Interview with Mario Gudelj, Chairman of the Management Board at Croatian TSO
Croatian Transmission System Operator Ltd. (HOPS) is a company which is 100% owned by the Croatian Utility Company (HEP). With our owner being a vertically integrated company, HOPS is certified under the ITO model and manages independently its own business policies. The owners’ rights are defined and transparent, with clear-cut business practices with HEP Group companies that are supervised by the Croatian Energy Regulatory Agency (HERA). This works perfectly, and we have an outstanding cooperation with both HEP Production and HEP Distribution companies, which together with HOPS make the energy backbone of Croatia.
According to 2017 asset data, HOPS owns and operates 7,683 km of overhead lines and cables, 1,247 km of which are 400 kV, 1,213 km are 220 kV and 5,223 km are 110 kV lines. We operate 175 transformer stations – six substations of 400/x kV, 14 substations of 220/x kV and 155 substations of 110/x kV.
Fleet of transformers
HOPS operates a fleet of 190 transformers with rated voltage of 110 kV up to 400 kV and a total installed capacity of 13,204 MVA. Specifically:
- 400/220 kV: 5 units, total capacity 2,000 MVA
- 400/110 kV: 8 units, total capacity 2,400 MVA
- 220/110 kV: 23 units, total capacity 3,550 MVA
- 220/35 kV: 1 unit, with capacity of 20 MVA
- 120/110 kV: 1 unit, with capacity of 80 MVA
- 110/35 (30) kV: 152 units, total capacity 5,154 MVA
The average lifetime of power transformers is approximately 26 years. Over the last four years, we have intensively been acquiring new units, mainly to replace the units reaching the end of their operating life, which are mostly older than 40 years. The average transformer’s life expectancy is 39.2 years, according to CIGRÉ Working Group 37-27 and the published material Ageing of the System – Impact on Planning, Year 2000. In addition, the average life expectancy corresponds very well to our administrative depreciation rate of 2.5 % per annum (full depreciation in 40 years’ time).
The average load of power transformers in the transmission network is relatively low – approximately 19% for units of 400/x kV, around 10% for 220/x kV units and about 23% for units of 110/x kV. The main reason the load is low is that we follow the requirement of the security of electricity supply, adhering to the so-called N-1 criterion in our facilities. The N-1 criterion means that if there is a case of unavailability of one of the units, the entire load is taken over by other units in the facility. Another reason the average load is low is the fact that the typical load in our power system is extremely uneven, meaning that the maximum daily load is over 100 % higher than the daily minimum.