A method for voltage stabilization of an electrical power network system including a producing power network system side, a consuming power network side including a power load, a power transmission line with an impedance Z LN , a transformer and an on-line tap changer added to the transformer. The impedance of the line is measured in case of dynamic instabilities. A transformer ratio n is controlled by changing a voltage reference V ref of the on-line tap changer. The voltage reference is changed according to a feed forward compensation from the impedance of the line.
1. A method for voltage stabilization of an electrical power network system comprising a producing power network system side and a consuming power network side comprising a transformer, a power transmission line with an impedance (ZLN) connected to a primary side of the transformer, a power load connected to a secondary side of the transformer, and an on-line tap changer added to the transformer, wherein a transformer ratio n is controlled through the on-line tap changer trying to keep the voltage (V2) on the secondary side of the transformer at a voltage reference (Vref) the method comprising: measuring the impedance of the line in case of dynamic instabilities; and changing the controlling a transformer ratio n by changing a voltage reference (Vref) of the on-line tap changer, according to a feed forward compensation from the impedance of the line.
2. The method according to claim 1 , wherein the feed forward compensation drives the power network system to a stable equilibrium point in a stable region, and wherein the stable region lies below a loci for maximum power transfer n 2 Y LD Z LN =1, where Y LD is power load admittance, Z LN is transmission line impedance and n is the transformer ratio.
3. The method according to claim 1 , wherein the feed forward compensation is provided by a first order filter H ff (s)=sT d /(sT+1), where T and T d are tuning parameters.
4. The method according to claim 1 , wherein a feedback controller is provided according to an equation Vfb=−max (0,a(n 2 Y LD −1/Z LN ), where n is the transformer rato, Y LD is power load admittance, Z LN is transmission line impedance and a is a tuning parameter that is influencing a region of attraction of an equilibrium point.