Harmonic voltage compensation and harmonic current equalization strategy for grid-forming inverter

Abstract

The invention discloses a harmonic voltage compensation and harmonic current equalization strategy for a grid-forming inverter. In a multi-machine parallel connection environment, because line impedance is not matched, a scheme for solving an electric energy quality problem by utilizing an inverter composite control strategy tends to aggravate a system circulation problem, and then the stable operation of the system is influenced. In order to give consideration to both power equalization and harmonic suppression, modeling analysis is carried out on a micro-grid system containing background harmonic, and a control strategy of a network-forming inverter with harmonic voltage compensation and harmonic current equalization functions is provided. The harmonic voltage compensation control is used for performing frequency division control on the output signal, the fundamental wave domain adopts network construction type control, and the harmonic wave domain adopts harmonic voltage compensation control. The band-pass filter is used for collecting specific harmonic current to synthesize harmonic voltage, so that harmonic voltage drop on a line is just compensated, and the harmonic voltage quality of a common coupling point is improved. In order to solve the problem of equipartition caused by mismatching of line impedance, virtual impedance control is introduced, harmonic impedance is redesigned according to the adjusted output impedance, and harmonic output impedance before and after adjustment is measured and compared.

Description

Technical Field

[0001] The present invention relates to the field of power system and power electronics technology, and in particular to a grid-forming inverter control strategy for a microgrid system, which has the functions of harmonic voltage compensation and harmonic current equalization, and aims to improve the power quality of the microgrid. Background Art

[0002] With the continuous development and utilization of new energy, microgrids, as an important platform for the access of new energy, play a vital role in the reliability of the entire energy system in terms of stability and power quality. Distributed power sources (DG) in microgrids are mostly connected to the grid using inverters to achieve interconnection with the power grid. However, in a microgrid system with multiple machines in parallel, due to the mismatch of line impedance, the traditional control strategy of the inverter may introduce or aggravate the system circulation problem when solving the power quality problem, which not only affects the stable operation of the system, but also limits the microgrid's ability to absorb new energy. In addition, nonlinear loads and power electronic equipment in microgrids will also generate a large amount of harmonic currents, deteriorating the voltage quality of the point of common coupling (PCC), thereby affecting the power quality of the power grid.

[0003] In order to improve the power quality and system stability of microgrids, studies have proposed a variety of harmonic control methods, including the use of special devices such as active filters and static VAR generators, as well as the improvement of power quality through the combination of inverter control strategies. However, these methods often require additional hardware equipment or complex control algorithms, and in a multi-machine parallel environment, how to achieve effective harmonic current sharing and PCC voltage harmonic control is still a technical challenge. Summary of the invention

[0004] Aiming at the harmonic problem existing in the grid-connected inverter in the multi-machine parallel environment in the microgrid system, the present invention proposes a comprehensive control strategy, which integrates the harmonic voltage compensation and harmonic current sharing functions, and effectively improves the power quality of the microgrid.

[0005] The present invention collects the inverter outlet current, divides the collected system current into two parts of fundamental wave and harmonic wave by using a second-order bandpass filter, and then extracts the phase angle and amplitude of each harmonic current.

[0006] In the fundamental wave domain, traditional grid-type control is adopted to provide inertia damping support for the microgrid system and perform power distribution.

[0007] Virtual synchronous motor control is a type of grid-type inverter control method, which mainly simulates the rotor mechanical equation and excitation regulation equation of the synchronous motor to provide inertia and damping support for the system. The rotor mechanical equation mainly controls active power and frequency. The rotor mechanical equation is as follows:

[0008]

[0009]

[0010]

[0011] Where: J is the moment of inertia of the virtual motor, kg·m 2 ; ω, ω0 are the mechanical angular velocity and the grid synchronization angular velocity, rad / s; T m 、T e are mechanical torque and electromagnetic torque, N·m, respectively; D is the damping coefficient, N·m·s / rad; P e is the electromagnetic power; P ref is the active power setting value; f is the frequency of the grid-forming generator terminal voltage; f0 is the grid reference frequency.

[0012] The excitation regulation equation mainly controls reactive power and voltage. The excitation regulation equation is as follows:

[0013] E=E0+K Q (Q ref -Q)

[0014] Where: Q ref is the reactive power setting value; E0 is the reference voltage, K Q is the reactive power-voltage droop coefficient.

[0015] Harmonic voltage compensation control is adopted in the harmonic domain. The three-phase harmonic voltage is synthesized by the extracted harmonic phase angle and amplitude, and then added to the voltage output by the fundamental wave grid-type control to be used as a reference voltage.

[0016] The harmonic voltage synthesis method is as follows:

[0017]

[0018] In order to solve the power sharing problem caused by different line impedances, a virtual impedance Z is introduced. vir Adjust the output impedance Z out .

[0019] Once the virtual impedance is introduced into the system, the reference voltage of the grid-type control will be subtracted from the virtual impedance link to generate a new reference voltage signal u ref After the introduction of virtual impedance, the total equivalent impedance characteristics have changed.

[0020] When the introduced inductance value is large enough, the total equivalent output impedance of the branch is inductive.

[0021] There are two main functions of virtual impedance. One is that in some low-voltage microgrid environments, the line resistance is relatively large. Even with the influence of the filter, the output impedance is still resistive or the influence of the resistance value cannot be ignored, making it impossible to decouple power from frequency and voltage, increasing the difficulty of controlling power. At this time, virtual impedance is needed to adjust it so that the system control still satisfies the relationship between active power-frequency and reactive power-voltage; at the same time, the harmonic voltage compensation strategy proposed in the present invention also assumes compensation under inductive conditions.

[0022] Secondly, in a microgrid environment with multiple machines connected in parallel, the different spatial distributions of distributed power sources will lead to different line impedances, which will affect the power sharing effect. It is necessary to add virtual impedance to adjust the output impedance of each inverter to eliminate the impact of line impedance differences.

[0023] Since the introduction of virtual impedance will cause the voltage drop to further increase, the virtual impedance should not be set to a too large value. On the other hand, if the virtual impedance is set too small, it will hardly affect the output value, so an appropriate value needs to be set.

[0024] The voltage drop from the DG output voltage of the microgrid system to the PCC point is:

[0025]

[0026] If the line impedance of the ith DG is specified as the reference value, the above formula can be rewritten as:

[0027]

[0028] The virtual impedance that needs to be compensated is:

[0029] Δ Z j =(R j - R i )+j(X j - X i )=Δ R j +jΔ X j

[0030] From the above formula, it can be deduced that the virtual impedance Z vir In order to achieve power sharing, the influence of the difference between the j-th DG line impedance and the reference impedance must be eliminated.

[0031] Since the virtual impedance adjusts the equivalent output impedance, using the original impedance size for compensation in the harmonic reference voltage calculation will cause the compensation amplitude and phase to shift. The harmonic impedance in the formula needs to be redesigned to take into account the harmonic power current sharing and harmonic voltage compensation effects.

[0032] In order to ensure the harmonic voltage compensation effect, the design of harmonic impedance parameters and virtual impedance in harmonic voltage compensation control needs to take two points into consideration:

[0033] First, the harmonic impedance modulus value needs to be matched with the equivalent output impedance of the line to ensure that the amplitude of the compensation current is equal;

[0034] Second, because the impedance angle used in synthetic harmonic compensation is unified under inductive conditions, the phase angle of the output impedance should also be as close to inductive as possible, that is, X is much larger than R, to ensure that the compensation current tracked by the current is in the opposite phase.

[0035] Through the above control strategy, the inverter actively generates harmonic voltage, realizes harmonic compensation at the common coupling point, and also realizes the equalization of harmonic current. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 is the equivalent circuit model of two machines in parallel, Figure 1 (a) is the fundamental wave equivalent circuit model, Figure 1 (b) is the fundamental wave equivalent circuit model.

[0037] Figure 2 It is the harmonic current amplitude and phase angle extraction.

[0038] Figure 3 This is the block diagram of harmonic voltage compensation control.

[0039] Figure 4 It is a network control block diagram with harmonic voltage compensation and harmonic current balancing functions. DETAILED DESCRIPTION

[0040] In the following statements, some specific system architectures and technical details are introduced for the purpose of explanation rather than limitation, in order to deepen the understanding of the embodiments of the present invention. However, it should be noted by those skilled in the art that the present invention can still be implemented even in the absence of these detailed descriptions. In order to highlight the innovation and features of the present invention, the explanation of some well-known systems, devices, circuits and methods will be shortened. Based on the technology and patent requirements mentioned in the uploaded files, the following is a complete description of the specific implementation method of the present invention.

[0041] The present application is described in detail below with reference to the accompanying drawings and specific implementation methods.

[0042] A case study is conducted on a two-machine parallel microgrid with nonlinear loads, which operates in off-grid mode.

[0043] Frequency division modeling is performed on a two-machine parallel microgrid with background harmonics.

[0044] In the fundamental wave domain, due to the use of grid-type control to output fundamental wave power, the inverter can be equivalent to a voltage source, and the power consumed by the nonlinear load is equivalent to a load;

[0045] In the harmonic domain, since the nonlinear load will force the circuit to output harmonic current, it can be equivalent to a harmonic current source. In this article, the inverter is set with harmonic voltage compensation control, and the corresponding harmonic voltage will be generated at the inverter output. It can still be regarded as a voltage source in the corresponding harmonic domain.

[0046] Figure 1 It is the equivalent circuit model of the fundamental domain and the harmonic domain.

[0047] Depend on Figure 1 The KVL and KCL of (b) can be derived as follows:

[0048]

[0049] Combined to get:

[0050]

[0051] Where z1 = z out1h +z L1h ,z2=z out2h +z L2h , z1 and z2 are the equivalent impedances from the inverter to the PCC point.

[0052] The control goal is to eliminate the voltage harmonics at the PCC point. PCCh is 0, then we have

[0053]

[0054] It can be seen that if the common point voltage harmonics are to be eliminated, the sum of the ratios of the harmonic reference voltage of each inverter to the equivalent impedance needs to be equal to the harmonic current emitted by the nonlinear load.

[0055] Collect the inverter output current, use a second-order bandpass filter to divide the collected system current into two parts: fundamental wave and harmonic wave, and then extract the phase angle and amplitude of each harmonic current. The method for extracting the phase angle amplitude is as follows Figure 2 As shown, the harmonic order h in the figure is 3, 5, 7...

[0056] The collected current fundamental wave is controlled by meshing type. The harmonic part uses the extracted harmonic phase angle and amplitude to synthesize the three-phase harmonic voltage, which is then added to the voltage output by the fundamental wave meshing type control to serve as the reference voltage.

[0057] The harmonic impedance Z hThe value of is the modulus of the sum of the filter and line impedances in the harmonic domain when the averaging problem is not considered.

[0058] The block diagram of harmonic voltage compensation control is as follows: Figure 3 shown.

[0059] Since harmonic voltage compensation control will increase harmonic power, when there is inconsistent line impedance, the harmonic components in the circulating current will be greatly aggravated, which will affect the stability of the system. The problem of harmonic current balancing also needs to be considered.

[0060] When the two harmonic voltage sources are open circuit, the current can be obtained by the parallel circuit splitting. Obviously, only when the equivalent impedance from each DG to the PCC point is equal, can the harmonic current source i 0h Evenly divided, that is

[0061] z1=z2

[0062] When the harmonic current source is short-circuited, if the control target requires the current from the two voltage sources to the PCC point to be consistent, then,

[0063]

[0064] It can be seen from the above formula that the harmonic current can be evenly divided only when the above two equations are true at the same time.

[0065] The virtual impedance in the full frequency range can be set to achieve the equal distribution of harmonic currents.

[0066] The overall control block diagram of the system is as follows: Figure 4 shown.

[0067] The voltage and current are collected for frequency division control. The fundamental power is controlled by mesh control in the fundamental domain. The harmonic voltage compensation is used in the harmonic domain to control the output of the required harmonic voltage. The final reference voltage is obtained by subtracting the voltage drop of the virtual impedance control link from the sum of the two.

Claims

1. A grid-type inverter harmonic voltage compensation and harmonic current balancing strategy, characterized in that: The strategy includes the following steps: Perform frequency division control on the inverter output signal to distinguish between fundamental wave and harmonic wave; In the fundamental wave domain, grid-type control is used to regulate power; In the harmonic domain, harmonic voltage compensation control is adopted to collect specific harmonic currents through bandpass filters and synthesize harmonic voltages to compensate for the harmonic voltage drop on the line; Virtual impedance control is introduced to redesign the harmonic impedance according to the adjusted output impedance to achieve equal distribution of fundamental power and harmonic current.

2. The harmonic voltage compensation and harmonic current balancing strategy of a grid-connected inverter according to claim 1, characterized in that: The harmonic voltage compensation control described above can theoretically compensate the voltage harmonic distortion at the common coupling point to zero.

3. The harmonic voltage compensation and harmonic current balancing strategy of a grid-connected inverter according to claim 1, characterized in that: Grid-type control provides inertial damping support and distributes fundamental power.

4. The harmonic voltage compensation and harmonic current balancing strategy of a grid-connected inverter according to claim 1, characterized in that: The functions of the virtual impedance control include: Correct the line impedance to the inductive state; Correct the impedance difference of each inverter line.

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