Control method of AC / DC microgrid interconnected converter based on triple phase shift plus frequency conversion control

Abstract

The present invention relates to the field of AC / DC microgrids, and specifically to an AC / DC microgrid interconnected converter control method based on triple phase shift plus frequency conversion control. The present invention solves the problems of nonlinear power control, difficult control link design, low equipment power density, large size of voltage-stabilizing electrolytic capacitors, and severe switching losses in traditional solutions. The isolated grid-connected three-phase converter is composed of a dual active bridge converter and a full-bridge inverter. The control flexibility is increased through a triple phase shift modulation method, and the linear control of the output power by the control coordinate is realized, thereby achieving a better control effect; the voltage-stabilizing electrolytic capacitor on the DC side of the converter is replaced with a smaller film capacitor, which reduces the design cost and hardware cost of the converter while improving the system operation reliability, and improves the converter power density; the 33% PWM modulation method designed for the rear-stage full-bridge circuit of the three-phase converter greatly reduces the switch tube loss compared to the traditional PWM modulation method, thereby improving the system operation efficiency.

Description

Technical Field

[0001] The present invention relates to the field of AC / DC microgrids, and in particular to an AC / DC microgrid interconnected converter control method based on triple phase shift plus frequency conversion control. Background Art

[0002] As an important part of the global energy Internet system, electric energy plays a pivotal role in it. In recent years, renewable energy represented by wind energy and photovoltaics has been widely valued as a series of clean energy sources worldwide. The AC / DC hybrid microgrid that connects new energy power to the traditional AC grid is the main form of the current power grid. In the AC / DC microgrid, the bidirectional isolated AC / DC microgrid interconnected converter plays a key role in the transmission of energy between the DC distribution network and the AC grid system in the power system. At the same time, it is widely used in uninterruptible power supply, battery energy storage system and distributed power generation, and has become a hot topic of research. The AC / DC converter structure with dual active bridges has attracted widespread attention from experts and scholars at home and abroad due to its advantages such as high power density, modularization, symmetrical structure and relatively simple control. In the study of the control strategy of the dual active bridge converter, the single phase shift control has the disadvantages of large current stress and the inability to eliminate the reflux power. In the triple phase shift control, the converter has three control variables, and the control dimension and flexibility are improved, which can achieve better control effect.

[0003] In the control strategy for isolated AC / DC microgrid interconnected converters, traditional two-stage control is a common control method, including the phase-shift control method of the front-stage dual active bridge converter and the SPWM control method of the rear-stage three-phase full-bridge topology. This scheme has three disadvantages: the nonlinear relationship between the control coordinates of the traditional phase-shift control and the output power increases the difficulty of power control; the switch tubes that make up the rear-stage three-phase inverter always work in a high-frequency state, which increases the switching loss; the voltage-stabilizing electrolytic capacitor is large in size, which reduces the power density of the converter and increases the probability of failure during system operation, reducing the reliability of system operation. Summary of the invention

[0004] In order to solve the technical problems of nonlinear transmission power control of traditional isolated AC / DC microgrid interconnected converters, high switching frequency of three-phase inverters, and large size of DC-side electrolytic capacitors, the present invention designs an AC / DC microgrid interconnected converter control method based on triple phase shifting plus frequency conversion control in view of the defects of the existing technology.

[0005] The present invention is realized based on the following technical scheme: a control method of an AC / DC microgrid interconnected converter based on triple phase shift plus frequency conversion control, wherein the AC / DC microgrid interconnected converter adopts an isolated AC / DC three-phase converter; the isolated AC / DC three-phase converter topology structure is composed of a DAB converter and a three-phase full-bridge inverter SI, and the DAB converter at the front end is composed of four primary switches S1 , S 2 , S 3 , S 4 The inverter full bridge FB 1 , a high frequency transformer T r , leakage inductance L and four secondary switches S 5 , S 6 , S 7 , S 8 The rectifier full bridge FB 2 The DAB converter is composed of a filter capacitor C 2 Cascaded back-end inverter. The three-phase full-bridge inverter SI contains six active switches Q 1 , Q 2 , Q 3 , Q 4 , Q 5 , Q 6 , SI through the output side filter inductor L 0 And output side filter capacitor C 0 Connect to AC power supply. The duty cycle of all switch device drive pulses of DAB dual active bridge converter is 50%. T is half a switching cycle, satisfying T =1 / (2 f s ), f s is the actual switching frequency. The DC power supply size is u dc , the AC power supply size is u g , FB 1 The voltage difference at the midpoint of the bridge arm is v p , FB 2 The voltage difference at the midpoint of the bridge arm is v s , the AC current at the SI output side is i ac . Compared with the inverter bridge moving inward D 1 Represents S 1 and S 4 Compared with the shift between the two, the shift outside the two D 2 Represents S 1 and S 5 Compared with the shift between the two, the shift inside the rectifier bridge is D 3 Represents S 5 and S 8 When power is transmitted in the forward direction, D1 、D 2 and D 3 The domain of is [0,1]. Voltage control is used to maintain the output three-phase voltage stability without adding additional current sensors. The details are as follows:

[0006] By driving the switch tube to modulate the FB 2 The 300Hz DC waveform is output, and then the six switches of the back-end full-bridge inverter are modulated to output a three-phase balanced voltage waveform on the AC side. Since the dual-active converter switches operate at high frequency, the rated switching frequency is f s =100kHz, and film capacitor C 2 The capacity is very small, so the output voltage waveform of the dual active bridge converter has less harmonic content. The control block diagram of the isolated AC / DC microgrid interconnected converter is shown in Figure 2 In order to improve the nonlinear relationship between transmission power and control quantity in the traditional scheme, variable frequency control is introduced and virtual frequency is defined. f n , which is a fixed value, sets the size of the virtual frequency to

[0007] f n = f s / (1-2 α )(1)

[0008] u g_ref Indicates the output voltage reference value. Since the design is to improve the performance of the converter in steady state, a fixed input voltage is used. u g_ref Instead of the actual output voltage u g to avoid u g The value is very small, resulting in inaccurate calculation of the output current reference value. Define the phase shift control amount α In order to achieve linear control of the control variable on the transmission power, the inner shift ratio and the outer shift ratio must satisfy

[0009] D 1 =0.5-2 α , D 2 =0.5- α , D 3 =0(2)

[0010] Output voltage reference value ug_ref The actual output voltage u g The difference is output by the PI controller as the control quantity auxiliary modulation signal Δ α ; P ref is the transmission power reference value, that is

[0011] P ref = u g i ac (3)

[0012] The calculation formula of the shift phase control amount in the control method is:

[0013] α * =i ac f n L / NU dc (4)

[0014] The above two equations can be used to calculate the phase shift control amount main modulation signal α * , which is related to the control quantity auxiliary modulation signal Δ α The difference is used as the final shift control quantity to modulate the converter drive signal and control the DAB switching frequency to change over time. The main modulation signal derived from formula (4) is not affected by the power transmission direction, so the controller has excellent dynamic response performance when the system power direction changes. Modulation signal D 1 、D 2 and D 3 It can be obtained by solving formula (2). In summary, the adopted control strategy only needs the AC bus voltage as a feedback parameter, so the optimized control strategy can be more easily implemented in an embedded processor (such as a DSP chip).

[0015] Furthermore, the back-end three-phase full-bridge inverter of the present invention adopts a 6-pulse 33% PWM control strategy to output a three-phase voltage waveform: in order to improve the power density of the converter, a small-capacity film capacitor is used instead of a large-volume voltage-stabilizing electrolytic capacitor, and the driving signal of the two switch tubes in each bridge arm is a symmetrical square wave. At any time, only one bridge arm of the three-phase inverter adopts PWM modulation. For example, the switch device Q 1 , Q 2 In T 1 V ab / V cbAs the modulation pulse signal, the driving signals of the other four switching devices are ON or OFF. The three-phase full-bridge driving signal is as follows Figure 5 As shown, V m_x Q 1 , Q 2 The modulation signal, V m_y Q 3 , Q 4 The modulation signal, V m_z Q 5 , Q 6 Modulation signal. Divide one cycle into 6 equal time periods T 1 ~T 6 , with time period T 1 For example, Q 5 and Q 4 Open, Q 3 and Q 6 Shutdown, Q 1 and Q 2 By using V ab / V cb As the modulation signal, it is combined with the carrier waveform V c_ref The turn-on pulse of the device is obtained by comparison, and the turn-on signals of the remaining five time periods follow a similar rule.

[0016] The three-phase full-bridge adopts a 33% PWM control strategy to further reduce the converter switching loss. That is, the six switches of the back-end full-bridge inverter only work in the PWM stage for 1 / 3 of the time, and remain on or off for the rest of the time. Figure 5 shown.

[0017] Compared with the prior art, the control method of the AC / DC microgrid interconnected converter based on triple phase shift plus frequency conversion control provided by the present invention has the following advantages and positive effects: (1) the control flexibility is increased by the triple phase shift modulation method, and the linear control of the output power by the control coordinate is realized by frequency conversion control, thereby achieving a better control effect; (2) the voltage-stabilizing electrolytic capacitor on the DC side of the converter is replaced with a smaller film capacitor, which reduces the design cost and hardware cost of the converter while improving the system operation reliability, thereby improving the converter power density; (3) the 33% PWM modulation method based on the rear-stage full-bridge circuit of the three-phase converter greatly reduces the switch tube loss compared with the traditional PWM modulation method, thereby improving the system operation efficiency. The control method has good practicality. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 It is a structural diagram of an AC / DC microgrid interconnected converter based on current stress optimization of the topology diagram involved in the present invention;

[0019] Figure 2It is a simplified voltage closed-loop control block diagram of the AC / DC microgrid interconnected converter involved in the present invention;

[0020] Figure 3 It is a power forward transfer waveform diagram of the AC / DC microgrid interconnection converter involved in the present invention;

[0021] Figure 4 It is a power reverse transfer waveform diagram of the AC / DC microgrid interconnected converter involved in the present invention;

[0022] Figure 5 It is a schematic diagram of modulation signals of a rear-stage three-phase full-bridge converter involved in the present invention;

[0023] Figure 6 It is a schematic diagram of modulation waveform of the rear-stage three-phase full-bridge converter involved in the present invention;

[0024] Figure 1 Middle: The isolated AC-DC three-phase converter topology consists of a DAB converter and a three-phase inverter SI. The front-end DAB converter consists of four primary switches S 1 , S 2 , S 3 , S 4 Inverter full bridge FB 1 , a high frequency transformer T r , leakage inductance L and contains four secondary switches S 5 , S 6 , S 7 , S 8 The DAB converter is composed of a rectifier full bridge, the transformer ratio is 1:n, and the DAB converter is connected through the filter capacitor C 2 Cascaded back-end inverter. The three-phase full-bridge inverter SI contains six active switches Q 1 , Q 2 , Q 3 , Q 4 , Q 5 , Q 6 , SI through the output side filter inductor L 0 And output side filter capacitor C 0 Connect AC power. The DC power supply is u dc , the AC power supply size is u g , FB 1 The voltage difference at the midpoint of the bridge arm is v p , FB 2 The voltage difference at the midpoint of the bridge arm is v s , the AC current at the SI output side is i ac. i 1 is the DAB output current, i 2 It is the input current of the three-phase inverter full bridge.

[0025] Figure 2 Middle: Compared with the inverter bridge moving inward D 1 Represents S 1 and S 4 Compared with the shift between the two, the shift outside the two D 2 Represents S 1 and S 5 Compared with the shift between the two, the shift inside the rectifier bridge is D 3 Represents S 5 and S 8 When power is transmitted in the forward direction, D 1 and D 2 The domain of is [0,1]. u g Indicates the converter output voltage. u g_ref Indicates the output voltage reference value. Since the design is to improve the performance of the converter in steady state, a fixed output voltage is used. u g_ref Instead of the actual output voltage u g to avoid u g A very small value results in inaccurate calculation of the output current reference value. α For the control quantity of the shift ratio, the output voltage reference value u g_ref The actual output voltage u g The difference is output by the PI controller as the control quantity auxiliary modulation signal Δ α ; P ref The reference value of the transmission power is calculated by formula (3) to obtain the reference value of the output current, and then the main modulation signal of the phase shift control quantity is calculated by formula (4). α * , which is related to the control quantity auxiliary modulation signal Δ α The difference between the two is taken as the final phase shift control quantity, and the phase shift quantity is obtained by calculating formula (1) and (2): D 1 , D 2 , D 3 , and the switching frequency fs Thus, the input drive signal modulation module controls the switch tube to conduct so that the converter operates under a given control mode.

[0026] Figure 3 middle: D 1 Represents S 1 and S 4 The phase shift ratio between D 2 Represents S 1 and S 5 The phase shift ratio between v p For FB 1 The voltage difference at the midpoint of the bridge arm, v s For FB 2 The voltage difference at the midpoint of the bridge arm, T is a half switching period, i 1 is the DAB output current, i 2 The input current of the three-phase inverter full bridge is adjusted so that the conduction pulse of the DAB rear-stage full-bridge rectifier lags behind the conduction pulse of the front-stage full-bridge inverter so that the power is transferred from the DC side to the AC side.

[0027] Figure 4 Middle: When the converter transfers power in reverse, the power is transferred from the AC side to the DC side by adjusting the conduction pulse of the DAB rear-stage full-bridge rectifier to advance the conduction pulse of the front-stage full-bridge inverter.

[0028] Figure 5 Middle: V ab Represents the voltage between nodes a and b, V bc Represents the voltage between nodes b and c, V ca Represents the voltage between nodes c and a, V m_x Q 1 , Q 2 The modulation signal, V m_y Q 3 , Q 4 The modulation signal, V m_z Q 5 , Q 6 The modulation signal.

[0029] Figure 6 middle: Indicates that Q 2 The opposite signal, Indicates that Q 4 The opposite signal, Indicates that Q 6 The opposite signal, V mRepresents the modulation signal of the subsequent three-phase full-bridge converter, including the modulation signal V m_x 、V m_y 、V m_z The driving signals of the two switching tubes in each bridge arm are symmetrical square waves, and only one bridge arm of the three-phase converter adopts PWM modulation at any time. DETAILED DESCRIPTION

[0030] Based on the derivation of the control model and the establishment of the equivalent model of the three-phase converter, a method of introducing triple phase-shift plus frequency conversion control in the voltage closed-loop control of the AC / DC microgrid interconnected converter is proposed through mathematical deduction.

[0031] The isolated AC / DC microgrid interconnection converter consists of a dual active full-bridge converter DAB and a three-phase full-bridge inverter SI. DAB and SI are connected by thin film capacitors. C 2 Based on the triple phase shift plus frequency conversion control method, the control quantity is realized α The linear control of power simplifies the complexity of the control system. At the same time, the 33% PWM control method of the subsequent three-phase converter is designed to reduce the average switching frequency and reduce equipment losses.

[0032] The present invention is based on a control method for an AC / DC microgrid interconnected converter with triple phase shift plus frequency conversion control, and the method is specifically developed as follows:

[0033] The control method includes front-stage DAB triple phase-shift plus frequency conversion control and rear-stage three-phase converter 33% PWM control.

[0034] (1) Pre-stage DAB triple phase shift plus frequency conversion control

[0035] In order to improve the nonlinear relationship between transmission power and control quantity in the traditional scheme, a virtual frequency is defined f n , which is a fixed value, sets the size of the virtual frequency to

[0036] f n = f s / (1-2 α )(1)

[0037] u g_ref Indicates the output voltage reference value. Since the design is to improve the performance of the converter in steady state, a fixed output voltage is used. u g_ref Instead of the actual output voltage u g to avoid u gThe value is very small, resulting in inaccurate calculation of the output current reference value. Define the phase shift control amount α In order to achieve linear control of the control variable on the transmission power, the inner shift ratio and the outer shift ratio must satisfy

[0038] D 1 =0.5-2 α , D 2 =0.5- α , D 3 =0(2)

[0039] Output voltage reference value u g_ref The actual output voltage u g The difference is output through the PI controller as the auxiliary modulation signal Δ α ; P ref is the transmission power reference value, that is

[0040] P ref = u g i ac (3)

[0041] The calculation formula of the shift phase control amount in the control method is:

[0042] α * =i ac f n L / NU dc (4)

[0043] The above two equations can be used to calculate the phase shift control amount main modulation signal α * , which is related to the control quantity auxiliary modulation signal Δ α The difference between the two is taken as the final phase shift control quantity, and the phase shift quantity is obtained by calculating formula (1) and (2): D 1 , D 2 , D 3 and switching frequency f s, so that the input drive signal modulation module controls the switch tube to conduct, so that the converter operates under a given control mode. The main modulation signal derived from formula (4) is not affected by the power transmission direction, so the controller has excellent dynamic response performance when the system power direction changes.

[0044] (2) 33% PWM control of the subsequent three-phase converter

[0045] The back-end three-phase full-bridge inverter of the present invention adopts 33% PWM control strategy to output three-phase voltage waveform: in order to improve the power density of the converter, a small-capacity film capacitor is used instead of a large-volume voltage-stabilizing electrolytic capacitor, and the driving signal of the two switch tubes in each bridge arm is a symmetrical square wave. At any time, only one bridge arm of the three-phase inverter adopts PWM modulation. For example, the switch device Q 1 , Q 2 In T 1 The time period is V ab / V cb As a modulated pulse signal, V c_ref As the carrier signal, the driving signals of the other four switching devices are ON or OFF. The three-phase full-bridge driving signal is as follows Figure 5 As shown, V m_x Q 1 , Q 2 The modulation signal, V m_y Q 3 , Q 4 The modulation signal, V m_z Q 5 , Q 6 Modulation signal. Divide one cycle into 6 equal time periods T 1 ~T 6 , with time period T 1 For example, Q 5 and Q 4 Open, Q 3 and Q 6 Shutdown, Q 1 and Q 2 By using V ab / V cb As the modulation signal, it is compared with the carrier waveform to obtain the turn-on pulse of the device. The turn-on signals of the remaining five time periods follow a similar rule.

[0046] The voltage closed-loop control, triple phase-shift plus frequency conversion control and 33% PWM control described in the present invention are all implemented through a DSP chip.

Claims

1. A control method for an AC / DC microgrid interconnected converter based on triple phase shift plus frequency conversion control, wherein the AC / DC microgrid interconnected converter adopts an isolated AC / DC three-phase converter; the topology of the isolated AC / DC three-phase converter is composed of a DAB converter and a three-phase full-bridge inverter SI, and the DAB converter at the front end is composed of four primary switches S 1 , S 2 , S 3 , S 4 Inverter full bridge FB 1 , a high frequency transformer T r , leakage inductance L and four secondary switches S 5 , S 6 , S 7 , S 8 Full bridge rectifier FB 2 The transformer ratio is 1:n, and the DAB converter passes through the filter capacitor C 2 Cascaded back-end three-phase full-bridge inverter SI; the three-phase full-bridge inverter SI contains six active switches Q 1 , Q 2 , Q 3 , Q 4 , Q 5 , Q 6 , SI passes through the output side filter inductor L 0 And the output side filter capacitor C 0 Connect to AC power; The DC power supply size is u dc , the AC power supply size is u g , FB 1 The voltage difference at the midpoint of the bridge arm is v p , FB 2 The voltage difference at the midpoint of the bridge arm is v s , the AC current at the output side of SI is i ac ;i 1 is the DAB output current, i 2 is the SI input current; It is characterized in that Combining triple phase shifting with frequency conversion control, the linear control of the control variable on the transmission power is realized, as follows: Introduce variable frequency control and define virtual frequency f n , which is a fixed value, sets the size of the virtual frequency to f n =f s / (1-2α) (1) f s is the actual switching frequency of all switching devices in the DAB converter, α is the phase shift control amount; u g_ref Indicates the output voltage reference value, using the output voltage reference value u g_ref Instead of the actual output voltage u g ; The inverter bridge moves inward compared to D 1 Represents S 1 and S 4 Compared with the shift between, the shift outside is D 2 Represents S 1 and S 5 The shift ratio between the two is D 3 Represents S 5 and S 8 In order to realize the linear control of the control quantity on the transmission power, the inner shift ratio and the outer shift ratio must satisfy D 1 =0.5-2α, D 2 =0.5-α, D 3 =0(2) Output voltage reference value u g_ref The actual output voltage u g The difference is output by the PI controller as the control quantity auxiliary modulation signal Δα; ref is the transmission power reference value, that is P ref =in g and ac (3) The calculation formula of the shift phase control amount in the control method is: α * =i ac f n Yes / no dc (4) The shift phase control amount main modulation signal α is calculated by the above two equations (3) and (4): * The difference between the control value auxiliary modulation signal Δα and the phase shift control value is taken as the final phase shift control value. The phase shift value D is obtained by calculating formulas (1) and (2): 1 , D 2 , D 3 and the switching frequency f s , so that the input drive signal modulation module controls the switch tube to conduct, so that the converter operates under a given control mode.

2. The AC / DC microgrid interconnected converter control method based on triple phase shift plus frequency conversion control as claimed in claim 1, It is characterized in that Also includes the three-phase full-bridge inverter 33% PWM control strategy: switching device Q 1 , Q 2 , Q 3 , Q 4 , Q 5 , Q6 form a three-phase full-bridge inverter, the switch tube Q 1 The source and switch tube Q 2 The drain of the switch tube Q 3 The source and switch tube Q 4 The drain of the switch tube Q 5 The source and switch tube Q 6 The drain connection forms the third bridge arm, a, b, and c are the midpoints of each bridge arm respectively, and the driving signals of the two switching tubes in each bridge arm are symmetrical square waves. At any time, only one bridge arm of the three-phase inverter adopts PWM modulation, and the switching devices of the other two bridge arms are turned on or off respectively.

3. The AC / DC microgrid interconnected converter control method based on triple phase shift plus frequency conversion control as claimed in claim 2, It is characterized in that The switching cycle of each switching device is divided into 6 equal time periods T 1 , T 2 , T 3 , T 4 , T 5 , T 6 , in T 1 Time period, Q 5 and Q 4 Open, Q 3 and Q 6 Shutdown, Q 1 and Q 2 By using V ab / V cb As the modulation signal, it is compared with the carrier waveform to obtain the device's turn-on pulse; at T 2 Time period, Q 1 and Q 4 Open, Q 3 and Q 2 Shutdown, Q 5 and Q 6 By using V cb / V ab As the modulation signal, it is compared with the carrier waveform to obtain the device's turn-on pulse; at T 3 Time period, Q 6 and Q 2 Open, Q 5 and Q 1 Shutdown, Q 3 and Q 4 By using V ba / V ac As the modulation signal, it is compared with the carrier waveform to obtain the device's turn-on pulse; at T 4 Time period, Q 3 and Q 6 Open, Q 5 and Q 4 Shutdown, Q 1 and Q 2 By using V ac / V bc As the modulation signal, it is compared with the carrier waveform to obtain the device's turn-on pulse; at T 5 Time period, Q 3 and Q 2 Open, Q 1 and Q 4 Shutdown, Q 5 and Q 6 By using V ca / V ba As the modulation signal, it is compared with the carrier waveform to obtain the device's turn-on pulse; at T 6 Time period, Q 5 and Q 2 Open, Q 1 and Q 6 Shutdown, Q 3 and Q 4 By using V bc / V ca As the modulating signal, it is compared with the carrier waveform to obtain the on pulse of the device.

4. The AC / DC microgrid interconnected converter control method based on triple phase shift plus frequency conversion control as claimed in claim 3, It is characterized in that After adopting 33% PWM control scheme, C 2 Use small-capacity film capacitors instead of electrolytic capacitors, and take C 2 =2μF, it can output an ideal sinusoidal voltage waveform.

5. The AC / DC microgrid interconnected converter control method based on triple phase shift plus frequency conversion control according to any one of claims 2 to 4, Features: Voltage closed-loop control, triple phase-shift plus frequency conversion control and 33% PWM control are all implemented through DSP chips.

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