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NEK has made significant progress on key stages of the restoration of Chaira pumped-storage hydropower plant

National Electricity Company (NEK) has made significant progress on key stages in the process of restoring hydropower units 1 and 3 (U1 and U3) of Chaira PSHPP – the largest pumped storage hydropower plant in the Balkans. Two teams of highly qualified engineers from companies that are global leaders in the field continue to work in parallel on the restoration of U1 and U3. The entire expert capacity of NEK is involved in the process. With the implementation of the rehabilitation activities on U1 and U3, NEK is implementing its strategy for the gradual complete restoration of Chaira PSHPP.

A specially developed innovative solution has been applied to U3, which has proven its effectiveness in the successful restoration of U2 of Chaira PSHPP. The activities related to the rehabilitation of U3 turbine have been carried out as scheduled, and the final phase of generator installation is currently underway. Commissioning and start-up tests of the hydro unit are planned to begin at the end of March.

At the end of 2025, U1 received a positive opinion from the original equipment manufacturer, the Japanese corporation Toshiba, on the feasibility of repairing the hydro unit. Preparatory activities for the start of rehabilitation are currently underway.

Regarding the public procurement for the design and delivery of completely new equipment for U1 and U4, by the deadline for submission of bids – 9 January 2026, no proposals were received, despite the extended deadlines and the company's efforts to attract potential contractors.

In this regard, NEK is taking follow-up actions aimed at ensuring the implementation of the project by upgrading the initial plan and changing the approach in order to achieve a sustainable and long-term solution.

It should be noted that at the end of 2024, NEK successfully restored U2 of Chaira PSHPP, which last year operated smoothly for 1,203 hours in generator mode and 1,570 hours in pump mode, generating 222,575 MWh of electricity. In this way, the unit contributes significantly to the effective balancing and improvement of the security and stability of the country's electric power system.

Information as of 9 January 2026, on the condition of the dams and facilities managed by NEK EAD in the Arda cascade.

At 12:00 p.m. on 9 January 2026, the inflow to the Arda cascade had already decreased tenfold and amounted to 131 cubic meters per second. A water volume of 94 cubic meters per second passes through the spillway. The total volume of the high wave passing through the reservoir was 100 million cubic meters, which is more than the total usable volume of the reservoir, i.e., it is not possible to regulate the incoming water quantities without overflowing. During the high wave period, Ivailovgrad HPP operates around the clock.

Arda Cascade consists of Kardzhali, Studen Kladenets, and Ivailovgrad reservoirs. As of 9 January 2025, the cascade is operational under subsiding complex meteorological conditions, with high storage volumes and a passing high wave through the Ivailovgrad reservoir.

Arda River is characterized by significant fluctuations in discharge and high winter flows, requiring increased attention to dam management in view of the frequent passage of high waves through the cascade and the associated safe overflow through the spillways.

The volumes stored in the reservoirs are as follows:

1. Kardzhali Reservoir – 87% of its total volume
2. Studen Kladenets Reservoir – 94% of its total volume
3. Ivailovgrad Reservoir – full capacity with water overflowing through the spillway.

On 7 January 2025, a sharp increase in the inflow to the Ivailovgrad reservoir was observed as a result of intense torrential rains in the Krumovitsa River basin. After analyzing the data, a procedure was initiated to transfer the dam under high wave conditions. The necessary steps were taken in a timely manner to notify all institutions, agencies, and local authorities involved in the disclosure process. At 22:00 that same evening, a peak high wave of 1,156 cubic meters per second was recorded. With this volume of water, the reservoir quickly fills to capacity and at midnight begins to overflow through its relief facilities. The operational staff on duty at NEK EAD begins reporting information on the condition of the dam every two hours, around the clock.

In conclusion, Ivailovgrad dam fulfilled its purpose and played an important role in controlling the torrential nature of the Arda River, reducing the peak of a high wave of 1,100 cubic meters per second by about 70% to 330 cubic meters per second released downstream of the dam.

National Electricity Company (Natsionalna Elektricheska Kompania) approaches the water management of its multipurpose and significant dams with a high level of responsibility.