PURPOSE: To make possible the superposition and impression of a voltage without the possibility of the saturation of a magnetic circuit by superposing and impressing the voltage from one DC power source to an AC high voltage system via a common grounding bus.

CONSTITUTION: The grounding lines of grounding apparatus (voltage transformers 3, 3', 4, 4', 9, grounding resistance 10 at the neutral point of a generator) for parallel operating high voltage buses 2, 2' and generating bus 7 are connected to the common grounding bus 28 insulated from the ground and are connected from points P, P' via terminals Q, Q' to the stationary side terminal of a switch 30 by lines 29, 29'. The change-over position A of the switch 30 grounds the bus 28 in the normal position. The position C is the position where the measuring voltage is impressed and a battery 14' is inserted between the bus and the ground when the change-over contact piece of the switch 30 is driven by an electromagnetic coil 31 and is moved to the position C. The voltage of said battery is then superposed and impressed through the bus 28 and further all the apparatus for grounding to the AC voltage of the high voltage system. The DC current flowing through the apparatus for grounding is only the current sharing respectively the total of the transient current charging the electrostatic capacity to the ground of the high voltage system and the current flowing in an insulating resistance to the ground. Since the value thereof is small, there is no possibility of inducing the saturation of the magnetic circuit.