Why a CT may blast when its secondary is open, while nothing happens to a Power Transformer or a PT when its secondary is open ?
- CT: Current is fixed by the System Load Current and the Voltage depends upon the Load. Power Transformer: Voltage is fixed by the System. and the Current depends upon the Load. CT Primary is connected in Series with the Power System and its Primary Current is decided by the System Load Current and NOT by the CT Burden. A Power Transformer behaves approximately like a Constant Voltage Source because its primary is connected across a fixed system voltage. The system maintains nearly constant voltage at the transformer terminals. Therefore, the transformer tries to maintain approximately constant voltage on the secondary side also.
- CT Secondary Open versus Power Transformer LV Open: When the CT Secondary is Open, Current Continues to flows through the Primary. Secondary Current being zero, there is no Opposing Secondary MMF. Now the entire primary MMF acts on the core and produces very high magnetic flux. This excessive flux induces a very high voltage across the CT secondary terminals. The secondary voltage may rise to several kV depending on burden and current.
Now, in the case of Power Transformer, when the LV side is OPEN:
- There is no load connected on secondary
- Hence Secondary Current becomes nearly zero Because there is no secondary load, the primary does NOT need to draw heavy current from the system. So only a small current called Magnetizing Current flows in the HV winding. This small current is just enough to produce the normal working flux in the core. Hence:
- Flux remains normal
- Core does not saturate
- No excessive voltage is generated
- Transformer remains safe
