What Is a Voltage Transformer?
For a normal substation, the voltage often exceeds 10,000 volts. If you try to measure such high voltages by hand, you may have to say goodbay to the world, which is why there is a voltage transformer (voltage transformer, VT). Simply put, a voltage transformer is a special type of transformer that is designed to reduce high voltages to a safe, low voltage level. voltage transformers can be divided into the following categories according to their use and structure:
1. Electromagnetic voltage transformer: utilizes the principle of electromagnetic induction to convert high voltage to low voltage.
2. Capacitive voltage transformer: the use of capacitive voltage division principle, the high voltage is converted to low voltage, commonly used in ultra-high voltage and ultra-high voltage system.
What Causes Voltage to Be Induced in a Transformer?
voltage transformer based on Faraday’s law of electromagnetic induction work, in the closed core, wound with two different turns, mutually insulated windings (shown below)
N1:
● Primary winding (high-voltage side): connected to the high-voltage line, subject to system voltage.
N2:
● Secondary winding (low voltage side): connected to the measuring and protection equipment, outputs the standard voltage.
When an AC high voltage current flows in the primary winding, a varying magnetic field is generated in the iron core. This magnetic field is induced in the secondary winding, which in turn generates a voltage in the secondary winding. Since the number of turns in the secondary winding is much less than that in the primary winding, the voltage in the secondary winding will be much lower than that in the primary winding, according to the transformer principle. As the principle of voltage transformer is similar to ordinary transformers, so that the output voltage is proportional to the input voltage, the specific numerical relationship can be expressed by the following formula: N1/N2 = U1/U2, where N1 and U1 represent the turns and voltage of the primary winding, N2 and U2 represent the turns and voltage of the secondary winding.
What Is the Function of Voltage Transformer in Substation?
Substation is an important facility used in the power system to change the voltage and distribute power. It mainly plays two roles: one is the transmission lines to deliver high-voltage electrical energy into suitable for distribution to the user of low-voltage electrical energy; the second is the power of different transmission lines for shunting, merging and switching to ensure the stable operation of the power grid. Substations generate voltages that are supplied to several industrial and residential areas, so it is important to ensure that the voltages are in optimal condition and that they do not suffer significant losses during transmission. In order to achieve this, the voltage at each node must be measured, and this is where voltage transformers come into play.
voltage transformers not only reduce high voltages to safe low voltage levels, but also serve other important purposes. To illustrate a simple example, a 220kV substation is struck by lightning, resulting in a temporary voltage drop. At this point, the voltage transformer in the substation detects the voltage drop and immediately transmits this information to the relay protection device. According to the preset protection logic, the relay protection device quickly judged that the voltage drop might cause damage to the grid equipment or affect the quality of power supply, so it issued a timely command to cut off the power supply in the faulty area, which protected the stable operation of the grid and the safety of the equipment. At the same time, the operation and maintenance personnel through the voltage transformer to provide accurate voltage data, can quickly analyze the causes of voltage drop, and take timely measures to restore the voltage, to ensure the reliability of power supply.
In summary, the main role of the voltage transformer includes:
1. voltage Measurement: voltage transformer can transform the high voltage into low voltage, usually this transformation ratio is fixed, such as 220kV grid may be transformed into 100V through the voltage transformer, so that the low voltage can be safely accessed to the voltmeter or digital display for accurate measurement.
2. Relay Protection: In power systems, relay protection devices are required to make protective actions, such as over-voltage or under-voltage protection, in response to voltage changes in the grid. voltage transformers provide accurate voltage signals to the protection device, so that it can reliably determine the status of the power grid and implement the appropriate protection measures.
3. Simultaneous Detection: In grid-connected operation or grid reconfiguration, it is necessary to ensure the synchronization of voltage phase and frequency between each grid or generator, voltage transformers provide the voltage signals required for synchronization detection equipment to ensure stable and reliable operation of the power grid.
4. Electricity metering: Electricity meters require voltage signals to calculate the user’s electricity consumption, voltage transformers provide such signals and ensure the accuracy of metering.
