Causes and solutions for damage to distribution transformers

1. Causes of damage to distribution transformer

1. Overload

First, with the improvement of people's lives, electricity consumption has generally increased rapidly. The original distribution transformer has a small capacity, and it is a small horse pulling a big cart, which cannot meet the needs of users, causing the transformer to be overloaded. Second, due to seasonal and special weather reasons, the peak of electricity consumption causes the distribution transformer to be overloaded. Due to the long-term overload operation of the transformer, the internal components, coils, and oil insulation of the transformer are aged, and most of the transformer load is distributed seasonally and temporally. Especially in the busy farming season in rural areas, the distribution transformer will be used under overload or full load, and it is used under light load at night. The load curve difference is very large, the operating temperature is as high as 80 ℃, and the lowest temperature is 10 ℃. Moreover, according to the maintenance of rural transformers, the average moisture at the bottom of each transformer is more than 100 g, and this moisture is precipitated from the oil through the breathing air of the transformer oil through thermal expansion and contraction. Second, the lack of oil inside the transformer reduces the oil level, causing the contact surface between the insulating oil and the air to increase, accelerating the entry of moisture in the air into the oil surface, and reducing the insulation strength inside the transformer. When the insulation is reduced to a certain value, a breakdown short circuit fault occurs inside the transformer.

2. Illegal refueling of distribution transformers

An electrician refueled the distribution transformer in operation. After 1 hour, the fuse of the high-voltage drop switch of the transformer blew two phases and there was slight oil spray. After on-site inspection, it was found that it needed to be overhauled. The main reasons for the burning of the transformer were: first, the newly added transformer oil was inconsistent with the oil type in the transformer box. Transformer oil has several oil bases, and different types of oil bases cannot be mixed in principle; second, there was no power outage when refueling the distribution transformer, causing the cold and hot oil inside the transformer to mix, and the circulating oil flow accelerated, bringing the moisture at the bottom of the body to circulate inside the high and low voltage coils, causing the insulation to drop and causing breakdown and short circuit; third, unqualified transformer oil was added.

3. Improper reactive power compensation causes resonant overvoltage

In order to reduce line losses and improve equipment utilization, the "Rural Low Voltage Electric Power Technical Regulations" stipulates that distribution transformers with a capacity of more than 100 kVA should use reactive power compensation devices. If the compensation is improper and the total capacitive reactance and total inductive reactance are equal on the running line, ferromagnetic resonance will occur in the running line and equipment, causing overvoltage and overcurrent, burning distribution transformers and other electrical equipment.

4. System ferromagnetic resonance overvoltage

The 10 kV distribution lines in the rural power grid are inconsistent in length, distance to the ground, and conductor specifications. In addition, when the operating parameters of the distribution transformer, welding machine, capacitor, and large load switching change greatly, or the single-phase intermittent grounding of the 10 kV neutral point ungrounded system may cause resonant overvoltage in the system. Once the system resonant overvoltage occurs, the high-voltage fuse of the distribution transformer will be blown in the least, and the distribution transformer will be burned in the worst case. In some cases, it will cause flashover or explosion of the distribution transformer bushing.

5. Lightning overvoltage

Distribution transformers must be equipped with qualified lightning arresters on the high and low voltage sides according to regulations to reduce the damage caused by lightning overvoltage and ferromagnetic resonance overvoltage to the high and low voltage coils or bushings of the transformer. The following are the main reasons for the damage of distribution transformer due to overvoltage: First, the installation test of lightning arrester does not meet the requirements. Generally, three lightning arresters are grounded at one point when lightning arrester is installed. In long-term operation, it is seriously rusted due to disrepair, wind and rain, and climate change and other special circumstances cause the grounding point to be disconnected or in poor contact. When there is lightning overvoltage or system resonance overvoltage, the transformer is broken down because it cannot discharge and reduce the voltage to the earth in time; second, because most transformers are insured by insurance companies, the resulting heavy insurance company compensation and light maintenance management. Some users think that the transformer has participated in the insurance, and it doesn’t matter whether the lightning arrester is installed or not, or whether it is tested or not. Anyway, the insurance company is responsible for compensation if the transformer is broken. This is also an important factor in the serious damage of distribution transformers over the years; third, only the installation test of the lightning arrester on the high-voltage side of the transformer is emphasized, while the installation test of the lightning arrester on the low-voltage side is neglected. Because the lightning arrester is not installed on the low-voltage side of the transformer, when the low-voltage side of the transformer is struck by lightning, the inverter impacts the high-voltage side coil of the transformer, and the low-voltage side coil may also be damaged.

6. Secondary short circuit

When the secondary of the distribution transformer is short-circuited, a short-circuit current several times or even dozens of times higher than the rated current is generated on the secondary side, and a large current is also generated on the primary side to offset the demagnetization effect of the short-circuit current on the secondary side. Such a large short-circuit current will, on the one hand, cause huge mechanical stress inside the transformer coil, resulting in coil compression, loosening and shedding of the main and auxiliary insulation, and coil deformation. On the other hand, due to the existence of the short-circuit current, the temperature of the primary and secondary coils rises sharply. At this time, if the primary and secondary fuses are improperly selected or replaced with aluminum copper wire, the transformer coil may be burned out quickly.

7. Poor crimping of tap changer

First, the tap changer itself is of poor quality, with an unreasonable structure, insufficient spring pressure, incomplete contact between the moving and static contacts, and a smaller insulation distance between the dislocated moving and static contacts. Discharge or short circuit occurs between the two taps, which quickly burns the transformer tap coil or the entire winding. Second, there are human factors. Some electricians are not clear about the principle of no-load voltage regulation, which leads to partial contact between the moving and static contacts after voltage regulation, or due to long-term operation of the transformer tap changer contacts, dirt on the static contacts causes poor contact, resulting in discharge and sparking that burns the transformer.

8. The respirator hole is blocked

Generally, a "breather" is installed on the oil pillow of transformers above 50 kVA. The cover of the "breather" is generally a transparent glass cylinder with a "moisture absorbent" inside. It is easy to break during transportation, so under normal circumstances, the manufacturer does not install it before leaving the factory. A "small square iron plate" is sealed with screws at the position of the "moisture absorber" where the "breather" is installed on the transformer oil pillow to play a moisture-proof role. The "small square iron plate" must be removed in time when it is put into operation. If it is not removed in time and replaced with a "breather", due to the continuous generation of heat after operation, the insulating oil expands due to heat, the pressure in the transformer increases, the oil circuit cannot circulate, the heat cannot be dissipated, the heat of the iron core and the coil becomes higher and higher, the insulation performance decreases, and eventually the transformer burns out.

9. Others

There are many problems in the daily operation, maintenance and management of distribution transformers: First, during the inspection or installation process, when tightening or loosening the nuts of the transformer conductive rods, the conductive rods may rotate as they rotate, which may cause the soft copper sheets led out of the secondary side to collide with each other, causing phase short circuit or primary coil lead break; second, objects or tools may be accidentally dropped on the transformer during inspection and maintenance, damaging the bushing, which may cause flashover grounding at the least and short circuit at the worst; third, after the parallel transformers are inspected, tested or the cables are replaced, the phases are not checked, and the wiring is random, resulting in the wrong phase sequence. After the transformers are put into operation, a large circulating current will be generated, which will burn the transformer; fourth, an anti-theft metering box is installed on the low-voltage side of the transformer. Due to space problems and poor process crimping, some are directly wrapped with wires, resulting in excessive contact resistance of the low-voltage side wiring, which generates heat and sparks during heavy load operation, causing the conductive rods to burn out.

2. Solutions for damage to distribution transformers

According to the above analysis of the causes of distribution transformer damage, a considerable part of the distribution transformer damage can be avoided, and some transformer damage accidents can be eliminated in the bud as long as equipment inspections are strengthened and safety regulations and systems are strictly followed. The specific countermeasures are as follows.

1. Carry out inspection and testing before operation

Distribution transformers must be inspected on site before they are put into operation. The main contents are as follows.

Check whether the oil level gauge on the oil pillow is intact, and whether the oil level is clear and on the oil level line that matches the environment. If the oil level is too high, after the transformer is put into operation with load, the oil temperature rises, and the oil expands, which is likely to cause the oil to overflow from the breather connection pipe on the top of the oil pillow; if the oil level is too low, the oil level may drop to the point where the oil level gauge cannot see the oil level when the transformer is lightly loaded or shut down for a short period of time in winter.

Check whether the bushing, oil level gauge, oil drain valve, etc. are well sealed and whether there is oil leakage. Otherwise, when the transformer is loaded, more serious leakage will occur in the hot state.

Check whether the explosion-proof pipe (breathing airway) is unobstructed and intact, and whether the desiccant of the respirator is ineffective.

Whether the transformer casing grounding is firm and reliable, because it plays a direct protective role for the transformer.

Check whether the primary and secondary outlet bushings of the transformer and their connections with the wires are in good condition and whether the phase colors are correct.

Check whether the nameplate on the transformer is consistent with the transformer specifications required, such as the voltage level on each side, the capacity of the transformer and the position of the tap changer.

Measure the insulation of the transformer. Use a 1000~2500V megohmmeter to measure the insulation resistance of the primary and secondary windings of the transformer to ground (when measuring, the non-measured winding is grounded), as well as the insulation resistance between the primary and secondary windings, and record the ambient temperature during measurement. There is no hard and fast regulation for the allowable value of insulation resistance, but it should be compared with historical conditions or original data and not less than 70% of the factory value (when the temperature of the transformer being measured is different from the temperature during the manufacturer's test, it should be converted to the same temperature for comparison).

The mutual difference of the DC resistance of each phase of the transformer should be less than 4% of the average value, and the mutual difference of the DC resistance between lines should be less than 2% of the average value.

If all the above checks are qualified, insert an alcohol thermometer above 100 ℃ into the temperature measuring hole of the transformer to monitor the operating temperature of the transformer at any time, then air-drop the transformer (without load), check whether there is any abnormality in the electromagnetic sound, and measure whether the secondary side voltage is balanced. If balanced, it means that the transformer ratio is normal, there is no turn-to-turn short circuit, and the transformer can operate normally with load.

2. Precautions during operation

The following contents must be done in the operation and management of distribution transformers.

When using the distribution transformer, it is necessary to regularly check whether the three-phase voltage is balanced. If it is seriously unbalanced, timely measures should be taken to adjust it. At the same time, the oil level, temperature, oil color of the transformer should be checked regularly to see if there is leakage, and whether the color of the desiccant in the respirator has changed. If it has expired, it should be replaced in time, and defects should be eliminated in time.

Clean the dirt on the distribution transformer regularly, take anti-pollution measures when necessary, install bushing anti-pollution caps, check whether the bushing has flashover discharge, whether the grounding is good, whether there is broken wire, desoldering, or breakage, and measure the grounding resistance regularly.

If the screw of the transformer is rotated during disassembly and assembly, it must be strictly handled and put into operation only after confirmation. Reasonable selection of secondary side wire connection method, such as copper-aluminum transition wire clamp, etc. Apply conductive paste on the contact surface to increase the contact area and conductivity and reduce oxidation heat.

Lightning arresters are installed on the primary and secondary sides of the distribution transformer, and the arrester grounding lead, transformer casing, and secondary neutral point are grounded together. For transformers with a capacity of more than 100 kVA and more inductive equipment, the upper oil temperature should not often exceed 85 °C, and the maximum should not exceed 95 °C (inserting a thermometer into the temperature hole on the side of the distribution transformer can measure the instantaneous temperature of the operating transformer at any time), and long-term overload operation is not allowed. However, when the daily load factor is less than 1 (the ratio of the daily average load to the maximum load) and the upper oil temperature does not exceed the allowable value, it can be operated according to the normal overload regulations, and the total overload value should not exceed 20% for the transformer). When the aging rate of the insulation (insulating medium such as oil) in the transformer is doubled, the service life should be reduced accordingly. Therefore, long-term overload operation must be avoided.

Avoid unbalanced three-phase load operation. Unbalanced operation of the three-phase load of the transformer will cause unbalanced three-phase current, and the three-phase voltage will also be unbalanced. Transformers operating with unbalanced three-phase loads should be considered as the maximum current load. If the maximum unbalanced three-phase current or neutral current measured during the maximum load period exceeds 25% of the rated current, the load should be redistributed among the three phases.

Prevent secondary short circuit. The secondary short circuit of the distribution transformer is the most direct cause of transformer damage, and the key to the reasonable selection of the distribution transformer is here. In general, the fuse on the high-voltage side (drop insurance) of the distribution transformer is selected within 1.2 to 1.5 times the rated current of the high-voltage side, and the low-voltage side is selected according to the rated current. In this case, even if a low-voltage short circuit occurs, the fuse can still play a due protective role for the transformer.