The core of a power transformer is not electrically connected to the winding but is magnetically coupled. This means that the core can become charged with a high voltage due to the electromagnetic coupling, which can create excessive heating in the core unless it is grounded. Grounding the core provides a path for this induced voltage to be safely directed to the ground, which helps to protect the transformer and reduce the risk of electrical fires and shocks to people who may come into contact with the transformer.
Without grounding, the transformer's core can experience high eddy current losses, which can reduce the efficiency of the transformer. The increased eddy currents can also cause the transformer's core to overheat, leading to unhealthy operation and potentially damaging the transformer. Failing to ground the core can also increase the risk of insulation failures, which can lead to accidents.
Overall, it is important to properly ground the core of a power transformer to ensure safe and efficient operation.