Direct-current distribution line fault distance measuring device and method and application

Abstract

The invention provides a direct-current distribution line fault distance measuring device and method and application, and relates to the technical field of power system relay protection. The probe circuit comprises a probe circuit unit. The probe circuit unit comprises two groups of cascaded chopper circuits, and the first group of chopper circuit is formed by sequentially connecting a first capacitor C1, a first switch tube S1 and a first diode D1 in series. And the second group of chopper circuits is formed by sequentially connecting a second capacitor C2, a second switch tube S1 and a second diode D2 in series, and the cascade midpoint of the two groups of cascaded chopper circuits is grounded. According to the invention, full-automatic measurement is realized; the size is smaller, and the cost is lower; the required sampling frequency is low, and the method is simple and easy to implement; the method is easy to integrate in a relay protection device, can be used for repeated detection, is simple to operate, is convenient to use, effectively reduces errors in sampling calculation, and improves the fault positioning precision of a short-distance DC distribution line.

Description

Technical Field

[0001] The present invention relates to the technical field of relay protection of electric power systems, and in particular to a fault distance measuring device, method and application of a DC distribution line. Background Art

[0002] Relay protection technology for DC distribution networks has been a hot topic in DC system research in recent years. After a DC line fails, how to quickly and accurately locate the fault point is crucial for rapid maintenance of the DC system fault line and timely restoration of power supply. From the perspective of power supply reliability and safety of the power system, it is of great significance to study the fault location technology of DC distribution network lines.

[0003] At present, the existing fault distance measurement methods are mainly divided into three types: traveling wave method, transient method and injection method. The traveling wave method uses the time difference between the transmission and reflection of the traveling wave in the line to calculate the line fault distance. The principle of this method is relatively simple, but it requires a very high sampling rate, so it is often used for long-distance line fault distance measurement. The transient law calculates the fault distance based on the limited and complex transient quantity signal after the fault. Due to the influence of factors such as the sudden change and complexity of the transient stage of the fault, system noise, arc faults, etc., the positioning result may deviate greatly from the actual result, or GPS is required for data synchronization at both ends. The injection method usually injects a certain specific distance measurement signal into the fault line to locate the fault point, and the distance measurement accuracy is relatively high. The prior art has designed a fault distance measurement probe circuit, which uses the characteristics of the RLC or RL series circuit to calculate the fault distance, and has achieved certain results, but it is large in size, high in cost, and the operation process is cumbersome. Summary of the invention

[0004] The purpose of the present invention is to provide a DC distribution line fault distance measuring device that meets the requirements, has a low sampling frequency, a simple method, is easy to implement and easy to integrate into a relay protection device, can be operated repeatedly, and can effectively improve the positioning accuracy of short-distance DC distribution line faults.

[0005] To achieve the above object, the present invention is implemented through the following technical solutions:

[0006] A DC distribution line fault distance measuring device comprises a probe circuit unit; the probe circuit unit comprises two groups of cascaded chopper circuits, wherein the first group of chopper circuits is formed by a first capacitor C1, a first switch tube S1 and a first diode D1 connected in series in sequence, and the second group of chopper circuits is formed by a second capacitor C2, a second switch tube S1 and a second diode D2 connected in series in sequence, and the cascade midpoints of the two groups of cascaded chopper circuits are grounded.

[0007] Furthermore: the source of the first switch tube S1 and the cathode of the first diode D1 are connected to the positive pole of the DC line, and the drain of the second switch tube S2 and the anode of the second diode D2 are connected to the negative pole of the DC line.

[0008] Furthermore: according to the fault type (positive pole, negative pole or bipolar fault), the switch tube is controlled to emit a square wave voltage signal with a specific frequency and duty cycle.

[0009] Furthermore, the value range of the specific frequency f is: f>0, and the value range of the duty cycle k is: 0<k<1.

[0010] A DC distribution line fault distance measurement method is applied to any of the DC distribution line fault distance measurement devices described above, comprising: a probe circuit unit, which extracts the DC component and fundamental component of the voltage and current in sequence by sampling the voltage and current signals output by the probe circuit unit, and calculates the fault distance according to the fault distance measurement formula.

[0011] Furthermore: the fault distance measurement method extracts the voltage and current signals output by the sampling probe circuit unit, extracts the DC component and fundamental component of the voltage and current in sequence, and calculates the fault distance according to the fault distance measurement formula.

[0012] Further: the first fault distance measurement formula of the fault distance measurement method is:

[0013]

[0014] Where x1 and x2 are the line fault distances, U1 and I1 are the fundamental components of the voltage and current signals at the output of the probe circuit unit, U0 and I0 are the DC components of the voltage and current signals at the output of the probe circuit unit, f is the switching frequency of the switch S1 or S2, and l0 is the inductance per kilometer of the DC line.

[0015] Further: A fault distance measurement formula of the fault distance measurement method is:

[0016]

[0017] Where x1 and x2 are the line fault distances, U1 and I1 are the fundamental components of the voltage and current signals at the output of the probe circuit unit, and are the phase angles of the fundamental components of the voltage and current signals at the output end of the probe, f is the switching frequency of the switch tubes S1 and S2, and l0 is the inductance per kilometer of the DC line.

[0018] Furthermore: the calculation of the fault distance further includes: calculating a relative error of the fault distance, wherein the relative error is equal to a ratio of an absolute value of a difference between the fault distance and an actual fault distance to the actual fault distance.

[0019] An application of a DC distribution line fault distance measuring device in relay protection of a DC distribution system, wherein the fault distance measuring device is integrated in a relay protection device.

[0020] Compared with the prior art, the present invention has the following beneficial effects:

[0021] 1. The probe circuit unit designed in the present invention is composed of a capacitor, a diode and a switch tube, and is installed at one end of the DC line section. Compared with the existing module circuit containing inductor elements, it has a smaller size, lower cost, and realizes fully automatic measurement. There is no need to repeatedly open and close the switch for charging and discharging operations, and it is more convenient to use.

[0022] 2. The fault distance measurement method described in the present invention calculates the fault distance based on the DC component and fundamental component of the voltage and current, requires a low sampling frequency, and is simple and easy to implement.

[0023] 3. The present invention is easy to integrate into a relay protection device, can perform repeated detection multiple times, and can directly measure when a fault occurs without cutting off the circuit to isolate the fault area. The operation is simple and easy to use. The present invention adopts a method of repeated sampling and calculation to effectively reduce the errors in sampling and calculation, and improve the positioning accuracy of short-distance DC distribution line faults. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 is a schematic diagram of a probe circuit unit of the present invention;

[0025] Figure 2 It is a schematic diagram of an equivalent circuit of a DC line positive pole grounding fault of the present invention;

[0026] Figure 3 It is a schematic diagram of an equivalent circuit of a negative-pole grounding fault of a DC line according to the present invention;

[0027] Figure 4 It is a schematic diagram of a DC line bipolar fault equivalent circuit of the present invention;

[0028] Figure 5 A voltage waveform diagram and an amplitude-frequency response diagram output by the probe circuit unit of the present invention;

[0029] Figure 6 A current waveform diagram and an amplitude-frequency response diagram output by the probe circuit unit of the present invention;

[0030] Figure 7 The single-pole ground fault distance measurement result of the present invention;

[0031] Figure 8 is the bipolar short-circuit fault distance measurement result of the present invention;

[0032] In the figure:

[0033] C1, first capacitor; C2, second capacitor; D1, first diode; D2, second diode; S1, first switch tube; S2, second switch tube. DETAILED DESCRIPTION

[0034] The technical solution of the present invention will be clearly and completely described below in conjunction with the accompanying drawings.

[0035] like Figure 1 As shown, the present invention provides a DC distribution line fault distance measuring device, including a probe circuit unit; the probe circuit unit includes two groups of cascaded chopper circuits, wherein the first group of chopper circuits is formed by a first capacitor C1, a first switch tube S1 and a first diode D1 connected in series in sequence, and the second group of chopper circuits is formed by a second capacitor C2, a second switch tube S1 and a second diode D2 connected in series in sequence, and the cascade midpoint of the two groups of cascaded chopper circuits is grounded.

[0036] According to a specific embodiment of the present invention, the source of the first switch tube S1 and the cathode of the first diode D1 are connected to the positive pole of the DC line, and the drain of the second switch tube S2 and the anode of the second diode D2 are connected to the negative pole of the DC line.

[0037] According to a specific embodiment of the present invention, according to the fault type (positive pole, negative pole or bipolar fault), the switch tube is controlled to transmit a square wave voltage signal with a specific frequency and duty cycle.

[0038] According to a specific implementation of the present invention, the value range of the specific frequency f is: f>0, and the value range of the duty cycle k is: 0<k<1.

[0039] A DC distribution line fault distance measurement method is applied to any of the DC distribution line fault distance measurement devices described above, comprising: a probe circuit unit, which extracts the DC component and fundamental component of the voltage and current in sequence by sampling the voltage and current signals output by the probe circuit unit, and calculates the fault distance according to the fault distance measurement formula.

[0040] According to a specific implementation of the present invention, the first fault distance measurement formula of the fault distance measurement method is:

[0041]

[0042] Where x1 and x2 are the line fault distances, U1 and I1 are the fundamental components of the voltage and current signals at the output of the probe circuit unit, U0 and I0 are the DC components of the voltage and current signals at the output of the probe circuit unit, f is the switching frequency of the switch S1 or S2, and l0 is the inductance per kilometer of the DC line.

[0043] According to a specific implementation of the present invention, a fault distance measurement formula of the fault distance measurement method is:

[0044]

[0045] Where x1 and x2 are the line fault distances, U1 and I1 are the fundamental components of the voltage and current signals at the output of the probe circuit unit, and are the phase angles of the fundamental component of the voltage signal and the fundamental component of the current signal at the output end of the probe, respectively; f is the switching frequency of the switch tubes S1 and S2; and l0 is the inductance per kilometer of the DC line.

[0046] According to a specific embodiment of the present invention, the following steps are also included: by adjusting the duty cycle of the switching tube, the current amplitude output by the probe unit circuit is within the appropriate range of the current transformer, and repeated sampling and calculation are performed until the error between the calculation results is less than the threshold or reaches a preset maximum number of repetitions.

[0047] According to a specific embodiment of the present invention, the threshold value ranges from 0.1% to 1%, and the maximum number of repetitions ranges from 3 to 10 times.

[0048] An application of a DC distribution line fault distance measuring device in relay protection of a DC distribution system, wherein the fault distance measuring device is integrated in a relay protection device.

[0049] The working principle of the present invention is as follows:

[0050] like Figure 1 As shown, the probe circuit unit is connected to one end of the DC line section. When the DC line operates normally, the DC circuit breaker is in a closed state, the DC distribution network line performs normal power transmission, and the bus voltage charges capacitors C1 and C2 through the body diodes of the switch tubes S1 and S2. Capacitors C1 and C2 store energy to provide energy for fault distance measurement. When a fault occurs in the DC line, the DC circuit breaker trips quickly to terminate power transmission. Among them, the length of the DC distribution network line is mostly concentrated within 3km, and the distributed capacitance of the line is particularly small and can be ignored.

[0051] When a single-pole ground fault occurs in the DC line, the probe circuit, the positive DC line and the fault point form a closed loop. The equivalent circuit is as follows: Figure 2 When a negative ground fault occurs in the DC line, the probe circuit, the negative DC line and the fault point form a closed loop. The equivalent circuit is shown in Figure 3 In the figure, x1 is the distance between the probe unit and the fault point, r0 and l0 are the resistance and inductance per unit length of the DC line, respectively. f is the fault resistance, U Cis the capacitor voltage.

[0052] Depend on Figure 2 and Figure 3 It can be seen that the probe circuit and the single-pole grounding fault line form a generalized BUCK circuit, which generates a 0~U voltage on the DC line by controlling the switch tube S1 or S2. C Square wave voltage signal; by collecting the voltage and current signals at the output end of the probe circuit and using the time-frequency domain transformation tool, the DC component and fundamental component of the voltage and current can be obtained, then:

[0053]

[0054] Where u(t) and i(t) are the voltage and current at the output of the probe circuit, respectively; U1 and I1 are the fundamental components of the voltage u(t) and current i(t) signals, respectively; U0 and I0 are the DC components of the voltage u(t) and current i(t) signals, respectively; and FFT[] is the fast Fourier transform.

[0055] The voltage and current state equation of the equivalent circuit is:

[0056]

[0057] Where x1 is the line fault distance, f is the switching frequency of switch S1 or S2, r0 and l0 are the resistance and inductance per unit length of the DC line, respectively, and R f is the fault resistance.

[0058] According to the state equation (2), the line fault distance x1 and the fault resistance R can be obtained: f The expression is:

[0059]

[0060] When a bipolar short circuit fault occurs in the DC line, the probe circuit forms a closed loop with the positive and negative DC lines and the fault point. The equivalent circuit is as follows: Figure 4 shown.

[0061] At this time, the state equation (2) of the equivalent circuit can be corrected as follows:

[0062]

[0063] Therefore, according to equation (5), the expression of the DC line bipolar fault can be obtained as follows:

[0064]

[0065] Furthermore, according to the equivalent circuit, the phasor form of the voltage and current equation can be written as:

[0066]

[0067] From equation (8), we can calculate:

[0068]

[0069] In the formula, and They are respectively the phase angle of the fundamental component of the voltage signal at the output end of the probe and the phase angle of the fundamental component of the current signal.

[0070] Therefore, the line fault distance x1 and the fault resistance R f The second solution expression is:

[0071]

[0072] Similarly, the bipolar fault expression can be obtained as:

[0073]

[0074] A simulation model of a DC distribution network and the method of the present invention is established in Matlab / Simulink, and the simulation parameters are set as follows:

[0075] The length of the DC line is L = 2 km;

[0076] The DC line resistance and inductance parameters are r0=121mΩ / km, l0=0.97mH / km respectively;

[0077] The simulation sampling frequency is set to 100kHz;

[0078] The switching frequency of the switches S1 and S2 is 1kHz, and the duty cycle is 50%.

[0079] Assuming that the fault distance of the DC line single-pole grounding is 1 km and the fault resistance is 1Ω, the voltage waveform output by the probe circuit unit and its amplitude-frequency response are as follows: Figure 5 As shown, we can get U0 = 48.6558V, U1 = 62.448V; the voltage waveform and amplitude-frequency response of the probe circuit unit output are as follows Figure 6 As shown, it can be obtained that I0 = 43.4102A, I1 = 10.0639A, and the fault distance D1 is calculated to be 1.0019km according to the first distance measurement formula (3), and the relative error of the distance measurement result is 0.19%, as shown in formula (14):

[0080]

[0081] The voltage and current phase angles are According to the second distance measurement formula (10), the fault distance D2 is calculated to be 1.0013 km, and the relative error of the distance measurement result is 0.16%, as shown in formula (15):

[0082]

[0083] Similarly, when there is a bipolar fault, U0 = 48.807V, I0 = 39.3077A, U1 = 62.604V, I1 = 5.1025A. According to the first distance measurement formula (6), the fault distance D2 is calculated to be 1.0014km, with a relative error of 0.14%. At this time, the voltage and current phase angles are According to the second distance measurement formula (12), the fault distance D2 is calculated to be 1.0013 km with a relative error of 0.14%.

[0084] The above embodiments are only for illustrating the technical concept and features of the present invention, and their purpose is to enable people familiar with the technology to understand the content of the present invention and implement it accordingly, and they cannot be used to limit the protection scope of the present invention. Any equivalent transformation or modification made according to the spirit of the present invention should be included in the protection scope of the present invention.

Claims

1. A DC distribution line fault distance measuring device, characterized in that: It includes a probe circuit unit; the probe circuit unit includes two groups of cascaded chopper circuits, wherein the first group of chopper circuits is composed of a first capacitor C1, a first switch tube S1 and a first diode D1 connected in series in sequence, and the second group of chopper circuits is composed of a second capacitor C2, a second switch tube S1 and a second diode D2 connected in series in sequence, and the cascade midpoint of the two groups of cascaded chopper circuits is grounded.

2. The DC distribution line fault distance measuring device according to claim 1, characterized in that: The source of the first switch tube S1 and the cathode of the first diode D1 are connected to the positive pole of the DC line, and the drain of the second switch tube S2 and the anode of the second diode D2 are connected to the negative pole of the DC line.

3. The DC distribution line fault distance measuring device according to any one of claims 1 or 2, characterized in that: Depending on the fault type (positive, negative or bipolar fault), the switch tube is controlled to emit a square wave voltage signal with a specific frequency and duty cycle.

4. The DC distribution line fault distance measuring device according to claim 3, characterized in that: The value range of the specific frequency f is: f>0, and the value range of the duty cycle k is: 0<k<1.

5. A DC distribution line fault location method, characterized in that: The DC distribution line fault distance measuring device applied to any one of claims 1 to 4 comprises: a probe circuit unit, which extracts the DC component and fundamental component of the voltage and current in sequence by sampling the voltage and current signals output by the probe circuit unit, and calculates the fault distance according to the fault distance measurement formula.

6. The DC distribution line fault location method according to claim 5, characterized in that: A fault distance formula of the fault distance measurement method is: Where x1 and x2 are the line fault distances, U1 and I1 are the fundamental components of the voltage and current signals at the output of the probe circuit unit, U0 and I0 are the DC components of the voltage and current signals at the output of the probe circuit unit, f is the switching frequency of the switch S1 or S2, and l0 is the inductance per kilometer of the DC line.

7. The DC distribution line fault location method according to claim 5, characterized in that: A fault distance formula of the fault distance measurement method is: Where x1 and x2 are the line fault distances, U1 and I1 are the fundamental components of the voltage and current signals at the output of the probe circuit unit, and are the phase angles of the fundamental components of the voltage and current signals at the output end of the probe, f is the switching frequency of the switch tubes S1 and S2, and l0 is the inductance per kilometer of the DC line.

8. The DC distribution line fault location method according to any one of claims 5 to 7, characterized in that: The following steps are also included: By adjusting the duty cycle of the switch tube, the current amplitude output by the probe unit circuit is within the appropriate range of the current transformer, and repeated sampling and calculation are performed until the error between the calculation results is less than a preset threshold or reaches a preset maximum number of repetitions.

9. The DC distribution line fault location method according to claim 8, characterized in that: The threshold value ranges from 0.1% to 1%, and the maximum number of repetitions ranges from 3 to 10 times.

10. Application of the DC distribution line fault distance measuring device according to any one of claims 1 to 4 in relay protection of DC distribution system, characterized in that: The fault distance measuring device is integrated into the relay protection device.

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