What Is a Voltage Transformer?

Voltage transformer ^[1] (Potential Transformer for short PT, Voltage Transformer for short VT) is similar to a transformer and is an instrument used to transform voltage. However, the purpose of transforming the voltage of the transformer is to facilitate the transmission of electrical energy, so the capacity is very large, and it is generally calculated in kilovolt-ampere or mega-volt-ampere; while the purpose of the voltage transformer to convert the voltage is mainly used for measuring instruments and relay The protection device is used 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 voltage transformer is very small, generally only a few volts, Dozens of volt-amperes, the maximum does not exceed one thousand volt-amperes. The entry introduces its basic structure, working principle, main types, wiring methods, precautions, exceptions and treatments, and ferromagnetic resonance.

The basic structure of a voltage transformer is very similar to that of a transformer. It also has two windings, one called a primary winding and one called a secondary winding. Both windings are mounted on or around the core. There is insulation between the two windings and between the winding and the core, so that there is electrical isolation between the two windings and between the winding and the core. When the voltage transformer is running, the primary winding N1 is connected in parallel to the line, and the secondary winding N2

Its working principle is the same as that of the transformer ^[2]

1. According to the installation site, it can be divided into indoor and outdoor ^[4]

The voltage transformer model consists of the following parts ^[1]

The common wiring methods of voltage transformers are as follows ^[6]

1. Before the voltage transformer is put into operation, it should be tested and inspected according to the items specified in the regulations. For example, measuring polarity, connection group, shaking insulation, nuclear phase sequence, etc. ^[7]

The occurrence of magnet resonance is a jump process caused by the saturation of the iron core during operation or system failure. The ferromagnetic resonance that occurs in the power grid is divided into parallel ferromagnetic resonance and series ferromagnetic resonance. ^[11]

Main characteristics of voltage transformer

1) For ferromagnetic resonance circuits, the circuit may have more than one stable working state under the same power supply potential. What kind of working state the circuit is stable in depends on the transition process caused by external impact ^[10] .

2) The non-linear ferromagnetic characteristic of PT is the root cause of ferromagnetic resonance, but the saturation effect of the ferromagnetic element itself also limits the amplitude of the overvoltage. In addition, the loop loss also damps and limits the resonant overvoltage. When the loop resistance is greater than a certain value, no strong ferromagnetic resonance overvoltage will occur.

3) For a series resonance circuit, the necessary condition for generating a ferromagnetic resonance overvoltage is 0 = 1 / L0C <; . Therefore, ferromagnetic resonance can occur over a wide range.

4) The energy for maintaining the resonance oscillation and compensating the resistance loss of the circuit is supplied by the power frequency power supply. In order for the power frequency energy to be converted into energy of other resonance frequencies, the conversion process must be periodic and rhythmic, that is, 1/2 (1, 2, 3) times the frequency of resonance.

5) Damage of PT by ferromagnetic resonance. Electromagnetic resonance (frequency division) should generally have the following three conditions.

The non-linear effect of ferromagnetic voltage transformer (PT) is the main cause of ferromagnetic resonance.

The PT inductive reactance is within 100 times of the capacitive reactance, that is, the parameter matching is in the resonance range.

There must be excitation conditions, such as the sudden closing of PT, the sudden disappearance of single-phase grounding, external interference to the system, or overvoltage generated by system operation.

According to the test, the current of the frequency division resonance is more than 240 times the normal current, the power frequency resonance current is about 40 to 60 times the normal current, and the high frequency resonance current is smaller. Among these resonances, the frequency division resonance destroys the most. If the insulation of the PT is good, the power frequency and high frequency generally do not endanger the safety of the equipment, and the 6kV system has the above conditions.

Elimination method of voltage transformer

From a technical point of view, in order to avoid the occurrence of ferromagnetic resonance, the following measures can be taken: choose a TV with good excitation characteristics or use a capacitive TV; in the same 10kV power distribution system, the number of TVs should be minimized; in three-phase Connect the neutral point of the primary side of the TV in series with a single-phase TV or connect a damping resistor at the secondary opening triangle of the TV; connect a capacitor of a certain size on the bus to make the ratio of capacitive reactance to inductive reactance less than 0.01 to avoid resonance; system The neutral point is equipped with an arc suppression coil; the ground compensation device adopts the principle of automatic tuning, and it can solve such problems better through the over, full and under compensation operation modes. ^[11]