JP7492891B2 - Power conversion device and inverter output current control method - Google Patents

Description

The present invention relates to a power conversion device configured to be capable of independent operation and a method for controlling the output current of an inverter provided in the power conversion device.

Utilization of the independent operation function of solar power generation equipment, including storage batteries, is expected in preparation for long-term power outages during disasters. In conventional independent operation, power is generally supplied to a specific load, and a power conditioner supplies an independent output of about 100V/1500W of single-phase two-wire power to the load via a dedicated outlet, for example. However, with the recent expansion of the use of storage batteries, there is a demand to supply the independent output of a power conditioner to all loads within a home. In addition, systems that use an automatic switch that automatically switches from commercial power to the independent output of a power conditioner when a power outage occurs are becoming common.

Patent document 1 shows a technology for controlling the independent operation of distributed power sources, which estimates the rise of the load current and controls the rise time of the independent output voltage to prevent inrush current.

Japanese Patent No. 5938068

Incidentally, many home appliances use a so-called capacitor input type configuration, in which the input AC power supply voltage is converted to DC voltage using an electrolytic capacitor or film capacitor (hereafter referred to as the input capacitor). In addition, since automatic switches need to ensure a driving voltage, they are designed to switch connections after the power conditioner's independent output voltage is output in the event of a power outage and commercial voltage is no longer supplied.

In this way, if the connection is switched to the independent output after the power conditioner is ready for independent output, that is, after the inverter is able to provide sufficient output, a large inrush current may flow through the input capacitor immediately after the connection is switched. Here, the inrush current is calculated as Ip = V/R. R is the line resistance from the commercial power source to the input capacitor, and since its value is very small, at less than 1 Ω, the inrush current tends to be large. Note that this phenomenon can occur regardless of whether the connection is switched from the commercial power source to the independent output of the power conditioner automatically or manually. Particularly in a system that supplies independent output from the inverter to all loads in the home, there are many devices connected and the inrush current tends to be large.

In addition, when an automatic switch is used, as mentioned above, the power conditioner's independent output voltage is only connected to the load after it has risen, which creates the problem that it is not possible to start up the power conditioner using a so-called soft start, which gradually raises the output voltage of the power conditioner.

Generally, power conditioners have a mechanism that stops them from starting if a certain allowable current is exceeded, and if the inrush current becomes too large, there is a risk that the power conditioner will start and stop repeatedly.

In view of the above problems, the present invention aims to provide a power conversion device capable of stably supplying independent output.

The power conversion device according to the first aspect of the present invention includes an inverter that converts a DC input into an AC input and outputs the AC input, a PWM control unit that transmits a PWM pulse signal to the inverter to perform PWM control, and a current measurement unit that measures the output current of the inverter, and when the output current of the inverter measured by the current measurement unit exceeds a predetermined first threshold value in the case where the power conversion device switches to independent operation, the PWM control unit provides a predetermined pause period during which the density of the on-pulses of the PWM pulse signal is made smaller than that before the output current exceeded the first threshold value.

According to this aspect, when the output current of the inverter exceeds a predetermined first threshold, the density of the on-pulses of the PWM pulse signal is made smaller than before the output current of the inverter exceeds the first threshold, so that the output current from the inverter circuit is suppressed, and protection against overcurrent can be more reliably provided. In other words, a stable supply of independent output is possible.

In the first aspect, the pause period may be set based on the period from when the output current exceeds a predetermined first threshold to when the output current decreases to a predetermined second threshold that is smaller than the first threshold.

In this way, by setting the end of the pause period based on the actual measured current value, more precise current adjustment is possible.

In the first aspect, the PWM control unit may drop the PWM pulse signal for a predetermined period of time after the output current of the inverter exceeds the first threshold. The method of dropping the PWM pulse signal is not particularly limited, and may, for example, stop the PWM pulse signal for an arbitrary period of time after the first threshold is reached, or reduce the number of pulses in the predetermined period of time.

The second aspect of the present invention relates to a control method for controlling the output current of an inverter when the inverter is switched to independent operation after its output voltage reaches a predetermined value or more, and the control method sets the density of the on-pulses of a PWM signal for controlling the inverter to a predetermined first density until the output current of the inverter reaches a predetermined first threshold, and when the output current exceeds the first threshold, provides a pause period of a predetermined length in which the density of the on-pulses of the PWM signal is set to a second density or less that is smaller than the first density so that the voltage charged to the input capacitor of the load to which the inverter supplies power gradually increases.

As mentioned above, for example, in a system using an automatic switch, when switching to independent operation occurs after the inverter output reaches a predetermined level, there is a risk that a large inrush current will flow to the input capacitor of the load immediately after the connection is switched. Therefore, in this embodiment, a pause of a predetermined length is provided to reduce the density of the on-pulses of the PWM signal so that the voltage charged to the input capacitor of the load gradually increases. By providing such a pause, the inrush current flowing to the input capacitor of the load can be suppressed, enabling a stable supply of independent output.

In the second aspect, the pause period may be set based on the period from when the output current exceeds a predetermined first threshold to when the output current decreases to a predetermined second threshold that is smaller than the first threshold.

In this way, by setting the end of the pause period based on the actual measured value, more precise current adjustment is possible.

In the power conversion device of the present invention, a predetermined pause period is set when the output current of the inverter exceeds a predetermined first threshold, so that the inverter circuit can be more reliably protected against overcurrent and a stable supply of independent output is possible.

FIG. 1 is a block diagram showing a configuration of a grid interconnection system. 1 is a block diagram showing a configuration of a power conversion device; 4 is a flowchart showing the operation of a CPU. 3A to 3C are waveform diagrams illustrating a schematic diagram of changes in output of a power conversion device and changes in a load current. FIG. 5 is a partially enlarged view of FIG. 4 . FIG. 5 is a view corresponding to FIG. 4 according to a comparative example.

The following describes in detail an embodiment of the present invention with reference to the drawings. The following description of the preferred embodiment is merely exemplary in nature and is not intended to limit the present invention, its scope of application, or its uses.

Figure 1 shows an example of the configuration of a grid-connected system 1 in a state where power is being supplied from a power grid 70 (hereinafter also referred to as normal operation). This grid-connected system 1 includes a power conditioner 10, a grid-connected breaker 30, a switching circuit 40, and a switching control unit 50.

The power conditioner 10 includes a DC/DC converter 11, a power conversion device 20, and a grid-connection switch 12. The power conditioner 10 also includes a single-phase three-wire system terminal 13 for connecting to the power grid 70, and a single-phase three-wire independent terminal 14 for supplying power to the load 90 during independent operation.

The DC/DC converter 11 converts the output voltage of the distributed power source 60 to a predetermined voltage and outputs the converted DC power to the DC voltage lines DL1 and DL2. The distributed power source 60 is composed of power generation means such as solar power generation means, wind power generation means, fuel cells, cogeneration systems, etc., and power storage means such as storage batteries.

As shown in FIG. 2, the power conversion device 20 includes a smoothing capacitor C, a DC voltage measurement unit 21, a midpoint control circuit 22, an inverter circuit 23, a filter circuit 25, an AC current measurement unit 26, an AC voltage measurement unit 27, an input unit 28, and a CPU (Central Processing Unit) 29 as a PWM control unit.

The smoothing capacitor C is connected between the DC voltage lines DL1 and DL2. The output of the DC/DC converter 11 is smoothed by the smoothing capacitor C and input to the midpoint control circuit 22.

The midpoint control circuit 22 stabilizes the voltage (so-called intermediate voltage) of the neutral line NL of the power conversion device 20, and controls it so that no DC component is generated in the neutral line NL. In the example of FIG. 2, two series-connected capacitors C1 and C2 and two series-connected transistors Q7 and Q8 are connected in parallel between the DC voltage lines DL1 and DL2, and a reactor R1 is provided between the wiring between the capacitors C1 and C2 and the wiring between the transistors Q7 and Q8. The neutral line NL is connected to the wiring between the capacitors C1 and C2. The DC voltage measurement unit 21 includes voltage sensors M1 and M2 that measure the voltages of the respective capacitors C1 and C2.

The inverter circuit 23 converts the direct current input via the DC voltage lines DL1 and DL2 into an alternating current and outputs it to the AC voltage lines AL1 and AL2. In FIG. 2, an H-bridge type inverter is shown as an example of the inverter circuit 23. Although not shown, a bidirectional inverter may be used as the inverter circuit 23. The inverter circuit 23 in FIG. 2 has a conventionally known configuration, and detailed explanation of its configuration and operation will be omitted here. In FIG. 2, Q1 to Q4 are switching elements that constitute an H-bridge circuit and perform switching operations upon receiving a PWM signal from the CPU 29. The output of the inverter circuit 23 is output to the system terminal 13 and the independent terminal 14 via the filter circuit 25.

The filter circuit 25 includes a reactor R2 provided on the AC voltage line AL1, a reactor R3 connected to the AC voltage line AL2, and capacitors C3 and C4 connected in series between the AC voltage lines AL1 and AL2. The wiring between the capacitors C3 and C4 is connected to the neutral line NL.

The AC current measurement unit 26 includes a current sensor M3 for measuring the current flowing through the AC voltage line AL1, a current sensor M4 for measuring the current flowing through the AC voltage line AL2, and a current sensor M5 for measuring the current flowing through the neutral line NL. The AC voltage measurement unit 27 includes a voltage sensor M7 for measuring the voltage between the AC voltage line AL1 and the neutral line NL, and a voltage sensor M8 for measuring the voltage between the neutral line NL and the AC voltage line AL2.

During grid-connected operation in which the power generated by the distributed power source 60 flows in reverse, the CPU 29 controls the grid-connected switch 12 to be conductive by turning it on, and transmits a PWM signal to the switching elements Q1 to Q4 to perform PWM control.

In addition, during independent operation, which is performed when a power outage occurs, the CPU 29 controls the interconnection switch 12 to be turned off and cuts off the power, and transmits a PWM signal to the switching elements Q1 to Q4, the duty ratio of which is adjusted so that the AC output output from the independent terminal 14 is constant, thereby PWM controlling the inverter circuit 23. The control of the CPU 29 during independent operation will be described in detail later.

Returning to FIG. 1, the grid-connection breaker 30 is a manual breaker provided between the grid terminal 13 and the inlet 80. When the power conditioner 10 is connected to the grid, the grid-connection breaker 30 is set to a conductive state, providing electrical continuity between the grid terminal 13 and the inlet 80. On the other hand, when the power conditioner 10 is not connected to the grid, the grid-connection breaker 30 is set to a non-conductive state.

The switching circuit 40 includes a power generation unit side switch 41 and a grid side switch 42, and switches whether the load 90 is connected to the power conditioner 10 or the power grid 70. A distribution board 91 is provided between the switching circuit 40 and the load 90. The power generation unit side switch 41 is provided between the distribution board 91 and the self-sustaining terminal 14 of the power conditioner 10. The grid side switch 42 is provided between the distribution board 91 and the inlet 80. Here, a capacitor input load is generally used, and in the following description, an input capacitor 95 is assumed to be connected to the load 90. The input capacitor 95 is, for example, a film capacitor with a capacitance value of about 1000 [μF].

In addition, in FIG. 1, the wiring resistance from the independent terminal 14 to the load 90 is shown as a schematic diagram with the reference numeral 94. The resistance value R of the wiring resistance 94 is set to, for example, 1 Ω or less.

The switching control unit 50 controls the on/off of the power generation unit side switch 41 and the grid side switch 42. Under normal circumstances, the switching control unit 50 turns off the power generation unit side switch 41 and turns on the grid side switch 42 so that power is supplied from the power grid 70 to the load 90. On the other hand, during independent operation such as when the power grid 70 experiences a power outage, the switching control unit 50 turns on the power generation unit side switch 41 and turns off the grid side switch 42 so that independent power is supplied from the power conditioner 10 to the load 90.

- Operation of the grid-connected system -
Next, the operation of the grid-connected system 1 will be described with reference to Figs. 3 to 5. Here, the operation will be described when a power outage occurs during normal operation (hereinafter also referred to as normal operation) in which power is supplied from the power grid 70, the power supply from the power grid 70 is stopped, and the system is switched to independent operation. Here, it is assumed that the power conditioner 10 is not grid-connected during normal operation. That is, the grid-connected switch 12 is controlled to be off. Note that the switching control unit 50 is assumed to be external to the power conditioner 10, but may be provided in the power conditioner 10 or the switching circuit 40. Also, the function of the switching control unit 50 may be incorporated into the CPU 29, and the CPU 29 may control the switching circuit 40.

First, in step S1 during normal operation, the switching control unit 50 turns off the power generation unit side switch 41 and turns on the grid side switch 42. This causes power to be supplied from the power grid 70 to the load 90.

Next, in step S2 after a power outage occurs, when a stop in the power supply from the power grid 70 is detected, the switching control unit 50 turns off the grid-side switch 42. For example, the switching control unit 50 determines whether a power outage has occurred based on the measurement results of a current sensor (not shown) provided at the inlet 80 or the like. The power outage state of the power grid 70 is also monitored by the CPU 29, and when a power outage is detected, the power conditioner 10 is started for independent operation.

In step S3, when the power conditioner 10 is able to output power for independent operation, the voltage from the power conditioner 10 is supplied to the switching circuit 40 and the switching control unit 50, and the switching control unit 50 turns on the power generation unit side switch 41. This establishes electrical continuity between the independent terminal 14 and the load 90, and the rated voltage (e.g., 100V or 200V) is supplied to the load 90. In FIG. 4, the power generation unit side switch 41 is turned on at time tp0, and the output voltage of the power conditioner 10 before and after the time, the capacitor input voltage of the load after time tp0, and the load current are shown changing. The load current is measured, for example, using current sensors M3 to M5.

As mentioned above, the power generation unit side switch 41 is turned on after the power conditioner 10 is able to output voltage for independent operation, so a large inrush current flows as the load current, and the input voltage (capacitor input voltage) of the load 90 also rises significantly.

More specifically, if the inrush current flowing through the input capacitor 95 is Im, Im is expressed by the following equation 1.

Im = (Vo - Vc) / R (1)

Here, Vo is the output voltage of the inverter circuit 23, i.e., the voltage between the neutral line NL and the AC voltage line AL1 and between the neutral line NL and AL2. Also, Vc is the voltage applied to the input capacitor 95, and R is the resistance value of the wiring resistor 94.

For example, when the wiring resistance 94 is 1 [Ω] and the output voltage Vo of the inverter circuit 23 is 100 [V], when the power generation unit side switch 41 is turned on, immediately after the switch is turned on, Vc is 0 [V], so a large rush current of about 100 [A] flows as an inrush current. In the example of Figure 4, when the power generation unit side switch 41 is turned on and independent operation starts at time tp0, the load current IL rises sharply between time tp0 and time tp1, and the above-mentioned rush current flows.

In the next step S4, the CPU 29 determines whether the load current IL exceeds a predetermined first threshold It1. The first threshold It1 is not particularly limited, but is set arbitrarily to be lower than the rated current of the power conditioner 10 and the load 90, for example.

If the determination in step S4 is NO, i.e., if the load current IL is equal to or less than the first threshold value It1, the CPU 29 continues to output, as a PWM pulse signal, a signal whose duty ratio is adjusted so that the AC output output from the independent terminal 14 is constant. The density of the on-pulses of the PWM pulse signal at this time is referred to as the "normal on-pulse density."

Here, the term "density of on-pulses" is used as an index indicating the distribution ratio of on-pulses in a predetermined unit period or the degree of density of on-pulses. The "density of on-pulses" is adjusted, for example, by increasing or decreasing the number of pulses of the PWM pulse signal during a predetermined period after the load current IL exceeds the first threshold It1. That is, if the number of pulses in the predetermined period is increased, the on-pulse density increases. On the other hand, if the number of pulses in the predetermined period is reduced by, for example, missing waves during the above-mentioned predetermined period, the on-pulse density decreases. Here, missing waves is used as a term including reducing the number of pulses by stopping the PWM pulse signal and reducing the number of pulses by thinning out the on-pulses of the PWM pulse signal. Note that the adjustment of the on-pulse density is not limited to increasing or decreasing the number of pulses. For example, a method of adjusting the on-pulse width is exemplified by changing the pulse interval of the PWM pulse signal or changing the duty ratio of the PWM pulse signal.

On the other hand, if the determination in step S4 is YES, i.e., if the load current IL exceeds the predetermined first threshold It1, the flow proceeds to step S5, where the CPU 29 reduces the density of the on-pulses of the PWM pulse signal to be smaller than the normal on-pulse density (the state before the load current IL exceeds the first threshold It1). The method for reducing the on-pulse density is as described above. FIG. 5 shows an example in which the output of the PWM pulse signal for a predetermined period is stopped to reduce the on-pulse density when the load current IL exceeds the predetermined first threshold It1.

Specifically, FIG. 5 is a partially enlarged view of FIG. 4, and shows the waveforms of the load current IL and the PWM pulse signal enlarged for times tp1, tp3, and tp5. In the example of FIG. 5, the CPU 29 stops the PWM pulse signal at time t11 when the load current IL exceeds a predetermined first threshold It1. When the PWM pulse signal is stopped, the output of the power conditioner 10 stops, so that the load current IL stops increasing at time tp1, a short time after time t11, and then the load current IL decreases. Note that the first threshold It1 can be set arbitrarily, but is set, for example, based on the allowable current or rated current of the power conditioner 10.

In the next step S6, the CPU 29 determines whether the load current IL is smaller than a predetermined second threshold It2. The second threshold It2 is set so that the voltage charged to the input capacitor 95 of the load 90 is not completely discharged. When the load current IL becomes smaller than the predetermined second threshold It2 (YES in step S6), the flow proceeds to the next step S7. In step S7, the CPU 29 resumes output of the PWM pulse signal, thereby resuming charging to the input capacitor 95 of the load 90. In the example of FIG. 5, the load current IL falls below the second threshold It2 at time t12. Therefore, the CPU 29 resumes output of the PWM pulse signal from time t12. After a short time has passed since time t12, the load current IL starts to rise again. As a result, charging is resumed with a certain amount of voltage remaining in the input capacitor 95 (see time t13 in FIG. 4). In this case, the value of Vc in Equation 1 starts at a value greater than when it is 0 V, and the inrush current Im that flows toward the next time tp2 becomes smaller. In FIG. 5, T1 indicates the period from when the PWM pulse signal is stopped for the first time until the PWM pulse signal is restarted. T1 is an example of a "rest period."

When the output of the PWM pulse signal is resumed in step S7, the flow returns to step S4, and the CPU 29 determines whether the load current IL exceeds a predetermined first threshold value It1.

When the load current IL again exceeds the first threshold It1 (YES in step S4), the CPU 29 stops the PWM pulse signal (step S5). When the load current IL falls below the second threshold It2 (YES in step S6), the CPU 29 resumes the PWM pulse signal (step S7). In the second cycle of the flow, the voltage remaining in steps S4 to S7 of the first cycle is also added. Therefore, the voltage of the input capacitor 95 becomes higher than at the end of the first cycle (see time t23 in FIG. 4).

After that, the processes from step S4 to step S7 are repeated, and as shown at time t33 and time t43 in FIG. 4, the inversion position of the input voltage of the load gradually rises to the right. After a while, the load voltage settles to a predetermined value according to the output voltage of the power conditioner 10 (see time tpf in FIG. 4).

In addition, in FIG. 5, the state of the load current IL and the PWM pulse signal around time tp3 in FIG. 4 is shown in the center left and right. T3 indicates the "rest period" from the third time the PWM pulse signal is stopped until the PWM pulse signal is restarted. As described above, at the time of T3, Vc is larger than at the time of T1, so Im in formula (1) is smaller, and as a result, the period T3 is shorter than the period T1. The right side of FIG. 5 shows the state of the load current IL and the PWM pulse signal around time tp6 in FIG. 4. At the time tp6, the load current IL falls before reaching the first threshold value It1, so the CPU 29 does not stop the PWM pulse signal.

As described above, by setting the first threshold It1 and the second threshold It2 to arbitrary values, the voltage Vc across the input capacitor 95 of the load 90 gradually builds up, making it possible to output a stable output voltage while suppressing overcurrent.

--Comparative Example--
The operation of the grid-connected system 1 in the case where the "predetermined pause period" in this embodiment is not provided will be described with reference to Fig. 6. In the example of Fig. 6, when the load current IL exceeds a predetermined first threshold It1, a pause period is provided in which the CPU 29 stops the PWM signal to stop the inverter circuit 23, and the inverter circuit 23 is restored after a while.

Specifically, in the example of FIG. 6, as in the case of FIG. 4, the independent operation of the power conditioner 10 is started at time tp0. Then, at time t51, the load current IL exceeds the first threshold It1. Therefore, the CPU 29 stops the inverter circuit 23, and at time t52 after a predetermined stop period, the CPU 29 drives the inverter circuit 23 to resume the output of the power conditioner 10. However, when the control method as shown in FIG. 6 is used, the load current IL exceeds the first threshold It1, and the inverter circuit 23 is stopped repeatedly. That is, in this comparative example, there is a problem that the voltage across the input capacitor of the load is not charged, so that independent operation never starts, but by performing the control of the embodiment, such a problem does not occur.

As described above, the power conversion device 20 of this embodiment includes the inverter circuit 23 that converts the direct current output from the DC/DC converter 11 into alternating current and outputs it, the CPU 29 as a PWM control unit that transmits a PWM pulse signal to the inverter circuit 23 to perform PWM control, and the AC current measurement unit 26 that measures the output current of the inverter circuit 23. When the load current IL as the output current of the inverter circuit 23 measured by the AC current measurement unit 26 exceeds a predetermined first threshold value It1 when switching from a normal operation state to an independent operation state, the CPU 29 provides a predetermined pause period in which the density of the on-pulses of the PWM pulse signal is made smaller than before the load current IL exceeds the first threshold value It1.

This allows the voltage charged to the input capacitor 95 of the load 90 to be gradually increased while suppressing overcurrent, making it possible to supply a stable output voltage to the capacitor input load while reducing the inrush current flowing through the load 90. In other words, it is possible to output a stable output voltage while protecting the inverter circuit 23 from overcurrent.

In the above embodiment, the "predetermined pause period" of the inverter circuit 23 is the period from when the load current IL exceeds the first threshold to when it falls below the second threshold, but is not limited to this. For example, the predetermined pause period may be set in advance. For example, when the load current IL exceeds the first threshold It1, the PWM pulse signal may be stopped for a predetermined fixed period. The fixed period during which the PWM pulse signal is stopped may be common throughout the entire period, or may be changed (e.g., shortened) depending on the number of times the first threshold It1 is exceeded. In addition, the peak current of the rush current of the load current IL may be monitored, and the stop period of the PWM pulse signal may be set according to the peak current.

According to the present invention, in a power conditioner configured for independent operation, the voltage charged to the capacitor of the capacitor input load can be gradually increased after the start of independent operation to suppress inrush current, which is extremely useful.

20 Power conversion device 23 Inverter circuit (inverter)
26 AC current measuring unit (current measuring unit)
29 CPU (PWM control unit)
90 Load 95 Input capacitor It1 First threshold It2 Second threshold

Claims (4)

An inverter that converts DC input into AC output,
a PWM control unit that transmits a PWM pulse signal to the inverter to perform PWM control;
A current measuring unit that measures an output current of the inverter,
The power conversion device, when switching to independent operation, and when the output current of the inverter measured by the current measuring unit exceeds a predetermined first threshold, the PWM control unit provides a predetermined pause period in which the density of on-pulses of the PWM pulse signal is made smaller than before the output current exceeded the first threshold , and sets the pause period based on the period from when the output current exceeded the first threshold to when it decreased to a predetermined second threshold smaller than the first threshold so as to gradually decrease the pause period . The power conversion device according to claim 1 , further comprising: a step of repeatedly executing an operation of providing the pause period and an operation of returning the on-pulse density of the PWM pulse signal to a normal on-pulse density before the pause period. The power conversion device according to claim 1 , wherein the PWM control unit reduces an on-pulse density by interrupting the PWM pulse signal during the pause period compared to a normal on-pulse density before the pause period. A method for controlling an output current of an inverter when switching to an independent operation after an output voltage of the inverter becomes equal to or greater than a predetermined value, comprising the steps of:
setting a density of on-pulses of a PWM signal for controlling the inverter to a predetermined first density until an output current of the inverter reaches a predetermined first threshold;
A method for controlling an output current of an inverter, comprising: providing a pause period of a predetermined length in which a density of on-pulses of the PWM signal is set to a second density or less that is smaller than the first density, so that, when the output current exceeds the first threshold, a voltage charged to an input capacitor of a load to which the inverter supplies power gradually increases ; the pause period is set based on a period from when the output current exceeds the first threshold to when the output current decreases to a predetermined second threshold that is smaller than the first threshold .

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