Voltage transformer is an indispensable device in the power system. It has important functions and application value. Its function is to convert high voltage into lower voltage for measurement, metering, protection and control system. In order to ensure the safe, reliable, economical and stable operation of the power system, various electrical parameters need to be accurately measured and monitored, and voltage transformer is one of them. Below we will understand the structure, wiring, principle and application of voltage transformer, so that we can better understand and master its importance and usage methods, and provide guarantee for the safe, reliable, economical and stable operation of the power system.
Definition of voltage transformer
◆The voltage transformer (PT for Potential transformer and VT for Voltage transformer) is very similar to the step-down transformer, both of which are used to transform the voltage on the line or bus.
◆The voltage transformer is a transformer with an iron core. It is mainly composed of primary and secondary coils, an iron core and insulation. When a voltage U1 is applied to the primary winding, a magnetic flux is generated in the iron core. According to the law of electromagnetic induction, a secondary voltage U2 is generated in the secondary winding.
◆By changing the number of turns of the primary or secondary winding, different ratios of primary voltage to secondary voltage can be generated, which can form voltage transformers with different ratios.
◆The voltage transformer converts high voltage into low voltage in proportion, generally 100V. The primary side of the voltage transformer is connected to the primary system, and the secondary side is connected to measuring instruments, relay protection and other equipment.
Function, classification and structure of voltage transformer
1.The role of voltage transformers
◆Voltage transformers are sensors that isolate high voltages and provide relay protection, automatic devices and measuring instruments with primary voltage information. They convert high voltages into 100V or 100/3V standard secondary voltages in proportion for use by metering, instrumentation and relay protection.
◆Voltage transformers can be used to isolate high voltages from electrical workers, ensuring the safety of equipment and personnel.
2.Classification of voltage transformers
◆According to the installation location, they can be divided into indoor and outdoor types. 35kV and below are mostly indoor types, while 35kV and above are mostly outdoor types, and there is a significant difference in insulation.
◆ According to the number of phases, they can be divided into single-phase and three-phase types. 10kV and below use three-phase types.
◆According to the number of windings, they can be divided into double windings, three windings and four windings:
◆According to the insulation method, they can be divided into dry type, cast type, oil-immersed type and gas type.
◆According to the working principle, it can be divided into: electromagnetic type, capacitive type and new photoelectric voltage transformer. Among them, the electromagnetic type can be divided into: three-phase type and single-phase type; the three-phase type can be divided into: three-phase two-column type and two-phase five-column type.
3.Voltage transformer structure
◆Oil-immersed voltage transformer Oil-immersed voltage transformer is divided into single-stage and series-stage. Single-stage can be used for voltage levels of 220kV and below, and series-stage can be used for all voltage levels of 66kV and above. The primary and secondary windings of the single-stage type are wound on a common iron core, and the insulation is not graded. Energy conversion is achieved by magnetic coupling. The series-stage type consists of multiple primary windings with the same number of turns, which are assembled on the same iron core with half the number of windings, arranged from top to bottom, and connected between the high voltage and the ground.
◆SF6 gas-insulated voltage transformer The SF6 gas-insulated voltage transformer consists of a shell, an insulating sleeve, an iron core, a secondary winding, and installation accessories. The primary winding generally adopts a pagoda-shaped layer winding and is wound in a graded manner; the secondary winding is a layer winding, which is wound close to the iron core. The iron core usually adopts a single-phase double-column laminated iron core. SF6 gas insulated voltage transformer can be used with GIS or as an independent SF6 gas insulated voltage transformer.
◆Casting insulation voltage transformer Casting insulation uses unsaturated resin and epoxy resin. Casting insulation voltage transformer is usually used for 35kV voltage level, and can be made into indoor, outdoor, grounded, ungrounded and other structures. (Online training course for electrical designers of Tree Bird Education) For 10kV switch cabinet, almost all of them are cast
◆Capacitor voltage transformer The structure of capacitor voltage transformer is divided into two types according to the combination of capacitor voltage divider and electromagnetic unit: split type and single column type. The capacitor voltage divider and electromagnetic unit are installed separately, and the electromagnetic unit is connected to the central Asian terminal of the capacitor voltage divider through an external sleeve. The capacitor voltage divider and the electromagnetic unit are assembled together from top to bottom, and the two are connected inside the product, which is called single column type.
Configuration principles of voltage transformers
◆For main connections such as single bus, single bus segmented, double bus, etc., a three-phase voltage transformer shall be installed on the bus; when there is power on the outgoing line and reclosing is required to check synchronization or there is no voltage and synchronization paralleling is required, a single-phase or two-phase voltage transformer shall be installed on the line side.
◆For 3/2 main connection, three-phase voltage transformers are often installed on the line or transformer side, and single-phase transformers are installed on the bus for synchronous paralleling and reclosing to check no voltage and synchronization.
◆The voltage transformer of the internal bridge connection can be installed on the line side or on the busbar, but usually not at the same time. Different installation locations will affect the protection function.
◆For voltage levels of 220kV and below, voltage transformers generally have two to three secondary stages, one of which is connected in an open triangle and the other in a star. In the 500kV system, for the complete duplication of zero relay protection, three secondary voltage transformers are generally selected, two of which are connected in a star and one in an open triangle.
◆When there are special needs for the metering circuit, a voltage transformer secondary dedicated to metering can be added or a voltage transformer group dedicated to metering can be installed.
◆In a small grounding current system, when it is necessary to check the line voltage or synchronization, a two-phase voltage transformer or a voltage transformer wiring room voltage should be installed on the line side.
◆In a large grounding current system, when the line has a requirement to check the line voltage or synchronization, a voltage extraction device should be used first. 500kV lines are generally equipped with three capacitive line voltage transformers for protection, measurement and carrier communication.
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