Current Transformer (CT)
Function of a CT
used to step down line current to make it easier to measure by current measuring instruments like an ammeter.
used to measure high value of currents.
usually have only one winding.
The primary is connected in series with the line load.
When the primary has a large current rating, the primary winding may consist of a straight conductor passing through the center of the magnetic circuit.
Ratio of the current magnitude
Class of CT
Metering CT
Protection CT
Class 0.1 to Class 5.
Has ratings for accuracy class and accuracy limit factor (ALF).
Class 0.2 used for TNB Tariff Metering.
Class 0.5 and 1.0 used for ammeters and other
Consider 300/5A 5P10 15VA CT.
Perform correctly for fault currents and reasonable accuracy for load current.
P denotes protection CT.
5P denotes protection CT with 5% error at the accuracy limit.
5P10 denotes ALF of 10. That means the accuracy limit is 10 times the rated current.
CT Type
Slot/Window/Ring Type (ctnd)
Wound Type
Bar Type
CT Nameplate
Core
Ratio
Burden
Class
ISF/ALF
RCT
TYPE
INSULATION LEVEL
HSV
STC
INSULATION CLASS
Knee Point Voltage (Vkp)
measure the precious value of current when fault occur in the system.
The magnetization curve classified 3 zones. Magnetizing, linear, pre-saturation, saturation curves.
Normal operation CT operate at linear region then in this region called ankle point.
What happens above Vkp
The CT is said to be in saturation.
Saturation = Primary current is so high that the core cannot handle any more flux.
In saturation, there is no flux change when the primary current changes
Since there is no flux change there is no secondary current flow.
relay calibration in maintenance
Shorting Wire on Terminal Block
Test Terminal Block & Test Plug
How to Select CT Ratio of CT
determined by the maximum value of the load current to be measured.
Primary rating selected must be beyond the measured/defined maximum
Type Test of Current Transformer (CT)
DC Resistance Test
Polarity Testing/Flick Test
Magnetizing Curve Test
CT Ratio Test