What Is a Distribution Transformer?

Distribution transformer ^[1] , referred to as "distribution transformer" for short, refers to a type of static electrical equipment in the distribution system that converts AC voltage and current according to the law of electromagnetic induction to transmit AC energy. In some areas, power transformers with voltage levels below 35 kV (mostly 10KV and below) are called "distribution transformers", referred to as "distribution transformers". The place and place where "distribution transformers" are installed are both substations. The distribution transformer should be installed on a pole or on the ground in the open air. The installation method, installation precautions, supply and distribution methods, capacity selection, operation and maintenance, etc. are introduced in detail.

Distribution transformer refers to a kind of static electrical equipment used in power distribution system to transform AC voltage and current according to the law of electromagnetic induction to transmit AC energy. China's transformer products can be generally divided into ultra-high voltage (750KV and above), ultra-high voltage (500KV) transformer, 220-110KV transformer, 35KV and below transformer according to voltage level. Distribution transformers usually refer to those that operate in the distribution network with a voltage level of 10-35KV and a capacity of 6300KVA and below to directly supply power to end users.

Since 2000, due to urban and rural

Distribution transformers are divided into oil-immersed transformers and dry-type transformers according to the insulation medium; according to different voltage regulation methods, they are divided into non-excitation voltage transformers and on-load voltage regulators ^[4]

Installation method of distribution transformer

Under normal circumstances, the distribution transformer should be installed on a pole or installed in the open air. Distribution transformers in factories, workshops, and suburban living areas can be installed indoors according to specific conditions ^[5]

10KV high-voltage power grid adopts three-phase three-wire

In the operation of distribution transformers, there are cases where the load is under-loaded due to excessive capacity, and there are cases where the equipment is overheated or even burned due to overload or over-current operation. The improper selection of the capacity of this device affects the reliability and economics of power supply in the power system ^[6]

Overload operation of distribution transformer

Transformer overload operation means that the load current exceeds the rated current of the transformer. In general, when the transformer is running at low load, its insulation material cannot fully function, and during continuous overload operation, the transformer will generate high temperature, which will cause the winding insulation part to burn out and form a turn-to-turn short circuit; meanwhile, transformer oil will be generated. Oil sludge accumulates on the fuel tank plate, windings and iron core, resulting in poor heat dissipation of the transformer oil. This vicious cycle not only seriously affects the life of the transformer, but also causes accidents such as high-voltage breakdown and transformer burnout. Therefore, observe the three-phase load current frequently. The three-phase load current should be consistent. If there is deviation, it should not exceed 10%.

2. Abnormal sound of distribution transformer

When the alternating current passes through the transformer windings, a normal and uniform "buzz" sound will be generated due to the core's natural vibration. If an abnormal sound occurs, you need to find the cause and report it to the relevant department in time. The sound is also different when the transformer is under no load and after being loaded. According to the characteristics of the abnormal sound compared with the past, it can be put into operation after the cause is found.

3. Checking the distribution transformer temperature

The operating temperature of a transformer has a lot to do with its life. Transformer operating at normal temperature below 95 , its life is 20 a; if it is operated at 110 , its life will be shortened to 7 a; if it is operated at 130 , its life will be shortened to 2 a; transformer at 170 Continuous operation at temperature will cause scrapping around d. 10 The temperature exceeds the allowable value of the transformer. To find out the cause, take measures in time.

4. Is the oil level of the distribution transformer normal? Is there any seepage, oil leakage or abnormal oil color?

There are many reasons for the drop in oil level. Due to poor welding quality and poor sealing, heat sinks, valves, box edges, etc. are prone to seepage and oil leakage. When the oil level drops below the top cover of the transformer, the contact surface between oil and air increases, and it is easy to oxidize and deteriorate and absorb the moisture in the air, resulting in a reduction in the pressure resistance of the oil, thereby destroying the insulation performance of the winding. When the oil shortage is serious, the insulation of the conductive parts of the transformer to ground and each other is reduced, causing a phase-to-phase or ground breakdown discharge. If you continue to use it at this time, the transformer oil will not be able to circulate convection normally, which will cause the transformer oil temperature to rise, shorten the life and even burn it. ^[1]

1. Unbalanced three-phase load or seasonal overload

Three-phase load imbalances in distribution transformers exist from the survey results. Especially in rural areas, most of the power loads are single-phase loads, and the load changes greatly. Therefore, there are many three-phase load imbalances in distribution transformers. Make the three phases unable to run symmetrically and generate zero sequence current. On the one hand, the loss of the transformer is increased, and on the other hand, the effective capacity of the transformer is reduced. The above two conditions will cause the transformer to overheat, the insulation oil to deteriorate, the winding insulation level to be reduced, and eventually the transformer to be damaged. The following measures can be taken:

Investigate the load situation of the distribution transformer, including the load of 24 hours a day and the load of 4 seasons a year, find out the general situation of the load, and adjust the three-phase load as much as possible to make it close to symmetrical operation;

Adjust the peak and valley time of power consumption to reduce the overload situation; at the same time, increase the capacity of the transformer in time to avoid long-term overload operation of the transformer.

Poor grounding

Lightning protection of distribution transformers subjected to lightning strikes is generally done, but there are still two problems: the lightning arrester is not well grounded; only attach the lightning arrester on the high-voltage side, and ignore the problem of installing a lightning arrester on the low-voltage side (especially Dore). If the arrester is not well grounded, the surge arrester cannot discharge the current well when overvoltage occurs, and the transformer insulation will be damaged; if the surge arrester is not installed on the low voltage side, when the surge arrester on the high voltage side releases a large lightning current to the ground, A voltage drop is generated at the ground position. This voltage also acts on the neutral point of the low-voltage side winding while passing through the transformer casing, and the low-voltage side winding is grounded through the wave impedance of the low-voltage line. The following measures can be taken ^[1]

Distribution transformer structure

Transformer, coil structure, coil oil tank and iron core, oil insulation to transfer heat, and the heat dissipation pipe is installed on both sides ^[7] .

The no-load switch is adjustable in pressure, the porcelain sleeve is used to isolate the edge, and the oil standard oil temperature respirator is installed on the upper side of the oil pillow.

Transformers, voltage transformers, electromagnetism and magnetic magnetism, the primary coil passes electricity, and the secondary coil induces electricity.

Although the two coils are not connected, the closed magnetic circuit has an iron core. The larger the current, the thicker the conductor, and the voltage turns ratio becomes positive.

The transformer model has known performance, technical parameters for verification, voltage and current no-load current, and capacity KVA is rated.

Impedance voltage power loss, no-load loss is P0, 12 types of wiring groups, Y, yn0, Y, d11, YN, d11 type.

There are two methods for voltage adjustment. On-load and no-load voltage regulation. The voltage is lowered by one step, and the tap changer is at high voltage.

What is the principle of voltage regulation? The voltage of each winding is adjusted by plus or minus 5%. The pointer is in place.

The position of the distribution transformer is determined. The key to ensure the voltage is small capacity and multiple points, which is selected in the center of the load.

The high voltage does not exceed 15 kilometers, and the low voltage power supply is half a kilometer. The lighting radius is slightly enlarged, but the maximum is 1.5km.

Outdoor transformer type three, single pole double pole and three pole, 560 or more transformers, floor-standing stone bench.

In order to ensure the safety of operation, it is best to use indoor transformers, and the distribution transformers are built at the central point, and the power voltage is guaranteed.

Distribution transformer protection

The distribution transformer protection is relatively simple. The short-circuit fuse switch is used to neutralize the lightning by the ground wire. The key lies in the lightning arrester.

The height of falling down is four or five meters, and the tilt is not more than 30 degrees. Regardless of lightning protection and falling off, the distance remains half a meter.

Estimated Transformer Rated Current and Fusing Current

It is estimated that the distribution current is multiplied by the capacity. The high voltage is sixty-one, and the low voltage is doubled.

The principle of fuse selection is also selected according to capacity. High-voltage displacement is safe. Double the low-voltage capacity. Distribution transformer operation system.

Ground zero is standard, electrical grounding is safe, and transformers run multiple types, TT, TN, and IT.

The TN system is divided into three, TN-S, TN-C, and TN-C-S. You can choose the grounding system.

Lightning protection and grounding of distribution transformers

Lightning protection grounding electrode, the smaller the resistance, the better, eight pieces of 2.5m angle steel ground, the spacing of 2 meters is more appropriate.

2.5 Flat Gang connection, four hundred ohms meter, one hundred transformers, less than ten ohms can be.

p Distribution transformer loss (p)

There is loss of power transformer, copper loss, iron loss, temperature rise, core magnetization changes repeatedly, and hysteresis eddy current is iron loss.

Loss reduction and energy saving optional change, "S11" type is the most advanced, iron loss and copper loss are all equal, this operation is the most energy-saving.

Transformers can be side-by-side, the load size can be adjusted, and side-by-side operation has many advantages. There are four to meet the conditions.

The wiring groups must be the same, the ratio of the two units is not bad, the short-circuit voltage must be the same, and the capacity should not exceed three to one.

Electricity, electricity and light transfer station, transformer power distribution power supply, in order to continue to power, distribution transformer maintenance is also critical.

It is patrolled once a week and a half, the maintenance system is guaranteed, and the external cleaning and inspection are performed, and the safe power supply is the best.

Transformer load distribution conversion, 15 kilowatts is about 1 amp, 3 to 5 thousand volt substation, 1 amp current is 50 counts.

The voltage level is 110,000, and about 160 is about 1 amp. To get the exact number, please use the conventional formula.

Distribution transformer prevents burnout

1. Reasonably choose the installation location of the distribution transformer

The installation of the distribution transformer must meet the user's voltage requirements, and try to avoid installing the distribution transformer in wild mountains and mountains, which is vulnerable to lightning and away from residential areas. This is not convenient for regular maintenance by operators and management of workers. .

2. Reasonably choose the capacity of the distribution transformer

It is also very important to choose the capacity of the distribution transformer reasonably, which can neither cause the overload of the distribution transformer to burn out, nor the waste of large horse-drawn carts. The capacity of the distribution transformer should be selected reasonably based on the user load and statistical capacity. For example: a 100kV · A distribution transformer, with a power factor of 0.85, it can carry a load of 85kW.

3. Strengthen the measurement of electricity load

During the peak period of power load, the measurement of the load of each Satons distribution transformer should be strengthened, and the number of measurements should be increased if necessary. The distribution transformer with unbalanced three-phase current should be adjusted in time to prevent the neutral current from being too easy to burn out. Leads cause user equipment to burn out.

4.Avoid installing low-voltage metering boxes on distribution transformers

The low-voltage metering boxes installed on the distribution transformer stations in remote mountainous areas should be gradually removed and replaced with indoor installations as much as possible. For example, when our bureau eliminated the powerless villages in the past few years, in order to facilitate management and prevent power loss, low-voltage installations were installed in remote mountainous areas. Metering box, due to long-term operation, the glass of the metering box or the low-voltage pile head of the distribution transformer cannot be replaced in a timely manner, resulting in damage to the distribution transformer due to burning of the energy meter due to rain and other reasons.

5. Reasonably configure the high and low voltage fuses of the distribution transformer

The high- and low-voltage fuses of the distribution transformer have an unreasonable melt configuration, which is likely to cause severe overload of the distribution transformer and burn out the distribution transformer. High and low voltage melt configuration should follow: transformers with a capacity below 100kV · A are configured with fuses with 2.0 to 3.0 times the rated current; transformers with a capacity of 100kV · A and above are equipped with fuses with 1.5 to 2.0 times the rated current; The low-side fuse should be configured with a slightly larger rated current.

6. It is not advisable to adjust the tap-changer without permission.

Due to the difference in the power load in winter and summer, the voltage level is slightly different. In order to meet the voltage requirements, some rural electricians randomly adjust the tap-changer without performing related tests, resulting in the tap-switch not being in place, causing phase-to-phase short-circuits and burning out the distribution. Electric transformer.

7. Install insulation cover for distribution transformer

In order to prevent natural disasters and damage by external forces, high- and low-voltage insulation covers should be installed in narrow communities and forest protection areas when necessary to prevent low-voltage short-circuits from burning out transformers due to falling things on distribution transformers.

8, lightning arresters should be installed on the high and low voltage sides of the distribution transformer

Lightning arresters should be installed for the distribution transformers in multiple minefields. If the arrester quality is unqualified or the fault is not replaced in time, it is easy to be damaged by lightning. Every year during the thunderstorm season, the lightning arrester should be sent to the repair and testing department to be installed in time after passing the test, and the use of unqualified products is prohibited.

9. Regularly measure the grounding resistance of distribution transformers

After long-term operation of the distribution transformer (especially the ground wire is replaced by aluminum wire), severe oxidation occurs, the grounding resistance increases, and the buried ground body rusts and breaks, causing the neutral point potential to shift. When lightning or overvoltage occurs , Easy to cause accidents. According to the regulations, the grounding device should meet the following requirements: the grounding resistance of distribution transformers below 100kV · A is not greater than 10, and the grounding resistance of distribution transformers above 100kV · A is not greater than 4.

10. Strengthen daily management

Regularly inspect the lines and cut down the trees to prevent the branches from hitting the wires and causing low voltage short circuits to burn out the distribution transformer. The lack of daily management of the distribution transformer by the staff causes the transformer to run short of oil for a long time, and the silicone is not installed in the respirator or the silicone is not replaced in time. The distribution transformer itself has quality problems. As the installation site of the distribution transformer is far away, some construction workers put the untested distribution transformer into operation without authorization, temporarily using electricity, and burning out the transformer.

11. Regularly check the low-voltage leads of the distribution transformer

It is strictly forbidden to use the wire itself as the wire nose and directly go to the low-voltage pile head of the distribution transformer. Regularly check and solidify the connection between the lead and the low-voltage pile head of the distribution transformer to prevent the low-voltage pile head of the distribution transformer from being burned due to looseness.

To ensure the long-term safe operation of distribution transformers with fewer failures, as an operation manager, it is necessary to perform on-site inspections, maintenance, and measurements, and to identify problems and deal with them in time to avoid the expansion of the transformer and burnout of the distribution transformer.

Distribution transformer repair

1) Before carrying out maintenance, it is necessary to switch off the power switch of the previous level and suspend the power failure sign. Then check with an electric pen to confirm that the device being repaired is powered off before repairs can be performed;

2) During maintenance, two people must be present, one for maintenance and one for supervision;

3) Maintenance personnel must be professional electricians familiar with power distribution devices;

4) Maintenance personnel must wear protective equipment and use electrical insulation tools

5) The replacement of electrical appliances must be consistent with the original specifications, and the use of substandard substitutes is prohibited;

6) During maintenance, you must not arbitrarily change the wiring of the original power distribution device, and you cannot arbitrarily remove the original switchgear;

7) At the end of the repair, the power distribution device should be disconnected, and then the power supply side should be turned on and closed.

8) At the same time when the maintenance is over, remove the power-off sign from the upper level power switch and hang it.