Power-electronics-based online signaling techniques and their applications to power systems

The application of power electronics to power systems to facilitate the transmission or conversion of electric energy has been well known. This thesis reviews a different type of application in which power electronics circuits are deployed in power systems to create small but discernible signals online. These signals are utilized for communication, online monitoring, fault diagnosis and other information-oriented purposes. These power-electronics-based online signaling techniques have many potential applications in power systems. This thesis identifies a thyristor-based circuit named "zero-crossing-distortion" as a representative of these techniques, and applies it to solve the following three challenging problems.;Islanding detection of distributed generators (DGs) is a significant technical barrier for the emerging DG industry. This thesis proposes an innovative solution scheme based on power line communication. The scheme broadcasts a signal from a substation to downstream DGs by using the distribution feeders as the signal paths. A DG is considered as islanded if it cannot sense the signal. The zero-crossing-distortion technique is adopted to generate the signals. The signal generation device can also be located downstream of the substation at, for example, the upstream of an area with many DG installations to improve the flexibility in implementation. The proposed schemes have been verified through laboratory and field tests.;This thesis then presents an effective and easy-to-implement method for measuring power system harmonic impedances. The zero-crossing-distortion technique is utilized to create controlled short-circuits at the measurement point, producing currents and voltage disturbances for measuring the system impedance. The strength of the disturbance is controlled so that it can support precise measurement without causing power quality problems. The method can be implemented into a portable device for low voltage applications.;This thesis also investigates a method for identifying a line experiencing a single-phase grounded fault in noneffectively grounded systems. Faulted line identification in such systems is very difficult due to the small magnitudes of fault currents. The proposed method temporarily grounds the system and creates a fault current large enough for identifying the faulted line and yet small enough not to cause system problems. Lab experiments verified the effectiveness of the proposed method.