A current transformer (CT) is, in many respects, different from other transformers. The primary is connected in series with the network, which means that the primary and secondary currents are stiff and completely unaffected by the secondary burden. The currents are the prime quantities and the voltage drops are only of interest regarding exciting current and measuring cores.
One of the fundamental tests of CTs is the ratio and phase error test. Traditionally this test has been performed by injecting a current on the high voltage side and measured the secondary current. However, with modern test equipment it is possible to make this test from the secondary side of the CT. The advantage is that the test equipment is smaller and lighter since it requires less power.
Low-power current transformers (LPCTs)
The most common sensors for medium voltage applications use low-power passive current transformers. These do not contain any active interior electronics, thus not requiring any additional power supply, and set reliable and highly accurate devices with a broad dynamic range of measurements. The best technology for current measurement, given its size and wide range of measured current, is the Rogowski coil technology.
A Rogowski coil is a toroidal coil, without an iron core, placed around the primary conductor which can either be part of the sensor or be represented by other equipment (e.g., medium voltage cable or bus bar). The output signal of a Rogowski coil is a voltage that is then processed and integrated by the connected intelligent electronic devices. The requirements of low-power current transformers (LPCT) were firstly described in the standard IEC 60044-8, released in 2002, which has been progressively replaced by a new set of standards under IEC 61869 standard family. The new standard IEC 61869-10 covers the LPCTs without active electronics.
CTs its circuits diagnosis
Ratio/phase analysis; an assessment of the CT’s accuracy
Saturation test; identification of core type; protection or meter
Residual magnetisation/demagnetisation; a test to determine the level of residual magnetism in the CT and to demagnetize
Winding resistance; used for calculations of saturation voltage and accuracy with specific burden
CT burden test; ensures that burden is within the CT’s VA limits