DC line fault location method based on series hybrid circuit breaker

Abstract

The present invention discloses a DC line fault location method based on a series hybrid circuit breaker, comprising the following steps: S01: measuring the current limiting inductor voltage of the series hybrid circuit breaker in the zero-limit modulation stage V L3 , get the current limiting inductor voltage V L3 The maximum value V L3max With minimum value V L3min ; S02: According to the proposed fault location algorithm, combined with the obtained V L3max and V L3min , calculate the line parasitic inductance L line ; S03: Based on the approximately uniform distribution of DC line impedance, combined with the calculated line parasitic inductance L line , thereby determining the distance to the fault point. The present invention can achieve accurate positioning of the fault point and has the advantage of being able to withstand high transition resistance.

Description

Technical Field

[0001] The present invention relates to direct current (DC) grid fault location, and in particular to a DC line fault location method based on a series hybrid circuit breaker. Background Art

[0002] With the rapid development of distributed power sources such as photovoltaics and energy storage, DC distribution networks, with their advantages of low line losses, large power supply capacity, high power quality, flexible control, and strong ability to accommodate new energy sources, have garnered widespread attention. However, when a short-circuit fault occurs in a DC distribution network, the fault current rises extremely quickly due to the low system inertia, requiring DC circuit breakers to quickly interrupt the fault. Furthermore, the fault current reaches the converter blocking threshold very quickly. Once the converter station is blocked, it becomes difficult to collect effective fault information within the DC distribution network, making fault location difficult.

[0003] At present, the fault location of DC distribution networks mostly draws on the location algorithms of AC distribution networks, which are mainly divided into two categories: fault analysis method and signal injection method. The fault analysis method extracts the transient characteristics at the moment of the fault, calculates the fault line parameters based on the equivalent circuit topology after the fault, and then solves the fault distance to achieve fault location. The patent "A traveling wave ranging method and system taking into account the changes in comprehensive parameters" is based on the principle of traveling wave protection. It accurately measures the arrival time of the fault surge current at each end converter through wavelet transform, and determines the fault location based on the time difference measured at each converter station. This method has extremely high performance requirements for data acquisition tools and is limited in use in medium and low voltage short-distance DC distribution networks. Literature

[0004] "Short-Circuit and Ground Fault Analyses and Location in VSC-Based DC Network Cables" analyzes the distance characteristics of fault overcurrent values ​​and the critical time of the diode freewheeling effect, and performs online fault location during the fault transient process in the AC grid feed phase. The location accuracy is closely related to the fault loop resistance and is easily affected by transition resistance interference.

[0005] The signal injection method uses controllable or external components in the DC distribution network to inject a controllable signal into the fault line, locating the fault based on the fault response characteristics. The patent "A Method and System for Fault Location Using the Frequency Difference Ratio of Two-Terminal Traveling Waves in DC Transmission Lines" injects current into the DC distribution network using an external probe and calculates the fault distance based on the frequency response characteristics of the fault current. This method requires current injection after the fault is cleared, making it an offline fault location algorithm. The document "Embedded Fault Location in DC Microgrid Systems Based on a Lock-In Amplifier" injects duty cycle disturbances into the DC grid using a lock-in amplifier. The dq impedance components are calculated based on the output voltage and current response characteristics, thereby estimating the fault distance. This method is an online fault location algorithm, but small injected disturbances may affect the accuracy of protection. These methods can accurately achieve fault location in specific scenarios, but they require additional auxiliary circuitry or rely on complex algorithms to extract fault information. Furthermore, since DC distribution line faults, especially single-pole grounding faults, can occur through a large transition resistance, the fault location accuracy of these methods is highly susceptible to the transition resistance at the fault point. Summary of the Invention

[0006] In view of the defects of the prior art, the present invention provides a DC line fault location method based on a series hybrid circuit breaker.

[0007] A DC line fault location method based on a series hybrid circuit breaker includes the following steps: S01: measuring the current limiting inductor voltage V of the series hybrid circuit breaker in the zero-limit modulation stage L3 , get the current limiting inductor voltage V L3 The maximum value of V L3max With the minimum value V L3min ; S02: According to the proposed fault location algorithm, combined with the obtained V L3max With V L3min , calculate the line parasitic inductance L line ; S03: Based on the approximately uniform distribution of DC line impedance, combined with the calculated line parasitic inductance L line , thereby determining the distance to the fault point.

[0008] Optionally, the step S02 includes: L3max With V L3min Add them together to get V L3_total , in order to eliminate the influence of equivalent load resistance and short-circuit transition resistance on the estimated line inductance; the line parasitic inductance L line With V L3_total The following relationship is satisfied:

[0009]

[0010] Where, L3 is the inductance value of the current limiting inductor L3, U dc and U C are the DC system voltage and the voltage of the energy storage capacitor respectively, and n2 is the turns ratio of the secondary winding to the primary winding of the high turns ratio coupled inductor T2. In the zero-limit modulation stage, within one control cycle, the current-limiting inductor voltage V L3 The maximum value of V L3max With the minimum value V L3min The following relations are satisfied:

[0011]

[0012] Where H is the control bandwidth of hysteresis control, R L is the load resistance, R f is the transition resistance of short circuit fault.

[0013] Furthermore, the series hybrid circuit breaker includes a main branch and a voltage injection circuit, the voltage injection circuit includes a primary winding of a low-turns-ratio coupled inductor T1 and a high-turns-ratio coupled inductor T2 and an energy storage capacitor C, and the main branch includes a secondary winding of a low-turns-ratio coupled inductor T1 and a high-turns-ratio coupled inductor T2 connected in series; when the main branch current exceeds the protection threshold, the voltage injection circuit is turned on, the energy storage capacitor C discharges, and a voltage with a polarity opposite to that of the DC system is coupled on the main branch side through the coupled inductors T1 and T2; the main branch current is zero-limited modulated by the voltage injection circuit; the voltage injection circuit also includes fully-controlled power devices Q1 and Q2, and freewheeling diodes D1 and D2; when the main branch current drops to 0, the series hybrid circuit breaker enters the zero-limited modulation stage, Q1 is turned on, and Q2 controls the high-frequency switch based on the hysteresis loop.

[0014] The present invention provides a DC fault line protection method based on a series hybrid circuit breaker. This method leverages the current characteristics of the series hybrid circuit breaker in the zero-limited modulation mode to accurately locate faults. This method offers the advantage of withstanding high transition resistance and eliminates the need for additional auxiliary injection circuits and complex fault location algorithms. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 It is the topology of the series hybrid circuit breaker in this application;

[0016] Figure 2 It is a schematic diagram of a series hybrid circuit breaker interrupting a short circuit fault in the present application;

[0017] Figure 3 It is the equivalent circuit of the series hybrid circuit breaker in the fast interruption stage in this application;

[0018] Figure 4 It is the equivalent circuit of the series hybrid circuit breaker in the zero-limit modulation stage in this application;

[0019] Figure 5 This is the implementation step of DC line fault location in this application;. DETAILED DESCRIPTION

[0020] To make the above-mentioned objects, features, and advantages of the present invention more readily apparent, the following detailed description of specific embodiments of the present invention is provided in conjunction with the accompanying drawings, so that the above-mentioned and other objects, features, and advantages of the present invention will become more apparent. Like reference numerals denote like parts throughout the drawings. The drawings are not drawn to scale; emphasis is placed on illustrating the subject matter of the present invention.

[0021] The terms and words used in the following description and claims are not limited to the bibliographical meanings, but are merely used by the inventor to enable a clear and consistent understanding of the invention. Therefore, it will be apparent to those skilled in the art that the following description of various embodiments of the invention is provided for illustration purposes only and not for the purpose of limiting the invention as defined by the appended claims and their equivalents.

[0022] It should be understood that the singular forms "a," "an," and "the" include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to a "module" includes reference to one or more of such modules. The advantages and features of the present invention and methods of accomplishing the same may be more readily understood by reference to the detailed description of the embodiments below and the accompanying drawings. However, the present invention may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete and will fully convey the concepts of the invention to those skilled in the art.

[0023] The topology of series hybrid circuit breaker is as follows Figure 1 As shown, it mainly includes the main branch and voltage injection circuit, where T1 and T2 are low-turns-ratio coupled inductors and high-turns-ratio coupled inductors respectively. The main branch includes the secondary windings of T1 and T2 connected in series, mechanical switch S1 and current-limiting inductor L3. The voltage injection circuit includes the primary windings of T1 and T2, energy storage capacitor C, fully controlled power devices Q1 and Q2, and freewheeling diodes D1 and D2. L is the load resistance, R f is the transition resistance of short circuit fault, L line is the line inductance of the system. dc and U C are the DC system voltage and the voltage of the energy storage capacitor respectively.

[0024] The schematic diagram of a series hybrid circuit breaker interrupting a short circuit fault is as follows: Figure 2 As shown in the figure, when the main branch current exceeds the protection threshold, the voltage injection circuit activates, rapidly discharging the energy storage capacitor. This generates a voltage with opposite polarity to the DC system voltage on the main branch side through the coupled inductors (T1 and T2), forcing the main branch current to drop rapidly. This phase is called the rapid interruption phase. Simultaneously, the voltage injection circuit performs zero-limit modulation on the main branch current, modulating it into a near-zero high-frequency AC ripple. This expands the mechanical switch's operating window and active fault detection window. This phase is called the zero-limit modulation phase. A detailed analysis of the rapid interruption and zero-limit modulation phases is provided below.

[0025] When the series hybrid circuit breaker is in the fast interruption stage, Q1 and Q2 are turned on, and its equivalent circuit is as follows: Figure 3 According to Kirchhoff's law, the rate of change of the main branch current can be expressed as

[0026]

[0027] When the injection voltage of the coupled inductor (U L12 +U L22 ) is greater than the DC system voltage, that is,

[0028] U L12 +U L22 =n1U C +n2U C >U dc (1.2)

[0029] Where n1 and n2 are the turns ratios of the secondary winding to the primary winding of T1 and T2, respectively. Substituting equation (1.2) into equation (1.1), we obtain di2 / dt < 0. Therefore, this phase forces the main branch current to decrease rapidly.

[0030] When the main branch current drops to 0, the series hybrid circuit breaker enters the zero-limit modulation stage, Q1 is turned on, and Q2 controls the high-frequency switch based on the hysteresis loop. Its equivalent circuit is as follows: Figure 4 As shown. At this stage, the injected voltage (U L12 +U L22 ) consists of two high-frequency modulated voltage levels that satisfy the following equations

[0031]

[0032] According to equations (1.1) and (1.3), when the injected voltage is at level 1, di2 / dt < 0, and the main branch current decreases; when the injected voltage is at level 0, di2 / dt > 0, and the main branch current increases. During the zero-limit modulation stage, the injected voltage level switches between levels 1 and 2 at a high frequency, modulating the main branch current into a near-zero high-frequency AC ripple, thereby extending the mechanical switching window.

[0033] In summary, series hybrid circuit breakers achieve ultra-fast fault interruption by injecting reverse polarity voltage through coupled inductors without commutating the fault current. Furthermore, the main branch contains only low-impedance coupled inductor windings, resulting in low conduction losses. Furthermore, by modulating the fault current to zero, they achieve zero-current switching, similar to mechanical switches. Therefore, with its numerous advantages, series hybrid circuit breakers exhibit significant potential for DC grid fault protection.

[0034] It is worth noting that according to formula (1.1), under different line inductance L line Under these conditions, the slope of the main branch current (di2 / dt) will vary to a certain extent, meaning that series hybrid circuit breakers have the potential to identify system fault characteristics. However, if the line inductance is obtained based on the current slope of the main branch circuit, the following difficulties will arise. First, the main branch current contains a large amount of noise. If the slope of the main branch current is obtained by differentiation, the noise will be further amplified, making it difficult to obtain an accurate slope of the main branch circuit. Second, the slope of the main branch current is also greatly affected by the equivalent load resistance and short-circuit transition resistance. Therefore, it is difficult to infer the line inductance based solely on the slope of the main branch current.

[0035] The terminal voltage V of the current limiting inductor L3 in the series hybrid circuit breaker L3 It is directly proportional to the slope of the main branch current and satisfies the following formula

[0036]

[0037] According to formula (1.4), by detecting V L3 The slope of the main branch current (di² / dt) can be indirectly obtained. This method eliminates the need to differentiate the fault current and avoids the problem of increased noise. Based on this, an algorithm for fault location is proposed by detecting the voltage of the current-limiting inductor. The detailed analysis is as follows.

[0038] Combining equations (1.1), (1.3) and, we can see that the series hybrid circuit breaker operates in the zero-limit modulation stage. In one control cycle, V L3 The maximum and minimum values ​​of

[0039]

[0040] Where H is the control bandwidth of hysteresis control. According to the characteristics of formula (1.5), V L3max With V L3min Add them together to eliminate the influence of equivalent load resistance and short-circuit transition resistance on the estimated line inductance. L3max With V L3min The sum is defined as V L3_total , satisfying the following relationship

[0041]

[0042] Where, L3, U dc , n2 and U C These are the design parameters of the DC circuit breaker, so we only need to measure V L3_total , you can get the line inductance L line .

[0043] The specific implementation steps of the DC line fault protection method proposed in the present invention are as follows: Figure 5 As shown, there are mainly the following steps:

[0044] S01: Measure the current-limiting inductor voltage V of the series hybrid circuit breaker during the zero-limit modulation stage L3 , get the current limiting inductor voltage V L3 The maximum value of V L3max With the minimum value V L3min ;

[0045] S02: According to the proposed fault location algorithm, combined with the obtained V L3max With V L3min , calculate the line parasitic inductance L line ;

[0046] S03: Based on the approximately uniform distribution of DC line impedance and the calculated line parasitic inductance L line , thereby determining the distance to the fault point.

[0047] A DC line fault location method based on a series hybrid circuit breaker indirectly obtains the fault current slope by detecting the voltage of the current-limiting inductor, avoiding the errors caused by derivative calculations. This method then determines the line parasitic inductance and accurately locates the fault point. Furthermore, this method tolerates high transition resistances and eliminates the need for auxiliary circuitry or complex distance measurement algorithms.

[0048] Although the technology has been illustrated and described with respect to one or more embodiments, changes and / or modifications may be made to the illustrated examples without departing from the spirit and scope of the appended claims. In particular, with respect to the various functions performed by the components or structures (assemblies, devices, circuits, systems, etc.) described above, terms used to describe such components (including references to "means") are intended to correspond to any component or structure that performs the designated function of the described component (e.g., functionally equivalent), even if not structurally equivalent to the disclosed structure that performs the function in the example embodiments described herein, unless otherwise indicated. In addition, although a particular feature may have been disclosed with respect to one embodiment among several embodiments, such feature may be combined with one or more other features in other embodiments as may be desired and advantageous for any given or particular application. Furthermore, to the extent that the terms "comprising," "including," "having," "having," "containing," or variations thereof are used in the detailed description or claims, such terms are intended to be inclusive in a manner similar to the term "comprising."

[0049] In the above description, many specific details are set forth in order to fully understand the present invention. However, the above description is only a preferred embodiment of the present invention. The present invention can be implemented in many other ways different from those described herein, so the present invention is not limited to the specific implementation disclosed above. At the same time, any person skilled in the art can make many possible changes and modifications to the technical solution of the present invention using the methods and technical contents disclosed above without departing from the scope of the technical solution of the present invention, or modify it into an equivalent embodiment of equivalent changes. Any simple modification, equivalent change and modification made to the above embodiment based on the technical essence of the present invention without departing from the content of the technical solution of the present invention still falls within the scope of protection of the technical solution of the present invention.

Claims

1. A DC line fault location method based on a series hybrid circuit breaker, characterized in that: The following steps are involved: S01: Measure the current-limiting inductor voltage of the series hybrid circuit breaker during the zero-limit modulation stage V L3 , get the current limiting inductor voltage V L3 The maximum value V L3max With minimum value V L3min ; S 02: According to the proposed fault location algorithm, combined with the obtained V L3max and V L3min , calculate the line parasitic inductance L line ; S03: Based on the approximately uniform distribution of DC line impedance and the calculated line parasitic inductance L line , thereby determining the distance to the fault point.

2. The DC line fault location method according to claim 1, characterized in that: The step S02 includes: V L3max and V L3min Add together to get V L3_total , in order to eliminate the influence of equivalent load resistance and short-circuit transition resistance on the estimated line inductance.

3. The DC line fault location method according to claim 2, characterized in that: The line parasitic inductance L line and V L3_total The following relationship is satisfied: Where, L 3 is the current limiting inductor L 3 inductance value, U dc and U C are the DC system voltage and the voltage of the energy storage capacitor respectively, n 2 is a high turns ratio coupled inductor T The turns ratio of the secondary winding to the primary winding is 2.

4. The DC line fault location method according to claim 3, characterized in that: In the zero-limit modulation stage, within one control cycle, the current-limiting inductor voltage V L3 The maximum value V L3max With minimum value V L3min The following relations are satisfied: Where, H is the control bandwidth of hysteresis control, R L is the load resistance, R f is the transition resistance of short circuit fault.

5. The DC line fault location method according to claim 1, characterized in that: The series hybrid circuit breaker includes a main branch and a voltage injection circuit, wherein the voltage injection circuit includes a low turns ratio coupled inductor T 1 and high turns ratio coupled inductor T 2 primary winding and energy storage capacitor C , the main branch includes a low turns ratio coupled inductor in series T 1 and high turns ratio coupled inductor T 2's secondary winding.

6. The DC line fault location method according to claim 5, characterized in that: When the main branch current exceeds the protection threshold, the voltage injection circuit is turned on, the energy storage capacitor C discharges, and a voltage with a polarity opposite to that of the DC system is generated on the main branch side through coupling inductors T1 and T2.

7. The DC line fault location method according to claim 6, characterized in that: The main branch current is subjected to zero-limit modulation by the voltage injection circuit.

8. The DC line fault location method according to claim 5, characterized in that: The voltage injection circuit also includes a fully controlled power device Q 1 and Q 2, and freewheeling diode D 1 and D 2.

9. The DC line fault location method according to claim 8, characterized in that: When the main branch current drops to 0, the series hybrid circuit breaker enters the zero-limit modulation stage. Q 1 conduction, Q 2. Hysteresis-based control of high-frequency switching.

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