A real-time mutual inductance estimation method for LCL type bidirectional wireless power transmission system

Abstract

The application discloses a real-time mutual inductance estimation method of an LCL type bidirectional wireless power transmission system, and mutual inductance value is M=kL; wherein, L is an inductance value of a primary side of the LCL type bidirectional wireless power transmission system; k is a coupling coefficient, which is determined by the following formula: ω0 is a fundamental angular frequency; Zeq is a third harmonic equivalent impedance at a capacitor Cp of the primary side in the LCL type bidirectional wireless power transmission system. The real-time mutual inductance estimation method of the LCL type bidirectional wireless power transmission system is easy to implement and has high detection precision.

Description

Technical Field

[0001] This invention relates to a real-time mutual inductance estimation method for an LCL-type bidirectional wireless power transfer system. Background Technology

[0002] Bidirectional Wireless Power Transfer (WPT) systems utilize magnetic field coupling to achieve contactless power transfer. With its advantages of strong adaptability, convenience, flexibility, safety, and aesthetics, it effectively overcomes the problems of mechanical wear and leakage associated with traditional contact power supply methods. It is currently widely used in fields such as electric vehicle charging, medical electronic equipment charging, and consumer electronic equipment.

[0003] Bidirectional inductive power transfer systems have various topologies, among which the LCL topology has always been a research hotspot due to its excellent security. Existing research on wireless power transfer systems shows that the mutual inductance between the primary and secondary coils of a wireless charging system inevitably changes during application. This not only directly affects the system's maximum transfer power but also impacts the zero-voltage conduction (ZVS) of the internal switching transistors, reducing system efficiency. Therefore, mutual inductance identification is a crucial issue that cannot be ignored in inductive power transfer systems.

[0004] Currently, the main method for mutual inductance estimation is the steady-state method, which can be broadly divided into two types: those near the fundamental frequency band and those utilizing higher harmonics. Methods at the fundamental frequency band use fundamental frequency data for analysis. Because the fundamental frequency is easy to measure and requires no additional data processing, fundamental frequency methods generally have high accuracy. However, because the fundamental frequency estimation method operates at a frequency offset from the resonant frequency, it often cannot transmit energy simultaneously with mutual inductance estimation. Methods utilizing harmonics, on the other hand, use the inherent harmonic information generated by a single-phase full-bridge circuit to estimate mutual inductance, since the system's operating frequency is still the resonant frequency of the main circuit. This allows for estimation of mutual inductance typically simultaneously with energy transmission.

[0005] Harmonic estimation methods can achieve real-time mutual inductance estimation, and therefore have attracted much attention in practical applications. However, current harmonic estimation methods also have many problems, such as large current distortion, which affects the power transmission quality of the system; or too much measurement data, which introduces too much measurement error and increases circuit cost, etc.

[0006] Therefore, it is necessary to design a real-time mutual inductance estimation method for an LCL-type bidirectional wireless power transfer system. Summary of the Invention

[0007] The technical problem to be solved by the present invention is to provide a real-time mutual inductance estimation method for an LCL-type bidirectional wireless power transmission system. This real-time mutual inductance estimation method for an LCL-type bidirectional wireless power transmission system is easy to implement and has high accuracy.

[0008] The technical solution of the invention is as follows:

[0009] A real-time mutual inductance estimation method for an LCL-type bidirectional wireless power transfer system, wherein the estimated mutual inductance value is M = kL;

[0010] Where: L is the inductance value of the primary side of the LCL type bidirectional wireless power transfer system;

[0011] k is the coupling coefficient, determined by the following formula.

[0012] ω0 is the fundamental angular frequency;

[0013] Zeq is the third harmonic equivalent impedance at the primary-side capacitor Cp in an LCL-type bidirectional wireless power transfer system. The calculation formula is as follows:

[0014] V 13 For the input voltage, I 13 I1 is the input current; I1 is the current flowing through the coupling coil on the primary side, and V1 is the voltage of the coupling coil on the primary side. 13 V represents the third harmonic component of the current I1. 13 This represents the third harmonic component of voltage V1.

[0015] The primary and secondary parameters of the main circuit of the LCL-type bidirectional wireless power transfer system must be consistent, and the capacitance and inductance values ​​of the primary and secondary sides must be equal.

[0016] Beneficial effects:

[0017] This invention, based on third harmonic analysis, proposes a real-time mutual inductance estimation method for an LCL-type bidirectional wireless power transfer system. This method allows for high-precision mutual inductance estimation during energy transfer and real-time detection of coil coupling. Compared to existing methods, the proposed method offers higher measurement accuracy, requires fewer measurements, has lower cost, and is easier to implement. (After obtaining the mutual inductance, the system can achieve a wider range of zero-voltage switching (ZVS), enabling high-efficiency energy transfer. It can also promptly cut off the energy transfer channel in case of circuit abnormalities, improving system safety. Parameter identification accuracy can reach over 99%.)

[0018] After obtaining mutual inductance, the system can achieve a wider range of zero-voltage switching (ZVS), enabling high-efficiency energy transfer. It can also promptly cut off the energy transfer channel in case of circuit abnormalities, improving system safety. Parameter identification accuracy can reach over 99%. Attached Figure Description

[0019] Figure 1 This is a typical topology diagram of an LCL-type system;

[0020] Figure 2 To simplify the circuit diagram of the LCL type system;

[0021] Figure 3 The equivalent circuit diagram of the third harmonic circuit;

[0022] Figure 4 This is a schematic diagram showing the relationship between the input impedance magnitude and the coupling coefficient.

[0023] Figure 5 This is a schematic diagram illustrating the implementation of the mutual inductance estimation method.

[0024] Figure 6 This is a simulation model diagram;

[0025] Figure 7 This is a simulation waveform diagram;

[0026] Figure 8 This is a curve showing the accuracy of mutual inductance estimation. Detailed Implementation

[0027] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments:

[0028] Example 1:

[0029] The LCL-type bidirectional inductive power transmission system has attracted much attention due to its advantages in transmission stability and security. Figure 1 A typical topology diagram of an LCL-type BWPT system is given.

[0030] In a two-way wireless power transfer system, the primary and secondary sides have almost identical structures. When power is transferred in the reverse direction, the original energy receiving side becomes the energy output side. In the diagram, E1 and E2 are the corresponding DC voltage sources for the primary and secondary sides. When energy flows in the forward direction, the primary DC voltage source E1 provides the power output for the entire system, and the secondary DC voltage source E2 receives the power output from E1. S1-S4 constitute a high-frequency inverter circuit, whose AC output voltage is V. p S5-S8 constitute a high-frequency fully controlled rectifier circuit, and its AC side voltage is V. s Primary coils L1 and L P With resonant capacitor C P The primary side main circuit is formed, and the secondary side coils L2 and L3 are also included. s and resonant capacitor C s This forms the main circuit on the secondary side.

[0031] Since the H-bridges on both the primary and secondary sides of the bidirectional system use fully controlled switching transistors, their output AC voltage is completely controllable. An AC voltage source can be used to replace the AC voltage portion of both H-bridges. The equivalent circuit diagram is shown below. Figure 2However, pay attention to V. p With V s It is a rectangular wave that contains harmonic components.

[0032] System Modeling and Mutual Inductance Estimation Methods

[0033] In mutual inductance estimation, the influence of the load on circuit information is a major challenge. This section will briefly describe the principle of phase-shift control, analyze how to use phase-shift control to eliminate the influence of the load on mutual inductance estimation, and discuss the selection and analysis of subsequent data.

[0034] Third Harmonic Circuit Analysis

[0035] Phase-shift control is simple to operate and requires no additional DC / DC device, making it widely used in bidirectional inductive power transfer systems. In this system, both the primary and secondary full-bridge circuits use phase-shift control. The upper and lower switches of a single bridge arm are complementary, and their duty cycles are constant at 50%. The fully controlled bridge has a left and a right bridge arm; the phase shift in phase-shift control changes the phase angle difference between the left and right arms. Under non-full load conditions, the system's output waveform is a sawtooth wave. Fourier analysis of this waveform yields:

[0036]

[0037] in Phase shift angle Specifically, this manifests as the phase angle difference between the drive signals of the left and right bridge arms within the same full-bridge configuration. As can be seen from Equation 1, the output rectangular wave voltage inevitably carries higher harmonics beyond the fundamental frequency; this is determined by the characteristics of phase-shift control. However, by controlling the phase-shift angle, specific harmonics can be maximized or eliminated.

[0038] The voltage waveform output by a fully controlled full-bridge circuit is a rectangular wave, which, after Fourier analysis, can be equivalent to the superposition of the fundamental wave and other odd harmonics. Treating the AC voltage as the excitation, the AC current is the response generated by the fundamental and odd harmonic voltages; the AC current on either side contains both fundamental and odd harmonic components. Furthermore, because the mutual inductance of the coils connects the circuits on both sides, the AC current on either side is determined by the AC voltages on both sides.

[0039] The coils and capacitors in the main circuit are all linear components. The voltage waveform output by the fully controlled H-bridge is only related to the phase shift angle. Under steady state, it can be regarded as an independent AC voltage source, satisfying the conditions for using the superposition theorem. At this time, using the superposition theorem, the system circuit can be divided into the fundamental circuit and the circuits of each harmonic. The actual current is the sum of the excitation current generated in each harmonic circuit.

[0040] Both the fundamental frequency and the third harmonic contain mutual inductance information. However, the fundamental frequency is the channel for energy transmission. When the system is working, the energy transmission of the system is controlled by controlling the fundamental frequency component. It is relatively difficult to use the fundamental frequency information to estimate mutual inductance. The harmonic voltage and current components are low and hardly participate in energy transmission. This extra degree of freedom can be used to estimate mutual inductance.

[0041] The phase shift angle can be derived from Formula 1. When the phase shift angle is 120°, the output AC sawtooth voltage does not contain the third harmonic. Let the phase shift angle of the secondary-side full bridge be fixed at 120°, while the phase shift angle of the primary-side full bridge is unrestricted. At this point, the third harmonic circuit model of the system is as follows: Figure 3 As shown. I 13 The third harmonic component represents the current I1, and the rest are analogous. Applying Kirchhoff's voltage and current laws to the mutually inductant circuit in the diagram, we can obtain:

[0042]

[0043] In this invention, the system utilizes the fundamental component to transmit energy, and mutual inductance is estimated by analyzing the third harmonic voltage and current. The analysis of the fundamental component will not be elaborated upon here; the analysis of the third harmonic circuit will be the focus of subsequent analyses.

[0044] Mutual Inductance Estimation Method Based on Third Harmonic Circuit

[0045] System parameters are determined during the system design phase; therefore, in mutual inductance estimation, the main circuit parameters are generally assumed to be known, such as the coil inductance, compensation capacitor capacitance, and line impedance. Furthermore, bidirectional inductive power transmission systems are typically symmetrical systems, with the primary and secondary circuit parameters being identical.

[0046] C P =C S =C,L1=L2=L P =L S =L

[0047]

[0048] In the formula, w0 is the resonant angular frequency of the main circuit of the system.

[0049] In summary, we can calculate the value from capacitor C. P The third harmonic input impedance on the other side has an input voltage of V. 13 The input current is I 13 The specific third harmonic input impedance is shown in equation (3).

[0050]

[0051] In the formula, k = M / L, k is the coupling coefficient, and M is the mutual inductance. The estimation process of mutual inductance can be equivalent to the estimation process of coupling coefficient, which is beneficial to subsequent analysis.

[0052] The equations listed are complex equations. If the magnitude and phase angle of the input impedance are known, they can be directly substituted into the formula to solve for the coupling coefficient. However, solving for the phase angle not only places higher demands on the current sensor of the experimental platform but also increases the complexity of the program. Using only the magnitude to solve for the coupling coefficient is a more easily implemented method, as shown in Figure 4:

[0053]

[0054] It can be seen that the formula has an absolute value sign at this time, which can further simplify the formula. First is |Zeq|, which is a measured value, and its value is the amplitude of the third harmonic voltage of the coil divided by the amplitude of the third harmonic current; second is the absolute value sign of the numerator in the square root, which needs to be specifically divided according to the coupling coefficient of the system. The bidirectional magnetic induction wireless charging system is a loosely coupled system. Generally speaking, the coupling coefficient is less than 0.5. The coil with a high coupling coefficient is called a transformer. Substituting the possible range of the coupling coefficient in the system back into the relationship of the input impedance, it can be concluded that the phase angle of the input impedance is positive, and 2.14jω0L≤|Zeq|≤3ω0L. Now, let's analyze the absolute value problem of equation (4). The second absolute value in the square root only needs to be positive. The final estimation algorithm is shown in equation 5:

[0055]

[0056] In practice, each coupling coefficient k has a corresponding |Zeq|, but each |Zeq| does not correspond to only one k value. See the formula for details. Figure 4 When the coupling coefficient is too large, the amplitude of Zeq will drop to zero and then gradually increase. Near this zero point, the amplitude of one Zeq will correspond to two coupling coefficients. This is also the problem of the absolute value sign in formula (4). It can be seen that the formula and the graph correspond to each other. However, as mentioned above, the coupling coefficient of a loosely coupled system will not be too large. Within the actual coupling range, |Zeq| and the coupling coefficient k are in one-to-one correspondence.

[0057] In practical applications, the third harmonic input impedance |Zeq| is obtained by measuring the amplitudes of the harmonic voltage and current. Substituting |Zeq| back into equation (5) yields the coupling coefficient k of the system. The specific system flow is as follows: Figure 5 As shown.

[0058] It can be seen that the entire system has a simple structure and the operation process is easy to implement.

[0059] Simulation verification

[0060] To verify the accuracy of the proposed mutual inductance estimation method for a two-way wireless power transfer system based on third harmonics, refer to Figure 1 Establish a simulation model of the LCL-type system on the Matlab / Simulink platform, such as Figure 5 As shown in the table, the system parameters are as follows. The main modules used in the mutual inductance estimation algorithm are the Fast Fourier Analysis module and the M-Function module. The Fourier Analysis module is used to measure the third harmonic component in the system voltage and current, while the M-Function module is used to solve formula (5). In addition, the system also uses a delay module to achieve real-time control of the phase shift angle. The specific simulation model diagram is shown below. Figure 6 As described, the waveform is as follows Figure 7 As shown.

[0061] Table 1 System Parameters

[0062]

[0063]

[0064] Table 2. Mutual Inductance Estimation Results

[0065]

[0066] In the mutual inductance estimation simulation, the mutual inductance of the coupled system is changed by altering the system's parameters, while other parameters remain constant throughout the process. Specific experimental results are as follows: Figure 8 As shown, the mutual inductance identification model can achieve accurate mutual inductance estimation even under conditions of large mutual inductance variations. The maximum error occurs at a coupling coefficient of 0.25, where the mutual inductance value is 7.5 mH, and the relative error is 0.45%. In the simulation, measurement fluctuations caused by the Fourier analysis module may introduce errors, as may the internal resistance of passive components. However, overall, the estimation method proposed in this invention has high accuracy, is simple to implement, and has high practical value.

[0067] Conclusion: In current methods for estimating mutual inductance using harmonics, some models employing the least squares method result in excessive computational load and long calculation times. This invention, employing a steady-state equation calculation strategy, significantly reduces data processing time and enables rapid estimation. Other methods require numerous detection parameters, introducing excessive measurement errors and increasing hardware costs. This invention requires only two measurements, offering high accuracy and ease of implementation. Some methods introduce subharmonics to detect mutual inductance, leading to significant system current distortion and affecting power transmission quality. This invention utilizes the system's inherent third harmonic, resulting in minimal current distortion and high power transmission quality. In summary, this invention proposes a real-time mutual inductance estimation method for bidirectional wireless power transmission systems based on harmonics. This method offers high accuracy, simple calculations, requires few measurements, is easy to implement, and does not affect power transmission quality, making it valuable for practical applications.

Claims

1. A real-time mutual inductance estimation method for an LCL-type bidirectional wireless power transfer system, characterized in that, Both the primary and secondary full-bridge circuits use phase-shift control, with the upper and lower switches of a single bridge arm being complementary and having a constant duty cycle of 50%. Under non-full load conditions, the system's output waveform is a sawtooth wave. Fourier analysis of this waveform yields: (1)； Where φ is the phase shift angle, 0 < φ < 180, which specifically represents the phase angle difference between the drive signals of the left and right bridge arms within the same full bridge; E1 is the primary side DC voltage source; Let the phase shift angle of the secondary side of the full bridge be fixed at 120°, while the phase shift angle of the primary side of the full bridge is not limited; apply Kirchhoff's voltage and current laws to the mutual inductance circuit of the LCL type bidirectional wireless power transfer system, and obtain: (2)； The bidirectional inductive power transfer system is a symmetrical system, with primary coils L1 and L2. P With resonant capacitor C P The primary side main circuit is formed, and the secondary side coils L2 and L3 form the secondary side main circuit. s and resonant capacitor C s The main circuit that constitutes the secondary side has the same primary side parameters as the secondary side parameters: ； ； In the formula, w0 is the resonant angular frequency of the main circuit of the system; L is the inductance value of the primary side of the LCL type bidirectional wireless power transfer system; Find the capacitance C P The third harmonic input impedance on the other side has an input voltage of V. 13 The input current is I 13 The third harmonic input impedance is (3)； In the formula, k = M / L, k is the coupling coefficient, M is the mutual inductance, and the estimation process of the mutual inductance is equivalent to the estimation process of the coupling coefficient. Current I1 is the current flowing through the coupling coil on the primary side, and voltage V1 is the voltage of the coupling coil on the primary side; 13 V represents the third harmonic component of the current I1. 13 The third harmonic component represents voltage V1; The coupling coefficient is solved using the magnitude. (4); Further simplification yields the following final estimation algorithm: (5)； The mutual inductance value is M=kL.

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