In recent years, in the selection of the protection and control switch of the 10kV distribution transformer, the combination of the load switch-fuse combination electric appliance has the advantages of simple structure, convenient operation and maintenance, low cost, reliable operation, etc. Get a wide range of applications. In practical applications, how to correctly select the combination of electrical appliances, load switches, fuses and transformers how to properly select the parameters is related to whether the function of the combined electrical appliances can be played to ensure the safe operation of the system.
1 Transfer current verificationSince the three-phase fuse melt melting of the combined electric appliance has a time difference, one phase of the three-phase fuse is first disconnected, and the striker acts. At this time, the other two-phase fuse may not be extinguished and the striker is formed. The phenomenon that the fault current is cut off by the load switch, that is, the breaking task originally undertaken by the fuse is transferred to the load switch. Therefore, the transfer current refers to the three-phase symmetrical current when the fuse and the load switch are switched. Below this value, the first open phase current is interrupted by the fuse, and the other two phase currents are interrupted by the load switch. Above this value, the three-phase current is only interrupted by the fuse. The transfer current is an important indicator that we should pay attention to when selecting a combination of electric appliances. If the selection is improper, the transfer current that the load switch can withstand is not enough, and it will be unable to bear the task of breaking the two-phase short-circuit current and causing the explosion of the switch.
The load switch is usually divided into two types: general type and frequent type. The gas-generating and gas-pressure type load switches with air as the insulating medium are general type, vacuum and SF6 load switches are frequent type, different load switches and transfer current indicators are used. Different, the transfer current of the general type load switch is about 800~1000A, and the frequent type can reach 1500~3150A.
The capacity of the distribution transformer is different, and the corresponding transfer current is also different. The actual transfer current can be estimated by the transformer capacity. Generally, the transfer current of the S9-800/10 type distribution transformer is 978A.
According to the definition of the transfer current and the breaking time and characteristics of the load switch, the load switch transfer current should avoid the maximum short-circuit current and be controlled within 70% of the maximum short-circuit current, that is, the actual transfer current is about 978×70%=685A. On the basis of analyzing the technical coefficient of domestic load switch and fuse, taking into account the discreteness of the product, according to the verification result of the transfer current, the transformer with the capacity of 800kVA or less can be selected for the general load of air insulation according to the experience of our city. Switches, transformers with a capacity in the range of 800 to 1250 kVA, generally use vacuum or SF6 insulated frequent load switches. Transformers with a capacity greater than 1250kVA require circuit breakers for protection and control. Judging from the operation of the combined electrical appliances of the city for many years, it is safe and reliable, and the situation is good. There has been no accident due to improper matching.
2 Selection of handover current indicatorsSome load switches are equipped with a shunt release for protection tripping such as overload, that is, the overload switch protects the load switch by means of relay protection without burning the fuse, and the fuse is only short-circuit protected. The point at which the action characteristic of the relay protection operated by the shunt release and the time-current characteristic of the fuse intersect is referred to as "handover current". The transfer current is an overcurrent value, which is lower than the overcurrent of the transfer current. The load switch is disconnected by the action of the shunt release. When the transfer current is higher than the transfer current, the fuse is protected. For this reason, the load switch with higher transfer current parameter can be selected, which can effectively reduce the number of times of the fuse, thereby greatly reducing the number of replacement fuses, which has certain technical and economic significance. For vacuum and SF6 load switches, the relatively high transfer current value can improve the transfer current close to the transfer current, so as to give full play to the advantages of such frequent load switches.
3 Optional current limiting fusesIn the load switch-fuse combination electric appliance, the load switch is responsible for the breaking of the normal current or the transfer current, and the fuse bears the breaking of the overload current and the short-circuit current, and the breaking ability of the two electric appliances cooperates with each other to successfully complete the breaking task. Therefore, the selection of current limiting fuses is crucial. The selected current limiting fuse should have the characteristics of high breaking capacity, small minimum breaking current, low operating temperature, steep time-current characteristic curve and small characteristic curve error. At the same time, it should meet the requirements of anti-aging, various installation forms and appropriate dimensions. Also note that at an ambient temperature of 40 ° C, the power loss of the fuse must not exceed 75W.
When a fuse is selected, the rated current of the fuse is matched to the capacity of the transformer. Some people think that the fuse with a large rated current will be selected.
More safe is wrong, which not only causes economic waste, but also makes the "time-current characteristic" of the fuse worse, the protection speed is reduced, and the correct breaking protection of the fuse is affected. According to the IEC standard, in a 10kV system, the rated current of the fuse can be generally selected according to Table 1 for transformers of different capacities:
Table 1 fuse rated current
| 10kV transformer rated capacity (kVA) | 100 | 160 | 250 | 500 | 800 | 1250 |
| Fuse rated current (A) | 16 | 25 | 25 | 50 | 80 | 100 |
(1) For a system with multiple transformers and parallel operation, pay special attention to the verification problem of the transfer current when selecting the combination electric appliance. If the calibration calculation is as described above, if two matching models of the same model and capacity are juxtaposed Operation, if the secondary side terminal of the transformer is short-circuited, the transformer impedance will be only half of the single transformer system, so that the maximum three-phase short-circuit current on the high-voltage side is doubled, and the corresponding possible transfer current is doubled. . Therefore, for a system with multiple transformers and parallel operation, when the combination of electrical appliances is selected, the calculation of the transfer current should be carried out, so that the combined electrical appliances whose transfer current index meets the requirements are selected.
(2) If one of the following requirements is met, the combined electrical equipment shall be equipped with a shunt release to realize the rapid electric opening of the load switch:
1 Oil-immersed transformer with heavy gas protection is required. Under normal circumstances, oil-immersed transformers with capacities of 800kVA and above must be equipped with heavy gas trip protection.
2 Over-temperature trip protection of dry transformers.
3 Trip protection with mis-charged open door with dry-type transformer.
4 has remote operation control requirements.
5 ConclusionIn short, the selection of the load switch-fuse combination electric appliance should be based on the actual use occasion, according to the transformer capacity and operation mode, combined with the technical parameters and breaking capacity of various types of load switches, to obtain the transfer current and the transfer current, Load switches, fuses and transformers are reasonably matched with parameters to make the right choice for the combined appliance to ensure safe and reliable operation of the combined appliance.
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