The grid system leaving pier 13...

The UCTE Interim Report summarised here is the result of the first phase of its investigation. The Final Report is expected soon and promises a more comprehensive analysis of the sequence of events and recommendations for the improvement of procedures at UCTE and individual TSOs as well as of specific security standards at UCTE level.

On the night of 4 November 2006, at around 22:10, the UCTE interconnected grid was affected by a serious incident originating from the North German transmission grid that led to power supply disruptions for more than 15 millions European households and a splitting of the UCTE network into three islands.

The immediate and effective response of all TSOs (following UCTE security standards) prevented the event becoming a Europe-wide blackout, although it was still among the most severe disturbances Europe has experienced.

Immediately after the blackout, UCTE initiated the setting up of an Investigating Committee with the task of finding out the root causes and proposing recommendations ‘to avoid a repetition of such an event’. Its main purpose is to provide a transparent and complete explanation of events to UCTE members and to the general public. Its main tasks include investigating events and causes related to the disturbance, evaluating TSOs’ actions with respect to the UCTE Operational Handbook and the Multilateral Agreement in force from July 2005, examining their compliance with standards and application of temporary measures, assessing adequacy of those standards, and devising any necessary improvements.

UCTE report

Judging by the number of TSOs involved and the size of the frequency deviation, the November 4/5 event constituted the most severe disturbance in the history of UCTE. After the tripping of many HV lines, starting in north Germany, the UCTE grid was divided into three islands (west, NE and SE) resulting in significant power imbalances in each island. Those in the western area induced a severe frequency drop. In both under-frequency areas (west and SE), activating generation reserves and load shedding allowed the normal frequency to be restored in a relatively short time. In the over-frequency area (NE), the lack of control over generation units (fast reduction of schedules and automatic reconnection of wind generation) contributed to the deterioration of system conditions, namely long lasting over-frequency with severe line overloading.

Generally, the unco-ordinated operation of generation units (mainly wind and CHP) during the disturbance complicated the process of re-establishing normal conditions. After several unsuccessful attempts full resynchronisation was completed within 38 minutes of the system split. Although the individual TSOs did not have a complete and clear picture of events at any given moment, normal service in all European countries was re-established in less than two hours. However, the decentralised spread of responsibilities among TSOs demonstrated its efficiency: appropriate measures were taken against further deterioration, thus avoiding a blackout of the entire European continent.

Essential root causes

Non-fulfillment of the N-1 criterion. The evaluation of N-1 secure conditions was not based on results of an N-1 numerical analysis, nor on an analysis of possible changes in the system conditions during the following hours. The impact of the outage of the Conneforde-Diele line and the topology change in Landesbergen substation were not checked by E.ON Netz via a numerical simulation, only by an empirical evaluation. Preliminary security calculations confirm that the N-1 criterion was not fulfilled in the E.ON Netz grid and on some of its tie-lines to the neighbouring TSOs.

Inappropriate regional inter-TSO co-ordination. The initial planning for switching off the 380 kV Diele-Conneforde line foreseen on November 5 was duly prepared by the directly involved TSOs (E.ON Netz, RWE TSO and TenneT). However, a change in the timing of this switching manoeuvre was communicated by E.ON Netz to the other directly involved TSOs very late and was not checked to ensure the secure operation of the system. No specific attention was paid by E.ON Netz to the fact that the protection devices had different settings on each end of the Landesbergen-Wehrendorf line although this data was critical, given the very high flow on this line.

Further critical factors

Generator related issues During the disturbance, a significant number of generation units tripped due to the frequency drop in the west of the UCTE system. This certainly contributed to the deterioration of system conditions and to the delay in restoring normal conditions. In addition, most of the TSOs do not have access to the real time data of the power units connected to the distribution grids, which prevented them from performing a useful evaluation of system conditions. In the North East island, the uncontrolled reconnection of generation units induced very severe conditions.

Limited range of action available to dispatchers. German TSOs are required to take various grid related, market related, and other measures prescribed by the German Energy Law for the management of emergencies. The adequacy and effectiveness of such measures needs investigation.

TSO/DSO co-ordination during defence and restoration. In some control areas, DSOs without proper knowledge of the overall position of the UCTE system and without co-ordinating with their TSOs. This worsened the conditions. Actions taken by TSOs during the resynchronisation process were not coordinated in all cases.

Training of dispatchers to be improved. Two aspects have to be examined: procedural tools and inter-TSO co-ordination and consultation.