Impact of voltage phase angle changes on low-voltage ride-through performance of small scale hydro DG units

Computer simulations of a simplified radial 132, 66 and 22 kV system including a 5 MVA hydro power unit, has been performed in order to assess the hydro unit´s fault-ridethrough (FRT) capability, with special emphasis on the Critical Clearing Time (CCT) of the unit. The results show that there is a significant difference in CCT when the disturbance reflects a realistic fault in the system, i.e. a fault that reflects changes in both voltage magnitude and phase angle, compared with a disturbance for which only the change in voltage magnitude is taken into account. Since requirements regarding fault-ride-through (FRT) capability found in most of today´s national grid codes are defined via a specified transient voltage vs. time disturbance profile, the paper therefore suggests that these requirements should be extended to include also corresponding phase angle vs. time disturbance profiles.