Method for analyzing influence of reactive droop control on VSG large interference stability

Abstract

The invention relates to the technical field of electric power, and particularly discloses a method for analyzing the influence of reactive droop control on VSG large interference stability, which is based on a typical basic control structure of a second-order SG model-based amplitude-phase control VSG scheme realized under a dq coordinate system. The control structure comprises a distributed power supply, a three-phase voltage source type inverter, a filter inductor, an alternating current bus, a power control ring, a virtual impedance control ring, a current control ring and a PWM module, and gives an impedance large interference stability boundary under a balance point working condition in a voltage drop period when reactive droop is considered. The impedance large interference stability boundary of the system is widened, and the reactive droop control is beneficial to improving the large interference stability of the system.

Description

Technical Field

[0001] The invention relates to the field of electric power technology, and in particular to a method for analyzing the influence of reactive power droop control on the stability of VSG with large disturbance. Background Art

[0002] Virtual synchronous generator (VSG) controls the grid-connected converter as a voltage source by designing a power control loop, providing inertia, frequency and voltage regulation support for the power grid. Through the Thevenin equivalent circuit method, the grid-connected VSG system can be simplified as an output impedance series controlled voltage source. Among them, the output impedance plays an important role in the power response characteristics and system stability. The output impedance of the VSG can be reshaped by designing a virtual impedance to optimize the dynamic response characteristics and stability of the VSG.

[0003] In traditional methods, the design of grid-connected converter output impedance is based on the stability analysis of small disturbances. Even if the impact of large disturbances (such as short-circuit faults and line tripping) on ​​the output impedance is considered, the factors of the reactive control loop are ignored, resulting in little research on the impact of reactive power droop control on the stability of VSG large disturbances during voltage drops. In practical applications, when reactive power droop control is used, when a voltage drop disturbance occurs, the rapid response of the reactive loop causes the virtual internal potential to change, which in turn affects the stability performance of the system. Summary of the invention

[0004] The object of the present invention is to provide a method for analyzing the influence of reactive power droop control on the large disturbance stability of VSG, so as to improve the large disturbance stability of the system.

[0005] To achieve the above object, the present invention provides the following technical solutions:

[0006] An analysis method for the influence of reactive droop control on the stability of VSG under large disturbances is based on a basic control structure of a typical amplitude-phase control VSG scheme based on a second-order SG model implemented in a dq coordinate system. The control structure includes a distributed power supply, a three-phase voltage source inverter, a filter inductor, an AC bus, a power control loop, a virtual impedance control loop, a current control loop and a PWM module.

[0007] The active power loop of VSG can be given by:

[0008]

[0009] Where: M is the moment of inertia, P ref is the active power command, P is the output active power, k p is the active droop coefficient, D is the damping coefficient, and ω is the output frequency;

[0010] The reactive power loop can be expressed as:

[0011]

[0012] Where: Q ref is the reactive power instruction, E ref is the internal reference voltage, E is the virtual internal potential, Q is the output reactive power, k Q is the reactive power droop coefficient;

[0013] The reactive power droop control link can be expressed by the following expression:

[0014] E=E ref -(QQ ref ) / k Q

[0015] The calculation formula for reactive power output is:

[0016]

[0017] Where U is the grid voltage, E is the virtual internal potential, Z is the output impedance, θ is the output impedance angle, and δ is the power angle;

[0018] The calculation formula for active power output is:

[0019]

[0020] Preferably, the virtual internal potential at the initial moment is assumed to be E0, the corresponding reactive output is Q0, and E0 satisfies the following equation:

[0021]

[0022] make

[0023]

[0024] Solving the equation yields:

[0025]

[0026] Q0=Q ref +k Q (E ref -E0).

[0027] Preferably, the calculation process of the virtual internal potential and the output reactive power after the voltage drops is as follows:

[0028]

[0029] Where U sag is the fault grid voltage, E1 is the virtual internal potential after the voltage drops;

[0030] make

[0031]

[0032] , solving the equation, we can get the grid voltage drop after:

[0033]

[0034] Q1=Q ref +k Q (E ref -E1).

[0035] Preferably, in order to ensure the degree of current limiting, the following boundary conditions should be met:

[0036]

[0037] Where: t max is the moment of maximum current, δ0 is the power angle when the active power is commanded, k s I ref is the maximum current that the converter can withstand, k s is the flow coefficient, u g is the grid voltage, and ω is the output frequency.

[0038] Preferably, considering that the virtual internal potential is affected by the disturbance of the reactive ring, the power angle swing equation of the system before the fault is:

[0039]

[0040] The corresponding steady-state point at this time is:

[0041]

[0042] After the voltage drops, the power angle swing equation of the system becomes:

[0043]

[0044] The steady-state point after the voltage drops is: (δ Q,s ,Δω=0), the corresponding maximum critical point is: (π-2θ-δ Q,s ,Δω=0); where:

[0045]

[0046] Preferably, according to the existence constraint of the stable working point, it can be known that:

[0047]

[0048] δ Q,sSubstituting the expression into the above formula, we can further deduce that the boundary conditions that the output impedance needs to meet are:

[0049]

[0050] Preferably, the general expression of the energy function during reactive power droop control is:

[0051]

[0052] Preferably, (π-2θ-δ Q,s ,Δω=0) into the above general energy function expression, and the critical energy V can be obtained by simplification. cr The expression is:

[0053]

[0054] The initial condition is the power angle value and frequency disturbance at the instant when the grid voltage drops, that is, the initial condition is (δ Q,0 ,Δω Q,0 =0); Substituting it into the expression of the energy function, the initial energy value can be obtained:

[0055]

[0056] Preferably, the initial energy value is substituted into the stability criterion V cr -V(δ Q,0 ,0)>0, it can be obtained that the boundary condition that the output impedance should satisfy to ensure the transient power angle stability of the system is:

[0057]

[0058] The beneficial technical effects of this application mainly include:

[0059] (1) The introduction of reactive power droop control broadens the system's impedance large disturbance stability boundary, and reactive power droop control helps improve the system's large disturbance stability;

[0060] (2) Reactive power-voltage control can also be constant voltage control, constant reactive power control, inertia control, etc. Different reactive power or voltage control structures will also have different effects on the large disturbance stability of the VSG system. The results of this application can also be easily transplanted to other types of reactive power or voltage control structures, providing solutions for other types of reactive power or voltage control structures. BRIEF DESCRIPTION OF THE DRAWINGS

[0061] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the following briefly introduces the drawings required for use in the description of the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0062] Figure 1 It is a VSG control structure block diagram provided in an embodiment of the present application;

[0063] Figure 2 It is a control block diagram of a virtual steady-state synchronous impedance control method provided by an embodiment of the present application;

[0064] Figure 3 is a power angle curve diagram of type I large interference provided in an embodiment of the present application;

[0065] Figure 4 It is an impedance stability boundary diagram of a type I large interference provided in an embodiment of the present application;

[0066] Figure 5 This is a stable boundary diagram of large impedance interference with a balance point during a voltage drop provided by an embodiment of the present application. DETAILED DESCRIPTION

[0067] In order to make the purpose, technical solution and advantages of the embodiments of the present application clearer, the technical solution in the embodiments of the present application will be clearly and completely described below in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of this application.

[0068] The flowcharts shown in the accompanying drawings are only examples and do not necessarily include all the contents and operations / steps, nor must they be executed in the order described. For example, some operations / steps may also be decomposed, combined or partially merged, so the actual execution order may change according to actual conditions.

[0069] In the description of the present application, the terms "first / second" involved are merely used to distinguish similar objects and do not represent a specific order for the objects. It can be understood that "first / second" can be interchanged with a specific order or sequence where permitted, so that the embodiments of the present application described herein can be implemented in an order other than that illustrated or described herein.

[0070] It should be understood that the orientation or position relationship is based on the orientation or position relationship shown in the drawings. These orientation terms are only for the convenience of describing the present application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as limiting the present application.

[0071] In the prior art, the design of the output impedance of the grid-connected converter is all for the stability analysis of small disturbances. However, there is little research on the design of the output impedance under large disturbances (such as short-circuit faults and line tripping). In fact, large disturbances will bring more severe challenges to the stable operation of the system, and may cause serious problems such as impact current and power imbalance. During the occurrence of large disturbances, the performance index constraints that the system output impedance needs to meet are different. Only satisfying the small disturbance stability constraints often cannot guarantee that the system can still operate stably under large disturbance conditions.

[0072] Therefore, this application explores the large interference stability mechanism of VSG from the perspective of impedance optimization, derives the constraints of the output impedance under symmetrical voltage drop conditions, and improves the large interference stability of the VSG system from the perspective of optimizing the design of virtual impedance.

[0073] Figure 1 It is a basic control structure of a typical amplitude-phase control VSG scheme based on a second-order SG model implemented in a dq coordinate system. The distributed power source is connected to the AC bus through a three-phase voltage source inverter. The power control loop generates the amplitude and frequency of the output voltage command. The voltage / virtual impedance control loop simulates the electrical characteristics of the stator winding of the synchronous generator. The current control loop uses a PI regulator to achieve accurate current tracking and overcurrent protection. Figure 1 In, v abc and i abc are the three-phase output voltage and output current of VSG, v dq and i dq are the d-axis and q-axis components of the VSG output voltage and output current, respectively, P ref and Q ref are the active power and reactive power commands, respectively, E is the virtual internal potential of the VSG, simulating the internal potential of the synchronous generator, and ω is the angular frequency of the virtual internal potential generated by the active control loop. The control strategy is established in the dq coordinate system. The VSG algorithm generates the reference amplitude and phase of the output voltage. The virtual impedance is used to reshape the output impedance of the VSG. The current control loop improves the current control performance and overcurrent protection function of the system.

[0074] The active power loop of VSG can be given by:

[0075]

[0076] Where: M is the moment of inertia, Pref is the active power command, P is the output active power, k p is the active droop coefficient, D is the damping coefficient, and ω is the output frequency.

[0077] According to the above formula, the active power control loop of the virtual synchronous generator simulates the droop control and rotor swing equation of the synchronous generator. The former can provide a primary frequency regulation function to the power grid, and the latter can provide inertia and damping support.

[0078] Reactive power control uses QE droop to simulate the voltage regulation characteristics of the synchronous generator and generate the amplitude of the VSG virtual internal potential.

[0079] The reactive loop can be expressed as:

[0080]

[0081] Where: Q ref is the reactive power instruction, E ref is the internal reference voltage, E is the virtual internal potential, Q is the output reactive power, k Q is the reactive power droop coefficient.

[0082] Figure 1 The virtual impedance control in the embodiment adopts the virtual steady-state synchronous impedance (VSSI) control method. The control block diagram of the virtual steady-state synchronous impedance (VSSI) control method is as follows: Figure 2 shown.

[0083] Based on VSSI control, ignoring the delay of the current control loop, the output current is:

[0084]

[0085] Where: Z v , R v , X v are virtual impedance, virtual resistance and virtual reactance respectively; θ is the virtual impedance angle; u d and u q are the d and q axis components in the dq coordinate system. Therefore, the dq axis components of the output current can be expressed as:

[0086]

[0087] Where: δ is the power angle, u g is the grid voltage.

[0088] By Park inverse transformation, the current of the α axis can be obtained as:

[0089]

[0090] Taking the derivative of equation (5) we can get

[0091]

[0092] Order α / dt=0, the moment of maximum current

[0093]

[0094] The corresponding maximum current value is:

[0095]

[0096] Assume that the maximum current the converter can withstand is k s I ref , where k s is the overcurrent coefficient, then the current peak should satisfy the constraint condition: i αmax <k s I ref .

[0097] Furthermore, the impedance constraint condition of electromagnetic transient can be derived as:

[0098]

[0099] The rotor swing equation of VSG can be written as:

[0100]

[0101] Where: P ref is the active power command, P is the output active power, D is the damping coefficient, and ω is the output frequency.

[0102] When the voltage drops, in addition to the impact current, the output power P e It will also decrease, resulting in power imbalance, which will cause the power angle to be corrected according to equation (10). Among them, the power angle may cross the stability boundary and lose stability, which is the problem of electromechanical transient stability.

[0103] In order to suppress the impact current, it is necessary to increase the virtual impedance; and the larger the impedance value, the lower the power angle curve, and the more likely the power angle instability problem will occur. Therefore, this application derives the stable operating area of ​​the output impedance, so that the impact current is limited to a reasonable range, and the system remains stable during the power angle swing. If there is no such overlapping stable operating area, the system needs to take additional measures to ensure the stable operation of the VSG. For the convenience of analysis, large disturbances are divided into two categories: Category I is that there is still a balance point after the fault; Category II is that there is no balance point after the fault. This application mainly focuses on large disturbances of Category I.

[0104] The analysis process of type I large disturbance is as follows:

[0105] The power angle curve is as follows Figure 3 As shown, the mathematical expression is:

[0106]

[0107] Where U is the grid voltage, E is the VSG output voltage, Z is the output impedance, θ is the output impedance angle, and δ is the power angle.

[0108] Therefore, the swing equation of the power angle is

[0109]

[0110] like Figure 3 As shown in the figure, the power angle curve of the system before the fault is P, (δ0, Δω=0) and (π-2θ-δ0, Δω=0) are two steady-state points; after the fault, the power angle curve of the system drops to P1, and the system still has an equilibrium point: (δ s ,Δω=0) and (π-2θ-δ s ,Δω=0).

[0111] From (11), we can get the expression of power angle:

[0112]

[0113] In order to ensure the existence of the balance point, the power angle needs to satisfy:

[0114]

[0115] Combining equations (13) and (14), the constraints of the output impedance before and after the disturbance are:

[0116]

[0117] Where: U0 is the initial grid voltage, U sag is the fault grid voltage.

[0118] The constraints of electromagnetic transient current limiting are:

[0119]

[0120] Define the energy function during transient response:

[0121]

[0122] The energy function has clear physical meaning and can give a stability assessment close to the actual stable region.

[0123] The initial point (δ0, Δω=0) and the maximum critical point (π-2θ-δ s,Δω=0) are substituted into the energy function to obtain the initial energy V0 and critical energy V cr :

[0124]

[0125] According to Lyapunov's theorem, the constraint condition for system stability is V cr -V0>0. Therefore, the impedance constraint condition under the transient power angle stability constraint can be derived as follows:

[0126]

[0127] According to formula (15)(16)(19), Figure 4 The range of the system output impedance under large disturbance of type I is shown. When the output impedance of the VSG is reshaped within this stable region, the VSG system can maintain stable operation under large disturbance of type I, and the inrush current is also suppressed.

[0128] In the above analysis, the influence of reactive power control loop is not considered. In practical applications, when reactive power droop control is adopted, when voltage drop disturbance occurs, the rapid response of reactive power loop causes the virtual internal potential to change, which in turn affects the stability performance of the system.

[0129] When the reactive power control loop is considered, the system becomes a two-input and two-output system. It is necessary to establish a large signal model by combining the nonlinear equations of the two control loops. The steady-state operating points before and after the voltage sag fault are calculated through the large signal model.

[0130] The reactive power droop control link can be expressed by the following expression:

[0131] E=E ref -(QQ ref ) / k Q (20)

[0132] The calculation formula for reactive power output is:

[0133]

[0134] The calculation formula for active power output is:

[0135]

[0136] Assume that the virtual internal potential at the initial moment is E0, and the corresponding reactive output is Q0. Combining equations (20), (21) and (22), it can be deduced that E0 satisfies the following equation:

[0137]

[0138] make

[0139]

[0140] Solving the equation yields:

[0141]

[0142] Q0=Q ref +k Q (E ref -E0) (25)

[0143] Similarly, the calculation process of the virtual internal potential and output reactive power after the voltage drops is as follows:

[0144]

[0145] make

[0146]

[0147] , solving the equation, we can get the grid voltage drop after:

[0148]

[0149] Q1=Q ref +k Q (E ref -E1) (28)

[0150] At this time, in order to ensure the degree of current limiting, the following boundary conditions should be met:

[0151]

[0152] Where: t max The expressions of and δ0 can be found in equations (7) and (13).

[0153] Considering that the virtual internal potential is affected by the disturbance of the reactive ring, the power angle swing equation of the system before the fault is corrected to:

[0154]

[0155] The corresponding steady-state point at this time is:

[0156]

[0157] After the voltage drops, the power angle swing equation of the system becomes:

[0158]

[0159] The steady-state point after the voltage drops is: (δ Q,s,Δω=0), the corresponding maximum critical point is: (π-2θ-δ Q,s ,Δω=0). Where:

[0160]

[0161] According to the existence constraint of the stable working point, it can be seen that:

[0162]

[0163] δ Q,s Substituting the expression (33) into the above equation (34), we can further deduce that the boundary condition that the output impedance needs to satisfy is:

[0164]

[0165] According to the above energy function equation, the general expression of the energy function when considering reactive power droop control can be obtained as follows:

[0166]

[0167] Based on this energy function, the influence of reactive power droop control on transient stability is discussed below.

[0168] (π-2θ-δ Q,s ,Δω=0) into the above general energy function expression, and the simplification can be obtained as V cr The expression is:

[0169]

[0170] The initial condition is the power angle value and frequency disturbance at the instant when the grid voltage drops, that is, the initial condition is (δ Q,0 ,Δω Q,0 =0). Substituting it into the expression of the energy function, we can get the initial energy value:

[0171]

[0172] Substituting into the stability criterion V cr -V(δ Q,0 ,0)>0, it can be obtained that the boundary condition that the output impedance should satisfy to ensure the transient power angle stability of the system is:

[0173]

[0174] Where: Q,0 ,δ Q,s For the mathematical expressions of and E1, please refer to equations (31), (33) and (27), respectively.

[0175] The impedance large disturbance stability boundary under the condition of balance point during voltage drop with and without reactive power droop is drawn together Figure 5 In. Figure 5 As shown in the figure, the dotted line is the impedance large disturbance stability boundary when no reactive control strategy is added, and the solid line is the impedance large disturbance stability boundary when reactive power droop control is added. The same color represents the same constraint. It can be seen from the figure that the impedance stability area when no reactive control strategy is added is the purple part in the figure; and the stable area surrounded by the impedance boundary when reactive power droop control is added is the rose red area in the figure, which completely includes the purple area. This shows that the introduction of reactive droop control broadens the impedance large disturbance stability boundary of the system, and reactive droop control helps to improve the large disturbance stability of the system.

[0176] Reactive power-voltage control can also be constant voltage control, constant reactive power control, inertia control, etc. Different reactive power or voltage control structures will also have different effects on the large disturbance stability of the VSG system. The results of this application can also be easily transplanted to other types of reactive power or voltage control structures, providing solutions for other types of reactive power or voltage control structures.

[0177] It will be appreciated by those skilled in the art that all or some of the steps, systems, and functional modules / units in the methods disclosed above may be implemented as software, firmware, hardware, and appropriate combinations thereof. In a hardware implementation, the division between the functional modules / units mentioned in the above description does not necessarily correspond to the division of physical components; for example, a physical component may have multiple functions, or a function or step may be performed by several physical components in cooperation. Some or all physical components may be implemented as software executed by a processor, such as a central processing unit, a digital signal processor, or a microprocessor, or may be implemented as hardware, or may be implemented as an integrated circuit, such as an application-specific integrated circuit. Such software may be distributed on a computer-readable storage medium, which may include a computer-readable storage medium (or a non-transitory medium) and a communication medium (or a temporary medium).

[0178] The above is only a specific implementation of the present application, so that those skilled in the art can understand or implement the present application. It will be apparent to those skilled in the art that various modifications to these embodiments are possible, and the general principles defined in the present application can be implemented in other embodiments without departing from the spirit or scope of the present application. Therefore, the present application will not be limited to these embodiments shown in the present application, but will conform to the widest scope consistent with the principles and novel features applied for by the present application.

Claims

1. An analysis method for the influence of reactive droop control on the stability of VSG under large disturbances is based on a basic control structure of a typical amplitude-phase control VSG scheme based on a second-order SG model implemented in a dq coordinate system. The control structure includes a distributed power supply, a three-phase voltage source inverter, a filter inductor, an AC bus, a power control loop, a virtual impedance control loop, a current control loop and a PWM module, and is characterized in that: The active power loop of VSG can be given by: Where: M is the moment of inertia, P ref is the active power command, P is the output active power, k p is the active droop coefficient, D is the damping coefficient, and ω is the output frequency; The reactive power loop can be expressed as: Where: Q ref is the reactive power instruction, E ref is the internal reference voltage, E is the virtual internal potential, Q is the output reactive power, k Q is the reactive power droop coefficient; The reactive power droop control link can be expressed by the following expression: E=E ref -(Q-Q ref ) / k Q The calculation formula for reactive power output is: Where U is the grid voltage, E is the virtual internal potential, Z is the output impedance, θ is the output impedance angle, and δ is the power angle; The calculation formula for active power output is:

2. The method according to claim 1, characterized in that: Assume that the virtual internal potential at the initial moment is E0, the corresponding reactive output is Q0, and E0 satisfies the following equation: make Solving the equation yields: Q0=Q ref +k Q (E ref -E0).

3. The method according to claim 2, characterized in that: The calculation process of the virtual internal potential and output reactive power after the voltage drops is as follows: Where U sag is the fault grid voltage, E1 is the virtual internal potential after the voltage drops; make , Solving the equation, we can get the grid voltage drop after: Q1=Q ref +k Q (E ref -E1)。 4. The method according to claim 3, characterized in that: In order to ensure the degree of current limiting, the following boundary conditions should be met: Where: t max is the moment of maximum current, δ0 is the power angle when the active power is commanded, k s I ref is the maximum current that the converter can withstand, k s is the flow coefficient, u g is the grid voltage, and ω is the output frequency.

5. The method according to claim 4, characterized in that: Considering that the virtual internal potential is affected by the disturbance of the reactive ring, the power angle swing equation of the system before the fault is: The corresponding steady-state point at this time is: After the voltage drops, the power angle swing equation of the system becomes: The steady-state point after the voltage drops is: (δ Q,s ,Δω=0), the corresponding maximum critical point is: (π-2θ-δ Q,s ,Δω=0); where:

6. The method according to claim 5, characterized in that: According to the existence constraint of the stable working point, it can be seen that: δ Q,s Substituting the expression into the above formula, we can further deduce that the boundary conditions that the output impedance needs to meet are:

7. The method according to claim 6, characterized in that: The general expression of the energy function during reactive droop control is:

8. The method according to claim 7, characterized in that: (π-2θ-δ Q,s ,Δω=0) into the above general energy function expression, and the critical energy V can be obtained by simplification. cr The expression is: The initial condition is the power angle value and frequency disturbance at the instant when the grid voltage drops, that is, the initial condition is (δ Q,0 ,Δω Q,0 =0); Substituting it into the expression of the energy function, the initial energy value can be obtained:

9. The method according to claim 8, characterized in that: Substitute the initial energy value into the stability criterion V cr -V(δ Q,0 ,0)>0, it can be obtained that the boundary condition that the output impedance should satisfy to ensure the transient power angle stability of the system is:

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