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BACKGROUND OF THE INVENTION
The present invention relates to a power conversion apparatus and a control
method thereof More particularly, the present invention relates to a power conversion
apparatus which includes an arm configured by connecting one or a plurality of unit converters
5 in series and which is suitable to convert AC power into DC power or antily, and a control
method thereof
In recent years, a number of power conversion apparatus which convert AC into
DC or DC into AC are used. A power conversion apparatus which uses unit converters is
known as this kind of power conversion apparatus. For example, when a plurality of unit
10 converters are connected in series, a power conversion apparatus which withstands a high
voltage is relatively easily obtained. A unit converter has an energy storage element such as a
capacitor, and supplies a voltage of the energy storage element to an output terminal by operating
a semiconductor switching element.
This kind of power conversion apparatus is configured by connecting in parallel a
15 unit in which one or a plurality of unit converters are connected in series so as to perform power
conversion between DC and AC as an apparatus treating a plurality of phases.
In a circuit configuration of a unit converter, a power semiconductor device
controllable to be on/oflF such as an IGBT (Insulated-Gate Bipolar Transistor), a GTO (Gate
Tum-Off Thyristor), and a GCT (Gate-Commutated Thyristor) is generally used as a switching
20 element.
As the above-described example, some power conversion apparatus are
configured by connecting a secondary winding of a transformer in series to a plurality of unit
converters so as to cooperate with a three-phase power system therethrough. Specifically, the
secondary winding of the transformer is made to be open winding and one ends of series circuits
25 of respective unit converters are star-connected. The above-described technology is disclosed
in, for example, JP-A-2010-233411.
In the above-described technology, a magnetomotive force through a zero-phasesequence
current is canceled as one characteristic at the time of power conversion. In this way,
in this specification, a converter which can cancel a magnetomotive force through a zero-phase-
30 sequence current is referred to as a ZC-MMC (Zero-Sequence Cancelling Modular Multilevel
Converter).
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SUMMARY OF THE INVENTION
In the above-described conventional technology, an energy storage value for an
energy storage element needs to be controlled. When an energy storage element will be
described by using as an example a capacitor, an energy storage value needs to be controlled to a
5 predetermined voltage for performing a normal operation.
In a unit in which one or a plurality of unit converters are connected in series
(hereinafter, referred to as an arm), when a capacitor voltage of each unit converter is uneven, a
voltage balance of the entire arm is broken. For example, descriptions will be made by using
the fact that the total sum of capacitor voltages of arms is controlled in a tolerance level. In
10 unevenness of the capacitor voltages of each unit converter, when a zero-phase voltage is
superimposed on an output voltage command value of each arm, different active power is
intentionally generated in each arm. As a result, among arms, a method for balancing a
capacitor voltage average value in each arm is considered (in the specification, this method for
arm balance control is referred to as a zero-phase-sequence voltage superimposition method).
15 However, there is a problem that when a zero-phase sequence voltage is
superimposed on an output voltage command value of each arm against unevenness of capacitor
voltages in each unit converter, the superimposed zero-phase sequence voltage appears on a DC
side terminal.
In particular, for example, two DC terminals are mutually connected and an
20 operation is performed as a frequency converter (FC), a high-voltage direct current (HVDC)
system, and a back-to-back (BTB) system. In this case, one zero-phase sequence voltage
through arm balance control of a ZC-MMC appears on a common DC terminal and interferes
with the other control.
It is an object of the present invention to provide a power conversion apparatus
25 which can withstand unevenness of capacitor voltages while preventing a superimposed voltage
from appearing on a DC side terminal, and a control method thereof
In order to achieve the above object, in the present invention, a power conversion
apparatus to connect arms and windings in series on a predetermined order side of a transformer
to configure a serial circuit, connect the serial circuits in parallel to configure connection parts as
30 DC terminals, connect one or a plurality of unit converters in series to configure the arms, and
connect a plurality of phase power sources or phase loads to winding on another order side of the
transformer,, the unit converter being configured as an at least two-terminal converter by
including the at least one energy storage element and the at least one switching element
controlling an output depending on a vohage of the energy storage element, wherein the power
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conversion apparatus controls imbalance of storage energy or voltage of the energy storage
element so that a current flowing through each of the arms has a phase and amplitude different
from each other to be approximated to balance.
A power conversion apparatus to connect a plurality of arms in parallel, mutually
5 connect some arms of a plurality of the parallel-connection arms to be formed as a first DC side,
mutually connect other arms of a plurality of the parallel-connection arms to be formed as a
second DC side, connect one or a plurality of unit converters in series to configure each of the
plurality of arms, configure a winding provided between the first DC side and the second DC
side or a winding connected to a terminal provided between the first DC side and the second DC
10 side as a winding on a predetermined order side of a transformer, and electrically connect a
winding on the predetermined order side to a winding on another order side, the power
conversion apparatus includes at least one energy storage element; and at least one switching
element, the unit converter including the at least one energy storage element and the at least one
switching element for outputting a voltage depending on a voltage of the energy storage element,
15 wherein the power conversion apparatus controls a current flowing through each of the arms to
have a phase and amplitude different from each other and approximate imbalance of a voltage or
storage energy of the energy storage element to balance.
Specifically, in the power conversion apparatus in which at least one energy
storage element and at least one switching element are included, a plurality of series circuits of a
20 transformer winding and an arm in which one or a plurality of at least two-terminal unit
converters which depend on ON/OFF of the switching element and supply a zero voltage or a
voltage depending on a voltage of the energy storage element are connected in series are
connected in parallel, and a multi-phase power source or a multi-phase load is connected to
another winding of the transformer, and the parallel-connection point is set as a DC terminal, the
25 power conversion apparatus includes means for controlling a current flowing through each of the
arms to have a phase and amplitude different from each other so that each of the arms may give
and receive different active power.
Further, an FC, an HVDC system, or a BTB system in which at least one power
conversion apparatus described above is provided and at least another power conversion
30 apparatus is connected directly or through a DC transmission line to a DC terminal of the power
conversion apparatus is provided.
According to the present invention, the power conversion apparatus can bring a
balance against unevenness of a capacitor voltage while preventing a superimposed vohage for a
balance from appearing on a DC terminal. More specifically, an effect that energy or a voltage
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of the energy storage device can be balanced or an arbitrary imbalance can be generated without
causing one power conversion apparatus to interfere with control of another power conversion
apparatus is obtained in an FC, an HVDC system, or a BTB system.
Other objects, features and advantages of the invention will become apparent
5 from the following description of the embodiments of the invention taken in conjunction with the
accompanying drawings.
BRffiF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates a power conversion apparatus according to first and second
embodiments of the present invention;
10 FIG. 2 illustrates a configuration of a unit converter in a bidirectional chopper
circuit system;
FIG. 3 illustrates a configuration of a unit converter in a full-bridge circuit system;
FIG. 4 illustrates one example of a control block diagram of the power conversion
apparatus according to the present invention;
15 FIG. 5 illustrates one example of an arm balance control block diagram of the
power conversion apparatus according to the present invention;
FIG. 6 illustrates a gate signal generating unit of a control block of the power
conversion apparatus according to the present invention;
FIG. 7 illustrates a conceptual waveform of the power conversion apparatus
20 according to the present invention;
FIG. 8 illustrates a configuration example of a transformer of the power
conversion apparatus according to the first and second embodiments of the present invention;
FIG. 9 illustrates a power conversion apparatus according to a third embodiment
of the present invention; and
25 FIG. 10 illustrates a configuration example of a transformer of the power
conversion apparatus according to the third embodiment of the present invention.
DESCRIPTION OF THE EMBODIMENTS
[First Embodiment]
A first embodiment of the present invention will be described below with
30 reference to the accompanying drawings of the embodiment.
The present invention has a configuration in which in a power conversion
apparatus in which a bidirectional chopper circuit is used as a unit converter, three circuits in
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which three arms in which a plurality of the bidirectional chopper circuits are connected in series
and three secondary windings of a transformer are connected in series, respectively, are
connected in parallel, one terminal of the parallel connection point is set as a DC positive side
terminal, the other terminal thereof is set as a DC negative side terminal, and a primary winding
5 of the transformer is connected to a three-phase power system, when a negative-phase-sequence
current flows through the arm according to a difference of an average value for each arm of a
voltage in the capacitor, a control means for having a function of balancing an average value for
each arm of a vohage in the capacitor is included.
Further, the present embodiment has a configuration of an FC (fi-equency
10 converter), an HVDC (High-Voltage Direct Current) system, or a BTB (Back-To-Back) system
in which DC terminals of two power conversion apparatus described above are mutually
connected, or one DC terminal of one power conversion apparatus described above and another
DC terminal of one power conversion apparatus in another system are mutually connected.
According to the present embodiment, an effect that the one power conversion
15 apparatus can balance an average value in each arm of voltage in the capacitor without
interfering with control of the other power conversion apparatus is obtained.
First, the entire configuration of the first embodiment will be described with
reference to FIG. 1.
The power conversion apparatus 102 is connected to the three-phase power
20 system 101 via the transformer 103. A configuration of the transformer 103 will be described
later. AU-phase arm 104U, a V-phase arm 104V, and a W-phase arm 104W are connected in
series to respective phases (points u, v, and w) of secondary windings of the transformer 103.
Further, the other ends of respective arms 104U, 104 V, and 104W are connected to a DC positive
side terminal (point P). In addition, a neutral point of the secondary winding of the transformer
25 is connected to a DC negative side terminal (point N).
That is, circuits in which the secondary windings (802) of the transformer and the
respective arms 104U, 104V, and 104W are connected in series with each other are connected in
parallel at points P and N.
An other-side power conversion apparatus 112 is connected directly or via a DC
30 transmission line (not illustrated) to the DC positive side terminal (point P) and the DC negative
side terminal (point N). Further, the other-side power conversion apparatus 112 is connected to
another three-phase AC system 113. Here, the other-side power conversion apparatus 112 may
have the same configuration as or a configuration different from that of the power conversion
apparatus 102.
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Each of the arms 104U, 104 V, and 104W is a circuit in which a plurality of unit
converters 105 are connected in series to each other. An internal configuration of the unit
converter 105 will be described later.
Hereinafter, voltages and currents illustrated in FIG. 1 are defined.
5 A U-phase phase voltage, a V-phase phase voltage, and a W-phase phase voltage
of the three-phase power system 101 are referred to as VSU, VSV, and VSW, respectively.
Further, a current which flows through a point u of the transformer 103 and the Uphase
arm 104U is referred to as lU, a current which flows through a point v of the transformer
103 and the V-phase arm 104V is referred to as IV, and a current which flows through a point w
10 of the transformer 103 and the W-phase arm 104W is referred to as IW.
Further, the sum of output voltages produced from one or the plurality of unit
converters 105 included in the U-phase arm 104U is referred to as an output voltage VU of the
U-phase arm 104U. Similarly, the sum of output voltages produced from one or the plurality of
unit converters 105 included in the V-phase arm 104V is referred to as an output voltage W of
15 the V-phase arm 104 V, and the sum of output voltages produced from one or the plurality of unit
converters 105 included in the W-phase arm 104W is referred to as an output vohage VW of the
W-phase arm 104W.
A direct-current voltage between the DC positive side terminal (point P) and the
DC negative side terminal (point N) is referred to as a VDC. Further, a direct current which
20 flows from the power conversion apparatus 102 to the DC positive side terminal (point P) of the
other-side power conversion apparatus 112 is referred to as an IDC.
Further, a capacitor voltage of each unit converter 105 is referred to as VCjk.
Here, j represents the arm 104U, 104V, or 104W to which the unit converter belongs, for
example, j=U, V, or W Further, k represents a number in the arm 104U, 104y, or 104W, for
25 example, k=l, 2, ..., Nc. Here, Nc represents the number of the unit converters 105 included in
the arm 104U, 104 V, or 104W
Next, a voltage detecting means, a current detecting means, and a control means
illustrated in FIG. 1 will be described.
The voltage detecting means 107 detects the phase vohages VSU, VSV, and VSW
30 of the three-phase power system 101, and transmits them to the control means 106. Note that in
the present embodiment, a case where the voltage detecting means 107 detects a phase vohage
will be described, and further the voltage detecting means 107 may detect a line voltage.
The current detecting means 108U, 108 V, and 108W detect currents lU, IV, and
IW which flow through each of the arms 104U, 104 V, and 104W, and transmit them to the
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control means 106.
The voltage detecting means 109 detects a voltage of a capacitor (203 and/or 301)
provided on each unit converter 105, and transmits it to the control means 106 via a capacitor
voltage detection line 110.
5 The control means 106 generates gate signals given to switching elements 20IH
and 201L, and/or 201XH, 201XL, 201YH, and 201YL of each unit converter 105 (FIGS. 2 and
3), and transmits them through gate signal transmission lines 111 by using the voltages VSU,
VSV, and VSW obtained from the voltage detecting means 107, the currents lU, IV, and IW
obtained from the current detecting means 108U, 108V, and 108W, and the capacitor vohage
10 VCjk of each unit converter 105 obtained from the voltage detecting means 109.
Here, the control means 106 is illustrated as one rectangle in FIG. 1. The control
means 106 can be divided into a plurality of components and the plurality of components can be
installed in places physically separated from each other or potentials electrically different from
each other. The present embodiment is supposed to include the above case.
15 Further, FIG. 1 illustrates a case where the control means 106 detects the VSU,
VSV, VSW, lU, rv, and IW, and the capacitor voltage VCjk of each unit converter 105 by using
the voltage detecting means 107, the current detecting means 108U, 108 V, and 108W, and the
voltage detecting means 109. Further, the control means 106 can detect other electric amounts
such as VDC and IDC, and the present embodiment is supposed to include the above case.
20 One example of an internal configuration of the unit converter 105 will be
described below with reference to FIG. 2. As the unit converter 105, a unit converter 105a in a
bidirectional chopper circuit system capable of producing a unipolar voltage and a unit converter
105b in a full-bridge circuit system can be used. First, a circuit configuration of the unit
converter 105a in the bidirectional chopper circuit system will be described with reference to
25 FIG. 2.
First, a circuit configuration of the unit converter 105a in the bidirectional
chopper circuit system will be described with reference to FIG. 2. Note that FIG. 3 (the unit
converter 105b) will be described in a second embodiment.
A circuit in which the high-side switching element 20IH and the high-side
30 freewheeling diode 202H are anti-parallel-connected to each other and a circuit in which the
low-side switching element 20IL and the low-side freewheeling diode 202L are anti-parallelconnected
to each other are connected in series at a point a. The serial-connection circuit is
connected in parallel to the capacitor 203.
In the specification, the high-side switching element 20IH and the low-side
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switching element 20IL, and the X-phase high-side switching element 201XH, the X-phase lowside
switching element 201 XL, the Y-phase high-side switching element 201YH, and the Y-phase
low-side switching element 201YL illustrated later in FIG. 3 are collectively referred to simply as
the switching element 201.
5 In FIGS. 2 and 3, a symbol of an IGBT is illustrated as the switching element 201.
When a switching element is a power semiconductor device controllable to be on/off, a
switching element of a type different from that of an IGBT can be used, for example, a GTO, a
GCT, and a MOSFET (Metal-Oxide-Semiconductor Field Effect Transistor).
A voltage between the point a and one end (point n) of the capacitor 203 is
10 referred to as an output voltage Vjk of the unit converter 105. Note that, j=U, V, and W, k=l, 2,
..., Nc, and Nc represents the number of the unit converters 105 included in each of the arms
104U, 104 V, and 104W.
The voltage detecting means 109 detects the voltage VCjk, and transmits it to the
control means 106 through the capacitor voltage detection line 110.
15 Based on gate signals GHjk and GLjk transmitted from the control means 106, a
gate driver 205 controls ON/OFF of the high-side switching element 20IH and the low-side
switching element 20IL.
A relationship between the output voltage Vjk of the unit converter 105 in the
bidirectional chopper circuit system and an ON/OFF state of the switching elements 20IH and
20 201L will be described below.
In the case where the high-side switching element 201H is ON and the low-side
switching element 20IL is OFF, the output voltage Vjk can be controlled so as to be
approximately equal to the capacitor vohage VCjk regardless of the current Ij (j=U, V, and W).
In the case where the high-side switching element 201H is OFF and the low-side
25 switching element 201L is ON, the output voltage Vjk can be controlled so as to be
approximately equal to zero regardless of the current Ij.
One example of an internal configuration of the transformer 103 will be described
below with reference to FIG. 8.
A primary winding 80 lU and secondary divided windings 802Un and 802Up are
30 wound onto a core 804U. Similarly, a primary winding 801V and secondary divided windings
802Vn and 802Vp are wound onto a core 804V. Further, a primary winding 801W and
secondary divided windings 802Wn and 802Wp are wound onto a core 804W.
The primary windings 80 lU, 801V, and 80IW are star-connected at a point M.
Further, the secondary divided windings 802Un, 802Vn, and 802Wn are starW6853
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connected at a point N.
Further, one ends of the respective secondary divided windings 802Up, 802 Vp,
and 802Wp are connected to the arms 104U, 104V, and 104W via points u, v, and w,
respectively.
5 The other ends of the secondary divided windings 802Up, 802 Vp, and 802Wp on
the side opposite to the arms 104U, 104V, and 104W are connected to ends on the side opposite
to the point N of the secondary divided windings 802 Vn, 802Wn, and 802Un, respectively.
In other words, six secondary divided windings 802Up, 802Un, 802Vp, 802Vn,
802Wp, and 802Wn are zigzag-connected. In the present embodiment, the six secondary
10 divided windings 802Up, 802Un, 802Vp, 802Vn, 802Wp, and 802Wn are collectively referred to
simply as a secondary winding. Here, the number of windings of the secondary divided
windings 802Up, 802Un, 802Vp, 802Vn, 802Wp, and 802Wn is approximately equal to each
other.
FIG. 8 illustrates a case where the primary windings 80 lU, 801V, and 801W are
15 star-connected. Further, the present invention can be applied also to a case where the primary
windings 80 lU, 801V, and 801W are delta-connected, and the present embodiment is supposed
to include the above case.
As a case where the secondary divided windings 802Up, 802Un, 802Vp, 802Vn,
802Wp, and 802Wn are connected, for example, FIG. 8 fiarther illustrates a case where the
20 secondary divided windings 802Up and 802Vn are connected. Further, the present invention
can be applied also to a case where the secondary divided windings 802Up, 802Un, 802Vp,
802 Vn, 802Wp, and 802 Wn are zigzag-connected by using a combination of the secondary
divided windings different from that of FIG. 8. The present embodiment is supposed to include
the above case.
25 Also to a case where the secondary divided windings 802Up, 802Un, 802Vp,
802Vn, 802Wp, and 802Wn are not zigzag-connected, the present invention can be further
applied also to, for example, a transformer illustrated in FIGS. 24 and 25 of Patent Literature 1
(IP-A-2010-233411), and the present embodiment is supposed to include the above case.
Further, in the specification, in the windings of the transformer 103, a winding
30 connected to the three-phase power system 101 is referred to as the primary winding, and
windings connected to the arms 104U, 104V, and 104W are referred to as the secondary divided
winding. "Primary" and "Secondary" are politic appellation for explanation. Even if the
appellation is opposite, the present invention can be applied.
A control method performed by using the control means 106 will be described
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below with reference to FIGS. 4 to 6. Then, a principle in which an average value of voltages
of one or a plurality of capacitors 203 included in each of the arms 104U, 104 V, and 104W can
be balanced among the arms will be described with reference to FIG. 7. In the present
embodiment, descriptions will be made on the premise that a transformation ratio of the
5 transformer 103 is 1:1.
FIG. 4 is a block diagram illustrating one example of the control method
performed by using the control means 106. The control means 106 is configured by an AC
vohage command value generating unit 412 and a DC voltage command value generating unit
413.
10 First, the AC voltage command value generating unit 412 will be described.
The AC vohage command value generating unit 412 calculates AC components
VUac*, Wac*, and VWac* of command values VU*, VV*, and VW* of the output voUages
VU, W, and VW produced from the respective arms 104U, 104V, and 104W, based on a
capacitor vohage command value VC*, the capacitor vohage VCjk of each unit converter 105
15 obtained through the capacitor voltage detection line 110, the currents lU, IV, and IW obtained
via the current detecting means 108U, 108V, and 108W, the voUages VSU, VSV, and VSW
obtained via the vohage detecting means 107, and negative phase current command values Id2*
and Iq2* calculated by using an arm balance unit (FIG. 5) to be hereinafter described.
Hereinafter, the AC components VUac*, Wac*, and VWac* are collectively referred to as an
20 AC voltage command value.
The AC vohage command value generating unit 412 has at least two functions.
The total average capacitor vohage control function being one function controls an average value
VC (referred to as the total average capacitor vohage) of the vohages VCjk in the capacitors 203
of all the unit converters 105 so as to be approximately matched with the capacitor vohage
25 command value VC*. In addition, a current control function being another function controls
the currents lU, IV, and IW from the arms 104U, 104V, and 104W so as to be matched with the
command values Id* and Iq* of Id and Iq converted on a (d-q) coordinate system.
The total average capacitor vohage control function will be described below.
Based on the capacitor vohage VCjk in each unit converter 105 obtained through
30 the capacitor vohage detection line 110, the AC vohage command value generating unh 412
calculates the total average capacitor voltage VC by using the average value calculator 401.
The AC vohage command value generating unit 412 subtracts the obtained total
average capacitor vohage VC from the capacitor voltage command value VC*, and feedbackcontrols
h by using a gain 403. The AC vohage command value generating unit 412 adds a
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feed-forward item IdfF* to a value produced from the gain 403 by using an adder 404 so as to
obtain a positive phase d-axis current command value Idl *. Here, the feed-forward item Idff*
is a d-axis current command value to feed-forward control active power to be flowed to the
power conversion apparatus 102 from the three-phase power system 101.
5 Here, the gain 403 is a proportional gain (P), a proportional and integral gain (PI),
or a proportional, integral, and derivativegain (PID).
In the description of the present embodiment, a d-axis and a q-axis are supposed
to be set so that active power may flow in the power conversion apparatus 102 from the AC
system 101 in the case where the d-axis current Id is positive, and so that active power may flow
10 in the AC system 101 from the power conversion apparatus 102 in the case where the d-axis
current Id is negative. Note that even if a definition method of the d-axis and the q-axis is
changed, the present invention can be applied and the present embodiment is supposed to include
the above case.
The d-axis current Id is controlled so as to be approximately matched with the
15 positive phase d-axis current command value Idl * by using a current control function to be
hereinafter described. In this case, the AC voltage command value generating unit 412 can
control active power to flow in or out of the power conversion apparatus 102 from the threephase
power system 101 according to shortage or excess of the total average capacitor voltage
VC, and the total average capacitor voltage VC to be approximately matched with the capacitor
20 voltage command value VC*.
Next, the current control fixnction will be described.
A (a-P) conversion and a (d-q) conversion are performed to the currents lU, IV,
and IW detected by using the current detecting means 108U, 108V, and 108W so as to obtain the
d-axis current Id and the q-axis current Iq. Here, a phase angle 9 for use in the (d-q) conversion
25 is a phase angle detected by using a phase detector 411 from among the phase voltages VSU,
VSV, and VSW of the three-phase power system 101 and, for example, is synchronized with a
phase of the VSU. That is, the phase angle 9 is synchronized with a phase at the moment of the
VSU and, for example, is a signal which changes from 0 rad to 27t rad.
Based on formulae (1) and (2), a d-axis voltage command value Vd* and a q-axis
30 voltage command value Vq* given to the power conversion apparatus 102 are calculated so that
the d-axis current detection value Id may be matched with the d-axis current command value Id*
and so that the q-axis current detection value Iq may be matched with the q-axis current
command value Iq*.
[MATH. 1]
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Vd*-VSd-Gain(Id*-Id)-R-Id-X-Iq ... (1)
[MATH. 2]
Vq*=VSq-Gain(Iq*-Iq)+X-Id-R-Iq ... (2)
Here, VSd and VSq in the formulae (1) and (2) are a system d-axis voltage and a
5 system q-axis voltage of the three-phase power system obtained by performing the (a-P)
conversion and the (d-q) conversion to the voltages VSU, VSV, and VSW of the three-phase
power system 101, respectively. Further, the d-axis current command value Id* and the q-axis
current command value Iq* are obtained by using formulae (5) and (6) to be hereinafter
described.
10 Further, the gain 407 (Gain in the formulae (1) and (2)) is a proportional gain (P),
a proportional and integral gain (PI), or a proportional, integral, and derivative gain (PID).
Preferably, R in the formulae (1) and (2) is set as a winding resistance of the
transformer 103, and X therein is set as a leakage reactance of the transformer 103.
Further, the AC voltage command value generating unit 412 performs an inverse
15 (d-q) conversion and an inverse (a-P) conversion to Vd* and Vq* obtained by using the formulae
(1) and (2) so as to calculate the AC voltage command values VUac*, Wac*, and VWac*.
The DC vohage command value generating unit 413 generates a DC voltage
command value VDC*.
Further, arm voltage command values VU*, W*, and VW* are calculated by
20 using formulae (3) to (5).
[MATH. 3]
[MATH. 4]
25 [MATH. 5]
VU*=VUac*+VDC* ... (3)
W*=Wac*+VDC* ... (4)
VW*=VWac*+VDC* ...(5)
Based on the arm voltage command values VU*, W*, and VW*, a gate signal
given to each unit converter 105 is calculated by using a gate signal generating unit (FIG. 6) to be
hereinafter described.
30 An arm balance unit which is characteristic in the present invention will be
described below with reference to FIG 5. Then, a principle in which an average value of
voltages of one or a plurality of unit converters 105 included in the respective arms 104U, 104 V,
and 104W can be balanced among the arms will be described with reference to FIG. 7.
In FIG. 5, the arm balance unit calculates the negative phase current command
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values Id2* and Iq2* on the (d-q) coordinate system based on capacitor voltages VCUk, VCVk,
and VCWk of each phase obtained through the capacitor voltage detection line 110 and the phase
angle 0 detected by using the phase detector 411. A calculation method to be described below
is one example of the calculation method for obtaining the Id2* and Iq2*. As the calculation
5 method for obtaining the Id2* and the Iq2* of securing a balance among the arms of the
capacitor voltage VCjk, other calculation methods mathematically equivalent to the following
description are used, and the present embodiment is supposed to include the above methods.
First, an average value VCU of the capacitor voltage VCUk of each unit converter
105 included in the U-phase arm 104U is calculated by using an average value calculator 501.
10 Similarly, an average value VCV of the capacitor voltage VCVk of each unit converter 105
included in the V-phase arm 104V is calculated by using the average value calculator 501.
Further, an average value VCW of the capacitor voltage VCWk of each unit converter 105
included in the W-phase arm 104W is calculated by using the average value calculator 501.
Next, an average value VC=(VCU+VCV+VCW)/3 of the VCU, VCV, and VCW
15 is calculated by using an average value calculator 502. Further, VCU-VC, VCV-VC, and
VCW-VW are calculated by using a subtracter 402.
A moving average during a duration of one period or integer periods of the threephase
power system 101 is calculated to the obtained VCU-VC, VCV-VC, and VCW-VW by
using a moving average calculator 405 so as to obtain AVCU, AVCV, and AVCW. In addition,
20 a low-pass filter can be used in place of the moving average and the present embodiment is
supposed to include the above case.
Note that a (a-P) converter of FIG. 5 can double as functions of the average value
calculator 502 and the subtracter 402 of FIG. 5. Therefore, the average value calculator 502 and
the subtracter 402 of FIG. 5 are not necessarily provided.
25 Further, the (a-P) conversion is performed to the obtained AVCU, AVCV, and
AVCW so as to obtain AVCa and AVCp. Further, negative phase current command values I2a*
and I2P* on the (a-P) coordinate system are calculated based on formulae (6) and (7).
[MATH. 6]
I2a*-GainxAVCa ...(6)
30 [MATH. 7]
I2p*=-GainxAVCp ...(7)
By using the phase angle 29, the (d-q) conversion is performed to the I2a* and the
I2p* obtained by using the formulae (3) and (4) so as to obtain the negative phase current
command values Id2* and Iq2* on the (d-q) coordinate system. The (d-q) conversion is
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performed by using the phase angle 20 being twice the phase angle 0 of the VSU detected by
using the phase detector 411, thereby being equivalent to the (d-q) conversion at a double
frequency. Accordingly, the I2d* and the I2q* mean negative phase components in the three
phase coordinate system.
5 As illustrated in FIG. 4 and formulae (8) and (9), a d-axis current command value
Id* is calculated as the sum of the positive phase d-axis current command value Idl* and the
negative phase d-axis current command value Id2* obtained in FIG. 5. Further, a q-axis current
command value Iq* is calculated as the sum of the positive phase q-axis current command value
Iql* and the negative phase q-axis current command value Iq2* obtained in FIG. 5.
10 [MATH. 8]
Id*=Idl*+Id2* ... (8)
[MATH. 9]
Iq*=Iql*+Iq2* ...(9)
Based on the formulae (8) and (9), the d-axis current command value Id* and the
15 q-axis current command value Iq* include a positive phase component and a negative phase
component. The above-described current control function feedback-controls so as to match
actual Id and Iq with Id* and Iq* as much as possible, respectively.
According to the control method of the present embodiment, a zero-phase
component is not included in a signal produced from an inverse (a-p) converter 410. Therefore,
20 the command value VDC* being a signal produced from the DC voltage command value
generating unit 413 does not interfere with the command values Vuac*, Vvac*, and Vwac* being
signals produced from the current control fiinction including the command values Id2* and Iq2*
for use in arm balance control. Accordingly, in the arm balance control, variation does not
occur at the voltage VDC between the point P and the point N. Accordingly, the average values
25 VCU, VCV, and VCW can be balanced without interfering with control of the other-side power
conversion apparatus 112.
In the above description, a case where the current control function of FIG. 4 is
performed on the (d-q) coordinate system is described. When a positive phase component and
a negative phase component of the arm currents lU, IV, and IW can be controlled, the present
30 invention can be applied even if other systems, for example, a method for individually
controlling currents in three phases is used.
A method for calculating a gate signal given to each unit converter 105 based on
the arm voltage command values VU*, W*, and VW* will be described below with reference to
FIG 6. Here, as an example, FIG. 6 illustrates a case of using a triangular wave comparison
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carrier system PWM (Pulse Width Modulation) for comparing a triangular wave carrier and a
voltage command value. When there is used a system in which the arm voltage command
values VU*, W*, and VW* and actual arm voltages VU, W, and VW are controlled so as to be
matched with each other as much as possible, the above method is not limited to the triangular
5 wave comparison carrier system PWM. Other modulation system such as a space vector
modulation can be applied and the present embodiment is supposed to include the above case.
Gate signals GHUk and GLUk given to each unit converter 105 of the U-phase
arm 104U are calculated based on the U-phase arm voltage command value VU* and a triangular
wave carrier TriUk (k=l, 2, ..., Nc) corresponding to each unit converter 105 of the U-phase arm
10 104U. Here, the triangular wave carrier TriUk is a signal generated by a carrier generating unit
602.
Further, gate signals GHVk and GLVk given to each unit converter 105 of the Vphase
arm 104V are calculated based on the V-phase arm voltage command value W * and a
triangular wave carrier TriVk (k=l, 2, ..., Nc) corresponding to each unit converter 105 of the V-
15 phase arm 104V. Here, the triangular wave carrier TriVk is a signal generated by the carrier
generating unit 602.
In addition, gate signals GHWk and GLWk given to each unit converter 105 of
the W-phase arm 104W are calculated based on the W-phase arm voltage command value VW*
and a triangular wave carrier TriWk (k=l, 2, ..., Nc) corresponding to each unit converter 105 of
20 the W-phase arm 104W. Here, the triangular wave carrier TriWk is a signal generated by the
carrier generating unit 602.
The control means 106 transmits the obtained gate signals GHUk, GLUk, GHVk,
GLVk, GHWk, and GLWk to the gate driver 205 of each unit converter through the gate signal
transmission lines HI.
25 A principle in which the average values VCU, VC V, and VCW can be balanced
by using a control method of the arm balance unit illustrated in FIG. 5 will be described below
with reference to FIG 7.
FIG. 7 illustrates conceptual waveforms of phase voltages VSU, VSV, and VSW
of the three-phase power system 101, the arm voltages VU, W, and VW, positive-phase-
30 sequence components lUl, IVl, and IWl of the arm currents lU, IV, and IW, negative-phasesequence
components IU2, IV2, and IW2 of the same arm currents lU, IV, and IW, the arm
currents lU, IV, and IW, power PU, PV, and PW in the respective arms 104U, 104V, and 104W,
and capacitor vohage average values VCU, VCV, and VCW in the respective arms 104U, 104V,
and 104W.
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Note that IU=IU1+IU2, IV-IV1+IV2, and IW=IW1+IW2 hold.
Here, a unit of each waveform in the vertical axis of FIG. 7 is arbitrary. Further,
one scale in the horizontal axis is equal to one period of the power source of the three-phase
power system 101.
5 Here, through a wire connection of the transformer 103 illustrated in FIG. 8, the
phase vohages VSU, VSV, and VSW advance by about BOD as compared to AC components of
the arm vohages VU, W, and VW.
Further, in power of the respective arms 104U, 104V, and 104W, PU=VUxIU,
PV=WxIV, and PW=VWxIW hold. Based on a definition of a direction in the vohages VU,
10 W, and VW and the currents lU, IV, and IW of FIG. 1, the direction to which power PU, PV, and
PW are produced from the respective arms 104U, 104V, and 104W is positive.
For explaining effects of the present invention in a simplified manner, FIG. 7
illustrates a state in which an imbalance occurs in the average values VCU, VCV, and VCW at
the time TO. Note that the arm balance control illustrated in FIG. 5 is not operated at this
15 moment.
When the arm balance control of FIG. 5 is operated at the time Tl, the current
control function of the AC voltage command value generating unit 412 controls the negativephase-
sequence components IU2, IV2, and IW2 of the arm currents lU, IV, and IW to flow
through each arm.
20 At the time Tl of FIG. 7, the capacitor voltage average value VCU in the arm of
the U-phase arm 104U is lower than the VCV and the VCW
Under the above conditions, a calculation of FIG. 5 and the current control
function of FIG. 4 permit an AC component of the VU and the negative-phase-sequence current
IU2 of a negative phase to flow to the U-phase arm 104.
25 As a resuh, the power PU, PV, and PW can be made to be unbalanced
intentionally. The process permits energy to flow into the U-phase arm 104U from the V-phase
arm 104V and the W-phase arm 104W.
Energy flowing in and out is charged and discharged in capacitors of the unit
converters 105 included in the respective arms 104U, 104V, and 104W. As a resuh, the
30 capacitor voltage average values VCU, VCV, and VCW can be balanced.
As described above, when a negative-phase-sequence component of an arm
current is controlled, an effect in which the capachor voltage average values VCU, VCV, and
VCW in the respective arms 104U, 104V, and 104W can be balanced is obtained.
That is, a current which flows through each arm is controlled to have amplitude
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and a phase different from each other so as to be balanced.
In the above description, a method for balancing the capacitor vohage average
values VCU, VCV, and VCW in the respective arms 104U, 104V, and 104W is described. An
average value of electrostatic energy in capacitors of the respective arms 104U, 104 V, and 104W
5 can be controlled so as to be balanced in place of the capacitor voltage average values VCU,
VCV, and VCW. In this case, it suffices that when electrostatic energy of each capacitor 203 is
equal to C, electrostatic energy WCjk=Cx VCjk2/2 is calculated based on the voltage VCjk of
each capacitor 203, and fixrther calculations of FIGS. 4 and 5 are performed by using the WCjk
in place of the VCjk.
10 [Second Embodiment]
A second embodiment of the present invention will be described.
In the first embodiment, the unit converter 105a (FIG. 2) in the bidirectional
chopper circuit system is used in each of the arms 104U, 104 V, and 104W. The present
embodiment has a configuration in which a part or all of unit converters 105 are replaced by the
15 unit converters 105b (FIG. 3) in the fiiU-bridge circuit system illustrated in FIG. 3.
According to the present embodiment, in addition to the effects to be obtained in
the first embodiment, an effect in which a voltage VDC between a DC positive side terminal
(point P) and a DC negative side terminal (point N) can be controlled so as to be positive, zero,
or negative is obtained.
20 Only points in which the present embodiment differs from the first embodiment
will be described below.
First, an internal configuration of the unit converter 105b in the fiiU-bridge circuit
system will be described with reference to FIG. 3.
A main circuit of the unit converter 105b in the fiill-bridge circuit system has a
25 configuration in which a circuit in which an anti-parallel connection circuit including the Xphase
high-side switching element 201XH and the X-phase high-side freewheeling diode 202XH
and another anti-parallel connection circuit including the X-phase low-side switching element
201 XL and the X-phase low-side freewheeling diode 202XL are connected in series, another
circuit in which an anti-parallel connection circuit including the Y-phase high-side switching
30 element 201YH and the Y-phase high-side freewheeling diode 202YH and another anti-parallel
connection circuit including the Y-phase low-side switching element 201YL and the Y-phase
low-side freewheeling diode 202YL are connected in series, and the capacitor 301 are connected
in parallel.
Here, a series connection point between the anti-parallel connection circuit
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including the X-phase high-side switching element 201XH and the X-phase high-side
freewheeling diode 202XH and the another anti-parallel connection circuit including the X-phase
low-side switching element 201 XL and the X-phase low-side freewheeling diode 202XL is
referred to as a point X.
5 Further, a series connection point between the anti-parallel connection circuit
including the Y-phase high-side switching element 201YH and the Y-phase high-side
freewheeling diode 202YH and the another anti-parallel connection circuit including the Y-phase
low-side switching element 201YL and the Y-phase low-side freewheeling diode 202YL is
referred to as a point Y.
10 A voltage Vjk between the point X and the point Y is referred to as an output
voltage of the unit converter 105b in the full-bridge circuit system.
The unit converter 105b in the full-bridge circuit system includes the vohage
detecting means 109 for detecting the capacitor voltage VCjk (j=U, V, and W, and k=l, 2, ...,
Nc), and is connected to the control means 106 through the capacitor voltage detection line 110.
15 In addition, the unit converter 105b in the fiill-bridge circuit system includes a
gate driver 302 which applies a gate voltage between a gate and an emitter of each of the
switching elements 201XH, 201 XL, 201YH, and 201 YL based on the gate signals GXHjk,
GXLjk, GYHjk, and GYLjk transmitted from the control means 106 through the gate signal line
111.
20 A relationship between the output voltage Vjk of the unit converter 105b in the
full-bridge circuit system and an ON/OFF state of the switching elements 201XH, 201 XL,
201YH, and 201YL will be described below.
In the case where the X-phase high-side switching element 201XH is ON, the Xphase
low-side switching element 201 XL is OFF, the Y-phase high-side switching element
25 201YH is ON, and the Y-phase low-side switching element 201YL is OFF, the output voltage
Vjk can be controlled so as to be approximately equal to zero regardless of the current Ij.
In the case where the X-phase high-side switching element 201XH is ON, the Xphase
low-side switching element 201 XL is OFF, the Y-phase high-side switching element
201 YH is OFF, and the Y-phase low-side switching element 201YL is ON, the output voltage
30 Vjk can be controlled so as to be approximately equal to the capacitor voltage VCjk regardless of
the current Ij.
In the case where the X-phase high-side switching element 201XH is OFF, the Xphase
low-side switching element 201 XL is ON, the Y-phase high-side switching element
201YH is ON, and the Y-phase low-side switching element 201YL is OFF, the output vohage
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Vjk can be controlled so as to be approximately equal to a vohage -VCjk having a polarity
opposite to that of the capacitor voltage VCjk regardless of the current Ij.
In the case where the X-phase high-side switching element 201XH is OFF, the Xphase
low-side switching element 201 XL is ON, the Y-phase high-side switching element
5 201YH is OFF, and the Y-phase low-side switching element 201YL is ON, the output voltage
Vjk can be controlled so as to be approximately equal to zero regardless of the current Ij.
The unit converter 105b in the full-bridge circuit system generates the gate signals
GXHjk, GXLjk, GYHjk, and GYLjk corresponding to the above-described switching elements
201XH, 201 XL, 201YH, and 201YL. For this purpose, in the present embodiment, in the case
10 where the corresponding unit converter 105 is the unit converter 105b in the full-bridge circuit
system, the triangular wave comparison PWM pulse generator 601 illustrated in FIG. 6 supplies
the gate signals GXHjk, GXLjk, GYHjk, and GYLjk in place of the gate signals GHUk, GLUk,
GHVk, GLVk, GHWk, and GLWk.
The unit converter 105b in the full-bridge circuit system supplies a negative
15 voltage, namely, -VCjk as described above. Therefore, when the VDC* produced from the DC
voltage command value generating unit 413 illustrated in FIG 4 is set to be zero or negative, an
effect in which an actual value of the VDC can be controlled so as to be zero or negative is
obtained.
[Third Embodiment]
20 A third embodiment of the present invention will be described with reference to
FIG. 9.
The present embodiment differs from the first and second embodiments in that a
transformer 901 in which an open winding is used as the secondary winding is used in place of
the transformer 103 and in that the arms 104U, 104V, and 104W are divided so as to be arms
25 903U, 903V, and 903W.
In the present embodiment, an effect that a potential of a transformer can be freely
changed is obtained in addition to the same effects as those of the first and second embodiments.
Points in which the present embodiment differs from the first and second
embodiments will be described below.
30 The secondary winding of a transformer 902 is an open winding, and the
transformer 902 includes at least six terminals at points ul, u2, vl, v2, wl, and w2. An internal
configuration of the transformer 902 will be described later.
One part of the U-phase arm 903U is connected to a DC positive side terminal
(point P) at the point ul. An output voltage of this part of the U-phase arm 903U is supposed to
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beVUl.
Further, another part of the U-phase arm 903U is connected to a DC negative side
terminal (point N) at the point u2. An output voltage of this part of the U-phase arm 903U is
supposed to be VU2.
5 Similarly, one part of the V-phase arm 903 V is connected to the DC positive side
terminal (point P) at the point vl. An output voltage of this part of the V-phase arm 903 V is
supposed to be W l .
In addition, another part of the V-phase arm 903 V is connected to the DC negative
side terminal (point N) at the point V2. An output voltage of this part of the V-phase arm 903 V
10 is supposed to be W2.
Further, one part of the W-phase arm 903W is connected to the DC positive side
terminal (point P) at the point wl. An output voltage of this part of the W-phase arm 903 W is
supposed to be VWl.
In addition, another part of the W-phase arm 903 W is connected to the DC
15 negative side terminal (point N) at the point w2. An output voltage of this part of the W-phase
arm 903W is supposed to be VW2.
The present embodiment differs from the first and second embodiments in that the
secondary winding (802) of the transformer is inserted into each of the arms 903U, 903 V, and
903W on the way. Also in the present embodiment, circuits in which the secondary winding
20 (802) of the transformer and each of the arms 90 lU, 901V, and 90IW are connected in series are
connected in parallel at the points P and N.
Here, when VU=VU1+VU2 holds in the U-phase arm 903U, the power
conversion apparatus of the present embodiment can be controlled similarly to those of the first
and second embodiments by using the control method illustrated in FIGS. 4 to 6.
25 Amplitude of the VUl and that of the VU2 may be equivalent or different.
When a ratio of the amplitude of the VUl and that of the VU2 are changed, a DC component at
potential of the U-phase secondary windings (802Up and 802 Vn) of the transformer 103 can be
changed between potential at the point P and that at the point N. That is, an effect that a DC
component at potential of the U-phase secondary windings (802Up and 802 Vn) can be freely
30 changed is obtained.
Similarly, when W=W1+W2 holds in the V-phase arm 903 V, the power
conversion apparatus of the present embodiment can be controlled similarly to those of the first
and second embodiments by using the control method illustrated in FIGS. 4 to 6.
Amplitude of the W l and that of the W 2 may be equivalent or different.
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When a ratio of the amplitude of the W l and that of the W 2 are changed, a DC component at
potential of the V-phase secondary windings (802 Vp and 802Wn) of the transformer 103 can be
changed between potential at the point P and that at the point N. That is, an effect that a DC
component at potential of the V-phase secondary windings (802 Vp and 802Wn) can be freely
5 changed is obtained.
Further, when VW=VW1+VW2 holds in the W-phase arm 903 W, the power
conversion apparatus of the present embodiment can be controlled similarly to those of the first
and second embodiments by using the control method illustrated in FIGS. 4 to 6.
Amplitude of the VWl and that of the VW2 may be equivalent or different.
10 When a ratio of the amplitude of the VWl and that of the VW2 are changed, a DC component at
potential of the W-phase secondary windings (802 Wp and 802Un) of the transformer 103 can be
changed between potential at the point P and that at the point N. That is, an effect that a DC
component at potential of the W-phase secondary windings (802 Wp and 802Un) can be freely
changed is obtained.
15 Further, in the same manner as in the first embodiment, the unit converter 105 can
be changed into the unit converter 105a in the bidirectional chopper circuit system. In this case,
the same effect as that of the first embodiment is obtained.
In addition, in the same manner as in the second embodiment, the unit converter
105 can be formed into the arms 903U, 903 V, and 903W in which the unit converter 105a in the
20 bidirectional chopper circuit system and the unit converter 105b in the full-bridge circuit system
are mixedly used. In this case, the same effect as that of the second embodiment is obtained.
One example of an internal configuration of the transformer 902 wall be described
below with reference to FIG. 10.
The primary winding 80 lU and the secondary divided windings 802Un and
25 802Up are wound onto the core 804U. Similarly, the primary winding 801V and the secondary
divided windings 802Vn and 802Vp are wound onto the core 804V. Further, the primary
winding 801W and the secondary divided windings 802Wn and 802Wp are wound onto the core
804W.
The primary windings 80lU, 801V, and 801W are star-connected at the point M.
30 One ends of the secondary divided windings 802Up, 802 Vp, and 802Wp are
connected to portions near to the point P of the arms 90 lU, 901V, and 901W via the points ul,
vl, and wl, respectively.
Further, one ends of the secondary divided windings 802Un, 802 Vn, and 802 Wn
are connected to portions near to the point N of the arms 90 lU, 901V, and 90IW via the points
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w2, u2, and v2, respectively.
The other ends of the secondary divided windings 802Up, 802 Vp, and 802Wp on
the side opposite to the arms 903U, 903V, and 903W are connected to ends of the secondary
divided windings 802Vn, 802Wn, and 802Un on the side opposite to the arms 903U, 903V, and
5 903 W, respectively.
In other words, six secondary divided windings 802Up, 802Un, 802Vp, 802Vn,
802Wp, and 802Wn are open-zigzag-connected. In the present embodiment, the six secondary
divided windings 802Up, 802Un, 802Vp, 802Vn, 802Wp, and 802Wn are collectively referred to
simply as a secondary winding. Here, the number of windings of the secondary divided
10 windings 802Up, 802Un, 802Vp, 802Vn, 802Wp, and 802Wn is approximately equal to each
other
FIG. 10 illustrates a case where the primary windings 80 lU, 801V, and 80IW are
star-connected. Further, the present invention can be applied also to a case where the primary
windings 80 lU, 801V, and 80IW are delta-connected, and the present embodiment is supposed
15 to include the above case.
As a case where the secondary divided windings 802Up, 802Un, 802Vp, 802Vn,
802Wp, and 802Wn are connected, for example, FIG. 10 further illustrates a case where the
secondary divided windings 802Up and 802Vn are connected. Further, the present invention
can be applied also to a case where the secondary divided windings 802Up, 802Un, 802Vp,
20 802Vn, 802Wp, and 802Wn are zigzag-connected by using a combination of the secondary
divided windings different from that of FIG. 7. The present embodiment is supposed to include
the above case.
Further, in the specification, among windings of the transformer 103, a winding
connected to the three-phase power system 101 is referred to as the primary winding, and
25 windings connected to the arms 104U, 104V, and 104W are referred to as the secondary divided
winding. "Primary" and "Secondary" are politic appellation for explanation. Even if the
appellation is opposite, the present invention can be applied.
Here, an example where a secondary divided winding of a transformer is
connected in series to an arm is described. Similarly, the present invention can be applied to the
30 power conversion apparatus in which an arm is configured by two arms of an upper arm and a
lower arm, an AC terminal is provided between the upper arm and the lower arm, and a
secondary divided winding of a transformer is connected to this AC terminal.
It should be further understood by those skilled in the art that ahhough the
foregoing description has been made on embodiments of the invention, the invention is not
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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.

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CLAIMS:
1. A power conversion apparatus (102) comprising:
unit converters being configured as an at least two-terminal converter (105);
arms (104U, 104 V, 104W) configured by connecting one or a plurality of unit
converters in series;
a serial circuit configured by connecting the arms (104U, 104V, 104W) and
windings on a predetermined order side of a transformer (103) in series;
DC terminals as connection parts when connecting the serial circuits in parallel;
and
a plurality of phase power sources or phase loads connected to the winding on
another order side of the transformer (103);
wherein the unit convertor has at least one energy storage element and at least
one switching element controlling an output depending on a voltage of the energy storage
element;
wherein the power conversion apparatus (102) controls imbalance of storage
energy or voltage of the energy storage element so that a current flowing through each of the
arms (104U, 104V, 104W) has a phase and amplitude different from each other to be
approximated to balance.
2. The power conversion apparatus (102) according to claim 1, wherein the unit
converter (105) is a bidirectional chopper circuit or a full-bridge circuit including a capacitor as
an energy storage element.
3. The power conversion apparatus (102) according to claim 1, wherein a
bidirectional chopper circuit and a full-bridge circuit having a capacitor as an energy storage
element are mixedly used in the arms (104U, 104V, 104W).
4. The power conversion apparatus (102) according to claim 1, wherein the plurality
of phase power sources are set as a power system.
5. The power conversion apparatus (102) according to claim 1, wherein to an arm
(104U, 104 V, 104W) in which an average value in the arm (104U, 104 V, 104W) of a voltage or
energy of the energy storage element is short or excessive, average power formed by a current
flowing through the arm (104U, 104 V, 104W) and an output vohage of the arm (104U, 104 V,
104W) is a direction of flowing in or out of the arm (104U, 104 V, 104W).
6. The power conversion apparatus (102) according to claim 1, wherein the power
conversion apparatus (102) controls a positive-phase-sequence current and a negative-phasesequence
current flowing through the arms (104U, 104V, 104W).
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7. The power conversion apparatus (102) according to claim 1, wherein when a
negative-phase-sequence current is controlled on a (d-q) axis and a command value of the
negative-phase-sequence current is set as Id2* and Iq2*, the power conversion apparatus (102)
calculates an average value in each of the plurality of arms (104U, 104 V, 104W) of a voltage or
energy of the energy storage element and a difference of the plurality of average values of
voltages or energy of all the energy storage elements, performs a moving average or a low-pass
filter to the obtained difference, performs a (a-|3) conversion, multiplies a signal obtained by
inverting polarities of the obtained a-axis component and p-axis component by a gain, and
performs an inverse (d-q) conversion so as to obtain the Id2* and the Iq2*.
8. The power conversion apparatus (102) according to claim 1, wherein upon
configuring a series circuit of a transformer winding and an arm (104U, 104 V, 104W) configured
by connecting one or a plurality of the unit converters (105) in series, the transformer winding is
connected in series so as to be interposed between the one or a plurality of the unit converters
(105).
9. A power conversion apparatus (102) comprising:
a plurality of arms (104U, 104 V, 104W) connected in parallel; and
a winding on a predetermined order side of a transformer (103);
wherein some arms of a plurality of the parallel-connection arms (104U, 104 V,
104W) are formed as a first DC side mutually connected;
wherein other arms of a plurality of the parallel-connection arms (104U, 104 V,
104W) are formed as a second DC side mutually connected;
wherein each of the plurality of arms (104U, 104 V, 104W) is connected to one or
a plurality of unit converters (105) in series;
wherein the unit converter (105) has at least one energy storage element and at
least one switching element for outputting a voltage depending on a voltage of the energy storage
element;
wherein the winding on a predetermined order side of a transformer (103) is a
winding provided between the first DC side and the second DC side or a winding connected to a
terminal provided between the first DC side and the second DC side;
wherein the winding on a predetermined order side of a transformer (103) is
electrically connected to a winding on another order side; and
wherein the power conversion apparatus (102) controls a current flowing through
each of the arms (104U, 104 V, 104W) to have a phase and amplitude different fi"om each other
and approximate imbalance of a voltage or storage energy of the energy storage element to
- 2 6 -
balance.
10. A method for controlling a power conversion apparatus (102); the power
conversion apparatus (102) comprising: a plurality of arms (104U, 104V, 104W) connected in
parallel; and a winding on a predetermined order side of a transformer (103); wherein some arms
of a plurality of the parallel-connection arms (104U, 104V, 104W) are formed as a first DC side
mutually connected; wherein other arms of a plurality of the parallel-connection arms (104U,
104V, 104W) are formed as a second DC side mutually connected; wherein each of the plurality
of arms (104U, 104V, 104W) is connected to one or a plurality of unit converters (105) in series;
wherein the unit converter (105) has at least one energy storage element and at least one
switching element for outputting a voltage depending on a voltage of the energy storage element;
wherein the winding on a predetermined order side of a transformer (103) is a winding provided
between the first DC side and the second DC side or a winding connected to a terminal provided
between the first DC side and the second DC side; and wherein the winding on a predetermined
order side of a transformer (103) is electrically connected to a winding on another order side, the
method comprising the steps of:
calculating a command value to control a current flowing through each of the
arms (104U, 104V, 104W) to have a phase and amplitude different from each other and
approximate imbalance of storage energy or a voltage of the energy storage element to balance;
and
controlling a current flowing through each of the arms (104U, 104V, 104W) based
on the command value.
11. A power conversion apparatus, substantially as herein described with reference to
accompanying drawings and examples.
12. A method for controlling a power conversion apparatus, substantially as herein
described with reference to accompanying drawings and examples.