A flexible harmonic control method for ship photovoltaic inverter

Abstract

The present invention provides a method for flexible harmonic control of a ship photovoltaic inverter. The method adopts a control structure in which a grid-connected current command current link and a compensation command current link are connected in parallel, so as to realize flexible switching between a harmonic compensation mode and a harmonic suppression mode. The voltage and current dual closed-loop control is adopted. The voltage outer loop adopts PI control to stabilize the DC side capacitor voltage and output the grid-connected active command current. The current inner loop adopts a parallel PI+repetitive controller to realize zero-static error tracking of the command current. In the harmonic compensation mode, the inverter output current includes the grid-connected active current and the compensation current to compensate for the harmonic current generated by the nonlinear load of the power grid. In the harmonic suppression mode, the inverter's ability to suppress power frequency harmonics is effectively improved, high-quality grid-connected current is output, and the total harmonic distortion rate of the grid-connected current is reduced. The present invention realizes the use of the redundant capacity of the inverter to compensate for grid harmonics when the inverter is lightly loaded; when the inverter is heavily loaded or fully loaded, the inverter operation stability is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of photovoltaic grid-connected inverter control, and in particular to a method for flexible harmonic control of a ship photovoltaic inverter. Background Art

[0002] Ships carry 90% of the current global trade volume with their low freight rates and large transport volumes. At the same time, as a transport carrier with huge energy consumption, ships are also one of the main sources of carbon dioxide emissions. In order to reduce carbon dioxide emissions in the shipping industry, it is encouraged to research and adopt cutting-edge technologies to promote energy conservation and consumption reduction in ships. In addition, various anti-pollution regulations have been issued and come into effect, indicating that ship emissions, design and operation requirements are shifting towards green ship standards. It can be seen that the study of green ship technology will have important practical significance, and the development of new energy technology for ships is an important development trend for green ships. New energy technology refers to the technology that develops and utilizes renewable energy such as solar energy, wind energy and tidal energy based on new technologies. Among the several popular new energy technologies today, photovoltaic power generation technology has become the best choice for the application of new energy on ships due to its mature technology and reduced cost of photovoltaic components. The ship photovoltaic inverter is the core equipment of the ship photovoltaic power generation system, and its operating performance directly determines the reliability and stability of the ship photovoltaic power generation system.

[0003] Since the ship power grid is different from the large land power grid, it is a strongly coupled, nonlinear island system with small capacity, frequent changes in operating conditions, short transmission lines and harsh working environment. The power quality problems such as three-phase imbalance and harmonic pollution in the ship power system are more prominent. With the increasing proportion of nonlinear loads in the ship power grid and the large-scale use of power electronic equipment in photovoltaic power generation systems, the harmonic pollution problem of the ship power grid is becoming increasingly serious. In order to reduce the impact of harmonics on ship operation, an active power filter can be installed to improve the power quality of the ship power grid, but this treatment method has problems such as high cost and relatively single equipment functions.

[0004] In the low-carbon economic environment, the control technology of grid-connected inverters is becoming more mature, with higher control freedom. It can have more tasks and functions while realizing the photovoltaic grid-connected function. The active power filter has similarities with the photovoltaic grid-connected inverter in the main circuit topology and control strategy. It is easy to know that the inverter has the potential to realize active filtering function while photovoltaic grid-connected power generation. On the other hand, due to the intermittent and uncertain nature of distributed photovoltaic power sources, the output power of ship photovoltaic grid-connected inverters often does not reach the rated power, has a large power margin, and does not work at night. It can be seen that ship photovoltaic grid-connected inverters have a certain capacity margin in most operating conditions. If these capacity margins can be used to compensate for the harmonics of the ship power grid, the equipment utilization rate of the ship photovoltaic grid-connected inverter can be improved while reducing the cost investment of power quality equipment and saving space in the ship equipment cabin. It is of great significance to promote the application of photovoltaic power generation systems on ships and ensure the stability and reliability of ship power grids. Summary of the invention

[0005] According to the technical problem raised above, a method for flexible harmonic control of a ship photovoltaic inverter is provided. The invention aims to increase the inverter operation function and provide the ship photovoltaic inverter with anti-interference ability to the background harmonic voltage of the ship power grid.

[0006] The technical means adopted by the present invention are as follows:

[0007] A method for flexible harmonic control of a ship photovoltaic inverter includes: a compensation current calculation link, a function switching link, and a double closed-loop control link, so that the inverter can control grid harmonics while suppressing the negative impact of grid harmonic voltage on the inverter output current; wherein:

[0008] The compensation current calculation link is used to detect the harmonic current of the ship's nonlinear load. p -i q and FBD composite harmonic detection method;

[0009] In the function switching stage, the system flexibly switches the working mode according to the redundant capacity of the inverter and the control target. The working modes include: harmonic compensation mode and harmonic suppression mode;

[0010] The dual closed-loop control link adopts the voltage and current dual closed-loop control method, in which the voltage outer loop is used to stabilize the DC side capacitor voltage and obtain the grid-connected active command current; the current inner loop adopts parallel PI+repetitive control technology to achieve zero-static-error tracking of the command current.

[0011] Furthermore, the improved p -i q In the FBD composite harmonic detection method, the first half of the link adopts the FBD link, which reduces the i p -i qThe matrix operation caused by coordinate transformation in the method simplifies the operation process; the latter stage adopts i p -i q link, providing DC side voltage regulation control instructions.

[0012] Furthermore, the improved p -i q In the FBD composite harmonic detection method, a sliding average filter is used to replace the low-pass filter to address the problem that it is difficult to achieve a balance between filtering accuracy and response speed due to the low-pass filter. At the same time, a dual second-order generalized integral frequency-locked loop is used to enable the phase-locked loop to accurately provide phase and frequency information when the grid voltage is unbalanced or the frequency fluctuates.

[0013] Furthermore, the function switching link specifically includes:

[0014] Non-linear load current i Mabc Improved i p -i q and FBD composite harmonic detection method to extract harmonic components i habc ;

[0015] When the inverter is running under light load, the compensation command current harmonic i href =i habc , the inverter is in harmonic compensation mode;

[0016] When the inverter is running at full load or overload, the compensation command current harmonics i href =0, the inverter is in harmonic suppression mode;

[0017] The compensation command current harmonic i href and grid-connected active power command current i pv * In the command current generation link, the reference command current i is superimposed and output ref .

[0018] Furthermore, the double closed-loop control link specifically includes:

[0019] DC link capacitor voltage command value U dc * And collect the DC side capacitor voltage U dc After the difference is made, it enters the voltage outer loop PI controller and outputs the grid-connected active command current i pv * And complete the voltage stabilization control of the DC side capacitor of the photovoltaic inverter;

[0020] Grid-connected inverter output current i sabcAs the control feedback, the command current error value outputs the real-time control voltage set value through the current inner loop parallel PI+ repetitive controller, and is converted into the corresponding IGBT switching signal through the PMW generation link, so that the inverter output current can track the reference command current without static error, realizing flexible control of the photovoltaic inverter.

[0021] Furthermore, the current inner loop parallel PI+repetitive controller specifically includes:

[0022] A repetitive controller is introduced to form a parallel repetitive PI controller for use in the current inner loop, wherein PI control is used to track the grid-connected active current command signal, and repetitive control is used to track the harmonic current command signal.

[0023] Compared with the prior art, the present invention has the following advantages:

[0024] 1. The flexible harmonic control method of the ship photovoltaic inverter provided by the present invention, in order to integrate the harmonic compensation function into the ship photovoltaic grid-connected inverter, and at the same time improve the inverter's anti-interference ability to background harmonic voltage, constructs a flexible harmonic control strategy for the ship photovoltaic inverter, which can realize flexible switching between harmonic compensation and harmonic suppression.

[0025] 2. The method for flexible harmonic control of a ship photovoltaic inverter provided by the present invention adopts an improved i p -i q and FBD composite harmonic detection method, effectively solving the problem of i based on instantaneous reactive power theory p -i q The detection method has complex calculations and the FBD harmonic detection method cannot superimpose the DC side capacitor voltage control signal, which effectively improves the real-time performance and accuracy of the control strategy;

[0026] 3. The method for flexible harmonic control of a marine photovoltaic inverter provided by the present invention adopts PI+repetitive control parallel composite control technology as the current inner loop controller to solve the problem that the current inner loop PI controller cannot track the command current without static error. Among them, PI control is used to track the grid-connected active current command signal, and repetitive control is used to track the harmonic current command signal. This method improves the bandwidth of the current inner loop controller, so that the system has good steady-state performance while ensuring its dynamic performance.

[0027] Based on the above reasons, the present invention can be widely promoted in the fields of photovoltaic grid-connected inverter control and the like. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative labor.

[0029] Figure 1 This is a block diagram of the method for flexible harmonic control of a ship photovoltaic inverter according to the present invention.

[0030] Figure 2 The improved i of the present invention p -i q And FBD composite harmonic detection schematic diagram.

[0031] Figure 3 This is a block diagram of the dual closed-loop control strategy of the system of the present invention. DETAILED DESCRIPTION

[0032] It should be noted that, in the absence of conflict, the embodiments of the present invention and the features in the embodiments can be combined with each other. The present invention will be described in detail below with reference to the accompanying drawings and in combination with the embodiments.

[0033] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions 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 only part of the embodiments of the present invention, not all of the embodiments. The following description of at least one exemplary embodiment is actually only illustrative and is by no means intended to limit the present invention and its application or use. 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.

[0034] It should be noted that the terms used herein are only for describing specific embodiments and are not intended to limit exemplary embodiments according to the present invention. As used herein, unless the context clearly indicates otherwise, the singular form is also intended to include the plural form. In addition, it should be understood that when the terms "comprising" and / or "including" are used in this specification, it indicates the presence of features, steps, operations, devices, components and / or combinations thereof.

[0035] Unless otherwise specifically stated, the relative arrangement of the parts and steps described in these embodiments, the numerical expressions and numerical values ​​do not limit the scope of the present invention. At the same time, it should be clear that, for ease of description, the sizes of the various parts shown in the drawings are not drawn according to the actual proportional relationship. The technology, methods and equipment known to ordinary technicians in the relevant field may not be discussed in detail, but in appropriate cases, the technology, methods and equipment should be regarded as part of the authorization specification. In all examples shown and discussed here, any specific value should be interpreted as merely exemplary, rather than as a limitation. Therefore, other examples of exemplary embodiments may have different values. It should be noted that similar numbers and letters represent similar items in the following drawings, so once an item is defined in one drawing, it does not need to be further discussed in subsequent drawings.

[0036] In the description of the present invention, it is necessary to understand that the directions or positional relationships indicated by directional words such as "front, back, up, down, left, right", "lateral, vertical, perpendicular, horizontal" and "top, bottom" are usually based on the directions or positional relationships shown in the drawings. They are only for the convenience of describing the present invention and simplifying the description. Unless otherwise specified, these directional words do not indicate or imply that the device or element referred to must have a specific direction or be constructed and operated in a specific direction. Therefore, they cannot be understood as limiting the scope of protection of the present invention: the directional words "inside and outside" refer to the inside and outside relative to the contours of each component itself.

[0037] For ease of description, spatially relative terms such as "above", "above", "on the upper surface of", "above", etc. may be used here to describe the spatial positional relationship between a device or feature and other devices or features as shown in the figure. It should be understood that spatially relative terms are intended to include different orientations of the device in use or operation in addition to the orientation described in the figure. For example, if the device in the accompanying drawings is inverted, the device described as "above other devices or structures" or "above other devices or structures" will be positioned as "below other devices or structures" or "below their position devices or structures". Thus, the exemplary term "above" can include both "above" and "below". The device can also be positioned in other different ways (rotated 90 degrees or in other orientations), and the spatially relative descriptions used here are interpreted accordingly.

[0038] In addition, it should be noted that the use of terms such as "first" and "second" to limit components is only for the convenience of distinguishing the corresponding components. If not otherwise stated, the above terms have no special meaning and therefore cannot be understood as limiting the scope of protection of the present invention.

[0039] like Figure 1As shown, the present invention provides a method for flexible harmonic control of a ship photovoltaic inverter, including: a compensation current calculation link, a function switching link and a double closed-loop control link, so that the inverter can control the grid harmonics and suppress the negative impact of the grid harmonic voltage on the inverter output current; wherein:

[0040] The compensation current calculation link is used to detect the harmonic current of the ship's nonlinear load. p -i q and FBD composite harmonic detection method;

[0041] In the function switching stage, the system flexibly switches the working mode according to the redundant capacity of the inverter and the control target. The working modes include: harmonic compensation mode and harmonic suppression mode;

[0042] The dual closed-loop control link adopts the voltage and current dual closed-loop control method, in which the voltage outer loop is used to stabilize the DC side capacitor voltage and obtain the grid-connected active command current; the current inner loop adopts parallel PI+repetitive control technology to achieve zero-static-error tracking of the command current.

[0043] During specific implementation, as a preferred embodiment of the present invention, the improved i p -i q In the FBD composite harmonic detection method, the first half of the link adopts the FBD link, which reduces the i p -i q The matrix operation caused by coordinate transformation in the method simplifies the operation process; the latter stage adopts i p -i q In order to solve the problem that the low-pass filter is difficult to achieve both filtering accuracy and response speed, a sliding average filter is used to replace the low-pass filter. At the same time, in order for the phase-locked loop to accurately provide phase and frequency information when the grid voltage is unbalanced or the frequency fluctuates, a dual second-order generalized integral frequency-locked loop is used.

[0044] like Figure 2 As shown in the figure, it is the overall flow chart of the compensation specified current calculation link, which specifically includes the following steps:

[0045] S1. Obtain the frequency, phase and other signals of the ship power grid through DSOGI-FLL-PLL (dual second-order generalized integral frequency-locked phase-locked loop);

[0046] S2. According to FBD theory, the system power grid and current vector are assumed to be u=[u1,u2,...u n ···u m ] T ,i=[i1,i2,···i n ···i m ] T , where i1p ,…,i np ,i mp is the active current component of the current vector; i 1z ,…,i nz ,i mz To remove other current shunt components except the active current component. The relevant calculation formula is as follows:

[0047] The instantaneous active power is:

[0048] The instantaneous voltage is:

[0049] The active equivalent conductance is:

[0050] The active power current can be obtained by equivalent conductance as i p =G p (t)u, then we can infer that i p is the current component that produces the same instantaneous power as system circuit i, that is,

[0051] The DC component of the active equivalent conductance is:

[0052] Active power current i p The fundamental component of is: p1 =G p u

[0053] The load component that generates reactive current in the system is equivalent to the reactive equivalent conductance Among them, u q It is the reactive voltage, which lags behind the grid voltage u waveform by 90° phase angle.

[0054] The DC component of the reactive equivalent conductance is:

[0055] Reactive power current i q The fundamental component of is: q1 =G q u q

[0056] S3. Calculate the active equivalent conductance and reactive equivalent conductance according to the calculation formula provided in step S2. The active equivalent conductance is the active current. times, the reactive equivalent conductance is the reactive current times, the active current and reactive current can be calculated;

[0057] S4, active current and reactive current are filtered through a sliding average filter (MAF) to obtain active current and the DC component of reactive current

[0058] S5, After the inverse coordinate transformation, the fundamental component of the three-phase load current in the abc coordinate system is obtained, and the compensation command current i is obtained by subtracting it from the load current. href .

[0059] In specific implementation, as a preferred embodiment of the present invention, the function switching link specifically includes: nonlinear load current i Mabc Improved i p -i q and FBD composite harmonic detection method to extract harmonic components i habc ; When the inverter is lightly loaded, the compensation command current harmonic i href =i habc , the inverter is in harmonic compensation mode; when the inverter is overloaded or fully loaded, the compensation command current harmonic i href =0, the inverter is in harmonic suppression mode; the compensation command current harmonic i href and grid-connected active power command current i pv * In the command current generation link, the reference command current i is superimposed and output ref .

[0060] Specifically, according to the rated output current of the inverter is I, the grid-connected active power current of the inverter is I P , calculate the maximum harmonic current I allowed to be output by the inverter h , By comparing the allowed output harmonic current and compensation current as well as the inverter operating capacity, the control strategy control target is determined, and the two control targets of harmonic compensation and harmonic suppression are flexibly switched.

[0061] In the function switching link, when I P Greater than 0.8I, the inverter is in harmonic suppression mode, I href =0; when I P When it is less than 0.8I, the inverter is in harmonic compensation mode. In this mode, the harmonic current amplitude I detected by the harmonic detection algorithm is href Greater than the maximum harmonic current I allowed by the inverter h When all harmonic currents cannot be compensated, a specific harmonic current I can be compensated first according to the specified current compensation strategy. nh If the remaining capacity is not enough to fully compensate for a particular harmonic, the compensation coefficient k can be used to partially compensate for the harmonic. For a particular harmonic, the compensation coefficient k = I nh / I h . From this we can get the following formula:

[0062]

[0063] The sum of the compensation harmonic reference current and the grid-connected active current reference current is taken as the designated current, and the command current is tracked without static error through the parallel PI and repetitive controller. After pulse width modulation, the control signal for driving the IGBT switch is obtained, which is filtered into a sine wave that meets the requirements through the LC filter and finally connected to the ship's power grid through the isolation transformer. The inverter output current satisfies: its fundamental component is the grid-connected current sinusoidal reference signal, and its harmonic component is the harmonic component in the current flowing through the nonlinear load. Due to the introduction of the harmonic component in the load current, the grid-connected inverter can emit harmonic current, compensate for the nonlinear load, and thus improve the power quality of the ship's power grid. In the harmonic suppression working mode, the compensation harmonic reference current value is 0, which can effectively improve the inverter's ability to suppress power frequency harmonics and output high-quality grid-connected current;

[0064] In specific implementation, as a preferred embodiment of the present invention, the double closed-loop control link specifically includes: the DC side capacitor voltage command value U dc * And collect the DC side capacitor voltage U dc After the difference is made, it enters the voltage outer loop PI controller and outputs the grid-connected active command current i pv * And complete the voltage stabilization control of the DC side capacitor of the photovoltaic inverter; the grid-connected inverter output current i sabc As the control feedback, the command current error value is outputted as a real-time control voltage given value through the current inner loop parallel PI+repetitive controller, and converted into the corresponding IGBT switch signal through the PMW generation link, so that the inverter output current can track the reference command current without static error, and realize flexible control of the photovoltaic inverter. The current inner loop parallel PI+repetitive controller specifically includes: the PI controller has a poor tracking effect on the harmonic current signal due to bandwidth limitation, and the use of a single PI control can no longer meet the system control requirements. Therefore, a repetitive controller is introduced, and the repetitive controller has a natural advantage in tracking harmonic currents, forming a parallel repetitive PI controller for use in the current inner loop, wherein PI control is used to track the grid-connected active current command signal, and repetitive control is used to track the harmonic current command signal. This method improves the bandwidth of the current inner loop controller, so that the system has good steady-state performance while ensuring its dynamic performance.

[0065] like Figure 3 As shown in the figure, it is the overall flow chart of the system double closed-loop control, which specifically includes the following steps:

[0066] Step 1: The actual DC side capacitor voltage value U obtained by voltage sampling dc and DC link capacitor voltage reference value U dc_ref Difference ΔU dcThe voltage is sent to the outer loop PI controller to obtain the grid-connected active current command signal I pdref ;

[0067] Step 2: Superimpose the d-axis component I of the harmonic current command value hdref , and obtain the d-axis component I of the command synthesis current dref , minus the feedback current I Ld , enter the d-axis current inner loop and repeat the PI controller to obtain the control voltage signal V rd ;

[0068] Step 3: q-axis component I of harmonic current command value hqref Subtract the feedback current I Lq , enters the q-axis current inner loop PI controller to obtain the control voltage signal V rq ;

[0069] Step 4: Control voltage signal V rd 、V rq The dq-axis component of the grid voltage U sd , U sq The grid signal U to be modulated is obtained by superimposing the grid disturbance to suppress the influence of the grid disturbance on the system control. rd , U rq ;

[0070] Step 5: U rd , U rq After PMW modulation, the output voltage U od , U oq The dq-axis component of the grid voltage U sd , U sq After the difference is applied to the transfer function of the AC output filter, the output current I is obtained. Ld ,I Lq .

[0071] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for flexible harmonic control of a ship photovoltaic inverter, characterized in that: include: The compensation current calculation link, function switching link and dual closed-loop control link enable the inverter to control grid harmonics while suppressing the negative impact of grid harmonic voltage on the inverter output current; among them: The compensation current calculation link is used to detect the harmonic current of the ship's nonlinear load. p -i q and FBD composite harmonic detection method; the improved i p -i q In the FBD composite harmonic detection method, the first half of the link adopts the FBD link, which reduces the i p -i q The matrix operation caused by coordinate transformation in the method simplifies the operation process; the latter stage adopts i p -i q link, providing DC side voltage regulation control instructions; the improved i p -i q In the FBD composite harmonic detection method, the sliding average filter is used to replace the low-pass filter in order to solve the problem that the low-pass filter is difficult to achieve both filtering accuracy and response speed. At the same time, in order for the phase-locked loop to accurately provide phase and frequency information when the grid voltage is unbalanced or the frequency fluctuates, a dual second-order generalized integral frequency-locked loop is used. In the function switching stage, the system flexibly switches the working mode according to the redundant capacity of the inverter and the control target. The working modes include: harmonic compensation mode and harmonic suppression mode; The dual closed-loop control link adopts the voltage and current dual closed-loop control method, in which the voltage outer loop is used to stabilize the DC side capacitor voltage and obtain the grid-connected active command current; the current inner loop adopts parallel PI+repetitive control technology to achieve zero-static-error tracking of the command current.

2. The method for flexible harmonic control of a ship photovoltaic inverter according to claim 1, characterized in that: The function switching link specifically includes: Non-linear load current i Mabc Improved i p -i q and FBD composite harmonic detection method to extract harmonic components i habc ; When the inverter is running under light load, the compensation command current harmonic i href =i habc , the inverter is in harmonic compensation mode; When the inverter is running at full load or overload, the compensation command current harmonics i href =0, the inverter is in harmonic suppression mode; The compensation command current harmonic i href and grid-connected active power command current i pv * In the command current generation link, the reference command current i is superimposed and output ref .

3. The method for flexible harmonic control of a ship photovoltaic inverter according to claim 1, characterized in that: The double closed-loop control link specifically includes: DC link capacitor voltage command value U dc * And collect the DC side capacitor voltage U dc After the difference is made, it enters the voltage outer loop PI controller and outputs the grid-connected active command current i pv * And complete the voltage stabilization control of the DC side capacitor of the photovoltaic inverter; Grid-connected inverter output current i sabc As the control feedback, the command current error value outputs the real-time control voltage set value through the current inner loop parallel PI+ repetitive controller, and is converted into the corresponding IGBT switching signal through the PMW generation link, so that the inverter output current can track the reference command current without static error, realizing flexible control of the photovoltaic inverter.

4. The method for flexible harmonic control of a ship photovoltaic inverter according to claim 3, characterized in that: The current inner loop parallel PI+repetitive controller specifically includes: A repetitive controller is introduced to form a parallel repetitive PI controller for use in the current inner loop, wherein PI control is used to track the grid-connected active current command signal, and repetitive control is used to track the harmonic current command signal.

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