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Description
The increasing demand for renewable electricity is driving the deployment of larger and more powerful offshore wind turbines. Electrical machines employing second generation high temperature superconducting (2G HTS) field windings offer the potential for high magnetic loading, increased torque density and reduced active mass. However, conventional conductive energization of these 2G HTS windings requires high current leads and electrical joints. This introduces heat into the cryogenic environment through thermal resistive losses $[P_{\mathrm{joint}}]$ as described in equation (1). Moreover, the higher currents $[I_{\mathrm{HTS}}]$ of 2G HTS conductors can result in substantial resistive losses at these electrical joints.
$$
P_{\mathrm{joint}}
=
I_{\mathrm{HTS}}^2R_{\mathrm{joint}}.
\tag{1}
$$
This work investigates flux pump systems for efficient, non contact energization of 2G HTS field windings.
Two flux pump configurations; a rotary permanent magnet flux pump (PMFP) and a stationary travelling wave flux pump (TWFP) are designed, fabricated and experimentally evaluated. Finite element models developed in Ansys Maxwell are used to characterize the magnitude, spatial distribution and temporal waveform of the magnetic flux density in the HTS air gap. The PMFP produces a short unipolar magnetic-field pulse during each magnet passage, followed by a relatively long low-field interval. In contrast, the TWFP employs multiphase AC windings together with a DC bias winding to generate a continuous, sinusoidal unipolar travelling magnetic field $[B_y(x,t)]$ according to the equation (2). Here, $B_{\mathrm{dc}}$ and $B_{\mathrm{ac}}$ are the instantaneous magnetic flux densities resulting from the DC and the AC supply respectively. And in steady-state, the electric field introduced in the 2G HTS depends on this travelling magnetic wave and the vortex velocity of the 2G HTS according to Faradays electro-magnetic induction.
$$
B_y(x,t)
=
B_{\mathrm{dc}}
+
B_{\mathrm{ac}}\cos(kx-\omega t).
\tag{2}
$$
Although the reference PMFP demonstrated better initial charging performance, the TWFP is selected for further optimization because its stationary construction eliminates mechanically rotating components near the cryogenic system and therefore offers greater mechanical simplicity and potential reliability.
Preliminary experiments are conducted using 0.6m long, 12mm wide THEVA TPL4120 and TPL2100 2G HTS tapes. Closed HTS loop is formed using face-to-face soldered lap joints with an overlap length of 20 mm. The joints are manufactured using a purpose built soldering system with controlled dwell time, temperature, and applied pressure. Experimental testing identified the best performing parameters as 50 s, 423 K, and 83.3 kPa, respectively. The circulating current is measured using a calibrated Hall sensor coupled to an iron core current transducer. All preliminary flux pump experiments are conducted in a liquid-nitrogen bath at approximately 77 K. The HTS charging equation itself consists of an inductive voltage term coming from the inductance of the HTS loop $[L_{\mathrm{HTS}}]$ and a resistive voltage term because of the solder joint resistance $R_{\mathrm{j}}$ as in the equation (3):
$$
L_{\mathrm{HTS}}\frac{\mathrm{d}I_{\mathrm{HTS}}}{\mathrm{d}t}
+ R_{\mathrm{j}}I_{\mathrm{HTS}}
= \overline{V}_{\mathrm{FP}}.
\tag{3}
$$
The PMFP energize the HTS loop to a maximum current of approximately 129 A with a pumping rate of around 65A/s and has a steady-state peak to peak current oscillation of around 50A. Whereas, the reference TWFP reached approximately 100 A with a pumping rate of 17A/s at 15Hz input AC frequency. The steady state peak-peak ripple is also higher around 60A. Subsequent TWFP design and control studies shows the air gap, excitation frequency and current waveform as important performance parameters. Reducing the air gap to 0.5 mm increases the maximum HTS current by 16.7% relative to the 1mm air-gap design. Increasing the excitation frequency reduced the time required to charge the loop to the maximum possible current from around 15s at 5 Hz to around 5s at 15 Hz. Further experiments will investigate different supply parameters to optimize the TWFP performance.
The results demonstrate the feasibility of both flux pump configurations and identify electromagnetic geometry and excitation control as effective routes for improving TWFP performance. A temperature controlled cryogenic chamber has additionally been constructed and is being commissioned to extend the investigation over 60K - 90 K using full-scale 2G HTS double-pancake coils. This work provides an experimental basis for the future integration of stationary flux pumps with superconducting field windings for offshore wind-turbine generators.
Keywords: 2G high-temperature superconductors; Flux pump.
| Submitters Country | Germany |
|---|---|
| Are you a student? | No |
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