A method for diagnosing the location of faults causing common-mode oscillations in flexible DC systems

Abstract

This invention proposes a method for diagnosing the location of faults causing common-mode oscillations in flexible DC systems. Relay protection devices capable of monitoring zero-sequence current and zero-sequence voltage are installed at the neutral point of the AC side of the converter at each end of the flexible DC network. When the fundamental component of the zero-sequence voltage is abnormally large, the third harmonic component of the zero-sequence voltage is abnormally small, and the third harmonic component of the zero-sequence current is abnormally small, the fault causing the common-mode oscillations in the flexible DC system is determined to be located on the AC side of the converter at that end. This invention is applicable to flexible DC networks containing multiple converter ends, where the neutral point of the AC side of the converter is grounded and a third harmonic modulation strategy is implemented. When common-mode oscillations occur in the flexible DC system, the location of the fault causing the common-mode oscillations can be quickly determined, and a signal can be sent to the control system to disconnect the converter at that end, eliminating the common-mode oscillations in the flexible DC system and ensuring the safe and reliable operation of the flexible DC grid.

Description

Technical Field

[0001] This invention relates to the field of power system fault location technology, and in particular to a method for diagnosing the location of faults causing common-mode oscillations in flexible DC systems. Background Technology

[0002] Flexible DC system converters often employ third harmonic modulation strategies to improve voltage utilization. When the flexible DC system does not use a pseudo-bipolar connection, the DC system will not directly generate common-mode current to ground. If the flexible DC system uses an impedance-grounded AC side neutral point of the converter, a ground fault will occur on the AC side of any valve, causing common-mode oscillation in the DC system. Furthermore, the fundamental components of zero-sequence voltage and current will appear on the AC side of all valves. Therefore, it is necessary to quickly identify the fault location causing the common-mode oscillation in the flexible DC system and isolate the faulty end to eliminate the common-mode oscillation and prevent it from affecting the safe operation of the entire DC power grid.

[0003] Existing conventional AC-side single-phase grounding protection strategies for pseudo-bipolar flexible DC systems are only applicable to flexible DC systems using pseudo-bipolar wiring and are not applicable to flexible DC systems with other grounding methods. Furthermore, diagnostic methods for DC-side grounding are also unsuitable for determining the fault location of common-mode oscillations in flexible DC systems. Summary of the Invention

[0004] The purpose of this invention is to provide a method for diagnosing the fault location causing common-mode oscillation in a two-terminal flexible DC network with a third harmonic modulation strategy and a neutral point grounded. This solution utilizes the difference in third harmonic voltage between the faulty and non-faulty ends under the third harmonic modulation strategy. After determining the fault location causing common-mode oscillation in the flexible DC system by measuring and calculating the third harmonic components of the zero-sequence voltage and zero-sequence current at the neutral point of the AC side of the converter, a signal is sent to the control system to disconnect the converter at that end, thereby eliminating the common-mode oscillation of the flexible DC system and enabling the DC grid to continue to operate safely and reliably.

[0005] This scheme installs relay protection devices at the neutral point of the AC side of the converter at each end of the flexible DC network, capable of monitoring zero-sequence current and zero-sequence voltage. When the fundamental component of the zero-sequence voltage is abnormally large, the third harmonic component of the zero-sequence voltage is small, and the third harmonic component of the zero-sequence current is small, the location of the common-mode oscillation fault in the flexible DC system is determined to be on the AC side of the converter at that end. This invention is applicable to flexible DC networks with multiple converter ends, where the neutral point of the AC side of the converter is grounded and a third harmonic modulation strategy is implemented. When common-mode oscillation occurs in the flexible DC system, the location of the fault causing the common-mode oscillation can be quickly determined, and a signal can be sent to the control system to disconnect the converter at that end, eliminating the common-mode oscillation of the flexible DC system and ensuring the safe and reliable operation of the flexible DC grid.

[0006] The present invention specifically adopts the following technical solution:

[0007] A method for diagnosing the location of a fault causing common-mode oscillation in a flexible DC system is characterized by installing a relay protection device at the neutral point of the AC side of the converter at each end of the flexible DC network, capable of monitoring zero-sequence current and zero-sequence voltage. When the fundamental component of the zero-sequence voltage is abnormally large, the third harmonic component of the zero-sequence voltage is small, and the third harmonic component of the zero-sequence current is small, the location of the fault causing the common-mode oscillation in the flexible DC system is determined to be on the AC side of the converter at that end.

[0008] Furthermore, the flexible DC grid is assumed to include several converter terminals, and the neutral point of the AC side of the converter is grounded, and it has a third harmonic modulation strategy.

[0009] The zero-sequence fundamental voltage, zero-sequence third harmonic voltage, and zero-sequence third harmonic current are obtained by measuring the neutral point on the AC side of each converter. If the fundamental component of the zero-sequence voltage at the neutral point on the valve side of the i-th terminal is obtained by measurement and calculation as U... 0(1) The third harmonic component of the zero-sequence voltage is U. 0(3) The third harmonic component of the zero-sequence current is I. 0(3) The following three criteria are used to diagnose whether the AC side of the i-th converter is the fault location causing the common-mode oscillation of the flexible DC system. When all three inequalities are satisfied, the AC side of the i-th converter is determined to be the fault location causing the common-mode oscillation of the flexible DC system:

[0010] Criterion 1: The fundamental component of the zero-sequence voltage is abnormally large, i.e.: |U 0(1) |>ε1;

[0011] Criterion 2: The third harmonic component of the zero-sequence voltage is too small, i.e.: |U 0(3) |<k1KU N ;

[0012] Criterion 3: And the third harmonic component of the zero-sequence current is relatively small, that is:

[0013] Where ε1 is the maximum error of the neutral point zero-sequence voltage under normal operating conditions, k1 is the reliability coefficient to prevent voltage misjudgment, K is the ratio of the third harmonic amplitude injected into the converter flexible DC system to the phase voltage amplitude, and U N Z represents the amplitude of the rated voltage on the AC side. 1o Z 2o These are the impedances on the neutral line of the AC side of this converter and the parallel equivalent impedances on the neutral lines of the AC side of the other converters, Z. L K is the equivalent impedance of the DC line, and k2 is the reliability coefficient to prevent misjudgment of current.

[0014] Further, the fundamental component of the zero-sequence voltage at the neutral point of the AC side of the converter at the i-th terminal measured and calculated is U 0(1) , the third harmonic component of the zero-sequence voltage is U 0(3) , and the third harmonic component of the zero-sequence current is I 0(3) . The specific method is as follows: Install a voltage sensor and a current sensor at the neutral point of the AC side of the converter. The polarity of the current sensor is positive from the ground to the neutral point; the sampled voltage and current are the zero-sequence voltage and zero-sequence current, and the fundamental component of the zero-sequence voltage at this terminal is U 0(1) , the third harmonic component of the zero-sequence voltage is U 0(3) , and the third harmonic component of the zero-sequence current is I 0(3) .

[0015] Further, the parallel equivalent impedance of the impedance on the neutral line of the AC side of the converters at other terminals is: If there are N terminals in addition to this terminal, the neutral line impedances are: Z1', Z'2... Z' n , is the parallel equivalent impedance of the other terminal. If N = 1, then Z 2o = Z1' is the parallel equivalent impedance of the other terminal.

[0016] Further, the value range of ε1 is 1% < ε1 < 10% of the zero-sequence voltage at the neutral point under normal operation.

[0017] Further, the value range of k1 is 0 < k1 < 1, and the value range of k2 is 0 < k2 < 1.

[0018] Compared with the prior art, when the present invention and its preferred solution are used to find the fault location that causes the common-mode oscillation of the flexible DC system, only the zero-sequence voltage and zero-sequence current at the neutral point of the AC side of the converter need to be collected, without using the DC side voltage and current. The fundamental component and the third harmonic component of the zero-sequence voltage and current can be used to diagnose which converter's AC side causes the common-mode oscillation of the flexible DC system. This solution can at least be applied to a two-terminal flexible DC system with a neutral point or an artificial neutral point grounded and a third harmonic modulation strategy. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The present invention will be further described in detail below in conjunction with the drawings and specific embodiments:

[0020] Figure 1 is a schematic diagram of the installation of the protection device in the method for diagnosing the fault location that causes the common-mode oscillation of the flexible DC system according to the embodiment of the present invention;

[0021] Figure 2 is a logic block diagram for performing fault protection in the method for diagnosing the fault location that causes the common-mode oscillation of the flexible DC system according to the embodiment of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0022] In the following, specific embodiments of this application will be described in detail with reference to the accompanying drawings. Based on these detailed descriptions, those skilled in the art will be able to clearly understand and implement this application. Without departing from the principles of this application, features from various embodiments can be combined to obtain new implementations, or certain features from some embodiments can be substituted to obtain other preferred implementations.

[0023] It should be noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the exemplary embodiments according to this application. As used herein, the singular form is intended to include the plural form as well, unless the context clearly indicates otherwise. Furthermore, it should be understood that when the terms "comprising" and / or "including" are used in this specification, they indicate the presence of features, steps, operations, devices, components, and / or combinations thereof.

[0024] To make the features and advantages of this patent more apparent and understandable, specific embodiments are provided below for detailed explanation:

[0025] It should be noted that, unless otherwise defined, the technical or scientific terms used in this invention should have the ordinary meaning understood by one of ordinary skill in the art to which this invention pertains. The terms "first," "second," and similar terms used in this invention do not indicate any order, quantity, or importance, but are merely used to distinguish different components. Terms such as "comprising" or "including" mean that the element or object preceding the word encompasses the elements or objects listed following the word and their equivalents, without excluding other elements or objects. Terms such as "connected" or "linked" are not limited to physical or mechanical connections, but can include electrical connections, whether direct or indirect. Terms such as "upper," "lower," "left," and "right" are used only to indicate relative positional relationships; when the absolute position of the described object changes, the relative positional relationship may also change accordingly.

[0026] like Figure 1 As shown, in the flexible DC grid, the AC side of the converter is connected to the AC grid via a converter transformer, and the DC side of the converter is connected to a DC bus, which is also connected to several other DC lines. The primary side of a voltage sensor is connected to the neutral point and ground of the AC side of the converter, while the secondary side is connected to the analog signal acquisition module of the relay protection device. A current sensor is installed on the neutral line of the AC side of the converter, and its secondary side is connected to the analog signal acquisition module of the relay protection device. The polarity of the current sensor is from ground to the transformer neutral point. The protection device can sample the neutral point zero-sequence voltage U0 and the neutral point zero-sequence current I0. The fundamental component U of the zero-sequence voltage at this terminal can be obtained using the Fourier algorithm. 0(1) Zero-sequence voltage third harmonic component U0(3) and the third harmonic component I of the zero-sequence current 0(3) .

[0027] The protection logic block diagram for diagnosing the location of common-mode oscillation faults in flexible DC systems is as follows: Figure 2 As shown. When no faults causing common-mode oscillations in the flexible DC system occur on the AC side of any converter in the flexible DC grid, no zero-sequence voltage fundamental component will appear. The amplitude of the zero-sequence voltage fundamental component |U| calculated by the relay protection is... 0(1) The error will not exceed the maximum measurement error, and there will be no misjudgment; when a fault occurs on the AC side of other converters in the flexible DC grid that causes common-mode oscillation in the flexible DC system, the U value at the neutral point of the AC side of this converter will be... 0(3) and I 0(3) The magnitude of U is related to the phase of the AC system at each end and the DC power flow, but U 0(3) and I 0(3) They will not both be zero simultaneously, therefore there will be no misjudgment. When a ground fault occurs on the AC side of the local converter, the voltage of the fault phase drops to zero, and the fundamental component of the zero-sequence voltage appears on the AC side, i.e., |U 0(1) |>ε1. Since the third harmonic component only flows in the zero-sequence network, according to the superposition principle, the third harmonic fault component of the three-phase zero-sequence voltage at the neutral point of the fault end cancels out the normal operation component. Therefore, the U at the neutral point of the fault side... 0(3) and I 0(3) Both are zero, i.e., |U 0(3) |<k1KU N ,

[0028] The embodiments of the present invention have been described in detail above with reference to the accompanying drawings, but the present invention is not limited to the described embodiments. For those skilled in the art, various changes, modifications, substitutions, and variations can be made to these embodiments without departing from the principles and spirit of the present invention, and these variations still fall within the protection scope of the present invention.

[0029] The system and method provided in this embodiment can be stored in a computer-readable storage medium in the form of code, implemented as a computer program, and the basic parameter information required for calculation can be input through computer hardware, and the calculation results can be output.

[0030] Those skilled in the art will understand that embodiments of the present invention can be provided as methods, systems, or computer program products. Therefore, the present invention can take the form of a completely hardware embodiment, a completely software embodiment, or an embodiment combining software and hardware aspects. Furthermore, the present invention can take the form of a computer program product embodied on one or more computer-usable storage media (including, but not limited to, disk storage, CD-ROM, optical storage, etc.) containing computer-usable program code.

[0031] This invention is described with reference to flowchart illustrations and / or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of the invention. It will be understood that each block of the flowchart illustrations and / or block diagrams, and combinations of blocks in the flowchart illustrations and / or block diagrams, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, special-purpose computer, embedded processor, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, generate instructions for implementing the flowchart illustrations and / or block diagrams. Figure 1 One or more processes and / or boxes Figure 1 A device that provides the functions specified in one or more boxes.

[0032] These computer program instructions may also be stored in a computer-readable storage medium that can direct a computer or other programmable data processing device to function in a particular manner, such that the instructions stored in the computer-readable storage medium produce an article of manufacture including instruction means, which are implemented in a process Figure 1 One or more processes and / or boxes Figure 1 The function specified in one or more boxes.

[0033] These computer program instructions may also be loaded onto a computer or other programmable data processing equipment to cause a series of operational steps to be performed on the computer or other programmable equipment to produce a computer-implemented process, thereby providing instructions that execute on the computer or other programmable equipment for implementing the process. Figure 1 One or more processes and / or boxes Figure 1 The steps of the function specified in one or more boxes.

[0034] 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, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the specific implementation of the present invention. Any modifications or equivalent substitutions that do not depart from the spirit and scope of the present invention should be covered within the scope of protection of the claims of the present invention.

[0035] This patent is not limited to the above-described preferred embodiment. Anyone can derive other methods for diagnosing the location of faults causing common-mode oscillations in flexible DC systems based on the teachings of this patent. All equivalent variations and modifications made within the scope of this patent application shall fall within the scope of this patent.

Claims

1. A method for diagnosing the location of a fault causing common-mode oscillation in a flexible DC system, characterized in that, Relay protection devices for monitoring zero-sequence current and zero-sequence voltage are installed at the neutral points of the AC sides of the converters at each end of the flexible DC network. When the fundamental component of the zero-sequence voltage is abnormally large, the third-harmonic component of the zero-sequence voltage is small, and the third-harmonic component of the zero-sequence current is small, it is determined that the fault location causing the common-mode oscillation of the flexible DC system is on the AC side of the converter at this end; Suppose the flexible DC power grid contains several converter terminals, and the neutral points of the AC sides of the converters are grounded, with a third-harmonic modulation strategy; The zero-sequence fundamental voltage, zero-sequence third harmonic voltage, and zero-sequence third harmonic current are obtained by measuring the neutral point on the AC side of each converter. If the fundamental component of the zero-sequence voltage at the neutral point on the valve side of the i-th terminal is obtained by measurement and calculation as U... 0(1) The third harmonic component of the zero-sequence voltage is U 0(3) The third harmonic component of the zero-sequence current is I. 0(3) The following three criteria are used to diagnose whether the AC side of the i-th converter is the fault location causing the common-mode oscillation of the flexible DC system. When all three inequalities are satisfied, the AC side of the i-th converter is determined to be the fault location causing the common-mode oscillation of the flexible DC system: Criterion 1: The fundamental component of the zero-sequence voltage is abnormally large, i.e.: |U 0(1) |>ε1; Criterion 2: The third harmonic component of the zero-sequence voltage is too small, i.e.: |U 0(3) |<k1KU N ; Criterion 3: And the third harmonic component of the zero-sequence current is relatively small, that is: Where ε1 is the maximum error of the neutral point zero-sequence voltage under normal operating conditions, k1 is the reliability coefficient to prevent voltage misjudgment, K is the ratio of the third harmonic amplitude injected into the converter flexible DC system to the phase voltage amplitude, and U N Z represents the amplitude of the rated voltage on the AC side. 1o Z 2o These are the impedances on the neutral line of the AC side of this converter and the parallel equivalent impedances on the neutral lines of the AC side of the other converters, Z. L K is the equivalent impedance of the DC line, and k2 is the reliability coefficient to prevent misjudgment of current.

2. The method for diagnosing the fault location causing common-mode oscillation in a flexible DC system according to claim 1, characterized in that: The fundamental component of the zero-sequence voltage at the neutral point of the AC side of the converter at the i-th terminal is obtained by measurement and calculation as U. 0(1) The third harmonic component of the zero-sequence voltage is U 0(3) The third harmonic component of the zero-sequence current is I. 0(3) The specific method is as follows: A voltage sensor and a current sensor are installed at the neutral point on the AC side of the converter. The polarity of the current sensor is positive, pointing from the ground to the neutral point. The sampled voltage and current are the zero-sequence voltage and zero-sequence current. The fundamental component of the local zero-sequence voltage is obtained using the Fourier algorithm as U. 0(1) The third harmonic component of the zero-sequence voltage is U 0(3) and the third harmonic component of the zero-sequence current is I 0(3) .

3. The method for diagnosing the fault location causing common-mode oscillation in a flexible DC system according to claim 1, characterized in that: The parallel equivalent impedance of the neutral line impedance on the AC side of the other converter terminals is as follows: If there are N terminals besides this terminal, the neutral line impedances are respectively: Let Z be the parallel equivalent impedance at the other end. If N = 1, then Z 2o =Z1' is the parallel equivalent impedance at the other end.

4. The method for diagnosing the fault location causing common-mode oscillation in a flexible DC system according to claim 1, characterized in that: The value range of ε1 is 1% < ε1 < 10% of the zero-sequence voltage at the neutral point under normal operation.

5. The method for diagnosing the fault location causing common-mode oscillation in a flexible DC system according to claim 1, characterized in that: The value range of k1 is 0 < k1 < 1, and the value range of k2 is 0 < k2 < 1.

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