JP2016226224A - Modular multilevel converter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce the number of cells or to increase the number of levels of an output voltage in a module multilevel converter.SOLUTION: The modular multilevel converter comprises: DC voltage sources VDCH and VDCL that are connected between DC terminals P and N and common to each phase; (k) (k≥1) pieces of positive electrode side cells ce1-cek for each of phases being connected in series to the DC terminal P; (k) (k≥1) pieces of negative electrode side cells ce2k-cek+1 for each of phases being connected in series to the DC terminal N; two inductors Land L, for each of phases, connecting one-side ends to the k-th positive electrode side cell cek and the k-th negative electrode side cells cek+1; an H bridge circuit 1 for each of phases connecting a DC side to other-side ends of the two inductors Land L; and a high frequency transformer HFT for each of phases connecting one end of a primary coil and one end of a secondary coil to an AC side of the H bridge circuit 1 and defining a common connection point of the other end of the primary coil and the other end of the secondary coil as an output terminal O.SELECTED DRAWING: Figure 2

Description

  The present invention relates to a modular multilevel converter, and more particularly to a high-frequency transformer provided to increase the number of levels of output voltage.

  FIG. 7 is a circuit diagram showing a modular multilevel converter (MMC) disclosed in Non-Patent Document 1. The modular multilevel converter includes a plurality of phase units 102, 103, and 104 connected in parallel.

  As shown in FIGS. 7 and 8, in the phase unit 102, a plurality of cells 107 and 108 are connected in series to the upper arm and the lower arm. The same applies to the phase units 103 and 104.

  Each phase unit 102, 103, 104 is connected to a DC voltage source 202. As shown in FIG. 9, each cell 107-112 of each phase unit 102, 103, 104 is provided with a DC capacitor Vdc. .

  The modular multilevel converter includes a control unit 101 that controls the cells 107 to 112 of the phase units 102, 103, and 104, and the DC input voltage 105 is supplied to the DC of the cells 107 to 112 of the at least one phase unit 102, 103, 104. The capacitor Vdc is charged, and the DC input voltage 105 output from the DC voltage source 202 is converted into an AC output voltage.

Das Andararup, Nadem Hamed, Norum Lars, 'Modular multilevel converter', 2013. 12.25, EP 2677653 A1.

  The modular multilevel converter can increase the number of levels of the output voltage, but in order to increase the number of levels of the output voltage, many cells are required in each phase. Generally, in order to double the number of levels of the output voltage, the number of cells needs to be doubled. As a result, the control becomes complicated and the cost increases. In addition, increasing the number of cells has led to an increase in the size of the apparatus and a decrease in reliability.

  As described above, in the modular multilevel converter, it is a problem to reduce the number of cells or increase the number of levels of the output voltage.

  The present invention has been devised in view of the above-described conventional problems, and one aspect thereof is a DC voltage source common to each phase connected between a DC terminal on the positive electrode side and a DC terminal on the negative electrode side, Each of positive phase side cells connected in series with k (k ≧ 1) in series with respect to the DC terminal on the positive side and each phase connected in series with k units (k ≧ 1) with respect to the DC terminal on the negative side. Negative-phase cell, two inductors each having one end connected to the k-th positive-side cell from the positive-side DC terminal, and the k-th negative-side cell from the negative-side DC terminal, and two inductors And the other end of the primary winding, and one end of the primary winding and one end of the secondary winding are connected to the AC side of the H bridge circuit. A high-frequency transformer of each phase having a common connection point with the other end of the secondary winding as an output terminal, the positive-side cell, Pole cell, the inductor, the H-bridge circuit, the number of phases of the high frequency transformer, characterized in that 1 or more.

  Further, as one aspect thereof, the positive electrode side cell and the negative electrode side cell have a capacitor and two switching elements connected in series between one end and the other end of the capacitor, and are common to the two switching elements. A common connection point between the connection point and one of the switching elements and the capacitor is a connection terminal.

  According to the present invention, it is possible to reduce the number of cells or increase the number of levels of output voltage in a modular multilevel converter.

The figure which shows the high frequency transformer used for Embodiment 1,2. FIG. 3 is a circuit diagram showing a modular multilevel converter for one phase in the first embodiment. FIG. 3 is a circuit diagram showing a cell in the first embodiment. The time chart which shows the output voltage waveform in the case of K = 4 in the conventional modular multilevel converter. 4 is a time chart showing an output voltage waveform when K = 4 in the modular multilevel converter of Embodiment 1. FIG. FIG. 4 is a circuit diagram illustrating a three-phase modular multilevel converter according to a second embodiment. Schematic which shows the conventional modular multilevel converter. Schematic which shows the conventional phase unit. Schematic which shows the conventional cell.

  The modular multilevel converter of the present invention uses a high frequency transformer HFT as shown in FIG. As shown in FIG. 1, one connection terminal (● side) of the primary winding in the high-frequency transformer HFT is connected to the terminal A, and one connection terminal (● non-side) of the secondary winding in the high-frequency transformer HFT. Is connected to terminal B. The common connection point between the other connection terminal (● less side) of the primary winding and the other connection terminal (● side) of the secondary winding in the high-frequency transformer HFT is the output terminal O of the modular multilevel converter. .

  The turn ratio of the high frequency transformer HFT is 1: 1. Therefore, the following expressions (1) and (2) can be satisfied.

Here, V _AO is the potential difference between point A and point O with reference point O, V _OB is the potential difference between point O and point B with point B as the reference point, and V _AB is point B with the reference point. The potential difference between the points A and B, i _AO and i _BO are currents flowing through the path shown in FIG. 1, and i _O is an output current.

By equalizing the number of turns of the primary winding (V _AO side winding) of the high-frequency transformer HFT and the number of turns of the secondary winding (V _BO side winding), Equation (1) is established. Further, by satisfying the expression (1), the current i _AO of the primary winding becomes equal to the current i _BO of the secondary winding, and the expression (2) is satisfied.

The use of the high frequency transformer HFT in the modular multilevel converter has the following two advantages.
1. The number of voltage levels can be easily doubled with only the high-frequency transformer HFT and some components.
2. As shown in equation (2), the load current can be equally divided without current sharing control.

[Embodiment 1]
FIG. 2 is a circuit diagram illustrating the modular multilevel converter according to the first embodiment. As shown in FIG. 2, DC voltage sources VDCH and VDCL that are common to the respective phases are connected in series between a DC terminal P on the positive electrode side and a DC terminal N on the negative electrode side. A common connection point of the DC voltage sources VDCH and VDCL is a neutral point NP. As shown in FIG. 2, the voltages of the positive side DC voltage source VDCH and the negative side DC voltage source VDCL are each E / 2.

Further, k positive cells ce1 to cek are sequentially connected in series to the positive DC terminal P, and k negative cells ce2k to cek + 1 are sequentially connected in series to the negative DC terminal N. One ends of inductors L _P and L _N are connected between the kth positive electrode side cell cek and the kth negative electrode side cell cek + 1.

The DC side of the H bridge circuit 1 is connected to the other ends of the two inductors L _P and L _N. The H bridge circuit 1 includes a flying capacitor C connected between the inductors L _P and L _N , a series circuit of switching devices S1 and S2 connected in parallel to the flying capacitor C, and a flying capacitor C. And a series circuit of switching devices S3 and S4 connected in parallel. The common connection point of the switching devices S1 and S2 becomes the AC side terminal A, and the common connection point of the switching devices S3 and S4 becomes the AC side terminal B.

  One end of the primary winding of the high frequency transformer HFT is connected to the terminal A, and one end of the secondary winding of the high frequency transformer HFT is connected to the terminal B. A common connection point between the other end of the primary winding of the high-frequency transformer HFT and the other end of the secondary winding is an output terminal O.

The structure of each positive electrode side cell ce1 to cek and negative electrode side cell cek + 1 to ce2k will be described with reference to FIG. In FIG. 3, the positive electrode side cell cek will be described as a representative. As shown in FIG. 3, capacitors C and k and switching elements S _{k, 1} and S _{k, 2} connected in series between one end and the other end of the capacitors C and k are included. A common connection point of the switching elements S _{k, 1} and S _{k, 2 and} a common connection point of the switching elements S _{k, 2} and the capacitors C, k are used as connection terminals. The rated voltage of the capacitors C and k is V _{C, k} and the voltage between the connection terminals is V _k .

[Operating principle]
The rated voltages V _{C and K} of the capacitors C and k of the positive electrode side cell cek in FIG. 2 are expressed by the following equation (3). The same applies to other positive side cells and negative side cells. Note that 2K is the number of cells. For example, when the number of cells = 4, 2K = 4, that is, K = 2.

The rated voltage V _C of the flying capacitor C of the H-bridge circuit 1 in FIG. 2 is expressed by the following equation (4). Rated voltage V _C of the flying capacitor C is half the rated voltage V _{C, k} in the capacitor C, k a positive side cell cek.

The rated voltages V _{C, 1 to} V _{C, 2k of} the capacitors C _{, 1 to C, 2k of} the cells ce1 to ce2k and the rated voltage Vc of the flying capacitor C are set to (3), (4 ) Control to the value of the expression.

  The following formulas (5) to (8) can be obtained from FIG.

Here, V _DO is a potential difference between the D point and the O point with the O point as a reference point, and V _OE is a potential difference between the O point and the E point with the E point as a reference point. i _P , i _N , and i _O are currents flowing through the path shown in FIG.

Assuming that the two inductors L _P and L _N have the same inductance value (L _P = L _N ), the output voltage V _O can be expressed by the equations (5) to (8) to (9).

The output voltage V _{O in the} equation (9) is a potential difference between the output terminal O and the neutral point NP in FIG. That is, the output voltage of the modular multilevel converter of FIG.

Depending on the state of the switching devices S1 to S4 of the H-bridge circuit 1, V _DO -V _OE can be controlled as in the following equation (10).

  The expression (10) will be described.

[Switching pattern 1]
When the switching devices S2 and S4 of the H-bridge circuit 1 are turned on and the switching devices S1 and S3 are turned off, V _DO −V _OE is expressed by the following equation (11).

[Switching pattern 2]
When the switching devices S1 and S3 of the H-bridge circuit 1 are turned on and the switching devices S2 and S4 are turned off, V _DO −V _OE is expressed by the following equation (12).

[Switching pattern 3]
When the switching devices S1 and S4 of the H-bridge circuit 1 are turned on and the switching devices S2 and S3 are turned off, V _DO −V _OE is expressed by the following equation (13).

[Switching pattern 4]
When the switching devices S2 and S3 of the H-bridge circuit 1 are turned on and the switching devices S1 and S4 are turned off, V _DO −V _OE is expressed by the following equation (14).

(9) the first term and the second term _{^{(Σ 2K k = K + 1}} Vk-Σ K k = 1 V k) -1/2 (V DO -V OE) of formula, the first term (sigma ^2K _k a ^{_{= K + 1 Vk-Σ K}} k = 1 V k) is a voltage based on a conventional modular multi-level converter shown, the second term 1/2 (V _DO -V _OE) is voltage based on the high-frequency transformer HFT shown Subtracted.

The output voltage V _O in which the slope of the step of the waveform is reduced is created by the third term L _P / 2 · d / dt · i _{O in the} equation (9). Normally, the inductor L _P is much smaller than the load inductance including L in FIG. 2, so the third term in equation (9) is not large enough to affect the output voltage V _O. Therefore, it can be considered that the third vertex = 0 in the equation (9).

  The modular multilevel converter according to the first embodiment shown in FIG. 2 can output a voltage level of 4K + 3. Since the conventional modular multilevel converter does not include the second term of the equation (9), it can output a voltage level of 2K + 1.

  The output voltage Vo is calculated by substituting the equation (10) into the second term of the equation (9).

Some examples of these voltage levels can be demonstrated as follows. The values of the output voltage V _O, the values of the voltages V ₁ ,..., V _2K between the connection terminals of the positive and negative cells ce1 to ce2k at that time, and the switching states of the switching devices S1 to S4 are shown.

FIG. 4 and FIG. 5 show the simulation results of the waveform of the output voltage V _O when K = 4 (number of cells = 8).

FIG. 4 shows a waveform of the output voltage V _O when K = 4 in the conventional modular multilevel converter. As shown in FIG. 4, the waveform of the 9-level output voltage V _O can be output.

FIG. 5 is an output voltage waveform when K = 4 in the modular multilevel converter according to the first embodiment. As shown in FIG. 5, the waveform of the 19-level output voltage V _O can be output.

Thus, since the first embodiment (FIG. 5) has more output voltage levels than the conventional method (FIG. 4), the waveform of the output voltage V _{O is} a sinusoidal waveform with less harmonic components. You can see that

[Embodiment 2]
FIG. 6 is a schematic diagram showing a modular multilevel converter according to the second embodiment. In the second embodiment, the positive side cells ce1 to cek, the negative side cells cek + 1 to ce2k, the inductors L _P and L _N , the H bridge circuit 1, and the high frequency transformer HFT of the modular multilevel converter of the first embodiment are spread over three phases. It is provided so that a three-phase voltage can be output. As shown in FIG. 2, the positive electrode side cells ce1 to cek and the negative electrode side cells cek + 1 to ce2k have the same structure as that of the first embodiment.

[effect]
The effects of the modular multilevel converter in Embodiments 1 and 2 are shown below.

  The first and second embodiments can increase the number of levels of the output voltage in the same number of cells as compared with the conventional method shown in FIG. Table 1 shows a comparison when the number of cells per phase = 8.

Therefore, the waveform of the output voltage V _O can be made a waveform close to a sine wave with less harmonic components without increasing the number of cells. As a result, the harmonic current suppressing filter L connected between the modular multilevel converter and the load Load shown in FIG. 2 can be reduced in size.

Further, the first and second embodiments can reduce the number of cells at the same number of output voltage levels as compared with the conventional method shown in FIG. Table 2 shows a comparison of the number of cells, the number of capacitors, the number of switching devices and the number of switching elements (IGBTs) when the number of levels of the output voltage V _O = 19.

  Note that the number of capacitors in Table 2 includes the capacitors of each cell and the flying capacitors of the H bridge circuit 1. The number of IGBTs in Table 2 includes the switching elements of each cell and the switching devices of the H bridge circuit 1.

  In Embodiments 1 and 2, it can be seen that not only the number of cells but also the number of capacitors and the number of IGBTs can be reduced. The reduction in the number of IGBTs also leads to a reduction in the number of gate drive circuits that drive the on / off of the IGBTs. The first and second embodiments that reduce the number of cells, the number of capacitors, and the number of IGBTs lead to high reliability by reducing the cost and size of the entire apparatus and reducing the number of components.

  Although the present invention has been described in detail only for the specific examples described above, it is obvious to those skilled in the art that various changes and modifications are possible within the scope of the technical idea of the present invention. Such variations and modifications are naturally within the scope of the claims.

  In the first and second embodiments, only one switching device is shown. However, a part or all of each switching device may include at least one of a series number or a parallel number.

P, N ... DC terminals VDCH, VDCL ... DC voltage source Ce1～cek ... positive-electrode cell cek + 1~ce2k ... negative electrode cell L _P, L _N ... inductor HFT ... high frequency transformer 1 ... H-bridge circuit

Claims (4)

DC voltage source common to each phase connected between the DC terminal on the positive electrode side and the DC terminal on the negative electrode side;
The positive electrode side cell of each phase connected in series with k (k ≧ 1) with respect to the DC terminal on the positive electrode side,
A negative electrode side cell of each phase connected in series with k (k ≧ 1) with respect to the DC terminal on the negative electrode side;
Two inductors each having one end connected to the kth positive side cell from the positive side DC terminal and from the negative side DC terminal to the kth negative side cell;
An H-bridge circuit for each phase in which the DC side is connected to the other ends of the two inductors;
One end of the primary winding and one end of the secondary winding are connected to the AC side of the H-bridge circuit, and the common connection point between the other end of the primary winding and the other end of the secondary winding is used as an output terminal. A high-frequency transformer for each phase;
With
The modular multilevel converter characterized in that the positive electrode side cell, the negative electrode side cell, the inductor, the H bridge circuit, and the high frequency transformer have one or more phases. The positive electrode side cell and the negative electrode side cell are:
A capacitor, and two switching elements connected in series between one end and the other end of the capacitor, and a connection terminal between the common connection point of the two switching elements and the common connection point of the one switching element and the capacitor The modular multilevel converter according to claim 1, wherein:   3. The modular according to claim 1, wherein a positive side cell, a negative side cell, an inductor, an H-bridge circuit, and a high-frequency transformer are provided over three phases to output a three-phase voltage. Multi-level converter.   The modular multilevel converter according to claim 1 or 2, wherein a part or all of each switching device provided in the H-bridge circuit includes at least one of a series number and a parallel number.

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