A plug-and-play power filter device

Abstract

The present invention discloses a plug-and-play power filter device, which belongs to the field of active harmonic filtering of power systems. It includes: an isolation transformer, including a tubular iron core, a primary winding and a secondary winding; the primary winding is a power grid transmission line that passes through the inside of the tubular iron core along the axial direction of the tubular iron core, and the secondary winding is wound on the outer surface of the tubular iron core along the radial direction of the tubular iron core; the tubular iron core is an open structure, and is connected to the power grid transmission line in a plug-and-play manner when used; a control module is used to generate a control signal for controlling an IGBT inverter by detecting system current and compensation current; an IGBT inverter is used to generate a compensation current with the same frequency as the control signal, and inject it into the secondary winding of the isolation transformer. The present invention can filter out high-order harmonics in the power system, facilitate access to the system without changing the original structure of the system, and reduce the volume of the filter device.

Description

Technical Field

[0001] The invention belongs to the field of active harmonic filtering of power systems, and more specifically, relates to a plug-and-play power filtering device. Background Art

[0002] With the large-scale distributed renewable energy access to the power system, in order to achieve flexible control and efficient use of electric energy, the power system has a high proportion of access to power electronic devices. However, power electronic devices are nonlinear devices, and their large-scale use in power systems will bring much more serious harmonic problems than traditional power grids.

[0003] Existing filtering devices are not adequate for dealing with the problem of distributed renewable energy access to the grid, and there are still the following deficiencies in practical applications:

[0004] (1) Unable to filter out background harmonics; poor filtering effect on high-order / ultra-high-order harmonics in the power system caused by the access of a high proportion of power electronic devices; when a parallel active filter is used for harmonic compensation, harmonic current amplification will occur on the load AC side; the bus voltage of the active filter is relatively high, which reduces the reliability of the system and is generally only used in low-voltage systems; the compensation characteristics of the passive filter are easily affected by the grid impedance and operating status, and are prone to parallel resonance with the system, resulting in harmonic amplification, and can only compensate for specified harmonics.

[0005] (2) Active filters are expensive and require large initial investment costs. If they are to be widely used in distributed power grids, a large amount of money must be invested. Active filters have large capacity and size, and have high requirements for the installation environment, making them difficult to install at fixed points. Although passive filters are low-cost, they have poor filtering effects, and because the impedance parameters cannot be adjusted, they cannot effectively address the problem of distributed new energy access to the grid.

[0006] (3) Neither passive filters nor active filters can monitor line faults. In a large power system, adding additional monitoring equipment will undoubtedly greatly increase costs, which is not conducive to the promotion of intelligent power grid construction. Summary of the invention

[0007] In view of the defects and improvement needs of the prior art, the present invention provides a plug-and-play power filter device, which aims to provide a plug-and-play power filter device to filter out high-order harmonics in the power system and reduce the size of the filter device.

[0008] To achieve the above object, the present invention provides a plug-and-play power filter device, comprising:

[0009] An isolation transformer comprises a cylindrical core, a primary winding and a secondary winding; the primary winding is a power grid transmission line passing through the cylindrical core along the axial direction of the cylindrical core, and the secondary winding is wound on the outer surface of the cylindrical core along the radial direction of the cylindrical core and connected to the output end of the IGBT inverter; wherein the system current i1 flowing through the primary winding includes a fundamental current and a harmonic current;

[0010] A control module, used for generating a control signal for controlling the IGBT inverter by detecting the system current i1 and the compensation current i2 output by the IGBT inverter;

[0011] An IGBT inverter, used for generating a compensation current i2 having the same frequency as the control signal, and injecting the compensation current i2 into the secondary winding of the isolation transformer, wherein the compensation current i2 includes a reverse primary side fundamental current and a forward amplified primary side harmonic current;

[0012] The length d of the cylindrical core satisfies:

[0013]

[0014] Among them, r1 and r2 are the inner diameter and outer diameter of the cylindrical iron core respectively, μ is the magnetic permeability of the material, and L is the preset inductance.

[0015] Furthermore, the cylindrical iron core is an open structure and can be connected to the power transmission line of the power grid in a plug-and-play manner when in use.

[0016] Furthermore, the inner diameter and outer diameter of the cylindrical iron core satisfy:

[0017] r2≤2r1.

[0018] Furthermore, the inner diameter r1 satisfies:

[0019] r1>ρ

[0020] Among them, ρ is the wire diameter of the power grid transmission line.

[0021] Furthermore, the inner diameter r1 satisfies:

[0022] r1=ρ+5mm

[0023] Among them, ρ is the wire diameter of the power grid transmission line.

[0024] Furthermore, the magnetic potential F in the cylindrical iron core of the isolation transformer satisfies:

[0025]

[0026] Where k is the turns ratio of the primary and secondary windings, i1 is the system current, α is the fundamental control coefficient, β is the harmonic control coefficient, is the fundamental component of the system current, is the harmonic component of the system current.

[0027] Furthermore, it also includes a protection module connected in parallel to the secondary side of the isolation transformer;

[0028] The protection module includes a normally closed contactor and a bidirectional thyristor structure connected in parallel.

[0029] Furthermore, it also includes a wireless communication module;

[0030] The control module is also used to determine whether the detected fluctuation of the system current i1 is within a set threshold range. If not, a control signal is generated and the control signal is transmitted to a set control center through the wireless communication module to achieve fault monitoring.

[0031] Furthermore, there are three filtering devices, which are respectively connected to each phase of the three-phase transmission line of the power grid.

[0032] Furthermore, the voltage at both ends of the IGBT inverter bridge arm is controlled to be constant by self-powering;

[0033] The self-powering method includes:

[0034] Controlling the angle of the output voltage of the IGBT inverter to lag behind the grid voltage by an angle, so that the compensation current i2 output by the IGBT inverter includes both positive reactive current and negative active current;

[0035] The active current is used to supplement the consumption of the IGBT inverter, so that the voltage at both ends of the bridge arm of the IGBT inverter is constant.

[0036] In general, the above technical solutions conceived by the present invention can achieve the following beneficial effects:

[0037] (1) The plug-and-play power filter device provided by the present invention has a fundamental current of the compensation current injected into the secondary winding of the isolation transformer that is in antiphase with the fundamental current of the system current, and a harmonic current of the compensation current that is in phase with the harmonic current in the system current, so that the fundamental magnetic potential in the isolation transformer is zero and the harmonic magnetic potential is increased. Based on the length formula of the tubular iron core of the isolation transformer proposed by the present invention, the excitation impedance of the iron core can be made larger, the system harmonic impedance is increased, so that harmonics flow into the passive filter branch, and the possibility of various resonances is reduced. It can meet the requirement that under a preset inductance, the harmonic amount of the power grid is zero within a set error range, and the high-order harmonics in the power system are effectively filtered out, especially the high-order voltage harmonics.

[0038] (2) The primary side of the isolation transformer of the present invention is a transmission line, that is, a single-turn structure is used to reduce the excitation impedance to a relatively small level, thereby reducing the capacity of the isolation transformer and the inverter, reducing the volume of the entire device, and thus reducing the cost; and when in use, the isolation transformer is connected in series to the power grid. This series topology structure is conducive to reducing the harmonic amplification effect.

[0039] (3) Compared with the prior art where the primary winding of the transformer needs to be connected in series or in parallel in the power grid line, the isolation transformer in the present invention is an open structure and is connected to the power grid transmission line in a plug-and-play manner when in use. The isolation transformer is directly clamped in the power grid line, and the cable is equivalent to the primary winding. There is no need to cut off the power supply. The access is simple and has no impact on the structure of the power grid itself. It has low requirements on the installation environment and can achieve fixed-point access.

[0040] (4) The device has the ability to draw electricity by itself and wireless communication capabilities, which facilitates the monitoring and operation of transmission lines while maintaining its own stable operation, realizes real-time data updating and data access to the network, improves the intelligence of the power system, and does not require additional monitoring equipment, which can reduce costs and make the device more suitable for the application background of large-scale new energy access to the power grid.

[0041] In summary, this device not only overcomes the drawbacks of traditional passive filters and conventional parallel active filters, but also ensures the power quality of the power system, improves system stability and reliability, and avoids the harmonic pollution problem caused by the access of a large number of power electronic devices to the grid. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Figure 1 A schematic structural diagram of a plug-and-play power filter device provided by an embodiment of the present invention.

[0043] Figure 2 A schematic diagram of an iron core of an isolation transformer provided in one embodiment of the present invention.

[0044] Figure 3 A control block diagram of a plug-and-play power filter device control module provided by an embodiment of the present invention. DETAILED DESCRIPTION

[0045] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0046] like Figure 1As shown, a plug-and-play power filter device provided by the present invention mainly includes: an isolation transformer, an IGBT inverter and a control module; wherein the harmonic source in the figure is used to simulate the generation of harmonics in the power grid.

[0047] The isolation transformer includes a tubular core, a primary winding and a secondary winding, wherein the primary winding is a power transmission line along the axial direction of the tubular core and passes through the inside of the tubular core, and the secondary winding is wound on the outer surface of the tubular core along the radial direction of the tubular core and is connected to the output end of the IGBT inverter; wherein the current i1 flowing through the primary winding of the isolation transformer includes fundamental current and harmonic current; the tubular core is an open structure, and is connected to the power transmission line in a plug-and-play manner when in use;

[0048] A control module, used for generating a PWM gate drive pulse control signal for controlling the IGBT inverter by detecting the system current i1 flowing through the primary winding of the isolation transformer and the compensation current i2 output by the IGBT inverter;

[0049] The IGBT inverter is used to generate a compensation current i2 with the same frequency as the control signal, and inject it into the secondary winding of the isolation transformer to make the fundamental magnetic potential in the isolation transformer zero and increase the harmonic magnetic potential; wherein the compensation current i2 includes a reverse primary side fundamental current and a forward amplified primary side harmonic current.

[0050] Preferably, the tubular core is a round tube. Figure 2 As shown, the length d of the tubular core of the isolation transformer satisfies:

[0051]

[0052] Among them, r1 and r2 are the inner diameter and outer diameter of the cylindrical iron core respectively, μ is the magnetic permeability of the material, preferably a high magnetic permeability material, and L is the preset inductance.

[0053] Based on the length formula of the tubular core of the isolation transformer proposed in the present invention, the excitation impedance of the core can be made larger, the equivalent impedance on the grid side is increased, and the harmonic amount of the grid is made zero within a set error range under a preset inductance, thereby playing a harmonic isolation role.

[0054] At the same time, the length of the iron core designed in the present invention is much longer than that of the conventional iron core, so that the equivalent impedance on the grid side is sufficient to meet the set requirements to achieve the best harmonic isolation effect.

[0055] Based on the fact that the magnetic flux density at the outermost part of the core is not less than the magnetic flux density at the innermost part According to the formula:

[0056] Hl=NI

[0057] H=μB

[0058] Where H is the magnetic field strength, N is the number of turns of the coil, I is the current in the coil, and l is the circumference of the loop;

[0059] It can be obtained that: r2≤2r1.

[0060] In order to ensure that the power transmission cable can pass through the iron core smoothly and the gap is as small as possible, assuming that the wire diameter of the transmission cable is ρ, the inner diameter r1 satisfies:

[0061] r1>ρ, and r1 is as equal to ρ as possible.

[0062] Considering that the iron core needs to be insulated, which will slightly reduce r1, in this embodiment, the value is:

[0063] r1=ρ+5mm

[0064] Based on the inner diameter, outer diameter and core length of the cylindrical core obtained above, the size of the core can be designed.

[0065] In the embodiment of the present invention, the control module adopts a "double current compensation" method combining reverse fundamental compensation and forward harmonic compensation to detect the system current i1 and generate a reference signal i after processing it. ref , control the IGBT inverter to track the reference signal i ref Output the compensation current i2 that meets certain requirements and inject it into the secondary side of the transformer, while detecting the feedback current i in real time f To track the compensation current i2.

[0066] Specifically, the overall control process is as follows: Figure 3 As shown in the figure, i1 is the system current, that is, the primary current of the transformer, i f is the feedback current, i.e. the transformer secondary current or compensation current i2, is the fundamental component of the system current i1, is the harmonic component of the system current i1, α is the fundamental control coefficient, β is the harmonic control coefficient, i ref is the reference signal.

[0067] Let the turns ratio of the primary and secondary windings of the isolation transformer be k = W1 / W2. According to the transformer flux compensation principle, the magnetic potential acting in the core of the isolation transformer is:

[0068] F=W1i1+W2i2=W1i1+W1ki ′ twenty one)

[0069] Among them, i ′ 2 is the value of compensation current i2 converted to the primary side of the transformer.

[0070] The system current i1 contains fundamental wave and harmonic wave, which can be expressed as:

[0071]

[0072] Reference signal i ref It consists of two parts: fundamental component and harmonic component, which can be expressed as:

[0073]

[0074] The control module controls the IGBT inverter to track the ref Generate a compensation current i2 and inject it into the secondary side of the transformer, so that the compensation current i2 is converted to the value i on the primary side of the transformer ′ 2 is:

[0075]

[0076] From equations (1), (2) and (3), the magnetic potential F in the core of the isolation transformer after the compensation current i2 is injected is:

[0077]

[0078] In this example, W1=1. As long as kα=1, theoretically, the fundamental wave magnetic potential can be made zero, and at the same time, increasing β can increase the harmonic magnetic potential. At this time, the isolation transformer presents leakage impedance to the fundamental wave, which is equivalent to a short circuit; it presents high impedance to the harmonic, and its size is controlled by the harmonic control coefficient β, which is equivalent to an open circuit. That is, by compensating the fundamental wave and the harmonic wave at the same time, the fundamental wave impedance is very small and the harmonic impedance is very large, thereby achieving the harmonic isolation effect.

[0079] In addition, the output voltage of the inverter is controlled to lag behind the grid voltage U s An angle makes the compensation current i2 output by the inverter contain reactive current i q In addition, it also contains negative active current i p , that is, i2=i q -i p Active current i p Used to supplement the consumption of the inverter, so that the DC voltage U d Constant. Then the device can realize the self-powering function.

[0080] Preferably, the plug-and-play power filter device of the present invention further includes a protection module connected in parallel to the secondary side of the isolation transformer; the protection module includes: a normally closed contactor and a bidirectional thyristor structure connected in parallel; when the line is in a normal state, the control module controls the normally closed contactor to be in a closed state, and when a line fault occurs or a short circuit occurs in the system, the control module controls the normally closed contactor to be in an open state, so that the inverter is in a short-circuit state to prevent the inverter from burning out.

[0081] Preferably, the plug-and-play power filter device of the present invention further includes a wireless communication module for transmitting the current line operation status to the control center in real time to achieve fault monitoring. Specifically, the control module determines whether the system current i1 fluctuates too much or is within a set threshold range based on the detected system current i1. If not, the corresponding control signal is transmitted to the control center through the wireless communication module for monitoring and maintenance.

[0082] The primary winding in the present invention is a single turn, that is, a filter device is connected to a single-phase cable of a power grid cable. For a three-phase system, the filter devices of the present invention are respectively connected to the three-phase cables. The phase angle parameters of the current of the three-phase system collected by the control module are used to regulate the phase angle parameters of the system current in the three filter devices.

[0083] Compared with the primary winding of the transformer in the prior art, which needs to be connected in series or in parallel in the power grid line, the isolation transformer in the present invention is an open structure. When used, it is connected to the power grid transmission line in a plug-and-play manner. The isolation transformer is directly clamped in the power grid line, and the cable is equivalent to the primary winding. There is no need to cut off the power. The access is simple and has no effect on the structure of the power grid itself. The installation environment requirements are low, and fixed-point access can be achieved. At the same time, the device has the ability to self-collect electricity and wireless communication capabilities, which provides convenience for the monitoring and operation of the transmission line while maintaining its own stable operation, realizes real-time data update and data access, improves the intelligence of the power system, and does not require additional monitoring equipment, which can reduce costs and make the device more suitable for the application background of large-scale new energy access to the power grid.

[0084] The isolation transformer of the present invention is mainly used for isolating harmonics, and the primary side is a transmission line, that is, a single-turn structure is adopted to reduce the excitation impedance to a relatively small level, thereby reducing the capacity of the isolation transformer and the inverter, reducing the volume of the entire device, and thus reducing the cost; at the same time, the equivalent harmonic impedance of the system is greatly improved, which is very conducive to simplifying or eliminating the passive filter, and further reducing the cost.

[0085] Therefore, the device of the present invention can filter out voltage harmonics, grid background harmonics and higher harmonics, weaken the harmonic amplification effect, and reduce the device cost by reducing the capacity of the transformer and inverter, simplifying or even eliminating the passive filter.

[0086] It will be easily understood by those skilled in the art that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A plug-and-play power filter device, characterized in that: include: An isolation transformer, comprising a cylindrical iron core, a primary winding and a secondary winding; The primary winding is a power transmission line passing through the inside of the tubular core along the axial direction of the tubular core, and the secondary winding is wound on the outer surface of the tubular core along the radial direction of the tubular core and connected to the output end of the IGBT inverter; wherein the system current i1 flowing through the primary winding includes fundamental current and harmonic current; A control module, used for generating a control signal for controlling the IGBT inverter by detecting the system current i1 and the compensation current i2 output by the IGBT inverter; An IGBT inverter, used for generating a compensation current i2 having the same frequency as the control signal, and injecting the compensation current i2 into the secondary winding of the isolation transformer, wherein the compensation current i2 includes a reverse primary side fundamental current and a forward amplified primary side harmonic current; The length d of the cylindrical core satisfies: Wherein, r1 and r2 are the inner diameter and outer diameter of the cylindrical iron core, μ is the magnetic permeability of the material, and L is the preset inductance; The cylindrical iron core is an open structure and is connected to the power transmission line of the power grid in a plug-and-play manner when in use.

2. The plug-and-play power filter device according to claim 1, characterized in that: The inner diameter and outer diameter of the cylindrical iron core satisfy: r2≤2r1。 3. The plug-and-play power filter device according to claim 2, characterized in that: The inner diameter r1 satisfies: r1>ρ Among them, ρ is the wire diameter of the power grid transmission line.

4. The plug-and-play power filter device according to claim 3, characterized in that: The inner diameter r1 satisfies: r1=ρ+5mm Among them, ρ is the wire diameter of the power grid transmission line.

5. The plug-and-play power filter device according to claim 1, characterized in that: The magnetic potential F in the cylindrical iron core of the isolation transformer satisfies: Where k is the turns ratio of the primary and secondary windings, i1 is the system current, α is the fundamental control coefficient, β is the harmonic control coefficient, is the fundamental component of the system current, is the harmonic component of the system current.

6. The plug-and-play power filter device according to claim 1, characterized in that: It also includes a protection module connected in parallel to the secondary side of the isolation transformer; The protection module includes a normally closed contactor and a bidirectional thyristor structure connected in parallel.

7. The plug-and-play power filter device according to claim 1, characterized in that: Also includes a wireless communication module; The control module is also used to determine whether the detected fluctuation of the system current i1 is within a set threshold range. If not, a control signal is generated and the control signal is transmitted to a set control center through the wireless communication module to achieve fault monitoring.

8. The plug-and-play power filter device according to claim 1, characterized in that: There are three filtering devices, which are respectively connected to each phase of the three-phase transmission line of the power grid.

9. The plug-and-play power filter device according to claim 1, characterized in that: Controlling the voltage at both ends of the IGBT inverter bridge arm to be constant by self-powering; The self-powering method includes: Controlling the angle of the output voltage of the IGBT inverter to lag behind the grid voltage by an angle, so that the compensation current i2 output by the IGBT inverter includes both positive reactive current and negative active current; The active current is used to supplement the consumption of the IGBT inverter, so that the voltage at both ends of the bridge arm of the IGBT inverter is constant.

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