The working principle, function, and parameter meaning of current transformers

As is well known, voltage transformers are important secondary equipment in power supply systems, widely used in secondary circuits such as measurement, measurement, and relay protection. In current or high-voltage locations, the current of the circuit cannot be directly measured using an ammeter, and can only be measured on the secondary side of the voltage transformer for safety. The following is a brief analysis of the working principle, efficacy, and parameter meanings of voltage transformers for your reference. The principle of voltage transformers

Like transformers, voltage transformers are instruments that convert large primary currents into small secondary currents based on the principle of electromagnetic induction. The voltage transformer consists of a closed iron core and winding. In the current circuit that needs to be detected, the number of winding turns on its primary side is very small. There are many winding resistance turns on the secondary side, which are connected in series in the protection circuit of the testing instrument. When the voltage transformer is working, the secondary side circuit is always closed. Therefore, the impedance of the series coil in the protection circuit of the testing instrument is small, and the working state of the voltage transformer is close to a short circuit.

Basic parameters of voltage transformers

1. Rated voltage ratio

The rated voltage ratio refers to the ratio of the primary rated voltage to the secondary rated voltage (sometimes referred to as the current ratio). The rated voltage ratio is generally expressed in the form of an approximate fraction. If the primary rated voltage I1e and the secondary rated voltage I2e are 100 and 5A respectively, then

KI=I1e/I2e=100/5

The so-called rated voltage refers to the current at which the transformer can operate for a long time without being damaged by heat. When the load current exceeds the rated voltage, it is called overload. If the transformer is overloaded for a long time, it will burn out its winding resistance or reduce the service life of the insulation material.

2. Accuracy level

Due to the existence of certain deviations in voltage transformers, the accuracy levels of voltage transformers are classified based on their allowable errors. The accuracy levels of domestic voltage transformers are 0.01, 0.02, 0.05, 0.1, 0.2.0.5, 1.0, 3.0, 5.0, 0.2S, and 0.5S.

Voltage transformers above 0.1 level are mainly used for precise measurement in the laboratory, or as a standard for detecting low-level transformers. They can also be used in conjunction with standard instruments for testing instruments, also known as standard voltage transformers. For some special requirements, the customer's electrical energy metering device generally uses voltage transformers of 0.2 and 0.5 levels (it is hoped that the energy meter is between 0.05 levels) and 6A, that is, voltage transformers of 1% to 0.2S level and 0.5S level of rated voltage 5A can accurately measure a certain current between 120%.

3. Short circuit capacity

The short-circuit capacity of a voltage transformer is the rated secondary current I2E, which is considered as power S2E consumed based on the secondary rated value Z2E. Therefore

S2e=I2e2Z2e

In general, I2e=5A, so S2e=52Z2e=25Z2e, and the short-circuit capacity can also be represented by the rated impedance Z2e.

When using voltage transformers, the total impedance of the secondary connecting wire and instrument current coil shall not exceed the short-circuit capacity specified on the nameplate, and shall not be less than 1/4 of the short-circuit capacity to ensure its accuracy. The rated secondary load calibrated on the manufacturer's nameplate is generally represented by short-circuit capacity, and the derived standard values are 2.5, 5, 10, 15, 25, 30, 50, 60, 80, 100V · A, etc.

4. Rated current

The rated current of a voltage transformer refers to the maximum voltage (effective value) of the long-term winding to ground. It only displays the insulation strength of the voltage transformer and is independent of its short-circuit capacity. It is marked behind the voltage transformer model. For example, LCW-35, where "35" refers to the rated current based on KV.

5. Polarity marking

The terminals of the primary and secondary windings of voltage transformers should be marked with polarity markings to ensure correct wiring for measurement and calibration checks.

(1) The first end of the primary winding is marked with L1, and the last end is marked with L2. When there are taps in the multi limit primary winding, the first end is marked with L1, and L2 and L3 are marked from the first tap.

(2) The first end of the secondary winding is marked as K1, and the last end is marked as K2. When the secondary winding contains an intermediate tap, the first end is marked as K1. Starting from the first tap, the following markings are K2 and K3.

(3) For voltage transformers with multiple secondary windings, data should be added before the outgoing terminals of multiple secondary windings are marked with "K" 1K1, 1K2, 1K3, etc.; 2K1, 2K2, 2K3.

(4) The arrangement of the symbols should ensure that the primary current flows from L1 to L2, the secondary current flows out from K1, and flows back to K2 through the external circuit.

From the polarity terminals of the primary and secondary windings of the voltage transformer, the directions of currents I1 and I2 are opposite. This polarity association is called subtractive polarity, and conversely, it is called additive polarity. Voltage transformers are usually represented as reduced polarity.

The function of current transformers

1. One of the functions of current transformers is to measure, usually used for charging or measuring the current of equipment during operation. When measuring high alternating current, in order to facilitate instrument measurement and reduce the risk of directly measuring high voltage, it is often necessary to use voltage transformers to convert it into a more uniform current. Here, the voltage transformer plays a role in current transformation and electrical isolation. Voltage transformers convert high current proportionally to low current according to requirements, and are used to measure the primary side connection system, secondary side connection detection instruments, or relay protection devices of voltage transformers.

2. The second function of current transformers is maintenance, usually used in conjunction with relay equipment. When there is a short circuit or load fault in the circuit, the voltage transformer sends a signal to the relay equipment, cutting off the faulty circuit, in order to maintain the safety of the power supply system. Maintaining voltage transformers is different from measuring current transformers. Only when the current is several times or tens of times higher than the normal current can it work effectively. In addition, it also stipulates that the insulation is reliable, the precise limit index is large enough, and the thermal and dynamic stability is strong enough.

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