Results of Investigations

Analysis indicates 2 root causes and 6 critical factors.

Root Causes

• N-1 criterion security rule
  Switching off the 380kV Diele - Conneforde line resulted in non N-1 secure conditions in the E.ON Netz grid and on some tie-lines to neighboring TSOs. E.ON Netz relied on an empirical evaluation and did not perform a numerical analysis of N-1 criteria.

E.ON Netz coupled the busbars in the Landesbergen substation in an effort to reduce the loading on the Landesbergen-Wehrendorf line. This was done without numerical analysis of the impact of this change on the power flow. The result was contrary to E.ON Netz's empirical assessment. It caused the line to trip triggering the cascade tripping that split the grid.

• Inter TSO co-ordination
  Relevant neighboring TSOs checked N-1 compliance with the proposed 380kV Conneforde Diele line switching during the planning stage but a late switching change was not communicated in a timely manner by E.ON Netz to other TSOs.  Despite the fact that the network was highly loaded, E.ON Netz did not prepare efficient remedial action to keep a minimum safety margin and prevent possible power flow increases.

Prior to the triggering event at the Landesbergen substation E.ON Netz did not fully coordinated with TSOs. No account was taken of different protection settings on either side of the Landesbergen-Wehrendorf line (i.e. on E.ON Netz and RWE TSO systems).

Critical Factors

• Generation related issues
  A significant amount of generation (wind power, CHP) that tripped was connected to the distribution grid. Most TSOs do not have the real time data for these units. Frequency restoration requires sufficient means for rescheduling generation in individual control areas. Lack of control over generation units during the disturbance complicated the process of re-establishing normal conditions and was key to long lasting frequency deviations in the North-East and for re-synchronization failures.

• Range of possible actions for the dispatchers to handle grid congestions
  A limited number of measures are available to TSOs to manage emergency situations while also ensuring compliance with national regulations, market requirements and internal procedures. The adequacy of these measures needs further scrutiny.

• Defense and restoration plans
  In some control areas, the DSOs started the re-energization of customers without proper knowledge of the overall situation in the UCTE system and without coordination with TSOs thereby hampering attempts to restore normal system conditions.

• Resynchronization
  Actions taken by TSOs during the resynchronization process were not sufficiently coordinated. There were several unsuccessful attempts to resynchronize with only a partial view of the whole grid. Protection devices performed correctly and prevented further negative consequences.

• Training of dispatchers
  The accuracy and completeness of dispatchers training was considered particularly in relation to (i) procedures and tools; and (ii) inter-TSO co-ordination under normal and emergency conditions. It was noted that dispatchers are not always trained on incidents that originate in external networks but affect their TSO network. It is not common practice to have joint simulation training with neighboring TSOs although there are discussions on procedures and exchanges of experience.

• Communication among TSOs
  The communication among UCTE TSOs was as expected in an emergency situation given the current framework of co-operation, the scope of responsibilities of individual TSOs and on-line data available at individual control centers. However information about the actual state of the UCTE system after splitting was not quickly available to all TSOs. The lack of information made it difficult for dispatchers to quickly and efficiently identify the source of the problem.