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Current transformers are used alongside power measuring devices to enable stepping down of primary current for measurement with ammeters. Stepping down of the high-voltage current from grid-lines by the CTs also makes it possible for the loads such as appliances and machines to use the power. You could explore more here about current transformer working. Mtecviet has the required experience and expertise to provide quality current transformers at scale to diverse business sectors. Considering the critical role CTs play in power measurement and use, we suggest that you consider the following factors when choosing a current transformer

Factors to consider when choosing a current transformer

Accuracy class

Accuracy is the core of a CTs performance. The international standard IEC 61869-1 defines various classes of current transformers according to their accuracy. These include 0.1, 0.2s, 0.2, 0.5, 0.5s, 1 and 3. The primary to secondary current error ratio of class 1 CT is 1% while in 0.5s, it stands at 0.5 or less.

Accuracy limit factor

Checking the accuracy limit factor forms a vital part of CT selection calculations when selecting a protective type current transformer. This value is calculated with the formula maximum fault current/rated primary current. Protective current transformers need to transform maximum fault currents in its service life. Thus, considering ALF is the key. When 5 P 10 is the stated measure in specification, 10 is the ALF.

Knee point voltage

Knee point voltage is the measure of secondary voltage above which the output current no longer linearly follows the input current within the limit of declared accuracy. The knee point is the voltage point at which a 10% rise in applied voltage leads to an increase of 50% in magnetizing current. Knee voltage is an especially important factor to consider in case of protection type transformers because they are subjected to a fault current that is 20-30 times their rating.


The resistive impedance to the secondary winding in the current transformer is called the burden. Items such as switch blocks, meters, or current conductors add to the burden of a current measurement circuit. Similarly, the conductor between the meter and the CT can also contribute to the burden. Lastly, in case meter cabinets are situated at a long distance from substation meters, the long cable length leads to high resistance. Deploying thicker cables, and using CTs with lower secondary current can redress this problem.

Short time current rating

Short time current rating is the measure of maximum fault current that a CT can withstand for a short time. This value is determined by a number of other factors such as peak asymmetric value of fault current, rms value of the fault current over a short period, the duration of short current, and its peak asymmetric value, and the measure of transient voltage at the point when the fault occurred as well as the point of extension of arc by the circuit breaker.