A VSC-HVDC transmission system is the network to meet all these challenges due to its operational flexibility, such as voltage support to ac networks, its ability to operate independent of ac network such as offshore wind farms, suitability for multi terminal HVDC network realization as active power reversal is achieved without dc link voltage polarity change. Present VSC-HVDC transmission systems rely on their converter station control systems and effective impedance between the point-of-common-coupling (PCC) and the converter terminals to ride through dc side faults. With present converter technology, the dc fault current comprises the ac networks contribution through converter free-wheeling diodes and discharge currents of the dc side capacitors. The magnitude of the dc-side capacitors discharge current decays with time is larger than the ac networks contribution.

THE dynamic voltage restorer (DVR) is a custom power device utilized to counteract voltage sags . It injects controlled three-phase ac voltages in series with the supply voltage, subsequent to a voltage sag, to enhance voltage quality by adjusting the voltage magnitude, wave shape, and phase angle . The main components of a DVR i.e., a series transformerT_S, a voltage- source converter (VSC), a harmonic filter, a dc-side capacitorC_DC, and an energy storage device . The line-side harmonic filter consists of the leakage inductance of the series transformer L_fand the filter capacitorC_f.The DVR is conventionally bypassed during a downstream fault to prevent potential adverse impacts on the fault and to protect the DVR components against the fault current

The Ability to control power flow in an electric power system without generation rescheduling or topology changes can improve the power system performance using controllable components, the line flows can be changed in such a way that thermal limits are not exceed, losses are minimized, stability margins are increased and contractual requirements are fulfilled without violating the economic generation dispatch. Flexible AC Transmission systems (FACTS) technology is the ultimate tool for getting the most out of existing equipment via faster control action and new capabilities. Voltage regulation and line loss minimization in distribution networks are challenging problems, particularly when it is not economic to upgrade the entire feeder system. This project presents a new method for achieving line loss minimization and all nodes voltage regulation in the loop distribution systems, simultaneously, by using unified power flow controller (UPFC), one of the most important FACTS devices.

Now a days renewable energy is widely used such as wind energy and solar energy, due to their extreme abundance. So they should be properly interfaced with the power grid with suitable devices. In this paper a system of offshore wind with low- frequency ac (LFAC) transmission system which increases the power capacity and transmission distance is proposed along with a photovoltaic integration on the offshore itself results more stability in the power system. A cycloconverter is designed for the low frequency transmission and also to interface with the main power grid. The wind power plant collection system is dc based, and connects to the LFAC transmission line with a 12-pulse thyristor converter. Simulation results are observed to analysis the output voltages and currents of the two renewable sources and to illustrate the performance of the system