Converter control method and system with weak network access capability and grid-connected current quality optimization and improvement

Abstract

The invention relates to a converter control method and system with weak network access capability and grid-connected current quality optimization and improvement, and belongs to the technical field of inverter and related converter control. The method comprises the following steps: acquiring voltage components Ud and Uq under a d axis and a q axis by utilizing a stator voltage equation of a virtual synchronous generator according to a three-phase voltage and a three-phase current at a grid side of a converter; calculating a modulation degree M and an angle difference value delta theta according to Ud and Uq; according to the modulation degree M and a pre-obtained corresponding relation between each switching angle used for controlling the converter under corresponding harmonic waves and the modulation degree M, obtaining each corresponding switching angle, and superposing the angle difference value delta theta and the position of a motor rotor to obtain a wave sending angle; each switching angle is compared with the wave sending angle, and the comparison result is used as a trigger pulse of a drive circuit of the converter and is given to the inverter; the method is used for solving the problems that the power grid shows the characteristic of no damping inertia due to the access of the new energy power generation system and the power grid fluctuates and even loses stability due to the power change of the load side.

Description

Technical Field

[0001] The present invention relates to a converter control method and system with weak network access capability and grid-connected current quality optimization and improvement, belonging to the technical field of inverter and related converter control. Background Art

[0002] Under the background of the "3060 target", new energy power generation systems are connected in a large scale, and the installed capacity of traditional synchronous generators will gradually decrease. Distributed power sources, such as energy storage, wind power, photovoltaics, electric vehicles, etc., are mainly connected to the grid through grid-connected inverters. Compared with traditional synchronous generators, they have the advantages of flexible control and rapid response, but they will also cause the grid to present the characteristics of undamped inertia. In this context, load-side power changes will cause grid fluctuations, and in severe cases, the system will become unstable.

[0003] When dealing with the above problems, the existing strategy directly uses the virtual synchronous generator (VSG) solution to resist the impact of the power grid. However, the modulation strategy is mainly based on the sinusoidal pulse width modulation strategy, which does not further improve the system's anti-interference ability.

[0004] The Chinese patent application publication document with publication number CN110474585A discloses a control modulation method for a high-power direct-drive permanent magnet synchronous motor. The method adopts a SHEPWM modulation strategy, which can eliminate specific harmonics and reduce the low-order harmonic content of the current. However, the method is only used for traditional synchronous generators and does not solve the problem of the grid presenting undamped inertia characteristics due to the access of new energy power generation systems and grid fluctuations or even instability caused by load-side power changes. Summary of the invention

[0005] The purpose of the present invention is to provide a converter control method and system with weak network access capability and grid-connected current quality optimization and improvement, so as to solve the problem of grid fluctuation or even instability caused by the undamped inertia characteristics of the grid due to the access of new energy power generation systems and load-side power changes.

[0006] To achieve the above object, the solution of the present invention includes:

[0007] The present invention provides a converter control method with weak grid access capability and grid-connected current quality optimization and improvement, comprising the following steps: obtaining a voltage component under the d-axis and a voltage component under the q-axis according to the three-phase voltage and three-phase current on the grid side of the converter using the stator voltage equation of a virtual synchronous generator; calculating the modulation index and the angle difference according to the voltage component under the d-axis and the voltage component under the q-axis; obtaining corresponding switch angles according to the modulation index and the correspondence between each switch angle for controlling the converter under the corresponding harmonic obtained in advance and the modulation index, superimposing the angle difference with the motor rotor position to obtain a wave transmission angle; and then comparing each switch angle with the wave transmission angle to obtain a trigger pulse as a drive circuit of the converter and give it to the inverter.

[0008] Furthermore, the corresponding relationship is obtained by the following steps: based on different modulation indices, the corresponding harmonic elimination equations are used to calculate each switching angle as the corresponding relationship between the switching angle and the modulation indices M under the corresponding harmonics.

[0009] Furthermore, the modulation index M is calculated by the following formula:

[0010]

[0011] Among them, V dc is the DC side voltage value, U d is the voltage component under the d-axis, U q is the voltage component under the q axis.

[0012] Furthermore, the angle difference Δθ is calculated by the following formula:

[0013]

[0014] Among them, U d is the voltage component under the d-axis, U q is the voltage component under the q axis.

[0015] Furthermore, the stator voltage equation of the synchronous generator is simplified to the second-order stator voltage equation, and the voltage component U under the d-axis is d and the voltage component U under the q axis q Calculated by the following formula:

[0016]

[0017] Where, s is the differential operator, s = d / dt; is the flux linkage of the corresponding d and q axis stator winding; i d 、i q is the current of the corresponding d and q axis stator winding; R d , X d are the resistance and reactance of the d-axis stator winding; Ue is the terminal voltage.

[0018] Furthermore, the three-phase current i a 、i b 、i c With three-phase voltage u a 、u b 、u c .

[0019] Furthermore, according to the three-phase current i a 、i b 、i c With three-phase voltage u a 、u b 、u c , calculate the active power P and reactive power Q, and then get the machine-end voltage Ue through the excitation regulator.

[0020] The present invention provides a converter control system with weak network access capability and grid-connected current quality optimization and improvement, including a processor, which is used to execute a computer program to implement the steps of the above converter control method with weak network access capability and grid-connected current quality optimization and improvement.

[0021] The beneficial effects of the present invention are as follows: the present invention provides a converter control method and system with weak grid access capability and grid-connected current quality optimization and improvement, and obtains the voltage component U under the d-axis by using the stator voltage equation of the virtual synchronous generator according to the three-phase voltage and three-phase current on the grid side of the converter. d and the voltage component U under the q axis q ; According to the voltage component U under the d axis d and the voltage component U under the q axis q Calculate the modulation index M and the angle difference Δθ; according to the modulation index M and the corresponding relationship between the switch angles used to control the converter under the corresponding harmonics obtained in advance and the modulation index M, obtain the corresponding switch angles, superimpose the angle difference Δθ with the motor rotor position to obtain the wave angle; then compare the switch angles with the wave angle, and use them as the trigger pulse of the converter drive circuit to give it to the inverter. This solves the problem of the grid showing the characteristics of undamped inertia due to the access of the new energy power generation system and the grid fluctuation or even instability caused by the power change on the load side. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 It is a converter control diagram with weak grid access capability and grid current quality optimization and improvement;

[0023] Figure 2 is the harmonic content of the output waveform of a specific harmonic elimination strategy;

[0024] Figure 3is the harmonic content of the output waveform of the sinusoidal pulse width modulation strategy;

[0025] Figure 4 It is the active power-frequency waveform diagram when the power changes;

[0026] Figure 5 It is the reactive power-voltage waveform diagram when the power changes;

[0027] Figure 6 It is the change of current amplitude in the off-grid / grid switching environment;

[0028] Figure 7 It is the harmonic content of the output waveform in the off-grid / grid switching environment. DETAILED DESCRIPTION

[0029] In order to make the objectives, technical solutions and advantages of the present invention more clear, the present invention is described in detail below in detail in combination with the accompanying drawings, embodiments and beneficial effects.

[0030] The idea of ​​the present invention is to obtain the voltage components under the d and q axes by the VSG control strategy, and then calculate the modulation index and the angle difference by processing through the SHEPWM modulation strategy, obtain the corresponding switching angle according to the table lookup method, superimpose the angle difference with the motor rotor position to obtain the wave transmission angle, and then compare each switching angle with the wave transmission angle, and use this as the trigger pulse of the converter drive circuit to give it to the inverter.

[0031] Method Example:

[0032] like Figure 1 The converter control method shown in the figure has weak grid access capability and grid current quality optimization and improvement, mainly including a main circuit and a control strategy. Among them, the main circuit is a conventional grid-connected inverter topology, including a DC voltage source, an inverter and a filter circuit, etc.; the control strategy is mainly a VSG control strategy and a SHEPWM modulation strategy.

[0033] The specific steps are as follows:

[0034] (1) Computer terminal voltage Ue and angular velocity ω:

[0035] The three-phase current i of the inverter grid side is collected by using Hall sensor a 、i b 、i c With three-phase voltage u a 、u b 、u c , calculate the active power P and reactive power Q, and then get the machine-end voltage Ue and angular velocity ω through the excitation regulator and power frequency regulator.

[0036] (2) The voltage component U under the d and q axes is obtained by the synchronous generator equationd and U q :

[0037] Considering the steady-state operation process, the stator voltage equation of the synchronous generator is simplified to the second-order stator voltage equation:

[0038]

[0039] Where, s is the differential operator, s = d / dt; is the flux linkage of the corresponding d and q axis stator winding; i d 、i q is the current of the corresponding d and q axis stator winding; R d , X d is the resistance and reactance of the d-axis stator winding; U e is the voltage at the machine end.

[0040] (3) Calculate the modulation degree M and the angle difference Δθ:

[0041] According to the voltage components under the d and q axes obtained in step (2), the modulation degree M and the angle difference Δθ are calculated by the following formula:

[0042]

[0043] Among them, V dc is the DC side voltage value.

[0044] (4) Obtain the corresponding switch angles according to the table lookup method:

[0045] The method for establishing the table is: referring to the patent application publication documents with publication numbers CN102983768A and CN104022667A, each switching angle is calculated based on different modulation indices using the corresponding harmonic elimination equations, and this is used as the corresponding relationship between the switching angle and the modulation indices M under the corresponding harmonics.

[0046] Taking N=5 as an example, let the amplitude of the 5th, 7th, 11th, and 13th harmonic components be 0 (that is, the 4 harmonics in the output phase voltage can be eliminated), so the SHEPWM nonlinear transcendental equation group based on Fourier series decomposition is:

[0047]

[0048] Where bn is the amplitude corresponding to each frequency waveform, m is the modulation degree, α i For each switch angle.

[0049] Since the equations are transcendental, solving them through a processor is complex and cannot meet real-time requirements. Therefore, Matlab or other computing tools are used to solve the transcendental equations offline, and the calculation results are stored in the memory in the form of a table. When using them, the table can be looked up to read the switch angles.

[0050] (5) Drive inverter:

[0051] The angle difference Δθ obtained in step (3) is superimposed on the motor rotor position θ to obtain the wave transmission angle θ1, and then the switching angles obtained in step (4) are compared with the wave transmission angle to obtain a trigger pulse for the converter drive circuit and give it to the inverter.

[0052] The following is combined with Figures 2 to 7 The effect of the method of this embodiment is described:

[0053] Attached Figure 2 , attached Figure 3 The harmonic content of the output waveform of the specific harmonic elimination strategy and the sinusoidal pulse width modulation strategy is compared.

[0054] In the picture: Figure 2 It can be seen that under the switching frequency of 500Hz, the phase voltage fundamental amplitude is 263.2V, THD=47.85%, and the line voltage fundamental amplitude is 452.9V, THD=30.79% for the specific harmonic elimination strategy; under the carrier frequency of 500Hz for the SPWM strategy, the phase voltage fundamental amplitude is 262.6V, THD=46.35%, and the line voltage fundamental amplitude is 458.2V, THD=27.48%. The THD difference between the two strategies does not exceed 1.5%, but the first 20 harmonics of the scheme of the present invention are smaller and cleaner. The 5th, 7th, 11th, and 13th harmonic contents under the specific harmonic elimination strategy are less than 1%, and the harmonics are concentrated in the high-frequency band, which solves the interference problem of low-frequency harmonics, is beneficial to the design of the filter system, and can reduce the volume and cost.

[0055] Attached Figure 4 , attached Figure 5 It is the active power-frequency waveform and reactive power-voltage waveform when power changes.

[0056] In the picture: Figure 4 1500W of active power is injected into the system at 0.2s and 0.6s respectively, and it only takes 0.05s for the entire system to reach a stable value. Figure 5 In the middle, 1500Var reactive power is injected into the system at 0.2s and 0.6s respectively, and the system voltage is well supported and recovers quickly. Therefore, this strategy can provide additional inertia and damping to the system, and has a good supporting effect on voltage and frequency.

[0057] Attached Figure 6, attached Figure 7 In order to further verify the effectiveness of the control strategy proposed in this paper, it is verified under grid-connected conditions.

[0058] In the figure: Figure 6 At the initial moment, the system is running with load in an off-grid condition, and the grid connection is switched at 0.4s; Figure 7 The following are the analysis results of the A-phase current and harmonic content under the off-grid / grid-connected conditions of the system. From the comparison results, it can be seen that the THD of the A-phase output current under the off-grid condition is 1.04%, while the THD of the A-phase output current under the grid-connected condition is 1.78%, which is larger than that under the off-grid condition, but both still meet the requirement that the grid-connected current THD is less than 5%. The simulation results confirm that the system operates well in the off-grid / grid-connected switching environment.

[0059] System Example:

[0060] The present invention provides a converter control system with weak network access capability and optimized grid-connected current quality. The system includes a processor, and the processor executes computer program instructions to implement a converter control method with weak network access capability and optimized grid-connected current quality introduced in a method embodiment. The method has been introduced clearly enough in the method embodiment and will not be repeated here.

Claims

1. A converter control method with weak grid access capability and grid current quality optimization and improvement, characterized in that: The method comprises the following steps: obtaining a voltage component under the d-axis and a voltage component under the q-axis according to the three-phase voltage and the three-phase current at the grid side of the converter by using the stator voltage equation of the virtual synchronous generator; calculating the modulation index and the angle difference according to the voltage component under the d-axis and the voltage component under the q-axis; obtaining corresponding switching angles according to the modulation index and the corresponding relationship between each switch angle for controlling the converter under the corresponding harmonic obtained in advance and the modulation index, superimposing the angle difference with the motor rotor position to obtain the wave transmission angle; and then comparing each switch angle with the wave transmission angle to obtain a trigger pulse as a drive circuit of the converter and give it to the inverter.

2. The converter control method with weak network access capability and grid-connected current quality optimization and improvement according to claim 1 is characterized in that: The corresponding relationship is obtained by the following steps: based on different modulation degrees, the corresponding harmonic elimination equations are used to calculate each switching angle as the corresponding relationship between the switching angle and the modulation degree M under the corresponding harmonic.

3. The converter control method with weak grid access capability and grid-connected current quality optimization and improvement according to claim 1 is characterized in that: The modulation index M is calculated by the following formula: Among them, V dc is the DC side voltage value, U d is the voltage component under the d-axis, U q is the voltage component under the q axis.

4. The converter control method with weak grid access capability and grid-connected current quality optimization and improvement according to claim 1 is characterized in that: The angle difference Δθ is calculated by the following formula: Among them, U d is the voltage component under the d-axis, U q is the voltage component under the q axis.

5. The converter control method with weak grid access capability and grid-connected current quality optimization and improvement according to claim 1 is characterized in that: The stator voltage equation of the synchronous generator is simplified to a second-order stator voltage equation, and the voltage component U under the d-axis is d and the voltage component U on the q axis q Calculated by the following formula: Where, s is the differential operator, s = d / dt; is the flux linkage of the corresponding d and q axis stator winding; i d 、i q is the current of the corresponding d and q axis stator winding; R d , X d are the resistance and reactance of the d-axis stator winding; Ue is the terminal voltage.

6. The converter control method with weak grid access capability and grid-connected current quality optimization and improvement according to claim 1 is characterized in that: The three-phase current i of the inverter grid side is collected by using Hall sensor a 、i b 、i c With three-phase voltage u a 、u b 、u c .

7. The converter control method with weak grid access capability and grid-connected current quality optimization and improvement according to claim 5 is characterized in that: According to the three-phase current i a 、i b 、i c With three-phase voltage u a 、u b 、u c , calculate the active power P and reactive power Q, and then obtain the machine-end voltage Ue through the excitation regulator.

8. A converter control system with weak grid access capability and grid current quality optimization and improvement, comprising a processor, characterized in that: The processor is used to execute a computer program to implement the steps of a converter control method having weak grid access capability and grid-connected current quality optimization and improvement as described in any one of claims 1 to 7.

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