VSG transient active support recovery control method considering virtual impedance

Abstract

The invention discloses a VSG transient active support recovery control method considering virtual impedance, and the method comprises the steps: 1, adding the virtual impedance needed for realizing current limiting in a fault transient state of a VSG single machine-infinite bus system, thereby obtaining a minimum value of line equivalent impedance needed for meeting current limiting; and 2, resetting an active power reference value and a reactive power reference value of the VSG in a fault transient state so as to realize consistency of grid-connected point voltage and power angle in a steady state and a transient state. According to the method, the virtual impedance is added during the fault to limit the current, and the transient VSG power reference value is calculated and reset, so that the voltage and the power angle of the grid-connected point can be kept stable before and after the fault, the synchronization of the VSG in the fault transient state is ensured, and the active support recovery of the VSG in the transient state is realized.

Description

Technical Field

[0001] The present invention relates to the field of transient stability control and analysis of VSGs, and in particular to a VSG transient active support recovery control method considering virtual impedance. Background Art

[0002] The centralized or decentralized integration of a high proportion of renewable energy sources has profoundly changed the structure, form, and stability mechanisms of traditional power grids, which primarily rely on synchronous generators. The synchronization stability of power grids has gradually evolved from "physical synchronization," dominated by the electromechanical transient characteristics of synchronous generators, to "controlled synchronization," dominated by the multi-timescale control of renewable energy converters. In recent years, the virtual synchronous generator (VSG) in grid-connected converters has become a research hotspot, with a focus on improving its control performance in weak or isolated grid environments and analyzing transient synchronization stability.

[0003] By approximately simulating the physical characteristics of synchronous generators, VSGs can provide inertia for the power grid, providing a certain degree of active support capability and improving the transient synchronization stability of the grid-connected system to a certain extent. They can also draw on traditional transient and steady-state analysis methods for synchronous generators to study issues related to grid-connected converters. When a grid fault occurs, a sudden drop in grid voltage often causes the VSG to lose power synchronization, resulting in angle instability and overcurrent. When designing VSG transient control methods, it is necessary to balance stability and current limiting requirements.

[0004] Therefore, how to design a VSG control method under fault transient conditions so that the VSG can effectively perform transient synchronization and current limiting during the fault process and realize active support control of the VSG under transient conditions is an urgent problem that technicians in this field need to solve. Summary of the Invention

[0005] In response to the problems of existing VSG in transient synchronization stability and active support control, the present invention proposes a VSG transient active support recovery control method considering virtual impedance, in order to achieve transient synchronization and current limiting of VSG in fault transient, thereby improving the transient synchronization stability of VSG.

[0006] In order to achieve the above-mentioned object, the present invention adopts the following technical solutions:

[0007] The VSG transient active support recovery control method considering virtual impedance of the present invention is characterized in that it is applied to a VSG stand-alone-infinite system composed of a VSG and an infinite power grid connected via an LC filter and line impedance, and includes the following steps:

[0008] S1: In the fault transient of the VSG single-machine-infinite system, the virtual impedance required to achieve current limiting is added to obtain the minimum value of the line equivalent impedance required to meet the current limiting requirements:

[0009] S1.1: In steady state, use equation (1) to obtain the voltage E and power angle δ at the common coupling point of the VSG single-machine-infinite system:

[0010] (1)

[0011] In formula (1), E0 represents the rated output voltage of VSG; U g represents the steady-state voltage of the infinite power grid; X g Indicates line reactance; D q is the droop coefficient of the reactive power loop; P0 and Q0 are the steady-state active power reference value and steady-state reactive power reference value of VSG respectively.

[0012] S1.2: When the VSG single-machine-infinite system enters a fault transient state, use formula (2) to calculate the equivalent impedance of the VSG single-machine-infinite system after adding virtual impedance current limiting in the transient state:

[0013] (2)

[0014] In formula (2), R gs and X gs are the equivalent resistance and equivalent reactance of the VSG single-machine-infinite system after adding virtual impedance current limiting in transient state; r v and x v are the virtual resistance and virtual reactance added in transient state respectively;

[0015] S1.3: Calculate the steady-state output current I of the VSG using equation (3) o and transient output current I os :

[0016] (3)

[0017] In formula (3), U gs Represents the transient voltage of the infinite power grid; E s represents the voltage at the common coupling point of the VSG single-machine-infinite system under transient conditions; δ s is the power angle of the common coupling point of the VSG single-machine-infinite system under transient conditions;

[0018] S1.4: Set the maximum current that VSG can withstand to I M , calculate the minimum value of the line equivalent impedance required to meet the current limiting requirement through formula (4) :

[0019] (4)

[0020] S2: Use formula (5) to reset the active power reference value P of VSG in fault transient state s0and reactive power reference value Q s0 , in order to achieve consistency of grid connection point voltage and power angle in steady state and transient state:

[0021] (5)

[0022] The electronic device of the present invention includes a memory and a processor, and is characterized in that the memory is used to store a program that supports the processor to execute the VSG transient active support recovery control method described in claim 1, and the processor is configured to execute the program stored in the memory.

[0023] The present invention provides a computer-readable storage medium, and the computer-readable storage medium stores a computer program, which is characterized in that when the computer program is run by a processor, the steps of the VSG transient active support recovery control method according to claim 1 are executed.

[0024] Compared with the prior art, the present invention has the following beneficial effects:

[0025] The present invention addresses the problems of existing VSG in transient synchronization stability and active support control. By adding virtual impedance to limit current during a fault, and calculating and resetting the transient VSG power reference value, the voltage and power angle of the grid connection point can be kept stable before and after the fault, ensuring the synchronization of the VSG in the fault transient state, and realizing the active support recovery of the VSG under transient conditions, thereby improving the transient synchronization stability of the VSG. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 This is a flow chart of the VSG transient active support recovery control method considering virtual impedance of the present invention;

[0027] Figure 2 Schematic diagram of a VSG stand-alone-infinite grid-connected system considering virtual impedance decoupling in the present invention;

[0028] Figure 3 This is the equivalent circuit diagram of the VSG stand-alone-infinite grid-connected system after adding virtual impedance in the present invention;

[0029] Figure 4 This is a block diagram of the VSG power loop control using the active support recovery control method proposed in this invention.

[0030] Figure 5 This is a simulation result diagram of the output current and power changes before and after a VSG fault without using any fault recovery control method in the present invention;

[0031] Figure 6 This is a simulation result diagram of the voltage and power angle changes at the common coupling point before and after a VSG fault without using any fault recovery control method in the present invention;

[0032] Figure 7 This is a simulation result diagram of the output current and power changes before and after a VSG fault using the proposed fault recovery method of the present invention;

[0033] Figure 8 This is a simulation result diagram of the voltage and power angle changes at the common coupling point before and after a VSG fault using the proposed fault recovery method in the present invention. DETAILED DESCRIPTION

[0034] In this embodiment, a VSG transient active support and restoration control method considering virtual impedance is applied to a VSG stand-alone-infinite system consisting of a VSG and an infinite power grid connected via an LC filter and line impedance, and includes the following steps:

[0035] S1: In the fault transient of the VSG single-machine-infinite system, the virtual impedance required to achieve current limiting is added to obtain the minimum value of the line equivalent impedance required to meet the current limiting requirements:

[0036] Figure 1 Flowchart of the VSG transient active support recovery control method considering virtual impedance of the present invention. In this embodiment, the schematic diagram of the VSG single-machine-infinite grid-connected system considering virtual impedance decoupling is as follows Figure 2 As shown in Figure 1, the VSG stand-alone-infinite system is a system consisting of the VSG and the infinite grid connected through an LC filter and line impedance. For ease of analysis, the resistance part of the line impedance is ignored. Using formula (1), the power loop equation of the VSG can be obtained:

[0037] (1)

[0038] In formula (1), is the derivative of the power angle with respect to time; ω and ω0 are the VSG angular frequency and the grid angular frequency respectively; P0 and Q0 are the steady-state active power reference value and steady-state reactive power reference value of the VSG respectively; P and Q are the active power and reactive power output of the VSG in steady state respectively; J is the moment of inertia of the VSG; is the derivative of the VSG angular frequency with respect to time; E represents the voltage at the common coupling point of the VSG stand-alone-infinite system in steady state; E0 represents the rated output voltage of the VSG; K represents the time derivative of the voltage at the common coupling point of the VSG single-machine-infinite system in steady state; q is the integral coefficient of the reactive power loop; D q is the droop coefficient of the reactive power loop; D is the damping coefficient of the active power loop.

[0039] Using formula (2), we can get the active power P and reactive power Q output by VSG:

[0040] (2)

[0041] In formula (2), δ is the power angle of the common coupling point of the VSG single-machine-infinite system in steady state; U g represents the steady-state voltage of the infinite power grid; X g Indicates the line reactance.

[0042] S1.1: In steady state, use Equation (3) to obtain the voltage E and power angle δ of the common coupling point of the VSG single-machine-infinite system:

[0043] (3)

[0044] When the VSG stand-alone-infinite system enters a transient fault state, the drop in the infinite grid voltage will inevitably increase the voltage difference between the common coupling point and the grid. If the line impedance remains unchanged at this time, the output current of the VSG will also increase. Adding transient virtual impedance can adjust the line equivalent impedance to limit overcurrent. The equivalent circuit diagram of the VSG stand-alone-infinite grid-connected system after adding virtual impedance is shown in the figure below. Figure 3 shown.

[0045] S1.2: When the VSG single-machine-infinite system enters a fault transient state, use formula (4) to calculate the equivalent impedance of the VSG single-machine-infinite system after adding virtual impedance current limiting in the transient state:

[0046] (4)

[0047] In formula (4), R gs and X gs are the equivalent resistance and equivalent reactance of the VSG single-machine-infinite system after adding virtual impedance current limiting in transient state; r v and x v are the virtual resistance and virtual reactance added in transient state respectively;

[0048] S1.3: Calculate the steady-state output current I of the VSG using equation (5) o and transient output current I os :

[0049] (5)

[0050] In formula (5), U gs Represents the transient voltage of the infinite power grid; E s represents the voltage at the common coupling point of the VSG single-machine-infinite system under transient conditions; δ s is the power angle of the common coupling point of the VSG single-machine-infinite system under transient conditions.

[0051] S1.4: Set the maximum current that VSG can withstand to I M, calculate the minimum value of the line equivalent impedance required to meet the current limiting requirement through formula (6) :

[0052] (6)

[0053] In the fault transient state, the selected r v and x v Must be able to , otherwise effective current limiting cannot be achieved.

[0054] S2: Reset the active power reference value P of VSG in fault transient state s0 and reactive power reference value Q s0 , in order to achieve consistency of grid connection point voltage and power angle in steady state and transient state:

[0055] Figure 4 This is the VSG power loop control block diagram of the active support recovery control method proposed in this invention. The grid voltage drop and the addition of transient virtual impedance change the working state of the converter. The active power P output by the VSG during the fault transient can be obtained using formula (7): s and reactive power Q s :

[0056] (7)

[0057] If the power reference value of the fault transient is not reset, the power may be unbalanced, causing VSG transient instability. By resetting the active power reference value and reactive power reference value of the VSG during the fault transient, the common coupling point voltage E s , power angle δ s Keeping the common coupling point voltage E and power angle δ consistent with those before the fault, the active power reference value P of VSG in the fault transient state is reset using formula (8): s0 and reactive power reference value Q s0 :

[0058] (8)

[0059] At this time, E can be achieved s =E,δ s =δ, formula (6) can be rewritten as formula (9):

[0060] (9)

[0061] 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-mentioned VSG transient active support recovery control method. The processor is configured to execute the program stored in the memory.

[0062] In this embodiment, a computer-readable storage medium stores a computer program, and when the computer program is executed by a processor, the steps of the above-mentioned VSG transient active support recovery control method are executed.

[0063] In order to verify the effectiveness of the active support recovery control method proposed in the technical solution of the present invention, the above-mentioned Figure 1 The simulation model shown in Figure 2 is shown. A three-phase short circuit fault occurs in the power grid at t = 2s, causing the grid voltage to drop to 0.2 pu and lasting for 4s. The active power reference value for the VSG in steady state is set to 0.8 pu, and the reactive power reference value is set to 0. A comparative analysis is conducted on the transient synchronization stability and current limiting of the system without any fault recovery control method and with the fault recovery method proposed in this invention.

[0064] Working condition 1: No fault recovery control method is adopted;

[0065] Before t=2s, the system is in a steady state, the output current, power of the VSG and the voltage and power angle of the common coupling point are stable, and the system operates normally. At t=2s, a three-phase short circuit fault occurs, and the grid voltage drops to 0.2pu. Since virtual impedance current limiting is not adopted, and the power reference value maintains the state before the fault, the output current of the VSG increases, the power cannot correctly track the rated value, and the voltage and power angle of the common coupling point cannot be stabilized. At t=6s, the fault is removed, but the output current, power, and voltage and power angle of the VSG at the common coupling point are in a state of continuous change and cannot be restored to the level before the fault. The simulation results of the output current and power changes before and after the VSG fault without any fault recovery control method are shown as follows. Figure 5 Part (a) of Figure 5 As shown in part (b) of the figure, the simulation results of the voltage and power angle changes at the common coupling point before and after the VSG fault without any fault recovery control method are as follows: Figure 6 Part (a) of Figure 6 As shown in part (b) of .

[0066] Working condition 2: adopting the fault recovery method proposed in the present invention;

[0067] Before t=2s, the system is in a steady state, the output current, power of the VSG and the voltage and power angle of the common coupling point are stable, and the system operates normally. At t=2s, a three-phase short circuit fault occurs, and the grid voltage drops to 0.2pu. Due to the adoption of the fault recovery control method proposed in this patent, the addition of virtual impedance current limiting, and the change of the transient power reference value, the output current of the VSG rises at the moment of the fault and then recovers to the pre-fault level. The power correctly tracks the rated value given by the transient, and the voltage and power angle of the common coupling point remain stable, and the difference from the steady-state value is not much. At t=6s, the fault is removed, and the output current, power, and voltage and power angle of the common coupling point of the VSG can be successfully restored to the pre-fault level. The simulation results of the output current and power changes before and after the fault of the VSG using the fault recovery method proposed in this patent are as follows: Figure 7 Part (a) of Figure 7 As shown in part (b), the simulation results of the voltage and power angle changes at the common coupling point before and after the VSG fault using the fault recovery method proposed in this patent are as follows: Figure 8 Part (a) of Figure 8 As shown in part (b) of .

[0068] Simulation results show that the proposed VSG transient active support and recovery control method, which considers virtual impedance, can effectively achieve synchronization and active support during VSG fault transients, ensuring that the voltage and power at the common coupling point remain essentially consistent between steady-state and transient states. This effectively improves the transient synchronization stability of the VSG single-machine-infinite system. The effectiveness of the proposed method has been well demonstrated.

Claims

1. A VSG transient active support recovery control method considering virtual impedance, characterized in that: It is applied to a VSG stand-alone-infinity system formed by connecting the VSG and the infinite power grid through an LC filter and line impedance, and includes the following steps: S1: In the fault transient state of the VSG single-machine-infinite system, the virtual impedance required to achieve current limiting is added to obtain the minimum value of the line equivalent impedance required to meet the current limiting: S1.1: In steady state, the voltage E and power angle δ of the common coupling point of the VSG single-machine-infinite system are obtained using equation (1): (1) In formula (1), E0 represents the rated output voltage of VSG; U g represents the steady-state voltage of the infinite power grid; X g Indicates line reactance; D q is the droop coefficient of the reactive power loop; P0 and Q0 are the steady-state active power reference value and steady-state reactive power reference value of VSG respectively; S1.2: When the VSG single-machine-infinite system enters a fault transient state, the equivalent impedance of the VSG single-machine-infinite system after adding virtual impedance current limiting in the transient state is calculated using formula (2): (2) In formula (2), R gs and X gs are the equivalent resistance and equivalent reactance of the VSG single-machine-infinite system after adding virtual impedance current limiting in transient state; r v and x v are the virtual resistance and virtual reactance added in transient state respectively; S1.3: Calculate the steady-state output current I of VSG using equation (3) o and transient output current I os : (3) In formula (3), U gs Represents the transient voltage of the infinite power grid; E s represents the voltage at the common coupling point of the VSG single-machine-infinite system under transient conditions; δ s is the power angle of the common coupling point of the VSG single-machine-infinite system under transient conditions; S1.4: The maximum current that VSG can withstand is I M , the minimum value of the line equivalent impedance required to meet the current limiting requirement is calculated by formula (4) : (4) S2: Use equation (5) to reset the active power reference value P of VSG in fault transient state s0 and reactive power reference value Q s0 , in order to achieve the consistency of grid connection point voltage and power angle in steady state and transient state: (5)。 2. 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 the VSG transient active support recovery control method according to claim 1, and the processor is configured to execute the program stored in the memory.

3. A computer-readable storage medium having a computer program stored thereon, characterized in that: When the computer program is executed by the processor, the steps of the VSG transient active support recovery control method according to claim 1 are executed.

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