CN107769389A - A kind of battery energy storage system for isolating symmetrical expression series connection circuit of reversed excitation - Google Patents

Abstract

The invention discloses a battery energy storage system of an isolated symmetrical series flyback circuit, which comprises a battery unit composed of battery cells connected in series; a main power converter is used to control the main current of the battery unit, and the main power converter and the battery unit connected in parallel, and connected to the three-phase AC power grid; and the isolated flyback converter unit, which is composed of multiple isolated flyback converters with the same structure in series, and is respectively connected to each battery cell for closed-loop independent current control. It is used to control the difference between the charging and discharging current of each battery cell and the main current. The present invention realizes the independent control of the differential current part of the battery cells through the main power converter and the isolated flyback converter unit, without independent control of all battery currents, and uses a symmetrical flyback circuit junction to flexibly realize bidirectional work , improve the energy utilization rate of the battery cell; at the same time, increase the RCD absorption circuit to ensure that the switch tube avoids peak voltage and ensures safe and reliable work.

Description

A battery energy storage system with isolated symmetrical series flyback circuit

technical field

The invention relates to the fields of energy storage systems and power electronic converters, in particular to a battery energy storage system with isolated symmetrical series flyback circuits.

Background technique

With the increasingly severe global energy security and climate change issues, ecological and environmental protection issues are becoming more and more prominent, and a new round of energy technology revolution marked by new energy and smart grids is constantly gestating and developing. In order to solve the energy fluctuation problem caused by intermittent energy generation, energy storage technology is one of the best technologies to solve the problem. Among them, battery energy storage has been widely used due to its advantages of convenient and fast installation, short construction period, and convenient expansion and transformation (modularization).

Existing modular battery energy storage systems usually use full-power independent control flexible group energy storage systems. It mainly has three topologies: H-bridge cascaded flexible group energy storage system, modular multilevel converter (MMC, modular-multilevel-converter) battery energy storage system and DC-DC cascaded flexible group system. In the existing three full-power independent control flexible group systems, regardless of the size of the module battery current difference, all the charging and discharging currents of the battery modules must flow through their respective converter switching devices, resulting in large current stress on the devices. Power loss such as large conduction loss, especially when the system capacity increases to a large base, the problem becomes more prominent. Considering that the capacity of each battery module basically fluctuates within a certain range at this stage, even if the battery is used in cascades, the capacity difference will not be too large, and it is not necessary to independently control the current of all batteries. Therefore, a more efficient, economical and safe flexible group energy storage system with independent current control of part of the power of the single battery is proposed, which not only adds a snubber circuit to absorb the peak voltage on the traditional circuit structure, but also ensures the safe operation and safety of the switch tube. The service life is long, and the topology design is simple, reducing the loss and cost of the converter, so as to improve the energy utilization rate of the battery module.

Contents of the invention

In view of the above problems, the purpose of the present invention is to provide a single battery energy storage system with an isolated symmetrical series flyback circuit to solve the problem of all charging of the switching devices flowing through the battery modules in the existing full-power modular flexible grouping system. The discharge current leads to large current stress and high cost of the device, and at the same time reduces the peak voltage generated by the closing of the switching device to ensure the safe operation and service life of the switching device.

The purpose of the present invention can be achieved by taking the following technical solutions:

A battery energy storage system with an isolated symmetrical series flyback circuit, said battery energy storage system comprising:

A battery unit, the battery unit includes several battery cells connected in series;

The main power converter is connected in parallel with the battery unit, the main power converter is used to control the main current of the battery unit, and the main power converter is connected to the three-phase AC grid, wherein the main current refers to all the same portion of the battery cell current; and

The isolated flyback converter unit is connected to the battery cells in one-to-one correspondence, and is used for independent closed-loop control of the difference between the charging and discharging current of each battery cell and the main current, wherein the difference between the charging and discharging current and the main current 5%-20% of the main current to adapt to the differences between different battery cells, so that each battery cell can work in the best state;

Wherein, the main current is independently controlled through vector control through the main power converter;

Wherein, the isolated flyback converter unit adopts a symmetrical flyback circuit structure to realize bidirectional control, and by adding an RCD absorption circuit on the basis of the traditional flyback circuit, the peak voltage is reduced and the service life is improved; the described The isolated flyback converter unit is connected to the DC side voltage of the main power converter in the form of the primary side in series, and the transformation ratio is about 1; the isolated flyback converter unit introduces a PWM control strategy for each battery cell The difference between the charging and discharging current and the main current is independently closed-loop controlled, and the PWM control switch tube duty cycle generation method includes one of the voltage type and the peak current.

Further, the main power converter is a traditional energy storage grid-connected inverter structure, including:

a first capacitor connected in parallel with said battery cell; and

The _first switch tube Q1 to the sixth switch tube _Q6 , each switch tube of the _first switch tube Q1 to the _sixth switch tube Q6 is respectively anti-parallel connected with a first diode, the first The first end of the switch tube Q1 is connected to the _first end of the first inductor, and the second end of the _first switch tube Q1 is connected to the positive pole of the battery cell; the second end of the second switch tube _Q2 is connected to the The first end of an inductor, and the first end of the second switching tube _Q2 is connected to the negative pole of the battery cell; the first end of the third switching tube _Q3 is connected to the first end of the second inductor, and the third The second terminal of the switching tube _Q3 is connected to the positive pole of the battery cell; the second terminal of the _fourth switching tube _Q4 is connected to the first terminal of the second inductor, and the first terminal of the fourth switching tube Q4 is connected to the battery cell The negative pole of the cell; the first end of the fifth switching tube _Q5 is connected to the first end of the third inductor, and the second end of the fifth switching tube _Q5 is connected to the positive pole of the battery cell; the sixth switching tube _Q6 The second terminal is connected to the first terminal of the third inductor, and the first terminal of the sixth switching tube _Q6 is connected to the negative pole of the battery cell, and the second terminals of the first inductor, the second inductor and the third inductor They are respectively connected to one phase U _a , U _b , and U _c of the three-phase AC power grid.

Further, the _first switching transistor Q1 to the sixth switching transistor _Q6 are insulated gate bipolar transistors or metal-oxide semiconductor field effect transistors, and the _first switching transistors Q1 to the sixth switching transistors are The first end of the sixth switching tube Q6 is the emitter terminal of the insulated gate bipolar transistor or the source terminal of the metal-oxide semiconductor field effect transistor, and the _first switching tube Q1 to the _sixth The second terminal of the switch tube _Q6 is the collector terminal of the IGBT or the drain terminal of the Metal-Oxide Semiconductor Field Effect Transistor.

Further, the vector control includes one of sinusoidal pulse width modulation and space vector pulse width modulation.

Further, the isolated flyback converter unit includes:

Several flyback high-frequency isolation transformers, the flyback high-frequency isolation transformers as a pair of mutually coupled inductances, which include a primary winding W ₁ and a secondary winding W ₂ ;

Several primary-side converters are connected to the primary winding W1 in one-to- _one correspondence, and the primary-side converters are connected in series, and each primary-side converter has the same structure; and

Several secondary converters are correspondingly connected to the secondary winding W ₂ and each battery cell 110 . Independent closed-loop control of the difference between the charging and discharging current of each battery cell 110 and the main current is realized.

Further, the isolated flyback converter unit is a bidirectional isolated symmetrical flyback converter, wherein the primary side converter includes:

The seventh switching tube Q7, the _seventh switching tube _Q7 is anti-parallel connected with a first diode, the first end of the _seventh switching tube Q7 is connected to the second terminal _of the primary winding W1, so The second end of the _seventh switching tube Q7 described above is connected to one end of the primary side capacitor of the next isolated flyback converter unit;

The primary side RCD snubber circuit is used to limit the maximum reverse peak voltage of the seventh switch tube Q ₇ to ensure that the switch tube works in a safe area. The primary side RCD snubber circuit includes a first snubber capacitor C _s1 , a first snubber resistor R _s1 and the first absorbing diode VD1, the first absorbing capacitor C _s1 is connected in parallel with the first absorbing resistor R _s1 , connected between the anode of the battery cell 100 and the first absorbing diode VD1 Between the cathodes, the anode of the _first absorption diode VD1 is connected to the second terminal of the primary winding W1; and

The second capacitor is connected between the positive pole of the battery unit 100 and the primary side capacitor of the next series isolated flyback converter unit.

Further, the _seventh switching transistor Q7 is an insulated gate bipolar transistor or a metal-oxide semiconductor field effect transistor, and the first terminal of the _seventh switching transistor Q7 is the collector of the insulated gate bipolar transistor terminal or the drain terminal of the metal-oxide semiconductor field effect transistor or, the second terminal of the _seventh switching tube Q7 is the emitter terminal of the insulated gate bipolar transistor or the source of the metal-oxide semiconductor field effect transistor Extreme.

Further, the isolated flyback converter unit is a bidirectional isolated symmetrical flyback converter, wherein the secondary side converter includes:

a third capacitor connected in parallel with the battery unit;

The eighth switch tube Q8, the _eighth switch tube _Q8 is anti-parallel connected with a first diode, the first end of the _eighth switch tube Q8 is connected to the _second terminal of the secondary winding W2, The second end of the _eighth switch tube Q8 is connected to the negative pole of the battery cell through the third capacitor; and

The secondary side RCD snubber circuit is used to limit the maximum reverse peak voltage of the eighth switch tube Q ₈ to ensure that the switch tube works in a safe area. The secondary side RCD snubber circuit includes a second snubber capacitor C _s2 , a second snubber resistor R _s2 and the second absorbing diode VD2, the second absorbing capacitor C _s2 is connected in parallel with the second absorbing resistor R _s2 , connected between the positive electrode of the battery cell and the Between the cathodes of the second absorption diode VD2, the anode of the second absorption diode VD2 is connected to the _second terminal of the secondary winding W2.

Further, the _eighth switching transistor Q8 is an insulated gate bipolar transistor or a metal-oxide semiconductor field effect transistor, and the first terminal of the _eighth switching transistor Q8 is the collector of the insulated gate bipolar transistor terminal or the drain terminal of the metal-oxide semiconductor field effect transistor, and the second terminal of the _eighth switching tube Q8 is the emitter terminal of the insulated gate bipolar transistor or the source terminal of the metal-oxide semiconductor field effect transistor son.

Further, the battery cells include newly produced lithium-ion battery modules, lead-acid battery modules, supercapacitor modules, nickel-metal hydride battery modules and returned batteries.

Compared with the prior art, the present invention has the following advantages and effects:

1. In the battery energy storage system of the present invention, the main power converter bears most of the power, controls the main current of the battery unit, and improves the efficiency of the whole machine; the isolated flyback converter unit only controls 5%-20% of the total battery current. % to carry out independent control to adapt to the inconsistency and improve the energy utilization rate of the battery; compared with other circuits which are divided into two levels of module equalization and single equalization, this circuit has only one level, simple structure, and the special primary side is connected in series The total voltage has a transformation ratio of about 1, which simplifies the complexity of transformer design and reduces design and production costs.

2. At the same time, the battery energy storage system of the present invention also adds an RCD absorption circuit on the basis of the traditional flyback circuit, which weakens the peak voltage that the switch tube bears when it is turned off, and ensures the safe operation and service life of the switch tube. The symmetrical circuit The structure makes the control of the converter simple and flexible. The isolated flyback converter unit and the main power converter can work simultaneously or separately without affecting each other.

3. The battery energy storage system of the present invention can be applied to energy storage system application occasions with low voltage, medium and small power levels, and high utilization rate requirements, or it can be retrofitted on the original system.

Description of drawings

Fig. 1 is a construction topology scheme diagram of the battery energy storage system disclosed in the present invention;

Fig. 2 is a main circuit diagram of a preferred embodiment of the battery energy storage system disclosed in the present invention;

Fig. 3 is the working waveform of the isolated flyback converter unit disclosed in the present invention.

Detailed ways

In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments It is a part of embodiments of the present invention, but not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by persons of ordinary skill in the art without making creative efforts belong to the protection scope of the present invention.

Example

Figure 1 shows the construction topology diagram of the battery energy storage system disclosed in this embodiment. As shown in Figure 1, the construction topology diagram of the battery energy storage system disclosed in this embodiment includes: an isolated flyback converter unit, The battery unit, the main power converter and the three-phase alternating current, through the main power converter and the isolated flyback converter unit, perform partial power independent current control on the battery unit.

Fig. 2 shows the main circuit diagram of the preferred embodiment of the battery energy storage system of the present invention, as shown in Fig. 2, the battery energy storage system of the present invention includes:

A battery unit 100, the battery unit includes several battery cells 110 connected in series;

The main power converter 200 is connected in parallel with the battery unit 100, the main power converter 200 is used to control the main current of the battery unit 100, and the main power converter 200 is connected to the three-phase AC grid, wherein the main current means the same fraction of all battery cell currents; and

The isolated flyback converter unit 300 is connected to the battery cells 110 in one-to-one correspondence, and is used for independent closed-loop control of the difference between the charge and discharge current and the main current of each battery cell 110, wherein the charge and discharge current and the main current The difference is preferably 5%-20% of the main current to adapt to the differences between different battery cells, so that each battery cell can work in the best state; only a small part of the current in the battery cell passes through each cell. The switching device of the power converter corresponding to the battery cell reduces the current stress and conduction loss of the device, reduces the cost, and realizes the improvement of the energy utilization rate of the battery module.

The main power converter 200 and the isolated flyback converter unit 300 respectively control the main current and the differential current, thereby realizing independent control of the charging and discharging current of the battery cell 110 and improving the energy utilization rate of the battery cell 110 .

Among them, the battery cells 110 can be newly produced lithium-ion batteries, lead-acid batteries, supercapacitors or nickel-metal hydride batteries, etc., or can be returned batteries in other systems, so as to realize the step-by-step utilization of returned batteries and make full use of the remaining batteries. The utilization value of the battery is conducive to environmental protection and resource conservation.

As shown in Figure 2, the main power converter 200 is preferably a traditional energy storage grid-connected inverter structure, including:

The first capacitor 210 is connected in parallel with the battery unit 100; and

The first switching tube Q ₁ to the sixth switching tube Q ₆ , each of the first switching tube Q ₁ to the sixth switching tube Q ₆ is connected in antiparallel with a first diode 220 respectively, and the first switching tube Q 1 to the sixth switching tube Q 6 The first end of a switch tube Q1 is connected to the _first end of the first inductor 230, and the second end of the _first switch tube Q1 is connected to the positive pole of the battery cell 100; the second end of the second switch tube _Q2 connected to the first end of the first inductor 230, and the first end of the second switching tube _Q2 is connected to the negative pole of the battery unit 100; the first end of the third switching tube _Q3 is connected to the first end of the second inductor 240 One end, and the second end of the third switching tube _Q3 is connected to the positive pole of the battery cell 100; the second end of the fourth switching tube Q4 is connected to the first end of the second inductor 240, and the _fourth switching tube Q ₄ is connected to the negative pole of the battery cell 100; the first end of the fifth switch tube _Q5 is connected to the first end of the third inductor 250, and the second end of the fifth switch tube _Q5 is connected to the battery cell The positive pole of 100; the second end of the sixth switching tube _Q6 is connected to the first end of the third inductor 250, and the first end of the sixth switching tube _Q6 is connected to the negative pole of the battery unit 100, the first inductor 230 , the second ends of the second inductor 240 and the third inductor 250 are respectively connected to one phase U _a , U _b , U _c of the three-phase AC grid.

The _first switching tube Q1 to the sixth switching tube _Q6 can be insulated gate bipolar transistors (IGBT, Insulated Gate Bipolar Transistor) or metal-oxide semiconductor field effect transistors (MOSFET, Metal-Oxide-Semiconductor Field Effect Transistor) , according to the different voltage levels of the application, the appropriate switch tube can be selected. Preferably IGBT, the first end of the _first switching tube Q1 to the _sixth switching tube Q6 is the emitter terminal of the IGBT, and the second end of the _first switching tube Q1 to the _sixth switching tube Q6 is the collector terminal of the IGBT .

The main power converter 200 controls the main current through vector control, and the vector control includes one of sinusoidal pulse width modulation (SPWM) and space vector pulse width modulation (SVPWM).

The isolated flyback converter unit 300 of the present invention adopts a symmetrical flyback circuit structure to realize bidirectional control, and adds an RCD absorbing circuit on the basis of a traditional flyback circuit to reduce peak voltage and increase service life. The isolated flyback converter unit 300 is connected to the DC side voltage of the main power converter 200 in series with the primary side, and the transformation ratio is about 1, which simplifies the complexity of the transformer design. The isolated flyback converter unit 300 performs independent closed-loop control on the difference between the charging and discharging current of each battery cell 110 and the main current by introducing a PWM control strategy, wherein, the duty ratio generation method of the PWM control switching tube includes a voltage type and (peak) current.

The isolated flyback converter unit 300 includes:

A plurality of flyback high-frequency isolation transformers 330. The flyback high-frequency isolation transformers 330 can be regarded as a pair of mutually coupled inductors, which include a primary winding W ₁ and a secondary winding W ₂ ;

A plurality of primary-side converters 310 are connected to the primary winding W1, and the primary _- side converters are connected in series, and each primary-side converter has the same structure; and

A plurality of secondary converters 320 are correspondingly connected to the secondary winding W ₂ and each battery cell 110 . Independent closed-loop control of the difference between the charging and discharging current of each battery cell 110 and the main current is realized.

On the basis of the main current of the battery unit 100 , the difference between the charging and discharging current of each battery unit 110 and the main current can be positive or negative, so the isolated flyback converter unit 300 is preferably a bidirectional isolated DC-DC converter.

As shown in Figure 2, the bidirectional isolated DC-DC converter is a bidirectional isolated symmetrical flyback converter, the primary side converter 310 includes a seventh switching tube Q ₇ , and the seventh switching tube Q ₇ is connected in antiparallel with a first Diode 220, the first terminal of the _seventh switching tube Q7 is connected to the second terminal of the primary winding W1, and the second terminal _of the _seventh switching tube Q7 is connected to the next isolated flyback converter One end of the primary side capacitor of the device unit;

The primary side RCD snubber circuit 311 is used to limit the maximum reverse peak voltage of the seventh switch tube Q ₇ to ensure that the switch tube works in a safe area. The primary side RCD snubber circuit 311 includes a first snubber capacitor C _s1 , a first The absorption resistor R _s1 and the first absorption diode VD1, the first absorption capacitor C _s1 and the first absorption resistor R _s1 are connected in parallel, connected between the anode of the battery unit 100 and the first absorption diode Between the cathodes of VD1, the anode of the _first absorption diode VD1 is connected to the second terminal of the primary winding W1; and

The second capacitor 312 is connected between the positive pole of the battery unit 100 and the primary side capacitor of the next series isolated flyback converter unit.

The _seventh switching tube Q7 can be an insulated gate bipolar transistor (IGBT, Insulated Gate Bipolar Transistor) or a metal-oxide semiconductor field effect transistor (MOSFET, Metal-Oxide-Semiconductor Field Effect Transistor), which can be selected according to different applications. Choose the appropriate switching tube for the voltage level and the required switching frequency. MOSFET is preferred, the first terminal of the _seventh switching tube Q7 is the drain terminal of the MOSFET, and the second terminal of the _seventh switching tube Q7 is the source terminal of the MOSFET.

As shown in Figure 2, the secondary converter 320 adopts the same topology as the primary converter 310 to ensure the flexibility of bidirectional operation of the system. The secondary converter 320 includes:

The third capacitor 322 is connected in parallel with the battery cell 110;

The eighth switch tube Q8, the _eighth switch tube _Q8 is anti-parallel connected with a first diode 220, the first end of the _eighth switch tube Q8 is connected to the _second terminal of the secondary winding W2 , the second end of the _eighth switch tube Q8 is connected to the negative pole of the battery cell 110 through the third capacitor 322; and

The secondary side RCD snubber circuit 321 is used to limit the maximum reverse peak voltage of the eighth switch tube Q ₈ to ensure that the switch tube works in a safe area. The secondary side RCD snubber circuit 321 includes a second snubber capacitor C _s2 , a second snubber capacitor Resistor R _s2 and the second absorption diode VD2, the second absorption capacitor C _s2 is connected in parallel with the second absorption resistance R _s2 , connected between the anode of the single cell 110 and the cathode of the second absorption diode VD2 through the third capacitor 322 Between, the anode of the second absorption diode VD2 is connected to the _second terminal of the secondary winding W2.

The eighth switching tube Q ₈ may be an insulated gate bipolar transistor (IGBT, Insulated Gate Bipolar Transistor) or a metal-oxide semiconductor field effect transistor (MOSFET, Metal-Oxide-Semiconductor Field Effect Transistor), which may vary depending on the application Choose the appropriate switching tube for the voltage level and the required switching frequency. MOSFET is preferred, the first terminal of the _eighth switching transistor Q8 is the drain terminal of the MOSFET, and the second terminal of the _eighth switching transistor Q8 is the source terminal of the MOSFET.

The DC side of the isolated flyback converter unit 300 can be directly connected to both ends of the internal battery unit 100 of the battery energy storage system as mentioned above, or can be connected to both ends of other external DC power sources.

The specific control process of the battery energy storage system of the present invention will be described in detail below with reference to FIG. 2 and FIG. 3 , taking the PWM control of a bidirectional isolated symmetrical flyback converter as an example.

As shown in Figure 2, the main power converter can obtain the main current I _m under the condition of the required power level and voltage level through the three-phase grid-connected control strategy.

Assume that P _di (i=1,2,...,n) is the partial power control of the i-th battery cell, that is, the power corresponding to the differential current, and P _∑s is the sum of the total differential power of the n battery cells. According to the output power of the isolated flyback converter, it can be known that the differential power of each battery is:

P _d2 = v _d2 I _d2

...

P _dn = v _dn I _dn

Where: D＝t _on /(t _on +t _off )

In the formula, v _d1 , v _d2 , ..., v _dn are the voltages of n battery cells, I _d1 , I _d2 , ..., I _dn are the voltages between the main current and the charging and discharging current of each module The differential current generally takes 5%-20% of the main current as the differential current, L2 is the inductance value of the secondary winding, T is a switching cycle, _f is the frequency of the switching tube, and D is the duty cycle of the switching tube in one cycle . The size of the duty cycle is obtained by PWM controlling the switching on and off of the switching tube.

As shown in Figure 2, the voltage difference of the battery cells we selected is not large, we can assume that v _d1 =v _d2 =...=v _dn , power conservation shows that the power P _p of the primary side converter is equal to the secondary side conversion The sum of the difference power of the device is:

P _p ＝P _∑s ＝P _d1 +P _d2 +...+P _dn

＝v _d1 I _d1 +v _d2 I _d2 +...v _dn I _dn

＝v _d1 (I _d1 +I _d2 +...I _dn )

v _dp is the DC side voltage of the isolated flyback converter unit, which is equal to the voltage of all battery cells, then the DC side current I _dp of the isolated flyback converter unit is:

It can be seen from this that, through reasonable optimization control, the main current I _M of the battery cell is the average current of all battery currents, then the differential currents such as I _d1 , I _d2 , ..., I _dn have positive and negative, and the positive and negative offset make the differential current and is 0, we can get

Since I _dp is 0, the total control power is small, the absolute loss is small, and the cost is also reduced.

To sum up, the battery energy storage system with independent power control of this part can greatly reduce the control power of the isolated flyback converter unit converter of the battery energy storage system by giving a reasonable control current, reduce the absolute loss, and improve the Energy utilization of the battery.

The above-mentioned embodiment is a preferred embodiment of the present invention, but the embodiment of the present invention is not limited by the above-mentioned embodiment, and any other changes, modifications, substitutions, combinations, Simplifications should be equivalent replacement methods, and all are included in the protection scope of the present invention.

Claims (10)

1. A battery energy storage system with an isolated symmetrical series flyback circuit, characterized in that the battery energy storage system includes: A battery unit, the battery unit includes several battery cells connected in series; The main power converter is connected in parallel with the battery unit, the main power converter is used to control the main current of the battery unit, and the main power converter is connected to the three-phase AC grid, wherein the main current refers to all the same portion of the battery cell current; and The isolated flyback converter unit is connected to the battery cells in one-to-one correspondence, and is used for independent closed-loop control of the difference between the charging and discharging current of each battery cell and the main current, wherein the difference between the charging and discharging current and the main current 5%-20% of the main current to adapt to the differences between different battery cells, so that each battery cell can work in the best state; Wherein, the main current is independently controlled through vector control through the main power converter; Wherein, the isolated flyback converter unit adopts a symmetrical flyback circuit structure to realize bidirectional control, and by adding an RCD absorption circuit on the basis of the traditional flyback circuit, the peak voltage is reduced and the service life is improved; the described The isolated flyback converter unit is connected to the DC side voltage of the main power converter in the form of the primary side in series, and the transformation ratio is about 1; the isolated flyback converter unit introduces a PWM control strategy for each battery cell The difference between the charging and discharging current and the main current is independently closed-loop controlled, and the PWM control switch tube duty cycle generation method includes one of the voltage type and the peak current. 2. A battery energy storage system with an isolated symmetrical series flyback circuit according to claim 1, wherein the main power converter is a traditional energy storage grid-connected inverter structure, comprising: a first capacitor connected in parallel with said battery cell; and The _first switch tube Q1 to the sixth switch tube _Q6 , each switch tube of the _first switch tube Q1 to the _sixth switch tube Q6 is respectively anti-parallel connected with a first diode, the first The first end of the switch tube Q1 is connected to the _first end of the first inductor, and the second end of the _first switch tube Q1 is connected to the positive pole of the battery cell; the second end of the second switch tube _Q2 is connected to the The first end of an inductor, and the first end of the second switching tube _Q2 is connected to the negative pole of the battery cell; the first end of the third switching tube _Q3 is connected to the first end of the second inductor, and the third The second terminal of the switching tube _Q3 is connected to the positive pole of the battery cell; the second terminal of the _fourth switching tube _Q4 is connected to the first terminal of the second inductor, and the first terminal of the fourth switching tube Q4 is connected to the battery cell The negative pole of the cell; the first end of the fifth switching tube _Q5 is connected to the first end of the third inductor, and the second end of the fifth switching tube _Q5 is connected to the positive pole of the battery cell; the sixth switching tube _Q6 The second terminal is connected to the first terminal of the third inductor, and the first terminal of the sixth switching tube _Q6 is connected to the negative pole of the battery cell, and the second terminals of the first inductor, the second inductor and the third inductor They are respectively connected to one phase U _a , U _b , and U _c of the three-phase AC power grid. 3. The battery energy storage system of an isolated symmetrical series flyback circuit according to claim 2, wherein the _first switching tube Q1 to the sixth switching tube _Q6 are insulated barrier Bipolar transistors or metal-oxide semiconductor field effect transistors, the first terminals of the _first switching transistor Q1 to the _sixth switching transistor Q6 are emitter terminals of insulated gate bipolar transistors or metal-oxide The source terminal of the object semiconductor field effect transistor, the second terminal of the _first switching tube Q1 to the _sixth switching tube Q6 is the collector terminal of the insulated gate bipolar transistor or the metal-oxide semiconductor field The drain terminal of the effect transistor. 4. A battery energy storage system with an isolated symmetrical series flyback circuit according to claim 1, wherein the vector control includes one of sinusoidal pulse width modulation and space vector pulse width modulation. 5. The battery energy storage system of an isolated symmetrical series flyback circuit according to claim 1, wherein the isolated flyback converter unit comprises: Several flyback high-frequency isolation transformers, the flyback high-frequency isolation transformers as a pair of mutually coupled inductances, which include a primary winding W ₁ and a secondary winding W ₂ ; Several primary-side converters are connected to the primary winding W1 in one-to- _one correspondence, and the primary-side converters are connected in series, and each primary-side converter has the same structure; and Several secondary converters are correspondingly connected to the secondary winding W ₂ and each battery cell 110 . Independent closed-loop control of the difference between the charging and discharging current of each battery cell 110 and the main current is realized. 6. The battery energy storage system of an isolated symmetrical series flyback circuit according to claim 5, wherein the isolated flyback converter unit is a bidirectional isolated symmetrical flyback converter, wherein, The primary converter includes: The seventh switching tube Q7, the _seventh switching tube _Q7 is anti-parallel connected with a first diode, the first end of the _seventh switching tube Q7 is connected to the second terminal _of the primary winding W1, so The second end of the _seventh switching tube Q7 described above is connected to one end of the primary side capacitor of the next isolated flyback converter unit; The primary side RCD snubber circuit is used to limit the maximum reverse peak voltage of the seventh switch tube Q ₇ to ensure that the switch tube works in a safe area. The primary side RCD snubber circuit includes a first snubber capacitor C _s1 , a first snubber resistor R _s1 and the first absorbing diode VD1, the first absorbing capacitor C _s1 is connected in parallel with the first absorbing resistor R _s1 , connected between the anode of the battery unit 100 and the first absorbing diode VD1 Between the cathodes, the anode of the _first absorption diode VD1 is connected to the second terminal of the primary winding W1; and The second capacitor is connected between the positive pole of the battery unit 100 and the primary side capacitor of the next series isolated flyback converter unit. 7. The battery energy storage system of an isolated symmetrical series flyback circuit according to claim 6, wherein the _seventh switching tube Q7 is an insulated gate bipolar transistor or a metal-oxide semiconductor field effect transistor, the first terminal of the _seventh switching tube Q7 is the collector terminal of the insulated gate bipolar transistor or the drain terminal of the metal-oxide semiconductor field effect transistor or, the _seventh switching tube Q7 The second terminal of is the emitter terminal of the insulated gate bipolar transistor or the source terminal of the metal-oxide semiconductor field effect transistor. 8. The battery energy storage system of an isolated symmetrical series flyback circuit according to claim 5, wherein the isolated flyback converter unit is a bidirectional isolated symmetrical flyback converter, wherein, The secondary converter includes: a third capacitor connected in parallel with the battery unit; The eighth switch tube Q8, the _eighth switch tube _Q8 is anti-parallel connected with a first diode, the first end of the _eighth switch tube Q8 is connected to the _second terminal of the secondary winding W2, The second end of the _eighth switch tube Q8 is connected to the negative pole of the battery cell through the third capacitor; and The secondary side RCD snubber circuit is used to limit the maximum reverse peak voltage of the eighth switch tube Q ₈ to ensure that the switch tube works in a safe area. The secondary side RCD snubber circuit includes a second snubber capacitor C _s2 , a second snubber resistor R _s2 and the second absorbing diode VD2, the second absorbing capacitor C _s2 is connected in parallel with the second absorbing resistor R _s2 , connected between the positive electrode of the battery cell and the Between the cathodes of the second absorption diode VD2, the anode of the second absorption diode VD2 is connected to the _second terminal of the secondary winding W2. 9. The battery energy storage system of an isolated symmetrical series flyback circuit according to claim 8, wherein the _eighth switching tube Q8 is an insulated gate bipolar transistor or a metal-oxide semiconductor field effect transistor, the first terminal of the _eighth switching tube Q8 is the collector terminal of the insulated gate bipolar transistor or the drain terminal of the metal-oxide semiconductor field effect transistor, the _eighth switching tube Q8 The second terminal is the emitter terminal of the insulated gate bipolar transistor or the source terminal of the metal-oxide semiconductor field effect transistor. 10. A battery energy storage system with an isolated symmetrical series flyback circuit according to claim 8, wherein said battery cells include newly produced lithium-ion battery modules, lead-acid battery modules, supercapacitors Modules, NiMH Modules, and Return Batteries.