The structural principles, advantages and disadvantages of currently used disconnectors are analyzed and compared. According to the characteristics of my country's power grid and the mechanism of arrester failure and damage, the technical performance requirements that the disconnector should meet are put forward. A thermal explosion decoupler with a new structural principle is designed and its application is explained. This type of decoupler has reasonable structural principles, high operational reliability, and perfect technical performance, and can be popularized and used.
As a supporting product of the arrester, the disconnector is used in series with the arrester. When the arrester manufacturer fails and is damaged, it can disconnect. The faulty and damaged arrester can be isolated from the system in time and clearly marked, thereby ensuring the normal operation of the system and facilitating the search for fault points and the maintenance and replacement of the arrester.
Abroad, distribution type, power station type and line type arresters running in power grids in countries and regions such as Europe, America, Japan and Southeast Asia have generally used disconnectors. However, in China, they are currently only used in a few areas, and are mostly used for distribution type arresters. There are two reasons for this phenomenon: on the one hand, the performance and function of the current disconnector are not recognized and familiar to users; on the other hand, the current disconnector has shortcomings and defects in many aspects such as the rationality of the structural principle, the reliability of operation, the applicability to the characteristics of my country's power grid, and the matching with the arrester failure damage mechanism, etc., and cannot meet the requirements of use.
In order to make the disconnector have a reasonable structural principle, reliable operation, be suitable for the characteristics of my country's power grid and match the fault damage mechanism of the arrester, so as to make the promotion and application of the disconnector possible and expand its application scope, it is very necessary to design and apply a disconnector with a new structural principle to be used in conjunction with the arrester.
1. Structural principles and characteristics
There are two types of separators: thermal explosion type and hot melt type. Among them, the most common internationally is the thermal explosion type separator.
1.1 Thermal explosion disconnector
Since the disconnector used in GE's silicon carbide valve arrester was introduced into my country in 1984, it has developed from an integrated type to an accessory type, but its structural principle has not been improved.
Its structural principle is: a capacitor is connected in parallel to the discharge gap, and the thermal detonator is placed in the lower electrode of the discharge gap. When the arrester is working normally, the voltage drop on the capacitor due to lightning and operating impulse current is not enough to cause the discharge gap to break down and discharge, and the disconnector does not operate; when the arrester fails and is damaged, the voltage drop of the power frequency fault current on the capacitor causes the discharge gap to break down and discharge, and the arc continues to heat the thermal squib. When the thermal squib heats up to about 400°C, the disconnector operates.
The silicon carbide valve arrester must have a series gap due to the nonlinearity of the silicon carbide valve plate, and the power frequency aftercurrent value is large after the arrester operates. Its failure means the complete failure of the arrester. Therefore, the failure and damage of the silicon carbide valve arrester has mutation characteristics. As for metal oxide arresters, due to the excellent nonlinearity of the metal oxide resistor, the power-frequency freewheeling value after the arrester is activated is extremely small. The fault damage is mainly manifested in two aspects: the arrester collapses due to the gradual aging of the metal oxide resistor, and the internal insulation of the arrester decreases due to poor sealing, resulting in internal insulation flashover. It can be seen that the failure damage of metal oxide arresters has both gradual and sudden characteristics.
Therefore, the thermal explosion disconnector with this structural principle of the European cable joint matches the mutation characteristics of the failure damage mechanism of the silicon carbide valve arrester and the failure damage mechanism of the metal oxide arrester, but does not match the gradual change characteristics of the failure damage mechanism of the metal oxide arrester, and is not suitable for the characteristics of my country's power grid.
Most power systems of 35 kV and below in my country are neutral-point non-effectively grounded systems, which is different from foreign countries, which are mostly neutral-point effectively grounded systems. The single-phase ground fault capacitance current of the neutral point non-effectively grounded system generally does not exceed 20 A, most of which are below 10 A, and even below 2 A for small-capacity systems, while the ground fault current of the neutral point effectively grounded system generally reaches 100~1000A [1]. The ampere-second characteristic curves specified by national standards [2, 3], international standards [4] and American standards [5] all start from 20A. Obviously, this only applies to neutral point effective grounding systems. The thermal explosion disconnector with this structural principle cannot ensure operation under small power frequency fault current, so it is not suitable for neutral point non-effectively grounded systems.
1.2 Hot melt separator
The structural principle of the hot-melt disconnector is: use solder to weld the action rod to the metal oxide resistor piece, and cooperate with the preload spring to form an action mechanism, using the resistor piece as a heat source. When the arrester is working normally, the lightning and operating impulse current is not enough for the energy absorbed by the resistor to melt the solder joints, so the disconnector does not operate. When the arrester fails and is damaged, the power frequency fault current continues to heat the resistor. When the resistor reaches about 200°C, the solder joints melt and the disconnector operates.
The hot-melt disconnector with this structural principle uses a metal oxide resistor as the heat source, which can achieve successful operation under small power frequency fault current. Therefore, it is suitable for the characteristics of the neutral point non-effective grounding system and matches the gradual change characteristics of the fault damage mechanism of the metal oxide arrester; however, Under large power frequency fault currents, it takes a certain amount of time for the resistor to heat up to the temperature required to melt the solder joints, and the requirement for rapid action cannot be achieved. Therefore, it is not suitable for the characteristics of the neutral point effective grounding system, and does not match the mutation characteristics in the fault damage mechanism of metal oxide arresters.
1.3 New thermal explosion separator
The thermal explosion type disconnector has the advantages of rapid disconnection once activated, good arc extinguishing effect and high operational reliability. If the action function under small power frequency fault current can be added to the design of the structural principle, the technical performance of the disconnector can be improved.
The new thermal explosion type disconnector uses a resistor made of conductive silicone rubber connected in parallel to the discharge gap. It is not only the leakage current channel of the arrester, but also the heat source for the operation of the disconnector under small power frequency fault current. This makes the disconnector structurally reasonable and reliable in operation. It is suitable for the characteristics of my country's power grid and matches the fault damage mechanism of the arrester.
2. Technical performance requirements
When the disconnector and arrester are used in series, their technical performance should meet:
a) The normal operating conditions should be the same as those of surge arresters used in series;
b) Should have the same withstand characteristics as the arrester;
c) It should have good operating ampere-second characteristics, be suitable for the characteristics of my country's power grid and match the fault damage mechanism of the arrester;
d) Should have appropriate mechanical strength.
3. Design and calculation
3.1 Structural design
The new thermal explosion separator uses a resistor made of conductive silicone rubber connected in parallel to the discharge gap. In order to miniaturize the size of the separator, the resistor is made into a ring shape. The upper and lower electrodes of the discharge gap are placed on the upper and lower end surfaces of the resistor respectively, and the resistor is compressed by 30% to 40% to achieve close electrical contact and sealing, and a discharge gap is formed at the axis of the ring-shaped resistor. The thermal detonator is placed in the lower electrode of the discharge gap and is in close contact with the bottom surface of the lower electrode to facilitate heat transfer.
The structural design achieves miniaturization and can be easily made into integrated or accessory products.
3.2 Electrical calculations
3.2.1 Discharge gap distance value
The distance value of the discharge gap is proportional to the arc energy after breakdown of the discharge gap. For the tolerance characteristics of the disconnector, it is better for the distance value of the discharge gap to be smaller. For the operating characteristics of the disconnector, it is better for the distance value of the discharge gap to be larger. The distance value of the discharge gap is generally 0.5 to 3 mm. After determining the distance value of the discharge gap, the heat transfer amount between the lower electrode and the thermal detonator can be controlled through structural design, thereby achieving the unification of the tolerance characteristics and operating characteristics of the disconnector.
3.2.2 Action starting current value
The following factors should be considered when determining the starting current value of the disconnector:
a) Under the continuous operation voltage of a metal oxide arrester, if the power frequency current flowing through its resistor column reaches several milliamperes, the resistor begins to heat up and shows signs of aging. When it reaches tens of milliamperes, it begins its irreversible aging. When it reaches hundreds of milliamperes, its aging speed is significantly accelerated, and when it reaches several amperes, it rapidly collapses;
b) When a metal oxide arrester is subjected to the transient power frequency overvoltage of the system and the equivalent salt density is 0.03 g/mm2 (level III pollution), the leakage current value on the surface of the jacket can be as high as hundreds of milliamperes;
c) The single-phase ground fault capacitance current of the small-capacity neutral point non-effective grounding system is small, which can be below 2A.
Based on the above factors, it is appropriate for the starting current value of the disconnector to be 0.5 A.
3.2.3 Resistor resistance
Tests have shown that the resistor's calorific value of 300W is the minimum calorific value required for reliable operation of the disconnector. Calculated according to P=I2R, in order for the disconnector to operate reliably under a power frequency fault current of 0.5 A, the resistance of the resistor should be 1.2 kΩ. When the distance value of the discharge gap is 1mm (the power frequency breakdown voltage is about 2.4 kV), calculated according to I=U/R, the power frequency fault current of the discharge gap breakdown discharge is 2A. In this way, all power frequency fault currents below 2A flow through the resistor, and only the resistor is used as a heat source to heat the thermal squib, achieving reliable operation of the disconnector under small power frequency fault currents; while the voltage drop of the power frequency fault current exceeding 2A on the resistor causes the discharge gap to breakdown and discharge, and the gap arc is used as the main heating source to quickly heat up the thermal squib, so that the disconnector can operate quickly under large power frequency fault currents.
The resistance value of the resistor is related to the structural size of the resistor and the volume resistivity of the conductive silicone rubber used. After the structural size design is determined, the required volume resistivity can be obtained by adjusting the formulation process of the conductive silicone rubber, thereby obtaining the required resistance value.
4. Apply European cable connectors
The series connection of the disconnector and the arrester can ensure the safe operation of the system and realize the maintenance-free operation of the arrester. The new thermal explosion separator has perfect technical performance, making it possible to promote its application.
TLB-1 type disconnector is suitable for use with voltage level arresters of 10kV and below and outlet arresters with rated voltage of 36kV and below; TLB-2 type disconnector is suitable for use with 35kV, 110kV, 220kV voltage level power station type and line type arresters.
The disconnector should be installed so that the disconnector can move freely and form a sufficient air gap so that the faulty and damaged arrester can be reliably isolated from the system, thereby ensuring that the normal operation of the system is not affected.
(a) For lightning arresters with voltage levels of 10kV and below, and outlet arresters with rated voltages of 36kV and below;
(b) For power station type surge arresters with voltage levels of 35kV, 110kV, and 220kV;
(c) Used for 35kV, 110kV, 220kV voltage level line arresters
The application of the disconnector can avoid system accidents caused by arrester failure and damage, and realize the maintenance-free arrester, which is a necessary equipment to ensure the safe operation of the system. The designed new thermal explosion disconnector has reasonable structural principles and high operational reliability. It is suitable for the characteristics of my country's power grid and matches the mechanism of arrester failure and damage. It has perfect technical performance and can be promoted for use. Suitable disconnectors should be selected for arresters with different voltage levels and used in different occasions, and appropriate installation methods should be adopted.
Yichang Hengyuan Technology Co., Ltd. specializes in the production of European-style cable joints, plug-and-pull heads, cable branch boxes, American-style elbow joints, high-voltage vacuum circuit breakers, high-voltage load switches, arresters, zinc oxide arresters, tank arresters, over-voltage protectors, counters, monitors, heat-shrinkable cable accessories, Manufacturer of cold shrink cable accessories, T-heads, wall bushings, European cable accessories, American cable joints, transformer neutral point protection devices, fuses, isolating switches, PT cable heads, elbow cable joints, 10kV outdoor terminals, switch cabinets, cable intermediate joints, cable accessories and other products
