1. what is voltage transformer
1, voltage transformer (Potential transformer referred to as PT,Voltage transformer also referred to as VT) and step-down transformer are very similar, are used to transform the voltage on the line or bus.
2. The voltage transformer is a transformer with iron core. It is mainly composed of 1. secondary coil, core and insulation. When a voltage U1 is applied to the primary winding, a magnetic flux φ is generated in the core and, according to the law of electromagnetic induction, a secondary voltage U2 is generated in the secondary winding.
3. Changing the number of turns of the primary or secondary winding can produce different primary voltage and secondary voltage ratios, which can form voltage transformers with different ratios.
4. The voltage transformer converts high voltage into low voltage proportionally, generally 100V. The primary side of the voltage transformer is connected to a system, and the secondary side is connected to measuring instruments, relay protection and other equipment.
Function of 2. voltage transformer
1. The voltage transformer is a sensor that isolates high voltage for relay protection, automatic devices and measuring instruments to obtain primary voltage information. High voltage is proportionally converted to 100V or 100/3V standard secondary voltage for metering, instrumentation and relay protection.
At the same time, the use of voltage transformers can isolate high voltage from electrical workers to ensure the safety of equipment and personnel.
Classification of 3. voltage transformers
1. According to the installation location, it can be divided into indoor type and outdoor type. 35kV and below are mostly indoor type, 35kV and above are mostly outdoor type, the insulation has obvious gap.
2. According to the number of phases, it can be divided into single-phase type and three-phase type. Three-phase type is adopted for 10kV and below.
3. According to the number of windings, it can be divided into two windings, three windings and four windings.
4. According to the insulation method, it can be divided into dry type, pouring type, oil immersion type and air type.
5. According to the working principle, it can be divided into: electromagnetic, capacitive and new photoelectric voltage transformers. The electromagnetic type can be divided into: three-phase type and single-phase type; three-phase type can be divided into: three-phase two-column type and two-phase five-column type.
4. voltage transformer structure
1. Oil-immersed voltage transformer
Oil-immersed voltage transformers are divided into single-stage and cascade-type single-stage, single-stage can be used for 220kV and below voltage levels, and cascade-type can be used for all voltage levels of 66kV and above.
The single-stage secondary winding is wound on a common iron core, insulation is not graded, and energy conversion is realized by magnetic coupling.
The cascade type consists of a plurality of primary windings with the same number of turns mounted on the same iron core with half the number of windings, arranged from top to bottom, connected between high voltage and ground.
2. SF6 gas insulated voltage transformer
SF6 gas insulated voltage transformer is composed of shell, insulating sleeve, iron core, 1. secondary winding and installation accessories. The primary winding generally adopts 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, and the iron core usually adopts single-phase double-column laminated iron core.
SF6 gas insulated voltage transformer can be matched with GIS, can also be used as an independent SF6 gas insulated voltage transformer.
3. Pouring insulation voltage transformer
Cast insulation using unsaturated resin and epoxy resin. Casting insulation voltage transformer is usually used for 35kV voltage level, can be made into indoor, outdoor, grounding, grounding and other structures.
For 10kV switchgear, almost all of them are cast.
4. Capacitor voltage transformer
Capacitor voltage transformer structure, according to the combination of capacitive voltage divider and electromagnetic unit is divided into: sub-mounted and single column two.
The capacitive voltage divider and the electromagnetic unit are installed separately, and the electromagnetic unit is connected to the central sub-terminal of the capacitive voltage divider through an external bushing.
The capacitive voltage divider and the electromagnetic unit are assembled together up and down, and the two are connected inside the product, which is called single-column type.
Wiring Method of 5. Voltage Transformer
1. Single-phase wiring is often used in large grounding current systems to judge that the line has no voltage or the same period. Any phase can be connected, but the corresponding phase of bus voltage should be used for another criterion.
2. A voltage transformer connected between two-phase voltages is mainly used for small current grounding system to judge whether the line is voltage-free or synchronous, because the small grounding current system allows single-phase grounding. If only one single-phase grounding voltage transformer is used, if the voltage transformer is just in the grounding phase, the measured voltage to ground of this phase is zero, it is impossible to verify whether the line is voltage-free, and if wrong judgment is made, it may cause non-synchronous closing.
3. V/V wiring is mainly used for bus voltage measurement of small grounding current system. It can complete three-phase voltage measurement as long as two voltage transformers connected to line voltage, thus saving investment. However, this wiring cannot measure the zero sequence voltage of the system.
Star connection and delta connection are the most widely used, and are often used to measure three-phase voltage and zero sequence voltage.
5. The neutral point is equipped with star connection of arc suppression voltage transformer. In the current grounding system, when the single-phase grounding is allowed to continue to run for 2h, the voltage of the non-grounded phase rises to the line voltage, and there is transient overvoltage for intermittent grounding.
This may cause the voltage transformer core saturation, causing ferromagnetic resonance, so that the system produces resonance overvoltage, so in the small current grounding system voltage transformer to consider harmonic elimination problem.
6. Capacitive type or voltage extraction device is adopted for line voltage transformer used for verification of the same period or line without voltage.
Wiring of secondary circuit of 6. voltage transformer
1. The following figure shows the wiring schematic diagram of the secondary circuit of the voltage transformer during the main wiring of the typical double bus or single bus section. As can be seen from the figure, the use of two secondary voltage transformers, a group of secondary three-phase connection into a star, a group of open triangle.
2. A group of star-shaped secondary buses are sent to secondary voltage small buses 1(2)WVa, 1(2)WVb, 1(2)WVc and WVN through the relay 1(2)K contacts of the small air switch 1(2)QA and the auxiliary contacts of the voltage transformer isolating switch. The small buses are used by protection devices and measuring equipment.
3. Due to the high precision requirement of the metering device, another set of voltages from the star connection outlet are sent to the special metering small bus 1(2)WVaj, 1(2)WVbj and 1(2)WVcj through fuse 3-5(6-8)FU and relay 1(2)K contacts.
4. The other group is connected to an open triangle, one end of which is directly connected to the small bus WVN, and the other end is connected to the small bus 1(2)WVL through the relay 1(2)K contact for protection devices requiring zero sequence voltage.
The secondary circuit of pressure transformer parallel is shown in the following figure:
5. Double bus voltage parallel circuit is shown in the figure above. The second parallel condition is that the parallel relays 1QJ and 2QJ can only operate when the bus coupler or sectional circuit breaker is in the closing position and the isolating switches on both sides are also in the closing position.
Configuration principle of 7. voltage transformer
1, for the main wiring is single bus, single bus section, double bus, etc., install three-phase voltage transformer on the bus; When there is power supply on the outgoing line, it is necessary to reclose the same period or no pressure, and it is necessary to parallel the same period, single-phase or two-phase voltage transformer shall be installed on the line side.
2. For 3/2 main wiring, three-phase voltage transformers are often installed on the line or transformer, while single-phase transformers are installed on the bus for simultaneous parallel and reclosing to check no pressure and simultaneous use.
3. The voltage transformer of the internal bridge connection can be installed on the line side or on the bus, generally not at the same time. Different installation locations have an impact on the protection function.
4. For voltage levels of 220kV and below, voltage transformers generally have two to three secondary levels, one group is connected to an open triangle and the other is connected to a star. In 500kV system, the complete duality of zero relay protection, three secondary voltage transformers are generally selected, of which two groups are connected to the star and one group is connected to the open triangle.
5. When the metering circuit has special needs, the secondary voltage transformer dedicated to metering can be added or the voltage transformer group dedicated to metering can be installed.
6, in the small grounding current system, need to check the line voltage or the same period, should be installed in the line side of the two-phase voltage transformer or install a voltage transformer wiring voltage.
7. In the large grounding current system, when the line has the requirement of checking the line voltage or the same period, the voltage extraction device should be selected first. Three capacitor line voltage transformers are generally installed on 500kV lines for protection, measurement and carrier communication.