At the Edge of Voltage Collapse in Slovenian Power System After Outage of NPP Krško and 400 kV Node Tumbri

A major disturbance that resulted in the tripping of the 400 kV line Tumbri (HR) – Hévíz (H) and the entire 400 kV node Tumbri in the Croatian system occurred on 27 August 2003 and almost led to a voltage collapse in that area. The disturbance was initiated by an accidental outage of a generator at NPP Krško (SLO) due to a human error during regular maintenance work. Its further development was the consequence of a hidden fault of the numeric protection at the 400 kV switchyard Tumbri. In the specific system operating conditions (network configuration and generator scheduling) at that moment, the systems of Slovenia and Austria in the central part of the UCTE system were brought near voltage collapse. At the same time the operational security level of the power systems of Croatia and BiH, then at the perimeter of the UCTE system, was also decreased. The sequence of events is described and discussed in the paper. Recordings of frequency and active power in representative transmission lines, taken at NDC Zagreb, are also given. A simulation analysis was performed first for the real disturbance scenario and then for the hypothetical scenario in which there was no failure of the protection at Tumbri substation and consequently the 400 kV node Tumbri would have remained in operation. The simulations showed that the loss of a NPP Krško generator would have only a local impact and would be barely noticeable at the UCTE level had the 400 kV node Tumbri remained in operation. Simulation results are in accordance with information obtained from the Slovenian power system where severely depressed voltages were recorded, particularly in the area around NPP Krško. It was also observed that the Slovenian system was acting as a reactive power sink while active power was flowing in the opposite direction, i.e. towards Croatia. Assessment of Croatian system security during the disturbance with respect to voltage stability, frequency and angle stability and congestion criteria was done on the basis of measurements and simulation results. The conclusion was that Croatian system security with respect to all the above mentioned criteria was preserved but it must be stressed that the initial system operating conditions were rather favorable from the stability point of view.