A midpoint clamped multi-level converter and modulation method, system and apparatus thereof

Abstract

The application discloses a midpoint clamping type multi-level converter and a modulation method, system and device thereof, and the mathematical relation for calculating the duty cycle of each switch of the multi-level converter is constructed based on the grid-side reference voltage of the multi-level converter, a midpoint voltage compensation term for controlling NPV balance and a duty cycle adjustment term for controlling the number of switching actions; when the multi-level converter enters work, the duty cycle corresponding to each switch of the multi-level converter is calculated according to the mathematical relation; and the on-off of each switch is controlled according to the duty cycle corresponding to each switch of the multi-level converter. It can be seen that the mathematical relation for calculating the duty cycle of the switch directly contains the midpoint voltage compensation term and the duty cycle adjustment term, the NPV balance can be ensured through the midpoint voltage compensation term, the modulation strategy has parameter robustness, and the NPV balance can be ensured when the capacitor parameters are not matched; and the number of switching actions can be controlled through the duty cycle adjustment term, so as to reduce the circuit loss.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of multi-level converter, in particular to a neutral point clamped multi-level converter and a modulation method, system and device thereof. BACKGROUND

[0002] Multi-level converter is widely concerned due to its ability to reduce switching stress and output more levels to improve the harmonic distortion rate of output voltage. Among them, NPC (Neutral point clamped) three-level converters such as T-type converter and diode clamped converter are widely used in wind and solar conversion, AC drive and other fields.

[0003] The key problem of NPC three-level converter operation is NPV (Neutral point voltage) balance, but under certain specific working conditions (such as non-linear, unbalanced load, high modulation index and low power factor), low-frequency NPV oscillation may exist. At present, the NPV balance control method is mainly divided into two categories: hardware method and software method; among them, the hardware method needs to modify the circuit topology to achieve; the software method needs to develop appropriate modulation strategy to achieve. The software method has more advantages because it does not increase the hardware cost, weight and size of the system.

[0004] In the prior art, the most commonly used modulation strategy of NPC three-level converter is SVM (Space Vector Modulation) strategy. For SVM strategy, the most commonly used NPV balance control method is adjacent three-vector method and virtual vector method; among them, the adjacent three-vector method is invalid under specific working conditions of high modulation index or low power factor, and the effect of eliminating low-frequency NPV oscillation is poor; although the virtual vector method is effective under different specific working conditions, the virtual vector method can only be applied to the scene of matching the parameters of two separate capacitors on the DC side of the converter (i.e. the capacitance values of the two separate capacitors are equal), once the parameters of the two separate capacitors are not matched, it will lead to NPV imbalance, so an additional control loop is needed to adjust the parameters of the two separate capacitors, which increases the control difficulty.

[0005] Therefore, how to provide a solution to the above technical problems is a problem that those skilled in the art need to solve at present. SUMMARY

[0006] The application aims to provide a midpoint clamping type multi-level converter and a modulation method, system and device thereof, wherein a midpoint voltage compensation term and a duty cycle adjustment term are directly included in a mathematical relationship for calculating the duty cycle of a switch, the midpoint voltage compensation term can ensure NPV balance, the modulation strategy has parameter robustness and can ensure NPV balance when the capacitor parameters are not matched, and the duty cycle adjustment term can control the number of switch actions, such as making at least one switch in each switch cycle not act, so as to reduce the number of switch actions and reduce circuit loss.

[0007] To solve the above technical problems, the application provides a modulation method of a midpoint clamping type multi-level converter, comprising:

[0008] Based on a grid-side reference voltage of a multi-level converter, a midpoint voltage compensation term for controlling NPV balance of the multi-level converter and a duty cycle adjustment term for controlling the number of switch actions of the multi-level converter, a mathematical relationship for calculating the duty cycle of each switch of the multi-level converter is constructed;

[0009] When the multi-level converter is in operation, the duty cycle corresponding to each switch of the multi-level converter is calculated according to the mathematical relationship;

[0010] According to the duty cycle corresponding to each switch of the multi-level converter, the on-off of each switch of the multi-level converter is controlled correspondingly.

[0011] Preferably, the multi-level converter is a three-level converter.

[0012] The process of constructing the mathematical relationship for calculating the duty cycle of each switch of the multi-level converter comprises:

[0013] The mathematical relationship for calculating the duty cycle of each switch of the three-level converter

[0014]

[0015] wherein, u dc u1+u2; u1, u2 are the voltages of two discrete capacitors on the DC side of the three-level converter; d ij (i=a, b, c; j=P, N) represents the duty cycle of the upper and lower switches in the a, b, c three-phase bridge arms of the three-level converter; u xn (x∈a, b, c) is the grid-side reference voltage; γ x (x∈a, b, c) is the midpoint voltage compensation term; m is the duty cycle adjustment term.

[0016] Preferably, the process of constructing the mathematical relationship for calculating the duty cycle of each switch of the three-level converter comprises:

[0017] The first relationship is obtained by averaging the grid-side reference voltage under switching states.

[0018] The sum of the duty cycles of the upper and lower arms of each phase in the three-phase bridge arm is used to construct the sum of the duty cycle adjustment term and the midpoint voltage compensation term, thus obtaining the second relational expression.

[0019] Based on the first and second relationships, mathematical relationships are obtained for calculating the duty cycles of each switch in the three-level converter.

[0020] Preferably, the process of constructing a midpoint voltage compensation term for controlling the NPV balance of the multilevel converter includes:

[0021] Based on the three-phase input phase currents on the grid side of the three-level converter and the voltages of the two discrete capacitors on the DC side, a midpoint voltage compensation term is constructed.

[0022] in, i i (i = a, b, c) represents the three-phase input phase current; |i i |(i=a,b,c) is the absolute value of the three-phase input phase current; sgn() is the sign function; ε is the proportional gain for neutral point voltage balance, ε>0; α is the finite-time convergence gain for the neutral point voltage, 0<α<1.

[0023] Preferably, the modulation method of the midpoint clamped multilevel converter further includes:

[0024] The first dynamic relationship for obtaining the voltage difference between the two discrete capacitors on the DC side is obtained. Wherein, C is the capacitance value of the two discrete capacitors on the DC side;

[0025] Substituting the midpoint voltage compensation term into the first dynamic equation, we obtain the second dynamic equation.

[0026] Based on (0 < α < 1, ε > 0), determine the convergence time of the second dynamic relation. in, The larger ε is, the longer the convergence time t is. z The smaller.

[0027] Preferably, the process of constructing a duty cycle adjustment term for controlling the number of switching operations of the multilevel converter includes:

[0028] Based on the physical constraints of the duty cycle of each switch in the three-level converter and the minimum number of switching operations required, a duty cycle adjustment term is constructed.

[0029] Preferably, the process of obtaining the grid-side reference voltage includes:

[0030] The grid-side voltage and current of the three-level converter are transformed by coordinates to obtain the voltage u in the d-axis and q-axis directions in a two-phase rotating coordinate system. d u q and current i d i q ;

[0031] according to Obtain the voltage reference value u d_ref u q_ref Among them, K P K is the proportional control coefficient. I i is the integral control coefficient; d_ref i q_ref ω is the reference current in the d-axis and q-axis directions; ω is the angular frequency; L is the inductance of the input filter inductor of the three-level converter;

[0032] The voltage reference value is transformed by coordinates to obtain the grid-side reference voltage u in a three-phase rotating coordinate system. xn (x∈a,b,c).

[0033] To address the aforementioned technical problems, the present invention also provides a modulation system for a midpoint clamped multilevel converter, comprising:

[0034] The construction module is used to construct a mathematical relationship for calculating the duty cycle of each switch of the multilevel converter based on the grid-side reference voltage of the multilevel converter, the midpoint voltage compensation term for controlling the NPV balance of the multilevel converter, and the duty cycle adjustment term for controlling the number of switching actions of the multilevel converter.

[0035] The calculation module is used to calculate the duty cycle of each switch of the multilevel converter according to the mathematical formula when the multilevel converter starts to work.

[0036] The control module is used to control the on / off state of each switch of the multilevel converter according to the duty cycle corresponding to each switch of the multilevel converter.

[0037] Preferably, the multilevel converter is a three-level converter;

[0038] The mathematical relation constructed by the construction module is:

[0039]

[0040] Among them, u dc= u1 + u2; u1 and u2 are the voltages of the two discrete capacitors on the DC side of the three-level converter; d ij (i = a, b, c; j = P, N) represents the duty cycle of the upper and lower switches in the a, b, and c phase bridge arms of the three-level converter; u xn (x∈a,b,c) is the grid-side reference voltage; γ x (x∈a,b,c) is the midpoint voltage compensation term; m is the duty cycle adjustment term.

[0041] Preferably, the midpoint voltage compensation term constructed by the construction module is:

[0042]

[0043] in, i i (i = a, b, c) represents the three-phase input phase current on the grid side of the multilevel converter; |i i |(i=a,b,c) represents the absolute value of the three-phase input phase current; sgn() is the sign function; ε is the proportional gain for neutral point voltage balance; and α is the finite-time convergence gain for the neutral point voltage.

[0044] Preferably, the duty cycle adjustment term constructed by the construction module is:

[0045]

[0046] Preferably, the process of obtaining the grid-side reference voltage includes:

[0047] The grid-side voltage and current of the three-level converter are transformed by coordinates to obtain the voltage u in the d-axis and q-axis directions in a two-phase rotating coordinate system. d u q and current i d i q ;

[0048] according to Obtain the voltage reference value u d_ref u q_ref Among them, K P K is the proportional control coefficient. I i is the integral control coefficient; d_ref i q_ref ω is the reference current in the d-axis and q-axis directions; ω is the angular frequency; L is the inductance of the input filter inductor of the three-level converter;

[0049] The voltage reference value is transformed by coordinates to obtain the grid-side reference voltage u in a three-phase rotating coordinate system. xn (x∈a,b,c).

[0050] To address the aforementioned technical problems, the present invention also provides a modulation device for a midpoint clamped multilevel converter, comprising:

[0051] The drive circuit connected to the control terminals of each switch of the multilevel converter;

[0052] The processor connected to the drive circuit is used to implement the steps of the modulation method of any of the above-described midpoint clamped multilevel converters when executing a computer program stored in itself; wherein the processor controls the on / off state of each switch of the multilevel converter through the drive circuit.

[0053] Preferably, the processor is a DSP processor.

[0054] To address the aforementioned technical problems, the present invention also provides a midpoint clamped multilevel converter, including the modulation device of any of the above-mentioned midpoint clamped multilevel converters.

[0055] This invention provides a modulation method for a midpoint-clamped multilevel converter. Based on the grid-side reference voltage of the multilevel converter, a midpoint voltage compensation term for controlling the NPV balance of the multilevel converter, and a duty cycle adjustment term for controlling the number of switching operations of the multilevel converter, a mathematical formula is constructed to calculate the duty cycle of each switch in the multilevel converter. When the multilevel converter starts working, the duty cycle corresponding to each switch is calculated according to the mathematical formula. Based on the duty cycle of each switch, the on / off state of each switch in the multilevel converter is controlled accordingly. It can be seen that the mathematical formula for calculating the switch duty cycle directly includes the midpoint voltage compensation term and the duty cycle adjustment term. The midpoint voltage compensation term ensures NPV balance, and this modulation strategy has parameter robustness, maintaining NPV balance even when capacitor parameters are mismatched. The duty cycle adjustment term controls the number of switching operations, such as ensuring that at least one switch does not operate in each switching cycle, thereby reducing the number of switching operations and lowering circuit losses.

[0056] The present invention also provides a midpoint clamping type multilevel converter and its modulation system and apparatus, which have the same beneficial effects as the above-mentioned modulation method. Attached Figure Description

[0057] To more clearly illustrate the technical solutions in the embodiments of the present invention, the drawings used in the prior art and embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0058] Figure 1A flowchart illustrating a modulation method for a midpoint clamped multilevel converter provided in an embodiment of the present invention;

[0059] Figure 2 A topology diagram of a T-type three-level converter provided in an embodiment of the present invention;

[0060] Figure 3 A structural block diagram of a control system for a T-type three-level converter provided in an embodiment of the present invention;

[0061] Figure 4 A control algorithm block diagram of a control system provided in an embodiment of the present invention;

[0062] Figure 5 A flowchart of a control algorithm provided in an embodiment of the present invention;

[0063] Figure 6 This is a schematic diagram illustrating the convergence of the midpoint potential difference of a T-type three-level converter within a finite time, as provided in an embodiment of the present invention.

[0064] Figure 7(a) shows the simulated voltage waveforms of the two discrete capacitors on the DC side during the switching from rectifier mode to inverter mode operation of a T-type three-level converter provided in this embodiment of the invention.

[0065] Figure 7(b) shows the simulated waveforms of grid-side voltage and current during the switching process from rectifier mode to inverter mode of a T-type three-level converter provided in this embodiment of the invention. Detailed Implementation

[0066] The core of this invention is to provide a midpoint clamped multilevel converter and its modulation method, system, and device. The mathematical formula for calculating the switch duty cycle directly includes a midpoint voltage compensation term and a duty cycle adjustment term. The midpoint voltage compensation term can ensure NPV balance. This modulation strategy has parameter robustness and can still ensure NPV balance when capacitor parameters are mismatched. The duty cycle adjustment term can control the number of switching operations, such as ensuring that at least one switch does not operate in each switching cycle, thereby reducing the number of switching operations and reducing circuit losses.

[0067] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0068] Please refer to Figure 1 , Figure 1This is a flowchart illustrating a modulation method for a midpoint clamped multilevel converter, as provided in an embodiment of the present invention.

[0069] The modulation method of this midpoint clamped multilevel converter includes:

[0070] Step S1: Based on the grid-side reference voltage of the multilevel converter, the midpoint voltage compensation term for controlling the NPV balance of the multilevel converter, and the duty cycle adjustment term for controlling the number of switching operations of the multilevel converter, construct the mathematical relationship for calculating the duty cycle of each switch of the multilevel converter.

[0071] Specifically, this application pre-constructs mathematical formulas for calculating the duty cycles of each switch in a multilevel converter. These formulas include the grid-side reference voltage of the multilevel converter, a midpoint voltage compensation term for controlling the NPV balance of the multilevel converter, and a duty cycle adjustment term for controlling the number of switching operations of the multilevel converter. Thus, this application ensures the NPV balance of the multilevel converter through the midpoint voltage compensation term and controls the number of switching operations of the multilevel converter through the duty cycle adjustment term; both of these effects are achieved by adjusting the duty cycle of each switch in the multilevel converter.

[0072] Step S2: When the multilevel converter starts working, calculate the duty cycle of each switch of the multilevel converter according to the mathematical relationship.

[0073] Specifically, when the multilevel converter starts working, this application calculates the duty cycle of each switch of the multilevel converter according to the mathematical relationship constructed in advance for calculating the duty cycle of each switch of the multilevel converter, so as to provide a basis for controlling each switch of the multilevel converter.

[0074] Step S3: Control the on / off state of each switch of the multilevel converter according to the duty cycle of each switch.

[0075] Specifically, after calculating the duty cycle of each switch of the multilevel converter, this application controls the on / off state of each switch of the multilevel converter according to the duty cycle of each switch of the multilevel converter, so that the multilevel converter operates in a state of NPV balance and low circuit loss.

[0076] This invention provides a modulation method for a midpoint-clamped multilevel converter. Based on the grid-side reference voltage of the multilevel converter, a midpoint voltage compensation term for controlling the NPV balance of the multilevel converter, and a duty cycle adjustment term for controlling the number of switching operations of the multilevel converter, a mathematical formula is constructed to calculate the duty cycle of each switch in the multilevel converter. When the multilevel converter starts working, the duty cycle corresponding to each switch is calculated according to the mathematical formula. Based on the duty cycle of each switch, the on / off state of each switch in the multilevel converter is controlled accordingly. It can be seen that the mathematical formula for calculating the switch duty cycle directly includes the midpoint voltage compensation term and the duty cycle adjustment term. The midpoint voltage compensation term ensures NPV balance, and this modulation strategy has parameter robustness, maintaining NPV balance even when capacitor parameters are mismatched. The duty cycle adjustment term controls the number of switching operations, such as ensuring that at least one switch does not operate in each switching cycle, thereby reducing the number of switching operations and lowering circuit losses.

[0077] Based on the above embodiments:

[0078] As an optional embodiment, the multilevel converter is a three-level converter;

[0079] The process of constructing the mathematical relationships for calculating the duty cycles of the switches in a multilevel converter includes:

[0080] Construct mathematical relationships for calculating the duty cycles of each switch in a three-level converter.

[0081]

[0082] Among them, u dc = u1 + u2; u1 and u2 are the voltages of the two discrete capacitors on the DC side of the three-level converter; d ij (i = a, b, c; j = P, N) represents the duty cycle of the upper and lower switches in the a, b, and c phase arms of the three-level converter; u xn (x∈a,b,c) is the grid-side reference voltage; γ x (x∈a,b,c) is the midpoint voltage compensation term; m is the duty cycle adjustment term.

[0083] This embodiment will be described in detail in subsequent embodiments, and will not be elaborated further in this application.

[0084] As an optional embodiment, the process of constructing a mathematical relationship for calculating the duty cycles of the switches in a three-level converter includes:

[0085] After averaging the grid-side reference voltage under switching conditions, the first relationship is obtained.

[0086]

[0087] The sum of the duty cycles of the upper and lower arms of each phase in the three-phase bridge arm is constructed as the sum of the duty cycle adjustment term and the midpoint voltage compensation term, resulting in the second relational expression.

[0088] Based on the first and second relations, the mathematical relations for calculating the duty cycles of each switch in the three-level converter are obtained.

[0089] Specifically, based on mathematical construction principles, the mathematical relationships of the duty cycles of each switch in the three-level converter are designed (including grid-side reference voltage, midpoint voltage compensation terms, and duty cycle adjustment terms): The specific process is as follows:

[0090] 1) After averaging the grid-side reference voltage under switching conditions, the following relationship is obtained:

[0091]

[0092] Among them, u an ,u bn ,u cn d represents the AC phase voltage of the three-level converter with reference to the neutral point potential, i.e., the grid-side reference voltage; u1 and u2 are the voltages of the two discrete capacitors on the DC side of the three-level converter; ij (i = a, b, c; j = P, N) represents the duty cycle of the upper and lower switches in the a, b, and c phase arms of the three-level converter.

[0093] 2) Based on the idea of ​​mathematical construction, the sum of the duty cycles of the upper and lower arms of each phase in the three-phase bridge arm of the three-level converter is constructed as the sum of the duty cycle adjustment term and the midpoint voltage compensation term, resulting in the following relationship:

[0094]

[0095] Where, γ x (x∈a,b,c) is the midpoint voltage compensation term; m is the duty cycle adjustment term.

[0096] 3) Combining equations (1) and (2), the solution for the duty cycle of each switch in the three-level converter is as follows:

[0097]

[0098] Among them, u dc =u1+u2.

[0099] It can be seen that the solution for the duty cycle of each switch in the three-level converter includes three parts: the grid-side reference voltage u. xn (x∈a,b,c), midpoint voltage compensation term γ x (x∈a,b,c) and duty cycle adjustment term m.

[0100] As an optional embodiment, the process of constructing a midpoint voltage compensation term for controlling the NPV balance of a multilevel converter includes:

[0101] Based on the three-phase input phase currents on the grid side of the three-level converter and the voltages of the two discrete capacitors on the DC side, a neutral point voltage compensation term is constructed.

[0102] in, i i (i = a, b, c) represents the three-phase input phase current; |i i |(i=a,b,c) represents the absolute value of the three-phase input phase current; sgn() is the sign function; ε is the proportional gain for neutral point voltage balancing, ε>0; α is the finite-time convergence gain for the neutral point voltage, 0<α<1.

[0103] Specifically, the constructed midpoint voltage compensation term is as follows:

[0104]

[0105] in, i i (i = a, b, c) represents the three-phase input phase currents on the grid side of the three-level converter; |i i |(i=a,b,c) represents the absolute value of the three-phase input phase current; sgn() is the sign function, when f>0, sign(f)=1; when f=0, sign(f)=0; when f<0, sign(f)=﹣1; ε is the proportional gain for neutral point voltage balance, with a value of: ε>0; α is the finite-time convergence gain for the neutral point voltage, with a value of: 0<α<1.

[0106] As an optional embodiment, the modulation method for the midpoint clamped multilevel converter further includes:

[0107] The first dynamic equation for obtaining the voltage difference between the two discrete capacitors on the DC side. Where C represents the capacitance of the two discrete capacitors on the DC side;

[0108] Substituting the midpoint voltage compensation term into the first dynamic equation, we obtain the second dynamic equation.

[0109] Based on (0 < α < 1, ε > 0), determine the convergence time of the second dynamic relation. in, The larger ε is, the longer the convergence time t is. z The smaller.

[0110] Specifically, the dynamic relationship of the voltage difference between the two discrete capacitors on the DC side of the three-level converter is as follows:

[0111]

[0112] In this three-level converter, the two discrete capacitors on the DC side have the same capacitance value, denoted by C.

[0113] From relation (4) and relation (5), we can obtain:

[0114]

[0115] Choosing (0 < α < 1, ε > 0) allows equation (6) to converge in a finite time. That is, if there is a deviation in the voltage of the two discrete capacitors on the DC side of the three-level converter, the voltage of the two discrete capacitors can be made consistent in a finite time, and its convergence time t z for:

[0116]

[0117] in, The larger ε is, the longer the convergence time t is. z The smaller the value, the faster the voltage difference between the two discrete capacitors on the DC side converges to zero.

[0118] As an optional embodiment, the process of constructing a duty cycle adjustment term for controlling the number of switching operations of a multilevel converter includes:

[0119] Based on the physical constraints of the duty cycle of each switch in the three-level converter and the requirement for the minimum number of switching operations, a duty cycle adjustment term is constructed.

[0120] Specifically, construct the duty cycle adjustment term:

[0121] The selection range for the duty cycle adjustment term m is:

[0122]

[0123] Considering that the midpoint clamped three-level converter operates in the linear modulation region and does not experience overmodulation, we have:

[0124]

[0125] Therefore, within the linear modulation region, inequality (8) must contain a duty cycle adjustment term m such that each duty cycle element satisfies:

[0126] 1≥d aP ,d aN ,d bP ,d bN ,d cP ,d cN ≥0(10);

[0127] The duty cycle adjustment term m can then be selected as:

[0128]

[0129] It can ensure that at least one switch of the three-level converter does not operate in each switching cycle, thereby reducing the number of switching operations.

[0130] As an optional embodiment, the process of obtaining the grid-side reference voltage includes:

[0131] By performing coordinate transformation on the grid-side voltage and current of the three-level converter, the voltage u in the d-axis and q-axis directions in a two-phase rotating coordinate system is obtained. d u q and current i d i q ;

[0132] according to Obtain the voltage reference value u d_ref u q_ref Among them, K P K is the proportional control coefficient. I i is the integral control coefficient; d_ref i q_ref ω represents the reference current along the d-axis and q-axis; ω represents the angular frequency; and L represents the inductance of the input filter inductor of the three-level converter.

[0133] By performing a coordinate transformation on the voltage reference value, the grid-side reference voltage u in the three-phase rotating coordinate system is obtained. xn (x∈a,b,c).

[0134] As can be seen, in the solution of the duty cycle of each switch in the three-level converter, the grid-side reference voltage, the midpoint voltage compensation term, and the duty cycle adjustment term are all explicitly represented by the required grid-side reference voltage, the measured grid-side current, and the voltage of the DC-side discrete capacitors. The algorithm is simple, easy to understand, and easy to implement. Moreover, the modulation strategy of the three-level converter is universal and has guiding significance for the modulation strategies of other power electronic converters.

[0135] The present application will now be described in further detail with reference to the accompanying drawings and specific embodiments:

[0136] Please refer to Figure 2 , Figure 2This is a topology diagram of a T-type three-level converter provided in an embodiment of the present invention. The T-type three-level converter includes a power grid 1, an H-bridge circuit 2, a neutral point three-way bidirectional switch 3, a DC-side voltage divider capacitor 4, and a DC-side load 5. The H-bridge circuit 2 and the three-way bidirectional switch 3 are connected to the power grid 1 through a grid-side filter inductor L. The bidirectional switch is constructed by connecting the emitters of two IGBTs (Insulated Gate Bipolar Transistors) in series. The DC-side discrete capacitor 4 is directly connected to the DC-side load 5.

[0137] Please refer to Figure 3 , Figure 3 This is a structural block diagram of a control system for a T-type three-level converter provided in an embodiment of the present invention. Figure 3 In the main circuit, the components include, for example: Figure 2 The T-type three-level converter shown includes a control circuit comprising a processor 7 (which can be a DSP (Digital Signal Processing) processor), a drive circuit 8, and a corresponding sampling and conditioning circuit 6. The left end of each of the three-phase arm units of the H-bridge circuit 2 is connected to the grid-side filter inductor L, and finally connected to the 220V AC power grid 1. Part of the sampling circuit in the sampling and conditioning circuit 6 is responsible for sampling and conditioning the grid-side voltage and current, while another part is responsible for sampling and conditioning the voltage of the DC-side voltage divider capacitors C1 and C2. The processor 7 is responsible for important tasks such as calculation and modulation, and transmits each PWM (Pulse Width Modulation) switching signal to the drive circuit 8, thereby controlling each switch.

[0138] Please refer to Figure 4 , Figure 4 This is a control algorithm block diagram of a control system provided in an embodiment of the present invention. In this application, the modulation of the T-type three-level converter section adopts a modulation method based on geometric transformation:

[0139] By performing coordinate transformation on the grid-side voltage and grid-side current of the T-type three-level converter, we can obtain the u values ​​in the d-axis and q-axis directions in a two-phase rotating coordinate system. d u q and current i d i q By employing a PI (Proportional-Integral) control algorithm in the current loop to process the current deviation, and adding a feedforward value, the voltage reference value u can be obtained. d_ref u q_ref :

[0140]

[0141] Among them, K P K is the proportional control coefficient.I i is the integral control coefficient; d_ref i q_ref ω represents the reference current in the d-axis and q-axis directions; ω represents the angular frequency; and L represents the inductance of the input filter inductor of the three-level converter.

[0142] For the above voltage reference value u d_ref u q_ref By performing coordinate transformation, the grid-side reference voltage u in the three-phase rotating coordinate system is obtained. xn (x∈a,b,c), based on the obtained grid-side reference voltage u xn (x∈a,b,c) Construct the initial duty cycle:

[0143]

[0144] The value of the midpoint voltage compensation term is determined based on the sampled grid-side current and the voltages of the two discrete capacitors on the DC side of the T-type three-level converter.

[0145]

[0146] Among them, (0<α<1,ε>0) is selected so that the voltage difference between the two discrete capacitors on the DC side can converge to zero within a finite time.

[0147] Based on the physical constraints of the duty cycle of each switch in the T-type three-level converter and the requirement for the minimum number of switching operations, the duty cycle adjustment term is selected as follows:

[0148]

[0149] In the T-type three-level converter, the grid-side reference voltage, the midpoint voltage compensation term, and the duty cycle adjustment term have all been determined, and the value of each switch duty cycle can be completely solved.

[0150] By using the above modulation method, the midpoint potential difference can be converged to zero within a finite time, thereby achieving midpoint potential balance. In addition, the above modulation method ensures that at least one switch does not operate in each switching cycle, which can reduce the number of switching operations.

[0151] Please refer to Figure 5 , Figure 5 The flowchart illustrates a control algorithm provided in this embodiment of the invention. The input mains voltage of the T-type three-level converter is 110V / 50Hz, the inductance of the input filter inductor L is 3mH, the capacitance of the DC-side voltage divider capacitors C1 and C2 is 500μF, the reference value of the DC-side voltage is 400V, and both the sampling frequency and the switching frequency are 20kHz. The control method steps for the T-type three-level converter are as follows:

[0152] 1) Acquire the grid-side voltage u of the T-type three-level converterabc and grid-side current i abc And the DC side capacitor voltages u1 and u2, and the phase information θ of the grid side voltage is extracted through a three-phase PLL (Phase Locked Loop), θ=ωt;

[0153] 2) Using the phase information ωt of the grid-side voltage, the grid-side voltage u is... abc and grid-side current i abc Perform coordinate transformations from three-phase rotating to two-phase stationary and from two-phase stationary to two-phase rotating to obtain the voltage u in the two-phase rotating coordinate system. d u q and current i d i q ;

[0154] 3) Reference value i for AC side current d_ref i q_ref With AC side current i d i q Subtract them, and the difference is used as the input to the current loop PI controller, for u d +ωLi q u q -ωLi d Subtracting the output of the PI controller from the voltage reference value u in the two-phase rotating coordinate system yields the voltage reference value u. d_ref u q_ref ;

[0155] 4) Utilizing the phase information ωt of the grid-side voltage again, the voltage reference value u in the two-phase rotating coordinate system is... d_ref u q_ref Converted to voltage reference value u in a three-phase rotating coordinate system an u bn u cn ;

[0156] 5) The voltage reference value u obtained after processing the sampled signal an u bn u cn The DSP processor calculates the duty cycle of each switch signal, and generates a PWM pulse signal by comparing the duty cycle with a triangular carrier wave. This pulse signal is then transmitted to the drive circuit to control the on / off state of each switch.

[0157] The T-type three-level converter has three operating modes: taking the A-phase operation as an example, the first operating mode is: upper bridge arm switch S ap Open, bidirectional switch S ao and lower bridge arm switch S an When the circuit is off, the output voltage is u1, corresponding to state P; the second operating mode is the bidirectional switch S. ao Open, upper bridge arm switch Sap and lower bridge arm switch S an When the circuit is off, the output voltage is 0, corresponding to state O; in the third operating mode, the lower bridge arm switch S... an Open, bidirectional switch S ao and upper bridge arm switch S ap When turned off, the output voltage is -u2, corresponding to the N state.

[0158] Please refer to Figure 6 , Figure 6 This diagram illustrates the convergence of the midpoint potential difference of a T-type three-level converter within a finite time, as provided in an embodiment of the present invention (horizontal axis: time / s, vertical axis: midpoint potential difference / V). Experimental results confirm the correctness and feasibility of the proposed topology and control method.

[0159] Power flow flowing out of the grid is defined as positive power flow, and power flow into the grid is defined as negative power flow. Figures 7(a) (horizontal axis: time / s, vertical axis: voltage of the two discrete capacitors on the DC side / V) and 7(b) (horizontal axis: time, vertical axis: grid-side voltage and current) show that the converter's power flow is controlled to be positive starting from 0.1s, with the converter operating in rectifier mode; at 0.25s, the operating mode is switched to negative power flow, with the converter operating in inverter mode. As can be seen from the figures, this modulation strategy can control NPV balance under both positive and negative power flow conditions.

[0160] The above description is merely a preferred embodiment of this application and is not intended to limit this application in any other way. Any modifications or equivalent changes made based on the technical essence of this application shall still fall within the scope of protection claimed by this application.

[0161] This application also provides a modulation system for a midpoint clamped multilevel converter, comprising:

[0162] The module is used to construct mathematical relationships for calculating the duty cycles of each switch in the multilevel converter, based on the grid-side reference voltage of the multilevel converter, the midpoint voltage compensation term for controlling the NPV balance of the multilevel converter, and the duty cycle adjustment term for controlling the number of switching actions of the multilevel converter.

[0163] The calculation module is used to calculate the duty cycle of each switch of the multilevel converter according to mathematical formulas when the multilevel converter starts working.

[0164] The control module is used to control the on / off state of each switch of the multilevel converter according to the duty cycle of each switch.

[0165] As an optional embodiment, the multilevel converter is a three-level converter;

[0166] The mathematical relation for constructing modules is:

[0167]

[0168] Among them, u dc = u1 + u2; u1 and u2 are the voltages of the two discrete capacitors on the DC side of the three-level converter; d ij (i = a, b, c; j = P, N) represents the duty cycle of the upper and lower switches in the a, b, and c phase arms of the three-level converter; u xn (x∈a,b,c) is the grid-side reference voltage; γ x (x∈a,b,c) is the midpoint voltage compensation term; m is the duty cycle adjustment term.

[0169] As an optional embodiment, the midpoint voltage compensation term constructed by the construction module is as follows:

[0170]

[0171] in, i i (i = a, b, c) represents the three-phase input phase currents on the grid side of the multilevel converter; |i i |(i=a,b,c) represents the absolute value of the three-phase input phase current; sgn() is the sign function; ε is the proportional gain for neutral point voltage balancing; and α is the finite-time convergence gain for the neutral point voltage.

[0172] As an optional embodiment, the duty cycle adjustment term constructed by the construction module is:

[0173]

[0174] As an optional embodiment, the process of obtaining the grid-side reference voltage includes:

[0175] By performing coordinate transformation on the grid-side voltage and current of the three-level converter, the voltage u in the d-axis and q-axis directions in a two-phase rotating coordinate system is obtained. d u q and current i d i q ;

[0176] according to Obtain the voltage reference value u d_ref u q_ref Among them, K P K is the proportional control coefficient. I i is the integral control coefficient; d_ref i q_ref ω represents the reference current along the d-axis and q-axis; ω represents the angular frequency; and L represents the inductance of the input filter inductor of the three-level converter.

[0177] By performing a coordinate transformation on the voltage reference value, the grid-side reference voltage u in the three-phase rotating coordinate system is obtained. xn (x∈a,b,c).

[0178] For a description of the modulation system provided in this application, please refer to the embodiments of the modulation method described above; further details will not be repeated here.

[0179] This application also provides a modulation device for a midpoint clamped multilevel converter, comprising:

[0180] The drive circuit connected to the control terminals of each switch of the multilevel converter;

[0181] The processor, connected to the drive circuit, is used to implement the steps of the modulation method of any of the above-mentioned midpoint clamping multilevel converters when executing the computer program stored in it; wherein the processor controls the on / off state of each switch of the multilevel converter through the drive circuit.

[0182] As an optional embodiment, the processor is a DSP processor.

[0183] For a description of the modulation apparatus provided in this application, please refer to the embodiments of the modulation method described above; further details will not be repeated here.

[0184] This application also provides a midpoint clamped multilevel converter, including the modulation device of any of the above-mentioned midpoint clamped multilevel converters.

[0185] For a description of the multilevel converter provided in this application, please refer to the embodiments of the modulation device described above; further details will not be repeated here.

[0186] It should also be noted that, in this specification, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0187] The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A modulation method for a midpoint clamped multilevel converter, characterized in that, include: Based on the grid-side reference voltage of the multilevel converter, the midpoint voltage compensation term for controlling the NPV balance of the multilevel converter, and the duty cycle adjustment term for controlling the number of switching actions of the multilevel converter, a mathematical relationship for calculating the duty cycle of each switch of the multilevel converter is constructed. When the multilevel converter starts working, the duty cycle of each switch of the multilevel converter is calculated according to the mathematical relationship. The on / off state of each switch of the multilevel converter is controlled according to the duty cycle of each switch. The multilevel converter is a three-level converter; The process of constructing the mathematical relationship for calculating the duty cycle of each switch in the multilevel converter includes: Construct a mathematical relationship for calculating the duty cycle of each switch in the three-level converter. ; in, ; It is the voltage of the two discrete capacitors on the DC side of the three-level converter; The duty cycle of the upper and lower switches in the a, b, and c phase bridge arms of the three-level converter is represented. This is the grid-side reference voltage; This is the midpoint voltage compensation term; This is the duty cycle adjustment term.

2. The modulation method for the midpoint clamped multilevel converter as described in claim 1, characterized in that, The process of constructing the mathematical relationship for calculating the duty cycle of each switch in the three-level converter includes: The first relationship is obtained by averaging the grid-side reference voltage under switching states. ; The sum of the duty cycles of the upper and lower arms of each phase in the three-phase bridge arm is used to construct the sum of the duty cycle adjustment term and the midpoint voltage compensation term, thus obtaining the second relational expression. ; Based on the first and second relationships, mathematical relationships are obtained for calculating the duty cycles of each switch in the three-level converter. .

3. The modulation method for the midpoint clamped multilevel converter as described in claim 1, characterized in that, The process of constructing a midpoint voltage compensation term for controlling the NPV balance of the multilevel converter includes: Based on the three-phase input phase currents on the grid side of the three-level converter and the voltages of the two discrete capacitors on the DC side, a midpoint voltage compensation term is constructed. ; in, ; The three-phase input phase current; is the absolute value of the three-phase input phase current; sgn() is the sign function; The proportional gain is for midpoint voltage balance. ; Let be the finite-time convergence gain of the midpoint voltage. .

4. The modulation method for the midpoint clamped multilevel converter as described in claim 3, characterized in that, The modulation method of the midpoint clamped multilevel converter also includes: The first dynamic relationship for obtaining the voltage difference between the two discrete capacitors on the DC side is obtained. ;in, The capacitance values ​​of the two discrete capacitors on the DC side; Substituting the midpoint voltage compensation term into the first dynamic equation, we obtain the second dynamic equation. ; based on Determine the convergence time of the second dynamic relation. ;in, ; The larger the value, the longer the convergence time. The smaller.

5. The modulation method for a midpoint clamped multilevel converter as described in claim 1, characterized in that, The process of constructing a duty cycle adjustment term for controlling the number of switching operations of the multilevel converter includes: Based on the physical constraints of the duty cycle of each switch in the three-level converter and the minimum number of switching operations required, a duty cycle adjustment term is constructed. .

6. The modulation method of the midpoint clamped multilevel converter as described in any one of claims 1-5, characterized in that, The process of obtaining the grid-side reference voltage includes: The grid-side voltage and current of the three-level converter are transformed by coordinates to obtain the voltages along the d-axis and q-axis in a two-phase rotating coordinate system. , and current , ; according to Obtain voltage reference value , ;in, This is the proportional control coefficient; These are integral control coefficients; , The reference currents are located in the d-axis and q-axis directions; Angular frequency; The inductance of the input filter inductor of the three-level converter; The voltage reference value is transformed using coordinates to obtain the grid-side reference voltage in a three-phase rotating coordinate system. .

7. A modulation system for a midpoint clamped multilevel converter, characterized in that, include: The construction module is used to construct a mathematical relationship for calculating the duty cycle of each switch of the multilevel converter based on the grid-side reference voltage of the multilevel converter, the midpoint voltage compensation term for controlling the NPV balance of the multilevel converter, and the duty cycle adjustment term for controlling the number of switching actions of the multilevel converter. The calculation module is used to calculate the duty cycle of each switch of the multilevel converter according to the mathematical formula when the multilevel converter starts to work. The control module is used to control the on / off state of each switch of the multilevel converter according to the duty cycle of each switch of the multilevel converter. The multilevel converter is a three-level converter; The mathematical relation constructed by the construction module is: ； in, ; It is the voltage of the two discrete capacitors on the DC side of the three-level converter; The duty cycle of the upper and lower switches in the a, b, and c phase bridge arms of the three-level converter is represented. This is the grid-side reference voltage; This is the midpoint voltage compensation term; This is the duty cycle adjustment term.

8. The modulation system of the midpoint clamped multilevel converter as described in claim 7, characterized in that, The midpoint voltage compensation term constructed by the construction module is as follows: ； in, ; The three-phase input phase currents on the grid side of the multilevel converter; is the absolute value of the three-phase input phase current; sgn() is the sign function; The proportional gain is for midpoint voltage balance. Let be the finite-time convergence gain of the midpoint voltage.

9. The modulation system of the midpoint clamped multilevel converter as described in claim 7, characterized in that, The duty cycle adjustment term constructed by the construction module is: 。 10. The modulation system of the midpoint clamped multilevel converter as described in any one of claims 7-9, characterized in that, The process of obtaining the grid-side reference voltage includes: The grid-side voltage and current of the three-level converter are transformed by coordinates to obtain the voltages along the d-axis and q-axis in a two-phase rotating coordinate system. , and current , ; according to Obtain voltage reference value , ;in, This is the proportional control coefficient; These are integral control coefficients; , The reference currents are located in the d-axis and q-axis directions; Angular frequency; The inductance of the input filter inductor of the three-level converter; The voltage reference value is transformed using coordinates to obtain the grid-side reference voltage in a three-phase rotating coordinate system. .

11. A modulation device for a midpoint clamped multilevel converter, characterized in that, include: The drive circuit connected to the control terminals of each switch of the multilevel converter; A processor connected to the drive circuit is used to implement the steps of the modulation method of the midpoint clamped multilevel converter as described in any one of claims 1-6 when executing a computer program stored in itself; wherein the processor controls the on / off state of each switch of the multilevel converter through the drive circuit.

12. The modulation apparatus for the midpoint clamped multilevel converter as described in claim 11, characterized in that, The processor is a DSP processor.

13. A midpoint clamped multilevel converter, characterized in that, The modulation device includes the midpoint clamping multilevel converter as described in claim 11 or 12.

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