What is the difference between a current transformer and a voltage transformer?

September 17 , 2020
1. The structure is different:

The primary winding of the current transformer is wound with thick lines, usually only one or a few turns, and is connected in series with the load of the measured current; the voltage transformer is a step-down transformer, and its primary winding coil has many turns, which is consistent with the measured current load. The high-voltage power grid is connected in parallel; the secondary winding has a few turns and is connected to the working voltage coil of a voltmeter or power factor meter.

2. The working principle is different:

Current transformers and voltage transformers have very different operating states during normal operation, mainly as follows:

1) The secondary current transformer can cause a short circuit fault, but it cannot be opened; the voltage transformer secondary can open a circuit, but it cannot cause a short circuit fault.

2) Compared with the load on the secondary side, the primary internal characteristic impedance of the voltage transformer is small or even negligible. It can be considered that the voltage transformer is a voltage source; and the primary internal resistance of the current transformer is very large, even Think of it as a current source with infinite internal resistance.

3) The magnetic flux density of the voltage transformer in normal operation is close to the saturation value, and its magnetic flux density is reduced during common faults; the magnetic flux density is very low when the current transformer is in normal operation, and the short-circuit fault is caused by the primary side short-circuit fault The increasing current causes its magnetic flux density to increase greatly, sometimes even far exceeding the saturation value.

3. The function is different:

The main function of the current transformer is to ensure the stable and safe operation of the power engineering system. It is necessary to monitor and accurately measure the operating conditions of electrical equipment. However, general precision measurement and relay protection equipment cannot be directly connected to primary high-voltage machinery and equipment. Instead, the large current of the primary system must be converted into a small current proportionally to provide detection instruments and relay protection equipment for use.

The main function of the voltage transformer is to convert the high voltage into a regulated secondary working voltage of 100V or lower according to the proportion, and provide it to the relay protection equipment, metering equipment, and instrument panel equipment.

The functional difference between a current transformer and a voltage transformer depends on whether one is measuring current and the other is measuring working voltage. The current transformer is connected in series in the power circuit, the primary winding has fewer turns than the secondary winding, and the secondary winding cannot be opened; while the voltage transformer is connected in parallel in the power circuit, the primary winding has more turns than the secondary winding. Therefore, the secondary winding cannot be short-circuited.
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