Virtual synchronous machine low voltage ride through control method, system and medium

Abstract

The present application relates to the technical field of smart grid, and discloses a virtual synchronous machine low-voltage ride-through control method, system and medium, the method first determines the operating state of the virtual synchronous machine based on the electrical quantity of the virtual synchronous machine, and on the basis of taking into account the transient stability and the maximum output power capacity of the virtual synchronous machine, adjusts the active reference value during the fault to ensure that the operating point exists, which can maximize the current capacity of the virtual synchronous machine converter, improve the dynamic reactive voltage support capability of the virtual synchronous machine and the transient synchronization stability of the system. At the same time, during the fault recovery stage, based on the virtual internal electromotive force steady-state amplitude compensation control, the active power impact caused by the excessive internal electromotive force amplitude of the virtual synchronous machine reactive voltage integral control at the fault recovery time is avoided; the reactive power is quickly restored to the normal value, the over-release of the reactive power caused by the integral control is reduced, the voltage peak value at the end of the voltage fault is reduced, and the transient voltage stability of the system is improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of smart grid, in particular to the technical field of new energy power generation grid-connected control, and more particularly to a virtual synchronous machine low voltage ride through control method, system and medium based on power adjustment and internal potential compensation. BACKGROUND

[0002] High proportion, large scale and long distance grid-connected technology represented by wind power, photovoltaic and other new energy based on inverters has become one of the core technical features of the new generation of power system. This technical characteristic leads to the weakening of the source-grid connection strength of the power system.

[0003] At present, the grid-connected system of the follow-grid type inverter based on the phase-locked loop synchronization mechanism dominates the dynamic characteristics, which is easy to dynamically couple with the weak power grid and further cause system oscillation. At the same time, in the short-circuit fault and other large disturbance scenarios, there is a risk of transient instability. Unlike the follow-grid type power system, the self-synchronous grid type grid-connected based on virtual synchronous technology can provide active inertia and reactive voltage support by actively building voltage and frequency. The voltage source series impedance characteristic is equivalent to adding a parallel branch to the ground for the system, thereby improving the short-circuit ratio and reducing the voltage sensitivity, and to some extent, enhancing the stability of the power system.

[0004] Similar to synchronous generators, the grid type converter may also experience transient instability due to the absence of equilibrium points when subjected to large disturbances. In the reactive voltage control of the virtual synchronous generator (VSG), the excitation system and voltage regulation characteristics of the synchronous generator are simulated through the integral element, thereby obtaining the virtual internal potential amplitude. In order to provide sufficient dynamic voltage reactive power support, it is necessary to increase the voltage droop coefficient, which will cause the internal potential amplitude output by the integral element to continuously rise and reach the amplitude limit value during the fault period. Even in the low voltage ride through recovery stage, the amplitude value remains at a high level and needs to be restored to the steady state value for a long time. The above process will cause the dynamic recovery speed of active power and reactive power to be slow, and the inverter and the power grid will face a large power impact and overvoltage risk.

[0005] In the existing network-configuration-type low-voltage ride-through scheme design, the reactive current increment referring to the grid-connected device grid-connection technical specification requirement, the reactive power and active power reference during fault are switched according to the fault voltage and converter capacity. Although the transient stability is ensured by reducing the active power reference, the constant reactive power control will lead to that the virtual synchronous machine does not fully utilize the converter capacity, and the reactive voltage support capability is weakened, which is difficult to meet the functional positioning of providing sufficient dynamic voltage support (to improve the short-circuit ratio to above the critical value) for the grid-connected device. To solve this problem, the prior art proposes to lock the reactive voltage integrator during the fault and reset the integrator to zero after the fault is cleared. This method can keep the potential amplitude constant during the fault, but it will lose the excitation and voltage regulation characteristics, and resetting the integrator to zero will cause a large transient impact in the recovery stage. In the Chinese patent application (publication number CN110266048A) published on September 20, 2019, a VSG control method under grid voltage symmetrical drop fault is proposed, which generates an additional phase angle in the fault recovery process through active power deviation PI control, so as to realize smooth transition. However, this scheme will cause the phase angle of the virtual synchronous machine to suddenly change. If the internal potential voltage reference value is switched to the terminal voltage during the fault and the reference is exited at the recovery time, the dynamic integration is accelerated, but it still has a recovery process of nearly 100 milliseconds, and cannot avoid the power impact and transient overvoltage problem caused by the high internal potential.

[0006] Therefore, according to the above analysis, for the existing low-voltage ride-through control scheme of new energy power generation grid connection, on the one hand, the low-voltage ride-through scheme based on constant reactive power setting does not fully utilize the inverter capacity, and the reactive voltage support capability of the network-configuration-type power supply is weakened, which cannot meet the functional positioning requirements, and the stable operating point is lost due to the constant active power reference, which reduces the transient stability of the network-configuration-type system. On the other hand, the traditional control strategy, such as the reactive voltage integral control mechanism, will cause the internal potential amplitude to be adjusted slowly in the recovery stage after the fault is cleared, and the value is significantly higher than the steady-state value for a long time, which will cause active power impact and reactive power overloading in the recovery process, cause high voltage at the terminal, and affect the safety of the device, and reduce the transient voltage stability of the virtual synchronous machine. SUMMARY

[0007] In view of the problems and deficiencies of the prior art, the present application aims to provide a virtual synchronous machine low-voltage ride-through control method and system based on power adjustment and internal potential compensation, which can maximize the current capacity of the virtual synchronous machine to achieve the best voltage support under different degrees of voltage drop while ensuring the transient stability of the unit; at the same time, the recovery process can be accelerated based on the internal potential amplitude switching in the fault recovery stage, effectively reducing the active power impact and reactive power overloading of the virtual synchronous machine, and improving the transient voltage stability of the system.

[0008] According to a first aspect of the present application, a virtual synchronous machine low voltage ride through control method is provided, comprising the following steps:

[0009] Obtaining the machine terminal voltage and current of the grid connection point, and the virtual internal voltage and phase angle of the virtual synchronous machine, performing Park transformation on the three-phase current and calculating the grid connection output power of the virtual synchronous machine;

[0010] Based on the machine terminal voltage and the virtual internal voltage, the operating state of the virtual synchronous machine is identified, including normal operation stage, fault duration stage and fault recovery stage;

[0011] In the active power control link, the active power reference of the virtual synchronous machine is dynamically corrected based on the operating state of the virtual synchronous machine, and the virtual phase angle is adjusted based on the corrected active power reference of the virtual synchronous machine, so that the grid connection output active power of the virtual synchronous machine tends to be consistent with the active power reference;

[0012] In the reactive power-voltage control link, the virtual internal voltage of the virtual synchronous machine is dynamically corrected based on the operating state of the virtual synchronous machine, so as to obtain the virtual internal voltage of the virtual synchronous machine under different operating states;

[0013] According to the virtual internal voltage and the virtual phase angle of the virtual synchronous machine under different operating states, the three-phase reference voltage of the virtual synchronous machine is outputted through the virtual synchronous control;

[0014] According to the PWM modulation of the three-phase reference voltage of the converter, the gate signal for driving the converter is generated.

[0015] According to a second aspect of the present application, a computer system is also provided, comprising:

[0016] One or more processors;

[0017] A memory storing instructions operable to cause the one or more processors to perform operations when executed by the one or more processors, the operations comprising the operations of the virtual synchronous machine low voltage ride through control method of the preceding embodiments.

[0018] According to a third aspect of the present application, a computer readable medium storing software is also provided, the software comprising instructions executable by one or more computers, the instructions causing the one or more computers to perform operations when executed by the one or more computers, the operations comprising the operations of the virtual synchronous machine low voltage ride through control method.

[0019] Based on the above embodiment of the application, a virtual synchronous machine low voltage ride through control method based on power adjustment and internal potential compensation is proposed, which firstly judges the operation state of the virtual synchronous machine based on the electrical quantity of the virtual synchronous machine, and in different operation states, adjusts the active power reference to improve the transient stability, while taking into account the transient stability and the maximum output power capacity of the virtual synchronous machine, adjusts the active reference value during the fault to ensure the existence of the operating point, maximizes the current capacity of the virtual synchronous machine converter, and improves the dynamic reactive voltage support capability of the virtual synchronous machine and the system transient synchronization stability.

[0020] Meanwhile, during the fault recovery stage, based on the virtual internal potential steady-state amplitude compensation, the active power impact caused by the excessive internal potential amplitude of the virtual synchronous machine during the fault recovery is avoided, the reactive power is quickly restored to the normal value, the reactive power overdraw caused by the integral control is reduced, the voltage peak at the end of the voltage fault is reduced, and the system transient voltage stability is improved.

[0021] Compared with the prior art, the virtual synchronous machine low voltage ride through control method has the following advantages:

[0022] Based on the conventional virtual synchronous machine control, the virtual synchronous machine low voltage ride through control method detects the grid voltage and current, adjusts the active reference value during the fault, ensures the existence of the steady-state operating point of the virtual synchronous machine under voltage drop, reduces the acceleration area, avoids transient instability, switches the internal potential to the normal value based on the internal potential steady-state amplitude compensation mechanism during the fault recovery stage, overcomes the slow recovery speed and high internal potential caused by the integral control, suppresses the active / reactive power impact and terminal overvoltage, and has the following advantages:

[0023] The virtual synchronous machine low voltage ride through control method can effectively improve the system transient stability, optimize the power and voltage recovery performance, suppress the impact of power on the inverter during the recovery process, reduce the terminal transient overvoltage caused by fault recovery, speed up the power and voltage recovery, realize the low voltage ride through control of the virtual synchronous machine, improve the safety and reliability of the grid-connected inverter power supply, and improve the power system large disturbance stability. BRIEF DESCRIPTION OF DRAWINGS

[0024] Figure 1 is a flowchart of the virtual synchronous machine low voltage ride through control method according to the embodiment of the application.

[0025] Figure 2 is a control logic block diagram of the virtual synchronous machine low voltage ride through control method according to the embodiment of the application.

[0026] Figure 3is a state identification and compensation control strategy schematic diagram of a virtual synchronous machine low voltage ride through control method according to an embodiment of the present application.

[0027] Figure 4 is a transient instability mechanism schematic diagram based on the equal area method according to an embodiment of the present application.

[0028] Figure 5a , 5b are respectively low voltage ride through active power and power angle graphs without active power reference value adjustment according to an embodiment of the present application.

[0029] Figure 6a , 6b are respectively low voltage ride through active power and power angle graphs based on active power reference value adjustment according to an embodiment of the present application.

[0030] Figure 7a , 7b , 7c are respectively result comparison graphs of improved virtual synchronous machine low voltage ride through control and original control according to an embodiment of the present application, which respectively represent active power, reactive power and terminal voltage comparison, wherein the solid line is the control result of the method proposed in the present application, and the dashed line is the result of the original control. DETAILED DESCRIPTION

[0031] In order to better understand the technical content of the present application, specific embodiments are described below with reference to the accompanying drawings.

[0032] Aspects of the present application are described in this disclosure with reference to the accompanying drawings, in which a number of illustrative embodiments are shown. The embodiments of the present disclosure are not necessarily intended to include all aspects of the present application. It should be understood that various concepts and embodiments introduced above and described in more detail below can be implemented in any of numerous ways, as the disclosed concepts and embodiments are not limited to any particular manner of implementation. Additionally, some aspects of the present application can be utilized independently, or in any suitable combination with other aspects of the present application.

[0033] Embodiment 1

[0034] In combination Figure 1 , Figure 2 , Figure 3 As shown in the figure, the virtual synchronous machine low voltage ride through control method according to an embodiment of the present application includes the following steps:

[0035] Obtain the terminal voltage and terminal current of the grid connection point, and the virtual internal potential and phase angle of the virtual synchronous machine, perform Park transformation on the three-phase interflow and calculate the grid connection output power of the virtual synchronous machine;

[0036] The virtual synchronous machine's operating status is determined by identifying the terminal voltage and virtual internal potential, including the normal operation stage, the fault persistence stage, and the fault recovery stage.

[0037] In the active power control stage, the active power reference value of the virtual synchronous machine is dynamically corrected based on the operating status of the virtual synchronous machine, and the virtual phase angle is adjusted based on the corrected active power reference value of the virtual synchronous machine, so that the grid-connected output active power of the virtual synchronous machine tends to be consistent with the active power reference value.

[0038] In the reactive power-voltage control stage, the virtual internal potential of the virtual synchronous machine is dynamically corrected based on the operating state of the virtual synchronous machine to obtain the virtual internal potential of the virtual synchronous machine under different operating states.

[0039] Based on the virtual internal potential and virtual phase angle of the virtual synchronizer under different operating states, the three-phase reference voltage of the output converter is determined through virtual synchronization control; and

[0040] The gate signal for driving the converter is generated by PWM modulation based on the three-phase reference voltage of the converter.

[0041] Combination Figure 1 , 2 As shown, the terminal voltage and terminal current of the grid connection point are collected and expressed as terminal voltage. Terminal current All of them are three-phase AC variables.

[0042] The electrical quantities of the virtual synchronizer are collected, specifically the virtual internal potential and phase angle of the virtual synchronizer, which are expressed as: Virtual internal potential E And virtual phase angle θ v .

[0043] Combination Figure 2 As shown, Park transforms are performed on the terminal voltage and terminal current respectively to obtain the dq0-axis components. The Park transform formula is as follows:

[0044] ;

[0045] in, x Represents a three-phase AC variable. x a , x b , x c Three-phase AC variables x The abc three-phase components, x d , x q , x 0 represents the three-phase AC variablex The component on the dq0 axis.

[0046] Further, the grid-connected output power of the virtual synchronous machine can be calculated as:

[0047] ;

[0048] wherein, P s , Q s Pd and Qd represent the grid-connected active power and reactive power of the virtual synchronous machine, respectively; U Td , U Tq Vd and Vq are the d-axis and q-axis components of the terminal voltage, respectively; I Td 、I Tq Id and Iq are the dq-axis components of the terminal current, respectively.

[0049] As an optional embodiment, in combination with the state recognition based on the terminal voltage and the virtual internal potential, the operation state of the virtual synchronous machine is determined, including: Figure 2 , Figure 3 As shown in the foregoing, the operation state of the virtual synchronous machine is determined, including:

[0050] The operation state flags S1 and S2 of the virtual synchronous machine are configured;

[0051] According to the terminal voltage, the virtual internal potential and the threshold value of the virtual synchronous machine entering the low-voltage ride-through U th , the operation state of the virtual synchronous machine is recognized:

[0052] When U T ≤ U th , it is determined that the virtual synchronous machine is in the fault duration phase, the flags S1=1 and S2=0 are set, and the virtual internal potential E is sampled, and the steady-state value before the fault is E 0; wherein U T represents the voltage amplitude, and Vd and Vq are the vector sum of the d-axis and q-axis components of the terminal voltage; U T = ( U Td 2 + U Tq 2 ) 1 / 2 ;

[0053] When U T > Uth and E 0- E | ε When the virtual synchronous machine is in the low voltage ride through stage, the flag bit S1=1 and S2=0 are set.

[0054] When U T > U th and E 0- E | ε When the virtual synchronous machine is in the fault recovery stage, the flag bit S1=0 and S2=1 are set, wherein ε is a preset deviation threshold.

[0055] As an optional implementation, in combination with 2 and 3, in the active control link, the active reference quantity of the virtual synchronous machine is dynamically corrected based on the operating state of the virtual synchronous machine, and the virtual phase angle is adjusted based on the corrected active reference quantity of the virtual synchronous machine, so that the grid-connected output active power of the virtual synchronous machine tends to be consistent with the active reference quantity, comprising:

[0056] According to the flag bits S1 and S2 of the operating state of the virtual synchronous machine, the operating state of the virtual synchronous machine is determined.

[0057] According to different operating states, different active reference quantities of the virtual synchronous machine are dynamically selected in the active control link P ref ; and

[0058] According to the deviation of the active reference quantity of the virtual synchronous machine under different operating states P ref and the grid-connected output active power of the virtual synchronous machine P s , the virtual phase angle θ v is adjusted to make the active reference quantity P ref and the grid-connected output active power P s tend to be consistent, so as to control the transient stability, and the active control dynamic equation is as follows:

[0059] T J × (d ω v / dt)= P ref - P s - D ( ω v -ω ref );

[0060] d θ v / dt = ω v ;

[0061] wherein, T J represents an inertia time constant, D represents a damping coefficient, ω v and ω ref respectively represent the actual value and the reference value of the angular velocity of the virtual synchronous machine;

[0062] wherein the active reference quantity of the virtual synchronous machine in different operating states P ref is corrected as follows:

[0063] In the normal operating stage, the active reference quantity of the virtual synchronous machine P ref is set as: P ref = P N , and in the steady state P ref = P s , ω ref = ω v ;

[0064] In the fault duration stage, P ref ≠ P s , ω v and θ v according to the change of the active control dynamic equation, the active reference quantity of the virtual synchronous machine P ref is set as:

[0065] ;

[0066] wherein, P N represents the rated value of the active reference quantity of the virtual synchronous machine, Q s represents the grid-connected output reactive power of the virtual synchronous machine, I lim represents the current limiting value of the converter;

[0067] During the fault recovery phase, the active power reference value of the virtual synchronizer... P ref Set as: P ref = P N .

[0068] Therefore, during the fault persistence phase, the actual value of the virtual synchronizer's angular velocity... ω v And virtual phase angle θ v According to the changes in the active power control dynamic equation, when the fault is relatively severe, the grid-connected active power output... P s Smaller, if P ref If the initial value remains unchanged, the virtual power angle will continue to increase, leading to transient instability. Therefore, in the embodiments of the present invention, when a fault persistence phase is detected, while considering both the transient stability of the virtual synchronous machine and the maximum output power capacity, the active power reference is reduced to ensure the existence of the operating point and reduce unbalanced power. The active power reference value of the virtual synchronous machine is adjusted as follows:

[0069] .

[0070] Therefore, by maximizing the current capacity of the virtual synchronous machine converter, ensuring that the virtual synchronous machine has a steady-state operating point under voltage drops, reducing the acceleration area, avoiding transient instability, and improving the dynamic reactive voltage support capability of the virtual synchronous machine and the transient synchronization stability of the system.

[0071] As an optional implementation method, combined with Figure 2 , Figure 3 As shown, in the reactive power-voltage control stage, the virtual internal potential of the virtual synchronous machine is dynamically corrected based on its operating state to obtain the virtual internal potential of the virtual synchronous machine under different operating states, including:

[0072] The virtual internal potential deviation is obtained by integrating the deviations between the reactive power output and terminal voltage of the virtual synchronous machine and their respective reference values. This deviation is then combined with the virtual internal potential reference value. E ref The reactive power and voltage control at the grid connection point are performed, and the dynamic equation for reactive power-voltage control is as follows:

[0073] E = E ref + [1 / ( T E × s )]×[ Kq ( Q ref - Q s )+ K u ( U ref – U T )]

[0074] In the formula, T E The virtual internal potential integral constant; Q ref This is a reference value for reactive power. U ref This is the reference value for the terminal voltage; K q and K u These are the reactive power droop control coefficient and the voltage droop control coefficient, respectively; parameters s This represents the Laplace operator, i.e., the Laplace complex variable.

[0075] In particular, during the normal operation phase and the fault duration phase, virtual internal potential steady-state amplitude compensation is performed according to the aforementioned reactive power-voltage control dynamic equation.

[0076] During the fault recovery phase, the virtual internal potential of the virtual synchronizer is... E Switch to steady state value E 0 is used to suppress reactive power and voltage dynamic surges caused by the integral element.

[0077] Therefore, through the virtual internal potential compensation method described above, during the fault persistence phase, according to the reactive power-voltage control dynamic equation, when the generator terminal voltage drops, the integral term increases the virtual internal potential, thereby outputting reactive power support power. However, during the fault recovery phase, the generator terminal voltage quickly recovers to its normal value, but the integral term causes the virtual internal potential to remain significantly higher than the steady-state value, resulting in excessive reactive and active power, triggering transient overvoltage at the generator terminal, and potentially even triggering cascading faults such as continuous high and low voltage ride-throughs, affecting system voltage stability. Therefore, in the embodiments of this invention, during the fault recovery phase, the internal potential amplitude is... E Switch to steady state value E 0, to reduce the dynamic power and voltage impacts caused by the integral stage.

[0078] Under normal operating conditions, the ratio of the terminal voltage deviation to the reactive power deviation of the virtual synchronous machine is equal to the ratio of the reactive power droop factor to the voltage droop factor. E = E 0.

[0079] Therefore, the application controls the compensation based on the virtual internal potential steady-state amplitude in the fault recovery stage, so as to avoid the active power impact caused by the excessive internal potential amplitude of the virtual synchronous machine reactive voltage integral control at the fault recovery time, and promote the quick recovery of the reactive power and voltage to the normal value through the internal potential steady-state amplitude switching, reduce the reactive power over-release caused by the integral control, reduce the voltage peak value at the voltage fault end time, and improve the system transient voltage stability.

[0080] As an optional embodiment, the three-phase reference voltage of the converter is output through the virtual synchronous control according to the virtual internal potential and the virtual phase angle of the virtual synchronous machine in different operating states, and can be realized based on the virtual admittance control and the current controller of the prior art. Figure 2 、 Figure 3 As shown in the figure, the virtual synchronous control process as an example includes:

[0081] According to the virtual internal potential and the virtual phase angle of the virtual synchronous machine in different operating states, the initial reference value of the converter current is obtained through the abc / dq conversion and the virtual admittance link;

[0082] Then, the current reference value is obtained through the ring current limiter, and the three-phase reference voltage of the converter is generated through the decoupling control and the dq / abc conversion.

[0083] As an optional embodiment, the virtual admittance link as an example is represented as:

[0084] ;

[0085] Among them, i Td,cmd 、 i Tq,cmd are the initial values of the dq-axis current instructions at the machine end, respectively; R v 、 X v are the virtual resistance and the virtual reactance, respectively;

[0086] E represents the virtual internal potential of the virtual synchronous machine in different operating states; U T represents the voltage amplitude, which is the vector sum of the d-axis and q-axis components of the machine end voltage.

[0087] As an optional embodiment, the control equation of the ring current limiter as an example is:

[0088] i Td,ref = min[1, I lim / ( iTd,cmd 2 + i Tq,cmd 2 ) 1 / 2 ]× i Td,cmd

[0089] i Tq,ref = min[1, I lim / ( i Td,cmd 2 + i Tq,cmd 2 ) 1 / 2 ] × i Tq,cmd

[0090] wherein, i Td,cmd 、 i Td,ref are the initial value of the d-axis current command of the machine terminal output and the reference current value output by the ring-shaped current limiter, respectively; i Tq,cmd 、 i Tq,ref are the initial value of the q-axis current command of the machine terminal output and the reference current value output by the ring-shaped current limiter, respectively.

[0091] In combination with the low-voltage ride-through control method of the virtual synchronous machine based on power adjustment and internal potential compensation in the above embodiments, when the virtual synchronous machine is identified to be in different operating states, the transient stability is improved based on active power reference adjustment, the transient stability and the maximum output power capacity of the virtual synchronous machine are taken into account, the working operating point exists by adjusting the active reference value during the fault, the current capacity of the virtual synchronous machine converter can be maximized, the dynamic reactive voltage support capability of the VSG and the transient synchronization stability of the system are improved; at the same time, based on the virtual internal potential steady-state amplitude compensation, the active power impact caused by the excessive internal potential amplitude of the VSG reactive voltage integral control at the fault recovery time is avoided, the reactive power can be quickly restored to the normal value, the over-discharge of the reactive power caused by the integral control is reduced, the voltage peak at the end of the fault is reduced, and the transient voltage stability of the system is improved.

[0092] Embodiment 2

[0093] In this embodiment, we will further illustrate it by combining a specific example.

[0094] The DC front end of the virtual synchronous machine inverter in the embodiment is connected to wind power, photovoltaic or energy storage, and is controlled by a constant DC voltage, so it is equivalent to a DC power source. In combination with Figure 2 As shown in the figure, the typical VSG inverter can be connected to the grid through an LC filter, connected to an infinite power source through a box transformer and a transmission line.

[0095] In this example, the grid-connected system parameters of the virtual synchronous machine are shown in the following table:

[0096] .

[0097] In this example, the transient synchronization of the virtual synchronous machine after a large disturbance is analyzed, and the transient instability mechanism is explained using the equal-area rule.

[0098] As Figure 4 shown is the transient instability mechanism based on the equal-area rule, wherein, t 1、 t 2、 t 3 are the corresponding times of fault start, fault clearance and fault recovery end, respectively, according to the low voltage ride through control method of the virtual synchronous machine in the preceding embodiment 1. During a serious fault, the terminal voltage is low, and if the active power reference value is higher than the virtual synchronous machine power angle curve, it will lead to the loss of the VSG stable operating point, and the virtual power angle θ v continuously accelerates until transient instability. By reducing the active power reference value, not only can the system power angle have a steady-state operating point, but also the acceleration area A 1. When the deceleration area A 2 is greater than A 1, the virtual synchronous machine can transition to a new operating point and maintain transient synchronization stability. In addition, reducing the acceleration area during a fault is also beneficial to the synchronization stability after the fault ends. During the fault process of the virtual synchronous machine, its grid-connected point output apparent power is forced to be limited to U T I lim , i.e. the static stability limit power of the power angle curve is U T I lim Therefore, based on the adjustment of the modified active power reference value, the existence of the steady-state operating point during the fault can be ensured.

[0099] In this example, the setting P ref = 0.9, the infinite power source voltage drops to 0.2 p.u. at 0.5 s, and the duration is 0.5 s. When the active power reference remains unchanged during the fault, its active power and power angle curve is as Figure 5a 、 5bAs shown. According to the aforementioned equal area rule, due to the lack of adjustment of the reference power leading to the absence of a steady-state operating point, the virtual power angle continues to increase during the fault period, and the acceleration area during the fault period is greater than the deceleration area during the recovery phase, ultimately causing transient instability of the virtual synchronous machine.

[0100] The active power and power angle curves obtained by the virtual synchronous machine low-voltage ride-through control method proposed in this invention are shown in Figures 6a and 6b. By reducing the active power reference value, the acceleration area is significantly reduced, and the power angle stabilizes at the operating point during the fault after a slight increase. During the fault recovery phase, the virtual internal potential is affected by the fault detection and sampling delay. E There is a certain error between 0 and the actual steady-state value. After a short period of oscillation, the power angle stabilizes at a power angle slightly lower than the operating point before the fault. θ The power angle is at 0; however, after the fault crossing process ends and the system switches to normal control mode, the power angle returns to 0. θ 0. It can be seen that the control based on active power reference adjustment proposed in this invention can effectively improve the transient stability of the virtual synchronous machine.

[0101] Under the same operating conditions, the low-voltage ride-through control method proposed in this invention is compared with the original control method without internal potential steady-state amplitude compensation during the fault recovery phase. The grid-connected active power during the entire VSG low-voltage ride-through process is as follows: Figure 7a As shown, the original control, without internal potential switching, experiences a peak active power recovery of 1.4 pu during the active power recovery process, requiring multiple damped oscillations and only returning to normal after 1.5 seconds of fault clearance. In contrast, the active power recovery peak of the method of this invention is only 1.03 pu, with no significant oscillations and rapid recovery. Reactive power and terminal voltage are as follows... Figure 7b and 7c As shown, the original control method requires 350ms to recover the reactive power to its initial value, during which time it is in a continuous over-generation state, resulting in a peak voltage of 1.19 pu at the generator terminals during the recovery phase. Furthermore, it is in a transient overvoltage state within 1 second after the fault is cleared, which can easily trigger VSG high-voltage ride-through control and even cause cascading faults. Through comparison of control simulation results, the low-voltage ride-through control method proposed in this invention can effectively suppress active power overshoot oscillations during the recovery phase, accelerate the power recovery speed, and significantly improve transient voltage stability.

[0102] Example 3

[0103] In conjunction with the implementation of the virtual synchronous machine low-voltage ride-through control method in the above embodiments, the present invention also discloses a computer system, comprising:

[0104] One or more processors;

[0105] Memory is used to store instructions that can be operated.

[0106] The instructions, when executed by the one or more processors, cause the one or more processors to perform operations comprising the operations of the virtual synchronous machine low voltage ride through control method of the preceding embodiments.

[0107] Embodiment 4

[0108] In conjunction with the implementation of the virtual synchronous machine low voltage ride through control method of the preceding embodiments, according to the present application, a computer readable medium storing software is also disclosed, the software comprising instructions executable by one or more computers.

[0109] The instructions, when executed by the one or more processors, cause the one or more processors to perform operations comprising the operations of the virtual synchronous machine low voltage ride through control method of the preceding embodiments.

[0110] While the present application has been disclosed in its preferred embodiments with reference to the drawings, it is to be understood that the application is not limited to the embodiments disclosed, but is intended to cover modifications, variations, and equivalents which fall within the spirit and scope of the application. Therefore, the protection scope of the present application is defined by the appended claims.

Claims

1. A virtual synchronous machine low voltage ride through control method, characterized in that, The method comprises the following steps: acquiring the terminal voltage and the terminal current of the grid-connected point, and the virtual internal potential and the phase angle of the virtual synchronous machine, performing Park transformation on the three-phase interflow and calculating the grid-connected output power of the virtual synchronous machine; based on the terminal voltage and the virtual internal potential, performing state identification to determine the running state of the virtual synchronous machine, including the normal running stage, the fault duration stage and the fault recovery stage; in the active control link, dynamically correcting the active reference quantity of the virtual synchronous machine based on the running state of the virtual synchronous machine, and adjusting the virtual phase angle based on the corrected active reference quantity of the virtual synchronous machine, so that the grid-connected output active power of the virtual synchronous machine is consistent with the active reference quantity; in the reactive-voltage control link, dynamically correcting the virtual internal potential of the virtual synchronous machine based on the running state of the virtual synchronous machine, to obtain the virtual internal potential of the virtual synchronous machine in different running states; outputting the three-phase reference voltage of the converter through the virtual synchronous control according to the virtual internal potential and the virtual phase angle of the virtual synchronous machine in different running states; and generating the gate signal for driving the converter through PWM modulation according to the three-phase reference voltage of the converter. The state identification based on the terminal voltage and the virtual internal potential to determine the running state of the virtual synchronous machine comprises: configuring the running state flag bits S1 and S2 of the virtual synchronous machine; According to machine terminal voltage, virtual internal potential and threshold value of virtual synchronous machine entering low voltage ride through U th Carrying out operation state identification of the virtual synchronous machine: When U T ≤ U th the virtual synchronous machine is in the fault duration phase, the flag bits S1=1 and S2=0 are set, and the virtual internal electric potential is sampled E the steady-state value before the fault is entered E 0, U T represents the voltage amplitude, which is the vector sum of the d-axis and q-axis components of the terminal voltage of the machine; When U T > U th And E 0- E |< The dynamic correction of the active reference quantity of the virtual synchronous machine in the active control link based on the running state of the virtual synchronous machine, and the adjustment of the virtual phase angle based on the corrected active reference quantity of the virtual synchronous machine, so that the grid-connected output active power of the virtual synchronous machine is consistent with the active reference quantity, comprises: When the virtual synchronous machine completes low voltage ride through and enters the normal operation stage, set the flag bits S1=0 and S2=0. When U T U th And E 0- E | determining the running state of the virtual synchronous machine according to the running state flag bits S1 and S2 of the virtual synchronous machine; When the difference between the actual value and the preset value is greater than the preset deviation threshold, it is judged that the virtual synchronous machine is in the fault recovery stage, and the flag bits S1=0 and S2=1 are set, wherein The dynamic correction of the virtual internal potential of the virtual synchronous machine in the reactive-voltage control link based on the running state of the virtual synchronous machine, to obtain the virtual internal potential of the virtual synchronous machine in different running states, comprises: is a preset deviation threshold.​ wherein, in the normal running stage and the fault duration stage, the virtual internal potential steady-state amplitude compensation is performed according to the reactive-voltage control dynamic equation; The outputting of the three-phase reference voltage of the converter through the virtual synchronous control according to the virtual internal potential and the virtual phase angle of the virtual synchronous machine in different running states comprises: According to different operating states, different active reference quantities of the virtual synchronous machine are dynamically selected in the active control link P ref ; and Based on the active power reference of the virtual synchronous machine under different operating conditions P ref Active power output in parallel with virtual synchronous machine P s Adjust the virtual phase angle to correct the deviation. after the abc / dq transformation according to the virtual internal potential and the virtual phase angle of the virtual synchronous machine in different running states, the initial reference value of the converter current is obtained through the virtual admittance link; v Make the active reference quantity P ref With grid-connected output active power P s To achieve uniformity and control transient stability, the active power control dynamic equation is as follows: T J ×( then the current reference value is obtained through the ring current limiter, and the three-phase reference voltage of the converter is generated through decoupling control and dq / abc transformation. v / dt)= P ref - P s - D ( The virtual admittance link is represented as: v - The control equation of the ring current limiter is: ref ); d comprise: v / dt = one or more processors; v ; wherein, T J denotes an inertia time constant, D denotes a damping coefficient, a memory storing instructions operable to, when executed by the one or more processors, cause the one or more processors to perform operations comprising the operations of the method of any one of claims 1-5. v and ​ ref denote the actual and reference values of the angular speed of the virtual synchronous machine, respectively. wherein the active reference quantity of the virtual synchronous machine in different operating states P ref Amend as follows: In the normal operation phase, the active reference quantity of the virtual synchronous machine P ref is set to: P ref = P N , in steady state P ref = P s , ​ ref = ​ v ; In the fault continuation phase, P ref ≠ P s , ​ v and ​ v According to the active control dynamic equation change, the active reference quantity of the virtual synchronous machine P ref is set to: ； wherein, P N represents the active reference quantity rating of the virtual synchronous machine, Q s represents the virtual synchronous machine grid-connected output reactive power, I lim represents the current limiting value of the converter. In the fault recovery phase, the active reference quantity of the virtual synchronous machine P ref is set to: P ref = 0 P N .

2. The VSM low voltage ride through control method of claim 1, wherein, ​ According to the deviation of the grid-connected output reactive power and the terminal voltage of the virtual synchronous machine from the respective reference values, integral operation is performed to obtain a virtual internal electromotive force deviation, and the virtual internal electromotive force reference value is combined E ref The grid-connected point output reactive power and voltage control are performed, and a reactive power-voltage control dynamic equation is: E = E ref + [1 / ( T E × s )] ×[ K q ( Q ref - Q s )+ K u ( U ref – U T )]； wherein T E is a virtual internal potential integration constant; Q ref is a reactive power reference value; U ref is a terminal voltage reference value; K q and K u are a reactive power droop control coefficient and a voltage droop control coefficient, respectively; parameters s denotes a Laplace operator; ​ During the fault recovery phase, the virtual internal potential of the virtual synchronizer is... E Switch to steady state value E 0 is used to suppress reactive power and voltage dynamic surges caused by the integral element.

3. The VSM low voltage ride through control method according to claim 1 or 2, characterized in that, ​ ​ ​ 4. The VSM low voltage ride through control method of claim 3, wherein, ​ ; wherein, i Td,cmd , i Tq,cmd are respectively the initial value of the machine-end output dq-axis current command; R v , X v are respectively the virtual resistance and virtual reactance; E represents the virtual internal voltage of the virtual synchronous machine in different operating states; U T represents the voltage amplitude, which is the vector sum of the d-axis and q-axis components of the terminal voltage of the machine; U Td , U Tq are the d-axis and q-axis components of the terminal voltage of the machine, respectively, U T = ( U Td 2 + U Tq 2 ) 1 / 2 .

5. The VSM low voltage ride through control method of claim 3, wherein, ​ i Td,ref = min[1, I lim / ( i Td,cmd 2 + i Tq,cmd 2 ) 1 / 2 ]× i Td,cmd ; i Tq,ref = min[1, I lim / ( i Td,cmd 2 + i Tq,cmd 2 ) 1 / 2 ]× i Tq,cmd ; In the formula, i Td,cmd , i Td,ref respectively are the initial value of the machine end output d-axis current instruction and the reference current value of the annular current limiter output. i Tq,cmd , i Tq,ref iq0 and iq1 are the initial value of the machine end output q-axis current command and the reference current value output by the ring current limiter, respectively.

6. A computer system, characterized by ​ ​ ​ 7. A computer readable medium storing software, characterized in that, The software includes instructions executable by one or more computers such that execution of the instructions by the one or more computers causes the one or more computers to perform operations comprising the operations of the method of any one of claims 1-5.

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