Energy storage system and island detection method

Abstract

The present application provides an energy storage system and an islanding detection method. The energy storage system includes an energy storage unit, an energy storage converter, a frequency detection circuit, and a controller. The controller performs a first / second frequency adjustment control on the energy storage converter, and adjusts the weight of the first / second frequency adjustment control according to the frequency difference between the frequency of the energy storage converter output voltage and the target frequency, controls the frequency of the energy storage converter output voltage to the target frequency, detects the occurrence of an islanding phenomenon in the energy storage converter, and adjusts the frequency of the energy storage converter output voltage in a first direction / second direction by the first / second frequency adjustment control. The first direction is the direction toward the grid reference frequency, and the second direction is the direction away from the grid reference frequency. After an energy storage converter in the energy storage system experiences an islanding phenomenon, the controller can pull the frequency of the output voltage of the energy storage converter that has experienced the islanding phenomenon to the target frequency, thereby detecting the islanding phenomenon and completing the anti-islanding protection action.

Description

Technical Field

[0001] The present application relates to the technical field of energy storage converter control, and in particular to an energy storage system and an islanding detection method. Background Art

[0002] In power grids with both high and low power consumption (i.e., a high proportion of renewable energy and power electronics), most renewable energy generation equipment currently uses grid-following (GFL) control. This creates urgent grid stability challenges, such as the N-1 minimum short-circuit ratio (SCR) scenario caused by commutation failure at the sending end of high-voltage direct current (HVDC) transmission, and fully electronic islanded power systems. Therefore, the introduction of grid-forming (GFM) control can effectively address the issue of renewable energy extremes and form an independent power system, proactively stabilizing grid frequency and voltage. However, when islanding occurs, the output voltage and frequency of power conditioning systems (PCSs) controlled by GFMs are relatively stable, making it difficult to use traditional methods to determine whether a PCS using GFLs is experiencing islanding. This, in turn, hinders relay protection for the entire grid. If the PCS is allowed to continue operating under grid-forming control, a significant surge current could occur upon grid recovery, potentially damaging equipment.

[0003] Therefore, it is necessary to propose an energy storage system and an islanding detection method to realize islanding detection of the energy storage converter. After the external power grid trips, the islanding phenomenon of the energy storage converter can be identified and a protective shutdown or off-grid operation can be performed. Summary of the Invention

[0004] The present application provides an energy storage system and an islanding detection method. After an energy storage converter in the energy storage system experiences islanding, a controller can pull the frequency of the output voltage of the islanding energy storage converter to a target frequency, thereby detecting the islanding phenomenon and completing anti-islanding protection action.

[0005] In a first aspect, the present application provides an energy storage system, comprising at least one energy storage unit, at least one energy storage converter, a frequency detection circuit, and a controller; at least one energy storage unit is connected to at least one energy storage converter in a one-to-one correspondence, and any one of the at least one energy storage converter is used to convert direct current input by the corresponding energy storage unit into alternating current and output it to a power grid; the frequency detection circuit is used to detect the frequency of the output voltage of the energy storage converter; the controller is used to perform a first frequency adjustment control and a second frequency adjustment control on the energy storage converter, and adjust the weights of the first frequency adjustment control and the second frequency adjustment control according to the frequency difference between the frequency of the output voltage of the energy storage converter and the target frequency to control the frequency of the output voltage of the energy storage converter to the target frequency, so as to detect the occurrence of an islanding phenomenon in the energy storage converter, wherein the first frequency adjustment control is used to adjust the frequency of the output voltage of the energy storage converter in a first direction, which is an adjustment direction toward the grid reference frequency, and the second frequency adjustment control is used to adjust the frequency of the output voltage of the energy storage converter in a second direction, which is an adjustment direction away from the grid reference frequency.

[0006] The present application does not rely on sudden changes in voltage amplitude and power to detect whether an islanding phenomenon occurs. The controller can pull the frequency of the output voltage of the energy storage inverter to the target frequency based on the inherent anti-droop characteristics, so that the controller can detect the occurrence of an islanding phenomenon in the energy storage inverter. During the pulling-up process and at the final operating frequency, the system operates stably. After the controller detects the occurrence of an islanding phenomenon, the controller can control the energy storage inverter to shut down, thereby completing the anti-islanding protection action, or switch the energy storage inverter to off-grid operation, thereby realizing on-grid and off-grid switching.

[0007] As a possible implementation, the energy storage system also includes: a power detection circuit for detecting the voltage and current on the side where the energy storage converter is connected to the grid to obtain power information of the energy storage converter; and a controller for controlling the frequency of the output voltage of the energy storage converter to a target frequency based on the power information of the energy storage converter.

[0008] The controller controls the frequency of the output voltage of the energy storage converter based on the power information of the energy storage converter. Specifically, the frequency of the output voltage of the energy storage converter can be adjusted according to the power information of the energy storage converter and the first frequency adjustment control and the second frequency adjustment control.

[0009] As a possible implementation manner, the controller is also used to: adjust the weights of the first frequency adjustment control and the second frequency adjustment control according to the frequency difference between the frequency of the output voltage of the energy storage converter and the target frequency; control the frequency of the output voltage of the energy storage converter to the target frequency according to the power information of the energy storage converter, the weight of the first frequency adjustment control and the weight of the second frequency adjustment control, so as to detect the occurrence of islanding phenomenon in the energy storage converter.

[0010] As a possible implementation method, the controller is specifically used to: adjust the size of the modulation wave signal according to the power information of the energy storage inverter, the weight of the first frequency adjustment control, and the weight of the second frequency adjustment control, and send the modulation wave signal to the drive circuit of the energy storage inverter to adjust the frequency of the output voltage of the energy storage inverter.

[0011] The drive circuit of the energy storage converter may include a virtual synchronous generator. Specifically, the controller may control the adjustment weights of the first and second frequency adjustment controls based on the frequency difference, and superimpose the weights with the power information of the energy storage converter. This weighted value can thereby obtain a voltage amplitude reference for the internal potential of the virtual synchronous generator in the drive circuit, thereby adjusting the frequency of the output voltage of the energy storage converter. The voltage amplitude reference of the internal potential of the virtual synchronous generator is then used to generate a modulated wave through closed-loop control of the internal potential and voltage amplitude. The modulated wave is compared with a carrier signal to generate a drive signal, which is then transmitted to the switching devices in the energy storage converter. Under the control of the drive signal, the switching devices in the energy storage converter adjust their conduction timing, thereby adjusting the active power output by the energy storage converter.

[0012] As a possible implementation manner, the controller is specifically used to: adjust the frequency of the output voltage of the energy storage converter in a first direction according to the voltage and current on the side where the energy storage converter is connected to the grid, so as to reduce the active load increased by the energy storage converter; or, adjust the frequency of the output voltage of the energy storage converter in a second direction according to the voltage and current on the side where the energy storage converter is connected to the grid, so as to increase the active load reduced by the energy storage converter.

[0013] The energy storage converter operates in a networked virtual synchronous generation mode, carrying a certain active load. As the active load increases, the grid frequency drops. The controller, through its droop characteristic, automatically increases the active power of the converter, thereby increasing its output. As the relative relationship between the converter's output and load gradually changes, the frequency of the converter's output voltage also gradually increases. As the frequency increases, the magnitude of the converter's increased active load also decreases, ultimately achieving a stable state. The controller, through its anti-droop characteristic, automatically reduces the converter's active power, thereby reducing its output. As the mismatch between the converter's output and load continues to increase, the converter's output voltage frequency continues to drop. As the frequency drops, the magnitude of the converter's reduced active load also increases, gradually bringing the converter's output voltage frequency closer to the grid reference frequency, ultimately achieving a stable state.

[0014] As a possible implementation manner, the controller is specifically used to: if the adjustment weight of the adjusted first frequency adjustment control is greater than the adjustment weight of the second frequency adjustment control, then adjust the frequency of the output voltage of the energy storage converter toward the first direction; if the adjustment weight of the adjusted first frequency adjustment control is less than the adjustment weight of the second frequency adjustment control, then adjust the frequency of the output voltage of the energy storage converter toward the second direction.

[0015] The first frequency regulation control is used to adjust the frequency of the energy storage converter's output voltage to the grid's base frequency. This can be considered an adjustment of the droop characteristic, stabilizing the frequency of the converter's final output voltage at the grid's base frequency. The second frequency regulation control is used to shift the frequency of the converter's output voltage away from the grid's base frequency. This can be considered an adjustment of the anti-droop characteristic, stabilizing the frequency of the converter's final output voltage to the target frequency.

[0016] As a possible implementation manner, the controller is specifically configured to: when the energy storage converter is electrically connected to the power grid, adjust the weight of the first frequency regulation control to be greater than the weight of the second frequency regulation control.

[0017] When the energy storage converter is connected to the power grid, in order to support the grid voltage and frequency of the power grid, the first weight of the first frequency regulation control adjusted by the controller is greater than the second weight of the second frequency regulation control, so that the frequency of the output voltage of the energy storage converter is adjusted in the first direction. Even if the frequency of the output voltage of the energy storage converter changes at this time, the droop characteristic adjustment of the controller can be used to assist in stabilizing the adjustment of the frequency of the output voltage of the energy storage converter, so that the frequency of the voltage finally output by the energy storage converter is close to the reference frequency of the power grid.

[0018] As a possible implementation manner, the controller is specifically configured to: when the energy storage converter is disconnected from the power grid, adjust the weight of the first frequency regulation control to be smaller than the weight of the second frequency regulation control.

[0019] After the energy storage converter is disconnected from the power grid, due to the loss of the power grid's clamping effect on the frequency of the energy storage converter's output voltage, the first weight of the first frequency regulation control adjusted by the controller is less than the second weight of the second frequency regulation control, thereby adjusting the frequency of the energy storage converter's output voltage toward the first direction. The controller can then adjust the frequency of the energy storage converter's output voltage to the target frequency to detect islanding in the energy storage converter, and the energy storage system including the energy storage converter is stable in both the frequency deviation process and the final operation at the target frequency.

[0020] As a possible implementation manner, the controller is further used to: shut down the target energy storage converter after detecting that the target energy storage converter has an islanding phenomenon, or switch the target energy storage converter to an off-grid operation state after detecting that the target energy storage converter has an islanding phenomenon.

[0021] In a second aspect, the present application provides an islanding detection method, which is applied to an energy storage system, the energy storage system including at least one energy storage unit, at least one energy storage converter, a frequency detection circuit, and a controller; at least one energy storage unit is connected to at least one energy storage converter in a one-to-one correspondence, and any one of the at least one energy storage converter is used to convert direct current input from the corresponding energy storage unit into alternating current and output it to the power grid. The method includes:

[0022] A first frequency adjustment control and a second frequency adjustment control are performed on the energy storage converter, and according to the frequency difference between the frequency of the output voltage of the energy storage converter and the target frequency, the weights of the first frequency adjustment control and the second frequency adjustment control are adjusted to control the frequency of the output voltage of the energy storage converter to the target frequency, so as to detect the occurrence of islanding phenomenon in the energy storage converter, wherein the first frequency adjustment control is used to adjust the frequency of the output voltage of the energy storage converter in a first direction, which is the adjustment direction toward the grid reference frequency, and the second frequency adjustment control is used to adjust the frequency of the output voltage of the energy storage converter in a second direction, which is the adjustment direction away from the grid reference frequency.

[0023] In a third aspect, the present application also provides a photovoltaic system, comprising: at least one photovoltaic component, at least one converter, a frequency detection circuit and a controller; at least one photovoltaic component is connected one-to-one with at least one converter, and any one of the at least one converter is used to convert the direct current input by the corresponding photovoltaic component into alternating current and output it to the power grid; the frequency detection circuit is used to detect the frequency of the converter output voltage; the controller is used to perform a first frequency adjustment control and a second frequency adjustment control on the converter, and adjust the weights of the first frequency adjustment control and the second frequency adjustment control according to the frequency difference between the frequency of the converter output voltage and the target frequency to control the frequency of the converter output voltage to the target frequency to detect the occurrence of islanding in the converter, wherein the first frequency adjustment control is used to adjust the frequency of the converter output voltage in a first direction, the first direction being the adjustment direction toward the grid reference frequency, and the second frequency adjustment control is used to adjust the frequency of the converter output voltage in a second direction, the second direction being the adjustment direction away from the grid reference frequency.

[0024] For the description of the technical effects that can be achieved from the second to the third aspects above, please refer to the description of the technical effects that can be achieved by any possible design in the first aspect above, and the repetitions will not be discussed. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 A schematic diagram of the structure of an energy storage system Figure 1 ;

[0026] Figure 2 A schematic diagram of the structure of an energy storage system Figure 2 ;

[0027] Figure 3 A schematic diagram of the structure of an energy storage system Figure 3 ;

[0028] Figure 4 Schematic diagram of the controller adjusting the frequency of the energy storage converter;

[0029] Figure 5 Schematic diagram of the waveform of the controller detecting the energy storage converter. DETAILED DESCRIPTION

[0030] In order to make the purpose, technical solutions and advantages of the present application clearer, the present application will be further described in detail below with reference to the accompanying drawings. However, the example embodiments can be implemented in various forms and should not be understood as being limited to the embodiments set forth herein; on the contrary, these embodiments are provided to make the present application more comprehensive and complete, and to fully convey the concepts of the example embodiments to those skilled in the art. The same figure marks in the figures represent the same or similar structures, and thus their repeated descriptions will be omitted. The words expressing position and direction described in this application are all explained using the accompanying drawings as examples, but changes can be made as needed, and all changes are included in the scope of protection of this application. The drawings in this application are only used to illustrate relative position relationships and do not represent true proportions.

[0031] In order to make the purpose, technical solutions and advantages of the present application clearer, the present application will be further described in detail below with reference to the accompanying drawings. The specific operating methods in the method embodiments can also be applied to device embodiments or system embodiments. It should be noted that in the description of the present application, "at least one" refers to one or more, wherein multiple refers to two or more. In view of this, "multiple" can also be understood as "at least two" in the embodiments of the present invention. "And / or" describes the association relationship of associated objects, indicating that three relationships can exist. For example, A and / or B can represent: A exists alone, A and B exist at the same time, and B exists alone. In addition, the character " / ", unless otherwise specified, generally indicates that the previous and next associated objects are in an "or" relationship. In addition, it should be understood that in the description of the present application, words such as "first" and "second" are only used to distinguish the purpose of description, and cannot be understood as indicating or implying relative importance, nor can they be understood as indicating or implying order.

[0032] There are two methods for controlling energy storage converters: grid-type control and following control. Grid-type control actively establishes the AC voltage amplitude and frequency, while following control tracks the grid voltage phase but cannot actively stabilize the voltage amplitude and frequency. Grid-type control enables the energy storage converter to have the inductive droop characteristics of a synchronous generator, thereby actively stabilizing the AC voltage.

[0033] At present, most renewable energy power generation and energy storage systems use following control for energy storage converters. Following control tracks the voltage phase of the power grid and cannot actively stabilize the voltage amplitude and frequency. Therefore, grid-forming control is introduced to form an independent power system, which has the function of actively stabilizing the frequency and voltage of the power grid.

[0034] When connected to the grid, the energy storage converter operates in current source mode, actively detecting the frequency and phase information of the grid voltage to control the output current, and the internal impedance characteristic is high impedance; when off the grid, the energy storage converter operates in voltage source mode, actively determining the frequency, amplitude and phase information of the output voltage to control the output voltage, and the internal impedance characteristic is low impedance.

[0035] However, when an islanding phenomenon occurs, the energy storage converter, under existing follower control, tracks the voltage phase of the grid, and therefore needs to use the traditional follower control method to determine islanding (i.e., judging whether an islanding phenomenon has occurred based on the output voltage amplitude and frequency). In grid-type control, since the voltage and frequency output by the energy storage converter are relatively stable, the controller cannot determine whether the energy storage converter has experienced an unplanned islanding phenomenon. Therefore, the energy storage converter connected to the energy storage unit or power generation unit will continue to operate in an islanded state, which is not conducive to the relay protection of the entire grid system. If the energy storage converter under grid-type control is allowed to continue operating, once the grid is restored, the energy storage converter under grid-type control will generate a large inrush current, damaging other equipment.

[0036] By adding positive feedback, the voltage and frequency output of the energy storage converter are offset. Specifically, when the power variation approaches zero, the positive feedback effect is enhanced, causing the output frequency and voltage of the energy storage converter to change rapidly, thereby shortening the islanding detection time. However, if positive feedback is added to the loop control, the entire grid can be easily destabilized if islanding occurs or during grid-connected operation.

[0037] In view of this, it is necessary to propose an energy storage system in which a controller can pull the frequency of the output voltage of the energy storage converter where islanding occurs to the target frequency. The controller can detect the islanding phenomenon and complete the anti-islanding protection action.

[0038] See Figure 1 As shown, Figure 1An energy storage system includes at least one energy storage unit 101 , at least one energy storage converter 102 , a frequency detection circuit 103 and a controller 104 .

[0039] At least one energy storage unit 101 is connected to at least one energy storage converter 102 in a one-to-one correspondence. Any one of the at least one energy storage converter 102 is used to convert the direct current input by the corresponding energy storage unit 101 into alternating current and output it to the power grid 105 .

[0040] The frequency detection circuit 103 is used to detect the frequency of the output voltage of the energy storage converter 102. The controller is used to perform a first frequency adjustment control and a second frequency adjustment control on the energy storage converter 102, and adjust the weights of the first frequency adjustment control and the second frequency adjustment control according to the frequency difference between the frequency of the output voltage of the energy storage converter 102 and the target frequency to control the frequency of the output voltage of the energy storage converter 102 to the target frequency, so as to detect the occurrence of islanding in the energy storage converter 102, wherein the first frequency adjustment control is used to adjust the frequency of the output voltage of the energy storage converter 102 in a first direction, which is the adjustment direction toward the reference frequency of the power grid 105, and the second frequency adjustment control is used to adjust the frequency of the output voltage of the energy storage converter 102 in a second direction, which is the adjustment direction away from the reference frequency of the power grid 105.

[0041] Among them, after one or several energy storage converters 102 in the energy storage system 100 are disconnected from the power grid 105, the controller 104 can offset the frequency of the output voltage of the disconnected energy storage converter 102 through frequency-active power anti-droop control. When the frequency of the output voltage of the energy storage converter 102 is adjusted to the target frequency, the controller 104 can detect that the energy storage converter 102 has an islanding phenomenon.

[0042] When the power grid 105 is disconnected from all energy storage converters 102, the controller 104 offsets the frequency of the output voltage of all energy storage converters 102 through frequency-active power reverse droop control. When the frequency of the output voltage of all energy storage converters 102 is adjusted to the target frequency, the controller 104 detects that all energy storage converters 102 have experienced islanding.

[0043] The grid-type energy storage converter 102 can be connected to the energy storage unit 101. The energy storage unit 101 can include an energy storage system (ESS), which can include at least a photovoltaic (PV) power generation unit, a battery system, and a direct current-to-direct current (DC-DC) converter unit. The energy storage system can be connected to the energy storage converter 102 via either AC coupling or DC coupling.

[0044] The grid-type energy storage converter 102 can perform AC / DC conversion by controlling the charging and discharging of the energy storage unit 101. In the grid-connected mode, when the energy storage converter 102 operates in the current source mode, the energy storage converter 102 can detect the frequency and phase information of the voltage of the grid 105, thereby controlling the output current. In this case, the internal impedance characteristic of the energy storage converter 102 is high impedance. In the off-grid mode, when the energy storage converter 102 operates in the voltage source mode, the energy storage converter 102 can detect the frequency, amplitude, and phase information of the output voltage, thereby controlling the output voltage. In this case, the internal impedance characteristic of the energy storage converter 102 is low impedance.

[0045] In grid-based control, the energy storage converter 102 can perform frequency-active power droop control to stabilize the frequency. Specifically, the controller 104 can be configured to control the output voltage of the energy storage converter 102 based on the frequency difference between the grid 105 frequency and a reference frequency, which is the rated frequency value of the grid 105 during normal operation, thereby stabilizing the frequency.

[0046] In actual use, if the grid frequency is higher than the reference frequency, it may indicate that the active power currently transmitted on the grid 105 is higher than the active power actually required by the grid. When the frequency of the grid 105 is lower than the reference frequency, it may indicate that the active power currently transmitted on the grid is lower than the active power actually required by the grid. Based on the above correspondence between frequency and active power, a control relationship of the frequency-active power droop control strategy can be established.

[0047] Therefore, the voltage and frequency output by the energy storage converter 102 in the grid-type control are relatively stable, and it is impossible to use the sudden change of voltage amplitude and power to detect whether an islanding phenomenon has occurred. Therefore, this application introduces an anti-droop strategy. When an unplanned islanding phenomenon occurs, the output characteristics of the energy storage converter 102 with the islanding phenomenon are made to be anti-droop characteristics as a whole. The anti-droop characteristics can make the frequency of the output voltage of the energy storage converter 102 with the islanding phenomenon away from the grid reference frequency, rising or falling to the target frequency (non-grid frequency), thereby facilitating the detection of the islanding phenomenon of the energy storage converter 102.

[0048] Specifically, after the frequency detection circuit 103 detects the frequency of the output voltage of the energy storage inverter 102, the controller 104 can adjust the weights of the first frequency adjustment control and the second frequency adjustment control according to the frequency difference between the frequency of the output voltage of the energy storage inverter 102 and the target frequency, so as to control the frequency of the output voltage of the energy storage inverter 102 to the target frequency and detect the occurrence of islanding phenomenon in the energy storage inverter 102.

[0049] As a possible implementation manner, the controller 104 is specifically used to: if the adjustment weight of the first frequency adjustment control after adjustment is greater than the adjustment weight of the second frequency adjustment control, then adjust the frequency of the output voltage of the energy storage inverter 102 toward the first direction; if the adjustment weight of the first frequency adjustment control after adjustment is less than the adjustment weight of the second frequency adjustment control, then adjust the frequency of the output voltage of the energy storage inverter 102 toward the second direction.

[0050] Specifically, the first frequency adjustment control is used to adjust the frequency of the output voltage of the energy storage inverter 102 to the reference frequency of the power grid 105. The first frequency adjustment control can be regarded as an adjustment of the droop characteristic, that is, under the first frequency adjustment control, the frequency of the final output voltage of the energy storage inverter 102 can be stabilized at the reference frequency of the power grid 105.

[0051] The second frequency regulation control is used to make the frequency of the output voltage of the energy storage inverter 102 away from the reference frequency of the power grid 105. The second frequency regulation control can be regarded as the adjustment of the anti-droop characteristic, that is, under the second frequency regulation control, the frequency of the output voltage of the energy storage inverter 102 can be made away from the reference frequency of the power grid 105, so that the frequency of the voltage finally output by the energy storage inverter 102 is adjusted to the target frequency.

[0052] The controller 104 controls the energy storage converter 102 to ultimately exhibit the first frequency regulation control (droop characteristic) / second frequency regulation control (anti-droop characteristic) characteristics depending on the weight ratios thereof. When the first weight of the first frequency regulation control is greater than the second weight of the second frequency regulation control, after the controller 104 applies the first frequency regulation control of the first weight to the energy storage converter 102 and superimposes the second frequency regulation control of the second weight, the energy storage converter 102 ultimately exhibits a droop characteristic. However, if the weight of the first frequency regulation control is less than the weight of the second frequency regulation control, after the controller 104 applies the first frequency regulation control of the first weight to the energy storage converter 102 and superimposes the second frequency regulation control of the second weight, the energy storage converter 102 ultimately exhibits an anti-droop characteristic.

[0053] As a possible implementation, the controller 104 is specifically configured to: when the energy storage converter 102 is electrically connected to the grid 105 , adjust the weight of the first frequency regulation control to be greater than the weight of the second frequency regulation control.

[0054] When the energy storage inverter 102 is connected to the power grid 105, in order to support the grid voltage and frequency of the power grid 105, the first weight of the first frequency regulation control adjusted by the controller 104 is greater than the second weight of the second frequency regulation control, so that the frequency of the output voltage of the energy storage inverter 102 is adjusted in the first direction. Even if the frequency of the output voltage of the energy storage inverter 102 changes, the frequency of the output voltage of the energy storage inverter 102 can be adjusted by adjusting the droop characteristics of the controller 104, so that the frequency of the voltage finally output by the energy storage inverter 102 is close to the reference frequency of the power grid 105.

[0055] As a possible implementation manner, the controller 104 is specifically configured to: when the energy storage converter 102 is disconnected from the power grid 105 , adjust the weight of the first frequency regulation control to be smaller than the weight of the second frequency regulation control.

[0056] After the energy storage inverter 102 is disconnected from the power grid 105, the power grid 105 loses its clamping effect on the frequency of the output voltage of the energy storage inverter 102. The first weight of the first frequency regulation control adjusted by the controller 104 is less than the second weight of the second frequency regulation control, thereby adjusting the frequency of the output voltage of the energy storage inverter 102 in the first direction. The controller 104 adjusts the frequency of the output voltage of the energy storage inverter 102 to the target frequency to detect the occurrence of islanding in the energy storage inverter 102. In addition, the energy storage system 100 including the energy storage inverter 102 is stable in both the frequency deviation process and the final operation at the target frequency.

[0057] As a possible implementation, see Figure 2 As shown, Figure 2A schematic diagram of the structure of an energy storage system Figure 2 ; The power detection circuit 106 is used to: detect the voltage and current on the side where the energy storage converter 102 is connected to the power grid 105 to obtain the power information of the energy storage converter 102; the controller 104 is also used to: control the frequency of the output voltage of the energy storage converter 102 to the target frequency according to the power information of the energy storage converter 102.

[0058] The controller 104 is configured to control the frequency of the output voltage of the energy storage converter 102 based on the power information of the energy storage converter 102. Specifically, the controller 104 can adjust the frequency of the output voltage of the energy storage converter 102 based on the power information of the energy storage converter 102 and the first frequency adjustment control and the second frequency adjustment control.

[0059] As a possible implementation, see Figure 3 As shown, Figure 3 A schematic diagram of the structure of an energy storage system Figure 3 The energy storage system further includes: a power receiving circuit 107 for receiving power information sent by the energy storage converter 102; the controller 104 is further used to control the frequency of the output voltage of the energy storage converter 102 to a target frequency according to the power information of the energy storage converter 102.

[0060] In this way, there is no need to configure a corresponding power detection circuit for each energy storage inverter 102. Only the power receiving circuit 107 can be used to receive the power information sent by the energy storage inverter 102 and send the received power information to the controller 104. The controller 104 can adjust the frequency of the output voltage of the energy storage inverter 102 according to the power information of the energy storage inverter 102 and the first frequency adjustment control and the second frequency adjustment control, which can not only reduce the detection cost, but also identify the islanding phenomenon.

[0061] As a possible implementation, the controller 104 is specifically used to: adjust the size of the modulation wave signal according to the power information of the energy storage inverter 102, the weight of the first frequency adjustment control, and the weight of the second frequency adjustment control, and send the modulation wave signal to the drive circuit of the energy storage inverter 102 to adjust the frequency of the output voltage of the energy storage inverter 102.

[0062] The drive circuit of the energy storage converter 102 may include a virtual synchronous generator (VSG), which can simulate the operating mechanism of a synchronous generator. Specifically, it mainly simulates the main body model, active frequency regulation, and reactive voltage regulation of the synchronous generator, so that the voltage output by the energy storage converter 102 is comparable to that of a traditional synchronous generator in terms of operating mechanism and external characteristics. The essence of VSG is to simulate the working principle of a synchronous generator by controlling the energy storage converter 102, thereby obtaining operating characteristics similar to those of a synchronous generator. When the power grid 105 is operating normally, its load always fluctuates frequently, and the power of the virtual synchronous generator will also change accordingly. As the load fluctuates, the excitation current needs to be adjusted to maintain the voltage at the machine end or a certain point in the system at a given level.

[0063] The controller 104 can specifically control the adjustment weights of the first frequency adjustment control and the second frequency adjustment control based on the frequency error value. The frequency error value is then superimposed with the power information of the energy storage converter 102 to obtain a voltage amplitude reference for the internal potential of the virtual synchronous generator in the drive circuit, thereby adjusting the frequency of the output voltage of the energy storage converter 102.

[0064] Among them, after the voltage amplitude of the internal potential of the virtual synchronous generator is referenced, the internal potential voltage amplitude is closed-loop controlled to obtain a modulated wave. After the modulated wave is compared with the carrier signal, a drive signal can be generated, and the drive signal is sent to the switching device in the energy storage converter 102. The switching device in the energy storage converter 102 adjusts its conduction timing under the control of the above-mentioned drive signal, thereby adjusting the active power value output by the energy storage converter 102.

[0065] The above-mentioned energy storage converter 102 may include multiple switching devices, which may be one or more of various types of switching devices such as MOSFET, bipolar junction transistor (BJT), insulated gate bipolar transistor (IGBT), silicon carbide (SiC) power tube, etc., which are not listed one by one in the embodiments of the present application. If the switching device included in the energy storage converter 102 is a MOS tube, the energy storage converter 102 can be connected to the gate of the MOS tube to adjust the active power value output by the energy storage converter 102 by controlling the on-off of the MOS tube; if the switching tube in the energy storage converter 102 is a BJT, the energy storage converter 102 can be connected to the base of the BJT to adjust the active power value output by the energy storage converter 102 by controlling the on-off of the BJT.

[0066] As a possible implementation manner, the controller 104 is specifically used to: adjust the frequency of the output voltage of the energy storage converter 102 toward the first direction according to the voltage and current on the side where the energy storage converter 102 is connected to the grid, so as to reduce the active load increased by the energy storage converter 102; or, adjust the frequency of the output voltage of the energy storage converter 102 toward the second direction according to the voltage and current on the side where the energy storage converter 102 is connected to the grid, so as to increase the active load reduced by the energy storage converter 102.

[0067] Among them, see Figure 4 As shown, Figure 4 Schematic diagram of frequency regulation for an energy storage converter. The rated frequency of the grid 105 is represented by ωN, the frequency value of the output of the energy storage converter 102 is ωE (internal potential angular frequency), and the difference between the grid frequency and the frequency of the output voltage of the energy storage converter 102 is Δω. In the first processing unit 401, the difference between ωN and ωE is used as the first frequency difference signal, and then the frequency difference signal is multiplied by a specified coefficient (e.g., droop coefficient m). Then, based on the first weight of the processed first frequency regulation control, the first power reference value ΔP of the energy storage converter 102 is adjusted. ref In the second processing unit 402, after taking the difference between ωN and ωE as the second frequency difference signal, the frequency difference signal is multiplied by a specified coefficient (for example, an anti-droop coefficient F(w)), and then the second power reference value △P of the energy storage converter 102 is adjusted according to the second weight of the second frequency regulation control after the processing. anti .

[0068] Then the first power reference value △P ref and the second power reference value △P anti Input to the third processing unit 403, the first power reference value △P ref , the second power reference value △P anti Continue with the power information P of the energy storage converter 102 fed The difference is used as the input of the virtual inertia 1 / Js. The superposition of the output value and ωN is then clipped to obtain a clipping result, denoted by ω'E. The clipping result is then integrated (the integration process is represented by 1 / s) to obtain the frequency reference signal θE for the output voltage of the energy storage converter 102. The difference between ω'E and the angular velocity ωpll of the phase-locked loop can also be used as the input of the damping Damp, and the output of the damping Damp is used as the feedback value of the damping power.

[0069] At the first power reference value △P ref Greater than the second power reference value △P anti When the first power reference value △P refThe corresponding first frequency regulation control plays a major role in frequency regulation. The output characteristic of the energy storage converter 102 is reflected as a droop characteristic to adjust the frequency of the output voltage of the energy storage converter 102 to the reference frequency of the power grid. ref Less than the second power reference value △P anti When the second power reference value △P anti The corresponding second frequency adjustment control plays a major role in frequency regulation. The output characteristic of the energy storage converter 102 is embodied as an anti-droop characteristic to adjust the frequency of the output voltage of the energy storage converter 102 to the target frequency. This is merely an example, and this application does not limit the specific numerical requirements for the first weight, second weight, droop coefficient, and anti-droop coefficient of the above embodiment. Those skilled in the art may adjust them according to actual circumstances.

[0070] When the islanding phenomenon does not occur, the energy storage converter 102 exhibits a frequency-active power droop characteristic, and when the grid-connected frequency deviation is relatively small, the energy storage converter 102 still exhibits a frequency-active power droop characteristic; when the islanding phenomenon occurs, the off-grid frequency deviation is relatively large, causing the energy storage converter 102 to exhibit a frequency-active power reverse droop characteristic.

[0071] When the energy storage converter 102 operates in a networked manner using virtual synchronous power generation, it carries a certain active load. As the active load increases, the frequency of the power grid 105 decreases. The controller 104, through its droop characteristic, automatically increases the active power of the energy storage converter 102, thereby increasing its output. As the relative relationship between the output and load of the energy storage converter 102 gradually changes, the frequency of the output voltage of the energy storage converter 102 also gradually increases. As the frequency increases, the magnitude of the increased active load of the energy storage converter 102 also decreases, ultimately achieving a stable state.

[0072] At active load P load When it increases, it will cause the frequency ω of the power grid 105 to decrease. Through the effect of the droop characteristic, the energy storage converter 102P pcs The output will increase, therefore, the frequency of the output voltage of the energy storage converter 102 will gradually increase to achieve dynamic balance.

[0073] The controller 104 uses the anti-droop characteristic to automatically reduce the active power of the energy storage converter 102, thereby reducing the output. When the mismatch between the output of the energy storage converter 102 and the load continues to increase, the frequency of the output voltage of the energy storage converter 102 will continue to decrease. After the frequency decreases, the amplitude of the active load reduction of the energy storage converter 102 will also increase, thereby gradually approaching the frequency of the output voltage of the energy storage converter 102 to the reference frequency of the power grid 105, thereby finally forming a stable state.

[0074] At active load Pload When it increases, it will cause the grid frequency ω to decrease. Through the effect of anti-droop characteristics, the energy storage converter 102P pcs The output will decrease, therefore, the frequency of the output voltage of the energy storage converter 102 will gradually move away from the reference frequency of the power grid 105, thereby eventually forming a dynamic imbalance.

[0075] When the first frequency regulation control and the second frequency regulation control of the controller 104 are executed simultaneously, the energy storage converter 102 will exhibit the output characteristics of the frequency regulation function of whichever one has a stronger frequency regulation function.

[0076] As a possible implementation, the controller 104 is further configured to shut down the target energy storage converter after detecting that the target energy storage converter has an islanding phenomenon, or to switch the target energy storage converter to an off-grid operation state after detecting that the target energy storage converter has an islanding phenomenon.

[0077] Specifically, when the controller 104 detects that the target energy storage converter has an islanding phenomenon, the controller may select to shut down the target energy storage converter to complete the anti-islanding protection action; or the controller may switch the target energy storage converter to off-grid operation to achieve on-grid and off-grid switching.

[0078] For example, see Figure 5 As shown, Figure 5 This is a waveform diagram of the controller detecting the energy storage converter. When grid 105 trips, the sum of the generated power output by energy storage converter 102 balances with the load power, and the grid current can approach 0A. At the moment of tripping, the frequency and voltage remain unchanged, but under the control of controller 104, the frequency gradually deviates from the grid reference frequency. Furthermore, while the voltage output by energy storage converter 102 is adjusted to the target frequency, the voltage remains unchanged. When the frequency of the output voltage of energy storage converter 102 is adjusted to the target frequency, controller 104 can detect that an islanding phenomenon has occurred in energy storage converter 102.

[0079] This application is applicable to the grid-connected architecture of other new energy power generation systems, such as wind power generation systems and hydropower generation systems. The above new energy systems are all equipped with energy storage converters for grid-connecting the generated electricity. They are also applicable to large-scale energy storage application scenarios, small and medium-sized distributed energy storage application scenarios, etc. This is only an example, and this application does not limit specific application scenarios. Optionally, this application is also applicable to off-grid scenarios of new energy power generation systems.

[0080] The present application does not rely on sudden changes in voltage amplitude and power to detect whether an islanding phenomenon occurs. Instead, the controller 104 widens the frequency of the output voltage of the energy storage inverter to a target frequency according to a preset inherent anti-droop characteristic. During the widening process and at the final operating frequency, the operation of the entire energy storage inverter is stable. When an islanding phenomenon is detected, the controller can shut down the energy storage inverter to complete the anti-islanding protection action, or it can control the energy storage inverter to operate off-grid, thereby realizing on-grid and off-grid switching.

[0081] Based on the same concept, the present application provides an islanding detection method, which is applied to the energy storage system 100 described in the above embodiment. The energy storage system includes at least one energy storage unit, at least one energy storage converter, a frequency detection circuit, and a controller. The at least one energy storage unit is connected to the at least one energy storage converter in a one-to-one correspondence. Any of the at least one energy storage converter is used to convert direct current input by the corresponding energy storage unit into alternating current and output it to the power grid. The method includes: performing a first frequency adjustment control and a second frequency adjustment control on the energy storage converter, and adjusting the weights of the first frequency adjustment control and the second frequency adjustment control based on the frequency difference between the frequency of the output voltage of the energy storage converter and the target frequency to control the frequency of the output voltage of the energy storage converter to the target frequency, so as to detect the occurrence of an islanding phenomenon in the energy storage converter. The first frequency adjustment control is used to adjust the frequency of the output voltage of the energy storage converter in a first direction, which is an adjustment direction toward the power grid reference frequency, and the second frequency adjustment control is used to adjust the frequency of the output voltage of the energy storage converter in a second direction, which is an adjustment direction away from the power grid reference frequency.

[0082] Based on the same concept, the present application also provides a photovoltaic system, which includes: at least one photovoltaic component, at least one converter, a frequency detection circuit and a controller; the at least one photovoltaic component is connected to the at least one converter in a one-to-one correspondence, and any one of the at least one converter is used to convert the direct current input by the corresponding photovoltaic component into alternating current and output it to the power grid; the frequency detection circuit is used to detect the frequency of the converter output voltage; the controller is used to perform a first frequency adjustment control and a second frequency adjustment control on the converter, and adjust the weights of the first frequency adjustment control and the second frequency adjustment control according to the frequency difference between the frequency of the converter output voltage and the target frequency to control the frequency of the converter output voltage to the target frequency, so as to detect the occurrence of islanding in the converter, wherein the first frequency adjustment control is used to adjust the frequency of the converter output voltage in a first direction, which is the adjustment direction toward the grid reference frequency, and the second frequency adjustment control is used to adjust the frequency of the converter output voltage in a second direction, which is the adjustment direction away from the grid reference frequency.

[0083] In the above embodiments, it can be implemented in whole or in part by software, hardware, firmware or any combination thereof. When implemented using software, it can be implemented in whole or in part in the form of a computer program product. The computer program product includes one or more computer instructions. When the computer program instructions are loaded and executed on a computer, the process or function according to the embodiment of the present application is generated in whole or in part. The computer can be a general-purpose computer, a special-purpose computer, a computer network, or other programmable device. The computer instructions can be stored in a computer-readable storage medium or transmitted from one computer-readable storage medium to another computer-readable storage medium. For example, the computer instructions can be transmitted from one website, computer, server or data center to another website, computer, server or data center via a wired (e.g., coaxial cable, optical fiber, digital subscriber line (DSL)) or wireless (e.g., infrared, wireless, microwave, etc.) method. The computer-readable storage medium can be any available medium that a computer can access or a data storage device such as a server or data center that includes one or more available media integrated. The available medium can be a magnetic medium (e.g., a floppy disk, a hard disk, a tape), or a semiconductor medium (e.g., a solid-state drive (SSD)).

[0084] The present application also provides a readable storage medium for storing the methods or algorithms provided in the above embodiments, such as a random access memory (RAM), flash memory, read-only memory (ROM), EPROM, electronic programmable ROM (EPROM), register, hard disk, removable disk, or any other form of storage medium known in the art.

[0085] The method described in the embodiment of the present application or the step of the algorithm can be directly embedded in the control device. The control device may include a RAM memory, a flash memory, a ROM memory, an EPROM memory, a register, a hard disk, a removable disk or other arbitrary forms of storage media in this area, for storing the method provided by the embodiment of the present application or the step of the algorithm. Exemplarily, the storage medium can be connected with the processing module or the processor (or controller) in the control device so that the processing module, the processor (or controller) can read information from the storage medium, and can write information to the storage medium. Alternatively, the storage medium can also be integrated into the processing module, the processor (or controller).

Claims

1. An energy storage system, characterized in that: It includes at least one energy storage unit, at least one energy storage converter, a frequency detection circuit and a controller; The at least one energy storage unit is connected to the at least one energy storage converter in a one-to-one correspondence, and any one of the at least one energy storage converter is used to convert the direct current input by the corresponding energy storage unit into alternating current and output it to the power grid; The frequency detection circuit is used to: detect the frequency of the output voltage of the energy storage converter; The controller is used to: perform a first frequency adjustment control and a second frequency adjustment control on the energy storage converter, and adjust the weights of the first frequency adjustment control and the second frequency adjustment control according to the frequency difference between the frequency of the output voltage of the energy storage converter and the target frequency, so as to control the frequency of the output voltage of the energy storage converter to the target frequency, so as to detect the occurrence of an islanding phenomenon in the energy storage converter, wherein the first frequency adjustment control is used to adjust the frequency of the output voltage of the energy storage converter in a first direction, which is an adjustment direction toward the grid reference frequency, and the second frequency adjustment control is used to adjust the frequency of the output voltage of the energy storage converter in a second direction, which is an adjustment direction away from the grid reference frequency.

2. The energy storage system according to claim 1, characterized in that The energy storage system further includes: a power detection circuit, configured to detect the voltage and current at a side where the energy storage converter is connected to the power grid to obtain power information of the energy storage converter; The controller is further configured to control the frequency of the output voltage of the energy storage converter to a target frequency according to the power information of the energy storage converter.

3. The energy storage system according to claim 2, characterized in that: The controller is further configured to: adjusting the weights of the first frequency adjustment control and the second frequency adjustment control according to a frequency difference between the frequency of the output voltage of the energy storage converter and the target frequency; According to the power information of the energy storage converter, the weight of the first frequency adjustment control and the weight of the second frequency adjustment control, the frequency of the output voltage of the energy storage converter is controlled to the target frequency to detect the occurrence of islanding phenomenon in the energy storage converter.

4. The energy storage system according to claim 3, characterized in that The controller is specifically used for: According to the power information of the energy storage converter, the weight of the first frequency adjustment control and the weight of the second frequency adjustment control, the size of the modulation wave signal is adjusted, and the modulation wave signal is sent to the drive circuit of the energy storage converter to adjust the frequency of the output voltage of the energy storage converter.

5. The energy storage system according to claim 3 or 4, characterized in that: The controller is specifically used for: According to the voltage and current of the side of the energy storage converter connected to the power grid, the frequency of the output voltage of the energy storage converter is adjusted in the first direction so as to reduce the active load added by the energy storage converter; Alternatively, the frequency of the output voltage of the energy storage converter is adjusted toward the second direction according to the voltage and current at the side where the energy storage converter is connected to the grid, so as to increase the active load reduced by the energy storage converter.

6. The energy storage system according to any one of claims 1 to 5, characterized in that: The controller is specifically used for: If the adjustment weight of the first frequency adjustment control after adjustment is greater than the adjustment weight of the second frequency adjustment control, the frequency of the output voltage of the energy storage converter is adjusted toward the first direction; if the adjustment weight of the first frequency adjustment control after adjustment is less than the adjustment weight of the second frequency adjustment control, the frequency of the output voltage of the energy storage converter is adjusted toward the second direction.

7. The energy storage system according to any one of claims 1 to 6, characterized in that: The controller is specifically configured to adjust the weight of the first frequency regulation control to be greater than the weight of the second frequency regulation control when the energy storage converter is electrically connected to the power grid.

8. The energy storage system according to any one of claims 1 to 7, characterized in that: The controller is specifically configured to adjust the weight of the first frequency regulation control to be smaller than the weight of the second frequency regulation control when the energy storage converter is disconnected from the power grid.

9. The energy storage system according to any one of claims 1 to 8, characterized in that: The controller is further configured to: After detecting that the target energy storage converter has an islanding phenomenon, the target energy storage converter is shut down, or after detecting that the target energy storage converter has an islanding phenomenon, the target energy storage converter is switched to an off-grid operation state.

10. A method for detecting an islanding, applied to an energy storage system, wherein the energy storage system comprises at least one energy storage unit, at least one energy storage converter, a frequency detection circuit, and a controller; the at least one energy storage unit is connected to the at least one energy storage converter in a one-to-one correspondence, and any one of the at least one energy storage converter is configured to convert direct current input from the corresponding energy storage unit into alternating current and output it to a power grid, characterized in that: The method comprises: A first frequency adjustment control and a second frequency adjustment control are performed on the energy storage converter, and according to the frequency difference between the frequency of the output voltage of the energy storage converter and the target frequency, the weights of the first frequency adjustment control and the second frequency adjustment control are adjusted to control the frequency of the output voltage of the energy storage converter to the target frequency, so as to detect the occurrence of islanding phenomenon in the energy storage converter, wherein the first frequency adjustment control is used to adjust the frequency of the output voltage of the energy storage converter in a first direction, and the first direction is the adjustment direction toward the grid reference frequency; the second frequency adjustment control is used to adjust the frequency of the output voltage of the energy storage converter in a second direction, and the second direction is the adjustment direction away from the grid reference frequency.

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