A Method for Suppressing VSG Power Oscillation Based on Active Power Feedback

Abstract

The present invention discloses a method for suppressing VSG power oscillation based on active power feedback. The method includes the following steps: 1) Collect the output voltage u of the VSG abc and the grid-connected current i abc , and calculate the active power P of the VSG e ; 2) Feed P e into the active droop control to obtain the virtual mechanical power P m , add P m to the active reference value P ref of the VSG to obtain the active command P set of the rotor motion equation; 3) Subtract P set from P e , and the obtained difference is subjected to proportional and integral operations to obtain the initial angular frequency deviation Δω1 of the VSG; 4) Multiply P e by the feedback coefficient K to obtain the angular frequency deviation correction amount Δω2 of the VSG; 5) Subtract Δω2 from Δω1 to obtain the final angular frequency deviation Δω3 of the VSG; 6) Add Δω3 to the rated angular frequency ω n to obtain the angular frequency ω of the VSG, and integrate to obtain the output phase θ of the active loop; 7) Feed the amplitude E and the phase θ of the VSG reference voltage into the voltage-current double closed loop, and drive the inverter to operate through the PWM link. This solution can achieve the suppression of the output active power transient oscillation of the VSG under external disturbances.

Description

Technical Field

[0001] The present invention belongs to the technical field of virtual synchronous generators, and particularly relates to a method for suppressing VSG power oscillation based on active power feedback. Background Art

[0002] In the field of new energy distributed generation, power electronic converters are usually used for grid connection. With the increase of new energy penetration rate, the deficiencies of microgrids lacking damping and inertia become more and more obvious. Based on the idea of simulating synchronous generators to improve the inertia and damping of microgrids, the virtual synchronous generator technology has emerged. The VSG technology adds a rotor motion equation on the basis of simulating the external characteristics of synchronous generators by droop control, simulating the rotor inertia and damping characteristics of synchronous generators, which is beneficial to improving the frequency inertia level and voltage support ability of new energy microgrids.

[0003] While the VSG has damping and inertia by simulating synchronous generators, it also introduces the inherent power oscillation problem of synchronous generators. In fact, the active power control loop of the VSG belongs to a second-order system. When the active power reference value P ref changes or the grid frequency ω g fluctuates, the active power P e output by the VSG and the frequency ω will have transient oscillations, which is not conducive to the safe and stable operation of grid-connected inverters.

[0004] Currently, the suppression of transient power and frequency oscillations of the VSG mainly includes two schemes: VSG parameter adaptation and transient damping construction. The VSG parameter adaptation makes full use of the flexibility of VSG parameters, but it belongs to nonlinear control, and the adaptive function lacks a clear design method, making it difficult to conduct quantitative analysis. The transient damping construction scheme mainly uses a transient link constructed by a differential link or a first-order link to construct a transient variable and superimpose it on the active power loop of the VSG. However, the differential link is prone to introducing high-frequency noise and is difficult to implement. The scheme of constructing transient damping with a first-order link makes the VSG become a third-order system, introducing new poles and zeros, and the system parameter design is relatively complex. Summary of the Invention

[0005] Aiming at the deficiencies of the existing VSG transient damping scheme being complex and difficult to implement, the present invention provides a VSG control method that can suppress the transient oscillations of VSG active power and frequency, has a small number of parameters, and a simple system structure.

[0006] To achieve the above object, the present invention uses the following technical solutions:

[0007] A method for suppressing VSG power oscillation based on active power feedback, the main steps are as follows:

[0008] 1) Calculation of the active power output by the VSG, collecting the output voltage u of the VSG abcand grid-connected current i abc , calculate the active power P of the VSG e ;

[0009] 2) Calculate the active power command of the rotor motion equation. Send the active power P output by the VSG e to the active power droop link to obtain the virtual mechanical power P of the VSG m . Add P m to the active power reference value P of the VSG ref to obtain the active power command P of the VSG rotor motion equation set ;

[0010] 3) Calculate the initial angular frequency deviation of the VSG. Subtract the rated angular frequency ω from the angular frequency ω of the VSG n to obtain the angular frequency deviation Δω of the VSG. Multiply Δω by the damping coefficient Dω n to obtain the damping power P d . Subtract P set from P e and P d . The obtained difference ΔP passes through the proportional link and the integral link to obtain the initial angular frequency deviation Δω1 of the VSG;

[0011] 4) Calculate the angular frequency deviation correction amount of the VSG. Calculate the active power feedback coefficient K according to the damping ratio ξ of the active power loop of the VSG. Multiply P e by K to obtain the angular frequency deviation correction amount Δω2 of the VSG;

[0012] 5) Calculate the final angular frequency deviation of the VSG. Subtract the angular frequency deviation correction amount Δω2 from the initial angular frequency deviation Δω1 of the VSG to obtain the final angular frequency deviation Δω3 of the VSG;

[0013] 6) Calculate the output phase of the active power loop of the VSG. Add the final angular frequency deviation Δω3 of the VSG to the rated angular frequency ω n to obtain the angular frequency ω of the VSG, and obtain the output phase θ of the active power loop of the VSG through the integral link;

[0014] 7) Calculate the voltage reference value e of the VSG according to the output amplitude E of the reactive power loop of the VSG and the output phase θ of the active power loop abc . Send e abc to the voltage-current double closed loop, and drive the inverter to operate through the PWM link.

[0015] A further improvement of the present invention is that in step 1), according to the output voltage u of the VSG abc and grid-connected current i abc , the calculation formula for the active power P output by the VSG e is:

[0016] P e= u a i a + u b i b + u c i c

[0017] A further improvement of the present invention lies in that, in step 2), the active power command P of the VSG rotor motion equation set is calculated as follows:

[0018] P set = P ref - K p (ω - ω n )

[0019] where K p is the active power droop coefficient of the VSG.

[0020] A further improvement of the present invention lies in that, in step 3), the calculation formula for the initial angular frequency deviation Δω1 of the VSG is:

[0021]

[0022] where J is the virtual inertia of the VSG.

[0023] A further improvement of the present invention lies in that, in step 4), the calculation formula for the active power feedback coefficient K of the VSG is:

[0024]

[0025] where is the active power gain coefficient of the VSG.

[0026] A further improvement of the present invention lies in that, in step 4), the calculation formula for the angular frequency deviation correction amount Δω2 of the VSG is:

[0027] Δω2 = KP e

[0028] A further improvement of the present invention lies in that, in step 5), the formula for the final angular frequency deviation Δω3 of the VSG is:

[0029] Δω3 = Δω1 - Δω2

[0030] A further improvement of the present invention lies in that, in step 6), the calculation formula for the output phase θ of the VSG active power loop is:

[0031] θ = ∫(Δω3 + ω n - ω g )dt

[0032] Advantages of the present invention:

[0033] The present invention feeds back the active power P output by the VSG e , through a proportional link, to the angular frequency deviation Δω of the VSG output, achieving transient oscillation suppression of the active power and frequency output by the VSG. Compared with the traditional scheme of increasing the damping coefficient D of the VSG, when the grid angular frequency deviates from the rated value ω n of the present invention, there will be no steady-state error in the active power P e output by the VSG. The present invention does not use a differential link, avoiding the deficiencies of introducing high-frequency noise and difficult implementation in the differential link. The present invention enables the VSG to still remain a second-order system without introducing new zeros and poles, and the system structure is simple and easy to implement. Description of the Drawings

[0034] Figure 1 is the VSG topology structure and control schematic diagram of the embodiment of the present invention.

[0035] Figure 2 is the block diagram of the active power transfer function of the VSG of the embodiment of the present invention.

[0036] Figure 3 is the equivalent diagram of the block diagram of the active power transfer function of the VSG of the embodiment of the present invention.

[0037] Figure 4 is the block diagram for calculating the initial angular frequency deviation of the VSG of the embodiment of the present invention.

[0038] Figure 5 is the block diagram for calculating the correction amount of the angular frequency deviation of the VSG of the embodiment of the present invention.

[0039] Figure 6 is the block diagram for calculating the final angular frequency deviation of the VSG of the embodiment of the present invention.

[0040] Figure 7 is the block diagram for calculating the output phase of the active power loop of the VSG of the embodiment of the present invention.

[0041] Figure 8 is the simulation result diagram of the step response of the active power of the traditional VSG.

[0042] Figure 9 is the simulation result diagram of the active power of the VSG of the embodiment of the present invention. Detailed Embodiment

[0043] Next, in combination with the drawings and embodiments, the present invention will be further described, but the present invention is not limited to the given examples.

[0044] As Figure 2 shown, the present invention provides a method for suppressing the power oscillation of the VSG based on active power feedback, and the specific steps are as follows:

[0045] Step 1: Calculation of the active power command for the VSG rotor motion equation. The active power P output by the VSG is sent e to the active power droop control link to obtain the virtual mechanical power P of the VSG m . Add P m to the VSG active power reference value P ref to obtain the active power command P for the VSG rotor motion equation set .

[0046] In this step, the calculation formula for the active power command P of the VSG rotor motion equation is: set

[0047] P set = P ref - K p (ω - ω n )

[0048] where P ref is the VSG active power reference value, K p is the active power droop coefficient of the VSG, ω is the VSG output angular frequency, and ω n is the rated angular frequency.

[0049] Step 2: Calculation of the initial angular frequency deviation Δω1 of the VSG. Subtract the rated angular frequency ω from the VSG angular frequency ω n to obtain the VSG angular frequency deviation Δω. Multiply Δω by the damping coefficient Dω n to obtain the damping power P d . Subtract P set from P e and P d . The resulting difference ΔP passes through a proportional link and an integral link to obtain the initial angular frequency deviation Δω1 of the VSG, as shown in Figure 4 .

[0050] In this step, the calculation formula for the initial angular frequency deviation Δω1 of the VSG is:

[0051]

[0052] where J is the VSG virtual inertia, Dω n is the damping coefficient of the VSG. This part calculates the initial angular frequency deviation Δω1 of the VSG according to the VSG rotor motion equation.

[0053] Step 3: Calculate the VSG angular frequency deviation correction amount Δω2. Calculate the active power feedback coefficient K according to the damping ratio ξ of the VSG active loop. Multiply P e by K to obtain the VSG angular frequency deviation correction amount Δω2, as shown in Figure 5 .

[0054] In this step, the calculation formula for the active power feedback coefficient \(K\) of the VSG is:

[0055]

[0056] Where, is the active power gain coefficient of the VSG.

[0057] In this step, the calculation formula for the angular frequency deviation correction amount \(\Delta\omega_2\) of the VSG is:

[0058] \(\Delta\omega_2 = K_P\) e

[0059] Where, \(P\) e is the active power output by the VSG, and \(K\) is the active power feedback coefficient.

[0060] Fourth step: Calculate the final angular frequency deviation \(\Delta\omega_3\) of the VSG. Subtract the angular frequency deviation correction amount \(\Delta\omega_2\) from the initial angular frequency deviation \(\Delta\omega_1\) of the VSG to obtain the final angular frequency deviation \(\Delta\omega_3\) of the VSG, as Figure 6 shown.

[0061] In this step, the formula for the final angular frequency deviation \(\Delta\omega_3\) of the VSG is:

[0062] \(\Delta\omega_3=\Delta\omega_1 - \Delta\omega_2\)

[0063] Fifth step: Calculate the output phase \(\theta\) of the active power loop of the VSG. Add the final angular frequency deviation \(\Delta\omega_3\) of the VSG to the rated angular frequency \(\omega\) n , to obtain the angular frequency \(\omega\) of the VSG, and through the integral link, obtain the output phase \(\theta\) of the active power loop of the VSG, as Figure 7 shown.

[0064] In this step, the calculation formula for the output phase \(\theta\) of the active power loop of the VSG is:

[0065] \(\theta=\int(\Delta\omega_3+\omega\) n -\omega\) g )dt

[0066] Where, \(\omega\) g is the angular frequency of the grid voltage.

[0067] Sixth step: Calculate the voltage reference value \(e\) of the VSG according to the output amplitude \(E\) of the reactive power loop of the VSG and the output phase \(\theta\) of the active power loop, abc , and send \(e\) abc to the voltage-current double closed loop, and drive the inverter to operate through the PWM link.

[0068] Example:

[0069] Taking the grid-connected model of an inverter controlled by a single VSG as an example, when the active power command of the VSG undergoes a step change or the grid frequency fluctuates, the active power dynamic response performances of the traditional VSG that increases the damping coefficient to suppress active power oscillations and the VSG based on active power feedback proposed by the present invention are compared.

[0070] The control effects of the traditional VSG with an increased damping coefficient and the VSG of this embodiment are as Figure 8 and Figure 9 shown. Figure 8 and Figure 9 are the active power waveform diagrams of the traditional VSG and the VSG of this embodiment, respectively.

[0071] Figure 8 In [diagram], the active power command of the VSG changes stepwise from 5000 W to 10000 W at 3 s, and the grid frequency drops from 50 Hz to 49.94 Hz at 6 s. Figure 9 In [diagram], the active power command of the VSG changes stepwise from 5000 W to 10000 W at 3 s, and the grid frequency drops from 50 Hz to 49.94 Hz at 6 s.

[0072] As can be seen from Figure 8 , for the traditional VSG with an increased damping coefficient, as the damping coefficient increases, the active power oscillations of the VSG are gradually suppressed. When the grid frequency drops, the larger the damping coefficient of the VSG, the greater the deviation of the steady-state active power value of the VSG from the given value.

[0073] As can be seen from Figure 9 , for the VSG of this embodiment, as the damping ratio ξ of the active power loop of the VSG increases, the active power oscillations of the VSG output are gradually suppressed. When the grid frequency drops, the steady-state active power value of the VSG strictly outputs according to the active power droop curve. This embodiment will not cause an active power error outside the active power droop characteristic of the VSG.

[0074] As can be seen from Figure 8 and Figure 9 by comparison, although the traditional VSG with an increased damping coefficient can suppress the active power oscillations of the VSG, it will change the active power droop characteristic curve of the VSG, causing an active power deviation outside the active power droop characteristic of the VSG; the VSG strategy based on active power feedback proposed by the present invention, while effectively suppressing the VSG power oscillations, does not affect the active power droop characteristic of the VSG and will not cause an active power deviation outside the active power droop characteristic of the VSG.

[0075] In summary, this embodiment proves the effectiveness of the VSG power oscillation suppression strategy based on active power feedback proposed by the present invention. The parameters of this embodiment are shown in Table 1:

[0076] Table 1 Simulation Parameters of the VSG Embodiment

[0077]

Claims

1. A method for suppressing VSG power oscillation based on active power feedback, characterized in that, Including the following steps: 1) Calculation of the active power output of the VSG, sampling the three-phase output voltage u a , u b , u c and the three-phase grid-connected current i a , i b , i c , calculating the active power P output by the VSG e ; 2) Calculation of the active power command for the VSG rotor motion equation. Send P e to the active power droop link. Based on the VSG active power reference value P ref and the VSG active power droop coefficient K p calculate the active power command P set ; 3) Calculation of the initial angular frequency deviation of VSG. The initial angular frequency deviation of VSG is calculated based on the VSG damping coefficient D and the VSG virtual inertia J where ω n is the rated angular frequency of VSG, and ω is the output angular frequency of VSG; 4) Calculation of the VSG angular frequency deviation correction amount, multiply the active power P output by the VSG by the active power feedback coefficient of the VSG e where ξ is the damping ratio of the VSG active power loop, and S is the active power gain coefficient of the VSG, to obtain the VSG angular frequency deviation correction amount Δω2 = KP E ; e ; 5) Calculating the final angular frequency deviation of the VSG. Subtract the angular frequency deviation correction Δω2 from the initial angular frequency deviation Δω1 of the VSG to obtain the final angular frequency deviation Δω3 of the VSG, where Δω3 = Δω1 - Δω2; 6) Calculation of the output phase of the VSG active loop: Add the final angular frequency deviation Δω3 of the VSG to the rated angular frequency ω n , subtract the grid angular frequency ω from the obtained result g , and perform integration to obtain the output phase θ of the VSG active loop: θ = ∫(Δω3 + ω n - ω g )dt; 7) Calculate the VSG voltage reference value e according to the amplitude E output by the VSG reactive power loop and the phase θ output by the VSG active power loop abc , and send e abc to the voltage-current double closed loop, and drive the inverter to operate through the PWM link.

2. The method for suppressing VSG power oscillation based on active power feedback according to claim 1, characterized in that, In step 1), the active power P output by the VSG is calculated according to the three-phase output voltage and three-phase grid-connected current of the VSG e The formula for this is: P e = u a i a + u b i b + u c i c .

3. The method for suppressing VSG power oscillation based on active power feedback according to claim 1, characterized in that, In step 2), the active power command P of the VSG rotor motion equation set is calculated by the formula: P set = P ref - K p (ω - ω n )。

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