Energy storage device suitable for flow battery and control method of energy storage device

Abstract

The invention relates to an energy storage device suitable for a flow battery and a control method of the energy storage device. The energy storage device is composed of a three-phase H-bridge conversion circuit and a series active filtering elimination circuit. Wherein the three-phase H-bridge conversion circuit is connected with a battery pack and a power grid to realize energy interaction, an alternating-current side is provided with three phases and four wires, and a direct-current side can be independently connected with three groups of batteries; the series-connection type active filtering elimination circuit is connected with the battery pack in series and then provides direct-current voltage for the three-phase H-bridge conversion circuit, meanwhile, frequency doubling voltage is generated, and elimination of frequency doubling fluctuation is achieved. Compared with an existing similar energy storage device, one-cluster-one management of the battery pack is realized, and a DCDC converter does not need to be additionally arranged; by adopting the circuit, the output voltage of the bridge arm is converted from the line voltage to the phase voltage, the alternating current side access voltage grade is improved by # imgabs0 # times, the conversion efficiency is as high as 99%, the two problems of low alternating current side access voltage and low conversion efficiency in the application of the flow battery energy storage system are solved, and the design cost of the flow battery energy storage system is reduced.

Description

Technical Field

[0001] The present invention relates to the field of power electronics technology, and particularly relates to an energy storage device suitable for a flow battery and a control method thereof. Background Art

[0002] The proportion of new energy in the energy structure is gradually increasing. As a "balancer" of new energy, energy storage technology pays more and more attention to safety and long-term storage capacity. Flow battery energy storage is superior to lithium battery energy storage in terms of safety due to the characteristics of the separation of the positive and negative electrodes of the electrolyte, and due to the characteristics of the mutual independence of the electrolyte and the stack, the energy storage duration is only related to the size of the electrolyte storage tank. The currently common energy storage duration is generally 4-8 hours, which is more than twice that of lithium battery energy storage. It has good development prospects.

[0003] Compared with lithium batteries, the port voltage of flow batteries is about half that of lithium batteries. At the same power level, the charge and discharge current of flow batteries is twice that of lithium batteries. Existing energy storage devices are designed to adapt to the voltage level and over-current capacity of lithium batteries and are directly applied to flow battery energy storage systems, which mainly have the following three problems: First, there are two technical routes for flow batteries: multi-group series high-voltage type and conventional series low-voltage type. The DC port voltage of the multi-group series high-voltage type is the same as that of lithium batteries, and shares an AC integrated boost cabin (including the energy storage device) with lithium battery energy storage. Therefore, the grid connection part cost is the same as that of lithium batteries, and the battery part cost is higher than that of lithium batteries. The multi-group series high-voltage type is mainly represented by Dalian Rongke. Currently, the multi-group series high-voltage type is in the demonstration project stage, and there are still high technical difficulties in large-scale engineering applications, such as high-voltage insulation problems, current sharing problems, liquid leakage problems, etc. Both the technical difficulty and the design cost are higher than those of the conventional series low-voltage type. Currently, the vast majority of flow battery manufacturers use the conventional series low-voltage type and look for a matching AC integrated boost cabin (including the energy storage device) in the market.

[0004] Second, in the application of conventional series low-voltage flow batteries, the grid connection voltage on the AC side is limited by the lower limit of the DC voltage of the flow battery, and it is impossible to achieve high-voltage grid connection of AC690V. Only grid connection voltage levels of AC315V or even lower can be achieved, resulting in charge and discharge currents of up to 1000A or even more than 2500A. Energy storage devices on the market need to have a power capacity of 2 times or more to be applied, which increases the design cost of the energy storage device by more than 2 times, and the cost of customized small-batch step-up transformers is also much higher than that of AC690V large-scale mass-produced step-up transformers (commonly used in lithium battery energy storage).

[0005] Thirdly, in the application of a conventional series low-voltage flow battery, in order to reduce the costs of the energy storage device and the step-up transformer, the port voltage of the flow battery needs to be increased by more than twice, and a high-power and high-voltage DCDC converter is required to boost the port voltage of the flow battery. This additionally increases the design cost of the DCDC converter. At the same time, the two-stage conversion structure also reduces the energy conversion efficiency, resulting in the design cost of flow battery energy storage being much higher than that of lithium battery energy storage and the energy conversion efficiency being much lower than that of lithium battery energy storage, which is not conducive to the development of flow battery energy storage. Summary of the Invention

[0006] The purpose of the present invention is to overcome the deficiencies of the prior art. In the application of a conventional series low-voltage flow battery, a energy storage device applicable to a flow battery and its control method are proposed. The device includes: a three-phase H-bridge conversion circuit, an AC-side LCL filter circuit, an AC pre-charge circuit, an AC EMI filter, an AC fuse, three active harmonic elimination circuits, three DC EMI filters, and three DC pre-charge circuits.

[0007] The three-phase H-bridge conversion circuit is composed of three single-phase H-bridge circuits. The switching devices are all IGBTs. There are 4 ports, namely U', V', W', and N' on the AC side of the three-phase H-bridge conversion circuit. The U', V', and W' ports are the positive poles of the AC ends of the three single-phase H-bridge circuits respectively, and the N' port is connected to the negative poles of the AC ends of the three single-phase H-bridge circuits.

[0008] The 4 ports, namely U', V', W', and N' on the AC side of the three-phase H-bridge conversion circuit are successively connected to the AC-side LCL filter circuit, the AC pre-charge circuit, the AC EMI filter, and the AC fuse, and then connected to the power grid U, V, W, and N.

[0009] There are 3 pairs of ports on the DC side of the three-phase H-bridge conversion circuit, which are respectively composed of the DC ends of the three single-phase H-bridge circuits. A capacitor is also connected in parallel at the DC end of the single-phase H-bridge circuit.

[0010] The positive poles of the DC ends of the three single-phase H-bridge circuits are all connected in series with a series active harmonic elimination circuit and then connected to the positive pole on the right side of the DC EMI filter. The negative pole of each DC end of the single-phase H-bridge circuit is directly connected to the negative pole on the right side of the DC EMI filter; the positive and negative ports on the left side of the three DC EMI filters are respectively connected in series with DC fuses and then connected to the positive and negative ends on the right side of the DC pre-charge circuit. The positive and negative poles on the left side of the three DC pre-charge circuits are respectively connected to the positive and negative poles of the battery pack.

[0011] The series active harmonic elimination circuit includes: a secondary rectifier circuit, a DC filter capacitor C2, a bidirectional H-bridge circuit, and a DC-side LCL filter circuit. The AC terminal of the secondary rectifier circuit is connected to one secondary output of the high-frequency isolation transformer T, and the DC terminal is connected to the DC terminal of the bidirectional H-bridge circuit after being connected in parallel with the DC filter capacitor C2. The AC terminal of the bidirectional H-bridge circuit is connected to the left two ports of the DC-side LCL filter circuit. The two ports on the right side of the DC-side LCL filter circuit are respectively connected to the positive pole of the DC terminal of the single-phase H-bridge circuit and the positive pole on the right side of the DC EMI filter. The high-frequency isolation transformer T has 1 input terminal and 3 output terminals. The input terminal is connected to the AC terminal of the inverter bridge circuit. The DC terminal of the inverter bridge circuit is connected to the DC power supply after being connected in parallel with the DC bus capacitor. The 3 output terminals are respectively connected to the secondary rectifier circuits of three series active harmonic elimination circuits.

[0012] The power of the primary inverter bridge circuit of the high-frequency isolation transformer T is less than 10 times the power of the secondary circuit.

[0013] Further, the AC-side LCL filter circuit is composed of two inductors and a filter capacitor. After the two inductors are connected in series, one end is connected to the three-phase H-bridge conversion circuit, and the other end is connected to the AC pre-charge circuit. One end of the filter capacitor is connected to the middle of the two series inductors, and the other end is connected to the N line.

[0014] Further, the AC pre-charge circuit is composed of a main circuit breaker KM1 and a circuit breaker KM2 connected in parallel at both ends of the main circuit breaker on the three-phase main circuit.

[0015] Further, the DC-side pre-charge circuit is composed of a main circuit breaker KM3 and a circuit breaker KM4 connected in parallel at both ends of the main circuit breaker KM3.

[0016] The present invention also provides a control method for an energy storage device applicable to a flow battery, which is used to realize the operation control of the above energy storage device. The control method includes the following steps: S1: Control the start of the inverter bridge circuit at the input terminal of the high-frequency isolation transformer T, control the start of the secondary rectifier circuit, control the bidirectional H-bridge circuit on the secondary side, and connect the battery pack to the DC terminal of the single-phase H-bridge circuit.

[0017] S2: Set the working mode of the three-phase H-bridge conversion circuit to grid-connected operation or off-grid operation.

[0018] S3: After the setting is completed, start the pre-charge control of the capacitor connected in parallel at the DC terminal of the single-phase H-bridge circuit, which is divided into AC pre-charge control during grid-connected operation and DC pre-charge control during off-grid operation.

[0019] S4: After the pre-charge of the capacitor connected in parallel at the DC terminal of the single-phase H-bridge circuit is completed, control the IGBT of the three-phase H-bridge conversion circuit to enter the grid-connected operation or off-grid operation mode.

[0020] S5: When operating in grid-connected mode, set the three-phase H-bridge conversion circuit to control the charging or discharging of the battery pack; when operating in off-grid mode, control the charging or discharging according to the load and the power of the external power supply.

[0021] S6: Control the series active harmonic elimination circuit to eliminate the double-frequency fluctuations on the DC side generated during the charging or discharging process.

[0022] The control of the series active harmonic elimination circuit to eliminate the double-frequency fluctuations on the DC side generated during the charging or discharging process includes: S601: The DC side of the inverter bridge circuit on the primary side of the high-frequency isolation transformer T is connected to a DC power supply to provide energy for the entire series active power filter elimination circuit. Taking the power-frequency sinusoidal AC voltage as the modulation wave signal, through the PWM modulation of turning on and off the 4 IGBTs on 2 bridge arms of the inverter bridge circuit, a stable power-frequency sinusoidal AC voltage is generated on the primary side of the high-frequency isolation transformer T.

[0023] S602: Three identical power-frequency sinusoidal AC voltages are induced at the 3 output terminals of the high-frequency isolation transformer T to provide stable AC voltages for the 3 secondary rectifier circuits respectively.

[0024] S603: The secondary rectifier circuit generates a DC voltage at the DC port through the PWM modulation of turning on and off the 4 IGBTs on its 2 bridge arms, and filters out the high-frequency ripples through a filter capacitor to provide a DC voltage for the bidirectional H-bridge circuit on the secondary side.

[0025] S604: The bidirectional H-bridge circuit on the secondary side obtains the AC double-frequency fluctuation voltage signal by subtracting the DC bus voltage and the battery voltage. After performing phase-locked control on this signal, the amplitude and phase angle of the double-frequency fluctuation voltage are obtained, and after taking its inverse, it is used as the modulation wave signal of the bidirectional H-bridge circuit on the secondary side. In this way, the PWM modulation of turning on and off the 4 IGBTs on its 2 bridge arms is controlled to generate an AC voltage signal with the same magnitude as the target double-frequency and a phase difference of 180°, so as to cancel the double-frequency fluctuations in the circuit.

[0026] Compared with the existing energy storage devices, in the present invention, the output voltage of the bridge arm of the three-phase H-bridge conversion circuit is converted from the line voltage to the phase voltage, and the voltage level of the AC side access is increased. times, and the conversion efficiency is as high as 99%, solving the two major problems of low AC-side access voltage and low conversion efficiency existing in the application of the flow battery energy storage system, and reducing the design cost of the flow battery energy storage system. The beneficial effects are mainly reflected in: First, the power density of the energy storage device is improved, and the design cost of the energy storage device is reduced. The high-voltage output ability on the AC side of this device can output more than twice the power under the same AC current, improving its own power density and achieving a cost-reducing design for the energy storage device. Second, the design cost of the flow battery energy storage system is reduced, and the energy conversion efficiency of the system is improved. Without the need to additionally increase the boost of the DCDC converter, this device can access the AC690V large-scale mass boost transformer (commonly used in lithium battery energy storage) on the AC side, and the conversion efficiency is as high as 99%, achieving a cost-reducing design for the flow battery energy storage system and improving the energy conversion efficiency at the same time. Finally, cluster management improves the service life of the battery pack. The DC side of this device has 3 independent battery pack interfaces. This design reduces the number of series-connected stacks by one-third, weakens the influence of the "barrel" effect on the battery pack during the charge and discharge process, and at the same time reduces the insulation protection level of the flow battery, improving the service life of the battery pack. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 is a schematic topological diagram of an energy storage device applicable to a flow battery according to an embodiment of the present invention; Figure 2 is a schematic circuit topological diagram of a traditional two-level energy storage device according to an embodiment of the present invention; Figure 3 is a schematic diagram of the working principle of the three-phase H-bridge conversion circuit in the discharge state according to an embodiment of the present invention; Figure 4 is a schematic diagram of the working principle of the three-phase H-bridge conversion circuit in the charging state according to an embodiment of the present invention; Figure 5 is a schematic topological diagram of a series active harmonic elimination circuit according to an embodiment of the present invention; Figure 6 is a schematic diagram of the secondary rectification and elimination circuit according to an embodiment of the present invention; Figure 7 is a schematic diagram of the working principle of the primary side of the harmonic elimination circuit according to an embodiment of the present invention; Figure 8 is a schematic diagram of the working principle of the positive half-cycle of the secondary side discharge of the harmonic elimination circuit according to an embodiment of the present invention; Figure 9 is a schematic diagram of the working principle of the negative half-cycle of the secondary side discharge of the harmonic elimination circuit according to an embodiment of the present invention; Figure 10 is a schematic diagram of the working principle of the positive half-cycle of the secondary side charging of the harmonic elimination circuit according to an embodiment of the present invention; Figure 11Schematic diagram of the working principle of the negative half-cycle charging of the secondary side of the harmonic elimination circuit according to an embodiment of the present invention. Specific embodiments

[0028] The following will detail the specific functions in four aspects: the energy storage device, the grid-connected and off-grid control method for AC access to high voltage, the control method for eliminating the second harmonic fluctuation, and the control method for the charging and discharging energy flow: (I) Energy storage device The energy storage device includes a three-phase H-bridge conversion circuit and a series active filter elimination circuit, as Figure 1 shown.

[0029] The three-phase H-bridge conversion circuit is composed of three single-phase H-bridge circuits. The switching devices are all IGBTs. There are 4 ports, U', V', W', and N', on the AC side of the three-phase H-bridge conversion circuit. The U', V', and W' ports are the positive poles of the AC ends of the three single-phase H-bridge circuits respectively, and the N' port is connected to the negative poles of the AC ends of the three single-phase H-bridge circuits.

[0030] The 4 ports, U', V', W', and N', on the AC side of the three-phase H-bridge conversion circuit are sequentially connected to the AC side LCL filter circuit, the AC pre-charge circuit, the AC EMI filter, and the AC fuse, and then connected to the power grid U, V, W, and N.

[0031] There are 3 pairs of ports on the DC side of the three-phase H-bridge conversion circuit, which are respectively composed of the DC ends of the three single-phase H-bridge circuits. A capacitor is also connected in parallel at the DC end of the single-phase H-bridge circuit.

[0032] The positive poles of the DC ends of the three single-phase H-bridge circuits are all connected to the positive pole on the right side of the DC EMI filter after being serially connected with a series active harmonic elimination circuit. The negative pole of each single-phase H-bridge circuit DC end is directly connected to the negative pole on the right side of the DC EMI filter; the positive and negative ports on the left side of the three DC EMI filters are serially connected with DC fuses and then connected to the positive and negative ends on the right side of the DC pre-charge circuit. The positive and negative poles on the left side of the three DC pre-charge circuits are respectively connected to the positive and negative poles of the battery pack.

[0033] The series active harmonic elimination circuit includes: a secondary rectifier circuit, a DC filter capacitor C2, a bidirectional H-bridge circuit, and a DC-side LCL filter circuit. The AC terminal of the secondary rectifier circuit is connected to one secondary output of the high-frequency isolation transformer T. After the DC terminal is connected in parallel with the DC filter capacitor C2, it is connected to the DC terminal of the bidirectional H-bridge circuit. The AC terminal of the bidirectional H-bridge circuit is connected to the left two ports of the DC-side LCL filter circuit. The two ports on the right side of the DC-side LCL filter circuit are respectively connected to the positive pole of the DC terminal of the single-phase H-bridge circuit and the positive pole on the right side of the DC EMI filter. The high-frequency isolation transformer T has 1 input terminal and 3 output terminals. The input terminal is connected to the AC terminal of the inverter bridge circuit. The DC terminal of the inverter bridge circuit is connected to the DC power supply after being connected in parallel with the DC bus capacitor. The 3 output terminals are respectively connected to the secondary rectifier circuits of three series active harmonic elimination circuits.

[0034] The power of the primary inverter bridge circuit of the high-frequency isolation transformer T is less than 10 times the power of the secondary circuit.

[0035] The AC-side LCL filter circuit consists of two inductors and a filter capacitor. After the two inductors are connected in series, one end is connected to the three-phase H-bridge conversion circuit, and the other end is connected to the AC pre-charge circuit. One end of the filter capacitor is connected to the middle of the two series inductors, and the other end is connected to the N line.

[0036] The AC pre-charge circuit consists of a main circuit breaker KM1 and a circuit breaker KM2 connected in parallel at both ends of the main circuit breaker on the three-phase main circuit.

[0037] The DC-side pre-charge circuit consists of a main circuit breaker KM3 and a circuit breaker KM4 connected in parallel at both ends of the main circuit breaker KM3.

[0038] The three-phase H-bridge conversion circuit is connected to the battery pack and the power grid to realize the charging and discharging functions of the battery pack. The AC side is connected in a three-phase four-wire manner, and the DC side has 3 independent battery pack interfaces to realize the charge and discharge management of the battery pack; the series active filter elimination circuit is connected in series with the battery pack to provide DC voltage for the three-phase H-bridge conversion circuit, and at the same time generates a second-harmonic voltage to eliminate the second-harmonic fluctuation interference. Since the second-harmonic fluctuation after three-phase superposition is mainly a reactive component, the primary inverter bridge circuit in the series active filter elimination circuit only needs to provide the active energy for the power device itself loss. Therefore, the power of its primary inverter bridge circuit is within 200W, which is much smaller than the power of the secondary side of 2kW to 5kW.

[0039] The operation control method for the above energy storage device includes the following steps: S1: Control the startup of the inverter bridge circuit at the input terminal of the high-frequency isolation transformer T, control the startup of the secondary rectifier circuit, control the bidirectional H-bridge circuit on the secondary side, and connect the battery pack to the DC terminal of the single-phase H-bridge circuit.

[0040] S2: Set the operating mode of the three-phase H-bridge conversion circuit to grid-connected operation or off-grid operation.

[0041] S3: After the setting is completed, start the pre-charge control of the parallel capacitor at the DC side of the single-phase H-bridge circuit, which is divided into AC pre-charge control during grid-connected operation and DC pre-charge control during off-grid operation.

[0042] S4: After the pre-charge of the parallel capacitor at the DC side of the single-phase H-bridge circuit is completed, control the IGBTs of the three-phase H-bridge conversion circuit to enter the grid-connected operation or off-grid operation mode.

[0043] S5: During grid-connected operation, set the three-phase H-bridge conversion circuit to control the charging or discharging of the battery pack; during off-grid operation, perform charging or discharging control according to the load and external power supply power conditions.

[0044] S6: Control the series active harmonic elimination circuit to eliminate the DC-side second-harmonic fluctuations generated during the charging or discharging process.

[0045] The control of the series active harmonic elimination circuit to eliminate the DC-side second-harmonic fluctuations generated during the charging or discharging process includes: S601: Connect the DC side of the inverter bridge circuit on the primary side of the high-frequency isolation transformer T to a DC power supply to provide energy for the entire series active filter elimination circuit. Use the power-frequency sinusoidal AC voltage as the modulation wave signal, and through PWM modulation of the on-off of the 4 IGBTs in 2 bridge arms of the inverter bridge circuit, a stable power-frequency sinusoidal AC voltage is generated on the primary side of the high-frequency isolation transformer T.

[0046] S602: Three identical power-frequency sinusoidal AC voltages are induced at the 3 output terminals of the high-frequency isolation transformer T, respectively providing stable AC voltages for the 3 secondary rectifier circuits.

[0047] S603: The secondary rectifier circuit generates a DC voltage at the DC port through PWM modulation of the on-off of the 4 IGBTs in 2 bridge arms, and filters out the high-frequency ripple through a filter capacitor to provide a DC voltage for the bidirectional H-bridge circuit on the secondary side.

[0048] S604: The bidirectional H-bridge circuit on the secondary side obtains the AC second-harmonic fluctuation voltage signal by subtracting the DC bus voltage and the battery voltage. After performing phase-locked control on this signal, the amplitude and phase angle of the second-harmonic fluctuation voltage are obtained, and after inverting it, it is used as the modulation wave signal of the bidirectional H-bridge circuit on the secondary side. Then, PWM modulation of the on-off of the 4 IGBTs in 2 bridge arms is controlled to generate an AC voltage signal with the same magnitude as the target second-harmonic and a phase difference of 180°, canceling the second-harmonic fluctuations in the circuit.

[0049] (2) Grid-connected and off-grid control method for AC access to high voltage 1) AC-side high-voltage access principle In the application of a conventional series low-voltage flow battery, the voltage range for the flow battery to operate at rated power charge and discharge is generally between DC400V and DC850V. On the premise of ignoring AC voltage fluctuations, the circuit topology of a traditional two-level energy storage device is as follows Figure 2 shown. Each arm outputs a phase of AC voltage. The voltage between the arms is the line voltage. The output form of the other topology arms is similar to this topology. The calculation method for the highest value of the AC-side line voltage is as follows , According to the operating voltage range of the flow battery, the highest value of the AC-side line voltage of the traditional energy storage device is AC283V, which is much lower than AC690V. If AC690V AC line voltage is to be satisfied, the DC voltage needs to be increased to at least 1000V to achieve it

[0050] The voltage between the arms of each H-bridge of the energy storage device mentioned in the present invention is the phase voltage, and there is a times numerical relationship with the line voltage. The calculation method for the highest value of its AC-side line voltage is as follows , According to the operating voltage range of the flow battery, the highest value of the AC-side line voltage of the energy storage device mentioned in the present invention is AC490V. Due to the characteristics of the H-bridge conversion circuit, it has been proposed in the literature that the modulation degree can be increased by 1.15 times through the control algorithm. Therefore, after introducing the control algorithm, the highest value of the AC-side line voltage is AC563V. Its final calculation formula is as follows , If AC690V AC line voltage is to be satisfied, the DC voltage only needs to be increased to 500V to achieve it, that is, the DC voltage range is between DC500V and DC850V, then AC690V AC line voltage access can be achieved. The application of this device realizes the technical solution of sharing the same integrated boost cabin with lithium batteries without increasing the insulation voltage level of the flow battery, and greatly reduces the design cost of the flow battery energy storage system

[0051] 2) Grid-connected and off-grid control method The AC side of this device mainly uses a three-phase H-bridge conversion circuit as the core topology, which is extracted from the cascaded H-bridge circuit topology. The cascaded H-bridge circuit generally consists of multiple H-bridge circuits connected in series to form a certain phase in a three-phase circuit. Three sets of cascaded H-bridge circuits form a three-phase cascaded H-bridge circuit. Due to the existence of the problem of voltage and current sharing in series, the general control method is single-phase independent control, and the control strategy is relatively complex

[0052] To avoid the voltage and current sharing problems introduced by the cascaded H-bridge circuit, the device adopts a single-stage cascaded H-bridge circuit, i.e., a three-phase H-bridge conversion circuit, as Figure 1 shown. The AC part of this circuit and the Figure 2 traditional two-level energy storage device circuit shown are the same except for the difference in the source of the N line. The main difference between the two circuit topologies in control lies in the difference in the driving signal distribution of the power device IGBT, while the modulation wave required for generating the driving signal and the generation principle are the same. To simplify the control method of the three-phase H-bridge conversion circuit, the control methods of the two circuits and the driving methods of the power device IGBT are integrated.

[0053] The grid-connected control method takes the three-phase AC current as the control target to realize the charge and discharge functions of the battery pack. First, collect the voltages of the three-phase filter capacitors on the AC side, perform phase locking on this voltage to obtain the phase of the grid voltage, and perform abc / dq coordinate system transformation on the three-phase filter capacitor voltages; second, collect the three-phase AC currents on the AC side and perform abc / dq coordinate system transformation, and respectively control the target values of the d-axis - active current and q-axis - reactive current to realize active charge and discharge and reactive power control; third, perform dq / abc coordinate system inverse transformation on the active and reactive control values of the dq axis to obtain the a, b, and c three-phase power frequency sine modulation wave signals; finally, use the a, b, and c phase modulation waves as the left arm modulation wave signals, and take the inverse of the a, b, and c phase modulation waves as the right arm modulation wave driving signals, and control the power device IGBT through the modulation wave to generate the driving signal to realize the grid-connected charge and discharge control function. In addition, the grid-connected control theory is already very mature in the traditional two-level energy storage device, so it will not be elaborated here.

[0054] The off-grid control method takes the AC filter capacitor voltage as the control target to realize AC side voltage stabilization and the charge and discharge functions of the battery pack. First, generate a voltage phase by itself as the phase of the grid voltage, collect the voltages of the three-phase filter capacitors on the AC side, perform abc / dq coordinate system transformation on the three-phase filter capacitor voltages, and collect the three-phase AC currents on the AC side and perform abc / dq coordinate system transformation; second, take the components of the three-phase AC voltage on the dq axis as the control target, and use the voltage and current double closed-loop to control the d-axis - active component and q-axis - reactive component respectively for AC side voltage stabilization and charge and discharge control of the battery pack; third, perform dq / abc coordinate system inverse transformation on the active and reactive control values of the dq axis to obtain the a, b, and c three-phase power frequency sine modulation wave signals; finally, use the a, b, and c phase modulation waves as the left arm modulation wave signals, and take the inverse of the a, b, and c phase modulation waves as the right arm modulation wave driving signals, and control the power device IGBT through the modulation wave to generate the driving signal to realize off-grid voltage support and charge and discharge control functions. In addition, the off-grid control theory is already very mature in the traditional two-level energy storage device, so it will not be elaborated here.

[0055] 3) Working Principle of Three-Phase H-Bridge Conversion Circuit In the three-phase H-bridge conversion circuit, the power-frequency sine modulation waves of each phase have the same frequency and equal amplitude, and only the initial phase angles differ by 120°. The working principles of the power devices IGBTs in the three phases are the same, and only the control moments are staggered by 120° from each other. Therefore, the working principle of the H-bridge conversion circuit of phase a is taken as an example for illustration, and the same applies to phases b and c.

[0056] When the three-phase H-bridge conversion circuit is in the discharging state, the AC voltage and the AC current are in the same phase. The working state of the H-bridge conversion circuit of phase a can be divided into the following six processes, as Figure 3 shown.

[0057] When the power-frequency sine modulation wave of phase a is in the positive half-cycle, the H-bridge conversion circuit of phase a switches between the three states shown in Figure 3 (a), (b), and (c) below, where figure (b) is the intermediate transition state. Assume that the modulation degree of the power-frequency sine modulation wave of phase a at this time is M (0 < M < 1), and the switching period is , and the dead time is , then within the switching period : Figure 3 The duration of the state shown in (a) is equivalent to . In this state, the G1 and G4 tubes of the power device IGBT are turned on, and the G2 and G3 tubes are turned off. The conduction path is: positive pole of the battery → G1 → inductor L → AC source → G4 → negative pole of the battery. At this time, the battery voltage is conducted to the inductor L through the power device IGBT. Since it is in the discharging state and the battery voltage is higher than the AC source voltage, the discharging current on the inductor L gradually increases and lasts for time; Figure 3 The duration of the state shown in (b) is . In this state, the G1, G4, G2, and G3 tubes of the power device IGBT are all turned off. Due to the existence of the inductor L, the current on the inductor L cannot change suddenly and will continue to flow in the original current direction for freewheeling. Therefore, the current direction still remains flowing towards the AC power supply at this time. The freewheeling path is: negative pole of the battery → D2 freewheeling → inductor L → AC source → D3 freewheeling → positive pole of the battery. At this time, the freewheeling circuit is maintained by the energy stored in the inductor L, and the freewheeling circuit applies a battery voltage opposite to the current direction to the inductor L. Therefore, the discharging current on the inductor gradually decreases and lasts for time; Figure 3 The duration of the state shown in (c) is . In this state, the G2 and G3 tubes of the power device IGBT are turned on, and the G1 and G4 tubes are turned off. Due to The time is shorter and the inductor current does not decrease to 0. G2 and G3 are turned on, having no impact on the inductor current path. The conduction path is: negative pole of the battery → D2 freewheeling → inductor L → AC source → D3 freewheeling → positive pole of the battery. At this time, the freewheeling circuit is maintained by the energy stored in the inductor L. This circuit continues to apply the battery voltage opposite to the current direction to the inductor L, causing the discharge current on the inductor to continue to decrease and last for time.

[0058] The above is the working process of a single switching cycle when the a-phase power frequency sine modulation wave is in the positive half-cycle. According to the PWM modulation principle, by controlling the modulation index M of each switching cycle, the magnitude of the current in the inductor L can be adjusted to achieve the control of the discharge current.

[0059] When the a-phase power frequency sine modulation wave is in the negative half-cycle, the a-phase H-bridge conversion circuit switches between the three states shown in Figure 3 (d), (e), and (f) as shown, where Figure (e) is the intermediate transition state. Assume that the modulation index of the a-phase power frequency sine modulation wave is M (0 < M < 1) at this time, and the switching cycle is , and the dead time is , then within the switching cycle : Figure 3 The duration of the state shown in (d) is . In this state, G2 and G3 of the power device IGBT are turned on, and G1 and G4 are turned off. Its conduction path is: positive pole of the battery → G3 → AC source → inductor L → G2 → negative pole of the battery. At this time, the battery voltage is conducted to the AC source through the power device IGBT. Since it is in the discharge state and the battery voltage is higher than the AC source voltage, the discharge current on the inductor L gradually increases and lasts for time; Figure 3 The duration of the state shown in (e) is . In this state, G1, G4, G2, and G3 of the power device IGBT are all turned off. Due to the existence of the inductor L, the current on the inductor L cannot change suddenly and will continue to freewheel in the original current direction. Therefore, the current direction still remains flowing towards the power device IGBT at this time. Its freewheeling path is: negative pole of the battery → D4 freewheeling → AC source → inductor L → D1 freewheeling → positive pole of the battery. At this time, the freewheeling circuit is maintained by the energy stored in the inductor L, and the freewheeling circuit applies a battery voltage opposite to the current direction to the inductor L. Therefore, the discharge current on the inductor gradually decreases and lasts for time; Figure 3 The duration of the state shown in (f) is . In this state, G1 and G4 of the power device IGBT are turned on, and G2 and G3 are turned off. Since The time is short, and the inductor current does not decrease to 0. The G1 and G4 transistors are turned on, which has no impact on the inductor current path. The conduction path is: negative pole of the battery → D4 for freewheeling → AC source → inductor L → D1 for freewheeling → positive pole of the battery. At this time, the freewheeling circuit is maintained by the energy stored in the inductor L. This circuit continues to apply the battery voltage in the opposite direction of the current to the inductor L, causing the discharge current on the inductor to continue to decrease and continue for time.

[0060] The above is the working process of a single switching cycle when the a-phase power frequency sine modulation wave is in the negative half-cycle. According to the PWM modulation principle, by controlling the modulation degree M of each switching cycle, the magnitude of the current in the inductor L can be adjusted to achieve the control of the discharge current.

[0061] When the three-phase H-bridge conversion circuit is in the charging state, the phase difference between the AC current and the AC voltage is 180°. The working state of the a-phase H-bridge conversion circuit can be divided into the following six processes, as Figure 4 shown. Its working principle is similar to that in the discharge state, only there are differences in the direction of the inductor current flow and the freewheeling circuit when the power device IGBT is in the off state, so it will not be specifically described here.

[0062] (III) Control method for eliminating double-frequency fluctuations 1) Principle of double-frequency fluctuation generation Taking the a-phase H-bridge conversion circuit as an example, the mechanism of double-frequency fluctuation generation is illustrated. Assuming that the a-phase power factor is 1, the AC side voltage and current can be expressed as: , The total instantaneous active power of the a-phase H-bridge conversion circuit can be expressed as: , It can be seen from the above formula that the AC side instantaneous active power consists of a DC component and an AC component , and the AC component is a cosine waveform with a frequency of 100Hz, which is twice the power frequency of 50Hz. Without considering the energy conversion loss, the DC side power is equal to the AC side power . Therefore, the DC side power is also the superposition of a DC component and a double-frequency AC component. The DC side power can be expressed as: , Then the DC voltage of the double-frequency fluctuation can be expressed as: , Then the DC current of the double-frequency fluctuation can be expressed as: , 2) Principle of eliminating double - frequency fluctuation Through the analysis of the double - frequency power fluctuation, it can be known that the double - frequency fluctuation reflected on the DC side can be equivalent to the voltage fluctuation or current fluctuation on the DC side. The double - frequency fluctuation can be eliminated from two dimensions of DC voltage or DC current.

[0063] The control method proposed in this invention eliminates the double - frequency fluctuation in the dimension of DC voltage. If the DC component of the DC voltage is represented by the battery pack voltage , then the formula can be transformed into: , From this, it can be seen that if a double - frequency component with the same magnitude and opposite direction as the AC component can be generated and superimposed on the DC voltage in series, then the DC voltage will be equal to the battery voltage , and the double - frequency fluctuation will be eliminated.

[0064] On the premise that the AC voltage meets the requirements of the power quality standard, ignoring the unbalanced situation caused by control accuracy and sampling error of the AC voltage, each phase of the three - phase H - bridge conversion circuit has a phase difference of 120°. The expression of the double - frequency voltage fluctuation on the DC side of the three - phase H - bridge conversion circuit can be obtained as: , The above formula can be simplified to: , Adding , , together, we can get: , From this, it can be seen that if the three - phase double - frequency voltage elimination circuits are mutually coupled on the secondary side of the transformer, the double - frequency fluctuation theoretically does not consume active energy on the primary side of the transformer, mainly taking the reactive energy of voltage fluctuation. Therefore, the primary side of the transformer only needs to provide active energy within 200W of the power device's own loss (including a small part of the active energy consumption caused by AC voltage imbalance), which greatly reduces the power consumption of the double - frequency elimination circuit and minimizes the impact of this circuit on the overall efficiency of the energy storage device. Based on this, this invention proposes a series - type active harmonic elimination circuit to eliminate the double - frequency fluctuation generated by the three - phase H - bridge conversion circuit at the same time.

[0065] 3) Working principle of the series - type active harmonic elimination circuit The series - type active harmonic elimination circuit includes four parts: the primary - side inverter bridge circuit, the high - frequency isolation transformer, the secondary - side rectifier circuit, and the secondary - side harmonic elimination circuit, asFigure 5 As shown. The primary-side inverter bridge circuit is used to provide a stable AC source through inversion. Its operating state has no relation with the charge and discharge states of the three-phase H-bridge conversion circuit. However, the secondary-side rectifier circuit and the secondary-side harmonic elimination circuit have certain differences in their operating principles according to the different charge and discharge states of the three-phase H-bridge conversion circuit. Therefore, the series active harmonic elimination circuit will introduce its specific operating principle in detail from two aspects: the primary side and the secondary side.

[0066] The primary side of the series active harmonic elimination circuit is a standard single-phase bridge inverter circuit, and its operating principle is as Figure 7 shown. It drives the power device IGBT through the power frequency sine modulation wave signal generated by itself. The detailed working process within one power frequency sine modulation wave period is described as follows.

[0067] When the power frequency sine modulation wave is in the positive half cycle, the primary-side inverter bridge circuit switches among the three states shown in Figure 7 (a), (b), and (c) as shown, where Fig. (b) is the intermediate transition state. Assume that the modulation degree of the power frequency sine modulation wave is M (0 < M < 1) at this time, and the switching period is , and the dead time is . Then within the switching period : Figure 7 The duration of the state shown in (a) is . In this state, the S1 and S4 tubes of the power device IGBT are turned on, and the S2 and S3 tubes are turned off. Its conduction path is: the positive pole of the battery → S1 → the primary side of T → S4 → the negative pole of the battery. At this time, the battery voltage is conducted through the power device IGBT, and a positive voltage is applied to the primary side of the high-frequency isolation transformer T, generating a gradually increasing positive current on the primary side of the high-frequency isolation transformer T and lasting for time; Figure 7 The duration of the state shown in (b) is . In this state, the S1, S4, S2, and S3 tubes of the power device IGBT are all turned off. Due to the existence of the high-frequency isolation transformer T, the current on the high-frequency isolation transformer T cannot change suddenly and will continue to flow in the original current direction for freewheeling. Therefore, the current direction still maintains the direction flowing towards the high-frequency isolation transformer T at this time. Its freewheeling path is: the negative pole of the battery → D2 for freewheeling → the high-frequency isolation transformer T → D3 for freewheeling → the positive pole of the battery. At this time, the freewheeling circuit is maintained by the energy stored in the high-frequency isolation transformer T, and the freewheeling circuit causes a reverse voltage to be applied to the primary side of the high-frequency isolation transformer T. Therefore, the positive current on the high-frequency isolation transformer T gradually decreases and lasts for time; Figure 7 The duration of the state shown in , in this state, the S2 and S3 transistors of the power device IGBT are turned on, and the S1 and S4 transistors are turned off. Since the time is short and the current of the high-frequency isolation transformer T has not decreased to 0, the conduction of the S2 and S3 transistors has no effect on the current path of the high-frequency isolation transformer T. The conduction path is: negative pole of the battery → D2 freewheeling → high-frequency isolation transformer T → D3 freewheeling → positive pole of the battery. At this time, the freewheeling loop is still maintained by the energy stored in the high-frequency isolation transformer T. This loop continues to apply a reverse voltage to the primary side of the high-frequency isolation transformer T, causing the current to continue to decrease and lasting time.

[0068] The above is the working process of a single switching cycle when the power-frequency sinusoidal modulation wave is in the positive half-cycle. According to the PWM modulation principle, by controlling the modulation degree M of each switching cycle, an equivalent positive half-cycle sinusoidal AC voltage signal can be generated on the primary side of the high-frequency isolation transformer T.

[0069] When the power-frequency sinusoidal modulation wave is in the negative half-cycle, the primary-side inverter bridge circuit switches between the three states shown in Figure 7 (d), (e), and (f) as shown, where Figure (e) is the intermediate transition state. Assume that the modulation degree of the power-frequency sinusoidal modulation wave is M (0 < M < 1) at this time, and the switching cycle is , and the dead time is , then within the switching cycle : Figure 7 The duration of the state shown in (d) is . In this state, the S2 and S3 transistors of the power device IGBT are turned on, and the S1 and S4 transistors are turned off. Its conduction path is: positive pole of the battery → S3 → primary side of T → S2 → negative pole of the battery. At this time, the battery voltage conducts through the power device IGBT, applying a reverse voltage to the primary side of the high-frequency isolation transformer T, generating a reverse gradually increasing current on the primary side of the high-frequency isolation transformer T and lasting time; Figure 7 The duration of the state shown in (e) is In this state, all the S1, S4, S2, and S3 transistors of the power device IGBT are in the off state. Due to the presence of the high-frequency isolation transformer T, the current on the high-frequency isolation transformer T cannot change suddenly and will continue to freewheel in the original current direction. Therefore, the current direction still remains flowing towards the power device IGBT at this time. Its freewheeling path is: negative pole of the battery → D4 freewheeling → high-frequency isolation transformer T → D1 freewheeling → positive pole of the battery. At this time, the freewheeling loop is maintained by the energy stored in the high-frequency isolation transformer T, and the freewheeling loop causes a positive voltage to be applied to the primary side of the high-frequency isolation transformer T. Therefore, the reverse current on the high-frequency isolation transformer T gradually decreases and lasts time; Figure 7 The duration of the (f) state shown is , in this state, the S1 and S4 transistors of the power device IGBT are turned on, and the S2 and S3 transistors are turned off. Since the time is short, the current of the high-frequency isolation transformer T does not decrease to 0. When the S1 and S4 transistors are turned on, it has no impact on the current path of the high-frequency isolation transformer T. The conduction path is: negative pole of the battery → D4 freewheeling → high-frequency isolation transformer T → D1 freewheeling → positive pole of the battery. At this time, the freewheeling circuit is still maintained by the energy stored in the high-frequency isolation transformer T. This circuit continues to apply a positive voltage to the primary side of the high-frequency isolation transformer T, causing the current to continue to decrease and last for time.

[0070] The above is the working process of a single switching period when the industrial-frequency sinusoidal modulation wave is in the negative half-cycle. According to the PWM modulation principle, by controlling the modulation degree M of each switching period, an equivalent negative half-cycle sinusoidal AC voltage signal can be generated on the primary side of the high-frequency isolation transformer T.

[0071] By repeating the control of each industrial-frequency sinusoidal modulation wave period, an industrial-frequency sinusoidal AC voltage can be generated on the primary side of the high-frequency isolation transformer T and induced to the secondary side of the high-frequency isolation transformer T to provide a stable AC source for the secondary circuit.

[0072] The secondary side of the series active harmonic elimination circuit is coupled with 3 AC outputs. Each AC output passes through a rectifier circuit and then an inverter circuit, and the required double-frequency voltage is output at the port and connected in series between the positive pole of the battery pack voltage and the positive pole of the DC bus. This path contains two types of energy flows: one is to obtain the AC double-frequency fluctuation voltage signal by taking the difference between the DC bus voltage and the battery voltage, and after performing phase-locked control on this signal, obtaining the amplitude and phase angle of the double-frequency fluctuation voltage, and then taking its inverse as the modulation wave signal of the secondary harmonic elimination circuit to generate a double-frequency AC energy flow with a double-frequency fluctuation equal in magnitude and 180° out of phase with the target double frequency; the other is a DC energy flow that is connected in series between the positive pole of the battery pack voltage and the positive pole of the DC bus and transfers charging or discharging energy between them during the charging and discharging process of the battery pack. Since the topologies and functions of the 3 AC paths are basically the same, one of them is taken as an example to elaborate on the working principles of the two energy flows in detail from four working conditions: discharge state - positive half-cycle of the modulation wave, discharge state - negative half-cycle of the modulation wave, charging state - positive half-cycle of the modulation wave, and charging state - negative half-cycle of the modulation wave, as Figures 8 - 11 shown, and its detailed working process is described as follows.

[0073] When working in the discharge state - positive half-cycle of the modulation wave, the double-frequency AC energy flow of the secondary elimination circuit is at Figure 8Switching occurs among the three states (a1), (b1), and (c1) shown, where figure (b1) is the intermediate transition state. Assume that the modulation degree of the double-frequency sinusoidal modulation wave is M (0 < M < 1) at this time, and the switching period is , and the dead time is . Then within the switching period : Figure 8 The duration of the state (a1) shown is . In this state, the power devices IGBT's S5, S8, S10, and S11 are turned on, and S6, S7, S9, and S12 are turned off. The conduction path is: the positive pole of the DC bus → L1 and L2 → S10 → the negative pole of the filter capacitor C2 → S8 → the secondary side of T → S5 → the positive pole of the filter capacitor C2 → S11 → the positive pole of the battery. At this time, the rectifier circuit part conducts through the power device IGBT, applying the induced voltage on the secondary side of the high-frequency isolation transformer T to both ends of the filter capacitor C2. The filter capacitor C2 is charged positively, and the voltage of the filter capacitor C2 increases, providing voltage support for the inverter circuit part; the inverter circuit part conducts through the power device IGBT, applying the positive voltage transmitted by the rectifier circuit to points a and b of the bridge arm, where point a is at a negative voltage and point b is at a positive voltage. At the same time, a current flowing out of the bridge arm at point b and flowing into the bridge arm at point a and gradually increasing is generated. The whole process lasts time; Figure 8 The duration of the state (b1) shown is . In this state, all the power devices IGBT's S5, S6, S7, S8, S9, S10, S11, and S12 are in the off state. Due to the existence of the high-frequency isolation transformer T in the rectifier circuit part and the existence of the inductor in the LCL filter in the inverter circuit part, the current on the high-frequency isolation transformer T and the inductor of the LCL filter cannot change suddenly and will continue to flow in the original current direction for freewheeling. Therefore, at this time, the current direction of the rectifier circuit part still remains from the high-frequency isolation transformer T to the bridge arm, and the current direction of the inverter circuit part still remains from the LCL filter to the bridge arm. The freewheeling path of its rectifier circuit part is: the positive pole of the high-frequency isolation transformer T → D5 freewheeling → the filter capacitor C2 → D8 freewheeling → the negative pole of the high-frequency isolation transformer T. At this time, the freewheeling circuit is maintained by the high-frequency isolation transformer T, and the freewheeling circuit makes the positive voltage continue to be applied to the filter capacitor C2. Therefore, the filter capacitor C2 continues to maintain the charging state and lasts time; the freewheeling path of its inverter circuit part is: the positive pole of the DC bus → L1 and L2 → D9 freewheeling → the filter capacitor C2 → D12 freewheeling → the positive pole of the battery. The freewheeling circuit makes the reverse voltage applied to points a and b. Therefore, the current of the LCL filter gradually decreases. The whole process lasts time; Figure 8The duration of the (c1) state shown is , in this state, the S6, S7, S9, and S12 transistors of the power device IGBT are conducting, and the S5, S8, S10, and S11 transistors are turned off. Its conduction path is: positive pole of the DC bus → L1 and L2 → D9 freewheeling → positive pole of the filter capacitor C2 → S7 → secondary side of T → S6 → negative pole of the filter capacitor C2 → D12 freewheeling → positive pole of the battery. At this time, the rectifier circuit part conducts through the power device IGBT, and the induced voltage on the secondary side of the high-frequency isolation transformer T is reversely applied across the filter capacitor C2. The filter capacitor C2 discharges reversely, and the voltage of the filter capacitor C2 decreases, but the voltage of the filter capacitor C2 does not reverse; the inverter circuit part conducts through the power device IGBT and continues to apply a reverse voltage at points a and b, so the current of the LCL filter continues to decrease. This entire process lasts time.

[0074] The above is the working process of a single switching period when working in the discharge state - positive half cycle of the modulation wave. According to the PWM modulation principle, by controlling the modulation degree M of each switching period, an equivalent positive half cycle double-frequency switching voltage signal can be generated at the output end, and the double-frequency switching voltage signal is converted into a double-frequency sinusoidal voltage signal through the LCL filter circuit, realizing the double-frequency AC energy flow of the secondary side elimination circuit.

[0075] When working in the discharge state - positive half cycle of the modulation wave, the DC energy flow of the battery pack discharge switches between the Figure 8 shown (a2), (b2), and (c2) three states, where Figure (b2) is the intermediate transition state. Assume that the modulation degree of the double-frequency sinusoidal modulation wave is M (0 < M < 1) at this time, and the switching period is , and the dead time is , then within the switching period : Figure 8 The duration of the (a2) state shown is , in this state, the S5, S8, S10, and S11 transistors of the power device IGBT are conducting, and the S6, S7, S9, and S12 transistors are turned off. The conduction path of the DC energy flow of the battery pack discharge is opposite to that of the double-frequency energy flow: positive pole of the battery → D11 → positive pole of the filter capacitor C2 → S5 → secondary side of T → S8 → negative pole of the filter capacitor C2 → D10 → L1 and L2 → positive pole of the DC bus.

[0076] Figure 8 The duration of the (b2) state shown is , in this state, all the S5, S6, S7, S8, S9, S10, S11, and S12 transistors of the power device IGBT are in the off state. At this time, there is no direct DC loop. Since the duration of this state It is very short and transfers less energy. Therefore, the energy stored in the filter capacitor C2 is used to convert the energy required for discharging, so as to realize the DC energy flow of the battery pack during discharging. Its conduction path is: the positive pole of the battery → D11 → filter capacitor C2 → D10 → L1 and L2 → the positive pole of the DC bus.

[0077] Figure 8 The duration of the (c2) state shown is , in this state, the S6, S7, S9, and S12 tubes of the power device IGBT are conducting, and the S5, S8, S10, and S11 tubes are turned off. The conduction path of the DC energy flow during the discharge of the battery pack is opposite to that of the double-frequency energy flow: the positive pole of the battery → S12 → the negative pole of the filter capacitor C2 → D6 → the secondary side of T → D7 → the positive pole of the filter capacitor C2 → S9 → L1 and L2 → the positive pole of the DC bus.

[0078] The above is the conduction path of the DC energy flow of the battery pack during a single switching period when working in the discharge state - the positive half-cycle of the modulation wave. By periodically repeating this path, the continuous transfer of the DC energy flow of the battery pack during the positive half-cycle of the modulation wave is realized.

[0079] When working in the discharge state - the negative half-cycle of the modulation wave, the double-frequency AC energy flow of the secondary side elimination circuit switches between the three states of Figure 9 shown in (a1), (b1), and (c1). Among them, Figure (b1) is the intermediate transition state, and the working process of its single switching period is the same as that of the positive half-cycle principle and will not be elaborated. According to the PWM modulation principle, by controlling the modulation degree M of each switching period, an equivalent negative half-cycle double-frequency switching voltage signal can be generated at the output end, and the double-frequency switching voltage signal is converted into a double-frequency sine voltage signal through the LCL filter circuit to realize the double-frequency AC energy flow of the secondary side elimination circuit.

[0080] When working in the discharge state - the negative half-cycle of the modulation wave, the DC energy flow of the battery pack switches between the three states of Figure 9 shown in (a2), (b2), and (c2). Among them, Figure (b2) is the intermediate transition state. The working process of its single switching period is the same as that of the positive half-cycle principle and will not be elaborated. By periodically repeating the conduction path of the DC energy flow of the battery pack, the continuous transfer of the DC energy flow of the battery pack during the negative half-cycle of the modulation wave is realized.

[0081] When working in the charging state - the positive half-cycle of the modulation wave, the double-frequency AC energy flow of the secondary side elimination circuit is in Figure 10The three states shown as (a1), (b1), and (c1) switch between each other, where Figure (b1) is an intermediate transition state. According to the PWM modulation principle, by controlling the modulation degree M of each switching cycle, an equivalent positive half-cycle double-frequency switching voltage signal can be generated at the output, and the double-frequency switching voltage signal is converted into a double-frequency sinusoidal voltage signal through the LCL filter circuit, realizing the double-frequency AC energy flow of the secondary side elimination circuit.

[0082] When working in the charging state - the positive half-cycle of the modulation wave, the DC energy flow of the battery pack discharging switches between the three states shown as (a2), (b2), and (c2), where Figure (b2) is an intermediate transition state. By periodically repeating the conduction path of the DC energy flow of the battery pack charging, the continuous transfer of the DC energy flow of the battery pack charging in the positive half-cycle of the modulation wave is realized. Figure 10 The three states shown as (a2), (b2), and (c2) switch between each other, where Figure (b2) is an intermediate transition state. By periodically repeating the conduction path of the DC energy flow of the battery pack charging, the continuous transfer of the DC energy flow of the battery pack charging in the positive half-cycle of the modulation wave is realized.

[0083] When working in the charging state - the negative half-cycle of the modulation wave, the double-frequency AC energy flow of the secondary side elimination circuit switches between the three states shown as (a1), (b1), and (c1), where Figure (b1) is an intermediate transition state. According to the PWM modulation principle, by controlling the modulation degree M of each switching cycle, an equivalent negative half-cycle double-frequency switching voltage signal can be generated at the output, and the double-frequency switching voltage signal is converted into a double-frequency sinusoidal voltage signal through the LCL filter circuit, realizing the double-frequency AC energy flow of the secondary side elimination circuit. Figure 11 The three states shown as (a1), (b1), and (c1) switch between each other, where Figure (b1) is an intermediate transition state. According to the PWM modulation principle, by controlling the modulation degree M of each switching cycle, an equivalent negative half-cycle double-frequency switching voltage signal can be generated at the output, and the double-frequency switching voltage signal is converted into a double-frequency sinusoidal voltage signal through the LCL filter circuit, realizing the double-frequency AC energy flow of the secondary side elimination circuit.

[0084] When working in the charging state - the negative half-cycle of the modulation wave, the DC energy flow of the battery pack discharging switches between the three states shown as (a2), (b2), and (c2), where Figure (b2) is an intermediate transition state. By periodically repeating the conduction path of the DC energy flow of the battery pack charging, the continuous transfer of the DC energy flow of the battery pack charging in the negative half-cycle of the modulation wave is realized. Figure 11 The three states shown as (a2), (b2), and (c2) switch between each other, where Figure (b2) is an intermediate transition state. By periodically repeating the conduction path of the DC energy flow of the battery pack charging, the continuous transfer of the DC energy flow of the battery pack charging in the negative half-cycle of the modulation wave is realized.

[0085] Through Figures 8 - 11 the periodic conversion between the working states shown, the transfer of two energy flows, namely the double-frequency AC energy flow of the secondary side elimination circuit and the DC energy flow of the battery pack charging or discharging, is finally realized.

[0086] Under the idea given in the present invention, the technical means in the above embodiments are transformed, replaced, and modified in a way that is easy to think of for those skilled in the art, and the functions achieved are basically the same as the corresponding technical means in the present invention, and the achieved invention purposes are also basically the same. The technical solutions formed in this way are fine-tuned from the above embodiments, and such technical solutions still fall within the protection scope of the present invention.

Claims

1. An energy storage device applicable to a flow battery, characterized in that, The device includes a three-phase H-bridge conversion circuit, an AC-side LCL filter circuit, an AC pre-charge circuit, an AC EMI filter, an AC fuse, three series active harmonic elimination circuits, three DC EMI filters, and three DC pre-charge circuits; The three-phase H-bridge conversion circuit consists of three single-phase H-bridge circuits. The switching devices are all IGBTs. There are 4 ports, namely U’, V’, W’, and N’ on the AC side of the three-phase H-bridge conversion circuit. The U’, V’, and W’ ports are the positive poles of the AC ends of the three single-phase H-bridge circuits respectively, and the N’ port is connected to the negative poles of the AC ends of the three single-phase H-bridge circuits; The 4 ports, namely U’, V’, W’, and N’ on the AC side of the three-phase H-bridge conversion circuit are sequentially connected to the AC-side LCL filter circuit, the AC pre-charge circuit, the AC EMI filter, and the AC fuse, and then connected to the power grid U, V, W, and N; There are 3 pairs of ports on the DC side of the three-phase H-bridge conversion circuit, which are respectively composed of the DC ends of the three single-phase H-bridge circuits. A capacitor is also connected in parallel at the DC end of the single-phase H-bridge circuit; The positive poles of the DC sides of the three single-phase H-bridge circuits are all connected to the positive pole on the right side of the DC EMI filter after being serially connected with a series active harmonic elimination circuit. The negative pole of each single-phase H-bridge circuit DC end is directly connected to the negative pole on the right side of the DC EMI filter; the positive and negative ports on the left side of the three DC EMI filters are respectively connected to the positive and negative poles on the right side of the DC pre-charge circuit after being serially connected with DC fuses. The positive and negative poles on the left side of the three DC pre-charge circuits are respectively connected to the positive and negative poles of the battery pack; The series active harmonic elimination circuit includes: a secondary rectification circuit, a DC filter capacitor C2, a bidirectional H-bridge circuit, and a DC-side LCL filter circuit. The AC end of the secondary rectification circuit is connected to one secondary output of the high-frequency isolation transformer T. After the DC end is connected in parallel with the DC filter capacitor C2, it is connected to the DC end of the bidirectional H-bridge circuit. The AC end of the bidirectional H-bridge circuit is connected to the left two ports of the DC-side LCL filter circuit. The two ports on the right side of the DC-side LCL filter circuit are respectively connected to the positive pole of the single-phase H-bridge circuit DC end and the positive pole on the right side of the DC EMI filter. The high-frequency isolation transformer T has 1 input end and 3 output ends. The input end is connected to the AC end of the inverter bridge circuit. The DC end of the inverter bridge circuit is connected in parallel with a DC bus capacitor and then connected to a DC power supply. The 3 output ends are respectively connected to the secondary rectification circuits of the three series active harmonic elimination circuits; The power of the primary inverter bridge circuit of the high-frequency isolation transformer T is less than 10 times the power of the secondary circuit.

2. The energy storage device according to claim 1, wherein, The AC-side LCL filter circuit consists of two inductors and a filter capacitor. The two inductors are connected in series, one end is connected to the three-phase H-bridge conversion circuit, and the other end is connected to the AC pre-charge circuit. One end of the filter capacitor is connected to the middle of the two series inductors, and the other end is connected to the N line.

3. The energy storage device according to claim 1, characterized in that, The AC pre-charge circuit consists of a main circuit breaker KM1 and a circuit breaker KM2 connected in parallel at both ends of the main circuit breaker on the three-phase main circuit.

4. The energy storage device according to claim 1, characterized in that, The DC-side pre-charge circuit consists of a main circuit breaker KM3 and a circuit breaker KM4 connected in parallel at both ends of the main circuit breaker KM3.

5. A control method for an energy storage device applicable to a flow battery, characterized in that, For realizing the operation control of the energy storage device described in any one of claims 1-4, the control method includes the following steps: S1: Control the startup of the inverter bridge circuit at the input end of the high-frequency isolation transformer T, control the startup of the secondary rectifier circuit, control the bidirectional H-bridge circuit on the secondary side, and connect the battery pack to the DC end of the single-phase H-bridge circuit; S2: Set the working mode of the three-phase H-bridge conversion circuit to grid-connected operation or off-grid operation; S3: After the setting is completed, start the pre-charge control of the parallel capacitor at the DC end of the single-phase H-bridge circuit, which is divided into AC pre-charge control during grid-connected operation and DC pre-charge control during off-grid operation; S4: After the pre-charge of the parallel capacitor at the DC end of the single-phase H-bridge circuit is completed, control the IGBT of the three-phase H-bridge conversion circuit to work and enter the grid-connected operation or off-grid operation mode; S5: During grid-connected operation, set the three-phase H-bridge conversion circuit to control the charging or discharging of the battery pack; during off-grid operation, control the charging or discharging according to the load and the power of the external power supply; S6: Control the series active harmonic elimination circuit to eliminate the DC-side second-harmonic fluctuation generated during the charging or discharging process.

6. The control method according to claim 5, characterized in that The control of the series active harmonic elimination circuit to eliminate the DC-side second-harmonic fluctuation generated during the charging or discharging process includes: S601: Connect a DC power supply to the DC side of the inverter bridge circuit on the primary side of the high-frequency isolation transformer T to provide energy for the entire series active filter elimination circuit. Use the power-frequency sinusoidal AC voltage as the modulation wave signal, and through the PWM modulation of turning on and off the 4 IGBTs on the 2 bridge arms of the inverter bridge circuit, make the primary side of the high-frequency isolation transformer T generate a stable power-frequency sinusoidal AC voltage; S602: The 3 output terminals of the high-frequency isolation transformer T sense and generate 3 identical power-frequency sinusoidal AC voltages, which respectively provide stable AC voltages for the 3 secondary rectifier circuits; S603: The secondary rectifier circuit generates a DC voltage at the DC port through the PWM modulation of turning on and off the 4 IGBTs on its 2 bridge arms, and filters out the high-frequency ripple through the filter capacitor to provide a DC voltage for the bidirectional H-bridge circuit on the secondary side; S604: The bidirectional H-bridge circuit on the secondary side obtains the AC second-harmonic fluctuation voltage signal by subtracting the DC bus voltage and the battery voltage. After performing phase-locked control on this signal, obtain the amplitude and phase angle of the second-harmonic fluctuation voltage, take its inverse as the modulation wave signal of the bidirectional H-bridge circuit on the secondary side, and use this to control the PWM modulation of turning on and off the 4 IGBTs on its 2 bridge arms to generate an AC voltage signal with the same magnitude as the target second-harmonic and a phase difference of 180°, so as to cancel the second-harmonic fluctuation in the circuit.

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