JP2004236418A - Power converter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a power converter capable of positively detecting individual operation by making effective use of generated power of a DC power supply. <P>SOLUTION: This power converter includes a current measuring means for measuring a current flowing-in from a commercial power system and/or a current flowing into the commercial power system, and performs output control so that a current value of the current measuring means may not be almost zero. Moreover, the power converter performs output control so that output fluctuations may occur periodically only when the current value of the current measuring means approaches almost zero, and so that the output fluctuations may become larger as the current value of the current measuring means approaches zero, thus positively detecting the individual operation and making effective use of the generated power by a solar battery. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a power converter that converts DC power obtained from a DC power supply such as a solar cell into AC power, and more particularly to an islanding detection technology.
[0002]
[Prior art]
2. Description of the Related Art In recent years, a system interconnection system that converts an output of a solar cell into an AC using an inverter and interconnects with a commercial power system has been put to practical use. In this system interconnection system, as shown in FIG. 6, DC power generated by the solar cell power supply 2 is converted to, for example, 200 V AC by a power converter 21, and the power converter 21 and the commercial power system 12 Are interconnected.
[0003]
As shown in FIG. 7, a conventional power converter 21 converts a power generated by the solar cell power supply 2 into a voltage different from the voltage of the solar cell power supply 2 and a DC voltage conversion circuit 23. And an inverter circuit 24 that converts the converted DC power into AC power. The inverter circuit 24 converts the maximum power generated by the solar cell power supply 2 into an AC output, and connects the load 13 to the commercial power system 12 to interconnect the power supply. Running (maximum output tracking control).
[0004]
Here, the power converter 21 has a switch 16 such as a relay, and when there is an abnormality in the commercial power system 12 due to a power failure or the like, the power conversion is immediately stopped so that the recovery operation can be performed safely and promptly. Means for shutting off the switch 16 and disconnecting from the commercial power system 12 is employed (single operation detection function).
[0005]
As means for detecting such an isolated operation, there are a passive method and an active method, and it is required to combine them in order to increase the detection accuracy. Generally, passive systems include a voltage phase jump detection system that detects a sudden change in the voltage phase of the system, a third harmonic voltage distortion detection system that detects a sudden increase in the third harmonic of the system, and a sudden change in the frequency of the system. There is a frequency change rate detection method to detect, etc.The active methods include a frequency shift method that applies a frequency bias to the output, an active power fluctuation method that fluctuates the active power of the output, and a reactive power fluctuation method that fluctuates the reactive power of the output. Etc.
[0006]
In addition, a method has been proposed in which the output current of the inverter is changed at random to detect islanding operation (for example, see Patent Document 1).
[0007]
[Patent Document 1]
JP-A-11-136865 (page 9, FIG. 1)
[0008]
[Problems to be solved by the invention]
However, in the conventional method, when the output power of the power conversion device and the power consumption of the load are in a balanced state, even when the commercial power system loses power or the like, the phase and frequency of the output portion of the power conversion device, etc. Since the fluctuation is small and the detected change amount is small, there is a problem that the independent operation cannot be detected and the isolated operation is continued.
[0009]
Also, in a method in which the output current of the inverter is randomly varied to detect islanding operation, the output power of the power converter is always larger than the maximum power generated by the solar cell because the output always has a variable factor. And the generated power cannot be used effectively.
[0010]
The present invention has been made in view of the above-described conventional problems, and has as its object to provide a power conversion device that effectively utilizes generated power of a DC power supply and reliably detects islanding operation. is there.
[0011]
[Means for Solving the Problems]
The present invention is a power converter that converts DC power obtained from a solar cell into AC power and is connected to a commercial power system and a load system to perform an interconnected operation. It has a current measuring means for measuring a current flowing in and / or a current flowing out to the commercial power system, and performs output control so that a current value of the current measuring means does not become substantially zero.
[0012]
Further, the present invention is characterized in that output control is performed so that the output is periodically changed only when the current value of the current measuring means approaches substantially zero.
[0013]
Further, the present invention is characterized in that output control is performed such that the output fluctuation increases as the current value of the current measuring means approaches zero. Thus, it is possible to reliably detect the islanding operation and to provide a power converter that can effectively use the power generated by the solar cell.
[0014]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings schematically illustrating a solar power generation device in which a plurality of solar cell modules are arranged on a roof.
[0015]
As shown in FIG. 1, a power conversion device 1 includes a DC voltage conversion circuit 3 that boosts the voltage of a solar cell power supply 2 and converts the voltage to a different voltage, and converts the DC power converted by the DC voltage conversion circuit 3 into an AC power And an inverter circuit 4 for converting the data into
[0016]
The DC voltage conversion circuit 3 comprises a reactor 5, a semiconductor switch element 6, a diode 7, a smoothing capacitor 8, and an output control means 9 for controlling the semiconductor switch element 6, thereby performing a voltage conversion.
[0017]
The inverter circuit 4 includes a bridge circuit 10 including a plurality of semiconductor switch elements, and a filter circuit 11 including a reactor and a capacitor, and converts the DC power output from the DC voltage conversion circuit 3 into a sine wave commercial AC waveform. The AC power output from the inverter circuit 4 is connected to the commercial power system 12 and the load 13, and power is supplied to the load 13 and / or power is sold to the commercial power system 12. Further, the power conversion device 1 includes an AC output detector 14 which is a current measuring means for measuring an AC output current supplied or supplied from or supplied to the commercial power system 12, detects an independent operation state, and disconnects from the commercial power system 12. The information is provided to the islanding operation detection control unit 15 for performing the operation.
[0018]
Next, the operation of the power converter of the present invention will be described. DC power obtained from the solar cell power supply 2 is input to the power converter 1. In the DC voltage conversion circuit 3, the semiconductor switch element 6 is turned on and off at high speed by the output control means 9, energy is stored in the reactor 5 when the semiconductor switch 6 is on, and the energy is stored when the semiconductor switch 6 is off. By being output to the smoothing capacitor 8 through the diode 7, it is possible to perform a step-up conversion to a DC voltage different from the DC voltage input from the solar cell power supply 2. It is desirable that the semiconductor switch element 6 is controlled by a PWM method for controlling the duty of a pulse according to the converted voltage. It is desirable that the converted and output DC voltage has a voltage value at which the inverter circuit 4 operates efficiently.
[0019]
The DC power input to the inverter circuit 4 is subjected to PWM (pulse width modulation) control by the high-speed switching operation of the bridge circuit 10 and passes through a filter circuit 11 which is an LPF (low-pass filter), thereby providing a sine wave AC output. Is obtained. The output AC power is supplied to a load 1, which is an AC device such as a motor or a light. However, if the amount of power generated by the solar cell power supply 2 exceeds the load power, the surplus power is returned to the commercial power system 12. Sell electricity with the tide.
[0020]
Further, an AC output detector 14 as current measuring means for detecting a sine-wave AC output current is attached to the commercial power system 12 and a transmission line between the load 13 and the power converter 1. Information (data) is provided to the isolated operation detection control unit 15 of the power converter 1 so that the power receiving current from the power converter 12 or the power selling current due to the reverse power flow can be measured. In the islanding operation detection control unit 15, as means for detecting the islanding state of the power converter 1, at least one active system and one or more passive systems (for example, a voltage phase jump detection system and a frequency shift system) are performed. However, the detection is performed with a certain sensitivity regardless of the power generation state or the load state.
[0021]
As shown in FIG. 2, the power generated by the solar cell constantly fluctuates depending on the sunshine condition and the temperature. In order to obtain the maximum power generated, the operating point should be changed to the maximum output point of the solar cell at that time. Controlling the power converter. For example, the operating point is moved from P2 to P1 when the output power of the solar cell increases due to solar radiation fluctuation, and is moved from P2 to P3 when the output power decreases. If the power is balanced, even when the commercial power system loses power, fluctuations in the phase and frequency of the output section of the power conversion device are small, and the amount of detected change is small. It will be difficult.
[0022]
Therefore, in the present invention, when the current value flowing through the commercial power system detected by the AC output detector is approaching substantially zero, the output power fluctuation of the power converter is stopped or reduced and the detected current value is reduced. The operation is performed so as not to become substantially zero.
[0023]
Specifically, as shown in FIG. 3, when the current I of the commercial power system approaches zero (the period of the shaded area), the maximum output point of the solar cell is tracked so that the current I does not fall below the detection level. Is stopped, and control is performed to generate electric power at a constant power so that the output of the output power P of the solar cell does not increase any more, and the power supply from the commercial power system is maintained. This control is performed by the output control means 9 including a microcomputer and a driving element for driving the semiconductor switching element 6. By such control, the output power of the power conversion device and the power consumption of the load do not become in a balanced state, and if there is an abnormality in the commercial power system such as a power failure, the detected change amount such as the phase and frequency is reduced. Because of the large size, it is possible to reliably detect the isolated operation state, and it is possible to provide a safe and highly reliable power converter.
[0024]
Next, a second embodiment of the present invention will be described. The configuration of the power converter is the same as that of FIG. 1 described above, but the operation is as follows. The isolated operation detection control unit 15 of the power conversion device 1 detects the isolated operation state, and outputs an output such that the output periodically fluctuates only when the current value detected by the AC output detector 14 approaches substantially zero. Perform control.
[0025]
That is, as shown in FIG. 4, under the control of the output control means 9, when the current of the commercial power system approaches zero, the following of the maximum output point is stopped, and the output is periodically increased or decreased to reduce the load consumption. Since the operation is performed so that the power and the output power are not balanced, even if the set value of the detection level is made closer to zero, it is possible to maintain the detection change amount such as the phase and the frequency.
[0026]
With such control, the output fluctuates periodically in the event of an abnormality in the commercial power supply system such as a power outage, and even if the power consumption of the load and the output power of the power converter are close to a balanced state, the output fluctuates. It is easy to detect the rate of change of the voltage, frequency, and the like, and it is possible to reliably detect the isolated operation state.
[0027]
In addition, since the output is changed periodically only when the current detected by the AC output detector approaches substantially zero, the output fluctuation may be reduced if the power consumption of the load and the output power of the power converter are not in a balanced state. By eliminating the loss due to the output fluctuation, the power generated by the solar cell can be effectively used, and a safe and efficient power converter can be provided.
[0028]
Further, in the present invention, the above-described islanding operation detection controller 15 detects the islanding operation state, and periodically changes the output only when the current value detected by the AC output detector 14 approaches substantially zero. In addition to performing output control as described above, it is also possible to perform control so that the ratio of output fluctuation increases as the detected current value approaches zero.
[0029]
That is, as shown in FIG. 5, under the control of the output control means 9, when the current of the commercial power system is within a certain range from zero, the output fluctuation is small, and as the current approaches the zero, the output increase / decrease ratio is increased. The operation is performed so that the power consumption of the load and the output power are not balanced.
[0030]
With such control, the output is periodically changed only when the current detected by the AC output detector approaches zero, so that when the power consumption of the load and the output power of the power converter are not in a balanced state. Does not fluctuate the output, and the amount of fluctuation increases stepwise until it approaches the balance state, so that the power generated by the solar cell can be effectively used.
[0031]
Note that, in the present embodiment, the control method in the case where the power generated by the solar cell is equal to or less than the power consumption of the load has been described, and when the power generated by the solar cell exceeds the power consumption, the current of the commercial power system is substantially reduced. It is preferable to perform control such that reverse power flow (power selling) is performed in a range where the power flow does not become zero.
[0032]
【The invention's effect】
As described above, according to the control method of the power conversion device of the present invention, DC power is converted into AC power, connected to the commercial power system and the load system to perform the interconnection operation, and flows from the commercial power system. Current measuring means for measuring the amount of current flowing into and / or out of the commercial power system, and control means for controlling the output so that the current value of the current measuring means does not become substantially zero. The output power of the device and the power consumption of the load do not become in a balanced state, and it is possible to reliably detect the isolated operation state when there is an abnormality in the commercial power system due to a power failure or the like.
[0033]
In addition, when the current value of the current measuring unit is close to substantially zero, the output is controlled so as to periodically change the output. Since the output fluctuates, the isolated operation state can be reliably detected, and the power generated by the solar cell can be effectively used.
[0034]
In addition, since the output control is performed so that the output fluctuation increases as the current value of the current measuring means approaches zero, the output periodically fluctuates in the event of an abnormality in the commercial power supply system such as a power failure. In addition, the isolated operation state can be reliably detected, and the power generated by the solar cell can be more effectively used.
[Brief description of the drawings]
FIG. 1 is a schematic configuration diagram of an example showing an embodiment of a power conversion device of the present invention.
FIG. 2 is a schematic explanatory diagram showing a change in an operating point of the power conversion device of the present invention.
FIG. 3 is a schematic explanatory diagram showing a correlation between output power and current received from a commercial power system in a control operation of the power conversion device of the present invention.
FIG. 4 is a schematic explanatory diagram showing a correlation between output power and a current received from a commercial power system in a control operation of a power converter according to another embodiment of the present invention.
FIG. 5 is a schematic explanatory diagram showing a correlation between output power and a current received from a commercial power system in a control operation of a power converter according to another embodiment of the present invention.
FIG. 6 is a schematic configuration diagram showing a form of a conventional grid interconnection system.
FIG. 7 is a schematic configuration diagram of an example showing a form of a conventional power converter.
[Explanation of symbols]
1: power conversion device 2: solar cell power supply 3: DC voltage conversion circuit 4: inverter circuit 5: reactor 6: semiconductor switch element 7: diode 8: smoothing capacitor 9: output control means (control means)
10: Bridge circuit 11: Filter circuit 12: Commercial power system 13: Load 14: AC output detector (current measuring means)
15: islanding operation detection control section 16: switch 21: power converter 23: DC voltage converter 24: inverter circuits P1, P2, P3: operating point P: solar cell output power I: commercial power system current

Claims (3)

Current measurement for converting DC power to AC power, performing connection operation by being connected to a commercial power system and a load system, and measuring a current flowing from the commercial power system and / or a current flowing to the commercial power system. And a control means for controlling the output so that the current value of the current measuring means does not become substantially zero. 2. The power converter according to claim 1, wherein the control unit performs output control for periodically changing an output when the current value of the current measuring unit approaches substantially zero. 3. The power converter according to claim 2, wherein the control unit performs output control so that a change in output increases as the current value of the current measurement unit approaches substantially zero.

Images