Power conversion device

1. A power conversion device, comprising:

an inverter circuit including a plurality of semiconductor switching elements, to convert a direct-current (DC) power on a DC side of the power conversion device to an alternating-current (AC) power and output the AC power from an AC side of the power conversion device;

a current detector to detect an AC current output from the AC side;

a control circuit to control operation of the inverter circuit, based on the AC current detected by the current detector; and

a drive circuit to control on and off of the plurality of semiconductor switching elements, in accordance with a control signal from the control circuit, wherein

when the control circuit is in a first control mode of controlling the inverter circuit so that the AC current follows an AC current command value, if the AC current increases greater than a predetermined overcurrent threshold, the control circuit switches from the first control mode to a second control mode to control the inverter circuit under the second control mode, wherein the first control mode is a PWM control mode and the second control mode is an overcurrent mode,

when the control circuit is in the second control mode, the control circuit controls the inverter circuit so that current decrease periods are provided in which the on and off of the plurality of semiconductor switching elements are controlled so that the AC current monotonically decreases, by stopping a first switching operation in which on and off of the plurality of semiconductor switching elements are controlled so that the AC current follows the AC current command value, wherein all of the plurality of switching elements are turned off in each of the current decrease periods of the second control mode,

wherein:

in the second control mode, a current upper limit is set within a range less than the overcurrent threshold and greater than the AC current command value, and

the control circuit provides the current decrease periods in response to a transition from the first control mode to the second control mode, and further provides in the second control mode current increase periods which each end when the AC current reaches the current upper limit at different times, alternately with the current decrease periods in the second control mode.

2. The power conversion device according to claim 1 , wherein

based on a comparison of the AC current with the current upper limit, determines whether to end the second control mode and switch to the first control mode.

3. The power conversion device according to claim 2 , wherein

in the first switching operation, the control signal is generated in accordance with a cyclic control pulse signal for causing the AC current to follow the AC current command value, the cyclic control pulse signal having a duty ratio which varies for each cycle,

in the second control mode, when the AC current reaches the current upper limit during the first switching operation, the control circuit stops the first switching operation and starts a current decrease period of the current decrease periods, and

the control circuit

generates the control pulse signal based on the AC current which is detected by the current detector throughout the first control mode and the second control mode, and

when the AC current is less than the current upper limit at a first timing synchronized with the control pulse signal in the current decrease period during the second control mode, ends the current decrease period and resumes the first switching operation.

4. The power conversion device according to claim 3 , wherein

in the first switching operation when the control circuit is in the second control mode, if the AC current is less than the current upper limit throughout one or more cycles of the control pulse signal, the control circuit ends the second control mode and switches to the first control mode, to control the inverter circuit under the first control mode.

5. The power conversion device according to claim 1 , wherein

when in the second control mode, at a second timing corresponding to a zero-cross point of the AC current command value or an AC voltage on the AC side, the control circuit ends the second control mode, and switches to the first control mode, to control the inverter circuit under the first control mode.

6. The power conversion device according to claim 5 , wherein

the control circuit

generates the control signal throughout the second control mode so that the plurality of semiconductor switching elements are all turned off, and

at a timing corresponding to the zero-cross point of the AC current command value or the AC voltage on the AC side, ends the second control mode, and switches to the first control mode, to control the inverter circuit under the first control mode.

7. The power conversion device according to claim 1 , wherein

the AC side is electrically connected to a commercial system, and

the AC current command value is set in accordance with a sine wave having a same frequency as the commercial system.

8. The power conversion device according to claim 1 , wherein the control circuit ends the second control mode and switches to the first control mode in accordance with the timing corresponding to the zero-cross point of the voltage or current on the AC side.

9. The power conversion device according to claim 1 , wherein the AC current command value cyclically changes.

10. The power conversion device according to claim 1 , wherein the AC current command value is a sine wave.