Distribution Box Fault Improvement

(1) Cause of failure

  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 action protectors, capacitors and meters. These low-voltage electrical appliances are designed and manufactured according to GB1497 "Basic Standards for Low-Voltage Electrical Appliances", and their normal working conditions are stipulated accordingly: the upper limit of the ambient air temperature does not exceed 40°C; the average value of the ambient air temperature for 24 hours does not exceed 35°C ; The lower limit of ambient air temperature is not lower than -5°C or -25°C.

The distribution box transformed from the rural power grid operates outdoors. It is not only exposed to direct sunlight to generate high temperature, but also generates heat itself during operation. Therefore, in the midsummer high temperature season, the temperature inside the box will reach above 60°C. The temperature greatly exceeds the ambient temperature specified by these electrical appliances, so failures caused by overheating of electrical components in the distribution box will occur.

  2. Failure caused by product quality

During the transformation of the rural power grid, due to the large number of distribution boxes required and the short construction period, the distribution box factory needed to supply low-voltage electrical appliances in a short time and in large quantities. Some products failed shortly after being put into operation. For example, some types of AC contactors cannot operate due to the burnout of the contactor closing coil shortly after the distribution box is put into operation.

3. Faults caused by improper selection of electrical appliances in the distribution box

Due to the inappropriate selection of AC contactor capacity during manufacture, AC contactors of the same capacity are installed for different outlet circuits, and the unbalanced three-phase load is not considered, and the current level of some outlet contactors cannot be adjusted to normal On the basis of the model selection, a current level selection is increased, which leads to the burning of the AC contactor during operation in the high temperature season in summer.

(2) Improvement plan

  1. For the distribution box with a distribution transformer capacity of 100kV A and above, the temperature control relay (JU-3 or JU-4 ultra-small temperature relay) and shaft The flow fan is installed on the box on the left side above the control board, so that when the temperature in the box reaches a certain value (such as 40°C), the exhaust fan can be automatically activated to forcibly discharge heat to dissipate heat from the box.

  2. A protection circuit is used to prevent the occurrence of failure of the external circuit powered by the distribution box. Choose a smaller intelligent phase loss protector, such as the DA88CM-II motor phase loss protection module (Shanghai product) installed in the distribution box to prevent the motor from being burned due to low voltage phase loss operation.

3. Improve the wiring method of the low-voltage capacitor bank of the original distribution box, and change its installation position from the pile head of the AC contactor to be connected between the low-voltage incoming line of the distribution box and the meter. To prevent the inaccurate measurement of the metering device due to a phase failure of the capacitor circuit during operation or damage to the capacitor. In addition, the capacitor selection model should be BSMJ series products to ensure reliable quality and safe operation of components.

4. If a pole-mounted distribution station rack is added, when making the distribution box shell, a 2 mm thick stainless steel plate can be selected, and the size of the distribution box will be appropriately enlarged in proportion (JP4-100/3W foundation used in the agricultural reform project In addition, the width of the original box is increased by about 100 mm, that is, the original 680 mm is changed to 780 mm. The overall size of the improved distribution box is: 1300 mm×780 mm×500 mm), in order to increase the number of branches The electrical safety distance between the outgoing wires, the outgoing wires and the box shell is conducive to the operation and maintenance of agricultural electricians and the replacement of fuses, and it can also dissipate heat.

5. Choose an energy-saving AC contactor (similar to CJ20SI type), and pay attention to the AC contactor coil voltage is connected to the corresponding terminal of the selected remaining current-operated protector, and pay attention to correct load matching. When selecting an AC contactor, you should choose a product with an insulation class of A and above, and you must ensure that the rated current of the main circuit contact should 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, and the coil is allowed to be used within the range of 80% to 105% of the rated voltage.

6. Selection of residual current operating protector. Products that meet the standards and have been certified must be selected. The breaking time of the leakage protector, when the leakage current is the rated leakage current, its action time should not be greater than 0.2s.

7. Low-voltage cables are used for the incoming and outgoing lines of the distribution box, and the selection of cables should meet the technical requirements. For example, the incoming lines of the distribution boxes of 30kVA and 50kVA transformers use VV22-35×4 cables, and the branch outgoing lines use VLV22-35×4 cables of the same specification; the incoming lines of the distribution boxes of 80kVA and 100kVA transformers respectively use VV22-50 ×4, VV22-70×4 cables, VLV22-50×4 and VLV22-70×4 cables are used for branching out, and the cables are crimped with the copper-aluminum wiring nose and then connected with the wiring pile head in the distribution box with bolts .

  8. Selection of fuses (RT, NT type). The rated current of the total overcurrent protection fuse on the low-voltage side of the distribution transformer should be greater than the rated current on the low-voltage side of the distribution transformer, generally 1.5 times the rated current, and the rated current of the melt should be in accordance with the allowable overload multiple of the transformer and fusing The device characteristics are determined. The rated current of the melt of the overcurrent protection fuse of the outlet circuit should not be greater than the rated current of the total overcurrent protection fuse. The rated current of the melt is selected according to the normal maximum load current of the circuit, and should avoid the normal peak current. The rated current of the parallel capacitor bank fuse can generally be selected according to 1.5 to 2.5 times the rated current of the capacitor.