Power converting apparatus, photovoltaic module, and photovoltaic system including the same

1. A power converting apparatus for converting direct current (DC) power into alternating current (AC) power and outputting the AC power, the apparatus comprising:

an inverter configured to convert the DC power input from a solar cell module or a converter into the AC power and output the AC power to a grid; and

an islanding operation detector configured to calculate a first frequency variation of the AC power output from the inverter, selectively inject reactive power of the solar cell module based on the first frequency variation, and perform control to turn off the inverter based on whether a second frequency variation of a grid voltage of the grid after the injection of the reactive power of the solar cell module is greater than or equal to a preset value,

where the islanding operation detector is configured to:

calculate the first frequency variation corresponding to a difference between a frequency of a current output voltage and a frequency of a previous output voltage; and

not inject the reactive power based on whether the first frequency variation is less than or equal to a reference value.

2. The power converting apparatus of claim 1 , wherein the islanding operation detector is further configured to change a level of the injected reactive power based on the first frequency variation.

3. The power converting apparatus of claim 1 , wherein the islanding operation detector is further configured to change a level of the injected reactive power during a first period of the inverter.

4. The power converting apparatus of claim 3 , wherein the first period of the inverter is determined randomly.

5. The power converting apparatus of claim 3 , wherein the first period of the inverter does not overlap with a first period of an adjacent power converting apparatus.

6. The power converting apparatus of claim 3 , wherein the islanding operation detector is configured to perform control to terminate injecting the reactive power and to output, during a second period, an AC voltage of which the reactive power is not injected based on whether a second frequency variation corresponding to a difference between a frequency of a grid voltage after the injection of the reactive power and a frequency of a grid voltage before the injection of the reactive power is less than the preset value.

7. The power converting apparatus of claim 6 , wherein the islanding operation detector is configured to perform control to maintain output power at a constant level or within a predetermined range based on the constant level during the first period and the second period based on whether the inverter operates.

8. The power converting apparatus of claim 3 , wherein the islanding operation detector is configured to perform control to maintain output power at a constant level or within a predetermined range based on the constant level during the first period based on whether the inverter operates.

9. The power converting apparatus of claim 1 , wherein the islanding operation detector is further configured to randomly inject the reactive power of the solar cell module.

10. The power converting apparatus of claim 1 , wherein the injected reactive power of the solar cell module is different from a reactive power injected by an adjacent power converting apparatus.

11. The power converting apparatus of claim 1 , wherein the islanding operation detector is further configured to:

randomly inject the reactive power of the solar cell module based on the first frequency variation.

12. The power converting apparatus of claim 1 , wherein the islanding operation detector is further configured to perform control to turn off the inverter based on whether a second frequency variation corresponding to a difference between a frequency of a grid voltage after the injection of the reactive power and a frequency of a grid voltage before the injection of the reactive power is greater than or equal to the preset value.

13. The power converting apparatus of claim 1 , wherein the islanding operation detector is configured to perform control to inject the reactive power into the output voltage based on whether the inverter restarts after a turn-off state.

14. The power converting apparatus of claim 1 , further comprising:

a converter configured to convert a level of the DC power input from the solar cell module and output the DC power with the converted level; and

a voltage detector configured to detect a voltage output from the inverter.

15. The power converting apparatus of claim 1 , further comprising a communication unit configured to receive grid voltage information of the grid.

16. A power converting apparatus for converting direct current (DC) power into alternating current (AC) power and outputting the AC power, the apparatus comprising:

an inverter configured to convert the DC power input from a solar cell module or a converter into the AC power and output the AC power to a grid; and

an islanding operation detector configured to calculate a first frequency variation of the AC power output from the inverter, selectively inject reactive power of the solar cell module based on the first frequency variation, and perform control to turn off the inverter based on whether a second frequency variation of a grid voltage of the grid after the injection of the reactive power of the solar cell module is greater than or equal to a preset value,

wherein the islanding operation detector is further configured to perform control to turn off the inverter based on whether a second frequency variation corresponding to a difference between a frequency of a grid voltage after the injection of the reactive power and a frequency of a grid voltage before the injection of the reactive power is greater than or equal to the preset value, and whether the second frequency variation is calculated a predetermined number of times or more to be greater than or equal to the preset value.

17. A photovoltaic module comprising:

a power converting apparatus for converting direct current (DC) power into alternating current (AC) power and outputting the AC power,

wherein the power converting apparatus comprises:

an inverter configured to convert the DC power input from a solar cell module or a converter into the AC power and output the AC power to a grid; and

an islanding operation detector configured to calculate a first frequency variation of the AC power output from the inverter, selectively inject reactive power of the solar cell module based on the first frequency variation, and perform control to turn off the inverter based on whether a second frequency variation of a grid voltage of the grid after the injection of the reactive power of the solar cell module is greater than or equal to a preset value,

wherein the islanding operation detector is configured to:

calculate the first frequency variation corresponding to a difference between a frequency of a current output voltage and a frequency of a previous output voltage; and

not inject the reactive power based on whether the first frequency variation is less than or equal to a reference value.

18. A photovoltaic system comprising:

a plurality of photovoltaic modules connected in parallel with one another and configured to output an alternating current (AC) voltage to a grid,

wherein the photovoltaic module includes a power converting apparatus, and

wherein the power converting apparatus comprises:

an inverter configured to convert DC power input from one solar cell module of the plurality of solar cell modules or a converter into AC power and output the AC power to the grid; and

an islanding operation detector configured to calculate a first frequency variation of the AC power output from the inverter, selectively inject reactive power of the one solar cell module based on the first frequency variation, and perform control to turn off the inverter based on whether a second frequency variation of a grid voltage of the grid after the injection of the reactive power of the one solar cell module is greater than or equal to a preset value,

wherein the islanding operation detector is configured to:

calculate the first frequency variation corresponding to a difference between a frequency of a current output voltage and a frequency of a previous output voltage; and

not inject the reactive power based on whether the first frequency variation is less than or equal to a reference value.

19. The photovoltaic system of claim 18 , further comprising a gateway configured to transmit information regarding the grid voltage of the grid to the power converting apparatus.