Active filter protection method and system

Abstract

The present invention discloses an active filter protection method, comprising: obtaining the current flowing through the capacitor branch of the filter under normal circumstances; obtaining the action value of the double-ended differential protection based on the capacitor branch current; and determining whether to activate the differential protection based on the relationship between the action value and the action range. The present invention can protect the entire filter, eliminating dead zones in the protection range and operating conditions. Simultaneously, the use of a two-side differential algorithm provides stronger braking for out-of-zone faults and higher sensitivity for in-zone faults. Compared with multi-side differential protection, this method significantly improves the sensitivity and reliability of the protection, reduces the computational complexity and difficulty of setting coordination, and has wide applicability and high practicality.

Description

Technical Field

[0001] The present invention belongs to the technical field of filter fault identification and protection, and in particular relates to an active filter protection method and system. Background Art

[0002] Active power filters (APFs) are a new type of electrical device based on power electronics technology, capable of operating in parallel with harmonic sources in the power grid. They monitor harmonic currents in the grid and filter them according to set filtering targets. They filter harmonics by injecting currents equal in magnitude and opposite in phase to the harmonic currents of each frequency. This superior dynamic performance makes active filters more precise and reliable than passive filters, and they can simultaneously filter multiple and higher-order harmonics.

[0003] Capacitors are a crucial component of active power filters. The voltages they withstand can easily damage them, leading to avalanches and explosions. To effectively protect these capacitors, they are typically divided into four equal-sized groups connected using H-type wiring. To protect these capacitors, the filter is equipped with unbalance protection, which increases the computational complexity and setting coordination. Normally, since the four groups of capacitors have identical capacitive reactance, no current flows through the central bridge wire. However, if a capacitor breaks down or a fuse blows, an unbalanced current flows through the bridge, allowing capacitor failure detection. A problem with relying on unbalanced current is that the capacitor voltage varies with the system's operating mode and the number of AC filters in operation, causing the unbalanced current flowing through the bridge branches to change accordingly. This affects the sensitivity of the capacitor unbalance protection. Differential protection alone cannot detect capacitor failures. Summary of the Invention

[0004] In order to solve the problems existing in the prior art, the present invention provides an active filter protection method, which can implement differential protection for the filter.

[0005] The technical problem to be solved by the present invention is achieved through the following technical solutions:

[0006] In a first aspect, a method for protecting an active power filter is provided, comprising:

[0007] Obtain the current flowing through the capacitor branch of the filter under normal circumstances;

[0008] The action value of the double-terminal differential protection is obtained according to the capacitor branch current;

[0009] Determine whether to start differential protection based on the relationship between the action amount and the action area.

[0010] In combination with the first aspect, further, the current flowing through the capacitor branch of the filter under normal circumstances is obtained by formula (1):

[0011]

[0012] in, is the current flowing through the capacitor branch under normal circumstances, is the measured voltage of the capacitor branch, X c1 It is the equivalent capacitive reactance of the capacitor under normal circumstances.

[0013] In combination with the first aspect, further, the action amount of the double-terminal differential protection obtained according to the capacitor branch current includes:

[0014] According to Kirchhoff's law and the current of the capacitor branch, we can get formula (2):

[0015]

[0016] in, are the transformer currents flowing through the upper and lower ends of the filter, is the current flowing through the capacitor branch under normal circumstances;

[0017] The capacitance of the filter is regarded as the equivalent distributed capacitance C in the line differential protection m and C n , let C m =C n , we can get

[0018]

[0019]

[0020] in, The current flowing through the equivalent capacitor C m 、C n The current, for Subtract the current flowing through the equivalent capacitance C m The current component following the current, for minus the current flowing through the equivalent capacitance C n The current component following the current;

[0021] According to Kirchhoff's law, we can get equations (5) and (6):

[0022]

[0023]

[0024] According to the differential protection principle, we can get

[0025]

[0026]

[0027] Among them, I op is the differential protection action quantity, I res The braking amount.

[0028] In combination with the first aspect, further, determining whether to start the differential protection according to the relationship between the action amount and the action area includes:

[0029] When the differential protection action quantity satisfies the action equation, the differential protection is started. The braking equation is:

[0030]

[0031] Among them, I res.0 is the inflection point current, K is the slope, I set.0 is the minimum starting current.

[0032] In a second aspect, an active filter protection system is provided, comprising:

[0033] A current acquisition module, used to obtain the current flowing through the capacitor branch of the filter under normal circumstances;

[0034] A protection action quantity acquisition module is used to obtain the action quantity of the double-terminal differential protection according to the capacitor branch current;

[0035] The protection module is used to determine whether to start differential protection based on the relationship between the action amount and the action area.

[0036] In a third aspect, an active filter protection system is provided, comprising a memory and a processor;

[0037] The memory is used to store instructions;

[0038] The processor is configured to operate according to the instructions to execute the steps of the method according to any one of the first aspects.

[0039] Beneficial effects: The present invention utilizes the high sensitivity and high reliability of double-end differential protection and the calculation method of line capacitance current compensation, regards the capacitor as the equivalent distributed capacitance in the line, calculates the current value flowing through the capacitor by the nominal parameters and voltage of the capacitor, and then fixes the compensation in the differential current. When the capacitor fails, the capacitor parameters change, the capacitor branch current is not equal to the normal operating current value, and the differential protection will have a differential current. The differential protection can reflect the capacitor failure. Therefore, the differential protection using this method can complete the protection of the entire filter, and there is no dead zone in the protection range and working conditions. At the same time, the two-side differential algorithm is adopted, which has stronger braking performance in the event of an out-of-zone fault and higher sensitivity in the event of an in-zone fault. The sensitivity and reliability of the original multi-side differential protection are greatly improved, the calculation amount and the difficulty of setting and coordination are reduced, and it has wide applicability and high practicality. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Figure 1 A schematic diagram of the normal operation of the active filter in the present invention;

[0041] Figure 2 This is the equivalent circuit diagram of capacitance current compensation in the present invention;

[0042] Figure 3 This is a schematic diagram of a grounding fault in an active filter of the present invention;

[0043] Figure 4 Schematic diagram of the action area in the present invention. DETAILED DESCRIPTION

[0044] In order to further illustrate the technical features and effects of the present invention, the present invention is further described below with reference to the accompanying drawings and specific implementation methods.

[0045] As attached Figure 1-4 As shown, a specific active filter protection method includes the following steps:

[0046] Step 1: Calculate the current flowing through the capacitor branch under normal operating conditions using the capacitor nominal parameters and the branch voltage collected by the voltage transformer PT, as shown in Equation 1.

[0047]

[0048] in, is the current flowing through the capacitor branch under normal circumstances, is the measured voltage of the capacitor branch, X c1 It is the equivalent capacitive reactance of the capacitor under normal circumstances.

[0049] Reference Figure 1 , the current transformers Tm and Tn in the AC filter form a differential protection. Based on Kirchhoff's law, the following equation can be listed, as shown in formula (2):

[0050]

[0051] in, are the currents flowing through the transformers Tm and Tn at the upper and lower ends of the filter respectively, is the current flowing through the capacitor branch under normal circumstances.

[0052] Step 2: Obtain the action value of the double-ended differential protection

[0053] Refer to the calculation method of double-terminal differential protection and line capacitance current compensation, and regard the capacitance as the equivalent distributed capacitance in the line differential protection. Figure 2 The following equation can be listed

[0054]

[0055]

[0056] in, The current flowing through the equivalent capacitor C m 、C n The current, for Subtract the current flowing through the equivalent capacitance C m The current component following the current, for Subtract the current flowing through the equivalent capacitance C n The current component following the current.

[0057] In the presence of capacitive current, Subtract the corresponding capacitor current from the current at both ends You can get Thus, Kirchhoff's law can be satisfied. Compensate the capacitor current of the active filter to eliminate its influence on the protection. It constitutes differential protection and can accurately cut off faults.

[0058]

[0059]

[0060] Adopt 2-stage differential protection, and set the action quantity of differential protection to be I op , the braking amount is I res

[0061]

[0062]

[0063] Step 3: Differential protection

[0064] The minimum starting current setting is like Figure 4 As shown, I op , I res If it falls within the differential action area, the protection is turned on, that is, I op , I res If the action equation shown in formula (9) is satisfied, it means that it falls within the action area.

[0065]

[0066] Among them, I res.0 is the inflection point current, K is Figure 4 Middle I res.0 The initial slope, I set.0 is the minimum starting current.

[0067] When the active filter is operating normally or an out-of-zone fault occurs, The theoretical value is 0, through reasonable adjustment To prevent the protection from false operation. Figure 3 As shown, there are

[0068]

[0069]

[0070] Protection can determine faults. is the fault current. Compared with the braking current required for multi-side differential The braking current is smaller and the protection is more sensitive and reliable.

[0071] When a capacitor fails, the capacitive reactance changes to X c2 , the branch current also changes to At this time there

[0072]

[0073] Using multi-side differential protection, the current flowing in and out is still equal and the fault cannot be determined. In this method, the differential protection still uses the nominal capacitive reactance X c1 Calculated current Differential protection will have differential current, which can be used to identify filter faults without unbalance protection.

[0074] Example 2

[0075] The present invention also provides an active filter protection system, comprising:

[0076] A current acquisition module, used to obtain the current flowing through the capacitor branch of the filter under normal circumstances;

[0077] A protection action quantity acquisition module is used to obtain the action quantity of the double-terminal differential protection according to the capacitor branch current;

[0078] The protection module is used to determine whether to start differential protection based on the relationship between the action amount and the action area.

[0079] Example 3

[0080] The present invention also provides an active filter protection system, comprising a memory and a processor;

[0081] The memory is used to store instructions;

[0082] The processor is configured to operate according to the instructions to execute the steps of the method.

[0083] Those skilled in the art will appreciate that the embodiments of the present application can be provided as methods, systems, or computer program products. Therefore, the present application can adopt the form of a complete hardware embodiment, a complete software embodiment, or an embodiment in combination with software and hardware. Moreover, the present application can adopt the form of a computer program product implemented on one or more computer-usable storage media (including but not limited to magnetic disk storage, CD-ROM, optical storage, etc.) that contain computer-usable program code.

[0084] The present application is described with reference to the flowcharts and / or block diagrams of the methods, devices (systems), and computer program products according to the embodiments of the present application. It should be understood that each process and / or box in the flowchart and / or block diagram, as well as the combination of the processes and / or boxes 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 produce a machine, so that the instructions executed by the processor of the computer or other programmable data processing device generate instructions for implementing the steps in the process. 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.

[0085] These computer program instructions may also be stored in a computer readable memory that can direct a computer or other programmable data processing device to work 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 The function specified in one or more boxes.

[0086] These computer program instructions can also be loaded onto a computer or other programmable data processing device so that a series of operational steps are executed on the computer or other programmable device to produce a computer-implemented process, thereby providing the instructions executed on the computer or other programmable device for implementing 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.

[0087] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to the above embodiments, ordinary technicians in the field should understand that the specific implementation methods of the present invention can still be modified or replaced by equivalents. Any modification or equivalent replacement that does not depart from the spirit and scope of the present invention should be covered by the scope of protection of the claims of the present invention.

Claims

1. An active filter protection method, characterized in that: include: Obtain the current flowing through the capacitor branch of the filter under normal circumstances; The action value of the double-terminal differential protection is obtained according to the capacitor branch current; Determine whether to start differential protection based on the relationship between the action amount and the action area; The action quantity of the double-terminal differential protection obtained according to the capacitor branch current includes: According to Kirchhoff's law and the current of the capacitor branch, we can get formula (2): （2） in, 、 are the transformer currents flowing through the upper and lower ends of the filter, is the current flowing through the capacitor branch under normal circumstances; Consider the filter capacitance as the equivalent distributed capacitance in line differential protection and ,make , we can get: （3） （4） in, 、 The equivalent capacitance 、 The current, for Subtract the equivalent capacitance The current component following the current, for Subtract the current flowing through the equivalent capacitance The current component following the current; According to Kirchhoff's law, we can get equations (5) and (6): （5） （6） According to the differential protection principle, we can get: （7） （8） in, is the differential protection action quantity, is the braking amount; The determining whether to start differential protection according to the relationship between the action amount and the action area includes: When the differential protection action quantity satisfies the action equation, the differential protection is started, and the braking equation is: （9） in, is the knee point current, is the slope, is the minimum starting current.

2. The active filter protection method according to claim 1, characterized in that: The current flowing through the capacitor branch of the filter under normal circumstances is obtained by formula (1): （1） in, is the current flowing through the capacitor branch under normal circumstances, is the measured voltage of the capacitor branch, It is the equivalent capacitive reactance of the capacitor under normal circumstances.

3. An active filter protection system, characterized in that: include: A current acquisition module, used to obtain the current flowing through the capacitor branch of the filter under normal circumstances; A protection action quantity acquisition module is used to obtain the action quantity of the double-terminal differential protection according to the capacitor branch current; The protection module is used to determine whether to start the differential protection based on the relationship between the action amount and the action area; The action quantity of the double-terminal differential protection obtained according to the capacitor branch current includes: According to Kirchhoff's law and the current of the capacitor branch, we can get formula (2): （2） in, 、 are the transformer currents flowing through the upper and lower ends of the filter, is the current flowing through the capacitor branch under normal circumstances; in, 、 are the transformer currents flowing through the upper and lower ends of the filter, is the current flowing through the capacitor branch under normal circumstances; Consider the filter capacitance as the equivalent distributed capacitance in line differential protection and ,make , we can get: （3） （4） in, 、 The equivalent capacitance 、 The current, for Subtract the equivalent capacitance The current component following the current, for Subtract the current flowing through the equivalent capacitance The current component following the current; According to Kirchhoff's law, we can get equations (5) and (6): （5） （6） According to the differential protection principle, we can get: （7） （8） in, is the differential protection action quantity, is the braking amount; The determining whether to start differential protection according to the relationship between the action amount and the action area includes: When the differential protection action quantity satisfies the action equation, the differential protection is started, and the braking equation is: （9） in, is the knee point current, is the slope, is the minimum starting current.

4. An active filter protection system, characterized in that: including memory and processor; The memory is used to store instructions; The processor is configured to operate according to the instructions to execute the steps of the method according to any one of claims 1 to 2.

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