The 8 January ‘system separation’

Figure 1. Decoupling of two busbars in the Ernestinovo substation

The initial event was the tripping of a 400 kV busbar coupler in the Ernestinovo substation (Croatia) by overcurrent protection at 14:04:25.9. This resulted in the decoupling of the two busbars in the Ernestinovo substation, which in turn separated north-west and south-east power flows in this substation. As shown in Figure 1, north-west bound lines, which remained connected to one busbar, connect Ernestinovo to Zerjavinec (Croatia) and Pecs (Hungary), while south-east bound lines, which remained connected to the other busbar, connect Ernestinovo to Ugljevik (Bosnia- Herzegovina) and Sremska Mitrovica (Serbia).

The separation of flows in the Ernestinovo substation led to the shifting of electric power flows to neighbouring lines, which were subsequently overloaded. At 14:04:48.9, the Subotica – Novi Sad (Serbia) line tripped due to overcurrent protection. This was followed by the further tripping of lines due to distance protection, as shown in Figure 2, leading eventually to system separation into two parts, at 14:05:08.6.

The two parts of the continental Europe synchronous area post separation are shown in Figure 3.

The system separation resulted in a deficit of power (about 6.3 GW) in the north-west area and a surplus of power (about 6.3 GW) in the south-east area, resulting in turn in a frequency decrease in the north-west area and a frequency increase in the south-east area. See Figure 4.

At 14:05 CET, the frequency in the north- west area decreased to 49.74 Hz within around 15 seconds before quickly reaching a steady state value of approximately 49.84 Hz. At the same time, the frequency in the south- east area increased to 50.6 Hz before settling at a steady state frequency between 50.2 Hz and 50.3 Hz.

Due to the low frequency in the north-west area, contracted interruptible services in France and Italy (in total around 1.7 GW) were disconnected in order to reduce the frequency deviation. These services are provided by large customers who are contracted by the respective transmission system operators (TSOs) to be disconnected if frequency drops under a certain threshold. In addition, 420 MW and 60 MW of supportive power were automatically activated from the Nordic and Great Britain synchronous areas respectively. These countermeasures ensured that already at 14:09 CET the frequency deviation from the nominal value of 50 Hz had been reduced to around 0.1 Hz in the north-west area.

In order to reduce the high frequency in the south-east area, automatic and manual countermeasures were activated, including a reduction in power generation (eg, automatic disconnection of a 975 MW generator in Turkey at 14:04:57). As a consequence, the frequency in the south-east area returned to 50.2 Hz at 14:29 CET and remained within control limits (49.8 and 50.2 Hz) until the resynchronisation of the two separated areas took place at 15:07:31.6 CET.

Between 14:30 CET and 15:06 CET the frequency in the south-east area was fluctuating between 49.9 Hz and 50.2 Hz due to the relatively small size of this area, where also several generating units were disconnected. During this period, the frequency in the north- west area fluctuated far less and remained close to the nominal value, due to the area’s large size.

The automatic response and the co-ordinated actions taken by the TSOs in continental Europe ensured that the situation was quickly restored. The contracted interruptible services in Italy and in France were reconnected at 14:47 CET and 14:48 CET, respectively, prior to the resynchronisation of the north-west and south- east areas at 15:08 CET.

Lessons from 2006

A separation of the synchronous area with a much larger disturbance and impacts on customers took place in Continental Europe on 4 November 2006. This event was extensively analysed and led to a number of substantial developments, like the European Awareness System (EAS) which is a platform allowing TSOs to exchange operational information in real time, enabling them to react immediately in case of unusual system conditions. The TSOs are therefore well prepared to co- ordinate and manage such events and limit the consequences. This preparedness and continuous observation of the system frequency allowed resynchronisation of the two separated areas in a very short time on 8 January.

In continental Europe, procedures are thus in place to avoid system disturbances and especially large frequency deviations with the risk of unco-ordinated disconnection of customers or generation. The TSOs Amprion (Germany) and Swissgrid (Switzerland) are responsible for these procedures in their joint role as synchronous area monitor (SAM) in continental Europe. The SAM continuously monitors the system frequency. In case of large frequency deviations, they inform all TSOs via the European Awareness System (EAS) and launch an extraordinary procedure for frequency deviations to co-ordinate countermeasures in a fast and effective manner in order to stabilise the system.

One step of this procedure is a telephone conference involving Amprion, Swissgrid, RTE (France), Terna (Italy) and REE (Spain). This teleconference took place at 14:09 CET on 8 January 2021. In the telephone conference, the situation was evaluated and information was provided about countermeasures already activated. The TSOs of the north-west and south- east areas also co-ordinated the actions needed for reconnection and restoration of the single synchronous area in continental Europe.

The 8 January event resulted in about 70 MW of [non-contracted] end user customer disconnections in the north-east area and 163 MW in the south-east area. But due to the high resilience of the interconnected network and the rapid response of European TSOs, the security of operation and electricity supply was not endangered further.

An important contribution to stabilising the system was delivered by the previously contracted interruptible services, which were activated in France and Italy, as already noted. Such contracts, pre-agreed with customers, allow the TSO to temporarily and automatically reduce electricity consumption depending on the real time status of the electric power system.

Root causes

According to Article 15 of European Commission regulation (EU) 2017/1485, for a Scale 2 event such as that of 8 January 2021, an expert investigation panel must be set up composed of TSO experts and regulators. The panel will produce a report describing in detail the sequence of events, root causes and any actions needed to contribute to preventing similar incidents in the future.