For Power Generation Plants, regardless of their nature, the performance of an electrical failure in the plant can have different impacts depending on the system of protection against electrical failures. Of course, the way of selecting the protection system against electrical faults will affect different aspects of the installation: cost, life, design time, execution times, etc.

We are currently analysing in detail several generation plants with TN-S earth system, where the equivalent circuits to evaluate the different types of earth faults correspond to the following diagram:

Picture - 1

When a short-circuit fault occurs in the generation plant the earth system in TN-S connection acts as shown below;

Picture - 2

The earth fault does not affect downstream of the evacuation connection point to the network (step-up transformer, previously mentioned in Picture 1), since there is the same common point where all the elements of the plant are connected by the earthing system. In this way, the fault value (Short-circuit current) would be the short-circuit fault that the NETWORK is able to supply (in Picture 1, it is the short-circuit value at the 220kV level). This current generates step and contact voltages, endangering people’s lives and the usefulness of machines.

Having said the above, in order to evaluate which is the grounding installation necessary in a case similar to the one described, we have to evaluate the short of the plant taking into account the failure value that is capable of supplying the transmission line.