Method for adjusting minimum current stress of high-capacity three-level bidirectional DC-DC converter

Abstract

The invention discloses a method for adjusting the minimum current stress of a high-capacity three-level bidirectional DC-DC converter. According to the method, firstly, the converter is divided into different working modes, and converter transmission power and current stress mathematical models in the different working modes are analyzed; and then establishing a Lagrange multiplier equation by taking the current stress as an optimization target, and solving a partial derivative of the Lagrange multiplier equation to obtain a combination relationship between duty ratio control quantities D1, D2 and D3 and a phase shift angle D phi, wherein the combination relationship enables the current stress to be minimum in different power intervals. And the switching time of the high-voltage side switching devices S1 to S8 and the switching time of the low-voltage side switching devices Q1 to Q4 are adjusted through the combination relation until the output voltage is stabilized to a voltage reference value Vref. The method is suitable for adjusting the minimum current stress of the three-level bidirectional DC / DC converter, online adjustment can be efficiently, accurately and conveniently carried out on a control system of the three-level bidirectional DC / DC converter in real time, the current stress in a resonant inductor is reduced, and the overall efficiency of the system is improved.

Description

Technical Field

[0001] The present invention relates to the field of high-power power conversion system design, and can be used as an effective means for regulating DC power distribution in a large-capacity new energy energy storage system, especially more applicable when it comes to the minimum current stress regulation method of a three-level bidirectional DC-DC converter. Background Art

[0002] As a key device used between the high-voltage DC side of new energy power generation and the low-voltage side of energy storage, the bidirectional DC / DC converter can effectively regulate the unstable output and power quality of new energy power generation. Compared with traditional non-isolated converters, the full-bridge bidirectional isolated DC / DC converter has greater advantages in terms of safety, anti-interference performance, and the ability to transmit electrical energy across voltage levels, and can achieve electrical isolation between the faulty side and the non-faulty side during a fault. However, when the power of the bidirectional DC / DC converter gradually increases, the current amplitude in the resonant inductor will rise rapidly, and at the same time, the winding current of the high-frequency isolation transformer will increase, resulting in an increase in the losses of parts such as inductors, transformers, and switching devices, and then causing some devices to heat seriously, ultimately leading to a reduction in performance, a decrease in insulation ability, and even the occurrence of thermal breakdown accidents.

[0003] Currently, the methods for regulating current stress in bidirectional DC / DC converters mainly focus on parameter selection and topology selection. There is no very effective method available for achieving the minimum current stress of bidirectional DC / DC converters through a combination of actual parameter detection and control strategy adjustment. Therefore, in order to further improve the working reliability of bidirectional DC / DC converters and the efficiency of the converter control system, relevant research needs to be carried out on its minimum current stress regulation method, and a minimum current stress regulation method for a full-bridge bidirectional DC / DC converter is proposed. Summary of the Invention

[0004] The purpose of the present invention is to provide a minimum current stress regulation method for a large-capacity three-level bidirectional DC-DC converter.

[0005] The technical solution for achieving the purpose of the present invention is as follows:

[0006] 1. A minimum current stress regulation method for a large-capacity three-level bidirectional DC-DC converter, used for flexible regulation and optimal control of electrical energy in a DC power supply system, characterized by including the following steps:

[0007] The first step: Installation of the DC / DC converter

[0008] 1.1 A three-level bidirectional DC / DC converter under four-degree-of-freedom control, characterized by comprising: input filter capacitors C1(1) and C2(2), a DC high-voltage module (3), an inductor L(4), a high-frequency isolation transformer T(5), a DC low-voltage module (6), an output filter capacitor C3(7), a signal acquisition unit, a control microprocessor (9), a high-voltage side DC bus (12), and a low-voltage side energy storage load (13); wherein:

[0009] The DC high-voltage module (3) is a diode-clamped three-level full-bridge circuit. Among them, fully-controlled devices S1 to S4 and diodes D1 and D2 form a diode-clamped three-level bridge arm; fully-controlled devices S5 to S8 and diodes D3 and D4 form a diode-clamped three-level bridge arm; the positive pole of the DC high-voltage module (3) is connected to the positive pole of the input filter capacitor C1(1) and the positive pole of the high-voltage side DC bus (12), and the negative pole is connected to the negative pole of C2(2) and the negative pole of the high-voltage side DC bus (12); the diode neutral points of the two bridge arms are connected to the series midpoint of C1 and C2; the AC side of the DC high-voltage module (3) is connected to the primary side of the transformer T(5) through the inductor L(4);

[0010] The DC low-voltage module (6) includes fully-controlled devices Q1 to Q4, forming a basic H-bridge structure. The positive pole of the DC low-voltage module (6) is connected to the positive pole of the output filter capacitor C3(7) and the positive pole of the low-voltage side energy storage load (13); the negative pole is connected to the negative pole of the output filter capacitor C3(7) and the negative pole of the low-voltage side energy storage load (13); the AC side of the DC low-voltage module (6) is connected to the secondary side of the transformer T(5);

[0011] The signal acquisition unit includes a high-voltage side voltage detector (8), a high-voltage side current detector (9), a low-voltage side voltage detector (10), and a low-voltage side current detector (11);

[0012] The turns ratio of the high-frequency isolation transformer T(5) is N:1;

[0013] The signal acquisition unit is connected to the corresponding signal input terminals of the control microprocessor (8);

[0014] The control signal output terminal of the control microprocessor (8) is connected to the control signal input terminals of fully-controlled devices S1 to S8 and Q1 to Q4;

[0015] 1.2 By applying control signals to the switching devices in the DC high-voltage module and the DC low-voltage module respectively through an embedded microprocessor, S1 and S3, S2 and S4, S5 and S7, S6 and S8, Q1 and Q2, Q3 and Q4 conduct complementarily. The duty ratios of S1, S4 are D1, the duty ratios of S5, S8 are D2, the duty ratios of Q1, Q3 are D3, and the phase difference between devices S1 and Q1 is named the phase-shift angle ( Figure 2 ) where D1, D2, D3 ∈ [0, 1];

[0016] Step 2: Design of the minimum current stress control method for the converter

[0017] 2.1 Division of operating modes

[0018] According to the duty cycle D 1, D2, D3 and the phase shift angle , the converter can be divided into the following three operating modes:

[0019] Mode 1, the corresponding phase shift angle constraint condition is

[0020] Mode 2, the corresponding phase shift angle constraint condition is

[0021] Mode 3, the corresponding phase shift angle constraint condition is

[0022] 2.2 Define the voltage conversion ratio k = NV2 / V1, where V2 is the output voltage of the low-voltage side and V1 is the input voltage of the high-voltage side; analyze the mathematical models of the transmission power and current stress of the converter as follows:

[0023] The calculation expression of the inductor current is as follows:

[0024]

[0025] In the formula, v ab (t), v cd (t) are the midpoint voltages of the primary and secondary bridge arms;

[0026] Take the maximum value of the inductor current at different times as the current stress i p , as follows:

[0027]

[0028] The per-unit power calculation formula is as follows:

[0029]

[0030] In the formula, P N = nV1V2 / (8f s L) is the maximum transmission power, and f s is the switching frequency;

[0031] According to the per-unit power calculation formula, the per-unit powers of the three operating modes can be calculated as follows:

[0032]

[0033] 2.3 Compare the inductor current stresses in different operating modes within the full power range through numerical analysis, and select the operating mode with the minimum current stress at the same transmission power. By numerically analyzing and comparing the local minimum current stresses of operating modes 1, 2, and 3, the power boundaries P A =(1 - k)(3k - 1), P B =2(1 - k 2 ) / (2 - k) 2 Divide the entire power range into three intervals. When 0 < P0 ≤ P A , the local optimal solution of the current peak value in mode 1 is less than that of other modes. Similarly, when P A < P0 ≤ P B and P B < P0 ≤ 1, the global minimum current peak value is in modes 2 and 3;

[0034] Based on the power intervals 0 < P0 ≤ P A , P A < P0 ≤ P B , P B < P0 ≤ 1, take the partial derivatives of the Lagrangian multiplier equation respectively. When 0 < k < 0.5, in different power intervals, the combined relationship between the duty cycle and the phase shift ratio is as follows:

[0035]

[0036]

[0037] When 0.5 < k < 1, in different power intervals, the combined relationship between the duty cycle and the phase shift ratio is as follows:

[0038]

[0039] Step 3: Collect the high-voltage side voltage V1 and the low-voltage side voltage V2 through the signal sampling unit, calculate the voltage conversion ratio k in real time, subtract the output feedback voltage V2 from the output voltage reference value V ref to obtain the voltage deviation, input the voltage deviation into the microprocessor, and calculate the phase shift angle through formula (8)

[0040]

[0041] In the formula, K p is the proportional constant, K i is the integral constant, and limit the magnitude of the phase shift angle to be between 0 and 1 / 2. If the phase shift angle is greater than 1 / 2, then make equal to 1 / 2; conversely, if the phase shift angle If less than 0, then make equal to 0;

[0042] Step 4: According to the phase-shift angle obtained in Step 3 and the voltage conversion ratio k, and according to the magnitude of the phase-shift angle and the voltage conversion ratio k corresponding to different working modes in Step 2, obtain the combined relationship between the phase-shift angle and the duty cycle under three different working modes, and further obtain the control quantities of the duty cycles D1, D2, and D3;

[0043] Step 5: Input the phase-shift angle obtained in Step 3 and the duty cycles D1, D2, and D3 obtained in Step 4 into the microcontroller, and output corresponding drive signals by the internal timer to adjust the switching times of the high-voltage side switching devices S1~S8 and the low-voltage side switching devices Q1~Q4. According to the actual adjustment effect, repeat Steps 3 and 4 until a relatively stable control effect is obtained. Description of the Drawings

[0044] Figure 1 is the internal structure connection diagram of the full-bridge bidirectional DC / DC converter of the present invention.

[0045] Figure 2 is the drive signal timing diagram of the three-level bidirectional DC-DC converter of the present invention.

[0046] Figure 3 is the control flow chart of the minimum current stress adjustment method of the full-bridge bidirectional DC / DC converter of the present invention. Detailed Embodiments

[0047] The following further describes the present invention with reference to the drawings.

[0048] Figure 1 is the internal structure and connection diagram of the high-capacity bidirectional DC / DC converter of the present invention, and can be used in combination with Figure 3 the control flow chart of the minimum current stress adjustment method shown to carry out the flexible adjustment and optimal control of the electric energy of the DC power supply system. Its usage method includes the following content:

[0049] Step 1: Installation of the DC / DC converter

[0050] 1.1 As Figure 1The internal structure and connection diagram of the large-capacity bidirectional DC / DC converter shown, a three-level bidirectional DC / DC converter under four-degree-of-freedom control, characterized in that it includes: input filter capacitors C1(1) and C2(2), a DC high-voltage module (3), an inductor L(4), a high-frequency isolation transformer T(5), a DC low-voltage module (6), an output filter capacitor C3(7), a signal acquisition unit, a control microprocessor (12), a high-voltage side DC bus (13), and a low-voltage side energy storage load (14); where:

[0051] The DC high-voltage module (3) is a diode neutral-point clamped three-level full-bridge circuit. Among them, fully controlled devices S1 to S4 and diodes D1 and D2 form a diode-clamped three-level bridge arm; fully controlled devices S5 to S8 and diodes D3 and D4 form a diode-clamped three-level bridge arm; the positive pole of the DC high-voltage module (3) is connected to the positive pole of the input filter capacitor C1(1) and the positive pole of the high-voltage side DC bus (12), and the negative pole is connected to the negative pole of C2(2) and the negative pole of the high-voltage side DC bus (12); the diode neutral points of the two bridge arms are connected to the series midpoint of C1 and C2; the AC side of the DC high-voltage module (3) is connected to the primary side of the transformer T(5) through the inductor L(4);

[0052] The DC low-voltage module (6) includes fully controlled devices Q1 to Q4, forming a basic H-bridge structure. The positive pole of the DC low-voltage module (6) is connected to the positive pole of the output filter capacitor C3(7) and the positive pole of the low-voltage side energy storage load (13); the negative pole is connected to the negative pole of the output filter capacitor C3(7) and the negative pole of the low-voltage side energy storage load (13); the AC side of the DC low-voltage module (6) is connected to the secondary side of the transformer T(5);

[0053] The signal acquisition unit includes a high-voltage side voltage detector (8), a high-voltage side current detector (9), a low-voltage side voltage detector (10), and a low-voltage side current detector (11);

[0054] The turns ratio of the high-frequency isolation transformer T(5) is N:1;

[0055] The signal acquisition unit is connected to the corresponding signal input terminals of the control microprocessor (8);

[0056] The control signal output terminals of the control microprocessor (8) are connected to the control signal input terminals of the fully controlled devices S1 to S8, Q1 to Q4.

[0057] 1.2 As Figure 2The driving signal timing diagram of the three-level bidirectional DC-DC converter is shown. Control signals are applied to the switching devices in the high-voltage DC module and the low-voltage DC module respectively through an embedded microprocessor. S1 and S3, S2 and S4, S5 and S7, S6 and S8, Q1 and Q2, Q3 and Q4 conduct complementarily. The duty cycles of S1 and S4, S5 and S8, Q1 and Q3 are D1, D2, and D3 respectively. The phase difference between devices S1 and Q1 is named the phase-shift angle. ( Figure 2 ) Among them, D1, D2, D3 ∈ [0, 1]; Step 2: Design of the minimum current stress control method for the converter

[0058] 2.1 Division of operating modes

[0059] According to the magnitude relationship of the duty cycles D1, D2, D3 and the phase-shift angle , the converter can be divided into the following three operating modes:

[0060] Mode 1, the corresponding phase-shift angle constraint condition is

[0061] Mode 2, the corresponding phase-shift angle constraint condition is

[0062] Mode 3, the corresponding phase-shift angle constraint condition is

[0063] 2.2 Define the voltage conversion ratio k = NV2 / V1, where V2 is the output voltage of the low-voltage side and V1 is the input voltage of the high-voltage side; analyze the mathematical models of the transmission power and current stress of the converter as follows:

[0064] Calculation expression of inductor current

[0065]

[0066] In the formula, v ab (t), v cd (t) are the midpoint voltages of the primary and secondary bridge arms;

[0067] Take the maximum value of the inductor current at different times as the current stress i p , that is

[0068]

[0069] The per-unit power calculation formula is as follows:

[0070]

[0071] In the formula, P N = nV1V2 / (8f s L) is the maximum transmission power, f sis the switching frequency;

[0072] According to the per-unit power calculation formula, the per-unit powers of the three operating modes can be calculated as follows:

[0073]

[0074] 2.3 By numerically analyzing the inductor current stress under different operating modes within the full power range, comparing the current stress magnitudes of different operating modes at the same transmission power, and selecting the operating mode with the minimum current stress; through numerical analysis and comparison of the local minimum current stresses of operating modes 1, 2, and 3, the power boundaries P A =(1 - k)(3k - 1), P B =2(1 - k 2 ) / (2 - k) 2 The entire power range is divided into three intervals. When 0 < P0 ≤ P A , the local optimal solution of the current peak value of mode 1 is less than that of other modes. Similarly, when P A < P0 ≤ P B and P B < P0 ≤ 1, the global minimum current peak value is in modes 2 and 3;

[0075] Based on the power intervals 0 < P0 ≤ P A , P A < P0 ≤ P B , P B < P0 ≤ 1, respectively taking partial derivatives of the Lagrangian multiplier equation, when 0 < k < 0.5, in different power intervals, the combination relationship between the duty cycle and the phase shift ratio is as follows:

[0076]

[0077] When 0.5 < k < 1, in different power intervals, the combination relationship between the duty cycle and the phase shift ratio is as follows:

[0078]

[0079] Step 3: Collect the high-voltage side voltage V1 and the low-voltage side voltage V2 through the signal sampling unit, calculate the voltage conversion ratio k in real time, subtract the output feedback voltage V2 from the output voltage reference value V ref to obtain the voltage deviation amount, input the voltage deviation amount into the microprocessor, and calculate the phase shift angle through formula (8)

[0080]

[0081] where K p is the proportional constant, K i is the integral constant, and the phase shift angle is restricted The magnitude is between 0 and 1 / 2. If the phase-shift angle is greater than 1 / 2, then make equal to 1 / 2; conversely, if the phase-shift angle is less than 0, then make equal to 0;

[0082] Step 4: According to the phase-shift angle obtained in Step 3 and the voltage conversion ratio k, and according to the magnitude of the phase-shift angle and the voltage conversion ratio k corresponding to different working modes in the second step, obtain the combined relationship between the phase-shift angle and the duty cycle under three different working modes, and further obtain the control quantities of the duty cycles D1, D2, and D3;

[0083] Step 5: Input the phase-shift angle obtained in Step 3 and the duty cycles D1, D2, and D3 obtained in Step 4 into the control microprocessor (8), and the internal timer outputs corresponding drive signals to adjust the switching times of the high-voltage side switching devices S1~S8 and the low-voltage side switching devices Q1~Q4. According to the actual adjustment effect, repeat Step 3 and Step 4 until the output voltage V2 is stabilized to the set voltage reference value V ref .

[0084] In addition, the present invention can also implement the following test functions:

[0085] For the minimum current stress regulation method of the large-capacity three-level bidirectional DC-DC converter, the values of the control parameters D1, D2, and D3 can be adjusted according to the needs of the actual system. When the electric energy transmission is from the low-voltage side to the high-voltage side, the expression in Equation (8) can be taken as negative to achieve the same function.

Claims

1. A method for regulating the minimum current stress of a high-capacity three-level bidirectional DC-DC converter, used to carry out flexible regulation and optimal control of electrical energy in a DC power supply system, characterized by the following steps: The first step: Installation of the DC / DC converter 1.1 Three-level bidirectional DC / DC converter under four-degree-of-freedom control, characterized in that, Including: Input filter capacitors C1(1) and C2(2), DC high-voltage module (3), inductor L(4), high-frequency isolation transformer T(5), DC low-voltage module (6), output filter capacitor C3(7), signal acquisition unit, control microprocessor (9), high-voltage side DC bus (12), low-voltage side energy storage load (13); where: The signal acquisition unit includes a high-voltage side voltage detector (8), a high-voltage side current detector (9), a low-voltage side voltage detector (10), and a low-voltage side current detector (11); The turns ratio of the high-frequency isolation transformer T(5) is N:1; The signal acquisition unit is connected to the corresponding signal input terminals of the control microprocessor (8); The control signal output terminal of the control microprocessor (8) is connected to the control signal input terminals of the fully-controlled devices S1~S8, Q1~Q4; 1.2 Apply control signals to the switching devices in the DC high-voltage module and the DC low-voltage module respectively through the embedded microprocessor. S1 and S3, S2 and S4, S5 and S7, S6 and S8, Q1 and Q2, Q3 and Q4 conduct complementarily. The duty cycles of S1 and S4, S5 and S8, Q1 and Q3 are D1, D2, and D3 respectively. The phase difference between devices S1 and Q1 is named the phase-shift angle (Figure 2); where D1, D2, D3 ∈ [0, 1]; Step 2: Design of the minimum current stress control method for the converter 2.1 Division of working modes According to the magnitude relationship of the duty cycles D1, D2, D3 and the phase shift angle , the converter can be divided into the following three operating modes: Mode 1, and the corresponding phase-shift angle constraint is Mode 2, and the corresponding phase-shift angle constraint is Mode 3, and the corresponding phase-shift angle constraint is 2.2 Define the voltage conversion ratio k = NV2 / V1, where V2 is the low-voltage side output voltage and V1 is the high-voltage side input voltage; analyze the mathematical models of the transmission power and current stress of the converter as follows: The calculation expression of the inductor current is as follows: where, v ab (t), v cd (t) is the voltage at the midpoint of the secondary side bridge arm; Take the maximum value of the inductor current at different times as the current stress ip, as follows: The per-unit power calculation formula is as follows: Where P N = nV1V2 / (8f s L) is the maximum transmission power, and f s is the switching frequency; According to the per-unit power calculation formula, the per-unit powers of the three working modes can be calculated as follows: 2.3 Compare the inductor current stress under different operating modes within the full power range through numerical analysis, compare the current stress magnitudes of different operating modes at the same transmission power, and select the operating mode with the minimum current stress; through numerical analysis and comparison of the local minimum current stress of operating modes 1, 2, and 3, the power boundary P A =(1 - k)(3k - 1), P B =2(1 - k 2 ) / (2 - k) 2 Divide the entire power range into three intervals. When 0 < P0 ≤ P A , the local optimal solution of the current peak of mode 1 is less than that of other modes. Similarly, when P A < P0 ≤ P B and P B < P0 ≤ 1, the global minimum current peak is in modes 2 and 3; 2.4 Based on the mathematical models of the transmission power and current stress of each working mode of the converter described in 2.2, establish the Lagrange multiplier equation as follows: where \(L(D,\lambda,u i )\) is the Lagrange polynomial, \(I P * (D)\) is the current stress optimization expression, \(P_0 * (D)-P_0 * \) is the reference transmission power equality constraint, \(f i (D)\) is the inequality constraint; Based on the power range 0 < P0 ≤ P described in 2.3 A , P A < P0 ≤ P B , P B < P0 ≤ 1, taking the partial derivatives of the Lagrange multiplier equations respectively, when 0 < k < 0.5, in different power ranges, the combined relationship between the duty cycle and the phase shift ratio is as follows: Mode 1: Mode 2: Mode 3: When 0.5 < k < 1, in different power intervals, the combination relationship between the duty cycle and the phase shift ratio is as follows: Mode 1: Mode 2: Mode 3: Step 3: Collect the high-voltage side voltage V1 and the low-voltage side voltage V2 through the signal sampling unit, calculate the voltage conversion ratio k in real time, and obtain the output voltage reference value V ref Compare and subtract it from the output feedback voltage V2 to obtain the voltage deviation. The voltage deviation is input into the microprocessor, and the phase shift angle is calculated through formula (8) where K p is a proportional constant, K i is an integral constant, and the phase shift angle is restricted to be between 0 and 1 / 2. If the phase shift angle is greater than 1 / 2, then is made equal to 1 / 2; conversely, if the phase shift angle is less than 0, then is made equal to 0; Step 4: According to the phase-shifting angle obtained in Step 3 and the voltage conversion ratio k, and corresponding to different working modes in Step 2 according to the magnitudes of the phase-shifting angle and the voltage conversion ratio k, obtain the combined relationship between the phase-shifting angle and the duty cycle under three different working modes, and further obtain the control quantities of the duty cycles D1, D2, and D3; Step 5: Input the phase-shifting angle obtained in Step 3 and the duty cycles D1, D2, and D3 obtained in Step 4 into the microcontroller. The internal timer outputs corresponding drive signals to adjust the switching times of the high-side switching devices S1~S8 and the low-side switching devices Q1~Q4. According to the actual adjustment effect, repeat Step 3 and Step 4 until a relatively stable control effect is achieved.

Images