How a current transformer works
The principle of current transformer is based on the principle of electromagnetic induction. A current transformer is composed of a closed core and windings. Its primary side winding turns are very small, stringed in the line of the current that needs to be measured, so it often has all the current of the line flowing through, the number of turns of the secondary side winding is more, connected in series in the measuring instrument and the protection loop, when the current transformer is working, its secondary side loop is always closed, so the impedance of the series coil of the measuring instrument and the protection loop is very small, and the working state of the current transformer is close to short circuit. The current transformer converts the large current on the primary side into a small current on the secondary side for measurement, and the secondary side cannot be opened.
The main use of current transformer is to convert the large current in the AC circuit into a certain proportion of small current (5 amperes in China's standard) for measurement and relay protection. We should know that in the process of power generation, substation, transmission and distribution, due to the different electrical equipment, the current is often from tens of amperes to tens of thousands of amperes, and these circuits may also be accompanied by high voltage. In order to be able to monitor and measure the circuits of these lines, and at the same time solve the dangers caused by high voltage and high current, current transformers are needed. Some of you may have seen an electrician's clamp meter, a device used to measure alternating current, and its "clamp" is a heart-piercing current transformer.
The current transformer is composed of a primary coil, a secondary coil, an iron core, an insulating support and an outlet terminal. The core of the current transformer is made of silicon steel sheets, and its primary coil is connected in series with the main circuit, and through the measured current I1, it generates alternating magnetic flux in the core, so that the secondary coil induces the corresponding secondary current I2. If the excitation loss is ignored, then I1n1 = I2n2, where n1 and n2 are the turns of the first and second coils respectively. The converter ratio of the current transformer is K=I1/I2=n2/n1.
Since the primary coil of the current transformer is connected in the main circuit, the primary coil must be insulated with the primary line voltage to the ground to ensure the safety of the secondary circuit and personal personnel. The secondary circuit is composed of the secondary coil of the current transformer, the current coil of the instrument and the relay in series. Current transformers can be broadly divided into two categories, current transformers for measurement and current transformers for protection.
The principle of the current transformer is based on the principle of electromagnetic induction, its primary winding often has all the current of the line flowing, when the current transformer is working, its secondary circuit is always closed, so the impedance of the measuring instrument and the series coil of the protection loop is very small, and the working state of the current transformer is close to a short circuit.
In an ideal current transformer, if the no-load current I.0=0 is assumed, then the total magnetodynamic potential I.0N0=0, according to the law of conservation of energy, the primary winding magnetokinetic potential is equal to the secondary winding magnetokinetic potential, i.e., I.1NI=-I.2N2. That is, the current of the current transformer is inversely proportional to its number of turns, and the ratio of the primary current to the secondary current I.1/I.2 is called the current ratio of the current transformer. When the secondary current is known, the primary current can be found by multiplying the current ratio, and the phasor of the secondary current is 1800 different from the primary current.
Features of current transformers
The so-called current transformer is generally composed of a closed core and windings according to the principle of electromagnetic induction. It has a relatively small number of turns on the primary side winding and needs to be connected in series in the current line, through which all current flows. However, the number of turns of the secondary side winding is more, and when it is connected in series for loop protection and instrument measurement, the secondary circuit form of the current transformer is closed. Therefore, it can be seen that the lower impedance of the series coil is present in the measuring instrument and the protection loop, similar to the short-circuit operation of a transformer. In fact, it is equivalent to converting the large current on the primary side into a small current on the secondary side through the current transformer, and at the same time it can be fully utilized, and the open circuit problem on the secondary side is not allowed. Usually a rated current of 5A or 1A is present in the secondary side of the current transformer. Due to the high voltage generated by the open circuit on the secondary side, which has a direct impact on the operating personnel and equipment, the secondary circuit of the current transformer in operation is not allowed to open the circuit.
In order to ensure the safety of the staff in contact with the relay and measuring instrument, it is necessary to place a reliable grounding in the secondary side of the current transformer, and in general, before disconnecting the secondary circuit of the current transformer, it is necessary to short the secondary terminal with copper wire. The current transformer is usually divided into two forms, one is electromagnetic and the other is capacitive, its primary winding is directly connected in series in the power line, and the current of the measured line directly affects the current in the primary winding. The power department should reserve the short-circuit switch on the secondary side, if there is an open circuit, the circuit load needs to be removed in time, and then the power failure is handled, and it can be used only after the treatment. At the same time, according to the series principle, the wiring of the current transformer must connect the secondary windings in series in the current coil of the relay or measuring instrument.
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