JPH0921834A - System linkage system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To detect a d.c. component flowing into a transformer from a.c. output of an inverter by a simple means. SOLUTION: In a system linkage system in which a transformer 3 is connected to the output side of an inverter 2 for converting a direct current from a d.c. power source 1 into an alternating current and the inverter 2 is interconnected to a system power source 4 through the transformer 3, a current detection device 5 is provided on the system power source side of the transformer 3, and an exciting current Ib flowing through the system power source side of the transformer 3 is measured by the current detection device 5. A peak detection circuit 6 for detecting a peak (a) appearing in the exciting current Ib due to drift excitation of the transformer 3 caused by that a d.c. component Ia flows into the transformer 3 and a computing circuit 7 for computing the peak value (m) to detect the d.c. component Ia corresponding to it are provided.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a grid interconnection system, and more specifically, it connects a DC power source such as a fuel cell or a solar cell and a distributed power source including an inverter to a grid through a transformer, and outputs the output of the inverter. The present invention relates to a system interconnection system capable of detecting a DC component flowing into a transformer connected to the side.

[0002]

2. Description of the Related Art For example, as shown in FIG. 8, a distributed power source composed of a DC power source 1 such as a solar cell and an inverter 2 for converting the DC power from the DC power source 1 into an AC power is converted into a transformer 3 such as a pole transformer. There is a system interconnection system in which the DC power source 1 and the system power source 4 supply electric power to a load [not shown] via the system power source 4 via a system.

Some of the inverters 2 of the distributed power source used in the system interconnection system have a built-in insulation transformer on the output side so that no DC component flows out. For example, Japanese Patent Laid-Open No. 2-307374. Some have been disclosed.

In the power converter disclosed in Japanese Patent Laid-Open No. 2-307374, the current on the inverter side of the transformer connected to the output side of the inverter and the current on the system power source side are measured, and the exciting current is calculated from the difference between them. When the DC component is flowing into the inverter output side of the transformer, the DC component to the transformer is controlled by controlling the inverter to cancel the biased excitation. I try to prevent the inflow of.

[0005]

By the way, in the above-mentioned inverter having a built-in isolation transformer, since the size of the inverter is increased due to the built-in isolation transformer, the cost is increased. Being developed. In this type of inverter, control is performed so as to cancel the biased excitation, but unlike an inverter with a built-in insulation transformer, insulation cannot be obtained between the DC side of the inverter and the commercial system, and the outflow of DC components is completely prevented. Therefore, it is necessary to detect the DC component flowing into the transformer and stop the inverter if the detected value exceeds the allowable value.

Further, even in the case of an inverter incorporating the insulating transformer, there is no particular problem during normal operation, but the DC component may flow out due to malfunction of the switch, deterioration, failure, or the like. Even in such a case, it is not possible to prevent the DC component from flowing out only by controlling the inverter, so if the DC component is detected and the detected value exceeds the allowable value, it is necessary to stop the inverter. There is.

Therefore, the present invention has been proposed in view of the above problems, and an object of the present invention is to provide a system in which the DC component flowing from the output of the inverter to the transformer can be detected by a simple means. It is to provide an interconnection system.

[0008]

As a technical means for achieving the above object, the first feature of the present invention is to connect a transformer to the output side of an inverter for converting a direct current from a direct current power source into an alternating current, In a system interconnection system in which the inverter is connected to a system power supply via the transformer, a current detector is provided on the system power supply side of the transformer, and the excitation that flows to the system power supply side of the transformer by the current detector. A peak detection circuit for measuring a current, detecting a peak appearing in the exciting current due to biased excitation of the transformer caused by a direct current component flowing into the inverter output side of the transformer, and calculating a peak value thereof. That is, an arithmetic circuit for detecting a DC component corresponding thereto is provided.

A second feature of the present invention is that a transformer is connected to the output side of an inverter for converting direct current from a direct current power source into an alternating current, and the inverter is connected to a system power source through the transformer. In the system interconnection system, a current detector is provided on the system power supply side of the transformer, the exciting current flowing to the system power supply side of the transformer is measured by the current detector, and the exciting current is frequency analyzed to transform the transformer. An arithmetic circuit is provided for calculating the content rate of the harmonics appearing in the exciting current due to the biased excitation of the transformer caused by the direct current component flowing into the inverter output side of the transformer and detecting the corresponding DC component. That is.

[0010]

In the first aspect of the present invention, the exciting current flowing to the system power source side of the transformer is measured by the current detector provided on the system power source side of the transformer. When a biased excitation occurs in the transformer because a DC component flows into the inverter output side of the transformer, a peak appears in the exciting current flowing to the system power source side of the transformer. This peak is detected by the peak detection circuit, and the peak value is calculated by the calculation circuit to detect the DC component corresponding thereto.

In the second invention, the exciting current is measured by the current detector provided on the system power source side of the transformer, and the exciting current is subjected to frequency analysis by the arithmetic circuit. When a biased excitation occurs in the transformer because a DC component flows into the inverter output side of the transformer, harmonics appear in the excitation current flowing to the system power source side of the transformer. The harmonic content is calculated by the arithmetic circuit to detect the DC component.

[0012]

EXAMPLE An example of a system interconnection system applied to interconnection of a distributed power source and a system power source such as a solar power generation system will be described below with reference to FIGS. 1 to 7.

In the embodiment shown in FIG. 1, a distributed power source including a DC power source 1 such as a solar cell and a fuel cell, and an inverter 2 for converting the DC power from the DC power source 1 into an AC voltage is a transformer such as a pole transformer. Connect to the system power supply 4 via 3,
Electric power is supplied from the DC power supply 1 and the system power supply 4 to a load [not shown]. The inverter 2 may or may not have an insulation transformer built therein.

The feature of this embodiment is that a current detector 5 is provided on the system power supply side of the transformer 3 connected to the output side of the inverter 2, and the current detector 5 connects the system power supply side of the transformer 3 to the system power supply side. The exciting current Ib that flows is measured, and a peak a appears in the exciting current Ib due to the biased excitation of the transformer 3 caused by the inflow of the DC component Ia into the inverter output side of the transformer 3 (see FIG. 2B). ], And a calculation circuit 7 for calculating the peak value m and detecting the corresponding DC component Ia.

In the above embodiment, the current detector 5 provided on the system power supply side of the transformer 3 measures the exciting current Ib flowing on the system power supply side of the transformer 3. At this time, when the DC component Ia flows from the inverter 2 to the inverter output side of the transformer 3, and thus biased excitation occurs in the transformer 3, as shown in FIG. A peak a appears in the flowing exciting current Ib. If the DC component Ia does not flow into the inverter output side of the transformer 3, the peak a does not appear in the exciting current Ib flowing to the system power source side of the transformer 3 as shown in FIG. 2 (a). .

Here, in general, when the transformer 3 is biasedly excited by the DC component Ia, the peak value appearing in the exciting current Ib flowing to the system power source side of the transformer 3 flows into the inverter output side of the transformer 3. It is proportional to the DC current. as a result,
As shown in FIG. 3, the peak value [peak / rated peak] (%) appearing in the exciting current Ib and the direct current (A) flowing into the inverter output side of the transformer 3 have a linear correspondence. . Therefore, this peak a is detected by the peak detection circuit 6
The peak value m is calculated by the arithmetic circuit 7 and the DC component Ia corresponding to the peak value m is detected. When the detected value of the DC component Ia exceeds the allowable value, the inverter 2 may be stopped based on the comparison result between the detected value and the allowable value.

In the peak detection circuit 6, the exciting current Ib
Is taken out as the system power supply side voltage, and the detection signal generation circuit 8
With reference to FIG. 1, the peak a of each cycle of the exciting current Ib is detected. At this time, since the exciting current phase is delayed by about 90 ° from the voltage phase, the peak a of the exciting current Ib is the voltage zero point. Since it appears in the vicinity [see FIG. 2 (b)], the peak of the exciting current Ib can be reliably detected regardless of the magnitude of the load current by setting the detection timing of the peak a to, for example, about ± 1 msec of the voltage zero cross point. It can be carried out.

Incidentally, FIG. 3 shows three types of transformers [capacity: 10 k.
VA, 50 kVA, 133 kVA], and FIG. 4 shows the experimental value and the calculated value in relation to the peak value of the exciting current Ib and the direct current in the transformer having a capacity of 10 kVA [the same figure (Fig. a)] and the excitation current waveform, it is clear that the experimental waveform and the calculated waveform [(b) in the figure] are almost completely identical, whereby the peak value m is calculated by the arithmetic circuit 7 and corresponds to it. DC component Ia
Can be detected.

The embodiment shown in FIG. 5 has the same system configuration as that of the embodiment shown in FIG. 1, that is, a distributed power source including a DC power source 1 and an inverter 2 is connected to a transformer 3 such as a pole transformer.
It is assumed that the system configuration is linked to the system power supply 4 via the.

The feature of this embodiment is that a current detector 5 is provided on the system power supply side of the transformer 3 connected to the output side of the inverter 2, and the current detector 5 causes the current to flow to the system power supply side of the transformer 3. The exciting current Ib is measured, the exciting current Ib is subjected to frequency [Fourier] analysis, and the exciting current Ib is converted into the exciting current Ib by the biased excitation of the transformer 3 caused by the direct current component Ia flowing into the inverter output side of the transformer 3. An arithmetic circuit 9 is provided for calculating the content rate of the expressed [for example, second and third] harmonics b and detecting the DC component Ia corresponding to the content rate as waveform distortion. It should be noted that the third harmonic is steadily present, but the second harmonic is rarely present in the system. Therefore, if the second harmonic is detected, the DC component Ia can be easily detected. In addition, the degree of waveform distortion can be clearly understood by calculating the content rate of the harmonic b.

In the above embodiment, the exciting current Ib is measured by the current detector 5 provided on the system power source side of the transformer 3, and the exciting current Ib is analyzed by the frequency [Fourier] analysis by the arithmetic circuit 9. At this time, if the biased excitation in which the DC component Ia flows into the inverter output side of the transformer 3 occurs, FIG.
As shown in FIG. 5, a harmonic wave b appears in the exciting current Ib flowing on the system power supply side of the transformer 3. If the direct current component Ia does not flow into the inverter output side of the transformer 3, a harmonic wave b appears in the exciting current Ib flowing to the system power source side of the transformer 3 as shown in FIG. 6 (a). do not do.

Here, in general, when the transformer 3 is biasedly excited by the DC component Ia, the harmonic b appearing in the exciting current Ib flowing to the system power source side of the transformer 3 flows into the inverter output side of the transformer. It is proportional to the DC current. Fig. 7 shows three types of transformers [capacity: 10 kVA, 50 kVA, 133 kV
A] is the result of the experiment performed, the figure (a) is the second harmonic, and the figure (b) is the result of the total harmonic. In the figure, the horizontal axis shows the direct current flowing into the transformer as a percentage (%) of the rated alternating current, and the vertical axis shows the second harmonic or the total harmonic when the transformer is in the rated load operating state. Shown as a percentage (%).

As a result, as shown in the figure, the harmonics appearing in the exciting current (second harmonic or total harmonic) and the direct current flowing into the transformer 3 have a linear correspondence. Therefore, the content rate of the higher harmonic wave b is calculated by the calculation circuit 9 to detect the DC component Ia as a waveform distortion.
When the detected value of the DC component Ia exceeds the allowable value, the inverter 2 may be stopped based on the comparison result between the detected value and the allowable value.

[0024]

According to the present invention, the exciting current is measured by the current detector provided on the system power supply side of the transformer, and the bias of the transformer caused by the direct current component flowing into the inverter output side of the transformer is measured. A peak or a harmonic appearing in the exciting current is detected by the excitation, and the content of the peak value or the harmonic is calculated by an arithmetic circuit. As a result, not only the inverter control, but even if the DC excitation flows into the inverter output side of the transformer due to the biased excitation due to other causes such as switch malfunctions, deterioration, and failures, It is possible to provide a grid interconnection system that can be reliably detected and that has high reliability and practical value.

[Brief description of drawings]

FIG. 1 is a circuit block diagram showing an embodiment of a grid interconnection system according to the present invention.

FIG. 2A is a waveform diagram showing a voltage and an exciting current on an inverter output side when a DC component does not flow into a transformer,
(B) Waveform diagram showing voltage and exciting current on the inverter output side when a DC component flows into the transformer

FIG. 3 is a characteristic diagram showing a relationship between a direct current flowing through an inverter output side of a transformer and a peak value appearing in an exciting current on a system power supply side.

FIG. 4A is a characteristic diagram showing data of experimental values and calculated values in relation to a peak value of an exciting current with respect to a DC current,
(B) is a waveform diagram showing the relationship between the experimental waveform of the exciting current and the calculated waveform

FIG. 5 is a circuit block diagram showing another embodiment of the present invention.

FIG. 6A is a waveform diagram showing a voltage and an exciting current at an inverter output side when a DC component does not flow into a transformer,
(B) Waveform diagram showing voltage and exciting current on the inverter output side when a DC component flows into the transformer

FIG. 7 is a characteristic diagram showing the relationship between the DC current flowing on the inverter output side of the transformer and the harmonics appearing in the excitation current on the system power supply side, where (a) is the second harmonic and (b) is the total. It is a harmonic.

FIG. 8 is a circuit block diagram showing an example of a grid interconnection system such as a solar power generation system.

[Explanation of symbols]

 1 DC power supply 2 Inverter 3 Transformer 4 System power supply 5 Current detector 6 Peak detection circuit 7 Arithmetic circuit 9 Arithmetic circuit Ia DC component Ib Exciting current a Peak b Harmonic m Peak value

 ─────────────────────────────────────────────────── ─── Continued Front Page (72) Inventor Kazuto Shibahara 47 Umezu Takaunecho, Ukyo-ku, Kyoto City, Kyoto Prefecture Nissin Electric Co., Ltd.

Claims (2)

[Claims] 1. A system interconnection system in which a transformer is connected to an output side of an inverter for converting a direct current from a direct current power source into an alternating current, and the inverter is connected to a system power source through the transformer. A current detector is provided on the system power supply side of the transformer, the exciting current flowing to the system power supply side of the transformer is measured by the current detector, and the transformer is generated because a DC component flows into the inverter output side of the transformer. A system interconnection system comprising: a peak detection circuit for detecting a peak appearing in the exciting current due to the partial excitation and a calculation circuit for calculating the peak value and detecting a DC component corresponding to the peak value. 2. A system interconnection system in which a transformer is connected to an output side of an inverter for converting a direct current from a direct current power source into an alternating current, and the inverter is interconnected with the system power source through the transformer. A current detector is provided on the side of the system power supply, the exciting current flowing to the side of the system power supply of the transformer is measured by the current detector, and the exciting current is subjected to frequency analysis to obtain a DC component at the inverter output side of the transformer. The system interconnection is characterized in that a computing circuit is provided for computing the content rate of the harmonics appearing in the exciting current by the biased excitation of the transformer caused by the inflow of the current and detecting the DC component corresponding thereto. system.