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Current Transformer And Its Application
Jan 19, 2018

Current transformer and its application

1. On current transformer


The current transformer is a special kind of transformer, its working principle is similar to the transformer. It is different from the iron core of the transformer that the alternating main flux is generated by the current of the AC voltage added at both ends of the primary winding. The alternating main magnetic flux in the core inducts the corresponding two EMF and two current in the two winding of the current transformer. The current and the turn number of the last winding and the two winding should be equal, that is, I1n1=I2n2, so I1/I2=n2/n1=K, K is called the ratio of the current transformer.


The current transformer is mainly used to convert the high voltage and large current in the power system to a low voltage and a small current at a certain rate. That is, no matter the current of the primary side is large, the two time side (usually) is 5A. Supply instruments, instruments, relays, etc. for measurement and protection.


Two, the current transformer should pay attention to the use of the current transformer.


1. The two side of the current transformer in operation will never be allowed to open, and the fuse and knife switch can not be installed on the two side.  This is because the number of turns on the two side winding of the current transformer is much larger than that of the primary side turns. The current transformer is equivalent to a step-up transformer under the open circuit condition. According to the relevant data, the voltage value can reach to about 1000V. It endangers the safety of the staff. Therefore, when there is a current on the primary side, the two side is short circuited. Besides, the current transformer is in short circuit state when it is working, so it can directly connect K1 and K2 to the ground. Remember.


2. When the current transformer is installed, the one end of the two side of the current transformer (generally K2), the iron core and the outer shell should be reliably grounded. In order to prevent the one or two side windings to be damaged by insulation, the one side voltage string to the two side, endangering the safety of the staff.


3. When the current transformer is installed, the precision grade should be considered. A high precision measuring instrument with low precision for protection. The choice should be paid attention to.


4, when installing the current transformer, we should pay attention to polarity (Tong Mingduan). The terminals on the primary side are L1, L2 (or P1, P2), and the primary side current flows from L1 to L2. The terminal of the two side is K1, K2 (or S1, S2), that is, the terminal of the two side is out of K1 and is inflow by K2. L1 and K1, L2 and K2 as polar (Tong Mingduan), may not be mistaken, or if it will reverse the watt hour meter.


5. The primary side winding of the current transformer is divided into one turn and multiple turns, and the LQG type is a single turn. While the use of type LMZ (through type), should pay attention to whether the core data on the nameplate, if it should be according to the requirements through the required number of turns. Note: the number of turns is to pass through the hollow root number as the standard, but not on the number of turns of the peripheral (or turn a calculation error).


6, the two winding of current transformer has a winding and two windings. If there are two windings, one of them is of high accuracy (small error value), generally used as metering, and the other is low accuracy (large error value), generally used for protection.


7, the connection line of the current transformer must be connected with the copper core insulated wire of 2.5mm2. Some electric department stipulates that 4mm2 copper conductor insulated wire must be used, but my view is not necessary.


Three. Application of current transformer


1. Measurement of AC current


It is necessary to set up the current measuring device if the equipment is running at the rated current value. For equipment above 40KW, an ammeter must be installed for monitoring.


The measurement of AC current has the direct measurement and the way of enlarging the measurement by the current transformer. The direct measurement is to connect the appropriate ampere to the current loop. The circuit shown below is measured by the current transformer access.



(a) a current transformer is used to measure the current in a three-phase balanced line. The current transformer is selected based on the current meter range, and the range of the ammeter is based on the actual value of the load current, which should occupy the 2/3 between the total range of the ammeter and the full scale value. For example, the rated current of a device is 100A, then an ammeter with a full value of 150A is selected. The current transformer transformer ratio of 150/5A should be chosen accordingly, current ratio is 150/5A the current meter. Readings are read directly with a table indicating value.


(b) two current transformers are used and an incomplete star is connected to measure three phase current in a three-phase balance or unbalanced line.


(c) a complete star connection is used to measure the three-phase current in a three-phase balance or unbalanced line, using three current transformers.


2. Measurement of alternating current


In order to expand the range of the AC watt hour meter, the most common thing in the factory is to use the method of current transformer to enlarge the range.



(1) the first diagram is the method of measuring the single phase watt hour meter and the current transformer. It can be used to measure three phase balance or single phase electric measurement.


The (2) and second graph is a method of measuring the three phase three wire active power meter with two elements and two current transformers. It can be used to measure the electrical measurement in a three-phase balance or unbalanced line.


The (3) and third graph is a three phase four line active power meter, and a measuring method of three current transformers. It can be used to measure the electrical measurement in a three-phase balance or unbalanced line.


3. Protection in electric power line


Overload protection in high power motors is often unable to get the corresponding thermal relay because of the large current. In such a case, the current transformer is usually used to solve it. The essence is to transform the large current into a small current with a thermal relay within 5A, which can meet the requirement of overload protection.


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