EARTfelt Welcome dear friends of protection and control engineering! Today we reveal the secret of remanence. What is remanence, how does it emerge and what impact does it have on the transfer behavior of a current transformer?
The phenomenon of remanence, also referred to as residual magnetism, describes the remaining magnetization of an iron core after the actual cause, an external magnetic field, has been removed. In the case of a current transformer, this means that the core remains magnetized, although no more current flows.
The magnetic field of the current-carrying primary coil generates a magnetic flux in a previously magnetically neutral iron core. Due to the magnetic flux or the magnetic flux density acting in the core, the iron core is considerably magnetized. If we now interrupt the flowing current again, the magnetic flux density in the iron core does not return to zero, but remains at its remanent value.
To understand the phenomenon, we need to look at the iron core in detail.
The iron core is a magnetizable material, which consists in its interior of many small elementary magnets.
Elemental magnets are the units that have a fixed-size magnetic dipole and a variable direction. If the elementary magnets are equally represented in all directions, their magnetic fields cancel each other out and the iron core appears non-magnetic.
On the other hand, if the elementary magnets are increasingly oriented along a certain direction, the sum of their magnetic fields forms an overall magnetic field measurable on the outside of the core and the core is magnetized.
Since iron belongs to the ferromagnetic materials, the elementary magnets are set parallel to each other even without the magnetic field of the coil in small areas. These areas are called domains and may also be known to us as "Weiss domains" from physics lessons. These are about 10 microns to 1 mm large areas, which are separated by the so-called Bloch walls.
Let us now consider the whole cycle of a current transformer at the level of elementary magnets. Initially, we start with a completely demagnetized current transformer core.
We put the CT into operation and switch on a primary current flow. Thereby, the elementary magnets are aligned along the direction of the magnetic field lines.
If we interrupt the current again, the external field is removed and the elementary magnets do not completely return to their neutral starting positions. Instead, the domains described now each form the same directional orientation. These Weiss domains are the reason for the remaining residual magnetism in the iron core.
It becomes particularly interesting when we look at the remanence with respect to the sinusoidal alternating current. In our characteristic below, the point of remanence is marked in the ordinate. As we can see, the magnetization of the core does not fall back to zero at the moment of the current zero crossing, but insists on the value of the residual magnetization. Since no jumps can occur, the magnetization always starts from the last point.
High operating currents as well as every single short circuit add remanence to a CT. The remanence accumulated over time causes the CT to saturate even in the event of errors with a low short-circuit current. The higher the remanence, the less short-circuit current is sufficient to drive the CT into saturation. The remanence is decisive for the transient transmission behavior of a current transformer.
🌐 Even if no more current flows through the CT, ferromagnetic core materials, e.g. iron consist of a remanence flux.
🌐 Short circuits with high DC components in particular add remanence to iron-core CT's.
🌐 Remanence reduces the transmission capability of the converter as it tends to saturate.
🌐 Remanence can be limited or eliminated with one or more air gaps in the core.