Discussion on the control and protection function

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Discussion on the control and protection function of the combined apparatus for 10kV transformer

in the control and protection of the distribution transformer of 10kV small and medium capacity end users, the combined apparatus composed of high-voltage load switch and high-voltage fuse has replaced the high-voltage circuit breaker used in the past. Due to the advantages of small size, compact structure, simple system wiring and low equipment cost, this kind of combined electrical appliance is widely used in 10kV substation in Jinan testing machine factory, especially in box type substation. This paper discusses the problems and solutions existing in the control, protection function, working characteristics and application of the distribution transformer of 10kV small and medium capacity end users by the combination appliance

high voltage circuit breakers are mainly used to break the normal load current and the short-circuit current in case of fault in the power system. In order to realize the function of automatically breaking the short-circuit current in case of fault, complex relay protection system devices must be equipped. The high-voltage switch of 10kV medium and small capacity distribution transformer terminal users frequently interrupts the normal load current. The combination appliance is used to replace the high-voltage circuit breaker, the high-voltage load switch is used to cut off the normal load current, and the fuse is used to cut off the short-circuit current, which can greatly simplify the relay protection device and reduce the equipment cost

in the full breaking process of cutting off the short-circuit current, the high-voltage circuit breaker must go through the action time of relay protection, the inherent opening time, arc burning and arc extinguishing time of the operating mechanism. These three parts of time are about three cycles of more than 60ms, and the fuse can quickly cut off all kinds of serious short-circuit fault currents within 10ms, quickly and effectively isolate the influence of the fault area on electricity, thus shortening the fault duration and influence range. The combined apparatus composed of the multi-function testing machine of Jinan testing machine factory, the fuse with fast fusing characteristics, and the corresponding auxiliary devices equipped with the load switch can fully meet the protection requirements of the GB specification for the 10kV distribution transformer of small and medium-sized end users for the control and protection of the distribution transformer of small and medium-sized end users

high voltage fuses rely on the melt (i.e. fuse) to effectively protect the good anti delay ampere second characteristics of overload and various short-circuit faults in the protected line. It does not have any operating mechanism and control circuit, and completely adopts the protection mode of phase failure operation. If there is a fault in any phase, the melt of any phase melts itself and the power is cut off, the phase will be lost. The characteristic of this melt phase loss operation protection is that the anti delay ampere second characteristic of melt fusing is highly dispersed within the qualified range, which determines that the protection of high-voltage fuse against fault has the objective existence of a certain phase fuse link fusing. This phase loss operation caused by a phase failure and fuse fusing will directly affect the abnormal voltage of the corresponding two phases on the low-voltage side of the distribution transformer. For the distribution transformer of Y, yn0 connection group, the central point will shift due to the asymmetry of three-phase voltage. For the distribution transformer of D, yn11 connection group, the corresponding two-phase voltage on the low-voltage side will be 1/2 of the rated voltage, making the load operate under voltage, which constitutes a 1-dimensional R & D and verification experiment plan. In order to avoid the undervoltage operation of load, the fuse can be changed to the method of internal connection, so that the voltage on the low-voltage side corresponding to the fault phase is zero, and the other two-phase voltages are normal. However, this method is only effective for the protection of single-phase load in single-phase power supply system. China adopts three-phase power supply system, and three-phase power and single-phase lighting load are often mixed power supply. The power system has restrictions on three-phase asymmetric load, and phase failure operation is not allowed. Lack of phase operation will cause the three-phase electrical equipment to fail to start, and the three-phase electrical equipment in slightly pearl luster operation may also be burned due to lack of phase operation, so this is not a complete solution to the problem

the method to completely solve the lack of phase operation: install a striker on the fuse. When the fuse is fused, the striker strikes the high-voltage load switch. The tripper adopts the method of mechanical tripping, or the shunt tripper that sends an electrical signal to start the high-voltage load switch adopts the method of electrical tripping, so that the load switch disconnects the three-phase high-voltage power supply at the same time. At this time, the breaking current of the load switch is the transfer current. After the transition process between the transmission time of the linkage and the inherent breaking time of the load switch, the current when the load switch breaks and extinguishes the arc is obviously much larger than the load current, which constitutes the whole process of breaking the fault current of the load switch fuse combination

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