As a professional manufacturer of power & distribution transformers over 20 years, we have seen some common faults of these transformers, and helped our clients to analyze and successfully solved the problems they have.
we think, it is necessary to list all these common faluts and its handling methods and share them with all of you.
Firstly, it is the failure of the copper or aluminum windings.
The transformer winding is the heart of the transformer, forming the electrical circuit for the input and output of electrical energy. Its fault modes can be divided into: winding short circuit, winding open circuit, winding loosening, deformation, displacement, and winding burning.
Transformer winding faults are mostly caused by the unreasonable structure and insulation of the winding itself, except for other factors. The occurrence rate of winding short circuits is the highest, which not only affects the winding itself, but also has a great impact on the iron core, leads, insulation layer, etc. This type of fault is fatal, and local high temperature or high-energy discharge may occur inside the transformer. If not handled in a timely manner, it can cause complete damage to the transformer winding. In severe cases, the oil temperature and sound speed may increase, the volume may expand, and even cause the transformer to explode, upgrading to a catastrophic fault.
For the loosening, deformation, instability, and insulation damage of transformer windings, although the transformer can operate in this situation, the internal part of the transformer is essentially damaged and has poor short-circuit resistance. If the external short circuit or lightning strike further causes the winding to loosen, the internal field strength distribution is uneven, which is prone to partial discharge and damage to the wire. In addition, loose wires are also prone to vibration under electromagnetic force, causing mutual friction and scratching of insulation. Winding burnout refers to the carbonization of the insulation part of the winding, which ultimately forms a winding short circuit and develops into a fatal fault. Therefore, this type of fault belongs to the category of monitoring faults.
The general handling method for this is:
1. Repair the deformed parts and replace the windings if necessary;
2. Tighten the compression ring screw, tighten the loose gasket and support bar;
3. Repair and improve the structure, increase mechanical strength, repair insulation, and apply immersion paint drying treatment.
When there is a phase break on the high voltage side of the winding, the transformer will not operate in a full phase state. The three-phase voltage and current on the low voltage side of the transformer are unbalanced, and the three-phase DC resistance is also unbalanced; If there is a two-phase circuit, the transformer cannot operate; When there is a two-phase open circuit on the low-voltage side, the transformer operates under single-phase load, and there is no voltage output from the two phases of the open circuit. Therefore, the transformer open circuit is a fatal fault, and the winding needs to be replaced or repaired.
Secondly, it is the iron core of a transformer.
The iron core and winding of a transformer are the main components for transmitting and exchanging electromagnetic energy. To ensure the reliable operation of a transformer, in addition to the qualified quality of the winding, the quality of the iron core is the key to determining normal operation. The fault modes of iron core can be divided into: multi-point grounding of iron core, poor grounding of iron core, and short circuit between iron core pieces. The multi-point grounding of the iron core can be divided into dynamic multi-point grounding of the iron core and reliable multi-point grounding.
Transformer core faults often occur with multiple grounding points in the core, accompanied by local overheating of the core. Prolonged operation time can cause aging of the oil paper insulation, carbonization of the insulation pad, aging of the insulation layer of the core sheet, and even breakage of the grounding lead of the core. These types of faults are critical faults. Short circuits between iron core pieces will form eddy currents in a strong magnetic field, causing local overheating of the iron core. Poor grounding of the iron core will also cause local overheating of the iron core, and at the same time, excessive dielectric loss will occur. The phenomenon of local overheating can easily burn out the insulation between the iron core pieces, amplifying the fault of the iron core, so they are also critical faults. The dynamic grounding situation of the iron core will be different. It is mainly formed by impurities under the action of electric field force to form a conductive small bridge (composed of impurity fibers and metal powder), which is sometimes destroyed under the impact of high current. The situation is unstable and generally does not affect the operation of the transformer. However, periodic local overheating can cause damage to the internal insulation, which is a mild fault. The insulation strength of the transformer core should be regularly tested, and the insulation value measured with a 1000 volt megohmmeter should not be less than 2 megohms. If the insulation strength is found to be below the standard, the bolt sleeve and insulation pad should be replaced in a timely manner, or the damaged silicon steel sheet should be insulated it again.
Thirdly, it is the failure of tap changer of a transformer.
The internal transmission structure of the on load tap changer is relatively complex, and it is often operated and switched. Its faults directly affect the normal operation of the transformer. Due to the influence of high temperature and insulation oil, the tap changer is easily oxidized on the surface of the contacts, producing an oxide film and increasing the contact resistance between the contacts. Due to poor contact, it can cause local high temperatures in the circuit, which can damage the contact surface. Its fault modes mainly include cylinder explosion, contact burning, gear disorder, and gear damage. Cylinder explosion can even cause the transformer to catch fire, which is a fatal fault. The operation of switch gear disorder, gear damage, and contact burning in a fault state will expand the fault. It is a critical fault.
The general handling method for this is:
1. Replace or repair the contact spring;
2. Tighten the loose bolts;
3. Correct the misplacement of the tap changer position;
4. If the installation or adjustment of the on load voltage regulating device is improper, the voltage regulating device should be adjusted according to the requirements.
Fouthly, it is the failure of lead wires of a transformer.
The lead wire is the intermediate link between the internal winding output of a transformer and the external wiring, and its joint is formed by welding. Therefore, the quality of welding directly affects the occurrence of bow line faults. The main fault modes include: lead short circuit, lead open circuit, and poor lead contact. Short circuit between leads and failure to handle it in a timely manner can lead to short circuit between windings,
Belonging to a fatal malfunction, the accident can escalate into a catastrophic failure. Poor contact of the leads can cause local high temperature to burn out the leads and cause the transformer to stop running, which is a critical fault.
Poor contact of leads can be caused by the following reasons:
1. Loose bolts;
2. Welding is not firm;
3. The contact of the tap changer is damaged.
In response to the above reasons, the following measures should be taken:
1. When the transformer is shut down for maintenance, the bolts with poor contact should be re tightened;
2. During maintenance, the welding surface must be cleaned thoroughly before welding, and the quality of the welding points must be carefully checked after welding to prevent accidents caused by welding points falling off during operation;
3. The switches should be switched in place, and the bolts should be tightened one by one to ensure that they are cut correctly before they can be put into operation.
Fifthly; it is the failure of bushings of a transformer.
Bushing is an important protective device for connecting the internal winding of a transformer to the external connection of the oil tank. It is susceptible to long-term effects such as electric fields, wind and rain, pollution, etc., which can cause porcelain glaze cracking and insulation aging, making it a common location for transformer faults. The main types of faults include: casing rupture, casing displacement, welding failure, and partial discharge. The explosion of the casing will cause the transformer to shut down or even burn out, making it a fatal fault; The displacement and welding of the bushing will cause water to enter the transformer body along the bushing, which is highly likely to cause short circuits or phase to phase short circuits in the transformer winding, partial discharge or local overheating, and easy to cause insulation breakdown inside the bushing, which is a critical fault.
The general handling method is:
1. Remove dust and dirt from the outer surface of the porcelain sleeve
2. If the sealing of the sleeve is not tight or the insulation deteriorates due to moisture, the sleeve should be replaced.
Six，it is the insulation failure of a transformer.
The internal insulation of transformers is the key to their quality, and most faults are caused by poor insulation performance. Therefore, the quality of insulation is the basic guarantee for the long-term, safe and reliable operation of transformers. The insulation fault modes can be divided into:
Insulation damage and excessive dielectric loss.
Insulation damage and excessive dielectric loss can still operate normally in the short term, but these faults can cause partial discharge or mild local overheating inside the transformer.
The general handling methods include:
1. Drying treatment should be carried out for insulation that is damp;
2. If the transformer oil deteriorates, it needs to be replaced or treated;
3. Check if the oil passage is blocked and remove any debris in the oil passage;
4. If the oil level is too low, check for leaks and increase the oil level to the oil level line.
Seven, it is the sealing failure of a transfomer.
Poor sealing of transformers is mainly caused by poor joint treatment, such as poor welding quality, disorderly bolt fastening, and uneven flange. The consequence is oil and gas leakage, with a wide range of impacts. The fault modes include aging of the sealing ring, detachment or rupture of the porcelain sleeve, cracks in the welding points of the box, and air leakage at the submersible pump.
This type of malfunction is usually not easy to detect, and special attention should be paid during inspection. Problems should be promptly addressed:
1. Replace the sealing ring for aging;
2. If the porcelain sleeve is broken, it needs to be replaced;
3. The welding points on the box body have cracks and need to be repaired by welding;
4. All fastening bolts must be tightened.
Eight, it is special noice of a transformer.
Due to changes in load and ambient temperature, the oil surface line of the conservator changes. Therefore, water vapor is sucked into the conservator along with air, condensing into water droplets, promoting internal oxidation and rust. As the accumulation intensifies, it will fall to the lower part of the conservator. Rust accumulates on some yoke iron through the connecting pipe between the oil conservator and oil cover, causing vibration and emitting special noise under the action of electromagnetic force. This will also lead to an increase in mechanical impurities in the operating oil of transformers, resulting in a deterioration of oil quality.
Handling method:
The cleaning of the oil conservator and sludge collector is carried out simultaneously and should be determined based on the load and temperature rise of the transformer. Experience has shown that cleaning once every two years is better.
Nine, it is Continuous discharge sound in a transformer.
The grounding of the transformer’s iron core is generally achieved by welding the lifting ring to the oil cover or using iron foot pads. When desoldering or oil stains are present on the contact surface, it can lead to poor contact at the connection point, and the iron core and its clamp metal are both in the electric field of the coil, which induces a constant potential. During high-voltage testing or operation, the induced potential difference exceeds its limit When asked about the discharge voltage, intermittent discharge sounds will be generated.
Handling method:
Suspended core inspection. Clean the grounding detachment surface, re weld or remove the oil sludge until it is clean, and maintain a good contact state. At the same time, a 500V megger should be used for testing, and the iron core and transformer casing should be well grounded.
Ten, Abnormal temperature of the transformer.
When the load, heat dissipation conditions, and ambient temperature remain unchanged, the temperature of the transformer is higher than the original temperature under the same conditions, and there is a continuous upward trend, which is also an abnormal temperature rise of the transformer. Like the extreme temperature rise, it is also a symbol of transformer failure. The causes of abnormal temperature rise include:
Short circuits between turns, layers, and strands of transformers; Partial short circuit in transformer core; Heating of fuel tanks, tank covers, etc.
Handling method:
If the temperature of the transformer is found to be abnormal during operation, the cause should be identified first, and corresponding measures should be taken to eliminate it. The temperature should be lowered. If it is caused by an internal fault of the transformer, the operation should be stopped for maintenance.
Eleven, Fuel spray & explosion of the transformer.
The reason for oil spray & explosion is that the fault short circuit current and high-temperature arc inside the transformer cause rapid aging of the transformer oil, and the relay protection device fails to cut off the power in time, causing the fault to persist for a long time, resulting in a continuous increase in pressure inside the box. High pressure oil and gas spray out from the explosion-proof pipe or other weak areas of the box, forming an accident.
Insulation damage:
Insulation damage caused by local overheating such as inter turn short circuits; Transformer water ingress causes moisture damage to the insulation; Lightning strikes and other overvoltage are the basic factors that cause insulation damage and internal short circuits.
Arc generation due to breakage of power cable : Poor welding of wire groups, loose lead connections, and other factors may cause wire breakage under high current impact. High temperature arcs are generated at the breakpoint, causing oil gasification and increasing internal pressure.
Voltage tap changer malfunction:
The regulating section coil of the high-voltage winding of a distribution transformer is connected together through a tap changer. The tap changer contacts are connected in series in the high-voltage winding circuit, and together with the winding, they pass through the load current and short-circuit current. If the dynamic and static contacts of the tap changer heat up, the ignition and arc start, causing the regulating section coil to short-circuit.
Twelve, Significant decrease in oil level and severe oil leakage of the transformer
The normal increase or decrease in oil level is caused by temperature changes, and the changes are not too significant. When the oil level drops significantly and even cannot be seen from the oil level gauge, it may be due to oil leakage or seepage in the transformer. The phenomenon of oil leakage during transformer operation is quite common, and even if the oil level is within the specified range, it can still continue to operate or be scheduled for maintenance.
Finally, Abnormal oil color with a burnt odor in a transformer.
The new transformer oil is slightly transparent and light yellow, and after running for a period of time, the oil color will turn into light red. If the oil color darkens, it indicates insulation aging of the transformer; If the oil turns black (containing carbon in the oil) or even has a burnt smell, it indicates that there is a fault inside the transformer (partial burning of the iron core, short circuit between windings, etc.), which will lead to serious consequences. The transformer should be stopped for maintenance, and the transformer oil should be treated or replaced with qualified new oil. Transformer oil plays an insulation and cooling role in transformers,
If the oil quality deteriorates, it will not play its due role. To prevent serious consequences caused by deterioration of oil quality, oil samples should be taken regularly for chemical analysis during normal operation of transformers
Verify to promptly identify issues.
Conclusion, The above-mentioned judgment on the sound, temperature, oil level, appearance, and other phenomena of transformers for distribution transformer faults can only be used as a preliminary on-site intuitive judgment. Because internal faults in transformers are not only a direct reflection of a single aspect, but also involve many factors, sometimes even leading to false appearances. When necessary, transformer characteristic tests and comprehensive analysis must be carried out to accurately and reliably identify the cause of faults and propose solutions. According to statistics, critical faults occur most frequently in large power transformers, with the lowest occurrence rate of catastrophic faults but the greatest harm to the program. The occurrence rate of fatal faults is relatively high, and the rate of mild faults is not high; Faults with low severity may progress to become faults with high severity, either by level or beyond,
Catastrophic or fatal failures of large power transformers not only bring huge losses to themselves, but also seriously affect the safe operation of the power grid. Therefore, an online monitoring device should be established for power transformers, closely monitoring their critical state to determine whether maintenance is necessary. This can effectively prevent the transformation of fault states, reduce or avoid losses caused by the development of power transformer faults, and improve the possibility of power transformer operation.
Meanwhile, choosing a professional manufacturer of the transformer and its excellent after-sale service is extemely important. PowerTel and his dedicated engineers are able to provide you a full range of power transformer up to 500 kV and a full range of distribution transformers, including single phase/three phase polr mounted, pad mounted transformer.
