AC soft start control method of three-phase four-leg topology

Abstract

The invention provides an AC soft start control method of a three-phase four-bridge-arm topology. The method comprises the following steps: S1; the power grid access state of the energy storage inverter is obtained, the power grid access state is identified according to alternating current incoming line modes, and the alternating current incoming line modes comprise a single-phase incoming line mode and a three-phase incoming line mode; a single-phase incoming line mode is adopted to control bus voltage, and the bus voltage is gradually increased to a target value; and after the bus voltage is stable, the three-phase incoming line mode is switched to continuously control the bus voltage to be output to a set value. Firstly, by slowly increasing the bus voltage and the output voltage in stages, the current peak value in the system starting process is remarkably reduced, and the impact current can be reduced by 30%-50%, so that a power device and a bus capacitor are effectively protected, and device damage caused by large current impact is avoided. And secondly, a dynamic neutral point compensation technology is introduced, so that the waveform quality of the output voltage is remarkably improved, the total harmonic distortion (THD) is lower than 3% and is superior to the performance which is usually higher than 5% in a traditional method, and the electric energy quality and the operation stability of the system are improved.

Description

Technical Field

[0001] The present invention relates to the technical field of power equipment, and more specifically, to an AC soft-start control method for a three-phase four-leg topology. Background Art

[0002] With the rapid development of the new energy industry, energy storage systems are increasingly widely used in power systems. As the core device connecting the AC-DC side energy conversion, the performance of the energy storage bidirectional inverter directly affects the stability and efficiency of the system. The current market's functional requirements for energy storage inverters are no longer limited to traditional grid-connected operation and off-grid operation to achieve peak-valley arbitrage, but are further extended to new application scenarios such as substation interconnection, photovoltaic energy storage charging integration, and AC-DC hybrid microgrids. These scenarios require the energy storage inverter to have more flexible operating modes and stronger control capabilities, especially the need to have the characteristics of a DC voltage source and be able to provide stable voltage support for DC loads or DC buses.

[0003] Currently, the soft-start circuit of voltage-source inverters mostly adopts a combination scheme of soft-start resistors, series soft-start switches (relays), and three-phase full-bridge inverters. During startup, this scheme first charges the DC bus capacitor by closing the soft-start relay KM1. After the bus capacitor voltage reaches a specific threshold, the main relay KM2 is then closed. However, this traditional soft-start method has obvious defects. At the moment when the soft-start switch is closed, a large inrush current will be generated in the main circuit loop. Frequent large-current impacts will seriously shorten the normal service life of power devices, increasing equipment maintenance costs and operation risks.

[0004] In terms of the inverter topology structure, due to the lack of neutral-point voltage control ability in traditional three-phase three-leg inverters, the DC bus capacitor will experience instantaneous charging during startup. The large-current impact generated thereby may not only damage power devices but also cause irreversible damage to the connected loads. Although the four-leg topology improves the three-phase imbalance problem to a certain extent by adding a neutral-point leg, there are still many challenges during the soft-start stage: the neutral-point voltage fluctuates violently, seriously affecting the output waveform quality and reducing the power quality; the system's dynamic response ability is insufficient, making it difficult to quickly adapt to load mutations, resulting in a decline in the operating stability of the equipment; at the same time, its control algorithm is complex, occupying a relatively high amount of computing resources, increasing the hardware cost and system development difficulty. These problems restrict the efficient application and performance improvement of energy storage inverters in multiple scenarios, and there is an urgent need for a more optimized AC startup control technical solution. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide an AC soft-start control method for a three-phase four-leg topology that can effectively improve the operating reliability and control performance of an energy storage inverter under complex working conditions while ensuring the safe startup of the system, in view of the deficiencies in the above technical solutions.

[0006] On the one hand, the present invention provides an AC soft start control method for a three-phase four-leg topology, and the control method includes the following steps:

[0007] S1; Obtain the grid connection status of the energy storage inverter, and identify the grid connection status according to the AC incoming line mode, where the AC incoming line mode includes a single-phase incoming line mode and a three-phase incoming line mode;

[0008] S2; Based on the identified AC incoming line mode, control the energy storage inverter to execute the external relay closing logic to establish the DC bus voltage and complete the soft start, and the energy storage inverter performs charge and discharge operations on the battery according to the set power.

[0009] In the AC soft start control method for the three-phase four-leg topology of the present invention; when the AC incoming line mode is the single-phase incoming line mode in the step S1, the external relay closing logic executed by the energy storage inverter PCS includes the following steps:

[0010] a; First, judge whether the single-phase voltage is within the normal range value E. If it is not within the normal range value, the single-phase voltage is abnormal. If it is within the normal range value, close the outer C AC relay and the soft start relay, and then close the AC main N relay.

[0011] In the AC soft start control method for the three-phase four-leg topology of the present invention; the external relay closing logic executed by the energy storage inverter PCS further includes the following steps:

[0012] b; Judge whether the uncontrolled rectifier voltage of the bus is greater than the set threshold Q. If it is not greater than the set threshold Q, further judge whether the soft start time exceeds 60S. If it exceeds, the AC soft start fails. If not, judge again whether the uncontrolled rectifier voltage of the bus is greater than the set threshold Q. If it is not greater than the set threshold Q, set the single-phase uncontrolled rectifier voltage on the DC bus side to V, and raise the bus voltage to be greater than the single-phase uncontrolled rectifier voltage by controlling the single-phase wave generation.

[0013] In the AC soft start control method for the three-phase four-leg topology of the present invention; the external relay closing logic executed by the energy storage inverter PCS further includes the following steps:

[0014] c; When the bus voltage completes the soft start, close the DC side contactor. After the DC side contactor is closed, switch to the single-phase current control algorithm according to the DC input type, and the energy storage inverter performs charge and discharge operations on the battery according to the set power.

[0015] In the AC soft start control method of the three-phase four-leg topology described in the present invention; in step S1, when the AC incoming line mode is a three-phase incoming line mode, the external relay closing logic executed by the energy storage inverter PCS includes the following steps:

[0016] d; First, judge whether the three-phase voltage is within the normal range value E. If it is not within the normal range value, the three-phase voltage is abnormal. If it is within the normal range value, close the outer C AC relay and the soft start relay, and then close the AC main N relay.

[0017] In the AC soft start control method of the three-phase four-leg topology described in the present invention; the external relay closing logic executed by the energy storage inverter PCS further includes the following steps:

[0018] e; Judge whether the uncontrolled rectifier voltage of the bus is greater than the set threshold Q. If it is not greater than the set threshold Q, further judge whether the soft start time exceeds 60S. If it exceeds, the AC soft start fails. If not, judge again whether the uncontrolled rectifier voltage of the bus is greater than the set threshold Q. If it is not greater than the set threshold Q, set the three-phase uncontrolled rectifier voltage to V on the DC bus side, and raise the bus voltage to be greater than the three-phase uncontrolled rectifier voltage by controlling single-phase wave generation.

[0019] In the AC soft start control method of the three-phase four-leg topology described in the present invention; the external relay closing logic executed by the energy storage inverter PCS further includes the following steps:

[0020] f; When the soft start of the bus voltage is completed, close the DC side contactor. After the DC side contactor is closed, switch to the three-phase current control algorithm according to the DC input type, and the energy storage inverter performs charge and discharge operations on the battery according to the set power.

[0021] The AC soft start control method of the three-phase four-leg topology of the present invention controls the bus voltage by adopting a single-phase incoming line mode and gradually raises it to the target value; after the bus voltage is stable, switch to the three-phase incoming line mode to continue controlling the bus voltage output to the set value. First, by gradually raising the bus voltage and the output voltage in stages, the current peak value in the system startup process is significantly reduced, and the inrush current can be reduced by 30% - 50%, thus effectively protecting the power devices and the bus capacitor and avoiding device damage caused by large current impact.

[0022] Secondly, the dynamic neutral point compensation technology is introduced, which significantly improves the output voltage waveform quality, makes the total harmonic distortion rate (THD) lower than 3%, better than the performance of the traditional method which is usually greater than 5%, and improves the power quality and operation stability of the system.

[0023] In addition, the present application has good load mutation adaptation ability, can cope with extreme working conditions such as motor blockage, and at the same time has strong anti-interference ability against grid voltage fluctuations, enhancing the robustness of the overall control. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 is a flowchart of the AC soft start control method for the three-phase four-leg topology of the present invention;

[0025] Figure 2 is a circuit topology structure diagram of the AC soft start control method for the three-phase four-leg topology of the present invention;

[0026] Figure 3 is a schematic diagram of the working principle of the AC soft start control method for the three-phase four-leg topology of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0027] In order to make the objectives, technical solutions and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0028] It should be noted that the terms "first", "second", etc. in the specification and claims of the present invention and the above-mentioned drawings are used to distinguish similar objects and do not necessarily need to describe a specific order or sequence. It should be understood that such data can be interchanged under appropriate circumstances so that the embodiments of the present invention described herein can be implemented in an order other than those illustrated or described herein. In addition, the terms "comprising" and "having" and any variations thereof are intended to cover non-exclusive inclusion. For example, a process, method, system, product or device comprising a series of steps or units does not necessarily have to be limited to those steps or units clearly listed, but may include other steps or units not clearly listed or inherent to these processes, methods, products or devices.

[0029] As Figures 1-3 shown, Figure 1 is a schematic flowchart of an embodiment of the AC soft start control method for a three-phase four-leg topology of the present invention. A three-phase four-leg topology AC soft start control method is provided, and the control method includes the following steps:

[0030] In step S1, obtain the grid access status of the energy storage inverter and identify the access grid status according to the AC incoming line mode, where the AC incoming line mode includes a single-phase incoming line mode and a three-phase incoming line mode;

[0031] In step S2, based on the recognized AC incoming line mode, the energy storage inverter is controlled to execute the external relay closing logic to establish the DC bus voltage and complete the soft start, and the energy storage inverter performs charge and discharge operations on the battery according to the set power.

[0032] In one embodiment, when the AC incoming line mode is the single-phase incoming line mode in step S1, the steps for the energy storage inverter PCS to execute the external relay closing logic are as follows:

[0033] In step a, first judge whether the single-phase voltage is within the normal range value E. If it is not within the normal range value, the single-phase voltage is abnormal. If it is within the normal range value, close the outer C AC relay and the soft start relay, and then close the AC main N relay. Among them, the normal range value E of the single-phase voltage is 230V * 0.85 to 230V * 1.1, and the voltage on the DC bus is 325V.

[0034] In one embodiment, the steps for the energy storage inverter PCS to execute the external relay closing logic further include the following steps:

[0035] In step b, judge whether the bus uncontrolled rectifier voltage is greater than the set threshold Q. If it is not greater than the set threshold Q, further judge whether the soft start time exceeds 60S. If it exceeds, the AC soft start fails. If not, judge again whether the bus uncontrolled rectifier voltage is greater than the set threshold Q. If it is not greater than the set threshold Q, set the single-phase uncontrolled rectifier voltage to V on the DC bus side, and raise the bus voltage to be greater than the single-phase uncontrolled rectifier voltage by controlling the single-phase wave generation. Among them, the set threshold Q of the bus uncontrolled rectifier voltage is 230V * 1.144 - 30V.

[0036] In one embodiment, the steps for the energy storage inverter PCS to execute the external relay closing logic further include the following steps:

[0037] In step c, when the bus voltage completes the soft start, close the DC side contactor. After the DC side contactor is closed, switch to the single-phase current control algorithm according to the DC input type, and the energy storage inverter performs charge and discharge operations on the battery according to the set power.

[0038] In one embodiment, when the AC incoming line mode is the three-phase incoming line mode in step S1, the steps for the energy storage inverter PCS to execute the external relay closing logic are as follows:

[0039] In step d, first judge whether the three-phase voltage is within the normal range value E. If it is not within the normal range value, the three-phase voltage is abnormal. If it is within the normal range value, close the outer C AC relay and the soft start relay, and then close the AC main N relay.

[0040] In one embodiment, the steps for the energy storage inverter PCS to execute the external relay closing logic further include the following:

[0041] In step e, it is judged whether the uncontrolled rectifier voltage of the bus is greater than the set threshold Q. If it is not greater than the set threshold Q, it is further judged whether the soft start time exceeds 60S. If it exceeds, the AC soft start fails. If not, it is judged again whether the uncontrolled rectifier voltage of the bus is greater than the set threshold Q. If it is not greater than the set threshold Q, the three-phase uncontrolled rectifier voltage is set to V on the DC bus side, and the bus voltage is raised to be greater than the three-phase uncontrolled rectifier voltage by controlling single-phase wave generation.

[0042] In one embodiment, the steps for the energy storage inverter PCS to execute the external relay closing logic further include the following:

[0043] In step f, after the soft start of the bus voltage is completed, the DC side contactor is closed. After the closing of the DC side contactor is completed, the three-phase current control algorithm is switched according to the DC input type, and the energy storage inverter charges and discharges the battery according to the set power.

[0044] It should be noted that in the three-phase incoming line mode of AC soft start, the specific operation process is as follows:

[0045] Based on the single-phase incoming line mode of the AC incoming line mode and the operation steps of the external relay closing logic executed by the energy storage inverter PCS, the system starts to control the bus voltage through single-phase incoming line mode wave generation, and raises the bus voltage to a target value higher than the three-phase uncontrolled rectifier voltage; after the bus voltage is established, the following operations are sequentially performed: First, the thyristors SCRs inside the three phases A, B, and C are closed, and then the three-phase internal relays are closed (the SCRs and the relays can send drive signals simultaneously). After 70ms, the thyristors SCRs are disconnected to complete the smooth switching from the soft start stage to the main circuit power supply; then it enters the three-phase incoming line mode combined control stage, and the bus voltage is precisely regulated by three-phase simultaneous wave generation to make it stable at the set target value; when the bus voltage reaches and is maintained within the target voltage range, the DC side contactor is closed; at this time, the energy storage converter (PCS) can complete the startup process and be put into operation as a stable DC voltage source to provide support for subsequent battery charging and discharging or other DC loads.

[0046] Specifically, in this application, the bus voltage is controlled by adopting the single-phase incoming line mode and gradually raised to the target value; after the bus voltage is stable, it is switched to the three-phase incoming line mode to continue controlling the bus voltage output to the set value. First, by gradually increasing the bus voltage and the output voltage in stages, the current peak value in the system startup process is significantly reduced, and the inrush current can be reduced by 30% - 50%, thereby effectively protecting the power devices and the bus capacitors and avoiding device damage caused by large current impact.

[0047] Secondly, the dynamic neutral point compensation technology is introduced, significantly improving the quality of the output voltage waveform, making the total harmonic distortion rate (THD) less than 3%, which is better than the performance of traditional methods that is usually greater than 5%, thus enhancing the power quality and operation stability of the system.

[0048] In addition, this application has good adaptability to sudden load changes, can handle extreme working conditions such as motor stall, and at the same time has strong anti-interference ability against grid voltage fluctuations, enhancing the robustness of the overall control.

[0049] At the hardware level, through the application of the soft start control strategy, the impact current is effectively resisted, avoiding the problem of contact damage caused by large current when the relay closes, thereby improving the reliability of equipment operation and extending the service life. At the same time, this solution also saves the number of AC soft start switches, not only solving the technical problem of large impact current at the moment of soft start switch closing, but also realizing the reduction of system cost and the decrease of inverter volume, improving the competitiveness of the product in the market.

[0050] When the inverter operates in the single-phase input mode, when the battery energy is depleted, the inverter can be started from the single-phase of the AC side to charge the battery, and the single-phase power can reach 41Kw; thus, the battery can be supplemented with energy.

[0051] When the inverter operates in the three-phase input mode, it operates in the DC source mode, can provide a constant voltage support of 600V to 1000V, and can be applied to application fields such as substation interconnection, photovoltaic energy storage charging, and AC / DC microgrid.

[0052] It should be noted that for the foregoing method embodiments, for the sake of simple description, they are all expressed as a series of action combinations. However, those skilled in the art should know that the present invention is not limited by the described action sequence, because according to the present invention, certain steps can be performed in other sequences or simultaneously. Secondly, those skilled in the art should also know that the embodiments described in the specification are all preferred embodiments, and the actions and modules involved are not necessarily essential to the present invention.

[0053] Through the description of the above embodiments, those skilled in the art can clearly understand that the method according to the above embodiments can be implemented by means of software plus a necessary general hardware platform. Of course, it can also be implemented by hardware, but in many cases, the former is a better implementation method. Based on such an understanding, the technical solution of the present invention, in essence, or the part that contributes to the prior art, can be embodied in the form of a software product. This computer software product is stored in a storage medium (such as ROM / RAM, magnetic disk, optical disk), and includes several instructions to enable a terminal device (which can be a mobile phone, computer, server, or network device, etc.) to execute the methods described in various embodiments of the present invention.

[0054] Therefore, as described above, only the preferred specific embodiments of the present invention are provided, and the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed by the present invention should be covered within the protection scope of the present invention. The protection scope of the present invention should be subject to the protection scope of the claims.

Claims

1. An AC soft start control method for a three-phase four-leg topology, characterized in that The method includes the following steps: S1; Obtain the grid connection status of the energy storage inverter, and identify the grid connection status according to the AC incoming line mode, where the AC incoming line mode includes a single-phase incoming line mode and a three-phase incoming line mode; S2; Based on the identified AC incoming line mode, control the energy storage inverter to execute the external relay closing logic to establish the DC bus voltage and complete the soft start, and the energy storage inverter performs charge and discharge operations on the battery according to the set power.

2. The AC soft start control method for a three-phase four-leg topology according to claim 1, characterized in that In step S1, when the AC incoming line mode is the single-phase incoming line mode, the external relay closing logic executed by the energy storage inverter PCS includes the following steps: a; First, determine whether the single-phase voltage is within the normal range value E. If it is not within the normal range value, the single-phase voltage is abnormal. If it is within the normal range value, close the outer C AC relay and the soft start relay, and then close the AC main N relay.

3. The AC soft start control method for a three-phase four-leg topology according to claim 2, characterized in that The external relay closing logic executed by the energy storage inverter PCS further includes the following steps: b; Determine whether the rectifier voltage without bus control is greater than the set threshold Q. If it is not greater than the set threshold Q, further determine whether the soft start time exceeds 60S. If it exceeds, the AC soft start fails. If not, determine again whether the rectifier voltage without bus control is greater than the set threshold Q. If it is not greater than the set threshold Q, set the single-phase uncontrolled rectifier voltage to V on the DC bus side, and raise the bus voltage to be greater than the single-phase uncontrolled rectifier voltage by controlling the single-phase wave generation.

4. The AC soft start control method for a three-phase four-leg topology according to claim 2, characterized in that, The external relay closing logic executed by the energy storage inverter PCS further includes the following steps: c; When the bus voltage completes the soft start, close the DC side contactor. After the DC side contactor is closed, switch to the single-phase current control algorithm according to the DC input type, and the energy storage inverter performs charge and discharge operations on the battery according to the set power.

5. The AC soft start control method for a three-phase four-leg topology according to claim 1, characterized in that, In step S1, when the AC incoming line mode is the three-phase incoming line mode, the external relay closing logic executed by the energy storage inverter PCS includes the following steps: d; First, determine whether the three-phase voltage is within the normal range value E. If it is not within the normal range value, the three-phase voltage is abnormal. If it is within the normal range value, close the outer C AC relay and the soft start relay, and then close the AC main N relay.

6. The AC soft start control method for a three-phase four-leg topology according to claim 5, characterized in that, The external relay closing logic executed by the energy storage inverter PCS further includes the following steps: e; Determine whether the rectifier voltage without bus control is greater than the set threshold Q. If it is not greater than the set threshold Q, further determine whether the soft start time exceeds 60S. If it exceeds, the AC soft start fails. If not, determine again whether the rectifier voltage without bus control is greater than the set threshold Q. If it is not greater than the set threshold Q, set the three-phase uncontrolled rectifier voltage to V on the DC bus side, and raise the bus voltage to be greater than the three-phase uncontrolled rectifier voltage by controlling the single-phase wave generation.

7. The AC soft start control method for the three-phase four-leg topology according to claim 6, characterized in that, The external relay closing logic executed by the energy storage inverter PCS further includes the following steps: f; When the bus voltage completes the soft start, close the DC side contactor. After the DC side contactor is closed, switch to the three-phase current control algorithm according to the DC input type, and the energy storage inverter performs charge and discharge operations on the battery according to the set power.

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