Transformer overheating is one of the most common faults in power transmission and distribution systems, which seriously threatens the safe and stable operation of equipment. Especially for 500 kva distribution transformer and oil immersed power transformer, once overheating occurs, it may lead to insulation aging, winding short circuit and even equipment burnout, resulting in huge economic losses. This article will comprehensively analyze the main causes of transformer overheating and propose targeted solutions, providing professional reference for relevant practitioners. JINSHANMEN TECHNOLOGY CO., LTD, as a professional manufacturer of power transmission and distribution equipment, has rich experience in the research and production of oil immersed power transformers and 500 kva distribution transformers, and its products are widely used in various power engineering fields.
1. Main Causes of Transformer Overheating
1.1 Transformer Winding Overheating
In recent ten years, to reduce transformer losses, manufacturers have successively adopted transposed conductors with continuous insulation to wind transformer windings. For oil immersed power transformers, the quality of winding insulation directly affects the heat dissipation effect. Due to the fact that domestic production technology of transposed conductors was not fully mastered in the early stage, the continuous insulation of transformers using transposed conductors expands after about ten years of operation. This expansion will block the oil passage between sections, resulting in poor oil flow. The turn insulation cannot be fully cooled, leading to severe aging, such as charring and embrittlement. Under long-term electromagnetic vibration, the insulation falls off, and local copper is exposed, forming inter turn (inter section) short circuit, which eventually causes transformer burnout accidents. In addition, poor quality of the winding itself, such as uneven winding density, will also lead to local overheating, which is particularly obvious in 500 kva distribution transformers under long-term full load operation.
1.2 Overheating Caused by Poor Contact of Dynamic and Static Contacts of Tap Changer
In on-load tap changing transformers, especially those with frequent voltage regulation and large load current such as 500 kva distribution transformer, frequent adjustment will cause mechanical wear, electric corrosion and contact pollution between contacts. The thermal effect of current will weaken the elasticity of the spring, thus reducing the contact pressure between dynamic and static contacts. The decrease of contact pressure will increase the contact resistance between contacts, leading to increased heat generation. The heat generation will accelerate the oxidation, corrosion and mechanical deformation of the contact surface, forming a vicious cycle. If not handled in time, it will often cause transformer damage accidents. For off-load tap changing transformers, poor contact of tap changers will also cause surface corrosion and oxidation, or reduce contact pressure between contacts, increasing contact resistance, thus forming overheating faults. This problem is more likely to occur in oil immersed power transformers that have been in operation for a long time.
1.3 Overheating Faults Caused by Lead Wire Failures
Lead wire failure is another important cause of transformer overheating, mainly including lead wire joint overheating and lead wire strand breakage. For lead wire joint overheating, it is a frequent fault in many transformers such as oil immersed power transformer. For example, in a main transformer of a bureau in Northeast Power Grid, the total hydrocarbon content was 455.9 ppm and acetylene was 4.23 ppm. Hoisting inspection found that the 66KV A-phase bushing through-cable lead wire overheated, and solder flowed out to the clamping parts and pressing parts; Another example is that the head of the B-phase bushing of a main transformer was overheated. Inspection found that the thread of the general cap was poorly matched, and 5-6 threads were burned out, causing overheating. For lead wire strand breakage, take a DFL-6000/220 single-phase transformer as an example. Abnormal chromatographic analysis results were found in May 1990, and the hot spot temperature may be as high as 1000 ℃. It was not until the overhaul in May 1993 that it was found that two strands of the lead wire in the neutral point bushing of the transformer were burned out and three strands were burned (35 strands in total, 240mm²). The cause was that during the maintenance in May 1989, when replacing the neutral point bushing, the lead wire (copper braid) was pulled upward with difficulty, causing the semi-lap wrapped white cloth tape on the outer layer of the lead wire to fall off. The bare braid lead wire contacted the inner wall of the copper tube in the bushing, resulting in shunting, discharge and overheating. This kind of fault will also cause serious hidden dangers to the 500 kva distribution transformer.
1.4 Overheating Caused by Abnormal Cooling Device
The cooling device is an important part to ensure the normal operation of the transformer, and its abnormality will directly lead to overheating. This problem is particularly prominent in oil immersed power transformers that rely on oil cooling and air cooling. It mainly includes two situations: blockage of the air path of the cooling device and abnormal operation of the fan. For the blockage of the air path of the cooling device, there have been many reports of overheating phenomena. For example, an OSFPSL-120000/220 transformer operated normally for 11 years. On August 28, 1992, the oil temperature suddenly rose from about 42 ℃ to about 90 ℃. Compared with transformers of the same capacity, the temperature rise was very different, but the electrical test results were normal. Visual inspection found that the fins of the radiating tubes of the air cooler were full of dust (never cleaned during long-term operation), which blocked the gaps. The wind from the electric fan could not blow to the radiating tubes, resulting in the continuous rise of the transformer temperature. After flushing, the oil temperature remained at about 40 ℃. Another example is a DFL-90000/220 transformer with high upper oil temperature, which once reached 80-90 ℃. Inspection found that the air duct gap of the radiator was blocked by debris, affecting normal heat dissipation. After flushing with high-pressure water gun, the oil temperature dropped to 60 ℃ and returned to normal. For the abnormal operation of the fan, it mainly includes fan reversal, wrong setting value of starting fan and power loss of fan. For example, due to the reverse connection of the power supply during the maintenance of the cooling system of a main transformer in a bureau, the fan reversed, reducing the cooling effect. The oil temperature was 15 ℃ higher than that of another main transformer with the same load. After identifying the cause and correcting it, the temperature returned to normal (the temperature difference between the two main transformers was only 1 ℃). For the 500 kva distribution transformer, the abnormal operation of the cooling fan will also lead to the rapid rise of the operating temperature under full load conditions.
2. Handling Countermeasures for Transformer Overheating
Aiming at the different causes of overheating faults, corresponding handling countermeasures should be taken to ensure the safe operation of transformers such as 500 kva distribution transformer and oil immersed power transformer.
1. For the low-voltage winding overheating caused by the winding structure, the low-voltage winding of the transformer should be changed to a double helix structure. This improvement can effectively optimize the oil flow channel, ensure sufficient cooling of the winding, and reduce the occurrence of overheating faults. This transformation scheme is also applicable to the 500 kva distribution transformer and oil immersed power transformer.
2. For overheating faults caused by blockage of cooling unit tubes, the cooling unit tubes should be cleaned with compressed air or water regularly (1-3 years). The cleaning process is as follows: (1) Before cleaning, stop the operation of the cooler, remove the fan protective cover and fan blades, so that both the front and rear of the cooler can be thoroughly cleaned. (2) First, use a vacuum cleaner to suck dust and debris from top to bottom on the air inlet side, and then blow the unit tubes with compressed air with a pressure of 0.1 MPa on the air outlet side, blowing and sucking at the same time, which can double the cleaning efficiency. (3) After removing the dust, rinse with tap water. During rinsing, rinse from the air outlet side to the air inlet side to prevent debris from entering the middle tube cluster and falling into the dead zone. Regular cleaning of the cooling system is particularly important for the heat dissipation stability of oil immersed power transformers.
3. Correctly connect the lead wires and tap changers, and tighten the nuts to avoid overheating due to loosening. During the installation and maintenance of 500 kva distribution transformer and oil immersed power transformer, special attention should be paid to the connection quality of lead wires and tap changers to ensure good contact and reduce contact resistance.
4. To avoid overheating after the connection between the lead wire and the bushing copper tube, the following measures can be taken: (1) Do not change the current insulation wrapping method of the lead wire, but accurately cut the length of the lead wire cable during the trial installation of each product to achieve accurate assembly of the lead wire length and the bushing. This can eliminate the adverse consequences of the cable being too long and leaning against the inner wall of the copper tube, but it will cause difficulties in the accurate assembly of the replacement bushing in the future. (2) Change the insulation wrapping method of the lead wire cable. For example, change the current method of only semi-lap wrapping one layer with white cloth tape to first semi-lap wrapping one layer with 0.1mm*30mm crepe paper in both positive and negative directions, and then semi-lap wrapping one layer with white cloth tape. During the final assembly of the bushing, the integrity of the lead wire cable insulation should be maintained, and no insulation loosening and copper exposure are allowed. In this way, even if the lead wire is in contact with the copper tube after assembly, the circuit will be separated by insulation and difficult to close, preventing current flow and overheating.
5. Strengthen management to avoid overheating faults caused by poor management. The forced oil circulation cooling system must have two reliable power supplies, with automatic switching devices, and regular switching tests should be carried out to ensure that the signal devices are complete and reliable. For the daily operation and maintenance of 500 kva distribution transformer and oil immersed power transformer, a sound management system should be established to conduct regular inspections and tests, and potential faults should be found and handled in a timely manner.
3. About JINSHANMEN TECHNOLOGY CO., LTD
JINSHANMEN TECHNOLOGY CO., LTD mainly produces oil immersed power transformers, dry-type power transformers, oil immersed three-dimensional coiled power transformers, dry-type three-dimensional coiled power transformers, mining explosion-proof dry-type transformers, mining explosion-proof mobile substations, amorphous alloy power transformers, on load capacity regulating power transformers, locomotive dry-type transformers, as well as prefabricated substations, modular substations, wind energy box type substations, high and low voltage switchgear and other transmission and distribution equipment. With advanced production technology and strict quality control system, the company provides high-quality and reliable power transmission and distribution equipment for global customers, and has rich experience in solving various operational problems of transformers such as 500 kva distribution transformer and oil immersed power transformer. If you have relevant needs, you can contact us for professional technical support and product consultation.