Distribution transformers in distribution systems based on the law of electromagnetic induction transform AC voltage and current transmission of alternating current energy of a static device. Usually installed in the poles, cradles or power distribution, the general 6 ~ 10 kV voltage dropped to about 400 V input users. Transformer operation is normal, a direct impact on user production and living electricity, and related to the safety of electrical equipment. In order to ensure that users spend quality, safety electricity, must ensure that the normal operation with variable. Therefore, it is necessary for us to talk about the protection measures of distribution transformers and their precautions from two aspects of protection configuration technology and daily operation and management:

First, the protection of configuration technology

1, the installation of arrester protection against lightning over-voltage: with variable lightning protection, the installation of gap-free metal oxide surge arresters as over-voltage protection to prevent intrusion of high voltage lines from high voltage lightning wave caused by the internal transformer insulation Breakdown, resulting in a short circuit, to prevent the occurrence of lightning damage accident. The use of arrester protection with variable, first, through the normal channels to purchase qualified products, put into operation before installation put into operation after rigorous testing requirements and then put into operation; Second, the preventive operation of equipment on a regular basis for the leakage current value exceeds Standard values ​​of substandard products to be replaced in time; Third, regular testing of transformer grounding resistance, 100KVA and above distribution transformers required grounding resistance must be less than 4Ω, 100KVA the following distribution transformers, grounding resistance must be less than 10Ω .

If the test value is not within the specified range, should be taken to extend the ground wire to increase the grounding and physical, chemical and other measures to achieve the required value, in April and July each year twice the grounding resistance of retest to prevent the weld off , Environmental and other factors lead to grounding resistance exceeded. If the transformer grounding resistance exceeds the lightning current lightning flow can not flow into the earth, but by grounding the lightning voltage will be added to the low voltage side of the distribution transformer and then reverse to high voltage, the transformer will burn down; Fourth, the installation location selection should be appropriate, High-voltage surge arresters installed in the near by the high voltage bushing with the lead to minimize the chance of direct lightning invade transformer with low voltage surge arresters installed in the nearest low-voltage bushing with the Department to ensure the thunder and lightning waves before the match with the right Action, installation of electrical equipment required by the standard installation requirements to prevent the blind installation of the meaning of loss of protection.

2, the installation of fast, over-current protection, to ensure selective removal of faulty lines: distribution of short-circuit protection and overload protection installed by the distribution transformer high voltage side of the fuse and the low voltage side of the total leakage protection (the device has Leakage protection and distribution with low voltage over-current protection) to achieve. In order to effectively protect the distribution transformer must be the correct choice of fuse (fuse, fuse, etc.) and low voltage over-current protection setting. High-voltage side of the fuse selection, should be able to ensure that the transformer inside or outside the casing short circuit is blown.

Fuse selection principle: (1) With the capacity of 100kVA and below distribution transformer, high-voltage fuses according to 2 ~ 2.5 times the rated current selection; (2) With capacity above 100kVA distribution transformer, high voltage fuse by 1.5 to 2 times the rated current select. The over-current action value of the low-voltage side leakage total protector shall be taken as 1.3 times of the rated value of the substation variable low-voltage side. The over-current protection setting of each branch line with substandard low voltage shall not be greater than the over-current protection value of the total protection, Side rated current, generally according to the maximum current carrying capacity to select the value of the flow to ensure that the outlet circuit in the short circuit or the output load is too large, causing the distribution transformer overload timely action, cut off the load and fault lines, to achieve the purpose of protection with distribution .

At the same time meet the selectivity of protection at all levels. When the low-voltage branch circuit short-circuit fault, the branch circuit action, the leakage current protection device over-current protection does not act, the low-voltage side of the total circuit failure or short circuit, the low voltage side leakage protection device overcurrent protection action, high side melt should not be blown; When the internal fault is short-circuited, the melt at the high-pressure side is blown. The high-voltage line protection device at the upper level substation should not be tripped to ensure the correct grading action of the distribution network protection device. Distribution of high-pressure side of the melt protection material must be equipped with a standard, and resolutely eliminate the use of copper, aluminum and other metal conductors to replace the fuse melt.

Second, the daily operation and management

1, to strengthen daily inspection, maintenance and regular testing:

(1) Routine maintenance, timely cleaning and erasing the oil on the distribution transformer and the dust on the high and low voltage casing to prevent flashover when the weather is wet or rainy, resulting in short-circuit casing, high-voltage fuse fuse, normal operation;

(2) Observe the oil level and oil color of the distribution transformer in a timely manner, and regularly check the oil temperature. Especially when the load changes greatly, the temperature difference is big, and the weather is bad, the inspection times should be increased. For the top oil in the oil-immersed distribution transformer Temperature not higher than 95 ℃, the temperature rise shall not exceed 55 ℃, in order to prevent the deterioration of the winding and oil over-speeding, top oil temperature should not often exceed 45 ℃;

(3) shake test with variable insulation resistance, check the leads are solid, with particular attention to the connection of the low-voltage outlet connection is good, the temperature is abnormal;

(4) Strengthen the measurement of electricity load, in the peak of electricity, strengthen the load measurement for each transformer, if necessary, increase the number of measurements, three-phase current imbalance in distribution transformers timely adjustment to prevent the neutral The current is too high to break the lead, resulting in damage to the user equipment, with variable damage. Connection group Yyn0 distribution, three-phase load should be balanced, not only with one or two phase power supply, neutral current should not exceed 25% of the low-voltage side of the rated current, and strive to make the distribution does not overload, not partial load run;

2, to prevent external damage: (1) a reasonable choice of distribution transformer installation site, transformer installation not only to meet the requirements of the user voltage, but also try to avoid installing it in the barren hills, easily struck by lightning, can not be installed away from residents District where to prevent theft of lawless elements. Installation location is too remote is not conducive to the regular maintenance of operating personnel, not easy for staff management; (2) to avoid the installation of low-voltage distribution transformer in the metering box, due to prolonged operation, metering box glass damage or distribution of variable pressure pile damage Can not be replaced in time, resulting in due to rain and other causes burning energy meter damage caused by distribution changes; (3) does not allow unauthorized adjustment of tap changer to prevent the tap changer is not in place, resulting in phase short circuit burned distribution transformer; 4) Install insulation cover on the distribution transformer high and low voltage to prevent natural disasters and foreign objects from damaging. Install high and low voltage insulation covers in narrow roads and animals entering and leaving the forest, to prevent the distribution transformer wiring pile from falling off Low voltage short circuit and burned distribution; (5) Regular patrol lines, felling line access to prevent the branches hit the wire caused by low voltage short circuit burned distribution transformer accident.

To sum up: For distribution transformer to maintain long-term safe and reliable operation, in addition to strengthening the technical level of protection and configuration, in the daily operation and management is also very important. As a distribution transformer management staff, we must do ground inspection, ground maintenance, ground measurement, timely detection of problems in time, to take various measures to strengthen the protection of distribution transformers to prevent failures or accidents in order to ensure distribution network security , Stable and reliable operation.

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