We all know that voltage transformers cannot be operated in a short circuit, and current transformers cannot be operated in an open circuit. If a voltage transformer is short-circuited or a current transformer is operated in an open circuit, the transformer will be damaged or danger will be created.
In principle, we all know that both voltage transformers and current transformers are transformers; the only difference is the parameters we focus on. So why is it that one type of transformer cannot operate with a short circuit while the other cannot operate with an open circuit?
During normal operation, the secondary winding of a voltage transformer is essentially open-circuited, with a very high impedance ZL. If a short circuit occurs in the secondary circuit, the impedance ZL rapidly decreases to almost zero. This generates a large short-circuit current in the secondary circuit, which can damage secondary equipment and even endanger personal safety. Voltage transformers can be protected by installing fuses on the secondary side to prevent damage from secondary-side short circuits. Where possible, fuses should also be installed on the primary side to protect the high-voltage power grid from being compromised by faults in the transformer's high-voltage winding or leads.
When a current transformer is operating normally, its impedance ZL is very small, equivalent to the secondary coil operating under a short-circuit condition. The magnetomotive force generated by the secondary current demagnetizes the magnetomotive force generated by the primary current. The excitation current is very small, and the total magnetic flux in the core is very small; the induced electromotive force of the secondary winding does not exceed tens of volts. If the secondary side is open-circuited, the secondary current becomes zero, the demagnetizing effect disappears, but the ε1 of the primary coil remains unchanged. Its primary current becomes entirely the excitation current, causing a sharp increase in the magnetic flux Φ in the core. The core is in a highly saturated state, and given the large number of turns in the secondary winding, a very high voltage (even reaching several thousand volts) will be generated across the secondary winding. This could not only damage the insulation of the secondary winding but also seriously endanger personal safety. Therefore, opening the secondary side of a current transformer is absolutely prohibited. Voltage transformers and current transformers are both transformers in principle. Voltage transformers focus on voltage changes, while current transformers focus on current changes. So why can't current transformers operate with an open circuit, and why can't voltage transformers operate with a short circuit?
During normal operation, ε1 and ε2 remain constant. The primary side of the voltage transformer is connected in parallel in the circuit, with a relatively high voltage and a very small current. During normal operation, the current on the secondary side is also very small, almost zero, forming a relative balance with the open-circuit infinite impedance in the secondary circuit. When the secondary impedance rapidly decreases to the point of short circuit, because ε2 remains constant, the secondary current will inevitably increase rapidly, burning out the secondary coil.
Similarly, during normal operation, ε1 and ε2 remain constant. The primary side of the current transformer is connected in series in the circuit, with a relatively high current and a very small voltage. During normal operation, the voltage on the secondary side is also very small, almost zero, forming a balance with the infinitesimal impedance of the short circuit in the secondary circuit. When the impedance of the secondary circuit rapidly increases to the point of being open-circuited, the secondary current rapidly drops to zero, and the primary current is entirely converted into the excitation current, causing the magnetic flux to rapidly increase to saturation and burn out the transformer.
Therefore, the same transformer can produce different results depending on its application.
