JPS6349455B2 - - Google Patents

Description

[Detailed Description of the Invention] The present invention detects a DC ground fault in a power supply circuit that connects a solar cell, fuel cell, storage battery, or other DC power source with a relatively large ground capacity to an AC system via an inverter for grid-connected operation. This relates to a device for

Taking solar cells as an example, the power obtained from them is unstable due to fluctuations in solar radiation, so as mentioned above, it is necessary to connect to the existing commercial AC system to supply stable power to the load. A driving method has been proposed.

In such an operating system, if there is a ground fault in the outdoor solar cell, it is necessary to interrupt the ground fault current by disconnecting the solar cell from the grid or short-circuiting the solar cell.

By the way, in order to detect a ground fault in a DC circuit at an early stage when a ground fault occurs, the following methods have been considered.

First, as shown in FIG. 1, the AC system 6 whose one end is grounded and the inverter 4 are insulated by the output transformer 5, and the midpoint M of the voltage dividing resistors 2a and 2b connected in parallel to the solar cell 1 Relay 3 between and the earth
Connect it. When a ground fault occurs in the solar cell 1, a potential difference is generated between the voltage dividing point M and the ground, and as a result, the relay 3 is activated. In this way, in the conventional system using voltage dividers 2a, 2b and relay 3, insulation by transformer 5 is essential. In other words, the transformer 5 serves to separate the potentials of the solar cell 1 so that the two poles of the solar cell 1 are not connected to the ground point on the grid 6 side due to the operation of the inverter 4. However, since this increases the size of the system, increases the price, and reduces operational efficiency, it is required to omit the transformer 5, especially in small-scale systems for general household use.

Therefore, various methods have been studied that can detect DC ground faults without problems even though a transformerless inverter is used as the inverter 4.

By the way, as shown in FIGS. 2a and 2b, a single-phase transformerless inverter 13 is connected to the solar cell 11 via a backflow prevention diode 12, and the power converted into DC by this inverter is passed through an AC (alternating current) filter 14. In the circuit that feeds the AC system 15 via the ground potentials X, Y, and
Regarding X' and Y', X is as shown in A in FIG. 3, Y is in B in the same figure, X' is in a in FIG. 4, and Y' is in b in the same figure. In addition, in FIG. 2, a shows the case where the filter reactor is not inserted into the ground side of the system 15, and b shows the case where the filter reactor is inserted.

As can be seen from FIGS. 2 to 4, the ground potential of the solar cell 11 changes due to the switching operation of the inverter 13.

On the other hand, the solar cell 11 has a certain ground capacity because objects at ground potential, such as the panel frame and mounting frame, are close to the cell. Therefore, when switching the inverter 13, the solar cell There is a current flowing from 11 to the earth.

Therefore, in order to detect a DC ground fault, it is necessary to distinguish between a current caused by ground capacity and a current caused by an accident such as electric shock or leakage, and to extract and detect only the latter.

Further, to explain this in more detail with reference to FIG. 5, if a ground fault occurs at the midpoint A of the solar cells 11 and 11' connected in series, the current flowing to the point B of the grounding resistor 19 will flow to the point A. is determined by the ground potential and ground resistance, and a differential current is generated at point C in the DC circuit.

However, as described above, since the circuit of the solar cell 11 has a ground capacity, the current at point C is a composite value of the ground fault current and the current passing through the capacity.

Figure 6A shows the voltage to ground at point A of the circuit shown in Figure 5 above, B shows the ground fault current at point B, and the difference current at point C in Figure 5 is as shown in Figure 6C when there is a ground fault, and when it is normal. It becomes as shown in Fig. 6 C'.

Of the current waveforms C and C', the current near the switching time of the inverter 13 is due to the capacitance and flows regardless of the presence or absence of an accident. It is necessary to remove the current caused by the capacitance before making a judgment.

As an example, as shown in FIG.
A conceivable method is to insert a detector 16 made of a current transformer at the point, and apply the difference current waveform at point C detected by the detector 16 to a low-pass filter (LPF) 17 before guiding it to the comparator 18.

However, if the low-pass filter 17 is made to completely detect only the DC component in order to avoid the influence of the capacitance passing current, a ground fault will occur at point A, which is near the voltage midpoint of the solar cells 11 and 11' group. Since the direct current component of the current also becomes zero, it becomes undetectable.

The purpose of the present invention is to eliminate such inconveniences and to avoid the influence of capacitance passing current even when a transformerless inverter is used in a power supply circuit that connects a DC power supply with a large ground capacity to an AC system via an inverter. It is an object of the present invention to provide a detection circuit for a DC ground fault, which is capable of detecting a ground fault and has no point where the ground fault cannot be detected.

However, according to the present invention, the purpose of the lever is to connect a DC power source with a relatively large stray capacitance to the ground to an AC system, which is a ground system, via an inverter that does not have an output transformer. In order to detect a ground fault, a current detector detects the difference in current between power supply lines on the DC side or the AC side, and an abnormality judgment circuit receives the output signal of the current detector and performs a ground fault determination. The circuit is inserted before the abnormality judgment circuit, and according to the signal from the drive pulse forming circuit of the inverter, the output signal of the current detector is activated for a short time each time a switching operation is performed in the inverter. This is achieved by including a switching circuit that prevents the signal from being transmitted to the abnormality determination circuit.

Embodiments of the present invention will be described in detail below with reference to the drawings.

FIG. 7 is a circuit diagram showing an embodiment of the present invention.
Components that are the same as those in FIG. 1 and FIG. 5 are given the same reference numerals.

In the figure, 11 is a solar cell, 12 is a backflow prevention diode, 13 is an inverter, 14 is an AC filter, 1
5 shows a commercial AC system, and in the power supply circuit that converts the power of the solar cell 11 from DC to AC with the inverter 13 and sends it to the AC system 15 via the AC filter 14, a transformerless inverter is used for the inverter 13, Also, during the input wiring from the solar cell 11 to the inverter 13, a detector 1 consisting of a DC current transformer is used to detect the difference current between the two polar wirings.
Insert 6.

In addition, this detector 16 uses an AC current transformer as another example, and connects two wires at once during wiring between the inverter 13 and the AC filter 14 or between the AC filter 14 and the AC system 15. It may also be arranged so that it passes through.

This is because, in the case of a transformerless system, a ground fault current in a DC circuit not only becomes an unbalanced current in the DC circuit, but also an unbalanced current in an AC circuit.

In the figure, reference numeral 21 denotes a pulse forming circuit for driving the inverter 13. From the pulse forming circuit 21, a signal corresponding to the on/off time of all the switching elements in the inverter 13 is obtained, and the switching circuit 22 is opened and closed using this signal. do.

The switching circuit 22 is formed using a field effect transistor or an analog switch, and the opening/closing command signal from the pulse forming circuit 21 is, for example, compared with a control signal and a carrier wave signal in a PWM (pulse width modulation) inverter, and this output is sent to each switching element. It is sufficient to obtain it from the output of such a PWM modulator, or it may be obtained by combining the signals after pulse distribution as the drive signal for each arm, or by adding a timer circuit and changing the time width. be.

A detection signal of the difference current between the bipolar wires of the DC side circuit or the difference current between the two wires of the AC side circuit detected by the detector 16 is led to the abnormality judgment circuit 23 via the switching circuit 22. The switching circuit 22 is turned off for a short period of time corresponding to the switching period of the inverter 13 and the charging/discharging period due to the ground capacitance immediately thereafter in order to prevent the transmission of the current component flowing through the stray capacitance. The abnormality determination circuit 23 issues a ground fault detection signal, for example, when the input absolute value exceeds a certain value. Based on this ground fault detection signal, a trip command is given to a switch disposed between the system 15 and the inverter 13, for example.

Figure 8 shows the operating waveforms of the circuit in Figure 7.
A and B are transistor drive signals output from the pulse forming circuit 21, and C is the transistor driving signal output from the pulse forming circuit 2.
1 to the switching circuit 22, D is the output signal from the detector 16 at the time of a ground fault, and E is the output signal from the switching detection 22 to the abnormality determination circuit 23.

As a result of adopting such a configuration in the detection circuit of the present invention, the current caused by the operation of the switching elements of the inverter and other fault currents are temporally differentiated, so that an accident may occur if a person comes into contact with the circuit. Even when current flows, it is known that there is an unbalanced current in the DC circuit during periods other than the switching period and its vicinity, and necessary protection operations can be performed appropriately and quickly.

Further, since the switching operation of the inverter occurs at least twice in one cycle of the system voltage, abnormality can be determined in a period of less than one cycle of the system frequency.

Although the above-mentioned embodiment shows an example of a single-phase inverter using transistors, the present invention can be applied regardless of the type of switching element, the inverter method of self-excitation or separately excitation, or the difference in the number of output phases. In addition, the present invention can also be applied to a three-phase system (three-phase four-wire system) in which the midpoint between two phases is grounded, in addition to a single-phase system with one wire grounded.

As described above, the DC ground fault detection circuit of the present invention can be used in a power supply circuit that connects a DC power source with a large ground capacity such as a solar battery to an AC system via an inverter, by adopting a transformerless inverter as an inverter. Since the current generated due to fluctuations in the ground potential of the solar cells, etc., is distinguished from the fault current, it is possible to properly detect grounding faults.Also, since the ground potential of the solar cells fluctuates due to the switching operation of the inverter, DC circuits It is capable of detecting grounding at any point.

[Brief explanation of the drawing]

Figure 1 is a circuit diagram of a conventional DC ground fault detection circuit.
Figure 2 is a circuit diagram of a power supply circuit using a transformerless inverter, Figures 3 and 4 are waveform diagrams showing the ground potential of the circuit in Figure 2, and Figure 5 is a circuit diagram showing another conventional example. , FIG. 6 is an operational waveform diagram of the circuit shown in FIG. 5, FIG. 7 is a circuit diagram showing an embodiment of the detection circuit of the present invention, and FIG. 8 is an operational waveform diagram of the same. 11... Solar cell, 12... Backflow prevention diode, 13... Transformerless inverter, 14...
AC filter, 15...AC system, 16...detector, 21...pulse forming circuit, 22...switching circuit, 23...abnormality determination circuit.

Claims (1)

[Claims] 1 In order to detect a DC ground fault in a power supply system in which a DC power supply with a relatively large stray capacitance to the ground is connected to an AC system, which is a grounded system, via an inverter without an output transformer, A current detector that detects the difference in current between power supply lines on the AC side, an abnormality judgment circuit that receives the output signal of this current detector and determines a ground fault, and a circuit that is inserted before this abnormality judgment circuit. According to the signal from the drive pulse forming circuit of the inverter, the output signal of the current detector is prevented from being transmitted to the abnormality judgment circuit for a short time each time a switching operation is performed in the inverter. A DC ground fault detection device characterized by comprising a switching circuit.