Differences and characteristics of electromagnetic and capacitive voltage transformers

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Differences and characteristics of electromagnetic and capacitive voltage transformers

Posted Date: 2024-02-02

I believe many people are not very clear about the differences and characteristics of electromagnetic voltage transformers and capacitive voltage transformers. Now let’s give a detailed explanation of the electromagnetic voltage transformer using 35kv busbar and the capacitive voltage transformer using 200kv busbar. .

Electromagnetic voltage transformer, which is similar to a power transformer. The working principle of the electromagnetic voltage transformer is characterized by: the primary winding of the electromagnetic voltage transformer is directly connected in parallel to the primary circuit. The voltage on the primary winding depends on the voltage on the primary circuit. There is no electrical coupling between the secondary winding and the primary winding. It is through magnetic coupling. The secondary winding is usually connected to some external instruments, instruments and maintenance equipment. The capacity is generally tens to hundreds of volt-amperes, so the load is very small and constant. Therefore, the primary side of the voltage transformer can be regarded as a voltage source. Not affected by secondary loads at all. During normal operation, the secondary side of the voltage transformer is basically in an open circuit state due to the small load. The secondary voltage of the voltage transformer is basically equal to the induced electromotive force on the secondary side, which depends on the primary system voltage.

There are many classification methods for electromagnetic voltage transformers. They can be divided into dry type and oil type according to the insulation medium; they can be divided into single-phase and three-phase according to the number of phases; they can be divided into double windings and three windings according to the number of windings. , four-winding three types; according to the different voltages they operate, they can be divided into semi-insulated and fully insulated voltage transformers, etc. In practical applications, single-phase three windings or four windings are generally used the most.

If the 35kV bus voltage transformer is a single-phase cast-insulated electromagnetic voltage transformer, the excitation characteristics of the electromagnetic voltage transformer are nonlinear characteristics. When the neutral point of the 35kV power system is offset, instantaneous arc grounding or reverse Under the stimulation of gate operation, it may form ferromagnetic resonance with the capacitance distributed in the power system. Therefore, the selected electromagnetic voltage transformer adopts the harmonic elimination method.

As the transmission voltage of the power system increases, the volume of electromagnetic voltage transformers becomes larger and larger, and the cost increases accordingly. Therefore, capacitive voltage transformers should be used for the 220kV voltage level. According to this requirement, we choose 220kV bus capacitive voltage transformer.

The full name of capacitive type is capacitive voltage transformer. Its working principle is shown in Figure 1. There is a capacitor group between the measured secondary circuit and the earth. The capacitor group consists of C1 and C2. The two ends of C2 are connected in parallel with the secondary load Z2 of the voltage transformer. L is the compensation reactor. When the voltage transformer is running without load, U2 =U0=C1&TImes;U1/(C1+C2)=etaTVU1. Voltage U2 is proportional to the magnitude of its primary voltage U1 and has the same phase. When the voltage transformer has load Z2, Figure 2 is the equivalent circuit diagram, in which U0 is the no-load voltage, jwL is the impedance of the compensation reactor, Z0 is the internal impedance of the voltage transformer Z0=1/jw (C1+C2), When jwL=Z0, that is, 1/w(C1+C2)=wL, Z0+jwL=0, U2=U0=ηTVU1, the secondary output voltage U2 of the voltage transformer is proportional to the primary side voltage U1 and has the same phase. .

Compared with electromagnetic voltage transformers, capacitive voltage transformers have the following characteristics:

1) In addition to being used as a voltage transformer, it can also be used as a capacitor and a coupling capacitor for high-frequency carrier communication.

2) The impact insulation strength of capacitive voltage transformer is higher than that of electromagnetic type.

3) Small size, light weight, low cost and small footprint.

4) The error characteristics and transient characteristics are not as good as the electromagnetic type, and the output capacity is small.

The main difference between electromagnetic and capacitive voltage transformers

Electromagnetic voltage transformer is a voltage transformer that converts primary voltage into secondary voltage proportionally through electromagnetic induction. This type of transformer does not have other electrical components that change the primary voltage, such as capacitors. Capacitive voltage transformer is a voltage transformer that extracts voltage from a series capacitor and then transforms it through a transformer as a voltage source for meters, relay maintenance, etc. Capacitive voltage transformers can also couple the carrier frequency to transmission lines for remote use. Communications, remote measurement, selective line high-frequency maintenance, remote control, teletype, etc. Therefore, compared with conventional electromagnetic voltage transformers, capacitive voltage transformers not only prevent ferromagnetic resonance caused by the full core of the voltage transformer, but also have many advantages in terms of economy and safety.

Capacitive voltage transformer mainly consists of capacitive voltage divider and medium voltage transformer. The capacitive voltage divider consists of a porcelain sleeve and a number of series capacitors installed in it. The porcelain sleeve is filled with insulating oil that maintains a positive pressure of 0.1MPa, and a steel bellows is used to balance different environments to maintain the oil pressure. The capacitive voltage divider can be used as a coupling capacitor Connect carrier equipment. The medium-voltage transformer consists of a transformer, a compensation reactor and a damping device installed in a sealed oil tank. The space at the top of the oil tank is filled with nitrogen. The primary winding is divided into a main winding and a trimmer winding, and a low-loss reactor is connected in series between the primary side and the primary winding. Since the nonlinear impedance and inherent capacitance of the capacitive voltage transformer sometimes cause ferromagnetic resonance in the capacitive voltage transformer, damping equipment is used to suppress the resonance. The damping equipment consists of a resistor and a reactor and is connected across the secondary winding. Under normal circumstances, the damping equipment has a high impedance. When ferromagnetic resonance causes overvoltage, the reactor is already full before the medium voltage transformer is affected and only the resistive load is left, so that the oscillation energy is quickly reduced.

Review Editor: Huang Fei

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