Grid-connected inverter active and reactive power quasi-closed loop control method

Abstract

The invention discloses a grid-connected inverter active power and reactive power quasi-closed loop control method. The method comprises the following steps: measuring active power P and reactive power Q actually output by an inverter; obtaining active power P0 and reactive power Q0 calculated by discrete sampling of the inverter; fitting the obtained data to obtain a fitting formula of P0 to P and a fitting formula of Q0 to Q; respectively substituting P0 and Q0 obtained by the inverter into a fitting formula to obtain active power Pest and reactive power Qest currently output by the inverter; pest and Qest are used as feedback quantities to form quasi-closed-loop control with the active power reference Rref and the reactive power reference Qref, and the output power of the inverter is compensated. According to the method, a fitting formula does not contain complex operation items, and the occupied chip computing power and storage space are very small; a grid-connected current sensor does not need to be used, so that the production and design cost of the inverter is saved; for inverters produced in batches, a complex fitting process does not need to be repeated for each device, and only linear proportion fine tuning needs to be carried out on a fitting formula.

Description

Technical Field

[0001] The invention relates to a quasi-closed-loop control method for active and reactive power of a grid-connected inverter, belonging to the technical field of active and reactive power compensation and correction of inverters. Background Art

[0002] The inverter filter output circuit generally adopts LC or LCL circuit, and the system modulation frequency is generally between 5k and 100k. When discrete sampling is performed on the inverter current flowing through the inverter inductor, such as Figure 7 As shown in the figure, it is inevitable that there will be a phase difference between the phase of the discretely sampled inverter current fundamental component and the phase of the actual fundamental component. This phase difference is mainly caused by factors such as ADC sampling delay, filter circuit delay, sampling noise, sampling trigger time, PWM modulation strategy, etc. This phenomenon will affect the accuracy of the active and reactive power output of the inverter. According to laboratory measurement results, when the reactive power output function is turned on, such as Figure 8 and Fig. 9 As shown in the figure, the maximum deviation between the reactive power output by the equipment and the expected reactive power output reached more than 8% of the rated power. At the same time, the calculated deviation of the phase angle of the inverter current reached more than 6°. Therefore, it is very necessary to correct the active and reactive power output by the inverter in a convenient and efficient way.

[0003] Traditional inverters rely on grid-connected current sampling for active and reactive power correction, that is, they use grid-connected current sensors and need to cooperate with correction software to perform corresponding correction operations. Traditional correction software contains many complex operating items, which takes up more chip computing power and storage space. In addition, with the miniaturization and cost control of inverters, the grid-connected output current sampling of the inverter is omitted. At this time, it is difficult to achieve compensation correction, and it is not conducive to the debugging of batch inverters. Summary of the invention

[0004] The purpose of the present invention is to solve the above-mentioned deficiencies in the prior art, and to propose a quasi-closed-loop control method for active and reactive power of a grid-connected inverter in view of the many defects of the traditional grid-connected current sampling and correction software.

[0005] In order to achieve the above object, the technical solution adopted by the present invention is:

[0006] A quasi-closed-loop control method for active and reactive power of a grid-connected inverter, characterized by comprising the following steps:

[0007] S1: Measure the active power P and reactive power Q actually output by the inverter;

[0008] S2: Obtain the active power P0 and reactive power Q0 calculated by discrete sampling of the inverter;

[0009] S3: Fit the data obtained in step S1 and step S2 respectively to obtain the fitting formulas of P0 to P and Q0 to Q;

[0010] S4: Substitute P0 and Q0 obtained by the inverter into the fitting formula to obtain the active power P currently output by the inverter. est and reactive power Q est ;

[0011] S5: P est and Q est As feedback quantity, with active power reference P ref and reactive power reference Q ref A quasi-closed-loop control is formed to compensate the inverter output power.

[0012] Preferably, in step S2, the grid voltage is set to Inverter current fundamental component Where ω is the grid angular frequency, ψ is the phase difference between the grid voltage and the inverter current, with the grid voltage as the horizontal axis and the inverter current as the vertical axis, a Lissajous figure is formed;

[0013] According to Green's formula:

[0014]

[0015] If Q=x, P=0 in the above formula, then:

[0016]

[0017] According to the above formula, the area Q enclosed by the closed curve of the Lissajous figure L Calculated as follows:

[0018]

[0019] Convert the above formula into the integral of time within the entire cycle (T1, T2) and Substitute:

[0020]

[0021] In the above formula, Q0=U rms I rms sin(ψ) is the reactive power, so:

[0022]

[0023] For discrete data, assuming that the number of sampling times in one power frequency cycle is N, the above formula can be discretized as:

[0024]

[0025] Where ΔI i =I i -I i-1 It is the difference between the current value calculated at the discrete sampling moment i and the current value calculated at the previous sampling moment.

[0026] Preferably, in step S3, the abstraction is a mathematical expression:

[0027]

[0028] Taylor expansion is performed on the above formula, and higher-order terms are ignored:

[0029]

[0030] Among them A ijk , B ijk (i=0...2, j=0...2, k=0...2) are polynomial fitting coefficients.

[0031] The beneficial effects of the present invention are mainly reflected in:

[0032] 1. The fitting formula does not contain complex calculation items, and occupies very little chip computing power and storage space;

[0033] 2. No need to use grid-connected current sensor, saving the production and design cost of the inverter;

[0034] 3. For mass-produced inverters, there is no need to repeat the complex fitting process for each device. It is only necessary to fine-tune the linear proportion of the fitting formula. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Other features, objects and advantages of the present application will become more apparent by reading the detailed description of non-limiting embodiments made with reference to the following drawings:

[0036] Figure 1 It is a flow chart of the control method of the present invention.

[0037] Figure 2 It is the Lissajous figure of the grid voltage and the inverter current in the control method of the present invention.

[0038] Figure 3 It is a discrete signal graph with Gaussian white noise in the control method of the present invention.

[0039] Figure 4 It is a schematic diagram of the deviation between the reactive power actually output by the inverter and the reference value after the quasi-closed-loop control of the present invention.

[0040] Figure 5 It is a schematic diagram of the deviation between the apparent power actually output by the inverter and the reference value after the quasi-closed-loop control of the present invention.

[0041] Figure 6 The present invention is a schematic diagram of the system structure of a grid-connected inverter control method.

[0042] Figure 7 This is a schematic diagram of discrete sampling points of traditional inverter current.

[0043] Figure 8 This is a graph showing how the inverter current fundamental phase error changes with the voltage-current phase difference and apparent power when the grid voltage is 180V in the prior art.

[0044] Fig. 9 This is a graph showing how the inverter current fundamental phase error changes with the voltage-current phase difference and apparent power when the grid voltage is 260V in the prior art. DETAILED DESCRIPTION

[0045] In order to make the purpose, technical solution and advantages of the embodiments of the present invention clearer, the technical solution in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0046] The present application will be further described in detail below in conjunction with the accompanying drawings and embodiments. It is to be understood that the specific embodiments described herein are only used to explain the relevant inventions, rather than to limit the inventions. It should also be noted that, for ease of description, only the parts related to the relevant inventions are shown in the accompanying drawings. It should be noted that, in the absence of conflict, the embodiments in the present application and the features in the embodiments can be combined with each other.

[0047] In the prior art, due to cost considerations, the grid-connected output current of some inverters is not sampled. In this case, the phase correction of the inverter current fundamental wave calculated by sampling is particularly important. From the user's perspective, the phase correction of the inverter current fundamental wave is equivalent to correcting the phase of the grid-connected current.

[0048] The inverter can calculate the active and reactive power of the current grid-connected output in real time through sampling. However, due to the influence of sampling phase deviation, the power output of the inverter will deviate. If the inverter can accurately calculate the actual output power through a certain method, the active power and reactive power can be compensated and output through automatic control.

[0049] The present invention proposes a method for performing software correction on the active and reactive power output by an inverter without relying on grid-connected current sampling. In the absence of a grid-connected current sensor, the inverter can use the currently calculated power to estimate the actual output power in real time by fitting, thereby completing quasi-closed-loop control, and limiting the deviation value of the active and reactive power actually output by the inverter to within 2% of the rated power.

[0050] Specifically, refer to Figures 1 to 6 As shown, a quasi-closed-loop control method for active and reactive power of a grid-connected inverter is provided, and the steps taken are as follows:

[0051] Determine the active power P and reactive power Q actually output by the inverter;

[0052] Obtain the active power P0 and reactive power Q0 calculated by discrete sampling of the inverter;

[0053] The acquired data are fitted respectively to obtain the fitting formulas of P0 to P and Q0 to Q;

[0054] Substitute the P0 and Q0 obtained by the inverter into the fitting formula to estimate the active power P currently output by the inverter. est and reactive power Q est ;

[0055] P est and Q est As feedback quantity, with active power reference P ref and reactive power reference Q ref Formed as Figure 6 The quasi-closed-loop control shown is used to compensate the inverter output power.

[0056] In a specific embodiment, the data of active and reactive power output by the grid-connected inverter can be collected in a laboratory by means of a power analyzer. This data collection belongs to the prior art and will not be described in detail here.

[0057] In a specific embodiment, the reactive power Q0 is calculated using the following method:

[0058] Taking single-phase electricity as an example, ignoring high-order harmonics, let the grid voltage Inverter current fundamental component Where ω is the grid angular frequency, ψ is the phase difference between the grid voltage and the inverter current, with the grid voltage as the horizontal axis and the inverter current as the vertical axis, the following is formed: Figure 2 The Lissajous figure shown.

[0059] According to Green's formula:

[0060]

[0061] If Q=x, P=0 in the above formula, then:

[0062]

[0063] According to the above formula, Figure 1 The area Q enclosed by the closed curve L It can be calculated as follows:

[0064]

[0065] Convert the above formula into the integral of time within the entire cycle (T1, T2) and Substitute:

[0066]

[0067] In the above formula, Q0=U rms I rms sin(ψ) is the reactive power, so:

[0068]

[0069] For discrete data, assuming that the number of sampling times in one power frequency cycle is N, the above formula can be discretized as:

[0070]

[0071] Where ΔI i =I i -I i-1 It is the difference between the current value calculated at the discrete sampling moment i and the current value calculated at the previous sampling moment.

[0072] In a specific embodiment, by analyzing the collected data, it is found that the corresponding relationship between P0 and P, and Q0 and Q is not completely linear. The sliding filter method is used to filter out the random high-frequency fluctuations caused by ADC discrete sampling, and it is found that the corresponding relationship presents a stable nonlinear time-invariant feature. The main influencing factors can be summarized as the grid voltage U0, active power P0 and reactive power Q0, which can be abstracted into a mathematical expression:

[0073]

[0074] Taylor expansion is performed on the above formula, and higher-order terms are ignored:

[0075]

[0076] Among them A ijk , B ijk (i=0...2, j=0...2, k=0...2) are polynomial fitting coefficients.

[0077] When compensating the inverter output power, Figure 6 As shown, there are reference data for active power and reactive power. est and Q est As the feedback quantity, the PID increase and decrease operations are used to control the compensation output. The compensation operation between the two reference data belongs to the existing technology and will not be described in detail here. The main purpose of this case is to obtain effective feedback information without sampling data, so as to meet the quasi-closed-loop control requirements.

[0078] The above fitting process often calculates different polynomial coefficients due to different initial values ​​of fitting coefficients, which may cause the fitting result to enter a local optimal solution with large errors. Therefore, the items that are infinitely close to 0 in the polynomial coefficients can be assigned 0 during the fitting process and iterated again. By collecting a large amount of data for fitting, the analysis results show that the polynomial fitting error can be controlled within 2%, among which the reactive power is within the range of -60% to 60%, and the active power is within the range of 20% to 100%, and the error can be controlled within 1.5%.

[0079] In order to fully verify the feasibility of this method, the actual output power of the inverter is tested under AC180V, AC220V, and AC260V conditions in different apparent output power ranges. The test data are as follows: Figure 4 and Figure 5 As shown in the figure, the deviation of the actual reactive power output of the inverter did not exceed 1% of the rated output power of 16.5kVA, and the actual output apparent power was also within 1.5% of the rated output power, exceeding the expected power control accuracy of 2%, verifying that this method can meet the inverter's requirements for output power control accuracy.

[0080] Since the ADC inverter current sampling can be corrected by zero point and proportion, the inverter current fundamental phase error caused by factors such as the filter circuit, ADC sampling delay, and PWM modulation strategy of the same model inverter can be uniformly summarized in the above fitting formula. Therefore, the same model inverters produced in batches can be linearly adjusted based on the same fitting formula to correct the active and reactive power output of the inverter.

[0081] From the above description, it can be found that the fitting formula does not contain complex calculation items, and occupies very little chip computing power and storage space; there is no need to use a grid-connected current sensor, which saves the production and design costs of the inverter; for mass-produced inverters, there is no need to repeat the complex fitting process for each device, and only linear proportional fine-tuning of the fitting formula is required.

[0082] The term "comprise" or any other similar term is intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus / device that includes a list of elements includes not only those elements but also other elements not expressly listed, or also includes elements inherent to such process, method, article, or apparatus / device.

[0083] So far, the technical solutions of the present invention have been described in conjunction with the preferred embodiments shown in the accompanying drawings. However, it is easy for those skilled in the art to understand that the protection scope of the present invention is obviously not limited to these specific embodiments. Without departing from the principle of the present invention, those skilled in the art can make equivalent changes or substitutions to the relevant technical features, and the technical solutions after these changes or substitutions will fall within the protection scope of the present invention.

Claims

1. A quasi-closed-loop control method for active and reactive power of a grid-connected inverter, characterized in that The steps include: S1: Measure the active power P and reactive power Q actually output by the inverter; S2: Obtain the active power P0 and reactive power Q0 calculated by discrete sampling of the inverter; S3: Fit the data obtained in step S1 and step S2 respectively to obtain the fitting formulas of P0 to P and Q0 to Q; S4: Substitute P0 and Q0 obtained by the inverter into the fitting formula to obtain the active power P currently output by the inverter. est and reactive power Q est ; S5: P est and Q est As feedback quantity, with active power reference P ref and reactive power reference Q ref A quasi-closed-loop control is formed to compensate the inverter output power.

2. A quasi-closed-loop control method for active and reactive power of a grid-connected inverter according to claim 1, characterized in that: In step S2, the grid voltage is set to Inverter current fundamental component Where ω is the grid angular frequency, ψ is the phase difference between the grid voltage and the inverter current, with the grid voltage as the horizontal axis and the inverter current as the vertical axis, a Lissajous figure is formed; According to Green's formula: If Q=x, P=0 in the above formula, then: According to the above formula, the area Q enclosed by the closed curve of the Lissajous figure L Calculated as follows: Convert the above formula into the integral of time within the entire cycle (T1, T2) and Substitute: In the above formula, Q0=U rms I rms sin(ψ) is the reactive power, so: For discrete data, assuming that the number of sampling times in one power frequency cycle is N, the above formula can be discretized as: Where ΔI i =I i -I i-1 It is the difference between the current value calculated at the discrete sampling moment i and the current value calculated at the previous sampling moment.

3. The quasi-closed-loop control method for active and reactive power of a grid-connected inverter according to claim 2, characterized in that: In step S3, it is abstracted into a mathematical expression: Taylor expansion is performed on the above formula, and higher-order terms are ignored: Among them A ijk , B ijk (i=0...2,j=0...2,k=0...2) are polynomial fitting coefficients.

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