Read the text. Extra high voltage alternators
Extra high voltage alternators
The number of generators operating directly at extra high voltage is relatively small. The majority of stations generate in the region of 6.6 or 11 kV. The main recommendation for generation at high voltage is the opportunity it presents of direct connection to the distribution system without the added cost of step-up transformers and the additional switchgear and auxiliaries they involve. The operating technique for high voltage generators is essentially the same as for machines of lower voltage. Also the same considerations regarding earthing of neutrals, precautions against third harmonic circulating currents, and schemes of differential protection apply. Short circuit drying out may not be required due to the special construction of windings and precautions taken in manufacture, but great care in the removal of surface dampness is necessary.
The chief characteristic of the high voltage alternator is the special slot and end winding insulation and grading necessary to cope with the higher stresses imposed. At the same time currents are smaller and short circuit stresses in conductors will be reduced. It is correct to say, however, that with H.V. alternators the condition of the ventilation system, its correct operation and the maintenance of a high degree of cleanliness are of first importance. The possibility of the formation of free moisture in the machine spaces due to air leaks in the ventilation system together with unsuitable air cooler temperatures has also to be watched, as cases have been met where, for example, due to too low an air temperature, water has been precipitated in the machine following upon the ingress of damp air. These factors make it necessary that H.V. alternators should be subjected to a periodical insulation test, which should be carried out under as nearly similar conditions as possible on each occasion and the results logged.
The prevention of brush discharging has been accomplished by suitable grading of insulation and the coating of end connector surfaces with semi-conducting preparations. The screening effect produced by the latter eliminates free brush discharge from points on the end winding surfaces where these are in an air space. Coated winding for the slot portions have also been used with the object of reducing discharge in localized voids. Brush discharge will set up oscillations which may be detected with receiving apparatus using suitable probes.