The transformation of a three-phase voltage system can be realized using three single-phase transformers. But at the same time, an apparatus of considerable weight and impressive dimensions will be used. A three-phase transformer is devoid of these disadvantages, since its windings are located on the rods of a common magnetic circuit. Therefore, in networks with a capacity of up to 60 thousand. kVA its application is the best option.
Content
- Purpose of a three-phase transformer
- Definition and types of device
- Operating principle
- Transformer structure
- Schemes and groups of connection of windings
- Losses and efficiency
-
Special purpose transformers
- Measuring voltage transducers
- welding equipment
- Continuously variable voltage autotransformer
- Parallel work
Purpose of a three-phase transformer
The main function of transformers is to transmit electricity over long distances. AC electrical energy is generated in power plants. During the transmission of electricity, losses for heating the wires appear. They can be reduced by lowering the amperage. To do this, it is necessary to increase the voltage so that its value is in the range from 6 to 500 kV.
The magnification factor depends on the value of the transmitted power and the distance to the destination. The power that is transmitted in this case depends on two parameters: voltage and current. The main characteristic influencing the change in the losses of wires associated with heating is the value of the current strength. Accordingly, in order to reduce heating losses, it is necessary to reduce the amperage. By decreasing the current, the voltage value must be increased accordingly. Then the value of the power that is being transmitted will remain unchanged.
After the voltage is delivered to consumers, it should be reduced to the required value. Accordingly, the main task of three-phase transformers is to increase the voltage before the transmission of electricity and decrease it after it.
Definition and types of device
A three-phase transformer is a static apparatus with three pairs of windings. The device is designed to convert voltage when transmitting power over long distances.
Phase classification:
- single-phase;
- three-phase.
Single-phase transformers are low power. Their main areas of application are everyday life and special-purpose work (welding, measurements, testing).
The power range of three-phase transformers varies widely. That's why their area of application is very diverse:
- for power supply of pantographs of special purpose;
- for connecting measuring devices;
- to change the voltage value during testing;
- to increase or decrease the voltage when connecting lighting or power loads.
Operating principle
The core of a three-phase transformer is a magnetic circuit and windings. Each phase has its own step-up and step-down winding. Since there are three phases, respectively, there are six windings. They are not connected to each other.
The principle of operation of a three-phase transformer, like a single-phase one, is based on the law of electromagnetic induction. When the primary winding is connected to the network, alternating current begins to flow in it. Because of it, the main magnetic flux appears in the core of the steel magnetic circuit, which covers the windings in each phase. An electromotive force of the same value and magnitude appears in each turn.
If the number of turns of the secondary winding is less than the number of turns of the primary, then the output will be a voltage of a lower value than at the input and vice versa.
The fact that the value of the electromotive force depends only on the number of turns of a certain winding, confirm the formulas:
E 1 = 4, 44f 1 Ф W 1
E 2 = 4.44 f 1 Ф W 2
E 1, E 2 - the value of the electromotive force in the primary and secondary windings, respectively, V;
f 1 - frequency of the current in the network, Hz;
Ф is the maximum value of the main magnetic flux, Wb;
W 1, W 2 - the number of turns in the primary and secondary windings, respectively.
Transformer structure
The main parts of the voltage converter are:
- magnetic circuit;
- high and low voltage windings;
- tank;
- inputs and outputs.
Additional equipment includes:
- expansion tank;
- exhaust pipe;
- breakdown fuse;
- devices for control and signaling.
The magnetic circuit is necessary for fastening all parts of the apparatus. It is a kind of a skeleton of a voltage converter. Its second task is to create a direction of movement for the main magnetic flux. Depending on the features of fastening the windings to the core, the magnetic circuit of the transformer can be of three types:
- armored rod;
- armored;
- pivotal.
For the manufacture of windings of low-power transformers, a copper wire with a rectangular or circular cross-section is used.
Transformer oil is a very important element in the apparatus. It is not used in low-power transformers (dry). At medium and high power, its use is mandatory. Have transformer oil two main tasks:
- cooling the windings heating up due to the current flowing through them;
- increased isolation.
The expansion tank is installed in transformers with a high voltage winding of more than 6 kV and an apparatus power of more than 75 kVA. By taking heat away from the windings, the transformer oil gradually heats up and expands. Its surplus enters the expansion tank. The function of the expander is to protect the oil from oxidation and moisture.
In high-power transformers, the expander pipeline is equipped with a gas relay and a valve that disconnects the expander from the tank if necessary.
Inputs and outputs are needed to connect the ends of the windings to power lines. They are located on the tank lid. They are made of glass or porcelain insulator with a conductive copper rod inside. The primary winding is attached to the inputs, and the secondary winding is attached to the terminals.
There is a voltage switch (anzapfa) on the tank lid. With the help of this device, you can change the number of connected turns of the windings at the same time in three phases. This manipulation allows you to increase or decrease the output voltage as needed.
The function of the exhaust pipe is to prevent damage to the tank in the event of an emergency. In the event of a breakdown, short circuit, the oil heats up rapidly and gases appear. Due to the presence of an exhaust pipe, the tank does not burst under significant pressure, and only the glass membrane in the pipe is damaged. This allows oil and gases to escape.
A breakdown fuse is installed next to the inputs and outputs. Its purpose is to protect low voltage networks from the appearance of high voltage in them.
A thermometric signaling device is required to control the temperature of the transformer oil, as well as to give an alarm in case of overheating.
Schemes and groups of connection of windings
In three-phase transformers, it is necessary to interconnect the primary windings in phases and the secondary ones. There are three connection schemes:
- star;
- triangle;
- zigzag.
When the windings are connected with a star, the linear voltage - between the beginnings of the phases - will be 1.73 times greater than the phase voltage (between the beginning and end of the phase). When the transformer windings are delta-connected, the phase and line-to-line voltages will be the same.
It is more profitable to connect the windings with a star at high voltages, and with a triangle at significant currents. The zigzag connection of the windings makes it possible to smooth out the asymmetry of the magnetizing currents. But the disadvantage of this connection method is the increased waste of winding material.
The group of connecting the windings shows the lag of the vector of the electromotive force of the step-down winding from the vector e. etc. with. step-up winding. Indicate a connection group with a series of numbers from 0 to 11.
Losses and efficiency
A transformer is a type of electrical machine with a minimum amount of losses. Their number is negligible and amounts to 1-2%.
Electrical losses go to heating the windings of the apparatus and fluctuate in direct proportion to the change in load. Magnetic losses appear due to the magnetization reversal of the core of the magnetic circuit and depend only on the value of the voltage that is supplied to the primary winding. Therefore, connecting the transformer to a higher voltage leads to an increase in magnetic losses.
The efficiency (efficiency) of an electric machine is the ratio of the useful power at the output of an electric machine to that supplied at the input. The efficiency of the transformer takes on a maximum value when the apparatus is loaded by 45-65%.
Special purpose transformers
Voltage converters that are not designed to supply lighting and power loads are special transformers. They are of several types: measuring, welding, autotransformers.
Measuring voltage transducers
Instrument transformers are used to turn on metering devices in high voltage circuits. Their use allows:
- expand the boundaries of measurement of alternating current installations;
- to increase the protection of persons serving the apparatus;
- use devices of small size and weight for measuring.
Subdivided into current transformers and voltage transformers.
The primary winding of the instrument transformers is connected to the network, and the measuring instruments are connected to the secondary.
welding equipment
Welding transformers reduce the mains voltage (220 V or 380 V) to the required 60-70 V. The low welding voltage ensures the safety of the welder. Lowering the voltage value below 60 V is unacceptable due to the fact that the arc may simply not ignite.
Welding transformers are not afraid of short circuits, since in this mode of operation the current strength is kept within the permissible values for a long time.
Continuously variable voltage autotransformer
In machine rooms, autotransformers are used to start high-power engines, as well as in laboratories when conducting various types of tests.
The main distinguishing feature of autotransformers is the presence of an electrical connection between the primary and secondary windings. In step-down autotransformers, this fact is a disadvantage, since with insufficient adherence to the technique safety, in case of emergency mode, breakdown of the device, the life and health of the operating personnel may be under threat.
Parallel work
For the reliable operation of a large number of pantographs, one power transformer is not enough. Therefore, at the substations, several voltage converters are connected to work. Connecting transformers to a group of the same consumers is called parallel operation. It is impossible to include any voltage converters for parallel operation. Some special requirements need to be met.
The invention of the transformer gave alternating current a chance to firmly enter the development of industry and take its place in everyday life and agriculture.
