The dam and the hydro power plant Kozjak are located on the river Treska, about 25 km upstream of the river inflow in the river Vardar and about 16 km upstream of HPP Matka, nearby Skopje and are part of the first level in the cascade of dams and hydropower plants of the river Treska.

With the construction of this dam large accumulation is formed and as front accumulation it provides regulated water for the downstream hydropower plants as well as water for irrigating of Skopsko Pole after the construction of the hydro meliorate system Skopsko Pole and the technical water for the industry in Skopje. One of the basic allocations of this accumulation is at the same time retention of the flood wave of river Treska in times of great waters where there is a provided area of 100.000.000 m³ water.

The first ideas for construction of this HPP arise after the flood of Skopje in 1962, which were later elaborated in the Study for regulation and control of river Vardar, elaborated by the company Norconsult A.C. – Oslo, Norway and financed by the United Nations. The accumulation has volume of 550.000.000 m³ water, out of which useful volume amounts to 260.000.000 m³ water.

With the completion of one of the most complex projects  – realization of the second phase of the rehabilitation of hydropower plants, installation of the new equipment for monitoring of the dams “Mavrovo”, “Globochica”, “Spilje”, “Tikves”, “Kozjak “and” Sveta Petka” was finished. Within the project additional equipment was installed, for the automation of electricity production process and for auscultation and observing the conditions, stability and safety of dams and associated facilities.

History
Studies and Preliminary Designs from 1962
Main project from 1989.
Start of construction in July 1994.
Commissioning of HPP Кozjak in 2004.
The total average annual inflows are 661.000.000 m³ water, while the average flow is 20,95 m³/s.

Inflow system
The inflow tunnel is with circular intersection and length of 460 m, and diameter of 5,00 m. The sill of the entrance object is 420,00 maSL.

Dam and accumulation
HPP Kozjak has rock filled dam with height of 126,1 meter, which forms accumulation with total volume of 550.000.000 m³ water, out of which the useful volume is 260.000.000 m³ water. The maximal operation level of the accumulation is 459,00 maSL, and the minimal level is 432,00 maSL.

Hydropower plant
Hydropower plant Kozjak is with installed capacity of 82 MW and annual generation of 130 GWh. The power house is located in the base of the dam. In the power house there are two units equipped with Francis turbines with vertical axis, as well as three – phase synchronous generators. The transformers are located outside the power house. The location of the plant 110 kV is external.