Achieving the EU’s climate neutrality ambitions requires a next generation of power electronics capable of reliable, fast and efficient operation in the new energy generation and distribution environment.
As part of the EU funded NOVETROL project, the Eaton ERL team in Prague is developing new current control and limitation techniques to enhance monitoring, protection and dynamic regulation of high current systems. In our previous contribution [1], we have described the current limiting concept, which integrates the variable extraordinary magnetoresistance (EMR) element with a magnetic field generator.
In this contribution, we are presenting the steps followed during the design of the magnetic field generator.
NOVETROL methodology
For the concept described in [1], the magnetic field generator is placed in line with the main current, thus triggering the increase in the resistance of the EMR element, in case of a short circuit or fast rise of the current from any cause. Therefore, for normal operation, the field generator should introduce a very low inherent resistance, and it should keep the magnetic field low enough (i.e. 0.1T), such that the EMR element is also maintained at a low resistance value.
In addition, for the current limiting operation, the magnetic field in the air gap of the generator should reach a value that is high enough (i.e. 1T) such that the EMR resistance can efficiently limit the current. As a constraint, the inductance L, which is also placed in series with the main current, has to be kept low enough such that it does not introduce unwanted delays in current dynamics, preventing the current limiter from operating efficiently.
We used as a first example a system with 20A maximum operating current, and we employed classical electromagnetic theory with modern CAD driven simulation. Starting with required field conditions – 0.1T at 20A and 1T at 264A (as the designed limited current value, as presented in [1]) – the team defined constraints governing inductance, airgap behaviour and winding/core parameters (number of turns N, cross section area for the core A).
Magnetic circuit fundamentals
The design starts from the standard relationships [2] between magnetic flux (Φ), flux density (B), magnetomotive force (N·I) and circuit reluctance (Rm) for the device shown in Figure 1 below:
B = Φ/A (1)
Φ = N·I/Rm (2)
Rm = l/μ0μrA for core and gap segments (3)