JP2015015778A - Power conversion system for system interconnection - Google Patents

Abstract

PROBLEM TO BE SOLVED: To resolve a problem that a device becomes large in size and high in costs when a voltage balance circuit for a capacitor series circuit is separately installed in a case where a plurality of inverters for system interconnection each producing an AC having a commercial frequency by an inverter operation for generating a three-level AC voltage from a voltage of a DC circuit in which two capacitors are connected in series and that has a positive electrode, a negative electrode, and a neutral electrode, and each injecting the AC power to a system are connected in parallel.SOLUTION: At system interconnection, control having an inverter operation for generating a three-level AC voltage from a DC power supply and a balancer operation for balancing respective voltages of two capacitors, is performed. At an autonomous operation, in any one of a plurality of inverters for system interconnection, control having an inverter operation for generating a three-level AC voltage based on an output current command value and a balancer operation for balancing the respective voltages of two capacitors, is performed, and in the remaining inverters, control having the inverter operation for generating the three-level AC voltage is performed.

Description

  In the present invention, two capacitors are connected in series, and an alternating current of commercial frequency is generated by an inverter operation that generates an alternating voltage combining a three-level voltage from a voltage of a direct current circuit having a positive electrode, a negative electrode, and a neutral electrode, A grid-connected power conversion system configured by connecting in parallel a plurality of grid-connected inverters that inject AC power into a power system via a grid-connected switch and a grid-connected reactor. The present invention relates to a control technique that balances the voltages of capacitors of a DC circuit connected in series during self-sustained operation.

  FIG. 8 shows a circuit configuration for balancing the voltage of each capacitor in a series connection circuit of capacitors C1 and C2 connected to a DC circuit in the neutral point clamp type three-level conversion circuit described in Patent Document 1. Indicates. The DC circuit part of the three-level conversion circuit includes a positive electrode P, a negative electrode N, and a neutral electrode M in a configuration in which capacitors C1 and C2 are connected in series.

  The configuration of the three-level conversion circuit is such that the configuration for one phase is connected between a positive electrode P and a negative electrode N of a DC circuit in which four (first to fourth) IGBTs each having a diode connected in antiparallel are connected in series. The first diode is connected in series between the third IGBT and the fourth IGBT between the series connection point of the first IGBT and the second IGBT and the neutral pole M which is the series connection point of the capacitor. A second diode is connected between the connection point and the neutral pole M, which is a series connection point of capacitors. A series connection point between the second IGBT and the third IGBT is an AC terminal.

  A three-phase circuit is formed by connecting three circuits for one phase in parallel, and a single-phase circuit is formed by connecting two circuits in parallel. As a conversion method, when an AC power supply is connected to each AC terminal via a reactor, a converter circuit that converts AC to DC is supplied to the DC circuit from the outside, and when a load is connected to the AC terminal, an inverter circuit is , Each configured.

  A circuit for balancing the voltage of each capacitor is formed by connecting a series circuit of IGBTDB1 and DB2 with diodes connected in antiparallel between the positive and negative electrodes of a DC circuit, and a series connection point of IGBTDB1 and DB2. And the reactor L1 is connected to the neutral pole M, which is a series connection point between the capacitors C1 and C2 of the DC circuit. The operation for balancing the voltages of the capacitors is to detect the voltage EDC1 of the capacitor C1 and the voltage EDC2 of the capacitor C2, and to control the on / off ratio of the IGBT DB1 and DB2.

  That is, when the voltage EDC1 of the capacitor C1 is higher than the voltage EDC2 of the capacitor C2, the IGBT DB1 is turned on, a current is passed from the capacitor C1 to the reactor L1, the energy is accumulated, and the IGBT DB1 is turned off to be accumulated in the reactor L1. The charged energy is charged in the capacitor C2. Further, when the voltage EDC1 of the capacitor C1 is lower than the voltage EDC2 of the capacitor C2, the IGBT DB2 is turned on, a current is passed from the capacitor C2 to the reactor L1, the energy is accumulated, and the IGBT DB2 is turned off to be accumulated in the reactor L1. The charged energy is charged in the capacitor C1. By repeating such an operation while controlling the ON / OFF ratio of IGBTDB1 and DB2, the voltage EDC1 of the capacitor C1 and the voltage EDC2 of the capacitor C2 can be balanced.

  FIG. 9 shows another circuit configuration of the three-level conversion circuit described in Patent Document 2. The DC circuit has a configuration in which a series circuit of capacitors Cp and Cn is connected in parallel with the DC power supply Ed, and includes a positive electrode P, a negative electrode N, and a neutral electrode M. A three-level conversion circuit includes a series circuit of IGBT Qu and Qx each having a diode connected in antiparallel between a positive electrode P and a negative electrode N of a DC circuit, and a series circuit of IGBT Qv and Qy each having an antiparallel connection of a diode. A series circuit of IGBTs Qw and Qz each having a diode connected in antiparallel is connected in parallel, and a bidirectional switch Qsu is connected between the series connection point of IGBT Qu and Qx and the neutral pole of the DC circuit, and IGBT Qv A bidirectional switch Qsv is provided between the series connection point of Qy and the neutral pole of the DC circuit, and a bidirectional switch Qsw is provided between the series connection point of IGBT Qw and Qz and the neutral pole of the DC circuit. Each is a connected configuration. A series connection point of each IGBT series circuit is an AC terminal, and is connected to an electric motor LM as a load. In this circuit example, the neutral point of the motor LM and the neutral pole of the DC circuit are connected.

  The balance control between the voltage of the capacitor Cp and the voltage of the Cn in the DC circuit is a method of detecting the voltage of each capacitor and controlling the neutral point current of the motor LM so that these voltages are balanced. That is, with the motor LM as an inductance, a mode for simultaneously turning on the three IGBTs (Qu, Qv, Qv) of the upper arm and a mode for simultaneously turning on the three IGBTs (Qx, Qy, Qz) of the lower arm. When the voltage of the capacitor Cp is higher than the voltage of the capacitor Cn, energy is accumulated from the capacitor Cp to the inductance of the motor LM when the upper arm is simultaneously turned on, and energy accumulated in the inductance is obtained by simultaneously turning off the upper arm. The capacitor Cn is charged. As a result, the voltage of the capacitor Cp decreases and the voltage of the capacitor Cn increases. When the voltage of the capacitor Cp is lower than the voltage of the capacitor Cn, energy is accumulated from the capacitor Cn to the inductance of the motor LM when the lower arm is simultaneously turned on, and energy accumulated in the inductance is obtained by simultaneously turning off the lower arm. The capacitor Cp is charged. As a result, the voltage of the capacitor Cn decreases and the voltage of the capacitor Cp increases. By repeating such an operation, the voltage of the capacitor Cp and the voltage of Cn are balanced.

JP 2002-199738 A JP 2011-109789 A

  As described above, the method of Patent Document 1 requires a semiconductor switch and a reactor for balancing the capacitor voltage in the conversion circuit, and the apparatus is large and expensive. Moreover, the method of patent document 2 is a control method in the case of having an inductance component such as an electric motor as a load, and its application range is limited.

  Accordingly, an object of the present invention is to provide an inverter system for grid interconnection that can reduce the number of parts added to a conversion circuit as much as possible, achieve downsizing and cost reduction, and is not affected by load conditions.

  In order to solve the above-described problem, in the first invention, an AC voltage obtained by combining two capacitors in series and combining a voltage of three levels from a voltage of a DC circuit having a positive electrode, a negative electrode, and a neutral electrode is provided. A grid-connected system consisting of a plurality of grid-connected inverters connected in parallel to create AC at commercial frequencies by the generated inverter operation and inject AC power into the power grid via the grid-connected switch and grid-connected reactor In the system power conversion system, during grid connection, an inverter operation that generates an AC voltage combining the three levels of voltages based on the output current command values of the plurality of grid connection inverters and the DC circuit A control having both a balancer operation for balancing the voltages of the two capacitors, and during the self-sustaining operation not linked to the power system, the plurality of systems In order to balance the voltage of each of the two capacitors of the DC circuit and the inverter operation that generates an AC voltage combining the three levels of voltages based on the output current command value in any one of the inverters for interconnection The control is combined with the balancer operation, and the remaining number of grid interconnection inverters are controlled so as to generate an alternating voltage combining the three levels of voltages.

  In the second invention, during the self-sustained operation in the first invention, any one of the plurality of grid interconnection inverters has the output current command value set to zero. The control is combined with the operation and the balancer operation for balancing the voltages of the two capacitors of the DC circuit, and the remaining number of grid interconnection inverters generate an AC voltage combining the three levels of voltages. Control of inverter operation.

  In the third invention, during the self-sustained operation in the first invention, the AC side of any one of the plurality of grid-connected inverters connected in parallel is connected to the remaining number of grid-connected inverters. The grid interconnection inverter is disconnected from the AC side of the grid interconnection inverter, and the single grid interconnection inverter is used as a balancer operation control for balancing the voltages of the two capacitors of the DC circuit, and the remaining number of systems is connected. The interconnection inverter controls the operation of an inverter that generates an alternating voltage combining the three levels of voltages.

  According to a fourth invention, in the grid interconnection power conversion system according to any one of the first to third inventions, an inverter operation for generating an AC voltage combining the three levels of voltages and two DC circuits are provided. The balancer operation for balancing the voltages of the two capacitors is a balancer operation control for balancing the voltages of the two capacitors of the DC circuit to the output voltage command value of the grid interconnection inverter. The control is to add a correction with the output of the controller.

In the present invention, AC at a commercial frequency is generated by an inverter operation that generates an AC voltage combining three levels of voltage from a DC circuit voltage having a positive electrode, a negative electrode, and a neutral electrode. In a grid-connected power conversion system configured by connecting in parallel a plurality of grid-connected inverters that inject AC power into a power system via a system reactor. Control that combines the inverter operation for generating the AC voltage combining the three levels of voltages based on the output current command value and the balancer operation for balancing the voltages of the two capacitors of the DC circuit. And during independent operation that is not linked to the power system, any one of the plurality of grid interconnection inverters is based on an output current command value. Control that combines an inverter operation that generates an AC voltage combining level voltages and a balancer operation to balance the voltages of the two capacitors of the DC circuit, and the remaining number of grid interconnection inverters are DC circuits Since the inverter operation is controlled so as to generate an AC voltage that is a combination of three levels of voltage, the voltage of the capacitor of the DC circuit can be balanced without adding new components.
As a result, it is possible to provide a grid interconnection inverter system that is small in size and low in price and is not affected by load conditions.

1 is a main circuit block diagram showing a first embodiment of the present invention; It is a control circuit block diagram at the time of the independent operation in the structure of FIG. It is a main circuit block diagram which shows the 2nd Example of this invention. It is a control circuit block diagram at the time of the independent operation of FIG. It is a main circuit block diagram which shows the 3rd Example of this invention. It is a control circuit block diagram at the time of the independent operation of FIG. It is a main circuit diagram of the grid connection inverter which this invention makes object. It is a circuit diagram which shows a 1st prior art example. It is a circuit diagram which shows a 2nd prior art example.

FIG. 7 shows an example of a main circuit diagram of a single grid interconnection inverter. The conversion circuit has the circuit configuration described in Patent Document 2. The present invention can be applied to the circuit shown in FIG. 8 as long as the conversion circuit is a three-level inverter having a DC circuit including a capacitor series circuit. An AC filter FLT composed of a reactor and a capacitor is connected to the AC output of the inverter via a current detection circuit Cdt, and a grid connection reactor Lo and a grid connection contactor CTT are connected to the output of the AC filter FLT. Terminals U, V, and W are connected to the power system and the load. Further, a voltage detector Vdt1 is connected to the output of the AC filter FLT, a voltage detector Vdt2 is connected to the AC terminals U, V, and W, and the output of each voltage detector is input to the changeover switch SW.
In such a configuration, the output of the current detection circuit Cdt and the output of the changeover switch SW are input to a control circuit not shown, and are used for voltage control and current control. The change-over switch SW is switched to the voltage detector Vdt2 side during grid connection and to the voltage detector Vdt1 side during self-sustained operation.

  The gist of the present invention is that an alternating current of a commercial frequency is generated by an inverter operation that generates an alternating voltage combining three levels of voltage from a voltage of a direct current circuit having a positive electrode, a negative electrode, and a neutral electrode, In a grid interconnection power conversion system configured by connecting in parallel a plurality of grid interconnection inverters that inject AC power into a power grid through a grid interconnection reactor, the grid interconnection power system is configured to connect the plurality of grid interconnections. Control of each system inverter has both an inverter operation for generating an AC voltage combining the three levels of voltages based on the output current command value and a balancer operation for balancing the voltages of the two capacitors of the DC circuit. And control of the balancer operation of any one of the plurality of grid interconnection inverters during independent operation not linked to the power system. And the remaining system interconnection inverter in number is the point that the control of the inverter operation.

  FIG. 1 shows a first embodiment of the present invention. In this circuit configuration, n grid interconnection inverters INV (1) to INV (n) are connected in parallel, a storage battery as a DC power source is connected to a DC input, and a system is connected to an AC output. Each of the n grid interconnection inverters is a combination of three levels of voltage from the voltage of a DC circuit comprising a positive electrode, a negative electrode, and a neutral electrode composed of a series connection circuit of two capacitors connected in parallel with the storage battery. It also has a balancer control function for balancing the voltages of two capacitors connected in series with an inverter control that generates an AC voltage.

  A control circuit block diagram of each of the grid interconnection inverters INV (1) to INV (n) at the grid interconnection is shown in FIG. The output voltage of the inverter circuit (reactor current of the filter circuit) is set to the first voltage command value obtained from a voltage regulator not described for adjusting the output voltage to the system voltage. The current command value (1 / n of the system output current) The output of the voltage regulator AVR which makes the deviation between the voltage EDC1 of the capacitor C1 and the voltage EDC2 of the capacitor C2 zero to the second voltage command value obtained by correcting the output of the current regulator ACR for This is a control method in which correction is applied.

  The correction amount of the first output voltage command value is obtained by a current adjuster ACR that obtains the deviation between the output current command value of the inverter circuit and the detected current value by the adder AD2, and adjusts this deviation to be zero. Further, the first voltage command value is corrected by the output of the current regulator ACR by the adder AD1, and becomes the second voltage command value. The output of the second voltage command value is corrected by the output of the voltage regulator AVR that makes the deviation between the voltage EDC1 of the capacitor C1 and the voltage EDC2 of the capacitor C2 zero by the adder AD4. The output of the adder AD4 is converted into an on / off signal by a pulse width modulation (PWM) circuit, and supplied to a switching element (in this case, an IGBT) of an inverter circuit through a pulse distribution circuit and a gate drive circuit which are not described. With such a control circuit, each of the n inverters performs both inverter control for converting the DC voltage to the system voltage based on the current command value and balance control for balancing the capacitor voltages.

  During independent operation not connected to the grid, one inverter performs the control operation shown in FIG. 2 (a), and the remaining inverters perform the inverter control operation for converting the DC voltage shown in FIG. 2 (b) into the grid voltage. Do. As described above, at the time of grid connection, each inverter performs inverter operation control in which a DC voltage is converted into a grid voltage and balancer operation control in which two capacitor voltages are balanced. During independent operation, one unit performs inverter operation control that converts DC voltage to system voltage and balancer operation control that balances two capacitor voltages, and the remaining inverters perform inverter operation control that converts DC voltage to system voltage. Do. As a result, it is not necessary to add a balancer circuit for balancing the capacitor voltage constituted by a reactor, a switching element, or the like as in the prior art. In addition, the capacitor voltage can be balanced without being affected by the load conditions.

  FIG. 3 shows a second embodiment of the present invention. The grid interconnection inverters INV (1) to INV (n) are connected in parallel, a storage battery as a DC power source is connected to the DC input, and a load is connected to the AC output. The control during grid connection is the same as in the first embodiment, but the operation during independent operation is different. During stand-alone operation, one of the n grid interconnection inverters performs balancer control to balance the voltages of two capacitors connected in series, and the remaining (n-1) inverters are storage batteries. An inverter control operation is performed to generate an AC voltage combining three levels of voltage from a DC circuit voltage including a positive electrode, a negative electrode, and a neutral electrode configured by a series connection circuit of two capacitors connected in parallel.

FIG. 4A shows a control circuit block diagram of one grid interconnection inverter. Although the control circuit configuration is the same as that of the first embodiment, the command value of the inverter output current (reactor current of the filter circuit) is set to zero and the AC voltage is output, but the output current is not supplied and the capacitor C1 Control for balancing the voltage EDC1 and the voltage EDC2 of the capacitor C2 is performed.
FIG. 4B shows control circuits of other (n−1) inverters. An inverter control operation is performed to generate an AC voltage that combines three levels of voltage from the voltage of a DC circuit having a positive electrode, a negative electrode, and a neutral electrode that are configured by a series connection circuit of two capacitors connected in parallel with the storage battery.
As described above, in this embodiment, one inverter controls the balance between the two capacitor voltages with the inverter output current command value set to zero. As a result, it is not necessary to add a balancer circuit for balancing the capacitor voltage constituted by a reactor, a switching element, or the like as in the prior art. In addition, the capacitor voltage can be balanced without being affected by the load conditions.

FIG. 5 shows a third embodiment of the present invention. The grid interconnection inverters INV (1) to INV (n) are connected in parallel, and a load is connected to the AC output. The difference from the other embodiments is that the AC output of one inverter is connected in parallel with another inverter via a switch S. The control during grid connection is the same as in the other embodiments, but the operation during independent operation is different. During independent operation, the AC output of one of the n grid interconnection inverters is disconnected from the other inverters by switch S, and balancer control is performed to balance the voltages of the two capacitors connected in series. The remaining (n-1) inverters combine three levels of voltage from the voltage of a DC circuit having a positive electrode, a negative electrode, and a neutral electrode, which are composed of a series connection circuit of two capacitors connected in parallel with the storage battery. The inverter is controlled to generate AC voltage. FIG. 6 shows a control block diagram of each inverter. One isolated inverter performs balancer control for balancing the voltage EDC1 of the capacitor C1 and the voltage EDC2 of the capacitor C2. FIG. 6A is a balancer control block diagram of one inverter in this embodiment. It is the same as other embodiments. In the case of this embodiment, one inverter at the time of self-sustained operation is responsible only for basin control, and the power duty is smaller than that of the other embodiments, which is suitable when the number of inverters connected in parallel is large.
In the above embodiment, an example in which one of the inverters connected in parallel performs the balancer control of the capacitor voltage is shown, but the same can be realized when a plurality of inverters perform the balancer control. Further, although an example of a three-level inverter in which two capacitors are connected in series has been shown, any multi-level conversion circuit having two or more capacitors in series can be similarly applied.

  The present invention is a capacitor voltage balance control technology in a grid-connected inverter system in which a plurality of conversion circuits each having a capacitor connected in series are connected in parallel, and is applied to grid-connected inverters such as solar inverters and wind power inverters. Applicable.

Ed, DP: Storage battery LM: Electric motor Cp, Cn, C1, C2 ... Capacitor L1, Lo ... Reactor FLT ... Filter CTT ... Contactor SW ... Changeover switch S ... Switch Cdt ... Current detection circuit Vdt1, Vdt2 ... Voltage detector Dpt ... Voltage detection circuit CONT ... Control circuit Qu, Qv, Qw, Qx, Qy, Qz, DB1, DB2 ... IGBT
Qsu, Qsv, Qsw: Bidirectional switch AD1-AD4: Adder AVR: Voltage regulator ACR: Current regulator PWM: Pulse width modulation circuit

Claims (4)

Connects two capacitors in series, creates AC at commercial frequency by inverter operation that generates AC voltage combining three levels of voltage from DC circuit voltage with positive, negative and neutral electrodes. In the grid-connected power conversion system configured by connecting in parallel a plurality of grid-connected inverters that inject AC power into the power system via the switch and grid-connected reactor,
At the time of grid connection, inverter operation for generating an AC voltage combining the three levels of voltages based on the output current command values of the plurality of grid connection inverters and the voltages of the two capacitors of the DC circuit Control with balancer operation for balancing,
During independent operation not linked to the power system, an inverter that generates an AC voltage by combining any one of the plurality of grid interconnection inverters with the three-level voltage based on an output current command value The control is combined with the operation and the balancer operation for balancing the voltages of the two capacitors of the DC circuit, and the remaining number of grid interconnection inverters generate an AC voltage combining the three levels of voltages. A power conversion system for grid interconnection characterized by controlling inverter operation.   2. The grid interconnection power conversion system according to claim 1, wherein any one of the plurality of grid interconnection inverters sets an output current command value to zero during the independent operation. The control is combined with the inverter operation and the balancer operation for balancing the voltages of the two capacitors of the DC circuit, and the remaining number of grid interconnection inverters are supplied with an AC voltage combining the three levels of voltages. A power conversion system for grid interconnection characterized by controlling the generated inverter operation.   2. The grid interconnection power conversion system according to claim 1, wherein during the independent operation, the AC side of any one of the plurality of grid interconnection inverters connected in parallel remains in the grid interconnection inverter. The number of grid interconnection inverters is disconnected from the AC side, and the one grid interconnection inverter is a balancer operation control for balancing the voltages of the two capacitors of the DC circuit, and the remaining number The grid interconnection inverter controls the operation of an inverter that generates an alternating voltage combining the three levels of voltages. In the grid connection power conversion system according to any one of claims 1 to 3,
The control having both an inverter operation for generating an AC voltage combining the three levels of voltages and a balancer operation for balancing the voltages of the two capacitors of the DC circuit is the output voltage of the grid interconnection inverter. A power conversion system for grid interconnection, characterized in that control is performed by correcting the output of a balancer operation control regulator for balancing the voltages of the two capacitors of the DC circuit with the command value.