Surge Protection of Unit-Connected Generators

In a unit-connected generator scheme where the transformer is protected by lightning arresters installed adjacent to its high voltage terminals, the maximum surge voltage that can appear at the highvoltage terminals is limited to the discharge voltage of the arresters given in Fig. 2. The surges can be transferred electrostatically and electromagnetically through the transformer winding to the generator terminals. The electrostatic transfer is governed by the capacitive network of the transformer windings; see Fig. 20. Formulas for an equivalent circuit are derived for simple computations of the effect of the terminal conditions. This component is of subtransient nature. For the two cases studied, the crest magnitude of the electrostatic component does not exceed 21 kv. This voltage decays rapidly and vanishes within 0.4 ¿sec. Its effect is negligible. The electromagnetic transfer is analyzed for the unit-connected scheme and formulas are derived for the transferred surge voltage at the generator terminals. Twelve cases most commonly met in practice were analyzed with and without terminal capacitance. The result is an envelope voltage of 10 kv per ¿sec linear front and 40-kv crest magnitude which is not exceeded in any of the investigated cases. The voltage distribution along the generator winding is analyzed by classical transmission line theory. An actual distribution computed for a 100-mva generator with the above envelope voltage at the terminals showed that no points within the winding attain a voltage higher than the terminal voltage of 40-kv crest.