What is Voltage?
Voltage transformer, Is a reducing transformer to reach the corresponding voltage in the initial of this transformer. The secondary voltage in these transformers is proportional and in phase with the primary voltage. These transformers are placed parallel to the initial voltage and ground.
This transformer also has different types and sizes, different power and different buildings. Voltage transformers are also available in single-phase, two-phase and multi-phase types. At high voltages, these transformers use capacitors that are built-in to save costs and reduce their building volume. trans It is installed and is known as capacitive voltage transformers.
In addition to measuring high voltage voltage and voltage sampling for protection transformers of voltage transformers in substations for communications PLC It is also used that in some cases, communication devices (line trap) are installed on these transformers, which we will discuss in the following.
Types of voltage transformers:
Inductive voltage transformer (VT or PT)
Capacitive Voltage Transformer
– Inductive voltage transformer
Voltage transformers consist of two windings, which depending on the type of transformer and the transformer requested in the second can be more than the number of windings (blind). Inside these transformers there is also lubricating oil and it causes the transformer to cool down. In the beginning, this transformer is connected to the nominal voltage of the substation and includes only one terminal (although in some of those primary terminals there are also input and output).
The output power of the voltage transformer is equal to the sum of the power of the secondary cores. The power that is inserted on the transformer plate is the power that the transformer can give permanently in the circuit. The voltage transformer is designed for Frequency 50 Hz can continue to be used at 60 Hz without loss of rated power.
– Capacitive voltage transformer
Today, due to the lower costs of this type of transformers and the type of their use, this type of transformers is used more, and in this category, we will pay more attention to this type of transformers; Since the insulating properties at higher voltages in inductive voltage transformers become relatively harder and more voluminous, it is initially reduced by capacitors, which are paper-type capacitors with aluminum conductors that are sequentially closed. Depending on the voltage, the number of capacitors varies and at higher voltages the number of series capacitors increases. After reducing this voltage by using a core and coil to the nominal value of the voltage in the mentioned second is reduced. Capacitive voltage transformers are less accurate but are more reasonably priced, and since the PLC system is also installed to prevent the installation of coupling capacitors, these transformers are used more.
Important specifications of a voltage transformer are as follows, which are taken into account when ordering or designing:
- The highest system voltage
- Nominal frequency
- Conversion ratio
- Tip and class
- Capacitive capacity Between the primordial and the earth
- Creepage Distance
- Maximum thermal load
Voltage transformers at lower voltages consist of only primary and secondary windings in which the insulation used is epoxy resin which is formed into molds. In the primary input of these transformers, a protective fuse is placed and their primary is connected to the ground from the other side (in single phase) and in the secondary it is the same, ie the end of the secondary winding is grounded. The accuracy class of most transformers used in substations is 3P, which indicates that there is a 3% error in the transformer conversion ratio.
In transformers of 63 kV and above, fuses are installed in the output of these transformers. These fuses can also be inside the transformer box itself or in another panel, which, if it is in the panels, comes with an auxiliary contact to send an alarm in case the fuse operates.
In external voltage transformers, during installation, care must be taken that the connection wire of its body is installed carefully and Resistance Have a bottom. During the operation time, except for visiting the connections and checking the appearance of the transformer, there is no need for special testing and testing. But after a connection or the time defined for the transformer by the manufacturer, the oil inside it must be tested. Also, in case of possible leakage, it must be repaired with oil equal to its oil class.
It is necessary to clean the capacitive insulators and wrench the secondary terminals at certain times, depending on the environmental situation of the transformer installation.
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