JP6430230B2 - Power converter and control method of power converter - Google Patents

Description

The present invention relates to a power converter and a method for controlling the power converter, and more particularly to an AC-DC comprising an arm and a transformer configured by connecting one or more unit converters each having a switching element and an energy storage element in series. The present invention relates to a power converter suitable for detecting an exciting current of a transformer in a power converter and a method for controlling the power converter.

An MMCC (modular multilevel converter) in which unit converters are connected in series is known as a technology for mutually converting direct current and alternating current at a high voltage used for direct current power transmission and the like. As this MMCC technology, MMCC-DSCC (modular multilevel converter based on double-star chopper cells) is a multi-level waveform close to a sine wave and a high voltage higher than the breakdown voltage of a switching element such as an IGBT (insulated-gate bipolar transistor). Is an AC / DC power conversion circuit that can directly output. Such a technique is described in Non-Patent Document 1, for example.
In MMCC-DSCC, two buffer reactors are required for each phase. Therefore, a unit converter provided with a switching element and an energy storage element as an AC / DC power conversion circuit that does not require the buffer reactor required in MMCC-DSCC. An arm constructed by connecting one or more in series with the winding of the transformer operates in series.
In this technique, when AC-DC power conversion is performed using a power converter, a DC current is passed through the secondary winding (winding connected to the arm) of the transformer in order to exchange DC power. Generally, when a direct current is passed through a transformer, a direct current magnetic flux is generated in the iron core of the transformer. This is referred to as DC bias. If the DC bias is excessive, the magnetic flux density will exceed the saturation magnetic flux density of the iron core, impeding normal operation of the transformer.

Therefore, the secondary winding of the transformer is divided into two, a positive secondary winding and a negative secondary winding. Due to the DC current, a DC magnetomotive force is generated in the positive secondary winding and the negative secondary winding, but the positive secondary winding and the negative secondary winding are opposite to the iron core. Since they are wound in the directions, the DC magnetomotive forces cancel each other. Therefore, the DC magnetic flux caused by the DC current for transmitting / receiving DC power is not generated in the transformer core. As a result, AC / DC power conversion is possible without causing DC bias. Such a technique is described in Patent Document 1, for example.

JP 2013-099054 A

H. Akagi, “Classification, terminology, and application of the modular multilevel cascade converter (MMCC),” IEEE Transactions on Power Electronics, vol. 26, no. 11, Nov. 2011, pp. 3119-3129.

In the above-described prior art, a DC component is generated in the excitation current of the transformer due to component variations, IGBT on / off control timing variations, offset errors of current / voltage sensors, etc. May occur.

The direct current component of the excitation current is different from the “direct current for transmitting and receiving DC power” mentioned in the above explanation, and its magnetomotive force is canceled by the positive secondary winding and the negative secondary winding. is not. If the DC bias becomes excessive, it will also hinder the normal operation of the transformer.

In order to detect the DC bias of the transformer, the “DC component of the excitation current” must be detected separately from the “DC current for transmitting and receiving DC power”.

An object of the present invention is to provide a power converter capable of detecting a DC component of an exciting current and a method for controlling the power converter in view of the above problems.

In order to achieve the above object, the present invention passes the current flowing in the primary winding through one current detection means and the two currents flowing in the two other windings as the other current detection means. In addition, it is configured such that the direct currents flow in opposite directions, and the outputs of one and the other current detection means are connected to one detection resistor for each phase.
Alternatively, the current flowing in the primary winding is passed through one current detection means, and the two currents flowing in the two other windings are passed through the other current detection means, and the direct currents flow in opposite directions. Thus, analog-digital conversion means are provided at the outputs of one and the other current detection means, respectively, and the function of subtracting the output digital signal of the analog-digital conversion means is provided.
Alternatively, the current flowing in the primary winding is passed through the first current detection means, and the current flowing in the two other windings is passed through the second and third current detection means, respectively. The current detection means is connected to one detection resistor.
Alternatively, the current flowing in the primary winding is passed through the first current detection means, and the current flowing in the two other windings is passed through the second and third current detection means, respectively. An analog-digital conversion means is provided at each output of the current detection means, and a function of adding or subtracting the output digital signal of the analog-digital conversion means is provided.
Alternatively, based on the current value of the current flowing in the first other side winding, the current value of the current flowing in the second other side winding, and the current value of the current flowing in the one side winding, the transformer The direct current component of the excitation current was detected.

According to the present invention, it is possible to obtain an effect that the DC component of the exciting current of the transformer can be detected separately from the DC current for transmitting and receiving DC power.

1st form of the power converter device by this invention Bidirectional chopper type unit converter Full-bridge type unit converter Transformer with excitation current detection means (Part 1) Relationship between magnetic flux and exciting current in transformer (Φ-I characteristics) Schematic waveform explaining excitation current detection method When the current detection means is an ideal transformer When the current detection means is an actual AC current transformer Transformer with excitation current detection means (Part 2) Transformer with excitation current detection means (Part 3) Transformer with excitation current detection means (Part 4)

Hereinafter, embodiments for carrying out the present invention will be described with reference to the drawings.

First, a first embodiment of the present invention will be described.

In the first embodiment, one current detecting means for detecting the current flowing in the primary winding of the transformer in the power converter is connected to each phase, and the sum of the current flowing in the two secondary windings is detected. One current detection means is connected to each phase.

In the first embodiment, the DC component of the exciting current of the transformer can be detected separately from the DC current for transmitting and receiving DC power.

The overall configuration of the first embodiment will be described below with reference to FIG.

The power conversion device 103 is a device that transmits power from the AC system 101 to the DC device 113 or vice versa.

Here, the DC device 113 is a device corresponding to a load that consumes DC power or a power source that supplies DC power, and is, for example, another AC / DC converter, a DC transmission network, a load resistor, a storage battery, or the like. .

The power conversion device 103 is connected to the AC system 101 via the circuit breaker 102 and the transformer 104. Further, it is connected to the DC device 113 via the DC power transmission lines 112P and 112N.

In addition, the power conversion device 103 includes a control means 106, and has a function of detecting at least voltage and current of each unit described later and controlling on / off of switching elements included in the power conversion device 103.

Hereinafter, the connection of the terminals a, b, c of the primary winding of the transformer 104, the terminals u, v, w of the positive secondary winding, and the terminals r, s, t of the negative secondary winding will be described. To do.

Points a, b, and c of the transformer 104 are connected to the power system 101 via the circuit breaker 102.

Also, one end of the arm 105u, v, w is connected to the u, v, w point of the transformer. The other ends of the arms 105u, v, and w are star-connected at point P and connected to the positive side DC power transmission line 112P.

Similarly, one ends of the arms 105r, s, and t are connected to the r, s, and t points of the transformer. The other ends of the arms 105r, s, and t are star-connected at the N point and connected to the negative-side DC transmission line 112N.

In the present specification, the arms 105u, v, w, r, s, and t are collectively referred to simply as “arm 105” unless it is necessary to distinguish between them.

Each arm 105 has a configuration in which one or a plurality of unit converters 111 are connected in series. The internal configuration of the unit converter 111 will be described later.

The voltage / current detection values of the respective parts input to the control means 106 and the switching element on / off commands output from the control means 106 will be described below.

As shown in FIG. 1, the currents flowing through the arms 105 are expressed as Iu, Iv, Iw, Ir, Is, It and are called arm currents.

In order to detect each arm current, current detection means 109 is connected to each arm 105. The current detection means 109 transmits the arm current detection value to the control means 106.

Further, the phase voltage of the AC system 101 is expressed as VSa, VSb, VSc, and the line voltage is expressed as VSab, VSbc, VSca. When it is not particularly necessary to distinguish, the phase voltage or the line voltage of the AC system 101 is generically referred to as “system voltage”.

In order to detect the phase voltage or the line voltage of the AC system 101, the voltage detection means 110 is connected to the AC system 101. The voltage detection unit 110 transmits the detected value of the phase voltage or line voltage of the AC system 101 to the control unit 106.

As will be described later with reference to FIGS. 2 and 3, each unit converter 111 includes a capacitor 203 and voltage detection means 205 that detects the voltage of the capacitor 203. The voltage detection means 205 transmits the detected value of the voltage of the capacitor 203 to the control means 106 via the control communication line 107. Hereinafter, the voltage of the capacitor 203 is simply referred to as “capacitor voltage”.

Furthermore, the transformer 104 includes excitation current detection means that is a feature of the present invention, and transmits detection values VBab, VBbc, VBca of excitation current to the control means 106. The excitation current detection means will be described later with reference to FIG.

The control means 106 performs processing based on the detected arm current, system voltage, and capacitor voltage, and the switching element of each unit converter 111 (switching described with reference to FIGS. 2 and 3) via the control communication line 108. An ON / OFF command for the elements 201XP, XN, YP, YN) is transmitted.

Due to the action of the control means 106, the power conversion device 103 can exchange desired active power and reactive power with the AC system 101. Further, the power conversion device 103 can supply a desired voltage to the DC device 113 or exchange desired power with the DC device 103 by the function of the control means 106.

The voltage at point P with reference to point N is denoted as VDC and is referred to as the system DC voltage. In addition, the current flowing from the power conversion device 103 to the DC device 113 at the point P is denoted as IDC and is referred to as a system DC current.

The principle that the power converter 103 exchanges desired active power and reactive power with the AC system 101, and the power converter 103 supplies a desired voltage to the DC device 113, or the DC device 113 and the desired power are supplied. The principle of exchange will be described after the description of the internal configuration of the unit converter 111.

Hereinafter, the internal configuration of the bidirectional chopper type unit converter 111c, which is an example of the unit converter 111, will be described with reference to FIG.

However, the effect of the present invention can be obtained as long as it is not limited to FIG. 2 and is a single-phase voltage source converter including a switching element and an energy storage element.

As shown in FIG. 2, the antiparallel circuit of the switching element 201XP and the freewheeling diode 202XP and the reverse parallel circuit of the switching element 201XN and the freewheeling diode 202XN are connected in series at the point x.

Further, the series circuit and the capacitor 203 which is a kind of energy storage element are connected in parallel at the p point and the n point.

When there is no need to distinguish between them, the switching elements 201XP and XN and switching elements 201YP and YN of FIG. 3 to be described later are collectively referred to as “switching element 201”. Similarly, the free-wheeling diodes 202XP and XN and the free-wheeling diodes 202YP and YN of FIG. 3 to be described later are collectively referred to simply as “free-wheeling diodes”.

The voltage of the capacitor 203 is denoted as VCjk and is simply referred to as “capacitor voltage”. In addition, the voltage at the point x with respect to the point n is denoted as Vjk and is referred to as “the output voltage of the unit converter”. Here, j represents the arm to which each unit converter belongs, and j = u, v, w, r, s, t. K is the number of the unit converter in each arm 105, and k = 1, 2,..., Nc. Nc is the number of unit converters 111 in each arm 105.

Hereinafter, it will be described that the output voltage Vjk of the bidirectional chopper type unit converter 111c can be controlled by controlling the on / off state of the switching element 201.

When the switching element 201XP is on and the switching element 201XN is off, the point x and the point p are substantially at the same potential regardless of the direction of the current Ij, so that Vjk = VCjk.

When the switching element 201XP is off and the switching element 201XN is on, the point x and the point n are substantially at the same potential regardless of the direction of the current Ij, so that Vjk = 0.

Therefore, by controlling the on / off state of the switching element 201, the output voltage of the bidirectional chopper type unit converter can be controlled to Vjk = VCjk or Vjk = 0.

The unit converter 111c includes a bidirectional chopper type unit converter control means 204. The bidirectional chopper type unit converter control means 204 receives an on / off command gjk received from the control means 106 via the control communication line 107. On / off state is controlled by controlling the gate-emitter voltage of the switching elements 201XP and XN based on the above.

Further, the bidirectional chopper type unit converter control means 204 detects the voltage VCjk of the capacitor 203 by the voltage detection means 205 and transmits it to the control means 106 via the control communication line 107.

Hereinafter, with reference to FIG. 3, an internal configuration of a full bridge type unit converter 111f, which is another configuration example of the unit converter 111, is shown.

As shown in FIG. 3, the antiparallel circuit of switching element 201XP and freewheeling diode 202XP and the reverse parallel circuit of switching element 201XN and freewheeling diode 202XN are connected in series at point x. This is referred to as a first series circuit.

Similarly, an antiparallel circuit of the switching element 201YP and the freewheeling diode 202YP and an antiparallel circuit of the switching element 201YN and the freewheeling diode 202YN are connected in series at the point y. This is referred to as a second series circuit.

The first and second series circuits and the capacitor 203 are connected in parallel at points p and n.

The voltage at the point x with respect to the point y is denoted as Vjk and will be referred to as “unit converter output voltage”. Here, j represents the arm to which each unit converter belongs, and j = u, v, w, r, s, t. K is the number of the unit converter in each arm 105, and k = 1, 2,..., Nc. Nc is the number of unit converters 111 in each arm 105.
Hereinafter, it will be described that the output voltage Vjk of the bidirectional chopper type unit converter 111c can be controlled by controlling the on / off state of the switching element 201.

When the switching element 201XP is on, the switching element 201XN is off, the switching element 201YP is on, and the switching element 201YN is off, both the point x and the point y are approximately the same potential as the point p regardless of the direction of the current Ij. Therefore, Vjk = 0 in general.

When the switching element 201XP is on, the switching element 201XN is off, the switching element 201YP is off, and the switching element 201YN is on, the x point is approximately the p point and the y point is approximately the n point regardless of the direction of the current Ij. Since the potential is the same, Vjk = VCjk.

When the switching element 201XP is off, the switching element 201XN is on, the switching element 201YP is off, and the switching element 201YN is on, the x point and the y point are approximately the same potential as the n point regardless of the direction of the current Ij. Therefore, Vjk = 0 in general.

When the switching element 201XP is off, the switching element 201XN is on, the switching element 201YP is on, and the switching element 201YN is off, the x point is approximately the n point and the y point is approximately the p point regardless of the direction of the current Ij. Since the potential is the same, Vjk = −VCjk.

Therefore, by controlling the ON / OFF state of the switching element 201, the output voltage of the bidirectional chopper type unit converter can be controlled to Vjk = VCjk, Vjk = 0, or Vjk = −VCjk.

The unit converter 111f includes a full bridge type unit converter control means 301. The full bridge type unit converter control means 301 is based on an on / off command gjk received from the control means 106 via the control communication line 107. Thus, the on / off state is controlled by controlling the gate-emitter voltage of the switching elements 201XP, XN, YP, YN.

Further, the full bridge unit converter control means 301 detects the voltage VCjk of the capacitor 203 by the voltage detection means 205 and transmits it to the control means 106 via the control communication line 107.

2 and 3, the symbol of IGBT (insulated-gate bipolar transistor) is drawn as the switching element 201. However, if the switching element can be controlled to be turned on / off, it is a MOSFET (metal-oxide semiconductor field effect transistor), GCT. The effects of the present invention can be obtained even with other types of switching elements such as (gate-commutated thyristor) and BJT (bipolar junction transistor).

Hereinafter, the internal configuration of the transformer 104 will be described with reference to FIG.

The transformer 104 includes iron core legs 401ab, bc, ca, primary windings 402ab, bc, ca, positive secondary windings 403u, v, w, and negative secondary windings 403r, s, t. .

In addition, current detection means 404 and 405, a detection resistor 406, and a voltage detection means 407 are provided as excitation current detection means.

First, focusing on the iron core leg 401ab, the connection of the wound winding will be described. The current detection means 404 and 405, the detection resistor 406, and the voltage detection means 407 will be described later.

A primary winding 402ab, a positive secondary winding 403u, and a negative secondary winding 403r are wound around the iron core leg 401ab.

The number of turns of the positive side secondary winding 403u and the negative side secondary winding 403r is substantially the same, and they are connected to each other at the M point. At this time, the connection is made so that the electromotive force at the point u based on the point M and the electromotive force at the point r based on the point M have the same polarity.

A primary winding 402bc, a positive secondary winding 403v, and a negative secondary winding 403s are wound around the iron core leg 401bc.

The number of turns of the positive side secondary winding 403v and the negative side secondary winding 403s is substantially the same, and they are connected to each other at the M point. At this time, the connection is made so that the electromotive force at the point v with respect to the point M and the electromotive force at the point s with respect to the point M have the same polarity.

Similarly, a primary winding 402ca, a positive secondary winding 403w, and a negative secondary winding 403t are wound around the iron core leg 401ca.

The number of turns of the positive secondary winding 403w and the negative secondary winding 403t is substantially the same, and they are connected to each other at the M point. At this time, the electromotive force at the point w with respect to the point M and the electromotive force at the point t with respect to the point M are connected so as to have the same polarity.

Hereinafter, when there is no need to distinguish between them, the iron core legs 401ab, bc, ca are collectively referred to as “iron legs 401”. Similarly, the primary windings 402ab, bc, ca are collectively referred to simply as “primary winding 402”, and the positive secondary windings 403u, v, w are collectively referred to simply as “positive secondary winding”, The negative secondary windings 403r, s, and t are collectively referred to simply as “negative secondary windings”. Further, the positive secondary winding and the negative secondary winding are collectively referred to simply as “secondary winding”.

Hereinafter, the principle by which the power converter 103 exchanges AC active / reactive power with the AC system 101 will be described.

First, the output voltages of the arms 105u, v, w, r, s, and t will be expressed as Vu, Vv, Vw, Vr, Vs, and Vt, respectively.

Further, Vur, Vvs, and Vwt are defined as in the equations (1) to (3).
[Equation 1]
Vur = (− Vu + Vr) / 2 (1)
Vvs = (− Vv + Vs) / 2 (2)
Vwt = (− Vw + Vt) / 2 (3)

Further, Iur, Ivs, and Iwt are defined as in the equations (4) to (6).
[Equation 2]
Iur = Iu + Ir (4)
Ivs = Iv + Is (5)
Iwt = Iw + It (6)

Here, when the numbers of turns of the primary winding 402, the positive secondary winding, and the negative secondary winding are N1, N2, and N2, respectively, equations (7) to (9) are established.
[Equation 3]
Iab = (N2 / N1) Iur + Imab (7)
Ibc = (N2 / N1) Ivs + Imab (8)
Ica = (N2 / N1) Iwt + Imab (9)

However, Imab, Imbc, and Imca are excitation currents converted into primary windings of the iron core legs 401ab, bc, and ca, respectively.

In the following description, the excitation currents Imab, Imbc, and Imca are ignored because they are sufficiently small. The means for detecting the excitation currents Imab, Imbc, and Imca will be described after explaining the principle that the power converter 103 exchanges active / reactive power with the AC system 101.

Equation (1) means the sum of voltages applied to the positive secondary winding 403u and the negative secondary winding 403r of the transformer 104. Equation (2) means the sum of voltages applied to the positive secondary winding 403v and the negative secondary winding 403s of the transformer 104. Similarly, equation (3) means the sum of voltages applied to the positive secondary winding 403w and the negative secondary winding 403t of the transformer 104.

Since the output voltage of each arm 105 is the sum of the output voltages Vjk of the unit converters 111 included in the arm 105, Vu, v, w, r, s, and t can be controlled by controlling Vjk. Moreover, Vur, vs, wt can be controlled by controlling Vu, v, w, r, s, t.

First, paying attention to the primary winding 402ab, the positive secondary winding 403ur, and the negative secondary winding 403vs wound around the iron core leg 401ab, the active / reactive power of the AC system 101 and the power conversion device 103 is considered. Explaining the exchange of However, for simplicity, the turns ratio of the transformer 104 will be described as 1: 1: 1.

When the circuit breaker 102 is turned on, the line voltage VSab of the AC system 101 is applied to the primary winding 402ab.

In this state, when the frequency of Vur shown in the equation (1) is controlled to be equal to VSab and the amplitude and phase are controlled as described below, the valid / invalid output between the AC system 101 and the power converter 103 is obtained. Can be controlled.

When control is performed so that the frequency, amplitude, and phase of Vur and VSab are all equal, the voltage applied to the leakage inductance of the transformer 401 becomes zero. Accordingly, since no current flows constantly, the active / reactive power between the AC system 101 and the power converter 103 is zero.

When the frequency and amplitude of Vur and VSab are equal and the phase of Vur is controlled so as to be delayed from the phase of VSab, the voltage applied to the leakage inductance of the transformer 401 is a voltage advanced by 90 ° from VSab. Therefore, the phase of the current Iur becomes equal to VSab. In other words, active power flows from the AC system 101 to the power converter 103. That is, the power converter 103 operates as a rectifier.

When the frequency and amplitude of Vur and VSab are equal and the phase of Vur is controlled to advance from the phase of VSab, the voltage applied to the leakage inductance of the transformer 401 is a voltage delayed by 90 ° from VSab. Therefore, the phase of the current Iur is opposite to that of VSab. In other words, active power flows out from the power converter 103 into the AC system 101. That is, the power converter 103 operates as an inverter.

When the frequency and phase of Vur and VSab are equal and the amplitude of Vur is controlled to be larger than the amplitude of VSab, the voltage applied to the leakage inductance of the transformer 401 is a voltage having a phase opposite to that of VSab. Therefore, the phase of the current Iur advances by 90 ° from VSab. In other words, capacitive reactive power is supplied from the power converter 103 to the AC system 101. That is, the power converter 103 operates as a phase advance capacitor.

When control is performed so that the frequency and phase of Vur and VSab are equal and the amplitude of Vur is smaller than the amplitude of VSab, the voltage applied to the leakage inductance of the transformer 401 is a voltage having the same phase as VSab. Therefore, the phase of the current Iur is delayed by 90 ° from VSab. In other words, inductive reactive power is supplied from the power converter 103 to the AC system 101. That is, the power converter 103 operates as a shunt reactor.

In the above, description has been given focusing on the windings wound around the iron core legs 401ab, but the same principle holds true for the windings wound around the iron core legs 401bc and ca.

In the foregoing, the principle of transferring active / reactive power between the AC system 101 and the power converter 103 has been described.

Next, the principle that the power conversion device 103 supplies a desired DC voltage to the DC device 113 will be described.

If the voltage applied to the leakage inductance of the transformer 104 is ignored, the system DC voltage Vdc can be expressed as in equation (10).
[Equation 4]
Vdc = {(Vu + Vr) + (Vv + Vs) + (Vw + Vt)} / 3 (10)

Therefore, by controlling the output voltages Vu, Vv, Vw, Vr, Vs, and Vt of each arm 105, Vdc can be controlled to a desired voltage.

For example, when the DC device 113 is a resistor, the system DC current Idc flows in accordance with Vdc controlled by the above principle and the resistance value of the DC device 113. At this time, the power converter 103 supplies the DC power Vdc × Idc to the DC device 113.

Next, the principle that the power conversion device 103 exchanges desired DC power with the DC device 113 will be described.

For example, when the DC device 113 is a DC voltage source and the voltage is Vdc ′, the DC transmission line 112 is controlled by controlling the voltage Vdc ′ of the DC device 113 and the difference Vdc−Vdc ′ of the system DC Vdc. In addition, the voltage applied to the transformer 104 can be controlled, and as a result, the system DC current Idc can be controlled. If Idc can be controlled, DC power Vdc × Idc can be controlled.

When the power converter 103 and the DC device 113 exchange the DC power Vdc × Idc, the system DC current Idc is shunted by 1/3 to each phase and flows to each phase of the secondary winding of the transformer 104. The current obtained by dividing the system DC current by 1/3 in each phase, that is, Idc / 3 is the “DC current for transferring DC power” mentioned in the above description.

In the following, even if a DC current for transmitting and receiving DC power flows through the transformer secondary winding by connecting the two secondary windings on the positive and negative sides of the transformer 104, the transformer core is The principle of AC / DC power conversion without magnetizing will be described.

Description will be made by focusing on the iron core leg 401ab.

Idc / 3 flowing from the r point toward the M point generates a DC magnetomotive force in the negative secondary winding 403r. The direction is upward in FIG. 4, and the magnitude is N2Idc / 3, where N2 is the number of turns of the negative secondary winding 403r.

On the other hand, Idc / 3 flowing from the M point toward the u point generates a DC magnetomotive force in the positive secondary winding 403u. The direction is downward in FIG. 4, and the magnitude is N2Idc / 3 if the number of turns of the positive secondary winding 403u is N2.

As described above, the number of turns of the positive secondary winding and the negative secondary winding is equal to N2. Accordingly, since the magnitudes of the DC magnetomotive forces caused by the positive secondary winding 403u and the negative secondary winding 403r are equal and opposite in direction, they cancel each other.

Therefore, the DC magnetic flux generated in the iron core leg 401ab can be made substantially zero. Therefore, even if a direct current is passed through the windings of the transformer, AC / DC power conversion can be performed without causing the transformer core to be DC-biased.

Hereinafter, the exciting current detecting means and the exciting current detecting method of the transformer 104, which are characteristic parts of the present invention, will be described with reference to FIG.

First, the description will be made focusing on the primary winding 402ab, the positive secondary winding 403u, and the negative secondary winding 403r wound around the iron core leg 401ab.

The current detection means 404 detects the current Iab flowing through the primary winding 402ab and outputs a current Iab '.

Here, the detection ratio of the current detection means 404 is set so that Iab ′ satisfies the expression (11). However, N1 is the number of turns of the primary winding 402ab, and N0 is an appropriate positive reference value.
[Equation 5]
Iab ′ = N1 × Iab / N0 (11)

The current detection means 405 detects the sum Iur = Iu + Ir of the currents Iu and Ir flowing through the positive secondary winding 403u and the negative secondary winding 403r, and outputs a current Iur '.

Here, the detection ratio of the current detection means 405 is set so that Iur ′ satisfies the equation (12). However, N2 is the number of turns of the positive secondary winding 403u and the negative secondary winding 403r, and N0 is an appropriate positive reference value.
[Equation 6]
Iur ′ = N2 × Iur / N0 (12)

The two current detection means 404 and 405 are connected to a common detection resistor 406. Assuming that the resistance value of the detection resistor is R, a difference current between Iab ′ and Iur ′ flows through R, and thus the voltage VBab at both ends thereof is expressed by the following equation (13).
[Equation 7]
VBab = (N1 × Iab−N2 × Iur) / N0 (13)

Here, equation (13) is rewritten as equation (14).
[Equation 8]
VBab = N1 × {Iab− (N2 / N1) × Iur} / N0 (14)

Further, the equation (15) is obtained by comparing the equations (7) and (14).
[Equation 9]
VBab = (N1 / N0) × Imab (15)

Since N1 and N0 are constants, a voltage signal proportional to the excitation current Imab can be obtained as the voltage VBab across the detection resistor 406.

Primary winding 402bc wound around the iron core leg 401bc, positive side secondary winding 403v, exciting current Imbc of the negative side secondary winding 403s, primary winding 402ca wound around the iron core leg 401ca, Similarly, the equations (16) and (17) can be obtained for the exciting current Imca of the positive secondary winding 403w and the negative secondary winding 403t.
[Equation 10]
VBbc = (N1 / N0) × Imbc (16)
VBca = (N1 / N0) × Imca (17)

The voltage detection unit 407 transmits the detected VBab, VBbc, VBca to the control unit 106.

From the above, it has been explained that a voltage signal proportional to the exciting current of the transformer 104 can be obtained by using the current detection means 404, 405, the detection resistor 406, and the voltage detection means 407.

However, the above description assumes that the current detection means 404 and 405 are ideal transformers or current output type direct current CT (current transformer) having characteristics sufficiently close thereto.

The case where the current detection means 404 and 405 are, for example, AC CT will be described later with reference to FIGS.

Hereinafter, a method for detecting the DC component of the excitation currents Imab, Imbc, and Imca from the detected VBab, VBbc, and VBca will be described with reference to FIGS.

FIG. 5 is an example of the Φ-I characteristic showing the excitation characteristics of the transformer 104, that is, the relationship between the flux linkage number Φ and the excitation current I. However, hysteresis is ignored for simplicity.

In the following description with reference to FIG. 6, when the transformer 104 is operating in the range of the thick line in FIG. 5, that is, the DC components Φdc and Idc are superimposed on the flux linkage number Φ and the excitation current I, respectively. Assume that the maximum value of the number of linkages exceeds the saturation flux linkage number Φsat. In other words, a case where the transformer 104 is DC-biased is assumed.

FIG. 6 is a schematic waveform when the power converter is operating as a rectifier.

A current Iab having substantially the same phase as the line voltage VSab of the AC system 101 flows through the primary winding 402ab.

The alternating current components of the currents Iu and Ir flowing in the positive secondary winding 403u and the negative secondary winding 403r have the same phase as Iab, and the direct current components are Idc / 3 and -Idc / 3, respectively.

Therefore, Iur, which is the sum of Iu and Ir, is an alternating current having the same phase as Iab.

The magnetic flux linkage number Φab of the iron core leg 401ab is approximately proportional to the time integration of the applied voltage VSab. Therefore, if VSab is a sine wave, the AC component of Φab has a waveform delayed by 90 ° from VSab.

Hereinafter, the case where Φab contains the DC component Φabdc, that is, the case where the transformer 104 is DC-magnetized will be described as an example.

Since Φabdc is contained in Φab, the maximum value of Φab exceeds the saturation flux linkage number Φsat.

When Φab exceeds Φsat, the excitation current Imab also exceeds the saturation current Isat. Along with the Φ-I characteristic shown in FIG. 5, the positive peak value of the excitation current Imab increases rapidly and distortion occurs.

FIG. 7 shows a schematic waveform of VBab when the current detection means 404 and 405 is an ideal current transformer or a current output type direct current CT having characteristics sufficiently close thereto.

Since VBab has a waveform proportional to Imab, the DC component VBabdc is detected through, for example, a low-pass filter or a moving average filter to detect that the core leg 401ab of the transformer 104 is DC-biased. it can.

FIG. 8 shows a schematic waveform of VBab when the current detection means 404 and 405 are AC CT.

Although VBab has a waveform similar to Imab, the AC component cannot detect the DC component Imabdc. However, using the fact that the waveform of Imab and the waveform of VBab proportional to it are distorted along with the DC bias, for example, the equation (18) is calculated as the difference between the absolute values of the positive peak VBabp and the negative peak VBapn of VBab. Thus, it can be detected that the iron core leg 401ab of the transformer 104 is DC-biased.
[Equation 11]
VBabpb = | VBabp | − | VBabp | (18)

Alternatively, by utilizing the fact that the waveform of VBab is distorted and becomes asymmetric between positive and negative with DC bias, and detecting even-order harmonic components such as the second-order harmonic component of VBab, the core leg 401ab of the transformer 104 is also detected. Can be detected as being dc-biased.

In the above description, the explanation has been given focusing on the iron core leg 401ab, but the iron core legs 401bc and ca can also detect DC bias magnetism based on the same principle.

In the above description, the case where the number of magnetic flux linkages of the iron core leg 401ab and the excitation current are DC-biased in the positive direction has been described as an example. DC bias can be detected in principle.

Hereinafter, with reference to FIG. 12, an example of the internal configuration of the control means 106 provided with means for suppressing the DC bias in the transformer 104 will be described.

The control means 106 detects the capacitor voltage VCjk of each unit converter 111, the arm currents Iu, v, w, r, s, t, and the line voltages VSab, VSbc, VSca of the AC system 101.

The capacitor voltage control means 1204 calculates the d-axis current command value Id * so that the detected capacitor voltage VCjk is substantially constant.

The adder 1208 calculates the sum Iur = Iu + Ir of the detected arm currents Iu and Ir. Similarly, Ivs and Iwt are calculated.

The obtained Iur, Ivs, and Iwt are coordinate-transformed by an α-β conversion computing unit 1201 to obtain Iα and Iβ.

Further, Iα and Iβ are coordinate-converted by a dq conversion calculator 1203 to obtain a d-axis current Id and a q-axis current Iq. Here, the phase angle θ used for the coordinate conversion is the phase angle of the line voltage VSab of the AC system 101 detected by the phase angle detection means 1202.

Further, the line voltages VSab, VSbc, VSca of the AC system 101 are coordinate-converted by the α-β conversion calculator 1201 to obtain VSα, VSβ.

Further, VSα and VSβ are coordinate-converted by a dq conversion calculator 1203 to obtain an AC system d-axis voltage VSd and an AC system q-axis voltage VSq. Here, the phase angle θ used for the coordinate conversion is the phase angle of the line voltage VSab of the AC system 101 detected by the phase angle detection means 1202. In this case, VSq is approximately zero.

From the d-axis current Id, the q-axis current Iq, the d-axis current command value Id *, the q-axis current command value Iq *, the AC system d-axis voltage VSd, and the AC system q-axis voltage VSq, the equations (19) and (20) D-axis voltage command value Vd * and q-axis voltage command value Vq * are calculated.
[Equation 12]
Vd * = VSd−G1 × (Id * −Id) −RId + ωLIq (19)
Vq * = VSq−G1 × (Iq * −Iq) −RIq−ωLIq (20)

Here, G1 is the current control gain, R is the winding resistance of the transformer, and ωL is the leakage inductance of the transformer.

The obtained Vd * and Vq * are coordinate-transformed by an inverse dq conversion calculator 1209 and an inverse α-β conversion calculator 1210 to obtain AC voltage command values Vur *, Vvs *, and Vwt *.

From the obtained Vur *, Vvs *, and Vwt *, DMPur, vs, and wt, which are outputs of a DC demagnetization suppression control unit 114, which will be described later, are subtracted by a subtractor 1205, and the corrected AC voltage command value Vur * ' , Vvs * ′, Vwt * ′.

From the obtained Vur *, Vvs *, Vwt *, the output voltage command values Vu *, Vr *, Vv *, Vs *, Vw *, Vt * of each arm are calculated by the equations (21) to (26). .
[Equation 13]
Vu * = − Vur * + Vdc * / 2 (21)
Vr * = Vur * + Vdc * / 2 (22)
Vv * = − Vvs * + Vdc * / 2 (23)
Vs * = Vvs * + Vdc * / 2 (24)
Vw * = − Vwt * + Vdc * / 2 (25)
Vt * = Vwt * + Vdc * / 2 (26)

The obtained Vu *, Vr *, Vv *, Vs *, Vw *, Vt * are distributed to each unit converter 111, and each of the units is subjected to, for example, pulse-width modulation by the modulation unit 1213. An on / off command gjk (j = u, v, w, r, s, t, k = 1, 2,..., N) of the switching element 201 of the unit converter 111 is obtained.

The obtained gjk is transmitted to each unit converter via the control communication line 108.

Hereinafter, the direct-current bias suppression control unit 1214, which is a characteristic part of the present invention, will be described.

A DC component is detected from the VBab, VBbc, and VBca detected by the current detection means 404, 405, the detection resistor 406, and the voltage detection means 407 in FIG. 4 using, for example, a low-pass filter and a moving average filter. Alternatively, the difference between the absolute values of the positive peak value and the negative peak value is detected. The gain G2 is multiplied by the difference between the DC component or the absolute value of the positive peak value and the negative peak value to obtain correction signals DMPur, DMPvs, and DMPwt for DC bias suppression control.

The obtained DMPur, DMPvs and DMPwt are subtracted from Vur *, Vvs * and Vwt * as described above to obtain corrected AC voltage command values Vur * ', Vvs *' and Vwt * '.

As a result, a voltage in the direction opposite to the direct-current bias generated in the iron core leg 401 of the transformer 104 is applied to the secondary winding of the transformer 104.

The magnetic flux linkage number Φab, bc, ca of the iron core leg 401 is a time integral of the voltage applied to each winding of the transformer 104.

By using the DC demagnetization suppression control unit 1214 of the present invention, a voltage is applied to each arm so as to generate a magnetic flux in the opposite direction to the DC magnetic flux that has been generated for some reason. Can be attenuated.

Next, with reference to FIG. 13, another method of DC bias suppression control will be described. However, only the differences between FIG. 12 and FIG. 13 will be described.

In FIG. 13, DMPur, DMPvs, and DMPwt, which are the outputs of the demagnetization suppression control 1214, are not the AC voltage command values Vur * ′, Vvs * ′, and Vwt * ′, but the sums Iur, Ivs, Subtracted from Iwt.

As a result, Iur, Ivs, and Iwt can be corrected. Therefore, each arm flows a current that cancels the DC bias of the transformer 104 by the current control shown in equations (19) and (20). Will be controlled.

Therefore, even if the DC demagnetization suppression control is configured as shown in FIG. 13, it is possible to obtain the effect that the DC demagnetization can be attenuated as a result.

Although not explicitly stated in the above description, the iron core legs 401ab, bc, ca may constitute an independent magnetic path, a tripod iron core, or a pentapod iron core. The effects of the present invention can be obtained.

A self-excited high-voltage direct current power transmission system (HVDC), a multi-terminal HVDC, or a DC grid can be configured by using at least one of the power conversion devices described above.
In this embodiment, the positive secondary winding 403u and the negative secondary winding 403r are connected, the positive secondary winding 403v and the negative secondary winding 403s are connected, and the positive secondary winding is connected. Although an example in which the wire 403w and the negative secondary winding 403t are connected is shown, the present invention is not limited to the connection of these wires, and a combination of different secondary divided windings is used to form a so-called staggered connection. For example, the positive secondary winding 403u and the negative secondary winding 403s are connected, and the positive secondary winding 403v and the negative secondary winding 403t are connected. The side secondary winding 403w and the negative secondary winding 403r may be connected.
In this embodiment, the arm 105u and the arm 105r are connected to one end and the other end of the combination of the positive secondary winding 403u and the negative secondary winding 403r, respectively, and the positive secondary winding 403v. The arm 105v arm 105s is connected to one end and the other end of the combination of the negative secondary winding 403s and one end of the combination of the positive secondary winding 403w and the negative secondary winding 403t, respectively. Although the example in which the arm 105w and the arm 105t are connected to each of the other ends has been shown, other modified examples may be used. For example, there is one combination of the positive side secondary winding 403u and the negative side secondary winding 403r. Only the arm 105u is connected to only the end (the other end arm 105r is omitted), and only the arm 105v is connected to only one end of the combination of the positive side secondary winding 403v and the negative side secondary winding 403s (the other side The end arm 105s Karel), may be positive secondary winding 403w and the negative secondary winding 403t and one end only to the arm 105w combination of connected (the arms 105t of the other end is omitted).

Hereinafter, a second embodiment of the present invention will be described.

The difference between the second embodiment and the first embodiment is that an analog-digital converter is connected to current detection means 404 and 405 for detecting the current of each winding of the transformer 104.

In the second embodiment, since the direct current component of the exciting current of the transformer can be obtained by digital calculation, it is possible to improve the noise tolerance when transmitting the detected exciting current to the control means 106.

The overall configuration of the second embodiment is the same as that of the first embodiment except that the transformer 104 is replaced with the transformer 109 of FIG. Therefore, the internal configuration of the transformer 901 according to the second embodiment will be described below with reference to FIG. However, only differences from the transformer 104 (FIG. 4) of the first embodiment will be described.

In the second embodiment, a detection resistor 406 is provided in each of the current detection means 404 and 405, and the voltage across the detection resistor 406 is converted into a digital signal by the analog-digital converter 902.

A signal obtained by analog-digital conversion of the sum Iur of the secondary windings 403u and 403r is subtracted by a subtractor 903 from a signal obtained by analog-digital conversion of the current Iab of the primary winding 402ab to obtain VBab.

Further, a signal obtained by analog-digital conversion of the current Ibc of the secondary windings 403v and 403s is subtracted by a subtractor 903 from a signal obtained by analog-digital conversion of the current Ibc of the primary winding 402bc, and VBbc is obtain.

Similarly, a signal obtained by analog-digital conversion of the current Ica of the secondary windings 403w and 403t is subtracted by a subtractor 903 from a signal obtained by analog-digital conversion of the current Ica of the primary winding 402ca, and VBca Get.

Since the subtractor 903 and the signal path are included in the digital operation unit 904, the noise immunity can be improved as compared with the first embodiment in which the analog signal is used.

The third embodiment of the present invention will be described below.

The difference between the third embodiment and the first embodiment is that current detecting means 405 is provided in each of the positive secondary windings 403u, v, w and the negative secondary windings r, s, t of the transformer 104. It is.

In the first embodiment, since one current detection unit 405 detects the sum of the currents of the two secondary windings, for example, it is necessary to pass two electric wires through an annular DC CT or AC CT.

In the third embodiment, since only one electric wire is passed through one current detection unit 405, an effect of simplifying the routing of the electric wire to the current detection unit can be obtained.

The overall configuration of the third embodiment is the same as that of the first embodiment except that the transformer 104 is replaced with the transformer 1001 of FIG. Therefore, the internal configuration of the transformer 1001 according to the second embodiment will be described below with reference to FIG. However, only differences from the transformer 104 (FIG. 4) of the first embodiment will be described.

Current detection means 404 is provided for each of the primary lines 402ab, bc, and ca to obtain detection values Iab ', Ibc', and Ica 'of the arm currents Iab, Ibc, and Ica.

Current detection means 405 is provided in each of the positive side secondary windings 403u, v, and w to obtain detection values Iu ′, Iv ′, and Iw ′ of arm currents Iu, Iv, and Iw.

Current detection means 405 is provided for each of the negative side secondary windings 403r, s, and t to obtain detected values Ir ', Is', It' of the arm currents Ir, Is, It.

The current detection means 405 connected to the positive side secondary windings 403u, v, w and the current detection means 405 connected to the negative side secondary windings 403r, s, t are provided for each iron leg 401ab, bc, ca. Connected to obtain the sum Iur ′, Ivs ′, Iwt ′ of the current detection values.

Differences between the current detection values Iu ′, Iv ′, Iw ′ of the current detection unit 404 of the primary winding 402 and Iur ′, Ivs ′, Iwt ′ flow to the detection resistor 406. Therefore, as in the first embodiment, voltage signals approximately proportional to the exciting current of the transformer 1001 can be obtained as the voltages VBab, VBbc, VBca across the detection resistor 406.

In the third embodiment, since one current detection unit 404 and 405 detects only the current of one electric wire, the effect of simplifying the routing of the electric wire to the current detection unit 404 and 405 can be obtained.

The fourth embodiment of the present invention will be described below.

In the fourth embodiment, an analog-digital converter is connected to each of the current detection means 404 and 405 as in the second embodiment, and the positive secondary winding and the negative secondary winding are connected as in the third embodiment. Each is provided with a current detection means 405.

In the fourth embodiment, as in the second embodiment, the DC component of the excitation current of the transformer can be obtained by digital calculation, so that the noise immunity when the detected excitation current is transmitted to the control means 106 can be improved. Can be obtained.

Further, in the fourth embodiment, as in the third embodiment, since only one electric wire is passed through one current detection unit 405, an effect that the wiring of the electric wire to the current detection unit can be simplified can be obtained.

The overall configuration of the fourth embodiment is the same as that of the first embodiment except that the transformer 104 is replaced with the transformer 1101 of FIG. Therefore, the internal configuration of the transformer 1101 of the fourth embodiment will be described below with reference to FIG. However, only differences from the transformer 104 (FIG. 4) of the first embodiment will be described.

In the fourth embodiment, a detection resistor 406 is provided in each of the current detection means 404 and 405, and the voltage across the detection resistor 406 is converted into a digital signal by the analog-digital converter 902.

A signal obtained by analog-digital conversion of the currents Iu and Ir of the secondary windings 403u and 403r from the signal obtained by analog-digital conversion of the current Iab of the primary winding 402ab is subtracted by a subtractor 903 to obtain VBab. .

Further, a signal obtained by analog-digital conversion of the currents Iv and Is of the secondary windings 403v and 403s is subtracted by a subtractor 903 from a signal obtained by analog-digital conversion of the current Ibc of the primary winding 402ab, and VBbc. Get.

Similarly, the subtracter 903 subtracts the signals obtained by analog-digital conversion of the currents Iw and It of the secondary windings 403w and 403t from the signal obtained by analog-digital conversion of the current Ica of the primary winding 402ca, Obtain VBca.

In the fourth embodiment, since the subtractor 903 and the signal path are included in the digital operation unit 904, the noise immunity can be improved as compared with the first embodiment in which the analog signal is used.

Further, in the fourth embodiment, since one current detection unit 404 and 405 detects only the current of one electric wire, the effect of simplifying the routing of the electric wire to the current detection unit 404 and 405 can be obtained.

DESCRIPTION OF SYMBOLS 101 ... AC system 102 ... Circuit breaker 103 ... Power converter 104 ... Transformer 105 provided with exciting current detection means Arm 106 ... Control means 107, 108 ... Control Communication line 109 ... Current detection means 110 ... Voltage detection means 111 ... Unit converter 112P, N ... DC power transmission line 113 ... DC device 201XP, XN, YP, YN ... Switching element 202 XP, XN, YP, YN... Freewheeling diode 203... Capacitor 204... Bidirectional chopper type unit converter control means 205... Voltage detection means 301. ... Iron core leg 402 ... Primary windings 403u, v, w ... Positive secondary windings 403r, s, t ... Negative secondary windings 404, 405 ... Current detection means 40 ... Detection resistor 407 ... Voltage detection means 901 ... Second form of transformer 902 provided with excitation current detection means ... Analog-digital converter 903 ... Subtractor 1001 ... Third form of transformer 1001 provided with exciting current detecting means. Fourth form of transformer provided with exciting current detecting means.

Claims (4)

One or more unit converters are connected in series to each phase, and each transformer has a primary winding and at least two secondary windings for each phase. One of two other-side windings and one of the other two-side windings for each phase are connected to the arm for each phase to constitute a circuit for each phase. , The current flowing through the primary winding is passed through one current detection means, the two currents flowing through the two other windings are passed through the other current detection means, and the direct currents of each other are reversed. The power conversion device is characterized in that the output of the one and the other current detection means is connected to one detection resistor for each phase. One or more unit converters are connected in series to each phase, and each transformer has a primary winding and at least two secondary windings for each phase. One of two other-side windings and one of the other two-side windings for each phase are connected to the arm for each phase to constitute a circuit for each phase. , The current flowing through the primary winding is passed through one current detection means, the two currents flowing through the two other windings are passed through the other current detection means, and the direct currents of each other are reversed. A power converter having a function of subtracting the output digital signal of the analog-digital conversion means, provided with analog-digital conversion means at the outputs of the one and other current detection means, respectively. 3. The power conversion device according to claim 1, wherein the unit converter is a bidirectional chopper circuit. 4. 3. The power conversion device according to claim 1, wherein the unit converter is a full bridge circuit. 4.