Current transformer installation

Common accuracies and burden powers, as well as error limits, according to IEC Standard are indicated in Table 1. Table 1 — Common accuracies and burden powers of CT and error limits. Common value for RF is 1. Rating factor of a CT is largely dependent upon ambient temperature. Also important to consider in a CT is the magnetizing curve that is similar to the one shown in Figure 9.

For this CT to operate satisfactorily at maximum fault currents , it must operate on the linear part of the magnetizing curve , i. The knee-point voltage is less applicable for metering current transformers as their accuracy is generally much tighter but constrained within a very small bandwidth of the current transformer rating , typically 1. However, the concept of knee point voltage is very pertinent to protection current transformers , since they are necessarily exposed to currents of 20 or 30 times rated current during faults , and is most critical on differential protection that will be discussed later.

The point on the magnetizing curve at which the CT operates is dependent upon the resistance of the CT secondary circuit. Good to Know:. Your email address will not be published. Notify me of follow-up comments by email. Notify me of new posts by email. Electrical Technology 2 5 minutes read. Show More. For best accuracy, these parts should not be interchanged with other CTs.

Solid-core CTs require that the phase conductor being measured be disconnected at one end so that it can be passed it through the opening in the CT.

This is not difficult when the wire gauge is small but becomes impractical with larger wire gauges and multi parallel conductors. For correct measurements, CTs must be installed on the phase conductor that corresponds to the voltage input connection.

The voltage input connections are on the green, five position, screw terminal block. It may help to use colored tape or labels to identify the wires.

To reduce magnetic interference between CTs on adjacent phases, it is a good practice to separate them by about 1 inch 25 mm. This also helps to prevent dust and debris from forming a bridge across the phase conductor terminals or bus bars and possibly causing a flashover arc.

If the CT opening is much bigger than the conductor, position the conductor in the center in the CT opening. Plastic cable ties can be used to secure the position of the CT on the phase conductor. A cable tie can also be secured around the perimeter of some models of CTs to prevent them from accidentally opening. CTs are marked with a symbol arrow or label which indicates the correct mechanical orientation of the CT on the conductor being measured.

In addition to installing CTs with the correct mechanical orientation, electrical polarity, as indicated by their white and black wires must also be correct. Each pair of CT wires connects to the appropriate terminal on the black six position screw terminal block. The polarity of each pair of terminals is indicated by a white and black dot on the label. Be sure to connect the white wire to the phase terminal aligned with the white dot, and the black wire to the terminal with the black dot.

Remember that both the physical orientation and the electrical polarity of each phase must be correct for proper operation. If a phase is reversed either electrically or mechanically, and current flows in the reverse direction, the WattNode meter will measure, depending on the model, zero or negative energy for that phase.

If the CT lead wires are longer than necessary, they can be shortened. With low voltage it is possible to dispense with grounding if the current transformers do not possess large metal contact surfaces.

However, common practice is to ground low voltage transformers too. Customary is grounding on S1. However, grounding can also take place on the S1 k terminal or S2 k terminals. Important: Always ground on the same side! Installation orientation Determine the flow direction of the energy in the cable that you wish to measure. Line length and cross-section The power consumption in W caused by the line losses is calculated as follows: specific resistance for CU: 0.

Grounding of current transformers According to VDE , current and voltage transformers should be secondary grounded from a series voltage of 3. These equipments and power analysers help you gain a comprehensive overview of your energy supplies and introduce the correct measures. The power quality is also monitored according to the general valid standards e.

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