Off-line analysis of the load response during large voltage variations

The load response during and after a severe fault is critical for transient stability. This risk increases with the presence of induction motors in the load area. This paper provides analysis of the load voltage characteristic under large voltage variations. Field measurements from a noneffectively earthed system, from phase-to-phase faults at the 50 kV-level in Sweden, have been used. The time-varying load parameters, resistance R(t) and reactance X(t), have been determined from measurements of phase-voltages and currents. Estimates of the active, reactive and apparent power have been calculated. The results show that commonly used models are inaccurate during large voltage variations. The load uncertainty critically increases with the size of the voltage changes since its nonlinear behavior accentuates for more system stressing conditions. This fact is more noticeable for the reactive load characteristic.