T-type three-level-based bidirectional wireless charging system and parameter design method thereof

Abstract

The invention provides a T-type three-level-based bidirectional wireless charging system and a parameter design method thereof, which are used for solving the technical problem of large harmonic pollution on an alternating current power grid side when an existing bidirectional wireless charging system is connected to a grid. The system comprises a bidirectional wireless charging circuit and a T-type three-level bidirectional AC-DC converter, the bidirectional wireless charging circuit comprises a primary side sub-circuit and a secondary side sub-circuit, and one end of the primary side sub-circuit is magnetically coupled with one end of the secondary side sub-circuit for electric energy transmission; the other end of the primary side circuit is connected with a direct-current power supply, the other end of the secondary side circuit is connected with one end of the T-type three-level bidirectional AC-DC converter, and the other end of the T-type three-level bidirectional AC-DC converter is connected with an alternating-current power grid. The T-type three-level bidirectional AC-DC converter comprises a T-type three-level bidirectional AC-DC sub-circuit which is a three-phase circuit, and each phase circuit comprises a bridge arm sub-circuit and a middle clamping sub-circuit. The T-type three-level AC-DC converter is used in the bidirectional wireless charging system, and has the advantages of small output harmonic wave and large power factor.

Description

Technical Field

[0001] The present invention relates to the field of wireless power transmission, and in particular to a T-type three-level based bidirectional wireless charging system and a parameter design method thereof. Background Art

[0002] With the improvement of the commercialization level of electric vehicles and the development of wireless charging technology, the research on wireless charging and discharging technology for electric vehicle-grid interaction (V2G) is becoming increasingly important. The wireless charging device for electric vehicles does not have a plug, is easy to operate, and avoids many shortcomings of traditional charging methods. However, in the process of V2G wireless charging, a large number of electric vehicles connected to the power grid and unreasonable charging and discharging control will lead to harmonic pollution. If the harmonics cannot be effectively controlled during the wireless transmission of electric energy, it will have a serious impact on the power grid. In addition, due to the widespread application of wireless charging technology in recent years, wireless power transmission technology has gradually tended to the field of high-power equipment such as urban rail vehicles and high-speed railways. Therefore, it is necessary to study how to increase the output power of the bidirectional converter in the bidirectional wireless charging system of electric vehicles.

[0003] Bidirectional AC-DC converter is the key equipment to realize wireless charging technology of electric vehicles. At present, the grid-side bidirectional converters in the wireless charging and discharging system of electric vehicles mostly use traditional three-phase full-bridge bidirectional converters, and the high-frequency inverters in the inductive power transmission system mostly use two-level single-phase full-bridge inverters. In order to reduce the grid-side harmonic pollution and improve the output power of the wireless power transmission system, it is necessary to study the bidirectional converters and high-frequency inverters in the V2G wireless charging and discharging system. Summary of the invention

[0004] The purpose of the present invention is to provide a T-type three-level bidirectional wireless charging system and a parameter design method thereof, so as to solve the technical problem of large harmonic pollution on the AC power grid side when the existing bidirectional wireless charging system is connected to the grid.

[0005] A bidirectional wireless charging system based on a T-type three-level circuit comprises a bidirectional wireless charging circuit and a T-type three-level bidirectional AC-DC converter, wherein the bidirectional wireless charging circuit comprises a primary subcircuit and a secondary subcircuit, and one end of the primary subcircuit is magnetically coupled with one end of the secondary subcircuit to transmit electric energy;

[0006] The other end of the primary circuit is connected to a DC power supply, the other end of the secondary circuit is connected to one end of a T-type three-level bidirectional AC-DC converter, and the other end of the T-type three-level bidirectional AC-DC converter is connected to an AC power grid.

[0007] Optionally, the T-type three-level bidirectional AC-DC converter includes a T-type three-level bidirectional AC-DC sub-circuit, the T-type three-level bidirectional AC-DC sub-circuit is a three-phase circuit, each phase circuit includes a bridge arm sub-circuit and an intermediate clamping sub-circuit, and the three bridge arm sub-circuits are arranged in parallel;

[0008] The bridge arm subcircuit includes two switch tubes connected in series in the same direction, and the middle clamp subcircuit includes two switch tubes connected in series in the opposite direction. The connection point of the two switch tubes of the bridge arm subcircuit is defined as the midpoint of the bridge arm, and one end of the middle clamp subcircuit is connected to the midpoint of the bridge arm.

[0009] Optionally, the T-type three-level bidirectional AC-DC converter further includes a bus capacitor subcircuit;

[0010] The bus capacitor subcircuit includes a first bus capacitor and a second bus capacitor arranged in series. The bus capacitor subcircuit is connected to the other end of the secondary circuit after being arranged in parallel with the bridge arm subcircuit. The connection point of the first bus capacitor and the second bus capacitor is defined as the midpoint of the capacitor, and the other ends of the three intermediate clamping subcircuits are all connected to the midpoint of the capacitor.

[0011] Optionally, the T-type three-level bidirectional AC-DC converter further includes three LCL filter subcircuits, one end of the three LCL filter subcircuits are respectively connected to the midpoints of the three bridge arms, and the other ends of the three LCL filter subcircuits are respectively connected to the three phases of the AC power grid;

[0012] The LCL filter subcircuit includes a filter capacitor and a first filter inductor and a second filter inductor connected in series. The connection point between the first filter inductor and the second filter inductor is defined as a filter midpoint. One end of the filter capacitor is connected to the filter midpoint, and the other end of the filter capacitor is grounded.

[0013] Optionally, the primary sub-circuit includes a primary high-frequency inverter, a primary compensation network and a primary coil connected in sequence, and the secondary sub-circuit includes a secondary coil, a secondary compensation network and a secondary high-frequency inverter connected in sequence, and the primary coil and the secondary coil are magnetically coupled to transmit electric energy.

[0014] Optionally, the primary side compensation network and the secondary side compensation network are SS topology structures.

[0015] A parameter design method for a T-type three-level bidirectional wireless charging system is used to design the parameters of the T-type three-level bidirectional wireless charging system. The specific steps are as follows:

[0016] Filter capacitor parameter design: calculate the reactive power of the filter capacitor, set the percentage threshold of the reactive power of the filter capacitor to the rated power of the system, and calculate the upper limit of the filter capacitor parameters;

[0017] Filter inductor parameter design: construct a vector relationship diagram of the AC side of the grid-connected inverter in steady state, determine the upper limit of the total inductance value of the LCL filter subcircuit, calculate the upper and lower limits of the inductance value of the second filter inductor, set the inductance ratio parameters of the first and second filter inductors, and calculate the inductance values ​​of the first and second filter inductors;

[0018] Resonant frequency parameter design: Determine the upper and lower limits of the resonant frequency of the LCL filter subcircuit according to the switching frequency of the grid-connected inverter and the grid base frequency, and calculate the resonant angular frequency of the LCL filter subcircuit according to the inductance values ​​of the first and second filter inductors and the capacitance value of the filter capacitor.

[0019] Optionally, the specific steps for designing the filter capacitor parameters are:

[0020] S1.1: The useless work of a single filter capacitor is obtained by multiplying the filter capacitor voltage, the system angular frequency and the capacitance value of the filter capacitor, and the total reactive power of the filter capacitor in the LCL filter subcircuit is obtained by summing the useless work of the three filter capacitors;

[0021] S1.2: Set the percentage threshold of the total reactive power of the filter capacitor to the rated power of the system, and calculate the upper limit of the total reactive power of the filter capacitor according to the percentage threshold and the rated power parameter of the system;

[0022] S1.3: Construct an inequality between the total reactive power of the filter capacitor and the upper limit value of the total reactive power of the filter capacitor, and solve it to obtain the upper limit value of the capacitance value of the filter capacitor.

[0023] Optionally, the specific steps for designing the filter inductor parameters are:

[0024] S2.1: In steady state, the total inductance of the LCL filter subcircuit is the sum of the inductance values ​​of the first and second filter inductors, and a vector relationship diagram of the AC side of the grid-connected inverter in steady state is constructed, including the AC side voltage vector of the grid-connected inverter, the AC grid side voltage vector, the filter inductor voltage vector, and the filter inductor current vector;

[0025] S2.2: According to the vector relationship diagram, the fundamental peak value of the output phase voltage of the grid-connected inverter, the peak value of the AC grid side voltage and the peak value of the filter inductor current are obtained, and the upper limit value of the total inductance of the LCL filter subcircuit is calculated;

[0026] S2.3: The lower limit of the filter inductance is limited by the ripple size. The current ripple is the largest at the zero crossing. The lower limit of the total inductance of the LCL filter subcircuit is calculated based on the maximum current ripple value.

[0027] S2.4: Calculate the upper limit of the inductance of the second filter inductor according to the harmonic distortion reduction ratio parameter of the LC link in the LCL filter subcircuit, the capacitance value of the filter capacitor and the switching frequency of the grid-connected inverter;

[0028] S2.5: setting the inductance proportionality coefficient between the second filter inductor and the first filter inductor, and calculating the inductance value of the first filter inductor according to the resonant frequency of the LCL filter subcircuit, the inductance proportionality coefficient and the inductance value of the filter inductor;

[0029] S2.6: Calculate the product of the inductance value of the first filter inductor and the inductance proportional coefficient to obtain the inductance value of the second filter inductor.

[0030] Optionally, the specific steps of resonant frequency parameter design are:

[0031] S3.1: Setting the switching frequency ratio parameter of the grid-connected inverter and the grid base frequency ratio parameter, the resonant frequency of the LCL filter subcircuit is greater than the product of the switching frequency of the grid-connected inverter and the switching frequency ratio parameter, and less than the product of the grid base frequency and the grid base frequency ratio parameter;

[0032] S3.2: Calculate the resonant angular frequency of the LCL filter subcircuit according to the inductance values ​​of the first and second filter inductors and the capacitance value of the filter capacitor.

[0033] Due to the adoption of the above technical solution, the present invention has the following advantages:

[0034] This application uses a T-type three-level AC-DC converter in a bidirectional wireless charging system, replacing the traditional two-level bidirectional converter. The T-type three-level converter has the advantages of small output harmonics and large power factor. Combining the T-type three-level converter with the bidirectional wireless charging system can better achieve forward and reverse transmission.

[0035] Other advantages, objectives and features of the present invention will be described in the following description to some extent, and to some extent, will be obvious to those skilled in the art based on the following examination and study, or can be taught from the practice of the present invention. The objectives and other advantages of the present invention can be realized and obtained through the following description. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] The accompanying drawings of the present invention are as follows.

[0037] Figure 1 The circuit diagram of the T-type three-level bidirectional wireless charging system of the present invention is shown in FIG.

[0038] Figure 2 1 is a circuit diagram of a T-type three-level bidirectional AC-DC converter according to the present invention.

[0039] Figure 3It is a vector relationship diagram of the AC side of the grid-connected inverter of the present invention.

[0040] Figure 4 It is a waveform diagram of the three-phase voltage and current switching process on the grid side of the present invention.

[0041] Figure 5 It is a waveform diagram of the single-phase voltage and current switching process of the present invention.

[0042] Figure 6 This is a voltage and current waveform diagram of the primary and secondary sides of the coupling mechanism before and after switching of the present invention.

[0043] Figure 7 This is an output voltage waveform diagram of the T-type topology in the grid-to-battery charging mode of the present invention.

[0044] Figure 8 This is a voltage and current waveform diagram on the grid side when the grid of the present invention charges the battery.

[0045] Fig. 9 This is a THD analysis diagram of the current on the grid side when the grid charges the battery according to the present invention.

[0046] Fig.10 This is a voltage and current waveform diagram on the grid side when the battery of the present invention feeds back to the grid.

[0047] Fig.11 This is a current THD analysis diagram on the grid side when the battery of the present invention feeds back to the grid. DETAILED DESCRIPTION

[0048] The present invention will be further described below in conjunction with the accompanying drawings and embodiments.

[0049] Embodiment 1:

[0050] like Figure 1 and Figure 2 A T-type three-level bidirectional wireless charging system is shown, characterized in that it includes a bidirectional wireless charging circuit and a T-type three-level bidirectional AC-DC converter, wherein the bidirectional wireless charging circuit includes a primary sub-circuit and a secondary sub-circuit, and one end of the primary sub-circuit is magnetically coupled with one end of the secondary sub-circuit to transmit electric energy;

[0051] The other end of the primary circuit is connected to a DC power supply, the other end of the secondary circuit is connected to one end of a T-type three-level bidirectional AC-DC converter, and the other end of the T-type three-level bidirectional AC-DC converter is connected to an AC power grid.

[0052] like Figure 1 and Figure 2As shown, the primary sub-circuit includes a primary high-frequency inverter, a primary compensation network and a primary coil connected in sequence, and the secondary sub-circuit includes a secondary coil, a secondary compensation network and a secondary high-frequency inverter connected in sequence, and the primary coil and the secondary coil are magnetically coupled to transmit electric energy. The primary compensation network and the secondary compensation network are SS topology structures.

[0053] In this embodiment, if Figure 1 As shown, Figure 1 The primary side high frequency inverter is composed of 4 switch tubes S1-S4, and the primary side compensation network includes the primary side compensation capacitor C p , the primary coil is L p , the secondary coil is L s , the secondary compensation network includes the secondary compensation capacitor C s The secondary side high frequency inverter is composed of 4 switching tubes T1-T4.

[0054] In this embodiment, the DC power supply is an electric vehicle power supply. When the AC power grid charges the electric vehicle, the T-type three-level bidirectional AC-DC converter connected to the power grid has both rectification and power factor correction functions, and rectifies the three-phase AC power on the AC power grid side to obtain a stable output DC power. The DC power is inverted at high frequency by the secondary high-frequency inverter to obtain high-frequency AC power, and the energy is transmitted to the primary coil through the coupling mechanism. The high-frequency AC power on the primary side is rectified and output as DC power by the primary high-frequency inverter, and then charges the vehicle-side battery after passing through the LC filter network.

[0055] When electric vehicles transmit electric energy to the AC grid, the primary high-frequency inverter inverts the DC power at high frequency, transmits energy to the secondary coil through the coupling mechanism, and outputs DC power through the secondary high-frequency inverter. The DC power is converted to AC power through the T-type three-level bidirectional AC-DC converter, and the high-frequency harmonics are filtered out through the LCL filter network, and the AC power is injected into the grid.

[0056] In this embodiment, as another embodiment of the present application, an LC filtering network is provided between the primary high-frequency inverter and the DC power supply and between the secondary high-frequency inverter and the T-type three-level bidirectional AC-DC converter.

[0057] like Figure 1 and Figure 2 As shown, the T-type three-level bidirectional AC-DC converter includes a T-type three-level bidirectional AC-DC sub-circuit, and the T-type three-level bidirectional AC-DC sub-circuit is a three-phase circuit, each phase circuit includes a bridge arm sub-circuit and an intermediate clamping sub-circuit, and the three bridge arm sub-circuits are arranged in parallel;

[0058] The bridge arm subcircuit includes two switch tubes connected in series in the same direction, and the middle clamp subcircuit includes two switch tubes connected in series in the opposite direction. The connection point of the two switch tubes of the bridge arm subcircuit is defined as the midpoint of the bridge arm, and one end of the middle clamp subcircuit is connected to the midpoint of the bridge arm.

[0059] In this implementation, if Figure 1 As shown, the three-phase circuits are defined as phase a, phase b and phase c. The bridge arm subcircuit of phase a includes switch tubes S arranged in series in the same direction A1 And switch tube S A4 The intermediate clamping subcircuit of phase a includes S A2 And switch tube S A3 The bridge arm subcircuit of phase a intersects with the middle clamp subcircuit at point A. The bridge arm subcircuit of phase b includes switch tubes S arranged in series in the same direction. B1 And switch tube S B4 The intermediate clamping subcircuit of phase b includes S B2 And switch tube S B3 The bridge arm subcircuit of phase b intersects with the middle clamp subcircuit at point B. The bridge arm subcircuit of phase c includes switch tubes S arranged in series in the same direction. C1 And switch tube S C4 The intermediate clamping subcircuit of phase a includes S C2 And switch tube S C3 ; The bridge arm sub-circuit of phase c intersects with the middle clamp sub-circuit at point C.

[0060] like Figure 1 and Figure 2 As shown, the T-type three-level bidirectional AC-DC converter also includes a bus capacitor subcircuit;

[0061] The bus capacitor subcircuit includes a first bus capacitor and a second bus capacitor arranged in series. The bus capacitor subcircuit is connected to the other end of the secondary circuit after being arranged in parallel with the bridge arm subcircuit. The connection point of the first bus capacitor and the second bus capacitor is defined as the midpoint of the capacitor, and the other ends of the three intermediate clamping subcircuits are all connected to the midpoint of the capacitor.

[0062] In this implementation, if Figure 1 and Figure 2 As shown, C d1 and C d2 They are the first bus capacitor and the second bus capacitor respectively.

[0063] like Figure 1As shown, the T-type three-level bidirectional AC-DC converter further includes three LCL filter subcircuits, one end of the three LCL filter subcircuits are respectively connected to the midpoints of the three bridge arms, and the other ends of the three LCL filter subcircuits are respectively connected to the three phases of the AC power grid;

[0064] The LCL filter subcircuit includes a filter capacitor and a first filter inductor and a second filter inductor connected in series. The connection point between the first filter inductor and the second filter inductor is defined as a filter midpoint. One end of the filter capacitor is connected to the filter midpoint, and the other end of the filter capacitor is grounded.

[0065] In this implementation, if Figure 1 and Figure 2 As shown, e a 、e b and e c are the three phases of three-phase alternating current, L 1 and L 2 are the first filter inductor and the second filter inductor respectively, C f For the filter capacitor.

[0066] In this embodiment, the T-type topology structure uses a pair of switch tubes for midpoint clamping, which makes it have higher efficiency, smaller volume and better EMI performance, and is widely used in photovoltaic inverters and other power conversion occasions. At the same time, each phase of the T-type three-level topology has two reverse series power switches, which clamp the output potential to the DC side midpoint potential, output low, medium and high levels, and improve the inverter efficiency. The T-type three-level topology has the following characteristics: (1) Each phase of the T-type three-level topology is composed of only four switch tubes, and no clamping diode is required to clamp the midpoint potential. Each phase has two fewer clamping diodes, which can reduce the integrated volume of the switch tube module; (2) There is no series connection of switch tubes on each phase bridge arm of the T-type three-level topology, and there will be no uneven voltage distribution of the series switch devices. Considering that all switches have the same withstand voltage level in actual application, the requirements for the withstand voltage level of the switches are the same; (3) When outputting positive voltage or negative voltage, the current only flows through one switch tube, and the conduction loss is half that of the diode clamped topology; and the switch loss distribution is more uniform, which improves the overall efficiency of the inverter. From a practical point of view, the T-topology has the advantages of easy integration, smaller size and lower cost.

[0067] Considering that T-type three-level is mainly used in high-power occasions in general engineering applications, a dual-tube parallel topology can be adopted. Compared with the traditional T-type three-level topology, it has the following advantages: (1) Usually, T-type three-level is a single-phase four MOS tubes. The use of a dual MOS tube parallel circuit can reduce the current load of each switch tube, thereby reducing power loss and temperature rise, and at the same time can increase power output. (2) In addition, dual-tube parallel connection can also improve circuit reliability. When one of the circuits fails, the other circuit can continue to work to ensure the normal operation of the system. (3) In actual work, the problem of midpoint potential being difficult to control often occurs. The dual-tube parallel connection can effectively control the voltage on the DC side at a constant value, making the DC side voltage more stable.

[0068] Embodiment 2:

[0069] A parameter design method for a T-type three-level bidirectional wireless charging system is used to design the parameters of the T-type three-level bidirectional wireless charging system according to any one of claims 1 to 6, and the specific steps are:

[0070] S1 filter capacitor parameter design: calculate the reactive power of the filter capacitor, set the percentage threshold of the reactive power of the filter capacitor to the rated power of the system, and calculate the upper limit of the filter capacitor parameters; the specific steps are:

[0071] S1.1: The useless work of a single filter capacitor is obtained by multiplying the filter capacitor voltage, the system angular frequency and the capacitance value of the filter capacitor, and the total reactive power of the filter capacitor in the LCL filter subcircuit is obtained by summing the useless work of the three filter capacitors;

[0072] S1.2: Set the percentage threshold of the total reactive power of the filter capacitor to the rated power of the system, and calculate the upper limit of the total reactive power of the filter capacitor according to the percentage threshold and the rated power parameter of the system;

[0073] S1.3: Construct an inequality between the total reactive power of the filter capacitor and the upper limit value of the total reactive power of the filter capacitor, and solve it to obtain the upper limit value of the capacitance value of the filter capacitor.

[0074] In this embodiment, the larger the filter capacitor in the LCL filter, the stronger its filtering ability for high-frequency current, but the reactive power generated will also increase. Taking the percentage threshold as 5%, we have:

[0075]

[0076] Where P n is the rated power of the grid-connected inverter, u c is the capacitor voltage. When the voltage drop on the filter inductor on the AC grid side is small, the capacitor voltage can be approximated to the grid voltage, then:

[0077]

[0078] In the formula, u g is the grid voltage.

[0079] S2 filter inductor parameter design: construct a vector relationship diagram of the AC side of the grid-connected inverter in steady state, determine the upper limit of the total inductance value of the LCL filter subcircuit, calculate the upper and lower limits of the inductance value of the second filter inductor, set the inductance ratio parameters of the first and second filter inductors, and calculate the inductance values ​​of the first and second filter inductors; the specific steps are:

[0080] S2.1: In steady state, the total inductance of the LCL filter subcircuit is the sum of the inductance values ​​of the first and second filter inductors, and a vector relationship diagram of the AC side of the grid-connected inverter in steady state is constructed, including the AC side voltage vector of the grid-connected inverter, the AC grid side voltage vector, the filter inductor voltage vector, and the filter inductor current vector;

[0081] In this embodiment, in steady state, the vector relationship of the AC side of the grid-connected inverter is as follows: Figure 3 As shown, Figure 3 Where U is the AC side voltage vector of the grid-connected inverter, E is the AC grid side voltage vector, V L is the filter inductor voltage vector and I L is the filter inductor current vector. It is usually required that the inverter can be controlled to run at any point on the circle in the figure (four-quadrant operation), that is, the amplitude and phase of the AC side voltage vector U of the grid-connected inverter must meet certain control requirements.

[0082] On the one hand, the amplitude and phase of U are related to E and V L Yes, E is constant, then the amplitude and phase of U are affected by V L The impact of V L The amplitude of (i.e. the radius of the circle in the figure) is proportional to the inductance and the current flowing through it. When the grid-connected inverter meets the rated current output, the size of the filter inductance directly affects V L The size of the amplitude affects the value of the amplitude of U.

[0083] On the other hand, the amplitude of U is proportional to the DC side voltage of the inverter, and its maximum value depends on the DC side voltage and the voltage utilization rate of PWM modulation. Therefore, when the DC side voltage of the inverter is constant and the PWM modulation method is constant, the maximum amplitude of U is also determined. Then V L The amplitude of must be limited to a certain extent, that is, the size of the filter inductor must be limited to a certain extent.

[0084] S2.2: According to the vector relationship diagram, the fundamental peak value of the output phase voltage of the grid-connected inverter, the peak value of the AC grid side voltage and the peak value of the filter inductor current are obtained, and the upper limit value of the total inductance of the LCL filter subcircuit is calculated;

[0085] In this embodiment, from Figure 3 As can be seen from the figure, when the inverter runs at point a on the circumference, the amplitude of U will reach its maximum value, which is equal to the amplitude of E and V L If the inverter only operates in the unity power factor power generation state (point b in the figure), then, and the PWM modulation mode is SVPWM modulation, the fundamental peak value of the output phase voltage of the grid-connected inverter is but:

[0086]

[0087] In the formula, E p is the peak value of the AC grid voltage, I LP is the peak value of the filter inductor current.

[0088] S2.3: The lower limit of the filter inductance is limited by the ripple size. The current ripple is the largest at the zero crossing. The lower limit of the total inductance of the LCL filter subcircuit is calculated based on the maximum current ripple value.

[0089] In this embodiment, the current ripple is maximum at the zero crossing point, which is:

[0090]

[0091] The grid current harmonic standard IEEE519-1992 stipulates that the upper limit of switching frequency distortion is 0.3%. Since the LC link of LCL will further filter out harmonics, the upper limit of harmonics is enlarged to 5%, and the lower limit of L is:

[0092]

[0093] The total inductance of LCL is 1 / 5 of the inductance of the single inductor filter, leaving a margin of 1 / 3, then:

[0094]

[0095] S2.4: Calculate the upper limit of the inductance of the second filter inductor according to the harmonic distortion reduction ratio parameter of the LC link in the LCL filter subcircuit, the capacitance value of the filter capacitor and the switching frequency of the grid-connected inverter;

[0096] In this embodiment, the first filter inductor L on the bridge arm side is generally 1 The value is relatively large, the AC grid side inductance L 2 The value is relatively small to better suppress the ripple of the bridge arm current.

[0097] For the AC grid side inductance L 2, considering that the LC link in the LCL filter subcircuit reduces the harmonic distortion from 5% to 0.3%, the expression can be obtained:

[0098]

[0099] S2.5: setting the inductance proportionality coefficient between the second filter inductor and the first filter inductor, and calculating the inductance value of the first filter inductor according to the resonant frequency of the LCL filter subcircuit, the inductance proportionality coefficient and the inductance value of the filter inductor;

[0100] S2.6: Calculate the product of the inductance value of the first filter inductor and the inductance proportional coefficient to obtain the inductance value of the second filter inductor.

[0101] In this embodiment, L 2 =γL 1 Where γ is the inductance ratio coefficient of the second filter inductor to the first filter inductor, which is usually less than or equal to 0.5. First determine the resonant frequency f according to the range res and γ, and then calculate the inductance value of the first filter inductor:

[0102]

[0103] According to the inductance value L of the first filter inductor 1 The inductance value of the second filter inductor is calculated using the inductance proportional coefficient γ.

[0104] S3 resonant frequency parameter design: determine the upper and lower limits of the resonant frequency of the LCL filter subcircuit according to the switching frequency of the grid-connected inverter and the grid base frequency, and calculate the resonant angular frequency of the LCL filter subcircuit according to the inductance values ​​of the first and second filter inductors and the capacitance value of the filter capacitor; the specific steps are:

[0105] S3.1: Setting the switching frequency ratio parameter of the grid-connected inverter and the grid base frequency ratio parameter, the resonant frequency of the LCL filter subcircuit is greater than the product of the switching frequency of the grid-connected inverter and the switching frequency ratio parameter, and less than the product of the grid base frequency and the grid base frequency ratio parameter;

[0106] S3.2: Calculate the resonant angular frequency of the LCL filter subcircuit according to the inductance values ​​of the first and second filter inductors and the capacitance value of the filter capacitor.

[0107] In this embodiment, the switching frequency ratio parameter of the grid-connected inverter is set to 20, and the grid base frequency ratio parameter is set to 0.2, then:

[0108] 20×f g ≤f res ≤0.2×f sw

[0109] In the formula, f swis the switching frequency of the grid-connected inverter, f g is the grid base frequency, f res is the resonant frequency of the LCL filter subcircuit;

[0110] The resonant angular frequency ω of the LCL filter subcircuit r for:

[0111]

[0112] Where, L 1 and L 2 are the inductance values ​​of the first and second filter inductors respectively, C f is the capacitance value of the filter capacitor.

[0113] S4 simulation verification:

[0114] S4.1 parameter setting: pass Figure 1 Build simulation models, Figure 1 On the basis of the above, an LC filter circuit is added between the primary high-frequency inverter and the DC power supply and between the secondary high-frequency inverter and the T-type three-level bidirectional AC-DC converter. The LC filter circuit includes a charging filter capacitor C connected in series. dc and charging filter inductor L dc , the LC filter circuit is connected in parallel with the primary high-frequency inverter or the secondary high-frequency inverter. According to the above derivation and analysis of the key parameters of the system, the parameter values ​​are shown in Table 1. In the simulation model, the PQ method modulation is adopted in the bidirectional wireless power transmission system, and the T-type three-level topology adopts SVPWM modulation. The positive and negative control of the bidirectional charging system is achieved by changing the positive and negative of the external phase shift angle of the wireless link, and the rectification / inversion working state of the T-type three-level topology is controlled by controlling the positive and negative of the given voltage of the bus voltage loop, and the two are simultaneously switched forward and reversely through the integrated switch.

[0115] Table 1 Simulation parameters of T-type three-level and bidirectional wireless power transmission system

[0116]

[0117]

[0118] S4.2 simulates the switching of the battery feedback to the grid to charge the battery: Figure 4 (a) is the waveform of the three-phase voltage switching process on the grid side. Figure 4 (b) is the waveform of the three-phase current switching process on the grid side; Figure 5 is the waveform during the single-phase voltage and current switching process, Figure 5 The middle gray line is the voltage waveform. Figure 5 The red line in the middle is the voltage waveform; Figure 6The voltage and current waveforms of the primary and secondary sides of the coupling mechanism before and after switching are as follows: Figure 6 The square wave in the middle is the primary and secondary voltage waveform. Figure 6 The cosine wave in the figure is the original secondary current waveform. Taking single-phase as an example, the integrated switch is switched at 0.08s, and the current is reversed after switching. A current peak will appear in the first cycle, and the current waveform will stabilize into a sine wave after 0.115s. At this time, the current and voltage are reversed, indicating that the grid has been switched to charge the battery.

[0119] S4.3 simulates the charging of the battery by the power grid: Figure 7 The figure shows the output voltage waveform of the T-type topology in charging mode. It can be seen that there will be a certain voltage fluctuation at the beginning, and it stabilizes at 900V after 0.08s, with a faster response speed. Figures 8-9 Analyze the voltage and current waveforms and current THD of the grid side when the grid charges the battery. Figure 8 The middle gray line is the voltage waveform. Figure 8 The red line in the middle is the voltage waveform; Figures 8-9 It can be seen that the fluctuating current reaches its peak value at 0.015s, and after 0.04s, the grid current waveform is a sine wave, and the THD is 1.13%.

[0120] S4.4 Battery feedback grid simulation: Take single-phase voltage and current as an example. Figures 10-11 Analysis of grid-side voltage and current waveforms and current THD when the battery feeds back to the grid. Fig.10 The middle gray line is the voltage waveform. Fig.10 The red line in the middle is the voltage waveform; Figures 10-11 It can be seen that the fluctuating current reaches its peak at 0.01s, and after 0.035s the current stabilizes into a sine wave, which is in phase with the grid-side voltage, and the current THD is 2.24%, which meets the grid-connected requirements.

[0121] In summary, this application studies and designs a bidirectional switching system that combines a T-type three-level with a bidirectional wireless power transmission system, introduces a T-type three-level topology and a dual-tube parallel connection method, and designs T-type three-level LCL filter self-circuit parameters. The range of inductance values ​​is obtained by current tracking voltage phase capability and grid ripple current requirements. In order to ensure a high power factor, the reactive power of the capacitor is limited to obtain the upper limit of the capacitance value. The simulation results show that when a bidirectional converter with a T-type three-level topology is used, the current response speed on the grid side is faster when the car is charging and when the battery is fed back to the grid, and unity power factor operation can be achieved. The current THD on the grid side is low, which can meet the grid connection requirements.

[0122] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the above embodiments, ordinary technicians in the relevant field should understand that the specific implementation methods of the present invention can still be modified or replaced by equivalents. Any modification or equivalent replacement that does not depart from the spirit and scope of the present invention should be covered within the scope of protection of the claims of the present invention.

Claims

1. A bidirectional wireless charging system based on T-type three-level, characterized in that: It includes a bidirectional wireless charging circuit and a T-type three-level bidirectional AC-DC converter, wherein the bidirectional wireless charging circuit includes a primary subcircuit and a secondary subcircuit, and one end of the primary subcircuit is magnetically coupled with one end of the secondary subcircuit to transmit electric energy; The other end of the primary circuit is connected to a DC power supply, the other end of the secondary circuit is connected to one end of a T-type three-level bidirectional AC-DC converter, and the other end of the T-type three-level bidirectional AC-DC converter is connected to an AC power grid.

2. According to claim 1, a T-type three-level bidirectional wireless charging system is characterized in that: The T-type three-level bidirectional AC-DC converter comprises a T-type three-level bidirectional AC-DC sub-circuit, wherein the T-type three-level bidirectional AC-DC sub-circuit is a three-phase circuit, each phase circuit comprises a bridge arm sub-circuit and an intermediate clamp sub-circuit, and the three bridge arm sub-circuits are arranged in parallel; The bridge arm subcircuit includes two switch tubes connected in series in the same direction, and the middle clamp subcircuit includes two switch tubes connected in series in the opposite direction. The connection point of the two switch tubes of the bridge arm subcircuit is defined as the midpoint of the bridge arm, and one end of the middle clamp subcircuit is connected to the midpoint of the bridge arm.

3. A T-type three-level bidirectional wireless charging system according to claim 2, characterized in that: The T-type three-level bidirectional AC-DC converter also includes a bus capacitor subcircuit; The bus capacitor subcircuit includes a first bus capacitor and a second bus capacitor arranged in series. The bus capacitor subcircuit is connected to the other end of the secondary circuit after being arranged in parallel with the bridge arm subcircuit. The connection point of the first bus capacitor and the second bus capacitor is defined as the midpoint of the capacitor, and the other ends of the three intermediate clamping subcircuits are all connected to the midpoint of the capacitor.

4. According to claim 2, a T-type three-level bidirectional wireless charging system is characterized in that: The T-type three-level bidirectional AC-DC converter further includes three LCL filter subcircuits, one end of the three LCL filter subcircuits are respectively connected to the midpoints of the three bridge arms, and the other ends of the three LCL filter subcircuits are respectively connected to the three phases of the AC power grid; The LCL filter subcircuit includes a filter capacitor and a first filter inductor and a second filter inductor connected in series. The connection point between the first filter inductor and the second filter inductor is defined as a filter midpoint. One end of the filter capacitor is connected to the filter midpoint, and the other end of the filter capacitor is grounded.

5. The T-type three-level bidirectional wireless charging system according to claim 1, characterized in that: The primary sub-circuit includes a primary high-frequency inverter, a primary compensation network and a primary coil connected in sequence, and the secondary sub-circuit includes a secondary coil, a secondary compensation network and a secondary high-frequency inverter connected in sequence. The primary coil and the secondary coil are magnetically coupled to transmit electric energy.

6. A T-type three-level bidirectional wireless charging system according to claim 5, characterized in that: The primary side compensation network and the secondary side compensation network are SS topology structures.

7. A parameter design method for a T-type three-level bidirectional wireless charging system, characterized in that: The method is used to design parameters of a T-type three-level bidirectional wireless charging system according to any one of claims 1 to 6, wherein the specific steps are as follows: Filter capacitor parameter design: calculate the reactive power of the filter capacitor, set the percentage threshold of the reactive power of the filter capacitor to the rated power of the system, and calculate the upper limit of the filter capacitor parameters; Filter inductor parameter design: construct a vector relationship diagram of the AC side of the grid-connected inverter in steady state, determine the upper limit of the total inductance value of the LCL filter subcircuit, calculate the upper and lower limits of the inductance value of the second filter inductor, set the inductance ratio parameters of the first and second filter inductors, and calculate the inductance values ​​of the first and second filter inductors; Resonant frequency parameter design: Determine the upper and lower limits of the resonant frequency of the LCL filter subcircuit according to the switching frequency of the grid-connected inverter and the grid base frequency, and calculate the resonant angular frequency of the LCL filter subcircuit according to the inductance values ​​of the first and second filter inductors and the capacitance value of the filter capacitor.

8. The parameter design method of a T-type three-level bidirectional wireless charging system according to claim 7, characterized in that: The specific steps for designing filter capacitor parameters are: S1.1: The useless work of a single filter capacitor is obtained by multiplying the filter capacitor voltage, the system angular frequency and the capacitance value of the filter capacitor, and the total reactive power of the filter capacitor in the LCL filter subcircuit is obtained by summing the useless work of the three filter capacitors; S1.2: Set the percentage threshold of the total reactive power of the filter capacitor to the rated power of the system, and calculate the upper limit of the total reactive power of the filter capacitor according to the percentage threshold and the rated power parameter of the system; S1.3: Construct an inequality between the total reactive power of the filter capacitor and the upper limit value of the total reactive power of the filter capacitor, and solve it to obtain the upper limit value of the capacitance value of the filter capacitor.

9. The parameter design method of a T-type three-level bidirectional wireless charging system according to claim 7, characterized in that: The specific steps for designing the filter inductor parameters are: S2.1: In steady state, the total inductance of the LCL filter subcircuit is the sum of the inductance values ​​of the first and second filter inductors, and a vector relationship diagram of the AC side of the grid-connected inverter in steady state is constructed, including the AC side voltage vector of the grid-connected inverter, the AC grid side voltage vector, the filter inductor voltage vector, and the filter inductor current vector; S2.2: According to the vector relationship diagram, the fundamental peak value of the output phase voltage of the grid-connected inverter, the peak value of the AC grid side voltage and the peak value of the filter inductor current are obtained, and the upper limit value of the total inductance of the LCL filter subcircuit is calculated; S2.3: The lower limit of the filter inductance is limited by the ripple size. The current ripple is the largest at the zero crossing. The lower limit of the total inductance of the LCL filter subcircuit is calculated based on the maximum current ripple value. S2.4: Calculate the upper limit of the inductance of the second filter inductor according to the harmonic distortion reduction ratio parameter of the LC link in the LCL filter subcircuit, the capacitance value of the filter capacitor and the switching frequency of the grid-connected inverter; S2.5: setting the inductance proportionality coefficient between the second filter inductor and the first filter inductor, and calculating the inductance value of the first filter inductor according to the resonant frequency of the LCL filter subcircuit, the inductance proportionality coefficient and the inductance value of the filter inductor; S2.6: Calculate the product of the inductance value of the first filter inductor and the inductance proportional coefficient to obtain the inductance value of the second filter inductor.

10. The parameter design method of a T-type three-level bidirectional wireless charging system according to claim 7, characterized in that: The specific steps of resonant frequency parameter design are: S3.1: Setting the switching frequency ratio parameter of the grid-connected inverter and the grid base frequency ratio parameter, the resonant frequency of the LCL filter subcircuit is greater than the product of the switching frequency of the grid-connected inverter and the switching frequency ratio parameter, and less than the product of the grid base frequency and the grid base frequency ratio parameter; S3.2: Calculate the resonant angular frequency of the LCL filter subcircuit according to the inductance values ​​of the first and second filter inductors and the capacitance value of the filter capacitor.

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