Power converter and method of controlling a power converter
By flexibly switching between voltage imbalance and DC-side power fluctuation suppression control within the state of charge range of the energy storage battery, the problem of overcurrent in the energy storage battery under grid voltage imbalance is solved, and stable power supply of the photovoltaic-energy storage system is achieved.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- HUAWEI TECH CO LTD
- Filing Date
- 2026-03-31
- Publication Date
- 2026-07-24
Smart Images
Figure CN122456840A_ABST
Abstract
Description
Technical Field
[0001] This application relates to the field of power electronics, and more particularly to a power converter and a control method for the power converter. Background Technology
[0002] With the continuous expansion of new energy grid connection and the increasing complexity of distribution network load structures, factors such as unbalanced loads, unbalanced line parameters, and single-phase distributed power source access have made grid voltage imbalance a more prominent issue. In grid voltage imbalance scenarios, controlling the inverters or converters connected to the grid in the energy storage system to actively adjust the positive and negative sequence components of the three-phase voltage can improve the voltage imbalance at the point of common coupling and enhance power quality. However, since energy storage systems provide energy through DC-side batteries or supercapacitors, while suppressing voltage imbalance, the fluctuating power borne by the inverters or converters is fed back to the DC side, causing fluctuations in DC-side power and current. Excessive power fluctuations can lead to battery overcurrent, battery state of charge exceeding limits, and even triggering protection shutdowns, severely impacting the system's continuous power supply capability and operational safety. Summary of the Invention
[0003] This application provides a power converter and a control method for the power converter, which can prevent overcurrent in the energy storage battery during the suppression of negative sequence voltage in the power grid and improve the reliability of power supply in the photovoltaic-energy storage system.
[0004] In a first aspect, this application provides a power converter. The input terminal of the power converter is connected to an energy storage battery, and the power converter converts direct current (DC) from the energy storage battery into alternating current (AC). The power converter is further configured to reduce the negative-sequence active power at its output terminal when the state of charge (SOC) of the energy storage battery is greater than a first SOC threshold and less than a second SOC threshold. The power converter is also configured to reduce the amplitude of the AC component of the active power at its output terminal when the SOC of the energy storage battery is less than a third SOC threshold or greater than a fourth SOC threshold, wherein the first SOC threshold is less than the second SOC threshold, the third SOC threshold is less than or equal to the first SOC threshold, and the fourth SOC threshold is greater than or equal to the second SOC threshold.
[0005] In this application, while suppressing negative-sequence voltage, the power converter feeds back the fluctuating power it handles to the DC side, causing fluctuations in DC power and current. When the energy storage battery connected to the power converter is in the final stage of charging / discharging, its internal material structure and chemical potential are on the verge of qualitative change. In other words, compared to when the energy storage battery is in the middle stage of normal operation, it is more sensitive to power fluctuations. Even slightly large power and current fluctuations can trigger overcurrent protection or cause irreversible damage to the battery. Therefore, when the energy storage battery's state of charge (SOC) is less than the third SOC threshold or greater than the fourth SOC threshold, the power converter prioritizes DC-side power fluctuation suppression, i.e., reducing the amplitude of the AC component of the active power at the power converter output to prevent power and current fluctuations from damaging the energy storage battery. By simultaneously considering grid voltage quality and safe operation of the energy storage side within different SOC ranges, overcurrent damage to the energy storage battery caused by continuous negative-sequence voltage suppression in the final stage of charging / discharging is avoided, thus improving the reliability of the photovoltaic-energy storage system's power supply.
[0006] In one possible implementation, when a voltage imbalance occurs in the power grid and the energy storage battery is in the middle range of normal operation, the negative sequence voltage at the control output of the power converter is reduced, and the negative sequence active current at the control output is less than or equal to the product of the rated current and the proportional threshold, thereby reducing the negative sequence active power at the output and achieving grid voltage imbalance suppression.
[0007] In one possible implementation, when a voltage imbalance occurs in the power grid and the energy storage battery is in the middle range of normal operation, the phase of the negative-sequence voltage at the power converter control output leads the phase of the negative-sequence current by 90 degrees. Here, since the negative-sequence active current is the component of the negative-sequence current parallel to the negative-sequence voltage, when the phase of the negative-sequence voltage at the control output leads the phase of the negative-sequence current by 90 degrees, the component of the negative-sequence current parallel to the negative-sequence voltage becomes zero, and the negative-sequence active current becomes zero. This reduces the negative-sequence active power at the output to zero, resulting in good suppression of negative-sequence active power and achieving grid voltage imbalance suppression.
[0008] In one possible implementation, when a voltage imbalance occurs in the power grid and the energy storage battery is in the final charging / discharging range, the power converter controls the negative-sequence current at the output terminal. This causes the AC component generated by the combination of the negative-sequence current and the positive-sequence voltage at the output terminal to be in the opposite direction to the AC component generated by the combination of the positive-sequence current and the negative-sequence voltage at the output terminal. This cancels out the generated AC component, thereby reducing the amplitude of the AC component of the active power at the output terminal of the power converter to suppress DC-side power fluctuations, prevent overcurrent damage to the energy storage battery in the final charging / discharging range, and improve the reliability of the power supply of the photovoltaic-energy storage system.
[0009] In one possible implementation, the first state-of-charge threshold is less than or equal to 20%, the second state-of-charge threshold is greater than or equal to 80%, the difference between the third state-of-charge threshold and the first state-of-charge threshold is greater than 5% and less than 10%, and the difference between the fourth state-of-charge threshold and the second state-of-charge threshold is greater than 5% and less than 10%.
[0010] In one possible implementation, the power converter is used to convert the DC power from the energy storage battery into AC power and output it to the power grid. The power converter is used to control the negative sequence active power at the output terminal to decrease when the state of charge of the energy storage battery is greater than a first state of charge threshold and less than a second state of charge threshold and the power grid experiences voltage imbalance. The voltage imbalance in the power grid includes the occurrence of an asymmetrical fault in the power grid or the connection of an asymmetrical load to the power grid.
[0011] Secondly, this application provides a control method for a power converter, the input of which is connected to an energy storage battery. The method includes controlling the negative-sequence active power at the output of the power converter to decrease when the state of charge (SOC) of the energy storage battery is greater than a first SOC threshold and less than a second SOC threshold. When the SOC of the energy storage battery is less than a third SOC threshold or greater than a fourth SOC threshold, the AC component amplitude of the active power at the output is controlled to decrease. The first SOC threshold is less than the second SOC threshold, the third SOC threshold is less than or equal to the first SOC threshold, and the fourth SOC threshold is greater than or equal to the second SOC threshold.
[0012] In this application, when the state of charge (SOC) of the energy storage battery is less than the third SOC threshold or greater than the fourth SOC threshold, the power converter prioritizes DC-side power fluctuation suppression. This means reducing the amplitude of the AC component of the active power at the power converter output to prevent power and current fluctuations from damaging the energy storage battery. By simultaneously considering both grid-side voltage quality and safe operation of the energy storage side within different SOC ranges, overcurrent damage to the energy storage battery caused by continuous negative sequence voltage suppression during the final stages of charging and discharging is avoided, thus improving the reliability of the photovoltaic-energy storage system's power supply.
[0013] In one possible implementation, the negative-sequence active power at the output of the power converter is reduced by controlling the negative-sequence voltage at the output and controlling the negative-sequence active current at the output to be less than or equal to the product of the rated current and the proportional threshold, thereby reducing the negative-sequence active power at the output and achieving grid voltage imbalance suppression.
[0014] In one possible implementation, reducing the negative-sequence active power at the output of the power converter involves controlling the phase of the negative-sequence voltage at the output to lead the phase of the negative-sequence current by 90 degrees. Here, since the negative-sequence active current is the component of the negative-sequence current parallel to the negative-sequence voltage, when the phase of the negative-sequence voltage at the output leads the phase of the negative-sequence current by 90 degrees, the component of the negative-sequence current parallel to the negative-sequence voltage becomes zero, and the negative-sequence active current becomes zero. This results in a reduction of the negative-sequence active power at the output to zero, effectively suppressing negative-sequence active power and achieving grid voltage imbalance suppression.
[0015] In one possible implementation, the amplitude of the AC component of the active power at the output terminal is reduced, including the negative sequence current at the output terminal. This causes the AC component generated by the combination of the negative sequence current and the positive sequence voltage at the output terminal to be in the opposite direction to the AC component generated by the combination of the positive sequence current and the negative sequence voltage at the output terminal, thereby canceling out the generated AC components. This reduces the amplitude of the AC component of the active power at the output terminal of the power converter to suppress DC-side power fluctuations, prevent overcurrent damage to the energy storage battery at the end of the charge and discharge cycle, and improve the reliability of the power supply of the photovoltaic-energy storage system.
[0016] In one possible implementation, the first state-of-charge threshold is less than or equal to 20%, the second state-of-charge threshold is greater than or equal to 80%, the difference between the third state-of-charge threshold and the first state-of-charge threshold is greater than 5% and less than 10%, and the difference between the fourth state-of-charge threshold and the second state-of-charge threshold is greater than 5% and less than 10%. Attached Figure Description
[0017] Figure 1 This is a schematic diagram of the framework structure of a photovoltaic energy storage system; Figure 2 This is a schematic diagram of the grid imbalance control of the power converter provided in this application; Figure 3 This is a schematic diagram of virtual synchronous generator control; Figure 4 This is a schematic diagram of current loop control; Figure 5 This is a flowchart illustrating the control method for the power converter provided in this application. Detailed Implementation
[0018] The power converter provided in this application is applicable to energy storage power supply scenarios, photovoltaic-storage hybrid power supply scenarios, UPS power supply scenarios, and electric vehicle battery swapping station scenarios. The following explanation uses a photovoltaic-storage hybrid power supply scenario as an example. (See also...) Figure 1 , Figure 1 This is a schematic diagram of the framework structure of a photovoltaic energy storage system, such as... Figure 1As shown, the photovoltaic-energy storage system 100 includes a photovoltaic system and an energy storage system. In the photovoltaic system, photovoltaic modules 10 directly convert solar energy into electrical energy using the photovoltaic effect. The photovoltaic modules 10 typically include multiple cells connected in series or parallel to achieve a certain output power. A photovoltaic inverter 301 converts the direct current (DC) from the photovoltaic modules 10 into alternating current (AC) and transmits the AC to the grid 40 or other loads. In the energy storage system, the energy storage battery 20 stores unstable electrical energy and converts DC to AC through a power conversion system (PCS) 302 to supply stable electrical energy to the grid 40 or other loads. Furthermore, the power conversion system 302 can also convert AC from the grid 40 into DC to charge the energy storage battery 20, and then store the electrical energy within the energy storage battery 20. The power converter provided in this application includes the aforementioned photovoltaic inverter 301 and power conversion system 302.
[0019] The above Figure 1 In the photovoltaic-storage system shown, the power converter can be a grid-connected energy storage converter. This type of converter, by simulating the operating characteristics of a traditional synchronous generator, actively provides voltage and frequency support to the grid, enabling it to independently provide stable power to the load even when the grid is absent or unstable. Essentially, the grid-connected energy storage converter is a voltage source, capable of internally setting voltage parameters to output stable voltage and frequency. Figure 1 In the photovoltaic-storage system shown, voltage imbalance occurs on the grid side due to factors such as asymmetrical grid faults, asymmetrical loads, asymmetrical line parameters, and single-phase distributed power source access, resulting in a certain proportion of negative sequence voltage from the grid. Under grid voltage imbalance scenarios, the power converter, due to its voltage source characteristics, can actively adjust the positive and negative sequence components of its output three-phase voltage through control strategies to suppress negative sequence voltage, thereby improving the voltage imbalance at the power converter output and enhancing power quality. However, since the photovoltaic-storage system provides energy through DC-side batteries or supercapacitors, the fluctuating power handled by the power converter while suppressing voltage imbalance will be fed back to the DC side, causing fluctuations in DC-side power and current. Specifically, if the power converter always prioritizes suppressing negative sequence voltage, it will generate significant negative sequence current and dual-frequency power oscillations (typically twice the power frequency component) on the converter side. These power oscillations will couple to the energy storage battery through the DC side of the power converter, causing periodic fluctuations in the DC-side power and current of the energy storage battery. Excessive power and current fluctuations can damage energy storage batteries, especially when the battery is nearing full charge or depletion. These fluctuations can easily cause the instantaneous current flowing through the battery to exceed the maximum protection current range, triggering overcurrent protection or accelerating battery aging. Severe overcurrent in the energy storage battery can also trigger the power converter's protection shutdown, seriously affecting the continuous power supply capability and operational safety of the photovoltaic-energy storage system.
[0020] To prevent overcurrent in the energy storage battery during the suppression of negative-sequence voltage from the grid when voltage imbalance occurs in the power converter, thereby improving the reliability of the power supply of the photovoltaic-energy storage system, the power converter provided in this application includes a controller that can acquire the state of charge (SOC) of the energy storage battery connected to the power converter. When the controller detects that the energy storage battery is in the middle range of normal operation, it prioritizes voltage imbalance suppression, that is, the controller controls the power converter to operate with the primary objective of suppressing negative-sequence voltage from the grid. Specifically, when the controller acquires that the SOC of the energy storage battery is greater than a first SOC threshold and less than a second SOC threshold, it controls the negative-sequence active power at the output of the power converter to decrease in order to suppress negative-sequence voltage from the grid. The first SOC threshold is less than the second SOC threshold; for example, the first SOC threshold is SOC_min + ΔSOC, and the second SOC threshold is SOC_max - ΔSOC, where SOC_min and SOC_max are the minimum and maximum SOC of the energy storage battery, respectively. ΔSOC is a preset safety margin, which can be taken as 5% to 10%. The middle range of normal operation for an energy storage battery can be represented as [SOC_min + ΔSOC, SOC_max - ΔSOC]. When the controller detects that the energy storage battery is in the final stage of charging / discharging, the internal material structure and chemical potential energy of the battery are on the verge of a qualitative change. In other words, compared to the middle range of normal operation, the energy storage battery is more sensitive to power fluctuations at this time. Even slightly large power or current fluctuations can trigger overcurrent protection or cause irreversible damage to the battery. Therefore, when the controller detects that the state of charge (SOC) of the energy storage battery is less than the third SOC threshold or greater than the fourth SOC threshold, the controller prioritizes DC-side power fluctuation suppression, that is, reducing the amplitude of the AC component of the active power at the output of the power converter to avoid damage to the energy storage battery caused by power and current fluctuations. Where the third state-of-charge (SOC) threshold is less than or equal to the first SOC threshold, and the fourth SOC threshold is greater than or equal to the second SOC threshold, the energy storage battery being in the final charging phase can be represented as (SOC_max - ΔSOC, SOC_max], and the energy storage battery being in the final discharging phase can be represented as [SOC_min, SOC_min + ΔSOC]. Here, the controller flexibly selects the preferred control mode based on the SOC magnitude of the energy storage battery, and adaptively switches between voltage imbalance suppression and DC-side power fluctuation suppression control objectives based on the SOC of the energy storage battery. By simultaneously considering the grid-side voltage quality and the safe operation of the energy storage side within different SOC ranges, the overcurrent damage to the energy storage battery caused by continuous negative sequence voltage suppression when the energy storage battery is in the final charging / discharging phase is avoided, thus improving the reliability of the photovoltaic-energy storage system's power supply.
[0021] In some feasible implementations, if a voltage imbalance occurs in the power grid, the power converter output receives a negative-sequence voltage from the grid. When the controller detects that the energy storage battery is in the middle range of normal operation, it controls the negative-sequence voltage at the output to decrease, and controls the negative-sequence active current at the output to be less than or equal to the product of the rated current and a proportional threshold. For example, by controlling the decrease of the negative-sequence voltage at the output and controlling the negative-sequence active current at the output to be less than or equal to 5% of the rated current, the negative-sequence active power at the output is reduced, thus suppressing the grid voltage imbalance.
[0022] In some feasible implementations, when a voltage imbalance occurs in the power grid, the controller detects that the energy storage battery is in the middle range of normal operation and controls the phase of the negative-sequence voltage at the output of the power converter to lead the phase of the negative-sequence current by 90 degrees. Here, since the negative-sequence active current is the component of the negative-sequence current parallel to the negative-sequence voltage, when the phase of the negative-sequence voltage at the control output leads the phase of the negative-sequence current by 90 degrees, the component of the negative-sequence current parallel to the negative-sequence voltage becomes 0, and the negative-sequence active current becomes 0. This reduces the negative-sequence active power at the output to 0, resulting in good suppression of negative-sequence active power and achieving grid voltage imbalance suppression.
[0023] In some feasible implementations, the power converter can employ a control mode with an outer voltage loop and an inner current loop, see [link to relevant documentation]. Figure 2 , Figure 2 This is a schematic diagram of the grid imbalance control of the power converter provided in this application. For example... Figure 2 As shown, the controller acquires the grid voltage and current, which can be obtained by acquiring the voltage and current at the grid connection point connected to the output terminal of the power converter. Next, the controller uses a dual-synchronous coordinate system positive and negative sequence decomposition method to decompose the grid voltage and current, obtaining positive and negative sequence voltage and current. Then, the positive and negative sequence power is calculated using instantaneous power theory.
[0024] On the one hand, the voltage loop is combined with the voltage amplitude reference. and positive sequence voltage Positive sequence power After calculation using algorithms such as PID, the positive-sequence reference current i is output. +re To the current loop. Specifically, the above voltage amplitude references... This is obtained through a Virtual Synchronous Generator (VSG) control strategy. See also... Figure 3 , Figure 3 This is a schematic diagram of virtual synchronous generator control. (For example...) Figure 3 As shown, the virtual synchronous generator control consists of two parts: virtual speed regulation and virtual excitation. The virtual speed regulation part includes the rated angular frequency... and grid connection point angular frequency The difference, after the active frequency modulation coefficient The product of these two factors yields the active power reference regulation. Active power reference adjustment Combined with active power reference value Actual output active power A common input power synchronization control module. This module simulates the swing equations of a synchronous generator; its output is combined with the rated angular frequency. Obtain the internal potential angular frequency Internal potential angular frequency The internal potential phase θ is obtained through integration (1 / s corresponds to integration in the frequency domain). The rated voltage amplitude in the virtual excitation section... and grid connection point voltage amplitude The difference, after passing through the reactive power voltage regulation coefficient The product of these two values yields the reactive power reference regulation. reactive power reference adjustment Combined with reactive power reference value Actual output reactive power Common input voltage control module. The output of the voltage control module is combined with the rated voltage amplitude. Obtain voltage amplitude reference .
[0025] On the other hand, the negative sequence voltage suppression module is based on negative sequence voltage. A negative sequence reference current is obtained. The DC fluctuation suppression module is based on negative sequence voltage. Positive sequence voltage and positive sequence reference current i +re Another negative sequence reference current is obtained. The controller acquires the State of Charge (SOC) of the energy storage battery and uses the SOC to select a negative-sequence reference current. When the SOC is greater than the first SOC threshold and less than the second SOC threshold, the negative-sequence reference current is selected and output as a negative-sequence reference current. When the State of Charge (SOC) is greater than the first state of charge threshold and less than the third state of charge threshold, or greater than the fourth state of charge threshold, the negative sequence reference current is selected to output the negative sequence reference current i. -reb Finally, the current loop is based on the positive-sequence reference current i. +re Negative sequence reference current selection: output negative sequence reference current i -re (i -re It can be i -rea Or i -reb The output current of the power converter generates a modulated wave signal. For details, see [link to relevant documentation]. Figure 4 , Figure 4 This is a schematic diagram of current loop control. (For example...) Figure 4 As shown, the current loop input positive sequence reference current i +re and negative sequence reference current i -re The positive sequence reference current i +re Decomposed into positive-sequence active reference current and positive sequence reactive reference current i +req Negative sequence reference current Decomposed into negative sequence active reference current and negative sequence reactive reference current i -req Positive-sequence active reference current i +red With positive sequence active current The difference, positive sequence reactive reference current With positive sequence reactive current The difference is fed into the PI controller. Positive sequence reactive current. and positive sequence active current The positive-sequence active power reference voltage is obtained by taking the inductive reactance X1 of the inverter filter inductor and the difference between it and the output of the PI controller. and positive sequence reactive power reference voltage u +req Negative-sequence active reference current With negative sequence active current The difference, negative sequence reactive reference current i -req With negative sequence reactive current The difference is fed into the PI controller. Negative sequence reactive current. and negative sequence active current The negative-sequence active power reference voltage U is obtained by taking the inductive reactance X1 of the inverter filter inductor and the difference between it and the output of the PI controller. -red and negative sequence reactive power reference voltage Positive-sequence active power reference voltage and positive sequence reactive power reference voltage Generates a positive-sequence modulated wave in dq coordinates, and a negative-sequence active power reference voltage U. -red and negative sequence reactive power reference voltage Generate a negative-order modulated wave in dq coordinates, and combine the modulated wave in dq coordinates with the above. Figure 3 The phase θ of the internal potential generated by the virtual speed regulation part is converted to form the final modulation wave signal under the abc three-phase coordinate system.
[0026] Understandably, when the current loop is based on a negative-sequence reference current... A modulation wave signal is generated. The controller then drives the switching transistors in the power converter based on this signal, reducing the negative-sequence active power at the power converter output to suppress negative-sequence voltage from the grid. When the current loop is based on the negative-sequence reference current i... -rebA modulation wave signal is generated, and the controller adjusts the switching on / off state of the power converter according to the modulation wave signal. After the switching transistors are driven and controlled, the amplitude of the AC component of the active power at the output of the power converter is reduced to suppress DC-side power fluctuations. Here, the controller drives and controls the switching transistors in the power converter according to the modulation wave signal, achieving adaptive switching control of voltage imbalance suppression and DC-side power fluctuation suppression based on the state of charge of the energy storage battery. This avoids overcurrent damage to the energy storage battery caused by continuous negative sequence voltage suppression when the battery is in the end of its charge / discharge range, thus improving the reliability of the power supply of the photovoltaic-energy storage system.
[0027] In some feasible implementations, when the SOC of the energy storage battery is greater than a first state-of-charge threshold and less than a second state-of-charge threshold, the negative-sequence reference current selects and outputs a negative-sequence reference current i. -rea Negative sequence reference current i -rea The expression is:
[0028] in, and These are the negative-sequence active reference current and the negative-sequence reactive reference current at the output terminal, respectively. and These are the negative-sequence active voltage and negative-sequence reactive voltage at the output terminals, respectively. and These represent the virtual resistance and virtual reactance of the power converter, respectively. The value is 0 or less than the set threshold. Negative sequence reference current selection output negative sequence reference current. Then, the current loop is based on the positive-sequence reference current i +re Negative sequence reference current i -rea A modulation wave signal is generated, and the controller drives the switching transistors in the power converter according to the modulation wave signal, so that the negative sequence current at the output terminal is equal to the aforementioned negative sequence reference current. The expression for the negative sequence current at the output terminal is:
[0029] in, and These are the negative-sequence active current and negative-sequence reactive current at the output terminal, respectively. The output terminal is controlled according to the above expressions. The negative sequence current causes the phase of the negative sequence voltage at the output terminal to lead the phase of the negative sequence current by 90 degrees. Since the negative sequence active current is the component of the negative sequence current that is parallel to the negative sequence voltage, when the phase of the negative sequence voltage at the control terminal leads the phase of the negative sequence current by 90 degrees, making the component of the negative sequence current that is parallel to the negative sequence voltage zero, the negative sequence active current becomes zero, thereby reducing the negative sequence active power at the output terminal to zero, thus achieving grid voltage imbalance suppression.
[0030] In some feasible implementations, when the SOC of the energy storage battery is less than the third state of charge threshold or greater than the fourth state of charge threshold, the negative sequence reference current is selected to output a negative sequence reference current. Sequence reference current The expression is:
[0031] in, , These are the negative-sequence active reference current and the negative-sequence reactive reference current at the output terminal, respectively. and These are the negative-sequence active voltage and negative-sequence reactive voltage at the output terminals, respectively. and These are the positive-sequence active voltage and positive-sequence reactive voltage at the output terminals, i +dre and i +qre These are the positive-sequence active reference current and the positive-sequence reactive reference current at the output terminal, respectively. The above i +dre and i +qre To pass the positive sequence reference current i output by the voltage loop +re It is obtained through positive and negative order decomposition.
[0032] Negative sequence reference current selection output negative sequence reference current Then, the current loop is based on the positive-sequence reference current i +re Negative sequence reference current i -reb A modulation wave signal is generated, and the controller drives the switching transistors in the power converter according to the modulation wave signal, so that the negative sequence current at the output terminal is equal to the aforementioned negative sequence reference current i. -reb The expression for the negative sequence current at the output terminal is:
[0033] By controlling the negative-sequence current at the output terminal according to the above expression, the AC component generated by the combination of the negative-sequence current and the positive-sequence voltage at the output terminal is opposite in direction to the AC component generated by the combination of the positive-sequence current and the negative-sequence voltage, thereby canceling out the generated AC component. This reduces the amplitude of the AC component of the active power at the output terminal of the power converter to suppress DC-side power fluctuations, avoid overcurrent damage to the energy storage battery at the end of the charge and discharge range, and improve the reliability of the power supply of the photovoltaic-energy storage system.
[0034] In some feasible implementations, the first state-of-charge threshold is less than or equal to 20%, and the second state-of-charge threshold is greater than or equal to 80%. The difference between the third state-of-charge threshold and the first state-of-charge threshold is greater than 5% and less than 10%, and the difference between the fourth state-of-charge threshold and the second state-of-charge threshold is greater than 5% and less than 10%. For example, the first state-of-charge threshold can be 20%, and the second state-of-charge threshold can be 80%. If the third state-of-charge threshold is less than the first state-of-charge threshold and the difference is 5%, and the fourth state-of-charge threshold is greater than the second state-of-charge threshold and the difference is 5%, then the third state-of-charge threshold is 15%, and the fourth state-of-charge threshold is 85%. Therefore, when the controller detects that the state of charge (SOC) of the energy storage battery is greater than 20% and less than 80%, it determines that the battery is in the middle range of normal operation and reduces the negative-sequence active power at the output of the power converter to suppress negative-sequence voltage from the grid. When the controller detects that the SOC is less than 15% or greater than 85%, indicating that the battery is in the end range of charging and discharging, it prioritizes DC-side power fluctuation suppression, i.e., reducing the amplitude of the AC component of the active power at the output of the power converter to avoid damage to the battery caused by power and current fluctuations. The controller adaptively switches between voltage imbalance suppression and DC-side power fluctuation suppression control targets based on the SOC of the energy storage battery, avoiding overcurrent damage to the battery caused by continuous negative-sequence voltage suppression when the battery is in the end range of charging and discharging, thus improving the reliability of the photovoltaic-energy storage system's power supply.
[0035] See Figure 5 , Figure 5 This is a flowchart illustrating the control method for the power converter provided in this application. The control method for the power converter provided in this application is applicable to the above-mentioned... Figures 1 to 2 Any of the power converters shown. For example... Figure 5 As shown, the control method for the power converter provided in this application includes the following steps: S101, obtain the state of charge of the energy storage battery.
[0036] S102, determine whether the state of charge of the energy storage battery is greater than the first state of charge threshold and less than the second state of charge threshold. If the determination result is yes, then proceed to step S103. If the determination result is no, then proceed to step S104.
[0037] In some feasible implementations, when the state of charge (SOC) of the energy storage battery is greater than a first SOC threshold but less than a second SOC threshold, the battery is in the middle range of normal operation. When the SOC is less than a third SOC threshold or greater than a fourth SOC threshold, the battery is in the end range of charge / discharge. At this point, the internal material structure and chemical potential energy of the battery are on the verge of qualitative change, making it prone to overcurrent. Therefore, by flexibly selecting the preferred control mode based on the SOC of the energy storage battery and adaptively switching the control targets of voltage imbalance suppression and DC-side power fluctuation suppression based on the SOC, the goal of simultaneously considering grid-side voltage quality and safe operation of the energy storage side can be achieved within different SOC ranges. This avoids overcurrent damage to the energy storage battery caused by continuous negative sequence voltage suppression when it is in the end range of charge / discharge, thus improving the reliability of the photovoltaic-energy storage system's power supply.
[0038] In some feasible implementations, the first state-of-charge threshold is less than or equal to 20%, and the second state-of-charge threshold is greater than or equal to 80%. The difference between the third state-of-charge threshold and the first state-of-charge threshold is greater than 5% and less than 10%, and the difference between the fourth state-of-charge threshold and the second state-of-charge threshold is greater than 5% and less than 10%. For example, the first state-of-charge threshold can be 20%, and the second state-of-charge threshold can be 80%. If the third state-of-charge threshold is less than the first state-of-charge threshold and the difference is 5%, and the fourth state-of-charge threshold is greater than the second state-of-charge threshold and the difference is 5%, then the third state-of-charge threshold is 15%, and the fourth state-of-charge threshold is 85%.
[0039] S103 controls the reduction of negative sequence active power at the output of the power converter.
[0040] In some feasible implementations, when the state of charge (SOC) of the energy storage battery is greater than a first SOC threshold and less than a second SOC threshold, the negative-sequence voltage at the control output is reduced, and the negative-sequence active current at the control output is reduced to or equal to the product of the rated current and a proportional threshold. For example, by reducing the negative-sequence voltage at the control output and reducing the negative-sequence active current at the control output to or equal to 5% of the rated current, the negative-sequence active power at the control output is reduced, thereby suppressing grid voltage imbalance.
[0041] In some feasible implementations, when the state of charge of the energy storage battery is greater than a first state of charge threshold and less than a second state of charge threshold, the expression for the negative sequence current at the control output is:
[0042] in, and These are the negative-sequence active current and negative-sequence reactive current at the output terminal, respectively. and These are the negative-sequence active voltage and negative-sequence reactive voltage at the output terminals, respectively. and These represent the virtual resistance and virtual reactance of the power converter, respectively. The value is 0 or less than a set threshold. By controlling the negative sequence current at the output terminal according to the above expression, the phase of the negative sequence voltage at the output terminal leads the phase of the negative sequence current by 90 degrees. Since the negative sequence active current is the component of the negative sequence current that is parallel to the negative sequence voltage, when the phase of the negative sequence voltage at the output terminal is controlled to lead the phase of the negative sequence current by 90 degrees, making the component of the negative sequence current that is parallel to the negative sequence voltage zero, the negative sequence active current is zero, thereby reducing the negative sequence active power at the output terminal to zero, thus achieving grid voltage imbalance suppression.
[0043] S104 controls the reduction of the AC component amplitude of the active power at the control output.
[0044] In some feasible implementations, when the state of charge of the energy storage battery is less than the third state of charge threshold or greater than the fourth state of charge threshold, the expression for the negative sequence current at the control output is:
[0045] By controlling the negative-sequence current at the output terminal according to the above expression, the AC component generated by the combination of the negative-sequence current and the positive-sequence voltage at the output terminal is opposite in direction to the AC component generated by the combination of the positive-sequence current and the negative-sequence voltage, thereby canceling out the generated AC component. This reduces the amplitude of the AC component of the active power at the output terminal of the power converter to suppress DC-side power fluctuations, avoid overcurrent damage to the energy storage battery at the end of the charge and discharge range, and improve the reliability of the power supply of the photovoltaic-energy storage system.
Claims
1. A power converter, characterized in that, The input terminal of the power converter is used to connect to the energy storage battery, and the power converter is used to convert the DC power from the energy storage battery into AC power. The power converter is also used to control the negative sequence active power at the output terminal to decrease when the state of charge of the energy storage battery is greater than a first state of charge threshold and less than a second state of charge threshold. The power converter is further configured to, when the state of charge of the energy storage battery is less than a third state of charge threshold or greater than a fourth state of charge threshold, control the amplitude of the AC component of the active power at the output terminal to decrease, wherein the first state of charge threshold is less than the second state of charge threshold, the third state of charge threshold is less than or equal to the first state of charge threshold, and the fourth state of charge threshold is greater than or equal to the second state of charge threshold.
2. The power converter according to claim 1, characterized in that, The power converter controls the reduction of negative-sequence active power at the output terminal, including: The power converter controls the negative sequence voltage at the output terminal to decrease, and controls the negative sequence active current at the output terminal to be less than or equal to the product of the rated current and the proportional threshold.
3. The power converter according to claim 1, characterized in that, The power converter controls the reduction of negative-sequence active power at the output terminal, including: The power converter controls the phase of the negative sequence voltage at the output terminal to lead the phase of the negative sequence current by 90 degrees, so as to control the negative sequence active current at the output terminal to be 0.
4. The power converter according to any one of claims 1-3, characterized in that, The power converter controls the reduction of the AC component amplitude of the active power at the output terminal, including: The power converter controls the negative sequence current at the output terminal, so that the AC component generated by the combination of the negative sequence current and the positive sequence voltage at the output terminal is in the opposite direction to the AC component generated by the combination of the positive sequence current and the negative sequence voltage at the output terminal.
5. The power converter according to any one of claims 1-4, characterized in that, The first state of charge threshold is less than or equal to 20%, the second state of charge threshold is greater than or equal to 80%, the difference between the third state of charge threshold and the first state of charge threshold is greater than 5% and less than 10%, and the difference between the fourth state of charge threshold and the second state of charge threshold is greater than 5% and less than 10%.
6. The power converter according to any one of claims 1-5, characterized in that, The power converter is used to convert the DC power from the energy storage battery into AC power and output it to the power grid. The power converter is used to control the negative sequence active power at the output terminal to decrease when the state of charge of the energy storage battery is greater than a first state of charge threshold and less than a second state of charge threshold and the power grid experiences voltage imbalance. The voltage imbalance in the power grid includes the occurrence of an asymmetrical fault in the power grid or the connection of an asymmetrical load to the power grid.
7. A control method for a power converter, characterized in that, The input terminal of the power converter is used to connect to an energy storage battery, and the method includes: When the state of charge of the energy storage battery is greater than the first state of charge threshold and less than the second state of charge threshold, the negative sequence active power at the output of the power converter is reduced. When the state of charge of the energy storage battery is less than the third state of charge threshold or greater than the fourth state of charge threshold, the amplitude of the AC component of the active power at the output terminal is reduced, wherein the first state of charge threshold is less than the second state of charge threshold, the third state of charge threshold is less than or equal to the first state of charge threshold, and the fourth state of charge threshold is greater than or equal to the second state of charge threshold.
8. The method according to claim 7, characterized in that, The control of reducing the negative-sequence active power at the output of the power converter includes: The negative sequence voltage at the output terminal is reduced, and the negative sequence active current at the output terminal is controlled to be less than or equal to the product of the rated current and the proportional threshold.
9. The method according to claim 7, characterized in that, The control of reducing the negative-sequence active power at the output of the power converter includes: The phase of the negative sequence voltage at the output terminal is controlled to lead the phase of the negative sequence current by 90 degrees, so as to control the negative sequence active current at the output terminal to be 0.
10. The method according to any one of claims 7-9, characterized in that, The reduction of the AC component amplitude of the active power at the output terminal includes: The negative sequence current at the output terminal is controlled such that the AC component generated by the combination of the negative sequence current and the positive sequence voltage at the output terminal is in the opposite direction to the AC component generated by the combination of the positive sequence current and the negative sequence voltage at the output terminal.
11. The method according to any one of claims 7-10, characterized in that, The first state of charge threshold is less than or equal to 20%, the second state of charge threshold is greater than or equal to 80%, the difference between the third state of charge threshold and the first state of charge threshold is greater than 5% and less than 10%, and the difference between the fourth state of charge threshold and the second state of charge threshold is greater than 5% and less than 10%.