On 18 May 2025, an overvoltage episode occurred in the power system of North Macedonia, leading to the separation of the 400 kV and 110 kV levels. This resulted in a loss of stability, power supply interruptions, and a total blackout of the entire 110 kV transmission network, to which domestic and industrial consumers in the country are connected. The 400 kV transmission network remained in operation throughout the duration of the event. System restoration by the operator MEPSO was completed three hours later on the same day.

The incident is classified as a Scale 3 (highest level) event according to the Incident Classification Scale (ICS) methodology, as it meets criterion OB3 for loss of load at the 110 kV level. In accordance with Article 15 of Regulation (EU) 2017/1485 (SO GL), ENTSO-E formed an Expert Panel consisting of 18 experts—nine from ACER and national regulators, and nine from European TSOs and ENTSO-E. The panel was led by an expert from 50Hertz (Germany), an operator not directly affected by the incident.

The final report builds upon the report of 11 November 2025, which traced the factual development of the incident, and presents an analysis of the root causes, conclusions, and recommendations aimed at preventing a recurrence of similar events in South-East Europe (SEE).

Pre-Incident Situation

From February to May 2025, a continuous and systematic increase in voltage levels was observed in MEPSO’s control area, particularly during night hours of low consumption. The upper limit for normal operation (420 kV) was regularly exceeded, and from mid-April onwards, values above the short-term limit of 440 kV were recorded almost daily. On the day of the incident, voltages reached nearly 450 kV in several substations.

This condition was not isolated to North Macedonia but reflected a regional phenomenon in SEE, driven by low night-time consumption, high capacitive reactive generation from transmission and distribution networks, limited active power transfer, insufficient reactive power compensation means, and reactive energy flows through interconnections from neighbouring areas with higher voltages.

Coordinated security analysis performed by the regional coordination centres SEleNe CC and SCC did not identify any violations of the loading limits of elements—the grid was assessed as N-1 secure regarding active power flows. Simultaneously, however, the analysis predicted that all 400 kV nodes in the MEPSO area would exceed permissible operational voltage limits. MEPSO did not provide an Individual Grid Model (IGM) for 18 May 2025 due to resource and process constraints, which forced SCC to trigger a backup procedure and use a model from 13 May 2025 to prepare the Common Grid Model (CGM).

Additionally, in April 2025, MEPSO suffered a UPS system failure, resulting in the SCADA information system remaining inaccessible at the time of the incident, despite efforts by the provider and the operator to restore it.

Development of the Incident

Overvoltage in the 400 kV network activated the overvoltage protection of the 400/110 kV transformers. At 02:26, they tripped at the Bitola 2 and Skopje 5 substations, being restored to operation a few minutes later. Repeated trips followed at 02:59 in Bitola 2 and at 03:34 in Skopje 5, with reconnection attempts proving unsuccessful. Following the series of transformer trips, the final separation of the 400 kV and 110 kV networks occurred at 04:59, followed by a total blackout at the 110 kV level.

The total loss of generation amounted to 313 MW, and the loss of load was approximately 485 MW. The Kriva Palanka substation remained energised solely thanks to the interconnection with Bulgaria. The balancing market and imbalance settlement were suspended between 05:00 and 07:00, with a total imbalance of 250 MWh in the 110 kV network (05:00–06:00) and 140 MWh in the 400 kV network (06:00–07:00).

The incident directly affected only ESO EAD—the Bulgarian transmission system remained in an alert state for eight hours due to high voltage levels in the western part of the country. Other TSOs in the Continental Europe synchronous area did not report significant disturbances, though voltage levels exceeding 420 kV were observed across all neighbouring operators on the day of the incident, confirming the regional nature of the problem.

Restoration and Communication

MEPSO triggered the restoration plan in accordance with Regulation (EU) 2017/2196 (NC ER) and national operational procedures. During restoration, it was found that reactive power was flowing into MEPSO’s 400 kV network from the neighbouring systems of EMS (Serbia) and KOSTT (Kosovo), further increasing voltage levels. To stabilise the situation, two 400 kV interconnections—Skopje 5 (MEPSO) – Ferizaj (KOSTT) and Štip (MEPSO) – Vranje (EMS)—were disconnected.

During the incident, communication with the regional coordination centre was not established, and the ENTSO-E Awareness System (EAS) and the corresponding procedure were not used to inform European TSOs. As no frequency deviation with an impact on the synchronous area was recorded, no contact was made with the Synchronous Area Monitor (SAM). Communication between MEPSO and neighbouring TSOs was conducted via mobile phones—a circumstance the Expert Panel noted as an indication of the need to strengthen effective communication during incidents.

Root Causes and Contributing Factors

The Expert Panel identified five primary causes for the incident and one contributing factor:

1. System operation above defined voltage limits – the 400 kV network regularly operated at values above 420 kV during low-load night hours without timely structural management of the overvoltage.

2. Reduced awareness of overvoltage risks in operational planning – available coordinated assessments did not signal violations of loading regimes, and the exceeding of voltage limits remained the sole indicator of developing system vulnerability.

3. Insufficient reactive power reserves with appropriate activation times and limited reactive support from system users (including producers).

4. Lack of effective preventive measures against high voltage levels and limited means for voltage regulation—temporary operational measures were insufficient to handle the structural nature of the problem.

5. Insufficient capacity for voltage regulation due to a lack of installed assets – there are no shunt reactors or other specialised reactive power compensation devices in the MEPSO area.

The overvoltage protection of the 400/110 kV transformers was cited as a contributing factor—upon reaching thresholds, it triggered successive trips that led to the final separation between the two network levels and the subsequent blackout.

Recommendations of the Expert Panel

The recommendations are structured into three areas:

Strengthening of previously adopted recommendations from the 2024 SEE incident

• Recommendation 4 (revised): Development of key indicators to assess the risk of reduced voltage stability and rapid voltage changes.

  • Delegated to: ENTSO-E system stability specialists.

  • Priority: Low.

• Recommendation 7 (revised): ENTSO-E to prepare a best-practice guide for voltage maintenance measures.

  • TSOs to ensure that plans for sufficient MVAr maintenance are up to date and, if necessary, integrated into TYNDP and national network development plans.

  • Priority: High.

New recommendations for the SEE region

• R250518_1: Systematic monitoring of operational voltage limit violations between 400 kV and 110 kV levels in the SEE region.

  • Delegated to: ENTSO-E and regional TSOs.

  • Priority: High.

• R250518_2: Development of an action plan to improve the situation regarding overvoltages above 420 kV in SEE. The plan should include regular reporting of voltage incidents under the ICS methodology, potential updates to the methodology itself, a review of overvoltage protection philosophy, short-, medium-, and long-term operational measures, appropriate voltage regulation modes for inverters (moving away from fixed power factors for RES), assessment of structural investment measures, evaluation of the role of RES and low-demand modes, and a review of bilateral operational agreements between TSOs.

  • Priority: High.

• R250518_3: Commitment from SEE TSOs regarding operational voltage regulation measures, including the establishment of a sustainable working format for coordination.

  • Priority: High.

Recommendations to MEPSO for the System Defence Plan

• R250518_4: Expansion of analysed scenarios in the defence plan to cover operation under low consumption, high RES share, reduced synchronous generation, and low inertia.

  • Priority: Medium.

• R250518_5: Operationalisation of protection and restoration procedures by accounting for Load Frequency Dependent Disconnection (LFDD) at the distribution network level.

  • Priority: Medium.

• R250518_6: Clear allocation of responsibilities, signalling requirements, and feedback between distribution operators and the National Control Centre.

  • Priority: Medium.

• R250518_7: Development of a dedicated system-level protection strategy, separate from individual asset protection, with coordinated settings to maintain stable island operation of the 110 kV network.

  • Priority: Medium.

Conclusions and Significance for the Region

The Expert Panel concludes that the incident was the result of a structural phenomenon characteristic of the entire SEE region, rather than an isolated technical problem in North Macedonia. High voltages during night-time hours, amid low consumption and a growing share of RES, are transitioning from an exception to a regular operational mode that existing system defence plans—focused primarily on low-frequency and system separation scenarios—do not sufficiently cover.

A sustainable solution requires simultaneous action in three areas:

1. Coordinated regional measures between SEE TSOs with support from regional coordination centres and ENTSO-E.

2. Targeted investments in specialised assets for reactive power compensation (shunt reactors, static compensators).

3. Regulatory changes ensuring systematic activation of producers’ capacity for voltage regulation, including inverter-based RES.

For the Bulgarian energy sector and ESO, these conclusions are particularly relevant. Beyond Bulgaria being the only neighbouring country whose system was affected, its western 400 kV region daily faces elevated voltage levels, which will require both operational and investment attention in the medium term.

The full text of the report is available here.