<p>An interconnecting apparatus for electric power systems wherein two electric power systems are connected to each other with a series transformer and the secondary winding of the series transformer is energized with a compensating voltage whose value and polarity are such that it cancels the voltage drop developed across said transformer when said electric power systems are normally operating. The compensating voltage is derived from the secondary winding of an exciting transformer inserted between the central point of the series transformer and the earth. The secondary winding of the series transformer is connected in delta and the secondary winding of the exciting transformer is star-connected. If it is necessary, the exciting transformer may be provided with a tertiary winding to control the voltages of the respective power systems by the voltage of this tertiary winding.</p>
