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The application of vacuum circuit breaker to reconstruct the

date:2019-06-05 22:32 viewed:
1. An overview of the
 
China's 10kV (6kV) power supply and distribution network still has little oil circuit breaker in operation, with the progress of science and technology and users put forward higher requirements for the reliability of power supply, this part of switchgear is facing technical transformation. Its transformation scheme can be divided into two categories; First, the switch cabinet overall update; The second is to retain the switch cabinet cabinet, only the internal electrical equipment for upgrading. The first scheme has the advantages of large investment, large reconstruction workload and long power failure time, and is suitable for substation transformation in other places. The second scheme has the advantages of less investment, less reconstruction work and short power failure time, etc.
 
Due to the advantages of long service life, frequent operation, strong breaking ability, simple structure and less maintenance work, vacuum circuit breaker should be the preferred switch in the transformation of switch cabinet with less oil. In order to realize the oil-free transformation of the transformer substation, the low-oil switch is reformed by stages. Using vacuum circuit breaker as a whole shall replace the less oil switch, all keep cabinet put oneself in another's position of the technical retrofit scheme, and the other contained in the electrical equipment according to its technical performance and investment situation, corresponding product cycles and technology upgrades, overall arrangement for the substation switchgear cabinet put oneself in another's position and spray paint, in order to improve the 10 kv switchgear (6 kv) interior electrical equipment technical level, also make the switch cabinet appearance also refreshed. That is to save the cost of technical transformation, and reduce the power failure due to the transformation of production time, is the implementation of the switchgear technical transformation should be promoted practical technical scheme.
 
2. Characteristics of vacuum circuit breaker
 
When the vacuum circuit breaker turns off the current, an arc will be generated between the two contacts. The temperature of the arc is very high, which can vaporize the contact material and form a lot of metal vapor between the two contacts. Since there is a "vacuum" around the contact, with very few gas molecules, the metal vapor quickly runs to the shield surrounding the contact, so that the contact gap returns to its original high "vacuum" state in a very short time (a few microseconds) after the current passes zero. Therefore, the arc extinguishing capacity of vacuum circuit breaker is much better than that of low-oil circuit breaker. Therefore, vacuum circuit breaker has the following characteristics:
 
(1) in the vacuum arc extinguishing, arc and hot gas is not exposed, do not splash on other objects.
 
(2) due to the high pressure strength in vacuum, the distance between the contacts is greatly shortened, the corresponding motion travel is much shorter, the inertia of the moving guide rod is small, suitable for frequent operation.
 
(3) due to the structure characteristics of vacuum circuit breaker, it has the advantages of short arc extinguishing time, low arc pressure, small arc energy, small contact loss, and many times of breaking.
 
(4) the operation mechanism is small and light weight, control power is small, no fire and explosion risk, so safe and reliable.
 
(5) contact seal in vacuum, will not be affected by moisture, dust and harmful gases and reduce its technical performance.
 
6 vacuum circuit breaker in the short circuit current, after troubleshooting, no need to repair the vacuum circuit breaker can be put into operation.
 
The characteristics mentioned above cannot be compared with other circuit breakers in terms of ensuring the continuity of production, improving operation rate and safety and reliability, so vacuum circuit breakers have become the most competitive switches in the medium voltage field (6kV ~ 10kV). Up to now, there have been dozens of professional manufacturers of vacuum circuit breakers in our country, some of which have introduced foreign patented technologies and production lines, making our vacuum circuit breaker manufacturing technology develop to the international advanced level and standardization, and some products have reached the international level.
 
3. Vacuum circuit breaker operation overvoltage suppression
 
The operating overvoltage of vacuum circuit breaker directly threatens the safe operation of electrical equipment, and corresponding countermeasures must be taken to suppress the operating overvoltage of vacuum circuit breaker. Based on the application practice of vacuum circuit breaker in laigang for more than ten years, the author thinks that it should be carried out from two aspects to restrain the operation overvoltage of vacuum circuit breaker. One is the design of the vacuum circuit breaker selection, because the manufacturer of vacuum circuit breaker with dozens of, even the same series of products is also has several production, the factory technical equipment level of each different, with the introduction of foreign production of patented technology, also has the introduction of foreign production lines, also has the cooperation with foreign companies. Therefore, in the design and selection of vacuum circuit breaker, it is necessary to combine the actual situation of vacuum circuit breaker production industry in China, and choose the production enterprise with advanced technology and equipment and perfect detection means. The selected product series should be the leading product with years of practical operation practice. So that the selected products have a low interception value, to reduce the operation of the interception overvoltage. Second, it is necessary to design and operate the over-voltage absorption device synchronously. The over-voltage absorption device widely used in China can be divided into two types, namely RC (resistance, capacitor combination) and zno varistors. In the transformation of the old switchgear should be preferred zinc oxide varistors, so small volume, easy to install, small maintenance workload.
 
Zinc oxide varistor has the ability to suppress overvoltage, low residue, the surge response fast, volt-ampere characteristics is symmetrical, in any positive and negative polarity surge voltage waveform can absorb, flow capacity is big, and have the advantages of no follow-up after discharge flow, and its small size ease of installation and is widely used for inhibiting operating overvoltage of vacuum breaker.
 
Zinc oxide varistor is zinc oxide as the main body, with tiny amounts of bismuth oxide, antimony oxide, cobalt oxide, manganese oxide by high temperature sintering polycrystalline ceramics such as nonlinear components, zinc oxide crystal is low resistance (1 ~ 10 Ω/cm) of n-type semiconductor, boundary layer is high resistance (1010 Ω/cm) of p-type semiconductors, under normal voltage (low field strength) boundary surfaces appear high resistance state, only a small leakage current through, and in high voltage, high electric field intensity level surface resistivity will suddenly drop, in low resistance state, and a two-way volt-ampere characteristics of voltage regulator tube. The parameters of zno varistors are selected as follows:
 
(1) nominal voltage; The nominal voltage shall be greater than the maximum power frequency voltage amplitude (Umax) in the using circuit, and the factor that the nominal voltage is allowed to decline by 10% shall be taken into account. The calculation formula is:
 
 
 
(2) the residual pressure UIA; The residual voltage value of zno varistors should be lower than the test voltage of the load, and the through-current capacity should generally be greater than or equal to 100A, so the residual voltage value is calculated as follows:
 
 
 
Where, K1 is the impact coefficient, K1=1.1; K2 is the aging coefficient of varistor, K2=1.05; Ue is the load rated working voltage.
 
(3) residual pressure ratio; The residual voltage ratio m of zno varistors is the ratio of residual voltage UIA to nominal voltage U1mA, i.e.
 
M = UIA/U1mA
 
Where: the smaller the m value is, the stronger the ability to suppress overvoltage is.
 
Zno varistors have no discharge characteristic, only U1mA (nominal voltage) characteristic. Japan calculates the U1mA value of ZS series zno varistors as the starting voltage of action. U1mA characteristics are zinc oxide varistors through the 1mA dc current, the varistors at both ends of the dc voltage. Residual voltage (UIA), an important technical indicator of zno varistors, is the voltage measured at both ends of the varistors when the discharge value is I under the action of high voltage peak. The ratio of residual voltage UIA to nominal voltage U1mA is called residual voltage ratio m. Another performance index of zno varistors is flow capacity iT, zno varistors to standard current waveform (10/20 mics), the interval time 5 minutes impact 2 times, nominal voltage U1mA value drop less than 10% of the maximum impact current amplitude, become its flow rate. Zno varistors are used to suppress the operating overvoltage of vacuum short-circuiter. The above three technical performance indexes are extremely important. Only when proper parameters are selected can the operating overvoltage of vacuum short-circuiter be effectively suppressed.
 
4. Transformation practice of switchgear
 
4.1. Transformation of gg-1a switchgear. In the transformation of high voltage switchgear of gg-1a series, since the external dimension of the switchgear of this series is 1200mm×1200mm×2280mm, most of its low-oil circuit breakers are sn10-10/1000-31.5 series, and the external dimension of the switch is 840mm×790mm×427mm. Zn5-10/1250-31.5 series vacuum circuit breakers can be replaced as a whole according to the rated current, breaking current and external dimension of the switch. The external dimension of the circuit breaker is 906mm×635mm×430mm. Due to the difference in the external size and installation size of the two switches, two 50mm×50mm×5mm Angle steels are welded at 1200mm from the bottom of the original cabinet to serve as supports for installing vacuum circuit breakers. Before the installation of the vacuum circuit breaker, a one-minute power-frequency voltage endurance test should be conducted on the vacuum arc extinguishing chamber to check its vacuum degree. The voltage endurance value is 42kV, and some discontinuous welding light and sparks are allowed in the test. Since the bottom of this series of vacuum circuit breakers has four bent plates installed near the roller, the installation can be carried out with the help of four bent plates under the chassis.
 
Due to the different lead positions of the low-oil circuit breaker and vacuum circuit breaker, it is necessary to re-make the hard bus of the circuit breaker to the top knife brake and the hard bus of the circuit breaker to the current transformer. Adjust the connecting rod of the circuit breaker and the operating mechanism, open and close the circuit slowly several times by hand, if there is no stagnation, use the electric mechanism to open and close the circuit. ZN5 vacuum circuit breaker itself is equipped with electromagnetic mechanism specially designed for it, without improper coordination, and the mechanism is easy to adjust.
 
4.2. Transformation of GFC-3A cabinet. Of GFC - 3 - a series of switchgear using only the handcart reform plan, to ensure the primary and secondary outlet of cabinet put oneself in another's position, so the vacuum circuit breaker in the Angle of the 238 mm from the bottom of the handcart, such not only ensure the vacuum circuit breaker connection bus to a safe distance from top to bottom, and ensure its good stability, low center of gravity position, push the handcart without forward backward situation. The low-level arrangement of circuit breakers is also conducive to the installation of mechanical and electrical interlocks that can prevent misoperation, and at the same time, the manual opening and closing position avoids the big shaft of handcart pressure, making manual opening and closing very convenient. After the transformation, the original mechanical interlock is retained, and the electrical interlock is added. The limit switch is placed on the lower right part of the big shaft under the pressure. As long as the pressure bar is moved 6mm, the switch will be electrically operated (the total travel distance is 26mm) to prevent the load from pushing in and pulling the hand truck. The zno varistors used to suppress the operating overvoltage are installed at the lower part of the transformer room of the cabinet. An Angle steel bracket is added in the transformer room to install the zno varistors. The lower end of the zno varistors should be reliably grounded by soft connection.
 
5. Operation and maintenance of vacuum circuit breaker
 
5.1. Periodically measure the out-of-travel of circuit breakers; Over travel of vacuum circuit breaker with less oil circuit breaker over travel concept is different, less oil circuit breaker over travel to the depth of the dynamic contact insert static contact, but the closing of vacuum circuit breaker trip for insulating rod end contact spring is compressed distance, the distance to keep within the scope of the requirements, contact between have enough pressure, can ensure the good contact contact. The overtravel of vacuum circuit breaker is generally 4mm (+0.5, -1; The permissible wear thickness of contact is generally 2 ~ 3mm. In the process of splitting and closing load current or fault current of vacuum circuit breaker, contact is constantly worn, thus overtravel is constantly reduced. Therefore, the overtravel of circuit breaker must be measured regularly, and timely adjustment should be made for those that do not meet the requirements, so as to ensure sufficient second pressure between contact heads to ensure good contact. For general vacuum circuit breakers, the out-of-travel measurement should be carried out once every 2000 times or twice of short-circuit current and 3 months of new operation.
 
5.2 periodically test the vacuum degree of arc extinguishing chamber. The vacuum degree of the circuit breaker's arc extinguishing chamber directly affects the breaking ability of the circuit breaker. Generally, the vacuum degree of arc extinguishing chamber should be tested every 2000 times or every year. The detection method is to increase the processing frequency voltage to 42KV at a rate not more than 12kV/s when the dynamic and static contact of vacuum circuit breaker is at the normal opening distance (13mm), and there should be no abnormal phenomenon after it is stabilized for one minute.
 
5.3 replacement conditions of arc extinguishing chamber. The arc extinguishing chamber whose service life has reached or has abnormal phenomena must be replaced, and the replacement conditions are generally as follows:
 
5.3.1 contact wear of vacuum circuit breaker has reached or exceeded the specified value.
 
5.3.2 the vacuum degree of arc extinguishing chamber cannot reach the standard required value.
 
5.3.3 its mechanical operation life has reached the specified value. The replacement of vacuum circuit breaker arc extinguishing chamber shall strictly follow the specific technical standards and relevant technical requirements of the manufacturer.
 
Conclusion 6.
 
Using vacuum circuit breaker to transform the old series switch cabinet with less oil circuit breaker, its technology is mature and feasible, and has the advantages of less investment, quick effect, short power outage time during the transformation, and so on, this technology is worth popularizing and applying in the technical transformation of city network and enterprise power grid.
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