VSG harmonic current impedance reshaping inhibition method

Abstract

According to the VSG harmonic current impedance remolding suppression method, a VSG is connected to a common bus end of a suburb power distribution network through an LC filter, a nonlinear load is connected to the common bus end, and a common bus is connected to the suburb power distribution network containing large harmonic voltage content through a power transmission line. The invention provides a VSG harmonic current impedance reshaping inhibition method, and solves the electric energy quality problem when a VSG is connected to a suburb power distribution network. By respectively carrying out negative and positive feedforward control on PCC voltage and additional voltage of an embedded small inductor, output impedance from the PCC to a VSG and impedance from a power grid side to the VSG are remodeled, specifically, equivalent harmonic impedance from the power grid side to the VSG is increased, output harmonic impedance from the PCC to the VSG is reduced, harmonic current generated by harmonic voltage of a power distribution network is weakened, and the harmonic current generated by the VSG is reduced. And meanwhile, the VSG absorbs more nonlinear load harmonic current, so that the harmonic content of the grid-connected current of the VSG is reduced, and the friendliness of accessing the VSG to the power distribution network is improved.

Description

Technical Field

[0001] This invention relates to the field of VSG power quality technology, and in particular to a method for suppressing VSG harmonic current impedance reshaping. Background Technology

[0002] Virtual Synchronous Generator (VSG) technology, by simulating the electromechanical transient characteristics of traditional synchronous generators, can provide the necessary voltage and frequency support for the power grid, and has become an effective technical approach to solve the stability problem of high-proportion renewable energy grid connection. However, most renewable energy power generation systems based on power electronic devices are located in remote suburban distribution networks, which are rich in harmonic voltage content. At the same time, an increasing number of nonlinear loads are connected to suburban distribution networks, introducing a large amount of harmonic current. Both of these factors can lead to severe distortion of the VSG grid-connected current, restricting the friendliness of VSG connection to suburban distribution networks.

[0003] For harmonic currents introduced by nonlinear loads, the traditional solution is to use current-controlled harmonic compensation methods with the same amplitude but opposite phase to cancel the harmonic current. However, most VSGs are voltage-controlled, lacking internal current and AC voltage control links, making traditional harmonic current compensation methods difficult to apply. While voltage-controlled harmonic compensation methods that introduce negative PCC voltage feedforward to reduce VSG output impedance can absorb a large amount of nonlinear load harmonic current, thus reducing grid harmonic current, the harmonic current generated by grid harmonic voltage will increase. Therefore, it can be seen that the grid harmonic voltage and nonlinear load harmonic current mutually restrict the requirements of VSG output harmonic impedance, and traditional harmonic current suppression methods struggle to resolve this contradiction. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to provide a method for suppressing the impedance reshaping of VSG harmonic current, which solves the contradiction between the harmonic voltage of the distribution network and the harmonic current of the nonlinear load on the grid-connected current quality when VSG is connected to the suburban distribution network, thereby improving the friendliness and adaptability of VSG connection to the suburban distribution network.

[0005] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is: a VSG harmonic current impedance reshaping suppression method, wherein the VSG is connected to the common bus terminal of the suburban distribution network via an LC filter, the nonlinear load is connected to the common bus terminal, and the common bus is connected to the suburban distribution network containing a large harmonic voltage content via a transmission line, specifically including the following steps: 1) Output voltage of VSG u o and output current i oSampling is performed, and then the actual active power output of the VSG is obtained through instantaneous power calculation. P e and reactive power Q e ; 2) The actual output active power P e and reactive power Q e The fundamental frequency command of the VSG output voltage is obtained through the VSG controller. u * VSGf ; 3) Place a small passive inductor between the public bus and the suburban distribution network. L s This is used to introduce an additional voltage variable. u s ; 4) Harmonic detection method based on instantaneous reactive power theory is used to detect the voltage of the common bus. u PCC and additional voltage u s The harmonic voltage content was detected, and the results were obtained respectively. u PCC Major subharmonic content u PCCh and u s Major subharmonic content u sh ; 5) For each u PCC Major subharmonic content u PCCh and u s Major subharmonic content u sh Perform negative and positive feedforward control, where, - m yes u PCCh The negative feedforward coefficient; n yes u sh The positive feedforward coefficient; the sum of the two yields the harmonic command of the VSG output voltage. u * VSGh ; ; 6) Output voltage fundamental frequency command u * VSGf and output voltage harmonic commands u *VSGh Add them together to obtain the control signal for the VSG. u * VSG ; ; 7) Control signal u * VSG The drive signal is obtained through PWM modulation.

[0006] Preferably, in step 4), a harmonic detection method based on instantaneous reactive power theory is used to detect... u PCC and u s The harmonic voltage content.

[0007] Preferably, the passive inductor L s The parameters are matched to the impedance characteristics of the transmission line and are only used to introduce additional voltage variables. u s It does not undertake the power transmission of the main circuit.

[0008] Preferably, the feedforward coefficients m and n satisfy the following relationship: ; Z out This is the equivalent impedance from VSG to the common bus terminal.

[0009] Preferably, in step 5), the output impedance from the common bus to the VSG is... and the impedance from the suburban power distribution network to the VSG They are respectively: .

[0010] Preferred, Z out Z is the equivalent impedance from VSG to the common bus terminal. out Approximately the impedance of an LC filter, Z out = sL f .

[0011] Preferred, Z total The equivalent impedance from VSG to the suburban distribution network is approximately: Z total = sL s + Z out .

[0012] Preferably, the PWM modulation in step 7) adopts a sinusoidal pulse width modulation method, and the modulation frequency is adapted to the working characteristics of the VSG switching device.

[0013] Preferably, the VSG controller includes an active power regulation module and a reactive power regulation module, which respectively regulate the active power and reactive power of the reactive power. P e , Q e Perform closed-loop control to generate u * VSGf .

[0014] Preferably, the nonlinear load includes one or more power electronic loads such as frequency converters, rectifiers, and electric arc furnaces.

[0015] This invention provides a VSG harmonic current impedance reshaping suppression method, solving the power quality problem of VSG connection to suburban distribution networks. By implementing negative and positive feedforward control on the PCC voltage and the additional voltage of the inserted small inductor, respectively, the output impedance from the PCC to the VSG and the impedance from the grid side to the VSG are reshaped. Specifically, the equivalent harmonic impedance from the grid side to the VSG is increased, and the output harmonic impedance from the PCC to the VSG is decreased, thereby weakening the harmonic current generated by the harmonic voltage of the distribution network. At the same time, the VSG absorbs more nonlinear load harmonic current, thereby reducing the harmonic content of the VSG grid-connected current and improving the friendliness of VSG connection to the distribution network. Attached Figure Description

[0016] The present invention will be further described below with reference to the accompanying drawings and embodiments: Figure 1 This is a schematic diagram of the VSG controller of the present invention; Figure 2 This is a schematic diagram of the VSG connection to the suburban power distribution network according to the present invention; Figure 3 This is a diagram of the VSG impedance reshaping type harmonic current suppression method of the present invention; Figure 4 This invention provides the harmonic domain equivalent circuit diagram of the proposed method under nonlinear loads only. Figure 5 This is the equivalent circuit diagram in the harmonic domain of the proposed method considering only the harmonic voltage of the power grid. Detailed Implementation

[0017] like Figure 1 As shown, the VSG is connected to the suburban distribution network common bus (PCC) via the transmission line impedance; the common bus is connected to a nonlinear load; the common bus is connected to the suburban distribution network via the line impedance; wherein, u g , ig This indicates the grid voltage and VSG grid-connected current. Z g Indicates the power grid impedance. Z line Indicates the line impedance. i Load Represents nonlinear load current; u PCC Represents the voltage at the common bus terminal; i o , U o These represent the actual output voltage and current of the VSG, respectively. L f , C f , R f These represent the filter inductance, filter capacitor, and filter equivalent resistance of the LC filter, respectively.

[0018] like Figure 2 As shown, the VSG impedance reshaping harmonic current suppression method mainly includes passive and active components to change the output impedance of the VSG and the grid impedance. The passive component involves connecting a small inductor in series between the common bus (PCC) and the suburban distribution network. L s Its main function is to introduce an additional voltage variable. u s The active part refers to the voltage at the PCC. u PCC Harmonic components and additional voltage u s The harmonic components are extracted separately, and then... u PCC harmonic components u PCCh Perform negative feedback to apply the additional voltage. u s harmonic components u sh Positive feedback is used to reshape the output impedance from PCC to VSG respectively. Z out and the impedance from VSG to the distribution network Z total This suppresses the VSG grid-connected current. i g The harmonic components are reduced, improving the grid friendliness and adaptability of VSG.

[0019] like Figure 3 As shown, the output voltage of the VSG u o (Three phases) uoabc and output current i o (Three phases) i oabc The sample is taken, and then the actual active power output of the VSG is obtained through instantaneous power calculation. P e and reactive power Q e The actual output active power P e and reactive power Q e The fundamental frequency command of the VSG output voltage is obtained through VSG control. u * VSGf ; Then, a harmonic detection method based on instantaneous reactive power theory is used to detect the voltage of the common bus. u PCC and additional voltage u s The harmonic voltage content was detected, and the results were obtained respectively. u PCC Major subharmonic content u PCCh and u s Major subharmonic content u sh ; To each u PCC Major subharmonic content u PCCh and u s Major subharmonic content u sh Perform negative and positive feedforward control, where, - m yes u PCCh The negative feedforward coefficient; n yes u sh The positive feedforward coefficient. Adding the two together yields the harmonic command of the VSG output voltage. u * VSGh ; (1) Then output voltage fundamental frequency command u * VSGf and output voltage harmonic commands u * VSGh Add them together to obtain the control signal for the VSG. u* VSG ; (2) control signal u * VSG The drive signal is obtained through PWM modulation.

[0020] The output impedance from the common bus to the VSG and the impedance from the suburban power distribution network to the VSG They are respectively: (3) in, Z out Z is the equivalent impedance from VSG to the common bus terminal. out Approximately the impedance of an LC filter, i.e.: Z out = sL f , Z total The equivalent impedance from VSG to the suburban distribution network is approximately: Z total = sL s + Z out .

[0021] m and n satisfy the following relationship: (4) like Figure 4 As shown, considering only the harmonic domain equivalent circuit of the proposed method under nonlinear load, voltage, current, and impedance variables with subscripts containing "h" are uniformly defined to represent the harmonic components of that variable. At this time, the harmonic components of the VSG grid-connected current are... i oh The specific derivation process is as follows: when u gh When =0, the VSG outputs harmonic current. i oh for: (5) (6) Combining equations (5) and (6), the equivalent harmonic impedance from PCC to VSG can be obtained: (7) From equation (7), it can be seen that the VSG harmonic domain model at this time is equivalent to... Z out Connect a virtual impedance in parallel, so that Z out PincerZ out / ( m - bn Under these circumstances, as long as a reasonable design is implemented... n and m， This reduces the VSG output harmonic impedance, allowing nonlinear load harmonic currents to flow to the VSG. At this point, the grid-side harmonic current is: (8) like Figure 5 As shown, in one embodiment of the present invention, only the harmonic domain equivalent circuit of the proposed method under grid harmonic voltage is considered. In this case, the specific derivation process of the VSG grid-connected current harmonic component ioh is as follows: at this time i Loadh =0, then the harmonic current on the grid side is: (9) In the formula, Z total = sL s + Z out .

[0022] According to KCL theorem: (10) By combining equations (1) and (10), we can obtain: (11) Combining equations (9) and (11), we obtain the equivalent harmonic impedance from the power grid to the VSG: (12) H Gain coefficient: (13) From equations (12) and (13), we can obtain i Loadh When = 0, the equivalent model of VSG in the harmonic domain is as follows: Figure 5 As shown. At this point, the equivalent harmonic impedance from the passive inductor connection point to the VSG is amplified. H Times. When satisfied n =1+ m hour, H Approaching infinity, that is Z total,eq As the voltage approaches infinity, there will be no path for grid harmonic voltages, resulting in almost no harmonic current. At this point, the grid-side harmonic current is: (14) comprehensive Figure 4 and Figure 5In both cases, by applying the superposition theorem and adding equations (8) and (14), we can obtain the total harmonic components of the grid-connected current: (15) To achieve a VSG grid-connected current distortion rate of less than 5%, m and n Must meet: (16) Under the conditions of equation (16), H When the value is infinite, equation (15) can be transformed into: (17) At this time, reasonable design n By adjusting the passive inductance value, the output impedance from the PCC to the VSG can be reduced, while the impedance from the VSG to the distribution network can be increased. This allows the VSG to absorb more harmonic currents from nonlinear loads, while suppressing harmonic currents generated by harmonic voltages in suburban distribution networks. This, in turn, improves the grid connection current quality of the VSG connected to the suburban distribution network.

[0023] This invention addresses the contradiction between the constraints of distribution network harmonic voltage and nonlinear load harmonic current on grid-connected current quality when VSG is connected to a suburban distribution network. It adopts a collaborative architecture of passive inductor series and dual harmonic feedforward control. By introducing an additional voltage variable through a small passive inductor connected in series between the common bus and the suburban distribution network, and then performing negative feedforward control on the main second harmonic voltage of the common bus and positive feedforward control on the main second harmonic components of the additional voltage, a bidirectional impedance reshaping is achieved, which reduces the output harmonic impedance from the common bus to the VSG and increases the equivalent harmonic impedance from the VSG to the suburban distribution network. This enhances the VSG's ability to absorb nonlinear load harmonic current and weakens the harmonic current caused by the distribution network harmonic voltage, ultimately improving the grid-connected current quality of the VSG and its grid-connection friendliness.

[0024] 1. Effectively resolves the core contradiction: It resolves the contradiction between the harmonic voltage of the distribution network and the harmonic current of the nonlinear load on the grid-connected current quality when VSG is connected to the suburban distribution network, and makes up for the shortcomings of traditional current-controlled harmonic compensation methods that are not applicable to VSG and traditional voltage-controlled harmonic compensation methods that will increase the harmonic current of the grid.

[0025] 2. Optimize impedance characteristics to achieve bidirectional suppression: By reshaping the bidirectional impedance, on the one hand, the output harmonic impedance from the common bus to the VSG is reduced, enhancing the VSG's ability to absorb harmonic currents from nonlinear loads and reducing the interference of nonlinear loads on the grid-connected current; on the other hand, the equivalent harmonic impedance from the VSG to the suburban distribution network is increased, weakening the harmonic currents caused by harmonic voltages in the distribution network, thereby reducing the harmonic content of the VSG grid-connected current from two aspects.

[0026] 3. Improve grid compatibility and friendliness: Significantly improve the grid-connected current quality of VSG, meet the access requirements of suburban distribution networks for new energy power generation systems, improve the adaptability and friendliness of VSG access to suburban distribution networks, and provide technical support for the stable grid connection of high proportion of new energy in suburban distribution networks.

[0027] 4. The technical solution is practical and reliable: The small passive inductor used is only used to introduce additional voltage variables and does not bear the power transmission of the main circuit. The structure design is simple and has strong compatibility with the voltage control characteristics of VSG. It does not require major modification of the existing VSG system and is easy to apply and promote in engineering.

[0028] The above embodiments are merely preferred technical solutions of the present invention and should not be considered as limitations on the present invention. The scope of protection of the present invention should be limited to the technical solutions described in the claims, including equivalent substitutions of the technical features described in the claims. That is, equivalent substitutions and improvements within this scope are also within the scope of protection of the present invention.

Claims

1. A method for suppressing the impedance reshaping of VSG harmonic currents, characterized in that, The VSG is connected to the common bus of the suburban distribution network via an LC filter. Nonlinear loads are connected to the common bus. The common bus is then connected to the suburban distribution network containing significant harmonic voltage content via transmission lines. The specific steps include: 1) Output voltage of VSG u o and output current i o Sampling is performed, and then the actual active power output of the VSG is obtained through instantaneous power calculation. P e and reactive power Q e ; 2) The actual output active power P e and reactive power Q e The fundamental frequency command of the VSG output voltage is obtained through the VSG controller. u * VSGf ; 3) Place a small passive inductor between the public bus and the suburban distribution network. L s This is used to introduce an additional voltage variable. u s ; 4) Harmonic detection method based on instantaneous reactive power theory is used to detect the voltage of the common bus. u PCC and additional voltage u s The harmonic voltage content was detected, and the results were obtained respectively. u PCC Major subharmonic content u PCCh and u s Major subharmonic content u sh ; 5) For each u PCC Major subharmonic content u PCCh and u s Major subharmonic content u sh Perform negative and positive feedforward control, where, - m yes u PCCh The negative feedforward coefficient; n yes u sh The positive feedforward coefficient; the sum of the two yields the harmonic command of the VSG output voltage. u * VSGh ; ； 6) Output voltage fundamental frequency command u * VSGf and output voltage harmonic commands u * VSGh Add them together to obtain the control signal for the VSG. u * VSG ; ； 7) Control signal u * VSG The drive signal is obtained through PWM modulation.

2. The VSG harmonic current impedance reshaping suppression method according to claim 1, characterized in that, In step 4), a harmonic detection method based on instantaneous reactive power theory is used to detect... u PCC and u s The harmonic voltage content.

3. The VSG harmonic current impedance reshaping suppression method according to claim 1, characterized in that, The passive inductor L s The parameters are matched to the impedance characteristics of the transmission line and are only used to introduce additional voltage variables. u s It does not undertake the power transmission of the main circuit.

4. The VSG harmonic current impedance reshaping suppression method according to claim 1, characterized in that, The feedforward coefficients m and n satisfy the following relationship: ； Z out This is the equivalent impedance from VSG to the common bus terminal.

5. The VSG harmonic current impedance reshaping suppression method according to claim 1, characterized in that, In step 5), the output impedance from the common bus to the VSG and the impedance from the suburban power distribution network to the VSG They are respectively: 。 6. The VSG harmonic current impedance reshaping suppression method according to claim 5, characterized in that, Z out Z is the equivalent impedance from VSG to the common bus terminal. out Approximately the impedance of an LC filter, Z out = sL f .

7. The VSG harmonic current impedance reshaping suppression method according to claim 5, characterized in that, Z total The equivalent impedance from VSG to the suburban distribution network is approximately: Z total = sL s + Z out .

8. The VSG harmonic current impedance reshaping suppression method according to claim 1, characterized in that, The PWM modulation in step 7) adopts sinusoidal pulse width modulation, and the modulation frequency is adapted to the working characteristics of the VSG switching device.

9. The VSG harmonic current impedance reshaping suppression method according to claim 1, characterized in that, The VSG controller includes an active power regulation module and a reactive power regulation module, which respectively regulate the active power and reactive power. P e , Q e Perform closed-loop control to generate u * VSGf .

10. The VSG harmonic current impedance reshaping suppression method according to claim 1, characterized in that, The nonlinear load includes one or more power electronic loads such as frequency converters, rectifiers, and electric arc furnaces.

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