Transformer
A transformer is a device that is used to convert energy and consists of two or more coils that are connected by a magnetic core and their flux is used to create a desired voltage
The basis of transformer work
When an electric current passes through a coiled wire, a magnetic or paramagnetic field is created around it, and if the magnetic field oscillates around a wire, an electric current is created in the coil, which is called electromagnetic induction
Therefore, if we place a second coil (secondary coil) next to a first coil (primary coil) and send an alternating current (alternating AC) in the first coil, an electric current will be created in the second coil
The voltage level can be determined according to the number of turns of the coils, the higher the number of turns of the second coil, the higher the secondary voltage level and the lower the current. The transformers used in the power distribution network are generally the following components are formed
Iron core
Coils
Tap Changer
Bushings
Radiator
Tank cover
Wheels
Source of expansion
Buchholz relay
Tap changers are used as a switch in the transformer to change the voltage level
Transformer iron core
In some transformers, there are two iron branches for each phase; One for the weak pressure part and the other for the strong pressure. The coils are mounted on these two iron branches. But there are other types of transformers where both coils are mounted on one iron branch and the space between them is the dispersion channel. It is covered with insulation
Transformer oil dehumidifier
In order to prevent moisture and dust from entering the transformer, any air exchange between the transformer and the external environment should be done through the dehumidifier chamber
Silica gel grains (usually cobalt element) are used inside the dehumidifier glass. When these seeds change color, it is time to replace the dehumidifier
Transformer bushing
Insulators are used on the transformer to connect weak pressure and strong pressure cables to transformer coils. Bushings are insulating equipment that one or more conductors can pass through and be isolated from the outside environment. Bushings are porcelain, glass or resin
Types of transformers based on the type of insulation
The transformers used in the power distribution network are divided into two main categories in terms of insulating material
Oil transformer
In these transformers, oil is used as insulation between the coils. Due to its high insulating properties, oil can withstand very high temperatures and prevent damage to the transmission
Oil transformers are divided into two categories: conservatory transformer and hermetic transformer
The difference between conservatory and hermetic transformer
In the trans-conservatory, the ambient air is in contact with the oil through the dehumidifier, and the change in the oil volume of the tank is compensated in the expansion source. But in the hermetic trans, the oil has no contact with the ambient air
Dry or resin transformer
Resin is used as insulation. The most important feature of dry transformers is that they do not catch fire. If the transformer room is inside the building, we must use dry transformers according to the national building regulations
Magnetic oil display
This equipment is used in oil transformers and determines the oil level in the transformer. There are two different types of oil sight
Non-contact oil display
In this type of oil display, only the trans oil level is displayed
Contact oil
In addition to determining the oil level, there are also contacts in the oil. If the oil level drops below a certain value, a contact is activated and an alarm sounds in the system
Oil thermometer
We use oil thermometers to display the temperature inside the transformer. These devices are installed on the surface of the trans and at the highest point
Buchholz transformer relay
Relay Buchholz in English is located only in oil transformers. It is a protective device that is placed between the oil source and the main body of the transformer. The gases created due to the decomposition of oil or solid insulating materials are collected in this chamber
In case of oil leakage from the transformer and breakdown of oil or insulating paper, the relay is detected and a contact is closed and the alarm sounds. If the amount of gas stored in the Buchholz relay does not exceed the specified limit, its first float (float siren) is activated and by connecting a special key, the alarm sounds
If we do not personally disconnect the transformer despite the alarm sounding, and the factor that caused gas emission from the transformer is not removed, the relay will be filled with gas to such an extent that it will find the force necessary to operate the second float (disconnect float). and as a result, the disconnection float automatically disconnects the said transformer from the network. If a big fault unexpectedly threatens the transformer, the emission of gas becomes so intense that the movement of gas is combined with the strong flow of oil, and these two cause the float to move and as a result, the transformer is immediately disconnected from the power grid
In transformers that produce gas without any technical defect while under load, such as transformers with automatic voltage regulator that produces an electric spark between the contacts during operation, it is not possible to bypass the Buchholz relay. Vari that has a place to store gas It can be used, but due to the creation of gas, which is not a sign of a defect in the transformer, only the movement of oil can be used to protect the transformer, and for this reason, in such transformers, the Buchholz relay with a breaker float is used. The place of installation of the Buchholz relay is in the connection pipe between the transformer and the oil expansion tank
Transformer cooling system
Loading from the transformer and reaching the nominal capacity of the transformer is associated with losses and as a result the internal temperature of the transformer increases. This increase is converted into heat due to the losses that occur in the core of the coils and is allowed as long as it does not damage the insulation. Therefore, the resulting heat must be transferred to the outside environment in a way to prevent overheating of the parts inside the transformer and its damage
According to IEC 76 standard, common cooling systems are defined, which will include
-ONAN (Oil Natural Air Natural)
It means that the transformer oil naturally circulates inside the radiator system and is cooled by the ambient air
ONAF (Oil Natural Air Forced)
In this case, the oil naturally circulates inside the radiator and transfers its heat to the surrounding environment by means of electric fans
OFAF (Oil Forced Air Forced)
In this case, the oil is circulated in the radiator by the pump and transfers its heat to the surrounding environment by electric fans
Transformers short impedance connection
The voltage that is required to be applied to the terminals of one coil to cause the nominal current to pass through the other coil, which is short-circuited, is called the short-circuit impedance. The value of this short-circuit impedance, which is also known as the percentage impedance of the transformer, is expressed as a percentage of the nominal voltage. Iron loss of the transformer is increased due to the reduction of the short circuit impedance and causes the core of the transformer to become larger. The reactance of the transformer needs reactive power, which must be provided by a generator or compensating devices. As can be seen, the decrease or increase of short circuit impedance has positive and negative effects. Therefore, in the optimal selection of this impedance, in addition to considering the voltage level and capacity and the costs of copper and iron, it is also necessary to pay attention to its side effects. Standard 75IEC-6 specifies the value of short-circuit impedance for different capacities, which are introduced as type numbers, as described in the table
Nominal capacitor of the transformer in KVA | Impedance percentage trans |
1250-631 | 40% |
3150-1251 | 5% |
Tap changer
A tap changer is a mechanism that can be used to change the voltage conversion ratio of the transformer. For example, if the output of the generator is fed to the network by a power transformer, when the network has a voltage drop, the drop in the network can be repaired by selecting a higher secondary voltage, and so on. In cases where the network has an increase in voltage, it is possible to compensate for the increase in the network voltage by reducing the secondary voltage. This operation can usually be done by hand or motor. Each time the Tap changes, it connects a certain combination of wire ends, so the number of wires turns that are placed in the circuit. Therefore, with a fixed voltage in the primary, the secondary voltage is changed.
Tap changers can be divided into two categories
1-Tap changer ON load
2-Tap changer Off load
The meaning of no-load tap changer is that first the transformer must be unloaded and then the tap changed, but in the under-load type, the tap can be changed under load, which of course requires a higher technique
Rated current of tap changer
To find the nominal current of the tap changer, the nominal current of the transformer is obtained and 15% is added to it, and the obtained number is also increased by 20% to obtain the nominal current of the tap changer
Transformer losses
In general, transformer losses are
loss load No
No-load loss is the reactive power absorbed by the transformer while a rated voltage with a rated frequency is applied to one side and the other side is open and unloaded. Foucault), copper losses are caused by no-load current
Copper losses caused by no-load current are very small, and in fact, hysteresis and Foucault losses make up no-load losses. No-load losses exist as long as the transformer is energized with constant frequency voltage, regardless of what load is taken from it
loss load
Load loss is the active power absorbed at the rated frequency by the transformer while one coil has current and the other coil is shorted
Losses of the load itself from copper losses caused by the load current, copper losses caused by the supply current, copper losses caused by the eddy current in the conductor of the coils due to wasted flux, losses caused in the cap and tank walls due to wasted flux. Is formed
The amount of load loss at the base temperature defined in the 76IEC-1 standard is divided according to the insulation class
The allowable temperature increase of windings and transformer oil
The maximum allowable temperature of the winding is a determining factor in the useful life of the transformer, and the allowable temperature of the oil is also of special importance if it is used as a coolant in transformers. Therefore, the allowable increase in temperature of winding and oil of transformers is determined according to IEC-2 standard 76 as described in the table below
Section
| Maximum allowed temperature increase in degrees Celsius
|
Coils
| 65 degrees for cooling systems other than ONAF 75 degrees for cooling wires ONAF
|
Oil
| 60 degrees for the case where the transformer does not have a conservator 55 degrees if the transformer has a conservator
|
Types of power transformer test
Power transformers are thoroughly tested in the manufacturer’s factory and are tested with nominal voltages and higher currents, but after transporting the transformer to its destination, in order to check and confirm the correct operation of the transformer and the absence of any type of defect during use, some tests are carried out. It is carried out in the place (post) with accurate but portable measuring devices, which are summarized below
Conversion ratio test
In this test, by giving voltage to the primary or secondary of the transformer, the voltage of the opposite side is accurately measured. In power-reducing transformers, the primary side is usually given a voltage of 380 volts
Tap changer continuity test
In this test, he gave 380 voltages to the primary and placed accurate analog voltmeters on the secondary side and checked the deviation of the hand in all three phases during the time of changing the taps so that the hand does not bounce back. At the time of changing the tap, it is necessary to act in the following order
Insulation resistance test
This test is performed with the help of a Megger device and it is measured in 15 seconds, 60 seconds, 5 minutes and 10 minutes. In this test, the primary heads are short circuited and the same is true in the secondary
No-load current test
In this test, by giving voltage to the primary and if the secondary is open circuit, we measure its current with an accurate ammeter. It is the same for secondary. In the star connection, the ratio of three-phase amperes is 1-8.0-1 and in the triangle connection, it is 1-1-3.1.
Magnetic flux test
In this test, we measure the current of each phase and read the voltage of the opposite coil by giving single-phase voltage to the ends of each phase and zero (in star connection)
Group test
In this test, similar heads are shorted in one phase (e.g., u-U) and three phase voltage is injected and we read the voltage for the temperature of the heads relative to each other
Short circuit test
We perform this test by shorting the secondary and reading and recording the current in the primary and secondary after connecting the 380 voltages to the primary.
Ohmic resistance test
In this test, we inject DC voltage (for example 12 volts) to the ends of each phase with the zero end in the star connection and both phases in the triangle connection and measure the passing current. (This test is better done in the last step)
Tangent delta test
In this test, it is possible to check the different modes in the transformer with the special device for this test and measure the capacitance between each point of the transformer.