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Separating distance between arrester and transformer in a power station

When the voltage is 400kV or above, the arrester is too large for the transformer box to easily support its volume and weight and must be mounted on a separate base. In addition, such an ‘extended’ distance is required if the transformer must be installed in a place where it is easy to move during maintenance. Access channels designed for this purpose can result in a distance of up to 30m between the arrester and the transformer.
Unfortunately, this magnitude of separation spacing (or protection zone) reduces protection when rapidly rising shock waves invade power stations along overhead transmission lines at close to the speed of light. When the impact hits the arrester, the voltage does decrease, but not to zero, but at best to the discharge voltage of the arrester. The impulse voltage propagates forward through the arrester and is reflected at the transformer. If the separation distance is large enough, the impulse voltage will be doubled. Although in most cases the reflected voltage is only a few percent higher than the impact of the incoming wave, it is this traveling-wave reflection phenomenon and the associated reflection factors that highlight the importance of separation spacing.