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Electrical & electronic design :Dr.K.Michailidis
 
THE VERTICAL SHAFT  DARRIEUS TYPE VARIABLE SPEED WIND GENERATOR
KA 140 (140 kW) OF SKYROS ISLAND
A BRIEF DESCRIPTION ( by  Prof.emer.N.Athanassiadis, Dr.Eng.K.Michailidis )
The Vertical axis Darrieus type wind generator KA 140, was designed in the Laboratory of Aerodynamics of the NTUA during the year 1987-89 and was manufactures in Greece. This WTG is the subject of Demonstration Project WE/209/85/IIE of  the EEC Directorate General XVII for Energy, with contractors the PPC of Greece and the NTUA.
The manufacturing of the WT as well as its preliminary factory tests were completed in June 1995.
From June 1995 the unit is ready and stored in the manufacturer's depot. Since then the manufacturer (BIOKAT industrial LtD) waits the order from PPC to transport and install the unit in Skyros island near the WTG OA/100 kW, which is installed and in service since Nov.1991. After the installation of the KA 140, so that entailing the fulfilment of the terms of the contract with the manufacturer,  the unit will belong PPC.
The main dimensions and the figure of the WTG KA140 are as follows :
Darrieus type rotor, with vertical shaft 26m height and maximum arc distance of each one of the two blades, in the middle of their Darrieus curve from the rotor axis, 9.6m. The aerodynamic section of the blade is constant along its length, of NACA 0015 type, with a chord of 714 mm.
The rotor shaft is formed from steel tube of 914 mm diameter and a length of 26 m. It will be supported on a heavy type spherical roller thrust bearing which is based on the top-cover of a specially formed steel structure and it is kept in its vertical position by a "cover bearing assembly" supported in its place by three suitable tensioned cables in a tetrahedron edges arrangement, anchored on three concrete blocks.
The rotating movement of the rotor will be transmitted by an intermediate shaft to the low speed shaft of the gear box. The gearbox is of 90o degrees angle transmission type, it has a speed ratio 1:32.611 and its horizontal high speed shaft is connected to the generator through a flexible coupling.
The disc of the two hydraulic brake systems is fixed on the low speed shaft of the gearbox. There are two pairs of brake callipers which constitute the service braking  assembly (hydraulically applied/spring retracted) and two pairs of brake callipers which form the fail-safe braking system (spring applied / hydraulically retracted).
The generator is of a DC type with independent excitation, 184 kW at 1595 RpM. Its cooling is achieved by a close circuit air/air exchanger system.
The innovation in the design of this unit is that it is able to work at variable speed according to the wind conditions, by means of an AC/DC converter placed electrically between the 3 phase  AC, 50 Hz electric network and the DC generator.
The speed is controlled through a digital controller, controlling the armature voltage. The speed reference and the whole control operation is achieved by a  PLC which acts on the converter side that controls the voltage of the above mentioned circuit, enabling thus the generator to run at variable speed. Through this action it is possible to obtain the best efficiency that can be achieved according to the wind conditions and the ability of the electric network to absorb the related power.
The PLC acts as a very capable operator but incomparably faster than a human operator. In such a manner PLC will be able to start the unit  from zero speed (the generator acts as a motor), increasing the speed from working conditions from about 30 to 48 RpM for the rotor, and monitoring the best possible efficiency and the safest possible operation of the system.
According to the aerodynamic characteristics of the rotor blades we estimate that the max. electric power of the KA140 will be 140 kW.

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