An optimized structure of LCL filter based on split capacitors

Abstract

The present invention discloses an optimized structure of an LCL filter based on split capacitors, which is composed of three groups of capacitors connected in a triangular shape to form three-phase terminals, and are respectively connected in parallel to three-phase power supply terminals connected in series with two groups of reactors. The capacitors are separated from the LCL filter, and at least one additional capacitor is connected in parallel to each branch of the triangular connection circuit after separation. The present invention solves the resonance frequency problem existing in the LCL third-order system, optimizes the filter performance, simultaneously improves the working environment of the filter capacitors, extends the working life of the capacitors, and is convenient for capacitor management and repair of damaged capacitors.

Description

Technical Field

[0001] The present invention relates to the technical field of filters, and particularly to an optimized structure of an LCL filter based on discrete capacitors. Background Art

[0002] In the process of frequency conversion rectification and inversion, it is equivalent to a high-speed switch, which will generate a large number of harmonics. These harmonics will interfere with the power grid, and at the same time, they will also cause false fault phenomena such as overvoltage, overcurrent, undervoltage, overload, and overheating in the frequency converter, which will further lead to a series of misoperations of the frequency converter. Therefore, a special filter must be installed at the input end of the frequency converter. The LCL filter is a kind of special filter for frequency conversion. As Figure 1 shown, the LCL filter is a third-order filtering system, which consists of a grid-side inductor L1, a filtering capacitor C, and a rectifier-side inductor L2. It can filter out high-order harmonics on the grid side of the frequency converter under the conditions of low switching frequency and small total inductance, and can stably feed back high-quality electric energy to the power grid.

[0003] The LCL filter is a structural form of the filter. The head is a group of inductors in series, the middle part is a parallel AC capacitor, and the tail is another group of inductors in series. The LCL filter is a key component of the IGBT power supply unit, which is used to isolate the high-frequency switching frequency from entering the power grid. Its relatively high AC inductance can smooth the line voltage waveform distortion caused by the high-frequency switching of the rectifier. The capacitive element of the filter can effectively filter out high-frequency (greater than 1 kHz) harmonics. The LCL filter is located on the input side of the frequency converter. L1 and L2 are reactors, and C is the filtering capacitor connected in parallel between the two reactors. Its position in the circuit can be seen in the appendix Figure 1 .

[0004] The capacitors of the existing LCL filter are composed of three groups of capacitors connected in a triangular shape to form 3-phase terminals, which are respectively connected to the three-phase power supply terminals in series with two groups of reactors. The filter capacitor is a component that will gradually age with the change of ambient temperature and humidity, and ages faster in a humid and high-temperature environment. According to its existing design, as Figure 2 shown, the capacitor 100 is located at the back of the integrated module and the innermost part of the electric cabinet. In a relatively enclosed space, the heat generated by the reactor needs to be transferred in time. The service life of the capacitor has a great relationship with the temperature. The higher the temperature, the shorter its life. With long-term use, with the accumulation of coal dust or abnormal temperature rise of the reactor, it will cause local overheating, which will accelerate the occurrence of phenomena such as bulging and leakage of the capacitor; secondly, due to the integrated design, it makes it impossible for equipment maintenance personnel to detect problems in time, and the maintenance workload will also increase significantly when problems occur.

[0005] Secondly, in the existing integrated module, if the filter capacitor is damaged, the filter will no longer be usable. When the capacitor is damaged, the device will generate a large amount of harmonic interference to the power grid, resulting in the device being unable to continue operating. This will undoubtedly have a greater impact on port production in particular, as there is not enough emergency time for staff to temporarily handle the device status, move the device position, etc. Summary of the Invention

[0006] Aiming at the above deficiencies, the present invention aims to provide an optimized structure of an LCL filter based on split capacitors, solve the resonance frequency problem existing in the LCL third-order system itself, optimize the filter performance, improve the working environment of the filter capacitor at the same time, extend its service life and facilitate management and maintenance.

[0007] To achieve the above object, the present invention provides the following technical solutions:

[0008] An optimized structure of an LCL filter based on split capacitors is composed of 3 groups of capacitors connected in a triangular shape to form 3-phase terminals, which are respectively connected in parallel to the 3-phase power supply terminals connected in series with two reactors. The capacitors are separated from the LCL filter, and at least one more capacitor is connected in parallel to each branch of the triangular connection circuit after separation.

[0009] As a preferred technical solution, the optimized structure of the LCL filter based on split capacitors further includes a filter capacitor cabinet. The capacitors after separation are installed in the filter capacitor cabinet and connected to the capacitor interface of the LCL filter through leads.

[0010] As a preferred technical solution, the optimized structure of the LCL filter based on split capacitors further includes an instrument transformer connected to the 3-phase terminal circuit, and the instrument transformer is installed inside the filter capacitor cabinet.

[0011] As a preferred technical solution, the optimized structure of the LCL filter based on split capacitors further includes a voltage monitoring display screen and a current monitoring display screen. The voltage monitoring display screen and the current monitoring display screen are respectively installed on the outer side of the cabinet door of the filter capacitor cabinet. The voltage monitoring display screen is connected to the capacitor, and the current monitoring display screen is connected to the instrument transformer, and are respectively used to monitor the voltage and current of the capacitor.

[0012] As a preferred technical solution, the optimized structure of the LCL filter based on split capacitors further includes a cooling fan connected in parallel with the capacitor. The cooling fan is installed on the top of the filter capacitor cabinet, and the air outlet direction thereof faces the inside of the filter capacitor cabinet.

[0013] As a preferred technical solution, the optimized structure of the LCL filter based on split capacitors further includes an indicator light, which is installed on the outer side of the cabinet door of the filter capacitor cabinet and is located below the voltage monitoring display screen and the current monitoring display screen.

[0014] As a preferred technical solution, there are a total of 5 indicator lights, 3 of which are respectively connected to three groups of capacitors for monitoring the power supply conditions of the three groups of capacitors; the remaining 1 is connected to the cooling fan for power supply monitoring of the cooling fan, and the other 1 is a spare indicator light.

[0015] The beneficial effects of the present invention are as follows:

[0016] 1. The operation of the equipment becomes more stable: By the method of splitting capacitors, the filter is more optimized in topological structure, which is beneficial to the improvement of filtering performance; after the power grid quality of the equipment is guaranteed, the impact on the rest of the components during operation will be reduced, and the overall stability is improved;

[0017] 2. Production is further guaranteed: In daily production, sudden failures often have a greater impact. For example, when the ship unloader at the port breaks down and stops, it will cause the ship unloader to occupy the large ship's cabin, resulting in other ship unloaders being unable to continue unloading the large ship, and then the production will be affected comprehensively. However, through the optimized structure of the LCL filter based on split capacitors of the present invention, the equipment will not stop immediately due to the damage of one capacitor, but there is enough time for the staff to carry out emergency treatment, and operations such as retracting the bridge can be performed on the equipment to avoid affecting production;

[0018] 3. The service life of the capacitor is increased: Through the newly added filter capacitor cabinet, the capacitor is migrated from the original relatively airtight and high-temperature environment to an independent capacitor cabinet, which greatly improves the working environment of the capacitor. At the same time, it is convenient for equipment management personnel to monitor and maintain. When the capacitor shows abnormal conditions, the management personnel can discover the fault quickly and carry out maintenance work. Description of the Drawings

[0019] Figure 1 is a schematic diagram of the position of the capacitor of the current LCL filter in the main circuit of the frequency converter;

[0020] Figure 2 is a schematic diagram of the installation position of the capacitor of the current LCL filter;

[0021] Figure 3 is a schematic diagram of the delta connection circuit of the capacitor of the current LCL filter;

[0022] Figure 4 is a schematic diagram of the connection of the capacitor of the current LCL filter in the filter circuit;

[0023] Figure 5 It is a schematic diagram of the triangular connection of capacitors for the optimized structure of the LCL filter based on split capacitors in the present invention;

[0024] Figure 6 It is a schematic diagram of the connection of the LCL filter capacitors in the filter circuit in the present invention;

[0025] Figure 7 It is a schematic diagram of the disassembled structure of the filter capacitor cabinet in the present invention;

[0026] Figure 8 It is a schematic diagram of the state of the filter capacitor cabinet used in cooperation with the electrical room in the present invention;

[0027] Figure 9 It is a circuit connection diagram of the optimized structure of the LCL filter based on split capacitors in the present invention;

[0028] Figure 10 It is a schematic diagram when the existing filter capacitor module is damaged after use;

[0029] Figure 11 It is a schematic diagram of the normal use state of the filter module in the present invention.

[0030] The description of the reference numerals is as follows:

[0031] 100 - Capacitor; 200 - Current transformer; 300 - Filter capacitor; 400 - Voltage monitoring display screen; 500 - Current monitoring display screen; 600 - Indicator light; 700 - Cooling fan; 800 - Filter capacitor cabinet; 900 - Electrical room. Detailed implementation manners

[0032] In order to further understand the content, features and effects of the present invention, the following embodiments are given and described in detail in conjunction with the accompanying drawings. It should be noted that the following embodiments are descriptive and not restrictive, and the protection scope of the present invention cannot be limited thereby.

[0033] In the description of the present invention, unless otherwise clearly defined and limited, the first feature being "on" or "under" the second feature may be that the first feature and the second feature are in direct contact, or the first feature and the second feature are indirectly in contact through an intermediate medium. Moreover, the first feature being "above", "over", "on top of" the second feature may be that the first feature is directly above or obliquely above the second feature, or merely indicates that the first feature is at a higher horizontal height than the second feature. The first feature being "under", "below", "beneath" the second feature may be that the first feature is directly below or obliquely below the second feature, or merely indicates that the first feature is at a lower horizontal height than the second feature.

[0034] In the description of the present invention, it should be understood that the terms "first", "second", etc. are only used for descriptive purposes and cannot be construed as indicating or implying relative importance or implicitly including one or more of the said features.

[0035] When it is said in the text that one element is "fixed" to another element, it can be directly fixed to the other element or can also be fixed through an intermediate element. When an element is described as "connected" to another element, it can be directly connected to the other element or connected through an intermediate element. The terms "upper", "lower", "left", "right", "top", "bottom", "left side", "right side" and similar expressions used herein are only for illustrative purposes and do not represent the only implementation.

[0036] The LCL filter is a structural form of the filter. The head is a group of inductors in series, the middle part is a shunt AC capacitor, and the tail is another group of inductors in series. The LCL filter is a key component of the IGBT power supply unit, used to isolate high-frequency switching frequencies from entering the power grid. Its relatively high AC inductance can smooth the line voltage waveform distortion caused by the high-frequency switching of the rectifier. The capacitive element of the filter can effectively filter out high-frequency (greater than 1 kHz) harmonics. The LCL filter is located on the input side of the frequency converter. L1 and L2 are reactors, and C is the filter capacitor shunted between the two reactors. See its position in the circuit Figure 1 .

[0037] Such as Figure 3 , Figure 4 As shown, the existing LCL filter capacitor is composed of three groups of capacitors connected in a triangular manner, forming 3-phase terminals, which are respectively connected to the three-phase power supply terminals connected in series with two groups of reactors. The filter capacitor is a component that will gradually age with the ambient temperature and humidity, and ages faster in a humid and high-temperature environment. According to its existing design, as Figure 2 shown, the capacitor 100 is located on the back of the integrated module, the innermost part of the electrical cabinet. In a relatively enclosed space, the heat generated by the reactor needs to be transferred in time. The service life of the capacitor is greatly related to the temperature. The higher the temperature, the shorter its life. With long-term use, with the accumulation of coal dust or abnormal temperature rise of the reactor, it will cause local overheating, thus accelerating the phenomenon of the capacitor bulging and leaking liquid; secondly, due to the integrated design, it makes it impossible for the equipment maintenance personnel to detect problems in time, and the maintenance workload when problems occur also increases significantly.

[0038] This embodiment provides an optimized structure of the LCL filter based on split capacitors, as Figure 5 , Figure 6As shown, it is composed of 3 groups of capacitors connected in a triangle, forming 3-phase terminals, which are respectively connected in parallel to the 3-phase power supply terminals connected in series with two groups of reactors. As an improvement of the present invention, first, the structure of the LCL filter is adjusted, and the capacitors are separated from the LCL filter (separated from the original module) to improve its working environment. Subsequently, for the capacitor connection circuit, in the form of capacitor separation, at least one more capacitor is connected in parallel to each branch of the separated triangular connection circuit. This solution can not only reduce the cost (when two capacitors are connected in parallel, their capacitance is C = C1 + C2, and the cost is less than that of a single large capacitor with a capacitance of C = C1 + C2). When one of the capacitors leaks liquid and bulges or is damaged during the operation of the equipment, it will not cause the equipment to stop immediately, thus facilitating the staff to repair or adjust the position of the equipment first. Moreover, importantly, for the deficiencies existing in the LCL filter itself, that is, the LCL filter belongs to a third-order system and there will be a sudden resonance peak within a certain frequency range, which will cause the system to oscillate and reduce the overall filtering effect. However, when this solution is implemented, it can significantly suppress its resonance state. The original capacitor C is divided into capacitors C1 and C2 connected in parallel, where C = C1 + C2 and C1 = ΒC. Define the current in the branch where C1 is located as i c1 , similarly define i c2 , then the current i 12 between the two capacitors can be deduced as i g = (1 - β)i + βi 12 . Taking the current i 12 flowing between the two capacitors as the feedback quantity, the equivalent series resistance of the filter inductor has a weak influence in the circuit and is ignored. The transfer function from the output voltage to the feedback current i

[0039]

[0040] is as follows: In the formula, L - total inductance value, L = L1 + L2; α - the ratio of the inductor value on the inverter side to the total inductance value, α = L1 / L; β = C1 / C. From the formula, it can be seen that when β + 1 = α, the zero-poles in the formula are cancelled out, and the original third-order system is simplified to a first-order system, thus eliminating the self-resonance problem existing in the third-order system.

[0041] Through the above implementation scheme, first, the LCL filter still retains its original functions of isolating the high-frequency switching frequency from entering the power grid and smoothing the line voltage waveform distortion caused by the high-frequency switching of the rectifier. At the same time, the problem of its own resonance is solved; second, when the capacitor leaks liquid and bulges or is damaged, the equipment can still operate normally. Although the filtering function is affected to a certain extent, it gives the management staff a certain amount of time for emergency adjustment and processing, which is of great significance to production operations, especially port production operations.

[0042] For the original LCL filter, the position of the capacitor is hidden, and since the capacitor life is significantly affected by temperature, its aging and damage will be accelerated at the original position. In addition, according to the application experience of the system design manufacturer (ABB crane industry), it is recommended to measure the LCL filter capacitor every 3 years and replace it every 5 years. This solution relocates the capacitor position and also adds monitoring of the capacitor.

[0043] Specifically, as Figure 7 、 Figure 8 、 Figure 9 shown, the optimized structure of the LCL filter based on split capacitors in this embodiment further includes a filter capacitor cabinet 800. By adding the filter capacitor cabinet 800, the split capacitors are installed in the filter capacitor cabinet 800 and connected to the capacitor interface of the LCL filter through leads.

[0044] The optimized structure of the LCL filter based on split capacitors in this embodiment further includes a transformer 200, and the transformer 200 is installed inside the filter capacitor cabinet 800. The transformer is connected to the three-phase terminal line for monitoring the magnitude of the current, facilitating maintenance and management personnel to promptly detect abnormal current phenomena and then perform planned maintenance in a timely manner.

[0045] The optimized structure of the LCL filter based on split capacitors in this embodiment further includes a voltage monitoring display screen 400 and a current monitoring display screen 500. The voltage monitoring display screen 400 and the current monitoring display screen 500 are respectively installed on the outer side of the cabinet door of the filter capacitor cabinet 800 for respectively monitoring the voltage and current of the capacitor. Among them, the voltage monitoring display screen 400 is connected to the filter capacitor 300, and the current monitoring screen 500 is connected to the transformer 200.

[0046] The optimized structure of the LCL filter based on split capacitors in this embodiment further includes an indicator light 600. The indicator light 600 is installed on the outer side of the cabinet door of the filter capacitor cabinet 800 and is located below the voltage monitoring display screen 400 and the current monitoring display screen 500. There are a total of 5 indicator lights 600, among which 3 are respectively connected to the three groups of capacitors for monitoring the power supply conditions of the three groups of capacitors. Another one is for monitoring the power supply of the cooling fan, and the last one is a spare indicator light.

[0047] The optimized structure of the LCL filter based on split capacitors in this embodiment further includes a cooling fan 700. The cooling fan 700 is installed on the top of the filter capacitor cabinet 800, and the air outlet direction thereof faces the inside of the filter capacitor cabinet 800. The cooling fan 700 is used to disperse the heat in the filter capacitor cabinet 800, enabling the filter capacitor 300 to be in a suitable temperature working environment for a long time, thereby extending the working life of the capacitor. The cooling fan 700 is connected in parallel with the capacitor and is also connected to the indicator light 600.

[0048] During use, as Figure 8 shown, the filter capacitor cabinet 800 can be placed near the electrical room 900, facilitating unified management and maintenance by the staff.

[0049] Through the above implementation, first, maintenance personnel can more intuitively check whether there is a phenomenon of bulging and leakage of the capacitor, and at the same time can make a preliminary judgment on the capacitor performance based on the current and voltage information; second, the working environment of the filter capacitor is improved, and when a fault occurs, it is more convenient for the maintenance personnel to replace the capacitor, saving maintenance time.

[0050] As Figure 10 shown, the existing filter capacitor is a whole-module design. One module contains 3 capacitors, and one module is used for one group of filters ( Figure 10 shown is the damaged module). Once a certain capacitor inside the module is damaged, the entire capacitor module cannot be used continuously and needs to be replaced as a whole. As Figure 11 shown, the present invention adopts a split capacitor design. One group of filters uses 6 capacitors, arranged side by side, and the capacitors are independently installed and connected by lines. When a capacitor is damaged, the damaged capacitor can be replaced separately.

[0051] The optimized structure of the LCL filter based on split capacitors provided in this embodiment: (1). By paralleling the filter capacitors, using multiple small capacitors in parallel to replace the original single large capacitor, once one of the capacitors is damaged, it will not cause the equipment to stop immediately, thus improving the operation stability of the equipment and providing emergency handling time for on-site production; (2). By splitting the capacitors, the resonance frequency problem existing in the current LCL filter third-order system is solved, the performance of the filter is optimized, and the power grid quality is significantly improved; (3). Through visualization processing, a dedicated filter capacitor cabinet is added, improving the working environment of the filter capacitor, increasing its service life, and at the same time facilitating the monitoring and maintenance by equipment management personnel. When a capacitor fails, the fault can also be processed faster, reducing the downtime.

[0052] Figure 9 It is the internal wiring diagram of the filter capacitor, which is connected according to the designed delta connection method and capacitor parallel connection method. At the same time, considering better distinguishing the three groups of capacitors, they are divided into 3 columns, with one column corresponding to one of the three-phase currents, making it more intuitive during maintenance and less likely to occur wrong connection or replacement.

[0053] The above is only a preferred specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art within the technical scope disclosed by the present invention, according to the technical solution and its concept of the present invention, makes equivalent substitutions or changes, and all should be covered by the protection scope of the present invention.

Claims

1. An optimized structure of an LCL filter based on split capacitors, which is composed of three groups of capacitors connected in a triangular shape to form three-phase terminals, and are respectively connected in parallel to the three-phase power supply terminals connected in series with two groups of reactors. The LCL filter is located on the input side of the frequency converter. The LCL filter is a third-order filtering system, and the LCL filter includes a grid-side inductor, a filtering capacitor, and a rectifier-side inductor, and is characterized in that , the capacitor and the LCL filter are separated, and at least one capacitor is connected in parallel to each branch of the separated delta-connected circuit; The optimized structure of the LCL filter further includes a filter capacitor cabinet and a cooling fan connected in parallel with the capacitor; the separated capacitors are installed in the filter capacitor cabinet and connected to the capacitor interface of the LCL filter through leads; the cooling fan is installed on the top of the filter capacitor cabinet, and the air outlet direction thereof faces the inside of the filter capacitor cabinet; The capacitor parameters of the optimized structure of the LCL filter are set to meet the resonance suppression condition: β + 1 = α; β = C1 / C, where C1 is the capacitance of one branch, C is the total capacitance value of the delta-connected circuit; L is the total inductance value, L = L1 + L2, L1 is the inductance value of the grid-side inductor, and L2 is the inductance value of the rectifier-side inductor; α = L1 / L.

2. The optimized structure of the LCL filter based on split capacitors according to claim 1, wherein It further includes a transformer connected to the three-phase terminal circuit, and the transformer is installed inside the filter capacitor cabinet.

3. The optimized structure of the LCL filter based on split capacitors according to claim 2, characterized in that, It further includes a voltage monitoring display screen and a current monitoring display screen. The voltage monitoring display screen and the current monitoring display screen are respectively installed on the outer side of the cabinet door of the filter capacitor cabinet. The voltage monitoring display screen is connected to the capacitor, and the current monitoring display screen is connected to the transformer, and are respectively used to monitor the voltage and current of the capacitor.

4. The optimized structure of the LCL filter based on split capacitors according to claim 3, characterized in that, It further includes an indicator light, and the indicator light is installed on the outer side of the cabinet door of the filter capacitor cabinet and is located below the voltage monitoring display screen and the current monitoring display screen.

5. The optimized structure of the LCL filter based on split capacitors according to claim 4, characterized in that, There are a total of 5 indicator lights, among which 3 are respectively connected to three groups of capacitors and are used to monitor the power supply conditions of the three groups of capacitors; the remaining 1 is connected to the cooling fan and is used for power supply monitoring of the cooling fan, and the other 1 is a spare indicator light.

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