Energy storage converter grid-connected and off-grid seamless switching control method based on current droop control

Abstract

The invention discloses an energy storage converter grid-connected and off-grid seamless switching control method based on current droop control, and the method comprises the steps: taking an active current and a reactive current as the droop amount, and adjusting the amplitude of the output voltage of an energy storage converter through the active current under the condition of a resistive line of a low-voltage power distribution network; the output voltage angular frequency of the energy storage converter is adjusted by using the reactive current, so that the control complexity is reduced, the dynamic performance and the steady-state precision are good, and the grid-connected and off-grid seamless switching of the energy storage converter can be realized; therefore, the technical defects that an energy storage converter PQ / VF dual-mode control algorithm is complex, voltage and current are uncontrollable in the mode switching process, and dynamic response of power droop control is slow are overcome, and safe and stable power supply to important loads is guaranteed.

Description

Technical Field

[0001] The present invention relates to a seamless switching control method for grid-connected and off-grid operation of an energy storage converter based on current droop control, and belongs to the field of converter control. Background Art

[0002] When the power grid is in a normal operating state, the energy storage converter converts the direct current stored in the battery into alternating current and coordinates the operation of the power grid according to requirements such as grid dispatching instructions and economy; when a fault occurs in the power system, the energy storage converter is required to operate independently to provide voltage and frequency support for important loads. Therefore, in order to ensure the stable power supply of important loads is not affected by the operating state of the power grid, the energy storage converter needs to develop a corresponding seamless switching algorithm for grid-connected and off-grid operation.

[0003] Some scholars have proposed a PQ / VF dual-mode control algorithm for an energy storage converter. It adopts VF control during island operation to provide voltage and frequency support for the islanded microgrid system; it adopts PQ control under grid-connected operation and delivers electric energy to the power grid according to dispatching instructions. When the energy storage converter adopts dual-mode control, it can adapt to multiple energy accesses, thereby improving the energy utilization efficiency. However, there are also some problems:

[0004] 1. The energy storage converter needs to include two sets of independent control algorithms, and also needs to include algorithms such as island detection, etc., with high control complexity.

[0005] 2. When the operating mode of the energy storage converter is switched, if the grid-connected switch switching instruction and the main power control mode switching signal are issued simultaneously, due to the delay in the grid-connected opening or closing process, this will cause the energy storage converter to appear in the VF control grid-connected mode and the PQ control island operation mode. During this transient switching, the voltage and current are uncontrollable, which is very likely to cause the failure of the operating mode switching or even equipment damage.

[0006] On this basis, some scholars have proposed a single-mode control algorithm for an energy storage converter using power droop control. It adopts power droop control both in grid-connected operation mode and island operation mode, reducing the control complexity and also avoiding the transient process of uncontrollable voltage and current during the switching of the operating mode and control algorithm. However, the digital filter at the power loop of the power droop control reduces the response speed of the energy storage converter, and there is a large delay in the process of operating mode switching, making it unable to adapt to scenarios that require fast power response. Summary of the Invention

[0007] The present invention aims to solve the deficiencies of the above-mentioned existing technologies, and proposes a seamless parallel-to-island switching control method for an energy storage converter based on current droop control, with the expectation of reducing the control complexity while having good dynamic performance and steady-state accuracy, which is conducive to realizing the seamless parallel-to-island switching of the energy storage converter, thereby ensuring the safe and stable power supply to important loads.

[0008] To achieve the above object, the present invention adopts the following technical solutions:

[0009] A seamless parallel-to-island switching control method for an energy storage converter based on current droop control according to the present invention is characterized by including the following steps:

[0010] Step 1: Define the voltage across the filter capacitor of the energy storage converter as the output voltage of the energy storage converter, and define the current flowing into the AC bus of the energy storage converter as the output current of the energy storage converter. Then, perform Thevenin equivalent on the energy storage converter according to the output voltage and output current of the energy storage converter to obtain the equivalent output impedance and output voltage of the energy storage converter.

[0011] Step 2: Write a set of equations for the balance of the active current magnitude and the reactive current magnitude according to the structure diagram when the Thevenin equivalent model of the energy storage converter is connected to the low-voltage distribution network.

[0012] Step 3: Combine the two equations of the set of equations to solve the expressions of the active current and the reactive current, and simplify the expressions of the active current magnitude and the reactive current magnitude under the condition that the output power angle is 0 to obtain the relationships between the active current magnitude of the energy storage converter and the output voltage amplitude, and between the reactive current magnitude and the output voltage angular frequency.

[0013] Step 4: Derive the control equation for the current droop control of the energy storage converter according to the relationships between the active current magnitude and the output voltage amplitude, and between the reactive current magnitude and the output voltage angular frequency.

[0014] Step 5: Combine the pre-synchronization control loop according to the control equation for the current droop control of the energy storage converter to obtain the control equation for the current droop control of the energy storage converter after adding the pre-synchronization control.

[0015] Step 6: Control the energy storage converter in the island operation mode according to the control equation for the current droop control of the energy storage converter after adding the pre-synchronization control to convert it into the energy storage converter in the grid-connected operation mode, and control the energy storage converter in the grid-connected operation mode to convert it into the energy storage converter in the island operation mode, thereby realizing seamless parallel-to-island switching.

[0016] The seamless parallel-to-island switching control method for an energy storage converter based on current droop control according to the present invention is also characterized in that the Step 1 includes:

[0017] Step 1.1: Construct the equivalent output impedance of the energy storage converter using Equation (1):

[0018] (1)

[0019] In Equation (1), s is the frequency domain, represents the equivalent output impedance of the energy storage converter, is the transfer function between the voltage reference and the load current input point in the forward path of the voltage control loop of the energy storage converter, is the transfer function between the load current input point and the output voltage in the forward path of the voltage control loop of the energy storage converter, and there is:

[0020] (2)

[0021] (3)

[0022] In Equations (2) and (3), is the equivalent gain of the bridge arm of the energy storage converter, is the transfer function of the voltage controller of the energy storage converter, is the transfer function of the current controller of the energy storage converter, is the value of the filter inductor of the energy storage converter, is the value of the filter capacitor of the energy storage converter;

[0023] Step 1.2: Construct the output voltage of the energy storage converter using Equation (4):

[0024] (4)

[0025] In Equation (4), is the equivalent output voltage of the energy storage converter in the frequency domain s, is the command value of the voltage loop of the energy storage converter.

[0026] Further, the said Step 2 includes:

[0027] Step 2.1: Based on the phase of the grid voltage, write the KVL equation of the equivalent voltage source of the energy storage converter passing through the equivalent output impedance, the line impedance between the energy storage converter and the grid, and from the energy storage converter to the grid using Equation (5):

[0028] (5)

[0029] In Equation (5): is the output current of the energy storage converter, is the modulus of the output current of the energy storage converter, is the phase angle of the output current of the energy storage converter; is the grid voltage, is the modulus of the grid voltage; is the equivalent output voltage of the energy storage converter, is the modulus of the equivalent output voltage of the energy storage converter, is the phase angle of the equivalent output voltage of the energy storage converter; is the sum of the equivalent output impedance of the energy storage converter and the line impedance, is the modulus of the sum of the equivalent output impedance of the energy storage converter and the line impedance, is the phase angle of the sum of the equivalent output impedance of the energy storage converter and the line impedance, and there is:

[0030] (6)

[0031] In formula (6), is the equivalent output impedance of the energy storage converter, represents the modulus of the equivalent output impedance of the energy storage converter, represents the phase angle of the equivalent output impedance of the energy storage converter; is the line impedance from the energy storage converter to the grid, represents the modulus of the line impedance from the energy storage converter to the grid, represents the phase angle of the line impedance from the energy storage converter to the grid;

[0032] Step 2.2: Expand formula (5) into formula (7):

[0033] (7)

[0034] In formula (7), j is the imaginary unit in the complex number field;

[0035] Step 2.3: After integrating formula (7) according to the real part and the imaginary part, obtain the system of equations shown in formula (8):

[0036] (8)

[0037] For formula (8), represents the modulus of the active current of the energy storage converter; represents the modulus of the reactive current of the energy storage converter.

[0038] Furthermore, the said step 3 includes:

[0039] Step 3.1, obtain the expressions of and using formula (9):

[0040] (9)

[0041] Step 3.2: Let , then the magnitude of the initially simplified active current is obtained using Equation (10) and the magnitude of the initially simplified reactive current are:

[0042] (10)

[0043] Step 3.3: Let the output power angle of the energy storage converter = 0, then the magnitude of the finally simplified active current and the magnitude of the finally simplified reactive current are obtained using Equation (11) :

[0044] (11).

[0045] Furthermore, in Step 4, Equation (12) is used to derive the control equation for the current droop control of the energy storage converter under the condition that the line impedance is resistive:

[0046] (12)

[0047] In Equation (12), is the reference value of the output voltage amplitude of the power loop; is the reference value of the output voltage angular frequency of the power loop; is the rated output active current of the energy storage converter, is the rated output reactive current of the energy storage converter; is the output active current of the energy storage converter, is the output reactive current of the energy storage converter; m is the droop coefficient of the active current - voltage amplitude, and n is the droop coefficient of the reactive current - voltage frequency; is the rated voltage amplitude of the energy storage converter, is the rated voltage angular frequency of the energy storage converter.

[0048] Furthermore, in Step 5, Equation (17) is used to obtain the control equation for the current droop control of the energy storage converter after adding pre - synchronization control:

[0049] (17)

[0050] In Equation (17), is the reference correction amount of the output voltage amplitude of the energy storage converter output by the pre - synchronization control loop; is the reference correction amount of the output voltage angular frequency of the energy storage converter output by the pre - synchronization control loop, and there is:

[0051] (18)

[0052] In formula (18), is the proportional coefficient of the PI controller in the pre-synchronization control loop; is the integral coefficient of the PI controller in the pre-synchronization control loop; s represents the frequency domain; is the q-axis component of the grid voltage; is the d-axis component of the grid voltage; is the q-axis component of the output voltage of the energy storage converter; is the d-axis component of the output voltage of the energy storage converter.

[0053] Furthermore, the said step 6 includes:

[0054] Step 6.1: Control the energy storage converter in the island operation mode to convert it into the energy storage converter in the grid-connected operation mode:

[0055] Step 6.1.1: Use a three-phase single synchronous coordinate system phase-locked loop to obtain the amplitude and phase of the grid voltage, and obtain the d-axis component and q-axis component of the grid voltage as well as the d-axis component and q-axis component of the voltage across the load of the energy storage converter according to the d-axis orientation of the grid voltage;

[0056] Step 6.1.2: After receiving the pre-synchronization enable signal from the superior, the pre-synchronization control loop respectively sends the difference between and as well as the difference between and into the PI controller for processing, and outputs the adjustment amount of the voltage amplitude and the adjustment amount of the voltage frequency to the voltage loop;

[0057] Step 6.1.3: When it is detected that the average value of within the set time is less than the proportional threshold of the rated voltage amplitude of the energy storage converter, it is considered that the output voltage amplitude of the energy storage converter reaches pre-synchronization;

[0058] When it is detected that the average value of within the set time is less than the proportional threshold of the rated voltage angular frequency of the energy storage converter, it is considered that the output voltage angular frequency of the energy storage converter reaches pre-synchronization;

[0059] When both pre-synchronizations are completed, close the grid connection switch, so that the energy storage converter is converted from the island operation mode to the grid-connected operation mode;

[0060] Step 6.2: After the energy storage converter operates stably in the grid-connected operation mode, the grid-connected switch is disconnected, and the energy storage converter adjusts the output of the power loop according to the load size according to Equation (13), so as to convert the energy storage converter in the grid-connected operation mode into the energy storage converter in the island operation mode.

[0061] An electronic device of the present invention includes a memory and a processor, characterized in that the memory is used to store a program that supports the processor to execute the seamless grid-connected and off-grid switching control method of the energy storage converter based on current droop control, and the processor is configured to execute the program stored in the memory.

[0062] A computer-readable storage medium of the present invention is characterized in that a computer program is stored on the computer-readable storage medium, and the computer program executes the steps of the seamless grid-connected and off-grid switching control method of the energy storage converter based on current droop control when run by a processor.

[0063] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0064] 1. The present invention proposes calculation formulas for the current droop coefficients m and n, which are universal, enabling the energy storage converter to operate stably under multiple working conditions such as grid-connected operation and island operation.

[0065] 2. The present invention uses current droop control as the control method of the energy storage converter. When its operation mode changes, the control algorithm does not need to be changed, reducing the complexity of control and avoiding the uncontrollable transient process of voltage and current during the switching process of the operation mode and the control algorithm, ensuring reliable power supply to the load.

[0066] 3. The present invention uses current droop control as the control method of the energy storage converter, omitting the three-phase power calculation and the digital filter at the power droop power outer loop, using the active component and reactive component of the current as the droop quantity, having good dynamic performance and being able to quickly respond to load demands. Description of the Drawings

[0067] Figure 1 It is the system structure diagram of a single energy storage converter in the present invention;

[0068] Figure 2 It is the equivalent circuit diagram of a single energy storage converter to the power grid in the present invention;

[0069] Figure 3 It is the control block diagram of the energy storage converter in the present invention;

[0070] Figure 4 It is the vector diagram of the output voltage and grid voltage of the energy storage converter in the grid voltage orientation coordinate system in the present invention;

[0071] Figure 5It is a graph of the change of the system frequency during the grid-connected and off-grid switching process of the energy storage converter in the present invention;

[0072] Figure 6 It is a three-phase voltage waveform diagram during the process of the energy storage converter in the present invention changing from island operation to grid-connected operation;

[0073] Figure 7 It is a diagram of the fundamental wave peak value and THD of the load voltage when the energy storage converter in the present invention operates in island mode and grid-connected mode. Detailed implementation manners

[0074] In this embodiment, a seamless grid-connected and off-grid switching control method for an energy storage converter based on current droop control includes the following steps:

[0075] Step 1, Figure 1 The figure shows the structure diagram of a single energy storage converter system, which is connected to the power grid through a grid-connected switch. When the grid-connected switch is closed, the energy storage converter system operates in the grid-connected mode; when the grid-connected switch is opened, the energy storage converter operates in the island mode. Now, define the voltage across the filter capacitor of the energy storage converter as the output voltage of the energy storage converter, and define the current flowing into the AC bus of the energy storage converter as the output current of the energy storage converter. Then, perform Thevenin equivalent on the energy storage converter according to the output voltage and output current of the energy storage converter to obtain the equivalent output impedance and output voltage of the energy storage converter; the Thevenin equivalent circuit is as Figure 2 shown.

[0076] Use Equation (1) to construct the equivalent output impedance of the energy storage converter:

[0077] (1)

[0078] In Equation (1), s is the frequency domain, represents the equivalent output impedance of the energy storage converter, is the transfer function between the voltage reference and the load current input point in the forward path of the voltage control loop of the energy storage converter, is the transfer function between the load current input point and the output voltage in the forward path of the voltage control loop of the energy storage converter, and there is:

[0079] (2)

[0080] (3)

[0081] In Equations (2) and (3), is the equivalent gain of the bridge arm of the energy storage converter, is the transfer function of the voltage controller of the energy storage converter, is the transfer function of the current controller of the energy storage converter, is the filtering inductance value of the energy storage converter, is the filtering capacitance value of the energy storage converter;

[0082] Construct the output voltage of the energy storage converter using Equation (4):

[0083] (4)

[0084] In Equation (4), is the equivalent output voltage of the energy storage converter in the frequency domain s, is the voltage loop command value of the energy storage converter.

[0085] Step 2: According to the structure diagram when the Thevenin equivalent circuit of the energy storage converter is connected to the low-voltage distribution network, write the equations for the balance of the active current modulus and the reactive current modulus;

[0086] Taking the phase of the grid voltage as the reference, use Equation (5) to write the KVL equation of the equivalent voltage source of the energy storage converter passing through the equivalent output impedance, the line impedance between the energy storage converter and the grid, and from the energy storage converter to the grid:

[0087] (5)

[0088] In Equation (5): is the output current of the energy storage converter, is the modulus of the output current of the energy storage converter, is the phase angle of the output current of the energy storage converter; is the grid voltage, is the modulus of the grid voltage; is the equivalent output voltage of the energy storage converter, is the modulus of the equivalent output voltage of the energy storage converter, is the phase angle of the equivalent output voltage of the energy storage converter; is the sum of the equivalent output impedance and the line impedance of the energy storage converter, is the modulus of the sum of the equivalent output impedance and the line impedance of the energy storage converter, is the phase angle of the sum of the equivalent output impedance and the line impedance of the energy storage converter, and there is:

[0089] (6)

[0090] In Equation (6), is the equivalent output impedance of the energy storage converter, represents the modulus of the equivalent output impedance of the energy storage converter, represents the phase angle of the equivalent output impedance of the energy storage converter; is the line impedance from the energy storage converter to the grid, represents the modulus of the line impedance from the energy storage converter to the power grid, and represents the phase angle of the line impedance from the energy storage converter to the power grid.

[0091] Expand Equation (5) into Equation (7):

[0092] (7)

[0093] In Equation (7), j is the imaginary unit in the complex number field;

[0094] After integrating Equation (7) according to the real and imaginary parts, the system of equations shown in Equation (8) is obtained:

[0095] (8)

[0096] Equation (8), represents the modulus of the active current of the energy storage converter; represents the modulus of the reactive current of the energy storage converter.

[0097] Step 3: Combine the two equalities of the system of equations to solve for the expressions of the active current and the reactive current, and simplify the expressions of the modulus of the active current and the modulus of the reactive current under the condition that the output power angle approaches 0, to obtain the relationships between the modulus of the active current of the energy storage converter and the amplitude of the output voltage, and between the modulus of the reactive current and the angular frequency of the output voltage;

[0098] Use Equation (9) to obtain and expressions:

[0099] (9)

[0100] In the low-voltage distribution network, it is generally considered that the line impedance is resistive. Let , then use Equation (10) to obtain the preliminarily simplified modulus of the active current and the preliminarily simplified modulus of the reactive current :

[0101] (10)

[0102] Usually, the modulus of the load impedance is much larger than the sum of the output impedance of the energy storage converter and the modulus of the line impedance. Then, at this time, the output power angle of the energy storage converter can be approximated as 0. Let the output power angle of the energy storage converter be 0. Then use Equation (11) to obtain the finally simplified modulus of the active current and the finally simplified modulus of the reactive current :

[0103] (11).

[0104] Step 4: As can be seen from Equation (11), under the condition that the line impedance is resistive, the active current output by the energy storage converter is positively correlated with the amplitude of its output voltage, and the reactive current output by the energy storage converter is positively correlated with the power angle. Since the power angle is the integral of the angular frequency of the energy storage converter, according to the relationship between the magnitude of the active current and the amplitude of the output voltage, and the magnitude of the reactive current and the angular frequency of the output voltage, the control equation of the current droop control of the energy storage converter is derived using Equation (12) under the condition that the line impedance is resistive;

[0105] (12)

[0106] In Equation (12), is the reference value of the amplitude of the output voltage of the power loop; is the reference value of the angular frequency of the output voltage of the power loop; is the rated output active current of the energy storage converter, is the rated output reactive current of the energy storage converter; is the output active current of the energy storage converter, is the output reactive current of the energy storage converter; m is the droop coefficient of the active current - voltage amplitude, and n is the droop coefficient of the reactive current - voltage frequency; is the rated voltage amplitude of the energy storage converter, is the rated voltage angular frequency of the energy storage converter.

[0107] According to the current droop control equation shown in Equation (12), its control block diagram is as shown in Figure 3 , observing Figure 3 it can be seen that the value of m in the resistive current droop control equation needs to ensure that when the active current changes , the change in its voltage amplitude is within ; similarly, the value of n needs to ensure that when the reactive current changes , the change in its frequency is within . Now, the expressions of m and n are defined as follows:

[0108] (13)

[0109] The energy storage converter adopting current droop control can provide voltage and frequency support for the system during island operation because it simulates the static characteristics of a synchronous generator. It can also coordinate the operation of the power grid according to dispatching instructions, economy, etc. during grid-connected operation. Since current droop control is a network-forming control, it can be equivalent to an independent voltage source. Its power outer loop will correct the output voltage of the energy storage converter according to the difference in output current between grid-connected operation and island operation, so that the amplitude and frequency of the output voltage gradually reach new stable values. Therefore, when the energy storage converter switches from the grid-connected mode to the island mode, there will be no obvious transient overshoot process in the output voltage.

[0110] The energy storage converter operating in the island mode will adjust the amplitude and frequency of its output terminal voltage according to the load condition according to the current droop curve. In most cases, there are deviations between the amplitude and frequency of the voltage across the filter capacitor of the energy storage converter operating in the island mode and the amplitude and frequency of the grid voltage. Taking phase A as an example, the output voltage of the energy storage converter in the island mode is defined and the grid voltage as:

[0111] (14)

[0112] (15)

[0113] In Equation (14), is the amplitude of the output voltage of the energy storage converter; is the angular frequency of the output voltage of the energy storage converter; is the initial phase of the output voltage of the energy storage converter; t is the time variable. In Equation (15) is the amplitude of the grid voltage; is the angular frequency of the grid voltage; is the initial phase of the grid voltage.

[0114] When the energy storage converter operates in the island mode, the amplitude of the output terminal voltage of the energy storage converter and the amplitude of the grid voltage differ very little and can be considered equal. Then and the instantaneous value difference between is:

[0115] (16)

[0116] It can be seen from Equation (16) that and The phase deviation will generate a voltage with an amplitude twice that of the grid voltage on both sides of the grid-connected switch. If the grid-connected switch is closed at this time, this voltage will act on the grid-connected switch that is approximately short-circuited, and a huge impact current will be generated, resulting in voltage waveform distortion and even equipment damage. Therefore, after receiving the grid connection command, the energy storage converter operating in island mode needs to perform voltage pre-synchronization before grid connection.

[0117] Step 5: According to the control equation of the current droop control of the energy storage converter, combined with the pre-synchronization control loop, use Equation (17) to obtain the control equation of the current droop control of the energy storage converter after adding pre-synchronization control;

[0118] (17)

[0119] In Equation (17), is the reference correction amount of the amplitude of the output voltage of the energy storage converter output by the pre-synchronization control loop; is the reference correction amount of the angular frequency of the output voltage of the energy storage converter output by the pre-synchronization control loop, and there is:

[0120] (18)

[0121] In Equation (18), is the proportional coefficient of the PI controller in the pre-synchronization control loop; is the integral coefficient of the PI controller in the pre-synchronization control loop; s represents the frequency domain; is the q-axis component of the grid voltage; is the d-axis component of the grid voltage; is the q-axis component of the output voltage of the energy storage converter; is the d-axis component of the output voltage of the energy storage converter.

[0122] The process of pre-synchronization is represented on the vector diagram as the output voltage vector of the energy storage converter constantly changing its own amplitude and speed, and finally coinciding with the grid voltage vector process. As Figure 4 shown, this coordinate system is oriented along the d-axis of the grid voltage vector. Project the output voltage of the energy storage converter onto this dq coordinate system as a vector, and the d-axis component and the q-axis component can be obtained. At this time, adjust the angular frequency of the energy storage converter so that and the included angle between them is 0, that is, , the frequency and phase pre-synchronization can be achieved; on this basis, adjust the amplitude of to be the same as , after meeting all the above conditions, the output voltage of the energy storage converter has been synchronized with the grid voltage.

[0123] Step 6: According to the control equation of the current droop control of the energy storage converter after adding pre-synchronization control, control the energy storage converter in the island operation mode to convert it into the energy storage converter in the grid-connected operation mode, and control the energy storage converter in the grid-connected operation mode to convert it into the energy storage converter in the island operation mode, so as to achieve seamless switching between grid connection and islanding.

[0124] Based on the above basic idea of the pre-synchronization of the energy storage converter for grid connection, a specific control method for the energy storage converter to switch from island operation to grid-connected operation based on current droop control is proposed:

[0125] Control the energy storage converter in the island operation mode to convert it into the energy storage converter in the grid-connected operation mode:

[0126] 1. Use a three-phase single synchronous coordinate system phase-locked loop to obtain the amplitude and phase of the grid voltage, and obtain the d-axis component of the grid voltage according to the d-axis orientation of the grid voltage and the q-axis component as well as the d-axis component of the voltage across the load of the energy storage converter and the q-axis component ;

[0127] 2. When receiving the pre-synchronization enable signal from the upper level, the pre-synchronization control loop respectively sends the difference between and the difference between and into the PI controller for processing, and outputs the adjustment amount of the voltage amplitude and the adjustment amount

[0128] of the voltage frequency to the voltage loop; 3. When it is detected that

[0129] the average value of within the set time (0.1 s) is less than the threshold (2%) of the rated voltage amplitude of the energy storage converter, it is considered that the output voltage amplitude of the energy storage converter reaches pre-synchronization;

[0130] When it is detected that

[0131] the average value of

[0130] within the set time (0.1 s) is less than the threshold (2%) of the rated voltage angular frequency of the energy storage converter, it is considered that the output voltage angular frequency of the energy storage converter reaches pre-synchronization;

[0131] When both pre-synchronizations are completed, close the grid connection switch, so that the energy storage converter switches from the island operation mode to the grid-connected operation mode;

[0131] After the energy storage converter operates stably in the grid-connected operation mode, the grid-connected switch is disconnected, and the energy storage converter adjusts the output of the power loop according to the load size according to Equation (13), so as to convert the energy storage converter in the grid-connected operation mode into the energy storage converter in the island operation mode.

[0132] Table 1

[0133]

[0134] To verify the seamless grid-connected and islanding switching control method of the energy storage converter based on current droop control proposed in the present invention, the algorithm is verified on the Simulink simulation platform. The main circuit and control parameters of the simulation are shown in Table 1.

[0135] Since the energy storage converter using current droop control has a significant improvement in dynamic response compared with power droop control, the frequency fluctuation during the pre-synchronization process is relatively higher than that of power droop control. However, this drawback can be limited by selecting reasonable pre-synchronization controller parameters. Figure 5 Figure shows the system frequency change during the grid-connected and islanding switching process of the energy storage converter. Observing this figure, it can be seen that the energy storage converter starts voltage pre-synchronization after receiving the pre-synchronization command and completes the pre-synchronization and switches to the grid-connected mode of operation at about 0.82 s. The maximum system frequency fluctuation is 0.1 Hz when the energy storage converter switches from the island mode to the grid-connected mode, meeting the requirement in the national standard that the maximum power fluctuation of the energy storage converter during the island-to-grid mode switching process does not exceed 0.2 Hz. A fault occurs on the grid side at 1.35 s, and the grid-connected switch is disconnected. The energy storage converter system switches from grid-connected operation to island operation. Since the energy storage converter transmits energy to both the load and the grid during grid-connected operation, when the grid-connected switch is disconnected and the system operates in island mode, the active power absorbed by the load suddenly increases, causing the frequency to rise. However, due to the regulation of the energy storage converter using current droop control, it resumes stable operation at about 1.42 s. The maximum frequency fluctuation during this process is 0.004 Hz, meeting the requirement in the national standard that the maximum power fluctuation of the energy storage converter during the grid-to-island mode switching process does not exceed 0.2 Hz.

[0136] Figure 6 The figure shows the three-phase voltage waveforms at both ends of the load during the process of the energy storage converter switching from island operation to grid-connected operation after completing pre-synchronization; Figure 7 Parts (a) and (b) in show the fundamental voltage peak value and THD when the energy storage converter operates in island mode and grid-connected mode respectively. Observing the two figures, it can be seen that when the energy storage converter operates in island mode, the fundamental voltage peak value is 312.5 V and the THD is 0.84%. When it operates in grid-connected mode, the fundamental voltage peak value is 310.3 V and the THD is 0.87%. The voltage peak value changes by 3.2 V during the entire grid-connected process, accounting for 1.3% of the rated voltage. At the same time, the voltage THD is lower than 5% during the entire process, meeting the national standard requirements.

[0137] In this embodiment, an electronic device includes a memory and a processor. The memory is used to store a program that supports the processor to execute the above method, and the processor is configured to execute the program stored in the memory.

[0138] In this embodiment, a computer-readable storage medium stores a computer program, and when the computer program is run by a processor, it executes the steps of the above method.

Claims

1. A method for seamless on-grid and off-grid switching control of an energy storage converter based on current droop control, characterized in that: The following steps are involved: Step 1, defining the voltage across the filter capacitor of the energy storage converter as the output voltage of the energy storage converter, and defining the current flowing into the AC bus of the energy storage converter as the output current of the energy storage converter, thereby performing Thevenin equivalent on the energy storage converter according to the output voltage and output current of the energy storage converter to obtain the equivalent output impedance and output voltage of the energy storage converter; Step 2: According to the structural diagram of the Thevenin equivalent model of the energy storage converter when it is connected to the low-voltage distribution network, write down the equation group of the active current modulus value and the reactive current modulus value balance; Step 3, the two equations of the system of equations are combined to solve the expressions of active current and reactive current, and under the condition that the output power angle is 0, the expressions of active current modulus and reactive current modulus are simplified to obtain the relationship between the active current modulus and the output voltage amplitude, and the reactive current modulus and the output voltage angular frequency of the energy storage converter; Step 4: derive the control equation of current droop control of the energy storage converter according to the relationship between the active current modulus and the output voltage amplitude, and the reactive current modulus and the output voltage angular frequency; Step 5, according to the control equation of the current droop control of the energy storage converter, combined with the pre-synchronization control loop, the control equation of the current droop control of the energy storage converter after adding the pre-synchronization control is obtained; Step 6: According to the control equation of the current droop control of the energy storage converter after adding the pre-synchronization control, the energy storage converter in the island operation mode is controlled to be converted into the energy storage converter in the grid-connected operation mode, and the energy storage converter in the grid-connected operation mode is controlled to be converted into the energy storage converter in the island operation mode, thereby realizing seamless switching between grid and off-grid.

2. The method for controlling the seamless on-grid and off-grid switching of an energy storage converter based on current droop control according to claim 1, characterized in that: The step 1 comprises: Step 1.1: Use equation (1) to construct the equivalent output impedance of the energy storage converter: (1) In formula (1), s is the frequency domain, Represents the equivalent output impedance of the energy storage converter, is the transfer function between the voltage reference in the forward channel of the voltage control loop of the energy storage converter and the load current input point, is the transfer function from the load current input point to the output voltage in the forward channel of the voltage control loop of the energy storage converter, and: (2) (3) In formula (2) and formula (3), is the equivalent gain of the bridge arm of the energy storage converter, is the transfer function of the voltage controller of the energy storage converter, is the transfer function of the current controller of the energy storage converter, is the filter inductance value of the energy storage converter, is the filter capacitance value of the energy storage converter; Step 1.2: Use equation (4) to construct the output voltage of the energy storage converter: (4) In formula (4), is the equivalent output voltage of the energy storage converter in the frequency domain s, is the voltage loop command value of the energy storage converter.

3. The method for controlling the seamless on-grid and off-grid switching of an energy storage converter based on current droop control according to claim 2 is characterized in that: The step 2 comprises: Step 2.1: Based on the phase of the grid voltage, use equation (5) to write the KVL equation from the equivalent voltage source of the energy storage converter through the equivalent output impedance, the line impedance between the energy storage converter and the grid, and the energy storage converter to the grid: (5) In formula (5): is the output current of the energy storage converter, is the modulus of the output current of the energy storage converter, is the phase angle of the output current of the energy storage converter; is the grid voltage, is the modulus of the grid voltage; is the equivalent output voltage of the energy storage converter, is the modulus of the equivalent output voltage of the energy storage converter, is the phase angle of the equivalent output voltage of the energy storage converter; is the sum of the equivalent output impedance of the energy storage converter and the line impedance, is the modulus of the sum of the equivalent output impedance of the energy storage converter and the line impedance, is the phase angle of the sum of the equivalent output impedance of the energy storage converter and the line impedance, and: (6) In formula (6), is the equivalent output impedance of the energy storage converter, Represents the modulus value of the equivalent output impedance of the energy storage converter, Represents the phase angle of the equivalent output impedance of the energy storage converter; is the line impedance from the energy storage converter to the grid, Represents the modulus of the line impedance from the energy storage converter to the grid, Represents the phase angle of the line impedance from the energy storage converter to the grid; Step 2.2: Expand equation (5) into equation (7): (7) In formula (7), j is the imaginary unit in the complex field; Step 2.3: After integrating equation (7) according to its real and imaginary parts, we get the equation system shown in equation (8): (8) Formula (8), Indicates the active current modulus value of the energy storage converter; Indicates the reactive current modulus value of the energy storage converter.

4. The method for controlling the seamless on-grid and off-grid switching of an energy storage converter based on current droop control according to claim 3 is characterized in that: The step 3 comprises: Step 3.1: Use formula (9) to get and The expression is: (9) Step 3.2, order , then the simplified active current modulus is obtained using formula (10): And the reactive current modulus after preliminary simplification : (10) Step 3.3: Set the output power angle of the energy storage converter =0, then the simplified active current is obtained by using formula (11): Modulus value and the final simplified reactive current modulus value : (11)。 5. The method for controlling the seamless on-grid and off-grid switching of an energy storage converter based on current droop control according to claim 4 is characterized in that: In step 4, equation (12) is used to derive the control equation for the current droop control of the energy storage converter under the condition that the line impedance is resistive: (12) In formula (12), is the reference value of the power loop output voltage amplitude; is the reference value of the power loop output voltage angular frequency; is the rated output active current of the energy storage converter, is the rated output reactive current of the energy storage converter; is the output active current of the energy storage converter, is the output reactive current of the energy storage converter; m is the droop coefficient of the active current-voltage amplitude, and n is the droop coefficient of the reactive current-voltage frequency; is the rated voltage amplitude of the energy storage converter, is the rated voltage angular frequency of the energy storage converter.

6. The method for controlling the seamless on-grid and off-grid switching of an energy storage converter based on current droop control according to claim 5 is characterized in that: In step 5, the control equation of current droop control of the energy storage converter after adding pre-synchronization control is obtained by using equation (17): (17) In formula (17), A reference correction value for the output voltage amplitude of the energy storage converter output by the pre-synchronization control loop; is the reference correction value of the output voltage angular frequency of the energy storage converter output by the pre-synchronization control loop, and has: (18) In formula (18), is the proportional coefficient of the PI controller in the pre-synchronization control loop; is the integral coefficient of the PI controller in the presynchronization control loop; s represents the frequency domain; is the q-axis component of the grid voltage; is the d-axis component of the grid voltage; is the q-axis component of the output voltage of the energy storage converter; is the d-axis component of the output voltage of the energy storage converter.

7. The method for controlling the seamless on-grid and off-grid switching of an energy storage converter based on current droop control according to claim 6, characterized in that: The step 6 comprises: Step 6.1 Control the energy storage converter in the island operation mode to convert it into the energy storage converter in the grid-connected operation mode: Step 6.1.

1. Use the three-phase single synchronous coordinate system phase-locked loop to obtain the amplitude and phase of the grid voltage, and obtain the d-axis component of the grid voltage according to the d-axis orientation of the grid voltage. and the q-axis component And the d-axis component of the voltage across the energy storage converter load and the q-axis component ; Step 6.1.2: After receiving the pre-synchronization enable signal from the upper level, the pre-synchronization control loop and The difference and and The difference is sent to the PI controller for processing and outputs the adjustment amount of the voltage amplitude and voltage frequency adjustment Give voltage loop; Step 6.1.3: When detected When the average value within the set time is less than the proportional threshold of the rated voltage amplitude of the energy storage converter, it is considered that the output voltage amplitude of the energy storage converter reaches pre-synchronization; When detected If the average value within the set time is less than the proportional threshold of the rated voltage angular frequency of the energy storage converter, it is considered that the output voltage angular frequency of the converter has reached pre-synchronization; When both pre-synchronizations are completed, the grid-connected switch is closed, so that the energy storage converter switches from the island operation mode to the grid-connected operation mode; Step 6.2: After the energy storage converter stably operates in the grid-connected operation mode, the grid-connected switch is disconnected, and the energy storage converter adjusts the output of the power loop according to the load size according to formula (13), thereby converting the energy storage converter in the grid-connected operation mode into the energy storage converter in the island operation mode.

8. An electronic device, comprising a memory and a processor, characterized in that: The memory is used to store a program that supports the processor to execute any one of the current droop control-based on-grid and off-grid seamless switching control methods for energy storage converters in claims 1-7, and the processor is configured to execute the program stored in the memory.

9. A computer-readable storage medium having a computer program stored thereon, characterized in that: When the computer program is executed by a processor, the steps of any one of the methods for controlling the seamless on-grid and off-grid switching of an energy storage converter based on current droop control in claims 1-7 are executed.

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