Analysis of Causes of Heat Failure of High-voltage Switch Cabinet and Analysis of Countermeasures

In the process of production, transmission, supply and distribution of electric energy and use, high-voltage switch cabinets are indispensable. It is one of the key electric equipments for dispatching and transmission of electric energy, and in particular, the stability of the electromagnetic melting environment inside high-voltage switch cabinets. It directly determines the effectiveness of its own function. Therefore, once the high-voltage switchgear has a heat-generating fault and cannot be processed and solved in a timely and effective manner, the fault occurrence rate of the high-voltage switchgear cabinet will be greatly increased, and at the same time, the reliability of the power supply will be affected. Years of work experience show that the location where the high-voltage switchgear is most prone to heat failure is its trolley contact. In the event of a fever failure, not only will the equipment's performance be seriously impaired, but it will also require a relatively long repair period, resulting in significant economic losses. The process leading to the formation of heat failures in high-voltage switchgear cabinets is not an overnight process, but a "step-by-step" process. That is to say, the temperature of the contact surface of the high-voltage switch cabinet rises in a bad working environment or an abnormal situation occurs in itself. At the same time, the continuous effect of the current heating effect causes the contact temperature to gradually increase. Once this temperature increase trend is too fast, the contact temperature is higher than the rated heat resistance standard of the current transformer inside the high voltage switch cabinet or higher than the heat resistance standard of the insulation sleeve, which will result in the current transformer and the insulation sleeve. Damage caused single-phase or two-phase short circuit, and ultimately caused the destructive power of the heating fault to be amplified and spread to other ancillary equipment. The stability of the final power supply system or power transformer was affected, and even a fire or even an explosion occurred.

Almost all high-voltage switchgear trolley car connections use copper bars, but metal copper not only has good electrical conductivity, but also has excellent thermal conductivity. The heat generated by the high-voltage switchgear in the working state will cause the copper row to be rapidly heated, resulting in serious impact on the opening and closing performance of the corresponding contacts inside the high-voltage switchgear. Once the high-voltage switchgear touches have a fever accident, it is not only troublesome but also difficult if the high-voltage switchgear is to be systematically repaired and repaired. Even after the maintenance and repair process, the electrical performance of the switch is basically not fully restored.

1 Causes of heat failure in high-voltage switchgear

There are many causes of heat failure in high-voltage switchgear cabinets, but they are mainly integrated in the following aspects:

(1) There is a problem with the installation process of the high-voltage switchgear. When installing a high-voltage switchgear, it is necessary to strictly follow the installation instructions of the equipment and related technical standards. If there is a problem in the installation process, the high voltage switchgear will be buried with hidden troubles. For example, the position of the trolley switch plug and the fixed plug inside the high-voltage switchgear does not meet the design requirements, resulting in a “virtual connection” of the trolley switch plug and the fixed plug, and the contact between the two is unreliable and causes a problem of heat generation. Therefore, professionals are required to supervise the installation and installation of the equipment, and carefully check the equipment during the test and acceptance of the installation, leaving no hidden danger for the equipment heating failure. In addition, in the processing, connection, and installation of busbars, the busbar surface is not completely smooth, uneven, and does not use special power grease, etc., to avoid leakage due to these errors and blowouts. The contact resistance value causes the occurrence of heat problems.

(2) Equipment test and maintenance personnel failed to perform their duties seriously. The test and overhaul of the high-voltage switchgear require the relevant staff to have a high degree of responsibility and the ability to earnestly fulfill their job responsibilities. Normally, test personnel and maintenance personnel need to disconnect some of the connection points of the high voltage switchgear in order to be able to accurately test and overhaul the equipment. After the test and overhaul work is completed, the high voltage switchgear equipment should be connected as it is. However, some test personnel and maintenance personnel have insufficient responsibilities and work attitudes are not correct, and they directly overlook certain details. For example, there are four joints for fastening bolts, but only three of them are installed after the work is completed, and even only one is installed; or the degree of tightening of the bolts is insufficient, resulting in too low pressure on the contact surfaces. If the line load is relatively small, generally no problem occurs, but once the line load suddenly increases, too small contact area pressure will directly lead to overheating of the connector, especially the connection point of the current transformer. If the pressure on the contact surface is too small, the chance of joint overheating is very high. After the implementation of the work responsibility system, this problem has been alleviated to some extent.

(3) The speed of equipment replacement is slow, and many equipments are out of service, and the problems of aging and deformation of components are prominent. Although China has vigorously applied numerous new technologies, new materials and new equipment in the power grid, and promoted the optimization and reform of the power grid, many old and outdated high-voltage switchgear equipment still stands at work. Although this measure on the surface reduces operating costs, the reliability of grid operation is infinitely reduced, and the possibility of failures and accidents is greatly increased. The purity of the conductor raw materials inside these old and obsolete high-voltage switchgear cabinets is insufficient, which reduces the conductivity of the conductor material. Although the operating conditions of the circuit breaker are normal, the diversion can still generate a very high heat, so that the internal temperature of the high voltage switch cabinet is in a process of rising. (4) The operating personnel did not master the operating essentials of the high-voltage switchgear. When operating the operator to operate the knife gate and circuit breaker, the conductive contacts will enter into the bus room, make the car in working position, the limit switch will close, and the green indicator light will be on. At this time, the operating personnel should gently shake the car to ensure that the car is fully closed and the movable contact and the static contact are in full contact. At this time, the resistance value of the contact surface is the lowest, and the heat problem is effectively solved. solve.

(5) The adverse effects of sudden changes in electrical loads. Power load is an important factor affecting the internal temperature of high-voltage switchgear equipment. If the power load can be kept within the normal range, the temperature rise of the high-voltage switchgear equipment caused by it will generally not exceed 75°C. However, a sharp increase in the current due to a short circuit or other causes will significantly increase the power load. A high load shock will accelerate the temperature rise in the weak link of the high-voltage switchgear. If the temperature is too high, problems such as oxidation, deformation, and aging of the connection point material may easily occur. If this problem is not discovered in time and effectively dealt with, the damage caused by the next high-load shock will accumulate on the basis of the previous damage. Once in a vicious circle, the connector's connection will continue to deteriorate, eventually causing the connector to melt.

2 Countermeasures for heat failure of high voltage switchgear

(1) The temperature monitoring method for the in-cab connector is frequently used. By monitoring the operating temperature of the contact points in the switchgear by means of contact temperature measurement (referred to as the temperature monitoring of the switchgear), the fire of the switchgear cabinet can be effectively prevented in time. However, due to the closed operation of the switchgear, the internal space is small and has exposed high pressure, which cannot be measured manually. Therefore, the passive wireless temperature measurement system of the switchgear is an important means to ensure the safe operation of the high voltage switchgear. The DCT-6G wireless communication high-voltage switchgear temperature monitoring system uses wireless transmission signals to isolate the high voltage in the switch cabinet. Fundamentally solves the problem that the operating temperature of the contacts in the high voltage switchgear is not easily monitored. (2) The presence or absence of joint heating failure is determined by monitoring the air temperature inside the switch cabinet. The overall solution is to install a temperature sensor on the exhaust hole at the top of each switch cabinet. If the air temperature reaches the specified value, an alarm signal is issued; or when the difference between the air temperature in the cabinet and the ambient temperature reaches a specified value, the alarm is issued. The specific alarm temperature setting value is determined by the test method. We first convert the temperature signal of the sensor to the computer after the analog-to-digital conversion, and then load the load data of the switch into the same computer. Determine the relationship between the switch load and the temperature through comparative analysis, and determine the alarm temperature value according to the temperature curve. . The alarm signal is issued by the computer and sent directly to the control room. The watchman in the control room can also query the real-time temperature value in the switch cabinet at any time through the computer. For an unattended substation, the overheat signal can be used as a remote measurement and sent directly to the monitoring center via the RTU. This temperature measurement method is easy and convenient to install, reliable in operation and low in investment. After the technology is mature, it can be used in a large number of substations. (3) The use of copper busbars. Busbar systems for 10 kV switchgears currently use rectangular copper busbars. This kind of copper busbar cannot be oversized by the size of the 10 kV switchgear in the substation switch. However, because the current is too large, multiple copper busbars must be used, which will naturally increase the copper consumption. If the use of copper tube busbars will bring such benefits: A: copper tube bus in the partial discharge, skin effect, electric field uniformity, heat dissipation capabilities (its surface area, there is enough space for air circulation within the tube), Dynamic heat stability is stronger than rectangular copper busbars. B: Reduce copper use and save costs. C: It is more convenient to install and use. D: If copper busbars are used in the 10kV switchgear busbar system, the insulation cost will be high. Since the switchgear is a sealed environment, it is possible to use a heat-shrinkable jacket. In this way, the price can be reduced. In combination with the guidance for the technical innovation of saving products during tendering, the unit cost can be further reduced. E: Insulated copper pipe busbars have many years of experience on the busbar bridge between the outdoor 10 kV bus bridge and the indoor cabinet. It is safe and reliable.

In short, high-voltage switchgears are very important substation equipment for power supply and distribution companies. They need to deal with their heat-failures in a timely manner to provide power supply system reliability and power supply economy. For high-temperature switch cabinet heating failures, it is necessary to perform effective processing based on clear causes of heat generation.

Shandong Zhengrui Electronics Co., Ltd.

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