Essential Knowledge for Power Engineers—A Detailed Explanation of Voltage Transformers
(This article is from: Electrical Engineering Technology, ID: )
I. Brief Introduction to Voltage Transformers
Voltage transformers (PTs, also known as TVs) are similar to transformers, both used to transform voltage on power lines. However, transformers transform voltage to transmit electrical energy, so their capacity is very large, typically measured in kilovolt-amperes (kVA) or megavolt-amperes (MVA). Voltage transformers, on the other hand, transform voltage primarily to power measuring instruments and relay protection devices, measuring line voltage, power, and electrical energy, or protecting valuable equipment, motors, and transformers in the event of a line fault. Therefore, voltage transformers have a very small capacity, generally only a few volt-amperes or tens of volt-amperes, with a maximum not exceeding one thousand volt-amperes.
Why are power presenters (PTs) needed on power lines ? This is because the voltage on power lines varies significantly depending on the generation, transmission, and consumption conditions. Some lines are low-voltage (220V and 380V), while others are high-voltage (tens of thousands or even hundreds of thousands of volts). To directly measure these low and high voltages, it would be necessary to manufacture corresponding low-voltage and high-voltage voltmeters, other instruments, and relays based on the line voltage. This not only presents significant challenges to instrument manufacturing, but more importantly, directly manufacturing high-voltage instruments to measure voltage directly on high-voltage lines is impossible and absolutely forbidden.
II. Working Principle of Voltage Transformers
Its working principle is the same as that of a transformer, and its basic structure is also an iron core and primary and secondary windings. Its characteristics are that its capacity is very small and relatively constant, and it is close to an unloaded state during normal operation.
Voltage transformers have very low impedance. If a short circuit occurs on the secondary side, the current will increase rapidly and burn out the coil. Therefore, a fuse is connected to the primary side of the voltage transformer, and the secondary side is reliably grounded to prevent personal injury and equipment accidents caused by a high potential to ground on the secondary side when the insulation between the primary and secondary sides is damaged.
III. Basic Structure of Voltage Transformers
Oil-immersed voltage transformers can be classified into ordinary type and cascade type according to their structure. Ordinary type is used for 3-35kV transformers, which is similar to ordinary small transformers. Electromagnetic voltage transformers of 110kV and above are generally made in cascade structure. Its characteristics are: the windings and iron core adopt graded insulation to simplify the insulation structure; the windings and iron core are placed in porcelain bushings to reduce weight and volume.
The basic structure of a voltage transformer is very similar to that of a transformer. It also has two windings: a primary winding and a secondary winding. Both windings are mounted on or wound around an iron core. Insulation exists between the two windings and between the windings and the iron core, providing electrical isolation between them. During operation, the primary winding N1 is connected in parallel to the line, and the secondary winding N2 is connected in parallel to an instrument or relay. Therefore, when measuring voltage on a high-voltage line, although the primary voltage is high, the secondary voltage is low, ensuring the safety of operators and instruments.
The coil impedance of a voltage transformer is relatively small, so the secondary winding of the voltage transformer cannot be short-circuited. If the secondary winding is short-circuited, the impedance that the winding bears will increase, and the winding will not be able to bear it, causing the PT to burn out.
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