Common fault causes and improvement of distribution box

reason
      1.1 faults caused by the influence of ambient temperature on low voltage apparatus
The low-voltage apparatus in the distribution box is composed of fuse, AC contactor, residual current action protector, capacitor and meter. The upper limit of ambient air temperature shall not exceed 40 ℃; The average value of ambient air temperature in 24h shall not exceed 35 ℃; The lower limit of ambient air temperature shall not be lower than - 5 ℃ or - 25 ℃.
The distribution box of rural power grid transformation operates outdoors. It is not only directly irradiated by the sun to produce high temperature, but also generates heat during operation. Therefore, in the high-temperature season in midsummer, the temperature in the box will reach more than 60 ℃, and the temperature at this time greatly exceeds the specified ambient temperature of these electrical appliances.
     1.2 faults caused by product quality
      In the transformation of rural power network, due to the large number of distribution boxes required and the short construction period, the distribution box factory needed the supply time and quantity of low-voltage appliances, resulting in the phenomenon of lax requirements for product quality, resulting in some products failing soon after they were put into operation. For example, some models of AC contactors cannot operate because the closing coil of the contactor is burned out shortly after the distribution box is put into operation.
      1.3 faults caused by improper selection of electrical appliances in the distribution box
Due to improper selection of AC contactor capacity during manufacturing, AC contactors with the same capacity are installed for different outgoing circuits, and the imbalance of three-phase load is not considered, so the current level of some outgoing contactors cannot be increased by one current level based on the normal selection model.
improvement
      2.1 for the distribution box with distribution transformer capacity of 100kV · A and above, the temperature control relay (ju-3 or ju-4 ultra small temperature relay) and axial flow fan shall be installed on the box on the left side above the control electrical board at the side wall of the heat dissipation window in the box, so that the exhaust fan can be started automatically when the temperature in the box reaches a certain value (such as 40 ℃), Forcibly expel heat to dissipate heat from the box.
      2.2 protection circuit shall be adopted to prevent the failure of external circuit powered by distribution box. Select a small intelligent phase loss protector. For example, da88cm-ii motor phase loss protection module (Shanghai Product) can be installed in the distribution box to prevent the motor from burning out due to low-voltage phase loss operation.
      2.3 improve the wiring mode of the low-voltage capacitor bank of the original distribution box, and change its installation position from the pile head on the AC contactor to between the low-voltage incoming line of the distribution box and the meter. Prevent inaccurate measurement of the metering device due to phase failure of the capacitor circuit or capacitor damage during operation. In addition, the selected model of capacitor shall be bsmj series products to ensure reliable component quality and safe operation.
      2.4 if a column mounted distribution stand is added, 2mm thick stainless steel plate can be selected when making the shell of the distribution box, and the size of the distribution box can be appropriately enlarged in proportion (on the basis of jp4-100 / 3W used in the agricultural reform project, the size in the width direction of the original box is increased by about 100mm, that is, it is changed from 680mm to 780mm. The overall dimension of the improved distribution box is 1300mm × 780mm × 500mm), so as to increase the electrical safety distance between each branch outgoing line and between the outgoing line and the box shell, which is conducive to the operation and maintenance of agricultural electricians and the replacement of melting parts, as well as heat dissipation.
     2.5 select energy-saving AC contactor (similar to cj20si), pay attention to the connection between the coil voltage of AC contactor and the corresponding terminal selected as current action protector, and pay attention to correct load matching. When selecting AC contactor, products with insulation grade of class A or above shall be selected, and the rated current of its main circuit contact must be greater than or equal to the load current of the controlled line. The rated voltage of electromagnetic coil of contactor is 380V or 220V.
      2.6 selection of residual current action protector. Products conforming to GB 6829 General requirements for residual current operated protectors and certified by China Electrotechnical Product Certification Committee must be selected. Time delay protector with low sensitivity similar to LJM (J) series can be selected. The mode of protector device shall comply with the national standard gb13955-2005 installation and operation of residual current operated protection device. The breaking time of leakage protector, when the leakage current is rated leakage current, its action time shall not be greater than 0.5 s.
      2.7 the incoming and outgoing lines of the distribution box shall be low-voltage cables, and the selection of cables shall meet the technical requirements. For example, vv22-35 is used for the incoming line of the distribution box of 30KVA and 50KVA transformers × 4. Vlv22-35 of the same specification shall be used for shunt outgoing line × 4 cable; The incoming line of distribution box of 80KVA and 100kVA transformer shall use vv22-50 respectively × 4、VV22-70 × 4. Vlv22-50 shall be used for cable and shunt outgoing line respectively × 4、VLV22-70 × 4. After the cable is crimped with the copper aluminum wiring nose, it is connected with the wiring pile head in the distribution box with bolts.
      2.8 selection of fuse (RT, NT type). The rated current of the total overcurrent protection fuse at the low-voltage side of the distribution transformer shall be greater than the rated current at the low-voltage side of the distribution transformer, generally 1.5 times of the rated current. The rated current of the melt shall be determined according to the allowable overload multiple of the transformer and the characteristics of the fuse. The rated current of the melt of the overcurrent protection fuse of the outgoing circuit shall not be greater than the rated current of the total overcurrent protection fuse. The rated current of the melt shall be selected according to the normal maximum load current of the circuit and shall avoid the normal peak current.
      2.9 in order to analyze the reactive power of rural low-voltage power grid, a DTS (x) series active and reactive two in one multi-functional electric energy meter (installed on the side of the meter board) is installed in the box to replace the three originally installed single-phase electric energy meters (DD862 series meters) to facilitate the on-line operation monitoring of the load.