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When a conductor is carrying dc current , it is uniformly distributed over the cross section of the conductor . However an alternting current flowing through the conductor does not distribute uniformly, rather it has the tendency to concetrate near the surface of the conductor. This is known as Skin effect.   Thus, skin effect is the tendency of an alternting current to concentrate near the surface of the conductor.     Due to skin effect the effective area of cross section of the conductor through which current flows is reduced consequently, the resistance of the conductor is slightly increased when carrying an alternating current. Cause of skin effect The cause of skin effect can be easily explained. A solid conductor may be thought to be consisting of a large number of strands, each carrying a small part of current. The inductance of each strand will vary according to its position. Thus, the strands near the centre are surrounded by a greater magnetic f...

A black start is the process of restoring an electric power station or a part of an electric grid to operation without relying on the external transmission network. Normally, the electric power used within the plant is provided from the station's own generators. If all of the plant's main generators are shut down, station service power is provided by drawing power from the grid through the plant's transmission line. However, during a wide-area outage, off-site power supply from the grid will not be available. In the absence of grid power, a so-called black start needs to be performed to bootstrap the power grid into operation. To provide a black start, some power stations have small diesel generators, normally called the black start diesel generator (BSDG), which can be used to start larger generators (of several megawatts capacity), which in turn can be used to start the main power station generators. Generating plants using steam turbines require station service ...