Energy storage system, off-grid switching method and energy storage converter

Abstract

The application discloses a kind of energy storage system, and the method and energy storage converter of off-grid switching, at least two energy storage converters PCS, the output of at least two PCSs is connected together and connected to alternating current grid;The input of at least two PCSs is connected to energy storage power supply;First PCS in at least two PCSs includes: voltage detection circuit, power conversion circuit and controller;First PCS is any one in at least two PCSs;Voltage detection circuit detects the grid voltage of the parallel point of the output of at least two PCSs;Controller judges when island occurs to alternating current grid according to the grid voltage of parallel point, adjusts the angle of the output voltage of first PCS to reference angle, so that the angle of the output voltage of at least two PCSs is all same;Reference angle is obtained according to the angular frequency of grid voltage when at least two PCSs are connected to grid, when switching off-grid, the angle of the output voltage of each PCS is same, and circulating current is inhibited.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of power systems, and particularly relates to a kind of energy storage system, method for switching between grid-connected and off-grid and energy storage converter. BACKGROUND

[0002] With the aggravation of global environmental pollution, green energy is more and more favored, such as photovoltaic power generation, wind power generation and water power generation. In order to better realize the cooperation of energy storage system and power grid, the energy storage system can participate in various regulation tasks of power grid, such as regulating reactive power.

[0003] Generally, the energy storage system includes multiple energy storage converters (PCS, Power Conversion System), and the output ends of the multiple PCSs are connected together to connect the power grid. In order to enable the PCS to participate in the regulation task of the power grid, the PCS operates in a current source mode. The current source has the characteristics that when connected to the grid, the PCS actively detects the frequency and phase of the grid voltage, controls the output current of itself according to the frequency and phase of the grid voltage, and the internal impedance characteristic is high impedance.

[0004] The energy storage system generally has a local load, and when the power grid jumps, the PCS needs to switch from the grid-connected mode to the off-grid mode to supply power to the local load, that is, the PCS performs grid-connected and off-grid switching. During the grid-connected and off-grid switching of the PCS, in consideration of the reliability and stability of the load power supply, the current supplied to the load needs to be ensured to be as little as possible. change, so as to realize seamless switching between grid-connected and off-grid.

[0005] During the grid-connected and off-grid switching caused by the abnormality of the power grid, there is no communication between the multiple PCSs, each PCS independently detects the grid voltage and independently performs grid-connected and off-grid switching, and the angles of the output voltages of the multiple PCSs are likely to have large differences, resulting in large circulating current between the PCSs, which cannot realize seamless switching between grid-connected and off-grid, and affects the power supply of the load. SUMMARY

[0006] In order to solve the above technical problems, the present application provides a kind of energy storage system, method for switching between grid-connected and off-grid and energy storage converter, which can make the angles of the output voltages of the multiple PCSs be the same when the PCS performs grid-connected and off-grid switching, suppress the large circulating current between the PCSs during grid-connected and off-grid, and stably supply power to the local load.

[0007] The embodiment of the present application provides a kind of energy storage system, comprising: multiple PCS, the output end of multiple PCS is connected together and connected to AC power grid;The input end of multiple PCS is connected to energy storage power supply;For example at least two PCS, at least two PCS each independently collects grid grid of parallel point, each independently realizes parallel and off-grid switching, and does not need mutual communication.Each PCS parallel and off-grid switching principle is same, i.e.at least two PCS first PCS includes: voltage detection circuit, power conversion circuit and controller;First PCS is for any one of at least two PCS;Power conversion circuit converts the electric energy provided by energy storage power supply and exports to AC power grid when parallel;Voltage detection circuit detects the grid voltage of parallel point of at least two PCS output end;Controller judges when AC power grid occurs island according to the grid voltage of parallel point, and the angle of the output voltage of first PCS is adjusted to reference angle, since reference angle is obtained according to the angular frequency of grid voltage of first PCS when at least two PCS parallel, therefore, the angle of the output voltage of at least two PCS is same.;In this way, multiple PCS when parallel and off-grid switching, the angle of the output voltage is switched to the same reference angle, so that the phase of output voltage is same, and there is no large circulating current between each PCS because of different phase, i.e.asynchronous, so that local load can be stably powered. It should be understood that when parallel and off-grid switching, the angle of the output voltage of each PCS is synchronized with the angle of the output current, i.e.the angle of the output voltage becomes the angle of the output current.

[0008] To solve the problem of large circulating current when multiple PCS parallel and off-grid switching, the energy storage system provided by the embodiment of the present application, when multiple PCS normal parallel operation, the output end of multiple PCS is connected with AC power grid, can obtain the angular frequency of grid voltage in real time, and obtains reference angle using the angular frequency of grid voltage;When AC power grid is abnormal, multiple PCS needs to operate off-grid, i.e.parallel to off-grid switching, each PCS adjusts the angle of its output voltage to reference angle. Since each PCS is connected with AC power grid when parallel, the reference angle obtained is obtained according to grid voltage, so the reference angle obtained by each PCS is same, when each PCS adjusts the angle of output voltage to reference angle, it can ensure that the angle of output voltage of all PCS is same, i.e.all is reference angle, and then there is no problem of different angle of output voltage between multiple PCS, and then there is no large circulating current between multiple PCS, so that multiple PCS can stably power local load when off-grid.

[0009] In a possible implementation, the controller is specifically configured to, when the AC power grid is normal, i.e., when at least two PCSs are connected to the grid, perform phase locking on the grid voltage to obtain an angular frequency of the grid voltage, then filter the phase-locked angular frequency, and integrate the filtered angular frequency to obtain the reference angle. For example, low-pass filtering or moving average filtering. When the grid is islanded, the angular frequency of the grid voltage will jump greatly. In order to continue to stably supply power to the local load, the PCS needs to supply the grid voltage before the phase jump to the local load. Therefore, the filtering is to make the angular frequency change slowly, i.e., when the input angular frequency changes transiently, the output angular frequency is as constant as possible. When the PCS is off-grid, the sudden change of the angular frequency is converted into a slow change, so as to eliminate the influence of the transient state. In addition, the filtering can also filter out interference signals.

[0010] Two different ways of obtaining the reference angle are introduced below, i.e., the filtered angular frequency can be filtered first, and then the sum of the filtered angular frequency and the reference angular frequency is obtained, or the sum of the angular frequency and the reference angular frequency can be obtained first, and then the sum is filtered:

[0011] Firstly, when the PCSs are connected to the grid, the controller performs coordinate transformation on the grid voltage to obtain the Q-axis component in the rotating coordinate system, phase locks the Q-axis component to obtain the angular frequency, low-pass filters the sum of the angular frequency and the reference angular frequency, and integrates the low-pass filtered angular frequency to obtain the reference angle.

[0012] Secondly, when the PCSs are connected to the grid, the controller performs coordinate transformation on the grid voltage to obtain the Q-axis component in the rotating coordinate system, phase locks the Q-axis component to obtain the angular frequency, low-pass filters the angular frequency, integrates the sum of the low-pass filtered angular frequency and the reference angular frequency to obtain the reference angle.

[0013] In a possible implementation, the controller is further configured to, when at least two PCSs are connected to the grid, obtain the phase of the grid voltage, correct the reference angle by using the phase of the grid voltage, and adjust the angle of the output voltage to the corrected reference angle when the AC power grid is islanded.

[0014] In a possible implementation, the controller obtains the phase of the grid voltage by integrating the sum of the angular frequency and the reference angular frequency when at least two PCSs are connected to the grid.

[0015] In a possible implementation, when the controller filters the sum of the angular frequency and the reference angular frequency, the filtered angular frequency will be attenuated, and the error will become larger and larger over time. Therefore, the reference angle obtained by integration also has an error. Therefore, in order to compensate for the error of the reference angle, the reference angle can be corrected at a fixed time. That is, the controller clears the reference angle at the zero-crossing point of the phase of the grid voltage, and then filters and integrates the phase-locked angular frequency of the grid voltage to obtain the corrected reference angle.

[0016] In a possible implementation, the controller is further configured to take the phase of the grid voltage as the angle of the output voltage of the first PCS when the at least two PCSs are connected to the grid.

[0017] In a possible implementation, the waveform of the reference angle is a sawtooth wave, and the angle of the sawtooth wave changes from 0 degrees to 360 degrees following a sine wave.

[0018] In a possible implementation, the controller detects the frequency or amplitude of the grid voltage at the parallel point, and determines that the AC grid is in island mode when the frequency of the grid voltage at the parallel point exceeds a preset frequency range or the amplitude of the grid voltage exceeds a preset amplitude range; the controller is further configured to set an island flag bit to 1 and set an angular frequency adjustment amount to zero when the AC grid is in island mode; and the controller is further configured to set the island flag bit to 2 and set the angular frequency adjustment amount according to the power angle characteristic to make the at least two PCSs achieve power sharing when the grid breaker is disconnected; the grid breaker is connected between the parallel point and the AC grid.

[0019] Based on the energy storage system introduced in the above embodiments, the present embodiment further provides a method for switching between grid-connected and off-grid of an energy storage system, wherein the energy storage system comprises: at least two energy storage converters (PCSs), the output ends of the at least two PCSs being connected together to connect an AC grid; and input ends of the at least two PCSs being connected to an energy storage power supply; the method is applicable to a first PCS of the at least two PCSs, the first PCS being any one of the at least two PCSs, and the method comprises the following steps: detecting a grid voltage at a parallel point of the output ends of the at least two PCSs; and adjusting an angle of an output voltage of the first PCS to a reference angle to make the angles of the output voltages of the at least two PCSs all the same when the AC grid is in island mode according to the grid voltage at the parallel point; the reference angle being obtained according to an angular frequency of the grid voltage when the at least two PCSs are connected to the grid.

[0020] In a possible implementation, the reference angle is obtained by filtering and integrating an angular frequency of the grid voltage phase-locked when the at least two PCSs are connected to the grid.

[0021] In a possible implementation, the reference angle is obtained by filtering and integrating an angular frequency of the grid voltage phase-locked when the at least two PCSs are connected to the grid, and specifically comprises:

[0022] In a possible implementation, the reference angle is obtained by filtering and integrating an angular frequency of the grid voltage phase-locked when the at least two PCSs are connected to the grid, and specifically comprises:

[0023] In a possible implementation, the reference angle is obtained by filtering and integrating an angular frequency of the grid voltage phase-locked when the at least two PCSs are connected to the grid, and specifically comprises:

[0024] The grid voltage is coordinate-transformed to obtain a Q-axis component in a rotating coordinate system when grid-connected, the Q-axis component is phase-locked to obtain an angular frequency, the angular frequency is low-pass filtered, and the sum of the reference angular frequency and the low-pass filtered angular frequency is integrated to obtain the reference angle.

[0025] In a possible implementation, the method further includes: obtaining a phase of the grid voltage when the at least two PCSs are grid-connected, correcting the reference angle by using the phase of the grid voltage, and adjusting the angle of the output voltage to the corrected reference angle when the AC grid is islanded.

[0026] In a possible implementation, the phase of the grid voltage is obtained when the at least two PCSs are grid-connected, and specifically includes: obtaining an angular frequency of the phase-locked grid voltage when the at least two PCSs are grid-connected, and integrating the sum of the angular frequency and a reference angular frequency to obtain the phase of the grid voltage.

[0027] In a possible implementation, the reference angle is corrected by using the phase of the grid voltage, and specifically includes: clearing the reference angle at a zero-crossing point of the phase of the grid voltage, and re-integrating the filtered angular frequency of the phase-locked grid voltage to obtain the corrected reference angle.

[0028] The application also provides a storage converter, the storage converter being a first PCS of at least two PCSs, the first PCS being any one of the at least two PCSs, and the output ends of the at least two PCSs being connected together in parallel to an AC grid; the first PCS including: a voltage detection circuit, a power conversion circuit, and a controller; the power conversion circuit being configured to convert and output electrical energy provided by a storage power supply to the AC grid when grid-connected; the voltage detection circuit being configured to detect a grid voltage at a parallel point of the output ends of the at least two PCSs; and the controller being configured to, when the AC grid is islanded, adjust an angle of an output voltage of the PCS to a reference angle, so that the angles of the output voltages of the at least two PCSs are the same, and determine the reference angle according to an angular frequency of the grid voltage when the at least two PCSs are grid-connected.

[0029] The energy storage converter provided by the embodiments of the present application collects the voltage of the parallel point in real time, and under the grid-connected state, the collected voltage signal passes through a phase-locked loop to obtain an angular frequency adjustment amount, the angular frequency adjustment amount and a reference angular frequency are summed to obtain an angular frequency which is integrated to obtain an output angle, and the output angle is input to a PCS control loop for grid-connected control. Meanwhile, the angular frequency is filtered and integrated, and then corrected to obtain a corrected reference angle. Since multiple PCSs have the same parallel point, the reference angle of each PCS is the same; when islanding is determined, the angular frequency adjustment amount is switched to 0, and the angles of the output voltages of the PCSs are aligned with the reference angle, and after alignment, the angle of the output voltage of each PCS is ensured to be the same, and then input to the PCS control loop for off-grid control.

[0030] In a possible implementation, the controller is specifically configured to perform coordinate transformation on the grid voltage to obtain a Q-axis component in a rotating coordinate system when the grid is connected, obtain an angular frequency of the grid voltage by phase locking, and perform low-pass filtering on the sum of the angular frequency and a reference angular frequency to obtain an integral of the low-pass filtered angular frequency to obtain a reference angle.

[0031] In a possible implementation, the controller is specifically configured to obtain an angular frequency of the grid voltage after phase locking when at least two PCSs are connected to the grid, integrate the sum of the angular frequency and a reference angular frequency to obtain a phase of the grid voltage, and correct the reference angle by using the phase of the grid voltage; and adjust the angle of the output voltage to the corrected reference angle when the AC grid is in islanding.

[0032] In a possible implementation, the controller clears the reference angle at the zero-crossing point of the phase of the grid voltage, and obtains the corrected reference angle by filtering and integrating the angular frequency after phase locking of the grid voltage.

[0033] When the AC grid is detected to be in islanding, the angular frequency adjustment amount is 0 during the on-off grid switching process, and the reference angle is corrected at the zero-crossing point of the phase obtained by integrating the reference angular frequency, and the reference angle is cleared at the zero-crossing point of the phase. That is, during the on-off grid switching process, the angular frequency does not need to be adjusted, the angular frequency adjustment amount output by the phase-locked loop (PLL) does not work, the angular frequency does not change at this time, and the reference angle is corrected by using only the phase obtained by integrating the reference angular frequency (for example, 50 Hz), so that each PCS is aligned to the same reference angle although the islanding is determined at different times, and the angles of the output voltages of the multiple PCSs are synchronized.

[0034] The present application has at least the following advantages:

[0035] The output ends of the plurality of PCSs in the energy storage system provided by the embodiment of the present application are connected together in parallel to the AC power grid. When the grid voltage is normal, each PCS normally operates in parallel with the grid, and the grid voltage is phase-locked to obtain an angular frequency. A reference angle is obtained according to the angular frequency and reserved for standby. When the AC power grid is in island mode, the output voltage of each PCS is adjusted to the reference angle obtained by each PCS. Since the reference angles obtained by each PCS are obtained by phase-locked to the grid voltage in the normal AC power grid, the reference angles obtained by each PCS are the same. When the AC power grid is in island mode, the output voltage of each PCS is switched to the reference angle obtained by each PCS. That is, the output voltage of each PCS is aligned with the reference angle, so that the output voltage of each PCS is the same, and the large circulating current between the PCSs is suppressed, and the local load can be stably powered when the PCSs are switched between grid-connected and off-grid modes. BRIEF DESCRIPTION OF DRAWINGS

[0036] Figure 1 A schematic diagram of an energy storage system provided by the embodiment of the present application;

[0037] Figure 2 A schematic diagram of a photovoltaic energy storage system provided by the embodiment of the present application;

[0038] Figure 3 A control architecture diagram of an energy storage system provided by the embodiment of the present application;

[0039] Figure 4A A control architecture diagram of an energy storage system provided by the embodiment of the present application;

[0040] Figure 4B A control architecture diagram of an energy storage system provided by the embodiment of the present application;

[0041] Figure 5 A waveform diagram of the grid voltage and the reference angle provided by the embodiment of the present application;

[0042] Figure 6 A control schematic diagram during grid-connected and off-grid switching provided by the embodiment of the present application;

[0043] Figure 7A An off-grid control schematic diagram after grid-connected and off-grid switching provided by the embodiment of the present application;

[0044] Figure 7B A schematic diagram of another energy storage system provided by the embodiment of the present application;

[0045] Figure 8 A method flowchart of grid-connected and off-grid switching of an energy storage system provided by the embodiment of the present application;

[0046] Figure 9Another method flowchart for switching between grid-connected operation and off-grid operation of an energy storage system is provided in embodiments of the present application.

[0047] Figure 10 A schematic diagram of an energy storage converter is provided in embodiments of the present application. DETAILED DESCRIPTION

[0048] The technical solutions in the embodiments of the present application will be described below with reference to the drawings in the embodiments of the present application.

[0049] The terms "first", "second", etc. in the following description are only used for the purpose of description, and cannot be understood as indicating or implying relative importance or implicitly indicating the number of the technical features indicated. Therefore, the features defined with "first", "second", etc. can explicitly or implicitly include one or more of the features. In the description of the present application, the meaning of "a plurality of" is two or more, unless otherwise specified.

[0050] In the present application, unless otherwise explicitly specified and limited, the term "connection" should be understood broadly, for example, "connection" can be fixed connection, or detachable connection, or integral; can be directly connected, or indirectly connected through an intermediate medium. In addition, the term "coupling" can be an electrically connected manner for realizing signal transmission. "Coupling" can be direct electrical connection, or indirect electrical connection through an intermediate medium.

[0051] Energy storage system embodiments

[0052] In order for those skilled in the art to better understand the technical solutions provided by the embodiments of the present application, the application scenario of the technical solutions will be introduced first. The embodiments of the present application relate to an energy storage system, which can operate in grid-connected mode, can supply power to a load together with an alternating current power grid, and can supply power to the load alone when the alternating current power grid fails. The embodiments of the present application do not limit the specific application scenarios of the energy storage system, for example, it can be used in large-scale energy storage application scenarios, small and medium-sized distributed energy storage application scenarios, or household energy storage application scenarios, etc.

[0053] Referring to Figure 1 The figure is a schematic diagram of an energy storage system provided in embodiments of the present application.

[0054] The energy storage system generally includes multiple PCSs, such as Figure 1As shown, the system includes n PCSs, n is a positive integer greater than or equal to 2, which are PCS1 to PCSn respectively, the input end of each PCS is connected to an energy storage system (ESS), each PCS can be connected to one ESS, or can be connected to multiple ESSs, for example, PCS1 is connected to one ESS, and PCSn is connected to one ESS, and the number of ESSs connected to each PCS is not limited in the embodiment of the application and can be set according to actual needs. For example, for a light storage system, the ESS can be an energy storage container, and the electrical energy of the battery cluster in the energy storage container can be obtained by photovoltaic power generation. For example, the ESS can include a photovoltaic assembly, a battery system, and a direct current-direct current converter, etc. Multiple PCSs can be independently connected to the first side of the transformer, or multiple PCSs can be AC coupled to the first side of the transformer, and the second side of the transformer is connected to a grid-connected switch, and the grid-connected switch can be specifically implemented by using a grid-connected circuit breaker.

[0055] In addition, the light storage system includes coupling of photovoltaic and energy storage, see Figure 2 The direct current power supply of the ESS can also come from wind power generation or hydroelectric power generation or a battery, etc.

[0056] That is Figure 1 The ESS in Figure 2 The ESS in Figure 2 The ESS in

[0057] Figure 1 The PPC in

[0058] When the PCS is in grid-connected operation, the PCS operates in a current source mode; when the AC power grid is tripped due to a fault, the energy storage system needs to be switched to off-grid operation to supply power to the local load, therefore, the PCS needs to operate in a voltage source mode to provide a stable voltage for the local load.

[0059] Since there is no communication among the multiple PCSs, each PCS independently samples, independently determines whether an island occurs according to the grid voltage, and performs off-grid control when the island occurs. Since the voltage detection circuits of the multiple PCSs are different in cable length at the parallel connection point, the cable length difference results in a difference in line impedance, the difference in line impedance results in a difference in voltage drop, and further results in a difference in the grid voltage detected by the multiple PCSs, thereby affecting the time at which the multiple PCSs determine the grid island, and further affecting the time at which the multiple PCSs switch from grid connection to off-grid, and a large difference in output angle, resulting in a large circulating current and affecting the stability of power supply to the local load.

[0060] To solve the problem of a large circulating current when multiple PCSs switch from grid connection to off-grid, the energy storage system provided in the embodiments of the present application is configured such that, when the multiple PCSs normally work in grid connection, the output ends of the multiple PCSs are connected to the AC grid, the angular frequency of the grid voltage can be obtained in real time, and the reference angle is obtained by using the angular frequency of the grid voltage; when the AC grid is abnormal and the multiple PCSs need to work in off-grid, that is, switching from grid connection to off-grid, each PCS adjusts the angle of the output voltage to the reference angle. Since each PCS is connected to the AC grid when it is in grid connection, the reference angle obtained by each PCS is obtained according to the grid voltage, and therefore the reference angles obtained by each PCS are the same, and when each PCS adjusts the angle of the output voltage to the reference angle, the angles of the output voltages of all the PCSs are the same, that is, the reference angle, and therefore there is no problem of different angles of the output voltages among the multiple PCSs, and further there is no large circulating current among the multiple PCSs, so that the multiple PCSs can stably supply power to the local load when they are in off-grid.

[0061] To enable those skilled in the art to better understand the technical solutions provided in the embodiments of the present application, the technical solutions provided in the embodiments of the present application are described in detail below with reference to the drawings.

[0062] Referring to Figure 3 FIG. 1 is a control architecture diagram of an energy storage system provided in the embodiments of the present application.

[0063] The energy storage system provided in the embodiments of the present application includes at least two energy storage converters PCSs, the output ends of the at least two PCSs are connected together to connect an AC grid, Figure 3 Only three PCSs are schematically shown in the figure, and the embodiments of the present application do not limit the specific number of PCSs; the input ends of the three PCSs are connected to an energy storage source, which can be from photovoltaic power generation, wind power generation, water power generation, or a battery, that is, the output of the ESS is DC power, and the PCS can convert the DC power into AC power consistent with the grid; the ESS connected to the input end of each PCS can be independent or can be connected to the same ESS by the multiple PCSs.

[0064] The multiple PCSs in the energy storage system provided by the embodiments of the present application independently complete the grid-connected and off-grid switching and are not controlled by other devices, and the multiple PCSs do not need to be synchronized, that is, do not need to send a signal of synchronization off-grid to each other.

[0065] Since each PCS independently completes its own grid-connected and off-grid switching in the embodiments of the present application, the principle of the grid-connected and off-grid switching is the same for each PCS. For the convenience of introduction, the grid-connected and off-grid control principle of any one of the multiple PCSs is introduced below, and the grid-connected and off-grid control principle of other PCSs is the same and is not described one by one. For the convenience of description, the first PCS represents any one of the multiple PCSs.

[0066] The first PCS includes a voltage detection circuit 200, a power conversion circuit (i.e., the PCS in the figure) and a controller 100; the power conversion circuit is configured to convert and output the power provided by the energy storage power supply to the AC power grid when the grid is connected.

[0067] The voltage detection circuit 200 is configured to detect the grid voltage at the parallel point of the output terminals of the at least two PCSs.

[0068] The parallel point is a point at which the output terminals of the multiple PCSs are connected together, the output terminals of the PCSs in the figure are connected to the first side of the transformer, the second side of the transformer is connected to the AC power grid, and the parallel point is a point at which the first side of the transformer is connected to the PCS.

[0069] The controller 100 is configured to, when the AC power grid is in islanding, adjust the angle of the output voltage of the first PCS to a reference angle so that the angles of the output voltages of the at least two PCSs are the same, and the reference angle is obtained according to the angular frequency of the grid voltage when the at least two PCSs are connected to the grid. It should be understood that since the reference angle is obtained when the multiple PCSs are normally connected to the grid, the reference angles obtained by the at least two PCSs are the same.

[0070] The islanding of the AC power grid indicates that the grid is abnormal and cannot continue to normally supply power to the local load, and at this time, the PCS needs to supply power to the local load. When the PCS is connected to the grid, the PCS is equivalent to a current source, and the AC power grid stably provides the voltage to the local load. When the PCS is off-grid, the PCS needs to be equivalent to a voltage source to provide power to the local load, and the PCS stably provides the voltage to the local load.

[0071] It should be understood that the control loop in the figure is also implemented by the controller 100.

[0072] For the convenience of understanding the working principle of the controller 100, Figure 3 a control link inside the controller 100 is illustrated.

[0073] To facilitate understanding of the technical solutions provided in the embodiments of this application, the control principle of the PCS is first introduced below when the AC power grid does not experience islanding (network jump).

[0074] In controller 100, solid lines represent the normal grid-connected operation process, and dashed lines represent the operation process of controller 100 when the AC grid becomes islanded.

[0075] First, an introduction Figure 3 The control process for grid connection corresponding to the solid line in the middle.

[0076] When the PCS is operating normally in grid connection, the voltage detection circuit 200 detects the three-phase voltage u at the output terminal of the PCS. abc The voltage DQ-axis components are transformed from a stationary coordinate system to a two-phase rotating coordinate system through coordinate transformation. Then, the Q-axis components are passed through a phase-locked loop (PLL) to obtain the angular frequency adjustment Δω, which represents the rotational speed of the grid voltage. pll The voltage in the rotating coordinate system is a vector, where the angular frequency is the frequency of rotation. In grid-connected mode, the angular frequency adjustment Δω = Δω pll That is, the angular frequency Δω output by the PLL. pll Superimposed reference angular frequency ω B (The fixed angular frequency of the power grid is 50Hz or 60Hz), the angular frequency Δω pll With reference angular frequency ω B Integrating the sum of the terms yields the phase θ of the grid voltage. real In the diagram, 1 / s represents the integral. At this point, the power grid is normal and no islanding has occurred; therefore, there is no islanding flag. AI in the diagram only illustrates islanding detection. AI1 is the output signal of AI and serves as the islanding flag, indicating the presence of an island. When AI1 = 0, it indicates that the AC power grid is normal and no islanding has occurred. This flag determines the final θ input to the control loop. pcs Islanding detection can be determined by grid voltage or grid current. For example, if the amplitude or frequency of the grid voltage deviates (becoming too large or too small), islanding can be identified. If the grid voltage is normal, islanding is not identified, and the PCS can operate normally in grid-connected mode. At this point, θ pcs =θ real , that is, θ real The output is directly supplied to the control loop. Here, the control loop refers to the grid-side control loop required for PCS grid connection, such as a current loop, voltage loop, or power loop. The control loop controls the PCS to perform power control or voltage control. These control loops are all for normal PCS grid connection and will not be elaborated further. It should be understood that during normal PCS operation, the voltage loop control detects its own output voltage.

[0077] In addition, when the PCS is running normally in grid connection, the controller of each PCS performs phase-locking with the voltage grid in real time to obtain the angular frequency, and obtains the reference angle based on the angular frequency. When the grid does not become islanded, the reference angle has no effect. It only plays a role during the grid connection to off-grid switching process.

[0078] At the moment when the AC power grid becomes islanded, the angular frequency regulation of the PLL output of each PCS may be different. Therefore, the angle of the output voltage of each PCS is different, and a phase difference occurs between the PCS. This will result in a large circulating current in the off-grid state.

[0079] The following is an introduction Figure 3 The dashed line in the middle represents the switching process from grid connection to off-grid operation.

[0080] When the controller determines that islanding has occurred in the AC power grid based on the grid voltage at multiple PCS parallel connection points, it adjusts its output voltage angle to the reference angle. (The last part, "θ", appears to be an error and doesn't need a direct translation.) B Assigned to θ real That is, θ is used in the control loop at this time. pcs =θ B Wherein, the reference angle θ B The reference angle is obtained based on the angular frequency of the grid voltage when at least two PCS are connected to the grid, and the reference angles obtained by at least two PCS are the same.

[0081] Because each PCS independently performs grid-connection / off-grid switching, each PCS obtains the angular frequency of the grid voltage in real time when connected to the grid, and obtains a reference angle based on the grid voltage's angular frequency. Therefore, the reference angle obtained by each PCS is the same. When the PCS performs grid-connection / off-grid switching, all PCS switch to their respective obtained reference angles. Since each PCS obtains the same reference angle, large differences in the angle of the output voltage between PCS can be avoided, thereby suppressing circulating currents between PCS. It should be understood that during grid-connection / off-grid switching, the angle of the PCS's output voltage is the same as the angle of its output current, that is, the output voltage and output current are synchronized.

[0082] One way for a controller to obtain a reference angle based on the angular frequency of the grid voltage is through filtering, such as low-pass filtering or moving average filtering. When the grid becomes islanded, the angular frequency of the grid voltage jumps dramatically. To continue supplying stable power to the local load, the PCS needs to supply the local load with the phase of the grid voltage before the jump. Therefore, filtering is used to make the angular frequency change slowly; that is, when the input angular frequency changes transiently, the output angular frequency remains as constant as possible. When the PCS goes offline, the abrupt change in angular frequency is converted into a gradual change, eliminating the effects of transients. Additionally, filtering can also remove interference signals.

[0083] The energy storage system provided in this application provides multiple PCSs whose output terminals are connected in parallel to the AC power grid. When the grid voltage is normal, each PCS operates normally in parallel with the grid, and each PCS obtains a reference angle based on the angular frequency of the grid voltage. When the AC grid becomes islanded, each PCS adjusts its output voltage angle to the reference angle it has obtained. Since the reference angles obtained by each PCS are obtained by phase-locking the grid voltage under normal AC grid conditions, the reference angles obtained by each PCS are the same. During islanding, each PCS switches its output voltage angle to the reference angle, that is, aligns it with the reference angle, ensuring that the output voltage angles of each PCS are the same. This can suppress large circulating currents between each PCS, and each PCS can stably supply power to the local load.

[0084] Furthermore, the energy storage system provided in this application embodiment does not rely on communication between the various PCS units, and can also achieve output voltage angle synchronization during grid-connected / off-grid switching because each PCS is aligned to the same reference angle, thereby truly achieving seamless switching during grid-connected / off-grid switching with minimal voltage distortion. Considering the voltage withstand capability of the local load during grid-connected / off-grid switching, during the switching transient process, for example, the amplitude of voltage distortion should not exceed 90% to 110% of the rated voltage, and the phase change should be less than 5 degrees.

[0085] The reference angle described in the above embodiments is obtained by first obtaining the sum of the angular frequency adjustment and the reference angular frequency, then filtering the sum of the angular frequency adjustment and the reference angular frequency, and finally integrating the filtered angular frequency to obtain the reference angle. Alternatively, there is another possible implementation method, see [link to implementation details]. Figure 4A This figure is another control architecture diagram of the energy storage system provided in the embodiments of this application.

[0086] The controller can first filter the angular frequency adjustment Δω, then calculate the sum of the reference angular frequency and the filtered angular frequency adjustment Δω1, and then adjust the reference angular frequency ω. B Integrating the sum of the filtered angular frequency adjustment Δω1, we obtain the reference angle θ. B .

[0087] It should be understood that Figure 4A The method for obtaining the reference angle shown is applicable to all technical solutions provided in the embodiments of this application. For ease of introduction, the following embodiments use... Figure 3 The method for obtaining the reference angle shown will be introduced.

[0088] The following section details the switching process when an AC power grid becomes islanded.

[0089] See Figure 4B This figure is another control architecture diagram of the energy storage system provided in the embodiment of this application.

[0090] Due to the controller's diagonal frequency Δω pll With reference angular frequency ω B When filtering the sum, the angular frequency will be attenuated after filtering. Over time, the error will increase, and the reference angle obtained after integration will also have errors. Therefore, in order to compensate for the error of the reference angle, the reference angle can be corrected periodically. A specific correction method is introduced below.

[0091] The controller is also used to obtain the phase of the grid voltage when at least two PCS are connected in parallel, and to correct the reference angle using the phase of the grid voltage; when the AC grid becomes islanded, the angle of the output voltage is adjusted to the corrected reference angle.

[0092] The phase of the grid voltage during normal grid-connected operation is obtained as follows: When at least two PCSs are connected to the grid, the controller obtains the angular frequency of the grid voltage after phase-locking. The phase of the grid voltage, θ, is obtained by integrating the sum of the angular frequency and the reference angular frequency. real .

[0093] The controller utilizes θ real For reference angle θ B The specific method for correction is as follows: at the phase θ of the grid voltage real The zero-crossing point will be the reference angle θ B After resetting to zero, the angular frequency of the grid voltage phase-locked loop is filtered again and integrated to obtain the corrected reference angle θ. B Because at θ real The reference angle is reset to zero at each zero crossing, therefore, the reference angle is a periodic sawtooth wave. The angle of the sawtooth wave follows the sine wave from 0 degrees to 360 degrees.

[0094] See Figure 5 The figure is a waveform diagram of the power grid voltage and reference angle provided in an embodiment of this application.

[0095] Figure 5 The sine wave in the image represents the waveform of the mains voltage. Figure 5 The sawtooth wave in the figure is the waveform of the reference angle provided in the embodiments of this application.

[0096] For example, if islanding occurs in the AC power grid at time t1, taking the parallel connection of the output terminals of two PCSs as an example, the angle of the output voltage of PCS1 is A, and the angle of the output voltage of PCS2 is B. As can be seen from the figure, both A and B deviate from the sawtooth wave corresponding to the reference angle. When PCS1 and PCS2 determine that islanding has occurred, the angle of the output voltage of PCS1 is adjusted to the reference angle, and the angle of the output voltage of PCS2 is adjusted to the reference angle, so that the angles of the output voltages of PCS1 and PCS2 are both the reference angle. This ensures that the angles of the output voltages of PCS1 and PCS2 are the same, thereby avoiding large circulating currents between PCS1 and PCS2 when they are connected in parallel, which would affect the stable power supply to the local load.

[0097] The controller also performs the calibration of the reference angle when the PCS is normally connected to the grid. That is, the calibration of the reference angle is continuously performed when the PCS is normally connected to the grid. The angle of the output voltage is switched to the calibration reference angle only when the PCS is switched to the grid connection.

[0098] It should be understood that the corrected reference angle obtained by each PCS is only used once during grid-to-offline handover, i.e., θ B Assigned to θ pcs After the switchover is completed, each PCS will be able to operate in an off-network state.

[0099] See Figure 6 This figure is a control diagram of on-grid and off-grid switching provided in an embodiment of this application.

[0100] Compare Figure 6 and Figure 4B , Figure 4B The islanding flag AI1 is 0, indicating that islanding has not yet occurred and the system is operating in grid-connected mode. Figure 6 The island flag AI1 = 1, that is Figure 6 The system has become isolated and is currently in a transitional state during the grid-to-offline handover process.

[0101] When the AI ​​determines that islanding has occurred in the AC power grid, it sets the islanding flag to 1. At this time, the angular frequency adjustment becomes 0, meaning that no angular frequency adjustment is needed during grid-to-grid switching, Δω = 0. The angular frequency adjustment Δω output by the phase-locked loop (PLL) is... pll It no longer has any effect; at this point, the angular frequency no longer changes, and only the reference angular frequency ω is used. B (e.g., 50Hz) The integrated phase relative to the reference angle θ B Perform correction, i.e., θ pcs =θ B This ensures that although each PCS identifies an island at different times, they all align to the same reference angle, thus ensuring the angle synchronization of the output voltage of multiple PCS.

[0102] To complete the complete grid-connection and off-grid switching, the switching process may take a period of time, such as tens to hundreds of milliseconds, waiting for all switches connected to the AC grid to be disconnected, i.e. disconnected from the AC grid. Only after the switch action is reliably completed can the complete grid-connection and off-grid switching be finished.

[0103] The energy storage system provided in this application mainly describes the process by which each PCS controls its own output voltage angle to switch to the same reference angle when islanding occurs in the AC grid. After the switching is completed, off-grid control is performed. The off-grid control is not described in detail here. That is, after off-grid operation, the control of the angular frequency adjustment amount belongs to the specific control of off-grid operation.

[0104] See Figure 7A This figure is a schematic diagram of off-grid control after on-grid switching provided in an embodiment of this application.

[0105] In the energy storage system provided in this embodiment, after each PCS aligns its output voltage angle with the reference angle, it completes the off-grid switching. At this time, the islanding flag AI1 is set to 2, and the off-grid voltage controls the VSG to control the PCS. Each PCS equally distributes power to supply local loads. It should be noted that at this time, the angular frequency adjustment is no longer 0, and the angular frequency begins to change again. Instead, the off-grid voltage controls the VSG to output the angular frequency adjustment Δω. VSG To perform off-grid control, the controller also determines when the grid-connected circuit breaker is open, sets the islanding flag AI1 to 2, and adjusts the angular frequency according to the power angle characteristics to ensure power distribution among the PCS units. The grid-connected circuit breaker is connected between the parallel point and the AC grid. It should be understood that during off-grid control, the off-grid voltage control VSG requires not only the output voltage u of the PCS but also... abc In addition, the output current i of the PCS is also required. abc .

[0106] The energy storage system provided in this application embodiment acquires the voltage at the parallel connection point of the energy storage converter in real time. In grid-connected mode, the acquired voltage signal passes through a phase-locked loop to obtain an angular frequency adjustment. The angular frequency obtained by summing the angular frequency adjustment and the reference angular frequency is integrated to obtain the output angle, which is then input to the PCS control loop for grid-connected control. Simultaneously, the angular frequency is filtered and integrated, and then corrected to obtain the corrected reference angle. Since multiple PCSs have the same parallel connection point, the reference angle of each PCS is the same. When islanding is detected, the angular frequency adjustment is switched to 0, and the angle of the output voltage of each PCS is aligned with the reference angle. After alignment, it is ensured that the angle of the output voltage of each PCS is the same, and then input to the PCS control loop for off-grid control.

[0107] The energy storage systems described in the above embodiments all involve directly connecting the output terminals of the PCS together in parallel, and then connecting to the AC power grid through a transformer. Furthermore, the grid-connected / off-grid switching technology provided in the above embodiments of this application is also applicable to situations where the output terminal of each PCS is connected to the first side of a corresponding transformer, and the second sides of all transformers are connected in parallel, i.e., connected at the parallel connection point.

[0108] See Figure 7B This figure is a control architecture diagram of another energy storage system provided in an embodiment of this application.

[0109] In this embodiment, we will continue to use three PCS as examples, namely PCS1, PCS2 and PCS3. The output terminal of PCS1 is connected to the first side of the first transformer T1, the output terminal of PCS2 is connected to the first side of the second transformer T2, and the output terminal of PCS3 is connected to the first side of the third transformer T3. The second sides of T1, T2 and T3 are connected in parallel to the AC power grid.

[0110] because Figure 7B The output terminals of each PCS in the system are not directly connected together, but are each connected to the AC power grid through their respective transformers. Therefore, the distance between each PCS and the parallel connection point may vary. This distance difference will cause different line impedances, resulting in different voltage drops. Consequently, the grid voltage detected by each PCS will differ, leading to variations in the grid voltage detected by each PCS. This, in turn, will affect the timing of each PCS's determination of grid islanding. However, using the technical solution provided in this application embodiment, each PCS obtains a reference angle. When islanding occurs, each PCS aligns with the reference angle. Therefore, even if the timing of islanding determination differs, there will not be a significant difference in the angle of the output voltage. Thus, there will be no large circulating current between the PCS.

[0111] Method Implementation Examples

[0112] Based on the energy storage system provided in the above embodiments, this application also provides a method for switching between grid connection and off-grid operation of the energy storage system, which will be described in detail below with reference to the accompanying drawings.

[0113] See Figure 8 The figure is a flowchart of a method for switching between grid connection and off-grid operation of an energy storage system provided in an embodiment of this application.

[0114] The grid-connected / off-grid switching method for energy storage systems provided in this embodiment is applied to energy storage systems, which include: at least two energy storage converters (PCS), the outputs of which are connected in parallel to the AC power grid; and the inputs of which are connected to an energy storage power source. This method is applicable to the first PCS among the at least two PCS, where the first PCS can be any one of the at least two PCS, and includes the following steps:

[0115] S801: Detects the grid voltage at the parallel connection point of the output terminals of at least two PCS units;

[0116] This method applies to any PCS, and the controller of each PCS executes the same method. Each PCS can obtain the grid voltage at the parallel point and independently determine whether the AC grid has become islanded.

[0117] S802: When the AC grid is islanded based on the grid voltage at the parallel connection point, the angle of the output voltage of the first PCS is adjusted to the reference angle so that the angles of the output voltages of at least two PCS are the same. The reference angle is obtained by the first PCS based on the angular frequency of the grid voltage when at least two PCS are connected to the grid, and the reference angles obtained by at least two PCS are the same.

[0118] Islanding in an AC power grid indicates an anomaly, preventing the grid from continuing to supply power to local loads. In this situation, a PCS (Power Supply System) is needed to supply power to the local loads. When the PCS is connected to the grid, it functions as a current source; when the PCS is disconnected from the grid, it functions as a voltage source to provide power to the local loads.

[0119] When the PCS is running normally in grid connection, the controller of each PCS locks the voltage grid in real time to obtain the angular frequency, and obtains the reference angle based on the angular frequency. When the grid does not become islanded, the reference angle has no effect. It only plays a role during the grid connection to off-grid switching process.

[0120] The method provided in this application embodiment operates normally in grid-connected mode when the grid voltage is normal, with each PCS obtaining a reference angle based on the angular frequency of the grid voltage. When the AC grid becomes islanded, each PCS adjusts the angle of its output voltage to the reference angle it has obtained. Since the reference angles obtained by each PCS are obtained by phase-locking the grid voltage under normal AC grid conditions, the reference angles obtained by each PCS are the same. During islanding, each PCS switches the angle of its output voltage to the reference angle, that is, aligns it with the reference angle, ensuring that the angles of the output voltages between each PCS are the same. This can suppress large circulating currents between each PCS, and each PCS can ensure stable power supply to the local load.

[0121] See Figure 9The figure is a flowchart of another method for switching between grid connection and off-grid operation of an energy storage system provided in an embodiment of this application.

[0122] S901: Detects the grid voltage at the parallel connection point of the output terminals of at least two PCS units;

[0123] S902: When the AC grid is normal based on the grid voltage, the angular frequency of the grid voltage after phase-locking is obtained. The sum of the angular frequency and the reference angular frequency is integrated to obtain the phase of the grid voltage. The PCS is then controlled for grid connection based on the phase of the grid voltage. At the same time, the sum of the phase-locked angular frequency and the reference angular frequency is filtered, and the filtered angular frequency is integrated to obtain the reference angle.

[0124] It should be understood that filtering can also be performed directly on the angular frequency after phase-locked loop (PLL). The filtered angular frequency can then be summed with the reference angular frequency, and the reference angle can be obtained by integrating the summed angular frequency. For details, please refer to [link to relevant documentation]. Figure 4A The control architecture shown.

[0125] Furthermore, angular frequency filtering introduces attenuation, and the error increases over time, leading to errors in the integrated reference angle. Therefore, to compensate for these errors, the reference angle can be periodically corrected. Specifically, the phase of the mains voltage can be used to correct the reference angle; when the AC grid experiences islanding, the angle of the output voltage is adjusted to the corrected reference angle. The correction process involves resetting the reference angle to zero at the zero-crossing point of the mains voltage phase, re-filtering the angular frequency of the mains voltage phase-locked loop, and then integrating to obtain the corrected reference angle. Because the reference angle is reset to zero at each zero-crossing point of the mains voltage phase, the reference angle is a periodic (0-360°) sawtooth wave, which is an AC signal.

[0126] Specifically, obtaining the angular frequency of the grid voltage involves: performing a coordinate transformation on the grid voltage to obtain the Q-axis component in a rotating coordinate system, and then using a phase-locked loop (PLL) to obtain the angular frequency from the Q-axis component. The filtering can be either a low-pass filter or a moving average filter.

[0127] S903: When the AC grid is islanded based on the grid voltage, each PCS switches its own output voltage angle to the reference angle to perform grid connection / disconnection switching.

[0128] During the switching process, the angular frequency adjustment is 0, and the reference angle is directly zeroed and corrected by using the zero-crossing point of the phase after integrating the reference angular frequency.

[0129] The method provided in this application embodiment acquires the voltage at the parallel connection point of the energy storage converter in real time. In grid-connected mode, the acquired voltage signal passes through a phase-locked loop to obtain an angular frequency adjustment. The angular frequency obtained by summing the angular frequency adjustment and the reference angular frequency is integrated to obtain the output angle, which is then input to the PCS control loop for grid-connected control. Simultaneously, the angular frequency is filtered and integrated, and then corrected to obtain the corrected reference angle. Since multiple PCSs have the same parallel connection point, the reference angle of each PCS is the same. When islanding is detected, the angular frequency adjustment is switched to 0, and the angle of the output voltage of each PCS is aligned with the reference angle. After alignment, it is ensured that the angle of the output voltage of each PCS is the same, and then input to the PCS control loop for off-grid control.

[0130] Energy storage converter examples

[0131] Based on the energy storage system and the method for switching between grid connection and off-grid provided in the above embodiments, this application also provides an energy storage converter PCS. The energy storage converter is installed in the energy storage system, which includes multiple PCS. The output terminals of the multiple PCS are connected in parallel to the AC power grid. When the AC power grid is normal, the multiple PCS operate normally in grid connection. However, when islanding occurs in the AC grid, multiple PCS units need to operate off-grid to continue supplying power to the local load. However, because multiple PCS units may not simultaneously detect islanding, or the switching times between grid connection and off-grid operation may differ, the output voltage angles of each PCS will vary. Since the output terminals of multiple PCS units are connected in parallel, significant circulating currents will occur between them when the output voltage angles differ. The PCS provided in this application embodiment allows each unit to obtain a reference angle based on the phase of the grid voltage during normal operation. Each PCS obtains the same reference angle, and during grid connection / off-grid operation, all units switch to the same reference angle. Therefore, the output voltage angles of all PCS units are the same reference angle, which can suppress significant circulating currents during grid connection / off-grid operation, thereby providing stable voltage and current to the local load. The PCS provided in this application embodiment is applicable to each PCS in an energy storage system. The following description is from the perspective of a single PCS; the grid connection / off-grid control process is the same for all PCS units.

[0132] The PCS provided in the embodiments of this application will be described in detail below with reference to the accompanying drawings.

[0133] See Figure 10 The figure is a schematic diagram of an energy storage converter provided in an embodiment of this application.

[0134] The energy storage converter PCS provided in this application embodiment is the first PCS among at least two PCS. The first PCS can be any one of the at least two PCS. The output terminals of the at least two PCS are connected in parallel to the AC power grid. The first PCS includes: voltage detection circuit 101, power conversion circuit 102 and controller 103.

[0135] The power conversion circuit 102 is used to convert the electrical energy provided by the energy storage power source and output it to the AC power grid under the control of the controller 103 when connected to the grid.

[0136] Voltage detection circuit 101 is used to detect the grid voltage at the parallel connection point of the output terminals of at least two PCS units;

[0137] The embodiments of this application do not specifically limit the specific implementation of the voltage detection circuit 101. For example, it can be implemented by a voltage sensor or by building a circuit.

[0138] The controller 103 is used to adjust the angle of the output voltage of the first PCS to a reference angle when the grid islanding occurs based on the grid voltage at the parallel connection point, so that the angles of the output voltages of at least two PCS are the same; the reference angle is obtained by the first PCS based on the angular frequency of the grid voltage when at least two PCS are connected to the grid, and the reference angles obtained by at least two PCS are the same.

[0139] Islanding in an AC power grid indicates an anomaly, preventing the grid from continuing to supply power to local loads. In this situation, a PCS (Power Supply System) is needed to supply power to the local loads. When the PCS is connected to the grid, it functions as a current source; when the PCS is disconnected from the grid, it functions as a voltage source to provide power to the local loads.

[0140] When the PCS is running normally in grid connection, the controller of each PCS locks the voltage grid in real time to obtain the angular frequency, and obtains the reference angle based on the angular frequency. When the grid does not become islanded, the reference angle has no effect. It only plays a role during the grid connection to off-grid switching process.

[0141] The PCS provided in this application embodiment obtains a reference angle based on the angular frequency of the grid voltage when the grid voltage is normal. When the AC grid becomes islanded, the PCS adjusts the angle of its output voltage to the reference angle it has obtained. Since the reference angles obtained by each PCS are obtained by phase-locking the grid voltage under normal AC grid conditions, the reference angles obtained by each PCS are the same. When islanded, each PCS switches the angle of its output voltage to the reference angle, that is, aligns it with the reference angle, to ensure that the angles of the output voltages of each PCS are the same, thereby suppressing large circulating currents between each PCS and ensuring that each PCS stably supplies power to the local load.

[0142] It should be understood that when the AC power grid becomes islanded, the power output of the power conversion circuit 102 will no longer be output to the AC power grid, but will instead provide off-grid power to the local load so that the local load can continue to operate stably.

[0143] The controller, specifically, is used to perform coordinate transformation on the grid voltage during grid connection to obtain the Q-axis component in a rotating coordinate system. The Q-axis component is then phase-locked to obtain an angular frequency. The sum of this angular frequency and a reference angular frequency is low-pass filtered. The low-pass filtered angular frequency is then integrated to obtain the reference angle. It should be understood that filtering can also be performed directly on the phase-locked angular frequency, and the filtered angular frequency can then be summed with the reference angular frequency. Integrating this summed angular frequency yields the reference angle. For details, please refer to [link to relevant documentation]. Figure 4A The control architecture shown.

[0144] The controller is specifically used to obtain the angular frequency of the grid voltage after phase-locking when at least two PCS are connected to the grid, integrate the sum of the angular frequency and the reference angular frequency to obtain the phase of the grid voltage, and use the phase of the grid voltage to correct the reference angle; when the AC grid becomes islanded, the angle of the output voltage is adjusted to the corrected reference angle.

[0145] The controller is specifically used to reset the reference angle to zero at the zero-crossing point of the grid voltage phase, and then integrate the angular frequency of the grid voltage phase-locked loop to obtain the corrected reference angle.

[0146] Furthermore, angular frequency filtering introduces attenuation, and the error increases over time, leading to errors in the integrated reference angle. Therefore, to compensate for these errors, the reference angle can be periodically corrected. Specifically, the phase of the mains voltage can be used to correct the reference angle; when the AC grid experiences islanding, the angle of the output voltage is adjusted to the corrected reference angle. The correction process involves resetting the reference angle to zero at the zero-crossing point of the mains voltage phase, re-filtering the angular frequency of the mains voltage phase-locked loop, and then integrating to obtain the corrected reference angle. Because the reference angle is reset to zero at each zero-crossing point of the mains voltage phase, the reference angle is a periodic (0-360°) sawtooth wave, which is an AC signal.

[0147] Specifically, obtaining the angular frequency of the grid voltage involves: performing a coordinate transformation on the grid voltage to obtain the Q-axis component in a rotating coordinate system, and then using a phase-locked loop (PLL) to obtain the angular frequency from the Q-axis component. The filtering can be either a low-pass filter or a moving average filter.

[0148] The PCS provided in this application embodiment acquires the voltage at the parallel connection point of the energy storage converter in real time. In grid-connected mode, the acquired voltage signal passes through a phase-locked loop to obtain an angular frequency adjustment. The angular frequency obtained by summing the angular frequency adjustment and the reference angular frequency is integrated to obtain the output angle, which is then sent to the PCS control loop for grid-connected control. Simultaneously, the angular frequency is filtered and integrated, and then corrected to obtain the corrected reference angle. Since multiple PCSs have the same parallel connection point, the reference angle of each PCS is the same. When islanding is detected, the angular frequency adjustment is switched to 0, and the angle of the output voltage of each PCS is aligned with the reference angle. After alignment, it is ensured that the angle of the output voltage of each PCS is the same, and then the angle is input to the PCS control loop for off-grid control.

[0149] It should be understood that in this application, "at least one (item)" means one or more, and "more than" means two or more. "And / or" is used to describe the relationship between related objects, indicating that three relationships can exist. For example, "A and / or B" can represent three cases: only A exists, only B exists, and both A and B exist simultaneously, where A and B can be singular or plural. The character " / " generally indicates that the preceding and following related objects are in an "or" relationship. "At least one (item) of the following" or similar expressions refer to any combination of these items, including any combination of single or plural items. For example, at least one (item) of a, b, or c can represent: a, b, c, "a and b", "a and c", "b and c", or "a and b and c", where a, b, and c can be single or multiple.

[0150] The above description is merely a preferred embodiment of this application and is not intended to limit the application in any way. Although this application has disclosed preferred embodiments above, it is not intended to limit the application. Any person skilled in the art can make many possible variations and modifications to the technical solutions of this application using the methods and techniques disclosed above, or modify them into equivalent embodiments with equivalent changes, without departing from the scope of the technical solutions of this application. Therefore, any simple modifications, equivalent changes, and modifications made to the above embodiments based on the technical essence of this application without departing from the content of the technical solutions of this application shall still fall within the protection scope of the technical solutions of this application.

Claims

1. An energy storage system, characterized by, The application relates to a power storage converter (PCS) system. The system comprises at least two PCSs, the output ends of the at least two PCSs being connected together in parallel to an AC power grid; The input ends of the at least two PCSs are connected to a power storage source; A first PCS among the at least two PCSs comprises a voltage detection circuit, a power conversion circuit and a controller; the first PCS is any one of the at least two PCSs; the power conversion circuit is used for converting and outputting the power provided by the power storage source to the AC power grid when the at least two PCSs are connected to the AC power grid; The voltage detection circuit is used for detecting the grid voltage at the parallel connection point of the at least two PCSs; The controller is used for adjusting the angle of the output voltage of the first PCS to a reference angle when the AC power grid is in island mode according to the grid voltage at the parallel connection point, so that the angles of the output voltages of the at least two PCSs are all the same; the reference angle is obtained according to the angular frequency of the grid voltage when the at least two PCSs are connected to the AC power grid.

2. The energy storage system of claim 1, wherein, The controller is specifically used for filtering and integrating the angular frequency of the phase-locked grid voltage to obtain the reference angle when the at least two PCSs are connected to the AC power grid.

3. The energy storage system of claim 2, wherein, The controller is specifically used for performing coordinate transformation on the grid voltage to obtain the Q-axis component in the rotating coordinate system, obtaining the angular frequency by phase locking the Q-axis component, performing low-pass filtering on the angular frequency, and integrating the sum of the low-pass filtered angular frequency and a reference angular frequency to obtain the reference angle when the at least two PCSs are connected to the AC power grid.

4. The energy storage system of claim 2, wherein, The controller is specifically used for performing coordinate transformation on the grid voltage to obtain the Q-axis component in the rotating coordinate system, obtaining the angular frequency by phase locking the Q-axis component, performing low-pass filtering on the angular frequency, and integrating the sum of the low-pass filtered angular frequency and a reference angular frequency to obtain the reference angle when the at least two PCSs are connected to the AC power grid.

5. An energy storage system according to claim 3 or 4, wherein, The controller is further used for obtaining the phase of the grid voltage when the at least two PCSs are connected to the AC power grid, and correcting the reference angle by using the phase of the grid voltage; the angle of the output voltage is adjusted to the corrected reference angle when the AC power grid is in island mode.

6. The energy storage system of claim 5, wherein, The controller is specifically used for obtaining the angular frequency of the phase-locked grid voltage, and integrating the sum of the angular frequency and a reference angular frequency to obtain the phase of the grid voltage when the at least two PCSs are connected to the AC power grid.

7. The energy storage system of claim 5, wherein, The controller is specifically used for clearing the reference angle at the zero-crossing point of the phase of the grid voltage, and re-filtering and integrating the angular frequency of the phase-locked grid voltage to obtain the corrected reference angle when the at least two PCSs are connected to the AC power grid.

8. The energy storage system of claim 5, wherein, The controller is further used for taking the phase of the grid voltage as the angle of the output voltage of the first PCS when the at least two PCSs are connected to the AC power grid.

9. The energy storage system of any one of claims 1-4, wherein, The waveform of the reference angle is a sawtooth wave, and the angle of the sawtooth wave changes from 0 degrees to 360 degrees following a sine wave.

10. The energy storage system of any one of claims 1-4, wherein, The controller is specifically used for detecting the frequency or amplitude of the grid voltage at the parallel connection point; when the frequency of the grid voltage at the parallel connection point exceeds a preset frequency range or the amplitude of the grid voltage exceeds a preset amplitude range, it is determined that the AC power grid is in island mode. The controller is further configured to set an island flag bit to 1 and set an angular frequency adjustment amount to 0 when the AC power grid is in island state; and further configured to set the island flag bit to 2 and set the angular frequency adjustment amount according to a power angle characteristic when a grid-connected breaker is disconnected, so that the PCSs achieve power sharing.

11. A method for on-grid and off-grid switching of an energy storage system, characterized in that, The energy storage system comprises at least two energy storage converters (PCSs), and the output ends of the at least two PCSs are connected together to connect an AC power grid; and the input ends of the at least two PCSs are connected to an energy storage power supply; and the method is applicable to a first PCS among the at least two PCSs, and the first PCS is any one of the at least two PCSs, and comprises the following steps: detecting a grid voltage at a parallel point of the output ends of the at least two PCSs; when the AC power grid is in island state, adjusting an angle of an output voltage of the first PCS to a reference angle according to the grid voltage at the parallel point, so that the angles of the output voltages of the at least two PCSs are all the same; and the reference angle is obtained according to an angular frequency of the grid voltage when the at least two PCSs are connected to the AC power grid.

12. The method of claim 11, wherein, Further comprising: filtering and integrating an angular frequency of the grid voltage after phase locking to obtain the reference angle when the at least two PCSs are connected to the AC power grid.

13. The method of claim 12, wherein, The filtering and integrating of the angular frequency of the grid voltage after phase locking to obtain the reference angle when the at least two PCSs are connected to the AC power grid specifically comprises: performing coordinate transformation on the grid voltage to obtain a Q-axis component in a rotating coordinate system, obtaining an angular frequency of the Q-axis component after phase locking, and performing low-pass filtering on the angular frequency, and integrating a sum of a reference angular frequency and the angular frequency after low-pass filtering to obtain the reference angle.

14. The method of claim 12, wherein, The filtering and integrating of the angular frequency of the grid voltage after phase locking to obtain the reference angle when the at least two PCSs are connected to the AC power grid specifically comprises: performing coordinate transformation on the grid voltage to obtain a Q-axis component in a rotating coordinate system, obtaining an angular frequency of the Q-axis component after phase locking, and performing low-pass filtering on the angular frequency, and integrating a sum of a reference angular frequency and the angular frequency after low-pass filtering to obtain the reference angle.

15. The method of claim 12, wherein, Further comprising: obtaining a phase of the grid voltage when the at least two PCSs are connected to the AC power grid, and correcting the reference angle by using the phase of the grid voltage; adjusting the angle of the output voltage to the corrected reference angle when the AC power grid is in island state.

16. The method of claim 14, wherein, The obtaining of the phase of the grid voltage when the at least two PCSs are connected to the AC power grid specifically comprises: obtaining an angular frequency of the grid voltage after phase locking when the at least two PCSs are connected to the AC power grid, and integrating a sum of the angular frequency and a reference angular frequency to obtain the phase of the grid voltage.

17. The method of claim 15, wherein, The correcting of the reference angle by using the phase of the grid voltage specifically comprises: clearing the reference angle at a zero-crossing point of the phase of the grid voltage, and re-filtering and integrating the angular frequency after phase locking to obtain the corrected reference angle.

18. An energy storage converter, characterized by The energy storage converter PCS is a first PCS among at least two PCSs, the first PCS being any one of the at least two PCSs, and output ends of the at least two PCSs being connected in parallel to an AC power grid; the first PCS comprises a voltage detection circuit, a power conversion circuit and a controller; the power conversion circuit is configured to convert and output power provided by an energy storage power source to the AC power grid when the AC power grid is connected; the voltage detection circuit is configured to detect grid voltage at a parallel point of the output ends of the at least two PCSs; the controller is configured to, when the AC power grid is in islanding, adjust an angle of an output voltage of the PCS to a reference angle, so that angles of output voltages of the at least two PCSs are all the same; the reference angle is obtained according to an angular frequency of the grid voltage when the at least two PCSs are connected to the AC power grid.

19. The energy storage converter of claim 18, wherein, The controller is specifically configured to, when the AC power grid is connected, perform coordinate transformation on the grid voltage to obtain a Q-axis component in a rotating coordinate system, obtain an angular frequency by phase locking the Q-axis component, perform low-pass filtering on a sum of the angular frequency and a reference angular frequency, and obtain the reference angle by integrating the low-pass filtered angular frequency.

20. The energy storage converter of claim 18 or 19, wherein, The controller is specifically configured to, when the at least two PCSs are connected to the AC power grid, obtain an angular frequency after phase locking of the grid voltage, perform integration on a sum of the angular frequency and a reference angular frequency to obtain a phase of the grid voltage, and correct the reference angle by using the phase of the grid voltage; when the AC power grid is in islanding, the angle of the output voltage is adjusted to the corrected reference angle.

21. The energy storage converter of claim 20, wherein, The controller is specifically configured to clear the reference angle at a zero-crossing point of the phase of the grid voltage, and obtain the corrected reference angle by filtering and integrating the angular frequency after phase locking of the grid voltage again.

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