Transformer basics introduction: the main functions and working principles of transformers

Transformer Guide A transformer is a device that uses the principle of electromagnetic induction to change AC voltage. Its main functions include: voltage conversion, current conversion, impedance conversion, isolation, voltage stabilization (magnetic saturation transformer), etc.

Classification of transformers

Transformers are divided into: distribution transformers, power transformers, fully sealed transformers, combined transformers, dry-type transformers, oil-immersed transformers, single-phase transformers, electric furnace transformers, rectifier transformers, reactors, anti-interference transformers, lightning protection transformers, box-type transformer test transformers, corner transformers, high current transformers, excitation transformers, etc.

Transformer components

The transformer components are mainly composed of iron core and coil, in addition to oil tank, oil pillow, insulating bushing and tap changer, etc.

The main function of the transformer

The transformer can not only increase the voltage to send electricity to the power consumption area, but also reduce the voltage to various levels of use voltage to meet the needs of electricity consumption. In short, both voltage increase and voltage reduction must be completed by the transformer.

The role of transformer oil pillow

When the volume of transformer oil expands or shrinks with the change of oil temperature, the oil pillow plays the role of oil storage and oil replenishment, which can ensure that the oil tank is full of oil. At the same time, the installation of the oil pillow reduces the contact surface between the transformer and the air, slowing down the deterioration of the oil. There is also an oil level gauge on the side of the oil pillow to monitor the change of oil level. There are three main types: corrugated type, capsule type, and diaphragm type.

Working principle of transformer oil purifier

The temperature difference between the upper and lower layers of the transformer in operation causes the oil to circulate in the oil purifier. Harmful substances in the oil, such as moisture, free carbon, oxides, etc., are absorbed by the silica gel in the oil purifier as the oil circulates, purifying the oil while maintaining good electrical and chemical properties, thus playing a role in regenerating the transformer oil.

How transformers transform voltage

From the working principle of the transformer, we know that the current enters from the primary winding and flows out from the secondary winding. The direction of the input alternating current changes continuously, which will generate a magnetic field that changes synchronously with the current. The size and direction of the magnetic field change continuously, thereby inducing current in the secondary coil. The voltage on each coil is equal. The more turns the secondary coil has, the higher the voltage output from the secondary coil.

If the primary coil has more turns than the secondary coil, the voltage on the secondary coil will decrease, which is a step-down transformer; conversely, if the primary coil has fewer turns than the secondary coil, the voltage on the secondary coil will increase, which is a step-up transformer.

Autotransformer

The autotransformer has only one set of coils, and the secondary coil is tapped from the primary coil. Its power transmission, in addition to electromagnetic induction transmission, also includes electrical transmission. This type of transformer has fewer silicon steel sheets and copper wires than ordinary transformers and is often used to regulate voltage.

Transformer voltage change rate

The voltage change rate of the voltage regulator is one of the main performance indicators of the transformer. When the transformer supplies power to the load, the voltage at the load end of the transformer will inevitably drop. The voltage drop value is compared with the rated voltage value, and the percentage is the voltage change rate. It can be expressed by the formula: voltage change rate = [(secondary rated voltage-load end voltage)/secondary rated voltage] × 100%. For ordinary power transformers, when connected to the rated load, the voltage change rate is 4-6%.

Transformer overload operation

When the transformer operates normally, the load factor of the daily load curve is mostly less than 1.

According to the principle of equivalent aging, as long as the extra life lost during overload and the less life lost during underload can compensate each other, the specified service life can still be achieved. The normal overload capacity of the transformer is formulated on the principle of not sacrificing normal life.

During the entire time interval, the transformer insulation aging rate only needs to be less than or equal to 1, and the following conditions must be met:

1. During overload, the temperature of the hottest point of the winding shall not exceed 140°C, and the upper oil temperature shall not exceed 95°C;

2. The maximum overload of the transformer shall not exceed 50% of the rated load;

Rated voltage of transformer

Voltage that is too high or too low will affect the normal operation and service life of the transformer, so voltage regulation is required.

Application scope of small transformers

Small transformers refer to single-phase transformers with a capacity of less than 1 kVA, which are mostly used as power transformers for electrical equipment control, power transformers for electronic equipment and power transformers for safety lighting.

Transformer losses during operation

1. Iron loss caused by the iron core. When the coil is energized, the magnetic lines of force are alternating, causing eddy current and hysteresis losses in the iron core.

2. Copper loss caused by the resistance of the coil itself. When current passes through the primary and secondary coils of the transformer, power loss occurs.

How to choose a transformer

1. Clarify the purpose. Is it used to increase or decrease the voltage?

2. Identify the number of power phases. Single phase or three phase;

3. Identify the use and environment. Select the cooling method for the transformer;

4. Determine the material of the coil (copper wire/aluminum wire) based on actual usage and budget;

5. Choose according to the rated parameters of the transformer, including rated voltage, rated current, and rated capacity.

Transformer core grounding

When the power transformer is operating normally, the core must be reliably grounded at one point. If it is not grounded, the floating voltage of the core to the ground will cause intermittent breakdown discharge of the core to the ground. Grounding the core at one point eliminates the possibility of forming a floating potential of the core.

However, when the core is grounded at more than two points, the uneven potential between the cores will form a loop current between the grounding points, causing a multi-point grounding heating fault in the core. The core grounding fault of the transformer will cause local overheating of the core. In severe cases, the local temperature rise of the core will increase, causing light gas to operate, and even heavy gas to operate and trip.

The difference between neutral point, zero point and zero line

The common connection point where the beginning (or end) of the three-phase winding is connected together is called the neutral point of the power supply. When the neutral point of the power supply has a good connection with the grounding device, the neutral point is called the zero point; and the wire leading from the zero point is called the zero line.

The difference between energy meter and power meter

The electric energy meter can simultaneously indicate active power, reactive power, counting, power factor, load curve, maximum and minimum load time, etc.

The power meter can only indicate active or reactive value.

Hazards of casing cracks

Cracks in the casing will reduce the insulation strength and cause further damage to the insulation until it is completely broken down. Water in the cracks may also cause the casing to rupture when it freezes.

The role of the central signaling device

The central signal device includes an accident signal and a warning signal, which are installed on the central signal panel in the main control room of the substation. When the circuit breaker of any distribution device in the substation trips due to an accident, the accident signal is activated; when an abnormal operation or an operating power failure occurs, the warning signal is activated. Both the accident signal and the warning signal have two signal devices, audio and light. The audio signal can arouse the attention of the on-duty personnel, and the light signal helps the on-duty personnel to determine the nature and location of the fault.

Internal overvoltage

Internal overvoltage is a sudden change in the state of the system due to operation, accident or other reasons. There will be a transition process from one stable state to another stable state. During this process, there may be dangerous overvoltage to the system.

The role of high voltage circuit breaker

1. Control function: When there is no-load current in the circuit, or the current load is relatively large, it can be cut off or closed in time, so as to control the entire circuit.

2. Protection function. When a circuit fails, such as a short circuit or an open circuit, the high-voltage circuit breaker can cut off the current. With this protection function, it can prevent the problem of the circuit from breaking.