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What is a Distribution Transformer

Distribution transformer refers to a static electrical device in the distribution system that transfers AC energy by converting AC voltage and current according to the law of electromagnetic induction. Usually installed on poles or in distribution substations, it can reduce the voltage from 6-10 kV to around 400 V or below and input it to residential house, industrial & commercial buildings.

Capacity selection of a distribution transformer

In the operation of distribution transformers, there are cases of under load operation due to excessive capacity, as well as cases of equipment overheating or even burning due to overload or overcurrent operation. The improper selection of capacity for a distribution transformer affects the reliability and economy of power supply in the power system.

The capacity of the transformer is selected based on load statistics. Due to the difficulty in accurately predicting the load, it is generally chosen based on the expected maximum load. The result of such selection is often set too high in capacity, which has adverse effects on the operation of the power system. If selected according to economic operation, it is to use the condition that the copper loss and iron loss of the transformer are equal, and derive the maximum economic load rate of the transformer and the rated capacity to maximum load ratio of the transformer. Due to the fact that the actual operating load is not necessarily the maximum load calculated by the load statistics, and the load is random, the operating efficiency is variable, and its economic operating benefits are difficult to achieve.

Currently, new low loss transformers are being used to replace high energy transformers in distribution systems, with a single iron loss reduction of approximately 40%. Due to the large number of distribution transformers and significant load fluctuations, their economic benefits are very significant.

Therefore, we believe that how to fully utilize the set capacity of transformers without damaging their normal service life should become the main basis for selecting the capacity of distribution transformers.

Our recommended method is to select the capacity of the distribution transformer based on the estimated maximum load Smax and typical daily load curve, and in accordance with the International Electrotechnical Commission (IEC) standard (1972) – Oil Immersed Transformer Warfare Load Guidelines. This standard has been adopted in China. The advantage of this method is that it takes into account the normal overload capacity of the transformer and fully utilizes the set capacity of the transformer without shortening its lifespan. This reduces investment, improves the operating conditions of the distribution network, and has significant economic benefits.

Operation and maintenance for a distribution transformer
1. Overload operation of distribution transformers
Transformer overload operation refers to the load current exceeding the rated current of the transformer. In general, during low load operation, the insulation material of a transformer cannot fully exert its function. However, during continuous overload operation, the transformer will generate high temperature, causing the insulation part of the winding to be burned and peeled off, forming a turn to turn short circuit; At the same time, transformer oil produces oil sludge which accumulates on the oil tank plate, winding and iron core, resulting in poor heat dissipation of transformer oil. This vicious cycle not only seriously affects the lifespan of transformers, but also causes accidents such as high-voltage breakdown and transformer burning. Therefore, it is necessary to regularly observe the three-phase load current. The three-phase load current should strive to be consistent, and if there is a deviation, it should not exceed 10%.
2. Abnormal sound from distribution transformers
When alternating current passes through the transformer winding, a normal and uniform “buzzing” sound will be generated due to the self vibration of the iron core. If there is an abnormal sound, it is necessary to investigate the cause and promptly report to the relevant department for handling. The sound of the transformer varies when it is unloaded and when it is loaded. Based on the characteristics of abnormal sound compared to the past, the cause can be identified before it can be put into operation.
3. Inspection of distribution transformer temperature
The operating temperature of a transformer is closely related to its lifespan. The transformer operates at a normal temperature below 95 ℃ and has a lifespan of 20 years; If operated at 110 ℃, the service life will be shortened to 7 years; If the temperature rises to 130 ℃ for operation, its lifespan will be shortened to 2 years; If the transformer continues to operate at a temperature of 170 ℃, it will be scrapped around d. If the temperature exceeds the allowable value of the transformer, the cause should be identified and countermeasures taken in a timely manner.
4. Whether the oil level of the distribution transformer is normal, whether there is leakage, oil leakage or abnormal oil color
There are many reasons for the decrease in oil level. Due to poor welding quality and sealing, the heat dissipation pipes, valves, and box edges are prone to leakage and oil leakage. When the oil level drops below the top cover of the transformer, the contact surface between the oil and air increases, making it easy to oxidize, deteriorate, and absorb moisture from the air, resulting in a decrease in the withstand voltage strength of the oil and damaging the insulation performance of the winding. When the oil shortage is severe, the insulation between the conductive parts of the transformer and the ground decreases, resulting in phase to phase or ground breakdown discharge. At this time, if it continues to be used, the transformer oil will not be able to circulate and convection normally, which will cause the temperature of the transformer oil to rise, shorten the service life and even burn out.
5. Is there any damage, rupture, or discharge marks on the insulation sleeve of the distribution transformer
If the insulation sleeve is not cleaned for a long time, or there are damage cracks and discharge traces, the leakage current of the insulation sleeve will increase due to the humidity of the air in rainy or foggy weather, and the insulation will decrease, leading to ground flashover. In addition, severe scaling of the insulation sleeve, as well as large fragments and cracks on the insulation sleeve, can also cause flashover or explosion accidents. To solve this problem, in addition to observing the insulation sleeve itself, attention should also be paid to the accumulation pattern of the sleeve, such as wind direction and surrounding environment, in order to do a good cleaning job.
6. Regular cleaning of distribution transformers
Regularly clean the dirt on the distribution transformer, check if there is flashover discharge on the casing, check if the grounding is good, and check if there are any broken wires, desoldering, or fractures. Regularly shake and measure the grounding resistance, which should not exceed 4 Ω (with a capacity of 100 kV · A or above) or 10 Ω (with a capacity of less than 100 kV · A), or take anti fouling measures and install anti fouling caps on the casing. When connecting and disassembling the outgoing lines of distribution transformers, it is necessary to strictly follow the process operation to avoid internal fractures of the outgoing lines.
5 Common Fault Analysis for a distribution transformer
1. Unbalanced three-phase load or seasonal overload
From the investigation results, it can be seen that there is a large amount of imbalance in the three-phase load of distribution transformers, especially in rural areas where the majority of the power load is single-phase and the load changes greatly. Therefore, there are many distribution transformers with unbalanced three-phase load, which makes the three-phase cannot operate symmetrically and generates zero sequence current. On the one hand, this increases the loss of the transformer, and on the other hand, it reduces the effective capacity of the transformer. The above two situations will cause overheating of the transformer, aging of the insulation oil, reducing the insulation level of the winding, and ultimately leading to transformer damage. The following measures can be taken:
① Investigate the load situation of distribution transformers, including the load of 24 hours a day and the load of 4 seasons a year, understand the approximate load situation, and try to adjust the three-phase load as much as possible to make it close to symmetrical operation;
② Adjust the peak and valley time of electricity consumption to reduce overload situations; At the same time, it is necessary to increase the capacity of the transformer in a timely manner to avoid long-term overload operation of the transformer.
2. Poor grounding
The lightning protection work for distribution transformers subjected to lightning strikes is generally done, but there are still two problems: ① poor grounding of lightning arresters; ② Only emphasizing the installation of lightning arresters on the high-voltage side, while neglecting the issue of installing lightning arresters on the low-voltage side (especially in areas with frequent lightning strikes). If the grounding of the lightning arrester is poor and overvoltage occurs, the lightning arrester cannot discharge current well, which will cause insulation damage to the transformer; If a lightning arrester is not installed on the low-voltage side, when a large amount of lightning current is discharged to the ground by the high-voltage side lightning arrester, a voltage drop is generated at the grounding position. This voltage also acts on the neutral point of the low-voltage side winding while passing through the transformer shell, and the low-voltage side winding is grounded through the wave impedance of the low-voltage line. The following measures can be taken:
① Identify the grounding defects related to the lightning arrester and re connect them as required. Note that the grounding wire of the lightning arrester should be directly connected to the shell of the transformer and the neutral point on the low-voltage side, and then the grounding device should be shared. The grounding resistance shall not exceed 4 Ω; ② For areas with multiple thunderstorms, a set of low-voltage lightning arresters should be added on the low-voltage side.
3. Oil leakage
There are also many leakage phenomena of transformer oil in distribution transformers. Due to leakage, the amount of oil in the transformer decreases, the oil level decreases, causing the infiltration of air and water vapor, accelerating the oxidation of the oil and causing it to deteriorate. This increases the viscosity of the oil, reduces the convection speed, affects the heat dissipation of the transformer, and causes a higher temperature rise, which further accelerates the deterioration of the oil. At the same time, the deteriorated oil acidity increases, leading to a decrease in the insulation resistance of the winding and even damaging the insulation. Over time, this will inevitably lead to transformer damage. The following measures can be taken: ① identify the location of oil leakage and handle it properly; ② Check whether the transformer oil is deteriorated and make a simple analysis of the oil. If the transformer oil gradually changes from light yellow to orange and brown at the initial stage, and the oil viscosity is large, it indicates that the transformer oil has deteriorated and must be purified or replaced; ③ When the transformer oil is not deteriorated, check whether the oil level is too low. If it is too low, add oil to the mark on the transformer oil storage tank; ④ Check the insulation resistance of the winding.