FPGA implementation of impedance-compensated phase-locked loop

Abstract

The Phase-Locked Loop (PLL) plays a key role in HVdc systems. Recently, a new type of PLL called the Impedance-Compensated Phase-Locked Loop (IC-PLL) was introduced to compensate for the voltage drop across the ac network's Thevenin impedance, making the phase locking more robust against transients and harmonics. The IC-PLL has an improved dynamic response as compared with the traditional approaches. However, prior to this thesis, studies of the IC-PLL were based on off-line simulations. In this thesis, an actual IC-PLL is constructed in hardware using a Field- Programmable Gate Array(FPGA). Paralleled structure is implemented on the FPGA to achieve high speed. The IC-PLL's performance is investigated by connecting it to a real-time model of the HVdc system simulated using a Real-Time Digital Simulator (RTDS). Different types of system disturbances such as sudden step change in power, voltage magnitude change and voltage distortion are applied to examine the IC- PLL's dynamic behaviour. Results are compared with the traditional Trans-vector PLL (TV-PLL). The results show the IC-PLL tracks the phase and frequency of the Point of Common Coupling(PCC) voltage in the steady-state and has a minimal error with load changes. Moreover, this thesis investigated potential sources of error when conducting the Controller Hardware-in-Loop (CHIL) simulation of the IC-PLL. The investigation was carried out by building a detailed off-line simulation of the CHIL simulation itself by including models of the interface delays, offsets and finite precision of ADC and DACs. The results show that for this type of interfacing, the constant interface delay and offsets have minimal impact on the accuracy, however, finite precision of ADC and DACs will have a significant impact on the CHIL simulation.