Coordination between the hottest load switch and f

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The coordination between load switch and fuse

1. The principle of current limiting circuit breaker

everyone knows that the breaking capacity of current limiting circuit breaker is very high, which can reach Ka, but the data given here is often the expected short-circuit current value, not the actual short-circuit current value through the current limiting circuit breaker. Therefore, in the distribution system protected by current limiting circuit breaker, the dynamic thermal stability is verified according to the estimated maximum load system, Pay attention to the selected short-circuit current parameters. Some manufacturers claim that their low-voltage circuit breakers are all current limiting circuit breakers. After understanding the current limiting principle of current limiting circuit breakers, we can distinguish the authenticity. The current limiting problem of low-voltage circuit breaker is proposed to break the large fault short-circuit current caused by low impedance large capacity transformer and the continuous development of distribution network. It requires that the breaking time of circuit breaker is short enough to make the short-circuit current break before reaching its expected peak. In the late 1950s, France first studied the current limiting breaking problem, which was widely used in various industries such as aerospace, automobile, shipbuilding and energy, and developed the current limiting air circuit breaker, so that the short-circuit breaking capacity of the air circuit breaker reached 100kA. For decades, many mature and effective current limiting technologies have been widely used in low-voltage circuit breakers, such as deionized grid arc extinguishing, current limiting resistance, self resetting current limiting elements, magnetic blowing, electric repulsion, VJC and solid insulation screen current limiting technology. At present, in the design of the advanced molded case circuit breaker, the air electromagnetic principle and current limiting principle are fully used to make its breaking capacity reach 200kA. In recent years, with the introduction of computer technology, control technology, new material technology and power electronics technology, current limiting technology has been updated, such as superconducting current limiter, GTO based current limiter, controllable impedance converter and fault detection technology. The research of these new current limiting breaking technologies will greatly improve the breaking capacity and current limiting capacity of circuit breakers

the principle of current limiting breaking of traditional low-voltage circuit breakers is that when a fault occurs, the contact opens quickly to produce an arc, which is equivalent to a rapidly increasing arc resistance in series in the circuit, so as to limit the short-circuit current. This rapidly growing arc resistance is often referred to as "dynamic arc resistance". Different from the arc extinguishing chamber of general circuit breaker, the arc extinguishing chamber of low-voltage current limiting circuit breaker adopts multiple arc extinguishing grids. In the process of breaking, firstly, the moving contact and the static contact separate to produce an arc, which moves to the arc extinguishing grid under the action of electromagnetic field, thermal field and flow field. When the arc enters the grid, due to the near pole voltage drop of several short arcs, the arc voltage rises rapidly, so as to achieve the purpose of current limiting. In order to have a higher arc voltage, the number of grids in the arc extinguishing chamber of the current limiting circuit breaker is more than that of the general circuit breaker, and it is arranged more closely. The faster the arc voltage rises, the better the current limiting effect. Finally, the arc voltage exceeds the value of the power supply voltage, making the power supply voltage unable to maintain the arc, so as to complete the arc extinguishing current limiting breaking. To make the arc voltage rise rapidly, there are two traditional methods: (1) magnetic blowing coil. In this case, the arc will be rapidly elongated, which not only increases the length of the arc, but also increases the heat conduction area of the arc. (2) Use the arc strike channel to quickly increase the arc voltage. When the contact is opened, the electromagnetic force along the arc striking path will lengthen the arc. When the arc is driven to the arc extinguishing chamber, it will be further divided and cooled. The premise of this method is: ① the arc must be forced out of the contact (it will occur only when the gap between the contacts is greater than about 1mm); ② The arc must be separated from the contact area very quickly, which reduces the loss of contact material and restores the contact gap at the same time; ③ The arc must move along the arc strike path at a very fast speed (about 100m/s), which is expensive, and then enter the deionized grid to improve the final arc voltage value

in the design of current limiting circuit breaker, there are four basic principles as follows:

a the contact opens quickly

b quickly increase the arc voltage

C make the final arc voltage value high

d fast dielectric strength recovery

the commonly used current limiting techniques are divided into three categories:

1) artificial zero point method. The arc is used to generate artificial zero, so that the current in the arc gap is zero, so that the arc is extinguished

2) improve the static volt ampere characteristic of arc. Deionized grid method, insulated grid method, narrow gap method and VJC method are usually used. Deionization grid method is to use metal grids to divide the arc into several short arcs connected in series, and use the voltage drop of the short arc to increase the arc voltage and extinguish the arc; Insulated grid method: that is, the grid is insulated, and its function is to derive the heat of the arc to improve the arc column voltage. At the same time, the grid divides the arc into several short arcs, and each grid is the electrode of the short arc, which produces many anode voltage drops and cathode voltage drops at the same time. For DC arc, the arc voltage drop near the pole and the voltage drop of the arc column are used to extinguish the arc together; The narrow slot method usually adopts multiple narrow slots, which can reduce the resistance of the arc entering the upper narrow slot. Therefore, when the electromagnetic force driving the arc movement is given, the gap smaller than the single narrow slot arc extinguishing chamber can be used. On the one hand, the arc diameter can be compressed to make the arc in close contact with the gap wall; On the other hand, it also increases the area and length of the arc, which further strengthens the role of cooling and de dissociation and extinguishes the arc; VJC method is mainly to cover a certain thickness of insulation or high resistance metal materials around the electrode, so as to control the arc column and achieve the purpose of increasing the arc voltage. The solid insulation screen method uses a solid insulation screen to quickly insert into the contact that breaks the fault current, so that the arc burning between the contacts is separated by the screen and quickly extinguished. The above products are a relatively popular new type of light-weight floor decoration materials in the world. Methods are usually used comprehensively, such as VJC and multi slit method, to achieve better current limiting and breaking effect

3) method of increasing contact breaking speed. Usually, a huge breaking spring or other acceleration device is used to open the contact, or an energy storage capacitor is used to discharge the repulsion coil, and eddy current is induced in the aluminum plate to generate a huge electric repulsion force, so as to open the moving contact. At the same time, the action of the release and the action of the mechanism are accelerated as much as possible to achieve the purpose of high-speed breaking. In this way, the smaller the time required for separation, the greater the current limiting effect. In the 1960s, power electronic devices were introduced into electrical appliances. At present, the contactless thyristor circuit breaker and the contact thyristor parallel hybrid circuit breaker have been developed in some countries and have been applied to a certain extent. However, due to the high conduction voltage drop of power electronic devices, the high energy consumption, the gap insulation distance of the breaker cannot be formed, the overload capacity is poor, the working parameters are lack of corresponding voltage levels and the cost is high, These make the contactless electrical apparatus composed of them can not be widely used. Of course, contactless electrical appliances have the characteristics of high operation rate, fast switching speed, low control power, low noise and long service life, which are suitable for some special workplaces. In the current limiting, the hybrid type with contacts is mainly used, such as the hybrid circuit breaker with parallel contacts and thyristors, which has the characteristics of low voltage drop and energy consumption when the contacts are normally connected, and then the characteristics of short breaking time of power electronic devices are used to further shorten the breaking time of current, so as to realize current limiting and breaking. In the design of circuit breaker, the parameters of current and voltage of power electronic devices should be considered. In the early days, thyristors were used, but they could not be turned off by themselves and needed to be switched off, resulting in the increase of the volume of electrical appliances. At present, self turning off devices are usually considered, such as IGBT (insulated gate bipolar transistor), GTO (turn off transistor), etc

2. Selective coordination between high-voltage current limiting fuse and low breaking capacity apparatus

in this case, the current limiting fuse acts under large fault current, and the low breaking capacity appliances can only break the small current it allows, so it needs to be realized according to the different time current curves of the two. 1/taking the intersection point of the curve as the boundary, the current limiting fuse bears the breaking of large fault current, and other electrical appliances bear the breaking of normal current and small fault current. 2/if other electrical appliances do not interlock and trip with the fuse impactor, the intersection point must be greater than the minimum action current of the high-voltage current limiting fuse. 3/the current at the intersection of the curve must be less than the breaking capacity of other electrical appliances. 4/when the high-voltage current limiting fuse is used to disconnect the circuit, other electrical appliances must have sufficient ability to pass the short-circuit current and close the short-circuit current (the thermal stability and dynamic stability of the appliance should be verified by its breaking current and closing current). These abilities should be compatible with the cut-off current and i2t value of the high-voltage current limiting fuse. 5/if other electrical appliances are interlocked and tripped with the fuse impactor, such as the load switch, the allowable transfer current value of the load switch is required to be greater than the maximum transfer current value of the fuse. (end)

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