Transient stability optimization control system and method for virtual synchronous machine

Abstract

The invention relates to a virtual synchronous machine transient stability optimization control system and method, and the method comprises the steps: sampling a three-phase power grid to detect the angular frequency of a power grid in real time, carrying out the subtraction of the angular frequency and the angular frequency outputted by an active-frequency control module of a virtual synchronous machine, multiplying a coefficient, and introducing into a reactive-voltage control module. The voltage sag dynamic state of the virtual synchronous machine under the power grid voltage drop is slowed down by means of angular frequency fluctuation caused by active power adjustment of the virtual synchronous machine under the power grid voltage drop, and the transient voltage supporting capacity of the virtual synchronous machine under the fault is improved. Under stable operation, the output angular frequency of the active power-frequency control module of the virtual synchronous machine is equal to the angular frequency of the power grid, so that the additional loop has no influence on the virtual synchronous machine during stable operation. According to the invention, the difference between the power grid angular frequency and the output angular frequency of the active-frequency control module is introduced into the reactive-voltage control module, so that the system can maintain good damping characteristics during the transient period, and the transient stability of the virtual synchronous machine system is improved.

Description

Technical Field

[0001] The invention relates to a grid-type converter system, and in particular to a method for optimizing transient stability control of a virtual synchronous machine during a grid voltage drop. Background Art

[0002] As the penetration rate of distributed power sources in the power system gradually increases, problems such as low inertia and poor frequency response of the power system have become important challenges faced by high-proportion renewable energy grid-connected power generation. Virtual Synchronous Generators (VSG) simulate the operation mechanism of synchronous generators, introduce virtual inertia and damping, and actively participate in the process of grid frequency and voltage regulation, so they have been widely used. However, the operating environment of distributed power sources is complex, and there may be grid short circuits, impedance changes and other faults. When the grid voltage drops, the VSG system has transient instability problems, which brings great challenges to the stable operation of distributed power generation systems. Therefore, the optimization problem of VSG transient stability during grid faults has attracted widespread attention, and scholars at home and abroad have carried out relevant research.

[0003] Relevant literature uses stability analysis methods such as the equal area method, phase diagram method, and Lyapunov method to discuss in detail the transient instability mechanism of VSG under grid voltage drop, indicating that one of the necessary conditions for the VSG system to maintain transient stability is the existence of a stability equilibrium point during the fault, and the existence of the equilibrium point is related to factors such as grid impedance, grid voltage, and inverter output voltage. Moreover, even if the system has a stable equilibrium point, due to the influence of the VSG inertial unit, when the system damping level is low, the VSG may still cross the unstable equilibrium point and cause transient instability. Therefore, some literature proposes to adaptively adjust the VSG control parameters during the fault period to improve the system damping performance during the transient period, thereby improving the transient stability of the system. However, the parameter design process of this method is complicated and cumbersome, and a reasonable fault detection scheme needs to be designed to achieve adaptive adjustment of control parameters. Summary of the invention

[0004] In order to solve the above technical problems, the purpose of the present invention is to provide a VSG transient stability optimization control method during grid voltage sag. This method can improve the transient stability of the VSG system by slowing down the VSG output voltage change speed and damping the system power angle divergence only during the grid voltage sag without affecting the steady-state operation characteristics of the VSG.

[0005] The technical solution of the present application is specifically: a method for optimizing the transient stability control of a VSG during a grid voltage drop, comprising a frequency detection module (Frequency Detection, FD) and an additional damping loop connected in series;

[0006] The three sampling point frequency detection module input is the three-phase grid voltage v g_abc , the output is the grid angular frequency ω g ;

[0007] The additional damping loop input is the grid angular frequency ω output by the three sampling point frequency detection module g The difference between the output angular frequency ω in the VSG active power-frequency control loop, multiplied by the damping coefficient K d Then the reactive power-voltage control module is introduced;

[0008] A method for optimizing the transient stability of VSG during a grid voltage drop comprises the following steps:

[0009] 1) Set the grid voltage v g The angular frequency ω output by the active power-frequency control module is used as the input of the transient damping module;

[0010] 2) Detect the grid voltage frequency ω based on the three-sampling point frequency detection method g ;

[0011] 3) Compare ω and ω g Subtract and multiply by the damping coefficient K d Introducing reactive power-voltage control module.

[0012] Preferably, the implementation process in step 2) is:

[0013] 2.1) T s is the sampling period, n is the sampling interval, and v is sampled respectively. g_abc (k-2n), v g_abc (kn), v g_abc (k) The grid voltage values ​​at three different times are used as input quantities of the frequency detection module at three sampling points;

[0014] 2.2) Perform Clark orthogonal transformation on the input three-phase voltage to obtain the α-axis components v at three different sampling times α (k-2n), v α (kn), v α (k), and the β-axis component v β (k-2n), v β (kn), v β (k);

[0015] 2.3) According to the discrete oscillator law, we can get

[0016] v α (k)+v α (k-2n)=2v α (kn)·cos(ω g nTs ) (1)

[0017] v β (k)+v β (k-2n)=2v β (kn)·cos(ω g nT s ) (2)

[0018] Let us multiply both sides of equation (1) by v α (kn), multiply both sides of formula (2) by v β (kn),

[0019]

[0020] At this time, the calculated angular frequency is:

[0021]

[0022] Preferably, the specific implementation process of step 3) is:

[0023] 3.1) After the additional damping loop is introduced into the reactive-voltage control module, the control equation of the reactive loop is:

[0024]

[0025] In the formula, K and Dq are the reactive loop integral coefficient and voltage droop coefficient of VSG, Q and Q0 are the actual value and command value of VSG reactive power, V and V0 are the actual value and command value of VSG output voltage, respectively.

[0026] 3.2) Based on the virtual synchronous machine active-frequency control module and reactive-voltage control module, a transient VSG mathematical model is established:

[0027]

[0028] In the formula, J and D p are the inertia coefficient and damping coefficient of the active loop of VSG, respectively, and P0 is the command value of the active power of VSG. δ is the phase difference between the output voltage of VSG and the grid voltage when the system is normally connected to the grid, which is defined as the power angle. 3.3) Define the state space vector:

[0029]

[0030] Among them, x1, x2, x3 are three state variables. Taking the first-order derivative of the state vector x, we get:

[0031]

[0032] 3.4) Setting all differential terms to 0 gives the equilibrium point x e =[δ e , Δω e , V e ] T , the superscript T represents the transpose of the matrix. Obviously, the existence of the equilibrium point means that the three state variables δ e , Δω e 、V e The nonlinear system of equations must be satisfied:

[0033]

[0034] It can be found that the equilibrium point does not depend on the transient damping coefficient K d , the transient damping scheme does not affect the existence characteristics of the steady-state equilibrium point.

[0035] Compared with the prior art, the present invention has the following significant effects:

[0036] The present invention uses a three-sampling point frequency detection technology to detect the grid angular frequency, has a faster detection speed and stability, and avoids the use of a traditional phase-locked loop. In addition, the present invention does not rely on any fault detection unit, has a simple control structure, does not affect the characteristics of the system during stable operation, and only improves the system stability during the grid voltage drop, thereby ensuring that the virtual synchronous machine can operate stably at the new equilibrium point, and improving the transient stability of the virtual synchronous machine grid-connected system. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 A schematic diagram of the virtual synchronous machine connected to the power grid and optimized control structure in the present invention;

[0038] Figure 2 This is a simulation waveform diagram comparing the performance of the three-sampling-point frequency detection used in the present invention with that of a traditional phase-locked loop;

[0039] Figure 3 The distribution diagram of characteristic roots of Jacobian matrix of the system under different damping coefficients;

[0040] Figure 4 is the phase plane diagram of the system under different damping coefficients;

[0041] Figure 5 The simulation waveform of the virtual synchronous machine system before the transient damping scheme is adopted when the grid voltage drops to 0.67pu;

[0042] Figure 6 The simulation waveform of the virtual synchronous machine system after the grid voltage drops to 0.67pu ​​and the transient damping scheme is adopted. DETAILED DESCRIPTION

[0043] The present invention is further described in detail below in conjunction with the accompanying drawings and specific implementation methods.

[0044] Embodiment 1:

[0045] Figure 1 This is a schematic diagram of the structure of the virtual synchronous machine connected to the power grid of the present invention, and the circuit part is consistent with the traditional three-phase inverter. dc is the DC side voltage; v PCC is the voltage at the circuit common coupling point; i PCC is the three-phase grid current; v g is the grid voltage; L f is the filter inductor; R f is the parasitic resistance, C f The present invention considers the worst case, that is, pure inductive grid impedance, that is, L g is the grid impedance. The VSG control part samples the three-phase PCC point voltage v PCC The three-phase grid current i PCC After dq transformation, dq axis voltage and current are generated respectively and input into the instantaneous power calculation module to calculate the real output active power P and reactive power Q of VSG. After active loop control, active power P outputs the phase θ required for coordinate transformation in control, and reactive power Q outputs the voltage amplitude command of VSG end (d axis voltage command of voltage and current closed loop control) after reactive loop control. The voltage and current closed loop controller generates the corresponding dq axis modulation wave command, performs PWM modulation after coordinate transformation, and forms closed loop control of VSG system.

[0046] Embodiment 2:

[0047] The present invention samples the three-phase grid voltage and uses a frequency detection technology based on multiple sampling points to accurately obtain the grid angular frequency ω g Among them, the specific implementation method based on the multi-sampling point frequency detection technology is as follows:

[0048] A virtual synchronous machine transient stability optimization control method includes a frequency detection module (FrequencyDetection, FD) and an additional damping loop connected in series;

[0049] The input of the multi-sampling point frequency detection module is the three-phase grid voltage v g_abc , the output is the grid angular frequency ω g ;

[0050] The additional damping loop input is the grid angular frequency ω output by the multi-sampling point frequency detection module. g The difference between the output angular frequency ω in the VSG active power-frequency control module and the damping coefficient K d Then the reactive power-voltage control module is introduced;

[0051] A method for optimizing transient stability control of a virtual synchronous machine comprises the following steps:

[0052] 1) Set the grid voltage v g The angular frequency ω output by the active power-frequency control module is used as the input of the transient damping module;

[0053] 2) Detect the grid voltage frequency ω based on the multi-sampling point frequency detection method g ;

[0054] 3) Compare ω and ω g Subtract and multiply by the damping coefficient K d Introducing reactive power-voltage control module.

[0055] Preferably, the implementation process in step 2) is:

[0056] 2.1) T s is the sampling period, n is the sampling interval, and v is sampled respectively. g_abc (k-2n), v g_abc (kn), v g_abc (k) The grid voltage values ​​at three different times are used as input quantities of the multi-sampling point frequency detection module;

[0057] 2.2) Perform Clark orthogonal transformation on the input three-phase voltage to obtain the α-axis components v at three different sampling times α (k-2n), v α (kn), v α (k), and the β-axis component v β (k-2n), v β (kn), v β (k).

[0058] 2.3) According to the discrete oscillator law, we can get

[0059] v α (k)+v α (k-2n)=2v α (kn)·cos(ω g nT s ) (7)

[0060] v β (k)+v β (k-2n)=2v β (kn)·cos(ω g nT s ) (8)

[0061] Let both sides of equation (7) be multiplied by v α (kn), multiply both sides of formula (8) by vβ (kn),

[0062]

[0063] At this time, the calculated angular frequency is:

[0064]

[0065] Preferably, the specific implementation process of step 3) is:

[0066] 3.1) After the additional damping loop is introduced into the reactive-voltage control module, the control equation of the reactive loop is:

[0067]

[0068] In the formula, K and Dq are the reactive loop integral coefficient and voltage droop coefficient of VSG, Q and Q0 are the actual value and command value of VSG reactive power, V and V0 are the actual value and command value of VSG output voltage, respectively.

[0069] 3.2) Based on the virtual synchronous machine active-frequency control module and reactive-voltage control module, a transient VSG mathematical model is established:

[0070]

[0071] In the formula, J and D p are the inertia coefficient and damping coefficient of the active-frequency control module of VSG, respectively. P0 is the command value of the active power of VSG. δ is the phase difference between the voltage on the output side of VSG and the grid voltage when the system is connected to the grid normally, which is defined as the power angle.

[0072] 3.3) Define the state space vector:

[0073]

[0074] Among them, x1, x2, x3 are three state variables. Taking the first-order derivative of the state vector x, we get:

[0075]

[0076] 3.4) Setting all differential terms to 0 gives the equilibrium point x e =[δ e , Δω e , V e ] T , the superscript T represents the transpose of the matrix. Obviously, the existence of the equilibrium point means that the three state variables δ e , Δω e 、V e The nonlinear system of equations must be satisfied:

[0077]

[0078] It can be found that the equilibrium point does not depend on the transient damping coefficient K d , the transient damping scheme does not affect the existence characteristics of the steady-state equilibrium point.

[0079] The phase-locked loop is the most widely used frequency detection module in grid-connected inverters. Figure 2 When the power grid has frequency fluctuations, the angular frequency measured by the phase-locked loop is compared with that measured by the multi-sampling point frequency detection technology. It can be seen that the frequency measured by the phase-locked loop has obvious oscillations and takes a long time to adjust. If used in an additional damping loop, it is easy to introduce other coupling information due to its complex structure, resulting in poor overall effect. The frequency detection technology based on multiple sampling points does not produce frequency measurement overshoot when the frequency fluctuates, and the adjustment speed is faster. It is a parametric method itself, with the advantages of simple structure, easy implementation, and fast detection speed.

[0080] Effect description of the present invention:

[0081] Based on equation (15), the Jacobian matrix J(x e ):

[0082]

[0083] Among them, [f1, f2, f3] T =[x1,x2,x3] T ; Then by solving the characteristic equation det[λI-J(x e )]=0, find the characteristic root λ of the Jacobian matrix 1，2，3 . where I is the identity matrix.

[0084] Figure 3 is the distribution of characteristic roots of the Jacobian matrix of the system under different damping coefficients. As the damping coefficient K d As the value of increases, the characteristic root moves closer to the imaginary axis, and the damping at the equilibrium point increases. Therefore, transient damping has an enhancing effect on the stability of the system and can effectively improve the damping characteristics of the system.

[0085] Figure 4 The phase diagram of the grid voltage dropping to 0.67pu ​​under different damping coefficients based on formula (13) is shown in Figure 13. d With the increase of , the system power angle changes from divergence to convergence. The proposed transient damping scheme can effectively improve the transient stability of the system.

[0086] Figure 5The simulation waveform of the virtual synchronous machine system before the transient damping scheme is adopted when the grid voltage drops to 0.67pu. When the grid voltage drops to 0.67pu ​​in 6s, if the transient damping scheme is not adopted, Figure 5 As shown, the system active power and reactive power are both oscillating, and the power angle diverges and becomes unstable.

[0087] Figure 6 The simulated waveform of the virtual synchronous machine system after the grid voltage drops to 0.67pu. When the grid voltage drops to 0.67pu ​​in 6s, the system can remain stable and converge to a new equilibrium point.

[0088] The virtual synchronous machine transient stability optimization control system and method proposed by the present invention can simply and quickly detect the grid voltage frequency ω g To ensure that the system transient stability is improved without affecting the steady-state working characteristics.

[0089] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A virtual synchronous machine transient stability optimization control system, characterized in that: The control system includes an instantaneous power calculation module, an active-frequency control module, a reactive-voltage control module, a voltage-current closed-loop control module and a transient damping module. The instantaneous power calculation module is connected in series with the active-frequency control module and the reactive-voltage unit, respectively, and then connected in series with the voltage-current closed-loop control module. The input of the instantaneous power calculation module is the three-phase voltage u at the grid connection point. pcc The d and q axis components v d 、v q The three-phase current i pcc The d and q axis components i d 、i q , the output is the instantaneous output active power P and reactive power Q of the virtual synchronous machine; The input of the active power-frequency control module is the active power given value P0 and the output instantaneous active power P of the instantaneous power calculation module, and the output is the adjusted coordinate transformation reference value θ; The input of the reactive power-voltage control module is the active power given value Q0 and the output of the instantaneous power calculation module, the instantaneous reactive power Q, and the output is the adjusted d-axis voltage reference value v d_ref ; The input of the voltage and current closed-loop control module is the three-phase voltage u pcc The d and q axis voltage reference value v d_ref 、v q_ref , the output is dq axis modulation wave information; The input of the transient damping module unit is the sampled grid voltage v g And the angular frequency ω output by the active-frequency control module, output damping information to the reactive-voltage control module.

2. A virtual synchronous machine transient stability optimization control system as claimed in claim 1, characterized in that: The active power-frequency control module is used to simulate the rotor motion characteristics of the synchronous generator, automatically balance the output angular frequency and generate phase angle information according to the active power set value and the actual output active power.

3. A virtual synchronous machine transient stability optimization control system as claimed in claim 1, characterized in that: The voltage reactive power-voltage control module is used to simulate the excitation voltage regulation characteristics of the synchronous generator, and controls the output terminal voltage to be constant according to the reactive power set value and the actual reactive power output.

4. A virtual synchronous machine transient stability optimization control system as claimed in claim 1, characterized in that: The voltage and current closed-loop control module is used to improve the tracking accuracy of the voltage at the output end of the PCC point and improve the current response speed.

5. A virtual synchronous machine transient stability optimization control system as claimed in claim 1, characterized in that: The transient damping module improves the damping effect of the VSG system and enhances the transient stability of the VSG system by introducing the VSG output angular frequency difference under transient conditions to the reactive power-voltage control module.

6. A method for optimizing transient stability control of a virtual synchronous machine, characterized in that: The following steps are involved: 1) Set the grid voltage v g The angular frequency ω output by the active power-frequency control module is used as the input of the transient damping module; 2) Sample the three-phase grid voltage and detect the grid voltage frequency ω g ; 3) The angular frequency ω output by the active power-frequency control module is compared with the grid voltage frequency ω g Subtract and multiply by the damping coefficient K d Introducing reactive power-voltage control module.

7. A virtual synchronous machine transient stability optimization control method as claimed in claim 6, characterized in that: Step 2) is specifically as follows: 2.1) T s is the sampling period, n is the sampling interval, and v is sampled respectively. g_abc (k-2n), v g_abc (kn), v g_abc (k) The grid voltage values ​​at three different times are used as input quantities of the input frequency detection module; 2.2) Clark orthogonal transformation is performed on the input three-phase voltage to obtain the α-axis component v of the grid voltage at three different sampling times. α (k-2n), v α (kn), v α (k), and the β-axis component v β (k-2n), v β (kn), v β (k); 2.3) The calculated grid voltage angular frequency is: Among them, ωg is the calculated grid voltage angular frequency, n is the sampling interval, Ts is the sampling period, and k is the sampling time.

8. A virtual synchronous machine transient stability optimization control method as claimed in claim 6, characterized in that: Step 3) is specifically: 3.1) After the additional damping loop is introduced into the reactive-voltage control module, the control equation of the reactive loop is: Where K and Dq are the integral coefficient and voltage droop coefficient of VSG reactive power-voltage control, Q and Q0 are the actual value and command value of VSG reactive power, V and V0 are the actual value and command value of VSG output voltage, ω and ω g They are respectively the angular frequency output by the active power-frequency control module and the grid voltage frequency calculated in step 2).

9. A virtual synchronous machine transient stability optimization control method as claimed in claim 7, characterized in that: The reactive power-voltage control module includes a VSG mathematical model, and the VSG mathematical model is: In the formula, J and D p are the inertia coefficient and damping coefficient of the active-frequency control of VSG, P0 is the active power given value; δ is the phase difference between the VSG output side voltage and the grid voltage when the system is normally connected to the grid, which is defined as the power angle; the state space vector is defined as: Among them, x1, x2, x3 are three state variables, and the first-order derivative of the state vector x is obtained: Set all differential terms in the formula to 0 and obtain the equilibrium point x e =[δ e , Δω e , V e ] T , the superscript T represents the transpose of the matrix; the three state variables δ in the steady state e , Δω e 、V e Satisfies the nonlinear system of equations: Among them, δ e is the power angle value of the system equilibrium point in steady state, V g is the grid voltage amplitude, X g is the inductive reactance of the power grid, V e is the output voltage amplitude of the system equilibrium point in steady state.

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