The load switch is mainly used to break and close the load current. The load switch can also be used in conjunction with a high-voltage fuse instead of a circuit breaker. Because load switches are easy to use and reasonable in price, load switches are widely used in 10kV distribution network systems. Reasonable selection of load switches in the design is of great significance to ensure the safe and reliable operation of the power grid.
1 Correct coordination of load switch and fuse
The fundamental difference between load switch and fuse is that the fuse has the ability to break short-circuit current, while the load switch is only used for switching the load current. It is generally believed that the load switch closes and divides the operating current, and the fuse breaks the short-circuit current. However, when a fault occurs, because the three-phase current is not necessarily the same, and the error allowed by the fuse, the fusing time difference between the three-phase fuse is unavoidable. After the first phase removes the fault, if the load switch cannot interrupt the load current in time, it will Causes transfer current and two-phase operation, causing damage to the power receiving equipment. A fuse with a striker and a load switch with a trip device can solve the problem of lack of phase operation. When the fuse of the fuse melts, the load switch trip device is immediately disconnected under the operation of the striker. Manufacturers mostly adopt a four-bar linkage mechanism. When the load switch is closed, the closing and opening springs are stored at the same time. When the four-bar linkage mechanism passes the dead point, the energy of the closing spring is released and the switch performs the closing operation. The energy of the brake spring is still maintained by the half-shaft mechanism. Once the striker strikes, the half-shaft is disassembled, the energy of the opening spring is released, and the switch performs opening operation. Therefore, in use, you must choose a fuse with a striker and a load switch with a mechanical trip device.
It should be pointed out that most of the fuses in use are used as backup protection fuses. This type of fuse has a minimum breaking current, which is 2.5 to 3 times the rated current of the fuse. When it is less than the breaking current, the backup fuse cannot Breaking this current is the difference between it and a full range fuse. The full-range fuse causes the melt to melt to the rated breaking current (40kA). Any current can be reliably disconnected, but it is expensive. When the fault current is less than the minimum breaking current of the backup fuse, although the fuse cannot be guaranteed to be broken, the fuse will blow, the impactor in its memory will be hit, and the impact load switch will be broken. For example, for a fuse with a rated current of 100A, its minimum breaking current is about 250-300A. In this current range, the fuse cannot be broken, but the fuse blows the striker and strikes the load switch to trip and break this current. The load switch of 600A can be used for reliable breaking.
Load switch-current-limiting fuse combination has good transformer protection characteristics, but only when the two are well coordinated can it be effective.
A area is the operating current range. I>InK, InK is the rated current of the combined electrical appliance. It is less than the rated current InHH of the fuse, because the temperature condition and heat loss dissipation during the installation of the fuse are limited, so that the combined electrical appliance cannot withstand the full current of the fuse. The rated current breaking of the combined electric appliance is completed by the load switch alone. The three load switches are opened at the same time, and the arc is extinguished at the same time.
B area is the overload range InHH<I<3InHH, within this range, the fuse is subjected to overcurrent exceeding the rated current. Starting from about 2InHH, the melt acts, but the fuse cannot yet extinguish the arc, and the striker of the fuse is triggered to make the load switch act, the three-phase open and the arc extinguish. Here, the meaning of melt action is that all melts are broken at least in one place. That is to say, in the overload range, the three-phase load switch opens and extinguishes the arc.
C area is the transfer current ITC range from about 3InHH, the arc can also be extinguished after the fuse is activated. In a three-phase circuit, one phase of the three-phase fuse acts first, 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 act, but the load switch sometimes moves faster. Therefore, in the current transfer area, 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 their respective power factors. It is between 5InHH (small fuse) and 1.5InHH (large fuse).
The d area is the current limit range. When the fault current is larger (from about 20InHH), the fuse has been activated in the first half wave of the current and limits the peak value of the fault current to its let-through current value ID. This is the current that the fuse extinguishes is greater than the transfer current ITC. Although the load switch operates under the action of the striker, it does not break the current.
Therefore, only the load switch and the fuse are matched properly, the combined electrical appliance can be broken: any load current of the load switch rated breaking current; any overcurrent of the combined electrical rated short-circuit breaking current. That is to say, the load switch plus the fuse can undertake the breaking task between the working current and the full short circuit.
2 Impactor operation and transfer current
The current passing through the fuse and the fusing time have inverse time characteristics, referred to as ampere-second characteristics. When an overcurrent occurs, the fuse will blow according to its ampere-second characteristics. The so-called transfer current means that one phase of the three-phase fuse is opened first, and the fusing time difference of the three-phase fuse is Dt. When the prime minister moves, the striker strikes out. At this time, the other two-phase fuses may not be extinguished and broken, and the striker strikes to form a load switch to cut off the fault current. The breaking task that should be undertaken 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 ampere-second characteristic of the fuse and the fixed breaking time of the load switch. The transfer current value can be determined by referencing the IEC-420 standard. On the fuse-second characteristic time axis, take 0.9 times the load switch fixed time and make a parallel line, and the corresponding current value is the transfer current. For example, a vacuum load switch, its inherent breaking time is 28ms, equipped with a 100A fuse, and the transfer current is calculated to be 1880A according to law, and the load switch should be able to break this current. When the fault current exceeds the transfer current, it is broken by a fuse. In fact, the transfer current is a current area. Due to the difference in the melting time between the three-phase fuses, there is a relative current difference, so it is a small current area, which is the transfer current area. It can be seen that the good cooperation between the load switch and the fuse can break any current. Obviously, different rated currents of the fuse have different ampere-second characteristics, so different rated currents and the same load switch will have different transfer currents. The rated transfer current refers to the transfer current of the largest fuse that can be equipped. Should pay attention to choosing load switch.
3 shunt release operating power and transfer current
With the promotion of "few people on duty" and "unattended" in substations, in order to meet the basic requirements of remote operation of operating units, when selecting load switches, it is necessary to configure shunt releases for protection tripping, that is, pass through when overloaded. The relay protection method opens the load switch, and the fuse is only for short-circuit protection. The action of the shunt release to disconnect the load switch in the combined electrical appliance is called the release operation. The time-current curve of the relay protection and the fuse will not be the same, and a cross point will inevitably occur when used together. The point of intersection between the action characteristic of the relay protection and the ampere-second characteristic of the fuse is called the "transfer current". In engineering, the method of determining the maximum transfer current according to IEC is: take the minimum opening time of the load switch on the time axis of the maximum arc front ampere-second characteristic of the fuse, plus the minimum action time of the external relay protection of 20ms, the corresponding current value is Maximum transfer current.
4 combination of electrical appliances selection
When selecting a combined electrical appliance, the value of the transfer current or the transfer current of the selected load switch must be appropriate. The selection of the 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, compressed gas, SF6 and vacuum according to its arc extinguishing principle. The gas-producing switch is heated by the arc extinguishing tube generated between the contacts of the opening, and the gas is generated to extinguish the arc. As the number of breaking times increases, the arc extinguishing tube gradually burns out, so the arc extinguishing tube must be replaced continuously. Gas-producing load switches can only break currents below 1000A. The compressed air switch relies on the gas generated during the opening movement of the movable contact rod to extinguish the arc. The movable contact rod is a hollow copper rod with a fixed piston inside, and the arc is extinguished by the gas generated during the opening movement. The approximate breaking current of the compressed air load switch is between 1350 and 1850A. A value below this range will greatly reduce the reliability of the product. The main advantage of the SF6 switch is three or four circuits. It is small in size and not affected by the external climate. However, SF6 gas consumes ozone resources and does not conform to the trend of environmental protection. Its current breaking capacity is between 2000～3500A. 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 ability. In 2000, the market used MINRONG/民Most melting production. Air and compressed air load switches are difficult to break transfer currents above 2000A, so it is recommended to use them in projects with a transformer capacity of 500kVA or less. For transformers with a capacity of 500kVA or more, it is recommended to use a vacuum load switch. The vacuum load switch adopts a vacuum interrupter. The static and dynamic contacts are in the interrupter. The vacuum interrupter has strong breaking capacity, stable performance, no fire and explosion hazards, and can be operated frequently, maintenance-free, and not only can be opened If the transfer current is interrupted, the transfer current can also be interrupted to reach over 2880A. In addition, choose a vacuum interrupter with a larger breaking capacity, such as between 10 and 25kA, which is most suitable for inverse time protection. This kind of vacuum load switch with high breaking capacity and equivalent secondary protection can actually be used as an economical vacuum circuit breaker.
Through the analysis of the selection of load switches in the 10kV distribution network system design, it provides experience on how to reasonably select load switches in the 10kV distribution network to ensure the safe and reliable operation of the power grid.
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