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What is a Power Transformer

A power transformer is an very important component of the power system and the core equipment of an electrical substation. A power transformer is a static electrical device that utilizes the principle of electromagnetic induction to convert one type of alternating current into another or several types of alternating current with the same frequency and different sizes, playing a role in transmitting electrical energy and changing voltage rate.

Basic Principle of  a Power Transformer

When the primary winding of a power transformer is connected to an AC power source, alternating current will flow through the winding and generate alternating magnetic flux in the core. This alternating magnetic flux is interconnected with the primary and secondary windings, and alternating induced electromotive force will be induced in them. The secondary winding has induced electromotive force, and if connected to the load, it can supply power to the load, transmit electrical energy, and achieve energy transfer from the primary side to the secondary side.

Classification of a Power Transformer

According to the number of phases: single-phase transformer, three-phase transformer

Depending on the number of windings

Double winding transformer, three winding transformer, auto transformer

According to different cooling modes: oil immersed self cooling transformer, oil immersed air-cooled transformer, oil immersed water-cooled transformer, forced oil circulation air-cooled transformer, dry-type transformer;

According to different purposes:

Power transformers (step-up transformers, step-down transformers, distribution transformers, etc.

Special transformers, like furnace transformer, rectifier transformer, welding transformer etc.

Instrument transformers, like voltage transformers and current transformers,and high-voltage transformers for testing.

Main technical parameters of a power transformer

Rated value of a power transformer

The maximum load power when operating under rated conditions. Generally: Small transformers with a capacity of no more than 630kVA; 800-6300kVA is a medium-sized transformer; 8000-63000kVA is a large transformer; over 90000kVA is a super large transformer.

Divided into primary and secondary voltages. Generally, the voltage levels are 400V, 3kV, 6kV, 10kV, 35kV, 66kV, 110kV, 220kV, 330kV, and 500kV. for tapping ± 5%.

The rated current I1N/I2N refers to the current that a transformer is allowed to pass through for a long time, and the unit of (line current) is A. The rated current can be calculated from the rated capacity and rated voltage.

The rated frequency f of industrial AC power is 50Hz or 50Hz.

No-load test:

Measure the voltage transformation ratio k, no-load current I0, and no-load loss p0

The no-load loss of transformers is mainly iron loss, which is the power loss caused by excitation current and can be divided into eddy current and hysteresis loss, roughly proportional to the square of the voltage.

The no-load current I0 is basically the excitation current of the transformer, which is generally 2-8% for small and medium-sized transformers, and less than 1% for large ones.

Short circuit test:

Measure the short-circuit voltage Uk and short-circuit power (i.e. short-circuit loss)

The main short-circuit load loss is copper loss (referred to as aluminum loss in aluminum wire transformers).

Copper loss refers to the ohmic resistance loss caused by winding current, which is roughly proportional to the square of the current.

Impedance voltage:

The percentage of the ratio between the actual value of short-circuit voltage Uk and the rated voltage U1N is called impedance voltage uk.

uk=(Uk/U1N)*100%

The impedance voltage uk is an important parameter of a transformer, and its size mainly depends on the design size of the transformer. The selection of uk involves factors such as transformer cost/efficiency/voltage stability/short-circuit current size.

During normal operation, it is hoped that the uk is smaller, so that the terminal voltage fluctuates less with the load. But when a sudden short circuit occurs, I hope the uk is larger to reduce the short-circuit current.

What components are there for a power transformer

The main components of a transformer are the winding and iron core . The winding is the circuit of a power transformer, and the iron core is the magnetic circuit of the transformer. The two constitute the core of the transformer, which is the electromagnetic part. In addition to the electromagnetic part, there are also accessories such as oil tanks, oil storage cabinets, tap changers, safety airways, gas relays, insulation sleeves, and oil purifiers. Its function is to ensure the safe and reliable operation of the transformer. The iron core is installed in the oil tank, which is full of transformer oil.Transformer oil is a kind of mineral oil with good insulation performance. Main functions of transformer oil: ① it plays an insulating role between transformer winding and winding, winding and iron core and oil tank. ② When heated, transformer oil will generate convection, which will heat the transformer core and winding. ③ When a discharge fault occurs inside the transformer, it has an arc extinguishing effect.The oil tank has many heat dissipation pipes to increase the heat dissipation area. In order to speed up heat dissipation, large transformers use internal oil pumps to force oil circulation, and use transformer fans to blow outside or use tap water to flush the transformer oil tank. These are all cooling devices for transformers.

Gas Relay for a Power Transformer

When there are inter turn short circuits, insulation damage, poor contact, and multiple grounding faults in the transformer, a large amount of heat energy will be generated, causing the oil to decompose into combustible gases and flow towards the oil storage tank. When the flow rate exceeds the set value of the gas relay, the baffle of the gas relay is impacted, causing the relay to trip, thereby avoiding the expansion of the accident. This situation is usually referred to as heavy gas protection action. When the gas rises along the oil surface and accumulates in the gas relay for more than 30ml, the signal contact of the gas relay can also be connected, giving an alarm, and light gas protection occurs. By analyzing transformer faults and identifying the causes of gas relay action, the nature and location of the faults in the transformer can be determined based on different reasons, and effective repairs can be carried out to restore normal operation as soon as possible.

Installation Guide for a Power Transformer

Conduct a visual inspection of the power transformer before installation. Verify whether the product model, capacity, voltage, connection group, short-circuit impedance, etc. of the transformer match the design.

Conduct a visual inspection of the transformer before installation. Check if the transformer is damaged during transportation; Whether there are signs of oil leakage and whether the oil level and color are normal; Whether the accessories are damaged and displaced; Whether the fasteners are loose; Is there any damage to the insulation.

Routine Inspections for a Power Transformer during its daily operation

(1) Check if the upper oil temperature of the transformer exceeds the allowable range. Due to the different load sizes, cooling conditions, and seasons of each transformer, the operating transformer should not only be based on the upper oil temperature not exceeding 85 ℃, but should also be compared with the previous oil temperature based on past operating experience and in the above situations. If the oil temperature suddenly increases, check whether the cooling device is normal and whether the oil circulation is damaged.

(2) Check the oil quality, it should be transparent and slightly yellow. The oil level should meet the standard line of the surrounding temperature. If the oil level is too low, the transformer should be checked for oil leakage, etc. If the oil level is too high, the use of the cooling device should be checked for internal faults.

(3) The sound of the transformer should be normal. During normal operation, there is generally a uniform buzzing electromagnetic sound. If there is a change in the sound, it should be carefully checked and promptly reported to the duty dispatcher, and the maintenance unit should handle it.

(4) The casing should be checked for cleanliness, cracks, and discharge marks, and the cooling device should be functioning properly.

(5) Special inspections should be conducted when the weather changes. When there is a strong wind, check if the leads swing violently, and there should be no debris at the top cover and bushing leads of the transformer; During heavy snowfall, all contacts should not immediately melt or discharge after snowfall; On foggy days, whether there are sparks or discharges in various parts, etc.

The newly installed power transformer operates at no load for 24 hours before being loaded; The high-voltage side voltage shall not exceed ± 5%; The maximum unbalanced current of low voltage shall not exceed 25%; Due to the use of Class A insulation in transformers, their maximum operating temperature is 105 ℃. So the temperature rise of the winding should not exceed 65 ℃, the upper oil temperature should not exceed 85 ℃, and the maximum temperature should not exceed 95 ℃; Transformers should not be overloaded for a long time, but overload is allowed under special circumstances, but the overload time must be adapted to the temperature and overload amount.

Protections for a Power Transformer

1. Lightning protection

Install lightning arresters to protect against immersion waves along the line;

a) While installing zinc oxide lightning arresters on the high-voltage side, zinc oxide lightning arresters that meet the requirements should also be installed on the low-voltage side according to regulations; b) The grounding terminals of the high and low voltage side lightning arresters, the neutral point on the low voltage side, and the distribution transformer casing must be short circuited nearby before being centrally connected to the grounding grid;

c) The high-voltage side lightning arrester should be installed on the load side of the drop type fuse.

2. Fuse protection:

① The main function of the high-voltage side fuse of the transformer is to protect the transformer and quickly fuse when there is an internal short circuit or a high-voltage lead short circuit in the transformer.

Principle of fuse selection: For transformers with a capacity of 100kVA or less, the fuse should be selected based on 2-3 times the rated current; When the transformer capacity is above 100kVA, the fuse should be selected according to 1.5~2 times the rated current.

② The high-voltage side fuse of the transformer is mainly used for short circuit protection and overload protection of the low-voltage main line of the transformer. The size of the fuse is selected based on the secondary load current or rated secondary current of the transformer.

3. Relay protection

Relay protection summary: An automated measure and equipment that can timely issue warning signals to the operating personnel on duty or directly issue tripping commands to the controlled circuit breakers to terminate the development of these events when faults occur in power components (such as generators, lines, etc.) or the power system itself, endangering the safe operation of the power system. It is generally referred to as a relay protection device.

Handling Guide for a power transformer when there is a fault

1. Abnormal phenomena during transformer operation:

(1) If there is oil leakage, high or low oil level, abnormal temperature, abnormal sound system, and abnormal cooling system during transformer operation, efforts should be made to eliminate them as soon as possible.

(2) When the load of the transformer exceeds the allowable normal overload value, the load of the transformer should be reduced according to regulations.

(3) The internal sound of the transformer is loud and abnormal, with a popping sound; The temperature is abnormal and constantly rising; Fuel injection from the conservator or safety airway; Severe oil leakage causes the oil level to drop below the indicated limit of the oil level gauge; The oil color changes too quickly, and carbon appears in the oil; If the casing is severely damaged or discharged, it should be immediately powered off for repair.

(4) When it is found that the oil temperature of the transformer is high and the oil level corresponding to the oil temperature has significantly decreased, oil should be added immediately. When refueling, follow the regulations. If the oil level drops rapidly due to a large amount of oil leakage, the gas protection should be changed to only act on the signal, and measures to block the oil leakage must be taken quickly, and oil should be added immediately.

(5) When the oil level of the transformer gradually rises due to temperature rise, if the oil level at the highest temperature may be higher than the oil level indicator, the oil shall be drained to reduce the oil level to an appropriate height to avoid oil overflow.

2. handling guide when transformer is fault

The common faults in the operation of transformers are faults in windings, bushings, and voltage tap changers, while faults in iron cores, oil tanks, and other accessories are relatively rare.

In order to handle accidents correctly, the following situations should be mastered: ① system operation mode, load status, and load type; ② The upper oil temperature, temperature rise, and voltage of the transformer; ③ Weather conditions at the time of the accident; ④ Is there any maintenance or other work around the transformer? ⑤ Whether the operating personnel have operated; ⑥ Is there any operation in the system? ⑦ What kind of protective action, accident situation, etc.

Winding faults for a power Transformer: mainly include inter turn short circuit, winding grounding, inter phase short circuit, wire breakage, and joint welding. The reasons for these faults are as follows:

① During manufacturing or maintenance, local insulation is damaged, leaving defects Due to poor heat dissipation or long-term overload during operation, debris may fall into the winding, resulting in excessive temperature and insulation aging The manufacturing process is poor, the compression is not tight, and the mechanical strength cannot withstand short circuit impact, causing deformation and insulation damage to the winding The winding is damp, and insulation expansion blocks the oil passage, causing local overheating. ⑤ The insulation oil deteriorates due to the mixing of water, or the contact area with the air is too large, resulting in a high acid value of the oil, a decrease in insulation level, or a low oil level. Some windings are exposed to the air and cannot be treated in a timely manner.

Due to the above reasons, if insulation breakdown occurs during operation, it can cause a short circuit or ground fault in the winding. The fault phenomenon during inter turn short circuit is that the transformer overheats and the oil temperature increases, the power side current slightly increases, the DC resistance of each phase is unbalanced, and sometimes there is a squeaking and bubbling sound in the oil. A slight inter turn short circuit can cause gas protection action; In severe cases, differential protection or overcurrent protection on the power side will also operate. Discovering inter turn short circuits should be dealt with promptly, as inter turn short circuits in windings often cause more serious faults such as single-phase grounding or inter phase short circuits.

Bushing failure for a Power Transformer

The common faults of this kind are explosion, flashover, and oil leakage, and the reasons are:

(1) Poor sealing and poor insulation due to moisture;

(2) Improper configuration of respirators or failure to handle inhalation of moisture in a timely manner.

Tap changer fault for a Power Transformer:

Common faults are surface melting and burning, discharge of interphase contacts or discharge of various joints. The main reasons are:

(1) Loose connection screws;

(2) Poor and improper adjustment of the load adjustment device;

(3) Poor insulation of the tap insulation board;

(4) Insufficient soldering of the joint, poor contact, poor manufacturing process, and insufficient spring pressure;

(5) The high acid value of the oil causes corrosion on the contact surface of the tap changer.

Iron Core failure of a Power Transformer

The majority of iron core failures are caused by insulation damage to the core screw of the iron core column or the clamping screw of the iron wheel. The consequences may result in two point connections between the core screw and the iron core lamination, causing circulation and local heating, and even causing local melting of the iron core. It may also cause local short circuits in the iron core laminations, generate eddy current overheating, cause damage to the insulation layer between the laminations, increase the no-load loss of the transformer, and deteriorate the insulation oil.

Gas protection fault for a Power Transformer

Gas protection is the main protection of transformers, with light gas acting on signals and heavy gas acting on tripping. The following is an analysis of the causes and handling methods of gas protection actions:

(1) After the light gas protection action, a signal is issued. The reason is that there is a slight fault inside the transformer; There is air inside the transformer; Secondary circuit failure, etc. The operating personnel should immediately inspect, and if no abnormalities are found, gas sampling analysis should be conducted.

(2) When the gas protection action trips, there may be a serious fault inside the transformer, causing a large amount of gas to be decomposed from the oil, or there may be a secondary circuit fault. In case of gas protection action tripping, the backup transformer should be put into operation first, and then external inspection should be conducted. Check the explosion-proof door of the oil conservator, whether each welding joint is cracked, and whether the transformer shell is deformed; Finally, check the flammability of the gas.

Transformer on fire

This is a very dangerous accident, as there are many combustible substances in the transformer, which may cause explosion or expand the fire if not handled in a timely manner. The main cause of transformer fire is: damage and flashover of the casing, oil flowing out under the pressure of the conservator and burning on the top cover; The internal fault of the transformer caused the shell or radiator to break and the burning transformer oil to overflow. When such accidents occur, the transformer protection should act to disconnect the circuit breaker. If the circuit breaker is not disconnected for some reason, manual action should be taken to immediately disconnect the circuit breaker, open the isolation switch that may lead to the transformer power supply, stop the cooling equipment, and extinguish the fire. When extinguishing a transformer, it is better to use a foam type extinguisher, and use sand when necessary.

Finally, When a power transformer trips automatically, it is necessary to investigate the protection action and conduct external inspections. After inspection, if it is not an internal fault but is caused by external faults (transient faults) or personnel malfunctions, power transmission can be put into operation without internal inspection. If differential protection is activated, all equipment within the protection range should be inspected. After the transformer malfunctions during operation, if it is determined that it is a winding or iron core fault, the core should be suspended for inspection. Firstly, measure the DC resistance of each phase winding and compare it. If the difference is significant, it indicates a winding fault. Then conduct an appearance inspection of the iron core, and measure the insulation resistance between the pieces using the DC voltage and ammeter method. If the damage is not significant, paint the damaged area.