Design, modeling and control of wireless power transfer system in domino structure for power grid online monitoring equipment

Abstract

With the development of the smart grid, various sensors and meters are increasingly installed on the transmission tower to detect the operation status of the power network. Such online monitoring equipment requires a continuous and stable power supply, which is grounded to the earth. In this thesis, a multi-coil wireless power transfer (WPT) system in domino structure is proposed to be integrated into the insulators. The energy is harvested by a current transformer from the strong alternating magnetic field around the power transmission line and transmitted to the monitoring devices over the insulation distance (typically, 1.1 m for 110-kV power network and 0.35 m for 35-kV power network). The resonators in the WPT system are manufactured on double-layer printed-circuit-board (PCB), making use of both winding inductance and parasitic capacitance to form a resonant circuit. The novel planar PCB resonators have smaller size, higher power density, more accurate model, and lower cost. They are easier to be manufactured and duplicated, and more suitable for integration into the insulator sheds. Most importantly, the PCB resonators can pass a series of rigorous high voltage tests for the insulator product as required by the national standard. The design, optimization, and implementation processes are introduced in Chapter 3 of this thesis. The characteristics of the WPT system integrated in the insulator are analyzed. The electrical devices usually require a constant voltage source no matter how the load condition changes. However, the equivalent load resistance for the WPT system has a great effect on the system performance such as the currents distribution on each coil, voltage gain, output power, and transmission efficiency. Also, the fast changes on load condition will cause a dynamic response in transient stage. A dynamic model is significant to express the system behavior and benefit the controller design for output voltage regulation. The phasor transformation method is employed in Chapter 4 to model the high-frequency AC variables in the WPT system. The time-varying phasors of the resonant voltages and currents are taken as state variables, rather than their rapid changing real-time values. Compared to traditional analytical methods, lower computational cost is significantly benefited from the system order reduction. Based on the dynamic model, a dynamic estimator for the WPT system is further derived in Chapter 5, which can identify the load information on the receiverside from the input voltage, input current, and their differentials on the transmitter. The estimator can not only recognize the small-signal variations of load conditions but also keep up with the step changes on load resistance. Both two-coil WPT system and multi-coil WPT system in domino structure are investigated for verification. With the help of this estimator, feedback information could be obtained without a wireless feedback communication channel. A single controller on the transmitter side of the WPT system could be designed to improve the dynamic response. In summary, this thesis synthesizes the resonator design, dynamic modeling, and control strategy for the multi-coil WPT system in domino structure to power the smart grid online monitoring equipment.