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This paper proposes a transformer-coupled series DC active power filter (DCAPF) applied to high performance magnet power supplies with large current and low ripples. In the condition of heavy magnet loads, this type of DCAPF has much better filtering performance compared with the parallel DCAPF. Firstly, the configuration of the magnet power supplies with the series DCAPF is proposed. Then a novel control strategy for the DCAPF, i.e. detecting the ripple voltage across the R-C branch filter as the reference signal of the DCAPF, is presented and analyzed in detail. After that, the most important component in the whole power system, the coupling transformer, is described in particular. The difference between it and ordinary transformers is introduced. Furthermore, the main steps of designing the coupling transformer are sketched out. With the novel control approach and the coupling transformer, a set of laboratory prototype of 5 kVA high performance magnet power supply system was developed. Simulation and experimental studies have been carried out. For 600 Hz components, the ripple voltage decreases to 1.14 V, only 6.52% of the value without the DCAPF. It is nearly the same case with other ripple components. The results verify the high performance of the transformer-coupled series DCAPF.