Reason

1.1 faults caused by the influence of ambient temperature on low-voltage electrical appliances

The low-voltage electrical appliances in the distribution box are composed of fuses, AC contactors, residual current operated protectors, capacitors and meters. The upper limit of ambient air temperature shall not exceed 40 ℃; the average value of ambient air temperature for 24 hours 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 not only generates high temperature by direct sunlight, but also generates heat in operation. Therefore, in the hot summer season, the temperature in the box will reach more than 60 ℃, which greatly exceeds the specified ambient temperature of these electrical appliances

1.2 faults caused by product quality

During the transformation of rural power grid, due to the large number of distribution boxes needed and the short construction period, the distribution box factory needed a large number of low-voltage electrical appliances, which resulted in the phenomenon that the requirements for product quality were not strict, resulting in the failure of some products shortly after they were put into operation. For example, some models of AC contactors cannot operate because the closing coil of contactors is burnt out soon after the distribution box is put into operation.

1.3 faults caused by improper selection of electrical appliances in distribution box

Due to improper selection of AC contactor capacity in manufacturing, AC contactor with the same capacity is installed for different outgoing circuits, and the imbalance of three-phase load is not considered, so it is not possible to improve the selection of current level of some outgoing contactors on the basis of normal selection model

Improvement

2.1 for distribution box with distribution transformer capacity of 100kV · A and above, 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 panel at the side wall of the cooling window of the box, so as to automatically start the exhaust fan and forcibly exhaust the heat when the box temperature reaches a certain value (such as 40 ℃) In order to make the box cool.

2.2 the protection circuit shall be adopted to prevent the occurrence of external circuit fault supplied by the distribution box. For example, da88cm-ii motor phase loss protection module (Shanghai Product) can be installed in the distribution box to prevent the motor from burning 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 the connection between the low-voltage incoming line of the distribution box and the meter. In order to prevent the inaccurate measurement of the metering device due to the phase failure of the capacitor circuit or the damage of the capacitor in operation. In addition, the capacitor selection model shall be bsmj series products to ensure reliable and safe operation of components.

2.4 if a new distribution stand on the column 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 according to the proportion (on the basis of jp4-100 / 3W type used in the agricultural reform project, the size of the original box in the width direction is increased by about 100mm, that is, from the original 680mm to 780mm). The external 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 the agricultural electrician and the replacement of fuse parts, as well as heat dissipation.

2.5 select energy-saving AC contactor (similar to cj20si) products, and pay attention to the connection between the coil voltage of AC contactor and the corresponding terminal of the selected residual current action protector, and pay attention to the correct load matching. When selecting AC contactor, its insulation grade shall be grade A or above, and the rated current of its main circuit contact shall be greater than or equal to the load current of the controlled line. The rated voltage of the electromagnetic coil of the contactor is 380V or 220V.

2.6 selection of residual current action protector. It is necessary to select products that meet the standard of GB 6829 General requirements for residual current operated protectors and are certified by CNEEC. Similar to LJM (J) series power-saving and low sensitivity time delay protector can be selected. The mode of protector device shall conform to the national gb13955-2005 installation and operation of residual current operated protective device. When the leakage current is the rated leakage current, the breaking time of the leakage protector shall not be greater than 0.s.

2.7 the incoming and outgoing lines of the distribution box shall be low-voltage cables, which shall meet the technical requirements. For example, vv22-35 × 4 cable is used for the incoming line of the distribution box of 30KVA and 50KVA transformers, and vlv22-35 × 4 cable of the same specification is used for the outgoing line; vv22-50 × 4 and vv22-70 × 4 cables are used for the incoming line of the distribution box of 80KVA and 100kVA transformers, and vlv22-50 × 4 and vlv22-70 × 4 cables are used for the outgoing line, and the cable is crimped with the copper aluminum terminal nose and then connected with the terminal 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, and the rated current of the melt shall be determined according to the allowable overload multiple and fuse characteristics of the transformer. The rated current of the fuse for overcurrent protection of the outgoing circuit shall not be greater than the rated current of the total overcurrent protection fuse. The rated current of the fuse 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 energy meter (installed on the meter board) is installed in the box