Testing the Megawatt-Scale Impedance Measurement Unit at Medium Voltage Levels

The Office of Naval Research is developing science and technology for advanced electric ships that utilize an integrated power system. This project was motivated by the success and feasibility of impedance measurements and impedance-based controls at the low voltage level. An extension of impedance measurement units (IMU) to the medium voltage (MV), megawatt-class converter levels would provide significant benefits to the U.S. Navy. The primary objective of this project was to evaluate the suitability of such a novel instrument for use of frequency domain characteri-zation of megawatt scale equipment, which will eventually be used for system integration studies. This project was the first test and use of an IMU at these voltage and power levels, following the design and preliminary testing of the MV IMU. The technical objectives included demonstration of the IMU design concept and impedance measurement capabilities within both AC- and DC-systems.
The testing approach was derived in coordination between the Center for Advanced Power Sys-tems (CAPS) at Florida State University and the Center for Power Electronics Systems (CPES) at Virginia Tech. The IMU’s power stage design is based on the PEBB (Power Electronic Building Block) concept, and individual stages were evaluated before operating the unit as an impedance measurement device. Test applications feasible at CAPS concerned 3.3 kV/60 Hz, and 4.16 kV/60 Hz, and 4 kV DC-systems. Guided by modeling and simulation efforts in the related ESRDC Impedance-based Controls task, commissioning and test plans were established. The pre-testing work addressed issues of experiment selection and design. One critical aspect was the pro-tection of experiments due to the uniqueness of the proposed tests. The system-based tests were conducted with the help of a Real Time Simulator (RTS) to facilitate Control Hardware-in-the-Loop simulations and control of the surrounding power system components.