T-type three-level photovoltaic grid-connected inverter control method based on harmonic compensation function

Abstract

The application provides a T-type three-level photovoltaic grid-connected inverter control method based on harmonic compensation function, applies harmonic compensation to a T-type three-level photovoltaic grid-connected inverter controller, adopts a phase-locked loop to analyze a grid voltage vector phase angle, calculates a fundamental output current of the inverter according to an effective value of three-phase grid voltage and photovoltaic array output power, calculates a harmonic component in a load current on the load side through a self-correcting filtering algorithm, calculates an inverter output current reference value from the fundamental output current and the harmonic component in the load current, further calculates an error of the inverter output current, and finally determines a switch conduction sequence of the inverter through an SVPWM control strategy. The control method adopted by the application realizes photovoltaic grid connection and active filtering dual functions without increasing equipment, effectively improves grid-side harmonic current, improves grid-connected current quality, and saves installation cost of a power filter.

Description

Technical Field

[0001] This invention belongs to the field of power quality control. Background Technology

[0002] With the rapid development of economy and technology, the widespread application of nonlinear loads can easily cause voltage and current distortion on the grid side, leading to power transmission and distribution equipment failures such as transformer overheating, capacitor overload, abnormal neutral current, and reduced power factor and efficiency. To reduce the harm to the power grid caused by harmonic distortion introduced at the consumer end, many scholars have conducted in-depth research on suppressing harmonic currents from grid-side loads. Practice has proven that active power filters can improve grid-side harmonic currents; however, active power filters are expensive and not suitable for low-voltage, low-power civilian applications.

[0003] Driven by the pursuit of low-carbon living, photovoltaic (PV) power generation systems have been widely adopted. PV grid-connected inverters and active power filters share similar topologies and control strategies. PV grid-connected inverters with filtering capabilities, i.e., multi-functional grid-connected inverters, have attracted attention and application, such as those capable of compensating for reactive power or adjusting the power factor. Some scholars have proposed traditional two-level PV inverters with harmonic filtering capabilities, employing deadbeat control to achieve unified control. This approach offers fast dynamic response and high accuracy, but requires discretization of the control equations, resulting in a large computational load. Another strategy is to use triangular wave comparison control for unified control. While this method maintains a fixed switching frequency, it suffers from high switching losses and slow response. Other scholars have employed fuzzy control and hysteresis comparison control strategies for unified control, but fuzzy control introduces a delay, leading to significant system latency. In conclusion, research on multi-functional PV inverters has significant engineering application value; however, the unified control methods are not yet mature and require further improvement. Furthermore, the aforementioned control methods for PV inverters with filtering capabilities are all designed for traditional two-level PV inverters.

[0004] Compared to three-phase PWM inverters, multilevel inverters offer numerous advantages, such as lower voltage stress on the switching transistors, thus attracting increasing attention. Diode-clamped inverters, cascaded H-bridge inverters, and T-type three-level inverters have been extensively studied. Unlike other multilevel inverters, the T-type three-level inverter has the significant advantage of not requiring clamping diodes, leading to successful applications in various industrial settings, including low-to-medium voltage, high-power motor drive control, power quality management, and photovoltaic grid-connected inverters. Currently, there is no research on control strategies for T-type three-level inverters with harmonic compensation capabilities. Summary of the Invention

[0005] Purpose of the invention: In order to solve the problems existing in the prior art, the present invention provides a control method for a T-type three-level photovoltaic grid-connected inverter based on harmonic compensation function.

[0006] Technical solution: This invention provides a control method for a T-type three-level photovoltaic grid-connected inverter based on harmonic compensation function, comprising the following steps:

[0007] Step 1: Obtain the phase angle θ of the grid voltage vector using phase-locked loop analysis;

[0008] Step 2: Calculate the fundamental output current of the T-type three-level photovoltaic grid-connected inverter based on θ and the output power of the photovoltaic array;

[0009] Step 3: Use a self-calibrating filtering algorithm to extract the harmonic components of the load current on the load side;

[0010] Step 4: Calculate the reference value of the output current of the T-type three-level photovoltaic grid-connected inverter based on the fundamental output current in Step 2 and the harmonic components of the load current in Step 3.

[0011] Step 5: Calculate the difference between the current reference value in Step 4 and the actual value of the output current of the T-type three-level photovoltaic grid-connected inverter. Use the SVPWM control strategy to calculate the switching sequence of the T-type three-level photovoltaic grid-connected inverter based on the difference, thereby realizing the control of the T-type three-level photovoltaic grid-connected inverter.

[0012] Furthermore, in step 2, the fundamental output current of the T-type three-level photovoltaic grid-connected inverter is calculated according to the following formula:

[0013]

[0014] Where P pv U represents the output power of the photovoltaic array. ga U gb U gc These are the effective values ​​of the three-phase grid voltages, i pa (t), i pb (t), i pc (t) represents the instantaneous value of the fundamental output current of the T-type three-level photovoltaic grid-connected inverter.

[0015] Furthermore, step 3 specifically includes:

[0016] Step 3.1: Measure the current on the load side of the T-type three-level photovoltaic grid-connected inverter using a current sensor and convert it to current in the dq coordinate system;

[0017]

[0018] Where T is the coordinate transformation formula. i la (t), i lb (t), i lc (t) represent the three-phase load currents, ild (t), i lq (t) represent the three-phase load current in the dq coordinate system;

[0019] Step 3.2: Calculate the fundamental component of the load current using the following self-calibrating filtering algorithm:

[0020]

[0021] Among them, i ld-f (s), i lq-f (s) represent the fundamental component of the load current, w is the angular frequency of the current; k is the proportionality coefficient, and s is the complex frequency.

[0022] Step 3.3: Extract the harmonic components of the load current on the load side according to the following formula:

[0023]

[0024] Among them, i ld-h (t), i lq-h (t) represent the harmonic components of the load current in the dq coordinate system; i lq-f (t) and i ld-h (t) respectively by i ld-f (s), i lq-f (s) is obtained by inverse Laplace transform.

[0025] Furthermore, in step 4, the reference value of the output current of the T-type three-level photovoltaic grid-connected inverter is calculated according to the following formula:

[0026]

[0027] in, i pa (t), i pb (t), i pc (t) represents the instantaneous value of the fundamental output current of the T-type three-level photovoltaic grid-connected inverter. These are the reference values ​​for the output current of a T-type three-level grid-connected inverter in the dq coordinate system.

[0028] Furthermore, in step 5, the difference between the current reference value and the actual value of the output current of the T-type three-level grid-connected inverter is calculated according to the following formula:

[0029]

[0030] Among them, i ca (t), i cb (t) and i cc (t) represents the actual output current of the T-type three-level grid-connected inverter.

[0031] Beneficial effects: The method proposed in this invention achieves dual functions of photovoltaic grid connection and active filtering without adding equipment, saving the installation cost of power filters. This is of significant value for the widespread application of T-type three-level photovoltaic grid-connected inverters. Furthermore, this invention can effectively suppress harmonic hazards introduced by nonlinear loads and improve the quality of grid-connected current. Attached Figure Description

[0032] Figure 1 This is a system block diagram of the control method of the present invention;

[0033] Figure 2 The waveform of the load current of a T-type three-level photovoltaic grid-connected inverter in the non-operating state is shown.

[0034] Figure 3 Figure 1 shows the grid-connected current waveform of a T-type three-level photovoltaic grid-connected inverter without harmonic compensation; Figure 2 shows the output current waveform of the T-type three-level photovoltaic grid-connected inverter; Figure 3 shows the grid-side current waveform of the T-type three-level photovoltaic grid-connected inverter.

[0035] Figure 4 Figure 1 shows the grid-connected current waveforms for harmonic compensation of a T-type three-level photovoltaic grid-connected inverter; Figure 2 shows the output current waveform of the T-type three-level photovoltaic grid-connected inverter; Figure 3 shows the grid-side current waveform of the T-type three-level photovoltaic grid-connected inverter. Detailed Implementation

[0036] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an improper limitation of the invention.

[0037] This invention provides a control method for a T-type three-level photovoltaic grid-connected inverter based on harmonic compensation function, such as... Figure 1 As shown, the three-phase load current at the power consumption terminal (i.e., the grid side and load side) is measured using sensors, and a self-calibrating filter algorithm is used to extract the load harmonic current, thereby obtaining the output current reference value of the T-type three-level grid-connected inverter. The controller with harmonic compensation function obtained using the method of this invention can effectively suppress the harmonic current components introduced by nonlinear loads and improve the waveform quality of the grid-connected current.

[0038] The aforementioned control method for a T-type three-level photovoltaic grid-connected inverter based on harmonic compensation includes the following steps:

[0039] Step 1: Use a phase-locked loop to analyze the phase angle θ of the grid voltage vector;

[0040] Step 2: Calculate the fundamental output current of the T-type three-level photovoltaic grid-connected inverter based on the effective value of the three-phase grid voltage and the output power of the photovoltaic array;

[0041] Step 3: Use a self-calibrating filter to extract the harmonic components of the grid-side load current;

[0042] Step 4: Calculate the reference value of the output current of the T-type three-level grid-connected inverter based on the fundamental output current and the load harmonic current;

[0043] Step 5: Calculate the error of the inverter output current from the current reference value and the actual measured output current of the T-type three-level grid-connected inverter, and use this error to determine the switching sequence of the inverter through the SVPWM control strategy to achieve control of the T-type three-level grid-connected inverter with harmonic compensation function.

[0044] Step 2 specifically involves: in a three-phase coordinate system, calculating the fundamental output current of the T-type three-level photovoltaic grid-connected inverter based on the effective value of the three-phase grid voltage and the output power of the photovoltaic array;

[0045]

[0046] In equation (1): P pv U represents the output power of the photovoltaic array. ga U gb U gc These are the effective values ​​of the three-phase grid voltage, i pa (t), i pb (t), i pc (t) represents the instantaneous value of the fundamental output current of the T-type three-level photovoltaic grid-connected inverter.

[0047] Based on the coordinate transformation formula, the fundamental output current in the three-phase coordinate system is converted into the current in the dq coordinate system;

[0048]

[0049] In formula (2): This is the coordinate transformation formula.

[0050] Step 3 specifically involves:

[0051] Step 1: Measure the three-phase load current using a current sensor and convert it to the current in the dq coordinate system using coordinate transformation;

[0052]

[0053] In equation (3); i la (t), i lb (t), i lc(t) represent the three-phase load currents, i ld (t), i lq (t) represents the three-phase load current in the dq coordinate system.

[0054] Step 2: Calculate the fundamental component of the load current in the dq coordinate system using a self-calibrating filter algorithm.

[0055]

[0056] In equation (4): i ld-f (s), i lq-f (s) are the complex domain forms of the fundamental component of the load current, w is the angular frequency of the current, k is the proportionality coefficient, and s is the complex frequency.

[0057] Step 3: Extract harmonic components from the load current and the fundamental component of the load current;

[0058]

[0059] In equation (5): its i ld-h (t), i lq-h (t) represent the time-domain form of the harmonic components of the load current in the dq coordinate system; i lq-f (t) and i ld-h (t) respectively by i ld-f (s), i lq-f (s) is obtained by inverse Laplace transform.

[0060] Step 4 specifically involves:

[0061] Calculate the reference value of the output current of a T-type three-level photovoltaic grid-connected inverter with harmonic compensation function in the dq coordinate system.

[0062]

[0063] In equation (6): These are the reference values ​​for the output current in the dq coordinate system.

[0064] Based on the coordinate transformation formula, the output current reference value in the dq coordinate system is converted into the output current reference value in the three-phase coordinate system.

[0065]

[0066] In equation (7): These are the reference values ​​for the output current in the three-phase coordinate system.

[0067] The error in the output current of the T-type three-level photovoltaic grid-connected inverter can be expressed as:

[0068]

[0069] In equation (8): Δi ca (t), Δi cb (t), Δi cc (t) represents the error of the output current of the T-type three-level photovoltaic grid-connected inverter.

[0070] To verify the effectiveness of this control strategy, the following simulation experiment was conducted. The simulation model parameters used in the experiment are shown in Table 1.

[0071] Table 1

[0072]

[0073] Figure 2 The waveform of the load current of the T-type three-level photovoltaic grid-connected inverter when it is not in operation is as follows: there is no load connected to the grid between 0 and 0.2 seconds; a linear load is connected at 0.2 seconds; and then a nonlinear load is connected at 0.4 seconds, thus adding harmonic components to the current.

[0074] Figure 3 The diagram shows the current waveform of a T-type three-level photovoltaic grid-connected inverter without harmonic compensation when the photovoltaic power generation system is in operation. Figure 3 (a) is the output current waveform of the grid-connected inverter; Figure 3 (b) is the current waveform on the grid side. Figure 4 This is the grid-connected current waveform of a T-type three-level photovoltaic grid-connected inverter with harmonic compensation control when the photovoltaic power generation system is in operation. Figure 4 (a) is the output current waveform of the grid-connected inverter; Figure 4 (b) shows the current waveform on the grid side. (Comparison) Figure 3 as well as Figure 4 It can be seen that, under the same nonlinear load conditions, compared with an inverter without compensation function, when using a T-type three-level grid-connected inverter with harmonic compensation function, the inverter output current contains harmonic current compensation components, the grid-side current waveform distortion is reduced, and the harmonic content is significantly reduced, which confirms the effectiveness of this control strategy.

[0075] The above description is merely an embodiment of the present invention and does not limit the patent scope of the present invention. Any equivalent structural or procedural transformations made based on the content of the present invention's specification and drawings, or direct or indirect applications in other related technical fields, are similarly included within the patent protection scope of the present invention.

Claims

1. A control method for a T-type three-level photovoltaic grid-connected inverter based on harmonic compensation function, characterized in that, Specifically, the steps include the following: Step 1: Obtain the phase angle θ of the grid voltage vector using phase-locked loop analysis; Step 2: Calculate the fundamental output current of the T-type three-level photovoltaic grid-connected inverter based on θ and the output power of the photovoltaic array; Step 3: Use a self-calibrating filtering algorithm to extract the harmonic components of the load current on the load side; Step 3.1: Measure the current on the load side of the T-type three-level photovoltaic grid-connected inverter using a current sensor and convert it to current in the dq coordinate system; ； in, Here is the coordinate transformation formula. , These are the three-phase load currents, These are the three-phase load currents in the dq coordinate system; Step 3.2: Calculate the fundamental component of the load current using the following self-calibrating filtering algorithm: ； in, These are the fundamental components of the load current. ω is the angular frequency of the current. s is the proportionality constant, and s is the complex frequency; Step 3.3: Extract the harmonic components of the load current on the load side according to the following formula: ； in, These are the harmonic components of the load current in the dq coordinate system; and respectively by Obtained through inverse Laplace transform; Step 4: Calculate the reference value of the output current of the T-type three-level photovoltaic grid-connected inverter based on the fundamental output current in Step 2 and the harmonic components of the load current in Step 3. Step 5: Calculate the difference between the current reference value in Step 4 and the actual value of the output current of the T-type three-level photovoltaic grid-connected inverter. Use the SVPWM control strategy to calculate the switching sequence of the T-type three-level photovoltaic grid-connected inverter based on the difference, thereby realizing the control of the T-type three-level photovoltaic grid-connected inverter.

2. The control method for a T-type three-level photovoltaic grid-connected inverter based on harmonic compensation function according to claim 1, characterized in that, In step 2, the fundamental output current of the T-type three-level photovoltaic grid-connected inverter is calculated according to the following formula: ； in The output power of the photovoltaic array. These are the effective values ​​of the three-phase power grid voltages. These are the instantaneous values ​​of the fundamental output current of the T-type three-level photovoltaic grid-connected inverter.

3. The control method for a T-type three-level photovoltaic grid-connected inverter based on harmonic compensation function according to claim 1, characterized in that, In step 4, the reference value of the output current of the T-type three-level photovoltaic grid-connected inverter is calculated according to the following formula: ； in, , These are the instantaneous values ​​of the fundamental output current of the T-type three-level photovoltaic grid-connected inverter. These are the reference values ​​for the output current of a T-type three-level grid-connected inverter in the dq coordinate system.

4. The control method for a T-type three-level photovoltaic grid-connected inverter based on harmonic compensation function according to claim 3, characterized in that, In step 5, the difference between the current reference value and the actual value of the output current of the T-type three-level grid-connected inverter is calculated according to the following formula: ； in, These are the actual values ​​of the output current of the T-type three-level grid-connected inverter. .

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