Cell-based multilevel converter with multiple operating modes and associated control method

Abstract

A multi-level converter (300, 310) comprises a plurality of power cells (302, 304) that receive power from a source and supply power to a plurality of output phases (U, V, W), wherein each output phase (U, V, W) comprises a high-voltage power cell (302) designed to output more than three voltage levels.

Description

Technical Field

[0001] This disclosure generally relates to a multilevel converter, and more particularly to a unit-based multilevel converter with multiple operating modes and one or more associated control methods. Such a multilevel converter can be, for example, a medium-voltage variable frequency drive. Throughout this specification, the terms "drive," "drive system," "multilevel power converter," "converter," "power supply," and "variable frequency drive (VFD)" are used interchangeably. Background Technology

[0002] Medium voltage (MV) inverter drives (such as, for example, multilevel power converters) are used in applications such as medium voltage AC drives, flexible AC transmission systems (FACTS), and high voltage DC (HVDC) transmission systems because single-power semiconductor devices cannot handle high voltages. Multilevel power converters typically include multiple power units for each phase, each power unit including an inverter circuit with semiconductor switches that can change the voltage output of the individual units. An example of a multilevel power converter is a cascaded H-bridge converter system with multiple H-bridge units, such as those described in U.S. Patent No. 5,625,545 to Hammond, the contents of which are incorporated herein by reference in their entirety.

[0003] Cascaded multilevel converters are a preferred topology in many medium and high voltage applications. However, in some cases, a large number of units may be required to achieve the desired output voltage level, leading to an increase in the overall system cost. One option to address this problem is to increase the voltage level of each individual unit, thereby reducing the number of units required for the system to achieve a given output voltage. One drawback of this approach is that systems built with higher voltage units are optimized for a specific output voltage level in terms of unit count, but they may not be optimized for other output voltage levels. Furthermore, the voltage output exhibits a large voltage step, which can be detrimental to coupled loads, such as motors, especially when the application involves large cables connecting the converter and the motor.

[0004] Cascaded multilevel converters are typically built using identical units, i.e., the same unit topology with the same voltage and current ratings. A different approach is to use units with different topologies and different voltage and current ratings, a so-called hybrid topology. In this case, units with higher voltage ratings typically switch at a lower (basic) frequency, while units with lower voltage ratings switch at a higher frequency. This traditional approach has two drawbacks: the high-voltage units will experience unequal losses between them, and depending on the modulation method and load conditions, the low-voltage units may have to absorb excess energy from the higher-voltage units. Furthermore, the converter may require an output filter, which increases the overall cost if the units are built with a higher voltage step (i.e., a step > 1000V). Summary of the Invention

[0005] In brief, aspects of this disclosure relate to a multilevel converter, and more particularly to a cell-based multilevel converter having multiple operating modes and a plurality of associated control methods.

[0006] A first aspect of this disclosure provides a multilevel converter including a plurality of power units that receive power from a source and supply power to a plurality of output phases, wherein each output phase includes at least one high-voltage power unit designed to output more than three voltage levels.

[0007] A second aspect of this disclosure provides an electric drive system including a power source, a load, and a multilevel converter coupled to the power source to generate a multiphase power supply for the load. The converter includes a plurality of power units that receive power from the power source and supply power to a plurality of output phases, wherein each output phase includes at least one high-voltage power unit designed to output more than three voltage levels. Attached Figure Description

[0008] Figure 1 shows a schematic diagram of a known basic configuration of a cascaded H-bridge converter system according to exemplary embodiments disclosed herein.

[0009] Figure 2 A schematic diagram of a known system including a standard cascaded H-bridge multilevel converter according to an exemplary embodiment of the present disclosure is shown.

[0010] Figure 3 A schematic diagram of a multilevel converter including different types of power units according to an exemplary embodiment of the present disclosure is shown.

[0011] Figure 4 A schematic diagram of a general-purpose power unit designed to output five voltage levels according to an exemplary embodiment of the present disclosure is shown.

[0012] Figure 5A schematic diagram of a first topology of a power unit designed to output five voltage levels is shown, and Figure 6 A schematic diagram of a second topology of a power unit designed to output five voltage levels according to an exemplary embodiment of the present disclosure is shown.

[0013] Figure 7 A graphical representation of the baseband (switching command) is shown, and Figure 8 A graphical representation of the details of a baseband according to an exemplary embodiment of the present disclosure is shown.

[0014] Figure 9 A graphical representation of the cell voltage of a power cell with capacitor Cf2 is shown, and Figure 10 A graphical representation of the cell voltage of a power cell without a capacitor Cf2 according to an exemplary embodiment of the present disclosure is shown.

[0015] Figure 11 A block diagram illustrating power unit control using a phase-shift PWM method according to an exemplary embodiment of the present disclosure is shown.

[0016] Figure 12 A graphical representation of the line-to-line voltage of five power units for each output phase of PS-PWM is shown according to an exemplary embodiment of the present disclosure, and Figure 14 A graphical representation of the line-to-line voltage of the four power units for each output phase using PS-PWM is shown.

[0017] Figure 13 A graphical representation of the DC link voltage and flying capacitor voltage of capacitor Cf1 according to an exemplary embodiment of the present disclosure is shown. Capacitor Cf1 is connected to five power units for each output phase (see [link to documentation]). Figure 12 ),and Figure 15 A graphical representation of the DC link voltage and flying capacitor voltage of capacitor Cf2 according to an exemplary embodiment of the present disclosure is shown, wherein capacitor Cf2 is connected to four power units for each output phase (see [link to documentation]). Figure 13 ).

[0018] According to exemplary embodiments of this disclosure, Figure 16 A graphical representation of the line-to-line voltage of the two power units for each output phase using the optimal PWM method is shown. Figure 17 A graphical representation of the output current of the two power units in each output phase using the optimal PWM method is shown, and Figure 18 A graphical representation of the DC link voltage and capacitor Cf1 voltage for the two power units of each output phase using the shift PWM method is shown.

[0019] Figure 19An exemplary embodiment of the present disclosure is shown for a multilevel converter (e.g.) Figure 3 A schematic diagram of an embodiment of the transformer winding arrangement of the converter 300 or converter 310 shown. Detailed Implementation

[0020] To facilitate understanding of the embodiments, principles, and features of this disclosure, they are explained below with reference to implementations in illustrative embodiments. In particular, they are described in the context of drive systems, especially medium-voltage (MV) frequency converters including multi-unit power supplies such as modular multilevel converter systems and cascaded H-bridge converter systems. However, the embodiments of this disclosure are not limited to use in the described devices or methods.

[0021] As used herein, "medium voltage" is a voltage greater than about 690V and less than about 69kV, and "low voltage" is a voltage less than about 690V. Those skilled in the art will understand that other voltage levels may be designated as "medium voltage" and "low voltage." For example, in some embodiments, "medium voltage" may be a voltage between about 3kV and about 69kV, and "low voltage" may be a voltage less than about 3kV.

[0022] The components and materials described below as constituting various embodiments are intended to be illustrative and not restrictive. Many suitable components and materials that will perform the same or similar functions as the materials described herein are intended to be included within the scope of embodiments of this disclosure.

[0023] Figure 1 shows a schematic diagram of a known multi-unit power supply 10, particularly a cascaded H-bridge converter system, which receives three-phase power from an AC source and delivers power to a load 12, such as a three-phase AC motor. An example of a cascaded H-bridge multilevel converter 310 is the Perfect Harmony manufactured by Siemens Industries. Driver.

[0024] Load 12 may include an AC motor, such as a synchronous, asynchronous, or permanent magnet motor, and may be rated for low, medium, or high voltage. For example, medium-voltage AC motors (such as those used in industrial process control) may operate in the range of 4.16 kV to 13.8 kV. Higher or lower voltages may be used. More than one motor may be connected. Other loads may be used in place of motors or in addition to motors. The motor responds to the voltage applied to the three phases by the multi-unit power supply 10, for example, to increase, decrease, or maintain speed or position.

[0025] Referring to Figure 1, the multi-unit power supply 10 includes a transformer 14, power circuitry 16, and a central control system 18, also referred to herein as a controller. The transformer 14 includes a primary winding that excites nine secondary windings, and the power circuitry 16 includes multiple printed circuit board (PCB) power units 26, referred to herein simply as power units 26 or power modules, each operatively coupled to the secondary windings of the transformer 14. Since the power supply 10 includes nine secondary windings, and power units 26 are operatively coupled to each secondary winding, the power supply 10 includes nine power units 26. Of course, depending on the type of the power supply 10 and / or the type of the load 12 coupled to the power supply 10, the power supply 10 may include more or fewer than nine power units 26 and / or more or fewer than nine secondary windings.

[0026] Power unit 26 is configured to provide a medium-voltage output to load 12. Each output phase A, B, C of power circuit 16 is fed by a set of power units 26 connected in series. The outputs of power units 26 are coupled in series in a first phase group 30, a second phase group 32, and a third phase group 34. The output voltage of each phase is the sum of the output voltages of the power units 26 in the corresponding phase groups 30, 32, and 34. For example, the first phase group 30 includes power units 26 labeled A1, A2, and A3, where the phase output voltage of output phase A is the sum of the output voltages of power units A1, A2, and A3. This also applies to output phase B and power units B1, B2, and B3, and output phase C and power units C1, C2, and C3. In this respect, power circuit 16 uses lower-voltage rated power units 26, including components rated for a lower voltage standard, to deliver a medium-voltage output to output load 12. Each power unit 26 is coupled to a central control system 18, for example, via an optical fiber communication link. The central control system 18 can use current feedback and voltage feedback to control the operation of the power unit 26.

[0027] It should be noted that in Figure 1, the number of power units 26 in each phase group 30, 32, 34 can be between 2 and 12 to provide different (medium voltage) outputs required by load 12. As shown in the embodiment of Figure 1, the number of secondary windings of transformer 14 matches the number of power units 26. Those skilled in the art will understand that other numbers of units and diode bridges may be used depending on the application, and the configurations shown and described herein are intended to be exemplary in nature.

[0028] Figure 2 A schematic diagram of a known system 200 is shown, including a standard cascaded H-bridge multilevel converter 210 comprising three phases having multiple power units 212, wherein each phase includes the same number of power units, such as three or four power units per phase. System 200 also includes a pulse width modulation (PWM) controller 230 and an AC motor 220.

[0029] exist Figure 2 In this example, system 200 is a medium-voltage drive, such as a 4160V medium-voltage drive, which includes a three-phase power supply providing an AC power input 202 via lines L1, L2, and L3. Each power unit 212 of the multilevel converter 210 is connected to the transformer secondary AC power 202 and generates a three-phase AC power supply as an output 204 via phase output lines U, V, and W. The AC output 204 via lines U, V, and W can be connected to a load, in this example, including an AC motor 220. The motor 220 can be operated by controlling the frequency and / or amplitude of the output voltage generated by the multilevel converter 210.

[0030] Each phase of the multilevel converter 210 includes multiple power units 212 arranged in a cascaded manner. Each power unit 212 has a three-phase input voltage and a single-phase output, and each unit 212 is configured to output three voltage levels.

[0031] Each power unit 212 of a phase is connected to the power input 202 via corresponding input lines L1, L2, and L3. Power can be supplied to the input lines L1, L2, and L3, for example, via a multiphase winding transformer. The power units 212 of a phase are connected in series and are respectively labeled as unit A-1, unit AN, unit B-1, unit B-2, unit BN-1, unit BN, unit C-1, unit C-2, unit CN-1, and unit CN. Each power unit 212 responds to a control signal from the PWM controller 230 to change the voltage level and / or frequency output, thereby generating a multi-level voltage waveform for each phase. The power unit 212 typically includes power semiconductor switching devices, passive components (inductors, capacitors), control circuitry, a processor, an interface, and other components for communicating with the controller 230. The power unit 212 operates based on signals from the controller 230.

[0032] Each power unit 212 includes a single-phase inverter circuit connected via input lines L1, L2, L3 to a separate DC source generated by rectification of the AC power input of each power unit 212. In this example, rectification is performed by diode rectifiers D1-D6 (unit A-1) and D7-D12 (unit AN) arranged in a bridge rectifier configuration. This example also uses a filter circuit including, for example, capacitors C1 and C2, to smooth the voltage ripple from the rectified DC power.

[0033] The inverter circuit of each power unit 212 includes power semiconductor switching devices Q1-Q4 (unit A-1) and Q5-Q8 (unit AN) arranged in an H-bridge (also known as a full-bridge) configuration. Switching devices Q1-Q8 may include, for example, but not limited to, power transistors such as insulated-gate bipolar transistors (IGBTs). Switching devices Q1, Q2 and Q5, Q6 are connected to unit output line 214a, while switching devices Q3, Q4 and Q7, Q8 are connected to unit output line 214b. The switching devices (i.e., transistors) Q1-Q8 receive, for example, a pulse-width modulated signal in the form of a gate input signal 216, which is controlled by controller 230 based on the pulse-width modulation.

[0034] Controller 230 selects either transistor Q1 or Q2 (and Q5 or Q6) to conduct via a first switching branch 218a, and selects either transistor Q3 or Q4 (and Q7 or Q8) to conduct via a second switching branch 218b, which allows power to be delivered to load 220 via lines 214a or 214b, respectively. In other words, a switching event triggered by the controller in switching branch 218a causes one of transistors Q1 or Q2 to be on and the other to be off. Similarly, a switching event triggered by the controller in switching branch 218b causes one of transistors Q3 or Q4 to be on and the other to be off. In the illustrated embodiment, the switching branches 218a and 218b of the individual unit 212 are simply referred to as switching branch A and switching branch B of that particular unit 212.

[0035] Motor 220 may include one type of AC motor, such as a synchronous, asynchronous, permanent magnet motor, and may be rated for low, medium, or high voltage. For example, medium-voltage AC motors (such as those used in industrial process control) may operate in the range of 4.16 kV to 13.8 kV. Higher or lower voltages may be used. More than one AC motor 220 may be connected. Other loads may be used in place of motor 220 or in addition to motor 220. AC motor 220 responds to the voltage applied to the three phases by a multilevel converter, for example, to increase, decrease, or maintain speed or position.

[0036] The PWM controller 230 may include, for example, a processor with memory capable of storing and executing specific instructions to implement the PWM control shown. The controller 230 may be implemented, for example, but not limited to, a microcontroller with internal or external memory, a fixed-point or floating-point digital signal processor (DSP), a programmable logic device (PLD), or any combination thereof. By pulse-width modulating the voltage reference of each phase, the controller 230 controls each power unit 212, and thus controls the amplitude and frequency of the voltage output between the output lines 214a and 214b of each power unit 212. The control circuitry or control board in the power unit 212 may receive the voltage reference and use appropriate vector control and pulse-width modulation to generate gate pulses for the power switching device. Alternatively, the controller 230 may provide gate pulses to the unit 212 based on the voltage reference output.

[0037] Figure 3 A schematic diagram of a multilevel converter 300, 310 including different types of power units according to an exemplary embodiment of the present disclosure is shown.

[0038] Compared with traditional methods (such as, for example, by Figure 2 Unlike the system 200 shown herein, the new multilevel converter described herein comprises multiple power units, which can be selected from various unit types and / or can operate in different operating modes. The power units can be identical in terms of rated voltage or topology, or the power units can have different topologies and different rated voltages. Since the power units for each phase are connected in series, they all have the same rated current. However, the rated voltages of (one or more) power units can be different.

[0039] A first exemplary configuration is shown by converter 300, wherein in each output phase U, V, W, there is at least one high-voltage (HV) power unit 302 and a low-voltage (LV) unit 304. In other examples, each output phase U, V, W may include two or more high-voltage units 302 and one low-voltage unit 304. Typically, there is one low-voltage unit 304 combined with multiple high-voltage units 302. Each power unit 302, 304 receives input power from power supply 306 and provides output power to load 308 via output phase U, V, W.

[0040] Specifically, the high-voltage power unit 302 has 2V dc The rated voltage, and the low-voltage power unit has V dc The rated voltage. Each high-voltage unit 302 is capable of outputting five voltage levels: -2V dc -V dc 0, Vdc and 2V dc Each low-voltage unit 304 can output three voltage levels: -V dc ,0,V dc In all cases, V dc and 2V dc These are the DC link voltages for the low-voltage power unit 304 and the high-voltage power unit 302, respectively. Typically, the DC link voltages for each power unit 302, 304 are obtained by a three-phase diode rectifier. In this example, the output voltage of the converter 300 may only include voltages with an amplitude of V. dc The voltage step.

[0041] Low-voltage power unit 304 can be, for example Figure 2 The configuration is as described in [reference 1], see unit A-1. Reference will be made later, for example. Figure 4 , Figure 5 and Figure 6 Describe the topology of the high-voltage power unit 302.

[0042] In the second exemplary configuration shown in converter 310, each output phase U, V, W includes the same high-voltage power unit 302, each high-voltage power unit 302 having a 2V rating. dc The rated voltage. Each high-voltage power unit 302 is capable of outputting five voltage levels: -2V dc -V dc 0, V dc 2V dc .

[0043] The converter 310, including the high-voltage unit 302, can operate in different operating modes. In the first operating mode, unit 302 is operated to output five voltage levels -2V. dc -V dc 0, V dc 2V dc Furthermore, the output voltage of converter 310 can include only voltages with an amplitude of V. dc The voltage step. In the second operating mode, unit 302 is operated to output only three voltage levels -2V. dc 0 and 2V dc In this mode, the output voltage of converter 310 includes an amplitude of 2V. dc The voltage step. In the third operating mode, there are (N-1) voltage steps with a rated voltage of 2V on each phase. dc The unit (high voltage unit 302), and a rated voltage of V dc The unit (low-voltage unit 304). Each high-voltage unit 302 can output 5 levels: -2V dc -V dc 0, Vdc and 2V dc Each low-voltage unit 304 can output three levels: -V dc ,0,V dc In all cases, V dc and 2V dc These are the DC link voltages for low-voltage unit 304 and high-voltage unit 304, respectively. The general topology of high-voltage unit 302 is as follows: Figure 4 As shown, the low-voltage unit 304 is in Figure 2 As shown in the diagram. In this third operating mode, the driver's output voltage will only contain the amplitude V. dc The voltage step.

[0044] One or more low-voltage units 304 and high-voltage units 302 both include the same type of power semiconductor components. Basically, the high-voltage unit 302 includes twice the number of power components (semiconductors) of the low-voltage unit 304.

[0045] Figure 4 A schematic diagram is shown of a general-purpose high-voltage (HV) power unit 400 designed to output up to five voltage levels according to an exemplary embodiment of the present disclosure.

[0046] The power unit 400 is connected to an AC power input via input lines L1, L2, and L3. Power can be supplied to the input lines L1, L2, and L3, for example, via a multiphase winding transformer. The power unit 400 typically includes power semiconductor switching devices, passive components (inductors, capacitors), control circuitry, a processor, interfaces, and other components. The power unit 400 operates based on signals from a controller (e.g., a PWM controller).

[0047] The power unit 400 includes a single-phase inverter circuit connected to a separate DC source generated by rectification of the AC power input. Rectification is performed by diode rectifiers D1-D6 arranged in a bridge rectifier configuration. This example also uses a filter circuit including, for example, capacitors C1 and C2, to smooth the voltage ripple from the rectified DC power.

[0048] The inverter circuit of the power unit includes power semiconductor switching devices Q1-Q4 and Q4-Q8 arranged in an H-bridge (also known as a full-bridge) configuration. Switching devices Q1-Q8 may include, for example, but not limited to, power transistors such as insulated-gate bipolar transistors (IGBTs). Switching devices (i.e., transistors) Q1-Q8 receive pulse-width modulated signals, for example, in the form of gate input signals, which are controlled by a controller based on the pulse-width modulation.

[0049] refer to Figure 4The following components can be considered auxiliary components because they have much lower rated power compared to other power semiconductors: transistor T1, clamp (freewheeling) diode FWD1, transistor T2, clamp diode FWD2, transistor T3, clamp diode FWD3, transistor T4, clamp diode FWD4, impedance Za1, impedance Za2, impedance Za3 and impedance Za4.

[0050] Figure 5 A schematic diagram of a first topology of a power unit 500 designed to output five voltage levels is shown, and Figure 6 A schematic diagram of a second topology of a power unit 600 designed to output up to five voltage levels according to an exemplary embodiment of the present disclosure is shown.

[0051] based on Figure 4 The general-purpose power unit 400 has at least two topologies for power units with five voltage levels, which are respectively in Figure 5 and Figure 6 As shown in the image.

[0052] about Figure 5 Although the Power Unit 500 appears to be a flyover capacitor converter in terms of topology, there are two key differences:

[0053] - The two capacitors Cf1 and Cf2 are not equal, with Cf2 being significantly smaller than Cf1. The relationship between Cf1 and Cf2 is described by equation (1), where K is greater than 2500:

[0054]

[0055] The switching devices Q1-Q4 in branch A and the switching devices Q5-Q8 in branch B are switched differently. Although pulse width modulation (PWM) technology is used to switch Q1-Q4 in branch A,

[0056] However, switches Q5-Q8 in branch B switch at the base frequency, for example... Figure 7 As shown.

[0057] about Figure 6 The converter topology of power unit 600 appears to be a flying capacitor topology for branch A and a neutral point clamp (NPC) topology for branch B. However, for branch B, switching devices Q5-Q8 switch at the base frequency instead of NPC modulation technology.

[0058] The power unit 600 also includes clamping diodes FWD3 and FWD4 (instead of...) Figure 5 The capacitor Cf2 of the topology. Figure 5 and Figure 6 The two topologies allow the voltage of branch B to be V when the branch voltage is V.dc In a short period of time, from 2V dc Switch to zero or switch from zero to 2V dc The voltage of branch B is defined as the voltage from point T2 to the negative terminal of the DC link bus (the negative terminals of capacitors C2 and C4).

[0059] Figure 7 A graphical representation of the baseband (switching command) is shown, and Figure 8 A graphical representation of the details of the baseband according to an exemplary embodiment of the present disclosure is shown. Specifically, Figure 7 and Figure 8 The following are shown for switching Figure 5 Switching commands for devices Q5, Q6, Q7, and Q8 in branch B of power unit 500.

[0060] exist Figure 8 The left portion shows details of the switching commands for Q5, Q6, Q7, and Q8 (Branch B), particularly when Branch B switches to the negative rail of the DC link. Figure 8 The right part shows the details of the switching commands for Q5, Q6, Q7 and Q8, especially when branch B switches to the positive rail of the DC link.

[0061] from Figure 7 It can be seen that the switching devices Q5, Q6, Q7, and Q8 in branch B switch simultaneously. However, as... Figure 8 As shown, the corresponding switching events are controlled differently. The switching devices Q5, Q6, Q7, and Q8 for switching branch B are shown in the figure. When the branch voltage is V... dc At that time, capacitor Cf2 allows the voltage in branch B to drop from 2V for a short period of time. dc Switch to zero or switch from zero to 2V dc The voltage in branch B is defined as the voltage from point T2 to the negative rail of the DC link (the negative rails of capacitors C2 and C4).

[0062] Figure 9 The illustration shows a graphical representation of the cell voltage of a power unit 500 having a capacitor Cf2 according to an exemplary embodiment of the present disclosure, and Figure 10 The illustration shows a graphical representation of the cell voltage of a power unit 500 without a capacitor Cf2 according to an exemplary embodiment of the present disclosure.

[0063] By comparison Figure 9 and Figure 10 It can be seen that, for example Figure 7 and Figure 8 The effect of the method of switching the base frequency in conjunction with capacitor Cf2 is shown.

[0064] exist Figure 9In the middle, there exists a capacitor Cf2, while Figure 10 There is no capacitor Cf2 in it. The result is that... Figure 9 The unit voltage is shown to have an amplitude of V. dc The step voltage, while Figure 10 In this context, the unit voltage includes 2V near the zero-crossing of the voltage. dc Narrow step voltages. These 2V dc Narrow step voltage amplitudes are undesirable because they increase the total harmonic distortion of the output voltage and increase the risk of motor insulation failure.

[0065] Compared to Cf1, capacitor Cf2 can be very small because the current flows into or out of capacitor Cf2 for a very short period of time near the zero-crossing of the voltage, such as... Figure 9 and Figure 10 As shown. At all other times, capacitor Cf2 is negligible because Q5 and Q6 are conducting, or Q7 and Q8 are conducting.

[0066] refer to Figure 5 and Figure 6 Unlike the switching devices Q5-Q8 (i.e., IGBTs) of branch B, which switch at the base frequency, the switching devices Q1-Q4 (i.e., IGBTs) of branch A use PWM technology to switch at a higher frequency. Many forms of PWM technology can be used, such as phase-shift PWM (PS-PWM), phase-configured PWM (PD-PWM), alternating phase configuration PWM (APOD-PWM), optimal PWM, etc. To date, the most popular and simplest method to implement is phase-shift PWM, although this method does not result in minimal total harmonic distortion (THD). A key advantage of phase-shift PWM is that, because this method results in equal capacitor current charging and discharging times, it allows for a natural balancing mechanism of the flying capacitor voltage for capacitor Cf1. Using this modulation method, pulses for each power unit 500, 600 are obtained as described below.

[0067] A reference voltage (typically a sine wave or a sine wave with a third harmonic component) is compared to a triangular waveform (carrier C1). When the reference voltage is higher than carrier C1, the turn-on time of Q1 is set. When the reference voltage is lower than carrier C1, the turn-off time of Q1 is set. Q4 always switches in the opposite manner; that is, when Q1 is on, Q4 is off, and when Q1 is off, Q4 is on.

[0068] A reference voltage (typically a sine wave or a sine wave with a third harmonic component) is compared to a triangular waveform (carrier C1N) phase-shifted by 180 degrees relative to C1. The turn-on timing of Q2 is set when the reference voltage is higher than the carrier C1N. The turn-off timing of Q2 is set when the reference voltage is lower than the carrier C1N. Q3 always switches in the opposite manner; that is, when Q2 is on, Q3 is off, and when Q2 is off, Q3 is on.

[0069] When there are N units in a phase, each unit has its own carrier Ck and its inverse carrier Ckn, where k is any number between 1 and N. There is a phase shift between the N carriers, as given by equation (2):

[0070]

[0071] For a three-phase converter, an additional phase shift is required between any two phases, as provided by formula (3):

[0072]

[0073] As previously referenced Figure 4 The following components of the power unit 400 can be considered auxiliary components because they have much lower rated power compared to other power semiconductors: transistor T1, clamp (freewheeling) diode FWD1, transistor T2, clamp diode FWD2, transistor T3, clamp diode FWD3, transistor T4, clamp diode FWD4, impedance Za1, impedance Za2, impedance Za3 and impedance Za4.

[0074] The purpose of the auxiliary components is to provide a way to charge capacitors Cf1 and Cf2 to V. dc The device ensures the correct voltage level. By switching T2 synchronously with Q1 and T1 synchronously with Q4, capacitor Cf1 is connected in parallel with C1 or C2 through impedance Za1 or Za2, thereby ensuring that Cf1 is charged to the appropriate voltage. dc Level. Similarly, by switching T4 synchronously with Q5 and switching T3 synchronously with Q8, capacitor Cf2 is connected in parallel with C3 or C4 through impedance Za3 or Za4, thereby ensuring that Cf4 is charged to the appropriate V. dc Level. Impedances Za1-Za4 can be designed using high-value resistors and / or inductors (including any parasitic inductance due to circuit layout). For example, impedances Za1-Za4 can be selected such that the current through these auxiliary components is 100-1000 less than the rating of the corresponding power unit.

[0075] Another purpose of the auxiliary components is to provide a slow voltage balancing mechanism for capacitors Cf1 and Cf2, especially when there is no load or a very light load coupled to the power unit. As mentioned earlier, the use of phase-shift PWM results in a natural voltage balance between capacitors Cf1 and Cf2. However, the balancing mechanism depends on the presence of load current, so if the load current is zero or very low, it is impossible to drive the capacitor voltage to V. dc Therefore, the auxiliary components provide a load-independent mechanism, through which the voltage of the flying capacitors Cf1 and Cf2 can be controlled to the correct voltage level V. dc .

[0076] Figure 11 A block diagram illustrating power unit control using a phase-shift PWM method according to an exemplary embodiment of the present disclosure is shown.

[0077] Based on the above description, a block diagram of power unit control using the phase-shift PWM method can be drawn, such as... Figure 11 As shown in the diagram. An additional optional box is shown in this block diagram, where small compensation terms can be added based on the voltage feedback from the flying capacitor VCf1 and the sign of the load current Iout.

[0078] To obtain the additional compensation term, equation (4) can be used, where Kp is the proportional gain used for regulating the voltage of the flying capacitor.

[0079]

[0080] Figure 12 A graphical representation of the line-to-line voltage of five power units for each output phase of PS-PWM is shown according to an exemplary embodiment of the present disclosure, and Figure 14 A graphical representation of the line-to-line voltage of the four power units for each output phase using PS-PWM is shown.

[0081] Figure 13 A graphical representation of the DC link voltage and flying capacitor voltage of capacitor Cf1 according to an exemplary embodiment of the present disclosure is shown. Capacitor Cf1 is connected to five power units for each output phase (see [link to documentation]). Figure 12 ),and Figure 15 A graphical representation of the DC link voltage and flying capacitor voltage of capacitor Cf2 according to an exemplary embodiment of the present disclosure is shown, wherein capacitor Cf2 is connected to four power units for each output phase (see [link to documentation]). Figure 13 ).

[0082] Perform simulations to examine and validate the PS-PWM technology for topologies including capacitor Cf1 in high-voltage power units 400, 500, and 600. Use the following simulation parameters:

[0083] Switching frequency 625Hz Output frequency 60Hz DC link voltage <![CDATA[2100V dc ]]> Flying capacitor voltage <![CDATA[1050V dc ]]> Peak output current 225A DC link total capacitance 2200μF Cf1 1100μF Cf2 22μF

[0084] Figure 16 , Figure 17 and Figure 18 An embodiment involving two power units for each output phase U, V, W, specifically includes a high-voltage power unit 302 and a low-voltage power unit 304. Specifically, according to exemplary embodiments of this disclosure, Figure 16 A graphical representation of the line-to-line voltage of the two power units for each output phase using the optimal PWM method is shown. Figure 17 A graphical representation of the output current of the two power units in each output phase using the optimal PWM method is shown, and Figure 18 A graphical representation of the DC link voltage and capacitor Cf1 voltage for the two power units of each output phase using the shift PWM method is shown.

[0085] Figure 16 , Figure 17 and Figure 18 Another simulation is shown, in which the multilevel converter 310 includes a high-voltage unit 302 and a low-voltage unit 304 on each output phase U, V, W. For example... Figure 16 As shown, a hybrid structure using high-voltage power unit 302 and low-voltage power unit 304 is employed, and the line-to-line voltage of the power unit includes only V. dc The voltage step, even if the high-voltage unit has 2V dc DC link bus voltage.

[0086] Figure 19 An exemplary embodiment of the present disclosure is shown for a multilevel converter (e.g.) Figure 3 A schematic diagram of an embodiment of the transformer winding arrangement of the converter 300 or converter 310 shown.

[0087] Each power unit of a multilevel converter is typically powered from the secondary winding of a multi-winding transformer. With all power units identical, the multi-winding transformer can be constructed with a phase shift between the secondary windings, which depends on the number of power units per phase.

[0088] In another example, an 18-pulse transformer 100 with multiple windings can be used to provide isolated input voltages to each power unit. This 18-pulse transformer can be used in converters 300, 310, which include one or more high-voltage units 302 and / or one or more low-voltage units 304 for each output phase.

[0089] In an exemplary embodiment, an 18-pulse transformer 100 with multiple windings provides isolated input voltages to each power unit A-1, AN, B-1, BN, C-1, CN. All power units A-1, AN, B-1, BN, C-1, CN of output phases A, B, and C are energized from transformer windings having the same phase shift via input lines L1, L2, and L3. In this example, all units on phase A (unit A-1, unit AN) include transformer windings with a 20-degree phase shift, denoted as . All units on phase B (unit B-1, unit BN) include the transformer winding with a 0-degree phase shift, denoted as... Furthermore, all units on phase C (unit C-1, unit CN) include transformer windings with a -20 degree phase shift, denoted as...

[0090] Compared to using only the high-voltage power unit 302, combining the low-voltage power unit 304 and one or more high-voltage power units 302 allows for further optimization and cost reduction. Therefore, the total cost of the multilevel converters 300 and 310 is reduced by decreasing the number of power units 302, 304 required for a specific output voltage.

Claims

1. A multilevel converter (300, 310), comprising: Multiple power units (302, 304) receive power from a source and supply power to multiple output phases (U, V, W). Each output phase (U, V, W) includes at least one high-voltage power unit (302), which is designed to output more than three voltage levels. The high-voltage power unit (302) includes a first switch branch containing semiconductor switches (Q1-Q4) and a second switch branch containing semiconductor switches (Q5-Q8). The first switching branch includes a first capacitor (Cf1), and the second switching branch includes a second capacitor (Cf2). The relationship between the first capacitor (Cf1) and the second capacitor (Cf2) is described by the following equation, where K is greater than 2500: ,or The first switching branch includes a flying capacitor topology, and the second switching branch includes a neutral clamp diode converter.

2. The multilevel converter (300, 310) according to claim 1. in, At least one high-voltage power unit (302) is designed to output five voltage levels.

3. The multilevel converter (300, 310) according to claim 1 or 2. in, Each output phase (U, V, W) includes multiple identical high-voltage power units (302).

4. The multilevel converter (300, 310) according to claim 1 or 2. in, At least one high-voltage power unit (302) is capable of operating in multiple operating modes, wherein in a first operating mode, at least one high-voltage power unit (302) outputs five voltage levels, and wherein in a second operating mode, at least one high-voltage power unit (302) outputs three voltage levels.

5. The multilevel converter (300, 310) according to claim 1 or 2. Each output phase (U, V, W) includes a combination of a high-voltage power unit (302) and a low-voltage power unit (304), wherein the low-voltage power unit (304) is designed to output three or fewer voltage levels.

6. The multilevel converter (300, 310) according to claim 5. in, The high-voltage power unit (302) and the low-voltage power unit (304) have different rated voltages.

7. The multilevel converter (300, 310) according to claim 1. in, The switches (Q1-Q4) of the first switch branch are switched using a different technique than that used for the switches (Q5-Q8) of the second switch branch.

8. The multilevel converter (300, 310) according to claim 1. It also includes a pulse width modulation (PWM) controller operablely coupled to the at least one high-voltage power unit (302), wherein the switches (Q1-Q4) of the first switching branch are controlled and switched by the PWM controller using PWM technology.

9. The multilevel converter (300, 310) according to claim 8. in, The PWM technology includes phase-shift PWM control, and wherein the phase-shift PWM control includes zero-crossing pulse processing logic to eliminate... The step voltage.

10. The multilevel converter (300, 310) according to claim 1. in, The switches (Q5-Q8) of the second switching branch are controlled and switched according to the base frequency, and include zero-crossing pulse processing logic to eliminate... The step voltage.

11. The multilevel converter (300, 310) according to claim 1. It also includes an 18-pulse transformer (100) that provides isolation voltage to the plurality of power units (302, 304), wherein the isolation voltage has a phase shift, and wherein power units (302, 304) with the same output phase (U, V, W) include transformer windings with the same phase shift.

12. The multilevel converter (300, 310) according to claim 11. in, The phase shift includes a 20-degree shift.

13. An electric drive system, comprising: Power supply (306). Load (308), and A multilevel converter (300, 310) coupled to the power supply for generating a multiphase power supply for the load, the converter (300, 310) comprising: Multiple power units (302, 304) receive power from the power source and supply power to multiple output phases (U, V, W). Each output phase (U, V, W) includes at least one high-voltage power unit (302), which is designed to output more than three voltage levels. The high-voltage power unit (302) includes a first switch branch containing semiconductor switches (Q1-Q4) and a second switch branch containing semiconductor switches (Q5-Q8). The first switching branch includes a first capacitor (Cf1), and the second switching branch includes a second capacitor (Cf2). The relationship between the first capacitor (Cf1) and the second capacitor (Cf2) is described by the following equation, where K is greater than 2500: ,or The first switching branch includes a flying capacitor topology, and the second switching branch includes a neutral clamp diode converter.

14. The electric drive system according to claim 13, in, Each output phase (U, V, W) includes a combination of a high-voltage power unit (302) and a low-voltage power unit (304), wherein the low-voltage power unit (304) is designed to output three or fewer voltage levels.

15. The electric drive system according to claim 14, in, The high-voltage power unit (302) is designed to output more than three voltage levels.

16. The electric drive system according to claim 13, It also includes an 18-pulse transformer (100) that provides isolation voltage to the plurality of power units (302, 304), wherein the isolation voltage has a phase shift, and wherein power units (302, 304) with the same output phase (U, V, W) include transformer windings with the same phase shift.

17. The electric drive system according to claim 16, in, The load includes a motor.

18. The electric drive system according to claim 17, in, The motors include medium-voltage AC motors operating in the range of 4.16kV to 13.8kV.

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