JP2014033565A - Power converter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress an unintended trip of an earth leakage breaker by avoiding a situation conflicting with a system interconnection rule.SOLUTION: A power converter 16 interconnected to a commercial power system 9 whose second line on the three-phase secondary side is grounded comprises: positive side and negative side bus capacitors 4 and 5 which are connected in series between positive and negative DC buses and whose neutral point is electrically connected to the second line; switch elements 12a and 12b connected in series between the positive and negative DC buses; a balancing circuit 12 having a reactor 12c connected between the neutral point 14 and a junction point between the switch elements 12a and 12b; and the like. A control circuit 15 performs control of activating either of the switch elements 12a and 12b, depending on voltage difference between the positive side and negative side bus capacitors 4 and 5.

Description

  The present invention includes an inverter circuit that converts direct current power from a direct current power source into alternating current power, and is connected to a commercial power system (hereinafter, abbreviated as “system” as appropriate) in which the second line on the three-phase secondary side is grounded. The present invention relates to a power conversion apparatus.

  In a power converter connected to a system in which the second line on the three-phase secondary side is grounded, an inverter circuit or the like may be configured by a method called a neutral point clamp method (NPC method). In the case of this method, the grounded second line is connected to the neutral point of a pair of bus capacitor connected in series between the positive and negative DC buses in the power converter. The neutral point is sandwiched between a positive electrode and a negative electrode of a DC power source such as a solar cell module or a fuel cell, and the current path of the DC power source is insulated from the ground.

  In the case of the power conversion device used for the three-phase second-line grounding system as described above, the voltage of the bus capacitor connected to the DC bus (bus capacitor voltage) is unbalanced due to harmonic distortion of the system voltage. For such an unbalance of the bus capacitor voltage, a method of adjusting the unbalance of the bus capacitor voltage by superimposing a zero-phase component voltage on each output line in the inverter circuit is disclosed ( For example, the following patent document 1).

Japanese Patent No. 3186369

  However, when the method of Patent Document 1 is applied to a power conversion device that is linked to a three-phase second-line grounding system, there are the following problems.

(1) When the second line of the system is grounded, a zero-phase current flows when a zero-phase component voltage is superimposed on each output line of the inverter circuit. On the other hand, there is a restriction on the current for the zero phase according to the grid connection regulations (JECC 9701-2010). Therefore, if the unbalanced amount of the bus capacitor voltage is increased, the grid interconnection regulation may not be satisfied.
(2) Although it is common to provide a leakage breaker on the output side of the inverter circuit, if the current for the zero phase increases, the leakage breaker trips, causing a problem that operation cannot be continued.

  The present invention has been made in view of the above, and in a power conversion device linked to a three-phase second-line grounded system, even when a countermeasure for imbalance of bus capacitor voltage is taken, An object of the present invention is to obtain a power conversion device that can avoid a situation that violates a system regulation and suppress an unintentional leakage breaker trip.

  In order to solve the above-described problems and achieve the object, the present invention provides a power converter that receives power from a DC power supply and is linked to a commercial power system in which a second line on the three-phase secondary side is grounded. The DC-DC converter for converting the output voltage of the DC power source to a desired voltage and a positive / negative DC bus are connected in series and are electrically connected to the second line with the connection point as a neutral point. Positive side and negative side bus capacitors, and an inverter clamped at the neutral point and converted to direct current power held by the positive side and negative side bus capacitors into desired AC power and supplied to the commercial power system The first and second switch elements connected in series between the positive and negative DC buses with the diodes connected in antiparallel, and the connection point between the first and second switch elements and the neutral point Rear And a control circuit that controls conduction of the first and second switch elements based on the voltages of the positive and negative bus capacitors, the control circuit including the positive bus Control is performed to operate one of the first and second switch elements in accordance with a voltage difference between a capacitor and the negative bus capacitor.

  According to the present invention, even when measures against imbalance of the bus capacitor voltage are taken, it is possible to avoid a situation that violates the grid connection regulations and to suppress an unintentional leakage breaker trip. Play.

FIG. 1 is a diagram illustrating a configuration of a power conversion device according to an embodiment of the present invention. FIG. 2 is a diagram illustrating a state in which the second harmonic voltage is superimposed on the system voltage during grid connection. FIG. 3 is a diagram illustrating an output voltage waveform and an output current waveform when a single-phase 100 V half-wave rectifier load is connected to a self-sustained operation load. FIG. 4 is a diagram illustrating an example of adjusting the unbalance of the bus voltage using a resistor circuit as an example of a conventional technique for adjusting the unbalance of the bus voltage.

  Hereinafter, a power converter according to an embodiment of the present invention will be described with reference to the accompanying drawings. In addition, this invention is not limited by embodiment shown below.

  FIG. 1 is a diagram illustrating a configuration example of a power conversion device 16 according to an embodiment of the present invention, in which power (DC power) supplied from a DC power supply 1 is converted into AC power and is a load system ( The desired power is supplied to the commercial power system) 9 and the self-sustained operation load 10.

  As shown in FIG. 1, the power converter 16 includes a DC-DC converter 2, a balance circuit 12, a P-side bus capacitor 4, an N-side bus capacitor 5, an inverter 3, an AC filter circuit 6, a system-side switch 7, a self-supporting device. The driving load side switch 8 and the control circuit 15 are provided.

  The DC power source 1 is a DC power supply source such as a solar power generation system, a fuel cell, and the like. The DC-DC converter 2 is, for example, a converter having an MPPT (Maximum Power Point Tracking) function or a boosting function, and changes the output voltage of the DC power supply 1 to a voltage value suitable for the inverter 3 and outputs it. The P-side (positive side) bus capacitor 4 and the N-side (negative side) bus capacitor 5 are capacitors that hold power supplied from the DC-DC converter 2 and are connected in series between the positive and negative DC buses. Moreover, the neutral point 14 which is those connection points is connected to the second line of the higher-order transformer (secondary transformer) of the system 9 which is a three-phase AC power supply, and is set to the same potential as the ground point.

  The balance circuit 12 is connected in series with a switch element 12a (first switch element) and a switch element 12b (second switch element) connected in series between positive and negative DC buses with diodes connected in antiparallel. A reactor 12c connected between the connection point of the switch elements 12a and 12b and the neutral point 14; The switch elements 12a and 12b are self-extinguishing semiconductor elements such as IGBTs and MOSFETs. The balance circuit 12 performs an operation for eliminating an unbalance of voltage difference between the P-side bus capacitor 4 and the N-side bus capacitor 5 (an unbalance of bus capacitor voltage). Detailed operation will be described later.

  The inverter 3 is a power conversion circuit called a neutral point clamp method (NPC method), and converts the DC power held in the P-side bus capacitor 4 and the N-side bus capacitor 5 into desired AC power. Then, the converted power is supplied to the commercial power system 9 and the independent operation load 10 via the AC filter circuit 6, the system side switch 7 and the independent operation load side switch 8. In FIG. 1, the inverter 3 is shown as a three-level circuit configuration, but it may be a two-level inverter or a multi-level inverter circuit configuration other than three levels.

  The AC filter circuit 6 is a circuit that reduces harmonic components and noise components included in the output of the inverter 3. The system side switch 7 and the independent operation load side switch 8 are switches for switching whether the output of the inverter 3 is supplied to the system 9 and the independent operation load 10, respectively.

  The control circuit 15 is a circuit unit that performs control for eliminating (adjusting) the unbalance of the bus capacitor voltage based on the bus capacitor voltage. In the example of FIG. 1, the difference unit 15a, the PI controller 15b, and the inversion 15c, reference signal generator 15d, comparators 15e and 15f, and dead time generators 15g and 15h.

  Next, the operation of the power conversion device according to the present embodiment will be described with reference to FIGS. 2 to 4 in addition to FIG. Here, FIG. 2 and FIG. 3 are diagrams for explaining a problem of balance control according to the conventional method. In particular, FIG. 2 shows a state in which the second harmonic voltage is superimposed on the system voltage during system interconnection. FIG. 3 is a diagram illustrating an output voltage waveform and an output current waveform when a single-phase 100 V half-wave rectifier load is connected to a self-sustained operation load. FIG. 4 is a diagram showing an example of adjusting the unbalance of the bus voltage with a resistor circuit as an example of a conventional method for adjusting the unbalance of the bus voltage. In the following description, the DC power source 1 is assumed to be a solar power generation system including a solar power generation panel.

  As a result, the current path of the DC power source 1 is insulated from the neutral point 14 grounded through the system 9. The ground voltage (potential difference from the ground point) of the N-side current path of the DC power supply 1 is a negative value of the N-side bus capacitor 5 voltage. The ground voltage of the P-side current path of the DC power source 1 is obtained by adding the ground voltage (negative voltage) of the N-side bus capacitor 5 to the output voltage of the DC power source 1. For example, when the voltage of the P-side bus capacitor 4 is Vcp [V], the voltage of the N-side bus capacitor 5 is Vcn [V], and the DC power supply voltage is Vi [V], the ground voltage of the N-side current path is −Vcn [ V], the ground voltage of the P-side current path, and Vi-Vcn (= Vcp) [V]. When the neutral point 14 is grounded, the voltage to ground of the current path is constant and there is almost no AC component, so the photovoltaic power generation panel and the casing of the power converter 16 (usually grounded) There is an advantage that the leakage current flowing through the floating capacitance between them is small.

  The generated power of the photovoltaic power generation panel varies depending on the solar radiation conditions. For this reason, the DC-DC converter 2 controls the input voltage from the photovoltaic power generation panel and the boosted voltage (the output voltage of the DC-DC converter 2) so that the output power of the inverter 3 is maximized.

  On the other hand, in a consumer (for example, a factory) that uses a rectifying load 10a (that is, a load connected via the rectifier 10b) as shown in the figure as the self-sustained operation load 10, harmonics are generated and the waveform of the system voltage is increased. Is often distorted. For example, when even-order harmonics are superimposed on the system voltage, the inverter output power is biased to either the positive side or the negative side (FIG. 2 is an example biased to the negative side), and the P-side bus capacitor 4 and N The voltage between the side bus capacitor 5 is unbalanced.

  In such a state, in Patent Document 1 described above, control for superimposing the voltage of the zero-phase component on each output line of the inverter circuit is performed by adding a zero-phase current or even-order current operation amount to the inverter current command. Do. However, as described in the above problem section, the system current is distorted or a zero-phase leakage current flows, so that a situation may occur in which the leakage breaker is activated and the operation cannot be continued.

  On the other hand, if the balance circuit 12 shown in FIG. 1 and the control circuit 15 for controlling the balance circuit 12 are used, the above-described problems can be solved. The control circuit 15 detects a voltage difference between the P-side bus capacitor 4 and the N-side bus capacitor 5 with a differentiator 15a and generates it as an operation amount with a PI (proportional integration) controller 15b. This manipulated variable is input to the comparators 15e and 15f (the inverted output of the manipulated variable is input to the comparator 15f by the inverter 15c).

  The comparators 15e and 15f generate a PWM signal based on the reference signal (for example, a triangular wave signal) generated by the reference signal generator 15d and the operation amount, and are switched by the dead time generators 15g and 15h. Are controlled so as not to conduct simultaneously, and then output to the balance circuit 12.

  As a result of the control, the control circuit 15 operates one of the switch elements according to the voltage difference between the P-side bus capacitor 4 and the N-side bus capacitor 5. For example, when the voltage of the P-side bus capacitor 4 is higher than the voltage of the N-side bus capacitor 5, the switch element 12a becomes conductive. When switch element 12a is turned on, the charge accumulated in P-side bus capacitor 4 is discharged and flows to reactor 12c. The energy stored in the reactor 12c flows through the diode of the switch element 12b and charges the N-side bus capacitor 5. By this series of operations, the voltage of the P-side bus capacitor 4 decreases, the voltage of the N-side bus capacitor 5 increases, and the bus capacitor voltage is balanced. When the voltage of the N-side bus capacitor 5 is higher than the voltage of the P-side bus capacitor 4, the operation is reversed.

  As described above, in the control according to the present embodiment, control that superimposes the voltage of the zero phase component on each output line of the inverter 3 or causes the leakage current of the zero phase to flow is not performed. For this reason, the situation which conflicts with grid connection regulation can be avoided, and it becomes possible to suppress the trip of the earth leakage breaker which is not intended.

  The above description is the operation when the power conversion device 16 is connected to the three-phase second-line grounded system 9, but this control is performed when the power conversion device 16 is connected to the autonomous operation load 10. It is also effective in some cases.

  As shown in FIG. 1, the self-sustained operation load 10 includes a 200V single-phase load 10c that hardly causes an imbalance of the bus capacitor voltage. On the other hand, a bus capacitor such as a rectifier load 10a connected via the rectifier 10b is present. Some loads are prone to voltage imbalance. In the case of the rectifying load 10a, as shown in FIGS. 3 (a) and 3 (b), an output current flows for each half circuit of the 100V single-phase output voltage waveform, so an unbalance between the bus capacitor voltages tends to occur. Even in such a case, the operation of the balance circuit 12 described above automatically eliminates the unbalance of the bus capacitor, so that a situation that violates the grid connection regulations and an unintentional leakage breaker trip, etc. It can be reliably avoided or suppressed.

  In addition, the bus capacitor voltage may be unbalanced before the operation is started. Conventionally, for example, a resistive load circuit 22 as shown in FIG. 4 is provided, and the control circuit 20 controls each switch element of the resistive load circuit 22 based on the deviation of the bus capacitor voltage. However, in order to quickly balance the voltage, the power capacity of the resistor has to be increased. When the power capacity of the resistor cannot be increased, there is a problem that the time until the start of operation is delayed. On the other hand, in the present embodiment, since the resistive load circuit 22 as shown in FIG. 4 is not used, it is realized by using the reactor 12c connected between the neutral point 14 and the connection point of the switch elements 12a and 12b. There is an advantage that the problem that the time until the start of operation becomes long is not caused.

  As described above, according to the power conversion device according to the present embodiment, one of the first and second switch elements according to the voltage difference between the positive bus capacitor and the negative bus capacitor. Therefore, it is possible to eliminate the imbalance between the bus capacitor voltages, to avoid a situation in which the grid connection regulations are violated, and to prevent an unintentional leakage breaker trip.

  In the above control, control may be performed from the time when a significant difference occurs in the voltage difference between the bus capacitors, or a predetermined threshold is provided, and control is started when this threshold is exceeded. You may make it do. Among these controls, when the control is performed from the time when a significant difference occurs, the control is performed before the voltage difference of the bus capacitor becomes large, so the imbalance between the bus capacitor voltages can be eliminated instantly. The effect is obtained. On the other hand, if the control is started when the threshold value is exceeded, the switching operation of the switch element can be reduced, the life of the switch element can be extended, and the switching loss and conduction loss are reduced. The effect that it can be obtained.

  Further, two different threshold values (first threshold value> second threshold value) are provided, and control is started when the voltage difference between the bus capacitor exceeds the first threshold value. In the process where the voltage difference becomes smaller, the control may be stopped when it falls below a second threshold value that is smaller than the first threshold value. By performing such control, it is possible to achieve both the speed of unbalance cancellation and the loss reduction. Even in such a case, control may be performed regardless of these threshold values when the apparatus is activated.

  The configuration shown in the above embodiment is an example of the configuration of the present invention, and can be combined with another known technique, and various modifications within the scope of the present invention are allowed. It goes without saying that it can be done.

  As mentioned above, this invention is useful as a power converter device which can avoid the situation which conflicts with grid connection regulation and can suppress the trip of the unintentional earth leakage breaker.

  1 DC power supply, 2 DC-DC converter, 3 inverter, 4 P side bus capacitor, 5 N side bus capacitor, 6 AC filter circuit, 7 system side switch, 8 self-sustained operation load side switch, 9 commercial power system, 10 Self-supporting operation load, 10a rectifier load, 10b rectifier, 10c 200V single phase load, 12 balance circuit, 12a switch element (first switch element), 12b switch element (second switch element), 12c reactor, 14 neutral point 15, 20 control circuit, 15a differencer, 15b PI controller, 15c inverter, 15d reference signal generator, 15e, 15f comparator, 15g, 15h dead time generator, 15e, 15f comparator, 16 power converter 22 Resistance load circuit.

Claims (6)

In the power converter that receives power supply from the DC power source and is linked to the commercial power system in which the second line on the three-phase secondary side is grounded,
A DC-DC converter that converts the output voltage of the DC power source to a desired voltage;
Positive and negative bus capacitors connected in series between the positive and negative DC buses and electrically connected to the second wire with the connection point as a neutral point;
An inverter that is clamped at the neutral point and that converts the DC power held in the positive and negative bus capacitors to desired AC power and supplies the AC power to the commercial power system;
First and second switch elements connected in series between the positive and negative DC buses with diodes connected in reverse parallel, and connected between the connection point of the first and second switch elements and the neutral point A balance circuit comprising a reactor,
A control circuit for controlling conduction of the first and second switch elements based on voltages of the positive and negative bus capacitors;
With
The control circuit performs control to operate any one of the first and second switch elements in accordance with a voltage difference between the positive bus capacitor and the negative bus capacitor. Power conversion device.   The power converter according to claim 1, wherein a stand-alone operation load is connected to the inverter in addition to the commercial power system.   The power converter according to claim 1, wherein the control circuit starts control from a point in time when a significant difference occurs in the voltage difference.   The power converter according to claim 1, wherein the control circuit performs control when the voltage difference exceeds a predetermined threshold value.   The control circuit starts control when the voltage difference exceeds a first threshold value, and in the process of decreasing the voltage difference, a second threshold value smaller than the first threshold value. The power converter according to claim 1, wherein the control is stopped when the value is lower than.   6. The power conversion according to claim 4, wherein the control circuit executes control regardless of the threshold value or the first and second threshold values when starting the device. apparatus.

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