As we all know, the current transformer is an important secondary equipment in the power system, in the measurement, measurement, relay protection and other secondary circuits are widely used, in the high current or high voltage occasions we can not directly use the ammeter to measure the current size of the circuit, only through the secondary side of the current transformer to measure, so as to be safe. The following is a brief analysis of the working principle, role and meaning of the parameters of the current transformer for your reference.
The principle of current transformer
Current transformer and transformer is based on the principle of electromagnetic induction to convert the primary side of the large current into the secondary side of the small current to measure the instrument. The current transformer is composed of a closed core and winding. It has a small number of turns on the primary side winding and is strung in the line where the current is to be measured. The secondary side winding turns are more, in series in the measuring instrument and protection circuit, the current transformer in the work, its secondary side circuit is always closed, so the measuring instrument and protection circuit series coil impedance is very small, the working state of the current transformer close to the short circuit.
Current transformer working principle, role and parameter meaning
The main parameters of the current transformer
1. Rated current ratio
Rated current ratio refers to the ratio of the primary rated current to the secondary rated current (sometimes referred to as current ratio). Rated current ratio is generally used not about the fraction of the form, such as a rated current I1e and the secondary rated current I2e are 100, 5A, then
KI=I1e/I2e=100/5
The so-called rated current is the current at which the transformer can operate for a long time without being damaged by heat. When the load current exceeds the rated current, it is called an overload. If the transformer is operated overload for a long period of time, it will burn out its windings or shorten the life of the insulation material.
2. Accuracy rating
As there is a certain error in the current transformer, the accuracy level of the transformer is divided according to the allowable error of the current transformer. The accuracy level of domestic current transformers are 0.01, 0.02, 0.05, 0.1, 0.2, 0.5, 1.0, 3.0, 5.0, 0.2S level and 0.5S level.
0.1 level above the current transformer, mainly used in the laboratory for precision measurement, or as a standard used to test the low-level transformer, but also with the standard instrument, used to test the instrument, so also called the standard current transformer. Users of electrical energy metering devices usually use 0.2 and 0.5 current transformers, for some special requirements (hope that the energy meter in 0.05 ~ 6A, that is, the rated current of 5A between 1% ~ 120% of a certain current can make accurate measurement) can use 0.2S and 0.5S current transformers.
3. Rated capacity
The rated capacity of the current transformer is the rated secondary current I2e through the secondary rated load Z2e when the apparent power S2e consumed, so
S2e=I2e2Z2e
General situation I2e = 5A, therefore, S2e = 52Z2e = 25Z2e, the rated capacity can also be expressed with the rated load impedance Z2e.
Current transformer in use, the secondary connection line and the total impedance of the instrument current coil, can not exceed the rated capacity specified on the nameplate and not less than 1/4 rated capacity to ensure its accuracy. The manufacturer's nameplate is usually marked with the rated secondary load with rated capacity, the output standard value of 2.5, 5, 10, 15, 25, 30, 50, 60, 80, 100V-A, etc.
4. Rated voltage
The rated voltage of the current transformer refers to the maximum voltage (rms) that the primary winding can withstand for a long time to the ground. It only indicates the insulation strength of the current transformer, and the rated capacity of the current transformer does not have any relationship. It is marked on the back of the current transformer model. For example, LCW-35, where "35" refers to the rated voltage, which is measured in kV.
5. Polarity marking
In order to ensure correct wiring for measurement and calibration work, the terminals of the primary and secondary windings of the current transformer should be marked with the polarity mark.
(1) the first winding is marked as L1, the end is marked as L2. When the primary winding with multiple volume limit is tapped, the first end is marked as L1, from the first tap onwards, it is marked as L2, L3"" in turn.
(2) The first end of the secondary winding is marked K1 and the end is marked K2. When the secondary winding has an intermediate tap, the first end is marked K1 and from the first tap onwards the following are marked K2 and K3"".
(3) for the current transformer with multiple secondary winding, should be respectively in each secondary winding out of the end of the mark "K" before adding numbers, such as 1K1, 1K2, 1K3 ""; 2K1, 2K2, 2K3 "".
(4) The symbols should be arranged so that when the primary current flows from the L1 end to the L2 end, the secondary current flows out of K1 and back to K2 via the external circuit.
From the current transformer primary winding and secondary winding of the same polarity terminal, the direction of the current I1, I2 is opposite, such a polarity relationship is called minus polarity, and vice versa is called plus polarity. Current transformers are generally expressed in terms of minus polarity.
The role of the current transformer
1, one of the roles of the current transformer is for measurement, often used for billing or measuring the current size of the equipment in operation. In the measurement of large alternating currents, in order to facilitate the measurement of instruments and to reduce the danger of direct measurement of high voltage electricity, it is often necessary to use current transformers to transform them into a more uniform current, where the current transformer plays the role of current transformation and electrical isolation. Current transformer will be high current according to the requirements of proportional conversion to low current, for measuring the primary side of the current transformer connected to the primary system, the secondary side of the measuring instrument or relay protection device, etc..
2, the role of the current transformer is used for protection, often used in conjunction with relay devices, when a short circuit or overload fault occurs in the line, the current transformer will send a signal to the relay device to cut off the fault circuit, so as to achieve the purpose of protecting the safety of the power supply system. Current transformers for protection are different from current transformers for measurement, which can only work effectively when the current is several times or tens of times greater than normal, and require reliable insulation, a large enough accurate limit factor, and sufficient thermal and dynamic stability.
