Patent Description:
There have been known power conversion devices such as: AC-DC conversion devices for converting AC input power into DC power and outputting the DC power; and DC-DC conversion devices for converting a DC input power into a DC power at a different voltage and outputting the DC power. In such power conversion devices, if an output terminal is short-circuited for some reason, excessive short-circuit current flows. The excessive short-circuit current causes failure of the power conversion device.

As a power conversion device capable of preventing short-circuit current from flowing, there is a power conversion device including a short-circuit current interruption unit that is composed of a semiconductor switching element and a diode. In this power conversion device, if the value of output current exceeds a preset threshold value, the semiconductor switching element of the short-circuit current interruption unit is turned off so that short-circuit current is interrupted (see, for example, Patent Document <NUM>).

<CIT> relates to a switching power supply circuit, and more particularly, to a switching power supply circuit having an overcurrent detecting function and an output short circuit detecting function.

<CIT> relates to a switching regulator for outputting a constant voltage, and more particularly, to a switching regulator including a short-circuit detection circuit.

<CIT> relates to a power conversion device and a three-phase power conversion device.

<CIT> relates to a motor driving apparatus for driving a feed axis and a main axis of a machine tool or arms and the like of an industrial machine and an industrial robot.

<CIT> relates to a converter system, a superior converter system, a renewable energy power station, as well as to a method and a controller for operating the converter system and superior converter system.

Patent Document <NUM>:
Japanese Laid-open Patent Publication (translation of PCT application) <CIT>.

Wind power generation systems, photovoltaic power generation systems, or the like have a plurality of windmills or a plurality of solar cell modules connected in parallel. In such a system, DC power is transmitted, and sub DC lines are in parallel to one another and connected to a main DC line serving as a main route for power transmission. The windmills or the solar cell modules are disposed on the sub DC lines. A power conversion device for converting power generated by each windmill or each solar cell module into DC power suitable for power transmission, is connected to the corresponding sub DC line.

If one power conversion device among a plurality of the power conversion devices connected in parallel experiences a short circuit, excessive current flows from the other power conversion devices having experienced no short circuit into the power conversion device having experienced the short circuit. This current is called sneak current. In the case of using conventional power conversion devices including short-circuit current interruption units, in a power conversion device thereamong having experienced a short circuit, the short-circuit current interruption unit thereof is operated so that short-circuit current can be interrupted.

However, also in the other power conversion devices having experienced no short circuit, the short-circuit current interruption units thereof are operated in response to excessive sneak currents so that current routes are disconnected. As a result, the other power conversion devices having experienced no short circuit are also stopped, and thus a problem arises in that the entire system is stopped for a long time.

The present invention, which is defined by independent claim <NUM>, has been made to solve the above problem. In the present invention, even when one power conversion device among a plurality of power conversion devices connected in parallel experiences a short circuit, the other power conversion devices having experienced no short circuit can be promptly restarted.

According to the invention, the problem is solved by the subject matter outlined in independent claim <NUM>. Advantageous further developments of the invention are set forth in the dependent claims.

The power conversion device according to the present invention includes the short circuit elimination determination unit configured to determine whether or not the short circuit at the output terminal has been eliminated. Thus, even when one power conversion device among a plurality of the power conversion devices connected in parallel experiences a short circuit, the other power conversion devices having experienced no short circuit can be promptly restarted.

Hereinafter, power conversion devices according to embodiments for carrying out the present invention will be described in detail with reference to the drawings. The same or corresponding portions in the drawings are denoted by the same reference characters.

<FIG> is a configuration diagram of a wind power generation system as an example of a system to which a power conversion device according to Embodiment <NUM> is applied. This example is a wind power generation system having three windmills connected in parallel. It is noted that the wind power generation system may have four or more windmills connected in parallel. In the wind power generation system, AC power generated by a generator <NUM> in each windmill is converted into DC power by a corresponding AC-DC conversion device <NUM>, and then the DC power is inputted to a corresponding power conversion device <NUM> of the present embodiment.

The power conversion device <NUM> is a DC-DC conversion device. It is noted that the AC-DC conversion device <NUM> has a function of converting AC input power into DC power and outputting the DC power. Meanwhile, the DC-DC conversion device has a function of converting a DC input power into a DC power at a different voltage and outputting the DC power.

A plurality of the power conversion devices <NUM> are in parallel to one another and connected to a main DC line <NUM> via respective sub DC lines <NUM>. As shown in <FIG>, each sub DC line <NUM> is provided with a circuit breaker <NUM>. The main DC line <NUM> is connected to a separate power grid via a large-capacity power conversion device <NUM>. It is noted that, as the system to which the power conversion device of the present embodiment is applied, there are a photovoltaic power generation system, a DC power distribution system that distributes DC power, and the like in addition to the wind power generation system.

<FIG> is a configuration diagram of each power conversion device <NUM> according to the present embodiment. The power conversion device <NUM> of the present embodiment includes: a DC-DC conversion unit <NUM>; an output terminal 3a from which the DC power obtained through conversion by the DC-DC conversion unit <NUM> is outputted to the corresponding sub DC line <NUM>; a current interruption unit <NUM> provided between the DC-DC conversion unit <NUM> and the output terminal 3a and having a function of interrupting current that flows from the DC-DC conversion unit <NUM> to the output terminal 3a; a short circuit occurrence determination unit <NUM> which determines, on the basis of a current value at the sub DC line <NUM>, whether or not a short circuit has occurred at the output terminal 3a; a short circuit elimination determination unit <NUM> which determines, on the basis of a current value and a voltage value at the sub DC line <NUM>, whether or not the short circuit at the output terminal 3a has been eliminated; and a first gate drive unit <NUM> which drives a semiconductor switching element of the current interruption unit <NUM>. It is noted that an output terminal 3b is a terminal at a reference potential for DC power to be outputted to the output terminal 3a.

The DC-DC conversion unit <NUM> includes: a first three-phase bridge circuit <NUM> which converts DC power inputted to a pair of input terminals 3c and 3d into AC power; and a second three-phase bridge circuit <NUM> which converts, into DC power, the AC power obtained through conversion by the first three-phase bridge circuit <NUM>.

The first three-phase bridge circuit <NUM> and the second three-phase bridge circuit <NUM> are connected to each other via a transformation circuit <NUM>. A capacitor <NUM> and a capacitor <NUM> are respectively connected to the input side of the first three-phase bridge circuit <NUM> and the output side of the second three-phase bridge circuit <NUM>. The power conversion device <NUM> further includes a second gate drive unit <NUM> which drives switching elements in the first three-phase bridge circuit <NUM> and the second three-phase bridge circuit <NUM> of the DC-DC conversion unit <NUM>.

The current interruption unit <NUM> includes: a semiconductor switching element <NUM>; a diode <NUM> connected in antiparallel to the semiconductor switching element <NUM>; and a flyback diode <NUM> connected in series to the semiconductor switching element <NUM>. Further, an inductance element <NUM> is connected to a connection point between the semiconductor switching element <NUM> and the flyback diode <NUM>. As the semiconductor switching element <NUM>, for example, an insulated-gate bipolar transistor (IGBT), a gate commutated turn-off thyristor (GCT), a metal-oxide-semiconductor field-effect transistor (MOSFET), or the like can be used.

The semiconductor switching element <NUM> of the current interruption unit <NUM> is constantly kept in an ON state during a normal operation (during power conversion) of the power conversion device <NUM>. Thus, current constantly flows in the semiconductor switching element <NUM> during a normal operation. Therefore, a unipolar element such as MOSFET in which voltage drop is small, is suitable as the semiconductor switching element <NUM>. It is noted that, since the current interruption unit <NUM> is configured with the semiconductor switching element, a switching operation is performed at high speed as compared to the case where the current interruption unit <NUM> is configured with a mechanical switch.

In addition, since the current interruption unit <NUM> includes the flyback diode <NUM>, current that flows to the inductance element <NUM> side can be caused to flow back to the flyback diode <NUM> side when the semiconductor switching element <NUM> is turned off. As a result, surge voltage to be applied to both ends of the semiconductor switching element <NUM> is suppressed, and reliability can be improved. Further, since the current interruption unit <NUM> includes the inductance element <NUM>, increase in short-circuit current that flows until the semiconductor switching element <NUM> is set to be turned off, can be made moderate. Thus, the short-circuit current can be assuredly interrupted.

The sub DC line <NUM> on the output side of the power conversion device <NUM> is provided with: a current sensor <NUM> which detects a current at the sub DC line <NUM>; and a voltage sensor <NUM> which detects a voltage at the sub DC line <NUM>. Specifically, the current value detected by the current sensor <NUM> is a current value at the output terminal 3a, and the voltage value detected by the voltage sensor <NUM> is a voltage value at the output terminal 3a. The current sensor <NUM> outputs the detected current value to the short circuit occurrence determination unit <NUM> and the short circuit elimination determination unit <NUM>. The voltage sensor <NUM> outputs the detected voltage value to the short circuit elimination determination unit <NUM>.

The short circuit occurrence determination unit <NUM> determines, on the basis of the information from the current sensor <NUM>, whether or not a short circuit has occurred at the output terminal 3a. Then, if the short circuit occurrence determination unit <NUM> determines that a short circuit has occurred, the short circuit occurrence determination unit <NUM> turns off the semiconductor switching element <NUM> of the current interruption unit <NUM> via the first gate drive unit <NUM>. By turning off the semiconductor switching element <NUM> of the current interruption unit <NUM>, current that flows from the DC-DC conversion unit <NUM> to the output terminal 3a is interrupted. In other words, the power conversion device <NUM> is stopped.

The short circuit occurrence determination unit <NUM> determines that a short circuit has occurred, if the current value detected by the current sensor <NUM> exceeds a preset overcurrent setting value. The overcurrent setting value is set to a value that is larger than a rated current value of the DC-DC conversion unit <NUM> and smaller than a maximum current value of a reverse bias safe operating area (RBSOA) of the semiconductor switching element <NUM>. By setting the overcurrent setting value to a value that is smaller than the maximum current value of the reverse bias safe operating area, failure of the semiconductor switching element <NUM> can be prevented.

The short circuit elimination determination unit <NUM> determines, on the basis of the information from the current sensor <NUM> and the information from the voltage sensor <NUM>, whether or not the short circuit at the output terminal 3a has been eliminated. Then, if the short circuit elimination determination unit <NUM> determines that the short circuit at the output terminal 3a has been eliminated, the short circuit elimination determination unit <NUM> turns on the semiconductor switching element <NUM> of the current interruption unit <NUM> via the first gate drive unit <NUM>.

By turning on the semiconductor switching element <NUM> of the current interruption unit <NUM>, the interruption of current that flows from the DC-DC conversion unit <NUM> to the output terminal 3a is canceled. It is noted that the phrase "the interruption of current is canceled" means that routes for current are connected to each other. By canceling the interruption of current at the current interruption unit <NUM>, the DC power obtained through conversion by the DC-DC conversion unit <NUM> is outputted to the sub DC line <NUM>. In other words, the power conversion device <NUM> is restarted.

The short circuit elimination determination unit <NUM> determines that the short-circuited state has been canceled, if the current value detected by the current sensor <NUM> is smaller than a preset first current value and the voltage value detected by the voltage sensor <NUM> is larger than a preset first voltage value.

The first current value that is set in the short circuit elimination determination unit <NUM> is set to be, for example, <NUM> % of a rated current value of the sub DC line <NUM>. Meanwhile, the first voltage value that is set in the short circuit elimination determination unit <NUM> is set to be, for example, <NUM> % of a rated voltage value of the sub DC line <NUM>. By setting to the above values, determination as to whether or not the short-circuited state has been canceled can be promptly made.

Next, operations of the power conversion device <NUM> of the present embodiment will be described.

<FIG> is a diagram for explaining sneak currents in a wind power generation system according to the present embodiment. This example is a wind power generation system having three windmills connected in parallel in the same manner as in <FIG>. AC-DC conversion devices 2A, 2B, and 2C are respectively connected in series to generators 1A, 1B, and 1C in the windmills, and power conversion devices 3A, 3B, and 3C of the present embodiment are also respectively connected in series to the generators 1A, 1B, and 1C.

Outputs of the power conversion devices 3A, 3B, and 3C are respectively connected to sub DC lines 6A, 6B, and 6C. The sub DC lines 6A, 6B, and 6C are respectively provided with circuit breakers 4A, 4B, and 4C. The sub DC lines 6A, 6B, and 6C are in parallel to one another and connected to the main DC line <NUM>. The main DC line <NUM> is connected to a separate power grid via the large-capacity power conversion device <NUM>.

It is assumed that, as shown in <FIG>, the power conversion device 3A has failed and experienced a short circuit. When the power conversion device 3A experiences a short circuit, excessive short-circuit currents (sneak currents) flow from the large-capacity power conversion device <NUM> and the other power conversion devices 3B and 3C toward the power conversion device 3A. In <FIG>, the broken arrows indicate the sneak currents. In addition, the flows of the excessive sneak currents lead also to reduction in voltages of the main DC line <NUM> and the sub DC lines 6A, 6B, and 6C.

Since the only device that has failed is the power conversion device 3A, opening of the circuit breaker 4A makes it possible to electrically separate the power conversion device 3A having experienced the short circuit from the main DC line <NUM>. However, sneak currents flow from the power conversion devices 3B and 3C into the power conversion device 3A during a time from a time point immediately after the power conversion device 3A has experienced the short circuit to a time point at which the circuit breaker 4A is opened.

At this time, the short circuit occurrence determination units <NUM> of the power conversion devices 3B and 3C determine that the sneak currents flowing through the sub DC lines 6B and 6C are short-circuit currents, and turn off the semiconductor switching elements <NUM> of the current interruption units <NUM>. Consequently, the sneak currents flowing through the sub DC lines 6B and 6C are interrupted at the current interruption units <NUM>, and thus the circuit breakers 4B and 4C provided to the sub DC lines 6B and 6C are not opened. However, the interruption causes each of the power conversion devices 3B and 3C to be in a stopped state.

After the power conversion device 3A is electrically separated from the main DC line <NUM> by opening the circuit breaker 4A, currents and voltages at the main DC line <NUM> and the sub DC lines 6B and 6C are restored. That is, the current value at the main DC line <NUM> and the current values at the sub DC lines 6B and 6C are returned to normal-state current values smaller than the values of short-circuit currents, and the voltage value at the main DC line <NUM> and the voltage values at the sub DC lines 6B and 6C are returned to normal-state voltage values.

At this time, the short circuit elimination determination units <NUM> of the power conversion devices 3B and 3C determine that the short-circuited states of the sub DC lines 6B and 6C have been canceled, and turn on the semiconductor switching elements <NUM> of the current interruption units <NUM> via the first gate drive units <NUM>. As a result, the power conversion devices 3B and 3C are promptly restarted, and power generated by each of the generators 1B and 1C can be transmitted to the main DC line <NUM>. By using the power conversion devices of the present embodiment in this manner, the period during which power cannot be generated owing to short-circuit failure or the like (hereinafter, written as loss of the opportunity of power generation) can be reduced to minimum in the wind power generation system.

Meanwhile, a case where the power conversion devices 3B and 3C do not include any short circuit elimination determination units, is as follows. The power conversion devices 3B and 3C can interrupt sneak currents by means of the short circuit occurrence determination units. As a result, unnecessary opening of the circuit breakers 4B and 4C provided to the sub DC lines 6B and 6C can be prevented.

However, in each of the power conversion devices 3B and 3C in which short-circuit currents have been interrupted, the semiconductor switching element of the current interruption unit is kept in an OFF state even though a normal operation can be performed. As a result, the power conversion devices 3B and 3C are not restarted, and thus power generated by each of the generators 1B and 1C cannot be transmitted to the main DC line <NUM>. Therefore, the loss of the opportunity of power generation becomes large.

By using the power conversion devices including the short circuit elimination determination units as described above, even when one power conversion device among the plurality of power conversion devices connected in parallel experiences a short circuit, the other power conversion devices having experienced no short circuit can be promptly restarted.

<FIG> is a configuration diagram of the first gate drive unit <NUM>. The first gate drive unit <NUM> receives signals from the short circuit occurrence determination unit <NUM> and the short circuit elimination determination unit <NUM>. If the short circuit occurrence determination unit <NUM> determines that a short circuit has occurred, the first gate drive unit <NUM> receives an OFF signal. Meanwhile, if the short circuit elimination determination unit <NUM> determines that the short circuit has been eliminated, the first gate drive unit <NUM> receives an ON signal.

As shown in <FIG>, the semiconductor switching element <NUM> of the current interruption unit <NUM> is driven by the first gate drive unit <NUM>. The semiconductor switching element <NUM> is constantly kept in an ON state during a normal operation (during power conversion). Thus, even if a gate resistance value <NUM> intrinsic to the first gate drive unit <NUM> is set to be large, increase in switching loss is not caused. Therefore, if the gate resistance value <NUM> intrinsic to the first gate drive unit <NUM> is set to be larger than a gate resistance value intrinsic to the second gate drive unit <NUM>, surge voltage at the time of turning off can be suppressed, and reliability can be improved.

In the power conversion device of the present embodiment, the short circuit occurrence determination unit <NUM> determines, on the basis of a current value at the sub DC line <NUM> detected by the current sensor <NUM>, whether or not a short circuit has occurred, and turns off the semiconductor switching element <NUM> of the current interruption unit <NUM> via the first gate drive unit <NUM>. At this time, a delay occurs among the current sensor <NUM>, the short circuit occurrence determination unit <NUM>, the first gate drive unit <NUM>, and the current interruption unit <NUM>.

Therefore, the semiconductor switching element <NUM> cannot be promptly turned off upon occurrence of a short circuit. The time from a time point at which the short circuit has occurred to a time point at which the semiconductor switching element <NUM> is turned off, is referred to as a delay time. Sneak current increases during the delay time. If the voltage at the sub DC line <NUM> is defined as V, the inductance value of the inductance element <NUM> of the current interruption unit <NUM> is defined as L, and the delay time is defined as T, the amount ΔI of increase in sneak current can be expressed with the following expression (<NUM>).

If the overcurrent setting value that is set in the short circuit occurrence determination unit <NUM> is defined as Ioc and the maximum current value of the reverse bias safe operating area (RBSOA) of the semiconductor switching element <NUM> is defined as Imax, when the inductance value L of the inductance element <NUM> is set so as to satisfy the following expression (<NUM>), the semiconductor switching element <NUM> can be prevented from being damaged.

For example, if the voltage V at the sub DC line <NUM> is assumed to be <NUM> V, the maximum current value Imax of the reverse bias safe operating area (RBSOA) of the semiconductor switching element <NUM> is assumed to be <NUM> A, the overcurrent setting value Ioc is assumed to be <NUM> A, and the delay time T is assumed to be <NUM>, the inductance value L of the inductance element <NUM> only has to be equal to or larger than <NUM>µH.

By using a power conversion device in which setting has been made as described above, the semiconductor switching element can be prevented from being damaged even when short-circuit current flows in the power conversion device.

It is noted that, although an example has been described in which each power conversion device of the present embodiment includes the three-phase bridge circuits for the power conversion unit, another power conversion unit may be used as long as the power conversion unit outputs DC power.

It is desirable that the overcurrent setting value that is set in the short circuit occurrence determination unit is set to a value that is smaller than a breaking current setting value of the circuit breaker provided to the sub DC line to which the power conversion device is connected. If the overcurrent setting value is set to a value that is smaller than the breaking current setting value of the circuit breaker, when sneak current flows, the current interruption unit is operated before the circuit breaker is operated. Thus, the sub DC line can be assuredly prevented from being disconnected by the circuit breaker.

<FIG> is a configuration diagram of a wind power generation system as an example of a system to which a power conversion device according to Embodiment <NUM> is applied. This example is a wind power generation system having the three windmills connected in parallel. In the wind power generation system, AC power generated by each of the generators <NUM> in the windmills is inputted to a corresponding power conversion device <NUM> of the present embodiment.

Each power conversion device <NUM> of the present embodiment is an AC-DC conversion device. The power conversion device <NUM> converts the AC power into DC power. The three power conversion devices <NUM> are in parallel to one another and connected to the main DC line <NUM> via the sub DC lines <NUM>. As shown in <FIG>, each sub DC line <NUM> may be provided with the corresponding circuit breaker <NUM>. The main DC line <NUM> is connected to a separate power grid via the large-capacity power conversion device <NUM>.

<FIG> is a configuration diagram of each power conversion device <NUM> according to the present embodiment. The power conversion device <NUM> of the present embodiment includes: an AC-DC conversion unit <NUM>; the output terminal 3a from which the DC power obtained through conversion by the AC-DC conversion unit <NUM> is outputted to the corresponding sub DC line <NUM>; the current interruption unit <NUM> provided between the AC-DC conversion unit <NUM> and the output terminal 3a and having a function of interrupting current that flows from the AC-DC conversion unit <NUM> to the output terminal 3a; the short circuit occurrence determination unit <NUM> which determines, on the basis of a current value at the sub DC line <NUM>, whether or not a short circuit has occurred at the output terminal 3a; the short circuit elimination determination unit <NUM> which determines, on the basis of a current value and a voltage value at the sub DC line <NUM>, whether or not the short circuit at the output terminal 3a has been eliminated; and the first gate drive unit <NUM> which drives the semiconductor switching element of the current interruption unit <NUM>. It is noted that the output terminal 3b is a terminal at a reference potential for DC power to be outputted to the output terminal 3a.

The AC-DC conversion unit <NUM> includes a two-level three-phase bridge circuit <NUM> which converts, into DC power, three-phase AC power inputted to three input terminals 3c, 3d, and 3e. A capacitor <NUM> is connected to the output side of the three-phase bridge circuit <NUM>. The power conversion device <NUM> further includes the second gate drive unit <NUM> which drives switching elements of the three-phase bridge circuit <NUM> of the AC-DC conversion unit <NUM>.

The sub DC line <NUM> on the output side of the power conversion device <NUM> is provided with: the current sensor <NUM> which detects a current at the sub DC line <NUM>; and the voltage sensor <NUM> which detects a voltage at the sub DC line <NUM>. The current sensor <NUM> outputs the detected current value to the short circuit occurrence determination unit <NUM> and the short circuit elimination determination unit <NUM>. The voltage sensor <NUM> outputs the detected voltage value to the short circuit elimination determination unit <NUM>.

The short circuit occurrence determination unit <NUM> determines, on the basis of the information from the current sensor <NUM>, whether or not a short circuit has occurred at the output terminal 3a. Then, if the short circuit occurrence determination unit <NUM> determines that a short circuit has occurred, the short circuit occurrence determination unit <NUM> turns off the semiconductor switching element <NUM> of the current interruption unit <NUM> via the first gate drive unit <NUM>. By turning off the semiconductor switching element <NUM> of the current interruption unit <NUM>, current that flows from the AC-DC conversion unit <NUM> to the output terminal 3a is interrupted. In other words, the power conversion device <NUM> is stopped.

The short circuit occurrence determination unit <NUM> determines that a short circuit has occurred, if the current value detected by the current sensor <NUM> exceeds a preset overcurrent setting value. The overcurrent setting value is set to a value that is larger than a rated current value of the AC-DC conversion unit <NUM> and smaller than the maximum current value of the reverse bias safe operating area (RBSOA) of the semiconductor switching element <NUM>.

The short circuit elimination determination unit <NUM> determines, on the basis of the information from the current sensor <NUM> and the information from the voltage sensor <NUM>, whether or not the short circuit at the output terminal 3a has been eliminated.

Then, if the short circuit elimination determination unit <NUM> determines that the short circuit at the output terminal 3a has been eliminated, the short circuit elimination determination unit <NUM> turns on the semiconductor switching element <NUM> of the current interruption unit <NUM> via the first gate drive unit <NUM>. By turning on the semiconductor switching element <NUM> of the current interruption unit <NUM>, the interruption of current that flows from the AC-DC conversion unit <NUM> to the output terminal 3a is canceled.

By canceling the interruption of current at the current interruption unit <NUM>, the DC power obtained through conversion by the AC-DC conversion unit <NUM> is outputted to the sub DC line <NUM>. In other words, the power conversion device <NUM> is restarted.

The first current value that is set in the short circuit elimination determination unit <NUM> is set to be, for example, <NUM> % of the rated current value of the sub DC line <NUM>. Meanwhile, the first voltage value that is set in the short circuit elimination determination unit <NUM> is set to be, for example, <NUM> % of the rated voltage value of the sub DC line <NUM>. By setting to the above values, determination as to whether or not the short-circuited state has been canceled can be promptly made.

It is assumed that, as shown in <FIG>, one power conversion device <NUM> among the three power conversion devices <NUM> connected in parallel has failed and experienced a short circuit. When the one power conversion device <NUM> experiences a short circuit, short-circuit currents (sneak currents) flow from the large-capacity power conversion device <NUM> and the other two power conversion devices <NUM> toward the power conversion device <NUM> having experienced the short circuit.

The power conversion device <NUM> having experienced the short circuit includes the current interruption unit <NUM> and the short circuit occurrence determination unit <NUM>, and thus can interrupt the short-circuit current. In addition, since short-circuit current flows from the large-capacity power conversion device <NUM> into the power conversion device <NUM> having experienced the short circuit, the circuit breaker <NUM> connected to the said power conversion device <NUM> is opened. The opening of the circuit breaker <NUM> leads to electrical separation of the power conversion device <NUM> having experienced the short circuit from the main DC line <NUM>.

However, sneak currents flow from the other two power conversion devices into the one power conversion device during a time from a time point immediately after the one power conversion device has experienced the short circuit to a time point at which the circuit breaker <NUM> is opened. At this time, the short circuit occurrence determination units <NUM> of the other two power conversion devices determine that the sneak currents flowing through the sub DC lines <NUM> are short-circuit currents, and turn off the semiconductor switching elements <NUM> of the current interruption units <NUM>. Therefore, the other two power conversion devices having experienced no short circuit are each in a stopped state.

After the power conversion device <NUM> having experienced the short circuit is electrically separated from the main DC line <NUM> by opening the circuit breaker <NUM>, currents and voltages at the main DC line <NUM> and the sub DC lines <NUM> are restored. The short circuit elimination determination units <NUM> of the other two power conversion devices determine that the short circuits at the output terminals 3a have been eliminated, and turn on the semiconductor switching elements <NUM> of the current interruption units <NUM> via the first gate drive units <NUM>.

As a result, the other two power conversion devices are promptly restarted, and power generated by each of the generators can be transmitted to the main DC line <NUM>. By using the power conversion devices of the present embodiment in this manner, the loss of the opportunity of power generation due to short-circuit failure or the like can be reduced to minimum in the wind power generation system.

By using such power conversion devices, even when one power conversion device among a plurality of the power conversion devices connected in parallel experiences a short circuit, the other power conversion devices having experienced no short circuit can be promptly restarted.

It is noted that, if the inductance value L of the inductance element <NUM> of the current interruption unit <NUM> is set so as to satisfy expression (<NUM>), the semiconductor switching element <NUM> can be prevented from being damaged, in the same manner as in Embodiment <NUM>.

In addition, if the gate resistance value intrinsic to the first gate drive unit <NUM> is set to be larger than the gate resistance value intrinsic to the second gate drive unit <NUM>, surge voltage at the time of turning off can be suppressed and reliability can be improved, in the same manner as in Embodiment <NUM>.

<FIG> is a configuration diagram of a power conversion device <NUM> according to Embodiment <NUM>. The power conversion device <NUM> of the present embodiment is a power conversion device used for a wind power generation system or the like, as the power conversion device shown in <FIG> for Embodiment <NUM> is. The power conversion device <NUM> of the present embodiment is a DC-DC conversion device, as the power conversion device of Embodiment <NUM> is. In the power conversion device <NUM> of the present embodiment, two DC-DC conversion units <NUM> and two current interruption units <NUM> are connected in series for the purpose of increasing output voltage.

First gate drive units <NUM> are respectively connected to the two current interruption units <NUM>. In addition, second gate drive units <NUM> are respectively connected to the two DC-DC conversion units <NUM>. The power conversion device <NUM> of the present embodiment further includes: the short circuit occurrence determination unit <NUM> which determines, on the basis of a current value at the sub DC line <NUM>, whether or not a short circuit has occurred at the output terminal 3a; and the short circuit elimination determination unit <NUM> which determines, on the basis of a current value and a voltage value at the sub DC line <NUM>, whether or not the short circuit at the output terminal 3a has been eliminated.

In the power conversion device <NUM> of the present embodiment, the configurations of each DC-DC conversion unit <NUM>, each current interruption unit <NUM>, the short circuit occurrence determination unit <NUM>, the short circuit elimination determination unit <NUM>, each first gate drive unit <NUM>, and each second gate drive unit <NUM> are the same as those in Embodiment <NUM>. It is noted that, although the two DC-DC conversion units <NUM> and the two current interruption units <NUM> are connected in series in the power conversion device shown in <FIG>, three or more DC-DC conversion units and three or more current interruption units may be connected in series.

The short circuit occurrence determination unit <NUM> determines, on the basis of the information from the current sensor <NUM>, whether or not a short circuit has occurred at the output terminal 3a. Then, if the short circuit occurrence determination unit <NUM> determines that a short circuit has occurred, the short circuit occurrence determination unit <NUM> turns off semiconductor switching elements <NUM> of the two current interruption units <NUM> via the first gate drive units <NUM>. By turning off the semiconductor switching elements <NUM> of the current interruption units <NUM>, currents that flow from the two DC-DC conversion units <NUM> to the output terminal 3a are interrupted. In other words, the power conversion device <NUM> is stopped.

The short circuit occurrence determination unit <NUM> determines that a short circuit has occurred, if the current value detected by the current sensor <NUM> exceeds a preset overcurrent setting value. The overcurrent setting value is set to a value that is larger than rated current values of the DC-DC conversion units <NUM> and smaller than maximum current values of reverse bias safe operating areas (RBSOAs) of the semiconductor switching elements <NUM>.

The short circuit elimination determination unit <NUM> determines, on the basis of the information from the current sensor <NUM> and the information from the voltage sensor <NUM>, whether or not the short circuit at the output terminal 3a has been eliminated. Then, if the short circuit elimination determination unit <NUM> determines that the short circuit at the output terminal 3a has been eliminated, the short circuit elimination determination unit <NUM> turns on the semiconductor switching elements <NUM> of the two current interruption units <NUM> via the first gate drive units <NUM>.

By turning on the semiconductor switching elements <NUM> of the current interruption units <NUM>, the interruption of currents that flow from the two DC-DC conversion units <NUM> to the output terminal 3a is canceled. By canceling the interruption of currents at the two current interruption units <NUM>, DC power obtained through conversion by each of the two DC-DC conversion units <NUM> is outputted to the sub DC line <NUM>. In other words, the power conversion device <NUM> is restarted.

By using such power conversion devices, even when one power conversion device among a plurality of the power conversion devices connected in parallel experiences a short circuit, the other power conversion devices having experienced no short circuit can be promptly restarted in the same manner as in Embodiment <NUM>.

<FIG> is a configuration diagram of another power conversion device <NUM> according to the present embodiment. In the power conversion device <NUM> shown in <FIG>, the two current interruption units <NUM> include respective inductance elements <NUM>. The two inductance elements <NUM> are connected in series. Meanwhile, as shown in <FIG>, one inductance element <NUM> may be provided, instead of the two inductance elements, between the output terminal 3a and the current interruption unit <NUM> that is located closer to the output terminal 3a. If the number of inductance elements is reduced in this manner, the size of the power conversion device is reduced. It is noted that the position of the one inductance element <NUM> may be any position on a current route connected to the output terminal 3a.

It is noted that, if the inductance value L of each of the two inductance elements <NUM> of the current interruption units <NUM> shown in <FIG> or the one inductance element <NUM> shown in <FIG> is set so as to satisfy expression (<NUM>), the semiconductor switching elements <NUM> can be prevented from being damaged, in the same manner as in Embodiment <NUM>.

<FIG> is a configuration diagram of a power conversion device <NUM> according to Embodiment <NUM>. The power conversion device <NUM> of the present embodiment is a power conversion device used for a wind power generation system or the like, as the power conversion device shown in <FIG> for Embodiment <NUM> is. The power conversion device <NUM> of the present embodiment is obtained by providing, in the power conversion device of Embodiment <NUM>, bypass switches <NUM> in parallel to the flyback diodes <NUM> of the current interruption units <NUM>.

The bypass switches <NUM> are constantly kept in opened states during a normal operation (during power conversion) of the power conversion device <NUM>. Each bypass switch <NUM> is closed upon detection of: a failure of the current interruption unit <NUM> including the said bypass switch <NUM>; or a failure of the DC-DC conversion unit <NUM> connected to the input side of the said current interruption unit <NUM>. By closing the bypass switch <NUM>, a current route bypassing the failed DC-DC conversion unit <NUM> or the failed current interruption unit <NUM> is formed. Consequently, the operation of the power conversion device <NUM> can be continued with the other DC-DC conversion unit <NUM> and the other current interruption unit <NUM> capable of normal operations.

The short circuit occurrence determination unit <NUM> determines, on the basis of the information from the current sensor <NUM>, whether or not a short circuit has occurred at the output terminal 3a. Then, if the short circuit occurrence determination unit <NUM> determines that a short circuit has occurred, the short circuit occurrence determination unit <NUM> turns off the semiconductor switching elements <NUM> of the two current interruption units <NUM> via the first gate drive units <NUM>. By turning off the semiconductor switching elements <NUM> of the current interruption units <NUM>, currents that flow from the two DC-DC conversion units <NUM> to the output terminal 3a are interrupted. In other words, the power conversion device <NUM> is stopped.

The short circuit occurrence determination unit <NUM> determines that a short circuit has occurred, if the current value detected by the current sensor <NUM> exceeds a preset overcurrent setting value. The overcurrent setting value is set to a value that is larger than the rated current values of the DC-DC conversion units <NUM> and smaller than the maximum current values of the reverse bias safe operating areas (RBSOAs) of the semiconductor switching elements <NUM>.

In addition, since each power conversion device of the present embodiment includes the bypass switches in parallel to the flyback diodes of the current interruption units, even if one of the DC-DC conversion units or one of the current interruption units fails, the operation can be continued with the other DC-DC conversion unit and the other current interruption unit capable of normal operations.

It is noted that, when either of the DC-DC conversion units <NUM> or either of the current interruption units <NUM> has failed, the corresponding bypass switch <NUM> is closed, and thus a mechanical switch that does not need any power in order to keep the closed state is desirable as the bypass switch <NUM>. Further, the power conversion device of the present embodiment may be a power conversion device including the one inductance element <NUM> shown in <FIG> for Embodiment <NUM>.

<FIG> is a configuration diagram of another power conversion device <NUM> according to the present embodiment. As shown in <FIG>, the bypass switches <NUM> may be connected in parallel to the flyback diodes <NUM> with the inductance elements <NUM> interposed therebetween. In the power conversion device <NUM> having such a configuration, even if either of the bypass switches <NUM> is closed when the semiconductor switching element <NUM> of the corresponding current interruption unit <NUM> has failed in a short-circuited state, discharge current from the capacitor <NUM> of the corresponding DC-DC conversion unit <NUM> flows through the bypass switch <NUM> via the inductance element <NUM>. Therefore, abrupt increase in the discharge current from the capacitor <NUM> can be inhibited.

It is noted that each of the short circuit occurrence determination units <NUM> and the short circuit elimination determination units <NUM> in the power conversion devices according to embodiments <NUM> to <NUM> may be composed of a processor <NUM> and a storage device <NUM> as shown in <FIG> which shows an example of hardware. Although not shown, the storage device <NUM> includes a volatile storage device such as a random access memory and a nonvolatile auxiliary storage device such as a flash memory.

Alternatively, the storage device <NUM> may include, as the auxiliary storage device, a hard disk instead of a flash memory. The processor <NUM> executes a program inputted from the storage device <NUM>. In this case, the program is inputted from the auxiliary storage device via the volatile storage device to the processor <NUM>. In addition, the processor <NUM> may output data such as a calculation result to the volatile storage device of the storage device <NUM> or save the data to the auxiliary storage device via the volatile storage device.

Although the invention is described above in terms of various exemplary embodiments, it should be understood that the various features, aspects, and functionality described in one or more of the individual embodiments are not limited in their applicability to the particular embodiment with which they are described, but instead can be applied, alone or in various combinations to one or more of the embodiments of the invention.

Claim 1:
A system comprising a plurality of power conversion devices (<NUM>, <NUM>) coupled in parallel
at the output side, each power conversion device (<NUM>, <NUM>) couplable to a respective AC power generator (<NUM>) at the input side, and
each power conversion device (<NUM>, <NUM>) comprising:
- a power conversion unit (<NUM>) configured to convert input power into DC power;
- an output terminal (3a) from which the DC power obtained through conversion by the power conversion unit (<NUM>) is output;
- a DC-line (6A, 6B, 6C) provided with a circuit breaker (4A, 4B, 4C);
- a current sensor (<NUM>) for detecting a current at the DC-line (6A, 6B, 6C);
- a voltage sensor (<NUM>) for detecting a voltage at the DC line (6A, 6B, 6C);
- a short circuit occurrence determination unit (<NUM>) configured to determine, on the basis of a current value detected by the current sensor (<NUM>), whether or not a sneak current exists, the sneak current being an excessive current that indicates that one of the other power conversion devices (<NUM>, <NUM>) is experiencing a short circuit;
- a short circuit elimination determination unit (<NUM>) configured to determine, on the basis of a current value detected by the current sensor (<NUM>) and a voltage value detected by the voltage sensor (<NUM>), whether or not a short circuit has been eliminated; and
- a current interruption unit (<NUM>) provided between the power conversion unit (<NUM>) and the output terminal (3a), the current interruption unit (<NUM>) being configured to
- interrupt current that flows from the power conversion unit (<NUM>) to the output terminal (3a) by turning OFF a semiconductor switching element (<NUM>) of the current interruption unit (<NUM>), if the short circuit occurrence determination unit (<NUM>) determines that a sneak currents exists, and
- cancel the current interrupt, by turning ON the semiconductor switching element (<NUM>), if the short circuit elimination determination unit (<NUM>) determines that the short circuit has been eliminated;
wherein the semiconductor switching element (<NUM>) is constantly kept in an ON state during a normal operation of the system, and the system is configured to control the circuit breakers (4A, 4B, 4C) such as to open the circuit breaker (4A, 4B, 4C) of a power conversion device (<NUM>, <NUM>) only if that device is experiencing a short circuit.