Wireless Power Supply Design for Precharging Circuit of SST in Railway Vehicles

Frequency modulation control-based wireless power supply (WPS) system adopted for the operating signals of a precharging circuit in solid-state-transformer (SST) is proposed in this article. To guarantee high voltage insulation capability between trolley line and railway vehicle and to decrease the voltage level of railway feeder, a large and heavy main transformer (MTr) operating at 50/60 Hz is conventionally used in railway vehicles. The SST, operating at high frequency for compact size and active control of bidirectional power flow, is an emerging technology in order to substitute a conventional MTr. At initial start of the SST, it is important to slowly charge the capacitor voltages by a precharging circuit connected in series with the trolley line. To operate a precharging circuit in the SST, the proposed WPS system can be utilized for high voltage insulation capability in this article. To transfer operating signals wirelessly through the air, long slim dipole coils for transmitter (Tx) and receiver (Rx) are adopted to fabricate a small size of the whole WPS system. The proposed WPS can be operated at a maximum power efficiency point w.r.t. the wide ranges of the coil distance and load conditions by the proposed frequency modulation control method in the primary side. A 3-30 W prototype of the Tx and Rx coils for the proposed WPS were fabricated and verified by simulation and experiments, showing that 84.0% and 80.2% of high dc-dc efficiencies for P
_L
= 30 and 12 W at d
_c
= 30 mm, respectively. As a result, the proposed control method without communication circuits between Tx and Rx sides has been successfully verified when coil distance and load resistance are widely varied.