Unbalanced current detection method and device

Abstract

The present invention relates to the technical field of current detection, and in particular to a method and device for detecting unbalanced current. When detecting unbalanced current, the present invention injects a zero-sequence voltage component into an SVG device to transfer the neutral point of the SVG device, thereby making the regenerated voltage vector perpendicular to the current vector, so as to realize the detection of three-phase unbalanced current, thereby avoiding the situation where the SVG device fails to compensate for the unbalanced current under special circumstances.

Description

Technical Field

[0001] The present invention relates to the technical field of current detection, and in particular to a method and device for detecting unbalanced current. Background Art

[0002] With the rapid development of society, the scale of power systems is increasing, and users are also putting forward higher requirements for power quality. The safe and stable operation of power systems is affected by many factors. For the unbalanced current in the power system, it is mainly caused by the asymmetry of three-phase components, line parameters or loads. The unbalanced current situation is also a key indicator for evaluating power quality. The load in the power system is constantly changing, and the three-phase voltage and current of the power supply point are easily affected by this and unbalanced, resulting in line losses. The safe operation of the power system is largely affected by the unbalanced current. Once the degree of imbalance exceeds the normal tolerance of the distribution network, the entire power system will be in an unsafe state.

[0003] The unbalanced three-phase load will not only affect the safety of the power grid, but also affect the safe operation of other electrical equipment, and increase the power loss of transmission lines and distribution transformers. In the power supply system, due to the existence of impedance, the current flowing through the line will cause loss, and the loss is proportional to the square of the current. When an unbalanced current occurs, the current on the neutral line will also cause loss, resulting in increased loss. As the main equipment for power supply in the distribution network, the loss of the distribution transformer itself also varies with the load itself. When the current is unbalanced, the greater the imbalance, the greater the zero-sequence current. The existence of zero-sequence current will cause the copper loss or iron loss of the transformer to increase. Therefore, when operating under unbalanced current, the loss of the distribution transformer will increase, affecting the temperature of the transformer itself and causing a reduction in life. With the continuous improvement of power quality requirements, in the distribution network, the detection and control of unbalanced current has become an urgent problem to be solved.

[0004] There are schemes for detecting three-phase unbalanced current in the prior art. For example, the patent with authorization announcement number CN106300399B discloses a three-phase unbalanced current compensation method based on SVG. The method uses the load current signal as the control input and determines whether to compensate the load current through the three-phase unbalanced state judgment link. When compensation is required, the frequency and phase of the grid voltage are obtained through the adaptive frequency locking and phase locking links, and then the harmonic current components in the load current are filtered out by a discrete second-order generalized integrator, and the negative-sequence current value required to be converted for each phase when the balanced state is reached is calculated. The current is sent as a reference current signal to the control instruction calculation module inside the SVG device to drive the IGBT to generate a current equal to and opposite to the fundamental negative-sequence component in the load current as the unbalanced current. However, when calculating the unbalanced current, the above scheme often encounters a situation where a large negative-sequence current causes the current of a certain phase of the SVG to be too large, and the device starts the overcurrent protection and exits operation, resulting in the failure of SVG compensation. Summary of the invention

[0005] In view of this, the present invention proposes a method and device for detecting unbalanced current, which are used in the case where SVG compensation fails during the detection and compensation of unbalanced current.

[0006] In order to achieve the above object, a method for detecting unbalanced current is provided, the method comprising the following steps:

[0007] S1: Collect the grid phase voltage of the three-phase three-wire system , , ;

[0008] S2: Collect the output current of static VAR generator in three-phase three-wire system , , and output voltage , , , determine whether the three-phase three-wire system is unbalanced, if so, enter S3;

[0009] S3: injecting a zero-sequence voltage component into the three-phase three-wire system;

[0010] S4: Calculating the unbalanced current in the three-phase three-wire system;

[0011] Wherein, the S4 is specifically:

[0012] S4.1: Set the grid phase voltage in the three-phase three-wire system , , The mathematical expression of is: ;

[0013] Among them, the U p is the positive sequence component of the phase voltage, the U N is the negative sequence component of the phase voltage; ω is the fundamental angular frequency of the phase voltage in the three-phase three-wire system, t is the time, and θ is the phase angle of the negative sequence component of the phase voltage;

[0014] S4.2: Perform negative sequence PARK transformation on the grid phase voltage in the three-phase three-wire system to obtain the d-axis component u d~ , q-axis component u q~ ;

[0015] S4.3: For the d-axis component u d~ , q-axis component u q~ Perform filtering operation to obtain the DC component u d 、u q ;

[0016] S4.4: Perform PARK inverse transformation on the d-axis component and the q-axis component after the filtering operation, and add them to the zero-sequence voltage injected in S3 to obtain the voltage component , , The mathematical expression of

[0017] S4.5: The voltage component obtained in S4.4 , , The grid phase voltage , , Subtracting the two, an unbalanced voltage in the three-phase three-wire system is obtained, and an unbalanced current of the three-phase three-wire system is obtained according to the unbalanced voltage.

[0018] Preferably, in S1, the grid phase voltage of the three-phase three-wire system is collected by a voltage sensor in the grid.

[0019] Preferably, in S2, the output voltage of the three-phase three-wire system is obtained by a static VAR generator. , , and output current , , .

[0020] Preferably, the static VAR generator is a star-connected cascade type static VAR generator, and each phase of the star-connected cascade type static VAR generator is composed of a filter inductor L and n H-bridge units.

[0021] Preferably, according to the output current of the static VAR generator in the power grid , , and output voltage , , Whether the grid is vertical or not determines whether there is an unbalanced situation in the grid.

[0022] Preferably, in S3, the constraint condition of the injected zero-sequence voltage component is: by injecting the zero-sequence voltage component, the new voltage component in the static VAR generator is , , Perpendicular to the current component, the With current vertical; With current vertical; With current vertical.

[0023] Preferably, the , , The phase difference is also 120°.

[0024] Preferably, in S4.3, the u d~ 、u q~ Perform filtering operation.

[0025] Preferably, in S4.5, the voltage component obtained in S4.4 is , , The grid phase voltage , , Subtracting the voltage component obtained in S4.4 to obtain the unbalanced voltage in the three-phase three-wire system is as follows: , , The grid phase voltage , , Subtracting the unbalanced voltage in the three-phase three-wire system to obtain an expression for the unbalanced voltage in the three-phase three-wire system, and obtaining the unbalanced voltage in the three-phase three-wire system according to the expression for the unbalanced voltage in the three-phase three-wire system;

[0026] Obtaining the unbalanced current of the three-phase three-wire system according to the unbalanced voltage specifically includes: obtaining the unbalanced current according to the unbalanced voltage and the load in the three-phase three-wire system.

[0027] According to another aspect of the present invention, a device for detecting an unbalanced current is provided. The device adopts the above-mentioned method for detecting an unbalanced current. The device comprises:

[0028] The grid phase voltage acquisition module is used to collect the grid phase voltage of the three-phase three-wire system. , , ;

[0029] The grid imbalance judgment module is used to collect the output current of the static VAR generator in the three-phase three-wire system. , , and output voltage , , Determine whether the three-phase three-wire system is unbalanced, and if so, enter the zero-sequence voltage component injection module;

[0030] A zero-sequence voltage component injection module, used for injecting a zero-sequence voltage component into the three-phase three-wire system;

[0031] The unbalanced current calculation module is used to calculate the unbalanced current in the three-phase three-wire system.

[0032] The advantages and beneficial effects of the present invention are:

[0033] When detecting unbalanced current, the present invention transfers the neutral point of the SVG device by injecting a zero-sequence voltage component into the SVG device, thereby making the regenerated voltage vector perpendicular to the current vector, thereby realizing the detection of three-phase unbalanced current, and avoiding the situation where the SVG device fails to compensate for the unbalanced current under special circumstances. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] In order to more clearly illustrate the technical solutions in the present invention or the prior art, the drawings required for use in the description of the present invention or 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 work.

[0035] Figure 1 A flow chart of a method for detecting unbalanced current provided by an embodiment of the present invention;

[0036] Figure 2 A schematic diagram of a circuit topology structure of a star-connected cascade static VAR generator provided in an embodiment of the present invention;

[0037] Figure 3 A schematic diagram showing that the neutral point of an SVG device provided by an embodiment of the present invention is transferred from point O to point N;

[0038] Figure 4 A schematic diagram of a balanced three-phase three-wire system provided by an embodiment of the present invention;

[0039] Figure 5 A schematic diagram of an unbalanced three-phase three-wire system provided by an embodiment of the present invention. DETAILED DESCRIPTION

[0040] 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.

[0041] Embodiment 1: The unbalanced current detection method of this embodiment is as follows: Figure 1 As shown, the method includes:

[0042] S1: Collecting the grid phase voltage of the three-phase three-wire system , , ;

[0043] Wherein, the grid phase voltage of the three-phase three-wire system is collected by a voltage sensor in the grid;

[0044] S2: Collect the output current of static VAR generator in three-phase three-wire system , , and output voltage , , Determine whether the three-phase three-wire system is unbalanced, and if so, enter S3;

[0045] The static VAR generator (SVG) has the advantages of a wide operating range and fast adjustment speed. In addition, the static VAR generator (SVG) can use PWM technology and multi-level, multiplexing and other methods to control the content of harmonics, so that the compensation current is fully utilized, thereby achieving compensation for unbalanced current conditions. The static VAR generator (SVG) is the main choice for the power department to perform reactive power compensation. Therefore, a static VAR generator (SVG) is generally installed in a three-phase three-wire system. Therefore, this embodiment obtains the three-phase voltage of the three-phase three-wire system through the static VAR generator (SVG). , , and three-phase current , , ;

[0046] Furthermore, the static VAR generator (SVG) is a star-connected cascade type static VAR generator. Figure 2 The circuit topology of the star-connected cascade static VAR generator is shown in FIG. Figure 2 As shown, each phase of the star-connected cascade static VAR generator is composed of a filter inductor L and n H-bridge units;

[0047] The star-connected cascade static VAR generator also includes an A-phase link, a B-phase link and a C-phase link, and its main function is to make the output capacitive or inductive reactive current continuously adjustable through the voltage source converter connected in parallel to the system to provide reactive power compensation.

[0048] When the three-phase three-wire system is balanced, the output current of the static VAR generator in the power grid is , , and output voltage , , Vertical, that is Figure 4 As shown, if the three-phase three-wire system is unbalanced, the output current of the static VAR generator in the power grid , , and output voltage , , In a non-vertical state, such as Figure 5 As shown, therefore, according to the output current of the static VAR generator in the power grid , , and output voltage , , Whether the grid is vertical or not determines whether there is an unbalanced situation in the grid.

[0049] S3: injecting a zero-sequence voltage component into the three-phase three-wire system;

[0050] When the three-phase three-wire system is unbalanced, if there are more positive-sequence components and negative-sequence components in the grid voltage. If the star-connected cascaded SVG only performs positive-sequence control and the converter only outputs positive-sequence voltage, the negative-sequence current is equal to the grid negative-sequence voltage divided by the filter impedance. At this time, a very small negative-sequence voltage in the grid will generate a large negative-sequence current, which will cause the current of a certain phase of the SVG to be too large, and the device will start the overcurrent protection and exit operation, resulting in SVG compensation failure. Therefore, compared with the prior art, this embodiment injects a zero-sequence component into the static VAR generator to transfer the neutral point of the SVG device from point O to point N; Figure 3 FIG. 4 is a schematic diagram showing the transfer of the neutral point of the SVG device from point O to point N; Figure 3 As shown,

[0051] The constraint condition of the injected zero-sequence voltage component is: by injecting the zero-sequence voltage component, the new voltage component in the SVG , , perpendicular to the current component, that is, With current vertical; With current vertical; With current vertical;

[0052] Furthermore, since the output current of the star-connected cascaded SVG is three-phase symmetrical, the phase difference between the phase currents is 120°, so , , The phase difference is also 120°.

[0053] In this step, a zero-sequence voltage component is injected into the SVG device to shift the neutral point of the SVG device, thereby making the regenerated voltage vector perpendicular to the current vector, so as to achieve subsequent detection of three-phase unbalanced current.

[0054] S4: Calculating the unbalanced current in the three-phase three-wire system;

[0055] In this step, since a zero-sequence voltage component is injected into the three-phase three-wire system in S3, a new zero-sequence voltage component is generated by superimposing the original zero-sequence voltage component in the three-phase three-wire system. In this embodiment, an extraction operation is performed on the new zero-sequence voltage component for detecting unbalanced current.

[0056] Wherein, the S4 is specifically:

[0057] S4.1: Set the grid phase voltage in the three-phase three-wire system , , The mathematical expression of

[0058] Wherein, the mathematical expression is: ;

[0059] Among them, the U p is the positive sequence component of the phase voltage, the U N is the negative sequence component of the phase voltage; ω is the fundamental angular frequency of the phase voltage in the three-phase three-wire system, t is the time, and θ is the phase angle of the negative sequence component of the phase voltage;

[0060] Among them, U p and U N The grid phase voltage through S1 , , Calculated;

[0061] S4.2: Perform negative sequence PARK transformation on the grid phase voltage in the three-phase three-wire system to obtain the d-axis component u d~ , q-axis component u q~ ;

[0062] PARK transformation is a coordinate transformation that simplifies the operation analysis of power grid signals by projecting the a, b, c three-phase currents of the power grid onto the rotating direct axis (d axis), quadrature axis (q axis) and zero axis (0 axis) perpendicular to the dq plane. This transformation transforms the abc coordinate system into the dq coordinate system, where the d axis rotates synchronously with the rotor flux and the q axis is perpendicular to the d axis. Through this transformation, the three-phase current can be converted into a two-phase orthogonal current.

[0063] PARK transformation is a coordinate transformation that simplifies the operation analysis of power grid signals by projecting the a, b, c three-phase currents of the power grid onto the rotating direct axis (d axis), quadrature axis (q axis) and zero axis (0 axis) perpendicular to the dq plane. This transformation transforms the abc coordinate system into the dq coordinate system, where the d axis rotates synchronously with the rotor flux and the q axis is perpendicular to the d axis. Through this transformation, the three-phase current can be converted into a two-phase orthogonal current.

[0064] S4.3: For the d-axis component u d~ , q-axis component u q~ Perform filtering operation to obtain the DC component u d 、u q ;

[0065] Among them, in this step, the sliding filtering method is used to filter the u d~ 、u q~ Perform filtering to remove the AC component and obtain the DC component u d 、uq ;

[0066] S4.4: Perform PARK inverse transformation on the d-axis component and the q-axis component after the filtering operation, and add them to the zero-sequence voltage injected in S3 to obtain the voltage component , , The mathematical expression of

[0067] S4.5: The voltage component obtained in S4.4 , , The grid phase voltage , , Subtracting the two, an unbalanced voltage in the three-phase three-wire system is obtained, and an unbalanced current of the three-phase three-wire system is obtained according to the unbalanced voltage.

[0068] The voltage component obtained by S4.4 , , The grid phase voltage , , Subtract the voltage components obtained in S4.4 to obtain the unbalanced voltage in the three-phase three-wire system. , , The grid phase voltage , , Subtracting the unbalanced voltage in the three-phase three-wire system to obtain an expression for the unbalanced voltage in the three-phase three-wire system, and obtaining the unbalanced voltage in the three-phase three-wire system according to the expression for the unbalanced voltage in the three-phase three-wire system;

[0069] Obtaining the unbalanced current of the three-phase three-wire system according to the unbalanced voltage specifically includes: obtaining the unbalanced current according to the unbalanced voltage and the load in the three-phase three-wire system.

[0070] Embodiment 2: This embodiment is a device for detecting unbalanced current. The device adopts the unbalanced current detection method of embodiment 1. The device includes:

[0071] The grid phase voltage acquisition module is used to acquire the grid phase voltage of the three-phase three-wire system. , , ;

[0072] The grid imbalance judgment module is used to collect the output current of the static VAR generator in the three-phase three-wire system. , , and output voltage , , , determine whether there is an unbalance in the three-phase three-wire system, and if so, enter the zero-sequence voltage component injection module;

[0073] A zero-sequence voltage component injection module, used for injecting a zero-sequence voltage component into the three-phase three-wire system;

[0074] The unbalanced current calculation module is used to calculate the unbalanced current in the three-phase three-wire system.

[0075] Embodiment 3: This embodiment includes a computer-readable storage medium, on which a data processing program is stored, and the data processing program is executed by a processor to perform various steps of a crew handover method in embodiment 1.

[0076] It should be understood by those skilled in the art that the embodiments of this article can be provided as methods, devices (equipment), or computer program products. Therefore, this article can adopt the form of a complete hardware embodiment, a complete software embodiment, or an embodiment combining software and hardware aspects. Including but not limited to RAM, ROM, EEPROM, flash memory or other memory technology, CD-ROM, digital versatile disk (DVD) or other optical disk storage, magnetic cassette, tape, disk storage or other magnetic storage device, or any other medium that can be used to store desired information and can be accessed by a computer. In addition, it is well known to those of ordinary skill in the art that communication media generally contain computer-readable instructions, data structures, program modules, or other data in modulated data signals such as carrier waves or other transmission mechanisms, and may include any information delivery medium.

[0077] This document is described with reference to flowcharts and / or block diagrams of methods, apparatuses (devices), and computer program products according to the embodiments of this document. It should be understood that each process and / or block in the flowchart and / or block diagram, as well as the combination of processes and / or blocks in the flowchart and / or block diagram, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, a special-purpose computer, an embedded processor, or other programmable data processing device to generate a machine, so that the instructions executed by the processor of the computer or other programmable data processing device generate instructions for implementing the processes in the flowchart and / or block diagram. Figure 1 A process or multiple processes and / or boxes Figure 1 A device that provides the functions specified in a block or multiple blocks.

[0078] These computer program instructions may also be stored in a computer-readable memory capable of directing a computer or other programmable data processing device to operate in a specific manner, so that the instructions stored in the computer-readable memory produce an article of manufacture comprising an instruction device, which implements the process Figure 1 A process or multiple processes and / or boxes Figure 1 A step that specifies a function in one or more boxes.

[0079] The embodiments and / or implementation methods described above are only used to illustrate the preferred embodiments and / or implementation methods of the technology of the present invention, and are not intended to impose any formal restrictions on the implementation methods of the technology of the present invention. Any person skilled in the art may make some changes or modifications to other equivalent embodiments without departing from the scope of the technical means disclosed in the content of the present invention, but they should still be regarded as the technology or embodiments that are essentially the same as the present invention. Note that the above are only preferred embodiments of the present invention and the technical principles used. Those skilled in the art will understand that the present invention is not limited to the specific embodiments described here, and that various obvious changes, readjustments and substitutions can be made to those skilled in the art without departing from the scope of protection of the present invention. Therefore, although the present invention is described in more detail through the above embodiments, the present invention is not limited to the above embodiments, and may also include more other equivalent embodiments without departing from the concept of the present invention, and the scope of the present invention is determined by the scope of the appended claims.

Claims

1. A method for detecting unbalanced current, characterized in that: The method comprises the following steps: S1: Collect the grid phase voltage of the three-phase three-wire system , , ; S2: Collect the output current of static VAR generator in three-phase three-wire system , , and output voltage , , , determine whether the three-phase three-wire system is unbalanced, if so, enter S3; According to the output current of the static VAR generator in the power grid , , and output voltage , , Whether it is vertical to determine whether the power grid is unbalanced; S3: injecting a zero-sequence voltage component into the three-phase three-wire system; in S3, the constraint condition of the injected zero-sequence voltage component is: by injecting the zero-sequence voltage component, the new voltage component in the static VAR generator , , Perpendicular to the current component, the With current vertical; With current vertical; With current vertical; , , The phase difference is 120°; S4: Calculating the unbalanced current in the three-phase three-wire system; Wherein, the S4 is specifically: S4.1: Set the grid phase voltage in the three-phase three-wire system , , The mathematical expression of S4.2: Perform negative sequence PARK transformation on the grid phase voltage in the three-phase three-wire system to obtain the d-axis component u d~ , q-axis component u q~ ; S4.3: For the d-axis component u d~ , q-axis component u q~ Perform filtering operation to obtain the DC component u d 、u q ; S4.4: Perform PARK inverse transformation on the d-axis component and the q-axis component after the filtering operation, and add them to the zero-sequence voltage injected in S3 to obtain the voltage component , , The mathematical expression of S4.5: The voltage component obtained in S4.4 , , The grid phase voltage , , Subtract the voltage components obtained in S4.4 to obtain the unbalanced voltage in the three-phase three-wire system. , , The grid phase voltage , , Subtracting the unbalanced voltage in the three-phase three-wire system to obtain an expression for the unbalanced voltage in the three-phase three-wire system, and obtaining the unbalanced voltage in the three-phase three-wire system according to the expression for the unbalanced voltage in the three-phase three-wire system; Obtaining the unbalanced current of the three-phase three-wire system according to the unbalanced voltage specifically includes: obtaining the unbalanced current according to the unbalanced voltage and the load in the three-phase three-wire system.

2. The method for detecting unbalanced current according to claim 1, characterized in that: In S1, the grid phase voltage of the three-phase three-wire system is collected by a voltage sensor in the grid.

3. The method for detecting unbalanced current according to claim 1, characterized in that: In S2, the output voltage of the three-phase three-wire system is obtained by a static VAR generator. , , and output current , , .

4. The method for detecting unbalanced current according to claim 3, characterized in that: The static VAR generator is a star-connected cascade type static VAR generator, and each phase of the star-connected cascade type static VAR generator is composed of a filter inductor L and n H-bridge units.

5. The method for detecting unbalanced current according to claim 1, characterized in that: In S4.3, the sliding filter method is used to filter the u d~ 、u q~ Perform filtering operation.

6. An unbalanced current detection device, the device adopting the unbalanced current detection method according to any one of claims 1 to 5, characterized in that: The device comprises: The grid phase voltage acquisition module is used to collect the grid phase voltage of the three-phase three-wire system. , , ; The grid imbalance judgment module is used to collect the output current of the static VAR generator in the three-phase three-wire system. , , and output voltage , , , determine whether there is an unbalance in the three-phase three-wire system, and if so, enter the zero-sequence voltage component injection module; A zero-sequence voltage component injection module, used for injecting a zero-sequence voltage component into the three-phase three-wire system; The unbalanced current calculation module is used to calculate the unbalanced current in the three-phase three-wire system.

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