Unit Power Factor Control Method for AC-DC Microgrid Interconnection Converter Based on Electric Spring

Abstract

The present invention relates to the field of AC-DC microgrids, and specifically to a unity power factor control method for an AC-DC microgrid interconnection converter based on a power spring. The present invention solves the technical problems of low transmission efficiency of the traditional two-stage topology of the converter, the distinction between critical loads and non-critical loads, and the increase in hardware costs due to external active devices and energy storage devices. In the present invention, the AC-DC microgrid interconnection converter adopts a single-stage power transmission topology, and realizes open-loop unity power factor correction by modulating the switching signal; after connecting the power spring, it does not distinguish between non-critical loads and critical loads, and improves the power supply quality provided to non-critical loads; the power spring device is connected to the converter by multiplexing the AC side bridge arm, without the need to connect active devices and energy storage devices, reducing the equipment volume and hardware costs. The unity power factor control method for the AC-DC microgrid interconnection converter based on the power spring of the present invention has good practicability.

Description

Technical Field

[0001] The present invention relates to the field of AC-DC microgrids, and specifically to a unity power factor control method for an AC-DC microgrid interconnection converter based on a power spring. Background Art

[0002] In recent years, renewable energy sources represented by wind energy and photovoltaic energy, as a series of clean energy sources, have received widespread attention worldwide. With the increasing perfection of new energy power generation theory and the increasing maturity of related technologies, new power technologies that are efficient and flexible and mainly based on new energy have developed rapidly. As an emerging form of power supply and consumption on the demand side, the AC-DC hybrid microgrid can more efficiently accommodate local AC-DC new energy power generation systems and energy storage units, and provide highly reliable power supply for local loads. It has become an important part of the future intelligent distribution network. In the AC-DC microgrid, the AC-DC microgrid interconnection converter plays a key role in the transmission of energy between the DC distribution network and the AC power grid system in the power system, and has become a research hotspot. The structure of the microgrid interconnection converter with a high-frequency transformer has received extensive attention from experts and scholars at home and abroad due to its advantages such as high power density, easy implementation of soft switching, and flexible control. In the research on the power spring control strategy, an additional energy storage device needs to be connected, which increases equipment losses and reduces the power density of the converter. The function multiplexing of the power spring interface and the AC-DC microgrid interconnection converter can be realized by adopting the method of multiplexing the AC full bridge, and its energy comes from the microgrid itself, which can better serve the future intelligent power distribution and consumption system.

[0003] The traditional two-way AC-DC converter with an externally connected power spring usually adopts a two-stage topology, including a front-stage dual-active bridge topology and a rear-stage full-bridge inverter topology, and voltage and current control strategies need to be designed for the front and rear stages respectively. This scheme has three disadvantages: the energy transmission efficiency of the two-stage topology in the isolated converter topology is low, and open-loop unity power factor correction cannot be achieved; the traditional power spring reduces the working voltage of non-critical loads to meet the stable operation of critical loads, and it is difficult to meet the power quality requirements of users on the user side after the concept of loads becomes blurred in the new power system; the traditional power spring device needs to connect active devices and energy storage devices, which increases the hardware cost and equipment volume, and has high requirements for the reliability of the energy storage device. Summary of the Invention

[0004] In order to solve the technical problems of low transmission efficiency of the two-stage topology of the AC-DC microgrid interconnection converter with a traditional externally connected power spring, distinguishing between critical loads and non-critical loads, and the increase in hardware cost and power density due to the externally connected active devices and energy storage devices, in view of the defects of the prior art, the present invention designs a unity power factor control method for an AC-DC microgrid interconnection converter based on a power spring.

[0005] The present invention is implemented based on the following technical solution: A unity power factor control method for an AC-DC microgrid interconnection converter based on a power spring. The topological structure corresponding to this method consists of an AC-DC microgrid interconnection converter and a power spring; the AC-DC microgrid interconnection converter is composed of a DC-side full-bridge circuit and an AC-side full-bridge circuit. The AC-side full-bridge is formed by back-to-back series connection of eight MOSFETs of the same model using the common-drain connection method. The full-bridge circuit is composed of Q 11 、Q 21 、Q 31 、Q 41 、Q 12 、Q 22 、Q 32 、Q 42 ; The DC-side full-bridge is composed of four MOSFET switches Q5, Q6, Q7, and Q8 of the same model; L f1 and C1 form an LC filter circuit on the AC input side, and the LC filter circuit on the DC side is composed of C2 and L f2 ; The transformer turns ratio is 1:n, where n is set to 1, and L is the sum of the auxiliary inductor and the transformer leakage inductance; The present invention designs a unity power factor control method to achieve single-stage power transmission and open-loop power factor correction, as follows:

[0006] The DC power supply voltage is v dc , the AC power supply voltage is v ac , the pulse timing of the AC full-bridge switching tubes is generated by the carrier u1. The modulation wave u dc carrying the DC duty cycle D Ddc and u2 and u2' jointly control the turn-on signals of the four switching tubes of the DC full-bridge. The carrier u2 is obtained after phase-shifting u1 by a phase-shift angle of φ. The pulse center interval duration between the voltages v p2 and v s across the inductor L is δ, satisfying δ = φ / 2πf s , where f s is the switching frequency of the AC-side full-bridge MOSFET. The carrier u2' can be obtained by inverting or phase-shifting u2 by 180°; V ac is the instantaneous value of the AC voltage, and V dc is the DC voltage value;

[0007] For resonance on the DC side, when u Ddc > u2, the drive of Q5 is set to high level, otherwise the drive of Q6 is set to high level; when u Ddc > u2', the drive of Q7 is set to high level, otherwise the drive of Q8 is set to high level. For the AC side, its working principle is that after zero-crossing comparison of v ac (t), it can be obtained that it contains v ac(t) The power-frequency square-wave signal of the symbol information. The pulse timing of the AC full-bridge switching tubes is generated by the carrier u1. When acting on the carrier u1, it can make the switching tubes perform a switching action with high and low frequencies cycling as follows: When v ac (t) The symbol makes a switching action with high and low frequencies cycling. Specifically, when v ac (t)>0, Q 11 、Q 21 、Q 31 、Q 41 remain conducting, and Q 12 、Q 22 、Q 32 、Q 42 switch states at the switching frequency f s ; when v ac (t)<0, Q 12 、Q 22 、Q 32 、Q 42 remain conducting, while Q 11 、Q 21 、Q 31 、Q 41 perform high-frequency switching state switching.

[0008] By using the fundamental component method and Ohm's law, it is easy to deduce that the complex power formula transmitted by the converter is

[0009]

[0010] where, M Q = nV ac sin(πD ac ) - V dc sin(πD dc )cosφ, P T and Q T are the active power and reactive power respectively.

[0011] When Q T = 0, the converter only transmits active power from the AC microgrid side backward, that is, the unity power factor correction is achieved. At this time, M Q = 0, and the expression of the duty cycle on the DC side is obtained as

[0012]

[0013] where, K = nV ac / V dc is defined as the voltage conversion ratio. Assuming that the power transmitted by the converter is P, the calculation formula for the phase-shift angle φ on the DC side is

[0014]

[0015] In the unity power factor control mode, assuming the AC duty cycle Dac Constantly being 1, the input current i of the AC microgrid is further calculated. ac (t).

[0016]

[0017] It can be seen from this that the AC-side current is in the same phase as the AC voltage, that is, the single-stage power factor correction is achieved.

[0018] Furthermore, the present invention adopts the method of multiplexing the AC full-bridge to expand a power spring interface for the AC-DC microgrid converter without adding active devices. To reduce the complexity of control, increase the load regulation range, and achieve fast response and flexible adjustment at the same time. In the present invention, the power spring adopts voltage feedback control based on RMS value calculation: the load voltage v collected by the sensor L passes through the PLL phase-locked loop to obtain the load voltage angular frequency ω L , passes through the Park transformation to obtain the direct-axis component U of the load voltage d and the quadrature-axis component U q , calculates the sum of the squared values and then takes the square root to obtain the RMS value of the load voltage. The reference voltage v at the input end Lref is the rated voltage of the AC load. By subtracting it from the actual RMS value of the load voltage and passing it through a proportional-integral regulator, the required power spring control signal can be obtained. After being modulated by the above-mentioned microgrid converter control module and power spring control module, a suitable voltage V is further output es , compensates for the possible overvoltage or undervoltage problems of the feeder, and finally ensures that the AC load always operates at the rated state.

[0019] The above-mentioned single-stage power factor control method for the AC-DC microgrid interconnection converter based on the power spring provided by the present invention, compared with the prior art, has the following advantages and positive effects: the AC-DC microgrid interconnection converter adopts a single-stage power transmission control method and realizes open-loop single-stage power factor correction through modulating the switching signal; after connecting the power spring, it does not distinguish between non-critical loads and critical loads, improving the power supply quality provided to non-critical loads; the power spring device is connected to the converter by multiplexing the AC-side bridge arm, without the need to connect active devices and energy storage devices, reducing the equipment volume and hardware cost. Description of the Drawings

[0020] Figure 1 is the topology diagram of the AC-DC microgrid interconnection converter based on the power spring involved in the present invention;

[0021] Figure 2 is the circuit waveform diagram of the forward working mode of the AC-DC microgrid interconnection converter involved in the present invention;

[0022] Figure 3It is the waveform diagram of the in-phase compensation mode circuit of the AC-DC microgrid interconnection converter based on the electric spring involved in the present invention;

[0023] Figure 4 It is the block diagram of the effective value voltage regulation control of the electric spring involved in the present invention;

[0024] Figure 1 Among them: The AC-DC microgrid interconnection converter is composed of a DC-side full-bridge circuit and an AC-side full-bridge circuit. The AC full-bridge is composed of eight MOSFETs of the same model connected in series back-to-back in a common-drain connection method. The DC-side full-bridge is composed of four MOSFET switching tubes Q5, Q6, Q7, and Q8 of the same model. v 11 、Q 21 、Q 31 、Q 41 、Q 12 、Q 22 、Q 32 、Q 42 The full-bridge circuit formed by series connection. The DC-side full-bridge is composed of four MOSFET switching tubes Q5, Q6, Q7, and Q8 of the same model. v dc Is the DC power supply voltage, v ac Is the AC power supply voltage, L f1 And C1 form an LC filter circuit on the AC input side to filter out the switching frequency and high-order harmonics. Similarly, the DC-side LC filter circuit is composed of C2 and L f2 Constitute. The transformer turns ratio is 1:n, L is the sum of the auxiliary inductance and the transformer leakage inductance, i Leq Is the current flowing through the filter inductor L. v p1 (t) and v s (t) respectively represent the voltages between the midpoints of the two full-bridge arms on the AC and DC sides. v p2 (t) is the voltage of v p1 (t) referred to the secondary side of the transformer. The electric spring interface and the interconnection converter share the AC-side full-bridge switch. To reduce the harmonic content on the output side, an LC filter including L es And C es Is used for filtering at the output of the electric spring. v es Is the output voltage of the electric spring, i Les Is the current flowing through the filter inductor L es , v L Is the load voltage, and the positive direction of the current is also marked in Figure 1 .

[0025] Figure 2 Among them: V L Is the amplitude of the AC voltage, f L =ω L / 2π = 50Hz is the frequency of the AC power grid, ω L Is the angular frequency of the AC power grid, T L Is the sine period of the AC power grid. For the DC side, when uDdc When u > u2, Q5 is driven to a high level, otherwise Q6 is driven to a high level; when u Ddc > u2′, Q7 is driven to a high level, otherwise Q8 is driven to a high level. When Q5 and Q8 are both at a high level, v s (t) is V dc , when Q6 and Q7 are both at a high level, v s (t) is -V dc , and at other times the value of v s (t) is zero. For the AC side, its working principle is that a power frequency square wave signal containing the sign information of v ac (t) can be obtained through zero-crossing comparison of v ac (t). The pulse timing of the AC full-bridge switching tubes is generated by the carrier u1. When acting on the carrier u1, it can make the switching tubes perform high-frequency and low-frequency cyclic switching actions according to the sign of v ac (t), specifically as follows: when v ac (t) > 0, Q 11 , Q 21 , Q 31 , Q 41 remain conducting, and Q 12 , Q 22 , Q 32 , Q 42 switch states at the switching frequency f s ; when v ac (t) < 0, Q 12 , Q 22 , Q 32 , Q 42 remain conducting, while Q 11 , Q 21 , Q 31 , Q 41 perform high-frequency switching state switching. T s = 1 / f s represents the AC side MOSFET switching period. In the present invention, f s is set to 100 kHz. The modulation wave u dc carrying the DC duty cycle D Ddc and u2 and u2′ jointly control the turn-on signals of the four switching tubes of the DC full-bridge. The carrier u2 is obtained by phase-shifting u1, and the phase-shift angle is φ. The pulse center interval duration of the voltages v p2 and v s across the inductor L is δ, satisfying δ = φ / 2πf s . The carrier u2′ can be obtained by inverting u2 or phase-shifting it by 180°. V dc is the DC voltage value.

[0026] Figure 3In the positive half-cycle in-phase compensation mode of the AC voltage, taking Q as an example 11 , Q 21 , Q 31 , Q 41 remain conducting. d es is the power spring control signal with slightly floating up and down added on the basis of unity power factor control. After being compared with the carrier wave u1, the PWM switch signal of the switching tubes Q ac carrying the AC duty cycle D 12 , Q 22 , Q 32 , Q 42 can be obtained. In the in-phase compensation mode, d es >0 makes the duty cycle D ac always remain greater than 1. This mode can be decomposed into two stages, namely, stage I is the inductor charging stage; stage II is the inductor discharging stage. V es is the output voltage of the power spring. In the inductor charging stage, starting from the moment t0, the pulses of Q 12 , Q 42 become high level, corresponding to the conduction of the switching tubes, and the voltage across the energy storage inductor L es of the power spring is V ac -V es >0, and L es starts to charge and lasts for the entire duration of stage I. In the inductor discharging stage, starting from the moment t1, the pulses of Q 22 , Q 32 become high level, the pulses of Q 12 , Q 42 become low level, making Q 22 , Q 32 conduct and Q 12 , Q 42 turn off. The voltage across the energy storage inductor L es of the power spring is -(V ac +V es )<0, and L es starts to discharge and lasts for the entire duration of stage II.

[0027] Figure 4 In: The load voltage v L obtains the load voltage angular frequency ω L through the PLL phase-locked loop, and obtains the direct-axis component U d and the quadrature-axis component U q of the load voltage through the Park transformation. After calculating the sum of the squared values and then taking the square root, the effective value of the load voltage is obtained. The reference voltage v Lrefis the rated voltage of the AC load. By subtracting it from the effective value of the actual load voltage and passing it through a proportional-integral regulator, the required power spring control signal can be obtained. After being modulated by the above-mentioned microgrid converter control module and power spring control module, a suitable voltage V is further output es , to appropriately compensate for the overvoltage or undervoltage problems that may occur in the feeder, and ultimately ensure that the AC load always operates at the rated state. Detailed implementation method

[0028] The isolated AC-DC microgrid interconnection converter consists of a dual-active-bridge interconnection converter and a power spring that multiplexes the AC side interface. Based on the unity power factor control method, single-stage power transmission and open-loop power factor correction are achieved, reducing system losses. At the same time, the power spring connected to the multiplexed AC full bridge suppresses the microgrid voltage fluctuations without adding additional power devices and energy storage devices.

[0029] The following further describes the present invention with reference to the accompanying drawings. A unity power factor control method for an AC-DC microgrid interconnection converter based on a power spring, which includes the unity power factor control of the AC-DC microgrid interconnection converter and the voltage feedback control of the power spring based on the effective value calculation; the corresponding topology of this method consists of a dual-active-bridge interconnection converter and a power spring that multiplexes the AC side interface, as shown in Figure 1 . The AC-DC microgrid interconnection converter consists of a DC side full-bridge circuit and an AC side full-bridge circuit. The AC side full-bridge is composed of Q 11 , Q 21 , Q 31 , Q 41 , Q 12 , Q 22 , Q 32 , Q 42 eight MOSFETs of the same type connected in series back-to-back in a common-drain connection method to form a full-bridge circuit. This connection method can prevent the current path from being cut off during the freewheeling period on the primary side. The DC side full-bridge is composed of four MOSFET switches Q5, Q6, Q7, and Q8 of the same type. The DC power supply voltage is v dc , and the AC power supply voltage is v ac = V L sin(ω L t), where V L is the amplitude of the AC voltage, f L = 50Hz is the frequency of the AC power grid, ω L is the angular frequency of the AC power grid, L f1 and C1 form an LC filter circuit on the AC input side to filter out the switching frequency and higher harmonics. Similarly, the LC filter circuit on the DC side is composed of C2 and L f2 . The transformer turns ratio is 1:n, L is the sum of the auxiliary inductor and the transformer leakage inductance, i Leqis the current flowing through the filtering inductor L of the power spring. Figure 1 where v p1 (t) and v s (t) represent the voltages between the midpoints of the two full-bridge arms on the AC and DC sides respectively. v p2 (t) is the voltage of v p1 (t) reduced to the secondary side of the transformer. The power spring interface multiplexes the AC-side full-bridge switches with the interconnection converter, thus not adding active components. Compared with the traditional AC / AC power spring, its energy comes from the microgrid system itself without the need for additional energy storage devices. To reduce the harmonic content on the output side, an LC filter containing L es and C es is used for filtering at the output of the power spring. v es is the output voltage of the power spring, i Les is the current flowing through the filtering inductor L es , v L is the load voltage, and the positive direction of the current is also marked in Figure 1 .

[0030] It is defined that the energy flowing from the AC side to the DC side is positive. Figure 2 is the circuit waveform of the positive working mode of the AC-DC microgrid interconnection converter when the power spring is not connected. Among them, T s =1 / f s represents the switching period of the AC-side MOSFET. In the present invention, f s is set to 100 kHz. To improve the disadvantages of low energy transfer efficiency and inability to achieve automatic unity power factor correction in the two-stage topology of the traditional scheme, the pulse timing of the AC full-bridge switching tubes is generated by the carrier u1. The modulation wave u dc carrying the DC duty cycle D Ddc acts together with u2 and u2' to control the turn-on signals of the four switching tubes of the DC full-bridge. The carrier u2 is obtained by phase-shifting u1, and the phase-shift angle is φ. The pulse center interval duration of the voltages v p2 and v s across the inductor L is δ, satisfying δ = φ / 2πf s . The carrier u2' can be obtained by inverting or phase-shifting u2 by 180°. V ac is the instantaneous value of the AC voltage, and V dc is the DC voltage value. Considering ω L <<ω s , it can be approximately considered that the AC microgrid voltage remains unchanged within one or two switching periods.

[0031] For the DC side, when u Ddc >u2, the drive of Q5 is set to high level, otherwise the drive of Q6 is set to high level; when u DdcWhen u2′ is greater than a certain value, Q7 drives to a high level, otherwise Q8 drives to a high level. For the AC side, its working principle is that after the zero-crossing comparison of v ac (t), a power-frequency square wave signal containing the sign information of v ac (t) can be obtained. The pulse timing of the AC full-bridge switching tubes is generated by the carrier u1. When acting on the carrier u1, it can make the switching tubes perform high-frequency and low-frequency cyclic switching actions according to the sign of v ac (t), as follows: When v ac (t) > 0, Q 11 、Q 21 、Q 31 、Q 41 remain conducting, and Q 12 、Q 22 、Q 32 、Q 42 switch states at the switching frequency f s ; When v ac (t) < 0, Q 12 、Q 22 、Q 32 、Q 42 remain conducting, while Q 11 、Q 21 、Q 31 、Q 41 perform high-frequency switching state transitions.

[0032] By using the fundamental component method and Ohm's law, it is easy to deduce that the complex power formula transmitted by the converter is:

[0033]

[0034] Where, M Q = nV ac sin(πD ac ) - V dc sin(πD dc )cosφ, P T and Q T are the active and reactive powers respectively.

[0035] When Q T is set to 0, the converter only transmits active power from the AC microgrid side backward, that is, unity power factor correction is achieved. At this time, M Q = 0, and the expression for the duty cycle on the DC side is obtained as

[0036]

[0037] Where K = nV ac / V dc is defined as the voltage conversion ratio, the power transmitted by the converter is P, and the calculation formula for the DC side phase shift angle φ is

[0038]

[0039] Under the unity power factor control mode, the AC duty ratio D is set ac to be constantly 1, and further the input current i of the AC microgrid is calculated ac (t).

[0040]

[0041] It can be seen therefrom that the AC side current is in the same phase as the AC voltage, that is, the single power factor correction is achieved.

[0042] The load connected to the output of the power spring is any load, without distinguishing between critical loads and non-critical loads. v L is the voltage across the load. It is equivalent that the power spring interface acts as an independent "AC power supply" and is connected in series between the load and the AC microgrid feeder. When the power generation power of the AC microgrid fluctuates or the load is unbalanced, etc., resulting in a change in the feeder voltage, the power spring can follow the fluctuation for corresponding compensation or cancellation and output an appropriate v es to keep the load voltage constant at the rated value. When the feeder voltage is maintained at the rated voltage value of the load, the power spring interface does not work. Therefore, the power spring system designed in the present invention has "initiative", which conforms to the development law of power technology. The power spring adjustment process can be expressed by the following formula:

[0043] v L = v ac + v es (5)

[0044] When the microgrid voltage drops and the power spring needs to output the same-phase voltage for compensation, this mode is called the in-phase compensation mode. On the contrary, when the microgrid has an overvoltage and the power spring needs to give a reverse voltage compensation, this mode is the anti-phase compensation mode. Taking the in-phase compensation mode as an example, Figure 3 the waveforms of the key circuit parameters after the power spring is connected to the AC-DC microgrid interconnection converter are given. The anti-phase compensation mode can be analyzed by the same method. When the AC voltage is in the positive half cycle, Q 11 , Q 21 , Q 31 , Q 41 remain conducting. d es is the power spring control signal with a slight up and down fluctuation added on the basis of the aforementioned unity power factor control. After being compared with the same carrier u1, the PWM switching signals of the high-frequency part carrying the AC duty ratio D ac information, Q 12 , Q 22 , Q 32 , Q 42 can be obtained. In the in-phase compensation mode, d es > 0 makes the duty ratio Dac Always remains greater than 1. This mode can be decomposed into two stages. Stage Ⅰ is the inductor charging stage; Stage Ⅱ is the inductor discharging stage. V es is the absolute value of the output voltage v of the power spring. During the inductor charging stage, starting from time t0, Q es , Q 12 , Q 42 pulses become high level, corresponding to the switching tube being turned on. The energy storage inductor L of the power spring es has a voltage of V ac -V es >0 at both ends. L es starts to charge and lasts for the entire duration of Stage Ⅰ. During the inductor discharging stage, starting from time t1, Q 22 , Q 32 pulses become high level, Q 12 , Q 42 pulses become low level, causing Q 22 , Q 32 to turn on while Q 12 , Q 42 turns off. The voltage at both ends of the energy storage inductor L of the power spring es is -(V ac +V es )<0. L es starts to discharge and lasts for the entire duration of Stage Ⅱ.

[0045] The power spring adopts voltage feedback control based on the effective value calculation. The control block diagram is as shown in Figure 4 . The load voltage v L passes through the PLL phase-locked loop to obtain the angular frequency ω L of the load voltage. After Park transformation, the direct-axis component U d and the quadrature-axis component U q of the load voltage are obtained. After calculating the sum of the squared values and taking the square root, the effective value of the load voltage is obtained. The reference voltage v Lref at the input end is the rated voltage of the AC load. By subtracting the effective value of the actual load voltage from it and passing through the proportional-integral regulator, the required power spring control signal can be obtained. After being modulated by the above-mentioned microgrid converter control module and power spring control module, a suitable voltage V es is further output to appropriately compensate for the overvoltage or undervoltage problems that may occur in the feeder, ultimately ensuring that the AC load always operates at the rated state.

[0046] The unity power factor control of the AC-DC microgrid interconnection converter and the voltage feedback control of the power spring based on the effective value calculation are both implemented through the DSP chip.

Claims

1. A unity power factor control method for an AC-DC microgrid interconnection converter based on an electric spring. The topology corresponding to this method consists of an AC-DC microgrid interconnection converter and an electric spring. The AC-DC microgrid interconnection converter is composed of a DC-side full-bridge circuit and an AC-side full-bridge circuit. The AC-side full-bridge is a full-bridge circuit formed by back-to-back series connection of eight MOSFETs of the same model using the common-drain connection method, which are 11 Q 21 Q 31 Q 41 Q 12 Q 22 Q 32 Q 42 . The DC-side full-bridge is composed of four MOSFET switches Q5, Q6, Q7, and Q8 of the same model. L f1 and C1 form an LC filter circuit on the AC input side. The LC filter circuit on the DC side is composed of C2 and L f2 . The transformer turns ratio is 1:n, and L is the sum of the auxiliary inductor and the transformer leakage inductance. It is characterized in that: A unit power factor control method is designed to achieve single-stage power transfer and open-loop power factor correction, which is specifically as follows: The DC power supply voltage is v dc , and the AC power supply voltage is v ac . The pulse timing of the AC full-bridge switching tubes is generated by the carrier u1 and carries the modulation wave u dc with the DC duty cycle D Ddc . Together with u2 and u2', it controls the turn-on signals of the four switching tubes of the DC full-bridge. The carrier u2 is obtained after phase-shifting u1 by the phase-shift angle φ. The pulse center interval duration of the voltages v p2 and v s at both ports of the inductor L is δ, and δ = φ / 2πf s , where f s is the switching frequency of the AC-side full-bridge MOSFET. The carrier u2' can be obtained by inverting u2 or phase-shifting it by 180°; V ac is the instantaneous value of the AC voltage, and V dc is the DC voltage value; For the DC side, when u Ddc > u2, the drive of Q5 is set to high level, otherwise the drive of Q6 is set to high level; when u Ddc > u2′, the drive of Q7 is set to high level, otherwise the drive of Q8 is set to high level; for the AC side, its working principle is that after the zero-crossing comparison of v ac (t), a power-frequency square-wave signal containing the sign information of v ac (t) can be obtained. The pulse timing of the AC full-bridge switching tubes is generated by the carrier u1. When acting on the carrier u1, it can make the switching tubes perform high- and low-frequency cyclic switching actions according to the sign of v ac (t). When v ac (t)> 0, Q 11 、Q 21 、Q 31 、Q 41 remain conducting, and Q 12 、Q 22 、Q 32 、Q 42 switch states at the switching frequency f s ; when v ac (t)<0, Q 12 、Q 22 、Q 32 、Q 42 remain conducting, while Q 11 、Q 21 、Q 31 、Q 41 perform high-frequency switching state transitions; Under the open-loop unit power factor control mode, the expression of the duty cycle on the DC side is where K = nV ac / V dc is defined as the voltage conversion ratio; The calculation formula of the phase-shift angle φ on the DC side is where P is the transmission power of the converter, and ω s is the high-frequency switching angular frequency on the AC side.

2. The unity power factor control method for the AC-DC microgrid interconnection converter based on the electric power spring according to claim 1, characterized in that This method adopts the way of multiplexing the AC full bridge to expand a power spring interface for the AC-DC microgrid converter without adding active devices.

3. The unity power factor control method for the AC-DC microgrid interconnection converter based on the electric power spring according to claim 2, wherein The power spring adopts voltage feedback control based on effective value calculation: the load voltage v collected by the sensor L passes through the PLL phase-locked loop to obtain the angular frequency ω of the load voltage L , and passes through the Park transformation to obtain the direct-axis component U of the load voltage d and the quadrature-axis component U q . The effective value of the load voltage is obtained by calculating the sum of the squared values and then taking the square root; the reference voltage v Lref at the input end is the rated voltage of the AC load. By subtracting it from the effective value of the actual load voltage and passing it through a proportional-integral regulator, the required power spring control signal can be obtained. After being modulated by the above-mentioned microgrid interconnection converter control module and power spring control module, a suitable voltage V es is further output to appropriately compensate for the overvoltage or undervoltage problems that may occur in the feeder, and finally ensure that the AC load always operates at the rated state.

4. The unity power factor control method for the AC-DC microgrid interconnection converter based on the electric power spring according to claim 3, wherein: The unit power factor control of the AC-DC microgrid interconnection converter and the voltage feedback control of the power spring based on the effective value calculation are both realized through the DSP chip.

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