Calculation and measurement of coupling loss in a no-insulation ReBCO racetrack coil exposed to AC magnetic field

Abstract

No-insulation coils are in general self-protecting and can therefore generally be operated at higher current densities. However, the electrical turn-to-turn connections may cause additional AC loss when charging the coil or when it is exposed to a time-dependent magnetic field. In this work, we study the case of a no-insulation ReBCO tape racetrack coil exposed to a uniform AC field applied parallel to the tape surface. We show that an anisotropic continuum model allows to formulate efficient analytical approximations for coupling loss in the low- and high-frequency limits. For intermediate frequencies, the continuum model needs to be evaluated numerically. The model was validated with representative measurements of AC loss in the coils, measured calorimetrically as well as magnetically using pick-up coils. The validation experiment nicely confirms the predicted frequency dependence of the coupling loss, which is P ∝ f 2 at low frequencies and P ∝ √f at high frequencies, due to the skin effect. The transition between low- and high-frequency regimes occurs around a characteristic frequency f c that is directly related to the characteristic time constant τ = 1/2πf c associated with the current decay in straightforward (dis)charge experiments.

Effect of a DC transport current on the AC loss in no-insulation ReBCO racetrack coils exposed to AC parallel magnetic field at 77 K and 4.2 K

“…Measurements are performed both at 77 K and 4.2 K. The DC current/AC field scenario is representative of what the DC stator coils in linear motors experience, except for the frequency, which can be much higher in real motor applications. This work expands on the results published earlier in [15,16]. We use a 2D numerical model to explain the findings and demonstrate that this computation method can be used very well as a design tool to compute AC loss in motor drives using ReBCO coils.…”

“…The induced magnetization current density J mag can reach this level at high frequencies, where the skin effect concentrates current in a shell at the outer surface. The magnitude of J mag decays inward over a characteristic length scale, the skin depth δ = √ ρ ⊥ sin 2 (α)/πµ 0 f [15]. Here, ρ ⊥ is the effective transverse resistivity of the coil, f is the frequency of the applied magnetic field and α = d turn /ℓ turn is the winding angle, i.e.…”

“…We have developed a 2D numerical model to predict AC loss generated in ReBCO racetrack coils. A detailed description of this computational method can be found in [15]. In brief, it calculates the current distribution in a 2D cross-section of a ReBCO racetrack coil by solving an integral form of Maxwell's equations [17,18].…”

Magnetization loss and transport current loss in ReBCO racetrack coils carrying stationary current in time-varying magnetic field at 4.2 K

“…A numerical model was developed to calculate the current distribution in a 2D cross-section of a racetrack coil. It is discussed briefly here, but is described in detail by Otten et al [23]. There, it is also used to calculate coupling loss in no-insulation ReBCO racetrack coils in a parallel applied magnetic field.…”

Study of the demagnetization behavior of no-insulation persistent-current mode HTS coils under external AC fields by 3D FEM simulation

“…Schnaubelt et al [54] built an NI HTS coil model based on the H−ϕ formulation with the turn-to-turn contact layers modeled as the shell elements. Mataira et al [55][56][57] proposed a method that models the NI coil as a homogeneous bulk set with an anisotropic resistivity; this method has been used and verified by quite a few groups in the world [58][59][60][61][62][63][64][65]. As a pure FEM model, this model is easy to realize, and very convenient for cohesive multi-physics coupling with other physical fields (e.g.…”