A real-time measurement method of grid impedance based on inverter control

Abstract

The present invention discloses a real-time grid impedance measurement method based on inverter control, which belongs to the technical field of renewable energy grid-connected power generation, and includes the following steps: S1: in the internal control of the inverter, the inverter grid-connected current and the absolute value of the grid-connected power factor angle are calculated by the current reference instruction, and the vector modulus value of the inverter grid-connected voltage is calculated by the phase-locked loop output; S2: according to the vector relationship of the inverter side of the grid-connected system, the real-time grid impedance value is calculated by the inverter grid-connected voltage, the current vector modulus value and the absolute value of the grid-connected power factor angle. On the basis of the original internal control of the inverter, the present invention does not require additional voltage and current sampling, and completely relies on the existing control signal of the inverter to realize the real-time measurement of the equivalent grid impedance of the system; and does not increase the equipment, does not affect the original control mode of the inverter, is simple and effective in implementation, and is convenient for directly applying the measurement results to the control optimization inside the inverter, and is worthy of being promoted and used.

Description

Technical Field

[0001] The invention relates to the technical field of renewable energy grid-connected power generation, and in particular to a real-time measurement method for grid impedance based on inverter control. Background Art

[0002] In order to cope with the global energy crisis and achieve sustainable development, my country has proposed the "dual carbon" goal. Renewable energy generation, especially the large-scale grid-connected application of new energy such as photovoltaics and wind power, has become an effective way for my country to achieve the "dual carbon" goal. However, due to the application of a large number of electronic devices, the new energy grid-connected system is prone to oscillation instability. The oscillation instability of the grid-connected system is mainly caused by the interaction between the grid-connected inverter and the power grid. The grid impedance is an important parameter for analyzing the stability of the system and proposing corresponding stability improvement methods. At present, a large number of studies on the stability of the grid-connected system are based on the premise of known grid impedance. Therefore, the real-time measurement of the grid impedance of the grid-connected system is of great significance to the stability analysis and optimal control of the grid-connected system.

[0003] At present, a large number of scholars have conducted research on the measurement technology of grid impedance. The mainstream idea is to inject harmonics of different frequencies into the grid-connected system, and then calculate the equivalent impedance of the grid-connected system based on the measured harmonic current and voltage. Although the existing technology can realize the measurement of impedance and the measurement results have good accuracy. However, most of the current impedance measurement technologies require additional equipment or change the control method of the inverter to achieve it. The implementation method is complicated, the performance requirements of the sampling element are high, and the reliability of the system and the quality of grid-connected power are reduced. Therefore, it is of great significance to study a simple and fast impedance measurement method to realize the real-time measurement of grid impedance based on the original control of the inverter, so as to facilitate the control and optimization of the system by using the impedance value inside the inverter. To this end, a real-time measurement method of grid impedance based on inverter control is proposed. Summary of the invention

[0004] The technical problem to be solved by the present invention is that the current grid impedance measurement technology is complex to implement, has high requirements on the performance of sampling elements, and reduces the reliability of the system and the quality of grid-connected power. A real-time grid impedance measurement method based on inverter control is provided. The method realizes real-time measurement of grid impedance on the basis of the original control of the inverter, greatly reduces the complexity of the measurement system, and is conducive to controlling and optimizing the system by using impedance values ​​inside the inverter.

[0005] The present invention solves the above technical problems through the following technical solutions, and the present invention comprises the following steps:

[0006] S1: In the internal control of the inverter, the inverter grid-connected current and the absolute value of the grid-connected power factor angle are calculated by the current reference command, and the vector modulus value of the inverter grid-connected voltage is calculated by the phase-locked loop output;

[0007] S2: According to the inverter side vector relationship of the grid-connected system, the real-time grid impedance value is calculated by the inverter grid-connected voltage, current vector modulus value and the absolute value of the grid-connected power factor angle.

[0008] Furthermore, in step S1, the inverter grid-connected current vector modulus is obtained by calculating the current reference instruction:

[0009]

[0010] Among them, I i is the inverter grid-connected current vector modulus, I d is the active current reference command inside the inverter, I q It is the internal reactive current reference instruction of the inverter.

[0011] Furthermore, in step S1, the absolute value of the grid-connected power factor angle is obtained by calculating the current reference instruction:

[0012]

[0013] in, is the absolute value of the inverter grid-connected power factor angle, I d is the active current reference command inside the inverter, I q It is the internal reactive current reference instruction of the inverter.

[0014] Furthermore, in step S1, the inverter grid-connected voltage vector modulus is obtained by calculating the output of the phase-locked loop:

[0015]

[0016] Among them, V i is the inverter grid-connected voltage vector modulus, V d is the direct-axis component of the inverter voltage output by the phase-locked loop, V q It is the quadrature-axis component of the inverter voltage output by the phase-locked loop.

[0017] Furthermore, in step S2, the equivalent grid inductance is calculated according to the vector relationship between the voltage and current on the inverter side of the grid-connected system:

[0018]

[0019] Among them, L g is the equivalent grid inductance, is the absolute value of the inverter grid-connected power factor angle, V iis the inverter grid-connected voltage vector modulus, I i is the vector modulus of the inverter grid-connected current, and ω0 is the fundamental angular frequency.

[0020] Furthermore, in step S2, the equivalent impedance, that is, the real-time grid impedance value, is calculated based on the equivalent grid inductance value:

[0021] Z=jωL g

[0022] Where Z is the equivalent grid impedance, L g is the equivalent grid inductance, ω is the angular frequency, and j is the imaginary unit.

[0023] Compared with the prior art, the present invention has the following advantages: the real-time measurement method of grid impedance based on inverter control does not require additional voltage and current sampling on the basis of the original internal control of the inverter, and completely relies on the existing control signal of the inverter to realize the real-time measurement of the system equivalent grid impedance; and does not increase equipment and does not affect the original control method of the inverter. It is simple and effective to implement, and it is convenient to directly apply the measurement results to the control optimization inside the inverter, and it is worthy of being promoted and used. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 is an inverter side equivalent circuit diagram of the grid-connected system in Embodiment 1 of the present invention;

[0025] Figure 2 is a vector diagram of the relationship between the voltage and current on the inverter side in the first embodiment of the present invention;

[0026] Figure 3 This is a schematic diagram of realizing system impedance measurement using the original control signal of the inverter in the first embodiment of the present invention;

[0027] Figure 4 This is a schematic diagram of a specific implementation of impedance measurement using Matlab / simulink in Embodiment 1 of the present invention;

[0028] Figure 5 The current reference instruction in the second embodiment of the present invention is I d =20,I q = 0 when the system impedance real-time measurement value curve;

[0029] Figure 6 is the current reference instruction I in the second embodiment of the present invention d =20,I q = -3 when the system impedance real-time measurement value curve.

[0030] Figure 7 is the current reference instruction I in the second embodiment of the present invention d =25,I q= -3 when the system impedance real-time measurement value curve. DETAILED DESCRIPTION

[0031] The following is a detailed description of an embodiment of the present invention. This embodiment is implemented on the premise of the technical solution of the present invention, and a detailed implementation method and a specific operation process are given, but the protection scope of the present invention is not limited to the following embodiment.

[0032] Embodiment 1

[0033] The grid-connected inverter is an important interface device that connects new energy sources to the power grid. For centralized large-scale power generation systems, on the one hand, since the grid-connected voltage is supported by the power grid, the grid-connected inverter mostly adopts the current control mode. Therefore, for the external circuit, the grid-connected inverter can be regarded as a current source. On the other hand, the grid-connected system is generally connected to a strong power grid by a high-voltage transmission line. Inductance is the main part of the impedance of the high-voltage transmission line and is the main factor affecting the stability of the system. Therefore, the grid impedance of the grid-connected system is mainly replaced by the inductance component. Based on this, the equivalent circuit structure of the inverter side of the grid-connected system can be obtained as follows Figure 1 As shown in the figure, since only the voltage-current vector relationship on the inverter side is considered, the internal control dynamics of the inverter are ignored and the inverter is replaced by a current source equivalent.

[0034] according to Figure 1 The equivalent relationship between voltage and current shown in the figure can be derived from Kirchhoff's circuit theorem as follows:

[0035]

[0036] in, Output grid-connected current vector for the inverter, is the inverter output voltage vector, is the ideal grid voltage vector; ω0 is the fundamental angular frequency, L g is the equivalent grid inductance, j is an imaginary unit.

[0037] According to the vector relationship shown in formula (1), the vector diagram of the relationship between the inverter side voltage and current can be drawn, as shown in Figure 2 As shown. According to the triangle relationship, it can be calculated:

[0038]

[0039] Among them, L g is the equivalent grid inductance, is the absolute value of the inverter grid-connected power factor angle, V i is the inverter grid-connected voltage vector modulus, I i is the vector modulus of the inverter grid-connected current, and ω0 is the fundamental angular frequency.

[0040] According to formula (2), the equivalent inductance L of the grid-connected system can be calculated by the grid voltage, inverter output voltage, inverter output current and power factor angle. g ; According to the equivalent inductance value, the equivalent impedance can be calculated:

[0041] Z=jωL g (3)

[0042] In the actual control of the grid-connected inverter, the inverter output current is obtained by the current reference instruction. Therefore, the grid-connected current vector modulus can be directly calculated by the active and reactive current reference instructions as follows:

[0043]

[0044] Among them, I i is the inverter grid-connected current vector modulus, I d is the active current reference command inside the inverter, I q is the reactive current reference command inside the inverter; similarly, the absolute value of the power factor angle can be calculated based on the active and reactive current reference commands:

[0045]

[0046] in, is the absolute value of the inverter grid-connected power factor angle, I d is the active current reference command inside the inverter, I q It is the internal reactive current reference instruction of the inverter.

[0047] Since the phase information of the grid-connected voltage needs to be obtained through the phase-locked loop in the inverter control, and the direct-axis and quadrature-axis components of the inverter output voltage can be extracted during the phase-locked loop phase-locking of the inverter output voltage. Therefore, the direct-axis and quadrature-axis components of the inverter voltage can be output through the phase-locked loop, and their vector modulus values ​​can be calculated as follows:

[0048]

[0049] Among them, V i is the inverter grid-connected voltage vector modulus, V d is the direct-axis component of the inverter voltage output by the phase-locked loop, V q It is the quadrature-axis component of the inverter voltage output by the phase-locked loop.

[0050] Through equations (2)-(6), the real-time measurement of grid impedance can be realized by using the internal control signal of the inverter. The Matlab / Simulink implementation of impedance measurement is as follows: Figure 3 and Figure 4 shown. Figure 3 The figure is a schematic diagram of realizing grid-connected system impedance measurement by using the original control signal of the inverter in an embodiment of the present invention. Figure 4 The figure is a schematic diagram of a specific implementation of impedance measurement using Matlab / simulink in an embodiment of the present invention.

[0051] Embodiment 2

[0052] In this embodiment, in order to verify the effectiveness of the proposed method (the method in the first embodiment), a grid impedance of 6.28Ω is added to the grid-connected line, and the system is simulated and verified for different grid-connected current instructions; Figure 5 The current reference command is I d =20,I q = Real-time measurement value of system impedance when 0; Figure 6 The current reference command is I d =20,I q = -3 when the real-time measurement value of the system impedance. Figure 7 is the current reference command I d =25,I q = -3 when the system impedance real-time measurement value; it can be seen from the simulation waveforms in the three cases that under different inverter output currents, the impedance measurement method of the present invention can accurately measure the equivalent impedance of the grid-connected system to be 6.28Ω, and the measurement result is consistent with the actual designed grid impedance value.

[0053] In summary, the real-time grid impedance measurement method based on inverter control of the above embodiment, based on the original control signal of the inverter, realizes simple and fast measurement of grid impedance through the voltage and current vector relationship on the inverter side; no additional equipment is added, the original control method of the inverter is not changed, and the system control complexity and grid-connected power quality are not affected; the measurement results can be directly applied to stability analysis and improvement such as inverter optimization control, and have important application value and are worthy of being promoted and used.

[0054] Although the embodiments of the present invention have been shown and described above, it is to be understood that the above embodiments are exemplary and are not to be construed as limitations of the present invention. A person skilled in the art may change, modify, replace and vary the above embodiments within the scope of the present invention.

Claims

1. A real-time measurement method of grid impedance based on inverter control, characterized in that: The following steps are involved: S1: In the internal control of the inverter, the inverter grid-connected current and the absolute value of the grid-connected power factor angle are calculated by the current reference command, and the vector modulus value of the inverter grid-connected voltage is calculated by the phase-locked loop output; In step S1, the inverter grid-connected current vector modulus is obtained by calculating the current reference instruction: Among them, I i is the inverter grid-connected current vector modulus, I d is the active current reference command inside the inverter, I q It is the internal reactive current reference instruction of the inverter; In step S1, the absolute value of the grid-connected power factor angle is obtained by calculating the current reference instruction: in, is the absolute value of the inverter grid-connected power factor angle, I d is the active current reference command inside the inverter, I q It is the internal reactive current reference instruction of the inverter; In step S1, the inverter grid-connected voltage vector modulus is obtained by calculating the output of the phase-locked loop: Among them, V i is the inverter grid-connected voltage vector modulus, V d is the direct-axis component of the inverter voltage output by the phase-locked loop, V q It is the quadrature-axis component of the inverter voltage output by the phase-locked loop; S2: According to the inverter side vector relationship of the grid-connected system, the real-time grid impedance value is calculated by the grid-connected voltage and current vector modulus values ​​of the inverter and the absolute value of the grid-connected power factor angle; In step S2, the equivalent grid inductance is calculated according to the vector relationship between the voltage and current on the inverter side of the grid-connected system: Among them, L g is the equivalent grid inductance, is the absolute value of the inverter grid-connected power factor angle, V i is the inverter grid-connected voltage vector modulus, I i is the inverter grid-connected current vector modulus, and ω0 is the fundamental angular frequency.

2. The method for real-time measurement of grid impedance based on inverter control according to claim 1, characterized in that: In step S2, the equivalent impedance is calculated according to the equivalent grid inductance value, that is, the real-time grid impedance value: Z=jωL g Where Z is the equivalent grid impedance, L g is the equivalent grid inductance, ω is the angular frequency, and j is the imaginary unit.

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