Synchronous Generator Brushless Field Excitation and Voltage Regulation via Capacitive Coupling Through Journal Bearings

Wound field synchronous generators (WFSG) are the standard electromechanical converter for back-up and utility scale power generation. Maintenance costs may be minimized by adopting noncontact or "brushless" technologies to replace sliding slip ring connections for rotor field excitation. This paper presents a brushless excitation approach using ceramic insulated sleeve (journal) bearings with oil lubrication to form capacitively coupled slip rings, in contrast to more traditional inductive brushless exciters and rotary transformers. This capacitive power transfer (CPT) approach exhibits advantages including low weight, low volume, and has a relatively simple construction using off-the-shelf components. Analysis, design, and prototype construction of the CPT system are presented. Experimental results demonstrate that 1.7 nF of capacitive coupling transfers 340 W to the rotor field winding of a 10 kW 208 V WFSG. Voltage regulation of a WFSG is demonstrated during steady state and 1 per unit load step changes yielding a NEMA-MG1 class G2 rating.