Controlling cascaded multilevel converters

Abstract

A cascaded multilevel converter is disclosed. The converter comprises a plurality of modules coupled together to form a limb, each of the modules comprising a switching circuit and a DC link for providing a DC voltage to the switching circuit. The converter further comprises a controller for controlling the switching circuit of each module to generate an AC voltage in the limb, wherein the controller is configured to determine, for each module, a voltage across a capacitor of the DC link of the module, determine, for each module, a reference power value for charging the capacitor of the DC link of the module to a reference voltage value of the module, determine a common reference AC current value for an AC current in the limb from the plurality of reference power values of the plurality of modules, determine a common reference AC voltage value for an AC voltage in the limb from the common reference AC current value, determine, for each module, a reference AC voltage value using the common reference AC voltage value and the reference power value of the module, and generate a pulse width modulation signal for controlling the switching circuit of each module from the reference AC voltage value of the module and the voltage across the capacitor of the module.

Description

Technical Field

[0001] This invention relates to a cascaded multilevel converter and a method for controlling the cascaded multilevel converter. Background Technology

[0002] A cascaded multilevel converter is a power electronic device used to provide an AC voltage level at the output by using multiple low-level DC voltages as inputs. One application of cascaded multilevel converters is as solid-state transformers in power transmission and distribution networks, replacing traditional line-frequency transformers for connecting different AC voltage levels.

[0003] A cascaded multilevel converter comprises multiple inverter modules, each including multiple power electronic switches operable to generate an AC output from a DC input by controlling the switching sequence of these switches. The AC outputs of multiple inverter modules are cascaded (i.e., connected in series) to form branches, such that the AC output voltage in a branch is synthesized by the sum of the outputs of the inverter modules. Each inverter module is supplied with DC voltage by a separate DC link, which can be coupled to the DC voltage output of a rectifier circuit, etc. This cascaded structure offers several advantages. First, since the AC output voltages of each module are added in the branches, each module only needs to be able to handle relatively low voltages and power, enabling the use of low-voltage power electronics at the module level in medium / high voltage converter applications. Furthermore, simply adding additional modules in the cascade makes it relatively easy to improve the power quality of the AC output signal in the branches, thereby increasing the number of different voltage levels available for synthesizing the AC output signal and reducing harmonic distortion in the output signal.

[0004] However, the aforementioned advantages of cascaded multilevel converters come at the cost of increased complexity, both in terms of circuit topology and the control of multiple inverter modules. Especially considering control, independent control of the individual DC link voltages supplying each inverter module is required. For example, independent control of the DC link voltages allows for maintaining equal charge in the DC links of each inverter module in a branch, which advantageously prevents modules from exceeding their operating range, even when load conditions and DC link capacitance values ​​are unequal between modules. Furthermore, independent control of the DC link voltages also advantageously allows for the detection and isolation of faulty modules, enabling the converter to continue operating even if one or more component modules fail. Summary of the Invention

[0005] The purpose of this invention is to provide a cascaded multilevel converter in which the switching sequence of the inverter stage is controlled based on the determined voltage of each DC link circuit. This advantageously achieves good dynamic decoupling of the DC link circuits, allows independent control of each DC link voltage, and / or facilitates fault detection at the individual module level, as well as corresponding module isolation.

[0006] The above and other objectives are achieved by the features claimed in the independent claims. Other implementations are apparent from the dependent claims, the specification, and the drawings.

[0007] A first aspect of the present invention provides a cascaded multilevel converter, the cascaded multilevel converter comprising: a plurality of modules coupled together to form a branch, each module including a switching circuit and a DC link for providing a DC voltage to the switching circuit; a controller for controlling the switching circuit of each module to generate an AC voltage in the branch; wherein the controller is configured to: determine the voltage across a capacitor of the DC link of the module for each module; determine a reference power value for each module for charging the capacitor of the DC link of the module to a reference voltage value of the module; determine a common reference AC current value of the AC current in the branch based on the plurality of reference power values ​​of the plurality of modules; determine a common reference AC voltage value of the AC voltage in the branch based on the common reference AC current value; determine a reference AC voltage value for each module using the following two values: the common reference AC voltage value and the reference power value of the module; and generate a pulse width modulation signal for controlling the switching circuit of each module based on the following two values: the reference AC voltage value of the module and the voltage across the capacitor of the module.

[0008] Therefore, the switching circuit of each module can be controlled by the controller to generate an AC voltage output in the branch based on the DC voltage input provided by the DC link. The controller calculates the duty cycle of the switching circuit by generating a suitable pulse width modulation signal. The pulse width modulation signal is generated based on the AC voltage reference value and the determined voltage across the capacitor of each individual DC link. Taking into account the individually determined DC link voltage when controlling the switching circuit advantageously achieves good dynamic decoupling of the DC link circuit and allows independent control of the DC link voltage. Furthermore, determining the individual DC link voltage can advantageously enable fault detection at the individual module level, thereby facilitating bypassing of the faulty module without causing the entire converter to fail. In this invention, only a single current control task is performed, recognizing that the current through each module in the module should be the same. In other words, only a single current reference and current control task is performed for all modules in the branch. The switching circuit of one or each module can be implemented by a half-bridge circuit, or more preferably by a full-bridge circuit.

[0009] For example, the voltage across the capacitor of the DC link of each module can be determined by measuring the voltage across the capacitor using a voltage sensor. The controller of the converter may include multiple devices, each performing a portion of a variety of tasks, or the tasks may be performed by one or more multitasking devices. The capacitor of the DC link may be formed by a single capacitor assembly, or alternatively may include multiple capacitors coupled in series; that is, the term “capacitor” as used herein should be understood to mean “equivalent capacitor”.

[0010] In one implementation, the reference voltage value for each module can be a predefined value stored in the memory of the controller. Using predefined reference voltage values, such as a manually defined set of values ​​in the controller, advantageously allows setting a reference voltage optimized for the specific application in which the converter is deployed. For example, the reference voltage value can be set considering the output AC voltage and the rated power of the converter components.

[0011] In one implementation, the reference power value of each module can be determined based on the difference between the square of the voltage across the capacitor of the DC link of the module and the reference voltage value of the module. Since the determined voltage value and the reference voltage value are squared before determining the reference power value of each module, the input to the control loop is linearly proportional to the energy stored in the capacitor of each DC link.

[0012] In one implementation, the controller may include a reference current control device for: receiving the reference power value of each of the plurality of modules and determining the common reference AC current value. In other words, the current control task can be performed by a single device, wherein the single device is capable of viewing all reference power values ​​in the branch. Performing all current control tasks on the same device, rather than on multiple physically distributed devices, can advantageously minimize communication latency between devices performing different tasks. Therefore, the response latency of the converter to instructions from the controller, and the response latency of the controller to changes in the operation of the converter, can be reduced.

[0013] In one implementation, the controller may include a plurality of switching circuit control devices, each of which is configured to: receive the reference AC voltage value of one of the plurality of modules and control the switching circuit of that module. The plurality of switching circuit control devices may be assigned to each module such that each device receives the reference AC voltage value of the module and calculates the duty cycle to generate a pulse width modulation signal to control the switching circuit of that module. In other words, the duty cycle calculation is performed by each control device at the module level. This can advantageously reduce the communication latency between the low duty cycle calculation device and the switching circuit of the module. Therefore, the response latency of the converter to instructions from the controller, and the response latency of the controller to changes in the operation of the converter, can be advantageously reduced.

[0014] In one implementation, the controller can be used to: control the switching circuit of each module according to an initial operating mode; determine the power requirement of each module in the module; determine whether each module in the module has a power requirement of the same sign; and if each module in the module has a power requirement of the same sign, control the switching circuit of each module according to the method. In other words, if the power requirements of the modules are different, the controller can control the switching circuit of the module according to a first operating mode. For example, in the first operating mode, the switching circuit can be controlled according to a conventional cluster operating mode, where the reference voltage of each DC link is set in the controller to the average value of the total branch voltage. This can advantageously avoid spurious operation of the switching circuit, in which one or more modules in the module have a power requirement of a different sign than other modules in the module.

[0015] In one implementation, the controller may further be configured to: compare the voltage of at least one of the modules with a predefined threshold voltage, and then, in response to the voltage of the module being higher or lower than the threshold voltage, configure the module to form an AC voltage path in the branch that bypasses the DC link of the module. The predefined threshold voltage value may be set to a voltage value expected for the module to function normally in the application. Accordingly, if the determined voltage does not match the predefined voltage value, it can be inferred that the module itself is faulty, or that the voltage determined for the module is incorrect, and the faulty module can be bypassed. This can advantageously enable the converter to continue operating even in the event of a failure of one or more component modules, thereby avoiding a complete system failure. For example, the DC link of the module can be bypassed by appropriately configuring the switching circuit so that the AC voltage path in the branch bypasses the DC link. However, this bypass mode depends on the switching circuit being sufficiently operable to employ the bypass configuration. Therefore, for example, it may be necessary to provide dedicated means for even bypassing the switching circuit of the module. The controller can actually be used to compare the voltage of a plurality of modules (preferably each module) in the module with a predefined threshold voltage and perform the bypass operation for each of the plurality of modules. Accordingly, failure of each of the modules can be tolerated, thereby improving resilience.

[0016] In one implementation, the module may include a relay operable by the controller to bypass the module's switching circuitry, wherein configuring the module to form an AC voltage path in the branch for the DC link bypassing the module includes configuring the relay to bypass the module's switching circuitry. Providing an auxiliary relay for bypassing the module's switching circuitry provides the converter with additional flexibility, as the module can be bypassed even if the switching circuitry is not sufficiently operable to employ a bypass configuration. Specifically, the relay is expected to be mechanically more robust than the switch of the switching circuitry and therefore potentially less prone to failure. Thus, in this implementation, the module can be effectively bypassed even if the switch of the switching circuitry is inoperable. Each module may preferably include a relay for bypassing the module's switching circuitry.

[0017] In one implementation, each of the switching circuits can be configured as an H-bridge. H-bridge circuits can advantageously exhibit relatively simple control characteristics. Specifically, the multiple switching devices of an H-bridge circuit can advantageously be controlled by a single common signal.

[0018] In one implementation, the cascaded multilevel converter includes: a plurality of other modules coupled together to form other branches, each of the plurality of other modules including a switching circuit and a DC link, the DC link being used to provide a DC voltage to the switching circuit; and a controller for controlling the switching circuit of each other module to generate an AC voltage in the other branches, wherein the other branches are arranged in parallel with the branch.

[0019] In other words, the multilevel converter can be configured as a multiphase multilevel converter with multiple branches for providing multiphase AC power. The converter may even include multiple other branches, such as two other branches, making the converter suitable for providing three-phase AC power. The controller can be used to control the switching circuits of the other branches in the same mode as the switching circuits of the module controlling the branches. The controller may include a common system for controlling the switching circuits of the branches and each of the other branches.

[0020] In one implementation, the branch and the one or more other branches may have the same number of modules. In a multiphase system, the power provided by each phase is typically the same. Using the same number of modules to meet the power requirements of each phase can advantageously simplify system design and control.

[0021] In one implementation, the reference power value for each module can be determined using a digital filter.

[0022] In one implementation, the controller may be configured to: compare the reference power value with a threshold power value; and if the reference power value exceeds the threshold power value, perform an anti-saturation operation. In other words, if the DC link controller reports an infeasible high power demand, it can be inferred that the operation of the DC link controller is impaired, and therefore the anti-saturation operation may involve resetting the memory of the digital filter. For example, the threshold power value may be a predefined value stored in the memory of the controller, representing the expected upper limit power demand for the particular application, and the controller may be configured to compare the reference power value of each module with the threshold.

[0023] A second aspect of the present invention provides a method for controlling a cascaded multilevel converter, the cascaded multilevel converter comprising: a plurality of modules coupled together to form a branch, each module comprising a switching circuit and a DC link, the DC link being used to provide a DC voltage to the switching circuit; the method comprising: determining for each module a voltage across a capacitor of the DC link of the module; determining for each module a reference power value for charging the capacitor of the DC link of the module to a reference voltage value of the module; determining a common reference AC current value for AC current in the branch based on the plurality of reference power values ​​of the plurality of modules; determining a common reference AC voltage value for AC voltage in the branch based on the common reference AC current value; determining a reference AC voltage value for each module using the following two values: the common reference AC voltage value and the reference power value of the module; generating a pulse width modulation signal for controlling the switching circuit of each module based on the following two values: the reference AC voltage value of the module and the voltage across the capacitor of the module.

[0024] A third aspect of the invention provides a controller for controlling a cascaded multilevel converter to generate an AC voltage in an output branch, the controller being configured to: determine the voltage across a capacitor in the DC link of each module of the converter; determine a reference power value for each module to charge the capacitor in the DC link of the module to a reference voltage value of the module; determine a common reference AC current value for the AC current in the branch based on the plurality of reference power values ​​of the plurality of modules; determine a common reference AC voltage value for the AC voltage in the branch based on the common reference AC current value; determine a reference AC voltage value for each module using the common reference AC voltage value and the reference power value of the module; and generate a pulse width modulation signal for controlling a switching circuit of each module based on the reference AC voltage value of the module and the voltage across the capacitor of the module.

[0025] These and other aspects of the invention will become apparent from the embodiments described below. Attached Figure Description

[0026] To facilitate a clearer understanding of the invention, embodiments of the invention will now be described by way of example in conjunction with the accompanying drawings, wherein:

[0027] Figure 1 An example of a solid-state transformer including a cascaded H-bridge (CHB) multilevel converter embodying one aspect of the present invention is illustrated schematically.

[0028] Figure 2 schematically shown Figure 1 The inverter module of the CHB multilevel converter shown includes a DC link for providing DC voltage to the switching circuit of the module.

[0029] Figure 3 schematically shown Figure 2 The equivalent circuit representation of the inverter module shown is illustrated.

[0030] Figure 4 A block diagram schematically illustrates a control scheme for controlling the operation of a multilevel converter;

[0031] Figure 5 A block diagram schematically illustrates a control scheme for regulating the voltage of the DC link of each inverter module.

[0032] Figure 6 It shows the way Figure 4 The control scheme shown is illustrated as a graphical representation of the simulated DC link voltage characteristics.

[0033] Figure 7 It shows the passage under fault conditions Figure 4 The control scheme shown is illustrated as a graphical representation of the simulated DC link voltage characteristics.

[0034] Figure 8 A block diagram schematically illustrates an alternative control scheme for regulating the voltage of the DC link of each inverter module. Detailed Implementation

[0035] The figure schematically illustrates a cascaded multilevel converter embodying one aspect of the invention.

[0036] First refer to Figure 1 The output of the solid-state transformer 101 is coupled to a three-phase power grid 102, which includes grid conductors 103, 104, and 105 for conducting phase currents "a", "b", and "c", respectively. The solid-state transformer 101 includes a multilevel converter 107 for converting multiple DC voltage sources (108, 109, and 110) provided by the discrete output of the initial rectifier stage, etc., into AC voltages for supplying to the power grid 102.

[0037] The multilevel converter 107 includes Q branches, three of which in this example are branches 111, 112, and 113, for supplying phase "a", "b", and "c" AC voltages to the grid conductors (103, 104, 105), respectively. Therefore, in this example, the multilevel converter 107 is configured as a three-phase converter for outputting three-phase current. Each branch (111, 112, 113) of converter 107 is substantially identical in structure. Therefore, for the sake of brevity, only branch 111, which supplies phase "a" current to grid conductor 103, will be described in detail herein; however, it should be understood that substantially the same description applies to branches 112 and 113, which are responsible for supplying phase "b" and "c" currents to grid conductors 104 and 105, respectively.

[0038] Branch 111 of the multilevel converter 107 includes multiple (M) inverter modules, such as module 1 (denoted by 114), module 2 (denoted by 115), and module M (denoted by 116). The outputs of these modules are coupled in series to a common conductor 117, such that the AC output voltages of each of the inverter modules 114 to 116 are summed across conductor 117. A controller 118 is provided for controlling the operation of each of the inverter modules 114 to 116 in branch 111. The multiple inverter modules 114 to 116 are substantially similar in structure, except for almost unavoidable differences due to imperfect tolerances of different components, and operate in substantially the same manner. Again, for the sake of brevity, only inverter module 114 is described in detail herein, but it should be understood that substantially the same description applies to each of the other M modules in the branch. In this example, the multilevel converter has a three-phase cascaded H-bridge configuration.

[0039] refer to Figure 2 and Figure 3 Each inverter module (e.g., inverter module 114) includes: a switching circuit, typically as shown in 201; a DC link circuit, typically as shown in 202, for providing DC voltage from DC source 108 to the switching circuit; and a bypass switching circuit, typically as shown in 204, for bypassing the switching circuit 201 of the module.

[0040] The switching circuit 201 includes multiple power electronic switches in an H-bridge configuration, specifically four power electronic switches 205, 206, 207, and 208 in this example. Output terminals (209, 210) are located on opposite branches of the H-bridge switching circuit. Those skilled in the art will understand that by appropriately controlling the switching sequence of switches 205 to 208, each module can provide three different voltage levels—+Vdc, -Vdc, and 0Vdc—to the common conductor 117. Therefore, by appropriately controlling the relative switching sequence of all modules in the branch, an AC voltage can be generated on conductor 117, the magnitude of which is the sum of the voltage outputs of each of the M inverter modules. In this example, each of the power electronic switches 205 to 208 is an insulated-gate bipolar transistor (IGBT). IGBTs have relatively high efficiency, fast switching capability, and high rated voltage, and are therefore a preferred type of power electronic switch in this application. However, those skilled in the art will understand that the utility of the present invention is not limited to any particular type of switch, and in alternative embodiments, alternative power electronic switches can be easily deployed to replace IGBTs 205 to 208.

[0041] DC link circuit 202 includes conductors (211, 212) arranged in series with DC source 108, and DC link capacitor 213 coupled in parallel with DC source 108 between conductors (211, 212). Therefore, capacitor 213 acts as a buffer to store energy for supplying to switching circuit 201 as needed. DC link circuit 202 also includes voltage sensor circuit 214, operable to measure the voltage across capacitor 213 and provide the voltage measurement to controller 118.

[0042] A bypass switch circuit 204 is coupled between the output terminals (209, 210) of switch circuit 201. The bypass switch circuit 204 includes a relay 215, operable under the control of controller 118, to controllably connect and disconnect the bypass switch circuit 204, thereby controllably bypassing the switch circuit 201 of module 114. Therefore, the bypass switch circuit 204 is operable to bypass switch circuit 201 and form an alternative path for current flow in conductor 117. In the event of a fault in switch circuit 201, for example, a failure of one or more of switches 205 to 208, it may be necessary to bypass switch circuit 201 to isolate the faulty switch circuit 201 from conductor 117.

[0043] The controller 118 is coupled to the power electronic switches 205 to 208 of the switching modules 201 of each of the M inverter modules 114 to 116 in each of the Q branches 111 to 113 of the multilevel converter 107. Therefore, the controller 118 is operable to control the switching circuit 201 of each module 114, thereby controlling the output AC voltage of each branch 111. In this example, the various functions of the controller 118 are performed by a common or local computing device; however, it should be understood that these functions can alternatively be performed by mutually remote computing devices communicating with each other.

[0044] Common Reference Figure 4 and Figure 5 ,use Figure 1 The naming convention shown schematically represents the proposed control scheme implemented by the controller.

[0045] Controller 118 executes four main control processes. In summary, in block 401, a reference power value is calculated for each module in each branch, the reference power value defining the amount of power required for each DC link to charge its capacitor to a predefined reference voltage; in block 402, a current reference value is generated for each of the Q branches of the converter, the current reference value defining the average current required for each module; in block 403, an individual AC voltage reference value is determined for each module; and in block 404, a pulse-width modulation signal is generated to control the switching circuitry of each module based on the individual AC voltage reference value for each module and the determined voltage across the capacitor of the module.

[0046] Block 401 of controller 118 represents a plurality of (Q x M) DC link energy controllers (i.e., the number of branches Q of the converter multiplied by the number of modules M in each branch) for controlling the energy stored in the capacitors 213 of the DC link of each of the M inverter modules in each of the Q branches of the multilevel converter 107. The function of each of the Q x M DC link energy controllers is as follows: Figure 5 As shown. Reference Figure 5 Each DC link energy controller 401 input includes: Q x M DC link voltage measurements (V... dc,q,m (Matrix), the voltage measurement value can be measured using the voltage sensor circuit 214 of each module 114, etc.; QxM reference values ​​of DC link voltages (v ref q,m Matrix); Grid voltage (e q Vector); AC current (i q (Vector). In this example, the reference value (v) of QxM DC link voltages. ref q,mThe matrix represents the input to the DC link energy controller 401. For example, the voltage reference value can be received from a dedicated memory of a controller 118 external to the DC link energy controller 401. However, the reference value can also be predefined in the internal memory of the DC link energy controller 401.

[0047] Special Reference Figure 5 Before closed-loop operation, the measured voltage signal (v) dc,q,m (matrix) and reference voltage signal (v) ref q,m The square of the matrix is ​​calculated. Through this operation, the input to the closed-loop operation is a variable linearly proportional to the energy stored in the DC link capacitor, which is expressed by equation E. dc =C / 2v dc 2 Therefore, C is the capacitance value.

[0048] It should be understood in this regard that, for example, due to the tolerances of the capacitor assembly, the actual value of C may not be known accurately. However, in the proposed closed-loop operation mode, this error is not a problem because the C / 2 term is included in the closed-loop operation K. qm (kT s In the loop gain of ), and the squared value w = v dc (t) 2 Or mathematically equivalent to v dc (t) 2 The variable is a controlled variable. Therefore, this feedback linearization technique calculates on demand how each DC link charges its capacitor to a reference voltage (V). ref q,m The power required for the matrix.

[0049] The output of each DC link controller 401 is a reference power value. This reference power value is used by block 402 for reference current generation and current control, and by block 403 for individual reference AC voltage calculation.

[0050] Next, referring to block 402, in the first step, the current reference value for each branch is constructed, that is, the individual power reference values ​​of all the working modules in the branch are added together to obtain the current reference value through P. ref q The total branch power demand is obtained. Then, the active current amplitude is calculated using the following equation: |i ref q |=G q P ref q / |e F |, where G q It is the gain factor, |e F| represents the amplitude of the AC mains voltage. In practice, G is chosen. q =2, because in steady state, the power of each branch is P. q (dc)=|i q ||e F | / 2.

[0051] Next, the current amplitude is multiplied by a sinusoidal signal in phase with the fundamental component of the voltage in the same branch, as defined in phase-locked loop (PLL) block 405. The angle of this signal can be defined in many equivalent forms, for example, (i) Q individual single-phase synchronization algorithms can be synchronized independently, or (ii) a synchronization technique using a dq-PLL that tracks the positive principal component can be used. The result of this operation is the AC reference current i. ref q The vector, which is fed into the current control loop. Various suitable alternative PLL synchronization methods for performing the same function are well known to those skilled in the art. The amplitude of the AC grid voltage |e F | It can also be estimated using PLL.

[0052] Therefore, the current controller block 402 serves as a closed-loop controller designed to follow its current reference values ​​(reference values ​​Q = 1, 2, 3, ... in single-phase / multi-phase systems). The control action variable defined for each branch is the total AC branch voltage v. q Reference value. The current control loop may optionally use information that defines the grid voltage as a feedforward.

[0053] v is calculated using a closed-loop control algorithm. q After obtaining the reference value, divide this value by p. ref q This generates a variable (i) that defines the reverse average current of each module. ref q,m ) -1 It should be understood that (i ref q,m ) -1 The value is proportional to the overall branch voltage, but completely independent of how that voltage is shared between modules. Therefore, this advantageously provides a natural dynamic decoupling between modules in the branch.

[0054] Next, referencing block 403, the reference AC voltage value is calculated for PWM block 404 using the power required by each module.

[0055] Individual voltage reference values ​​are obtained based on the reverse average current variable and the individual power requirement of each module, wherein the reverse average current variable is common to all modules in the same branch. The voltage reference value for each module is then derived using the following equation: v refq,m =(i ref q,m ) -1 p ref q,m Then, use v at the module level. ref q,m Set the module voltage as the PWM input. Represent it in matrix form as v ref dc,q,m The DC link voltage is typically used to calculate the module's duty cycle; that is, the AC voltage reference value is divided by the DC link voltage. This yields a normalized duty cycle, thus avoiding any cross-coupling in the circuit. In other words, in this way, each module uses only its own DC link voltage value.

[0056] This calculation takes into account that the power required by each DC link load is proportional to the AC voltage, and the current is common to all modules in the branch. Therefore, the only degree of freedom to provide the required power to the DC link is to apply the AC voltage reference value at the module level.

[0057] Finally, referring to box 406, the proposed converter operation can be called "active shared operation" because the module voltage is a function of real-time power consumption. However, it should be understood that this operating mode is only feasible if all modules in the same branch have power requirements of the same sign (i.e., all modules in the branch either require positive input power or all require negative input power).

[0058] This defines enabling. Figure 4 The control scheme shown here is subject to necessary conditions. In practice, these conditions are met during normal system operation because, under ideal conditions, the sizes of all modules are set to evenly distribute the total power available on the DC power supply side. However, in practice, system operation may deviate from ideal operation, therefore the system should be able to accommodate such deviations.

[0059] Therefore, in block 406, the power requirements of each module are compared to determine if each module has a power requirement of the same sign. If the determination is yes, i.e., if all modules have a power requirement of the same sign, then block 404 uses the voltage reference value v for each module. ref q,m Generate a pulse width modulation signal. Conversely, if the power requirements of the modules do not have the same sign—that is, if one module requires positive power and another requires negative power—then the voltage reference value v for each module is... ref q,m It is replaced by the value representing the average value of the total branch voltage, i.e., through the function v. ref q The / M setting, where M is the number of modules in the branch, avoids spurious operations of the converter.

[0060] Figure 6 It shows Figure 1 The simulation results of the multilevel converter are shown, where Q = 3 and M = 12.

[0061] In this simulation, the reference operating value for each DC link capacitor is set to 1000 volts, and the capacitor values ​​are randomly assigned within a 20% tolerance of the nominal capacitance value. When a load is connected, at T = 1 second, the load consumption is randomly assigned within a 20% tolerance.

[0062] During the initial operating cycle, from T=0 seconds to T=1 seconds, the initial sequence of pre-charging, first activation of the PWM controller, and disabling active-shared operation under light load was simulated. In subsequent operating cycles, starting from T=1 seconds, the active-shared operation mode was activated under full load conditions. As observed, zero average error between DC link voltages was achieved with active-shared operation enabled. Therefore, this simulation demonstrates the converter's rapid and robust adaptation when active-shared operation mode is enabled.

[0063] Figure 7 It shows Figure 1 The simulation results of the multilevel converter shown are used with Figure 6 The simulation shown uses the same parameters.

[0064] In this second simulation, after the initial cycle of light load operation, at T = 0.5 seconds, the load is connected and the active shared operation mode is enabled, wherein, during the light load operation, the DC link voltage controller reference value is set to v. ref q / M, where the reference value is the average value of the total branch voltage. As observed, upon initiating the active shared operation mode, the DC link voltage error rapidly approaches zero, reaching zero at approximately T = 0.6 seconds. At approximately T = 0.6 seconds, a simulated fault condition is introduced, under which the voltage sensor of module M = 1 reports a 0-volt measurement, leading to a fault operation of the associated DC link controller. The consequence of this fault condition is that the DC link controller demands power consumption above a threshold to charge the DC link capacitor of module M = 1.

[0065] Controller 118 is operable to perform a bypass operation in response to the fault condition, in which the switching circuit 201 of module 114 is bypassed. In this example, the bypass operation is achieved by closing relay 215 to bypass the switching circuit, thereby creating a current short circuit in the branch surrounding the switching circuit of the faulty module. It can be observed that the DC link capacitor of the faulty module subsequently discharges, and the operation of the remaining M-1 modules is unaffected. Therefore, it can be seen from the system's response to a fault in module M=1 that the operation of each module is highly separated from the operation of other modules in the branch.

[0066] Figure 8 This illustrates an implementation of a DC link energy controller that considers multiple tasks, in which two different control samples T are taken into account. s1 and T s2 T s1 <T s2 .

[0067] It should be understood that the nature of the digital controller implementation of multiple (QxM) DC link controllers 401 allows for the sharing of different control tasks among different physical devices. However, this task sharing can lead to communication delays between different devices, and consequently, delays in the control system. Therefore, in practice, it may be necessary for a device with information on all power reference values ​​in the same branch to perform the current control task. However, duty cycle calculation can be performed at the module level. Performing duty cycle calculation at the module level may be relatively faster due to the avoidance of communication between different devices.

[0068] While the invention and its advantages have been described in detail, it should be understood that various changes, substitutions, and modifications may be made without departing from the spirit and scope of the invention as defined in the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and "a" or "an" does not exclude a plurality.

Claims

1. A cascaded multilevel converter, characterized in that, include: Multiple modules, coupled together to form a branch, each module including a switching circuit and a DC link, the DC link being used to provide DC voltage to the switching circuit; a controller for controlling the switching circuit of each module to generate AC voltage in the branch; wherein the controller is used to: Determine the voltage across the capacitor of the DC link of each module; A reference power value is determined for each module to charge the capacitor of the DC link of the module to a reference voltage value of the module. The reference power value of each module is determined based on the difference between the square of the voltage across the capacitor of the DC link of the module and the reference voltage value of the module. The common reference AC current value of the AC current in the branch is determined based on the multiple reference power values ​​of the multiple modules; The common reference AC voltage value in the branch is determined based on the common reference AC current value; The following two values ​​are used to determine the reference AC voltage value for each module: the common reference AC voltage value and the reference power value of the module; A pulse width modulation signal for controlling the switching circuit of each module is generated based on the following two values: the reference AC voltage value of the module and the voltage across the capacitor of the module.

2. The cascaded multilevel converter according to claim 1, characterized in that, The reference voltage value for each module is a predefined value stored in the memory of the controller.

3. The cascaded multilevel converter according to any one of the preceding claims, characterized in that, The controller includes a reference current control device for: receiving the reference power value of each of the plurality of modules and determining the common reference AC current value.

4. The cascaded multilevel converter according to claim 1, characterized in that, The controller includes multiple switching circuit control devices, each of which is used to: receive the reference AC voltage value of one of the multiple modules and control the switching circuit of that module.

5. The cascaded multilevel converter according to claim 1, characterized in that, The controller is used for: The switching circuit of each module is controlled according to the initial operating mode; Determine the power requirements of each module in the module; Determine whether each module in the module has a power requirement of the same sign; If each module in the module has the same power requirement, then a pulse width modulation signal is generated using the voltage reference value of each module to control the switching circuit of each module.

6. The cascaded multilevel converter according to claim 1, characterized in that, The controller is further configured to: compare the voltage of at least one of the modules with a predefined threshold voltage, and then, in response to the voltage of the module being higher or lower than the threshold voltage, configure the module to form an AC voltage path in the branch that bypasses the DC link of the module.

7. The cascaded multilevel converter according to claim 6, characterized in that, The module further includes a relay operable by the controller to bypass the switching circuit of the module, wherein configuring the module to form an AC voltage path in the branch that bypasses the DC link of the module includes configuring the relay to bypass the switching circuit of the module.

8. The cascaded multilevel converter according to claim 1, characterized in that, Each of the switching circuits is configured as an H-bridge.

9. The cascaded multilevel converter according to claim 1, characterized in that, include: Multiple other modules, coupled together to form other branches, each of the multiple other modules including a switching circuit and a DC link for providing DC voltage to the switching circuit; a controller for controlling the switching circuit of each other module to generate AC voltage in the other branches, wherein the other branches are arranged in parallel with the branch.

10. The cascaded multilevel converter according to claim 9, characterized in that, The branch and the one or more other branches have the same number of modules.

11. The cascaded multilevel converter according to claim 1, characterized in that, The reference power value for each module is determined using a digital filter.

12. The cascaded multilevel converter according to claim 11, characterized in that, The controller is configured to: compare the reference power value with the threshold power value; and if the reference power value exceeds the threshold power value, perform an anti-saturation operation.

13. A method for controlling a cascaded multilevel converter, characterized in that, The cascaded multilevel converter includes: multiple modules coupled together to form a branch, each module including a switching circuit and a DC link, the DC link being used to provide DC voltage to the switching circuit; the method includes: Determine the voltage across the capacitor of the DC link of each module; A reference power value is determined for each module to charge the capacitor of the DC link of the module to a reference voltage value of the module. The reference power value of each module is determined based on the difference between the square of the voltage across the capacitor of the DC link of the module and the reference voltage value of the module. The common reference AC current value of the AC current in the branch is determined based on the multiple reference power values ​​of the multiple modules; The common reference AC voltage value in the branch is determined based on the common reference AC current value; The following two values ​​are used to determine the reference AC voltage value for each module: the common reference AC voltage value and the reference power value of the module; A pulse width modulation signal for controlling the switching circuit of each module is generated based on the following two values: the reference AC voltage value of the module and the voltage across the capacitor of the module.

14. A controller for controlling a cascaded multilevel converter to generate AC voltage in the output branch, characterized in that, The controller is used for: For each module of the converter, determine the voltage across the capacitor of the DC link of each module; A reference power value is determined for each module to charge the capacitor of the DC link of the module to a reference voltage value of the module. The reference power value of each module is determined based on the difference between the square of the voltage across the capacitor of the DC link of the module and the reference voltage value of the module. The common reference AC current value of the AC current in the branch is determined based on the multiple reference power values ​​of the multiple modules; The common reference AC voltage value in the branch is determined based on the common reference AC current value; The following two values ​​are used to determine the reference AC voltage value for each module: the common reference AC voltage value and the reference power value of the module; A pulse width modulation signal for controlling the switching circuit of each module is generated based on the following two values: the reference AC voltage value of the module and the voltage across the capacitor of the module.

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