Factors to be considered when choosing current transformers

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 current transformers also makes it possible for the loads such as appliances and machines to use the power. You could explore more here about current transformers work. Newtek Electricals has the required experience and expertise to provide quality current transformers at scale to diverse business sectors. Selecting the right current transformers (CTs) is crucial for accurate current measurement in electrical systems. To ensure you choose the appropriate CTs, consider the following key factors:

Factors to consider when choosing a current transformer

CT Accuracy Class

Accuracy is the core of a current transformer (CT) work performance. The international standard IEC 61869-1 defines various classes of current transformers testing 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 CTs 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 current transformer test 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.

Current Transformers 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.

Burden of CT Working

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.

Significance of 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.

Consultation:

If you’re unsure about CT selection, consult with a professional electrician, electrical engineer, or a representative from the CT manufacturer. They can provide expert guidance tailored to your specific application.

By carefully assessing these factors, you can make an informed decision and select the right current transformers for your electrical measurement needs.