Multi-strand litz wire beam splitting compensation method for wireless power transmission system

Abstract

The invention discloses a multi-strand litz wire beam splitting compensation method for a wireless power transmission system, and belongs to the technical field of wireless power transmission. The invention aims to solve the problem of non-uniform current distribution caused by unequal inter-beam coupling coefficients of multiple strands of litz wires in electric energy transmission. Comprising the following steps: connecting a compensation capacitor in series on each beam of Litz wire branch of a primary side coil and a secondary side coil of the loosely coupled transformer to obtain compensated branches, and then connecting all the compensated branches in parallel; and the compensation capacitance value of each compensation capacitor is selected, so that branch currents after compensation of all primary sides are equal, branch currents after compensation of all secondary sides are equal, and current sharing of each beam is realized. The Litz wire loss compensation method is used for Litz wire loss compensation.

Description

Technical Field

[0001] The invention relates to a multi-strand Litz wire beam splitting compensation method for a wireless power transmission system, belonging to the technical field of wireless power transmission. Background Art

[0002] With the continuous advancement of power electronics technology, the production, transmission and use of electric energy are undergoing rapid changes. Wireless power transfer (WPT) technology can transmit electric energy without using cables. It has the advantages of convenience, safety, low maintenance cost and strong environmental adaptability. It has broad application prospects in electric vehicles, consumer electronics, medical equipment, underwater operations and other fields. Inductive wireless power transmission is relatively mature and widely used. It relies on the law of electromagnetic induction and uses loosely coupled transformers (LCT) as energy transmission carriers. The operating frequency mainly refers to the Qi standard (medium and low frequency, tens to hundreds of kHz) and the ISM standard (high frequency, 6.78MHz or 13.56MHz, etc.). In the application fields of electric vehicles and rail transit, the power of the WPT system is very large, up to tens of kW or even MW. Therefore, in order to increase the power capacity of the system, the coil of the LCT needs to be wound with thick wires to carry larger currents.

[0003] In the field of medium and low frequency WPT, the traditional method is to use a single thick Litz wire, which is often made of multiple strands of mutually insulated thin wires twisted together multiple times. In fact, it is difficult to achieve complete twisting of the twisted structure. Common Litz wire twisting includes two-level (strand, bundle) or three-level (strand, sub-bundle, bundle) structure. For the convenience of analysis, strands, sub-bundles, and bundles are collectively referred to as branches. The twisting methods are divided into complete twisting and bundle twisting. Complete twisting often requires the number of branches to be ≤6, so that the branches can be completely transposed in both radial and axial directions, and the coupling coefficients between each branch are equal and close to 1. Twisting with seven branches or more belongs to bundle twisting, because the branch at the center of the twist does not participate in the twisting, so the overall difference in coupling between the central branch and other branches is relatively large. In addition, due to the limitations of the process level, even if the number of branches is ≤6, it is difficult to achieve truly complete transposition. Therefore, the current distribution inside the Litz wire is actually uneven, especially for branches with large overall coupling differences.

[0004] High-power WPT systems require thicker Litz wires, of which seven-bundle twisting is more common and belongs to bundle twisting. The coupling coefficients between bundles are not equal, and there is uneven current. And because the thick Litz wire has a larger cross-sectional area and more insulated thin wires, it is more difficult to completely twist, so the uneven current distribution effect is more serious, reducing the power capacity and efficiency of the coil. Summary of the invention

[0005] Aiming at the problem that the coupling coefficients between multiple strands of Litz wires are unequal during power transmission, resulting in uneven current distribution, the present invention provides a beam splitting compensation method for multiple strands of Litz wires in a wireless power transmission system.

[0006] A method for compensating for splitting multiple Litz wires in a wireless power transmission system of the present invention comprises:

[0007] A compensation capacitor is connected in series to each Litz wire branch of the primary coil and the secondary coil of the loosely coupled transformer to obtain a compensated branch, and then all the compensated branches are connected in parallel;

[0008] The compensation capacitance value of each compensation capacitor is selected so that the currents of all primary-side compensated branches are equal and the currents of all secondary-side compensated branches are equal, so as to achieve current balancing of each beam.

[0009] According to the wireless power transmission system multi-strand Litz wire splitting compensation method of the present invention, the method of selecting the compensation capacitance value of each compensation capacitor includes:

[0010] According to Kirchhoff's law, the relationship between the voltage and current of the compensated branch and the self-inductance and mutual inductance of the coil is established, and the equation for making the branch currents equal is obtained based on the relationship, so as to calculate the compensation capacitance value.

[0011] According to the wireless power transmission system multi-strand Litz wire beam splitting compensation method of the present invention, the relationship between the voltage and current of the compensated branch of the primary coil of the loosely coupled transformer and the coil self-inductance and coil mutual inductance is:

[0012]

[0013] In the formula is the inverter output voltage vector, ω is the system resonant angular frequency, L pn is the inductance of the nth Litz wire branch of the primary coil of the loosely coupled transformer, C pn The compensation capacitor connected in series with the nth Litz wire branch of the primary coil of the loosely coupled transformer, is the current vector of the nth compensated branch of the primary coil of the loosely coupled transformer, M p1n is the mutual inductance between the first and nth Litz wire branches of the primary coil of the loosely coupled transformer, M p(n-1)n is the mutual inductance between the n-1th Litz wire branch and the nth Litz wire branch of the primary coil of the loosely coupled transformer, M pnsn is the mutual inductance between the nth Litz wire branch of the primary coil and the nth Litz wire branch of the secondary coil of the loosely coupled transformer, is the current vector of the nth compensated branch of the secondary coil of the loosely coupled transformer.

[0014] According to the wireless power transmission system multi-strand Litz wire beam splitting compensation method of the present invention, the equation for equal branch currents after primary compensation of the loosely coupled transformer is:

[0015]

[0016] Where M pn It is the sum of the mutual inductance of the nth Litz wire branch and the other n-1 Litz wire branches of the primary coil of the loosely coupled transformer.

[0017] According to the wireless power transmission system multi-strand Litz wire splitting compensation method of the present invention, based on the equation that the branch currents are equal after the primary compensation of the loosely coupled transformer, C is obtained. pn for:

[0018]

[0019] According to the wireless power transmission system multi-strand Litz wire beam splitting compensation method of the present invention, based on the same calculation method as the primary coil of the loosely coupled transformer, the compensation capacitor C connected in series with the nth Litz wire branch of the secondary coil of the loosely coupled transformer is obtained. sn :

[0020]

[0021] Where L sn is the inductance of the nth Litz wire branch of the secondary coil of the loosely coupled transformer, M sn It is the sum of the mutual inductance of the nth Litz wire branch and the other n-1 Litz wire branches of the secondary coil of the loosely coupled transformer.

[0022] According to the multi-strand Litz wire beam splitting compensation method of the wireless power transmission system of the present invention, the primary coil and the secondary coil of the loosely coupled transformer are both formed by winding 7 bundles×172 strands of Litz wire.

[0023] Beneficial effects of the present invention: The method of the present invention performs beam splitting and parallel compensation on the Litz wire, which changes the connection mode between the traditional compensation capacitor and the Litz wire. No additional devices are required, and it is not limited by the type of compensation topology. It is simple, reliable and practical. In addition, the method of the present invention is not restricted by the manufacturing process. The Litz wire does not need to be completely twisted to achieve current sharing in each beam, thereby improving the system efficiency, and does not change the original output characteristics of the system. In addition to the WPT field, the method of the present invention is also expected to be applied to other power electronics fields such as LLC resonant converters and dual active bridge converters (Dual Active Bridge, DAB), and has a wide range of applications. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 Schematic diagram of the principle of the multi-strand Litz wire splitting compensation method of the wireless power transmission system of the present invention; V inis the DC input voltage, C in is the DC input filter capacitor, Q 1 -Q 4 are the four switch tubes of the full-bridge inverter, U in is the inverter output voltage, I in is the inverter output current, I pn is the current of the nth compensated branch of the primary coil of the loosely coupled transformer, I sn is the current of the nth compensated branch of the secondary coil of the loosely coupled transformer, M ps is the mutual inductance parameter matrix of the primary and secondary coils of the loosely coupled transformer, I re is the rectifier input current, U re is the rectifier input voltage, T 1 -T 4 are the four switching tubes of the synchronous rectifier, C F is the DC output filter capacitor, R L is the load resistance, U o is the load voltage;

[0025] Figure 2 yes Figure 1 Schematic diagram of the controlled source equivalent model; R E is the load fundamental equivalent input impedance,

[0026] Figure 3 It is a schematic diagram of the cross section of 5 bundles of three-level structure Litz wires;

[0027] Figure 4 It is a schematic diagram of the cross section of 7 bundles of two-stage structure Litz wires;

[0028] Figure 5 It is a schematic diagram of the traditional method of compensating the Litz wire loss in wireless power transmission systems;

[0029] Figure 6 is based on Figure 5 Schematic diagram of loosely coupled transformer; D 0 is the outer length of the loosely coupled transformer, D 1 is the inner side length of the loosely coupled transformer, and d is the transmission distance of the loosely coupled transformer;

[0030] Figure 7 This is the theoretical waveform of the primary coil current of the traditional method;

[0031] Figure 8 This is the theoretical waveform of the secondary coil current of the traditional method;

[0032] Fig. 9 is the variation diagram of beam coefficient of the traditional method after random perturbation of Litz wire parameters;

[0033] Fig.10 It is the variation diagram of coefficient of variation of the traditional method after random perturbation of Litz wire parameters;

[0034] Fig.11 It is a theoretical waveform diagram of the primary coil current using the method of the present invention;

[0035] Fig.12 It is a theoretical waveform diagram of the secondary coil current using the method of the present invention;

[0036] Fig.13 is a graph showing the variation of beam current coefficient after random disturbance of Litz wire parameters by the method of the present invention;

[0037] Fig.14 It is a graph showing the change of the current coefficient after the litz wire parameters are randomly disturbed by the method of the present invention;

[0038] Fig.15 is a schematic diagram of a prototype of a wireless power transmission system obtained based on the method of the present invention;

[0039] Fig.16 is based on Fig.15 The curve diagram of output current and efficiency as a function of load obtained by experiment on the prototype; the new type in the figure represents the method of the present invention;

[0040] Fig.17 The voltage and current experimental waveforms of the inverter and rectifier obtained based on the traditional method and the method of the present invention; in Equivalent to Figure 5 Middle I in 、u in Equivalent to Figure 5 Middle U in 、i re Equivalent to Figure 5 Middle I re 、u re Equivalent to Figure 5 Middle U re ;

[0041] Fig.18 This is the experimental waveform of the primary coil current of the traditional method;

[0042] Fig.19 This is the secondary coil current experimental waveform of the traditional method;

[0043] Fig. 20 It is the primary coil current experimental waveform diagram of the method of the present invention;

[0044] Fig.21 It is the secondary coil current experimental waveform diagram of the method of the present invention. DETAILED DESCRIPTION

[0045] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0046] It should be noted that, in the absence of conflict, the embodiments of the present invention and the features in the embodiments may be combined with each other.

[0047] The present invention will be further described below in conjunction with the accompanying drawings, but is not intended to be a limitation of the present invention.

[0048] Combination Figures 1 to 4 As shown, the present invention provides a method for beam splitting and compensating multiple Litz wires in a wireless power transmission system, comprising:

[0049] A compensation capacitor is connected in series to each Litz wire branch of the primary coil and the secondary coil of the loosely coupled transformer to obtain a compensated branch, and then all the compensated branches are connected in parallel;

[0050] The compensation capacitance value of each compensation capacitor is selected so that the currents of all primary-side compensated branches are equal and the currents of all secondary-side compensated branches are equal, so as to achieve current balancing of each beam.

[0051] This embodiment divides the capacitor of the transmitting coil or receiving coil in series in the traditional method into multiple capacitors, which are connected in series with each bundle of Litz wire respectively and then connected in parallel as a whole. After beam splitting compensation, the current waveforms of each bundle on the original secondary side roughly overlap, and current equalization can be achieved for each bundle.

[0052] Figure 3 and Figure 4 As shown, the current in a certain bundle of Litz wires is mainly determined by the sum of the mutual inductances between the bundle and other bundles. The currents of each bundle are not equal. The branch with a larger sum of self-inductance and mutual inductance has a smaller current, which will lead to reduced transmission efficiency.

[0053] The circuit schematic diagram of the n-beam WPT system based on beam splitting compensation in this embodiment is as follows: Figure 1 As shown, each beam is connected in series with a compensation capacitor. Figure 2 for Figure 1 The controlled source equivalent model. Figure 2 The mutual inductance between the primary coil and the secondary coil bundle i and j is expressed as M pij and M sij , i = 1 ~ n, j = 1 ~ n and i ≠ j; the mutual inductance between the primary and secondary sides is expressed as M pisj , i=1~n, j=1~n and i≠j.

[0054] Further, the method of selecting the compensation capacitance value of each compensation capacitor includes:

[0055] According to Kirchhoff's law, the relationship between the voltage and current of the compensated branch and the self-inductance and mutual inductance of the coil is established, and the equation for making the branch currents equal is obtained based on the relationship, so as to calculate the compensation capacitance value.

[0056] In this implementation, taking the primary coil as an example, the relationship between the voltage and current of the compensated branch of the primary coil of the loosely coupled transformer and the coil self-inductance and coil mutual inductance is:

[0057]

[0058] In the formula is the inverter output voltage vector, ω is the system resonant angular frequency, L pn is the inductance of the nth Litz wire branch of the primary coil of the loosely coupled transformer, C pn The compensation capacitor connected in series with the nth Litz wire branch of the primary coil of the loosely coupled transformer, is the current vector of the nth compensated branch of the primary coil of the loosely coupled transformer, M p1n is the mutual inductance between the first and nth Litz wire branches of the primary coil of the loosely coupled transformer, M p(n-1)n is the mutual inductance between the n-1th Litz wire branch and the nth Litz wire branch of the primary coil of the loosely coupled transformer, M pnsn is the mutual inductance between the nth Litz wire branch of the primary coil and the nth Litz wire branch of the secondary coil of the loosely coupled transformer, is the current vector of the nth compensated branch of the secondary coil of the loosely coupled transformer.

[0059] The beam splitting compensation method of this embodiment aims to make the currents of each beam branch on the primary side equal, that is, Since the coupling coefficient of the primary and secondary coils is low, the mutual inductance is small, and the mutual inductance of each beam on the primary side and each beam on the secondary side is not much different, that is, it can be considered that M pisi They are basically equal, so the current sharing condition of the beam-splitting compensation topology can be achieved.

[0060] The equation for equal branch currents after primary compensation of loosely coupled transformer is:

[0061]

[0062] Where M pn It is the sum of the mutual inductance of the nth Litz wire branch and the other n-1 Litz wire branches of the primary coil of the loosely coupled transformer.

[0063] The mutual inductance between the primary and secondary sides can transmit active power, while the mutual inductance between the primary beams will generate reactive power. Similar to the traditional method, the reactive component needs to be fully compensated to achieve the zero phase angle (ZPA) and constant current output (CCO) characteristics of the WPT system. Therefore, the compensation capacitor C of the i-th beam of the primary side pi It is necessary to compensate for the self-inductance L pi The sum of the mutual inductance of this beam and other beams on the primary side M pi The same is true for the secondary side, and the capacitance value of the beam compensation can be obtained.

[0064] Furthermore, based on the equation that the branch currents are equal after primary compensation of the loosely coupled transformer, we can get C pn for:

[0065]

[0066] Similarly, based on the same calculation method as the primary coil of the loosely coupled transformer, the compensation capacitor C connected in series with the nth Litz wire branch of the secondary coil of the loosely coupled transformer is obtained. sn :

[0067]

[0068] Where L sn is the inductance of the nth Litz wire branch of the secondary coil of the loosely coupled transformer, M sn It is the sum of the mutual inductance of the nth Litz wire branch and the other n-1 Litz wire branches of the secondary coil of the loosely coupled transformer.

[0069] When M pisj are equal, the ZPA and CCO characteristics of the system remain unchanged.

[0070] As an example, the primary coil and the secondary coil of the loosely coupled transformer are both formed by winding 7 bundles×172 strands of Litz wire.

[0071] Below, in order to verify the effect of the present invention, a comparative experiment is carried out:

[0072] Combination Figure 5 As shown in the figure, the traditional compensation method is analyzed. Taking the original side as an example, U p The sum of the self-inductance voltage of each bundle of inductance on the primary side and the mutual inductance voltage of other bundles on the primary side, the current of each bundle of coils can be calculated. ps Compared with the mutual inductance between the Litz wire bundles, it can be considered that the mutual inductance M between the primary and secondary bundles is pisj Basically the same, and then we can get U p All elements within are equal.

[0073] From the characteristics of the circuit equation, we can see that equation U p =jωL p I p It has a unique solution, that is, after knowing the self-inductance and mutual inductance parameters of each bundle, the current of each bundle can be solved. Therefore, the current sharing solution can be brought into the equation to reverse the current sharing conditions of each bundle of the primary coil:

[0074]

[0075] Similarly, the condition for equal current of each bundle of the secondary coil can be obtained, where M pi and M si Respectively represent the sum of the mutual inductance of the i-th bundle of the primary and secondary sides and other bundles in the Litz wire:

[0076]

[0077] Taking the primary side as an example, when formula (1) is not true, the currents of each beam are not equal, and the branch with a larger sum of self-inductance and mutual inductance has a smaller current. loss Higher, this will reduce the transmission efficiency.

[0078] In addition, the mutual inductance matrix L of the primary coil inductance p Under certain values ​​of , the solution of a beam current may be less than 0, that is, some beam currents are in antiphase with other beams, which is more serious. Because under certain working conditions, the sum of the currents of each beam of the coil is certain. The anti-phase current will increase the amplitude of the current of other beams, and the magnetic field generated will also cancel each other with other beams, which is not conducive to energy transmission, increases the loss of the coil, and seriously reduces the transmission efficiency of the coil.

[0079] Due to L p The inverse matrix of is too complex and the analytical solution of each beam current is difficult to express. Figure 3 and Figure 4 The analysis of the Litz wire stranding structure shows that L p With certain characteristics, the following takes seven bundles of Litz wire as an example, combined with Figure 6 As shown, the current conditions of each beam on the primary side are analyzed and discussed.

[0080] From the twisted structure of 7 bundles of Litz wire, it can be seen that under ideal conditions, the inductance mutual inductance matrix L p The elements of satisfy:

[0081]

[0082] Combining formula (1) we can get:

[0083]

[0084] According to the above analysis, we know that the third beam current I p3The amplitude is small, and the other six beams have equal current. And calculation can get I p3 The condition of anti-phase with other beam currents:

[0085]

[0086] In order to intuitively reflect the above uneven current effect, specific coil parameters are given and the above conclusions are verified by numerical calculation. Since the finite element simulation of Litz wire is too complicated and it is difficult to reflect the incomplete twisting caused by the process error of Litz wire, the experimental measurement parameters are directly used for numerical calculation. The coil used in the experiment is as follows Figure 6 The specific parameters are shown in Table 1. The primary and secondary sides are both wound into square coils with 7 bundles × 172 strands of Litz wire, with an outer diameter of 21.5 cm, an inner diameter of 10 cm, and a transmission distance of 9.5 cm. The specific numbers of each bundle of Litz wire on the primary and secondary sides are as follows Figure 4 As shown, the third beam always remains in the center position.

[0087] Table 1 Electrical parameters of WPT system prototype

[0088]

[0089]

[0090] Compensation capacitor C p and C s The values ​​are 69.72nF and 69.95nF respectively. After considering ESR, according to the parameters in Table 1, the current waveforms of each beam of the primary and secondary coils can be obtained, as shown in Figure 7 and Figure 8 As shown. The currents of each beam are obviously uneven, and the current of the third beam is opposite to the currents of the other beams. By calculation, the loss of the coil is 61.27W.

[0091] Incomplete twisting of Litz wires results in inconsistent mutual inductance between bundles, which in turn leads to uneven current in each bundle. In order to analyze the sensitivity of Litz wire parameters to current distribution, the self-inductance and mutual inductance parameters of each bundle in Table 1 are averaged to ensure that the inductance of each bundle and the mutual inductance between bundles are completely consistent, which are 40.82μH and 39.03μH respectively. At this time, each bundle has an equal current. p12 Reduced by 1%, it can be calculated that the first beam current i p1 and the second beam current i p2 is equal to the current of other beams and is 1.28 times that of other beams, that is, the current is seriously uneven, indicating that the Litz wire parameters are sensitive to the current distribution.

[0092] There are 28 parameters of self-inductance and mutual inductance of seven bundles of Litz wires, which is too complicated. In order to more comprehensively analyze the sensitivity of Litz wire parameters to current distribution, a numerical calculation method of random sampling and statistical analysis is adopted. That is, Monte Carlo simulation is used to apply random perturbations within the range of ±1% to the self-inductance and mutual inductance parameters. p1 Take the example to observe the amplitude change, such as Fig. 9 and Fig.10 shown.

[0093] Definition p1 The amplitude and i p1 ~i p7 The ratio of the total is the beam current coefficient. In the case of equal current, the coefficient is 0.1428. When the beam current coefficient is less than 0, it means that the current is reversed. p1 ~i p7 The ratio of the standard deviation to its mean value is the coefficient of variation. In the case of uniform current, the coefficient of variation is 0. When the coefficient of variation is close to or greater than 1, it means that i p1 ~i p7 The unevenness is very serious.

[0094] Depend on Fig. 9 and Fig.10 It can be seen that when the self-inductance and mutual inductance parameters of each beam change within the range of ±1%, i p1 The beam current coefficient and coefficient of variation vary greatly. The beam current coefficient is far from 0.1428 and even less than 0. The coefficient of variation is close to 1. This shows that the beam current is very sensitive to the Litz wire parameters. A parameter error of only 1% may lead to serious uneven beam current or even phase reversal.

[0095] The current sharing analysis of the beam splitting compensation system of the present invention is as follows: pisj The values ​​of the beam splitting compensation capacitors can be calculated based on the LCT parameters in Table 1, as shown in Table 2.

[0096] Table 2 Parameters of beam splitting compensation capacitors

[0097]

[0098] In order to intuitively reflect the current sharing effect, the same formula (2) is used for numerical calculation to obtain the theoretical waveform of each beam current of the primary and secondary coils, as shown in Fig.11 and Fig.12 As shown. Obviously, the currents of each beam are basically overlapped, and the current balancing effect is significant. By calculation, the coil loss is 40.51W, which is 20.76W lower than the traditional compensation method.

[0099] Similar to the traditional method analysis, random perturbations within the range of ±1% are applied to the self-inductance and mutual inductance parameters in the method of the present invention, and the i p1The beam coefficient and coefficient of variation are as follows: Fig.13 and Fig.14 As shown. Within 500 random perturbations, the beam current coefficient is very close to 0.1428, and the coefficient of variation is close to 0. This means that the beam splitting compensation method of the present invention can effectively reduce the sensitivity of each beam current to the Litz wire parameters and achieve equal current for each beam of the Litz wire.

[0100] Example:

[0101] In order to verify the beam splitting compensation method of the present invention, a 2kW WPT system prototype was built. Fig.15 The prototype is mainly composed of a full-bridge inverter, a primary compensation capacitor, a secondary compensation capacitor, a loosely coupled transformer LCT and a synchronous rectifier. Figure 6 As shown, the outer diameter is 21.5cm, the transmission distance is 9.5cm, and the coupling coefficient is 0.16.

[0102] The parameters of the WPT system are shown in Tables 1 and 2, with a DC input voltage of 100V, a frequency of 95kHz, and a load of 1 to 5Ω. Since the capacitors of the beam splitting compensation are not much different, and the total parallel capacitance is basically equal to the capacitance of the traditional compensation, in practical applications, seven identical capacitors are directly used in parallel, and their capacitance is equal to one-seventh of the capacitance of the traditional compensation capacitor. When comparing the experimental effects of the traditional and the compensation methods of the present invention, the working state remains unchanged, and only the connection method between the capacitor and the Litz wire is changed.

[0103] Under different load conditions, the output current and efficiency curves of the traditional centralized compensation and the split beam compensation of the present invention are as follows: Fig.16 As shown. When the load increases from 1Ω to 5Ω, under the traditional method, the output current drops from 21.2A to 20.21A, while under the method of the present invention, the current drops from 21.18A to 20.23A, and the current difference is less than 1%, both of which have good CCO characteristics. The method of the present invention can improve the efficiency by 3.1% and 1.1% under light load and heavy load conditions, respectively, and the effect is very significant, and the maximum efficiency can reach 91.09%.

[0104] Under the conventional compensation method and the present invention, the voltage and current experimental waveforms of the inverter and rectifier are as follows: Fig.17 As shown. The waveforms of the two are basically the same. in Slightly delayed re , the inverter switch tube achieves zero voltage switching (zero voltage switching, ZVS).

[0105] Under the traditional compensation method, the primary coil current waveforms under light load and heavy load are as follows: Fig.18 As shown, it can be seen that the currents of each beam are not equal, especially the current of the third beam i p3, which is in antiphase with other beam currents, which is consistent with the theoretical waveform analyzed above.

[0106] The secondary coil current waveform is as follows: Fig.19 As shown, similar to the primary current, the currents of each beam are not equal, and the third beam current i s3 The current of each beam of the secondary coil is in opposite phase with other beam currents. When the load changes, the current of each beam of the secondary coil remains basically constant, which conforms to the CCO output characteristics.

[0107] Under the compensation method of the present invention, the waveform of the primary and secondary coil current is as follows: Fig. 20 and Fig.21 As shown, it is obvious that each coil bundle achieves current sharing.

[0108] In summary, the split-beam parallel compensation method proposed in the present invention effectively solves the problem of uneven current in each beam of Litz wire in the WPT system, thereby improving the transmission efficiency of the system. The effectiveness of the method of the present invention has been proved through theoretical analysis. In addition, the experimental results of building a 2kW experimental prototype show that the CCO characteristics of the present invention remain unchanged under variable load conditions. Compared with the traditional method, the output current change of the method of the present invention is less than 1%. The light load efficiency is improved by 3.1%, and the full load efficiency is improved by 1.1%, with significant effects. The method of the present invention is not limited by the compensation topology, and may not even be limited to the WPT field. It also has broad research prospects in fields such as motors and power supplies that require Litz wires.

[0109] Although the present invention is described herein with reference to specific embodiments, it should be understood that these embodiments are merely examples of the principles and applications of the present invention. It should therefore be understood that many modifications may be made to the exemplary embodiments and that other arrangements may be devised without departing from the spirit and scope of the present invention as defined by the appended claims. It should be understood that the various dependent claims and features described herein may be combined in a manner different from that described in the original claims. It should also be understood that the features described in conjunction with a single embodiment may be used in other described embodiments.

Claims

1. A method for compensating for splitting of multiple Litz wires in a wireless power transmission system, characterized in that: include, A compensation capacitor is connected in series to each Litz wire branch of the primary coil and the secondary coil of the loosely coupled transformer to obtain a compensated branch, and then all the compensated branches are connected in parallel; The compensation capacitance value of each compensation capacitor is selected so that the currents of all primary-side compensated branches are equal and the currents of all secondary-side compensated branches are equal, so as to achieve current balancing of each beam.

2. The method for compensating for splitting multiple Litz wires in a wireless power transmission system according to claim 1, characterized in that: Methods for selecting the compensation capacitance value of each compensation capacitor include: According to Kirchhoff's law, the relationship between the voltage and current of the compensated branch and the self-inductance and mutual inductance of the coil is established, and the equation for making the branch currents equal is obtained based on the relationship, so as to calculate the compensation capacitance value.

3. The method for compensating for splitting multiple Litz wires in a wireless power transmission system according to claim 2, characterized in that: The relationship between the voltage and current of the compensated branch of the primary coil of the loosely coupled transformer and the coil self-inductance and coil mutual inductance is: In the formula is the inverter output voltage vector, ω is the system resonant angular frequency, L pn is the inductance of the nth Litz wire branch of the primary coil of the loosely coupled transformer, C pn The compensation capacitor connected in series with the nth Litz wire branch of the primary coil of the loosely coupled transformer, is the current vector of the nth compensated branch of the primary coil of the loosely coupled transformer, M p1n is the mutual inductance between the first and nth Litz wire branches of the primary coil of the loosely coupled transformer, M p(n-1)n is the mutual inductance between the n-1th Litz wire branch and the nth Litz wire branch of the primary coil of the loosely coupled transformer, M pnsn is the mutual inductance between the nth Litz wire branch of the primary coil and the nth Litz wire branch of the secondary coil of the loosely coupled transformer, is the current vector of the nth compensated branch of the secondary coil of the loosely coupled transformer.

4. The method for compensating for splitting multiple Litz wires in a wireless power transmission system according to claim 3, characterized in that: The equation for equal branch currents after primary compensation of loosely coupled transformer is: Where M pn It is the sum of the mutual inductance of the nth Litz wire branch and the other n-1 Litz wire branches of the primary coil of the loosely coupled transformer.

5. The method for compensating for splitting multiple Litz wires in a wireless power transmission system according to claim 4, characterized in that: Based on the equation that the branch currents are equal after primary compensation of the loosely coupled transformer, C pn for:

6. The method for compensating for splitting multiple Litz wires in a wireless power transmission system according to claim 5, characterized in that: Based on the same calculation method as the primary coil of the loosely coupled transformer, the compensation capacitor C connected in series with the nth Litz wire branch of the secondary coil of the loosely coupled transformer is obtained. sn : Where L sn is the inductance of the nth Litz wire branch of the secondary coil of the loosely coupled transformer, M sn It is the sum of the mutual inductance of the nth Litz wire branch and the other n-1 Litz wire branches of the secondary coil of the loosely coupled transformer.

7. The method for compensating for splitting multiple Litz wires in a wireless power transmission system according to claim 1, characterized in that: The primary coil and the secondary coil of the loosely coupled transformer are both wound by 7 bundles × 172 strands of Litz wire.

Images