A method and apparatus is described for generating a synchronizing signal for a firing-circuit subassembly to trigger a power converter valve of a controlled series compensator. According to the invention, a sinusoidal substitute variable is provided, which can be either a measured line current or the sum of a capacitor current and a valve current, such that the zero crossings of said substitute variable do not vary. This substitute variable is modified in terms of its phase position so that a synchronizing signal that corresponds to the phase position of the capacitor voltage of the series capacitor of the controlled series compensator is made available to the firing-circuit subassembly. The result, even in the bypassed state, is a sinusoidal synchronizing signal whose zero crossings do not vary, so that operating stability problems no longer occur in the transient state during operation close to resonance.