1. A power conversion device (I, 1 A, 1 B) comprising: a cluster (12uv, 12vw, 1 2 ~in) w hich one or more unit cells (121) are connected in series; and a transformer (1 1) that has at least three legs (1 13uv, 1 13vw, 1 13wu) on each of which a primary winding (1 1 1 uv, 1 1 1 vw, 1 1 1 wu) and a secondary winding (1 12uv, 1 12vw, and 112wu) are wound for magnetic coupling wherein both ends of the secondary winding (1 12uv, 1 12vw, 1 12wu) are connected respectively to both ends of the cluster (12uv, 12vw, 12wu).

2. The power conversion device (1, 1 A, 1 B) according to claim 1 wherein the primary windings (1 1 luv, 1 1 1 vw, 1 11 wu) are delta-connected.

3. The power conversion device (1, 1 A, 1B) according to claim 1 wherein the primary windings (1 1 1 uv, 1 1 1 vw, 1 1 1 wu) are star-connected or zigzag-connected.

4. The power conversion device (1, 1 A, 1 B) according to claim 1 wherein the unit cell (121) comprises: a full-bridge circuit in which at least four switching devices (121 1 to 1214) are connected; voltage application means connected to a DC side of the full-bridge circuit; and a positive-side terminal (1 2 1 8) and a negative-side terminal (1 2 19) connected to an AC side of the full-bridge circuit.

5. The power conversion device (1, 1 A, 1 B) according to claim 4 wherein the voltage application means is a capacitor (121 5).

6. The power conversion device (1, 1 A, 1 B) according to claim 1 wherein the transformer (1 1) is a three-phase transformer having the three legs (1 13uv, 113vw, 1 1 3 ~ ) .

7. The power conversion device (1, 1A, 1 B) according to claim 1 wherein the primary windings (1 1 luv, 1 1 1 vw, 11 lwu) of the transformer (1 1) are connected to phases of an electric power system (2) for linking to the electric power system (2).

8. The power conversion device (1, 1 A, 1 B) according to claim 7 wherein capacitive reactive power or inductive reactive power is supplied to the electric power system (2).

9. The power conversion device (1, 1 A, 1 B) according to claim 4 wherein the voltage application means is active power supply meails for supplying active power via the full-bridge circuit.

10. The power conversion device (1, 1 A, 1 B) according to claim 9 wherein the active power supply means is any one of a secondary cell, a solar cell, and a fuel cell.

11. The power coliversion device (1,1A, 1B) according to claim 9 wherein the primary windings (I11uv, 111vw, I11 wu) are connected to an AC load and power of any amplitude and any frequency is supplied to the AC load.

12. The power conversion device (1,1A, 1B) according to claim 9 wherein the primary windings (11luv, I1Ivw, 11I wu) of the transformer (11) are connected to an electric motor (3) and power of any amplitude and any frequency is supplied to tbe electric motor (3) to control a torque, a rotation speed, and a position of the electric motor (3).

BACKGROUND OF THE INVENTION
The present invention relates to a power conversion device that includes a
plurality of clusters in each of which one or more unit cells are connected in series.
A Modular Multilevel Converter (MMC) is a power converter that uses ordoff
5 controllable switching devices such as an Insulated Gate Bipolar Transistor (IGBT), to produce a
voltage, equal to or higher than the withstand voltage of these switching devices, to the output
side.
Paragraph 0002 of the specification of JP-A-20 1 1 - 176955 describes that "The
delta-connected cascade multilevel converter (hereinafter called CMC) employs a circuit method
10 that uses the ordoff controllable switching devices, such as an Insulated Gate Bipolar Transistors
(IGBT), to output a voltage equal to or higher than the withstand voltage of the switching
devices. According to "Classification and comparison of modular multilevel converters
(MMC)" by Makoto Hagiwara and Hirofumi Akagi, pp. 1-45,2008 IEE-Japan Industry
Application Society Conference, the delta-connected CMC is configured by delta-connecting
15 three series-connected members, each of which is composed of a cluster (a series-connected
member of a plurality of unit cells) and a single-phase reactor."
Paragraph 0003 01 the specification of JP-A-20 1 1 - 176955 describes that "Each
unit cell, a single-phase full-bridge circuit, includes switching devices and a DC capacitor. The
unit cell controls the ordoff of the switching devices to output the DC capacitor voltage, the
20 reversed-polarity voltage of the DC capacitor, or the zero voltage."
Paragraph 0004 of the specification of JP-A-2011-176955 describes that "Because
each cluster is a series-connected member of one or more unit cells, the output voltage of each
cluster (hereinafter called the cluster voltage) is the sum of the output voltages of one or more
unit cells included in the cluster. When each cluster includes a plurality of unit cells, the cluster
25 voltage may be in a multilevel waveform by appropriately shifting the switching times of the unit
cells included in the cluster. Therefore, by increasing the number of unit cells included in each
cluster, the higher harmonic components of the cluster voltage can be reduced."
Paragraph 0005 of the specification of JP-A-20 1 1 - 176955 describes that "The
delta-connected CMC linked to the electric power system can be operated, for example, as a self-
30 excited static synchronous compensator (STATCOM)."
The delta-connected MMC is thought of as a three-phase voltage source in which
each of the three clusters applies the sum of the output voltages of the unit cells. When the
delta-connected MMC is linked to the electric power system, the differential voltage between the
system voltage of the electric power system and the output voltage of the delta-connected MMC
is applied to the impedance between the electric power system and the delta-connected MMC
5 and, as a result, the electric current flows. This allows active power and reactive power to be
transferred between the electric power system and the delta-connected MMC.
SUMMARY OF THE INVENTION
The waveform of each cluster voltage of the delta-connected MMC is not an ideal
10 sine wave but is a multilevel waveform generated by the switching of the unit cells included in
each cluster.
As shown in FIG. 4 in the specification of JP-A-20 11 - 176955 or in FIG. 3 of the
present invention, the multilevel waveform generated by the switching of the unit cells includes
components that prevent the sum of three cluster voltages from becoming zero. In the
15 description below, the sum of the three cluster voltages is called a zero-phase voltage.
In the power conversion device described in JP-A-2011-176955, there is a
circulation circuit composed of the clusters and a single-phase reactor as shown in FIG. 1. If
the single-phase reactor is not connected in this power conversion device, the current that
circulates through the three clusters (hereinafter called the zero-phase current) becomes
20 excessive due to the zero-phase voltage, sometimes resulting in a possibility that the IGBTs of
the unit cells of a cluster are destroyed. To avoid this condition, the power conversion device
described in JP-A-2011-176955 must have a single-phase reactor to limit the zero-phase current.
In view of the foregoing, it is an object of the present invention to provide a
power conversion device that can limit the zero-phase current without using a reactor.
2 5 To solve the problems described above, a power conversion device of the present
invention is configured as follows.
That is, a power conversion device according to claim 1 includes a cluster in
which one or more unit cells are connected in series; and a transformer that has at least three legs
on each of which a primary winding and a secondary winding are wound for magnetic coupling
30 wherein both ends of the secondary winding are connected respectively to both ends of the
cluster.
Other means will be described in Detailed Description of the Exemplary
embodiment.
According to the present invention, there is provided a power conversion device
that limits the zero-phase current without using a reactor.
Other objects, features and advantages of the invention will become apparent
from the following description of the embodiments of the invention taken in conjunction with the
accompanying drawings.
5
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a general configuration diagram showing a power conversion device in a
first embodiment.
FIG. 2 is a diagram showing a configuration and an operation of a unit cell in the
10 first embodiment.
FIG. 3 is a diagram showing an example of the waveforms of the cluster voltages
in the first embodiment.
FIG. 4 is a diagram showing a path of the zero-phase current in the first
embodiment.
15 FIG. 5 is a general configuration diagram showing a power conversion device in a
second embodiment.
FIG. 6 is a diagram showing a configuration and an operation of a unit cell in the
second embodiment.
FIG. 7 is a general configuration diagram showing a power conversion device in a
20 third embodiment.
DETAILED DESCRIPTION OF THE EMBODIMENTS
Embodiments of the present invention are described in detail below with
reference to the drawings.
25 (First embodiment)
A power conversion device 1 in the first embodiment eliminates the need for a
single-phase reactor, which is required in the invention described in JP-A-2011-176955, and
reduces the number of parts of the power conversion device 1.
FIG. 1 is a general configuration diagram showing the power conversion device 1
30 in the first embodiment.
The power conversion device 1, connected to an electric power system 2, is linked
to this electric power system 2.
The power conversion device 1 includes a transformer 11, a uv-phase cluster
12uv, a vw-phase cluster 12vw, a wu-phase cluster 12wu, a control unit 13, and voltmeters 14uv,
- 5 -
14vw, and 14wu. The power conversion device 1 is a three-phase MMC.
In the description below, the connection sides of the transformer 11 are described
as follows. The side in which the electric power system 2 is connected is called a "primary
side", and the side in which the clusters 12uv, 12vw, and 12wu are connected is called a
5 "secondary side".
The transformer 11, a three-phase transformer, has a three-leg core. The
transformer 11 includes legs 11 3uv, 11 3vw, and 1 13wu. On the leg 11 3uv, a uv-phase primary
winding I 1 1 uv and a uv-phase secondary winding 1 12uv are wound around for magnetic
coupling. On the leg 1 13vw, a vw-phase primary winding 1 1 1 vw and a vw-phase secondary
10 winding 112vw are wound around for magnetic coupling. On the leg 113wu, a wu-phase
primary winding 1 1 lwu and a wu-phase secondary winding 112wu are wound around for
magnetic coupling.
In each of the uv-phase cluster 12uv, vw-phase cluster 12vw, and wu-phase
cluster 12wu, six unit cells 12 1 are connected in series. In FIG. 1, the two unit cells 121
15 represent the six unit cells. Note that the number of unit cells is not limited to six. In each of
the clusters 12uv, 12vw, and 12wu, one or any number of unit cells 12 1 may be connected.
The both ends of the uv-phase secondary winding 112uv of the transformer 1 1 are
connected to the both ends of the uv-phase cluster 12uv. The both ends of the vw-phase
secondary winding 112vw of the transformer 11 are connected to the both ends of the vw-phase
20 cluster 12vw. The both ends of the wu-phase secondary winding 1 12wu of the transformer 1 1
are connected to the both ends of the wu-phase cluster 12wu.
The uv-phase primary winding 11 luv of the transformer 1 1 is connected between
the u-phase and the v-phase of the electric power system 2. The vw-phase primary winding
1 1 1 vw of the transformer 1 1 is connected between the v-phase and the w-phase of the electric
25 power system 2. The wu-phase primary winding 1 1 1 wu of the transformer 1 1 is connected
between the w-phase and the u-phase of the electric power system 2. That is, the primary
windings 1 1 1 uv, 1 1 I vw, and 1 1 lwu of the transformer 1 1 are delta-connected for connection to
the electric power system 2.
The voltmeter 14uv is connected between the u-phase and the v-phase of the
30 electric power system 2 to measure the line-to-line voltage Vsuv. The voltmeter 14vw is
connected between the v-phase and the w-phase of the electric power system 2 to measure the
line-to-line voltage Vsvw. The voltmeter 14wu is connected between the w-phase and the uphase
of the electric power system 2 to measure the line-to-line voltage Vswu. In addition, the
voltmeters 14uv, 14vw, and 14wu are connected to the control unit 13 via an optical fiber (not
shown) to send the measured voltage information.
The control unit 13 is connected to the voltmeters 14uv, 14vw, and 14wu and to
the unit cells 12 1, which constitute the clusters 12uv, 12vw, and 12wu, via an optical fiber (not
shown). The control unit 13 controls cluster voltages Vuv, Vvw, and Vwu based on the line-to-
5 line voltages Vsuv, Vsvw, and Vswu measured by the voltmeters 14uv, 14vw, and 14wu.
The voltage and the cul-sent are defined as follows.
The voltage between the both ends of the uv-phase cluster 12uv is the cluster
voltage Vuv. The voltage between the both ends of the vw-phase cluster 12vw is the cluster
voltage Vvw. The voltage between the both ends of the wu-phase cluster 12wu is the cluster
10 voltage Vwu.
The secondary-side current Iuv2 flows between the uv-phase cluster 12uv and the
uv-phase secondary winding 112uv of the transformer 1 1. The secondary-side current Ivw2
flows between the vw-phase cluster 12vw and the vw-phase secondary winding 11 2vw of the
transformer 11. The secondary-side current Iwu2 flows between the wu-phase cluster 12wu
15 and the wu-phase secondary winding 1 12wu of the transformer 1 1.
The secondary-side voltage Vuv2 is applied to the both ends of the uv-phase
secondary winding 112uv of the transformer 11. The secondary-side voltage Vuv2 and the
cluster voltage Vuv are the same.
The secondary-side voltage Vvw2 is applied to the both ends of the vw-phase
20 secondary winding 1 12vw of the transformer 1 1. The secondary-side voltage Vvw2 and the
cluster voltage Vvw are the same.
The secondary-side voltage Vwu2 is applied to the both ends of the wu-phase
secondary winding 112wu of the transformer 11. The secondary-side voltage Vwu2 and the
cluster voltage Vwu are the same.
2 5 The voltage across the both ends of the uv-phase primary winding 11 1 uv of the
transformer 11 is the primary-side voltage Vuvl. The voltage across the both ends of the vwphase
primary winding I1 lvw of the transformer 11 is the primary-side voltage Vvwl. The
voltage across the both ends of the wu-phase primary winding 11 I wu of the transformer 11 is the
primary-side voltage Vwul .
30 The primary-side current Iuvl flows through the uv-phase primary winding 11 luv
of the transformer 11. The direction in which the primary-side current Iuvl flows is the same
as the positive direction of the primary-side voltage Vuvl.
The primary-side current Ivwl flows through the vw-phase primary winding
11 lvw of the transformer 11. The direction in which the primary-side current Ivwl flows is the
same as the positive direction of the primary-side voltage Vvwl.
The primary-side current Iwul flows through the wu-phase pri~naryw inding
11 lwu of the transformer 11. The direction in which the primary-side current Iwul flows is the
same as the positive direction of the primary-side voltage Vwul.
5 In the u-phase of the electric power system 2, the system current Isu flows in the
direction from the power conversion device 1 to the electric power system 2. In the v-phase of
the electric power system 2, the system current Isv flows in the direction from the power
conversion device 1 to the electric power system 2. In the w-phase of the electric power system
2, the system current Isw flows in the direction from the power conversion device 1 to the
10 electric power system 2.
The voltage between the u-phase and the v-phase of the electric power system 2 is
the line-to-line voltage Vsuv. The line voltage Vsuv is measured by the voltmeter 14uv.
The voltage between the v-phase and the w-phase of the electric power system 2
is the line-to-line voltage Vsvw. The line-to-line voltage Vsvw is measured by the voltmeter
15 14vw.
The voltage between the w-phase and the u-phase of the electric power system 2
is the line-to-line voltage Vswu. The line-to-line voltage Vswu is measured by the voltmeter
14wu.
FIG. 2(a) to (d) are diagrams showing the configuration and the operation of the
20 unit cells 121 in the first embodiment.
FIG. 2(a) is a general configuration diagram showing the unit cell 121 in the first
embodiment.
The unit cells 12 1 includes an x-phase high-side switching device 12 1 1 an xphase
low-side switching device 12 12, a y-phase high-side switching device 121 3, a y-phase
25 low-side switching device 12 14, a DC capacitor 12 15, a voltmeter 12 16, and a gate driver 12 17.
The switching devices 12 1 1 - 12 14 are odoff controllable devices, such as an IGBT or a GTO
(Gate Turn-Off thyristor) and, in each of the devices, a diode is connected in the reverse
direction.
The x-phase is connected to the positive-side terminal 12 18 of this unit cell 12 1
30 and to the emitter of the switching device 12 1 1 and the collector of the switching device 12 12.
The y-phase is connected to the negative-side terminal 12 19 of this unit cell 12 1
and to the emitter of the switching device 12 13 and the collector of the switching device 121 4.
The collector of thc switching device 121 1 and the collector of the switching
device 12 13 are connected to one end of the DC capacitor 12 15. The emitter of the switching
device 12 12 and the emitter of the switching device 12 14 are connected to the other end of the
DC capacitor 12 15 that works as voltage application means.
That is, the switching devices 12 1 1 - 12 14 form a full-bridge circuit. To the DC
side of this full-bridge circuit, the DC capacitor 12 15 that works as voltage application means is
5 connected. The DC capacitor 1215 is voltage application means that applies the voltage vckj to
the both ends. Note that k indicates one of the uv-phase, vw-phase, and wu-phase and that j
indicates the number of the unit cell 12 1.
The AC side of this full-bridge circuit is the x-phase and the y-phase. The xphase
is connected to the positive-side terminal 121 8. The y-phase is connected to the
10 negative-side terminal 121 9. The voltage between the positive-side terminal 121 8 and the
negative-side terminal 121 9 is the unit cell voltage vkj.
The gate of each of the switching devices 12 11 - 12 14 is connected to the gate
driver 12 17. The gate driver 12 17 is connected to the control unit 13 via an optical fiber (not
shown) (see FIG. 1). The control unit 13 sends the control signal to the gate driver 1217 via an
15 optical fiber (not shown) to control the on/off of the switching devices 12 1 1 - 12 14.
The voltmeter 12 16, connected to the both ends of the DC capacitor 12 15,
measures the voltage vckj and outputs the information on the measured voltage vckj to the
control unit 13 via an optical fiber (not shown).
FIG. 2(b) is a diagram showing a method for applying the positive voltage to the
20 unit cell 12 1.
In the case shown in FIG. 2(b), the switching device 12 11 on the x-phase high side
is on, the switching device 12 12 on the x-phase low side is off, the switching device 12 13 on the
y-phase high side is off, and the switching device 1214 on the y-phase low side is on. In this
case, the unit cell voltage vkj is approximately equal to the voltage vckj, applied by the DC
25 capacitor 121 5, regardless of the electric current flowing in this unit cell 121. In this
specification, the voltage vckj applied by the DC capacitor 12 15 is called a positive voltage.
FIG. 2(c) is a diagram showing a method for applying the zero voltage to the unit
cell 121.
In the case shown in FIG. 2(c), the switching device 121 1 on the x-phase high side
30 is on, the switching device 1212 on the x-phase low side is off, the switching device 121 3 on the
y-phase high side is on, and the switching device 1214 on the y-phase low side is off. In this
case, the unit cell voltage vkj is approximately equal to O[V] regardless of the electric current
flowing in this unit cell 121.
Similarly, when the switching device 121 1 011 the x-phase high side is off, the
switching device 12 12 on the x-phase low side is on, the switching device 12 13 on the y-phase
high side is off, and the switching device 1214 on the y-phase low side is on, the unit cell voltage
vkj is approximately equal to O[V] regardless of the electric current flowing in each unit cell 121.
FIG. 2(d) is a diagram showing a method for applying the negative voltage to the
5 unit cell 121.
In the case shown in FIG. 2(d), the switching device 12 11 on the x-phase high side
is off, the switching device 12 12 on the x-phase low side is on, the switching device 121 3 on the
y-phase high side is on, and the switching device 1214 on the y-phase low side is off. In this
case, the unit cell voltage vkj is approximately equal to the negative voltage (-vckj), which is the
10 inverse of the voltage applied by the DC capacitor 1215, regardless of the electric current
flowing in this unit cell 12 1.
Therefore, the unit cells 121 may be thought of as the voltage source that applies
the positive voltage vckj, zero voltage (0), or negative voltage (-vckj) regardless of the electric
current flowing in this unit cell 12 1.
15 Next, the cluster voltages Vuv, Vvw, and Vwu are described below.
The control unit 13 of the power conversion device 1 in the first embodiment uses
the carrier phase shift PWM (Pulse-Width Modulation) to control the on/off of the switching
device 12 1 1 - 12 14 included in each unit cell 12 1.
In the power conversion device 1 in the first embodiment, each of the clusters
20 12uv, 12vw, and 12wu includes six unit cells 12 1, and the frequency of the triangle wave carrier
signal of each unit cell 121 is nine times higher than the basic wave frequency. Note that the
number of unit cells 121 and the frequency of the carrier signal are not limited to those values
but that other values may also be used for them.
FIG. 3(a) to (d) are diagrams showing an example of the waveform of each cluster
25 voltage in the first embodiment.
FIG. 3(a) shows the waveform of the cluster voltage Vuv. The vertical axis in
the figure indicates the cluster voltage Vuv in arbitrary units [a. u.] (Arbitrary Unit).
FIG. 3(b) shows the waveform of the cluster voltage Vvw. The vertical axis in
the figure indicates the cluster voltage Vvw in arbitrary units [a. u.].
30 FIG. 3(c) shows the waveform of the cluster voltage Vwu. The vertical axis in
the figure indicates the cluster voltage Vwu in arbitrary units [a. u.].
FIG. 3(d) shows the waveform of the sum voltage Vuv + Vvw + Vwu. The
vertical axis in the figure indicates the sum voltage Vuv + Vvw + Vwu in arbitrary units [a. u.].
'I'he horizontal axis in each of FIG. 3(a) - 3(d) indicates the phase angle in radians
[sad] using n as the unit.
The power conversion device 1 in the first embodiment sets an equal amplitude
for the amplitude of the fundamental wave component of the cluster voltages Vuv, Vvw, and
Vwu, with the phase of the fundamental wave component shifted by (2n13) [sad] with respect to
5 each other.
As shown in FIG. 3(a) to 3(c), each of the cluster voltages Vuv, Vvw, and Vwu
has 13 levels of waveform (positive and negative levels (2) x number of unit cells 121 (6) + 1
(zero-voltage level)).
Here, each of the clusters 12uv, 12vw, and 12wu, in which one or more unit cells
10 121 are connected in series, may be thought of as a voltage source, which applies the sum of
output voltages of one or more unit cells 12 1, regardless of the secondary-side currents Iuv2,
Ivw2, and Iwu2 that flow in each of the clusters.
Each of the cluster voltages Vuv, Vvw, and Vwu includes the fundamental wave
component and the harmonic component.
15 The sum voltage Vuv + Vvw + Vwu of the cluster voltages Vuv, Vvw, and Vwu,
shown in FIG. 3(d), does not include the fundamental wave component. However, the sum
voltage Vuv + Vvw + Vwu does not become 0 but includes the zero-phase component. The
current proportional to the time integration of the zero-phase component is the zero-phase
current 10. Note that the waveforms in FIG. 3(a) to 3(d) are exemplary only. The waveforms
20 may be controlled so that the cluster voltages Vuv, Vvw, and Vwu include the zero-phase
fundamental wave component or a higher harmonic component with an order equal to or lower
than that of the switching frequency.
FIG. 4 is a diagram showing the path of the zero-phase voltage VO and the zerophase
current I0 in the first cmbodirnent. The same reference value is given to the same
25 member as that in FIG. 1.
The zero-phase voltage VO of the uv-phase cluster 12uv is applied to the
secondary winding 1 12uv of the transformer 11 and, in proportion to the time integration of this
zero-phase component, the zero-phase current I0 flows. The uv-phase zero-phase current 10,
which flows in the closed circuit formed by the cluster 12uv and the secondary winding 112uv of
30 the transformer 11, is limited by the inductance of the secondary winding 112uv.
Similarly, the zero-phase voltage VO of the vw-phase cluster 12vw is applied to
the secondary winding 112vw of the transformer 11 and, in proportion to the time integration of
this zero-phase component, the zero-phase current I0 flows. The vw-phase zero-phase current
10, which flows in the closed circuit formed by the cluster 12vw and the secondary winding
- 11 -
112vw of the transformer 11, is limited by the inductance of the secondary winding 11 2vw.
Sin~ilarlyt,h e zero-phase voltage VO of the wu-phase cluster 12wu is applied to
the secondary winding 1 12wu of the transformer 11 and, in proportion to the time integration of
this zero-phase component, the zero-phase current I0 flows. The wu-phase zero-phase cussent
5 10, which flows in the closed circuit formed by the cluster 12wu and the secondary winding
11 2wu of the transformer 1 1, is limited by the inductance of the secondary winding 112wu.
In this manner, the power conversion device 1 in the first embodiment can limit
the zero-phase current 10 with the use of the inductance of the secondary windings 112uv,
1 12vw, and 1 12wu of the transformer I 1. This configuration therefore enables the power
10 conversion device 1 to eliminate the need for a single-phase reactor, thus realizing compact size
and low cost of the device.
The method for controlling the currents Iuv, Ivw, and Iwu of the power
conversion device 1 in the first embodiment is described below with reference as necessary to
FIG. 1 and FIG. 2.
15 The control unit 13 of the power conversion device 1 determines the cluster
voltages Vuv, Vvw, and Vwu by controlling the voltage of the DC capacitor 121 5 of each unit
cell 12 1 of the clusters 12uv, 12vw, and 12wu and by controlling the on/off of the switching
devices 12 1 1 - 12 14 included in the unit cell 12 1. That is, the control unit 13 detects the
voltage of the DC capacitor 12 15 of each unit cell 12 1, included in the clusters 12uv, 12vw, and
20 12wu, and turns on or off the switching devices 12 1 1 to 12 14, included in the unit cell 12 1, for
controlling the cluster voltages Vuv, Vvw, and Vwu so that each cluster voltage becomes a
desired voltage value.
First, the following describes a case in which the power conversion device 1
supplies capacitive reactive power to the electric power system 2 as a static synchronous
25 compensator.
The control unit 13 of the power conversion device 1 detects the line voltages
Vsuv, Vsvw, and Vswu, matches the frequency and the phase of the fundamental wave
component of the cluster voltages Vuv, Vvw, and Vwu with the frequency and the phase of the
line-to-line voltages Vsuv, Vsvw, and Vswu, and performs control so that the amplitude of the
30 fundamental wave component of the cluster voltages Vuv, Vvw, and Vwu becomes smaller than
the amplitude of the line-to-line voltages Vsuv, Vsvw, and Vswu.
In this case, the primary-side currents Iuvl, Ivwl, and Iwul lag the line-to-line
voltages Vsuv, Vsvw, and Vswu by a phase angle of 90". Therefore, the power conversion
device 1 can supply capacitive reactive power to the electric power system 2.
- 12-
Next, the following describes a case in which the power conversion device 1
supplies inductive reactive power to the electric power system 2 as a static synchronous
compensator.
The control unit 13 of the power coi~versiond evice 1 detects the line voltages
5 Vsuv, Vsvw, and Vswu, matches the frequency and the phase of the fundamental wave
component of the cluster voltages Vuv, Vvw, and Vwu with the frequency and the phase of the
line voltages Vsuv, Vsvw, and Vswu, and performs control so that the amplitude of the
fundamental wave component of the cluster voltages Vuv, Vvw, and Vwu becomes larger than
the amplitude of the line voltages Vsuv, Vsvw, and Vswu.
10 In this case, the primary-side currents Iuvl, Ivwl, and Iwul lead the line voltages
Vsuv, Vsvw, and Vswu by a phase angle of 90". Therefore, the power conversion device 1 can
supply inductive reactive power to the electric power system 2.
The control method for use when the power conversion device 1 supplies
capacitive or inductive reactive power to the electric power system 2 as a static synchronous
15 compensator has been described.
As another control method, the power conversion device 1 may detect the voltage,
applied to the positive-phase inductance of the transformer 11, and controls the cluster voltages
Vuv, Vvw, and Vwu to control the primary-side currents Iuvl, Ivwl, and Iwul .
As a still another control method, the power conversion device 1 may directly
20 detect the primary-side currents Iuvl, Ivwl, and Iwul using the ammeters and perform feedback
control so that the current values becomes desired current values.
As yet another control method, the power conversion device 1 may directly detect
the secondary-side currents Iuv2, Ivw2, and Iwu2 using the ammeters and perform feedback
control so that the cuwent values becomes desired current values. The primary-side currents
25 Iuvl, Ivwl, and Iwul and the secondary-side currents Iuv2, Ivw2, and Iwu2 are determined by
the turn ratio of the transformer 11. Therefore, the primary-side currents Iuvl , Ivwl , and Iwul
can be controlled by controlling the secondary-side currents Iuv2, Ivw2, and Iwu2.
In place of the fundamental wave current of the primary-side currents Iuvl, Ivwl,
and Iwul, the power conversion device 1 may control the negative-phase current or the harmonic
30 current.
The power conversion device 1 may perform control so that the cluster voltages
Vuv, Vvw, and Vwu include the zero-phase voltage VO. The zero-phase current 10, which flows
in the clusters 12uv, 12vw, and 12wu, is proportional to the time ilitegration of the sum voltage
Vuv + Vvw + Vwu. Therefore, the power conversion device 1 can control the zero-phase
- 13 -
current I0 by controlling the cluster voltages Vuv, Vvw, and Vwu. The power conversion
device 1 can control the secondary-side currents Iuv2, Ivw2, and Iwu2 that flow in the clusters
12uv, 12vw, and 12wu.
(Effects of the first embodiment)
5 The first embodiment described above gives the following effects (A) to (D).
(A) The power conversion device 1 limits the zero-phase current I0 by the
inductance of the secondary windings 11 2uv, 1 12vw, and 1 12wu of the transformer 11. This
configuration therefore enables the power conversion device 1 to eliminate the need for a
current-limiting, single-phase reactor, thus realizing compact size and low cost of the device.
10 (B) Each unit cell 12 1 includes the full-bridge-configured switching devices 12 1 1
- 12 14 and, on the DC side of the full-bridge circuit, includes the DC capacitor 12 15. In each
unit cell 121, one of the positive voltage that is the voltage across the both ends of the DC
capacitor 121 5, the negative voltage that is the reverse of the voltage across the both ends of the
DC capacitor 12 1 5, and the zero voltage may be applied to the positive-side terminal 12 1 8 and
15 the negative-side terminal 12 19. This allows the clusters 12uv, 12vw, and 12wu, each of which
includes M series-connected unit cells 12 1, to apply the voltage at the level of (1 + 2 x M).
(C) The transformer 1 1 is a three-phase transformer with the legs 1 13uv, 1 13vw,
and 11 3wu. This configuration allows the power conversion device 1 to link to the three-phase
AC electric power system 2.
2 0 (D) The power conversion device 1, connected and linked to the electric power
system 2, can supply capacitive reactive power and inductive reactive power to the electric
power system 2. This configuration enables the power conversion device 1 to operate as a
static synchronous compensator.
(Second embodiment)
2 5 FIG. 5 is a general configuration diagram showing a power conversion device 1A
in a second embodiment. The same reference value is given to the same member as that of the
power conversion device 1 in the first embodiment shown in FIG. 1.
Unlike the power conversion device 1 in the first embodiment shown in FIG. 1,
the power conversion device 1A in the second embodiment has a configuration in which the six
30 unit cells 12 1, included in each of the clusters 12uv, 12vw, and 12wu, are replaced by unit cells
121A. The power conversion device 1 A in the second embodiment can continuously send and
receive active power to and from the electric power system 2.
FIG. 6(a) to 6(d) are diagrams showing the configuration and the operation of the
unit cells 121A in the second embodiment. The same reference value is given to the same
member as that of the unit cell 121 in the first embodiment shown in FIG. 2.
FIG. 6(a) is a diagram showing the configuration of the unit cell 121A in the
second embodiment.
Unlike the unit cell 12 1 in the first embodiment (see FIG. 2), the unit cell 12 1A in
5 the second embodiment includes a secondary battery 12 15A instead of the DC capacitor 12 15
(see FIG. 2). The secondary battery 12 15A is voltage application means for applying the
voltage vckj across the both ends and, at the same time, active power supply means for supplying
active power. Note that k indicates one of the uv-phase, vw -phase, and wu-phase and that j
indicates the number of the unit cell 121A.
10 FIG. 6(b) is a diagram showing a method for applying the positive voltage to the
unit cell 121A.
In the manner similar to that shown in FIG. 2(b), the unit cell 12 1A controls the
on/off of the switching devices 121 1 to 1214. In this case, the unit cell voltage vkj is
approximately equal to the voltage vckj, applied by the secondary battery 12 15A, regardless of
15 the electric current flowing in this unit cell 121A.
FIG. 6(c) is a diagram showing a method for applying the zero voltage to the unit
cell 121A.
In the manner similar to that shown in FIG. 2(c), the unit cell 12 1A controls the
onloff of the switching devices 121 1 to 1214. In this case, the unit cell voltage vkj is
20 approximately equal to O[V] regardless of the electric current flowing in this unit cell 121A.
FIG. 6(d) is a diagram showing a method for applying the negative voltage to the
unit cell 121A.
In the manner similar to that shown in FIG. 2(d), the unit cell 121 A controls the
odoff of the switching devices 12 11 to 1214. In this case, the unit cell voltage vkj is
25 approximately equal to the negative voltage (-vckj), which is the reverse of the voltage applied
by the secondary battery 12 15A, regardless of the electric current flowing in this unit cell 12 1A.
Therefore, the unit cells 121 may be thought of as the voltage source that applies
the positive voltage vckj, zero voltage (0), or negative voltage (vckj) regardless of the electric
current flowing in this unit cell 121A.
30 As with the power conversion device 1 in the first embodiment, the power
conversion device 1 A in the second embodiment detects the voltage of the secondary battery
12 15A of each unit cell 121 A included in each of the clusters 12uv, 12vw, and 12wu and controls
the on/off the switching devices 12 1 1 to 12 14 included in the unit cell 12 1 A. This
configuration allows the power conversion device 1A to control the cluster voltages Vuv, Vvw,
and Vwu so that the voltages become desired voltage values.
The control method used by the power conversion device 1 A to receive active
power from the electric power system 2 is described below with reference as necessary to FIG. 5.
The control unit 13 of the power conversion device 1A detects the line voltages
5 Vsuv, Vsvw, and Vswu, matches the frequency and the amplitude of the basic wave component
of the cluster voltages Vuv, Vvw, and Vwu with the frequency and the amplitude of the line
voltages Vsuv, Vsvw, and Vswu and, in addition, delays the phase of the fundamental wave
component of the cluster voltages Vuv, Vvw, and Vwu from the phase of the line voltages Vsuv,
Vsvw, and Vswu.
10 In this case, in the primary-side currents Iuvl, Ivwl, and Iwul, the current, whose
phase is the anti-phase of the phase of the line voltages Vsuv, Vsvw, and Vswu, flows.
Therefore, the power conversion device 1A can receive active power from the electric power
system 2.
Next, the control method used by the power conversion device 1A to send
15 (supply) active power to the electric power system 2 is described below with reference as
necessary to FIG. 5.
The power conversion device 1 A detects the line voltages Vsuv, Vsvw, and Vswu,
matches the frequency and the amplitude of the fundamental wave component of the cluster
voltages Vuv, Vvw, and Vwu with the frequency and the amplitude of the line voltages Vsuv,
20 Vsvw, and Vswu and, in addition, leads the phase of the fundamental wave component of the
cluster voltages Vuv, Vvw, and Vwu from the phase of the line voltages Vsuv, Vsvw, and Vswu.
In this case, in the primary-side currents Iuvl, Ivwl, and Iwul, the current, whose
phase is the in-phase of the phase of the line voltages Vsuv, Vsvw, and Vswu, flows. Therefore,
the power conversion device 1A can send (supply) active power to the electric power system 2.
25 As described above, the power conversion device 1A can continuously send and
receive active power to and from the electric power system 2 with the use of the unit cell 121A to
which the secondary battery 12 1 5A is connected. Therefore, the power conversion device 1 A
can be used as a power storage system that is linked to the electric power system 2.
(Effects of the second embodiment)
30 The second embodiment described above gives the following effects (E) and (F).
(E) The unit cell 12 1 A of the power conversion device 1 A has the secondary
battery 12 1 5A. This configuration allows the power conversion device 1 A to continuously send
and receive active power to and from the electric power system 2.
(F) The power conversion device 1A can store active power of the electric power
system 2 in the secondary battery 121 5A and supply active power, stored in the secondary
battery 12 15A, to the electric power system 2. Therefore, the power conversion device 1 A can
function as a power storage system.
(Third embodiment)
5 A power conversion device 1 B in a third embodiment uses the unit cells 121A to
each of which the secondary battery 12 15A that works as active power supply means, such as the
one shown in FIG. 5, is connected. The power conversion device 1B supplies power to an
electric motor 3 and to other AC loads. This configuration allows the power conversion device
1 B in the third embodiment to apply AC voltage of any amplitude and any frequency to the
10 electric motor 3 and to other AC loads.
FIG. 7 is a general configuration diagram showing the power conversion device
IB in the third embodiment. The same reference value is given to the same member as that of
the power conversion device 1A in the second embodiment shown in FIG. 5.
As shown in FIG. 7, unlike the power conversion device 1A in the second
15 embodiment, the power conversion device 1 B in the third embodiment is connected to the
electric motor 3. The other configuration is similar to that of the power conversion device 1A
in the second embodiment.
The power conversion device 1B in the third embodiment is connected directly to
the electric motor 3. Note that the power conversion device 1B need not always be connected
20 directly to the electric motor 3 but may be connected to the electric motor 3 via another
transformer, a reactor, or a filter.
The method for controlling the power conversion device 1B is described below
with reference as necessary to FIG. 7.
As described in the first embodiment, the cluster voltages Vuv, Vvw, and Vwu
25 across the both ends of the clusters 12uv, 12vw, and 12wu can be controlled by controlling the
on/off of the switching devices 12 1 1 - 12 14 of the unit cell 12 1 A included in each of the clusters
1 2uv, 1 2vw, and 1 2wu.
The amplitude of the cluster voltages Vuv, Vvw, and Vwu can be controlled to
any value in the range from (vckj x M) [V] to (vckj x M) [V] in increments of vckj [V],
30 including 0 [V], where the number of unit cells 12 1 A included in each of the clusters 12uv,
12vw, and 12wu is M and the voltage applied by the secondary battery 1215A of each unit cell
12 1 A is the voltage vckj.
In addition, the frequency of the cluster voltages Vuv, Vvw, and Vwu can be
controlled to any value in the range lower than the switching frequency. By feedback
controlling the currents Iu, Iv, and Iw, which flow to the electric motor 3 via the cluster voltages
Vuv, Vvw, and Vwu, either directly or via the current signal generated by coordinate conversion,
the torque, the rotation speed, and the rotation position of the electric motor 3 can be controlled.
As described above, the power conversion device 1B in the third embodiment,
5 which operates as a device for applying an AC voltage of any amplitude and any frequency, can
control the torque, the rotation speed, and the rotation position of the electric motor 3.
(Effects of the third embodiment)
The third embodiment described above gives the following effects (G) and (H).
(G) The power conversion device 1 B, connected to the electric motor 3, can apply
10 an AC voltage of any amplitude and any frequency to the electric motor 3. This configuration
therefore allows the power conversion device 1 B to control the torque, the rotation speed, and
the rotation position of the electric motor 3.
(H) The control unit 13 of the power conversion device 1 B can control the zero phase
voltage VO of the clusters 12uv, 12vw, and 12wu and the zero-phase current I0 that flows
15 in those clusters. This configuration therefore allows the phases of the electric motor 3 or an
AC load to be controlled even when they are unbalanced.
(Modifications)
It will be understood that the present invention is not limited to the embodiments
described above but that various modifications are included. For example, the above
20 embodiments are described in detail to make it easy to understand the present invention but the
embodiments should not be limited to those having all configurations that are described. It is
possible to replace a part of the configuration of one embodiment with the configuration of
another or to add a part of the configuration of one embodiment to the configuration of another.
It is also possible to add another configuration to, or to delete or replace, a part of the
25 configuration of each embodiment.
The configurations, functions, processing units, and processing means described
above may be implemented, in part or in whole, by hardware such as an integrated circuit. The
configurations and the functions described above may also be implemented by software by the
processor that interprets and executes the program created for implementing the function. The
30 information on the programs, tables, and files for implementing the functions may be stored in a
recording device such as a memory, a hard disk, or an SSD (Solid State Drive) or in a recording
medium such as a flash memory card or a DVD (Digital Versatile Disk).
In the description of each embodiment, it should be noted that only the control
lines and information lines necessary for the description are shown and that all control lines or
information lines necessary for the product are not shown. In practice, almost all the
comfigurations are considered interconnected with each other.
Examples of the modifications of the present invention are (a) to (e) given below.
(a) In the first to third embodiments, the primary windings 11 luv, 11 lvw, and
1 1 1 wu of the transformer 1 1 are delta-connected. However, the primary windings 1 1 1 uv,
11 lvw, and 11 1 wu may be connected not only in the delta connection but also in another line
connection method such as the star connection or the zigzag connection.
(b) In the first to third embodiments, the transformer 11 uses a three-leg core.
However, the core is not limited to a three-leg core. The transformer 11 may use a four-leg core
10 instead of a three-leg core. In addition, the transfer may use a multiple-leg core having four or
more legs.
(c) The power conversion device in the first to third embodiments is a three-phase
MMC. However, the power conversion device is not limited to a three-phase MMC, but a polyphase
MMC having four or more phases may be used.
15 (d) Instead of the secondary battery 12 1 5A, the unit cell 12 1 A in the second
embodiment may include another power supply device that supplies active power such as a solar
cell or a fuel cell. This allows the power conversion device 1A to continuously supply active
power to the electric power system 2.
(e) The power conversion device 1 B in the third embodiment is connected to the
20 electric motor 3 in the above description. The power conversion device 1 B need not always be
connected to the electric motor 3 but may be connected to a power generator or any other AC
load.
It should be further understood by those skilled in the art that although the
foregoing description has been made on embodiments of the invention, the invention is not
25 limited thereto and various changes and modifications may be made without departing from the
spirit of the invention and the scope of the appended claims.

WE CLAIM:
1. A power conversion device (I, 1 A, 1 B) comprising:
a cluster (12uv, 12vw, 1 2 ~in) w hich one or more unit cells (121) are connected
in series; and
a transformer (1 1) that has at least three legs (1 13uv, 1 13vw, 1 13wu) on each of
which a primary winding (1 1 1 uv, 1 1 1 vw, 1 1 1 wu) and a secondary winding (1 12uv, 1 12vw, and
112wu) are wound for magnetic coupling wherein
both ends of the secondary winding (1 12uv, 1 12vw, 1 12wu) are connected
respectively to both ends of the cluster (12uv, 12vw, 12wu).
2. The power conversion device (1, 1 A, 1 B) according to claim 1 wherein the
primary windings (1 1 luv, 1 1 1 vw, 1 11 wu) are delta-connected.
3. The power conversion device (1, 1 A, 1B) according to claim 1 wherein the
primary windings (1 1 1 uv, 1 1 1 vw, 1 1 1 wu) are star-connected or zigzag-connected.
4. The power conversion device (1, 1 A, 1 B) according to claim 1 wherein
the unit cell (121) comprises:
a full-bridge circuit in which at least four switching devices (121 1 to 1214) are
connected;
voltage application means connected to a DC side of the full-bridge circuit; and
a positive-side terminal (1 2 1 8) and a negative-side terminal (1 2 19) connected to
an AC side of the full-bridge circuit.
5. The power conversion device (1, 1 A, 1 B) according to claim 4 wherein
the voltage application means is a capacitor (121 5).
6. The power conversion device (1, 1 A, 1 B) according to claim 1 wherein
the transformer (1 1) is a three-phase transformer having the three legs (1 13uv,
113vw, 1 1 3 ~ ) .
7. The power conversion device (1, 1A, 1 B) according to claim 1 wherein
the primary windings (1 1 luv, 1 1 1 vw, 11 lwu) of the transformer (1 1) are
connected to phases of an electric power system (2) for linking to the electric power system (2).
8. The power conversion device (1, 1 A, 1 B) according to claim 7 wherein
capacitive reactive power or inductive reactive power is supplied to the electric
power system (2).
9. The power conversion device (1, 1 A, 1 B) according to claim 4 wherein
the voltage application means is active power supply meails for supplying active
power via the full-bridge circuit.
10. The power conversion device (1, 1 A, 1 B) according to claim 9 wherein
the active power supply means is any one of a secondary cell, a solar cell, and a
fuel cell.
11. The power coliversion device (1,1A, 1B) according to claim 9 wherein
the primary windings (I11uv, 111vw, I11 wu) are connected to an AC load and
power of any amplitude and any frequency is supplied to the AC load.
12. The power conversion device (1,1A, 1B) according to claim 9 wherein
the primary windings (11luv, I1Ivw, 11I wu) of the transformer (11) are
connected to an electric motor (3) and
power of any amplitude and any frequency is supplied to tbe electric motor (3) to
control a torque, a rotation speed, and a position of the electric motor (3).