the power conversion system 100 converts the DC power generated by the PV strings 120 and 122 into higher voltage AC power by the first power conversion device 110 without stepping down. It can be output to the wiring 176. Further, the power conversion system 100 can convert the DC power generated by the PV strings 124 and 126 into higher voltage AC power by the second power conversion device 112 without stepping down, and output the DC power to the wiring 186. The wirings 176 and 186 are connected at the connection node 178, and the electric power output to the wirings 176 and 186 is transmitted to the medium-distance trunk line 190. Therefore, the power loss can be reduced as compared with the conventional case (the input voltage is once stepped down and then converted to a high voltage). At this time, each of the first power conversion device 110 and the second power conversion device 112 aligns the phase of their own output voltage with the phase of the AC voltage of the medium-distance trunk line 190 detected by the detection device 142. Therefore, the output power of the first power conversion device 110 and the second power conversion device 112 can be supplied to the medium-distance trunk line 190 without any trouble.

The connection node 178 has a function of collecting AC power output from the first power conversion device 110 and the second power conversion device 112. As a result, a simple system is realized in which the output powers of the first power conversion device 110 and the second power conversion device 112 are collected at the connection node 178 and directly supplied to the medium-distance trunk line 190 constituting the trunk line system 196. can.

The PV string 120, PV string 122 and junction box 130 function as a DC power generation device that supplies DC power to the first power conversion device 110, and the PV string 124, PV string 126 and junction box 132 are second power conversion devices. It functions as a DC power generator that supplies DC power to 112. By setting the generated voltage of each of the PV strings 120 to 126 to a high voltage of 1500 V, the PV string and the junction box required to obtain the same power in the junction boxes 130 and 132 as compared with the conventional (lower voltage). The number of wires between them can be reduced. The generated voltage of each of the PV strings 120 to 126 is not limited to 1500V, and may be the same value of 1000V or more and 6600V or less. Even in that case, the number of wires between the PV string and the junction box can be reduced. In order to enable a design in which the current density and the withstand voltage are balanced, it is preferable that the generated voltage of each of the PV strings 120 to 126 is 1500 V or more and 3300 V or less. That is, by setting the generated voltage to 1500 V or higher, the current density at the same power can be reduced as compared with the case of lower voltage. By setting the generated voltage to 3300 V or less, the withstand voltage design between the directly connected PV modules becomes easy.

The number of PV strings connected to one junction box is not limited to two, and may be three or more. Further, the number of each of the junction box and the power conversion device is not limited to 2, and may be 3 or more.

In each of the first power conversion device 110 and the second power conversion device 112, the input 1500V is converted into a higher voltage 6600V (effective value) AC voltage without lowering the voltage, so that the power loss is further increased. Can be reduced. The output voltage (alternating current) of the first power conversion device 110 and the second power conversion device 112 is not limited to 6600V (effective value). Any AC voltage higher than the input voltage and high voltage (effective value more than 600V and 7000V or less) may be used. The output voltage (alternating current) of the first power conversion device 110 and the second power conversion device 112 is preferably 6600V (effective value) or less. This facilitates pressure resistance design than in the case of higher pressure.

By incorporating the communication units 150 and 152 capable of PLC communication in the first power conversion device 110 and the second power conversion device 112, respectively, the communication units 150 and 152 can be combined with the first power conversion device 110 and the second power conversion device 112. The external wiring of the first power conversion device 110 and the second power conversion device 112 can be reduced and made compact as compared with the case where the first power conversion device 110 and the second power conversion device 112 are configured separately. In addition, wiring for communication is not required, costs can be reduced, and installation work becomes easy.

By using a SiC MOSFET as a switching element constituting the first power conversion device 110 and the second power conversion device 112 (inverter 206, inverter 226, etc.), high-speed operation is possible, and power saving and compactness can be achieved. By using a high withstand voltage SiC MOSFET, it is easy to increase the withstand voltage of the first power conversion device 110 and the second power conversion device 112, and the power is input to the first power conversion device 110 and the second power conversion device 112. The generated voltage of the PV string can be 1500V or more.

Since the power conversion system 100 includes the power receiving cubicle 162, the low voltage generated by the power receiving cubicle 162 can be supplied to each device constituting the photovoltaic power generation system. This allows the power to maintain the photovoltaic system to be self-sufficient. The power conversion system 100 may further include a grid interconnection transformer 160. Further, the power conversion system 100 may include a grid interconnection transformer 160 and may not include a power receiving cubicle 162. When the power conversion system 100 includes the grid interconnection transformer 160, the generated power of the PV strings 120 to 126 can be transmitted to a longer distance, and the power can be supplied to a customer at a long distance from the power conversion system 100. The grid interconnection transformer 160 and the power receiving cubicle 162 may be provided outside the power conversion system 100.

(Operation of power converter)
The operation of the first power conversion device 110 and the second power conversion device 112 will be described more specifically with reference to FIGS. 4 and 5. The process shown in FIG. 4 is realized by the control unit 200 of the first power conversion device 110 reading a predetermined program from the storage unit 212 and executing the program. The process shown in FIG. 5 is realized by the control unit 220 of the second power conversion device 112 reading a predetermined program from the storage unit 232 and executing the program. Since the PV strings 120 to 126 receive sunlight to generate electricity, the first power conversion device 110 and the second power conversion device 112 operate during the time period (daytime) when the PV strings 120 to 126 can generate electricity. It has a function to operate automatically. For example, a timer (not shown in FIGS. 2 and 3) equipped with each of them activates the programs shown in FIGS. 4 and 5 at a preset time.

(Operation of the first power converter)
With reference to FIG. 4, in step 300, the control unit 200 starts power conversion. Specifically, the control unit 200 outputs a control signal (PWM control signal) of the switching element constituting each of the booster unit 204 and the inverter 206 in order to operate the booster unit 204 and the inverter 206. As a result, the booster unit 204 generates and outputs a booster voltage from the DC voltage (for example, about 1500 V) input from the first filter unit 202. The inverter 206 converts the DC voltage input from the booster unit 204 into an AC voltage (for example, an effective value of about 6600 V) having a predetermined frequency (for example, 60 Hz or 50 Hz) and outputs the DC voltage to the second filter unit 208. After that, control shifts to step 302.

In step 302, the control unit 200 acquires the phase information of the voltage of the trunk line system. Specifically, the control unit 200 acquires the phase information (for example, zero cross timing) of the AC voltage of the medium-distance trunk line 190 from the communication unit 158 in the detection device 142 via the communication unit 150. After that, control shifts to step 304.

The communication between the communication units 150 and 158 is performed by the PLC method as described above, and the signal is transmitted via the wiring 176. For example, the communication unit 150 includes a packet (for example, TCP /) including a code for requesting transmission of phase information (hereinafter referred to as a transmission request), a transmission address (address of the communication unit 150), and a destination address (address of the communication unit 158). IP packet) is output to the wiring 176. The packet (digital data) is output to the wiring 176 as an analog signal modulated and encoded by a predetermined modulation method and coding method. Upon receiving the transmission request, the communication unit 158 outputs a packet including phase information (zero cross timing), a transmission address (address of the communication unit 158), and a destination address (address of the communication unit 150) to the wiring 176.

In step 304, the control unit 200 adjusts the phase of the output voltage of the inverter 206 by using the phase information acquired in step 302. Specifically, the control unit 200 makes the phase of the output voltage of the inverter 206 (that is, the output voltage of the second filter unit 208) the same as the phase specified by the phase information acquired in step 302. Adjust the output timing of the control signal of the inverter 206. After that, control shifts to step 306.

In step 306, the control unit 200 transmits the phase information acquired in step 302 to the communication unit 152 of the second power conversion device 112 via the communication unit 150. After that, control shifts to step 308. At this time, the communication between the communication units 150 and 152 is performed by the PLC method, and the signal is transmitted via the wirings 176 and 186 connecting the first power conversion device 110 and the second power conversion device 112. For example, the communication unit 150 outputs a packet including phase information, a transmission address (address of the communication unit 150), and a destination address (address of the communication unit 152) to the wiring 176. The transmitted signal is transmitted from the wiring 176 to the wiring 186.

In step 308, the control unit 200 outputs the AC power generated by the inverter 206 to the outside of the first power conversion device 110, that is, to the wiring 176. Specifically, the control unit 200 short-circuits (turns on) the circuit breaker 210. As a result, AC power is output from the first power conversion device 110 to the wiring 176. Since the AC power output to the wiring 176 is supplied to the medium-distance trunk line 190 (reverse power flow), grid interconnection control, power factor control, and the like are executed as described above. After that, control shifts to step 310.

In step 310, the control unit 200 executes MPPT control according to the fluctuation of the power generation state of the PV strings 120 and 122. Specifically, the control unit 200 detects the input power voltage to the first filter unit 202 and the current flowing through the first filter unit 202 by the monitoring unit as described above, and maximizes the power. The control signals (PWM control signals) of the booster 204 and the inverter 206 are adjusted. After that, control shifts to step 312.

In step 312, the control unit 200 determines whether or not to terminate. Specifically, the control unit 200 acquires the current time from the timer and determines whether or not the preset stop time has been reached. When the stop time is reached, it is determined that the process is finished, and the control shifts to step 314. Otherwise, control returns to step 302 and the process described above is repeated. Further, it is also possible to determine whether or not to end depending on the amount of power generation (or the amount of solar radiation) of the PV strings 120 to 126.

In step 314, the control unit 200 stops the power conversion. Specifically, the control unit 200 opens (turns off) the circuit breaker 210 and stops the operation of the booster unit 204 and the inverter 206 (stops the output of the control signal). This ends this program.

(Operation of the second power converter)
The control unit 220 of the second power conversion device 112 executes the process shown in FIG. In step 400, the control unit 220 starts power conversion. Specifically, the control unit 220 outputs a control signal (PWM control signal) of the switching element constituting each of the booster unit 224 and the inverter 226 in order to operate the booster unit 224 and the inverter 226. As a result, the booster unit 224 generates and outputs a booster voltage from the DC voltage (for example, 1500 V) input from the first filter unit 222. The inverter 226 converts the DC voltage input from the booster unit 224 into an AC voltage having a predetermined frequency (for example, an effective value of 6600 V) and outputs the DC voltage to the second filter unit 228. Here, it is assumed that the frequency and voltage of the output voltage of the inverter 226 are preset to the same values as the output voltage of the inverter 206.

In step 402, the control unit 220 determines whether or not the phase information has been acquired. Specifically, the control unit 200 determines whether or not the communication unit 152 has received the phase information from the communication unit 150. If it is determined that the phase information has been acquired (received), the control shifts to step 404. Otherwise, step 402 is repeated. By repeatedly executing step 402 after starting the power conversion, the phase of the output voltage of the second power conversion device 112 can be surely aligned with the phase of the output voltage of the first power conversion device 110.

In step 404, the control unit 220 adjusts the phase of the output voltage of the inverter 226. Specifically, the control unit 220 makes the phase of the output voltage of the inverter 226 (that is, the output voltage of the second filter unit 228) the same as the phase specified by the phase information acquired in step 402. The output timing of the control signal of the inverter 226 is adjusted.

In step 406, the control unit 220 outputs the AC power generated by the inverter 226 to the outside of the second power conversion device 112, that is, to the wiring 186. Specifically, the control unit 220 short-circuits (turns on) the circuit breaker 230. As a result, AC power is output from the second power conversion device 112 to the wiring 186. The phase of the AC voltage output from the second power conversion device 112 is aligned with the phase of the AC voltage output from the first power conversion device 110. Since the AC power output to the wiring 186 is supplied to the medium-distance trunk line 190 (reverse power flow), grid interconnection control, power factor control, and the like are executed as described above. After that, control shifts to step 408.

In step 408, the control unit 220 executes MPPT control according to the fluctuation of the power generation state of the PV strings 124 and 126. Specifically, the control unit 220 detects the input power voltage to the first filter unit 222 and the current flowing through the first filter unit 222 by the monitoring unit as described above, and maximizes the power. The control signals (PWM control signals) of the booster 224 and the inverter 226 are adjusted. After that, control shifts to step 410.

In step 410, the control unit 220 determines whether or not to terminate. Specifically, the control unit 220 acquires the current time from the timer and determines whether or not the preset stop time has been reached. If it is determined that the stop time is reached, the control shifts to step 414. Otherwise, control proceeds to step 412. Further, it is also possible to determine whether or not to end depending on the amount of power generation (or the amount of solar radiation) of the PV strings 120 to 126.

In step 412, the control unit 220 determines whether or not the phase information has been acquired, as in step 402. If it is determined that the phase information has been acquired (received), the control returns to step 404. As a result, as described above, the phase of the output voltage of the inverter 226 is adjusted by step 404 using the acquired phase information. Otherwise, control returns to step 408.

In step 414, the control unit 220 stops the power conversion. Specifically, the control unit 220 opens (turns off) the circuit breaker 230 and stops the operation of the booster unit 224 and the inverter 226 (stops the output of the control signal). This ends this program.

As described above, the first power conversion device 110 periodically transmits the phase information to the second power conversion device 112, and the second power conversion device 112 that has received the phase information uses the received phase information to output the output voltage. Adjust the phase of. As a result, the phase of the output voltage of the first power conversion device 110 and the second power conversion device 112 can be aligned with the phase of the voltage of the medium-distance trunk line 190. Therefore, the output power of the first power conversion device 110 and the second power conversion device 112 can be supplied to the medium-distance trunk line 190 without any trouble.

In the above, the case where the program shown in FIGS. 4 and 5 is started by the timer provided in each of the first power conversion device 110 and the second power conversion device 112 has been described, but the present invention is not limited thereto. The method of invoking the program shown in FIGS. 4 and 5 is arbitrary. The program of either the first power conversion device 110 or the second power conversion device 112 may be started first, and the other program may be started. Further, both the programs of the first power conversion device 110 and the second power conversion device 112 may be started by receiving a start instruction from the outside. Further, controlling the start of operation of the first power conversion device 110 and the second power conversion device 112 according to the amount of power generation (or the amount of solar radiation) of the PV strings 120 to 126, that is, shown in FIGS. 4 and 5. It is also possible to determine when to start the program.

The first power conversion device 110 and the second power conversion device 112 may have a grid interconnection protection function (independent operation prevention function, etc.) and the like, similarly to the power conditioner of the conventional photovoltaic power generation system.

In the above, the case where the DC power input to the first power conversion device 110 and the second power conversion device 112 is generated by the PV module has been described, but the present invention is not limited to this. Not limited to the power generation device that generates DC power, the power generated by the power generation device that generates AC power may be used. For example, electric power generated by wind power generation may be used. In wind power generation, the generated AC power is once converted to DC power, so the converted DC voltage may be used instead of the output power of the PV string.

In the above, the case where the first power conversion device 110 becomes the master and the power information (voltage and current) and the phase information detected by the detection device 142 are transmitted to the second power conversion device 112 which is a slave has been described. Not limited to this. The detection device 142 may directly transmit the detected power information and phase information from the communication unit 158 to the communication unit 152.

In the above, the case where the power information (voltage and current) and the phase information detected by the detection device 142 are transmitted to the first power conversion device 110 by the communication of the PLC method has been described, but the present invention is not limited to this. For example, if the positions of the first power conversion device 110 and the connection node 178 are relatively close (if the wiring 176 is short), the first power from the detection device 142 by a communication method other than the PLC method (for example, RS232C or GPIB). Power information and phase information may be transmitted to the converter 110. Further, the first power conversion device 110 may be configured to include the detection device 142.

Although the present disclosure has been described above by explaining the embodiments, the above-described embodiments are examples, and the present disclosure is not limited to the above-described embodiments. The scope of the present disclosure is indicated by each claim of the scope of claims, taking into consideration the description of the detailed description of the invention, and all changes within the meaning and scope equivalent to the wording described therein. include.

100 Power conversion system 110 First power conversion device 112 Second power conversion device 114 PV module 120, 122, 124, 126 PV string 130, 132 Connection box 140 Operation device 142 Detection device 150, 152, 154, 156, 158 Communication unit 160 grid interconnection transformer 162 Power receiving cubicle 170, 172, 174, 176, 180, 182, 184, 186 Wiring 178 Connection node 190 Medium-distance trunk line 192 Long-distance trunk line 194 Short-distance wiring 196 Trunk line system 200, 220 Control unit 202, 222 First filter unit 204, 224 Booster unit 206, 226 Inverter 208, 228 Second filter unit 210, 230 Circuit breaker 212, 232 Storage unit 300, 302, 304, 306, 308, 310, 312, 314, 400 , 402, 404, 406, 408, 410, 412, 414 steps

Claims (12)

Each has a plurality of conversion devices that convert the DC power output from the DC power generation device into AC power and output it.
A connection node that collects the AC power output from each of the plurality of conversion devices and outputs the combined AC power to the first trunk line.
Including a step-down substation device that steps down and outputs the combined AC power transmitted to the first trunk line.
A power conversion system in which the effective value of the voltage of the AC power is equal to or higher than the voltage value of the DC power. The power conversion system according to claim 1, further comprising a step-up substation device that boosts the combined AC power transmitted to the first trunk line and outputs it to the second trunk line. Further including a phase detection device for acquiring the phase information of the AC voltage of the first trunk line,
Each of the plurality of converters includes a communication device that communicates via wiring that interconnects the outputs of the plurality of converters.
The first conversion device, which is one of the plurality of conversion devices, adjusts the phase of the output voltage of the first conversion device using the phase information.
The first conversion device further converts the phase information into a second conversion device other than the first conversion device among the plurality of conversion devices via the communication device included in the first conversion device. Send to the communication device included in
The electric power according to claim 1 or 2, wherein the second conversion device adjusts the phase of the output voltage of the second conversion device by using the phase information received from the first conversion device. Conversion system. It further includes a power detection device that detects power information representing power fluctuations on the first trunk line.
The first conversion device adjusts the output power of the first conversion device according to the power information.
The first conversion device transmits the power information to the communication device included in the second conversion device via the communication device included in the first conversion device.
The power conversion system according to claim 3, wherein the second conversion device adjusts the output power of the second conversion device according to the power information received from the first conversion device. Each of the plurality of conversion devices further includes a monitoring unit for monitoring the power generation state of the DC power generation device that supplies power to the conversion device.
Each of the plurality of conversion devices adjusts the AC power output by the conversion device according to the power generation state monitored by the monitoring unit included in the conversion device.
The power conversion system according to any one of claims 1 to 4. The DC power generation device includes a plurality of photovoltaic modules connected in series and parallel to each other.
The power conversion system according to any one of claims 1 to 5, wherein the voltage value of the DC power is 1000 V or more and 6600 V or less. The power conversion system according to claim 6, wherein the voltage value of the DC power is 1500 V or more and 3300 V or less. The power conversion system according to any one of claims 1 to 7, wherein the effective value of the voltage of the AC power is 6600V. The power conversion system according to any one of claims 1 to 8, wherein each of the plurality of conversion devices includes a SiC element used for converting the DC power into the AC power. The power conversion system according to any one of claims 1 to 9.
Including solar panels
In the power conversion system, the solar panel is used as the DC power generation device, the main line system is used as the first main line, and the DC power generated by the solar panel is converted into AC power and output to the main line system. Solar power system. A conversion unit that converts the input DC power into AC power and outputs the AC power to the main line system.
Includes a communication unit that communicates via wiring that transmits the AC power output from the conversion unit.
The communication unit receives phase information from the outside via the wiring, and receives phase information from the outside.
The conversion unit is a power conversion device that adjusts the voltage phase of the AC power using the phase information. A control method for a power conversion system including a plurality of conversion devices, a connection node connected to each output unit of the plurality of conversion devices, and a step-down substation.
A step of converting DC power input to each of the plurality of conversion devices into AC power,
The step-down substation includes a step of stepping down the combined AC power generated by collecting the AC power output from each of the plurality of conversion devices by the connection node.
A control method in which the effective value of the voltage of the AC power is equal to or higher than the voltage value of the DC power.

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