Voltage Transformer :
Reducing the value of voltage and current to suitable values that can be measured by measuring or protection devices.
Isolate the circuits on the secondary side (measuring devices - protection) from the primary side (high voltages).
Function of voltage Transformer:
The voltage transformers reduce the voltage to 100 - 110 volts to feed the measuring devices (KW-KV-KWH).
It also supplies the protective devices Directional O/C - Over/under voltage.
It also performs the process of feeding control systems Load shedding interlock.
Types of voltage transformers:
- Potential transformer .
- capacitive voltage transformer .
Installing a voltage Transformer :
- A closed magnetic circuit consisting of slides of silicon iron.
- A primary coil contains a large number of coils and is connected in parallel with the electrical circuit.
- A secondary coil contains a small number of coils and is also connected in parallel with the voltage coils to the measuring or protection devices .
- An insulating material that isolates the primary coils from the secondary coils , so when the the voltage , the isolation increase .
Specifications for the selection process of the transformer:
- Rated voltage: It is the value of the voltage on the primary coil, for example 12KV.
- Level Voltage: It means the voltage on the secondary coil, for example 110 - 100 - 75 - 28 - 12KV.
- Breakdown-Bules Voltage: The maximum value that the transformer can withstand for moments.
- Turns Ratio: It is the conversion ratio from the primary coil to the secondary coil.
- Accuracy class: It describes the error rate in reading the converter in terms of magnitude and angle.
The error rate as mentioned earlier in the current transformer in detail:
Error = KsVs-Vp / Vp * 100
How to connect voltage transformers:
In general, the voltage transformers are connected to measure the voltage on the three phases separately, where each of the VT is connected between Phase and Neutral, thus allowing the protection device to see the value of the voltages of the three phases separately, Va-Vb-Vc.
There are sometimes voltage transformers used to measure the total effort in the three phases and not the voltage on each of them on the
This method is called Resdual Connection.
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