What are the functions of current transformers?
What is a current transformer?
Current transformer is an instrument which converts the large current at the primary side into the small current at the secondary side according to the principle of electromagnetic induction. The current transformer consists of a closed core and windings. It has a small number of turns in the primary side winding and is strung in the line with the current to be measured.
So it often has all the current flows through the line, the secondary winding circle number is more, concatenated in measuring instrument and protection circuit, current transformer at work, its secondary side circuit is always closed, so the measuring instrument and protection circuit in series with the coil impedance is very small, the working state of the current transformer is close to a short circuit. The current transformer converts the large current at the primary side to the small current at the secondary side for measurement. The secondary side cannot be open circuit.
What are the functions of current transformers?
A current transformer (CT) is used to measure the current of another circuit. CT is used worldwide to monitor high-voltage lines in national power grids. The current transformer is designed to produce an ALTERNATING current in the secondary winding proportional to the current measured in the primary winding. In this case, current transformers reduce large currents to lower values, thus providing a safe way to monitor current flow in an AC transmission line.
Operational hazards can occur if the secondary loop of the CT is disconnected and the primary loop is energized. Open circuit conditions can occur unintentionally through the burden of routine maintenance or damaged secondary circuit leads. In these cases, high voltage transients may occur and damage the CT winding insulation; It may become inoperable. In addition, these transients cause high vortices in the CT core. These may be harmful to the magnetization characteristics of CT and lead to errors in measurement accuracy.
IEEE C57.13 recommends that voltage limiting devices be equipped with secondary windings to prevent dangerous voltages. It states that the voltage limiting device should withstand an open circuit condition for one minute without damaging the secondary circuit. Metrosil Current transformer Protection Units (CTPUs) provide this protection and, unlike other voltage limiting devices, do not require immediate replacement in abnormal conditions. They can stay in place without the need for user intervention.
Current transformer protection device
When connected to a load, the varistor is subjected to an applied voltage during normal operation or failure. It acts as a passive load and absorbs very little current, thus preventing inaccurate CT measurements. In the open circuit state, the varistor is subjected to an applied current and acts as an active load. Therefore, it limits the voltage between the CT terminals to prevent any damage. Thermostatic switches manage thermal circulation within the Metrosil CTPU when the CT is open. A second thermostatic switch can be mounted on the radiator plate for remote monitoring. A Metrosil varistor can control the size of the backemf by dissipating the energy stored in the coil to an appropriate load.
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