What Is a Zero Flux Current Transformer?
A zero flux current transformer is a device used to detect the current value called the zero phase current in a three-phase AC power supply.
The sum of the vectors of the currents flowing in each phase of a three-phase AC power supply is normally symmetrical and thus zero, but when a ground fault current flows, the balance is disturbed and a non-zero value is obtained.
In the event of a ground fault, a zero flux current transformer is activated to immediately disconnect the fault location from the surrounding electrical circuit.
For this reason, they are built into RCDs in advance and constantly play a role in detecting accidents.
Uses of Zero Flux Current Transformers
When an electrical circuit in an electrical facility is electrically connected to the earth, a large current flows into that point as an earth fault current, and there is a risk of a serious accident.
Zero flux current transformers are used to prevent this, and when a ground fault current is detected, the ground fault point is immediately disconnected from the surrounding area.
Although also used for low-voltage general instrumentation, the role of the zero flux current transformers becomes more important as it is essential for higher voltage circuits, where the severity of the accident may be greater.
Principle of the Zero Flux Current Transformers
In a three-phase AC power supply, if the currents flowing in each of the three phases are equal and symmetrical, the sum of their vectors is zero.
The average value of the sum of the vectors is called the “zero-phase current,” and in the symmetrical form shown above, the zero-phase current is zero.
If a ground fault current flows in any of the three phases, the three phases are unbalanced and the zero-phase current is not zero. This allows for the immediate detection of ground faults.
“Zero flux current transformers” are a device that detects this kind of zero-phase current in a three-phase AC power supply.
Like ordinary current transformers, it has a structure in which a coil is wound around a circular iron core, and a conductor penetrates through the center of the ring.
However, while a normal current transformer has only one through conductor, zero flux current transformers have three through conductors for each of the three phases.
If the current balance of the three phases is disturbed for any reason, a secondary current flows momentarily through the coil, and this is detected as a ground fault.
It is often built into RCDs together with relays and circuit breakers and plays an important role in preventing leakage currents that can lead to accidents, fires, and other disasters.