An energy storage device suitable for flow batteries and its control method

Abstract

This invention relates to an energy storage device and its control method suitable for flow batteries. The device consists of a three-phase H-bridge converter circuit and a series-type active filter elimination circuit. The three-phase H-bridge converter circuit connects the battery pack to the power grid to achieve energy exchange. The AC side is three-phase four-wire, and the DC side can independently connect to three battery groups. The series-type active filter elimination circuit, connected in series with the battery pack, provides DC voltage to the three-phase H-bridge converter circuit and simultaneously generates a second-harmonic voltage to eliminate second-harmonic fluctuations. Compared with existing similar energy storage devices, this invention enables one-cell-per-battery management without the need for an external DC-DC converter. The circuit bridge arm output voltage is converted from line voltage to phase voltage, increasing the AC side access voltage level by a factor of two, with a conversion efficiency of up to 99%. This solves the two major problems of low AC side access voltage and low conversion efficiency in flow battery energy storage systems, reducing the design cost of flow battery energy storage systems.

Description

Technical Field

[0001] This invention relates to the field of power electronics technology, and in particular to an energy storage device and its control method suitable for flow batteries. Background Technology

[0002] The proportion of new energy sources in the energy structure is gradually increasing. As a "modifier" of new energy, energy storage technology is increasingly emphasizing safety and long-term storage capacity. Flow battery energy storage, due to its separate positive and negative electrodes, is safer than lithium battery energy storage. Furthermore, the independent nature of its electrolyte and stack means that the storage time is only related to the size of the electrolyte storage tank. Currently, the common storage time is generally 4-8 hours, more than twice that of lithium battery energy storage. It has good development prospects.

[0003] Compared to lithium batteries, flow batteries have a port voltage of approximately half that of lithium batteries, and at the same power level, the charge / discharge current of flow batteries is twice that of lithium batteries. Existing energy storage devices are designed to be adapted to the voltage levels and overcurrent capabilities of lithium batteries. Directly applying these devices to flow battery energy storage systems presents three main problems:

[0004] Firstly, there are two technical routes for flow batteries: multi-series high-voltage type and conventional series low-voltage type. The multi-series high-voltage type has the same DC port voltage range as lithium batteries and shares an integrated AC booster chamber (including energy storage device) with lithium battery energy storage. Therefore, its grid-connected cost is comparable to that of lithium batteries, while the battery cost is higher. Dalian Rongke is the main manufacturer of the multi-series high-voltage type. Currently, the multi-series high-voltage type is in the demonstration project stage, and large-scale engineering applications still face significant technical challenges, such as high-voltage insulation, current sharing, and leakage. Its technical difficulty and design cost are higher than the conventional series low-voltage type. Currently, most flow battery manufacturers use the conventional series low-voltage type, searching the market for matching integrated AC booster chambers (including energy storage devices).

[0005] Secondly, in conventional series low-voltage flow battery applications, the AC side grid connection voltage is limited by the lower limit of the DC voltage of the flow battery, making it impossible to achieve a high-voltage grid connection of AC690V. It can only achieve a grid connection voltage level of AC315V or even lower, resulting in a charging and discharging current of up to 1000A or even 2500A or more. Commercial energy storage devices need twice the power capacity or even more to be used, which increases the design cost of energy storage devices by more than 2 times. Moreover, the cost of customized small-batch step-up transformers is much higher than the cost of AC690V large-scale step-up transformers (common for lithium battery energy storage).

[0006] Third, in conventional series-connected low-voltage flow battery applications, to reduce the cost of energy storage devices and step-up transformers, the flow battery port voltage needs to be increased by more than two times. This requires the use of high-power, high-voltage DC-DC converters to boost the flow battery port voltage. This additionally increases the design cost of the DC-DC converter, and the two-stage conversion structure also reduces energy conversion efficiency. Consequently, flow battery energy storage has a much higher design cost than lithium battery energy storage, and a much lower energy conversion efficiency, which is detrimental to the development of flow battery energy storage. Summary of the Invention

[0007] The purpose of this invention is to overcome the shortcomings of the prior art and propose an energy storage device and its control method suitable for flow batteries in conventional series low-voltage flow battery applications. The device includes: a three-phase H-bridge converter circuit, an AC-side LCL filter circuit, an AC pre-charge circuit, an AC EMI filter, an AC fuse, three active harmonic cancellation circuits, three DC EMI filters, and three DC pre-charge circuits.

[0008] The three-phase H-bridge converter circuit consists of three single-phase H-bridge circuits, and the switching devices are all IGBTs. The AC side of the three-phase H-bridge converter circuit has four ports: U', V', W', and N'. Ports U', V', and W' are the positive terminals of the AC terminals of the three single-phase H-bridge circuits, and port N' is connected to the negative terminal of the AC terminals of the three single-phase H-bridge circuits.

[0009] The four AC side ports U', V', W', and N' of the three-phase H-bridge converter circuit are sequentially connected to the AC side LCL filter circuit, the AC pre-charge circuit, the AC EMI filter, and the AC fuse before being connected to the power grid via U, V, W, and N.

[0010] The three-phase H-bridge converter circuit has three pairs of ports on its DC side, each consisting of the DC terminals of three single-phase H-bridge circuits. A capacitor is also connected in parallel to the DC terminal of each single-phase H-bridge circuit.

[0011] The positive DC terminals of the three single-phase H-bridge circuits are each connected in series with a series-type active harmonic cancellation circuit and then connected to the positive terminal on the right side of the DC EMI filter. The negative DC terminal of each single-phase H-bridge circuit is directly connected to the negative terminal on the right side of the DC EMI filter. The positive and negative terminals on the left side of the three DC EMI filters are connected in series with DC fuses and then connected to the positive and negative terminals on the right side of the DC pre-charge circuit. The positive and negative terminals on the left side of the three DC pre-charge circuits are respectively connected to the positive and negative terminals of the battery pack.

[0012] The series-type active harmonic cancellation circuit comprises: a secondary-side 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-side rectifier circuit is connected to one secondary-side output of the high-frequency isolation transformer T. The DC terminal is connected to the DC filter capacitor C2 in parallel and then to the DC terminal of the bidirectional H-bridge circuit. The AC terminal of the bidirectional H-bridge circuit is connected to the two left ports of the DC-side LCL filter circuit. The two right ports of the DC-side LCL filter circuit are respectively connected to the positive terminal of the DC terminal of the single-phase H-bridge circuit and the positive terminal of the right side of the DC EMI filter. The high-frequency isolation transformer T has one input terminal and three output terminals. The input terminal is the AC terminal of the inverter bridge circuit. The DC terminal of the inverter bridge circuit is connected to a DC power supply after being connected to a DC bus capacitor in parallel. The three output terminals are respectively connected to the secondary-side rectifier circuits of three series-type active harmonic cancellation circuits.

[0013] 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.

[0014] Furthermore, the AC-side LCL filter circuit consists of two inductors and one filter capacitor. The two inductors are connected in series, with one end connected to the three-phase H-bridge converter circuit and the other end connected to the AC pre-charge circuit. One end of the filter capacitor is connected to the middle of the two series-connected inductors, and the other end is connected to the neutral line.

[0015] Furthermore, the AC pre-charging circuit consists of a main circuit breaker KM1 and a circuit breaker KM2 connected in parallel across the three-phase main circuit.

[0016] Furthermore, the DC-side pre-charging circuit consists of a main circuit breaker KM3 and circuit breakers KM4 connected in parallel across both ends of the main circuit breaker KM3.

[0017] The present invention also provides a control method for an energy storage device suitable for flow batteries, for realizing the operation control of the above-mentioned energy storage device, the control method comprising the following steps:

[0018] S1: Controls the start of the inverter bridge circuit at the input terminal of the high-frequency isolation transformer T, controls the start of the secondary rectifier circuit, controls the bidirectional H-bridge circuit on the secondary side, and connects the battery pack to the DC terminal of the single-phase H-bridge circuit.

[0019] S2: Set the operating mode of the three-phase H-bridge converter circuit to grid-connected or off-grid operation.

[0020] S3: After setting, start the pre-charge control of the parallel capacitor on the DC end of the single-phase H-bridge circuit. It is divided into AC pre-charge control when running in grid-connected mode and DC pre-charge control when running off-grid mode.

[0021] S4: After the pre-charging of the parallel capacitor at the DC end of the single-phase H-bridge circuit is completed, the IGBT of the three-phase H-bridge converter circuit is controlled to operate, and enter the grid-connected or off-grid operation mode.

[0022] S5: When operating in grid-connected mode, a three-phase H-bridge converter circuit is set to control the charging or discharging of the battery pack; when operating off-grid mode, the charging or discharging is controlled according to the load and external power supply.

[0023] S6: Controls the series active harmonic cancellation circuit to eliminate DC-side second harmonic fluctuations generated during charging or discharging.

[0024] The controlled series active harmonic cancellation circuit eliminates DC-side second harmonic fluctuations generated during charging or discharging, including:

[0025] 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 filter elimination circuit. The power frequency sinusoidal AC voltage is used as the modulation wave signal. By performing PWM modulation on and off of the four IGBTs of the two 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.

[0026] S602: The three output ports of the high-frequency isolation transformer T induce three identical power frequency sinusoidal AC voltages, which provide stable AC voltages for the three secondary rectifier circuits respectively.

[0027] S603: The secondary rectifier circuit generates DC voltage at the DC port by PWM modulation of the four IGBTs in its two bridge arms to turn on and off. The high-frequency ripple is filtered out by the filter capacitor, providing DC voltage for the bidirectional H-bridge circuit on the secondary side.

[0028] S604: The secondary-side bidirectional H-bridge circuit obtains an AC second-harmonic fluctuation voltage signal by subtracting the DC bus voltage and battery voltage. After phase-locked loop control of this signal, the amplitude and phase angle of the second-harmonic fluctuation voltage are obtained, inverted, and used as the modulation wave signal of the secondary-side bidirectional H-bridge circuit. This signal is used to control the PWM modulation of the four IGBTs in the two bridge arms to generate an AC voltage signal with the same magnitude and 180° phase difference from the target second harmonic, thus canceling the second-harmonic fluctuation in the circuit.

[0029] Compared with existing energy storage devices, the output voltage of the bridge arm of the three-phase H-bridge converter circuit is converted from line voltage to phase voltage, thereby improving the AC side access voltage level. This device achieves twice the power output and a conversion efficiency of up to 99%, solving two major problems in flow battery energy storage systems: low AC-side input voltage and low conversion efficiency. This reduces the design cost of flow battery energy storage systems. The main benefits are: First, it increases the power density of the energy storage device and reduces its design cost. The high voltage output capability on the AC side allows it to output more than twice the power under the same AC current, increasing its power density and enabling cost-effective design. Second, it reduces the design cost of the flow battery energy storage system and improves the system's energy conversion efficiency. Without adding an additional DC-DC converter, the device can connect to a large-scale AC 690V step-up transformer (common for lithium-ion battery energy storage) on the AC side, achieving a conversion efficiency of up to 99%, thus reducing the cost of the flow battery energy storage system while improving energy conversion efficiency. Finally, cluster management improves battery lifespan. The device has three independent battery pack interfaces on the DC side. This design reduces the number of battery stacks connected in series by one-third, weakening the "weakest link" effect during charging and discharging, while also reducing the insulation protection level of the flow battery and improving its lifespan. Attached Figure Description

[0030] Figure 1 This is a schematic diagram of the topology of an energy storage device suitable for a flow battery according to an embodiment of the present invention;

[0031] Figure 2 This is a schematic diagram of the circuit topology of a conventional two-level energy storage device according to an embodiment of the present invention;

[0032] Figure 3 This is a schematic diagram illustrating the working principle of the three-phase H-bridge converter circuit in the discharge state according to an embodiment of the present invention;

[0033] Figure 4 This is a schematic diagram illustrating the charging state working principle of the three-phase H-bridge converter circuit according to an embodiment of the present invention;

[0034] Figure 5 This is a schematic diagram of the series-type active harmonic cancellation circuit topology according to an embodiment of the present invention;

[0035] Figure 6 This is a schematic diagram of the secondary-side rectification and elimination circuit according to an embodiment of the present invention;

[0036] Figure 7 This is a schematic diagram of the primary-side working principle of the harmonic elimination circuit according to an embodiment of the present invention;

[0037] Figure 8 This is a schematic diagram illustrating the working principle of the secondary discharge positive half-cycle of the harmonic elimination circuit according to an embodiment of the present invention.

[0038] Figure 9This is a schematic diagram illustrating the working principle of the secondary side discharge negative half-cycle of the harmonic elimination circuit according to an embodiment of the present invention.

[0039] Figure 10 This is a schematic diagram of the working principle of the secondary side charging positive half-cycle of the harmonic elimination circuit according to an embodiment of the present invention;

[0040] Figure 11 This is a schematic diagram illustrating the working principle of the secondary side charging negative half-cycle of the harmonic elimination circuit in an embodiment of the present invention. Detailed Implementation

[0041] The following sections will provide detailed explanations of the specific functions of four aspects: energy storage devices, grid-connected and off-grid control methods for AC high-voltage access, control methods for eliminating second harmonic fluctuations, and charge / discharge energy flow control methods.

[0042] (a) Energy storage devices

[0043] The energy storage device includes a three-phase H-bridge converter circuit and a series active filter elimination circuit, such as... Figure 1 As shown.

[0044] The three-phase H-bridge converter circuit consists of three single-phase H-bridge circuits, and the switching devices are all IGBTs. The AC side of the three-phase H-bridge converter circuit has four ports: U', V', W', and N'. Ports U', V', and W' are the positive terminals of the AC terminals of the three single-phase H-bridge circuits, and port N' is connected to the negative terminal of the AC terminals of the three single-phase H-bridge circuits.

[0045] The four AC side ports U', V', W', and N' of the three-phase H-bridge converter circuit are sequentially connected to the AC side LCL filter circuit, the AC pre-charge circuit, the AC EMI filter, and the AC fuse before being connected to the power grid via U, V, W, and N.

[0046] The three-phase H-bridge converter circuit has three pairs of ports on its DC side, each consisting of the DC terminals of three single-phase H-bridge circuits. A capacitor is also connected in parallel to the DC terminal of each single-phase H-bridge circuit.

[0047] The positive DC terminals of the three single-phase H-bridge circuits are each connected in series with a series-type active harmonic cancellation circuit and then connected to the positive terminal on the right side of the DC EMI filter. The negative DC terminal of each single-phase H-bridge circuit is directly connected to the negative terminal on the right side of the DC EMI filter. The positive and negative terminals on the left side of the three DC EMI filters are connected in series with DC fuses and then connected to the positive and negative terminals on the right side of the DC pre-charge circuit. The positive and negative terminals on the left side of the three DC pre-charge circuits are respectively connected to the positive and negative terminals of the battery pack.

[0048] The series-type active harmonic cancellation circuit comprises: a secondary-side 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-side rectifier circuit is connected to one secondary-side output of the high-frequency isolation transformer T. The DC terminal is connected to the DC filter capacitor C2 in parallel and then to the DC terminal of the bidirectional H-bridge circuit. The AC terminal of the bidirectional H-bridge circuit is connected to the two left ports of the DC-side LCL filter circuit. The two right ports of the DC-side LCL filter circuit are respectively connected to the positive terminal of the DC terminal of the single-phase H-bridge circuit and the positive terminal of the right side of the DC EMI filter. The high-frequency isolation transformer T has one input terminal and three output terminals. The input terminal is the AC terminal of the inverter bridge circuit. The DC terminal of the inverter bridge circuit is connected to a DC power supply after being connected to a DC bus capacitor in parallel. The three output terminals are respectively connected to the secondary-side rectifier circuits of three series-type active harmonic cancellation circuits.

[0049] 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.

[0050] The AC-side LCL filter circuit consists of two inductors and one filter capacitor. The two inductors are connected in series, with one end connected to the three-phase H-bridge converter circuit and the other end connected to the AC pre-charge circuit. One end of the filter capacitor is connected to the middle of the two series-connected inductors, and the other end is connected to the neutral (N) line.

[0051] The AC pre-charging circuit consists of a main circuit breaker KM1 and a circuit breaker KM2 connected in parallel across the three-phase main circuit.

[0052] The DC-side pre-charge circuit consists of the main circuit breaker KM3 and the circuit breaker KM4 connected in parallel across both ends of the main circuit breaker KM3.

[0053] The three-phase H-bridge converter circuit connects the battery pack to the power grid to enable charging and discharging of the battery pack. The AC side has a three-phase four-wire connection, while the DC side has three independent battery pack interfaces for battery charging and discharging management. A series-type active filter circuit, connected in series with the battery pack, provides DC voltage to the three-phase H-bridge converter circuit and simultaneously generates a second-harmonic voltage to eliminate second-harmonic fluctuation interference. Since the second-harmonic fluctuation after three-phase superposition is mainly reactive, the primary-side inverter bridge circuit in the series-type active filter circuit only needs to provide the reactive power that compensates for the power device's own losses. Therefore, the power of its primary-side inverter bridge circuit is less than 200W, far less than the 2kW to 5kW power of the secondary side.

[0054] The operation control method for the above-mentioned energy storage device includes the following steps:

[0055] S1: Controls the start of the inverter bridge circuit at the input terminal of the high-frequency isolation transformer T, controls the start of the secondary rectifier circuit, controls the bidirectional H-bridge circuit on the secondary side, and connects the battery pack to the DC terminal of the single-phase H-bridge circuit.

[0056] S2: Set the operating mode of the three-phase H-bridge converter circuit to grid-connected or off-grid operation.

[0057] S3: After setting, start the pre-charge control of the parallel capacitor on the DC end of the single-phase H-bridge circuit. It is divided into AC pre-charge control when running in grid-connected mode and DC pre-charge control when running off-grid mode.

[0058] S4: After the pre-charging of the parallel capacitor at the DC end of the single-phase H-bridge circuit is completed, the IGBT of the three-phase H-bridge converter circuit is controlled to operate, and enter the grid-connected or off-grid operation mode.

[0059] S5: When operating in grid-connected mode, a three-phase H-bridge converter circuit is set to control the charging or discharging of the battery pack; when operating off-grid mode, the charging or discharging is controlled according to the load and external power supply.

[0060] S6: Controls the series active harmonic cancellation circuit to eliminate DC-side second harmonic fluctuations generated during charging or discharging.

[0061] The controlled series active harmonic cancellation circuit eliminates DC-side second harmonic fluctuations generated during charging or discharging, including:

[0062] 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 filter elimination circuit. The power frequency sinusoidal AC voltage is used as the modulation wave signal. By performing PWM modulation on and off of the four IGBTs of the two 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.

[0063] S602: The three output ports of the high-frequency isolation transformer T induce three identical power frequency sinusoidal AC voltages, which provide stable AC voltages for the three secondary rectifier circuits respectively.

[0064] S603: The secondary rectifier circuit generates DC voltage at the DC port by PWM modulation of the four IGBTs in its two bridge arms to turn on and off. The high-frequency ripple is filtered out by the filter capacitor, providing DC voltage for the bidirectional H-bridge circuit on the secondary side.

[0065] S604: The secondary-side bidirectional H-bridge circuit obtains an AC second-harmonic fluctuation voltage signal by subtracting the DC bus voltage and battery voltage. After phase-locked loop control of this signal, the amplitude and phase angle of the second-harmonic fluctuation voltage are obtained, inverted, and used as the modulation wave signal of the secondary-side bidirectional H-bridge circuit. This signal is used to control the PWM modulation of the four IGBTs in the two bridge arms to generate an AC voltage signal with the same magnitude and 180° phase difference from the target second harmonic, thus canceling the second-harmonic fluctuation in the circuit.

[0066] (II) Grid-connected and off-grid control methods for AC high-voltage connections

[0067] 1) AC side high voltage connection principle

[0068] In conventional series-connected low-voltage flow battery applications, the flow battery typically operates within the rated power charge / discharge voltage range of DC 400V-DC 850V. Ignoring AC voltage fluctuations, the circuit topology of a traditional two-level energy storage device is as follows: Figure 2 As shown, each bridge arm outputs one phase of AC voltage, and the voltage between bridge arms is the line voltage. The output form of the other bridge arms is similar to this topology. The calculation method for the maximum value of the AC side line voltage is as follows:

[0069] ,

[0070] Based on the operating voltage range of flow batteries, the highest AC line voltage for traditional energy storage devices is AC283V, which is far lower than AC690V. To achieve an AC690V line voltage, the DC voltage would need to be increased to at least 1000V.

[0071] The voltage between the arms of each H-bridge in the energy storage device mentioned in this invention is the phase voltage, which differs from the line voltage. The calculation method for the maximum value of the AC side line voltage, based on the quantitative relationship of multiples, is as follows:

[0072] ,

[0073] Based on the operating voltage range of the flow battery, the highest AC line voltage of the energy storage device mentioned in this invention is AC490V. Since existing literature suggests that the modulation index can be increased by 1.15 times through a control algorithm, the highest AC line voltage after introducing the control algorithm is AC563V. The final calculation formula is as follows: ,

[0074] To meet the AC 690V line voltage requirement, the DC voltage only needs to be increased to 500V. That is, the DC voltage range is between DC 500V and DC 850V, thus enabling AC 690V line voltage access. This device achieves a shared integrated booster chamber with lithium batteries without increasing the insulation voltage level of the flow battery, significantly reducing the design cost of the flow battery energy storage system.

[0075] 2) On-grid and off-grid control methods

[0076] The AC side of this device primarily uses a three-phase H-bridge converter circuit as its core topology, which is extracted from the cascaded H-bridge circuit topology. A cascaded H-bridge circuit typically consists of multiple H-bridge circuits connected in series to form one phase of a three-phase circuit; three sets of cascaded H-bridge circuits constitute a three-phase cascaded H-bridge circuit. Due to the voltage and current sharing issues arising from its series connection, the typical control method is single-phase independent control, resulting in a relatively complex control strategy.

[0077] To avoid the voltage and current sharing problems introduced by cascaded H-bridge circuits, this device adopts a single-stage cascaded H-bridge circuit, i.e., a three-phase H-bridge converter circuit, such as... Figure 1 As shown. This circuit is related to... Figure 2 The AC section of the traditional two-level energy storage device circuit shown is identical except for the source of the N-line. The main difference in control between the two circuit topologies lies in the distribution of the drive signals for the IGBT power devices. However, the modulation waves required for generating the drive signals and the principles behind their generation are the same. To simplify the control of the three-phase H-bridge converter circuit, the control methods of the two circuits and the drive methods of the IGBT power devices are integrated.

[0078] The grid-connected control method uses the three-phase AC current as the control target to achieve the charging and discharging function of the battery pack. First, the voltage of the three-phase filter capacitors on the AC side is acquired, phase-locked, and the phase of the grid voltage is obtained. The three-phase filter capacitor voltage is then transformed into an abc / dq coordinate system. Second, the three-phase AC current is acquired and transformed into an abc / dq coordinate system. The target values ​​of the d-axis (active current) and q-axis (reactive current) are controlled respectively to achieve active charging and discharging and reactive power control. Third, the active and reactive power control values ​​on the dq axis are inversely transformed into a dq / abc coordinate system to obtain the a, b, and c phase power frequency sinusoidal modulation wave signals. Finally, the a, b, and c phase modulation waves are used as the modulation wave signal for the left bridge arm, and the a, b, and c phase modulation waves are inverted to serve as the driving signal for the right bridge arm modulation wave. The driving signal generated by the modulation waves controls the IGBT power devices, realizing the grid-connected charging and discharging control function. Furthermore, the grid-connected control theory is already very mature in traditional two-level energy storage devices, so it will not be elaborated upon here.

[0079] The off-grid control method uses the AC filter capacitor voltage as the control target to achieve AC side voltage stabilization and battery pack charging / discharging functions. First, it generates its own voltage phase as the grid voltage phase, acquires the three-phase AC filter capacitor voltage, and performs an abc / dq coordinate transformation on the three-phase AC current. Second, using the dq-axis components of the three-phase AC voltage as the control target, it employs a voltage-current dual closed-loop control to manage the d-axis active component and q-axis reactive component respectively, achieving AC side voltage stabilization and battery pack charging / discharging control. Third, it performs an inverse dq / abc coordinate transformation on the active and reactive control values ​​of the dq-axis to obtain the a, b, and c-phase power frequency sinusoidal modulation wave signals. Finally, it uses the a, b, and c-phase modulation waves as the left bridge arm modulation wave signal, and inverts the a, b, and c-phase modulation waves as the right bridge arm modulation wave drive signal. The modulation waves generate drive signals to control the IGBT power devices, achieving off-grid voltage support and charging / discharging control functions. Furthermore, the theory of off-grid control is already quite mature in traditional two-level energy storage devices, so it will not be elaborated here.

[0080] 3) Working principle of three-phase H-bridge converter circuit

[0081] In a three-phase H-bridge converter circuit, the power frequency sinusoidal modulation wave of each phase has the same frequency and amplitude, with only a 120° difference in initial phase angle. The working principle of the three-phase power devices IGBT is the same, with only the control timing being staggered by 120°. Therefore, the working principle of the phase a H-bridge converter circuit will be used as an example for explanation, while phases b and c are the same.

[0082] When the three-phase H-bridge converter circuit is in the discharge state, the AC voltage and AC current are in phase. The operating state of its phase a H-bridge converter circuit can be divided into the following six processes, such as... Figure 3 As shown.

[0083] When the a-phase power frequency sinusoidal modulation wave is in the positive half-cycle, the a-phase H-bridge converter circuit is in Figure 3 The three states (a), (b), and (c) shown are mutually switching, with state (b) representing an intermediate transition state. Assume that the modulation index of the phase a power frequency sinusoidal modulation wave is M (0 < M < 1) at this time, and the switching period is... Dead time is Then during the switching cycle Inside:

[0084] Figure 3 The duration of state (a) shown is equivalent to In this state, transistors G1 and G4 of the IGBT are turned on, while transistors G2 and G3 are turned off. The conduction path is: battery positive terminal → G1 → inductor L → AC source → G4 → battery negative terminal. At this time, the battery voltage is conducted to inductor L through the IGBT. Because it is in a discharging state, the battery voltage is higher than the AC source voltage, so the discharge current on inductor L gradually increases and continues... time;

[0085] Figure 3 The duration of state (b) shown is In this state, transistors G1, G4, G2, and G3 of the IGBT are all off. Due to the presence of inductor L, the current in inductor L cannot change abruptly and will maintain its original current direction for freewheeling. Therefore, the current direction remains towards the AC power supply, and its freewheeling path is: battery negative terminal → D2 freewheeling → inductor L → AC power source → D3 freewheeling → battery positive terminal. The freewheeling circuit is maintained by the energy stored in inductor L, and the freewheeling circuit applies a battery voltage opposite to the current direction to inductor L. Therefore, the discharge current on the inductor gradually decreases and continues to... time;

[0086] Figure 3 The duration of state (c) shown is In this state, transistors G2 and G3 of the power device IGBT are turned on, while transistors G1 and G4 are turned off. The time is short, and the inductor current has not decreased to 0. Transistors G2 and G3 are turned on, which does not affect the inductor current path. The conduction path is: battery negative terminal → D2 freewheeling → inductor L → AC source → D3 freewheeling → battery positive terminal. At this time, the freewheeling circuit is maintained by the energy stored in inductor L. This circuit continues to apply a battery voltage opposite to the current direction to inductor L, causing the discharge current on the inductor to continue to decrease and persist. time.

[0087] The above describes the working process of a single switching cycle when the a-phase power frequency sinusoidal 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 inductor L current can be adjusted to achieve the control of the discharge current.

[0088] When the a-phase power frequency sinusoidal modulation wave is in the negative half-cycle, the a-phase H-bridge converter circuit is in Figure 3 The three states (d), (e), and (f) shown in the figure switch between each other, with (e) representing an intermediate transition state. Assume that the modulation index of the phase a power frequency sinusoidal modulation wave is M (0 < M < 1) at this time, and the switching period is... Dead time is Then during the switching cycle Inside:

[0089] Figure 3 The duration of state (d) shown is In this state, transistors G2 and G3 of the IGBT are turned on, while transistors G1 and G4 are turned off. The conduction path is: battery positive terminal → G3 → AC source → inductor L → G2 → battery negative terminal. At this time, the battery voltage is conducted to the AC source through the IGBT. Since it is in a discharging state, the battery voltage is higher than the AC source voltage, so the discharge current on inductor L gradually increases and continues. time;

[0090] Figure 3 The duration of state (e) shown is In this state, transistors G1, G4, G2, and G3 of the IGBT are all off. Due to the presence of inductor L, the current in inductor L cannot change abruptly and will maintain its original current direction for freewheeling. Therefore, the current direction remains towards the IGBT, and its freewheeling path is: battery negative terminal → D4 freewheeling → AC source → inductor L → D1 freewheeling → battery positive terminal. The freewheeling circuit is maintained by the energy stored in inductor L, and the freewheeling circuit applies a battery voltage opposite to the current direction to inductor L. Therefore, the discharge current on the inductor gradually decreases and continues to... time;

[0091] Figure 3 The duration of state (f) shown is In this state, transistors G1 and G4 of the power device IGBT are turned on, while transistors G2 and G3 are turned off. The time is short, and the inductor current has not decreased to 0. Transistors G1 and G4 are turned on, which does not affect the inductor current path. The conduction path is: battery negative terminal → D4 freewheeling → AC source → inductor L → D1 freewheeling → battery positive terminal. At this time, the freewheeling circuit is maintained by the energy stored in inductor L. This circuit continues to apply a battery voltage opposite to the current direction to inductor L, causing the discharge current on the inductor to continue to decrease and persist. time.

[0092] The above describes the working process of a single switching cycle when the a-phase power frequency sinusoidal modulation wave is in the negative half-cycle. According to the PWM modulation principle, by controlling the modulation index M of each switching cycle, the magnitude of the inductor L current can be adjusted to achieve the control of the discharge current.

[0093] When the three-phase H-bridge converter circuit is in the charging state, the AC current and AC voltage are 180° out of phase. The operating state of its phase a H-bridge converter circuit can be divided into the following six processes, such as... Figure 4As shown. Its working principle is similar to that in the discharge state, with the only differences being the direction of inductor current flow and the freewheeling circuit when the power device IGBT is in the off state, so it will not be described in detail here.

[0094] (III) Methods for eliminating second harmonic fluctuations

[0095] 1) The principle of second harmonic wave generation

[0096] Taking the a-phase H-bridge converter circuit as an example, we will explain the mechanism of second harmonic ripple generation. Assuming the power factor of phase a is 1, its AC side voltage and current can be expressed as:

[0097] ,

[0098] The total instantaneous active power of the phase-a H-bridge converter circuit can be expressed as:

[0099] ,

[0100] As can be seen from the above formula, the instantaneous active power on the AC side is determined by a DC current. and a communication volume Furthermore, the AC quantity is a cosine waveform with a frequency of 100Hz, twice that of the power frequency of 50Hz. Without considering energy conversion losses, the DC-side power... With AC side power Since they are equal, the DC power is also the superposition of a DC quantity and a second harmonic AC quantity. The DC power can be expressed as:

[0101] ,

[0102] The DC voltage fluctuates at second harmonic frequency It can be represented as:

[0103] ,

[0104] The DC current fluctuating at second harmonic frequency It can be represented as:

[0105] ,

[0106] 2) Principle of eliminating second harmonic fluctuations

[0107] Analysis of the second harmonic power fluctuation shows that the second harmonic fluctuation on the DC side can be equivalent to the voltage fluctuation or current fluctuation on the DC side, and the second harmonic fluctuation can be eliminated from the two dimensions of DC voltage or DC current.

[0108] The control method proposed in this invention eliminates second-harmonic fluctuations in the dimension of DC voltage. If the DC voltage... The DC component is obtained using the battery pack voltage. Then the formula It can be transformed into:

[0109] ,

[0110] Therefore, it can be concluded that if a component related to communication can be generated... Second harmonic components of equal magnitude but opposite direction are superimposed on a DC voltage in series. Up, then DC voltage Regarding battery voltage If they are equal, the second harmonic fluctuation is eliminated.

[0111] Assuming the AC voltage meets the power quality standards, and ignoring the imbalance caused by control accuracy and sampling errors, the three-phase H-bridge converter circuit has a phase difference of 120° between each phase. Therefore, the expression for the DC side second-harmonic voltage fluctuation of the three-phase H-bridge converter circuit is:

[0112] ,

[0113] The above equation can be simplified to:

[0114] ,

[0115] Will , , Adding them together, we get:

[0116] ,

[0117] Therefore, if the three-phase second-harmonic voltage cancellation circuits are coupled together on the secondary side of the transformer, the second-harmonic fluctuations theoretically do not generate active power consumption on the primary side of the transformer; the main energy consumption is the reactive energy of voltage fluctuations. Thus, the primary side of the transformer only needs to provide active power within 200W of the power device's own losses (including a small portion of AC voltage imbalance energy consumption), which greatly reduces the power consumption of the second-harmonic cancellation circuit, minimizing its impact on the overall efficiency of the energy storage device. Based on this, this invention proposes a series-type active harmonic cancellation circuit that simultaneously eliminates the second-harmonic fluctuations generated by the three-phase H-bridge converter circuit.

[0118] 3) Working principle of series active harmonic cancellation circuit

[0119] A series-type active harmonic cancellation circuit consists of four parts: a primary-side inverter bridge circuit, a high-frequency isolation transformer, a secondary-side rectifier circuit, and a secondary-side harmonic cancellation circuit. Figure 5As shown. The primary-side inverter bridge circuit is used to provide a stable AC source through inversion. Its operating state is unrelated to the charging and discharging state of the three-phase H-bridge converter circuit. However, the secondary-side rectifier circuit and the secondary-side harmonic elimination circuit have certain differences in their operating principles depending on the charging and discharging state of the three-phase H-bridge converter circuit. Therefore, the series-type active harmonic elimination circuit will be described in detail from the perspectives of the primary and secondary sides.

[0120] The primary side of the series-type active harmonic cancellation circuit is a standard single-phase bridge inverter circuit, and its working principle is as follows: Figure 7 As shown, the IGBT power device is driven by a self-generated power frequency sinusoidal modulation wave signal. The detailed working process within one power frequency sinusoidal modulation wave cycle is described below.

[0121] When the power frequency sinusoidal modulation wave is in the positive half-cycle, the primary-side inverter bridge circuit is in Figure 7 The three states (a), (b), and (c) shown in the figure switch between each other, with state (b) representing an intermediate transition state. Assume that the modulation index of the power frequency sinusoidal modulation wave is M (0 < M < 1) and the switching period is... Dead time is Then during the switching cycle Inside:

[0122] Figure 7 The duration of state (a) shown is In this state, transistors S1 and S4 of the IGBT are turned on, while transistors S2 and S3 are turned off. The conduction path is: battery positive terminal → S1 → primary side of transformer T → S4 → battery negative terminal. At this time, the battery voltage is applied positively to the primary side of the high-frequency isolation transformer T through the IGBT, generating a gradually increasing positive current on the primary side of the high-frequency isolation transformer T, which continues... time;

[0123] Figure 7 The duration of state (b) shown is In this state, transistors S1, S4, S2, and S3 of the IGBT are all off. Due to the presence of the high-frequency isolation transformer T, the current on the transformer T cannot change abruptly and will maintain its original current direction for freewheeling. Therefore, the current direction remains towards the high-frequency isolation transformer T, and its freewheeling path is: battery negative terminal → D2 freewheeling → high-frequency isolation transformer T → D3 freewheeling → battery positive terminal. The freewheeling circuit is maintained by the energy stored in the high-frequency isolation transformer T, and the freewheeling circuit applies a reverse voltage to the primary side of the high-frequency isolation transformer T. Therefore, the forward current on the high-frequency isolation transformer T gradually decreases and continues to... time;

[0124] Figure 7The duration of state (c) shown is In this state, transistors S2 and S3 of the power device IGBT are turned on, while transistors S1 and S4 are turned off. The time is short, and the current in the high-frequency isolation transformer T has not decreased to 0. S2 and S3 are turned on, but this does not affect the current path of the high-frequency isolation transformer T. The conduction path is: battery negative terminal → D2 freewheeling → high-frequency isolation transformer T → D3 freewheeling → battery positive terminal. 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 reverse voltage to the primary side of the high-frequency isolation transformer T, causing the current to continue to decrease and remain so. time.

[0125] The above describes 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 index 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.

[0126] When the power frequency sinusoidal modulation wave is in the negative half-cycle, the primary-side inverter bridge circuit is... Figure 7 The three states (d), (e), and (f) shown in the figure switch between each other, with (e) representing an intermediate transition state. Assume that the modulation index of the power frequency sinusoidal modulation wave is M (0 < M < 1) at this time, and the switching period is... Dead time is Then during the switching cycle Inside:

[0127] Figure 7 The duration of state (d) shown is In this state, transistors S2 and S3 of the IGBT are turned on, while transistors S1 and S4 are turned off. The conduction path is: battery positive terminal → S3 → primary side of transformer T → S2 → battery negative terminal. At this time, the battery voltage is applied to the primary side of the high-frequency isolation transformer T through the IGBT, generating a gradually increasing reverse current on the primary side of the high-frequency isolation transformer T, which continues... time;

[0128] Figure 7 The duration of state (e) shown is In this state, transistors S1, S4, S2, and S3 of the IGBT are all off. Due to the presence of the high-frequency isolation transformer T, the current on the transformer T cannot change abruptly and will maintain its original current direction for freewheeling. Therefore, the current direction remains towards the IGBT, and its freewheeling path is: battery negative terminal → D4 freewheeling → high-frequency isolation transformer T → D1 freewheeling → battery positive terminal. The freewheeling circuit is maintained by the energy stored in the high-frequency isolation transformer T, and the freewheeling circuit applies a positive voltage 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 continues to... time;

[0129] Figure 7 The duration of state (f) shown is In this state, transistors S1 and S4 of the power device IGBT are turned on, while transistors S2 and S3 are turned off. The time is short, and the current in the high-frequency isolation transformer T has not decreased to 0. S1 and S4 transistors are conducting, which does not affect the current path of the high-frequency isolation transformer T. The conduction path is: battery negative terminal → D4 freewheeling → high-frequency isolation transformer T → D1 freewheeling → battery positive terminal. 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 persist. time.

[0130] The above describes the working process of a single switching cycle when the power frequency sinusoidal modulation wave is in the negative half-cycle. According to the PWM modulation principle, by controlling the modulation index M of each switching cycle, an equivalent negative half-cycle sinusoidal AC voltage signal can be generated on the primary side of the high-frequency isolation transformer T.

[0131] By repeatedly controlling each cycle of the power frequency sinusoidal modulation wave, a power frequency sinusoidal AC voltage can be generated on the primary side of the high-frequency isolation transformer T, and induced on the secondary side of the high-frequency isolation transformer T, providing a stable AC source for the secondary circuit.

[0132] The series-type active harmonic cancellation circuit couples three AC outputs on its secondary side. Each AC output passes through a rectifier circuit and then an inverter circuit, outputting the required second harmonic voltage at the port, which is then connected in series between the positive terminal of the battery pack voltage and the positive terminal of the DC bus. This path contains two types of energy flows: one is an AC second harmonic fluctuation voltage signal obtained by subtracting the DC bus voltage and the battery voltage, which is then phase-locked to obtain the amplitude and phase angle of the second harmonic fluctuation voltage. This amplitude and phase angle are then inverted and used as the modulation wave signal for the secondary harmonic cancellation circuit, generating a second harmonic AC energy flow that is equal in magnitude and 180° out of phase with the target second harmonic; the other is a DC energy flow connected in series between the positive terminal of the battery pack voltage and the positive terminal of the DC bus, transferring charging or discharging energy between the two during the charging and discharging process of the battery pack. Since the three AC paths have essentially the same topology and function, we will take one of them as an example and explain in detail the working principles of the two energy flows under four operating conditions: discharge state - positive half-cycle of modulation wave, discharge state - negative half-cycle of modulation wave, charging state - positive half-cycle of modulation wave, and charging state - negative half-cycle of modulation wave. Figure 8-11 As shown, its detailed working process is described below.

[0133] When operating in the discharge state—the positive half-cycle of the modulation wave—the second-side elimination circuit's frequency-doubled AC energy flow is... Figure 8 The diagram shows three states (a1), (b1), and (c1) that switch between each other, with (b1) representing an intermediate transition state. Assume the modulation depth of the second harmonic sinusoidal modulated wave is M (0 < M < 1), and the switching period is... Dead time is Then during the switching cycle Inside:

[0134] Figure 8 The duration of state (a1) shown is In this state, transistors S5, S8, S10, and S11 of the power IGBT are turned on, while transistors S6, S7, S9, and S12 are turned off. The conduction path is: DC bus positive terminal → L1 and L2 → S10 → filter capacitor C2 negative terminal → S8 → T secondary side → S5 → filter capacitor C2 positive terminal → S11 → battery positive terminal. At this time, the rectifier circuit, through the IGBT, applies the induced voltage from the secondary side of the high-frequency isolation transformer T to the filter capacitor C2, causing C2 to charge forward and its voltage to rise, providing voltage support for the inverter circuit. The inverter circuit, through the IGBT, applies the positive voltage from the rectifier circuit to points a and b of the bridge arm, where point a is negative and point b is positive. Simultaneously, a current flows out of the bridge arm at point b and into the bridge arm at point a, gradually increasing. The entire process continues... time;

[0135] Figure 8 The duration of state (b1) shown is In this state, transistors S5, S6, S7, S8, S9, S10, S11, and S12 of the IGBT power devices are all off. Due to the presence of the high-frequency isolation transformer T in the rectifier circuit and the inductor in the LCL filter in the inverter circuit, the current on the high-frequency isolation transformer T and the LCL filter inductor cannot change abruptly and will maintain their original current direction for freewheeling. Therefore, the current direction in the rectifier circuit remains from the high-frequency isolation transformer T to the bridge arm, and the current direction in the inverter circuit remains from the LCL filter to the bridge arm. The freewheeling path in the rectifier circuit is: positive terminal of high-frequency isolation transformer T → freewheeling through D5 → filter capacitor C2 → freewheeling through D8 → negative terminal of high-frequency isolation transformer T. The freewheeling circuit is maintained by the high-frequency isolation transformer T, and this circuit ensures that a positive voltage continues to be applied to the filter capacitor C2. Therefore, the filter capacitor C2 continues to charge and... The freewheeling path in the inverter circuit is as follows: DC bus positive terminal → L1 and L2 → D9 freewheeling → filter capacitor C2 → D12 freewheeling → battery positive terminal. The freewheeling circuit applies a reverse voltage to points a and b, thus gradually decreasing the current in the LCL filter. The entire process continues... time;

[0136] Figure 8 The duration of state (c1) shown is In this state, transistors S6, S7, S9, and S12 of the power IGBT are turned on, while transistors S5, S8, S10, and S11 are turned off. The conduction path is: DC bus positive terminal → L1 and L2 → D9 freewheeling → filter capacitor C2 positive terminal → S7 → T secondary side → S6 → filter capacitor C2 negative terminal → D12 freewheeling → battery positive terminal. At this time, the rectifier circuit, through the power IGBT, applies the induced voltage on the secondary side of the high-frequency isolation transformer T in reverse to the two ends of the filter capacitor C2. The filter capacitor C2 discharges in reverse, and its voltage decreases, but it does not reverse. The inverter circuit, through the power IGBT, continues to apply reverse voltage at points a and b, so the current of the LCL filter continues to decrease. The entire process continues. time.

[0137] The above describes the working process of a single switching cycle when operating in the discharge state - the positive half-cycle of the modulation wave. According to the PWM modulation principle, by controlling the modulation index M of each switching cycle, an equivalent positive half-cycle double frequency switching voltage signal can be generated at the output. The double frequency switching voltage signal is then converted into a double frequency sinusoidal voltage signal by the LCL filter circuit, realizing the double frequency AC energy flow of the secondary side elimination circuit.

[0138] When operating in the discharge state—the positive half-cycle of the modulation wave—the DC energy flow of the battery pack discharge is... Figure 8 The three states (a2), (b2), and (c2) shown in the figure switch between each other, with (b2) being the intermediate transition state. Assume that the modulation index of the second harmonic sinusoidal modulated wave is M (0 < M < 1) at this time.

[0139] And the switching cycle is Dead time is Then during the switching cycle Inside:

[0140] Figure 8 The duration of state (a2) shown is In this state, the S5, S8, S10, and S11 transistors of the power device IGBT are turned on, while the S6, S7, S9, and S12 transistors are turned off. The DC energy flow path of the battery pack discharge is opposite to that of the second harmonic energy flow: battery positive terminal → D11 → positive terminal of filter capacitor C2 → S5 → secondary side of T → S8 → negative terminal of filter capacitor C2 → D10 → L1 and L2 → positive terminal of DC bus.

[0141] Figure 8 The duration of state (b2) shown is In this state, transistors S5, S6, S7, S8, S9, S10, S11, and S12 of the IGBT power device are all in the off state. There is no direct DC circuit at this time, due to the duration of this state... Since the circuit is very short and transfers relatively little energy, the energy stored in the filter capacitor C2 is converted into the energy required for discharge to realize the DC energy flow for battery pack discharge. The conduction path is: battery positive terminal → D11 → filter capacitor C2 → D10 → L1 and L2 → DC bus positive terminal.

[0142] Figure 8 The duration of state (c2) shown is In this state, the S6, S7, S9, and S12 transistors of the power device IGBT are turned on, while the S5, S8, S10, and S11 transistors are turned off. The DC energy flow path of the battery pack discharge is opposite to that of the second harmonic energy flow: battery positive terminal → S12 → filter capacitor C2 negative terminal → D6 → T secondary side → D7 → filter capacitor C2 positive terminal → S9 → L1 and L2 → DC bus positive terminal.

[0143] The above describes the conduction path of the DC energy flow of the battery pack during a single switching cycle when operating in the discharge state - the positive half-cycle of the modulation wave. By periodically repeating this path, the continuous transmission of the DC energy flow of the battery pack during the positive half-cycle of the modulation wave is achieved.

[0144] When operating in the discharge state—the negative half-cycle of the modulation wave—the second-side elimination circuit's frequency-doubled AC energy flow is... Figure 9 The three states (a1), (b1), and (c1) shown in the figure switch between each other, with (b1) being the intermediate transition state. Its operation process in a single switching cycle is the same as that of the positive half-cycle, and will not be described again. According to the PWM modulation principle, by controlling the modulation index M of each switching cycle, an equivalent negative half-cycle double frequency switching voltage signal can be generated at the output. This double frequency switching voltage signal is then converted into a double frequency sinusoidal voltage signal by an LCL filter circuit, realizing the double frequency AC energy flow of the secondary-side elimination circuit.

[0145] When operating in the discharge state—the negative half-cycle of the modulation wave—the DC energy flow of the battery pack discharge is... Figure 9 The three states (a2), (b2), and (c2) shown in the figure switch between each other, with (b2) being the intermediate transition state. The working process of a single switching cycle is the same as that of the positive half-cycle, and will not be described again. By periodically repeating the conduction path of the DC energy flow of the battery pack discharge, the continuous transmission of the DC energy flow of the battery pack discharge in the negative half-cycle of the modulation wave is realized.

[0146] When operating in the charging state—the positive half-cycle of the modulation wave—the second-side elimination circuit's frequency-doubled AC energy flow is... Figure 10 The three states (a1), (b1), and (c1) shown in the figure switch between each other, with (b1) being the intermediate transition state. According to the PWM modulation principle, by controlling the modulation index M of each switching cycle, an equivalent positive half-cycle double frequency switching voltage signal can be generated at the output. After passing through the LCL filter circuit, the double frequency switching voltage signal is converted into a double frequency sinusoidal voltage signal, realizing the double frequency AC energy flow of the secondary side elimination circuit.

[0147] When operating in the charging state—the positive half-cycle of the modulation wave—the DC energy flow of the battery pack during discharge is... Figure 10 The diagram shows three states (a2), (b2), and (c2) that switch between each other, with (b2) representing an intermediate transition state. The continuous transmission of DC energy flow during the positive half-cycle of the modulation wave is achieved through the conduction path of the DC energy flow for periodically repetitive battery charging.

[0148] When operating in the charging state—the negative half-cycle of the modulation wave—the second-side elimination circuit's frequency-doubled AC energy flow is... Figure 11 The three states (a1), (b1), and (c1) shown in the figure switch between each other, with (b1) being the intermediate transition state. According to the PWM modulation principle, by controlling the modulation index M of each switching cycle, an equivalent negative half-cycle double frequency switching voltage signal can be generated at the output. After passing through the LCL filter circuit, the double frequency switching voltage signal is converted into a double frequency sinusoidal voltage signal, realizing the double frequency AC energy flow of the secondary side elimination circuit.

[0149] When operating in the charging state—the negative half-cycle of the modulation wave—the DC energy flow of the battery pack discharge is... Figure 11 The diagram shows three states (a2), (b2), and (c2) that switch between each other, with (b2) representing an intermediate transition state. The continuous transmission of DC energy flow during the negative half-cycle of the modulation wave is achieved through the conduction path of the DC energy flow for periodically repetitive battery charging.

[0150] pass Figure 8-11 The periodic switching between the operating states shown ultimately realizes the transfer of the double-frequency AC energy flow of the secondary-side elimination circuit and the DC energy flow of the battery pack during charging or discharging.

[0151] Under the framework of this invention, the technical means in the above embodiments can be transformed, replaced, or modified in a manner that is easily conceived by those skilled in the art, and the effect is basically the same as the corresponding technical means in this invention, and the purpose of the invention is also basically the same. The technical solution formed in this way is a fine-tuning of the above embodiments, and such technical solution still falls within the protection scope of this invention.

Claims

1. An energy storage device suitable for flow batteries, characterized in that, The device includes a three-phase H-bridge converter circuit, an AC-side LCL filter circuit, an AC pre-charge circuit, an AC EMI filter, an AC fuse, three series-connected active harmonic cancellation circuits, three DC EMI filters, and three DC pre-charge circuits; The three-phase H-bridge converter circuit consists of three single-phase H-bridge circuits. The switching devices are all IGBTs. The AC side of the three-phase H-bridge converter circuit has four ports: U', V', W', and N'. Ports U', V', and W' are the positive terminals of the AC terminals of the three single-phase H-bridge circuits, and port N' is connected to the negative terminals of the AC terminals of the three single-phase H-bridge circuits. The four AC side ports U', V', W', and N' of the three-phase H-bridge converter circuit are sequentially connected to the AC side LCL filter circuit, the AC pre-charge circuit, the AC EMI filter, and the AC fuse before being connected to the power grid via U, V, W, and N. The three-phase H-bridge converter circuit has three pairs of ports on its DC side, each consisting of the DC terminals of three single-phase H-bridge circuits. A capacitor is also connected in parallel to the DC terminal of each single-phase H-bridge circuit. The positive DC terminals of the three single-phase H-bridge circuits are each connected in series with a series-type active harmonic cancellation circuit and then connected to the positive terminal on the right side of the DC EMI filter. The negative DC terminal of each single-phase H-bridge circuit is directly connected to the negative terminal on the right side of the DC EMI filter. The positive and negative terminals on the left side of the three DC EMI filters are connected in series with DC fuses and then connected to the positive and negative terminals on the right side of the DC pre-charge circuit. The positive and negative terminals on the left side of the three DC pre-charge circuits are respectively connected to the positive and negative terminals of the battery pack. The series-type active harmonic cancellation circuit comprises: a secondary-side 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-side rectifier circuit is connected to one secondary-side output of the high-frequency isolation transformer T. The DC terminal is connected to the DC filter capacitor C2 in parallel and then connected to the DC terminal of the bidirectional H-bridge circuit. The AC terminal of the bidirectional H-bridge circuit is connected to the two left ports of the DC-side LCL filter circuit. The two right ports of the DC-side LCL filter circuit are respectively connected to the positive terminal of the DC terminal of the single-phase H-bridge circuit and the positive terminal of the right side of the DC EMI filter. The high-frequency isolation transformer T has one input terminal and three 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 to the DC bus capacitor in parallel. The three output terminals are respectively connected to the secondary-side rectifier circuits of three series-type active harmonic cancellation 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, characterized in that, The AC-side LCL filter circuit consists of two inductors and one filter capacitor. The two inductors are connected in series, with one end connected to the three-phase H-bridge converter circuit and the other end connected to the AC pre-charge circuit. One end of the filter capacitor is connected to the middle of the two series-connected inductors, and the other end is connected to the neutral (N) line.

3. The energy storage device according to claim 1, characterized in that, The AC pre-charging circuit consists of a main circuit breaker KM1 and a circuit breaker KM2 connected in parallel across 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 the main circuit breaker KM3 and the circuit breaker KM4 connected in parallel across both ends of the main circuit breaker KM3.

5. A control method for an energy storage device suitable for flow batteries, characterized in that, The control method for implementing operational control of the energy storage device according to any one of claims 1-4 includes the following steps: S1: Controls the start of the inverter bridge circuit at the input terminal of the high-frequency isolation transformer T, controls the start of the secondary rectifier circuit, controls the bidirectional H-bridge circuit on the secondary side, and connects the battery pack to the DC terminal of the single-phase H-bridge circuit. S2: Set the operating mode of the three-phase H-bridge converter circuit to grid-connected or off-grid operation; S3: After setting, start the pre-charge control of the parallel capacitor on the DC end of the single-phase H-bridge circuit. It is divided into AC pre-charge control when running in grid-connected mode and DC pre-charge control when running off-grid mode. S4: After the pre-charging of the parallel capacitor on the DC end of the single-phase H-bridge circuit is completed, the IGBT of the three-phase H-bridge converter circuit is controlled to work and enter the grid-connected or off-grid operation mode. S5: When operating in grid-connected mode, a three-phase H-bridge converter circuit is set up to control the charging or discharging of the battery pack; when operating off-grid mode, the charging or discharging is controlled according to the load and external power supply. S6: Controls the series active harmonic cancellation circuit to eliminate DC-side second harmonic fluctuations generated during charging or discharging.

6. The control method according to claim 5, characterized in that, The controlled series active harmonic cancellation circuit eliminates DC-side second harmonic fluctuations generated during charging or discharging, including: 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 filter elimination circuit. The power frequency sinusoidal AC voltage is used as the modulation wave signal. By performing PWM modulation on and off of the four IGBTs of the two 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. S602: The three output ports of the high-frequency isolation transformer T induce three identical power frequency sinusoidal AC voltages, which provide stable AC voltages for the three secondary rectifier circuits respectively; S603: The secondary rectifier circuit generates DC voltage at the DC port by PWM modulation of the four IGBTs in its two bridge arms to turn on and off. The high-frequency ripple is filtered out by the filter capacitor, providing DC voltage for the bidirectional H-bridge circuit on the secondary side. S604: The secondary-side bidirectional H-bridge circuit obtains an AC second-harmonic fluctuation voltage signal by subtracting the DC bus voltage and battery voltage. After phase-locked loop control of this signal, the amplitude and phase angle of the second-harmonic fluctuation voltage are obtained, inverted, and used as the modulation wave signal of the secondary-side bidirectional H-bridge circuit. This signal is used to control the PWM modulation of the four IGBTs in the two bridge arms to generate an AC voltage signal with the same magnitude and 180° phase difference from the target second harmonic, thus canceling the second-harmonic fluctuation in the circuit.

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