Method for improving power angle stability after system fault based on fractional order virtual synchronous converter

Abstract

The invention relates to a method for improving power angle stability after a system fault based on a fractional order virtual synchronous converter, and the method comprises the steps: modeling of the fractional order virtual synchronous converter, and increase of a limit clearing angle and limit clearing time of a system. The transient synchronization stability of the system is improved through modeling of the fractional order virtual synchronous converter and increasing of the limit cut-off angle and the limit cut-off time of the system.

Description

Technical Field

[0001] The present invention relates to a method, and in particular to a method for improving the power angle stability of a system after a fault based on a fractional-order virtual synchronous converter, and belongs to the technical field of stable operation of a new energy power system. Background Art

[0002] Under the background of "dual carbon", new energy power generation technology has developed rapidly. New energy units represented by wind power and photovoltaic power are connected to the power grid after converting electric energy through power electronic equipment. The control strategy of power electronic converter can be divided into two types: grid-following type and grid-building type, which are suitable for different scenarios. Grid-following control achieves synchronization by tracking the grid voltage phase through a phase-locked loop. New energy units usually operate in maximum power point tracking (MPPT) mode and do not have active support capabilities and island operation capabilities. Large-scale access will lead to a decrease in the inertia level of the grid, affecting the safe and stable operation of the grid. Grid-building control actively generates voltage amplitude and phase reference values ​​through external loop control, has voltage and frequency support capabilities, and is particularly suitable for island operation mode.

[0003] The virtual synchronous generator (VSG) provides inertia support for the system by simulating the rotor motion equation of the synchronous generator, which is a kind of grid-type control. However, the virtual inertia introduced by the VSG technology increases the order of the system, making the active power output of the inverter prone to oscillation when it is connected to the grid. On the one hand, the instantaneous anti-interference and overload capabilities of the VSG are poor, and the current shock caused by the long-term power oscillation will trigger the grid-connected inverter to shut down; on the other hand, the introduction of virtual inertia and damping also slows down the response speed of the system. The literature proposes a synchronous generator control strategy based on adaptive virtual inertia or damping to address the problem of power oscillation or frequency stability. These methods simulate the behavior of the synchronous generator, construct the relationship between the frequency or power change rate and the virtual inertia, and adaptively change the inertia or damping parameters of the VSG control, thereby improving the anti-interference and overload capabilities of the microgrid system. The above literature improves the performance of the system by changing the inertia parameters or damping parameters, but ignores the contradiction between the dynamic performance, steady-state performance and parameter adjustment of the output active power when the inverter is connected to the grid.

[0004] Due to the superiority of fractional-order controllers, more and more fractional-order controllers are applied to power electronics technology, making the control performance of power electronics better. In 1999, Igor Podlubny proposed the fractional-order PI λ D u Controller,In recent years, fractional-order controllers have achieved success in many fields.

[0005] However, the prior art has not studied the power angle stability of the system after a fault occurs. The present invention uses the fault limit removal time as an indicator to measure the large disturbance (transient) synchronous stability margin of the system, and analyzes the influencing factors of the transient synchronous stability of the system. The transient synchronous stability of the system is studied using the method proposed in the present invention. Combined with the analysis results of the factors affecting transient stability, a fractional-order virtual synchronous converter method is proposed, and this control method improves the transient synchronous stability of the system. Summary of the invention

[0006] The present invention aims at the technical problems existing in the prior art and provides a method for improving the power angle stability of a system after a fault based on a fractional-order virtual synchronous converter. The technical scheme is based on the method for improving the power angle stability of a system after a fault based on a fractional-order virtual synchronous converter. By modeling the fractional-order virtual synchronous converter and increasing the system's limit cut-off angle and limit cut-off time, the transient synchronization stability of the system is improved.

[0007] In order to achieve the above-mentioned purpose, the technical solution of the present invention is as follows: a method for improving the power angle stability of a system after a fault based on a fractional-order virtual synchronous converter, comprising two parts: modeling of the fractional-order virtual synchronous converter and increasing the system's limit cut-off angle and limit cut-off time.

[0008] The fractional-order virtual synchronous converter modeling can not only improve the dynamic performance of the traditional virtual synchronous converter system and smooth the power oscillation in the dynamic process, but also increase the order of the system due to the introduction of virtual inertia in the traditional virtual synchronous converter control strategy, resulting in problems such as slower system response speed and power oscillation. Therefore, during the design, the system order should not be higher than the second-order system.

[0009] The limit cut-off angle and limit cut-off time of the system are increased by changing the active control link of the virtual synchronous converter, simulating the rotor motion equation of the synchronous generator, and when a fault occurs, the limit cut-off angle and limit cut-off time of the system are increased by analyzing the rotor motion equation when the fault occurs.

[0010] The modeling of the fractional-order virtual synchronous converter includes the following steps:

[0011] Step 1-1: Analyze the model and operation mechanism of the traditional virtual synchronous converter, and point out the problems existing in the traditional virtual synchronous converter; the fractional-order virtual synchronous converter modeling introduces a fractional-order virtual inertia link and changes the transfer function of the active power control link, thereby improving the dynamic performance of the traditional virtual synchronous converter system and smoothing the power oscillation in the dynamic process. The increase in the system order will cause the system response speed to slow down.

[0012] Step 1-2: transform the active control link of the traditional virtual synchronous converter, change the order of the virtual inertia link to a fractional order, and express the fractional-order virtual synchronous converter active control link to simulate the synchronous machine rotor motion equation;

[0013] Compared with the traditional virtual synchronous converter power loop, the integral factor λ is added, so the active control loop of the fractional-order virtual synchronous converter simulates the SG rotor motion equation, which can be expressed as:

[0014]

[0015] Assuming that the grid-side voltage phase angle is 0 rad, θ=δ in the active loop of the fractional-order virtual synchronous converter, and the output electromagnetic power closed-loop transfer function is:

[0016]

[0017] Where: K p It is the gain from the voltage phase difference δ at both ends of the line to the output active power.

[0018] The introduction of virtual inertia in the traditional VSG control strategy will increase the order of the system, leading to problems such as slow system response speed and power oscillation. Therefore, during design, the system order should not be higher than the second-order system. The present invention limits the range of λ to 0<λ<1.

[0019] Step 1-3: According to the active control link of the above-mentioned fractional-order virtual synchronous converter, the synchronous machine rotor motion equation is simulated, and the output electromagnetic power closed-loop transfer function is written. The traditional virtual synchronous converter control strategy will increase the order of the system due to the introduction of virtual inertia, resulting in problems such as slower system response speed and power oscillation. Therefore, during the design, the system order should not be higher than the second-order system.

[0020]

[0021] Where: J is virtual inertia; D is virtual damping; P ref and P e are the reference active power and output active power respectively; ω and ω0 are the VSG angular frequency and reference angular frequency respectively; λ is the integration factor.

[0022] Wherein, increasing the limit resection angle and limit resection time of the system comprises the following steps:

[0023] Step 2-1: Based on the above-mentioned output electromagnetic power closed-loop transfer function, through Bode plot analysis, it is found that the fractional-order virtual synchronous converter adopts a fractional-order virtual inertia control strategy, which slows down the amplitude attenuation rate in the low-frequency band and high-frequency band in the open-loop system and reduces the phase lag. Compared with traditional methods, the fractional-order control strategy has significant advantages in dynamic response.

[0024] Step 2-2: According to the relationship between the open-loop frequency characteristics and the dynamic performance of the closed-loop system, the closed-loop system using the fractional-order virtual synchronous converter will show a smaller overshoot and a shorter rise time, making the system respond faster, and the dynamic performance and stability are significantly improved.

[0025] Step 2-3: By analyzing the rotor motion equation during the fault, according to the equal area law and simulation verification, increase the system's limit resection angle and limit resection time;

[0026] For a single-machine infinite power system of fractional-order virtual synchronous converter, ignoring the influence of line impedance angle, its output power is shown in equation (1).

[0027]

[0028] Where: X g =ω0L g ; V g is the phase voltage amplitude of the infinite power supply; V is the phase voltage amplitude of the grid connection point; δ is V g The phase angle difference with V; that is, the power angle of the fractional-order virtual synchronous converter single-machine grid-connected system,

[0029] The rotor motion equation of the virtual synchronous converter single machine infinite system during the fault period is:

[0030]

[0031] Combining equation (3) and equation (4), we can get:

[0032] (Jω0Δω)d 1-λ Δω=(P ref -P eF -Dω0Δω)dδ 1-λ (12)

[0033] Where λ is the above integration factor, where 0<λ<1.

[0034] Integrating Δω and δ on both sides of the equation, we can get:

[0035]

[0036] The acceleration and deceleration area of ​​the fractional-order virtual synchronous converter single-machine infinite system is shown in formula (7):

[0037]

[0038] Since the fractional-order integral order λ<1, its power angle characteristic curve will change. Compared with the traditional virtual synchronous converter control strategy, the fractional-order virtual synchronous converter control strategy improves the power angle characteristic curve and reduces the acceleration area when a fault occurs; after the fault is removed, the deceleration area is increased, thereby improving the transient stability of the system.

[0039] Step 2-4: Compared with the traditional virtual synchronous converter power loop, the integral factor λ is added, so the fractional-order virtual synchronous converter active control loop simulates the SG rotor motion equation, which can be expressed as:

[0040]

[0041] Assuming that the grid-side voltage phase angle is 0 rad, θ=δ in the active loop of the fractional-order virtual synchronous converter, and the output electromagnetic power closed-loop transfer function is:

[0042]

[0043] Where: K p It is the gain from the voltage phase difference δ at both ends of the line to the output active power.

[0044] The introduction of virtual inertia in the traditional VSG control strategy will increase the order of the system, leading to problems such as slow system response speed and power oscillation. Therefore, during design, the system order should not be higher than the second-order system. The present invention limits the range of λ to 0<λ<1.

[0045] Compared with the prior art, the present invention has the following advantages: the present invention uses the fault limit removal time as an indicator to measure the large disturbance (transient) synchronous stability margin of the system, and analyzes the influencing factors of the transient synchronous stability of the system. The transient synchronous stability of the system is studied using the method proposed in the present invention. Combined with the analysis results of the factors affecting transient stability, a fractional-order virtual synchronous converter method is proposed. Since the fractional-order integral order λ of the acceleration and deceleration area of ​​the fractional-order VSG single-machine infinite system is less than 1, its power angle characteristic curve will increase. Therefore, when a fault occurs in the system, the acceleration area of ​​the system is reduced and the deceleration area of ​​the system is increased. This control method improves the transient synchronous stability of the system. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] Figure 1 It is a schematic diagram of the active control link of the fractional-order virtual synchronous converter of the present invention.

[0047] Figure 2 It is a schematic diagram of the control link of the fractional-order virtual synchronous converter of the present invention.

[0048] Figure 3 Schematic diagram of the equal-area rule taking fractional orders into account. DETAILED DESCRIPTION

[0049] In order to deepen the understanding of the present invention, the present embodiment is described in detail below with reference to the accompanying drawings.

[0050] Embodiment 1: A method for improving the power angle stability of a system after a fault based on a fractional-order virtual synchronous converter, comprising two parts: modeling of a fractional-order virtual synchronous converter and increasing a limit cut-off angle and a limit cut-off time of the system.

[0051] The fractional-order virtual synchronous converter modeling can not only improve the dynamic performance of the traditional virtual synchronous converter system and smooth the power oscillation in the dynamic process, but also increase the order of the system due to the introduction of virtual inertia in the traditional virtual synchronous converter control strategy, resulting in problems such as slower system response speed and power oscillation. Therefore, during the design, the system order should not be higher than the second-order system.

[0052] The limit cut-off angle and limit cut-off time of the system are increased by changing the active control link of the virtual synchronous converter, simulating the rotor motion equation of the synchronous generator, and when a fault occurs, the limit cut-off angle and limit cut-off time of the system are increased by analyzing the rotor motion equation when the fault occurs.

[0053] The modeling of the fractional-order virtual synchronous converter includes the following steps:

[0054] Step 1-1: Analyze the model and operation mechanism of the traditional virtual synchronous converter, and point out the problems existing in the traditional virtual synchronous converter; the fractional-order virtual synchronous converter modeling introduces a fractional-order virtual inertia link and changes the transfer function of the active power control link, thereby improving the dynamic performance of the traditional virtual synchronous converter system and smoothing the power oscillation in the dynamic process. The increase in the system order will cause the system response speed to slow down.

[0055] Step 1-2: transform the active control link of the traditional virtual synchronous converter, change the order of the virtual inertia link to a fractional order, and express the fractional-order virtual synchronous converter active control link to simulate the synchronous machine rotor motion equation;

[0056] Compared with the traditional virtual synchronous converter power loop, the integral factor λ is added, so the active control loop of the fractional-order virtual synchronous converter simulates the SG rotor motion equation, which can be expressed as:

[0057]

[0058] Assuming that the grid-side voltage phase angle is 0 rad, θ=δ in the active loop of the fractional-order virtual synchronous converter, and the output electromagnetic power closed-loop transfer function is:

[0059]

[0060] Where: K pIt is the gain from the voltage phase difference δ at both ends of the line to the output active power.

[0061] The introduction of virtual inertia in the traditional VSG control strategy will increase the order of the system, leading to problems such as slow system response speed and power oscillation. Therefore, during design, the system order should not be higher than the second-order system. The present invention limits the range of λ to 0<λ<1.

[0062] Step 1-3: According to the active control link of the above-mentioned fractional-order virtual synchronous converter, the synchronous machine rotor motion equation is simulated, and the output electromagnetic power closed-loop transfer function is written. The traditional virtual synchronous converter control strategy will increase the order of the system due to the introduction of virtual inertia, resulting in problems such as slower system response speed and power oscillation. Therefore, during the design, the system order should not be higher than the second-order system.

[0063]

[0064] Where: J is virtual inertia; D is virtual damping; P ref and P e are the reference active power and output active power respectively; ω and ω0 are the VSG angular frequency and reference angular frequency respectively; λ is the integration factor.

[0065] Wherein, increasing the limit resection angle and limit resection time of the system comprises the following steps:

[0066] Step 2-1: Based on the above-mentioned output electromagnetic power closed-loop transfer function, through Bode plot analysis, it is found that the fractional-order virtual synchronous converter adopts a fractional-order virtual inertia control strategy, which slows down the amplitude attenuation rate in the low-frequency band and high-frequency band in the open-loop system and reduces the phase lag. Compared with traditional methods, the fractional-order control strategy has significant advantages in dynamic response.

[0067] Step 2-2: According to the relationship between the open-loop frequency characteristics and the dynamic performance of the closed-loop system, the closed-loop system using the fractional-order virtual synchronous converter will show a smaller overshoot and a shorter rise time, making the system respond faster, and the dynamic performance and stability are significantly improved.

[0068] Step 2-3: By analyzing the rotor motion equation during the fault, according to the equal area law and simulation verification, increase the system's limit resection angle and limit resection time;

[0069] For a single-machine infinite power system of fractional-order virtual synchronous converter, ignoring the influence of line impedance angle, its output power is shown in equation (1).

[0070]

[0071] Where: X g =ω0L g; V g is the phase voltage amplitude of the infinite power supply; V is the phase voltage amplitude of the grid connection point; δ is V g The phase angle difference with V; that is, the power angle of the fractional-order virtual synchronous converter single-machine grid-connected system,

[0072] The rotor motion equation of the virtual synchronous converter single machine infinite system during the fault period is:

[0073]

[0074] Combining equation (3) and equation (4), we can get:

[0075] (Jω0Δω)d 1-λ Δω=(P ref -P eF -Dω0Δω)dδ 1-λ (twenty one)

[0076] Where λ is the above integration factor, where 0<λ<1.

[0077] Integrating Δω and δ on both sides of the equation, we can get:

[0078]

[0079] The acceleration and deceleration area of ​​the fractional-order virtual synchronous converter single-machine infinite system is shown in formula (7):

[0080]

[0081] Since the fractional-order integral order λ<1, its power angle characteristic curve will change. Compared with the traditional virtual synchronous converter control strategy, the fractional-order virtual synchronous converter control strategy improves the power angle characteristic curve and reduces the acceleration area when a fault occurs; after the fault is removed, the deceleration area is increased, thereby improving the transient stability of the system.

[0082] Step 2-4: Compared with the traditional virtual synchronous converter power loop, the integral factor λ is added, so the fractional-order virtual synchronous converter active control loop simulates the SG rotor motion equation, which can be expressed as:

[0083]

[0084] Assuming that the grid-side voltage phase angle is 0 rad, θ=δ in the active loop of the fractional-order virtual synchronous converter, and the output electromagnetic power closed-loop transfer function is:

[0085]

[0086] Where: K p It is the gain from the voltage phase difference δ at both ends of the line to the output active power.

[0087] The introduction of virtual inertia in the traditional VSG control strategy will increase the order of the system, leading to problems such as slow system response speed and power oscillation. Therefore, during design, the system order should not be higher than the second-order system. The present invention limits the range of λ to 0<λ<1.

[0088] Example 2: See Figure 1 A method for improving the power angle stability of a system after a fault based on a fractional-order virtual synchronous converter is proposed, which includes two parts: modeling of the fractional-order virtual synchronous converter and increasing the limit cut-off angle and limit cut-off time of the system.

[0089] See also Figure 2 , 3 The fractional-order virtual synchronous converter modeling can not only improve the dynamic performance of the traditional virtual synchronous converter system, but also smooth out the power oscillation in the dynamic process. The traditional virtual synchronous converter control strategy will increase the order of the system due to the introduction of virtual inertia, resulting in problems such as slow system response speed and power oscillation. Therefore, during the design, the system order should not be higher than the second-order system.

[0090] The limit cut-off angle and limit cut-off time of the system are increased by changing the active control link of the virtual synchronous converter, simulating the rotor motion equation of the synchronous generator, and when a fault occurs, the limit cut-off angle and limit cut-off time of the system are increased by analyzing the rotor motion equation when the fault occurs.

[0091] The modeling of the fractional-order virtual synchronous converter includes the following steps:

[0092] Step 1-1: Analyze the model and operation mechanism of the traditional virtual synchronous converter, and point out the problems existing in the traditional virtual synchronous converter;

[0093] Step 1-2: transform the active control link of the traditional virtual synchronous converter, change the order of the virtual inertia link to a fractional order, and express the fractional-order virtual synchronous converter active control link to simulate the synchronous machine rotor motion equation;

[0094] Step 1-3: According to the active power control link of the above-mentioned fractional-order virtual synchronous converter, the synchronous machine rotor motion equation is simulated, and the output electromagnetic power closed-loop transfer function is written. The traditional virtual synchronous converter control strategy will increase the order of the system due to the introduction of virtual inertia, resulting in problems such as slow system response speed and power oscillation. Therefore, when designing, the system order should not be higher than the second-order system. The fractional-order virtual synchronous converter modeling introduces a fractional-order virtual inertia link to change the transfer function of the active power control link, thereby improving the dynamic performance of the traditional virtual synchronous converter system and smoothing the power oscillation in the dynamic process. The increase in the system order will cause the system response speed to slow down.

[0095] Wherein, increasing the limit resection angle and limit resection time of the system comprises the following steps:

[0096] Step 2-1: Based on the above-mentioned output electromagnetic power closed-loop transfer function, through Bode plot analysis, it is found that the fractional-order virtual synchronous converter adopts a fractional-order virtual inertia control strategy, which slows down the amplitude attenuation rate in the low-frequency band and high-frequency band in the open-loop system and reduces the phase lag. Compared with traditional methods, the fractional-order control strategy has significant advantages in dynamic response.

[0097] Step 2-2: According to the relationship between the open-loop frequency characteristics and the dynamic performance of the closed-loop system, the closed-loop system using the fractional-order virtual synchronous converter will show a smaller overshoot and a shorter rise time, so that the system responds faster, and the dynamic performance and stability are significantly improved. Step 2-3: When a fault occurs, analyze the fault.

[0098] It should be noted that the above embodiments are not intended to limit the protection scope of the present invention, and equivalent changes or substitutions made on the basis of the above technical solutions all fall within the protection scope of the claims of the present invention.

Claims

1. A method for improving the power angle stability of a system after a fault based on a fractional-order virtual synchronous converter, characterized in that: It includes two parts: modeling of fractional-order virtual synchronous converter and increasing the system's limit cut-off angle and limit cut-off time.

2. The method for improving the power angle stability of a system after a fault based on a fractional-order virtual synchronous converter according to claim 1 is characterized in that: The modeling of the fractional-order virtual synchronous converter comprises the following steps: Step 1-1: Analyze the model and operation mechanism of the traditional virtual synchronous converter, and point out the problems existing in the traditional virtual synchronous converter; Step 1-2: transform the active control link of the traditional virtual synchronous converter, change the order of the virtual inertia link to a fractional order, and express the fractional-order virtual synchronous converter active control link to simulate the synchronous machine rotor motion equation; Step 1-3: According to the active control link of the fractional-order virtual synchronous converter, the synchronous machine rotor motion equation is simulated, and the output electromagnetic power closed-loop transfer function is written out. The closed-loop transfer function is as follows: Where: J is virtual inertia; D is virtual damping; P ref and P e are the reference active power and output active power respectively; ω and ω0 are the VSG angular frequency and reference angular frequency respectively; λ is the integration factor.

3. The method for improving the power angle stability of a system after a fault based on a fractional-order virtual synchronous converter according to claim 2 is characterized in that: The method of increasing the limit resection angle and limit resection time of the system comprises the following steps: Step 2-1: Based on the above output electromagnetic power closed-loop transfer function, through Bode plot analysis, it is found that the fractional-order virtual synchronous converter adopts a fractional-order virtual inertia control strategy, which slows down the amplitude attenuation speed of the low-frequency band and high-frequency band in the open-loop system and reduces the phase lag. Step 2-2: According to the relationship between the open-loop frequency characteristics and the dynamic performance of the closed-loop system, the closed-loop system using the fractional-order virtual synchronous converter will show a smaller overshoot and a shorter rise time, making the system respond faster, and the dynamic performance and stability are significantly improved. Step 2-3: When a fault occurs, by analyzing the rotor motion equation at the time of the fault, according to the equal area law and simulation verification, increase the system's limit resection angle and limit resection time. For a single-machine infinite power system of a fractional-order virtual synchronous converter, ignoring the influence of the line impedance angle, its output power is shown in equation (1): Where: X g =ω0L g ; V g is the phase voltage amplitude of the infinite power supply; V is the phase voltage amplitude of the grid connection point; δ is V g The phase angle difference with V; that is, the power angle of the fractional-order virtual synchronous converter single-machine grid-connected system, The rotor motion equation of the virtual synchronous converter single machine infinite system during the fault period is: Combining equation (3) and equation (4), we can get: (Jω0Dω)d 1-λ Give=(P ref -P eF -Dω0Dω)dδ 1-λ (3) Where λ is the above integration factor, where 0<λ<1, Integrating Δω and δ on both sides of the equation, we can get: The acceleration and deceleration area of ​​the fractional-order virtual synchronous converter single-machine infinite system is shown in formula (7): Since the fractional integral order λ is less than 1, its power angle characteristic curve will change. In the formula: J is the virtual inertia; D is the virtual damping; P ref and P e are the reference active power and output active power respectively; ω and ω0 are the VSG angular frequency and reference angular frequency respectively; λ is the integration factor, Step 2-4: Compared with the traditional virtual synchronous converter power loop, the integral factor λ is added, so the fractional-order virtual synchronous converter active control loop simulates the SG rotor motion equation, which can be expressed as: Assuming that the grid-side voltage phase angle is 0 rad, θ=δ in the active loop of the fractional-order virtual synchronous converter, and the output electromagnetic power closed-loop transfer function is: Where: K p It is the gain from the voltage phase difference δ at both ends of the line to the output active power.

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