Protection of Transformer Neutrals against Destructive Transient Disturbances

The paper points out the danger to which the neutral point of transformers, connected to long distance transmission lines, are exposed through the building up of excessive potentials at this point under conditions of atmospheric lightning. This is due to the fact that the neutral point in a bank of Y-connected transformers acts as a reflection point for all the waves that are produced by induced lightning, which waves, at the place of origin, have the same amplitude and sign, and, therefore, travel with equal speed, considering that the impedance of line and transformer is the same for each phase. The three waves meeting attheneutral point build up to threetimes their individual potential at that place, thereby producing excessive potential stresses, and, according to experience, destruction of bushings and windings. The author recommends the installation of lightning arresters at the neutral point to provide a discharge path for the excess potential, which, according to measurement with a spark gap, may reach values of 350,000 volts, and higher.