WO2013135277A1

WO2013135277A1 - A clamped modular power converter

Info

Publication number: WO2013135277A1
Application number: PCT/EP2012/054419
Authority: WO
Inventors: Frans Dijkhuizen; Jyoti Sastry; Panagiotis Bakas; Konstantinos Papastergiou
Original assignee: Abb Technology Ltd.
Priority date: 2012-03-14
Filing date: 2012-03-14
Publication date: 2013-09-19

Abstract

A power converter (9) for transferring power between a high voltage DC connection ( DC⁺, DC^-) and a high voltage AC terminal (AC, AC₀). The power converter comprises a power converter assembly (9) comprising : a first switch (4a), a first converter arm (3a), a second converter arm (3b), and a second switch (4b), connected serially in the mentioned order between the positive and negative terminals of the DC connection; and a connection link (15a, 15b, 13) arranged from a point between the second converter arm (3b) and the second switch (4b) and a point between the first converter (3a) arm and the first switch (3b). Each one of the converter arms (3a, 3b) comprises a plurality of converter cells (32, 32a-d) and each one of the converter cells has at least one switching element (40) and an energy storage element (41); the high voltage AC terminal is provided between the first converter arm (3a) and the second converter arm (3b).

Description

A CLAMPED MODULAR POWER CONVERTER

TECHNICAL FIELD

The invention relates to a power converter for converting power between a high voltage DC (Direct Current) connection and a high voltage AC (Alternating Current) connection.

BACKGROUND

High voltage power conversion between DC and AC are known in the art for a variety of different applications. One such application is for links related to HVDC (high voltage DC). The article "Novel 3-level Hybrid Neutral-Point-Clamped Converter" by Soeiro et al, Power Electronic Systems Laboratory, ETG Ziirich, available at

http: / / www.pes.ee.ethz.ch /'uploads / tx ethpublications /R. Soeiro PID1846047.pdf at the time of filing this application, presents three level voltage source converter 3, see e.g. Fig 1 (b), showing a conventional Neutral-Point-Clamped Converter. However, it would be advantageous to achieve an AC/DC converter which provides a better power quality without introducing great losses.

SUMMARY

An object of embodiments herein is to eliminate or at least alleviate the problems discussed above. A first aspect is a power converter for transferring power between a high voltage DC connection and a high voltage AC connection. The power converter comprises a power converter assembly comprising: a first switch, a first converter arm, a second converter arm, and a second switch, connected serially in the mentioned order between the positive and negative terminals of the DC connection; and a connection link arranged from a point between the second converter arm and the second switch and a point between the first converter arm and the first switch, wherein the connection link is arranged to limit each one of the voltages across the first switch and second switch. Each one of the converter arms comprises a plurality of converter cells and each one of the converter cells comprises a switching element and an energy storage element; and the high voltage AC connection is provided between the first converter arm and the second converter arm. Using this hybrid structure of switches on the outer parts and converter arms in the centre a great AC waveform quality is provided at reasonable cost is achieved. The AC waveform of exceptional quality is achieved by the converter arms, while the use of the outer switches reduces the component costs compared to a pure converter arm structure for the same rating.

The connection link may be arranged to limit each one of the voltages across the first switch and the second switch to half of the voltage over the high voltage DC connection.

The first switch may be a first transistor and the second switch may be a second transistor. Transistors are easily controlled.

The first switch may be a first thyristor and the second switch may be a second thyristor. Thyristors typically produce lower losses than transistors.

The power converter may further compnse at least one capacitor arranged between the positive and negative terminals of the DC connection. The at least one capacitor allows an ac current to circulate to reduce or even eliminate any AC component on the DC connection.

The power converter may further comprise two capacitors serially arranged between the positive and negative terminals of the DC connection. Using two capacitors, a ground point can be provided in between the capacitors.

The connection link may compnse two serially connected diodes. A point between the two serially connected diodes may be connected to a point between the two serially arranged capacitors. Using such a structure, the rating of the switches can be reduced, since the switches at a maximum would need to withstand half the voltage between the terminals of the DC connection.

The connection link may comprise a capacitor.

The first converter arm and the second converter arm may each comprise at least one full bridge converter cell. The first converter arm and the second converter arm may each comprise at least one half bridge converter cell.

All converter cells may all be of the same structure.

Generally, all terms used in the claims are to be interpreted according to their ordinary meaning in the technical field, unless explicitly defined otherwise herein. All references to "a/an/the element, apparatus, component, means, step, etc." are to be interpreted openly as referring to at least one instance of the element, apparatus, component, means, step, etc., unless explicitly stated otherwise. The steps of any method disclosed herein do not have to be performed in the exact order disclosed, unless explicitly stated. BRIEF DESCRIPTION OF THE DRAWINGS

The invention is now described, by way of example, with reference to the accompanying drawings, in which:

Fig 1 is a schematic diagram of a power converter for converting between DC and AC;

Fig 2 is a schematic diagram of a three phase power converter for converting between DC and AC;

Fig 3 is a schematic diagram of a first embodiment of a power converter assembly of Figs 1-2;

Fig 4 is a schematic diagram of a second embodiment of the power converter assembly of Figs 1-2; Figs 5A-B are schematic diagrams illustrating embodiments of the switches of Figs 3-4;

Fig 6 is a schematic diagram illustrating possible converter cell arrangements of converter arms of Figs 3-4;

Figs 7A-C are schematic diagrams illustrating embodiments of converter cells of the converter arms of Fig 5; and Fig 8 shows some schematic graphs illustrating operation of the power converter assembly of Fig 3. DETAILED DESCRIPTION

The invention will now be described more fully hereinafter with reference to the accompanying drawings, in which certain embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided by way of example so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout the description.

Fig 1 is a schematic diagram of a power converter 1 for converting between DC and AC. The power converter 1 converts power in either direction between a high voltage DC connection and a high voltage AC connection. The high voltage DC connection comprises a positive terminal DC⁺ and a negative terminal DC^". The high voltage AC connection comprises a phase terminal AC and an AC ground terminal AC₀. Power can flow from DC to AC or vice versa. The power converter 1 compnses a power converter assembly 9 which performs the actual power conversion. This division between the power converter 1 and the power converter assembly does not need to be represented by physical objects, whereby the power converter 1 and the power converter assembly 9 can in practice be the same device.

Fig 2 is a schematic diagram of a three phase power converter 1 for converting between DC and AC. The three phase power converter 1 here comprises three power converter assemblies 9a-c. In this way, the AC connection here comprises three phase terminals AQ, AC, and AC₃ to be able to provide a three phase connection, e.g. to an AC grid, an AC power source or an AC power load. Optionally, an AC ground terminal AC₀ is also provided (not shown). Fig 3 is a schematic diagram of a first embodiment of a power converter assembly 9 of Figs 1-2. The power converter assembly 9 comprises a first switch 4a, a first converter arm 3a, a second converter arm 3b, and a second switch 4b, connected serially in the mentioned order between the positive and negative terminals DC⁺, DC^" of the DC connection. Two capacitors 12a-b are serially arranged between the positive and negative terminals DC⁺, DC) of the DC connection to allow a DC current to circulate with minimal effect on the DC terminals DC⁺, DC^". Optionally, if the power converter assembly is part of a three phase system, such as the one depicted in Fig 2, the capacitors 12a-b can be omitted.

Furthermore, a connection link is arranged from a point between the second converter arm 3b and the second switch 4b, and a point between the first converter arm 3a and the first switch 4a. In this embodiment, the connection link comprises a first diode 15a and a second diode 15b, serially arranged allowing a current to flow from the point between the second converter arm 3b and the second switch 4b, and the point between the first converter arm 3a and the first switch 4a. A mid point between the diodes 15a-b is connected to a mid point between the capacitors 12a-b. In this way, the voltage at the point between the first switch 4a and the first converter arm 3a is essentially the same as the voltage between the capacitors 12a-b, minus a voltage drop over the first diode 15a, which, in the case of a high voltage application, is essentially insignificant. In this way, voltage rating requirements for the first switch is essentially halved, compared to the full voltage across DC⁺ - DC^".

As will be shown in more detail below with reference to Fig 6, each one of the converter arms 3a-b comprises a plurality of converter cells. The converter cells can be individually controlled to achieve a finer granularity in the conversion, e.g. to achieve a more sinusoidal (or square, saw tooth shaped, etc.) power conversion.

Fig 4 is a schematic diagram of a second embodiment of the power converter assembly 9 of Figs 1-2. In this embodiment, the connection link comprises a flying capacitor 13, instead of the diodes shown in the embodiment of Fig 3. The capacitor 13 reduces a maximum voltage stress over each one of the first and second switches to half of the voltage over the DC terminals DC⁺, DC^".

Figs 5A-B are schematic diagrams illustrating embodiments of the switches of Figs 3-4. Either one of the switches 4a-b is here represented as a switch 4. In Fig 5A, the switch 4 comprises a transistor 20. The transistor 20 is any suitable high power transistor, e.g. an IGBT (Insulated Gate Bipolar Transistor) or power FET (Field Effect Transistor). The transistor operation is controlled by a controller 50 according to what is shown in Fig 8 and explained in more detail below.

In Fig 5B, the transistor is implemented using a thyristor 21, which is also controlled by the controller 50 according to what is shown in Fig 8 and explained in more detail below. The modulation of the power converter comprising the thyristor 21 will lead to the current eventually passing zero, whereby the thyristor 21 will stop conducting.

Fig 6 is a schematic diagram illustrating possible converter cell arrangements of converter arms of Figs 3-4. Fig 6 illustrates the structure of any one of the converter arms 3a-b, here represented by a single converter arm 3. The converter arm 3 comprises a plurality of converter cells 32a-d, wherein each converter cell 32a-d is controlled by the controller 50 according to what is shown in Fig 8 and explained in more detail below.

The converter cells 32a-d can be connected in series to increase voltage rating or in parallel to increase current rating. The serially connected converter cells 32a-d can optionally be individually controlled to achieve a finer granularity in the conversion, e.g. to achieve a more sinusoidal (or square, saw tooth shaped, etc.) power conversion. Also, by controlling the serially connected converter cells in this way, the switching frequency of each converter cell is relatively low, which results in low switching losses when compared to higher switching frequencies. While the converter arm 3 is here illustrated to have four converter cells 32a-d, any number of converter cells is possible, including one, two, three or more. In one embodiment, the number of converter cells in each converter arm 3 is in the range from 30 to 1000 converter cells.

Optionally, a smoothing inductor 33 is serially provided in the converter arm 3 to provide a smoother current Figs 7A-C are schematic diagrams illustrating embodiments of converter cells 32a-d of the converter arms of Fig 6. Any one of the converter cells 32a-d is here represented as a single converter cell 32. A converter cell 32 is a combination of one or more semiconductor switches, such as transistors, and one or more energy storing elements, such as capacitors, supercapacitors, inductors, batteries, flywheels etc. Optionally, a converter cell can be a multilevel converter structure such as a flying capacitor or MPC (Multi-Point-Clamped) or ANPC (Active - Neutral-Point-Clamped) multilevel structure.

Fig 7A illustrates a converter cell comprising an active component in the form of a switch 40 and an energy storage component 41 in the form of a capacitor. The switch 40 can for example be implemented using an insulated gate bipolar transistor (IGBT), Integrated Gate-Commutated Thyristor (IGCT), a Gate Turn-Off thyristor (GTO), or any other suitable high power semiconductor component. In fact, the converter cell 32 of Fig 7A can be considered to be to be a more general representation of the converter cell shown in Fig 7B, which will be described here next. Fig 7B illustrates a converter cell 32 implementing a half bridge structure. The converter cell 32 here comprises a leg of two serially connected active components in the form of switches 40a-b, e.g. IGBTs, IGCTs, GTOs, etc. A leg of two serially connected diodes 42a-b is connected with the leg of serially connected switches 40a-b as shown in the figure. An energy storage component 41 is also provided in parallel with the leg of transistors 40a-b and with the leg of diodes 42a-b. The voltage synthesized by the converter cell can thus either be zero or the voltage of the energy storage component 41.

Fig 7C illustrates a converter cell 32 implementing a full bridge structure. The converter cell 32 here comprises four switches 40a-d, e.g. IGBTs, IGCTs, GTOs, etc. An energy storage component 41 is also provided in parallel across a first leg of two transistors

40a-b and a second leg of two transistors 40c-d. Compared to the half bridge of Fig 7B, the full bridge structure allows the synthesis of a voltage capable of assuming both signs, whereby the voltage of the converter cell can either be zero, the voltage of the energy storage component 41, or a reversed voltage of the energy storage component 41. Fig 8 shows some schematic graphs illustrating operation of the power converter assembly 9 of Fig 3. A line 60 represents the control S_t of the first switch 4a, and a line 61 represents the control S₂ of the second switch 4b.

The remaining lines 62— 65 all represent voltages across vanous parts of the power converter assembly 9, where the direction of the voltage tor each one is measured from a lower point to an upper point in Fig 3. The line 62 represents a voltage v_t across the first switch 4a, the line 63 represents a voltage v₂ across the first converter arm 3a, the line 64 represents a voltage v₃ across the second converter arm 3b and the line 65 represents a voltage v₄ across the second switch 4b. The line 66 represents the AC phase voltage v_AC, relative to DC^", at point AC of Fig 3.

Now the operation over time will be explained. At a time t₀, the first switch is 4a is made to conduct using control signal s_l and the second switch 4b is made to block using control signal s₂. The voltage v_l across the conducting first switch 4a is thus zero in this half period. The voltage v₂ across the first converter arm 3a starts fully negative and successively builds up to its fully positive level and then drops down to its fully negative level. In this way, the full positive effect of DC⁺ is at first completely compensated and then gradually builds up to DC⁺, after which the positive effect of DC⁺ is again increasingly compensated. Hence, a positive sinusoidal AC phase voltage v_AC is generated on the AC connection for the first half period. In this half period, the second converter arm 3b is in full blocking mode, just like the second switch 4b.

At time t_l5 the first switch is 4a is made to block using control signal s_t and the second switch 4b is made to conduct using control signal s₂. The voltage v₃ across the conducting second switch 4b is thus zero in this half period. The voltage v₃ across the second converter arm 3b starts fully positive and successively drops down to its fully negative level and then builds up to its fully positive level. In this way, the full negative effect of DC^" is at first completely compensated and then gradually drops down to DC^", after which the negative effect of DC^" is again increasingly compensated. Hence, a negative sinusoidal AC phase voltage v_AC is generated on the AC connection for the second half period. In this half period, the first converter arm 3a is in full blocking mode, just like the first switch 4a.

Time t₂ corresponds to time t₀, after the operation presented above is repeated.

The first and second switches 4a-b are thus switched at the fundamental frequency, i.e. the frequency of the AC connection, e.g. 50Hz or 60Hz. It is to be noted that the waveform of the converter arms shown in Fig 8 is relatively coarse and in reality the actual waveform can be more sinusoidal.

The invention has mainly been described above with reference to a few embodiments. However, as is readily appreciated by a person skilled in the art, other embodiments than the ones disclosed above are equally possible within the scope of the invention, as defined by the appended patent claims.

Claims

1. A power converter (1) for transferring power between a high voltage DC connection (DC⁺, DC^") and a high voltage AC connection (AC), the power converter (1) comprising a power converter assembly (9) comprising:

a first switch (4a), a first converter arm (3a), a second converter arm (3b), and a second switch (4b), connected serially in the mentioned order between the positive and negative terminals of the DC connection (DC); and

a connection link arranged from a point between the second converter arm (3b) and the second switch (4b) and a point between the first converter arm (3a) and the first switch (4a), wherein the connection link is arranged to limit each one of the voltages across the first switch (4a) and second switch (4b);

wherein each one of the converter arms (3a-b) comprises a plurality of converter cells (32a-d, 32) and each one of the converter cells (32a-d, 32) comprises a switching element (40, 40a-d) and an energy storage element (41); and

the high voltage AC connection is provided between the first converter arm (3a) and the second converter arm (3b).

2. The power converter (1) according to claim 1, wherein the connection link is arranged to limit each one of the voltages across the first switch (4a) and the second switch (4b) to half of the voltage over the high voltage DC connection (DC⁺, DC).

3. The power converter (1) according to claim 1 or 2, wherein the first switch (4a) is a first transistor and the second switch (4b) is a second transistor.

4. The power converter (1) according to claim 1 or 2, wherein the first switch (4a) is a first thyristor and the second switch (4b) is a second thyristor.

5. The power converter (1) according to any one of the preceding claims, further comprising at least one capacitor (12a-b) arranged between the positive and negative terminals (DC⁺, DC) of the DC connection.

6. The power converter (1) according to any one of the preceding claims, further comprising two capacitors (12a-b) serially arranged between the positive and negative terminals (DC⁺, DC) of the DC connection.

7. The power converter (1) according to any one of the preceding claims, wherein the connection link comprises two serially connected diodes (15a-b).

8. The power converter (1) according to claim 8 when dependent on claim 7, wherein a point between the two serially connected diodes is connected to a point between the two serially arranged capacitors.

9. The power converter (1) according to any one of the preceding claims, wherein the connection link comprises a capacitor (13).

10. The power converter (1) according to any one of the preceding claims, wherein the first converter arm (3a) and the second converter arm (3b) each comprises at least one full bridge converter cell.

11. The power converter (1) according to any one of the preceding claims, wherein the first converter arm (3a) and the second converter arm (3b) each comprises at least one half bridge converter cell.

12. The power converter (1) according to any one of the preceding claims, wherein all converter cells (32, 32a-d) are all of the same structure.