A leakage protection method for low voltage distribution network based on double-terminal residual current variation

Abstract

A leakage protection method for a low-voltage distribution system based on the variation of residual current at both ends belongs to the field of relay protection for distribution networks. Traditional leakage protection methods only use the amplitude information of residual current at one end, which has poor selectivity, is prone to false operation, has slow action speed, and low sensitivity. The present invention proposes a leakage protection method for a low-voltage distribution network based on the variation of residual current at both ends, which can be implemented based on a distribution Internet of Things platform, and uses the amplitude of the variation of residual current at both ends of the protected section as a comprehensive protection criterion. When the criteria at both ends meet the requirements of their respective setting thresholds, the protection is activated to achieve leakage protection. This method comprehensively utilizes the phasor information of residual current at both ends of the section. On the one hand, it can eliminate the influence of factors such as unbalanced leakage current of the line during normal operation, thereby improving the protection utilization rate; on the other hand, it does not require multi-stage delay coordination, and has a fast action speed; at the same time, it has a low setting threshold and high sensitivity.

Description

Technical Field

[0001] The invention relates to the field of distribution network relay protection, in particular to a low-voltage distribution network leakage protection method based on double-terminal residual current variation. Background Art

[0002] With the rapid development of social economy and people's living standards, the existing safety protection level of low-voltage distribution network in my country can no longer meet the needs of users' production and life. In recent years, accidents such as electric shock and leakage fire caused by leakage faults in low-voltage distribution networks have occurred from time to time, causing huge losses of life and property. According to statistics, more than 85% of personal electric shock accidents in my country occurred in low-voltage distribution networks. In the rainy season of 2018, many electric shock accidents caused by leakage of public power facilities such as street lamps occurred in Fujian, Guangdong and other places in my country. After being reported by the media, they caused great public repercussions. According to statistics, more than 500,000 electrical fires occurred nationwide from 2011 to 2016, causing more than 3,000 deaths and more than 2,000 injuries, with direct economic losses of more than 9 billion yuan, exceeding 30% of the total number of fires and casualties in the country.

[0003] At present, the most widely used leakage protection device in low-voltage distribution network is residual current operated protection device (RCD), which adopts amplitude comparison principle, that is, protection action is performed when the amplitude of local residual current exceeds the setting threshold. It is easily affected by factors such as line unbalanced leakage current and terminal load leakage, which leads to malfunction of protection device or even failure to put into operation, thus losing the due protection effect. Taking the investigation of 6 distribution transformers in a city in southern my country as an example, the system grounding method is mainly TT method, with a total of 1,387 public transformers, 810 of which are installed, and the installation rate is only 58.40%; 399 of which are put into operation, and the operation rate is only 49.26%. The system has lost most of its leakage protection functions, and the safety protection level is low, which greatly increases the hidden dangers of electric shock and leakage fire.

[0004] In order to achieve safer and more effective leakage protection, domestic and foreign power researchers have conducted a lot of research. The document "Research on Pulsating DC Residual Current Protector" studies a pulsating DC residual current protector, which can monitor the residual current signal containing pulsating DC in real time and realize pulsating residual current leakage protection for distribution lines, but the hardware design is relatively complex; the document "Research on Additional DC Power Supply Leakage Protection" proposes a selective leakage protection method based on the additional DC power supply method, which can realize continuous detection of the insulation resistance of the power grid, but it is difficult to realize and adjust; the document "Electric Shock Feature Detection Method Based on Sliding Detrending Fluctuation Analysis" uses the calculated change trend of the scaling index to realize electric shock feature detection, but its practicality needs to be improved. In general, the problems existing in the leakage protection technology of low-voltage distribution networks have not been effectively solved, and it is of great significance to study a safer leakage protection method.

[0005] As a weathervane for power grid construction in the new era, the rapid development and application of the power distribution Internet of Things has provided convenient conditions for the protection method using multi-terminal information, and provided a new opportunity to solve the leakage protection problem of the existing low-voltage distribution network. Based on the key technical conditions such as intelligent perception, edge computing, information storage, and device interconnection provided by the power distribution Internet of Things platform, through the plug-and-play and remote control of the equipment, a more secure, automated, information-based, and intelligent leakage protection can be achieved.

[0006] The present invention comprehensively utilizes the amplitude of the residual current change at both ends of the section to reflect the characteristics of the leakage fault, and relies on the emerging power Internet of Things technology to realize the monitoring and protection of the leakage fault. It has unique advantages and is still applicable when the normal downstream leakage current of the system is large and the fault resistance is high. It provides a new idea for the research on leakage protection of low-voltage distribution networks and has broad practical application value. Summary of the invention

[0007] The purpose of the present invention is to further improve the leakage protection level of low-voltage distribution network. Based on the analysis of the amplitude characteristics of the double-terminal residual current change in the section before and after the leakage fault, a low-voltage distribution network leakage protection method based on the double-terminal residual current change is proposed.

[0008] The technical solution of the present invention is:

[0009] a. Install residual current monitoring terminals at both ends of the protected section, measure the residual current at both ends of the section in real time, and subtract the measured residual current phasors at both ends from the phasors n cycles ago to calculate the amplitude of the residual current changes at the beginning and end

[0010] b. The first and last terminals compare their respective change amplitudes in real time And the corresponding setting threshold I set1 ,I set2 ;

[0011] c. The terminal detects When the limit is exceeded, the limit-crossing information is immediately reported to the head-end terminal;

[0012] d. The head-end terminal detects When a time window of several cycles is immediately opened, waiting for the end terminal to send it a limit-crossing information;

[0013] e. Within the time window, if the head-end terminal does not receive the end-end over-limit information, it is judged that a leakage fault has occurred in the area, and the protection device is immediately controlled to operate and cut off the faulty section; otherwise, the protection returns.

[0014] In the above scheme:

[0015] Head end threshold I set1The setting method is:

[0016] I set1 =K rel I Zmax +I d1 (1)

[0017] In the formula, K rel is the reliability coefficient, ranging from 1.5 to 2, I Zmax I is the maximum leakage current of the protected section when there is no fault, which is 8mA~10mA; d1 is the DC bias, which is 20mA to 30mA. Therefore, I set1 Take 32mA~50mA.

[0018] End threshold I set2 The setting method is:

[0019] I set2 =I d2 (2)

[0020] In the formula, I d2 is the DC bias, which is 15mA to 20mA. set2 Take 15mA~20mA.

[0021] Compared with the prior art, the present invention has the following beneficial effects:

[0022] The present invention proposes a low-voltage distribution network leakage protection method based on the variation of double-end residual current, which can be implemented based on the distribution Internet of Things platform. It mainly solves the problems of poor selectivity, easy false operation, slow action speed and low sensitivity of the traditional leakage protection method. The variation of the residual current phasor at the head and end of the protected section is used as the monitoring quantity, and the amplitude information of the two is comprehensively used as the protection criterion. The present invention makes full use of the electrical quantity information during leakage faults. On the one hand, it can effectively overcome the influence of factors such as unbalanced leakage current downstream and reduce false protection operations; on the other hand, it does not require multi-stage delay coordination, has good selectivity and fast action speed; in addition, a lower setting threshold can be set, and the protection sensitivity is high. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 It is a simulation model of low-voltage distribution network based on the distribution point Internet of Things platform. The dotted line indicates the communication conditions provided by the distribution point Internet of Things platform.

[0024] Figure 2 Run the flow chart for the protection method;

[0025] Figure 3 The attached Figure 1 The waveform of the residual current variation at the beginning and end of L1 when a 2kΩ single-phase leakage fault occurs in L1 in the system shown;

[0026] Figure 4 The attached Figure 1 The waveform of the residual current variation at the beginning and end of L1 when a 2kΩ single-phase leakage fault occurs in L2 in the system shown; DETAILED DESCRIPTION

[0027] The protection technical solution in the present invention requires the protection equipment to realize the collection and real-time transmission of residual current change information and have a certain computing capability, which can be achieved by developing an intelligent residual current monitoring terminal and using the conditions provided by the distribution Internet of Things platform.

[0028] The protection technical solution of the present invention will be clearly and completely described below in conjunction with the accompanying drawings of the present invention.

[0029] Ⅰ) Combination Figure 1 The application scenarios of the equipment using the protection method of the present invention are as follows:

[0030] The novel residual current monitoring terminal developed according to the present invention can be installed at the beginning and end of each section for a TT system or a TN-S system to realize leakage protection for each protected section.

[0031] II) Combination Figure 1 , Figure 2 , Figure 3 , Figure 4 , a low voltage distribution network leakage protection method based on the double-terminal residual current variation, which works as follows:

[0032] (1) When L1 has a 2kΩ A-phase leakage fault

[0033] a. The monitoring terminal detects the change in residual current at the beginning and end of L1 as follows Figure 3 As shown, before the fault After the failure

[0034] b. The first and last terminals compare their respective change amplitudes in real time And the corresponding setting threshold I set1 ,I set2 , the comparison result before the fault is After the failure

[0035] c. The terminal was not detected before and after the fault Do not report the limit-crossing information to the head-end terminal;

[0036] d. After the failure, the head-end terminal detects Immediately open a time window of several cycles and wait for the end terminal to send it an out-of-limit information.

[0037] e. Within the time window, the head-end terminal does not receive the end-end over-limit information, and determines that a leakage fault has occurred in the area, and immediately controls the protection device to operate and cut off the faulty section.

[0038] (2) When L2 has a 2kΩ A-phase leakage fault

[0039] a. The monitoring terminal detects the change in residual current at the beginning and end of L1 as follows Figure 4 As shown, before the fault After the failure

[0040] b. The first and last terminals compare their respective change amplitudes in real time And the corresponding setting threshold I set1 ,I set2 , the comparison result before the fault is After the failure

[0041] c. The terminal detects the fault after Immediately report the limit-crossing information to the head-end terminal;

[0042] d. After the failure, the head-end terminal detects Immediately open a time window of several cycles and wait for the end terminal to send it an out-of-limit information.

[0043] e. Within the time window, the head-end terminal receives the end-end over-limit information and determines that no leakage fault has occurred in the area, and the protection returns.

[0044] The above is only a preferred embodiment of the present invention, and does not limit the present invention in other forms. Any technician familiar with the profession may use the above disclosed technical content to change or modify it into an equivalent embodiment with equivalent changes. However, any simple modification, equivalent change and modification made to the above embodiment according to the technical essence of the present invention without departing from the technical solution of the present invention still belongs to the protection scope of the technical solution of the present invention.

Claims

1. A method for leakage protection of low-voltage distribution network based on double-terminal residual current variation, which uses the amplitude of the double-terminal residual current variation at the beginning and end of the protected section to reflect the leakage current, thereby performing leakage protection on the low-voltage distribution system, which is implemented based on the distribution Internet of Things platform, and is characterized in that: The basic steps of protection work are: a. Install residual current monitoring terminals at both ends of the protected section, measure the residual current at both ends of the section in real time, and subtract the measured residual current phasors at both ends from the phasors n cycles ago to calculate the amplitude of the residual current changes at the beginning and end b. The first and last terminals compare their respective change amplitudes in real time And the corresponding setting threshold I set1 ,I set2 ; c. The terminal detects When the limit is exceeded, the limit-crossing information is immediately reported to the head-end terminal; d. The head-end terminal detects When a time window of several cycles is immediately opened, waiting for the end terminal to send it a limit-crossing information; e. If the head-end terminal does not receive the end-end over-limit information within the time window, it is determined that a leakage fault has occurred in the area, and the protection device is immediately controlled to operate and the faulty section is removed; Otherwise the protection returns; The head end threshold I in step b set1 The setting method is: I set1 =K rel I Zmax +I d1 In the formula, K rel is the reliability coefficient, ranging from 1.5 to 2, I Zmax I is the maximum leakage current of the protected section when there is no fault, which is 8mA~10mA; d1 is the DC bias, which is 20mA to 30mA. Therefore, I set1 Take 32mA~50mA; The terminal threshold I in step b set2 The setting method is: I set2 =I d2 In the formula, I d2 is the DC bias, which is 15mA to 20mA. set2 Take 15mA~20mA; The comparison result before the fault is After the failure 2. The low voltage distribution network leakage protection method based on the double-terminal residual current variation according to claim 1 is characterized in that: Considering the time required for the fault to stabilize, the number of cycles n in step a is 2 to 5.

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