Transient stability control method for enhancing grid-forming inverter adopting current-limiting protection

Abstract

The invention discloses a transient stability control method for enhancing a network-forming inverter during current-limiting protection, and belongs to the field of DC / AC converters. The current-limiting protection measures are a circular current limiter and an embedded virtual impedance, and when the inductive current reference value generated by the inverter exceeds the allowable maximum value, the circular current limiter only limits the inductive current reference value and keeps the angle unchanged. The combined action of the two can be equivalent to self-adaptive virtual impedance so as to limit the magnitude of current flowing through the inverter. According to the control structure, a power feed-forward part is introduced on the basis of traditional droop control, the power feed-forward part plays a role only when the current limiter works, and therefore the purposes that the power transmission characteristic of the grid-forming inverter is not changed when the grid-forming inverter encounters small faults, and the grid-forming inverter can still work normally and stably when the grid-forming inverter encounters large power grid faults are achieved.

Description

Technical Field

[0001] This invention belongs to the field of DC / AC converters, and more specifically, relates to a transient stability control method for an enhanced grid-type inverter when current limiting protection is used. Background Technology

[0002] Currently, my country's power system is characterized by large-scale centralized grid connection of renewable energy generation, represented by wind and solar power. This results in a power system exhibiting "high-frequency and high-voltage" characteristics. The low inertia, low short-circuit current, and weak immunity of power electronic equipment contribute to the nonlinear, time-varying, uncertain, and complex dynamic characteristics of this power system. Grid-connected inverters have received widespread attention due to their ability to support both frequency and voltage. However, due to the voltage source characteristics of grid-connected inverters, their output current is highly dependent on external system conditions. Grid-connected inverters based on power electronic semiconductor devices can typically only withstand 1.2-2 times the overcurrent. This means that in the event of a fault, grid-connected inverters must employ appropriate current-limiting methods to restrict the current flowing through the inverter bridge arms.

[0003] Currently, current limiting protection strategies for grid-connected inverters are mainly divided into two categories: virtual impedance method and current limiter method. The virtual impedance method has the advantage of maintaining voltage source characteristics even under severe faults, but it also carries the risk of transient overcurrent during faults. In comparison, the current limiter method has a faster response speed and better current limiting effect, which is beneficial for suppressing transient overcurrents in the inverter, but it may not be able to restore the inverter to normal operating conditions after a fault. The control method based on circular current limiters and embedded virtual impedance combines the advantages of both, effectively suppressing overcurrents and ensuring the voltage source characteristics of the inverter, thus having a wider range of application prospects.

[0004] The mainstream control methods for grid-connected inverters are droop control and Virtual Synchronous Generator (VSG) control. The basic idea is to support frequency and voltage by simulating the swing equations of a synchronous generator. However, grid-connected inverters can still experience transient instability during large grid faults. To ensure the transient stability of grid-connected inverters under different grid faults, one solution is to dynamically adjust control parameters to adapt to the fault. This method improves transient stability without adding extra control, but it depends on the speed and accuracy of fault detection. Another solution is to modify the control structure. This method improves stability under transient disturbances by changing the system's dynamic response, but it usually alters the system's steady-state response.

[0005] It is evident that grid-connected inverters still face considerable challenges when encountering severe grid faults. The key issue for grid-connected inverters is how to ensure transient stability under a wide range of grid faults while implementing current-limiting protection measures. Summary of the Invention

[0006] In view of the above-mentioned defects or improvement needs of the existing technology, the present invention provides a transient stability control method for grid-connected inverters when using current limiting protection, the purpose of which is to enable grid-connected inverters to maintain transient stability under a wide range of grid faults.

[0007] To achieve the above objectives, the present invention provides a transient stability control method for enhanced grid-connected inverters when current limiting protection is employed, comprising the following steps:

[0008] The difference between the inverter's output voltage reference and the actual output voltage, after passing through the virtual impedance forward path, results in the current i of the decoupled branch. dq +jω0C f v dq The inductor current reference i' at the voltage loop output is obtained by summing the values. Ldqref The virtual impedance forward channel introduces a forward channel virtual impedance R. v +jX v , where R v and X v These are the real and imaginary parts of the forward path virtual impedance, respectively. dq i represents the actual output voltage of the inverter. dq C is the output current of the inverter. f ω0 is the filter capacitor, and ω0 is the reference value of the inverter's angular velocity.

[0009] Actual inductor current reference i Ldqref Traffic limiting will be implemented according to the following rules:

[0010]

[0011] Among them, i' Ldqref I serves as the inductor current reference for the voltage loop output. max The maximum current that the inverter switching transistor can be allowed to flow through, ||i' Ldqref ||for i' Ldqref Size;

[0012] Define the current limiting coefficient Using the current limiting coefficient K lim The difference between 1 and 1 can be used for power feedforward, which is equivalent to reducing the active power reference or increasing the equivalent output power P. eq This is beneficial for transient stability after the current limiting is triggered.

[0013] (1) When K lim When = 1, current limiting is not triggered: the power of feedforward is 0;

[0014] (2) When K limWhen the value is greater than 1, current limiting is triggered: adaptive adjustment introduces virtual power (K). lim -1)·K m P ref Perform power feedforward, where K m P is the power feedforward coefficient. ref This serves as the active power reference for the inverter.

[0015] Furthermore, the grid-type inverter voltage loop control structure provided by this invention consists of a virtual impedance forward path and a decoupling branch, defining the inductor current reference i' at the voltage loop output. Ldqref Compared with the actual inductor current reference i Ldqref That is, the ratio of the input to the output of the circular current limiter is the current limiting coefficient K. lim .

[0016] When the current limiter is triggered, the reference value of the inductor current in the grid-connected inverter can be expressed as:

[0017]

[0018] Where E is the inverter's output voltage reference vector.

[0019] If we neglect the dynamic processes of capacitance and current loop, i.e., consider i Ldq =i dq +jω0C f and i Ldq =i Ldqref The above formula can be rewritten as:

[0020] Ev dq =(K lim -1)(R v +jX v )i Ldq

[0021] Among them, i Ldq This is the current in the inverter's current filter inductor.

[0022] Therefore, the voltage loop control structure of the grid-type inverter based on embedded virtual impedance can effectively introduce an adaptive virtual impedance to limit the inductor current, and the impedance is 0 when the current limit is not triggered, so it will not change the original output characteristics of the inverter.

[0023] Furthermore, the equivalent output power P of the grid-connected inverter eq =P+(K) lim -1)·K m P ref Where P is the actual active power output of the inverter, and K is increased. m The size of the variable can significantly improve the transient stability of grid-type inverters.

[0024] The present invention also provides an electronic device, comprising: a computer-readable storage medium and a processor;

[0025] The computer-readable storage medium is used to store executable instructions;

[0026] The processor is used to read executable instructions stored in the computer-readable storage medium and execute the above-described method.

[0027] The present invention also provides a computer-readable storage medium storing computer instructions for causing a processor to perform the above-described method.

[0028] The present invention also provides a computer program product, including a computer program or instructions that, when executed by a processor, implement the above-described method.

[0029] Compared with the prior art, the above-described technical solutions conceived in this invention can achieve the following beneficial effects:

[0030] (1) The voltage loop control structure of the grid-type inverter based on embedded virtual impedance provided by the present invention, together with the circular current limiter, can effectively introduce an adaptive virtual impedance to limit the inductor current. When the current limit is not triggered, the impedance is 0, which will not change the original output characteristics of the inverter, so that the grid-type inverter can still maintain transient stability under a wide range of grid faults.

[0031] (2) The control method for enhancing the transient stability of grid-type inverters when using current limiting protection provided by the present invention can enhance the transient stability of grid-type inverters after current limiting is triggered, and does not change the original power transmission characteristics when current limiting is not triggered. Attached Figure Description

[0032] Figure 1 This is the topology and control block diagram of the grid-type inverter system proposed in this invention;

[0033] Figure 2 In this embodiment, V g =0.4pu and V g P-δ plot at 0.8 pu;

[0034] Figure 3 This embodiment is in V g Simulation waveform at 0.4 pu;

[0035] Figure 4 This embodiment is in V g Simulation waveform at 0.8 pu;

[0036] Figure 5 This is the minimum K that can maintain stability under different grid voltages in this embodiment. m value;

[0037] Figure 6 In this embodiment, when using the control method proposed in this invention, V g =0.4pu and V g P-δ plot at 0.8 pu;

[0038] Figure 7 In this embodiment, when using the control method proposed in this invention, V g Simulation waveform at 0.4 pu;

[0039] Figure 8 In this embodiment, when using the control method proposed in this invention, V g Simulation waveform at 0.8 pu;

[0040] Figure 9 In this embodiment, when using the control method proposed in this invention, V g Simulation waveform at 0.05 pu. Detailed Implementation

[0041] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. Furthermore, the technical features involved in the various embodiments of this invention described below can be combined with each other as long as they do not conflict with each other.

[0042] This invention provides a transient stability control method for enhanced grid-connected inverters when current limiting protection is employed, comprising the following steps:

[0043] The difference between the inverter's output voltage reference and the actual output voltage, after passing through the virtual impedance forward path, results in the current i of the decoupled branch. dq +jω0C f v dq The inductor current reference i' at the voltage loop output is obtained by summing the values. Ldqref The virtual impedance forward channel introduces a forward channel virtual impedance R. v +jX v , where R v and X v These are the real and imaginary parts of the forward path virtual impedance, respectively. dq i represents the actual output voltage of the inverter. dq C is the output current of the inverter. fω0 is the filter capacitor, and ω0 is the reference value of the inverter's angular velocity.

[0044] Actual inductor current reference i Ldqref Traffic limiting will be implemented according to the following rules:

[0045]

[0046] Among them, i' Ldqref I serves as the inductor current reference for the voltage loop output. max The maximum current that the inverter switching transistor can be allowed to flow through, ||i' Ldqref ||for i' Ldqref Size;

[0047] Define the current limiting coefficient Using the current limiting coefficient K lim The difference between 1 and 1 can be used for power feedforward, which is equivalent to reducing the active power reference or increasing the equivalent output power P. eq This is beneficial for transient stability after the current limiting is triggered.

[0048] (1) When K lim When = 1, current limiting is not triggered: the power of feedforward is 0;

[0049] (2) When K lim When the value is greater than 1, current limiting is triggered: adaptive adjustment introduces virtual power (K). lim -1)·K m P ref Perform power feedforward, where K m P is the power feedforward coefficient. ref This serves as the active power reference for the inverter.

[0050] Specifically, the grid-type inverter voltage loop control structure provided by this invention consists of a virtual impedance forward path and a decoupling branch, defining the inductor current reference i' of the voltage loop output. Ldqref Compared with the actual inductor current reference i Ldqref That is, the ratio of the input to the output of the circular current limiter is the current limiting coefficient K. lim .

[0051] When the current limiter is triggered, the reference value of the inductor current in the grid-connected inverter can be expressed as:

[0052]

[0053] Where E is the inverter's output voltage reference vector.

[0054] If we neglect the dynamic processes of capacitance and current loop, i.e., consider i Ldq =i dq +jω0C f and iLdq =i Ldqref The above formula can be rewritten as:

[0055] Ev dq =(K lim -1)(R v +jX v )i Ldq

[0056] Among them, i Ldq This is the current in the inverter's current filter inductor.

[0057] Figure 1 The diagram shows the topology and control block diagram of a grid-connected inverter system. The actual inductor current references i. Ldqref Current limiting is performed according to the above rules, using a circular current limiter, where the current limiting coefficient is K. lim The voltage loop control consists of a virtual impedance forward path and a current decoupling branch. Together with the circular current limiter, it can effectively introduce an adaptive virtual impedance to limit the inductor current. When the current limiter is not triggered, the impedance is 0, which will not change the original output characteristics of the inverter.

[0058] Table 1 shows the structure and control parameters provided in this embodiment.

[0059] Table 1

[0060]

[0061]

[0062] Figure 2 In this embodiment, V g =0.4pu and V g P-δ plot at V = 0.8 pu, where V g When the voltage is 0.4 pu, the maximum active power output of the system is less than the active power reference value, therefore the grid-connected inverter experiences transient instability, while at V g When the active power output is 0.4 pu, the maximum active power output of the system is greater than the active power reference value, so the grid-type inverter can maintain transient stability.

[0063] Figure 3 and Figure 4 In this embodiment, V g =0.4pu and V g The simulation waveform at 0.8 pu is shown below. Figure 3 V g When the power consumption is 0.4 pu, the grid-connected inverter experiences transient instability. Figure 4 V gThe grid-type inverter is transiently stable when the power consumption is 0.8 pu.

[0064] Based on the P-δ diagrams under different grid voltages Figure 5 The minimum K that can maintain stability under different grid voltages m Value, and take K m =1.15, which makes the grid-connected inverter more resistant to extreme grid faults V. g It can still maintain transient stability when the value is 0.05 pu.

[0065] Figure 6 In this embodiment, when using the control method proposed in this invention, V g =0.4pu and V g P-δ plot at V = 0.8 pu, where V g When V = 0.4 pu, the control method proposed in this invention creates a new equilibrium point for grid-type inverters, indicating that the inverter can remain stable under the same conditions, while at V g When the power density is 0.8 pu, the control method proposed in this invention does not change the balance point of the inverter, indicating that it will not affect the power transmission characteristics of the system under minor faults.

[0066] Figure 7 and Figure 8 In this embodiment, when the control method proposed in this invention is used, V g =0.4pu and V g Simulation waveform at 0.8 pu. Figure 7 and Figure 8 All can maintain transient stability;

[0067] Figure 9 In this embodiment, when using the control method proposed in this invention, V g The simulation waveform at 0.05 pu shows that even under extreme grid fault conditions, the grid-connected inverter can still maintain transient stability.

[0068] Those skilled in the art will readily understand that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A current-limiting control method for an enhanced grid-connected inverter, characterized in that, Includes the following steps: The difference between the inverter's output voltage reference and the actual output voltage, after passing through the virtual impedance forward path, results in the current i of the decoupled branch. dq +jω0C f v dq Adding them together, we obtain the inductor current reference i' at the voltage loop output. Ldqref The virtual impedance forward channel introduces a forward channel virtual impedance R. v +jX v , where R v and X v These are the real and imaginary parts of the forward path virtual impedance, respectively. dq i represents the actual output voltage of the inverter. dq C is the output current of the inverter. f ω0 is the filter capacitor, and ω0 is the reference value of the inverter's angular velocity. Actual inductor current reference i Ldqref Traffic limiting will be implemented according to the following rules: Among them, i' Ldqref I serves as the inductor current reference for the voltage loop output. max The maximum current that the inverter switching transistor can be allowed to flow through, ||i' Ldqref ||for i' Ldqref Size; Define the current limiting coefficient Using the current limiting coefficient K lim Power feedforward is performed using the difference from 1: (1) When K lim When = 1, current limiting is not triggered: the power of feedforward is 0; (2) When K lim When the value is greater than 1, current limiting is triggered: adaptive adjustment introduces virtual power (K). lim -1)·K m P ref Perform power feedforward, where K m P is the power feedforward coefficient. ref This serves as the active power reference for the inverter.

2. The current limiting control method as described in claim 1, characterized in that, The actual inductor current reference i Ldqref Current limiting is achieved using a circular current limiter, the current limiting coefficient of which is K. lim .

3. The current limiting control method as described in claim 2, characterized in that, When K lim When the value is greater than 1, current limiting is triggered, and the voltage loop and circular current limiter adaptively adjust to introduce a virtual impedance (K). lim -1)(R v +jX v The actual inductor current references i. Ldqref Represented as: E is the inverter's output voltage reference.

4. The current limiting control method as described in claim 3, characterized in that, When K lim When the value is greater than 1, current limiting is triggered, and the equivalent output power P of the inverter is... eq =P+(K) lim -1)·K m P ref Where P is the actual active power output by the inverter.

5. An electronic device comprising a memory and a processor, wherein the memory stores a computer program, characterized in that, When the processor executes the computer program, it implements the steps of the method as described in any one of claims 1 to 4.

6. A computer-readable storage medium having a computer program stored thereon, characterized in that, When the computer program is executed by a processor, it implements the steps of the method as described in any one of claims 1 to 4.

7. A computer program product, comprising a computer program or instructions, characterized in that, When the computer program or instructions are executed by a processor, they implement the steps of the method as described in any one of claims 1 to 4.

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