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The Function Of Current Transformer
Jun 25, 2018

The role of a current transformer is to convert a larger current to a smaller two current through a certain variable, which is used for protection, measurement, and so on. For example, a current transformer with a ratio of 400/5 can convert the actual 400A current to 5A current. Ann is in the switchgear for the purpose of connecting meters and protective relays. Each instrument can not be connected to a wire or bus with a large actual value, so it is necessary to convert it to a two value of a smaller value by a transformer and reflect the actual value at a time by a variable ratio.

The principle of the current transformer and the equivalent circuit are the same as those of the general transformer, but the original side winding is connected in the measured circuit, and the number of turns is very few; the low impedance load, such as the auxiliary side winding connection ammeter, the relay current coil and so on, is approximately short circuited. The primary side current (the measured current) and the secondary side current depend on the load of the measured line, and have nothing to do with the secondary side load of the current transformer. When the current transformer is running, the secondary side is not allowed to open. Because in this case, the primary side current will become an exciting current, which will cause the flux and side voltage to exceed the normal value and jeopardize the safety of the body and equipment. Therefore, it is not allowed to connect fuses in the secondary circuit of the current transformer, nor to remove the ammeter and relays without bypass.

The characteristics of the current transformer are: (1) the primary coil is connected in a circuit with few turns, so the current in the primary coil is entirely dependent on the load current of the measured circuit. It has nothing to do with the two current; (2) the current coil impedance of the instrument and relay of the two coil of the current transformer is very small, so it is normal. Under the short-circuit condition, the current transformer operates.

The ratio of one or two times rated current of a current transformer is called the rated mutual inductance ratio of the current transformer: kn=I1n/I2n

Because the rated current I1n of one coil has been standardized, the rated current of the two coil is 5 (1 or 0.5), so the rated mutual inductance of the current transformer has also been standardized. Kn can also be approximately represented as the number of turns of the one or two coils of the transformer, that is, the turns of N1 and N2 in the kn kN=N1/N2 N1 are the turns of the coils. The purpose of the current transformer is to measure a relatively large current.

The current transformer changes the large current to a small current in a certain proportion, providing the current for various instruments and relay protection, and isolating the two system from the high voltage. It not only guarantees the safety of the body and equipment, but also simplification and standardization for the manufacture of meters and relays, and improves the economic benefits. Generally used in conjunction with ampere meters, its main purpose is to use a small ammeter to measure large current. One side side of the line is measured, the two side of the ammeter, the wiring must pay attention to the range, which is the maximum range of the ammeter.  There is also a grounding. ''

What are the basic wiring modes of the current transformer? (1) full star connection. (2) the two phase two relay is an incomplete star connection. (3) two phase one relay current difference connection. (4) triangle connection. (5) three phases are connected to obtain zero sequence current.


Add: No.38 South Taoyuan Road, Yaozhuang Town,Jiashan County, Jiaxing City, Zhejiang Province, China
Contact person: Elva Zhuang&Sunny Ni
Tel:+86-573-84775555
Fax:+86-573-84776699
Phone:+86-13732570078
E-mail:sales9@great-relay.com
       sunny.ni@great-relay.com