Survey Analysis Report

Investigation and Analysis of Single-phase Grounding Protection Line Selection in Small Grounding Current System

table of Contents:

First, the importance of single phase earthing protection line selection - 1

Second, domestic and international status in this technical field - 3

III. Analysis of Causes of Poor Reliability in Existing Technologies - 5

IV. Successful Solutions to Ensure Reliability - 6

Xiangfan High-tech Society

2008-8-6

Small earth current system single-phase grounding protection device

Survey Analysis Report

First, the importance of single-phase grounding protection line selection?

In the power substation (switching station), 35 kV, 10 kV and 6 kV power supply systems are the main components of the power grid. In this system, the neutral point of the main transformer is not grounded or passes through the arc suppression coil and the resistance is grounded. There are multiple transmission lines on the same voltage class bus, most of which are led by aluminum (or copper) row overhead, and some are led by high voltage cables; the number is generally fifty six, ten or twenty or thirty Different; Each power transmission line has many branches, and it is erected as “radiation” and is connected with numerous distribution transformers. The distribution transformer is reduced to “low voltage” and then used by a large number of users. In such transmission lines, fault phenomena such as phase-to-phase short-circuit, over-current (over-load), and single-phase grounding often occur. Among them, the occurrence rate of single-phase grounding is the most frequent, accounting for more than 70 % of the total failure rate of the system; short-circuit faults are mostly formed after single-phase grounding and evolved into multi-phase grounding. “Single-phase earthing” refers to the three phases of A, B, and C of the line. Any one of the phases of the wire is disconnected, dropped, dropped, or penetrated through the insulation material of trees, buildings, and electrical equipment. Indirect conductivity with the earth.

Because the neutral point of the main transformer in the system is not earthed or passed through the arc suppression coil, high resistance grounding. No matter how many output distribution lines there are, and any one of them has single-phase grounding, it cannot form a loop directly with the coil winding of the main transformer, and no short-circuit current and over-current phenomenon will occur in the line. Since the capacitor is formed between the line and the ground, zero-sequence current will appear in each line. This current is very small, ranging from a few milliamps to a few hundred milliamps or several amps, and is proportional to the length of the line; under normal conditions, an overhead line of about 15 milliamps per kilometer. In the power industry, this kind of power supply system is called: "small ground current system" or "small current grounding system."

In this system, the triangular opening of the secondary winding of the voltage transformer connected to the bus has a zero-sequence voltage and can only act on the alarm. Although, according to the “Power Operation Regulations”, it is allowed to operate for 2 hours in the single-phase grounding state. However, when other lines are grounded again, there will be two short-circuit accidents when they are connected at the same time, which will easily cause damage to the power equipment; as the non-grounding phase line voltage rises up to three times the phase voltage, when the system is accompanied by When ferromagnetic resonance occurs, it will increase the phase voltage 1-5 times, or even higher, resulting in overvoltage, which accelerates the aging of the insulation material of the power equipment, shortens the service life, and causes the insulation equipment to be broken down. There will be two or more identical grounding and short circuit accidents, increasing the degree of damage to electrical equipment. Therefore, accidents such as explosion of voltage transformers, circuit breakers, burning of distribution transformers, breakdown of power cables and porcelain bottles, etc., often occur in power systems. The "short-circuit protection", "over-current protection" and "zero-sequence current" protection in existing relay protection or integrated automatic protection devices are all high-current startup protection devices; a small current at single-phase grounding cannot drive this type of protection. The protection device operates, therefore, it cannot operate on the trip of the high voltage switch (circuit breaker), and the faulty line and the non-faulty line cannot be isolated.

In order to avoid the expansion of accidents, fault lines and non-fault lines need to be distinguished in a timely manner. In the substation (station), switchyard or power plant, if there is no reliable "single-phase grounding protection line selection device", manual power-off trials are required to manually select the faulty line and sometimes even connect the busbars. All power distribution lines are out of power before they can be identified. This will result in the interruption of trouble-free line power supply, resulting in a large area of ​​blackout; at the same time, it also increases the number of high-voltage switch (circuit breaker) actions, shortens the service life, and reduces the reliability and power supply of the power supply. To find the grounding point on the line, it is also necessary to disconnect numerous branch lines from the main line one by one, and then measure the insulation resistance value of each segment or each branch to the ground with an insulation resistance meter, and manually determine the fault point range. This process is very complicated and requires a lot of work; for personal safety, various safety measures need to be set up, which requires a lot of manpower, materials and time, increases the labor intensity of electric power workers, and also has hidden dangers for the human body.

The above situation shows that: in a neutral point system that is ungrounded or powered by an arc-suppression coil or a resistance grounding method, the time for tripping a faulty line during single-phase grounding can be prolonged, but it leads to many other non-faulty lines. The interruption of power supply has caused a wider range of blackouts and hidden dangers in the lives and property of the people.

Now the State Electricity Regulatory Commission and power grid companies and other relevant management departments are increasingly demanding the reliability of power supply, requiring rural power grids to reach 99.8 and urban power grids to be more than 99.9 to be qualified; for each line, power failures will cause power failures in each year. There are also restrictions on the number of times and time, and in some areas, it is stipulated that the number of trips exceeds the limit, and a fine of 200 yuan is imposed on the relevant management unit or individual person each time.

Although the application of "substation integrated automation protection devices" in power systems has become very popular, many substations are already unattended. Because the solution of “integrated automation” in the selection of single-phase grounding protection for the small ground current system is not perfect; therefore, when a single-phase grounding occurs, the ground fault information cannot be reported to the dispatch monitoring center in a timely manner. Line maintenance personnel find and repair the line faults. In some areas, there was an over-cause of single-phase earthing of 10KV high-voltage distribution lines in some areas. This failure resulted in the failure of timely power-off, resulting in major accidents involving electrocution of other social workers active near grounding points. The power supply department has caused great troubles, economic losses and bad social impact. Therefore, according to the national standard: GB50062-92 "Records for relay protection and automatic device design of electrical installations" requirements: Single-phase grounding protection line selection devices shall be installed in the protection devices of high-voltage distribution lines in power plants and substations.

Second, domestic and foreign technology in this area

The selection of single-phase grounding protection for small ground current systems is a worldwide problem; it has not been completely resolved in the power production process for over one hundred years. In the early days of the last century, many foreign power engineering and technical personnel and higher education institutions had conducted a lot of research on this project. The degree of recognition was not only in-depth, but also more and more technical solutions. Accuracy was gradually increasing. Among them, the representative of the German power engineer Bach's "first half wave" theory and Russia's "reactive power direction" theory. The devices developed based on these theories are used in the power system, and the accuracy of the line selection can reach about 50% . China began to develop a single-phase grounding automatic line selection device for small-current grounding systems in the 1980s. Although it was late in its development, it has developed at a rapid rate and it has reached the world's advanced level. Among them, Beijing Electric Power Research Institute, Nanjing Electric Power Automation Research Institute, and Xuchang Relay Plant, based on the principle of zero-sequence current, adopt sensitive relays and transistor electronic protection technologies to protect selected lines by setting zero-sequence current operating values. Has developed a variety of models of devices. After many years of use, the accuracy of its line selection has approached 50% . Later, the "first half wave " theory was combined with transistor electronic technology to produce several different specifications of the line selection device. It was used in the power system of our country to make the line selection more accurate than the former. Increase up to 60% . Since the 1990s, since the SCM has gained popularity in our country. Many high-tech companies began to apply this high-tech microcomputer technology to their fields; at the same time, according to Prof. Xu Yuanheng of the North China Electric Power Institute, "In a neutral power supply system that is not grounded (or arc-extinguishing coiled), faults occur. The theory of the zero-sequence harmonic current of the line is opposite to the zero-sequence harmonic current of the non-faulted line. There have been many

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