WO2019212257A1

WO2019212257A1 - Power conversion apparatus having scott transformer

Info

Publication number: WO2019212257A1
Application number: PCT/KR2019/005262
Authority: WO
Inventors: 박영호; 류승표; 김수남; 김성일; 슈테판밀로바노빅; 드라젠두짓
Original assignee: 현대일렉트릭앤에너지시스템(주)
Priority date: 2018-05-02
Filing date: 2019-05-02
Publication date: 2019-11-07

Abstract

The present invention may comprise: a direct current-to-alternating current conversion unit having at least two multi-level converters each converting input direct-current power into alternating-current power; a Scott transformer which operates at a medium frequency of hundreds of Hz to tens of thousands of Hz so that a voltage level of the alternating-current power from each of the at least two multi-level converters of the direct current-to-alternating current conversion unit is transformed into three-phase alternating-current power to be output; and an alternating current-to-direct current conversion unit for converting the three-phase alternating-current power from the Scott transformer into direct-current power.

Description

Power Converter with Scott Transformer

The present invention relates to a power conversion device having a Scott transformer.

In general, power conversion devices are used in structures such as ships, offshore plants, and railroad cars.

Meanwhile, structures such as ships, offshore plants, and railroad cars may require various power levels, and for this purpose, the power converter may be a high power DC-AC or a high power DC-DC converter.

As the above-described power converter, a transformer and a plurality of multi-level converters may be employed. However, a current increase of at least one of the plurality of multi-level converters may occur due to current imbalance of the transformer, causing a problem in that the power converter does not operate normally. Can be.

(Patent Document 1) European Patent Publication No. 2458725

(Patent Document 2) European Patent Publication No. 2637296

According to one embodiment of the present invention, a power conversion apparatus employing a scott transformer is provided.

In order to solve the above problems of the present invention, a power converter according to an embodiment of the present invention is a DC-AC converter having at least two multi-level converter for converting the input DC power into AC power, hundreds of Hz to Scott transformer operating at a medium frequency of several tens of kHz and converting the voltage level of AC power from each of the at least two multi-level converters from the DC-AC converter into three-phase AC power and outputting the three-phase AC power. It may include an AC-DC converter for converting three-phase AC power from the Scott transformer into DC power.

According to an embodiment of the present invention, even when a failure occurs in the multi-level converter, it is possible to perform a normal power conversion operation.

1 is a schematic configuration diagram of a power conversion apparatus according to an embodiment of the present invention.

2 is a voltage phase diagram of a Scott transformer employed in a power conversion apparatus according to an embodiment of the present invention.

3 is a schematic diagram of a power conversion apparatus according to another embodiment of the present invention.

4A to 4D are conceptual circuit diagrams and voltage-current waveform graphs showing schematic operations of a power conversion apparatus according to an embodiment of the present invention.

5 is a voltage waveform graph of a power conversion apparatus according to an embodiment or the other embodiment of the present invention.

6A to 6D are conceptual views illustrating an operation when an anode grid or a plurality of single pole grids are input to a power converter according to an embodiment of the present invention, and a partial operation when one pole fails.

7A to 7D are diagrams illustrating a partial operation of a positive pole of a positive electrode grid in a power converter according to an embodiment of the present invention and a power converter according to another embodiment.

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings so that those skilled in the art may easily implement the present invention.

Referring to FIG. 1, the power converter 100 according to an embodiment of the present invention may include a DC-AC converter 110, a Scott transformer 120, and an AC-DC converter 130.

The DC-AC converter 110 may include at least two

multi-level converters

111 and 112.

Each of the at least two

multi-level converters

111 and 112 may convert the input DC power into AC power. The

multi level converters

111 and 112 may be modular multi level converters.

Each of the first and second

multi-level converters

111 and 112 may include first DC terminals P1 and N1 and second DC terminals P2 and N2.

Each converter of the first and second

multi-level converters

111 and 112 is implemented with four arms, and each arm is implemented with a serial connection of a sub module SM connected in series with an ARM inductor. Can be. That is, the first arm connects terminals P1 and A1, the second arm connects terminals P1 and B1, the third arm connects terminals N1 and A1, and the fourth arm connects terminals N1 and B1. . Each of the first and second

multi-level converters

111 and 112 may have an A branch and a B branch. The A branch may be configured in series with an upper first arm and a lower second arm and female inductor. The B branch may include an upper third arm, a lower fourth arm and a female inductor in series.

The submodule SM may be implemented as a half bridge (HB) cell or a full bridge (FB) cell or a combination of a half bridge and a full bridge. The arm may be implemented as a series combination of submodules combining several types of submodules connected in series with an inductor in the same type.

When a bipolar MVDC (medium voltage direct current) network with a neutral wire is available on the MVDC side and there are three poles (DC +, DCo, DC-), the DC terminal N1 of the first multi-level converter 111 and the second multi The DC terminal P2 of the level converter 112 may be connected together. At this time, the DC terminal P1 of the first multi-level converter 111 is connected to the DC + pole, the DC terminal N2 of the second multi-level converter 112 is connected to the DC-pole, and the first multi-level converter 111 is connected. The DC terminal N1 and the DC terminal P2 of the second multi level converter 112 may be connected to DCo.

The AC terminal of the first multi level converter 111 is connected to the first primary windings P1 (AP1 ˜ AN1) of the first transformer T1 of the Scott transformer 120, and of the second multi level converter 112. The AC terminal may be connected to the second primary winding P2 of the second transformer T2 of the Scott transformer 120 (from AP2 to AN2). The first secondary winding S1 of the first transformer T1 has a first terminal BP1 and a second terminal BN1. The second secondary winding S2 of the second transformer T2 has a first terminal BP2, a second terminal B02 and a third terminal BN2, where the second terminal B02 is a first terminal ( It is the center tap connection between BP2) and the third terminal BN2. In addition, the second terminal BN1 of the first secondary winding S1 of the first transformer T1 is connected to the second terminal B02 of the second secondary winding S2 of the second transformer T2.

In order to interface the MVDC network, the first and second

multi-level converters

111 and 112 may employ a submodule SM. The number of submodules SM may vary depending on the available MVDC network voltage level, the selected voltage class of the semiconductor used to implement the submodule SM, and the particular control margin not related to the converter basic operating principle.

For example, the submodule SM may be implemented as an active semiconductor device, which allows for degrees of freedom associated with the selection of operating frequencies of the first and second transformers T1 and T2 used in the Scott transformer 120. can do. Accurate operating frequencies are subject to design optimization based on various parameters, but operating frequencies of hundreds of Hz to several kHz are typically achievable with state-of-the-art semiconductor devices.

The use of a high voltage semiconductor (eg, 6.5 kV) may result in a smaller number of sub-modules SM of the first and second

multi-level converters

111 and 112, but the allowable operating frequency may be limited to less than several tens of kHz. On the other hand, the use of a low voltage semiconductor (for example, 1.7 kV) increases the number of sub-modules SM of the first and second

multi-level converters

111 and 112, but enables a higher switching frequency.

2 is a voltage phase diagram of a Scott transformer employed in a power conversion apparatus according to an embodiment.

Referring to FIG. 2, the Scott transformer 120 may be implemented using two double winding transformers T1 and T2. The first and second transformers T1 and T2 may each comprise one primary winding Pk and secondary winding Sk, where k = 1 or 2 and the windings of T1 and T2. The second secondary winding S2 of the second transformer T2 is divided into two parts, both of which have the same number of turns as N / 2, which enables center tap connection and also the first secondary winding of the first transformer T1. It may be connected to the bottom of (S1).

The upper end of the first secondary winding S1 of the first transformer T1 and the upper end and the lower end of the second secondary winding S2 of the second transformer T2 are the secondary of the Scott transformer 120 indicated by A, B and C. Used as a vehicle side terminal. In order to ensure the normal operation of the Scott transformer 120, the number of turns of the first secondary winding S1 of the first transformer T1 is

Should be set to.

As a result, when the number of turns of the first primary winding P1 of the first transformer T1 and the second primary winding P2 of the second transformer T2 is the same, the turns ratio relation is

May be induced.

The secondary terminal line voltages (VAB, VBC, and VCA) of the Scott transformer 120 are balanced three phases with three phase 120 degree phase differences, while the primary terminal voltages VT1 and VT2 of the Scott transformer 120 have 90 degree phase differences. It has a single phase.

The Scott transformer 120 may operate at a medium frequency of several hundred Hz to several tens of kHz.

When the power conversion device of the present invention is used in the European railway, the operating frequency of the European railway is 16 and 2/3 Hz to 60 Hz, and the volume and weight may increase when the Scott transformer operates at a commercial frequency of 50 Hz to 60 Hz. There is no choice but to. In contrast, the Scott transformer 120 of the present invention operates at a medium frequency of several hundred Hz to several tens of kHz, so that volume and weight can be reduced to several ten percent or less compared to a Scott transformer operating at a commercial frequency. If Scott transformers operating at commercial frequencies operate at frequencies below hundreds of Hz or tens of kHz, smooth operation is difficult, and there is no sense of size and weight reduction. Therefore, it is necessary to newly develop by applying the appropriate core material or winding to Scott transformer so that it can operate at intermediate frequency. Hundreds of kHz will be developed when the characteristics of semiconductor devices in DC-AC converter or AC-DC converter as well as Scott transformer are developed in the future. Operation may be possible even at the high frequency.

The DC-AC converter 110 and the AC-DC converter 130 may also operate at a medium frequency of several hundred Hz to several tens of kHz.

The DC-AC converter 110 converts a bipole DC or unipole DC power into two single-phase AC powers having a 90 degree phase difference, and the Scott transformer 120 converts two single-phase AC powers into three-phase AC powers. Converting to electric power, the AC-DC converter 130 may convert three-phase AC power into unipolar DC power.

The AC-DC converter 130 may have two semiconductor switches S1, S4, S3, S6, and S5 and S2 connected in series with each other, and may include three legs connected in parallel with each other. .

The AC-DC converter 130 includes a DC terminal, a first DC terminal P3 and a second DC terminal N3, and an AC terminal, that is, the first AC terminal A and the second AC terminal AB and the first terminal. 3 may be provided with an AC terminal C, and the AC terminal of the AC-DC converter 130 may be connected to the secondary terminals of the Scott transformer 120, that is, BP1, BP2, and BN2. The DC terminal side of the AC-DC converter 130 may have a kind of filter, for example, a capacitive filter bank.

The AC-DC converter 230 operates with a square wave voltage to minimize switching loss.

When the AC-DC converter 230 is composed of three legs connected in parallel with each other having two semiconductor switches S1, S4, S3, S6, and S5, S2 connected in series with each other, The legs may each receive AC power of each phase of three-phase AC power from Scott transformer 220, and the three legs may be operated by six-step operation and may deliver power in both directions. Can be.

The AC-DC converter 130 may be composed of three legs by transferring three-phase AC power from the Scott transformer 120, so that the number of devices may be reduced in comparison with the case where two H bridges are formed. .

Referring to FIG. 1 along with FIG. 1, the power conversion apparatus 200 according to another embodiment of the present invention may provide an AC-contrast in comparison with the power conversion apparatus 100 according to the embodiment of the present invention illustrated in FIG. 1. It may include an AC-DC converter 230 having a different configuration from the DC converter 130.

The AC-DC converter 230 of the power converter 200 according to another embodiment of the present invention is an AC-DC converter of the power converter 100 according to the embodiment of the present invention shown in FIG. 1. In contrast to 130, three legs having two diodes D1, D4, D3, D6, and D5, D2 connected in series with each other may include three legs connected in parallel with each other. Accordingly, power can be transmitted in one direction. Similarly, the AC-DC converter 230 may be composed of three legs by transferring three-phase AC power from the Scott transformer 220, so as to reduce the number of devices in case of two H bridges. The efficiency can be increased by rectifying square waves.

Although there is no phase change for the MVDC side and Scott transformer in the power converter 200 according to another embodiment of the present invention, the operation principle of the bidirectional power converter 100 shown in FIG. 1 is slightly different.

That is, instead of generating a pulse voltage at the output, the first and second

multi-level converters

211, 212 now operate to provide sinusoidal voltages VMMC1 and VMMC2 to the primary winding of Scott transformer 220 at their respective AC terminals. The fundamental frequencies of the voltages VMMC1 and VMMC2 may define operating frequencies of the first and second transformers T1 and T2 of the Scott transformer 220. In order to control the output voltage, the magnitudes of the voltages applied to the first and second transformers T1 and T2 must be adjusted. The voltages VT1 and VT2 of the first and second transformers T1 and T2 should be generated with a phase shift equal to one quarter of the fundamental period (90 degree electrical diagram), and the line voltage of the secondary winding of the Scott transformer 320 ( VAB, VBC, VCA) are symmetrical.

In addition to the description of the DC-AC converter 210 and the Scott transformer 220, the DC-AC converter 110 and the Scott transformer 120 in the power converter 100 according to an embodiment of the present invention. Since it is the same as or similar to the detailed description thereof will be omitted.

4A to 4D are conceptual circuit diagrams and voltage-current waveform graphs illustrating an operation of a power conversion apparatus according to an embodiment of the present invention.

4A through 4D, as shown in FIG. 4A, equivalent circuits of the first and second

multi-level converters

211 and 212 connected to the first and second transformers T1 and T2, respectively, are illustrated in FIG. Can be seen.

As shown in FIG. 4A, the voltage waveform VMMC1 (between the A and B branches of the first multi-level converter 111) with respect to the voltages generated on the secondary side (provided by the AC-DC converter 230). The current of the first and second transformers T1 and T2 can be controlled by adjusting the phase shift of the voltage) and VMMC2 (the voltage between the A and B branches of the second multi-level converter 112).

Voltage VMC1 of first multi-level converter 111 and voltage VT1 of first primary winding P1 of first transformer T1 associated therewith, voltage VMMC2 of second multi-level converter 112 and associated voltage thereof 2 Phase difference between voltage VT2 of the second primary winding P2 of the transformer

Calculate Here, the voltage VMMC1 of the first multi-level converter 111 and the VMMC2 of the second multi-level converter 112 are waveforms generated by the AC-DC converter 130 at their AC terminals, that is, VAB, VBC, and VCA. To be synchronized to

The signal for the leg of the AC-DC converter 130 may be phase shifted from each other by 1/3 of the fundamental period. In this way, due to the phase of each leg of the AC-DC converter 130, the voltage waveforms of the AC terminals A, B and C are at two separate voltage levels (0, Vo) with respect to the DC terminal N3. Thus, the phases may be generated out of phase with each other. 3 levels of voltage (as a result of direct subtraction of both leg voltages) and voltage levels (Vo, 0, -Vo) to the AC terminals (VAB, VBC, VCA) of the AC-DC converter 130 of this reference line. It may have (see Fig. 4b).

This corresponds to the AC terminal voltage line voltages VAB, VBC, and VCA of the AC-DC converter 130. VT1 (voltage of the first primary winding of the first transformer T1) and VT2 (voltage of the second primary winding of the second transformer T2) may be expressed by the following equation.

(Equation)

Where m1 is the primary and secondary winding ratio of transformer T1 and m2 is the primary and secondary winding ratio of transformer T2.

4C and 4D, voltage waveforms generated by the first and second

multi-level converters

111 and 112 and respective primary windings P1 and P2 of the first and second transformers T1 and T2 are illustrated. The currents iT1 and iT2 flowing through can be seen, respectively.

Based on the turns ratio relationship of the transformer, the formula and Figure 4b,

And

Can be calculated. If the output voltage is Vo,

ego,

to be. If the energy is equal to the force transmitted through the first and second transformers T1 and T2, the phase

1 lesson

2 will be equivalent to each other.

Accordingly, the power converter of the present invention may have the following technical effects.

1. Reduced number of semiconductor devices: Instead of using two three-phase multi-level converters, the DC-AC converter of the present invention uses two single-phase multi-level converters. As a result, the total number of semiconductor devices can be reduced to two-thirds, thereby reducing the size of the converter. The current capacity of the device is increased, but this can be canceled out in large part by significantly reducing the complexity of the circuit of the AC-DC converter.

2. Small transformers for galvanic isolation can be used: Scott transformers can be used to make the system more compact by reducing the number of effective turns of the system by a quarter, which makes it suitable for specific applications such as railways. It also simplifies transformer design.

3. Reducing System Size: Operating power converters with higher fundamental frequencies reduces the size (volume and weight) of Scott transformers operating at intermediate frequencies.

4. Rectifier Operation: Since the Scott transformer of the present invention uses two transformers connected to different cores, the diode rectifier can be applied.

5 is a voltage waveform graph of a power conversion apparatus according to another embodiment of the present invention.

Referring to FIG. 5, even if the phase difference between the voltages VT1 and VT2 is not perfect and there is a slight phase difference, the voltage unbalance generated in each leg of the AC-DC converter 230 does not affect the output voltage. The topology of the unidirectional power converter does not affect the operation of transformer turn ratio or phase shift imbalance in actual implementation. The output voltage Vo has a typical waveform of a 6 pulse diode rectifier.

Referring to FIG. 6A, when a bipolar grid is provided, the first and second multi level converters are connected in series as shown. Referring to FIG. 6B, a redundancy principle is shown when one of two multi-level converters fails or one pole of an external grid fails to supply power. The faulty pole is isolated from the circuit and the other continues to run at half the rated power.

On the other hand, if two unipolar grids are available, as shown in Fig. 6C, the first and second multi-level converters operate independently of one another (synchronized with Scott transformer).

Referring to FIG. 6D, a redundancy principle is shown when one of the two monopole grids fails. The faulty pole is isolated from the circuit and the other continues to run at half the rated power.

7A to 7D, another important aspect of the configuration of the present invention is that in the case of the bidirectional or unidirectional power converter shown in FIG. 1 or 3, system level redundancy can be implemented on the MVDC side.

A bipolar network with a neutral wire can be used, in which case the DC terminal P1 of the first multi-level converter is connected to the DC + pole, the DC terminal N2 of the second multi-level converter is connected to the DC- pole, and the first multi-level DC terminal N1 of the converter and DC terminal P2 of the second multi-level converter are connected to DC0.

If one of the one or at least two multi-level converters fails on the MVDC side, or if one pole of the external grid fails to power, the other one pole can continue to operate in half of the system in abnormal mode. .

The failed multi-level converter is isolated from the rest of the circuit, so the associated transformer (second transformer T2 in Figures 7A and 7C) acts as a bypass connection circuit that provides a small winding leakage inductance. In the AC-DC converter, one of the three legs cannot function, but the other two legs continue to operate in a manner similar to a single-phase DAB.

As described above, according to the present invention, even when a failure occurs in the multi-level converter, it is possible to perform a normal power conversion operation.

The present invention described above is not limited to the above-described embodiment and the accompanying drawings, but is defined by the claims below, and the configuration of the present invention may be modified in various ways without departing from the technical spirit of the present invention. It will be apparent to those skilled in the art that the present invention may be changed and modified.

Claims

A DC-AC converter having at least two multi-level converters each converting input DC power into AC power;

Scott transformer operating at a medium frequency of several hundred Hz to several tens of kHz and converting the voltage level of AC power from each of at least two multi-level converters from the DC-AC converter into three-phase AC power and outputting the three-phase AC power. transformer); And

AC-DC converter for converting three-phase AC power from the Scott transformer into DC power

Power conversion device having a Scott transformer comprising a.
The method of claim 1,

The DC-AC converter converts input DC power of a medium DC voltage and a bipolar grid into two single-phase AC powers having an AC medium voltage and a medium frequency. Power converter having a transformer.
The method of claim 2,

And the Scott transformer has a Scott transformer for converting AC power from the DC-AC converter into three-phase AC power having a low voltage of several kV or less according to a preset winding ratio.
The method of claim 3,

And the AC-DC converter has a Scott transformer for converting AC power from the Scott transformer into DC power having a low voltage of 1500V or less.
The method of claim 1,

And the AC-DC converter has a Scott transformer each having two semiconductor switches connected in series with each other and including three legs connected in parallel with each other.
The method of claim 5,

The sets of legs of the AC-DC converter are operated by six-step operation,

The power converter has a Scott transformer which is a bidirectional power converter.
The method of claim 1,

And the AC-DC converter has a Scott transformer, each having two diodes connected in series with each other and including three legs connected in parallel with each other.
The method of claim 7, wherein

The power converter has a Scott transformer which is a unidirectional power converter.
The method of claim 1,

At least one of the DC-AC converter and the AC-DC converter has a Scott transformer operating at a medium frequency of several hundred Hz to several tens of kHz.
The method of claim 1,

The sub-module of the multi-level converter has a Scott transformer consisting of a half bridge, a full bridge or a combination of a half bridge and a full bridge.