Online Measurement of Three-phase AC Power System Impedance in Synchronous Coordinates
Over the last two decades there has been an increased use of three-phase AC power systems that may not be connected to the main power grid, such as the power systems on more-electric airplane and all-electric ships. Power-electronic converters are usually a significant part of these systems, which provide excellent performance. But their negative incremental impedance nature increases the possibility of system instability.
A small-signal analysis that uses interface impedances defined in the synchronous frame is developed by Belkhayat at Purdue in the mid-90s to access the system stability. The system impedance varies with the operating point. Thus the impedance has to be obtained online at the desired operating point, on even in situ.
Literature investigates its use with system models, but the lack of equipment to measure such impedance prevents its use in practical systems. Measurement of impedances of each component enables the prediction of system stability before building the real system. The impedance data can also be used to investigate the instability in the system after it is built. The capability of impedance measurement can save the cost and time of system integrators.
After reviewing the state-of-the-art development of impedance measurement systems, the dissertation analyzes several systematical error sources in the system, which includes the signal processing and sampling circuits, the phase estimation for coordinate transformation and the injection device connection, and proposes the solution to reduce their influence.
Improved algorithm and system architecture are proposed to increase the measurement speed and accuracy. Chirp signal is used as an excitation signal to extract impedances at a group of frequencies at one time. The use of both shunt current injection and series voltage injection improves the SNR of measured signal. Oversampling, cross-correlation and frequency domain averaging technique are used to further reduce the influence of noise.
An instrument is built based on the proposed solution. A voltage source inverter is used to generate the perturbation. A PXI computer is used for real-time signal processing. A PC is used for data post processing and measurement process control. Software is developed to fully automate the measurement. The designed unit is tested with various linear and nonlinear load. The test result shows the validity of the proposed solution.