The invention provides improved control of a power transmission system having a first group of measurement units in a first geographical area providing a first set of phasors and a second group of measurement units in a second geographical area providing a second set of phasors, where the phasors in the sets are generated at the same instant in time. In this system the power control device includes a phasor aligning unit that time aligns the first and second sets of phasors and a control unit that compares each set of phasors with a corresponding phasor number threshold, determines that a first control condition is fulfilled if each phasor number threshold has been exceeded and enables the provision of a common signal if the first control condition is fulfilled. The common signal is based on the obtained phasors in the first and second sets.