A method for calculating high-frequency circulating current in branch circuits of motor simulator considering nonlinear factors

Abstract

The present application provides a method for calculating the high-frequency circulating current of a motor simulator branch taking into account nonlinear factors. The inverter of the motor simulator adopts a cascade coupling topology structure of m-phase 2 parallel branches, including the following steps: S1, establishing an equivalent circuit model of the cascade coupling topology of any one-phase 2 parallel branch and obtaining model parameters; S2, determining an expression of the maximum branch high-frequency circulating current of the cascade coupling topology based on the equivalent circuit model of the cascade coupling topology; S3, obtaining the hysteresis loop of the magnetic ring of the cascade coupling topology, wherein the hysteresis loop includes a linear interval and a nonlinear interval; S4, correcting the expression of the maximum branch high-frequency circulating current of the cascade coupling topology based on the hysteresis loop. The method provided in the present application can effectively improve the calculation accuracy of the maximum branch high-frequency circulating current of the cascade coupling topology of the motor simulator.

Description

Technical Field

[0001] The present application belongs to the field of motor simulator control technology, and relates to motor simulator branch circulating current calculation technology. Specifically, a motor simulator branch high-frequency circulating current calculation method is provided that takes nonlinear factors into consideration. Background Art

[0002] As a key testing equipment and high-end development tool in the field of electric drive of new energy vehicles, the electric motor simulator (EME) can significantly improve the R&D iteration speed of the electric drive system of new energy vehicles. With the rapid increase in the speed and torque power indicators of automotive drive motors, higher requirements are placed on the power output capacity of the motor simulator.

[0003] Existing motor simulators have widely adopted parallel topology and multi-level modulation strategies of carrier phase shift to improve output current capability, but it also brings about a more serious problem of circulating current between branches. The increase of circulating current will not only increase the switching loss and copper loss of power devices, increase system loss, cause serious heating of devices, and reduce life; at the same time, it will also increase the pressure of the cascade coupling structure responsible for suppressing branch circulating current in the parallel topology of the motor simulator, making its magnetic ring prone to saturation and current surge, thus causing system instability. Therefore, if the size of the branch circulating current can be determined in advance at the beginning of the design, it can help to judge the system loss and optimize it, as well as help to select magnetic rings that will not saturate under the design conditions, thereby improving the performance of the motor simulator.

[0004] Existing studies on the calculation of circulating currents between branches of parallel multi-level inverter topology structures all assume that the inductor structure is an ideal structure with no parasitic parameters. However, the actual inductor structure is usually accompanied by parallel parasitic resistance and parasitic capacitance, which makes the actual circulating current have a larger error compared to the ideal structure. This error will be amplified when the inductor enters the nonlinear working area. If it is not reasonably compensated, it will cause the predicted circulating current and the actual circulating current to have multiple orders of magnitude errors, resulting in the final designed motor simulator failing to achieve the expected power and experiencing magnetic ring saturation and system instability. Therefore, how to predetermine the maximum circulating current between the branches of the motor simulator is an urgent problem to be solved. Summary of the invention

[0005] In order to solve the above problems in the prior art, the present application provides a method for calculating the high-frequency circulating current of a motor simulator branch taking into account nonlinear factors through an embodiment, wherein the inverter of the motor simulator adopts a cascade coupling topology structure of m-phase 2 parallel branches, comprising the following steps:

[0006] S1, establish an equivalent circuit model of the cascade coupling topology of any phase 2 parallel branches and obtain model parameters;

[0007] S2, determining an expression of a maximum branch high-frequency circulating current of the cascade coupling topology based on an equivalent circuit model of the cascade coupling topology;

[0008] S3, obtaining a hysteresis loop of the magnetic ring of the cascade coupling topology, wherein the hysteresis loop includes a linear interval and a nonlinear interval;

[0009] S4, modifying the expression of the maximum branch high-frequency circulating current of the cascade coupling topology based on the hysteresis loop.

[0010] Furthermore, the equivalent circuit model of the cascade coupling topology includes a first equivalent inductor L 1 , the second equivalent inductance L 2 , parasitic resistance R 1 And parasitic capacitance C p ; The first equivalent inductance L 1 With parasitic resistance R 1 The first parallel end is connected to the 1st branch of the cascade coupling topology, and the second parallel end is connected to the second equivalent inductor L 2 The first end is connected to the second equivalent inductor L 2 The second end is connected to the 2 branches of the cascade coupling topology, and the parasitic capacitor C p Connected between branch 1 and branch 2 of the cascade coupling topology.

[0011] Preferably, the expression of the maximum branch high-frequency circulating current of the cascade coupling topology is determined by the following steps:

[0012] S21, based on Kirchhoff's current law, determine the voltage U between branch 1 and branch 2 in the cascade coupling topology as shown in equation (1): 12 The expression is:

[0013]

[0014] Among them, t is the time variable, i 1 is the current flowing through the first equivalent inductor L 1 The current;

[0015] S22, time-integrate equation (1) to obtain equation (2):

[0016]

[0017] Among them, C 1 is the integration constant;

[0018] S23, transform equation (2) into equation (3):

[0019]

[0020] in,

[0021] S24, solve equation (3) and obtain i as shown in equation (4) 1 The expression is:

[0022]

[0023] S25, using R 1 , L 1 The parallel voltage equality relationship and formula (4) are combined to obtain the expression of the branch high-frequency circulating current i in the equivalent circuit model shown in formula (5):

[0024]

[0025] Among them, C 2 is the integration constant;

[0026] S26, based on the branch high frequency circulation maximization condition and R 1 The current change constraint condition is determined as shown in equation (6) below:

[0027]

[0028] Wherein, T is the actual working switching period of the cascade coupling topology, i j,pre (t) is the maximum branch high-frequency circulating current of the j-th phase cascade coupling topology, j∈[1,m], C 1 Determine from (7):

[0029]

[0030] Preferably, the branch high-frequency circulating current maximization condition is: up =t down =0.5T, where t up is the current rise time in an actual working switching cycle, t down is the current fall time in an actual working switching cycle; the R 1 The current change constraint condition is: 1 The current does not change suddenly.

[0031] Preferably, the motor simulator suppresses the branch DC bias of the cascade coupling topology through a current balancing control strategy.

[0032] Preferably, the hysteresis loop of the magnetic ring of the cascade coupling topology is obtained by the following steps:

[0033] S31, winding N on the magnetic ring 1 turns of the primary winding and N2 turns of the secondary winding;

[0034] S32, a voltage U with constant amplitude, opposite direction and acting time equal to half of the actual working switching cycle of the magnetic ring is applied to both ends of the primary winding. 1 ;

[0035] S33, measure the primary current I of the magnetic ring 1 and secondary voltage U 2 , based on formula (8), the hysteresis loop of the magnetic ring is obtained:

[0036]

[0037] Among them, B and H are the magnetic ring under the applied voltage U 1 The magnetic flux density and magnetic field strength H,l e A is the effective magnetic path length of the magnetic ring, e is the effective cross-sectional area of ​​the magnetic ring.

[0038] Preferably, the number of primary turns N 1 and the applied voltage U 1 is set to enable the magnetic ring to enter a saturated state, the secondary side turns N 2 Based on the secondary voltage U 2 The measuring range and accuracy are determined.

[0039] Preferably, the nonlinear characteristics of the magnetic ring are characterized by fitting a piecewise linear function to the hysteresis loop of the magnetic ring.

[0040] Preferably, the expression of the maximum branch high-frequency circulating current of the cascade coupling topology is corrected by the following steps:

[0041] S41, performing piecewise fitting on the hysteresis loop to obtain a piecewise fitting function of the magnetic flux density of the cascade coupling topology, wherein the piecewise fitting function includes 1 to n linear functions corresponding to the first half of the actual working switching cycle of the cascade coupling topology, and n+1 to 2n linear functions corresponding to the second half of the actual working switching cycle of the cascade coupling topology, and each of the 1 to n linear functions and the n+1 to 2n linear functions includes a linear function corresponding to the linear interval;

[0042] S42, based on formula (9), calculate the real-time magnetic flux density B of the cascade coupling topology real :

[0043]

[0044] Among them, ΔU is the voltage difference of the windings at both ends of the magnetic ring, N is the number of turns of the winding, and B 0 is the initial magnetic flux density;

[0045] S43, real-time magnetic flux density B based on the cascade coupling topology real and the real-time current direction to determine the number of segments k of the linear function in which the magnetic ring is located;

[0046] S44, determine L based on formula (10) 1 , L 2 The real-time correction value L in the nonlinear interval of the hysteresis loop 1,k , L 2,k :

[0047]

[0048] Among them, μ k is the slope of the kth linear function, μ linear is the slope of the linear function corresponding to the linear region;

[0049] S45, based on L 1,k , L 2,k OK j,pre (t) A real-time correction value in the nonlinear region of the hysteresis loop.

[0050] The embodiment of the present application provides a method for calculating the high-frequency circulating current of the branch of the motor simulator considering nonlinear factors. First, an accurate equivalent circuit model of the cascade coupling topology is established and the values ​​of each circuit element therein are determined to more accurately reflect the circulating current of the topology under the selected magnetic ring conditions; and the mathematical model for calculating the high-frequency circulating current of the branch of the topology and the bus voltage, switching cycle, and duty cycle is derived through the equivalent circuit model. Since each inductance parameter in its formula is customized, it can only reflect the high-frequency circulating current of the cascade coupling topology when the actual magnetic ring is in the linear working area; and then the hysteresis loop of the selected magnetic ring is determined by experiment, and the magnetic permeability of the magnetic ring of each working area is approximately determined by linear segmentation of the hysteresis loop, and then the circulating current calculation formula is compensated in the nonlinear working area of ​​the magnetic ring based on this, so as to help the algorithm to more accurately reflect the high-frequency circulating current of the cascade coupling topology in the nonlinear working area of ​​the magnetic ring. Thereby, it can help the motor simulator to more reasonably select the parameters such as the size, number of winding turns, and material of the magnetic ring in the design stage, and it can also help the motor simulator to more accurately estimate the system loss in the design stage. BRIEF DESCRIPTION OF THE DRAWINGS

[0051] Figure 1 A schematic diagram of the system architecture of a motor simulator;

[0052] Figure 2 for Figure 1 The schematic diagram of the connection between the cascade coupling topology and each branch in the motor simulator shown;

[0053] Figure 3 A flow chart of a method for calculating high-frequency circulating current in a motor simulator branch considering nonlinear factors according to an embodiment of the present application;

[0054] Figure 4 A schematic diagram of an equivalent circuit model of a cascade coupling topology provided according to an embodiment of the present application;

[0055] Figure 5a A Nyquist diagram of impedance fitting of a cascade coupling topology provided according to an embodiment of the present application;

[0056] Figure 5b A Bode diagram of impedance fitting of a cascade coupling topology provided according to an embodiment of the present application;

[0057] Figure 6a A simulation diagram of a branch high-frequency circulating current when the duty cycle is 0.1 according to an equivalent circuit model provided in an embodiment of the present application;

[0058] Figure 6b A simulation diagram of a branch high-frequency circulating current when the duty cycle is 0.3 according to an equivalent circuit model provided in an embodiment of the present application;

[0059] Figure 6c A simulation diagram of a branch high-frequency circulating current when the duty cycle is 0.5 according to an equivalent circuit model provided in an embodiment of the present application;

[0060] Figure 6d A simulation diagram of a branch high-frequency circulating current when the duty cycle is 0.8 according to an equivalent circuit model provided in an embodiment of the present application;

[0061] Figure 7 A physical diagram of measuring the magnetic field characteristics of a magnetic ring according to an embodiment of the present application;

[0062] Figure 8 A schematic diagram of a hysteresis loop of a magnetic ring provided according to an embodiment of the present application;

[0063] Fig. 9 A schematic diagram of performing segmented fitting on the hysteresis loop of a magnetic ring according to an embodiment of the present application;

[0064] Fig.10 A schematic diagram of the architecture of a motor simulator control system provided for verifying Example 1 according to the present application;

[0065] Fig.11 A schematic diagram of the maximum branch high-frequency circulating current when the bus voltage and the number of winding turns provided in Example 1 are 300V and 4 turns respectively to verify the present application;

[0066] Fig.12A schematic diagram of the maximum branch high-frequency circulating current when the bus voltage and the number of winding turns provided in Example 1 are 300V and 6 turns respectively for verifying the present application;

[0067] Fig.13 A schematic diagram of the maximum branch high-frequency circulating current when the bus voltage and the number of winding turns provided in Example 1 are 700V and 4 turns respectively for verifying the present application;

[0068] Fig.14 A schematic diagram of the maximum branch high-frequency circulating current when the bus voltage and the number of winding turns provided in Example 1 are 850V and 4 turns respectively, for verifying the present application. DETAILED DESCRIPTION

[0069] Hereinafter, the present application will be further described based on preferred embodiments with reference to the accompanying drawings.

[0070] In the description of the embodiments of the present application, it should be noted that if the terms "upper", "lower", "inside", "outside", etc. indicate an orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, or the orientation or positional relationship in which the products of the embodiments of the present application are usually placed when in use, it is only for the convenience of describing the present application and simplifying the description, and does not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation on the present application. In addition, in the description of the present application, in order to distinguish different units, the words first, second, etc. are used in this specification, but these are not limited by the order of manufacture, nor can they be understood as indicating or implying relative importance, and their names may be different in the detailed description and claims of the present application.

[0071] The vocabulary in this specification is used to illustrate the embodiments of the present application, but is not intended to limit the present application. It should also be noted that, unless otherwise clearly specified and limited, the terms "disposed", "connected", and "connected" should be understood in a broad sense. For example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection, a direct connection, an indirect connection through an intermediate medium, or a connection between the two elements. For those skilled in the art, the specific meanings of the above terms in this application can be specifically understood.

[0072] In order to clearly illustrate the technical solution of the present application, its application scenario is first introduced.

[0073] Figure 1 The system framework diagram of an electric machine simulator (EME) that can use the calculation method of the present application to calculate branch high-frequency circulating currents is shown in FIG. Figure 1As shown, the motor simulator adopts a cascade coupling structure of multiple parallel inverters. Specifically, the inverter unit includes m inverter circuits (m is generally greater than or equal to 3), each inverter circuit is composed of two half-bridges in parallel, and is output to the filter unit through a cascade coupling topology (also known as a cascade coupling inductor circuit); the filter unit is connected between the motor simulator and the motor driver, and filters the m-phase current output by the inverter unit and outputs it to the m-phase line of the motor control unit.

[0074] Furthermore, the voltage / current sensor collects the real-time voltage of each phase of the motor controller under test (DUT) and the real-time current of each phase output by the motor simulator, and inputs them into the motor model simulation & control module; the module calculates the target current of each phase of the real motor based on the collected real-time voltage and current signals, and determines the initial modulation wave of each phase based on the difference between the real-time current of each phase and the target current, and inputs it into the current balancing module; the current balancing module solves the compensation modulation wave that balances the current of each branch based on the real-time current of each branch collected by the sensor, and inputs it into the modulation wave integration & PWM module for integration; the modulation wave integration & PWM module determines the final target modulation wave of each phase and each branch based on the initial modulation wave of each phase and the compensation modulation wave of each branch, thereby determining the switching signal of the inverter of each phase and each branch, thereby realizing closed-loop control of the motor simulator.

[0075] Figure 2 Shows Figure 1 The connection method of the parallel branch of any phase in the motor simulator and the cascade coupling topology is shown in the figure. The cascade coupling topology of each phase includes an inductance circuit formed by cascading two magnetic rings with windings. The current direction and voltage distribution in the figure correspond to the situation where branch 1 (i.e., the branch corresponding to the upper and lower switching devices on the left) is high and branch 2 (i.e., the branch corresponding to the upper and lower switching devices on the right) is low.

[0076] In addition, if Figure 1 As shown, the control system of the motor simulator includes a current balancing module, whose function is to control the difference between the currents (low-frequency parts) of the same-phase branches to be near 0, that is, to suppress the low-frequency parts of the circulating currents between the same-phase branches.

[0077] Although adding a current balancing module can effectively suppress the DC part of the parallel branch circulating current, as analyzed in the background technology, existing studies on the calculation of circulating current between branches of parallel multi-level inverter topology structures all assume that the inductor structure is an ideal structure with no parasitic parameters. However, the actual inductor structure is usually accompanied by parallel parasitic resistance and parasitic capacitance, which makes the actual value of the high-frequency part of the circulating current have a large error compared to the predicted value under the ideal structure assumption. This error will be amplified when the inductance of the magnetic ring group structure enters the nonlinear working area. If it is not reasonably compensated, it will cause the predicted circulating current and the actual circulating current to have multiple orders of magnitude errors, resulting in the final designed motor simulator failing to achieve the expected power and experiencing magnetic ring saturation and system instability. Therefore, a method is needed that can take into account the nonlinear characteristics of the coupled topology to accurately calculate the high-frequency circulating current of the parallel branch.

[0078] Figure 3 Flow chart of a method for calculating high-frequency circulating current in a motor simulator branch considering nonlinear factors according to an embodiment of the present application, such as Figure 3 As shown, the method comprises the following steps:

[0079] S1, establish an equivalent circuit model of the cascade coupling topology of any phase 2 parallel branches and obtain model parameters;

[0080] S2, determining an expression of a maximum branch high-frequency circulating current of the cascade coupling topology based on an equivalent circuit model of the cascade coupling topology;

[0081] S3, obtaining a hysteresis loop of the magnetic ring of the cascade coupling topology, wherein the hysteresis loop includes a linear interval and a nonlinear interval;

[0082] S4, modifying the expression of the maximum branch high-frequency circulating current of the cascade coupling topology based on the hysteresis loop.

[0083] The method first establishes an equivalent circuit model of a cascade coupling topology consisting of two magnetic rings and windings through step S1, and then determines an expression of a maximum branch high-frequency circulating current of the cascade coupling topology through the equivalent circuit model in step S2. Since there is a deviation between the calculation result of the expression in a nonlinear region and the actual value of the high-frequency circulating current, and the deviation is caused by the nonlinearity of the magnetic field characteristics of the magnetic ring, the linear region and the nonlinear region of the magnetic ring are further determined in step S3 by drawing a hysteresis loop of the magnetic ring. Finally, in step S4, the hysteresis loop is used to correct the expression of the maximum branch high-frequency circulating current to improve the accuracy of the calculation result of the maximum branch high-frequency circulating current in the nonlinear region.

[0084] Each step is described in detail below.

[0085] A. Establish the equivalent circuit model of the cascade coupling topology and determine the parameters.

[0086] like Figure 2 As shown, for any phase parallel branch, its cascade coupling topology includes two cascaded magnetic rings + windings to form an inductor. At the same time, the above inductor structure will cause parasitic resistance and parasitic capacitance, so it can be established as follows: Figure 4 The equivalent circuit model of the cascade coupling topology shown in FIG. 1 is composed of two inductor structures (the first equivalent inductor L 1 , the second equivalent inductance L 2 ), a resistor structure (parasitic resistance R 1 ) and a capacitor structure (parasitic capacitance C p ), wherein the first equivalent inductance L 1 With parasitic resistance R 1 The first parallel end is connected to the 1st branch of the cascade coupling topology, and the second parallel end is connected to the second equivalent inductor L 2 The first end is connected to the second equivalent inductor L 2 The second end is connected to the 2 branches of the cascade coupling topology, and the parasitic capacitance C p The two ends are respectively connected between branch 1 and branch 2 of the cascade coupling topology.

[0087] After the model is established, the impedance analyzer can be used to perform impedance sweep frequency acquisition (1kHz~8MHz) using the inductor actually used, and the acquired impedance data can be fitted based on the established equivalent circuit model to determine the inductance L. 1 、Inductance L 2 With resistor R 1 The numerical value of Figure 5a and Figure 5b Schematic diagram of the fitting curve of the impedance data of the cascade coupling topology structure adopted by the motor simulator of the embodiment of the present application, wherein: Figure 5a is the Nyquist plot, Figure 5b It is a Bode plot, and it can be seen that the fitting curve is very close to the actual curve.

[0088] B. Determine the expression for the maximum branch high-frequency circulating current of the cascade coupled topology.

[0089] After the equivalent circuit model of the cascade coupling topology is obtained in step S1, an expression of the maximum branch high-frequency circulating current corresponding to the equivalent circuit model is further derived in step S2.

[0090] Since the purpose of accurately calculating the branch high-frequency circulating current of the cascade coupling topology in this application is to evaluate whether the hardware parameters of the motor simulator (such as the material properties of the magnetic ring, the number of windings, etc.) are saturated due to excessive circulating current under a given bus voltage and control strategy, so that the system becomes unstable and enters the saturation state prematurely, it is obviously necessary to calculate the maximum branch high-frequency circulating current. For a cascade coupling topology structure, when the actual switching period of its switching device is T, the maximum branch high-frequency circulating current appears when the duty cycle is 0.5: t up =t down =0.5T, where t up is the current rise time in an actual working switching cycle, t down is the current falling time in an actual working switching cycle. In the embodiment of the present application, the condition for the occurrence of the above-mentioned maximum branch high-frequency circulating current is also called the branch high-frequency circulating current maximization condition. Figures 6a to 6d The schematic diagrams of the simulation of high-frequency circulating current between branches of the equivalent circuit model under the same bus voltage and switching period with duty ratios of 0.1, 0.3, 0.5, and 0.8 are shown respectively. Figures 6a to 6d The simulation results also show that the high-frequency circulating current in the branch reaches its maximum value when the switch duty cycle is 0.5.

[0091] pass Figure 2 It can be seen that the equivalent circuit model of the cascade coupling topology established in the embodiment of the present application includes the first equivalent inductance L 1 The parallel parasitic resistance R 1 , because its impedance under the working condition (10kHz~800kHz) is equal to the first equivalent inductance L 1 The difference is within 10 times, so in actual use, it will have an impact on the current flowing through the first equivalent inductance L 1 The current has a great impact, thereby affecting the magnetic field strength passing through the magnetic ring, and causing the magnetic ring to enter the nonlinear working area earlier (or later). Therefore, it needs to be considered in the process of determining the branch high-frequency circulating current expression corresponding to the circuit model.

[0092] The parasitic capacitance of the inductor in the equivalent circuit model is mainly composed of the capacitance between the windings and the capacitance between the windings and the magnetic ring. For the completely independent equivalent parasitic capacitance connected in parallel with the inductor, it is generated by multiple parasitic capacitances in series, and its capacitance is small, so the pulse current caused is small; for the equivalent parasitic capacitance connected in series with the inductor, it is generated by multiple parasitic capacitances in parallel. Although its capacitance is large, due to the current stabilizing effect of the inductor, the actual oscillating current caused is small, and the impact on the current flowing through the inductor is small. In combination with the above two points, in the embodiment of the present application, the influence of parasitic capacitance is ignored in the process of determining the branch high-frequency circulating current expression.

[0093] Based on the above analysis, combined with Figure 2 From the equivalent circuit model shown in the figure, it can be seen that the high-frequency circulating current i of the branch is flowing through the first equivalent inductor L 1 The current i 1 and flows through the parasitic resistance R 1 The current i 2 Composition, that is: i = i 1 +i 2 Therefore, determining the maximum branch high-frequency circulating current is to determine the expression of i when the duty cycle is 0.5.

[0094] In some preferred embodiments of the present application, the expression of the maximum branch high-frequency circulating current of the cascade coupling topology is determined by the following steps:

[0095] S21, Determine the voltage U between branch 1 and branch 2 in the cascade coupling topology based on Kirchhoff's current law 12 expression.

[0096] Specifically, based on Kirchhoff's current law, U 12 It can be expressed as:

[0097]

[0098] Among them, t is the time variable, i 1 is the current flowing through the first equivalent inductor L 1 In the embodiments of the present application, for ease of explanation, L is used in the formula 1 , L 2 , R 1 Respectively represent the inductance or resistance value of their respective components.

[0099] S22, time-integrate equation (1) to obtain equation (2):

[0100]

[0101] Among them, C 1 is the integration constant.

[0102] S23, transform equation (2) into equation (3):

[0103]

[0104] in,

[0105] S24, solve equation (3) and obtain i as shown in equation (4) 1 The expression is:

[0106]

[0107] S25, using R1 , L 1 The parallel voltage equality relationship and formula (4) are combined to obtain the expression of the branch high-frequency circulating current i in the equivalent circuit model shown in formula (5):

[0108]

[0109] Among them, C 2 is the integration constant.

[0110] The specific derivation process of formula (5) is as follows:

[0111] Due to R 1 , L 1 In parallel, the voltages at both ends are equal, that is,

[0112] You can get 2 The expression is:

[0113]

[0114] Substituting it into equation (4), we can obtain equation (5).

[0115] S26, based on the branch high frequency circulation maximization condition and R 1 The current change constraint condition is determined as shown in equation (6) below:

[0116]

[0117] Wherein, T is the actual working switching period of the cascade coupling topology, i j,pre (t) is the maximum branch high-frequency circulating current of the j-th phase cascade coupling topology, j∈[1,m], C 1 Determine from (7):

[0118]

[0119] The specific derivation process of equations (6) and (7) is described below.

[0120] As analyzed above, j,pre (t) appears when the duty cycle is 0.5, that is, when the duty cycle is 0.5, C 1 , C 2 After the value of i j,pre (t) is an expression for

[0121] C 2 The calculation process is as follows:

[0122] By R 1 The current i 2It can be expressed as:

[0123]

[0124] Due to L 1 and L 2 The current flowing through R 1 The current i 2 It is also necessary to meet the current change constraint condition, that is, the current flowing through R 1 The current i 2 No mutation occurs. Under this constraint, we can deduce:

[0125]

[0126] After finishing, we get:

[0127]

[0128] C 1 The calculation process is as follows:

[0129] Since the constant part in the total current i formula only affects the current bias, and in the embodiment of the present application, the motor simulator adopts a current balancing control strategy, in steady state, the current is balanced, and the bias can be regarded as 0, so (5) can be written as follows:

[0130]

[0131] Among them C y is the constant part in (5). Since the current is balanced, the bias is 0, so C y is 0.5 times the current amplitude, and the calculation yields:

[0132]

[0133] Comparison C 1 With C y , and C 2 Substituting, we can finally get the expression of the first half cycle in (6) and (7), considering that the current waveform of the second half cycle of the branch circulation in one cycle is the offset of the current waveform of the first half cycle After inverting it, we finally get the complete expression of the maximum branch high-frequency circulating current of the cascade coupling topology when the duty cycle is 0.5.

[0134] C. Draw the hysteresis loop of the magnetic ring.

[0135] In step S3 of the present application, the hysteresis loop of the magnetic ring can be obtained by measuring the magnetic field characteristics of the magnetic ring. In some preferred embodiments, step S3 further includes the following steps:

[0136] S31, winding N on the magnetic ring 1 turns of the primary winding and N 2 turns of the secondary winding;

[0137] S32, a voltage U with constant amplitude, opposite direction and acting time equal to half of the actual working switching cycle of the magnetic ring is applied to both ends of the primary winding. 1 ;

[0138] S33, measure the primary current I of the magnetic ring 1 and secondary voltage U 2 , based on formula (8), the hysteresis loop of the magnetic ring is obtained:

[0139]

[0140] Among them, B and H are the magnetic ring under the applied voltage U 1 The magnetic flux density and magnetic field strength H,l e A is the effective magnetic path length of the magnetic ring, e is the effective cross-sectional area of ​​the magnetic ring.

[0141] Figure 7 The figure shows a physical picture of measuring the magnetic field characteristics of a magnetic ring in a specific embodiment, wherein the white Litz wire is the primary winding of the magnetic ring, and a current probe is put on the winding on this side of the wire to measure the primary current, and its two ends are connected to two half bridges. The signal board generates two sets of upper and lower bridge arm switch signals, and controls the conduction and shutdown of the two half-bridge switch devices through the drive board; the red DuPont wire is the secondary winding of the magnetic ring, and the open circuit voltage at both ends is measured by a differential voltage probe.

[0142] Primary side turns N 1 and the applied voltage U 1 The appropriate value is selected to make the magnetic ring enter the saturation state, and the secondary side turns N 2 According to the range of the differential voltage probe (number of turns N 2 Too large, the secondary induced voltage is too large, which will exceed the voltage probe range) and accuracy (number of turns N 2 Too small, the secondary induced voltage is small, the corresponding voltage when the magnetic ring enters saturation is too small, and the probe accuracy is insufficient) select a suitable value. Figure 8 In a specific embodiment, the hysteresis loop of the magnetic ring adopted by the cascade coupling topology structure of the motor simulator is drawn according to the above measurement results.

[0143] D. Use the hysteresis loop to correct the expression of the high-frequency circulating current of the maximum branch.

[0144] observe Figure 8From the hysteresis loop shown, it can be seen that the magnetic flux density of the magnetic ring changes with the magnetic field strength, showing alternating nonlinear and linear changes, and the first half cycle and the second half cycle hysteresis characteristics. In the linear region, during the calculation of the high-frequency circulating current of the maximum branch, L 1 , L 2 It is quite consistent with the impedance fitting result. Therefore, the maximum branch high-frequency circulating current calculated by formula (6) is consistent with the actual value. However, in the nonlinear region, the magnetic flux density shows a saturation characteristic with the change of magnetic field intensity. At this time, L 1 , L 2 The value of has deviated from the impedance fitting result. Obviously, if we still follow L 1 , L 2 The impedance fitting result of L will produce a great deviation from the actual value. Therefore, it is necessary to calculate the impedance fitting result of L 1 , L 2 The value of is corrected to obtain the accurate calculation result of the maximum branch high-frequency circulating current when the magnetic ring is in the nonlinear region.

[0145] In the embodiment of the present application, in order to balance calculation accuracy and calculation efficiency, in step S4, a piecewise linear function is used to fit the measured magnetic ring hysteresis loop, and in subsequent compensation, it is considered as a magnetic ring hysteresis loop to facilitate compensation algorithm calculation.

[0146] In some specific embodiments, step S4 further includes the following steps:

[0147] S41, performing piecewise fitting on the hysteresis loop to obtain a piecewise fitting function of the magnetic flux density of the cascade coupling topology, wherein the piecewise fitting function includes 1 to n linear functions corresponding to the first half of the actual working switching cycle of the cascade coupling topology, and n+1 to 2n linear functions corresponding to the second half of the actual working switching cycle of the cascade coupling topology, and each of the 1 to n linear functions and the n+1 to 2n linear functions includes a linear function corresponding to the linear interval.

[0148] In some preferred embodiments, n is an odd number not less than 3, wherein the magnetic field change curve of the magnetic field intensity H rising from the minimum value (when the current is reversed and the absolute value is maximum) to the maximum value (when the current is forward and the absolute value is maximum) is fitted to 1 to n linear functions including a linear function corresponding to the linear interval of the magnetic ring; accordingly, the magnetic field change curve of the magnetic field intensity decreasing from the maximum value to the minimum value is fitted to n+1 to 2n linear functions including a linear function corresponding to the linear interval of the magnetic ring.

[0149] Fig. 9 Shows the Figure 8The result of segmented fitting of the hysteresis loop is shown in FIG. 1 . In this embodiment, the hysteresis loop is divided into 10 segments, wherein the third segment and the eighth segment correspond to the linear intervals of the magnetic loop, respectively.

[0150] S42, based on formula (9), calculate the real-time magnetic flux density B of the cascade coupling topology real :

[0151]

[0152] Among them, ΔU is the voltage difference of the windings at both ends of the magnetic ring, N is the number of turns of the winding, and B 0 is the initial magnetic flux density.

[0153] Specifically, Figure 2 As shown, ΔU corresponds to the voltage difference between the two ends of the magnetic ring. When the duty cycle is 0.5, the initial magnetic flux density In the specific implementation process, ΔU can be considered to be equal to the bus voltage U dc One quarter of

[0154] S43, real-time magnetic flux density B based on the cascade coupling topology real and the real-time current direction to determine the number of segments k of the linear function in which the magnetic ring is located;

[0155] S44, determine L based on formula (10) 1 , L 2 The real-time correction value L in the nonlinear interval of the hysteresis loop 1,k , L 2,k :

[0156]

[0157] Among them, μ k is the slope of the kth linear function, μ linear is the slope of the linear function corresponding to the linear region;

[0158] S45, based on L 1,k , L 2,k OK j,pre (t) A real-time correction value in the nonlinear region of the hysteresis loop.

[0159] Specifically, when the magnetic ring is in the linear range (for example, Fig. 9 When the piecewise fitting function of the hysteresis loop is in the 3rd or 8th segment, L can be used directly. 1 , L 2 Substitute the impedance fitting result into formula (6) to calculate the maximum branch high-frequency circulating current. When the magnetic ring is in the nonlinear range, L is calculated by formula (10). 1 , L2 Scaling is performed to obtain its correction value L 1,k , L 2,k , and then use the above correction value to calculate the maximum branch high-frequency circulating current. Through the above correction, the calculation accuracy of the maximum branch high-frequency circulating current in the nonlinear area can be effectively improved.

[0160] [Verification Example 1]

[0161] In this embodiment, the system architecture is as follows Fig.10 As shown, it adopts a motor simulator with a 3-phase 2-branch parallel cascade coupling topology, and uses the motor simulator branch high-frequency circulating current calculation method considering nonlinear factors provided in this application to calculate the maximum branch high-frequency circulating current. Table 1 lists the experimental parameters of this verification embodiment.

[0162] Table 1 Verification Example Experimental Parameters

[0163]

[0164]

[0165] Fig.11 , 12 It shows the change of the circulating current obtained by the branch high-frequency circulating current calculation method proposed in this application in a complete working cycle under the conditions of the same bus voltage and different winding turns, and its comparison with the experimental circulating current. The orange curve in the figure is the circulating current between the a-phase 1 branches collected in the above-mentioned motor simulator cascade topology. Since the low-frequency circulating current between the branches has been eliminated by the current balancing module in the simulator control algorithm, the collected current is the high-frequency circulating current between the branches; the blue curve is the branch-to-branch circulating current curve obtained by the proposed high-frequency circulating current calculation method. The current fluctuations at the beginning of 0-2 microseconds in each figure are the interference to the current clamp acquisition data caused by the switching of the switching device. The current fluctuations after 2 microseconds are the oscillations caused by the charging and discharging of the topological structure inductance and parasitic capacitance. It can be seen that its amplitude is very small and can be ignored.

[0166] pass Fig.11 , Fig.12 It can be seen that the circulating current calculated under different numbers of winding turns is very close to the circulating current measured in the actual experiment. The peak-to-peak value difference between the calculated circulating current and the experimental circulating current of the cascade coupled topology structure with 4 turns of winding is 6.4%, and the peak-to-peak value difference between the calculated circulating current and the experimental circulating current of the cascade coupled topology structure with 6 turns of winding is 1.2%. It can be seen that the proposed calculation method has a high accuracy in predicting the circulating current in the linear working area of ​​the magnetic ring.

[0167] pass Fig.11 and Fig.12It can also be seen that the branch circulating current calculation results obtained by treating the inductor as an ideal inductor have large deviations from the calculation results of this method and the measured results, especially in the nonlinear region.

[0168] Table 2 shows the high-frequency circulating current calculation accuracy of the branch high-frequency circulating current calculation method under different bus voltages in this embodiment:

[0169] Table 2 Circulating current calculation accuracy under different bus voltages

[0170] Winding turns Bus voltage / V Calculation error / % 4 200 1.1％ 4 300 6.4％ 4 400 5.5％ 6 200 6.6％ 6 300 1.2％ 6 400 3.5％ 6 500 2.5％

[0171] Fig.13 , Fig.14 The following table shows the changes of the circulating current obtained by the branch high-frequency circulating current calculation method proposed in this application under the same number of winding turns and different bus voltage conditions in a complete working cycle and the comparison with the experimental circulating current. Fig.13 , Fig.14 It can be clearly seen that when the magnetic ring enters the nonlinear working area, the correction effect of the algorithm provided in this application is shown. When the bus voltage is 700V, the calculation error of the circulating current is 1.8%, and when the bus voltage is 850V, the calculation error of the circulating current is 5.2%. It can be seen that as the bus voltage increases, the correction effect is more obvious, and the calculation accuracy of the high-frequency circulating current is higher.

[0172] The above is a detailed introduction to the specific implementation methods of the present application. For those skilled in the art, several improvements and modifications may be made to the present application without departing from the principles of the present application. These improvements and modifications also fall within the scope of protection of the claims of the present application.

Claims

1. A method for calculating high-frequency circulating current in a motor simulator branch considering nonlinear factors, wherein the inverter of the motor simulator adopts a cascade coupling topology structure of m-phase 2 parallel branches, characterized in that: The following steps are involved: S1, establish an equivalent circuit model of the cascade coupling topology of any phase 2 parallel branches and obtain model parameters; S2, determining an expression of a maximum branch high-frequency circulating current of the cascade coupling topology based on an equivalent circuit model of the cascade coupling topology; S3, obtaining a hysteresis loop of the magnetic ring of the cascade coupling topology, wherein the hysteresis loop includes a linear interval and a nonlinear interval; S4, modifying the expression of the maximum branch high-frequency circulating current of the cascade coupling topology based on the hysteresis loop; The equivalent circuit model of the cascade coupling topology includes a first equivalent inductor L1, a second equivalent inductor L2, a parasitic resistor R1 and a parasitic capacitor C p ; The first equivalent inductor L1 is connected in parallel with the parasitic resistor R1, the first parallel end thereof is connected to the 1 branch of the cascade coupling topology, the second parallel end is connected to the first end of the second equivalent inductor L2, the second end of the second equivalent inductor L2 is connected to the 2 branch of the cascade coupling topology, and the parasitic capacitor C p connected between branch 1 and branch 2 of the cascade coupling topology; The expression of the maximum branch high-frequency circulating current of the cascade coupling topology is determined by the following steps: S21, based on Kirchhoff's current law, determine the voltage U between branch 1 and branch 2 in the cascade coupling topology as shown in equation (1): 12 The expression is: Wherein, t is the time variable, i1 is the current flowing through the first equivalent inductor L1; S22, time-integrate equation (1) to obtain equation (2): Where C1 is the integration constant; S23, transform equation (2) into equation (3): in, S24, solve equation (3) and obtain the expression of i1 as shown in equation (4): S25, using the parallel voltage equality relationship of R1 and L1 and combining equation (4), the expression of the branch high-frequency circulating current i in the equivalent circuit model is obtained as shown in equation (5): Where C2 is the integration constant; S26, based on the branch high-frequency circulating current maximization condition and the current change constraint condition of R1, determine the expression of the maximum branch high-frequency circulating current of the cascade coupling topology as shown in the following equation (6): Wherein, T is the actual working switching period of the cascade coupling topology, i j,pre (t) is the maximum branch high-frequency circulating current of the j-th phase cascade coupling topology, j∈[1,m], and C1 is determined by (7): The hysteresis loop of the magnetic ring of the cascade coupling topology is obtained by the following steps: S31, winding N1 turns of a primary winding and N2 turns of a secondary winding on the magnetic ring; S32, applying a voltage U1 with a constant amplitude, opposite direction and a duration equal to half of the actual working switching cycle of the magnetic ring to both ends of the primary winding; S33, measuring the primary side current I1 and the secondary side voltage U2 of the magnetic ring, and obtaining the hysteresis loop of the magnetic ring based on formula (8): Where B and H are the magnetic flux density and magnetic field strength H of the magnetic ring under the applied voltage U1, l e A is the effective magnetic path length of the magnetic ring, e is the effective cross-sectional area of ​​the magnetic ring.

2. The method for calculating high-frequency circulating current of a motor simulator branch considering nonlinear factors according to claim 1 is characterized in that: The branch high-frequency circulation maximization condition is: t up =t down =0.5T, where t up is the current rise time in an actual working switching cycle, t down It is the current falling time in an actual working switching cycle; The current change constraint condition of R1 is: the current flowing through R1 does not change suddenly.

3. The method for calculating high-frequency circulating current of a motor simulator branch considering nonlinear factors according to claim 1 is characterized in that: The motor simulator suppresses the branch DC bias of the cascade coupling topology through a current balancing control strategy.

4. The method for calculating high-frequency circulating current in a motor simulator branch considering nonlinear factors according to claim 1 is characterized in that: The primary side turns N1 and the applied voltage U1 are set to enable the magnetic ring to enter a saturation state, and the secondary side turns N2 is determined based on the measurement range and accuracy of the secondary side voltage U2.

5. The method for calculating high-frequency circulating current in a motor simulator branch considering nonlinear factors according to claim 1, characterized in that: The nonlinear characteristics of the magnetic ring are characterized by performing piecewise linear function fitting on the hysteresis loop of the magnetic ring.

6. The method for calculating high-frequency circulating current in a motor simulator branch considering nonlinear factors according to claim 1, characterized in that: The expression of the maximum branch high-frequency circulating current of the cascade coupling topology is modified by the following steps: S41, performing piecewise fitting on the hysteresis loop to obtain a piecewise fitting function of the magnetic flux density of the cascade coupling topology, wherein the piecewise fitting function includes 1 to n linear functions corresponding to the first half of the actual working switching cycle of the cascade coupling topology, and n+1 to 2n linear functions corresponding to the second half of the actual working switching cycle of the cascade coupling topology, and each of the 1 to n linear functions and the n+1 to 2n linear functions includes a linear function corresponding to the linear interval; S42, based on formula (9), calculate the real-time magnetic flux density B of the cascade coupling topology real : Among them, ΔU is the voltage difference of the windings wound on both ends of the magnetic ring, N is the number of turns of the winding, and B0 is the initial magnetic flux density; S43, real-time magnetic flux density B based on the cascade coupling topology real and the real-time current direction to determine the number of segments k of the linear function in which the magnetic ring is located; S44, based on formula (10), determine the real-time correction value L of L1 and L2 in the nonlinear interval of the hysteresis loop 1,k , L 2,k : Among them, μ k is the slope of the kth linear function, μ linear is the slope of the linear function corresponding to the linear region; S45, based on L 1,k , L 2,k OK j,pre (t) A real-time correction value in the nonlinear region of the hysteresis loop.

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