A potential transformer, hereinafter referred to as TV, is a device used to transform voltage in a power system. The difference between the potential transformer and the transformer is that the purpose of the transformer to transform the voltage is to facilitate the transmission of electric energy, so the capacity is large, and the unit is generally calculated in kV ampere or kVA. The purpose of potential transformer transformation is mainly to supply power to measuring instruments and relay protection devices to measure the voltage, power and electrical energy of the line, or to protect valuable equipment, motors and transformers in the line when the line fails , so the capacity of the potential transformer is very small, generally only a few volts, tens of volts, and the maximum does not exceed 1kVA.
1. Basic structure and classification of potential transformers
The basic structure of the potential transformer is very similar to that of the transformer. It also has two windings, one is called the primary winding and the other is called the secondary winding. Both windings are mounted or wound around the iron core. There is insulation between the two windings and between the unit and the iron core, so that there is electrical isolation between the two windings and between the winding and the iron core. When the potential transformer is running, the primary winding N1 is connected to the line in parallel, and the secondary winding N2 is connected to the instrument or relay in parallel. Therefore, when measuring the voltage on the high-voltage line, although the primary voltage is high, the secondary is low-voltage, which can ensure the safety of operators and instruments. According to different dimensions, potential transformers can be classified as follows:
(1) According to the installation location, it can be divided into indoor type and outdoor type: 35kV and below are mostly made into indoor type; 35kV and above are made into outdoor type.
(2) According to the number of phases: it can be divided into single-phase and three-phase type, 35kV and above cannot be made into three-phase type.
(3) According to the number of windings: it can be divided into two-winding and three-winding potential transformers. In addition to the primary side and the basic secondary side, the three-winding potential transformer also has a set of auxiliary secondary sides for grounding protection.
(4) According to the insulation method: it can be divided into dry type, pouring type, oil-immersed type and inflatable type.
Dry-type potential transformers have a simple structure, no fire and explosion hazards, but have low dielectric strength, and are only suitable for indoor installations below 6kv; cast-type potential transformers are compact in structure and easy to maintain, suitable for 3~35kV indoor installations Off-grid device; oil-immersed potential transformer has good insulation performance and can be used in outdoor power distribution devices above 10kV; gas-filled potential transformer is used in SF6 fully enclosed electrical appliances.
(5) According to the working principle: it can be divided into electromagnetic, capacitive and electronic potential transformers.
1) Electromagnetic potential transformer: It is a device that uses the principle of electromagnetic induction to convert voltage or current proportionally.
2) Capacitive potential transformers: Capacitive potential transformers are divided by series capacitors, and then step-down and isolated by electromagnetic transformers, as a potential transformer for meters, relay protection, etc., capacitive potential transformers The transmitter can also couple carrier frequencies to power lines for long distance communications, remote measurement, selective line high frequency protection, remote control, teletyping, etc.
3) Electronic potential transformer: a device consisting of one or more voltage or current sensors connected to the transmission system and the secondary converter, used to transmit a quantity proportional to the measured quantity, for measuring instruments, meters and relays. Electrical protection or control devices.
2. Potential transformer operation and maintenance specifications
In order to ensure the stable operation of the potential transformer, inspections should be strengthened before and during operation to ensure the operation with hidden dangers.
(1) The potential transformer shall be tested and inspected according to the items specified in the regulations before it is put into operation. For example, measuring polarity, connection group, shaking insulation, nuclear phase sequence, etc.
(2) The wiring of the potential transformer should ensure its correctness. The primary winding and the circuit under test should be connected in parallel, and the secondary winding should be connected in parallel with the voltage coil of the connected measuring instrument, relay protection device or automatic device. At the same time, pay attention to the polarity. correctness.
(3) The capacity of the load connected to the secondary side of the potential transformer should be appropriate, and the load connected to the secondary side of the potential transformer should not exceed its rated capacity, otherwise the error of the transformer will increase, and it is difficult to achieve the correctness of the measurement .
(4) Short circuit is not allowed on the secondary side of the potential transformer. Since the internal impedance of the potential transformer is very small, if the secondary circuit is short-circuited, a large current will appear, which will damage the secondary equipment and even endanger personal safety. The potential transformer can be equipped with a fuse on the secondary side to protect itself from being damaged by a short circuit on the secondary side. If possible, fuses should also be installed on the primary side to protect the high-voltage power grid from endangering the safety of the primary system due to the failure of the transformer’s high-voltage windings or lead wires.
(5) In order to ensure the safety of people when touching the measuring instruments and relays, the secondary winding of the potential transformer must be grounded at one point. Because after grounding, when the insulation between the primary and secondary windings is damaged, it can prevent the high voltage of the instrument and the relay from endangering personal safety.
(6) Short circuit is absolutely not allowed on the secondary side of the potential transformer.
(7) Whether the foundation of the potential transformer has cracks, subsidence or collapse.
(8) Whether the supporting steel frame is deformed, corroded, cracked, etc.
(9) Whether the potential transformerrmer number plate and safety sign are missing, falling off, or the font is seriously blurred and unrecognizable.
(10) Whether the potential transformer is loose or skewed, and whether the composite material or porcelain surface is dirty, damaged, corona and discharge traces.
(11) Whether the paint layer on the outside of the device is dirty, peeling and peeling.