Load Break Switch
It is usually equipped with a simple arc extinguishing device, but its structure is relatively simple. The figure shows a compressed air high-pressure load switch. The working process is: when the brake is opened, the spindle rotates clockwise under the action of the opening spring. On the one hand, the piston is moved upward by the crank slider mechanism to compress the gas; In the aspect, the transmission system consisting of two sets of four-bar linkages causes the main knife to open first, then pushes the arc-extinguishing knife to open the arc contact, and the compressed air in the cylinder blows out the arc through the nozzle.
1.1 Basic concept
The load switch is a kind of switching device between the circuit breaker and the isolating switch. It has a simple arc extinguishing device, which can cut off the rated load current and a certain overload current, but cannot cut off the short circuit current.
1.2 Load switch classification
According to the use voltage, it can be divided into a high voltage load switch and a low voltage load switch.
1.2.1 High voltage load switch
There are 6 kinds of high voltage load switches.
220.127.116.11 Solid gas-producing high-pressure load switch: The energy of the arc-breaking arc itself is used to generate gas from the gas-generating material of the arc chamber to blow out the arc. The structure is relatively simple and suitable for products of 35 kV and below.
18.104.22.168 Compressed high-pressure load switch: The arc is blown out by the air pressure of the piston during the breaking process, and the structure is also relatively simple, and is suitable for products of 35 kV and below.
22.214.171.124 Compressed air type high-voltage load switch: the use of compressed air to blow off the arc, can break the large current, its structure is more complex, suitable for products of 60 kV and above.
126.96.36.199 SF6 high-voltage load switch: using SF6 gas to extinguish the arc, its breaking current is large, the breaking capacitor current performance is good, but the structure is more complicated, suitable for products of 35 kV and above.
188.8.131.52 Oil-immersed high-pressure load switch: The energy of the arc itself is used to decompose and vaporize the oil around the arc and cool and extinguish the arc. The structure is simple, but the weight is large, and it is suitable for outdoor products of 35 kV and below.
184.108.40.206 Vacuum high-voltage load switch: use vacuum medium to extinguish the arc, long electrical life, relatively high price, suitable for products of 220 kV and below.
The working principle of the high voltage load switch is similar to that of the circuit breaker. It is usually equipped with a simple arc extinguishing device, but its structure is relatively simple. The figure shows a compressed air high-pressure load switch. The working process is: when the brake is opened, the spindle rotates clockwise under the action of the opening spring. On the one hand, the piston is moved upward by the crank slider mechanism to compress the gas; In the aspect, the transmission system consisting of two sets of four-bar linkages causes the main knife to open first, then pushes the arc-extinguishing knife to open the arc contact, and the compressed air in the cylinder blows out the arc through the nozzle. When the switch is closed, the main and the arc extinguishing knives are rotated clockwise by the spindle and the transmission system, and the arc contacts are closed first; the main shaft continues to rotate, so that the main contacts are subsequently closed. During the closing process, the opening spring simultaneously stores energy. Because the load switch can not break the short-circuit current, it is often used in combination with the current-limiting high-voltage fuse. By using the current limiting function of the current limiting fuse, not only the task of breaking the circuit but also the heat caused by the short-circuit current can be significantly reduced. And the role of electric power.
1.2.2 Low voltage load switch
The low voltage load switch is also called the switch fuse group. It is suitable for AC power frequency circuit to manually switch on and off the load circuit manually; it can also be used for line overload and short circuit protection. The switching circuit is completed by a knife, and the overload and short circuit protection is completed by the fuse. The rubber-capped knife switch and the iron-shell switch used before the 1970s belong to the low-voltage load switch. The small-capacity low-voltage load switch contact split speed is related to the handle operating speed. The large-capacity low-voltage load switch operating mechanism adopts the spring energy storage action principle, and the splitting speed has nothing to do with the speed of the handle operation. The structure is relatively simple, and a reliable mechanical interlock device is attached. After the cover is opened, the switch cannot be closed and the switch is closed. The lid cannot be opened after closing, which ensures safe work.
1.3 Purchasing points
The load switch is mainly used to break and close the load current, and the load switch can be used in combination with the high-voltage fuse instead of the circuit breaker. Due to the convenient use and reasonable price of the load switch, the load switch is widely used in the 10kV distribution network system. Reasonable selection of load switches in the design is of great significance for ensuring the safe and reliable operation of the power grid.
1.3.1 The correct cooperation of the load switch and the fuse
The fundamental difference between a load switch and a fuse is that the fuse has the ability to break short-circuit current, and the load switch only serves as a switching of the load current. It is generally believed that the load switch is divided into operating currents and the fuses open short-circuit currents. However, when a fault occurs, since the three-phase currents are not necessarily the same, and the fuses agree with the error, the fuse time difference between the three-phase fuses is inevitable. After the first phase cuts off the fault, if the load switch cannot break the load current in time, Causes the transfer current and two-phase operation, causing damage to the powered equipment. A fuse with a striker, combined with a load switch with a trip unit, solves the problem of phase loss operation. When the fuse of the fuse melts, the load switch trip device is immediately disconnected under the operation of the striker. The production plant often adopts a four-bar linkage mechanism. When the load switch is closed, the closing spring simultaneously stores energy. When the four-bar linkage is over dead, the energy of the closing spring is released, and the switch is closed. The energy of the brake spring is still maintained by the half shaft mechanism. Once the striker hits, the half shaft is disengaged, the energy of the trip spring is released, and the switch performs the opening operation. Therefore, it is necessary to select a fuse with a striker and a load switch with a mechanical trip device in use.
It should be pointed out that the fuses in use are mostly used as backup protection fuses. The fuses have a minimum breaking current, which is 2.5 to 3 times the rated current of the fuse. When the current is less than the breaking current, the backup fuse cannot This current is interrupted, which is the difference between it and the full range of fuses. The full range of fuses can cause a reliable disconnection of any current between the melt melting and the rated breaking current (40kA), but it is expensive. When the fault current is less than the minimum breaking current of the backup fuse, although the fuse can not guarantee its breaking, the fuse will be blown, the impactor of the memory will be hit, and the impact load switch will be broken. For example, a fuse with a rated current of 100A has a minimum breaking current of about 250 to 300 A. In this current region, the fuse cannot be broken, but the fuse blows the striker and hits the load switch to trip, breaking the current, such as With the 600A load switch, it can be reliably disconnected.
The load switch-current limiting fuse combination electrical protection transformer has good characteristics, but only when the two are matched can it be effective.
The a area is the operating current range. I>InK, InK is the rated current of the combined appliance. It is smaller than the rated current InHH of the fuse. This is because the temperature condition and heat loss dissipation of the fuse are limited, so that the combined electrical device cannot withstand the entire current of the fuse. The rated current breaking of the combined electrical device is performed by the load switch alone. The three phases of the load switch are simultaneously turned off, and the three phases are simultaneously extinguished.
The b region is the overload range InHH
The c-zone is the transfer current ITC range from about 3InHH, and the fuse can also be extinguished after the action. In a three-phase circuit, one of the three-phase fuses first acts, triggering the striker and extinguishing the arc. The load switch extinguishes the current in the other two phases, and the other two-phase fuses may also operate, but the load switch sometimes moves faster. Therefore, in the transfer current region, the fuse and the load switch cooperate to complete the breaking task. The transfer current is the maximum current that the load switch can break under its respective power factor. It is between 5InHH (small fuse) and 1.5InHH (large fuse).
The d region is the current limiting range. When the fault current is larger (about 20InHH), the fuse operates in the first half of the current and limits the peak value of the fault current to its let current value ID. This is the current that the fuse is extinguished by more than the transfer current ITC. The load switch operates under the action of the striker, but does not break the current.
Therefore, only when the load switch and the fuse are properly matched, the combined electric appliance can be disconnected: any load current of the rated breaking current of the load switch; any over current of the rated short-circuit breaking current of the combined electric appliance. That is to say, the load switch plus fuse can undertake the breaking task between the working current and the full short circuit.
1.3.2 striker operation and transfer current
The current passing through the fuse and the blow time are inverse-time characteristics, referred to as the An-second characteristic. When an over current occurs, the fuse is blown according to its An-second characteristic. The so-called transfer current means that one phase of the three-phase fuse is first broken, and the fuse time difference of the three-phase fuse is Dt. When the prime minister moves, the striker strikes. At this time, another two-phase fuse may not be extinguished, and the striker hits out to form a load switch to cut off the fault current. The breaking task that should be assumed by the fuse is now transferred. To the load switch. When the fuse and the load switch are switched off, the symmetrical current is called "transfer current". Obviously, the value of the transfer current is related to the fuse's amp-second characteristic and the load switch fixed breaking time. The transfer current value can be determined by reference to the IEC-420 standard. In the fuse-an-second characteristic time axis, take the load-time of 0.9 times the load switch and make a parallel line. The corresponding current value is the transfer current. For example, a vacuum load switch has a natural breaking time of 28 ms, and is equipped with a 100 A fuse. The transfer current is determined to be 1880 A according to law, and the load switch should be able to break the current. When the fault current exceeds the transfer current, the fuse is broken. In fact, the transfer current is a current region. Because there is a blow time difference between the three-phase fuses, there is a relative current difference, so it is a small current region, which is the transfer current region. It can be seen that the good cooperation of the load switch and the fuse can interrupt any current. Obviously, different rated currents of fuses have different amp-second characteristics. Then different rated currents are contracted with one load switch, and there are different transfer currents. The rated transfer current refers to the transfer current that can be used with the maximum fuse. Select the load switch should pay attention.
1.3.3 shunt release operating power and handover current
With the promotion of “small people on duty” and “unattended” in substation, in order to meet the basic requirements of remote operation of the operating unit, when selecting the load switch, it is necessary to configure the shunt release for protection tripping, that is, when overloading The relay protection method opens the load switch and the fuse is only short-circuit protected. The action of the shunt release device causes the load switch in the combined appliance to be disconnected, which is called trip unit operation. The time-current curve of the relay protection and the fuse will not be the same, and the intersection will inevitably occur. The point at which the action characteristic of the relay protection intersects with the safety-second characteristic of the fuse is called "handover current". The method for determining the maximum handover current according to the IEC in the project is: the minimum opening time of the load switch is taken in the time axis of the maximum arc front-second characteristic of the fuse, plus the minimum operating time of the external relay protection of 20ms, the corresponding current value is Maximum transfer current.
1.3.4 selection of combination appliances
When selecting a combination device, the value of the transfer current or the transfer current of the selected load switch must be appropriate. The choice of fuse should consider the rated current of the transformer, the overload factor is 1.5, the magnetizing inrush current is 12 times 0.1s, and the surrounding environmental factors. The load switch can be divided into gas production, gas pressure, SF6 and vacuum according to its arc extinguishing principle. The gas producing switch is an arc-extinguishing arc extinguishing tube generated between the contacts of the opening, and generates a gas to extinguish the arc. As the number of breaking times increases, the arc extinguishing tube gradually burns out, so the arc extinguishing tube is continuously replaced. The gas-generating load switch can only interrupt currents below 1000A. The gas-pressing switch generates gas by arcing when its moving rod is opened, and the moving rod is a hollow copper rod, which is provided with a fixed piston, and generates gas by arcing to extinguish the arc. The compressed air load switch has a breaking current of between 1350 and 1850 A. Below this range, the reliability of the product is greatly reduced. The main advantage of the SF6 switch is three or four circuits. It is a common box in the SF6 gas, small in size, and is not affected by the external climate. However, the SF6 gas consumes ozone resources and does not conform to the environmental protection trend. Its current breaking capacity is between 2000 and 3500A. After 2000, the general user engineering design requires the SF6 load switch to have zero gauge pressure and the ability to break the normal load. Even when the SF6 gas leaks, it must have a certain load breaking capacity. In 2000, the market used MINRONG/Minmin Most of the melting production. Air and compressed air load switches are difficult to break the transfer current of 2000A or more. Therefore, it is recommended to use in the following projects with a transformer capacity of 500kVA. For transformers with a capacity of 500kVA or higher, it is recommended to use a vacuum load switch. The vacuum load switch adopts a vacuum interrupter, and the dynamic and static contacts are all in the arc extinguishing chamber. The vacuum interrupter has strong breaking capacity, stable performance, no fire and explosion hazard, and can be frequently operated and maintenance-free. Breaking the transfer current, it is also possible to break the transfer current to more than 2880A. In addition, a vacuum interrupter with a larger breaking capacity, for example between 10 and 25 kA, is best suited for use with inverse time protection. This kind of high-breaking capacity and equivalent secondary vacuum load switch can be used as an economical vacuum circuit breaker.
Through the analysis of the load switch problem in the 10kV distribution network system design, it provides experience for how to properly select the load switch in the 10kV distribution network to ensure the safe and reliable operation of the power grid.
1.4.1 Vertical installation, switch frame, closing mechanism, cable sheath and protective steel tube should be grounded reliably (cannot be grounded in series).
1.4.2 Before operation, several empty load and closing operations should be carried out. There is no jamming in each rotating part, and the closing is in place. There is enough safety distance after opening.
1.4.3 The fuse melt used in series with the load switch should be properly selected, that is, the fault current should be greater than the breaking capacity of the load switch to ensure that the melt is first blown, and then the load switch can be opened.
1.4.4 The contact is good when the switch is closed, and there is no overheating at the connection. During the inspection, the porcelain bottle should be checked for dirt, cracks, porcelain, and flashing discharge; the switch cannot be flushed with water (indoor type). (When a high voltage cabinet controls a transformer, it is best to stop the circuit high voltage cabinet when replacing the fuse.)