JP3127250B2 - Isolation prevention device for grid-connected inverter - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a grid-connected inverter that converts generated power from a solar cell or the like into an alternating current and supplies the load to a load connected to a commercial power system. The present invention relates to an islanding prevention device for a grid-connected inverter that prevents occurrence of power generation.

[0002]

2. Description of the Related Art FIG. 3 shows a configuration example of a conventional grid-connected inverter. In FIG. 3, reference numeral 1 denotes a solar cell, 2 denotes a power converter, 3 denotes a voltage relay, 4 denotes a frequency relay, and 5 denotes a switch. , 6 is a commercial power system, and 7 is a load. Normally, DC power generated by the solar cell 1 is converted into AC by the power converter 2 and supplied to the load 7 in connection with the commercial power system 6. Since the power converter 2 has its AC output connected to a commercial power system having an extremely low impedance, the power converter 2 generally employs a current control method and operates as a so-called current source. That is, by directly controlling the amplitude of the AC current of the power converter 2 in accordance with the power generated by the solar cell 1, the power generated by the solar cell 1 is converted to AC power.

The alternating current output from the power converter 2 is:
A high power factor having a small reactive component is desired, and usually, control is performed so as to output a sinusoidal current that matches the voltage phase of the commercial power system 6.

In the meantime, in the commercial power system 6, maintenance,
When performing a power outage for inspection, etc., it is necessary to open the switch above the power outage section and make the power outage section no-voltage.
When the grid-connected inverter as shown in FIG. 3 is installed in the power failure section, the power converter 2 is stopped simultaneously with the power failure,
It is necessary to open the switch 5 and disconnect the grid-connected inverter from the commercial power grid. If the power converter 2 does not stop and the switch 5 does not open, a voltage is applied to the power failure section by the grid interconnection inverter, and a very dangerous state occurs.

In order to detect a power failure in the commercial power system and to disconnect the system interconnection inverter, the voltage and frequency of the commercial power system 6 (hereinafter referred to as system voltage, system frequency, and system frequency) are generally controlled by the voltage relay 3 and the frequency relay 4. ). For example, the voltage relay 3 is set to the nominal value of the system voltage ±
If 10% and the frequency relay are set to the nominal value of the system frequency ± 1%, the system voltage or system frequency deviates from this range due to the power failure, and the power failure can be detected.

[0006] However, in rare cases, when the commercial power system 6 fails, the output power of the power converter 2 and the load 7
If the power consumption of both the active and inactive components is balanced, the output of the grid-connected inverter, which should have been a power outage, hardly changes in both voltage and frequency, and charges the load 7 for a long time. There is a so-called islanding operation. In this case, the output voltage of the power converter 2 hardly changes when the effective components are balanced at the time of the power failure, and the output frequency hardly changes when the invalid components are balanced. Therefore, if both are balanced, it is not possible to detect a power failure with either the voltage relay 3 or the frequency relay 4.

Conventionally, in order to solve this,
A method of monitoring the harmonic voltage included in the system voltage (Japanese Patent Publication No. 63-50941) and a method of monitoring the periodic fluctuation of the system frequency (Japanese Patent Application Laid-Open No. 3-242977) have been devised. If it is made good, the problem that a normal malfunction easily occurs is inevitable.

SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned problems, to reliably detect a power outage in a commercial power system, to disconnect a grid-connected inverter, and to achieve a single operation that rarely causes a malfunction. It is to provide a prevention device.

[0009]

In order to achieve the above object, according to the present invention, there is provided a phase shift means for shifting the output current phase of a grid-connected inverter at predetermined time intervals, and said phase shift means being synchronized with the phase shift timing. Means for monitoring a voltage phase change of the commercial power system is provided, and when the voltage phase change exceeds a predetermined level, the grid-connected inverter is disconnected from the commercial power system.

[0010]

According to the isolated operation prevention device of the present invention configured as described above, the output current phase of the grid-connected inverter is constantly shifted at predetermined time intervals. Because it is extremely low, shifting the output current phase of the grid-connected inverter has no effect on the phase of the grid voltage. However, when a power failure occurs, a voltage phase change corresponding to the shift of the current phase occurs due to the impedance of the load. Therefore, if this is monitored, the occurrence of the power failure can be reliably detected.

Further, since the monitoring of the voltage phase change is performed in synchronization with the timing of the phase shift, and masking is performed during the other periods, it is almost impossible to detect a voltage phase change due to other causes and cause a malfunction. Absent. Further, if the power failure is detected only when the voltage phase change is detected continuously a plurality of times, it is possible to make virtually no malfunction occur.

[0012]

FIG. 1 is a diagram showing an embodiment of the present invention, and FIG. 2 is a diagram showing the operation thereof. FIG.
In the power converter 2, 11 is a sine wave generator, 12 is a frequency divider, 13 is a clock, 14 is a timer, 15 is a phase change detector, 16 is a PLL (phase locked loop), and the other components are the same as those of the conventional example. It is.

While the timer 14 is counting, the frequency divider 12 divides the clock 13 by the PLL 16 so that the frequency and phase of the output current of the power converter 2 match the system frequency. The phase change detector 15 is masked so as not to operate. Timer 14 is about 2
When counting up every second, the frequency division ratio of the frequency divider 12 changes, and the output current phase of the power converter shifts, and at the same time, the mask of the phase change detector 15 is released to enable detection.

For example, if the frequency division ratio of frequency divider 12 is always 10
If the division ratio is set to 9 during one cycle in accordance with the zero crossing of the system voltage as the timer 14 counts up, the maximum value of the phase shift is about 36 degrees (60 Hz
Time). However, when the commercial power system 6 is normal,
As mentioned above, its impedance is extremely low,
FIG. As shown in b, the change in the output current phase of the power converter 2 has no effect on the system voltage phase (T2 = T1),
As a result, the phase change detector 15 does not perform detection.

However, if a power failure occurs and a balance state occurs, the change in the output current phase of the power converter 2 is as shown in FIG. As shown by c, the output voltage phase change appears as it is by the load 7, and this can be detected by the phase change detector 15. In this case, the value of the phase change Δφ is obtained by 2π (T1−T2) / T1, and the detection threshold may be set to about 10 degrees.

When the load 7 is a transformer, a rotating machine, or the like, a large inrush current flows when these are turned on, and a phase change of more than 10 degrees may occur in the system voltage even in a normal state. However, the detectable period of the phase change detector 15 is as shown in FIG. As shown in FIG. 4D, only about one to two cycles after the count-up of the timer 14 is considered, and considering that the count-up is every two seconds, there is very little coincidence between the closing timing and the detection timing. It is thought that there is almost no outbreak. Also,
If the phase change detector 15 detects the power failure only when the phase change is detected twice or more consecutively every two seconds, it is possible to virtually eliminate the malfunction without slightly delaying the power failure detection. it can.

[0017]

As described above, according to the present invention, a power failure is detected by constantly shifting the output current phase of the grid-connected inverter every predetermined time and monitoring the change in the voltage phase that occurs at the time of the power failure. Therefore, there is an effect that the isolated operation occurring at the time of the balance power failure can be reliably prevented without malfunction.

[Brief description of the drawings]

FIG. 1 is a diagram showing an embodiment of the apparatus of the present invention.

FIG. 2 is a diagram showing the operation of the apparatus of the present invention.

FIG. 3 is a diagram showing a configuration example of a conventional general system interconnection inverter;

[Explanation of symbols]

 Reference Signs List 1 solar cell 2 power converter 3 voltage relay 4 frequency relay 5 switch 6 commercial power system 7 load 11 sine wave generator 12 frequency divider 13 clock 14 timer 15 phase change detector 16 PLL (phase locked loop)

Claims (1)

(57) [Claims] 1. A system interconnection inverter for supplying electric power to a load in connection with a commercial power system using power generated by a battery as a DC power supply, wherein an output current phase of the system interconnection inverter is shifted at predetermined time intervals. Phase shift means, means for monitoring the voltage phase change of the commercial power system in synchronization with the phase shift timing of the phase shift means, and when the voltage phase change exceeds a predetermined level once or more than once. And a switch for interrupting the connection to the commercial power system.