Self-adaptive virtual impedance control method in fault state of grid-connected inverter

Abstract

The invention relates to the technical field of energy storage converter control, and particularly discloses an adaptive virtual impedance control method for a grid-connected inverter in a fault state, which comprises the following steps of: acquiring an output voltage, and when the output voltage is smaller than a voltage threshold, determining that the grid-connected inverter is in a fault state; according to the output voltage, calculating the voltage drop of the rated voltage and the reactive current needing to be injected into the inverter; calculating a real part and an imaginary part of the virtual impedance according to the output voltage, the voltage drop of the rated voltage and the reactive current needing to be injected into the inverter; obtaining an output current, and calculating an adaptive virtual voltage according to the output current and the real part and the imaginary part of the virtual impedance; and revising an output voltage reference value by using the self-adaptive virtual voltage, and then carrying out double-loop control to generate a PWM (Pulse-Width Modulation) gate signal. According to the invention, the voltage drop is used as an action signal of the virtual impedance, and the inverter injects the reactive current proportional to the voltage drop to reduce the fault when the power grid has the fault.

Description

Technical Field

[0001] The present invention relates to the technical field of energy storage converter control, and particularly to a control method for adaptive virtual impedance in the fault state of a grid-connected inverter. Background Art

[0002] When there is a large renewable energy system (RESs) in the power system, the system is prone to instability. In a traditional power system using synchronous generators, the frequency of the system does not change rapidly, mainly because large rotating components provide inertia, which enables the power grid to withstand various disturbances and sudden changes in load. On the other hand, RESs rely on inverters to generate electricity.

[0003] Although inverter-based systems offer the advantages of fast control and flexible change of control parameters during operation, they do not provide any mechanical inertia. To solve the problem of the reduction of mechanical inertia in a power system mainly composed of power electronic devices, the concept of virtual inertia is proposed. Its basic idea is to use an energy storage system to control distributed generation units to simulate a synchronous generator by providing virtual inertia equivalent to the rotor inertia.

[0004] A grid-connected inverter is a voltage source that can operate in grid-connected and independent modes and support the inertia of a weak grid. Multiple control techniques such as droop control and virtual synchronous generator (VSG) control can be used to regulate the operation of the grid-connected inverter. However, different from a synchronous generator that can provide 5 to 7 times the rated current during a short circuit, the grid-connected inverter has a limited rated current capacity due to the limitation of semiconductor devices. Therefore, in the event of a fault, special attention must be paid to controlling their output current.

[0005] Different from a grid-following (GFL) inverter, a grid-connected inverter lacks the inherent ability to limit the output current during a short circuit. Therefore, the virtual impedance method is adopted to use the output current to simulate the physical impedance at the output, thereby limiting the current. Many studies have solved this technical problem and successfully limited the current within the specified range. However, some grid codes not only require current limiting but also require injecting reactive current proportional to the voltage drop. Existing control methods do not control the grid-connected inverter to give a current proportional to the voltage drop to support the stability of the grid during a grid fault. Summary of the Invention

[0006] The present invention aims to solve the problem of reactive current injection during a grid voltage fault. For this purpose, the present invention provides a control method for adaptive virtual impedance in the fault state of a grid-connected inverter, using the voltage drop as the action signal of the virtual impedance, and injecting reactive current proportional to the voltage drop by the inverter during a grid fault to mitigate the fault.

[0007] The present invention provides a control method for adaptive virtual impedance in the fault state of a grid-connected inverter. The technical solution adopted is as follows: It includes the following steps: S1: Obtain the output voltage. When the output voltage is less than the voltage threshold, the grid-connected inverter is in the fault state, and proceed to S2; S2: Calculate the voltage drop of the rated voltage and the reactive current to be injected by the inverter according to the output voltage; S3: Calculate the real part and the imaginary part of the virtual impedance according to the output voltage, the voltage drop of the rated voltage, and the reactive current to be injected by the inverter; S4: Obtain the output current, and calculate the adaptive virtual voltage according to the output current, the real part, and the imaginary part of the virtual impedance; S5: Revise the output voltage reference value by using the adaptive virtual voltage, and then perform double-loop control to generate the PWM gate signal.

[0008] Further, the value of the reactive current to be injected by the inverter is the product of the proportionality factor and the voltage drop.

[0009] Further, the value range of the proportionality factor is 2 to 7.

[0010] Further, the inverter adopts a virtual synchronous generator control strategy for control.

[0011] Further, the calculation formulas for the real part and the imaginary part of the virtual impedance are: Among them, is the real part of the virtual impedance, is the imaginary part of the virtual impedance, is the maximum allowable current, is the voltage drop, is the proportionality factor, is the d-axis component of the output voltage, is the q-axis component of the output voltage.

[0012] Further, the adaptive virtual voltage includes the d-axis component of the adaptive virtual voltage and the q-axis component of the adaptive virtual voltage, and the calculation formula is: Among them, is the output current 's d-axis component, is the output current 's q-axis component.

[0013] Further, the double-loop control is an external voltage loop and an internal current control loop.

[0014] Further, the output voltage reference value minus the adaptive virtual voltage gives the revised output voltage reference value.

[0015] Further, in the external voltage loop, according to the revised output voltage reference value and the output voltage, the voltage error is calculated. The voltage error is adjusted by a PI controller and decoupled by combining with the cross-coupling term to obtain the current reference value. After passing through a limiter, the current reference value is fed into the internal current control loop.

[0016] One or more of the above technical solutions in the embodiments of the present invention have at least one of the following technical effects: The present invention adopts virtual synchronous generator control to provide inertial support for the power grid, establishes an equivalent model of adaptive virtual impedance, uses the voltage drop as the action signal of the virtual impedance, and injects a current proportional to the voltage drop during grid faults to mitigate the severity of the faults and meet the grid specification requirements during voltage faults. The present invention can minimize the severity of faults, significantly improve the stability and reliability of the power system, and is of great significance for the research on the control of adaptive virtual impedance for reactive current control in the fault state of grid-connected inverters.

[0017] The additional aspects and advantages of the present invention will be partly given in the following description, partly will become obvious from the following description, or will be understood through the practice of the present invention. Description of the Drawings

[0018] In order to more clearly illustrate the technical solutions in the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the drawings in the following description are some embodiments of the present invention. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0019] Figure 1 is the flowchart of the example of the present invention.

[0020] Figure 2 is the characteristic diagram of the reactive current-voltage drop relationship of the example of the present invention.

[0021] Figure 3 is the control block diagram of the virtual synchronous generator of the example of the present invention.

[0022] Figure 4 is the reactive droop control block diagram of the example of the present invention.

[0023] Figure 5 is the equivalent circuit diagram of the grid-connected inverter adopting multi-loop control and virtual impedance of the embodiment of the present invention.

[0024] Figure 6It is a graph showing the functional relationship between the real and imaginary parts of the virtual impedance and the voltage drop when k = 2 and k = 3 in the embodiments of the present invention.

[0025] Figure 7 It is a graph showing the change trajectories of the inverter output power and power angle during three-phase short circuit in the embodiments of the present invention.

[0026] Figure 8 It is the effective voltage value output by the inverter when k = 2 and 3 in the embodiments of the present invention.

[0027] Figure 9 It is the values of the real and imaginary parts of the virtual impedance when k = 2 and 3 in the embodiments of the present invention.

[0028] Figure 10 It is the reference current and the active and reactive currents output when k = 2 and 3 in the embodiments of the present invention.

[0029] Figure 11 It is the three-phase grid current during low voltage ride-through when k = 2 in the embodiments of the present invention.

[0030] Figure 12 It is the three-phase grid current during low voltage ride-through when k = 3 in the embodiments of the present invention.

[0031] Figure 13 It is the active and reactive power during low voltage ride-through when k = 2 in the embodiments of the present invention.

[0032] Figure 14 It is the active and reactive power during low voltage ride-through when k = 3 in the embodiments of the present invention.

[0033] Figure 15 It is the active and reactive currents during three-phase short circuit at four short-circuit ratios in the embodiments of the present invention. Detailed implementation manners

[0034] To make the objectives, technical solutions and advantages of the present invention clearer, the technical solutions in the present invention will be clearly and completely described below with reference to the accompanying drawings in the present invention. Apparently, the described embodiments are some but not all of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art without making creative efforts based on the embodiments in the present invention belong to the scope protected by the present invention. The following embodiments are used to illustrate the present invention but cannot be used to limit the scope of the present invention.

[0035] In the description of this specification, the description with reference to terms such as "one embodiment", "some embodiments", "example", "specific example", or "some examples" means that the specific features, structures, or characteristics described in connection with the embodiment or example are included in at least one embodiment or example of the embodiments of the present invention. In this specification, the schematic expressions of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, or characteristics described can be combined in a suitable manner in any one or more embodiments or examples. In addition, without contradiction, those skilled in the art can combine and combine the different embodiments or examples described in this specification and the features of different embodiments or examples.

[0036] The following will further elaborate on the present invention in conjunction with Figures 1 to 15 to describe a control method for adaptive virtual impedance in the fault state of a grid-connected inverter of the present invention: Grid codes are undergoing significant revisions to adapt to the evolving power system. For grid-connected inverters, injecting reactive current plays a crucial role in mitigating the severity of faults. Grid standards stipulate that the inverter must inject reactive current proportional to the voltage drop, as Figure 2 shown in the chart describing reactive current control, which involves dead zones, voltage drop limits, and maximum reactive current limits, etc., aiming to ensure voltage stability and equipment safety while controlling reactive power. The slope of the straight line in the figure is determined by the proportionality factor k, and its range is usually between 2 and 7. Although initially proposed for grid-connected photovoltaic and wind turbines, the injection of reactive current during grid faults is also applicable to grid-connected inverters. The present invention controls the inverter using a virtual synchronous generator, establishes an equivalent model of adaptive virtual impedance, uses the voltage drop as the action signal of the virtual impedance, and injects current proportional to the voltage drop during grid faults.

[0037] In this embodiment, as Figure 1 shown, a control method for adaptive virtual impedance in the fault state of a grid-connected inverter is provided, including the following steps: S1: Obtain the output voltage. When the output voltage is less than the voltage threshold, the grid-connected inverter is in a fault state, and proceed to S2.

[0038] When the output voltage is less than the voltage threshold the inverter is in a fault state, and the virtual impedance controller designed by the present invention is activated. The virtual impedance controller is used to make the inverter inject reactive current to mitigate the severity of the fault, specifically, realized through the adaptive virtual voltage generated by steps S2 to S4.

[0039] S2: Calculate the voltage drop of the rated voltage and the reactive current to be injected by the inverter according to the output voltage, asFigure 5 as shown

[0040] Calculate the difference between the output voltage and the rated voltage to obtain the voltage drop of the rated voltage.

[0041] The value of the reactive current to be injected by the inverter is the product of the proportionality factor and the voltage drop. The value range of the proportionality factor is 2 to 7.

[0042] S3: Calculate the real part and the imaginary part of the virtual impedance according to the output voltage, the voltage drop of the rated voltage, and the reactive current to be injected by the inverter.

[0043] The calculation formulas for the real part and the imaginary part of the virtual impedance are as follows: where is the real part of the virtual impedance, is the imaginary part of the virtual impedance, is the maximum allowable current, is the voltage drop, is the proportionality factor, is the d-axis component of the output voltage, is the q-axis component of the output voltage.

[0044] S4: Obtain the output current, and calculate the adaptive virtual voltage according to the output current, the real part and the imaginary part of the virtual impedance.

[0045] The adaptive virtual voltage includes the d-axis component and the q-axis component of the adaptive virtual voltage, and the calculation formula is as follows: where is the output current of the d-axis component, is the output current of the q-axis component.

[0046] S5: Revise the output voltage reference value by using the adaptive virtual voltage, and then perform double-loop control to generate the PWM gate signal.

[0047] The double-loop control mentioned above is an external voltage loop and an internal current control loop.

[0048] In the external voltage loop, the output voltage reference value ( and ) minus the adaptive virtual voltage ( and ) to obtain the revised output voltage reference value ( and ). According to the revised output voltage reference value and the output voltage ( and ), the voltage error ( and ) is calculated. The voltage error is adjusted by a PI controller and decoupled by combining with the cross-coupling term to obtain the current reference values ( and ). After passing through a limiter, the current reference values are fed into the internal current control loop. The internal current control loop generates PWM gate signals.

[0049] In this embodiment, according to the requirements of grid standards, when the grid fails, the grid-connected inverter injects reactive current proportional to the voltage drop. The calculation formula for the reactive current is: (1) Wherein, is the reactive current to be injected by the inverter, is the proportionality factor, and its range is usually between 2 and 7, is the voltage drop.

[0050] For the inverter, a virtual synchronous generator control strategy is adopted for control. The rotor equation of the synchronous generator is simulated in the inverter to achieve inertia, which is given by the following formula: (2) In the formula, is the input power, is the output power, is the rotor inertia, is the rotor speed, t is time, is the damping coefficient, is the difference between the rotor speed and the grid frequency.

[0051] When the rotor equation is applied to the inverter, it approximates a synchronous generator. When the inertia decreases, the rate of change of frequency (ROCOF) of the system increases, resulting in a larger frequency deviation within the same time period. The control block diagram of the virtual synchronous generator (VSG) is as shown in Figure 3 . The VSG simulates the inertia and damping characteristics of an actual synchronous generator to help stabilize the frequency of the system. This control strategy combines the reference value of the active power and the actual output active power to correct the frequency deviation. Figure 3 In , is the virtual inertia constant, the reference phase angle is generated by the VSG controller, the reference frequency is generated by the reactive power controller, s is the Laplace operator, is the frequency deviation,

[0052] (3) Among them, is the rated capacity.

[0053] Figure 4 represents the control of the inverter's reactive power to ensure voltage stability. Through the filter and gain coefficient to adjust the voltage drop , achieving precise regulation of the voltage. The reactive power control starts from the reactive power reference value and adjusts to the actual output voltage to reach the desired voltage level. Figure 4 In is the angular frequency parameter related to reactive power control, is the actually output reactive power, is the externally given voltage.

[0054] Figure 3 and Figure 4 together constitute the active and reactive power regulation of the inverter.

[0055] On this basis, this embodiment establishes an equivalent model of virtual impedance, uses the voltage drop as the action signal of the virtual impedance, and adaptively adjusts the virtual impedance during grid faults. Specifically, this embodiment calculates the required power demand through the active and reactive power reference values. Through the current controller and voltage controller, the system can adjust the input current and output voltage according to the current and voltage reference values, thereby achieving precise control of the load and the grid. The inverter uses pulse width modulation (PWM) technology to regulate the voltage output. And adopts a multi-loop control method, as shown in Figure 5 . The external voltage loop uses a PI controller to control the capacitor voltage in the synchronous frame. The output of the PI controller serves as the reference inductor current, and then there is a current limiter to prevent extreme current reference values from exceeding the maximum limit of the inverter. The internal current control loop controls the inverter inductor current, and the PWM controller generates PWM gate signals to regulate the inverter output voltage.

[0056] This embodiment focuses on designing the adaptive virtual voltage ( and ), which revises the reference voltage when the virtual impedance controller is activated in case of a fault. It is defined as follows: (4) Among them, is the d-axis component of the adaptive virtual voltage, is the q-axis component of the adaptive virtual voltage, is the real part of the virtual resistance impedance, is the imaginary part of the virtual resistance impedance, is the input current The d-axis component, is the input current of the q-axis component. The virtual impedance controller determines the real and imaginary parts of the virtual impedance based on the voltage deviation.

[0057] To effectively control the active and reactive components of the output current during a fault, an adaptive virtual impedance adjusted according to the voltage drop level is adopted, and the voltage error signal is defined as: (5) where, is the d-axis voltage error, is the q-axis voltage error, is the d-axis component of the output voltage reference value, is the q-axis component of the output voltage reference value, is the d-axis component of the output voltage, is the q-axis component of the output voltage.

[0058] Based on Equation (5), the injected reactive current may be less than the maximum capacity of the inverter. Therefore, active current is injected to prevent a significant drop in active power within the system and ensure that the total current does not exceed the rated current of the inverter. The reference active current during a fault is: (6) where, is the maximum allowable current.

[0059] To maintain the ideal operating point and prevent the voltage and power controller windings, the voltage error in Equation (5) must be zero at steady state. Setting to zero and using the reference current equations in Equations (1) and (6), rearranging Equation (5), the values of and can be calculated as follows: (7) where, is the revised d-axis component of the output voltage reference value, is the revised q-axis component of the output voltage reference value.

[0060] Based on Equation (7), and as functions of the voltage drop are as Figure 6 shown. It can be observed that for small voltage drops, increases as the voltage decreases and then decreases to zero. For severe faults, the virtual impedance is mainly reactive. In addition, increasing the value of the k factor causes to quickly tend to zero even for small voltage drops.

[0061] After the virtual impedance is set, the active power flowing from the inverter to the grid can be calculated as: (8) where is the power angle, defined as the phase difference between the internal reference voltage and the grid voltage , is the transformer reactance, is the grid reactance.

[0062] Figure 7 Describes the variation trajectory of the output power and power angle during a three-phase short-circuit fault. The curve of corresponds to normal operating conditions, using Equation (6) and . When a fault occurs (three-phase short circuit), the power rapidly drops from point a to zero at point b, triggering the activation of the virtual impedance. When the fault is cleared and the output voltage begins to recover, the operating point transitions from point b to point c. It should be noted that during the voltage recovery phase,

[0063] When the inverter current reaches the maximum limit value, the inverter can be regarded as a current source, and the output active power is: (9) where is the maximum allowable value of the inverter current when the output active power.

[0064] In this case, the operating point follows the saturation curve and then returns to point a.

[0065] This embodiment verifies this control method through experiments.

[0066] Table 1

[0067] In the current control test experiment under fault conditions, such as Figure 8 the voltage mode shown, verify the ability of this method to control the reactive current during a fault. The parameters of the inverter are shown in Table 1. As Figure 8 shown, at about 2.4 seconds, the grid voltage drops to about 20%, rises to about 70% at 2.8 seconds, and recovers to 100% at about 3.6 seconds. Consider two values of the k factor: 2 and 3. The real and imaginary parts of the virtual impedance are as Figure 9 shown, and the reference active, reactive currents and output current are asFigure 10 as shown Figure 10 in (a) and Figure 10 in (b), when k = 3, the voltage is about 70%, and the reactive current injected by the inverter is higher than that when k = 2. The increase in reactive current helps to support the power grid, and the output voltage increases by about 6%, as Figure 8 shown

[0068] The results show that this method can control the output reactive current to follow the reference current during a fast fault. The reactive current control has a feed-forward path, while the active component of the current does not have a feed-forward signal. No active current needs to be injected during a fast fault, which is the reason for its relatively slow speed in Figure 10 it

[0069] Figure 11 and Figure 12 respectively show the three-phase (A, B, C) output currents when k = 2 and 3. It is confirmed that the output current during the fault is a sine wave with less distortion. The active and reactive power signals are as Figure 13 and Figure 14 shown

[0070] In the experiment on the influence of the short-circuit ratio on voltage recovery, different short-circuit ratios (SCR) are considered to check the trajectory of the operating point during the fault. The short-circuit ratio is defined as the reciprocal of the grid impedance and is used as an index of the grid strength. A lower short-circuit ratio indicates a higher grid line impedance and a weaker grid. The SCR is calculated as follows: (10) where is the grid impedance in p.u., is the grid voltage, is the rated power

[0071] The experiment studied four grid conditions, namely SCR = 10, 6, 3, and 2. The output active and reactive current components are as Figure 15 shown. During the voltage recovery, it can be observed that a higher short-circuit ratio (indicating a stronger grid) leads to an overshoot of the active current

[0072] In summary, according to the requirements of the power grid, the present method controls the reactive current as a function of the voltage drop without switching to the current control mode. The virtual impedance is calculated and adjusted according to the voltage drop, and a specific reactive current value is injected. The control of the virtual impedance is performed using a proportionality factor (k), which can be adjusted between 2 and 7, with a default value of 2. The larger the value of k, the greater the injected reactive current, and the stronger the power grid voltage support during the LVRT (Low Voltage Ride Through) event and the priority is given to the reactive current; if the total current is lower than the rated value, additional active current is injected. The present method can enhance the stability of the power grid during grid faults. The control strategy of the proposed virtual impedance is verified through simulation, and the effectiveness and feasibility of the method are verified.

[0073] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions recorded in the foregoing embodiments, or perform equivalent replacements for some of the technical features; and these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A control method for adaptive virtual impedance in the fault state of a grid-connected inverter, characterized in that It includes the following steps: S1: Obtain the output voltage. When the output voltage is less than the voltage threshold, the grid-connected inverter is in a fault state and proceed to S2; S2: Calculate the voltage drop of the rated voltage and the reactive current to be injected by the inverter according to the output voltage; S3: Calculate the real part and the imaginary part of the virtual impedance according to the output voltage, the voltage drop of the rated voltage, and the reactive current to be injected by the inverter; S4: Obtain the output current. Calculate the adaptive virtual voltage according to the output current, the real part, and the imaginary part of the virtual impedance; S5: Use the adaptive virtual voltage to revise the output voltage reference value, and then perform double-loop control to generate the PWM gate signal.

2. The control method of adaptive virtual impedance under the fault state of a grid-connected inverter according to claim 1, characterized in that, The value of the reactive current to be injected by the inverter is the product of the proportionality factor and the voltage drop.

3. The control method of adaptive virtual impedance under the fault state of a grid-connected inverter according to claim 2, characterized in that, The value range of the proportionality factor is 2 to 7.

4. The control method of adaptive virtual impedance under the fault state of a grid-connected inverter according to claim 2, wherein, The inverter is controlled by adopting the virtual synchronous generator control strategy.

5. The control method of adaptive virtual impedance under the fault state of a grid-connected inverter according to claim 4, characterized in that, The calculation formulas for the real part and the imaginary part of the virtual impedance are: Among them, is the real part of the virtual resistance impedance, is the imaginary part of the virtual resistance impedance, is the maximum allowable current, is the voltage drop, is the scaling factor, is the d-axis component of the output voltage, is the q-axis component of the output voltage.

6. The control method of adaptive virtual impedance under the fault state of a grid-connected inverter according to claim 5, characterized in that The adaptive virtual voltage includes the d-axis component of the adaptive virtual voltage and the q-axis component of the adaptive virtual voltage , and the calculation formula is as follows: Among them, is the d-axis component of the output current , is the q-axis component of the output current .

7. The control method of adaptive virtual impedance under the fault state of a grid-connected inverter according to claim 1, characterized in that, The double-loop control is an external voltage loop and an internal current control loop.

8. The control method of adaptive virtual impedance under the fault state of a grid-connected inverter according to claim 7, wherein Subtract the adaptive virtual voltage from the output voltage reference value to obtain the revised output voltage reference value.

9. The control method of adaptive virtual impedance under the fault state of a grid-connected inverter according to claim 8, wherein, In the external voltage loop, according to the revised output voltage reference value and the output voltage, calculate the voltage error. The voltage error is adjusted by a PI controller and decoupled by combining with the cross-coupling term to obtain the current reference value. After passing through the limiter, the current reference value is sent to the internal current control loop.

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