Monitoring of Power System Dynamics during Reconnection of 1st and 2nd UCTE Synchronous Zones

Reconnection of the 1st and the 2nd UCTE synchronous zones on 10th of October 2004 was not only a major event but also a technically very challenging task for all involved. Activities were coordinated by the UCTE Executive Team from the National Dispatching Centre (NDC) in Zagreb (Croatia). During the reconnection, power system dynamics was permanently monitored at the NDC in Zagreb using UCTE WAMS and Croatian system dynamics was monitored through a manually coordinated wide area measuring scheme (further: “HR-WAMS”) specially arranged for this occasion. Bus voltage frequencies and phasor angles representing dynamics of the 1st zone (at Mettlen, Switzerland) and the 2nd zone (at Ag. Stefanos, Greece) were observed through UCTE WAMS while relevant quantities at key substations in Croatian power system were monitored through HR-WAMS. System dynamics was monitored and recorded during synchronisation of the two zones (switching on of the 400 kV interconnection line Sandorfalva (H) – Arad (RO) ) as well as during reconnection of all other 400 kV tie lines between the two zones. A new lightly damped interarea mode with the period of about 4-5 seconds was observed after the reconnection, propagating through the Croatian power system and further into the UCTE system via the 400 kV line Melina (HR) – Divaca (SI). Detailed analysis of the measurements taken during reconnection, as well as analysis of some later recordings of spontaneously arising oscillations, has shown that there could be problems with the newly introduced interarea modes. Another aspect of interest is the use of WAMS measurements for validation of power system models. On this occasion simulation was done for the case of synchronisation of the two zones. The power system model included detailed representation of power systems of Croatia (HR) and Bosnia and Herzegovina (BiH) and appropriate dynamic models of the UCTE. Rather good match between simulation and measurement was achieved for the bus frequency at the 400 kV node Ernestinovo which represents the 1st zone dynamics, and for the bus frequencies at the 400 kV nodes Mladost (SCG) and Ag. Stefanos (GR) which represent the 2nd zone dynamics. These results are encouraging and WAMS measurements proved to be extremely useful in validation of the dynamic model while there is still much work to be done.