Power converter

Abstract

It can safely shut down when a load shedding occurs. [Solution] In the power conversion device 1, the inverter unit 20 converts DC power to AC power and supplies the converted AC power to the load. The control unit 40 controls the inverter unit 20. When the control unit 40 detects that the load has been disconnected from the inverter unit 20, it stops the operation of the inverter unit 20 near the zero-crossing point of the AC output voltage of the inverter unit 20. The control unit 40 may also determine that the load has been disconnected if the instantaneous value of the voltage measured by the voltage sensors VT1 / VT2 exceeds a first predetermined value.

Description

Technical Field

[0001] The present disclosure relates to a power conversion device that is interconnected.

Background Art

[0002] In recent years, solar power generation systems have become widespread. In Japan, a power conditioner for a grid-connected solar power generation system needs to obtain JET (Japan Electrical safety & Environment Technology laboratories) certification. One of the external accident tests during grid connection operation when obtaining JET certification is a load interruption test. As a criterion for the load interruption test, it is stipulated that the maximum value of the voltage after opening is 150% or less of the rated voltage, and the time exceeding 105% is 0.5 seconds or less.

[0003] A method of detecting load interruption using an overvoltage protection function can be considered. When load interruption occurs, the output of the power conditioner becomes high impedance, so the output voltage increases. When the output voltage of the power conditioner reaches the overvoltage protection threshold, the connection relay is disconnected and the inverter section is gate-blocked.

[0004] As another method of detecting load interruption, Patent Document 1 discloses a method of determining that load interruption has occurred when the integrated value of the output voltage of the power conditioner for a half cycle is equal to or greater than a predetermined threshold, disconnecting the switch, and gate-blocking the inverter section.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] If the inverter section is gate-blocked immediately after a load drop is detected, the power conditioner may shut down while the residual voltage is still high. In some cases, the residual voltage after gate blocking may exceed 150% of the rated voltage. This poses a risk of causing malfunctions in other equipment.

[0007] This disclosure is made in view of these circumstances, and its purpose is to provide a power converter that can safely shut down when a load shedding occurs. [Means for solving the problem]

[0008] To solve the above problems, a power converter according to one embodiment of the present disclosure comprises an inverter unit that converts DC power to AC power and supplies the converted AC power to a load, and a control unit that controls the inverter unit. When the control unit detects that the load has been disconnected from the inverter unit, it stops the operation of the inverter unit near the zero-crossing point of the AC output voltage of the inverter unit. [Effects of the Invention]

[0009] According to this disclosure, the power converter can be safely shut down when a load shedding occurs. [Brief explanation of the drawing]

[0010] [Figure 1] This is a diagram illustrating the configuration of a power conversion device according to an embodiment. [Figure 2] This diagram shows the functional blocks related to inverter output control during grid connection in the control unit. [Figure 3] This figure shows an example of the behavior of output current and output voltage before and after load shedding in a comparative example. [Figure 4] This figure shows an example of the behavior of the output current and output voltage before and after load shedding according to the embodiment. [Figure 5] This figure shows an example of the behavior of the output current, output voltage, positive half-wave RMS value of the output voltage, and negative half-wave RMS value of the output voltage before and after load shedding according to the embodiment. [Figure 6] This figure shows an example of the behavior of the voltage command value input to the limiter and the duty cycle value output from the limiter before and after load shedding. [Modes for carrying out the invention]

[0011] Figure 1 is a diagram illustrating the configuration of a power converter 1 according to an embodiment. The power converter 1 according to this embodiment is a power conditioner for a solar power generation system. The power converter 1 is installed between the solar cell 2 and the distribution board 3.

[0012] Solar cell 2 utilizes the photovoltaic effect to directly convert light energy into DC power. Examples of solar cells 2 include silicon solar cells, solar cells made from compound semiconductors, dye-sensitized solar cells, and organic thin-film solar cells.

[0013] The power conversion device 1 comprises, as its main components, a DC / DC converter unit 10, an inverter unit 20, a filter unit 30, relays RY1 and RY2, and a control unit 40.

[0014] The DC / DC converter unit 10 is a converter capable of adjusting the voltage of the DC power output from the solar cell 2. The DC / DC converter unit 10 includes, for example, a boost chopper and can perform MPPT (Maximum Power Point Tracking) control to maximize the output of the solar cell 2.

[0015] The inverter unit 20 converts the DC power output from the DC / DC converter unit 10 into AC power, and supplies the converted AC power to the load connected to the distribution board 3 via the filter unit 30. In this specification, the load refers to the general term for the commercial power system 4 and the in-house load 5. Load interruption refers to a state in which both the commercial power system 4 and the in-house load 5 are electrically disconnected from the power conversion device 1. For example, when the main breaker of the distribution board 3 is turned off, load interruption occurs. Even when a power outage occurs in the commercial power system 4, in a state where power is being supplied from the power conversion device 1 to the in-house load 5 (self-sustaining operation mode), it does not result in load interruption.

[0016] The inverter unit 20 includes a bridge circuit including the first switching element S1 - the fourth switching element S4, and a clamp circuit including the fifth switching element S5 - the sixth switching element S6.

[0017] The bridge circuit includes a first arm in which the first switching element S1 and the second switching element S2 are connected in series, and a second arm in which the third switching element S3 and the fourth switching element S4 are connected in series. The first arm and the second arm are connected in parallel.

[0018] Power semiconductor devices are used for the first switching element S1 - the fourth switching element S4. For example, MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) or IGBT (Insulated Gate Bipolar Transistor) can be used. SiC-MOSFET or GaN-MOSFET may also be used. Freewheeling diodes are formed / connected in antiparallel to the first switching element S1 - the fourth switching element S4, respectively. When an N-channel MOSFET is used, the parasitic diode formed in the direction from the source to the drain can be utilized. When an IGBT is used, it is necessary to externally attach a freewheeling diode in antiparallel. In addition, a Schottky barrier diode may be externally attached to the MOSFET or IGBT.

[0019] The clamping circuit can short-circuit between the first output terminal N1 and the second output terminal N2 of the bridge circuit, and is a circuit capable of switching the conduction direction during short-circuit. The clamping circuit includes a fifth switching element S5 and a sixth switching element S6 connected in series in opposite directions between the first output terminal N1 and the second output terminal N2 of the bridge circuit.

[0020] In the example shown in FIG. 1, an NPN transistor is used for the fifth switching element S5 - the sixth switching element S6. Freewheeling diodes are connected in anti-parallel to the fifth switching element S5 - the sixth switching element S6, respectively. Note that a MOSFET or an IGBT may be used instead of the NPN transistor. Note that in the inverter section 20, the clamping circuit can be omitted.

[0021] The filter section 30 includes a first reactor L1, a second reactor L2, and a capacitor C1, and attenuates the harmonic components of the output voltage and output current of the inverter section 20 to make the output voltage and output current of the inverter section 20 approach a sine wave. The first reactor L1 is inserted in series with the first current line connected to the first output terminal N1 of the inverter section 20. The second reactor L2 is inserted in series with the second current line connected to the second output terminal N2 of the inverter section 20.

[0022] The capacitor C1 is connected in parallel with the inverter section 20 between the first current line and the second current line. The capacitor C1 is connected to the output sides of the first reactor L1 and the second reactor L2.

[0023] In this embodiment, a single-phase three-wire system is adopted, and the power conversion device 1 includes an output terminal block including an L1 terminal, an L2 terminal, and an N terminal. The first current line is connected to the L1 terminal of the output terminal block, the second current line is connected to the L2 terminal of the output terminal block, and the neutral line is connected to the N terminal. The neutral line is grounded.

[0024] The first relay RY1 is connected between the filter section 30 of the first current line and the output terminal block. The second relay RY2 is connected between the filter section 3 of the second current line and the output terminal block.

[0025] The first current sensor CT1 is installed between the inverter section 20 and the first reactor L1 of the first current line. The second current sensor CT2 is installed between the first reactor L1 and the first relay RY1 of the first current line. Alternatively, the first current sensor CT1 and the second current sensor CT2 may be installed on the second current line.

[0026] The first current sensor CT1 measures the current IL flowing through the first reactor L1 and outputs the measured current IL to the control unit 40. The second current sensor CT2 measures the output current Iout of the power converter 1 and outputs the measured output current Iout to the control unit 40. The first current sensor CT1 and the second current sensor CT2 can each be current sensors using Hall elements or current sensors using shunt resistors, respectively.

[0027] The first voltage sensor VT1 measures the output voltage Vout of the L1 phase between the first current line and the neutral line, and outputs the measured L1 phase output voltage Vout to the control unit 40. The second voltage sensor VT2 measures the output voltage Vout of the L2 phase between the second current line and the neutral line, and outputs the measured L2 phase output voltage Vout to the control unit 40. The first voltage sensor VT1 and the second voltage sensor VT2 are each composed of a resistive voltage divider circuit and a differential amplifier.

[0028] The first voltage sensor VT1 and the second voltage sensor VT2 are connected between relays RY1 and RY2 and the output terminal block. When connected to the grid, the first voltage sensor VT1 and the second voltage sensor VT2 measure the grid voltage (= output voltage Vout of power converter 1), when relays RY1 and RY2 are disconnected, they measure the grid voltage, and when operating independently, they measure the output voltage Vout of power converter 1.

[0029] The control unit 40 controls the entire power conversion device 1. Specifically, the control unit 40 controls the on / off state of the switching elements (not shown) included in the DC / DC converter unit 10, the first switching elements S1 to the sixth switching elements S6, and relays RY1 and RY2 included in the inverter unit 20. The control unit 40 is composed of a microcontroller, a gate driver IC, and other analog elements.

[0030] When the control unit 40 detects a load interruption (the output of the inverter unit 20 is in a high impedance state), it interrupts the gate signals of the first switching element S1 to the sixth switching element S6 to stop the operation of the inverter unit 20 (gate block) and disconnects relays RY1 and RY2. Normally, power semiconductor elements have a faster response speed than relays, so relays RY1 and RY2 are disconnected after the gate block.

[0031] Figure 2 shows the functional blocks of the control unit 40 related to inverter output control when connected to the grid. The control unit 40 includes a subtraction unit 41, a PI compensation unit 42, a multiplication unit 43, an addition unit 44, a limiter 45, and a PWM generation unit 46.

[0032] The subtraction unit 41 subtracts the instantaneous value of the output current Iout measured by the second current sensor CT2 from the current command value Iref (current target value). The current command value Iref is set to, for example, 10A. The PI compensation unit 42 generates a control value by performing PI compensation based on the current command value Iref output from the subtraction unit 41 and the measured value Iout, as well as the cumulative value of past errors.

[0033] The multiplication unit 43 multiplies the instantaneous value of the output voltage Vout measured by the first voltage sensor VT1 or the second voltage sensor VT2 by a predetermined constant K. The addition unit 44 adds the control value generated by the PI compensation unit 42 to the output voltage Vout multiplied by the constant K to obtain the voltage command value V * Generates.

[0034] Limiter 45 has a voltage command value V * Set upper and lower limits for the voltage command value V* The voltage command value is limited to a certain range, and the voltage command value limited to a certain range is normalized to a range of 0 to 100% to generate a duty cycle value. The PWM generation unit 46 compares the generated duty cycle value with the carrier wave to generate a PWM signal for driving the first switching element S1 to the fourth switching element S4.

[0035] Figure 3 shows an example of the behavior of the output current Iout and output voltage Vout before and after load shedding in the comparative example. In the comparative example, gate blocking is performed after load shedding occurs due to the overvoltage protection function provided in the power converter 1.

[0036] Generally, the steady-state range for single-phase system voltage is considered to be between 80V and 120V in RMS. Therefore, the instantaneous peak voltage in the steady-state range is approximately 170V (=120 × √2). The overvoltage protection threshold of single-phase power converters 1 is often set in the range of 180V to 190V. The control unit 40 activates the overvoltage protection function and gate blocks the power converter 1 if the output voltage Vout of the power converter 1 exceeds the overvoltage protection threshold once or several times.

[0037] If the voltage rises sharply after a load shedding event and immediately exceeds the overvoltage protection threshold, the gate block will be activated in a short time. Conversely, if the load shedding occurs when the load is small and the voltage is low, the output voltage Vout may not exceed the overvoltage protection threshold and may remain flat at a low voltage for a long period. In this case, the effective voltage after load shedding may exceed 150% of the rated voltage.

[0038] Furthermore, if the gate is blocked immediately after exceeding the overvoltage protection threshold, the inverter unit 20 may stop while residual voltage remains, resulting in a prolonged period of high voltage. In this case as well, the effective voltage after load disconnection may exceed 150% of the rated voltage. Note that the LC resonant waveform of the filter unit 30 is superimposed on the waveform of the output voltage Vout after load disconnection.

[0039] Figure 4 shows an example of the behavior of the output current Iout and output voltage Vout before and after load shedding according to the embodiment. In this embodiment, when the control unit 40 detects load shedding, it stops the operation of the inverter unit 20 near the zero-crossing of the AC output voltage of the inverter unit 20. The control unit 40 stops the operation of the inverter unit 20 near the next zero-crossing after detecting load shedding. This makes it possible to gate block early while avoiding the output voltage Vout remaining at a high voltage for a long period of time, as shown in Figure 3.

[0040] The control unit 40 can determine that a load shedding has occurred if the instantaneous value of the output voltage Vout measured by the first voltage sensor VT1 or the second voltage sensor VT2 exceeds a first predetermined value (condition 1). The first predetermined value is set to a value (for example, 171Vpk) which is the peak value in the steady-state range of the single-phase system voltage plus a margin.

[0041] The control unit 40 calculates the effective value of the output voltage Vout measured by the first voltage sensor VT1 or the second voltage sensor VT2 for half a cycle, and if the calculated effective value exceeds a second predetermined value (condition 2), it can determine that a load shedding has occurred. The second predetermined value is set to a value (for example, 121Vrms) which is the upper limit of the effective value in the steady range of the single-phase system voltage plus a margin.

[0042] Figure 5 shows an example of the behavior of the output current Iout, output voltage Vout, positive half-wave RMS value of output voltage Vout, and negative half-wave RMS value of output voltage Vout before and after load shedding according to the embodiment. The negative half-wave RMS value of output voltage Vout for the half-cycle including the timing of load shedding is 117.46 Vrms, and the positive half-wave RMS value for the next half-cycle is 129.1 Vrms. Load shedding is determined to have occurred at the end of this half-cycle. Note that in Figure 5, the waveform of the gate block GB is drawn with a delay due to the response delay of the observation equipment, but in reality, the gate block is performed at the timing when the output voltage Vout becomes flat.

[0043] In Condition 2, by determining whether or not load shedding has occurred based on the RMS voltage value every half cycle, load shedding can be detected earlier compared to when load shedding has occurred based on the RMS voltage value every full cycle, and the risk of the RMS voltage value after load shedding exceeding 150% of the rated voltage can be reduced.

[0044] The control unit 40 can determine that a load interruption has occurred if the instantaneous value of the output current Iout measured by the second current sensor CT2 remains near zero for a predetermined period (condition 3). The range of the output current Iout near 0A may be set to a range of 1 / 10 or less of the rated current of the power converter 1. The predetermined period is set, for example, to about 1 / 4 of the system frequency. In this case, if the state in which the output current Iout is 1Arms or less continues for 1 / 4 of the system frequency, the control unit 40 determines that a load interruption has occurred. Note that when using condition 3 alone, it is assumed that the output current Iout before load interruption is 1.5Arms or more.

[0045] In condition 3, it is necessary to use the output current Iout measured by the second current sensor CT2. After the load is cut off and before the inverter unit 20 is gate-blocked, current flows between the first reactor L1 and the second reactor L2. Therefore, after the load is cut off and until the inverter unit 20 is gate-blocked, the current measured by the first current sensor CT1 will not be 0A.

[0046] In condition 3, the presence or absence of load shedding is determined based on whether the instantaneous current value remained near zero for approximately 1 / 4 of a cycle. Compared to the case where the presence or absence of load shedding is determined based on the effective current value for one cycle, this method allows for earlier detection of load shedding and reduces the risk that the effective voltage value after load shedding will exceed 150% of the rated voltage.

[0047] The control unit 40 may determine that a load interruption has occurred if both condition 1 and condition 3 are met. The first predetermined value (e.g., 171Vpk) is set to a value lower than the existing overvoltage protection threshold (e.g., 180V~190V), and a voltage anomaly is detected immediately when the output voltage Vout exceeds the peak value in the steady-state range of the system voltage. However, there is a possibility of misjudging a momentary voltage rise that is not a load interruption as a load interruption occurring.

[0048] Furthermore, in FRT (Fault Ride Through) testing, if the remaining voltage is 0%, a section will occur where the current becomes 0A due to the gate block. For short-term voltage drops or power outages, it is required to maintain grid connection for a certain period without disconnecting relays RY1 and RY2. If it is determined that load shedding has occurred simply by satisfying condition 3, and relays RY1 and RY2 are disconnected, it may be possible to fail to meet the FRT requirements.

[0049] If a load shedding is determined to have occurred only when both conditions 1 and 3 are met, the risk of misjudgment can be reduced. Since load shedding is accompanied by an increase in output voltage Vout, adding condition 1 to condition 3 allows it to be distinguished from short-term voltage drops or power outages.

[0050] The control unit 40 may determine that a load shedding has occurred if both conditions 2 and 3 are met. Determining that a load shedding has occurred when both conditions 2 and 3 are met also reduces the risk of misjudgment. Since load shedding is accompanied by an increase in the output voltage Vout, adding condition 2 to condition 3 makes it possible to distinguish it from a short-term voltage drop or power outage.

[0051] As shown in Figure 2, the control unit 40 calculates the duty cycle of the drive signals (specifically, PWM signals) for the first switching element S1 to the fourth switching element S4 included in the inverter unit 20 so that the instantaneous value of the current measured by the first current sensor CT1 or the second current sensor CT2 matches the current target value. The control unit 40 can determine that a load interruption has occurred if the calculated duty cycle value remains at its maximum value (specifically, 100%) for a predetermined period of time (condition 4).

[0052] Figure 6 shows the voltage command value V input to the limiter 45 in Figure 2 before and after load shedding. * This figure shows an example of the behavior of the duty cycle value output from limiter 45. Voltage command value V * In the intervals where the current exceeds the upper limit (lim) or falls below the lower limit (lim), the duty cycle remains fixed at 100%. When a load shedding occurs, the error between the instantaneous current value measured by the first current sensor CT1 or the second current sensor CT2 and the current target value increases, causing the duty cycle to reach 100% in a short period of time.

[0053] Condition 4 can also be used when the second current sensor CT2 is not installed. As described above, after load shedding, the current measured by the first current sensor CT1 will not be 0A until the inverter unit 20 is gate-blocked. Therefore, if the second current sensor CT2 is not installed, it is difficult to use condition 3. In this respect, condition 4 can detect the occurrence of load shedding early and with high accuracy by adjusting the predetermined period during which the duty cycle value remains at 100% to an optimal value.

[0054] The control unit 40 may determine that a load interruption has occurred if both condition 1 and condition 4 are met, or it may determine that a load interruption has occurred if both condition 2 and condition 4 are met.

[0055] The control unit 40 can determine that a load interruption has occurred if the instantaneous value of the output voltage Vout measured by the first voltage sensor VT1 or the second voltage sensor VT2 exceeds the overvoltage protection threshold (e.g., 180V to 190V) (condition 0). Condition 0 is an existing determination condition. If condition 2 is not used, there may be cases where a load interruption is determined to have occurred by satisfying condition 0.

[0056] As described above, according to this embodiment, after detecting load shedding, the power converter 1 can be safely stopped by gate blocking near the zero-crossing of the output voltage Vout. When conditions 1 and 3 are used as an AND condition, both early detection of load shedding and ensuring detection accuracy can be achieved. The same applies when conditions 2 and 3 are used as an AND condition.

[0057] The present disclosure has been described above based on embodiments. The embodiments are illustrative, and it will be understood by those skilled in the art that various modifications are possible in combinations of their components and processing processes, and that such modifications are also within the scope of the present disclosure.

[0058] In the embodiment described above, a power converter 1 connected to a solar cell 2 was assumed. However, instead of the solar cell 2, another type of DC power source, such as a storage battery or fuel cell, may be connected. When a storage battery is connected, a bidirectional DC / DC converter capable of charging and discharging the storage battery is used in the DC / DC converter unit 10.

[0059] The embodiments may be specified by the following items.

[0060] [Item 1] An inverter unit (20) that converts DC power to AC power and supplies the converted AC power to the loads (4 and 5), A control unit (40) that controls the inverter unit (20), Equipped with, The power converter (1) is characterized in that, when the control unit (40) detects that the loads (4 and 5) have been disconnected from the inverter unit (20), it stops the operation of the inverter unit (20) near the zero-crossing point of the AC output voltage of the inverter unit (20). According to this, the system can be safely shut down when a load shedding occurs. [Item 2] The inverter unit (20) is further equipped with voltage sensors (VT1 / VT2) for measuring the output voltage, The power conversion device (1) according to item 1, characterized in that the control unit (40) determines that the load (4 and 5) has been shut off when the instantaneous value of the voltage measured by the voltage sensor (VT1 / VT2) exceeds a first predetermined value. According to this, load shedding can be detected early. [Item 3] The inverter unit (20) is further equipped with voltage sensors (VT1 / VT2) for measuring the output voltage, The power conversion device (1) according to item 1, characterized in that the control unit (40) calculates the effective value of half a cycle of the voltage measured by the voltage sensors (VT1 / VT2), and determines that the loads (4 and 5) have been shut off if the calculated effective value exceeds a second predetermined value. According to this, load shedding can be detected early. [Item 4] The inverter unit (20) is further equipped with a current sensor (CT2) for measuring the output current, The power conversion device (1) according to item 1, characterized in that the control unit (40) determines that the load (4 and 5) has been shut off if the instantaneous value of the current measured by the current sensor (CT2) remains near zero for a predetermined period of time. According to this, load shedding can be detected early while ensuring detection accuracy. [Item 5] Voltage sensors (VT1 / VT2) for measuring the output voltage of the inverter unit (20), A current sensor (CT2) measures the output current of the inverter unit (20), Furthermore, The power conversion device (1) according to item 1, characterized in that the control unit (40) determines that the load (4 and 5) has been shut off when the instantaneous value of the voltage measured by the voltage sensor (VT1 / VT2) exceeds a first predetermined value and the instantaneous value of the current measured by the current sensor (CT2) remains near zero for a predetermined period of time. According to this, load shedding can be detected early while ensuring detection accuracy. [Item 6] Voltage sensors (VT1 / VT2) for measuring the output voltage of the inverter unit (20), A current sensor (CT2) measures the output current of the inverter unit (20), Furthermore, The power conversion device (1) according to item 1, characterized in that the control unit (40) calculates the effective value of half a cycle of the voltage measured by the voltage sensors (VT1 / VT2), and determines that the loads (4 and 5) have been shut off if the calculated effective value exceeds a second predetermined value and the instantaneous value of the current measured by the current sensor (CT2) remains near zero for a predetermined period of time. According to this, load shedding can be detected early while ensuring detection accuracy. [Item 7] The inverter unit (20) is further equipped with current sensors (CT1 / CT2) for measuring the output current, The power conversion device (1) according to item 1, characterized in that the control unit (40) calculates the duty cycle of the drive signals of the switching elements (S1-S4) included in the inverter unit (20) so that the instantaneous value of the current measured by the current sensors (CT1 / CT2) matches the target value, and determines that the load (4 and 5) has been shut off if the calculated duty cycle value remains at its maximum value for a predetermined period of time. According to this, even if the current sensor is not installed on the output side of the interconnection relay, load updraft can be detected based on the current measurement value. [Item 8] The power converter (1) according to item 1, characterized in that the control unit (40) stops the operation of the inverter unit (20) near the next zero-crossing after detecting that the loads (4 and 5) have been disconnected from the inverter unit (20). According to this, the operation of the inverter unit (20) can be stopped safely and early. [Explanation of Symbols]

[0061] 1 Power converter, 2 Solar cell, 3 Distribution board, 4 Commercial power grid, 5 In-house load, 10 DC / DC converter section, 20 Inverter section, 30 Filter section, 40 Control section, 41 Subtraction section, 42 PI compensation section, 43 Multiplication section, 44 Addition section, 45 Limiter, 46 PWM generation section, L1-L2 Reactor, C1 Capacitor, RY1-RY2 Relay, S1-S6 Switching element, CT1-CT2 Current sensor, VT1-VT2 Voltage sensor.

Claims

1. An inverter unit that converts DC power to AC power and supplies the converted AC power to the load, A control unit that controls the inverter unit, Equipped with, The power conversion device is characterized in that, when the control unit detects that the load has been disconnected from the inverter unit, it stops the operation of the inverter unit near the zero-crossing point of the AC output voltage of the inverter unit.

2. The inverter unit is further equipped with a voltage sensor for measuring the output voltage, The power conversion device according to claim 1, characterized in that the control unit determines that the load has been shut off when the instantaneous value of the voltage measured by the voltage sensor exceeds a first predetermined value.

3. The inverter unit is further equipped with a voltage sensor for measuring the output voltage, The power conversion device according to claim 1, characterized in that the control unit calculates the effective value of half a cycle of the voltage measured by the voltage sensor, and determines that the load has been shut off if the calculated effective value exceeds a second predetermined value.

4. The inverter section is further equipped with a current sensor for measuring the output current, The power conversion device according to claim 1, characterized in that the control unit determines that the load has been shut off when the instantaneous value of the current measured by the current sensor remains near zero for a predetermined period of time.

5. A voltage sensor for measuring the output voltage of the inverter section, A current sensor for measuring the output current of the inverter section, Furthermore, The power conversion device according to claim 1, characterized in that the control unit determines that the load has been shut off when the instantaneous value of the voltage measured by the voltage sensor exceeds a first predetermined value and the instantaneous value of the current measured by the current sensor remains near zero for a predetermined period of time.

6. A voltage sensor for measuring the output voltage of the inverter section, A current sensor for measuring the output current of the inverter section, Furthermore, The power conversion device according to claim 1, characterized in that the control unit calculates the effective value of half a cycle of the voltage measured by the voltage sensor, and determines that the load has been shut off if the calculated effective value exceeds a second predetermined value and the instantaneous value of the current measured by the current sensor remains near zero for a predetermined period of time.

7. The inverter section is further equipped with a current sensor for measuring the output current, The power conversion device according to claim 1, characterized in that the control unit calculates the duty cycle of the drive signal of the switching element included in the inverter unit so that the instantaneous value of the current measured by the current sensor matches a target value, and determines that the load has been shut off if the calculated duty cycle value remains at its maximum value for a predetermined period of time.

8. The power conversion device according to claim 1, characterized in that the control unit stops the operation of the inverter unit near the next zero-crossing after detecting that the load has been disconnected from the inverter unit.

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