Zero common mode voltage control method and system for two parallel converters based on single vector predictive control

Abstract

The invention discloses a zero common-mode voltage control method and system for two parallel converters based on single vector predictive control, and the method comprises the steps: building a prediction model and an evaluation function with parallel current, three-phase circulating current and common-mode voltage as control targets, and achieving the comprehensive control of output current and three-phase circulating current while guaranteeing the zero common-mode voltage; on the basis of the mapping relation between zero common-mode voltage and switch state combinations, 20 switch combinations with the common-mode voltage being 0 are selected, and an alternative switch combination library is formed; and a common-mode voltage penalty term considering a commutation process and a system parameter error is introduced into the evaluation function, and according to current polarity determination, alternative vectors generating common-mode voltage are rejected online, so that accurate control of zero common-mode voltage is realized. The invention provides an efficient and reliable common-mode voltage suppression scheme, and the common-mode voltage of the two parallel converters is successfully eliminated.

Description

Technical Field

[0001] The present invention belongs to the field of predictive control of two parallel converters, and in particular relates to a zero common mode voltage control method and system for two parallel converters based on single vector predictive control. Background Art

[0002] Two parallel three-phase converters have been widely used in motor drive, uninterruptible power supply, new energy power generation and other fields. In these fields, common-mode voltage is an important indicator affecting working performance. Excessive common-mode voltage will increase the voltage stress of the device, reduce the service life of the device, and reduce the reliability of the system. High common-mode voltage may induce a series of negative effects, such as ground leakage current, breakdown of insulating materials, electromagnetic interference, etc., affecting the normal operation of the system. In order to suppress high common-mode voltage, large-volume common-mode chokes are usually required, which will significantly increase the system volume and reduce the power density. Therefore, research on reducing common-mode voltage from the algorithm level is of practical significance.

[0003] The evaluation index of common-mode voltage can be mainly divided into two types: average value and instantaneous value. Compared with the average value of common-mode voltage, the suppression of instantaneous common-mode voltage is more important. The suppression of instantaneous common-mode voltage can optimize dv / dt and further suppress common-mode current and electromagnetic interference. To this end, the literature [Z.Quan and Y.W.Li, "A Three-Level Space Vector Modulation Scheme for Paralleled Converters to Reduce Circulating Current and Common-Mode Voltage," in IEEE Transactions on Power Electronics, vol.32, no.1, pp.703-714, Jan.2017, doi:10.1109 / TPEL.2016.2529959] analyzed all 64 available vectors and screened out vectors with smaller common-mode voltage. Based on this, the scheme defines a new set of vector sequences, which suppresses the peak value of the instantaneous common-mode voltage to 0 while keeping the average value of the common-mode voltage at zero. However, the method in this document only weakens the common-mode voltage change rate dv / dt, and cannot fundamentally eliminate the instantaneous common-mode voltage. To this end, the document [D. Jiang, Z. Shen and F. Wang, "Common-Mode Voltage Reduction for Paralleled Inverters," in IEEE Transactions on Power Electronics, vol. 33, no. 5, pp. 3961-3974, May 2018, doi: 10.1109 / TPEL.2017.2712369] proposes a zero common-mode voltage modulation scheme (ZCMV), which only selects switching vectors with an instantaneous common-mode voltage of 0 in the switching sequence, in order to always control the common-mode voltage change rate to 0. However, the instantaneous common-mode voltage is closely related to the instantaneous switching state. ZCMV cannot accurately control the instantaneous switching state and cannot completely eliminate the common-mode voltage. The remaining common-mode voltage still introduces a large dv / dt, and the system still needs to be equipped with a corresponding prefabricated common-mode circulating current choke. Due to the lack of precise control of the instantaneous common-mode voltage, there is currently no method to completely eliminate the common-mode voltage. Summary of the invention

[0004] The purpose of the present invention is to provide a method and system for controlling zero common-mode voltage of two parallel converters based on single vector predictive control, so as to solve the problem that the existing solutions lack the means for precise control of instantaneous common-mode voltage, achieve complete elimination of common-mode voltage, and optimize parallel current and three-phase circulating current.

[0005] The technical solution to achieve the purpose of the present invention is:

[0006] A method for controlling zero common-mode voltage of two parallel converters based on single vector predictive control comprises: establishing a prediction model with parallel current, three-phase circulating current and common-mode voltage as control targets, realizing comprehensive control of output current and three-phase circulating current while realizing zero common-mode voltage; selecting a switch combination with a common-mode voltage of 0 based on a mapping relationship between the common-mode voltage of the two parallel converters and the switch state combination to form a candidate switch combination library; introducing a common-mode voltage penalty term considering the commutation process and system parameter errors into an evaluation function, and based on the candidate switch combination library, online eliminating the candidate vectors that generate common-mode voltage according to current polarity judgment to obtain the optimal switch combination, thereby realizing precise control of zero common-mode voltage.

[0007] Furthermore, the method specifically comprises:

[0008] Collect the DC bus voltage, three-phase load voltage and three-phase currents of the two converters at the current time k, and determine the parallel current and three-phase circulating current based on Kirchhoff's current law;

[0009] Delay compensation is added, two-cycle prediction is adopted, and the prediction values ​​of parallel current and three-phase circulating current at time k+2 corresponding to all candidate vectors are obtained through the prediction model;

[0010] Consider the commutation process, obtain the instantaneous state of the switch and its common-mode voltage, consider the system parameter error, and obtain error compensation;

[0011] Based on the predicted values ​​of parallel current and three-phase circulating current at time k+2, the instantaneous state of the switch and its common-mode voltage, and error compensation, the optimal switch combination that minimizes the evaluation function is obtained by size comparison, and then the power switching devices of the two converters are controlled separately.

[0012] Furthermore, the mapping relationship between the common mode voltage and the switch state combination is:

[0013]

[0014] Among them, s A1 、s B1 、s C1 Indicates the switching state of the three-phase bridge arm switch of a converter, s A2 、s B2 、s C2 Indicates the switch state of the three-phase bridge arm switch tube of another converter; all switch states are 1 or 0. When 1 is taken, it means that the upper tube of the corresponding bridge arm is turned on and the lower tube is turned off. When 0 is taken, it means that the upper tube of the corresponding bridge arm is turned off and the lower tube is turned on. V DC is the DC bus voltage.

[0015] Furthermore, the candidate switch combination library is 20 switch combinations with a common mode voltage of 0, which are:

[0016]

[0017] In the table, the parameters outside the brackets represent the equivalent output voltage vector, and the two parameters inside the brackets represent the corresponding voltage vectors of the first and second converters, V i represents the i-th equivalent output voltage vector in the redundant vector combination.

[0018] Furthermore, the prediction model includes a parallel current discrete prediction model and a single-phase circulating current discrete prediction model, and the parallel current discrete prediction model is:

[0019]

[0020] The single-phase circulating current discrete prediction model is:

[0021]

[0022] Among them, i A (k+1), i B (k+1) and i C (k+1) is the predicted value of the parallel current at time k+1, i cirA (k+1), i cirB (k+1) and i cirC (k+1) is the predicted value of the three-phase circulation at time k+1, L e , R e is the equivalent inductance and resistance, L e =L+L1 / 2, R e =R+R1 / 2, L and R are the inductance and parasitic resistance of the AC side, L1 and R1 are the inductance and parasitic resistance of the bridge arm side, V DC Indicates the DC bus voltage, T s is the switching period, s A1 (k), s A2 (k), s B1 (k), s B2 (k), s C1 (k), s C2 (k) represents the switching states of the switches T1 to T6 of the two converters, u AN (k) and u BN (k) and u CN (k) represents the output voltage of the three-phase bridge arm at time k, i A (k), i B (k), i C (k) is the parallel current value at time k, i cirA (k), icirB (k) and i cirC (k) is the three-phase circulating current value at time k, e A (k), e B (k), e C (k) is the three-way back electromotive force at time k, i cirA (k), i cirB (k) and i cirC (k) is the three-phase circulating current value at time k.

[0023] Furthermore, delay compensation is added, two-cycle prediction is adopted, and the prediction values ​​of parallel current and three-phase circulating current at time k+2 corresponding to all candidate vectors are obtained through the prediction model, including:

[0024] First, the parallel current and three-phase circulating current at time k, as well as the output voltage and switch state combination of the selected vector at time k-1 are substituted into the discrete prediction model to obtain the parallel current and three-phase circulating current at time k+1;

[0025] Then substitute all the candidate vectors into the prediction model to solve the parallel current and three-phase circulating current at time k+2.

[0026] Furthermore, considering the commutation process, the instantaneous state of the switch and its common-mode voltage obtained include:

[0027]

[0028] f1(s xi (t k ))=[s xi (k)-s xi (k+1)]·sign(i xi (t k ))

[0029]

[0030] Among them, t d Indicates the dead time, in s xi，td represents the switch state of the ith x-phase in the dead time, x = A, B, C represents three phases; t k represents the switching time, s xi，td represents the switch state of the ith x-phase in the dead time, s xi (k) represents the switch state of the i-th x-phase at time k, s xi (k+1) represents the switch state of the ith x-phase at time k+1, f1(s xi (t k )) is the judgment formula for the commutation process. When the function value is -1, it indicates that an unexpected instantaneous switching state occurs. P1 represents the instantaneous common-mode voltage value.

[0031] Furthermore, considering the system parameter error, the error compensation is obtained as:

[0032]

[0033] i xi (k+1)=f xi (k)+i xi (k)+δf xi (k)

[0034] Among them, s xi，td It represents the switching state of the i-th x-phase in the dead time; δ represents the error compensation parameter.

[0035] Furthermore, the evaluation function is:

[0036]

[0037] Among them, i refα 、i refβ is the current reference value in the two-phase stationary coordinate system, i α 、i β is the current prediction value in the two-phase stationary coordinate system, λ1 is the weight factor; f(P1, P2) represents the common mode voltage penalty function, which satisfies:

[0038]

[0039] Among them, P1 represents the instantaneous common mode voltage value, and P2 represents the error compensation amount considering the system parameter error.

[0040] A zero common mode voltage control system for two parallel converters based on single vector predictive control, comprising:

[0041] A prediction model and evaluation function building unit is used to establish a prediction model and an evaluation function with parallel current, three-phase circulating current and common mode voltage as control targets, wherein a common mode voltage penalty term is introduced into the evaluation function to take into account the commutation process and system parameter errors;

[0042] The candidate switch combination library building unit selects a switch combination with a common mode voltage of 0 based on the mapping relationship between the common mode voltage of the two parallel converters and the switch state combination;

[0043] The acquisition unit acquires the DC bus voltage, the three-phase load voltage and the three-phase currents of the two converters at the current time k, and determines the parallel current and the three-phase circulating current according to the three-phase currents of the two converters;

[0044] The parallel current and three-phase circulating current prediction unit adds delay compensation and adopts two-cycle prediction to obtain the parallel current and three-phase circulating current prediction values ​​at time k+2 corresponding to all candidate vectors through the prediction model;

[0045] The switch instantaneous state and common mode voltage determination unit obtains the switch instantaneous state and common mode voltage thereof by taking into account the commutation process and system parameter errors;

[0046] The optimal switch combination determination unit obtains the optimal switch combination that minimizes the evaluation function through size comparison based on the parallel current and three-phase circulating current prediction values ​​at time k+2, the instantaneous state of the switch and its common mode voltage, and controls the power switch device.

[0047] Compared with the prior art, the present invention has the following beneficial effects:

[0048] (1) The present invention adopts model predictive control, establishes a prediction model and an evaluation function with parallel current, three-phase circulating current and common mode voltage as control targets, and realizes comprehensive control of output current and three-phase circulating current while ensuring zero common mode voltage;

[0049] (2) The present invention provides an efficient and reliable common-mode voltage suppression scheme. A common-mode voltage penalty term that takes into account the commutation process and system errors is introduced into the evaluation function. Alternative vectors that generate common-mode voltage are eliminated online based on current polarity, thereby achieving precise control of zero common-mode voltage and successfully eliminating the common-mode voltage of two parallel converters. BRIEF DESCRIPTION OF THE DRAWINGS

[0050] Figure 1 This is the topology diagram of two parallel converters.

[0051] Figure 2 The present invention provides a flow chart of a method for controlling zero common mode voltage of two parallel converters based on single vector predictive control.

[0052] Figure 3 It is the space vector plane diagram of two parallel converters.

[0053] Figure 4 (a) is the circuit commutation process diagram of the current outflow node, the upper IGBT and the lower diode.

[0054] Figure 4 (b) is the switching state diagram of the current outflow node.

[0055] Figure 5 (a) is the circuit commutation process diagram of the current flowing into the node, the lower IGBT and the upper diode.

[0056] Figure 5 (b) is the switching state diagram of the current flowing into the node.

[0057] Figure 6 (a) is the simulation result diagram of parallel current, zero-sequence circulating current and common-mode voltage of ZCMV when M=0.4.

[0058] Figure 6(b) is the simulation result diagram of the parallel current, zero-sequence circulating current and common-mode voltage of this patent when M=0.4.

[0059] Figure 7 (a) is the three-phase circulating current simulation result diagram of ZCMV when M=0.4.

[0060] Figure 7 (b) is the three-phase circulating current simulation result diagram of this patent when M=0.4.

[0061] Figure 8 (a) is the simulation result diagram of parallel current, zero-sequence circulating current and common-mode voltage of ZCMV when M=0.8.

[0062] Figure 8 (b) is the simulation result diagram of the parallel current, zero-sequence circulating current and common-mode voltage of this patent when M=0.8.

[0063] Fig. 9 (a) is the three-phase circulating current simulation result diagram of ZCMV when M=0.8.

[0064] Fig. 9 (b) is the three-phase circulating current simulation result diagram of this patent when M=0.8. DETAILED DESCRIPTION

[0065] The technical solutions in the embodiments of the present invention will be described below in conjunction with the accompanying drawings.

[0066] This embodiment provides a method for controlling zero common-mode voltage of two parallel converters based on single vector predictive control, including: establishing a prediction model and an evaluation function with parallel current, three-phase circulating current and common-mode voltage as control targets, realizing comprehensive control of output current and three-phase circulating current while ensuring zero common-mode voltage; based on the mapping relationship between zero common-mode voltage and switch state combination, 20 switch combinations with a common-mode voltage of 0 are selected to form a library of candidate switch combinations; a common-mode voltage penalty term considering the commutation process and system parameter errors is introduced into the evaluation function, and the candidate vectors that generate common-mode voltage are eliminated online according to the current polarity judgment, so as to realize precise control of zero common-mode voltage. The present invention provides an efficient and reliable common-mode voltage suppression scheme, which successfully eliminates the common-mode voltage of two parallel converters.

[0067] The topology of two parallel converters is as follows: Figure 1 As shown in Figure 1. L and R are the inductance and parasitic resistance of the AC side, and L1 and R1 are the inductance and parasitic resistance of the bridge arm side. A1 、i B1 、i C1 is the three-phase current output by the first parallel device, i A2 、i B2 、i C2 is the three-phase current output by the second parallel device, iA 、i B 、i C is the total three-phase current of the two parallel converters.

[0068] Due to the lack of precise control of instantaneous common-mode voltage, there is currently no method that can completely eliminate the common-mode voltage of parallel converters. To this end, this patent proposes a zero common-mode voltage control method for two parallel converters based on single vector predictive control, such as Figure 2 As shown, the specific implementation steps are as follows:

[0069] (1) Based on the mapping relationship between the common mode voltage of the two parallel converters and the switch state combination, 20 switch combinations with a common mode voltage of 0 are selected to form a candidate switch combination library, which specifically includes:

[0070] According to Kirchhoff's law, the common mode voltage can be derived as:

[0071]

[0072] Among them, V DC Indicates the DC bus voltage, s A1 、s A2 、s B1 、s B2 、s C1 、s C2 express Figure 1 The switch states of T1 to T6 in the figure. All switch states can only be 1 or 0. When 1 is taken, it means that the upper tube of the corresponding bridge arm is turned on and the lower tube is turned off. When 0 is taken, it means that the upper tube of the corresponding bridge arm is turned off and the lower tube is turned on. There are 64 switch combinations in the two parallel converters. Substituting the 64 switch combinations into formula (3) can obtain the common mode voltage corresponding to each combination, see Table 1. These 64 switch combinations constitute 19 basic voltage vectors, such as Figure 3 .

[0073] Table 164 Common mode voltage correspondence table of vectors

[0074]

[0075] In Table 1, the numbers in brackets after the vector names indicate the common-mode voltage, expressed in V DC / 6 as the benchmark for the standardization. The first converter uses V 000 (Subscript 000 indicates s A1 =0,s B1 = 0 and s C1 =0), the second converter selects vector V 100 (The subscript 100 indicates s A2 =1,s B2 = 0 and s C2=0) as an example, under the combined effect of the two, the output vector is V 13 , further substituting the switch state into equation (1), we get the common mode voltage as -V DC / 3, so we get the vector representation V 13 (-2), as shown in Table 1.

[0076] Furthermore, in order to achieve zero common mode voltage control, 20 switch combinations with a common mode voltage of 0 in Table 1 are selected to form a candidate switch combination library, as shown in Table 2.

[0077] Table 2 Alternative switch combination library

[0078]

[0079] (2) The DC bus voltage, three-phase load voltage and three-phase currents of the two converters at the current time (time k) are collected, and the parallel current and three-phase circulating current are determined according to the three-phase currents of the two converters, including:

[0080] According to Kirchhoff's current law, the relationship between the three-phase current and the parallel current of two parallel converters is:

[0081]

[0082] According to Kirchhoff's current law, the relationship between the three-phase circulating current and the parallel current of two parallel converters is:

[0083]

[0084] (3) Delay compensation is added, and two-cycle prediction is adopted to obtain the predicted values ​​of parallel current and three-phase circulating current at time k+2 corresponding to all candidate vectors, including:

[0085] according to Figure 1 The circuit topology of two parallel converters gives the discrete prediction model of parallel current:

[0086]

[0087] Similarly, according to the circuit topology of two parallel converters, the single-phase circulating current discrete prediction model is obtained:

[0088]

[0089] Due to the system delay in the microprocessor, after the vector combination selected by the controller at the kth moment is transmitted to the comparator and the power amplifier link, all power switch tubes will not produce corresponding actions until the k+1th moment, so a two-cycle prediction is required to eliminate the impact of system delay. Therefore, the load voltage, parallel current, and three-phase circulating current sampling values ​​obtained at the kth moment, as well as the output voltage and switch state of the vector selected by the predictive control algorithm at the k-1th moment are first substituted into equations (4) and (5), and the prediction model is obtained to calculate the parallel current and three-phase circulating current prediction values ​​i at the k+1th moment. A (k+1), i B (k+1) and i C (k+1) and the predicted value i of the three-phase circulation cirA (k+1), i cirB (k+1) and i cirC (k+1); then substitute the 20 alternative switch combinations in step (1) into the discrete prediction model equations (4) and (5), and calculate the predicted value i of the parallel current at time k+2 under the action of the 20 alternative vector combinations. A (k+2), i B (k+2) and i C (k+2) and the predicted value i of the three-phase circulation cirA (k+2), i cirB (k+2) and i cirC (k+2).

[0090]

[0091]

[0092] Substituting the switch combination at any time into the common mode voltage expression, the instantaneous common mode voltage at any time is 0. The predicted current value of a single unit satisfies:

[0093]

[0094] Among them, x=A, B, C represents three phases.

[0095] (4) According to the predicted current polarities of the two parallel converters, the commutation process during switch switching is determined to obtain the corresponding instantaneous switch state, and the common mode voltage during switch switching is further calculated, specifically including:

[0096] Taking the current outflow node as an example, the commutation process is shown in Figure 4 and Figure 5 The commutation process can be divided into two main types: 1. Commutation from diode to IGBT, which requires waiting for the dead time t d After that, the IGBT conduction can be completed. Since the commutation is not completed during the dead time, the instantaneous switch state is equivalent to the switch state before switching (or the previous moment), see Figure 4 Process 1 and Figure 5 4.2. The current is commutated from the IGBT to the diode. The current is commutated at the moment the IGBT is turned off. There is no need to wait for the dead time. The instantaneous switch state is equivalent to the switch state after switching (or the switch state selected at the current moment). See Figure 4 Process 2 and Figure 5 Process 3.

[0097] According to the analysis, the instantaneous switch state is related to the current polarity and the switch switching action. The switch state of any phase of the two converters can be calculated by the following formula:

[0098]

[0099] Among them, t d Denotes the dead time, t k represents the switching time, s xi (k) represents the switch state of the i-th x-phase at time k, s xi (k+1) represents the switch state of the ith stage x phase at time k+1. xi (t k )) is the judgment formula for the commutation process. When the function value is -1, it indicates that an unexpected instantaneous switching state has occurred.

[0100] Then, according to the instantaneous switch state, the common mode voltage during the commutation process can be considered as:

[0101]

[0102] According to formula (10), due to the commutation process and system parameters, the switch tube acts early or late, causing the corresponding instantaneous switch combination to be not in the candidate switch combination library, resulting in a non-zero common mode voltage, which needs to be eliminated.

[0103] (5) The evaluation function takes the parallel current, three-phase circulating current and common-mode voltage as the control targets. The parallel current command value at time k+2 is set according to the demand, and the command value of the three-phase circulating current is set to 0. The common-mode voltage penalty term is introduced. Due to the commutation process and system parameters, the switch tube is operated early or delayed, resulting in the corresponding instantaneous switch combination not being in the alternative switch combination library, resulting in a non-zero common-mode voltage. The penalty term at this time is set to infinity, and the corresponding evaluation function is infinite.

[0104] Considering the comprehensive optimization of parallel current, three-phase circulating current and common-mode voltage, the common-mode voltage penalty term is introduced and the evaluation function is designed as follows:

[0105]

[0106] Among them, i refα ,i refβis the current reference value in the two-phase stationary coordinate system, and the current command value i at time k+2 refα (k+2) and i refβ (k+2) According to the demand setting, the command value of the three-phase circulating current is set to 0. α ,i β is the current prediction value in the two-phase stationary coordinate system, λ1 is the weight factor; f(P1, P2) represents the common-mode voltage penalty function. P1 represents the instantaneous common-mode voltage value in the vector switching process considering the commutation process, s xi，td It represents the switching state of the i-th x-phase during the dead time, and P2 represents the compensation amount considering the system parameter error.

[0107] When considering parameter errors, the single-unit current prediction equation in equation (8) should be modified to

[0108] i xi (k+1)=f xi (k)+δf xi (k)+i xi (k),δf xi (k) is the compensation item due to parameter error. In practice, the deviation between the actual value of the parameter and the nominal value will not be too large, and it is set according to the actual working conditions. Based on this, the compensation amount P2 of the system parameter error satisfies:

[0109]

[0110] Among them, the function g xi Used to determine whether the parameter error affects the current polarity. xi When 1 is equal to 1, it means that there is a possibility of misjudgment of current polarity due to parameter error. If this compensation is not used, the parameter error affects the judgment of current polarity, unexpected instantaneous switching states will occur, and common-mode voltage will be introduced. Therefore, to avoid such a situation, the error compensation term δf is introduced xi (k).

[0111] The penalty function f(P1, P2) is defined as:

[0112]

[0113] If the commutation process and parameter errors lead to a non-zero common-mode voltage during the switching process, its penalty value is set to infinity.

[0114] (6) The optimal switch combination that minimizes the evaluation function is obtained by size comparison, and then the power switch devices of the two converters are controlled respectively, specifically including:

[0115] The evaluation function values ​​corresponding to the 20 vectors in the candidate vector library are compared, and the switch combination corresponding to the minimum evaluation function value is the optimal switch combination, which is used as the control quantity of the system. The obtained control quantity is used to generate the switch control signals of the corresponding bridge arms of the two converters through a comparator. These switch control signals are further driven and amplified, and then the power switch devices of the two converters are controlled respectively.

[0116] The experimental results of a specific case are given below. In this embodiment, the DC side voltage is 200V, the AC side is a pure resistive load with a resistance of 5Ω; the output inductance of the bridge side is 10mH, and the parasitic resistance value is negligible; the control frequency is 10kHz; the fundamental frequency is 50Hz; and the modulation index is 0.4 and 0.8.

[0117] Figure 6 (a) and Figure 6 (b) are the simulation results of parallel current, zero-sequence circulating current and common-mode voltage of ZCMV and the method proposed in this patent when M=0.4, Figure 7 (a) and Figure 7 (b) The simulation results of the three-phase circulation of ZCMV and the method proposed in this patent when M=0.4 respectively; Figure 8 (a) and Figure 8 (b) are the simulation results of parallel current, zero-sequence circulating current and common-mode voltage of ZCMV and the method proposed in this patent when M=0.8, Fig. 9 (a) and Fig. 9 (b) The simulation results of the three-phase circulating current of ZCMV and the method proposed in this patent when M=0.8. As can be seen from the figure, based on the control algorithm flow of the present invention, both experimental conditions achieve stable control of parallel current and three-phase circulating current. In addition, compared with the ZCMV modulation strategy, both achieve zero common mode voltage at any time.

[0118] The present invention also provides a zero common mode voltage control system for two parallel converters based on single vector predictive control, comprising:

[0119] A prediction model and evaluation function building unit is used to establish a prediction model and an evaluation function with parallel current, three-phase circulating current and common mode voltage as control targets, wherein a common mode voltage penalty term is introduced into the evaluation function to take into account the commutation process and system parameter errors;

[0120] The candidate switch combination library building unit selects a switch combination with a common mode voltage of 0 based on the mapping relationship between the common mode voltage of the two parallel converters and the switch state combination;

[0121] The acquisition unit acquires the DC bus voltage, the three-phase load voltage and the three-phase currents of the two converters at the current time k, and determines the parallel current and the three-phase circulating current according to the three-phase currents of the two converters;

[0122] The parallel current and three-phase circulating current prediction unit adds delay compensation and adopts two-cycle prediction to obtain the parallel current and three-phase circulating current prediction values ​​at time k+2 corresponding to all candidate vectors through the prediction model;

[0123] The switch instantaneous state and common mode voltage determination unit obtains the switch instantaneous state and common mode voltage thereof by taking into account the commutation process and system parameter errors;

[0124] The optimal switch combination determination unit obtains the optimal switch combination that minimizes the evaluation function through size comparison based on the parallel current and three-phase circulating current prediction values ​​at time k+2, the instantaneous state of the switch and its common mode voltage, and controls the power switch device.

[0125] In general, the method of the present invention adopts centralized control to achieve precise control of the overall performance of two parallel power converters; a prediction model and evaluation function are established with parallel current, three-phase circulating current and common-mode voltage as control targets, thereby achieving comprehensive control of output current quality, single-phase circulating current and zero common-mode voltage; the present invention introduces a common-mode voltage penalty term that takes into account dead zones and calculation errors in the evaluation function, and eliminates alternative vectors that generate common-mode voltage online, thereby achieving precise control of zero common-mode voltage.

[0126] The above description of the embodiments is a preferred implementation of the present invention, but the implementation of the present invention is not limited to the above embodiments. Based on the idea principle and technical solution of the present invention, various modifications or variations that can be made by technicians in this field without creative work are still within the protection scope of the present invention.

Claims

1. A method for controlling zero common mode voltage of two parallel converters based on single vector predictive control, characterized in that: include: A prediction model is established with parallel current, three-phase circulating current and common-mode voltage as control targets. While achieving zero common-mode voltage, the output current and three-phase circulating current can be comprehensively controlled. Based on the mapping relationship between the common-mode voltage of the two parallel converters and the switch state combination, the switch combination with a common-mode voltage of 0 is selected to form a library of alternative switch combinations. A common-mode voltage penalty term that considers the commutation process and system parameter errors is introduced into the evaluation function. Based on the library of alternative switch combinations, the alternative vectors that generate common-mode voltage are eliminated online according to the current polarity to obtain the optimal switch combination, thereby achieving precise control of zero common-mode voltage.

2. The method for controlling zero common mode voltage of two parallel converters based on single vector predictive control according to claim 1, characterized in that: Specifically include: Collect the DC bus voltage, three-phase load voltage and three-phase currents of the two converters at the current time k, and determine the parallel current and three-phase circulating current based on Kirchhoff's current law; Delay compensation is added, two-cycle prediction is adopted, and the prediction values ​​of parallel current and three-phase circulating current at time k+2 corresponding to all candidate vectors are obtained through the prediction model; Consider the commutation process, obtain the instantaneous state of the switch and its common-mode voltage, consider the system parameter error, and obtain error compensation; Based on the predicted values ​​of parallel current and three-phase circulating current at time k+2, the instantaneous state of the switch and its common-mode voltage, and error compensation, the optimal switch combination that minimizes the evaluation function is obtained by size comparison, and then the power switching devices of the two converters are controlled separately.

3. The method for controlling zero common mode voltage of two parallel converters based on single vector predictive control according to claim 1, characterized in that: The mapping relationship between the common mode voltage and the switch state combination is: Among them, s A1 、s B1 、s C1 Indicates the switching state of the three-phase bridge arm switch of a converter, s A2 、s B2 、s C2 Indicates the switch state of the three-phase bridge arm switch tube of another converter; all switch states are 1 or 0. When 1 is taken, it means that the upper tube of the corresponding bridge arm is turned on and the lower tube is turned off. When 0 is taken, it means that the upper tube of the corresponding bridge arm is turned off and the lower tube is turned on. V DC is the DC bus voltage.

4. The method for controlling zero common mode voltage of two parallel converters based on single vector predictive control according to claim 3, characterized in that: The candidate switch combination library is 20 switch combinations with a common mode voltage of 0, which are: In the table, the parameters outside the brackets represent the equivalent output voltage vector, and the two parameters inside the brackets represent the corresponding voltage vectors of the first and second converters, V i represents the i-th equivalent output voltage vector in the redundant vector combination.

5. The method for controlling zero common mode voltage of two parallel converters based on single vector predictive control according to claim 1, characterized in that: The prediction model includes a parallel current discrete prediction model and a single-phase circulating current discrete prediction model. The parallel current discrete prediction model is: The single-phase circulating current discrete prediction model is: Among them, i A (k+1), i B (k+1) and i C (k+1) is the predicted value of the parallel current at time k+1, i cirA (k+1), i cirB (k+1) and i cirC (k+1) is the predicted value of the three-phase circulation at time k+1, L e , R e is the equivalent inductance and resistance, L e =L+L1 / 2, R e =R+R1 / 2, L and R are the inductance and parasitic resistance of the AC side, L1 and R1 are the inductance and parasitic resistance of the bridge arm side, V DC Indicates the DC bus voltage, T s is the switching period, s A1 (k), s A2 (k), s B1 (k), s B2 (k), s C1 (k), s C2 (k) represents the switching states of the switches T1 to T6 of the two converters, u AN (k) and u BN (k) and u CN (k) represents the output voltage of the three-phase bridge arm at time k, i A (k), i B (k), i C (k) is the parallel current value at time k, i cirA (k), i cirB (k) and i cirC (k) is the three-phase circulating current value at time k, e A (k), e B (k), e C (k) is the three-way back electromotive force at time k, i cirA (k), i cirB (k) and i cirC (k) is the three-phase circulating current value at time k.

6. The method for controlling zero common mode voltage of two parallel converters based on single vector predictive control according to claim 2, characterized in that: Delay compensation is added, two-cycle prediction is adopted, and the prediction values ​​of parallel current and three-phase circulating current at time k+2 corresponding to all candidate vectors are obtained through the prediction model, including: First, the parallel current and three-phase circulating current at time k, as well as the output voltage and switch state combination of the selected vector at time k-1 are substituted into the discrete prediction model to obtain the parallel current and three-phase circulating current at time k+1; Then substitute all candidate vectors into the prediction model to solve the parallel current and three-phase circulating current at time k+2.

7. The method for controlling zero common mode voltage of two parallel converters based on single vector predictive control according to claim 1, characterized in that: Considering the commutation process, the instantaneous state of the switch and its common-mode voltage obtained include: f1(s xi (t k ))=[s xi (k)-s xi (k+1)]·sign(i xi (t k )) Among them, t d Indicates the dead time, in s xi，td represents the switch state of the ith x-phase in the dead time, x = A, B, C represents three phases; t k represents the switching time, s xi，td represents the switch state of the ith x-phase in the dead time, s xi (k) represents the switch state of the i-th x-phase at time k, s xi (k+1) represents the switch state of the ith x-phase at time k+1, f1(s xi (t k )) is the judgment formula for the commutation process. When the function value is -1, it indicates that an unexpected instantaneous switching state occurs, and P1 represents the instantaneous common-mode voltage value.

8. The method for controlling zero common mode voltage of two parallel converters based on single vector predictive control according to claim 7, characterized in that: Considering the system parameter error, the error compensation is obtained as: i xi (k+1)=f xi (k)+i xi (k)+δf xi (k) Among them, s xi，td It represents the switching state of the i-th x-phase in the dead time; δ represents the error compensation parameter.

9. The method for controlling zero common mode voltage of two parallel converters based on single vector predictive control according to claim 8, characterized in that: The evaluation function is: Among them, i refα 、i refβ is the current reference value in the two-phase stationary coordinate system, i α 、i β is the current prediction value in the two-phase stationary coordinate system, λ1 is the weight factor; f(P1, P2) represents the common mode voltage penalty function, which satisfies: Among them, P1 represents the instantaneous common mode voltage value, and P2 represents the error compensation amount considering the system parameter error.

10. A zero common mode voltage control system for two parallel converters implementing the method according to any one of claims 1 to 9, characterized in that it comprises: A prediction model and evaluation function construction unit is used to establish a prediction model and an evaluation function with parallel current, three-phase circulating current and common mode voltage as control targets, wherein a common mode voltage penalty term is introduced into the evaluation function to take into account the commutation process and system parameter errors; The candidate switch combination library building unit selects a switch combination with a common mode voltage of 0 based on the mapping relationship between the common mode voltage of the two parallel converters and the switch state combination; The acquisition unit acquires the DC bus voltage, the three-phase load voltage and the three-phase currents of the two converters at the current time k, and determines the parallel current and the three-phase circulating current according to the three-phase currents of the two converters; The parallel current and three-phase circulating current prediction unit adds delay compensation and adopts two-cycle prediction to obtain the parallel current and three-phase circulating current prediction values ​​at time k+2 corresponding to all candidate vectors through the prediction model; The switch instantaneous state and common mode voltage determination unit obtains the switch instantaneous state and common mode voltage thereof by taking into account the commutation process and system parameter errors; The optimal switch combination determination unit obtains the optimal switch combination that minimizes the evaluation function through size comparison based on the parallel current and three-phase circulating current prediction values ​​at time k+2, the instantaneous state of the switch and its common mode voltage, and controls the power switch device.

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