Multi-phase transformation energy storage system, grid-connected system and control method thereof

Abstract

The invention relates to the technical field of energy storage, in particular to a multi-phase conversion energy storage system, a grid-connected system and a control method of the multi-phase conversion energy storage system. The energy storage system comprises a battery cluster, an N-phase energy storage converter and a multi-winding transformer, n is an integer and is a multiple of 3; the battery cluster is connected with the multi-winding transformer through a plurality of N-phase energy storage converters; the battery cluster is inverted into alternating voltage through the N-phase energy storage converter, N-phase voltage output by the N-phase energy storage converter is divided into N / 3 groups, the N / 3 groups are respectively connected to different windings of the multi-winding transformer, and a deviation angle between the different windings of the multi-winding transformer is consistent with an output voltage phase deviation angle of the different groups of the N-phase energy storage converter; the N-phase energy storage converter comprises a multi-phase unit, the multi-phase unit adopts the same direct-current bus, and the midpoint of the direct-current bus serves as a common reference point of all the phase units. The problem of neutral-point voltage fluctuation of a traditional three-phase energy storage converter is solved.

Description

Technical Field

[0001] This invention relates to the field of energy storage technology, specifically to a multiphase conversion energy storage system, a grid-connected system, and a control method thereof. Background Technology

[0002] With the rapid increase in installed capacity of renewable energy and the continuous improvement of power grid requirements for power quality and stable operation, energy storage systems are being applied in power systems in an increasingly wide range of scenarios, and their technical routes and topologies are constantly evolving. The 1500V DC voltage level, due to its good balance between transmission loss, equipment cost, and system efficiency, relatively moderate cable loss, and controllable insulation costs, has gradually become the mainstream choice for large-scale energy storage power stations, demonstrating significant advantages in improving energy density and reducing initial investment.

[0003] However, in existing technical solutions, traditional 1500V energy storage systems using three-phase three-level converter topology generally face a prominent technical challenge—severe midpoint voltage fluctuation problem.

[0004] Specifically, widely used three-phase neutral point clamped (NPC) converters are highly dependent on the neutral point current, leading to significant third-harmonic fluctuations in the neutral point potential during operation due to charging and discharging imbalances. These fluctuations not only cause DC-side capacitor voltage imbalances, exacerbating voltage stress on power semiconductor devices and potentially accelerating device aging and affecting system lifespan over long-term operation, but also cause converter output level distortion, resulting in excessive electromagnetic interference (EMI) levels and a series of secondary problems. This poses a serious challenge to the power quality, safe and stable operation, and reliability of the entire energy storage system. Summary of the Invention

[0005] The purpose of this invention is to provide a multiphase energy storage system, a grid-connected system and its control method, to solve the problem of midpoint voltage fluctuation in traditional three-phase energy storage converters.

[0006] This invention is achieved through the following technical solution: This invention discloses a multiphase conversion energy storage system, characterized in that it includes a battery cluster, an N-phase energy storage converter, and a multi-winding transformer; N is an integer and a multiple of 3; The battery cluster is connected to a multi-winding transformer via multiple N-phase energy storage converters; The battery cluster is converted into AC voltage by an N-phase energy storage converter. The N-phase voltage output by the N-phase energy storage converter is divided into N / 3 groups. The N / 3 groups are connected to different windings of a multi-winding transformer. The offset angle between the different windings of the multi-winding transformer is consistent with the phase deviation angle of the output voltage of the different groups of the N-phase energy storage converter. The N-phase energy storage converter includes multiphase units, which share the same DC bus. The midpoint of this DC bus serves as the common reference point for the multiphase units.

[0007] Furthermore, N is an even multiple of 3.

[0008] Furthermore, the N-phase energy storage converter includes N single-phase three-level NPC structures, and these N single-phase three-level NPC structures constitute a multi-phase unit; each phase includes four active switching devices and two clamping diodes; the DC bus voltage is divided into Udc / 2, 0, and... through two bus support capacitors. Udc / 2 has three voltage levels; Udc is the DC bus voltage. The formula for calculating the output voltage of an N-phase energy storage converter is:

[0009] in, For the first Phase output voltage, m is the modulation coefficient. The reference phase angle is N, where N is the number of phases in the multiphase energy storage converter. The formula for calculating the output current of an N-phase energy storage converter is:

[0010] in, For the first Phase output current, The power factor angle, This refers to the current amplitude. The reference phase angle is N, and the number of phases in the multiphase energy storage converter is N.

[0011] Furthermore, the formula for calculating the midpoint current of the N-phase energy storage converter is:

[0012] in, It is the midpoint current of the N-phase energy storage converter, where N is the number of phases in the multiphase energy storage converter.

[0013] Furthermore, the two clamping diodes provide a bidirectional current path when the topology output is in a zero-level state, clamping the withstand voltage of the four active switching devices to Udc / 2.

[0014] Furthermore, this multiphase conversion energy storage system has the following characteristics:

[0015] in, For the first j Midpoint current generated by the phase For the first Phase output voltage, For the first Phase output current; For the first Midpoint current generated by the phase; In a symmetrical even-phase system, the effects of the first N / 2 phase bridge arms on the midpoint current cancel each other out with the effects of the last N / 2 phase bridge arms on the midpoint current. The average value of the total midpoint current during the carrier cycle is zero, and there is no low-frequency fluctuation in the midpoint voltage.

[0016] Furthermore, when N=6, the energy storage converter is six-phase. The output of each six-phase energy storage converter is divided into two groups: the first group consists of phases A, B, and C, and the second group consists of phases U, V, and W. The voltage phase difference between the two groups is [missing information]. In each six-phase energy storage converter, phases A, B, C are connected to the first winding of a multi-winding transformer, and phases U, V, and W are connected to the second winding of the multi-winding transformer. The angle between the two windings and the power grid differs by a certain amount. This ensures consistent output on the high-voltage side.

[0017] Furthermore, the expressions for the per-unit voltages are as follows:

[0018]

[0019]

[0020]

[0021]

[0022]

[0023] in, This is the per-unit voltage of phase A. This is the per-unit voltage of phase B. This is the per-unit voltage of phase C. This is the voltage after phase U is normalized. This is the per-unit voltage of phase V. The voltage after W phase is normalized.

[0024] The present invention also discloses a grid-connected system containing the multiphase conversion energy storage system, wherein the high-voltage side of the multi-winding transformer is connected to the power grid.

[0025] This invention also discloses a control method for the multiphase conversion energy storage system, which controls the midpoint voltage by injecting a zero-sequence voltage. The injected zero-sequence voltage is calculated according to the following formula:

[0026] in, Where C is the required zero-sequence voltage, and C is the capacitance of the DC bus. It is the midpoint voltage deviation, which is equal to half the voltage difference between the upper and lower bus capacitors. It is a control cycle; For the first Phase output voltage, For the first j Phase output current.

[0027] Compared with the prior art, the present invention has the following beneficial technical effects: This invention provides a multiphase energy storage system. By employing a multiphase converter output (a multiple of 3 phases) and rationally setting each three-phase output as a group of phases, and then connecting to the grid via a phase-shifting transformer, it solves the midpoint voltage fluctuation problem of traditional three-phase energy storage converters. This achieves the goals of reducing harmonic content and simplifying the control process, thereby improving the overall performance and reliability of the energy storage system and meeting the grid's requirements for energy storage systems. It is suitable for scenarios such as new energy generation side / grid side / user side / microgrid. Theoretically, this multiphase energy storage system design can avoid the midpoint voltage fluctuation problem of 1500V DC voltage level energy storage converters, reduce harmonic content, simplify the control process, and improve the overall performance and reliability of the energy storage system.

[0028] This invention provides a control method for a multiphase conversion energy storage system. In actual operation, the DC bus voltage varies with the battery's state of charge (SOC), and the capacitance values ​​of the upper and lower capacitors on the DC bus side may differ due to manufacturing processes, leading to deviations in the digital control process. Zero-sequence voltage injection can avoid midpoint voltage drift caused by suboptimal operating conditions, as theoretically there is no midpoint voltage fluctuation. The required zero-sequence current to maintain the midpoint voltage is small and can be calculated using the simplest method. This avoids the complex calculation process of traditional three-level PCS. Attached Figure Description

[0029] Figure 1 This is a topology diagram of a six-phase energy storage converter; Figure 2 This is a diagram of a multiphase conversion energy storage system (N=6 as an example). Detailed Implementation

[0030] To make the objectives, technical solutions, and advantages of the present invention clearer, the following detailed description is provided in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention; that is, the described embodiments are only a part of the embodiments of the present invention, and not all of them.

[0031] The components described and illustrated in the accompanying drawings and embodiments of this invention can be arranged and designed in various different configurations. Therefore, the detailed description of the embodiments of the invention provided in the following drawings is not intended to limit the scope of the claimed invention, but merely to illustrate one selected embodiment of the invention. All other embodiments obtained by those skilled in the art based on the accompanying drawings and embodiments of this invention without inventive effort are within the scope of protection of this invention.

[0032] Terminology Explanation: Per Unit (pu): A dimensionless method in the electrical field for representing electrical quantities (voltage, current, power, etc.). The core is to compare the actual physical quantity with a selected "reference value" to obtain the per-unit value (i.e., per-unit value = actual value / reference value). The purpose is to simplify the comparison and calculation of parameters of equipment with different voltage levels and capacities, and avoid the analysis complexity caused by the difference in absolute values.

[0033] The features and performance of the present invention will be further described in detail below with reference to embodiments.

[0034] like Figure 2 As shown, this invention discloses a multiphase conversion energy storage system, including a battery cluster, an N-phase (N is an integer and a multiple of 3) energy storage converter, and a multi-winding transformer. The battery cluster is inverted into AC voltage by the N-phase energy storage converter. The output N-phase voltage is divided into N / 3 groups, which are respectively connected to different windings of the multi-winding transformer. Different groups are respectively connected to multiple sets of low-voltage windings of the multi-winding phase-shifting transformer. The offset angle between different windings is consistent with the phase deviation angle of the output voltage of different groups.

[0035] For example, when N = 6, such as Figure 1 As shown, this six-phase power conversion system (PCS) consists of six single-phase three-level NPC structures, each phase containing four active switching devices and two clamping diodes. The DC bus voltage is divided into Udc / 2, 0, and Udc / 2 via two bus support capacitors Cdc1 and Cdc2. Udc / 2 has three voltage levels. It uses a single midpoint (i.e., the DC bus midpoint) as the common reference point for all phase units, forming a "single midpoint" structure. This saves on capacitor count and provides a current loop for fault-tolerant operation. Two clamping diodes provide a bidirectional current path when the topology output is at zero voltage, and to some extent clamp the withstand voltage of the four active switching devices to Udc / 2. The single-midpoint dual three-phase energy storage converter has more degrees of freedom in current control and stronger fault tolerance.

[0036] The calculation expressions for the output voltage, output current, and neutral point current of an N-phase energy storage converter are as follows: (1) (2) (3) in, For the first Phase output voltage, m is the modulation coefficient. The reference phase angle is N, where N is the number of phases in the multiphase energy storage converter. For the first Phase output current, The power factor angle, This refers to the current amplitude. It is the midpoint current of the N-phase energy storage converter.

[0037] This multiphase conversion energy storage system has the following characteristics:

[0038] in, For the first j Midpoint current generated by the phase For the first Phase output voltage, For the first Phase output current; For the first The midpoint current generated by the phase.

[0039] In a symmetrical even-phase system, the effects of the first N / 2 phase bridge arms on the midpoint current cancel each other out, resulting in a total average midpoint current of zero over the carrier cycle and eliminating low-frequency fluctuations in the midpoint voltage. Complete elimination requires an even-numbered three-phase combination. Therefore, N=6 is recommended, although N=12 can also be chosen depending on the selection of individual components.

[0040] like Figure 1 As shown, taking N=6 as an example, the output of each six-phase PCS is divided into two groups: the first group consists of phases A, B, and C, and the second group consists of phases U, V, and W. The voltage phase difference between the two groups is [missing information]. Different six-phase energy storage converters correspond to the ABC phases being connected to the first winding of a multi-winding transformer, and the UVW phases being connected to the second winding of the same transformer. The angles of these two windings to the power grid also differ. This ensures consistent output on the high-voltage side.

[0041] From formula (1), the expressions for the per-unit voltages are derived as follows:

[0042]

[0043]

[0044]

[0045]

[0046]

[0047] in, This is the per-unit voltage of phase A. This is the per-unit voltage of phase B. This is the per-unit voltage of phase C. This is the voltage after phase U is normalized. This is the per-unit voltage of phase V. The voltage after W phase is normalized.

[0048] The voltage phase difference between the two groups is .

[0049] Although the inherent properties of this energy storage system eliminate the midpoint voltage fluctuations present in traditional three-phase three-level systems, in actual operation, the DC bus voltage Udc varies with the battery's SOC, the capacitance values ​​of the upper and lower capacitors on the DC bus side may differ due to manufacturing processes, and there are deviations in the digital control process. Therefore, in practical systems, it is still necessary to control the midpoint current to prevent midpoint voltage drift.

[0050] The common neutral point of the multiphase energy storage converter brings new degrees of freedom. The neutral point N allows zero-sequence current to flow freely, enabling independent injection of zero-sequence components into the multiphase arms, thus achieving more flexible neutral point voltage control. Neutral point voltage control can be achieved simply by injecting zero-sequence voltage, which is calculated according to the following formula (4): (4) in, Where C is the required zero-sequence voltage, and C is the capacitance of the DC bus. It is the midpoint voltage deviation, which is equal to half the voltage difference between the upper and lower bus capacitors. It is a control cycle; For the first Phase output voltage, For the first j Phase output current.

[0051] Optionally, the voltage range after injecting zero-sequence voltage remains unchanged, thus enabling rapid calculation by constraining the range of injected zero-sequence voltage.

[0052] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the specific implementation of the present invention. Any modifications or equivalent substitutions that do not depart from the spirit and scope of the present invention should be covered within the protection scope of the present invention.

Claims

1. A multiphase conversion energy storage system, characterized in that, It includes battery clusters, N-phase energy storage converters, and multi-winding transformers; N is an integer and a multiple of 3; The battery cluster is connected to a multi-winding transformer via multiple N-phase energy storage converters; The battery cluster is converted into AC voltage by an N-phase energy storage converter. The N-phase voltage output by the N-phase energy storage converter is divided into N / 3 groups. The N / 3 groups are connected to different windings of a multi-winding transformer. The offset angle between the different windings of the multi-winding transformer is consistent with the phase deviation angle of the output voltage of the different groups of the N-phase energy storage converter. The N-phase energy storage converter includes multiphase units, which share the same DC bus. The midpoint of this DC bus serves as the common reference point for the multiphase units.

2. The multiphase conversion energy storage system according to claim 1, characterized in that, N is an even multiple of 3.

3. The multiphase conversion energy storage system according to claim 1, characterized in that, The N-phase energy storage converter comprises N single-phase three-level NPC structures, which together form a multi-phase unit. Each phase includes four active switching devices and two clamping diodes. The DC bus voltage is divided into Udc / 2, 0, and... through two bus support capacitors. Udc / 2 has three voltage levels; Udc is the DC bus voltage. The formula for calculating the output voltage of an N-phase energy storage converter is: in, For the first Phase output voltage, m is the modulation coefficient. The reference phase angle is N, where N is the number of phases in the multiphase energy storage converter. The formula for calculating the output current of an N-phase energy storage converter is: in, For the first Phase output current, The power factor angle, This refers to the current amplitude. The reference phase angle is N, and the number of phases in the multiphase energy storage converter is N.

4. The multiphase conversion energy storage system according to claim 3, characterized in that, The formula for calculating the midpoint current of an N-phase energy storage converter is: in, It is the midpoint current of the N-phase energy storage converter, where N is the number of phases in the multiphase energy storage converter.

5. The multiphase conversion energy storage system according to claim 1, characterized in that, Two clamping diodes provide a bidirectional current path when the topology output is in a zero-level state, clamping the withstand voltage of the four active switching devices to Udc / 2.

6. The multiphase conversion energy storage system according to claim 1, characterized in that, The multiphase conversion energy storage system has the following characteristics: in, For the first j Midpoint current generated by the phase For the first Phase output voltage, For the first Phase output current; For the first Midpoint current generated by the phase; In a symmetrical even-phase system, the effects of the first N / 2 phase bridge arms on the midpoint current cancel each other out with the effects of the last N / 2 phase bridge arms on the midpoint current. The average value of the total midpoint current during the carrier cycle is zero, and there is no low-frequency fluctuation in the midpoint voltage.

7. The multiphase conversion energy storage system according to claim 1, characterized in that, When N=6, the energy storage converter is six-phase. The output of each six-phase energy storage converter is divided into two groups: the first group consists of phases A, B, and C, and the second group consists of phases U, V, and W. The voltage phase difference between the two groups is [missing information]. In each six-phase energy storage converter, phases A, B, C are connected to the first winding of a multi-winding transformer, and phases U, V, and W are connected to the second winding of the multi-winding transformer. The angle between the two windings and the power grid differs by a certain amount. This ensures consistent output on the high-voltage side.

8. The multiphase conversion energy storage system according to claim 7, characterized in that, The expressions for the voltages after standardization are as follows: in, This is the per-unit voltage of phase A. This is the per-unit voltage of phase B. This is the per-unit voltage of phase C. This is the voltage after phase U is standardized. This is the per-unit voltage of phase V. The voltage after W phase is normalized.

9. A grid-connected system comprising the multiphase conversion energy storage system according to any one of claims 1-8, characterized in that, The high-voltage side of a multi-winding transformer is connected to the power grid.

10. The control method for the multiphase conversion energy storage system according to any one of claims 1-8, characterized in that, The midpoint voltage is controlled by injecting a zero-sequence voltage, which is calculated according to the following formula: in, Where C is the required zero-sequence voltage, and C is the capacitance of the DC bus. It is the midpoint voltage deviation, which is equal to half the voltage difference between the upper and lower bus capacitors. It is the control cycle; For the first Phase output voltage, For the first j Phase output current.

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