WO2013151542A1

WO2013151542A1 - Multilevel converter

Info

Publication number: WO2013151542A1
Application number: PCT/US2012/032097
Authority: WO
Inventors: Parag Kshirsagar
Original assignee: Otis Elevator Company
Priority date: 2012-04-04
Filing date: 2012-04-04
Publication date: 2013-10-10

Abstract

A three level converter comprises a first converter leg having first switches; a second converter leg having second switches; a DC bus having a neutral point node; two clamping diodes connected to the neutral point node, the first converter leg and the second converter leg both coupled to the two clamping diodes, the two clamping diodes clamping a voltage in both the first converter leg and the second converter leg.

Description

MULTILEVEL CONVERTER

FIELD OF INVENTION

[0001] The subject matter disclosed herein relates generally to the field of power conversion systems, and more particularly to a multi-level converter with a reduced number of diodes operable as an inverter or as a rectifier.

DESCRIPTION OF RELATED ART

[0002] Three-phase motors are used in various industrial applications and devices.

Elevator systems, for example, typically utilize three-phase AC voltage drives to power hoist motors that move the elevator cars. Because these hoist motors can consume large amounts of energy, energy efficient power control systems are desirable for use in such elevator systems.

[0003] In typical elevator systems, a building AC voltage source is supplied to a rectifier circuit where it is converted into a DC voltage. Inverters are then used to convert the DC voltage back into an AC voltage having desired characteristics. While inverters are well suited for such conversions, the resultant AC voltages typically contain various harmonic frequencies due to the power stage switching operations of the inverters. These harmonic frequencies are undesirable and can negatively affect the related elevator systems when present. The potential impact of harmonic frequencies can be estimated by considering the total harmonic distortion (THD) of a system, where the THD is a measure of the distortion that is present in a signal as it passes through the system. In general, systems with less THD are more desirable.

[0004] Three-phase two-level converters, known as six switch converters, are typically used in elevator systems. FIG. 1 is a schematic diagram of a conventional three phase two- level converter. Conventional three-phase two-level converters without output filters typically have a THD of approximately 72% in the line-line voltage. Because THD of this magnitude is undesirable or unacceptable in most elevator system related applications, significant filtering is generally required in the source side in order to achieve an acceptable THD. THD values below 5% may be acceptable. Because such filtering requires the use of many additional passive components, filtering can often increase the size and cost of the associated inverter devices and elevator systems. [0005] Additionally, typical three-phase two-level inverters also exhibit high dv/dt values (i.e., high transient voltages) and high switching losses. Continuous repetitive high transient voltages, when applied on the motor, can damage winding insulation (dielectric breakdown) and affect bearing life in a system. Higher switching losses due to higher switching voltages significantly reduces the efficiency of the drive system.

[0006] The use of multilevel inverters, such as diode-clamped, three-phase three-level inverters, has been proposed to overcome the deficiencies of three-phase two-level inverters. FIG. 2 is a schematic diagram of a conventional three-phase three-level inverter.

Conventional three-phase three-level inverters, such as that shown in FIG. 2, employ a large number switches and diodes and are therefore overly complex and expensive. FIGs. 3 and 4 depict alternate three-phase three-level inverters known in the art.

BRIEF SUMMARY

[0007] According to an exemplary embodiment of the invention, a three level converter comprises a first converter leg having first switches; a second converter leg having second switches; a DC bus having a neutral point node; two clamping diodes connected to the neutral point node, the first converter leg and the second converter leg both coupled to the two clamping diodes, the two clamping diodes clamping a voltage in both the first converter leg and the second converter leg.

[0008] According to another exemplary embodiment of the invention, an N-level converter, comprises a first converter leg having 2N-2 first switches; a second converter leg having 2N-2 second switches; (N-2) x 2 clamping diodes, the first converter leg and the second converter leg both coupled to the (N-2) x 2 clamping diodes, the (N-2) x 2 clamping diodes clamping a voltage in both the first converter leg and the second converter leg;

wherein N is an integer greater than 2.

[0009] Other aspects, features, and techniques of embodiments of the invention will become more apparent from the following description taken in conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Referring now to the drawings wherein like elements are numbered alike in the FIGURES:

[0011] FIG. 1 is a schematic diagram of a conventional three-phase two-level inverter; [0012] FIGs. 2-4 are a schematic diagrams of conventional three-phase three-level inverters;

[0013] FIG. 5 is a schematic diagram of a power conversion system including a three- phase three-level converter and a controller according to an exemplary embodiment of the invention;

[0014] FIG. 6 is a schematic diagram of a three-phase four-level inverter and a controller according to an exemplary embodiment of the invention; and

[0015] FIG. 7 is a schematic diagram of a three-phase N-level inverter and a controller according to an exemplary embodiment of the invention.

DETAILED DESCRIPTION

[0016] FIG. 5 is a schematic diagram of a power conversion system 200 including a three-phase three-level converter 201. The three-level converter 201 depicted in this embodiment uses a neutral point clamped (NPC) topology having three converter legs and two clamping diodes D7, D8. Switches S1-S4 provide a first three-level converter leg, switches S5-S8 provide a second three-level converter leg and switches S9-S12 provide a third three-level converter leg. In an exemplary embodiment, switches S1-S12 are IGBTs, although MOSFETs, IGCT's, or other similar types of high- voltage switches may be utilized without departing from the scope of the invention. When operating as an inverter, the three- level converter legs respectively provide AC power to AC nodes Va, Vb and Vc

corresponding to motor winding phases A, B and C of motor 217 as described herein. When operating as rectifier, each three-level converter leg converts an AC voltage applied at one of AC nodes Va, Vb and Vc, to a DC voltage across positive DC node +VDC and negative DC node -VDC.

[0017] Switches SI, S4, S5, S8, S9 and S12 are each associated with a diode, Dl, D2, D3, D4, D5 and D6, respectively. Each diode is connected with its cathode coupled to the collector and its anode coupled to the emitter of a switch, to serve as a freewheeling or flyback diode. Converter 201 also includes two clamping diodes D7 and D8. The anode of clamping diode D7 is coupled to the neutral node O and the cathode of clamping diode D7 is coupled to the collector of switches S2, S6 and S10. The cathode of clamping diode D8 is coupled to the neutral node O and the anode of clamping diode D8 is coupled to the emitter of switches S3, S7 and Sl l. Clamping diodes D7 and D8 serve to clamp the output voltage at AC nodes Va, Vb and Vc to one of the nodes on the DC bus, i.e., the positive DC node, the neutral node or the negative DC node. [0018] One aspect of the topology of FIG. 5 is that multiple three-level converter legs utilize common clamping diodes D7 and D8. As contrasted with FIG. 2, where each leg employs two clamping diodes, the topology of FIG. 5 reduces the number of clamping diodes from 6 to 2. Further, switches S2, S3, S6, S7, S10 and Sl l lack a diode across the emitter and collector, thus reducing the number diodes by 6 diodes. A reduction in the number of clamping diodes and diodes corresponds to a reduction in silicon, which reduces system cost, reduces the complexity of system control and increases system reliability. Because the impedance in the system 200 is reduced as a result of a reduction in the diode count, a reduction in the number of total diodes results in lower conduction losses in the power conversion system 200.

[0019] In the first three-level converter leg, a junction of a first pair of switches SI and S4 is connected to a junction of a second pair of switches S2 and S3. The junction of the first pair of switches SI and S4 is the AC node Vc. The two clamping diodes D7 and D8 are in series with each other. Clamping diodes D7 and D8 are in parallel with switches S2 and S3, also in series with each other. The other three-level converter legs are similarly configured with respect to the clamping diodes D7 and D8. In other words, clamping diodes D7 and D8 provide voltage clamping to each of the three-level converter legs.

[0020] The converter 201 is coupled to a DC bus having a positive DC node, +VDC, a neutral node O and a negative DC node, -VDC. DC voltage sources 207, 209 may be implemented by capacitors or other components connected over a DC bus or discreet DC power sources having terminals coupled together.

[0021] Also shown in FIG. 5, the converter 201 comprises six switches S2, S3, S6, S7, S10 and Sl l that are connected to the neutral node through clamping diodes D7 and D8. In an exemplary embodiment, +VDC and -VDC have the same magnitude to ensure that a zero average current flows through the neutral node O in a fundamental period.

[0022] When operating as an inverter, controller 225 applies control signals to switches SI -SI 2 to generate AC waveforms at AC nodes Va, Vb and Vc. AC nodes Va, Vb and Vc are coupled to phases A, B, and C of motor 217, which correspond to windings of the motor. Taking switches S1-S4, for example, the control signals from controller 225 open or close switches S1-S4 to produce a voltage at AC node Vc relative to neutral node O in accordance with the table below. SI S2 S3 S4 Vc

On On Off Off +VDC

Off On On Off 0

Off Off On On -Vdc

[0023] The other three-level converter legs are controlled in the same fashion. Controller 225 employs pulse width modulation (PWM) to produce control signals to turn switches Sl- S12 on and off to produce AC waveforms at AC nodes Va, Vb and Vc.

[0024] The converter 201 may also be used as a rectifier to convert AC voltage at AC nodes Va, Vb and/or Vc to a DC voltage across the positive DC node +VDC and the negative DC node -VDC. Taking node Vc, for example, when operated as an inverter, the current direction is positive such that the current flows from the DC bus to AC node Vc, with switch S2 ON, switch S3 OFF, diode D7 conducting. On the other hand, when operating as a rectifier, the current direction is negative such that the current flows from AC node Vc to the DC bus, with switch S3 ON, switch S2 OFF, and diode D8 conducting. Thus, the bidirectional current operation of the converter 201 at the neutral point can be realized.

[0025] FIG. 6 is a schematic diagram of a four-level converter 700 having a plurality of switches according to another embodiment of the invention. The four-level converter 700 is similar to the three-level converter 201 shown and described with reference to FIG. 5.

Switches S1-S6 provide a first four-level converter leg, switches S7-S12 provide a second four-level converter leg and switches S13-S18 provide a third four-level converter leg. In an exemplary embodiment, the switches S1-S18 are IGBTs, although MOSFETs, IGCT's, or other similar types of high- voltage switches may be utilized without departing from the scope of the invention. When operating as an inverter, the four-level converter legs provide respective AC waveforms to AC nodes Va, Vb and Vc corresponding to motor winding phases A, B and C of motor 711. When operating as rectifier, each four-level converter leg converts an AC voltage applied at AC nodes Va, Vb and Vc, to a DC voltage across positive DC node +VDC and negative DC node -VDC.

[0026] Switches SI, S6, S7, S12, S13 and S18 are each associated with a diode, Dl, D2, D3, D4, D5 and D6, respectively. Each diode is connected with its cathode coupled to the collector and its anode coupled to the emitter of a switch to serve as a freewheeling or flyback diode. Converter 700 also includes four clamping diodes D7, D8, D9 and D10, coupled to DC nodes and switches in a manner similar to that described with reference to FIG. 5.

[0027] Similar to FIG. 5, in the topology of FIG. 6 multiple converter legs utilize common clamping diodes D7, D8, D9 and D10. The converter 700 is coupled to DC voltage sources 705, 707 and 709 to establish four voltage levels. The DC voltage sources 705, 707 and 709 may be implemented by capacitors or other components connected over a bus or discreet DC power sources providing a designated voltage. Clamping diodes D7, D8, D9 and D10 serve to clamp the output voltage at AC nodes Va, Vb, and Vc at voltage level to one of the voltages at nodes P, Q, R, N of the DC bus.

[0028] When operating as an inverter, controller 725 applies control signals to switches SI -SI 8 to generate AC waveforms at AC nodes Va, Vb and Vc. AC nodes Va, Vb and Vc are coupled to phases A, B, and C of motor 711, which correspond to windings of the motor. Controller 725 employs pulse width modulation (PWM) to produce control signals to turn switches S1-S18 on and off to produce AC waveforms at nodes Va, Vb and Vc.

[0029] The converter 700 may also be used as a rectifier to convert AC voltage at AC nodes Va, Vb and/or Vc to a DC voltage across positive DC node +VDC and negative DC node -VDC of the DC bus. Reference is made to converter leg including switches S1-S6. Taking node Va, for example, when operated as an inverter, the current direction is positive such that the current flows from the DC bus to AC node Va, with switch S2 ON, switch S3 OFF, diode D7 conducting. On the other hand, when operating as a rectifier, the current direction is negative such that the current flows from AC node Va to the DC bus, with switch S3 ON, switch S2 OFF, and diode D8 conducting. Switches S4 and S5 and diodes D9 and D10 operate in a similar manner. Thus, the bi-directional current operation of the converter 700 can be realized.

[0030] FIG. 7 is a schematic diagram of a generalized N-level converter in an exemplary embodiment. Particularly, the N-level converter 800 depicted in this embodiment is substantially similar to the four-level converter 700 in FIG. 6. A number of voltage sources 805-1 to 805-(N-l) provide the N voltage levels. Each N-level converter leg uses the common clamping diodes to clamp the voltage at an AC node to one of the DC voltages at the DC bus nodes.

[0031] To generalize, embodiments use a total of (N-2) x 2 clamping diodes, where N is the number of DC voltage levels on the DC bus, N being an integer greater than 2. Further, each N-level converter leg includes 2N-2 switches, where N is an integer greater than 2. The N-level converter legs share common clamping diodes to reduce the number of clamping diodes in the converter. As noted above with reference to FIGs. 5 and 6, the N-level converter may be operated as an inverter or a rectifier under the control of PWM control signals from controller 825.

[0032] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. While the description of the present invention has been presented for purposes of illustration and description, it is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications, variations, alterations, substitutions, or equivalent arrangement not hereto described will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the invention. Additionally, while the various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as being limited by the foregoing description, but is only limited by the scope of the appended claims.

Claims

1. A three level converter, comprising:

a first converter leg having first switches;

a second converter leg having second switches;

a DC bus having a neutral point node;

two clamping diodes connected to the neutral point node, the first converter leg and the second converter leg both coupled to the two clamping diodes, the two clamping diodes clamping a voltage in both the first converter leg and the second converter leg.

2. The three level converter of claim 1 wherein:

the first switches include a first pair of switches and second pair of switches, a junction of the first pair of switches being connected to a junction of the second pair of switches.

3. The three level converter of claim 2 wherein:

the two clamping diodes are in parallel with the second pair of switches.

4. The three level converter of claim 3 wherein:

the second pair of switches are in series and the clamping diodes are in series.

5. The three level converter of claim 2 wherein:

the junction of the first pair of switches is an AC voltage node.

6. The three level converter of claim 2 wherein:

the second switches include a third pair of switches and fourth pair of switches, a junction of the third pair of switches being connected to a junction of the fourth pair of switches.

7. The three level converter of claim 6 wherein:

the two clamping diodes are in parallel with the fourth pair of switches.

8. The three level converter of claim 1 further comprising: a controller providing control signals to the first switches and the second switches to operate the converter as at least one of an inverter and a rectifier.

9. An N-level converter, comprising:

a first converter leg having 2N-2 first switches;

a second converter leg having 2N-2 second switches;

(N-2) x 2 clamping diodes, the first converter leg and the second converter leg both coupled to the (N-2) x 2 clamping diodes, the (N-2) x 2 clamping diodes clamping a voltage in both the first converter leg and the second converter leg;

wherein N is an integer greater than 2.

10. The N-level converter of claim 9 wherein:

11. The N-level converter of claim 10 wherein:

the two clamping diodes are in parallel with the second pair of switches.

12. The N-level converter of claim 11 wherein:

13. The N-level converter of claim 10 wherein:

the junction of the first pair of switches is an AC voltage node.

14. The N-level converter of claim 10 wherein:

15. The N-level converter of claim 14 wherein:

the two clamping diodes are in parallel with the fourth pair of switches.

16. The N-level converter of claim 9 further comprising:

a controller providing control signals to the first switches and the second switches to operate the converter as at least one of an inverter and a rectifier.