Current Transformer (CT) Characteristics
Current Transformer (CT) Characteristics.
The Current transformer core is made of Ferromagnetic material like Iron that shows saturation is composed of microscopic regions called magnetic domains that act as a tiny permanent magnet. Before an external magnetic field is applied to the material, the domains are oriented in random directions. Their tiny magnetic fields points in random directions and cancel each other out, so the material has no overall net magnetic field when an external magnetizing field H is applied to the material and aligns parallel to the external field adding together to create a large magnetic field which extends out from the material, this is called magnetization.
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Current Transformer (CT) Characteristics. |
CT Error
Some of the primary current or the primary ampere-turns are utilized for magnetizing the core thus leaving less than the actual primary ampere-turns to be transferred to the secondary ampere-turns. This naturally introduces an error in the transformation. The error is classified into two the Ratio error and Phase error.
Magnetization Curve
CT errors result from the excitation current so in order to check the CT function, it's essential to measure the excitation curve, The magnetizing current of a CT depends on the cross-section and the length of the magnetic circuit, the number of turns and the magnetic circuit material. This curve is the best method of determining a CT's performance. It is a graph of the amount of magnetizing current required to generate an open-circuit voltage at the terminals of the unit.
CT response in a saturated state
- If the load and the magnitude of the primary current are large enough to saturate the CT, neither the flux nor the induced voltage is sinusoidal. The excitation current can take large magnitudes and the resulting error will be large.
- The secondary current is no longer proportional to the primary current.
- The current error which corresponds to the magnetization current increases significantly.
CT Secondary
If the load and the magnitude of the primary current are large enough to saturate the CT, neither the flux nor the induced voltage is sinusoidal.
Since CT normally has many more turns in secondary compared to the primary, the voltage generated on the open-circuited. CT will be much more than the system volts, leading to flashovers.
This peak value may be as low as a few hundred volts in a small measuring c.t. with a 5A secondary winding, but it might reach many kilovolts in the case of, say a 2000/1A protective c.t. with a large core section.
Open circuit voltage at CT is almost:
V= (3.5*Zb*Ip/Ns)^.5 it will come to a very extremely high value of voltage.
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