The Impact of HVDC Links on Transmission System Collapse

Modern power systems are continually being expanded, are required to carry more power and are being increasingly interconnected. All of these increase the risk of wide area blackout.In 2003 the North America Blackout demonstrated that a HVDC link provides a ‘firewall’ against the system collapse propagating through a network. The HVDC link between Quebec and New York ensured that the system collapse did not progress beyond the HVDC interconnection interface. The objectives of this paper are to investigate contributions that integrate HVDC interconnections into AC networks. The simulation studies were performed using MATLAB

Phase-Locked Loop Control In Low-Inertia Grid-Connected Voltage-Source Converters

As the integration of renewable energy on the grid increases, the number of voltage-source converters (VSC) installed also does. VSC controls both switch turn-on and turn-off, allowing a dc voltage source to be switched between phases. For the converter to accurately synchronize with the grid, a phase-locked loop (PLL) is used for the frequency measurements of the grid. However, the implementation of PLL with measurement delay introduces harmonics, noise, high frequency, and voltage oscillation to the system due to its dynamics. The dynamics introduced to the grid can be ignored under stiff grid conditions, but power from renewable sources decreases the grid inertia creating a weak-grid condition. Older grids accommodate this by using generators that compensate for the rate of change of frequency (RoCoF). Modern grids have less generator to accommodate the RoCoF, so there is a desideratum to implore a robust controller that responds quickly to the RoCoF, disturbance/ distortion rejection, and noise immunity to the grid. In recent literature, the effect of the PLL dynamics on a weak grid has been of great concern because of its unmodeled dynamics that destabilizes the converter under the weak-grid condition. This thesis proposes showing the impact of the weak-grid on the VSC as the dynamic of the grid changes. It also provides remedies to the grid instability and high-power injection levels. The detailed PLL dynamics model, including the ac-bus voltage dynamics with constant frequency, is developed and linearized. Even at a fixed frequency, various factors play a role in grid instability, and this tremendously affects the ability of the VSC to control the grid efficiently. The effect of the PLL gain under the weak-grid condition is analyzed

Adaptive DC stabilizer with reduced DC fault current for active distribution power system application

This paper takes a systematic view on the control and protection of medium power DC networks in an active distribution power system considering fault current limiting, system control and converter design. Reduced terminal capacitance and extra DC impedance are used to limit DC fault current and reduce the required converter current rating for medium power DC networks. An adaptive DC power stabilizer is proposed to alleviate possible system instability brought by the fault current limiting settings in the presence of constant power load. The effect of the current limiting method and the proposed stabilizer on DC fault current and stability enhancement are validated by simulation studies using a simple two-converter DC network and a multi-terminal DC network in an active distribution power syste