Calculation Of The Windings Short-Circuit Force For Three-Winding Power Transformer

In the short-circuit process of large power transformer, poor mechanicalstrength of windings is one of the main factors that causes many short-circuitaccidents, so accurately calculation electric power force and optimization insulationdesign can effectively improve anti-short circuit ablility of transformer.At present, the short-circuit characteristics of double-winding is studied bymany scholars, but different vlotage levels of transmissions are connected by three-winding transformer often. For the transformer with “LV-MV-HV-TV” structure,generally, when the LV winding and HV winding supply power, short circuitoccurred in MV winding, the system total short-circuit impedance is minimum, andthe short circuit current is largest. So winding short-circuit characteristics are studiedon a180MVA/220kV three-winding power transformer when LV winding and HVwinding supply power, short circuit occurred in MV winding. According to theprinciple of transformer short-circuit experiment, the short-circuit impedance value iscalculated under “LV-MV”,“LV-HV”,“MV-HV” operation with three different taps,and then the equivalent short-circuit impedance of each windings is got. Accordingto the magnetic potential balance principle and equivalent short-circuit impedance ofeach windings, the current distribution ratio of LV winding and HV winding is got indifferent taps, the leakage magnetic field is redistributed finally. The two2D axis-symmetrical FEM model of “LV-MV”,“MV-HV” is presented with the field-circuitcoupling method, which considers oil duct size, disk duty ratio. The winding currentdistribution, electric density distribution, magnetic density distribution and short-circuit electric magnetic force distribution are got according to the established thefinite element model.In the vibration process of transformer winding, the characteristics andamplitude of dynamic force are largely depend on the winding natural frequency ofaxial vibration. The axial vibration process of winding short-circuit is equivalent to“quality-spring-damping”system. Applying axial short-circuit clectromagnetic forceto disc element, the influence of disk vibration displacement by different prestress is studied, each transformer winding suitable axial prestress should be the one that thedisplacement of the winding top part disc decreases gradually after the fisrt peak ofaxial short-circuit force. Then the vibration diaplacement, vibration velocity,vibration acceleration and force are got according to each winding prestress.Finally, each winding mechanical strength and stability are checked accordingto radial force and axial force of each winding disks.