CT Secondary Open Circuit Phenomenon

A current transformer (CT) is designed to operate with its secondary winding connected to a low impedance load (like an ammeter or relay). This ensures the secondary MMF almost perfectly balances the primary MMF.

Normal Operation:

• Primary current ($I_p$) creates a primary magnetomotive force (MMF): $MMF_p = N_p I_p$.
• This MMF drives magnetic flux ($\phi$) through the core.
• Secondary MMF ($MMF_s = N_s I_s$) opposes the primary MMF, keeping the net MMF low.
• The resulting flux density ($B$) is well below the core's saturation level.

Open Secondary Condition:

• If the secondary winding is open-circuited ($I_s = 0$), then $MMF_s = 0$.
• The primary MMF ($N_p I_p$) is no longer opposed.
• This large, unopposed MMF drives a significantly higher flux ($\phi'$) through the core.

Core Flux Density Increase

The increased flux ($\phi'$) directly results in a much higher flux density ($B'$) in the core, as $B$ is proportional to $\phi$.

• This elevated flux density ($B'$) greatly increases the likelihood of the core reaching its saturation point.
• Core saturation leads to a drastic increase in magnetizing current, potential core overheating, and dangerously high induced voltages across the open secondary terminals.

Therefore, the primary consequence is the significant increase in flux density in the core, pushing it towards saturation.