DAB converter backflow power suppression method based on Runge-Kutta method

Abstract

The invention discloses a DAB converter backflow power suppression method based on a Runge-Kutta method, and the method comprises the steps: firstly selecting an auxiliary inductance value and an input capacitance value, and constructing a DAB converter; secondly, through volt-second balance and inductive current symmetry, calculating a relational expression between the backflow power of the DAB converter and an intra-bridge shift ratio D1 and a relational expression between the backflow power and an inter-bridge shift ratio D2, obtaining a relational expression between the backflow power and the D1, and establishing a fourth-order Runge-Kutta equation about the backflow power and the D1; and according to a fourth-order Runge-Kutta equation of the backflow power and the D1, solving a global optimal solution and values of the D1 and the D2. And finally, the solved D1 and D2 values are used for PWM wave modulation, and the backflow power is reduced under the condition that the transmission power is not changed. According to the invention, faster dynamic response is realized, and the backflow power is reduced on the basis of not changing the transmission power, so that the heat production is reduced, and the efficiency of the converter is improved.

Description

Technical Field

[0001] The invention belongs to the technical field of power electronics, and in particular relates to a DAB converter backflow power suppression method based on the Runge-Kutta method. Background Art

[0002] Energy storage power stations have been vigorously developed because they can effectively respond to the fluctuations of renewable energy and enhance the utilization rate of renewable energy. As a key link in energy conversion, bidirectional DC-DC converters play a vital role in the battery management system (BMS) of energy storage power stations. The hybrid three-level dual-active full-bridge converter (DAB) can realize the bidirectional flow of power. Its application in BMS can significantly improve the flexibility and reliability of the system. Compared with traditional DC-DC converters, it has higher power density, is easier to achieve soft switching, and can also reduce costs due to the lower voltage stress on the primary side. Currently, one of the effective solutions to improve the efficiency of DAB converters is to reduce the return power. In traditional DAB circuits, the simplest control method is single phase shift control (SPS). However, this method will generate a large amount of return power when the power is mismatched, and the soft switch will be lost, resulting in a serious reduction in efficiency. In order to solve the above problems, extended phase shift control (EPS) was proposed. This method reduces the return power and realizes full-range soft switching by controlling an additional degree of freedom on the primary side. However, when calculating using the Lagrange multiplier method, it was found that when the voltage transfer ratio is constant using traditional extended phase shift control, the expressions of the inner shift ratio and the outer shift ratio show a linear function relationship. Therefore, if you want to reduce the return power, the transmission power will also be greatly reduced. Summary of the invention

[0003] In view of the above-mentioned problems, the present invention provides a DAB converter backflow power suppression method based on the Runge-Kutta method, which is solved by the fourth-order Runge-Kutta formula. By adding a degree of freedom to the secondary full bridge, the backflow power can be reduced without changing the transmission power, which is of great significance to improving the operating efficiency and reliability of the battery management system of the energy storage power station.

[0004] The present invention is achieved through the following technical solutions:

[0005] Step 1: Select the inductance of the auxiliary inductor L1 and the capacitance of the input capacitors C1 and C2 to build a hybrid three-level dual-active full-bridge DAB converter.

[0006] Step 2: By using volt-second balance and inductor current symmetry, calculate the relationship between the DAB converter return power and the bridge inner shift ratio D1 and the bridge inter-shift ratio D2, and derive the relationship between the return power and D1.

[0007] Step 3: According to the relationship between the return power and D1, the fourth-order Runge-Kutta equation for the return power and D1 is established.

[0008] Step 4: According to the fourth-order Runge-Kutta equation of return power and D1, solve the global optimal solution and the values ​​of D1 and D2.

[0009] Step 5: Use the solved values ​​of D1 and D2 for PWM wave modulation to achieve the effect of reducing the return power without changing the transmission power.

[0010] Advantages or beneficial effects of the present invention:

[0011] The present invention adds a degree of freedom on the basis of traditional extended phase shift control, which not only optimizes the performance, achieves faster dynamic response by more accurately adjusting the parameters of the controller, but also reduces the return power without changing the transmission power, thereby reducing heat production, reducing the thermal management burden, and greatly improving the efficiency of the converter. At the same time, reducing the return power reduces the stress of the power device, thereby extending its life and improving the stability and reliability of the entire system. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Figure 1 is a wireframe schematic diagram of the present invention;

[0013] Figure 2 It is the topology diagram of hybrid three-level dual active full-bridge converter;

[0014] Figure 3 It is the PWM pulse sequence diagram;

[0015] Figure 4 This is the expression diagram of the return power and D1 and D2 of the hybrid three-level dual active full-bridge converter. DETAILED DESCRIPTION

[0016] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is further described below in conjunction with the accompanying drawings and implementation methods.

[0017] A wireframe schematic diagram of a DAB converter backflow power suppression method based on the Runge-Kutta method provided by the present invention is shown in FIG. Figure 1 As shown, the backflow power suppression method of this topology includes the following five steps:

[0018] Step 1: Select the values ​​of key parameters such as the auxiliary inductor L1, the input capacitors C1 and C2 to build a hybrid three-level dual active full-bridge DAB converter;

[0019] Step 2: By using volt-second balance and inductor current symmetry, the relationship between the return power of the hybrid three-level dual-active full-bridge converter and the inner-bridge shift phase D1 and the inter-bridge shift phase D2 is calculated, and then the relationship between the return power and D1 is further obtained by Lagrange number multiplication;

[0020] Step 3: According to the relationship between the return power and the bridge displacement D1, the fourth-order Runge-Kutta equation about the return power and D1 is established;

[0021] Step 4: Solve the fourth-order Runge-Kutta equation of the return power and D1 to obtain the global optimal solution and the values ​​of D1 and D2;

[0022] Step 5: Use the solved D1 and D2 values ​​for PWM wave modulation and compare them with the original modulation method to reduce the return power without changing the transmission power.

[0023] The hybrid three-level dual active full-bridge converter is as follows Figure 2 As shown, it is composed of a DC voltage source, a primary diode clamped hybrid three-level full bridge H1, a secondary two-level full bridge H2, input capacitors C1-C2, an output capacitor C3 and a high-frequency isolation transformer;

[0024] The positive electrode of the input capacitor C1 is connected to the positive electrode of the DC voltage source, the negative electrode of the input capacitor C1 is connected to the positive electrode of the input capacitor C2, and the negative electrode of the DC voltage source is connected to the negative electrode of the input capacitor C2;

[0025] The primary diode clamped hybrid three-level full bridge comprises 8 fully controlled switch tubes Q1-Q8, clamping diodes D1-D4 and auxiliary inductor L1, wherein the emitter of switch tube Q1 is connected to the collector of Q2, the emitter of switch tube Q2 is connected to the collector of Q3, the emitter of switch tube Q3 is connected to the collector of Q4, the emitter of switch tube Q5 is connected to the collector of Q6, the emitter of switch tube Q6 is connected to the collector of Q7, the emitter of switch tube Q7 is connected to the collector of Q8, the collectors of switch tubes Q1 and Q5 are connected to the positive electrode of capacitor C1, and the emitters of switch tubes Q4 and Q8 are connected to the negative electrode of capacitor C2; one side of inductor L1 is connected to the connecting line between switch tube Q2 and switch tube Q3 The other side of the inductor L1 is connected to one end of the primary side of the high-frequency isolation transformer, and the other end of the primary side of the high-frequency isolation transformer is connected to the connection line between the switch tube Q6 and the switch tube Q7; the positive electrode of the clamping diode D1 is connected to the negative electrode of the clamping diode D2, the negative electrode of the clamping diode D1 is connected to the connection line between the switch tube Q1 and the switch tube Q2, and the positive electrode of the clamping diode D2 is connected to the connection line between the switch tube Q3 and the switch tube Q4; the positive electrode of the clamping diode D3 is connected to the negative electrode of the clamping diode D4, the negative electrode of the clamping diode D3 is connected to the connection line between the switch tube Q5 and the switch tube Q6, and the positive electrode of the clamping diode D4 is connected to the connection line between the switch tube Q7 and the switch tube Q8;

[0026] The secondary full bridge includes four fully controlled switch tubes S1 to S4, the emitter of switch tube S1 is connected to the collector of S2, the emitter of switch tube S3 is connected to the collector of S4, the collectors of switch tubes S1 and S3 are connected to the positive electrode of capacitor C3, and the emitters of switch tubes S2 and S4 are connected to the negative electrode of capacitor C3; the two ends of the secondary side of the high-frequency isolation transformer are respectively connected to the connecting line between switch tube S1 and switch tube S2 and the connecting line between switch tube S3 and switch tube S4;

[0027] As described in step 2, in one control cycle, 0~T hs The waveform and T hs ~T s The waveform is completely symmetrical, so the present invention only analyzes 0~T hs When the DAB converter works in a stable state, the terminal voltage and current of the inductor have the following relationship:

[0028]

[0029] Wherein, Vab is the output voltage of the primary bridge arm, and Vcd is the output voltage of the secondary bridge arm; i L (t) is the current flowing through the inductor L1 at time t, and the transformation ratio of the high-frequency isolation transformer is n:1.

[0030] According to the symmetry of the inductor current:

[0031] i(to)=—i(t4)

[0032] Combining the relationship between the inductor voltage and current, the inductor current expression at each moment can be obtained:

[0033]

[0034] Where Vin is the input voltage, Vo is the output voltage; fs is the switching frequency, D1 is the internal displacement of the primary full bridge H1 and the secondary full bridge H2; D2 is the external displacement of the secondary full bridge H1 relative to the primary full bridge H1. The transmission power is calculated by the average power calculation formula and normalized:

[0035]

[0036] T s For a switching cycle, Th s is half a switching cycle, P n The maximum transmission power of the converter under SPS control is:

[0037]

[0038] Combining the above formula, the expression of the converter's transmission power P* is:

[0039]

[0040] like Figure 3 As shown, according to the PWM waveform, there is a return power during t1 to t'1, and the normalized expression of the return power is calculated as:

[0041]

[0042] like Figure 4 As shown, the integral is:

[0043]

[0044] To suppress the return power while keeping the transmission power unchanged, a common method is the Lagrange multiplier method, which is effective in solving the problem of finding the extreme value under such constraints. Therefore, the present invention adopts this method for derivation. First, the Lagrange multiplier method equation relationship for finding the extreme value of the return power under the transmission power constraint is defined:

[0045]

[0046] Where P is the target transmission power and λ is the Lagrange multiplier. Taking the partial derivative of L with respect to D1 and D2 and setting it equal to 0, we can get:

[0047]

[0048] Eliminating the common term λ in the two equations gives the simplest expressions of D1 and D2:

[0049]

[0050] As described in step 3, let y be the return power P t * , x is the full bridge internal displacement D1, take the first-order derivative of the return power with respect to D1, and substitute the expression into it to get:

[0051]

[0052] Substituting the expression into the return power P t * Regarding the differential equation of the full-bridge inward shift relative to D1, let x0 and y0 represent the independent variable value and function value of the 0th iteration state classification, that is, the initial value.

[0053] The required value can be simplified to P * The fourth-order Runge-Kutta method is used to solve the optimization problem of the y function with constraints. This method is a high-precision single-step method for solving ordinary differential equations. By discretizing the solution domain of the differential equation, the value y at the current moment is obtained. nAdding the time interval h and the calculated slope to predict the value of the next moment, the slope of the fourth-order Runge Kutta method is calculated by weighted average of the four evaluation points. The calculation formula of the fourth-order Runge Kutta method is as follows:

[0054]

[0055] Among them, h is the step size after the state variable is discretized, K1, K2, K3, K4 represent the slopes of the four evaluation points respectively, and x m Indicates the independent variable value of the mth iteration state classification, y m Represents the function value of the state classification at the mth iteration.

[0056] As described in step 4, considering the actual achievable accuracy range, the present invention sets the step length h to 0.01, and obtains a more accurate prediction result with lower computational complexity. The global optimal solution is calculated At this time, the simplest expression of D1 and D2 is D1=0.45. Comparing the three-dimensional graphs of reflux power and D1 and D2, the results are relatively consistent.

[0057] As shown in step 5, the present invention uses common parameters of energy storage power stations for verification, and the parameters are: V in =720v, V o =24v, n=15; the simulation circuit verifies that the reflux power is It is basically consistent with the theoretical value calculated above, and the transmission power remains unchanged, successfully achieving the effect of suppressing the backflow power.

[0058] The present invention combines Lagrange number multiplication with the fourth-order Runge-Kutta method, obtains the first four data through simulation for initial parameter confirmation and model fitting, and uses the fourth-order Runge-Kutta method to calculate the state variable y(x), assuming x0=0, and the update step size is: x m+1 =x m +h, and keep iterating in this form to find the minimum value of the indicator function in the solution domain, so as to obtain the global optimal solution.

[0059] On the basis of the traditional extended phase-shift control strategy, a degree of freedom D3 is added to the secondary full bridge. During the calculation process, it is found that when D3=D1, the purpose of reducing the reflux power and improving the system control accuracy can be achieved. Therefore, in the present invention, D3=D1 is directly set, which improves the efficiency and reduces the complexity of control to the greatest extent. At the same time, since D1 can only be modified with an accuracy of 0.01 in practice, it is more suitable to adopt the fourth-order Runge-Kutta method, using a fixed step size to achieve high efficiency and high-precision results with the minimum amount of calculation.

Claims

1. A DAB converter backflow power suppression method based on Runge-Kutta method, characterized in that: The following steps are involved: Step 1: Select the inductance of the auxiliary inductor L1 and the capacitance of the input capacitors C1 and C2 to build a hybrid three-level dual active full-bridge DAB converter; Step 2: Calculate the relationship between the DAB converter return power and the bridge internal shift phase D1 and the bridge inter-shift phase D2, and obtain the relationship between the return power and D1; Step 3: According to the relationship between the return power and D1, the fourth-order Runge-Kutta equation about the return power and D1 is established to modulate the PWM wave and complete the return power suppression.

2. The DAB converter backflow power suppression method based on the Runge-Kutta method according to claim 1 is characterized in that: The specific implementation process of step 2 is as follows: When the DAB converter works in a stable state, the terminal voltage and current of the inductor have the following relationship: Among them, V ab is the primary bridge arm output voltage, V cd is the output voltage of the secondary bridge arm; i L (t) is the current flowing through the inductor L1 at time t, and the transformation ratio of the high-frequency isolation transformer is n:1; According to the symmetry of the inductor current: and L (t0)=-i L (t4) Combining the relationship between the inductor voltage and current, the inductor current expression at each moment is obtained: Where Vin is the input voltage, Vo is the output voltage; fs is the switching frequency, D1 is the internal displacement of the primary full bridge H1 and the secondary full bridge H2; D2 is the external displacement of the secondary full bridge H1 relative to the primary full bridge H1; the transmission power is calculated by the average power calculation formula and normalized: Where T hs is half a switching cycle, P n The maximum transmission power of the converter under SPS control is: Combining the above formula, the transmission power P of the converter is obtained * The expression is: According to the PWM waveform, there is reflux power during t1~t′1, and the normalized expression of reflux power is calculated as: Integrating gives: Define the Lagrange multiplier method equation for finding the extreme value of return power under the transmission power constraint: Where P is the target transmission power and λ is the Lagrange multiplier. Taking the partial derivative of L with respect to D1 and D2 and setting it equal to 0, we get: Eliminating the common term λ in the two equations, we can obtain the simplest expressions of D1 and D2:

3. The DAB converter backflow power suppression method based on the Runge-Kutta method according to claim 2 is characterized in that: According to the relationship between the return power and the bridge displacement D1, the specific implementation process of establishing the fourth-order Runge-Kutta equation about the return power and D1 is as follows: Let y be the return power P t * , x is the full bridge internal displacement D1, take the first-order derivative of the return power with respect to D1, and substitute the expression into it to get: Substituting the expression into the return power P t * Regarding the differential equation of the full-bridge inward shift phase D1, let x0 and y0 represent the independent variable value and function value of the 0th iteration state classification, i.e., the initial value; What we want is to simplify it to P * The optimization problem of the y function with constraints is solved by the fourth-order Runge-Kutta method. By discretizing the solution domain of the differential equation, the value y at the current moment is obtained. n The time interval h and the calculated slope are added to predict the value of the next moment. The slope of the fourth-order Runge Kutta method is calculated by weighted average of four evaluation points. The calculation formula of the fourth-order Runge Kutta method is as follows: Among them, h is the step size after the state variable is discretized, K1, K2, K3 and K4 represent the slopes of the four evaluation points respectively, and x m Indicates the independent variable value of the mth iteration state classification, y m Represents the function value of the state classification at the mth iteration.

4. The DAB converter backflow power suppression method based on the Runge-Kutta method according to claim 3 is characterized in that: The specific implementation process of performing PWM wave modulation and completing reflux power suppression described in step 3 is as follows: According to the fourth-order Runge-Kutta equation of return power and D1, the global optimal solution and the values ​​of D1 and D2 are obtained; The solved values ​​of D1 and D2 are used for PWM wave modulation to reduce the return power without changing the transmission power.

Images