UCTE: Interim Report of the Investigation Committee on the 28 September 2003 blackout in Italy

Brussels – Grid operators of the five countries involved in the blackout (Austria,
France, Italy, Slovenia and Switzerland) released a joint Interim Report investigat-ing
the reasons why the sequence of events evolved from a single line trip in
Switzerland into the impossibility for the Italian system to operate separately from
the European network.
The Committee was set up within UCTE, the association of transmission system
operators for the coordination of the electricity interconnection in mainland
Europe. The blackout is the main incident UCTE had to face since its creation in 1951.

In the immediate aftermath of the 28 September 2003 blackout in Italy, Transmission
System Operators’ (TSO) executives of the five involved countries (Austria, France, Italy,
Slovenia and Switzerland) met within the framework of UCTE and decided to set up an
independent UCTE Investigation Committee that was given the mission to bring a trans-parent
and complete explanation of the blackout to the national and European
authorities and to the general community.

It was agreed that all required data would be provided by the operators of the five coun-tries
to the Committee that should operate in full transparency. The Committee, with the
full cooperation of these operators, comprised, apart from representatives of the involved
countries, experts from Belgium, Germany, the Netherlands and Spain.

The interim report gives a factual description of the sequence of events, followed by a
technical analysis and the Committee’s findings on the main causes of the incident. Sev-eral
issues are listed that will be further investigated.

It must be emphasized that the original function of the interconnected systems is to form
the backbone for the security of supply and to reach its required high level at reasonable
costs. To this aim the system has been developed in the past 50 years with a view to
assure mutual assistance between national subsystems including common use of re-serve
capacities and, to some extent, to optimize the use of energy resources by allow-ing
exchanges between these systems. Today’s market development with its high level
of cross-border exchanges was out of the scope of the original system design. It has led
the TSOs to operate the system close to its limits as allowed by the security criteria. The
blackout must be seen in this general context.

Nevertheless, the transmission system operators have in the last few years steadily im-proved
the capability of the existing infrastructure to allow cross border exchanges; by
using several measures such as, for example, computerised control and data acquisi-tion,
phase shifting transformers, coordination mechanisms and electronic data ex-change
between operators.

The sequence of events was triggered by a trip of the Swiss 380 kV line Mettlen-Lavorgo
at 03:01 caused by tree flashover. Several attempts to automatically re-close the line
were unsuccessful. A manual attempt at 03:08 failed as well.

Meanwhile, other lines had taken over the load of the tripped line, as is always the case
in similar situations. Due to its proximity, the other Swiss 380 kV line Sils-Soazza was
overloaded. This overload was acceptable in such emergency circumstances, according
to operational standards, for a short period. The allowable time period for this overload
was about 15 minutes according to calculations by the experts.

At 03:11, a phone conversation took place between the Swiss co-ordination centre of
ETRANS in Laufenburg and the Rome control centre of GRTN, the Italian transmission
system operator. The purpose of the call was to request from GRTN countermeasures
within the Italian system, in order to help relieving the overloads in Switzerland and bring
the system back to a safe state. In essence, the request was to reduce Italian imports by
300 MW, because Italy imported at this time up to 300 MW more than the agreed
schedule.

The reduction of the Italian import by about 300 MW was in effect 10 minutes after the
phone call, at 03:21, and returned Italy close to the agreed schedule.

This import reduction, together with some internal countermeasures taken within the
Swiss system, was insufficient to relieve the overloads.

At 03:25, the line Sils-Soazza also tripped after a tree flashover. This flashover was
probably caused by the sag in the line, due to overheating of the conductors.

Having lost two important lines, the then created overloads on the remaining lines in the
area became intolerable. By an almost simultaneous and automatic trip of the remaining
interconnectors towards Italy, the Italian system was isolated from the European network
about 12 seconds after the loss of the line Sils-Soazza.

During these 12 seconds of very high overloads, instability phenomena had started in
the affected area of the system. The result was a very low system voltage in northern
Italy and consequently, the trip of several generation plants in Italy.

Countermeasures were implemented within Italy in order to face a disconnection of the
country and sudden loss of the import, for example automatic shedding of parts of the
load. These measures were automatically activated, but, due to the loss of generation
plants, it was impossible for the Italian system to operate separately from the UCTE net-work.
About 2 minutes and 30 seconds after the disconnection of the country, the black-out
was an unavoidable fact.

The operation of the European interconnected electricity system is subject to security
and reliability standards set within the framework of the UCTE cooperation.

A main principle underlying these standards is, that the system must be operated in such
a way, that any single incident, for example the loss of a line, should not jeopardize the
security of the interconnected operation. This is called the N-1 rule.

This rule also states that in case of loss of N-1 security the system must not only with-stand
the situation, but it is supposed to return to the N-1 secure state as soon as possi-ble
to resist a possible new event.

It implies that countermeasures must be identified and prepared at each moment and for
each single incident, enabling the system to be brought back to a safe state when an
incident occurs.

The Committee examined the state of the system just before the occurrence of the first
event and the countermeasures that had been identified and prepared to tackle the loss
of the Mettlen-Lavorgo line. The Committee’s finding in this respect is that the system
was complying with the N-1 rule at this time, ETRANS taking into account countermea-sures
available outside Switzerland.

In this specific case, the appropriate countermeasure for the loss of the line was the
shutting down of the pumps in the pump storage plants in Italy, which are located close
to the connection points of the Swiss tie-lines to Italy and therefore have a high influence
on their loading. The pumping load in Italy amounted to about 3500 MW.

Shutting down the pumps in mutual support, when requested under emergency condi-tions
by ETRANS, is operational practice, although there is no official procedure or spe-cial
agreement between ETRANS and GRTN on this subject.

The Committee identified 4 main reasons for the fact that things did not go as foreseen.

1. Unsuccessful re-closing of the Mettlen-Lavorgo line because of a too high
   phase angle difference

Due to the high loads on the remaining lines, an automatic device, aiming at protect-ing
the equipment, blocked, according to its design settings, the possibility of restor-ing
the line back into service.

2. Lacking a sense of urgency regarding the Sils-Soazza line overload and call for
   inadequate countermeasures in Italy

The operators were unaware of the fact that the overload on Sils-Soazza was only
sustainable for about 15 minutes. A single phone call by ETRANS took place 10
minutes after the trip of the first line. ETRANS asked for the imports to be decreased
by 300 MW. This measure was completed by GRTN within 10 more minutes. Even
together with the Swiss internal countermeasures, it was insufficient to relieve the
overloads.

3. Angle instability and voltage collapse in Italy.

As explained in the sequence of events, this was the reason why the Italian system
collapsed after its separation from the UCTE system. It was not the cause of the ori-gin
of the event.

4. Right-of-way maintenance practices.

Tree cutting, to maintain safe distances regarding flashover, is subject to national
regulation. Therefore, the Committee did not examine these practices.

Several issues are listed in the report, which will be examined in the next stage.
Apart from the measures to be undertaken as a result of the lessons drawn from the
blackout, further investigation will go into the issues dealt with in this report.
Attention will also be given to the not yet fully investigated phases of the blackout: the
period between disconnection and blackout in Italy and the behavior of the UCTE sys-tem
outside Italy.

Moreover, lessons learnt and action to be undertaken after the blackout will be part of
the already ongoing work of the various working groups of UCTE.

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