Optical storage system three-port converter integrated with switched capacitor equalizer

Abstract

The invention discloses an optical storage system three-port converter of an integrated switched capacitor equalizer, and relates to the field of photovoltaic power generation. The invention provides an optical storage system three-port converter integrated with a switched capacitor equalizer, and aims to solve the problem that the photovoltaic power generation power of an optical storage system is greatly reduced due to a partial shielding phenomenon and the problem that the volume, weight and complexity of the optical storage system are increased when an independent photovoltaic equalizer is introduced to improve the photovoltaic power. The size and the cost of the system are reduced through a device multiplexing mode. The converter is formed by integrating a non-isolated dual-active bridge converter, a star switch capacitor equalizer and a bidirectional Buck-Boost converter, the equalizer is integrated into a three-port converter in a switching tube multiplexing mode, and load output and storage battery charging are adjusted through phase shift control and PWM control respectively. According to the three-port converter provided by the invention, zero-voltage switching-on of all the switching tubes is realized, and the efficiency of the converter is effectively improved.

Description

Technical Field

[0001] The present invention belongs to the field of photovoltaic power generation, and particularly relates to a three-port converter for a photovoltaic energy storage system integrated with a switched-capacitor equalizer. Background Art

[0002] Generally, photovoltaic modules are connected in series by multiple photovoltaic sub-strings to obtain a higher voltage output (similarly, a photovoltaic string composed of multiple series-connected photovoltaic modules). When partial shading occurs in a photovoltaic sub-string, electrical characteristic mismatches will occur between the series-connected photovoltaic sub-strings, resulting in a significant decrease in the power generation of the photovoltaic system. This phenomenon widely exists in photovoltaic energy storage systems.

[0003] As an effective solution to improve the power generation of a photovoltaic energy storage system under partial shading conditions, the photovoltaic equalizer has received extensive attention. The photovoltaic sub-string with high light intensity transfers power to the photovoltaic sub-string with low light intensity through the photovoltaic equalizer, thereby changing the operating point of the shaded photovoltaic sub-string to increase the power generation. Since the photovoltaic equalizer only processes the differential power between the photovoltaic sub-strings with high and low light intensities, the rated power of the converter is much lower than that of traditional full-power converters, such as DC optimizers and micro-inverters. Therefore, the photovoltaic equalizer has a higher power density, smaller volume and weight compared to the former two. However, as an independent converter, the photovoltaic equalizer will also introduce new hardware costs, volume and weight, increasing the complexity of the photovoltaic energy storage system.

[0004] A traditional photovoltaic energy storage system requires a DC-DC (direct current - direct current) converter to achieve the maximum power point tracking of photovoltaic modules, a bidirectional DC-DC converter to achieve the charge and discharge regulation of the battery, and a photovoltaic equalizer to solve the problem of partial shading of photovoltaic modules. The circuit structure of this photovoltaic energy storage system is relatively complex due to the need for three independent converters, and has a large volume and weight. There is still considerable room for improvement from the perspective of system simplification. Summary of the Invention

[0005] The purpose of the present invention is to provide a three-port converter for a photovoltaic energy storage system integrated with a switched-capacitor equalizer, which can not only solve the problem of power generation decline caused by partial shading of photovoltaic modules, but also fully integrate multiple independent converters into an integrated three-port converter, in order to improve the power density and efficiency of the photovoltaic energy storage system. Taking n series-connected photovoltaic sub-strings as the photovoltaic input end as an example (n≥2), Figure 1 the system architecture of the three-port converter is given.

[0006] The three-port converter for a photovoltaic energy storage system integrated with a switched-capacitor equalizer includes 2n + 2 switching tubes Q1~Q 2n 、Q A ~Q B , n + 3 filter capacitors C PV1～n 、Cbat , C PS , C o , n flying capacitors C 1～n , 1 filter inductor L bat , 1 inductor L and 1 capacitor C. The photovoltaic input port of the three-port converter is n series-connected photovoltaic sub-strings PV1 to PV n 、The output port is the load (the load is represented by the resistor R o ), and the battery port is connected to the storage battery.

[0007] The three-port converter of the integrated switched-capacitor equalizer for the photovoltaic and energy storage system is composed of a star-shaped switched-capacitor equalizer, a non-isolated dual-active-bridge converter, and a bidirectional Buck-Boost converter integrated together. Among them, the star-shaped switched-capacitor equalizer is configured as follows: n photovoltaic sub-strings PV1 to PV n are respectively connected in parallel with the filter capacitor C PV1～n , and are respectively connected in parallel with the half-bridge structure composed of two complementary-conducting switching tubes, namely Q1 to Q2, Q3 to Q 4…… Q 2n-1 to Q 2n , and one end of the flying capacitors C1 to C n is respectively connected to the midpoint of the half-bridge corresponding to each photovoltaic sub-string, that is, one end of the flying capacitor C1 is connected to the midpoint of the switching tubes Q1 to Q2, and one end of the flying capacitor C2 is connected to the midpoint of the switching tubes Q3 to Q4 …… The flying capacitor C n One end is connected to the midpoint of the switching tubes Q 2n-1 to Q 2n , and the other ends of the n flying capacitors C1 to C n are connected in a star shape to form a bus.

[0008] Assume that under the action of voltage equalization of the star-shaped switched-capacitor equalizer, the voltages of the photovoltaic sub-strings have been unified, v PV1 to v PVn = V PV = V in / n. The bidirectional Buck-Boost converter is integrated with the star-shaped switched-capacitor equalizer by multiplexing the switching tubes Q 2i-1 and Q 2i (i = 1 to n), and connects one end (connection point A) of the filter inductor L bat to the midpoint of any switching tube half-bridge Q 2i-1 to Q 2i to adjust the voltage conversion ratio between the voltage of the photovoltaic module V in and the voltage of the storage battery V bat . The other end of the filter inductor L bat is connected to the filter capacitor C bat and the positive pole of the storage battery. By adjusting the switching tubes Q1 to Q 2nThe duty cycle can adjust the battery voltage or achieve the maximum power point tracking (MPPT) of the photovoltaic module. The non-isolated dual-active-bridge converter multiplexes the switching transistors Q1 to Q 2n Integrated with the star switch-capacitor equalizer, the bus in the star switch-capacitor equalizer is connected to the series AC impedance L-C, and the other end of the series AC impedance L-C is connected to the midpoint of the switching transistor half-bridge Q A ~Q B The midpoint of the switching transistor half-bridge Q A ~Q B Stacked on the photovoltaic sub-strings PV1 to PV k (k = 1 to n) to boost the output voltage v o , that is, the source (connection point B) of the switching transistor Q A is connected to the positive pole of any photovoltaic sub-string PV k to adjust the voltage conversion ratio between the photovoltaic module voltage V in and the output voltage v o . The filter capacitor C PS is connected in parallel with the complementary-conducting switching transistor half-bridge structure Q A ~Q B . By adjusting the phase-shift angle 2n between the switching transistors Q1 to Q A in the star switch-capacitor equalizer and the switching transistors Q B ~Q in the non-isolated dual-active-bridge converter, the main power transmission can be controlled, thereby adjusting the output voltage v o . The duty cycles D of the upper half-bridge switching transistors Q2, Q4... Q 2n and Q B are controlled by PWM, and the lower half-bridge switching transistors conduct complementarily. In the three-port converter of the integrated switch-capacitor equalizer for the photovoltaic-battery system, the positions of connection point A and connection point B are flexibly adjusted according to the proportional relationship between the photovoltaic module voltage V in , the battery voltage V bat and the load output voltage v o , and there is no fixed position relationship constraint between them.

[0009] Next, the working principle of the converter will be explained in detail in combination with the working modes of the three-port converter and the switching node voltages v X , v Y , v Z . Within one switching period T s , the converter has four working modes, namely mode A, B, C, and D.

[0010] Mode A: The switching transistors Q2, Q 4…… Q 2nTurn on under zero voltage condition, the switch Q in the non-isolated dual-active bridge converter A conducts. During this state, the switch node voltages v X and v Y are at high level, and the voltage at the switch node v Z is at low level. The flying capacitors C1, C 2…… C n in the star-connected switched-capacitor equalizer are connected in parallel with C PV2 、C PV3 ……C PVn through a star connection. In this mode, the photovoltaic sub-strings PV2, PV3……PV n charge the flying capacitors C1, C2……C n . The filtering inductor L bat in the Buck-Boost circuit bears a positive voltage, and the filtering inductor current i Lbat increases linearly. In the non-isolated dual-active bridge converter, the inductor L bears a positive voltage, and the photovoltaic module charges the inductor L through the switch node v Y , and the current i L rises rapidly.

[0011] Mode B: The switches Q2, Q4……Q 2n in the star-connected switched-capacitor equalizer conduct, and the switch Q B in the non-isolated dual-active bridge converter turns on under zero voltage condition. During this state, the switch node voltages v X , v Y , v Z are all at high level. In this mode, the photovoltaic sub-strings PV2, PV3……PV n continue to charge the flying capacitors C1, C2……C n , the filtering inductor current i Lbat continues to increase linearly, which is the same as Mode A. The inductor L still bears a positive voltage, and the photovoltaic module continues to charge the inductor L through the star-connected switched-capacitor equalizer, and the inductor current i L rises slowly.

[0012] Mode C: The switches Q1, Q3……Q 2n-1 in the star-connected switched-capacitor equalizer turn on under zero voltage condition, and the switch Q B in the non-isolated dual-active bridge converter conducts. During this state, at this time the switch node voltages v X , v Y are at low level, and v Z is at high level. The flying capacitors C1, C2……C n in the star-connected switched-capacitor equalizer are connected in parallel with C PV1 、C PV2……C PVn-1 are connected in parallel, and at this time, the flying capacitors C1, C2... C n charge the filter capacitor C PV1 , C PV2 ... C PVn-1 . In the Buck - Boost circuit, the filter inductor L bat has a reverse voltage across its two ends, and the filter inductor current i Lbat decreases linearly. In the non - isolated dual - active - bridge converter, the inductor L has a reverse voltage across its two ends, the inductor L is reversely charged, and the inductor current i L decreases rapidly.

[0013] Mode D: The switching transistors Q1, Q3... Q in the star - type switched - capacitor equalizer 2n-1 are turned on, and the switching transistor Q in the non - isolated dual - active - bridge converter A is turned on under zero - voltage conditions. During this state, the switching - node voltages v X , v Y , v Z are all at low levels. In this mode, the flying capacitors C1, C2... C n continue to charge the filter capacitor C PV1 , C PV2 ... C PVn-1 , and the filter inductor current i Lbat continues to decrease linearly, which is the same as Mode C. At this time, the AC impedance L - C is short - circuited, the inductor L has a reverse voltage across its two ends, and the inductor L is reversely charged.

[0014] Under steady - state conditions, the flying capacitors C1, C2... C n are respectively connected in parallel across the filter capacitors C PV2 , C PV3 ... C PVn and C PV1 , C PV2 ... C PVn-1 by star connection. Assuming that the charging and discharging speed of the capacitors is very fast, the flying capacitors charge and discharge different photovoltaic sub - strings in one cycle, transfer part of the energy of the normally - illuminated photovoltaic sub - string to the partially - shaded photovoltaic sub - string, and finally achieve the automatic unification of the voltages of all photovoltaic sub - strings to V PV . Therefore, the star - type switched - capacitor equalizer acts as a voltage equalizer. According to the volt - second balance of the inductor L bat , under steady - state conditions, the relationship between the battery voltage V bat and the photovoltaic sub - string voltage V PV is V bat =(i + D)V PV . Therefore, by controlling the switching transistors Q1~Q 2nThe duty cycle can adjust the battery voltage or achieve the MPPT tracking of the photovoltaic module. In the non-isolated dual-active-bridge converter circuit, by adjusting the phase shift angle between the switching transistors Q1 to Q 2n and the switching transistor Q A to Q B , the main power transmission can be controlled, thereby adjusting the output voltage v o .

[0015] The present invention proposes a three-port converter for a photovoltaic and energy storage system integrated with a switched-capacitor equalizer. Capacitors are used as the main energy storage elements, and the switched-capacitor equalizer and the three-port converter are integrated together by device multiplexing. Therefore, the volume and weight of the converter are smaller, and the zero-voltage turn-on of all switching transistors is achieved, effectively improving the efficiency of the converter. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 is a three-port converter for a photovoltaic and energy storage system integrated with a switched-capacitor equalizer proposed;

[0017] Figure 2 is a schematic diagram of a typical application of the three-port converter for a photovoltaic and energy storage system integrated with a switched-capacitor equalizer;

[0018] Figures 3(a) to 3(d) is a working mode diagram of the three-port converter for a photovoltaic and energy storage system integrated with a switched-capacitor equalizer;

[0019] Figure 3(a) Mode A;

[0020] Figure 3(b) Mode B;

[0021] Figure 3(c) Mode C;

[0022] Figure 3(d) Mode D;

[0023] Figure 4 Main waveforms of the three-port converter for a photovoltaic and energy storage system integrated with a switched-capacitor equalizer; DETAILED DESCRIPTION OF THE EMBODIMENTS

[0024] The present invention proposes a three-port converter for a photovoltaic and energy storage system integrated with a switched-capacitor equalizer. Taking a photovoltaic module composed of 3 series-connected photovoltaic sub-strings as an example, Figure 2 a schematic diagram of a typical application of the three-port converter is given. The technical solution of the present invention will be further described in detail below with reference to the drawings and specific embodiments.

[0025] The technical solution for achieving the object of the present invention is as follows: The three-port converter of the integrated switched-capacitor equalizer for the photovoltaic and energy storage system is composed of a non-isolated dual-active-bridge converter, a star-connected switched-capacitor equalizer, and a bidirectional Buck-Boost converter integrated together. Among them, the star-connected switched-capacitor equalizer is configured as follows: Three photovoltaic sub-strings PV1 to PV3 are respectively connected in parallel with a filtering capacitor C PV1～3 and are respectively connected in parallel with a half-bridge structure composed of two complementary-conducting switching tubes, namely Q1 to Q2, Q3 to Q4, and Q5 to Q6. One end of the flying capacitors C1 to C3 are respectively connected to the midpoints of the half-bridges corresponding to each photovoltaic sub-string. That is, one end of capacitor C1 is connected to the midpoint of switching tubes Q1 to Q2, one end of capacitor C2 is connected to the midpoint of switching tubes Q3 to Q4, and one end of capacitor C3 is connected to the midpoint of switching tubes Q5 to Q6. The other ends of the three capacitors C1 to C3 are connected in a star connection to form a bus.

[0026] The bidirectional Buck-Boost converter is integrated with the star-connected switched-capacitor equalizer by multiplexing switching tubes Q5 and Q6. One end of the filtering inductor L bat is connected to the midpoint of switching tubes Q5 to Q6, and the other end of the filtering inductor L bat is connected to the filtering capacitor C bat and the positive electrode of the storage battery. By adjusting the duty ratios of the switching tubes Q1 to Q6, the charging and discharging of the storage battery can be controlled. The non-isolated dual-active-bridge converter is integrated with the star-connected switched-capacitor equalizer by multiplexing switching tubes Q1 to Q6. The bus in the star-connected switched-capacitor equalizer is connected to a series AC impedance L-C. The other end of the series AC impedance L-C is connected to the midpoint of the switching tube half-bridge Q A to Q B . The switching tube half-bridge Q A to Q B is stacked on top of the photovoltaic sub-strings PV1 to PV2 to increase the output voltage v o , that is, the source electrode of the switching tube Q A is connected to the positive electrode of the photovoltaic sub-string PV2. The filtering capacitor C PS is connected in parallel with a switching tube half-bridge structure Q A to Q B with complementary conduction. By adjusting the phase-shift angle between the switching tubes Q1 to Q6 and the switching tubes Q A to Q B , the main power transmission can be controlled, thereby adjusting the output voltage v o . The duty ratios D of the upper half-bridge switching tubes Q2, Q4, Q6, and Q B of the converter are controlled by PWM, and the lower half-bridge switching tubes conduct complementarily. There is a phase-shift angle A between the switching tubes Q1 to Q6 and the switching tubes Q B

[0027] ​Next, combined with the working modes of the three-port converter and the switch node voltages v X 、v Y 、v Z ,the working principle of the converter will be explained in detail. Assume that the photovoltaic sub-string PV1 is affected by partial shading. Within a switching period T s , the converter has four working modes, namely modes A, B, C, and D.

[0028] Mode A: The switching transistors Q2, Q4, and Q6 in the star-shaped switched-capacitor equalizer are turned on under zero-voltage conditions, and the switching transistor Q A in the non-isolated dual-active-bridge converter is turned on. During this state, the switch node voltages v X and v Y are at high levels, and the voltage at the switch node v Z is at a low level. The flying capacitors C1, C2, and C3 in the star-shaped switched-capacitor equalizer are connected in parallel with C PV2 、C PV3 through a star connection. In this mode, the photovoltaic sub-strings PV2 and PV3 charge the flying capacitors C1, C2, and C3. The filter inductor L bat in the Buck-Boost circuit bears a positive voltage, and the filter inductor current i Lbat increases linearly. In the non-isolated dual-active-bridge converter, the inductor L bears a positive voltage, and the photovoltaic module charges the inductor L through the switch node v Y , and the current i L rises rapidly. The circuit mode is shown in Figure 3(a).

[0029] Mode B: The switching transistors Q2, Q4, and Q6 in the star-shaped switched-capacitor equalizer are turned on, and the switching transistor Q B in the non-isolated dual-active-bridge converter is turned on under ZVS. During this state, the switch node voltages v X 、v Y 、v Z are all at high levels. In this mode, the photovoltaic sub-strings PV2 and PV3 continue to charge the flying capacitors C1, C2, and C3, the filter inductor current i Lbat continues to increase linearly, the same as in mode A. The inductor L still bears a positive voltage, and the photovoltaic module continues to charge the inductor L through the star-shaped switched-capacitor equalizer, and the inductor current i L rises slowly. The circuit mode is shown in Figure 3(b).

[0030] Mode C: The switching transistors Q1, Q3, and Q5 in the star-shaped switched-capacitor equalizer are turned on under ZVS, and the switching transistor Q B in the non-isolated dual-active-bridge converter is turned on. During this state, at this time the switch node voltages v X 、v Yis at a low level, v Z is at a high level. The flying capacitors C1, C2, and C3 in the star switch-capacitor equalizer are connected in parallel with C PV1 , C PV2 through a star connection. At this time, the flying capacitors C1, C2, and C3 charge the filter capacitors C PV1 , C PV2 . In the Buck-Boost circuit, the filter inductor L bat has a reverse voltage across it, and the filter inductor current i Lbat decreases linearly. In the non-isolated dual-active-bridge converter, the inductor L has a reverse voltage across it, the inductor L is reverse-charged, and the inductor current i L decreases rapidly, and the circuit mode is shown in Fig. 3(c).

[0031] Mode D: The switching transistors Q1, Q3, and Q5 in the star switch-capacitor equalizer are turned on, and the switching transistor Q A in the non-isolated dual-active-bridge converter is turned on under ZVS. During this state, the switching node voltages v X , v Y , v Z are all at a low level. In this mode, the flying capacitors C1, C2, and C3 continue to charge the filter capacitors C PV1 , C PV2 , the filter inductor current i Lbat continues to decrease linearly, which is the same as Mode C. At this time, the AC impedance L-C is short-circuited, the inductor L has a reverse voltage across it, and the inductor L is reverse-charged. The circuit mode is shown in Fig. 3(d).

[0032] Under steady-state conditions, the flying capacitors C1, C2, and C3 are respectively connected in parallel with the filter capacitors C PV2 , C PV3 and C PV1 , C PV2 through a star connection. Assuming that the charging and discharging speed of the capacitors is very fast, the flying capacitors charge and discharge different photovoltaic sub-strings in one cycle, transfer part of the energy of the photovoltaic sub-strings PV2 and PV3 to the partially shaded photovoltaic sub-string PV1, and finally realize that the voltages of all photovoltaic sub-strings are automatically unified to V PV . Therefore, the star switch-capacitor equalizer acts as a voltage equalizer. The main waveforms of this converter in one cycle are as Figure 4 shown.

[0033] When partial shading occurs on the photovoltaic panel PV2 or PV3, the working state is similar and will not be elaborated here.

Claims

1. A three - port converter for an optical storage system integrated with a switched - capacitor equalizer, characterized by: Taking the series connection of n photovoltaic sub - strings as the photovoltaic input end as an example (n≥2), the three - port converter of the energy storage system integrated with a switched - capacitor equalizer includes 2n + 2 switching tubes Q1~Q 2n 、Q A ~Q B , n + 3 filter capacitors C PV1～n 、C bat 、C PS 、C o , n flying - capacitor C 1～n , 1 filter inductor L bat , 1 inductor L and 1 capacitor C. The photovoltaic input port of the three - port converter is n series - connected photovoltaic sub - strings PV1~PV n 、The output port is the load (the load is represented by the resistor R o ), and the battery port is the storage battery V bat .

2. A three-port converter for a photovoltaic energy storage system integrated with a switched-capacitor equalizer, characterized in that: The three-port converter of the energy storage system integrated with a switched-capacitor equalizer is composed of a star-connected switched-capacitor equalizer, a non-isolated dual-active-bridge converter, and a bidirectional Buck-Boost converter. The star-connected switched-capacitor equalizer is configured as follows: n photovoltaic sub-strings PV1 to PV n are respectively connected in parallel with filter capacitors C PV1～n , and are respectively connected in parallel with a half-bridge structure composed of two complementary-conducting switching tubes, namely Q1 to Q2, Q3 to Q 4…… Q 2n-1 to Q 2n , and the flying capacitors C1 to C n have one end respectively connected to the midpoint of the half-bridge corresponding to each photovoltaic sub-string. That is, one end of the flying capacitor C1 is connected to the midpoint of the switching tubes Q1 to Q2, and one end of the flying capacitor C2 is connected to the midpoint of the switching tubes Q3 to Q4 …… One end of the flying capacitor C n is connected to the midpoint of the switching tubes Q 2n-1 to Q 2n , and the other ends of the n flying capacitors C1 to C n are connected in a star connection to form a bus.

3. A three-port converter for a photovoltaic energy storage system integrated with a switched-capacitor equalizer, characterized in that: Assume that under the action of voltage equalization of the star-type switched-capacitor equalizer, the voltages of the photovoltaic sub-strings have been unified, v PV1 ~v PVn =V PV =V in / n. The bidirectional Buck-Boost converter is integrated with the star-type switched-capacitor equalizer by multiplexing the switching tubes Q 2i-1 and Q 2i (i = 1~n), and one end (connection point A) of the filter inductor L bat is connected to the midpoint of any half-bridge of the switching tubes Q 2i-1 ~Q 2i to adjust the voltage conversion ratio between the photovoltaic module voltage V in and the battery voltage V bat . The other end of the filter inductor L bat is connected to the filter capacitor C bat and the positive electrode of the battery. By adjusting the duty cycle of the switching tubes Q1~Q 2n , the battery voltage can be adjusted or the maximum power point tracking (MPPT) of the photovoltaic module can be achieved. The non-isolated dual-active-bridge converter is integrated with the star-type switched-capacitor equalizer by multiplexing the switching tubes Q1~Q 2n . The bus in the star-type switched-capacitor equalizer is connected to the series AC impedance L-C, and the other end of the series AC impedance L-C is connected to the midpoint of the half-bridge of the switching tubes Q A ~Q B . The half-bridge of the switching tubes Q A ~Q B is stacked on the photovoltaic sub-strings PV1~PV k (k = 1~n) to boost the output voltage v o , that is, the source electrode (connection point B) of the switching tube Q A is connected to the positive electrode of any photovoltaic sub-string PV k to adjust the voltage conversion ratio between the photovoltaic module voltage V in and the output voltage v o . The filter capacitor C PS is connected in parallel with the complementary-conducting half-bridge structure of the switching tubes Q A ~Q B . By adjusting the phase-shift angle between the switching tubes Q1~Q 2n in the star-type switched-capacitor equalizer and the switching tubes Q A ~Q B in the non-isolated dual-active-bridge converter, the main power transmission can be controlled, thereby adjusting the output voltage v o . The upper half-bridge switching tubes Q2, Q 4…… Q 2n and Q B The duty cycle D is controlled by PWM, and the lower half-bridge switch conducts complementarily. In the three-port converter of the optical storage system with the integrated switched-capacitor equalizer, the positions of connection point A and connection point B are flexibly adjusted according to the proportional relationship between the photovoltaic module voltage V in , the battery voltage V bat and the load output voltage v o , and there is no fixed position relationship constraint between the two.

4. A three-port converter for a photovoltaic energy storage system integrated with a switched-capacitor equalizer, characterized in that: Combined with the working modes of the three-port converter and the switch node voltages v X , v Y , v Z to explain the working principle of the converter in detail. Within one switching period T s , the converter has four working modes, namely modes A, B, C, and D: (4-1) Mode A: The switching transistors Q2 and Q in the star switch-capacitor equalizer 4…… Q 2n turn on under zero-voltage conditions, and the switching transistor Q in the non-isolated dual-active-bridge converter A conducts. During this state, the switch-node voltages v X and v Y are at a high level, and the voltage at the switch node v Z is at a low level. The flying capacitors C1 and C in the star switch-capacitor equalizer 2…… C n are connected in parallel to C PV2 and C PV3…… C PVn through a star connection. In this mode, the photovoltaic sub-strings PV2 and PV 3…… PV n charge the flying capacitors C1 and C 2…… C n The filtering inductor L in the Buck-Boost circuit bat is subjected to a positive voltage across it, and the filtering inductor current i Lbat increases linearly. In a non-isolated dual-active-bridge converter, the inductor L is subjected to a positive voltage at both ends, and the photovoltaic module charges the inductor L through the switching node v Y and the current i L rises rapidly; (4-2) Mode B: Switching transistors Q2 and Q in the star switch-capacitor equalizer 4…… Q 2n conduct, and the switching transistors Q in the non-isolated dual-active-bridge converter B turn on under zero-voltage conditions. During this state, the switch-node voltages v X , v Y , v Z are all at high levels. In this mode, the photovoltaic sub-strings PV2 and PV 3…… PV n continue to charge the flying capacitors C1, C2... C n , and the filter-inductor current i Lbat continues to increase linearly, the same as in Mode A. There is still a positive voltage across the inductor L, and the photovoltaic module continues to charge the inductor L through the star switch-capacitor equalizer, and the inductor current i L rises slowly; (4-3) Mode C: The switching transistors Q1, Q3... Q in the star switch-capacitor equalizer 2n-1 turn on under zero-voltage conditions, and the switching transistor Q in the non-isolated dual-active-bridge converter B conducts. During this state, at this time, the switch-node voltages v X , v Y are at a low level, and v Z is at a high level. The flying capacitors C1, C2... C in the star switch-capacitor equalizer n are connected in parallel with C PV1 , C PV2 ... C PVn-1 through a star connection. At this time, the flying capacitors C1, C2... C n charge the filter capacitors C PV1 , C PV2 ... C PVn-1 . The filter inductor L bat in the Buck-Boost circuit bears a reverse voltage, and the filter-inductor current i Lbat decreases linearly. In a non-isolated dual-active-bridge converter, a reverse voltage is applied across the inductor L, and the inductor L is reverse-charged, causing the inductor current i L to decrease rapidly; (4-4) Mode D: The switching transistors Q1, Q3... Q in the star switch-capacitor equalizer 2n-1 conduct, and the switching transistors Q in the non-isolated dual-active-bridge converter A turn on under zero-voltage conditions. During this state, the switch-node voltages v X , v Y , v Z are all at low levels. In this mode, the flying capacitors C1, C2... C n continue to charge the filter capacitors C PV1 , C PV2 ... C PVn-1 , and the filter-inductor current i Lbat continues to decrease linearly, the same as in Mode C. At this time, the AC impedance L-C is short-circuited, and the inductor L is subjected to a reverse voltage, and the inductor L is reverse-charged.

5. A three-port converter for a photovoltaic energy storage system integrated with a switched-capacitor equalizer, characterized in that: Under steady-state conditions, the flying capacitors C1, C2... C n are respectively connected in parallel to the filter capacitor C PV2 , C PV3…… C PVn and C PV1 , C PV2 ... C PVn-1 through a star connection. Assuming that the charging and discharging speed of the capacitors is very fast, the flying capacitors charge and discharge different photovoltaic sub-strings respectively in a cycle, transfer part of the energy of the photovoltaic sub-string with normal light intensity to the partially shaded photovoltaic sub-string, and finally automatically unify the voltages of all photovoltaic sub-strings to V PV . Therefore, the star-connected switched-capacitor equalizer acts as a voltage equalizer. According to the inductance L bat Under the steady-state condition, the battery voltage V can be obtained based on the volt-second balance bat and the voltage V of the photovoltaic sub-string PV The relationship between them is V bat =(i + D)V PV , so by controlling the duty cycle of the switching transistors Q1 to Q 2n , the battery voltage can be adjusted or the MPPT tracking of the photovoltaic module can be achieved. In the non-isolated dual-active-bridge converter circuit, by adjusting the phase-shift angle between the switching transistors Q1 to Q 2n and the switching transistors Q A to Q B , the main power transmission can be controlled, thereby adjusting the output voltage v o .

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