Abstract :
Most modern transmission systems operate at 66kV or higher voltage and are supplied from alternators whose voltage is stepped up by transformers, and when considering the effect of transmission-line lightning surges on a power station it is necessary to regard the transformer and alternator as a single unit. After a brief analysis of the phenomena likely to be produced when an impulse voltage is applied to a transformer coupled to an alternator, the results are confirmed by tests on three such units. These show that the principal transfer of voltage through the transformer is due to magnetic coupling between windings and causes an oscillatory voltage to be superimposed on the surge voltage in the l.v. winding. In addition, the distributed capacitance and inductance between the windings of the transformer cause an electromagnetic transfer of voltage with travelling waves in both the transformer and alternator windings. Approximate empirical methods are given to calculate the transformer h.v. voltages from the various circuit parameters. The results are compared with those obtained from the tests. Since in practice the transformer and alternator are excited at power frequencies, whenever a surge is likely to reach the windings it is necessary to allow for this in estimates of the voltages which may occur at the alternator terminals. On the assumption that the surge protection for the unit consists of a rod-gap or other voltage-limiting device at the transformer h.v. terminals, an analysis is made with a view to ascertaining the maximum voltage which may occur at the alternator terminals. In a typical case this is shown to add about 10% to the potential expected without allowing for the power-frequency excitation.
