Method of controlling a regulating transformer with a settable translation ratio, which is switched between a first alternating current mains and a second alternating current mains, wherein - the phasor data of phasors of the first and/or second alternating current mains (11, 12) are detected (a); - the phase data of phasors of the first and/or second alternating current mains (11, 12) are detected at the regulating transformer; - an equivalent circuit diagram with equivalent circuit diagram parameters is determined for the first alternating current mains (c); - a load model with load model parameters is determined for the second alternating current mains (d); - the equivalent circuit diagram parameters and the load model parameters are determined from the phasor data (e); - when switching over to a desired translation ratio is to take place (b) - a working point (A') of the second alternating current mains is predicted for the desired translation ratio (f); - a stability criterion in the second alternating current mains is checked for the predicted working point (A') (g); - switching over to the desired translation ratio is carried out if the stability criterion is fulfilled (h), but otherwise not; wherein - the stability margin is checked in that - a stability reserve ΔP is determined for the current state of the second alternating current mains; - for the predicted working point (A') it is checked whether P G − P ′ > Δ ⁢ P is applicable; - P G is the limit effective power of the P-U plot; - P' is the effective power of the predicted working point (A'); - as stability margin there is selected Δ ⁢ P = P G − P H; - P H is the Hopf effective power of the P-U plot. - the phasor data of phasors of the first and/or second alternating current mains (11, 12) are detected (a); - the phase data of phasors of the first and/or second alternating current mains (11, 12) are detected at the regulating transformer; - an equivalent circuit diagram with equivalent circuit diagram parameters is determined for the first alternating current mains (c); - a load model with load model parameters is determined for the second alternating current mains (d); - the equivalent circuit diagram parameters and the load model parameters are determined from the phasor data (e); - when switching over to a desired translation ratio is to take place (b) - a working point (A') of the second alternating current mains is predicted for the desired translation ratio (f); - a stability criterion in the second alternating current mains is checked for the predicted working point (A') (g); - switching over to the desired translation ratio is carried out if the stability criterion is fulfilled (h), but otherwise not; wherein - a working point (A') of the second alternating current mains is predicted for the desired translation ratio (f); - a stability criterion in the second alternating current mains is checked for the predicted working point (A') (g); - switching over to the desired translation ratio is carried out if the stability criterion is fulfilled (h), but otherwise not; wherein - the stability margin is checked in that - a stability reserve ΔP is determined for the current state of the second alternating current mains; - for the predicted working point (A') it is checked whether P G − P ′ > Δ ⁢ P is applicable; - P G is the limit effective power of the P-U plot; - P' is the effective power of the predicted working point (A'); - as stability margin there is selected Δ ⁢ P = P G − P H; - P H is the Hopf effective power of the P-U plot. - a stability reserve ΔP is determined for the current state of the second alternating current mains; - for the predicted working point (A') it is checked whether P G − P ′ > Δ ⁢ P is applicable; - P G is the limit effective power of the P-U plot; - P' is the effective power of the predicted working point (A'); - as stability margin there is selected Δ ⁢ P = P G − P H; - P H is the Hopf effective power of the P-U plot. - P H is the Hopf effective power of the P-U plot.