The Glossary introduces content relevant for the operation of the transmission network in more detail.

Transmission Network Operation

Safe Transmission Network Operation

The safe operation of the transmission network means that customers have sufficient amounts of electric power available at all times. To ensure this, ELES must take certain European (Operation Handbook) and domestic (System Operating Instructions for the Transmission Network, SOITN) safety standards into consideration.

In ensuring operational safety, ELES also takes part in the Transmission System Operator Security Cooperation – TSC. With the rise of the generation of power from renewable sources and the pan-European marketplace for power trading, safety concerns have also become transnational, i.e. their efficient handling requires transnational cooperation and the coordination of measures during the planning stages, as well as during real-time operation.

Transmission Line Outages and Shutdowns

The key element of most safety standards is the implementation of the so-called n-1 criterion, which enables the handling of any transmission line outages. In brief, meeting this criterion means that a contingency scenario has been implemented for cases of the outage of an individual transmission network element (transmission line or a distribution transformer station), as well as that there will be no interruptions in the supply of power to users or any continuous overloads of the operating elements of the transmission lines that could trigger a cascading outage or voltage instability and/or a complete failure of the network and consequently a power grid blackout.

Besides the handling of outages (criterion n-1), detailed shutdown planning is also part of the safe operation of the electric power system. Shutdowns of transmission lines and other electric power devices are necessary due to regular and periodic maintenance works, investment works, troubleshooting or the prevention of malfunctions, as well as safety shutdowns in order to prevent health and death hazards or damage risks. They are also necessary to ensure the optimal operation of the transmission network.

Based on the maintenance and investment work plan, ELES drafts a plan of shutdowns of transmission lines and other electric power devices at the beginning of the year for the year that has begun, which includes the planned dates and the duration of each shutdown. The annual shutdown plan is published in the Indicative Energy Balance.

Special attention is dedicated to information on outages (market-relevant outages) and shutdowns (market-relevant shutdowns) that impact cross-border transmission capacities.

The published dates of the shutdowns of electric power devices are indicative and therefore non-binding for ELES.

Electric Power System Balancing

Electric power system balancing ensures the technically safe and economical operation of the electric power system, as well as a reliable supply of quality electric power. The foundation for electric power system balancing is a working electricity market. Limitations with regard to generation or transmission capacities equal a risk of an unbalanced electric power system.

The role of ELES as a system operator is to provide seamless cross-border electricity trading under the consideration of the safety criteria, thus contributing to the more efficient operation of the electricity market. ELES only intervenes in the case of significant imbalances in the transmission system for which the market participants failed to restore balance in due time with the purchase or sale of power in the market. In this case, ELES will purchase or sell electricity in order to restore balance, as well as use system services* that are of special importance for balancing the electric power system at the second or minute level.

* System services are provided by the operators of the transmission and distribution network in order to ensure the safe and reliable operation of the electric power system, as they are mandatory for the operation of the system. They encompass frequency and power regulation, voltage regulation, and the start-up of the generator without an external power source. They are described in detail in the Network Code for the Electricity Transmission System of the Republic of Slovenia.

Smart Grids

Smart grids represent an upgrade of the existing system with up-to-date information technologies. Freeing the electric power market, deregulation and a growing share of dispersed sources have altered the conditions in the electric power system. In order to handle these altered conditions, mere investment in traditional infrastructure no longer suffices anymore, even though the construction of a new transmission line or transformer is still a key element for the implementation of the basic functions of a transmission network. The implementation time frame of such investments is too long for successfully handling fast-paced changes.

An alternative solution to the stated issues can also be achieved by introducing smart grids. A good example of such practice at ELES is the SUMO project for establishing the actual operational limits by means of dynamic transmission capacity monitoring technologies and thus the actual utilisation of the transmission lines.

The WAMPAC project plays a similar role. It is based on a synchronous phasor current and voltage measurement technology in order to allow the operator to monitor the system stability in real time, which is lately reduced by an increasing amount of dispersed sources throughout Europe.

ELES also takes part in other international smart grid projects:

Visualisation of Loads and Transmission Capacity of the Transmission Network within the SUMO Project
Visualisation of Loads and Transmission Capacity of the Transmission Network within the SUMO Project

Visualisation of the Voltage Angles, in Future Upgraded with PMU
Visualisation of the Voltage Angles, in Future Upgraded with PMU

Cross-Border Transmission Capacities

Determining the CTC

Cross-border transmission capacities are the capacities of the lines connecting electric power systems across national boundaries.

The CTC offered by the system operator for commercial purposes are limited. ELES calculates the CTC for all three borders (Slovenian-Austrian, Slovenian-Croatian, Slovenian-Italian) based on the methodology of determining the CTC agreed within the European Network of Transmission System Operators (ENTSO-E).

ENTSO-E Methodology for Determining the CTC

The methodology for determining the CTC takes into consideration the condition of the electric power system, which constantly changes due to load fluctuations, changes in the number of users (producers, consumers), as well as the network configuration, and the rated uninterrupted current of an individual electric power system, which changes depending on the season and depends especially on the permitted temperature of the electric power element and the surrounding temperature.

The latter is the reason why the indicative CTC or NTC (Net Transfer Capacity) values for individual limits are determined separately for the winter and the summer. The NTC calculations are based on the simulation models of the entire synchronous grid, e.g. the reference models for the winter and summer of the coming year (long-term calculations) or bilaterally prepared models that describe the forthcoming power conditions in more detail (short-term calculations).

The first step in determining the CTC is the simulation of the cross-border working power exchange. The entire transmission capacity between two neighbouring electric power systems is determined by means of a gradual increase in the exchange between the systems, as well as the constant monitoring of system operation safety. The simulation is executed using a model that takes into consideration the Base Case Exchanges).

The increase of the exchange shall be introduced by increasing the generation in one system and reducing it in the other. The increase can be executed in several ways, such as a proportional increase/reduction of generation or increase/reduction by means of weighting factors, however an increase or reduction of loads is also possible in case of a low generation capacity of the electric power system.

The increase in the exchange of working powers among individual electric power systems is carried out up to the moment the safe operation of the electric power system is at risk. In the process, the thermal load of the elements, the voltage conditions and the stability of the dynamic system are monitored.

If a violation of any of the safe operation criteria is established and cannot be resolved using certain measures, the operating condition represents the safe operation limit. Based on the safe operation limit determined this way, we can determine the entire transmission capacity between the electric power systems according to the following formula:

TTC = BCE + ∆E = NTC + TRM


  • TTC = Total Transfer Capacity. TTC is the highest possible safe power exchange between two neighbouring EESs under consideration of the safety standards in an individual EES;
  • BCE = Base Case Exchange;
  • ∆E = exchange increase. Together with the exchange in the Base Case Exchange, it represents the power that can be exchanged between EESs under the presumption of the safe operation of an EES;
  • NTC = Net Transfer Capacity or net cross-border transmission capacity (foreseen for commercial use) is the difference between the total transfer capacity and the transmission reliability margin;
  • TRM = Transmission Reliability Margin determines the transmission capacity that the system operator shall provide to compensate for the possibility of an outage of the most powerful generator within the regulation zone, due to angle or voltage stability issues, etc.

If there are already cross-border transmission capacities allocated (within the annual or monthly auctions for the allocation of cross-border transmission capacities), the available cross-border transmission capacities are calculated according to the following formula:



  • ATC = Available Transfer Capacity representing the non-allocated part of NTC for further commercial usage;
  • AAC = Already Allocated Capacity.

The figure below shows the fluctuation of the TTC, TRM, NTC, ATC and AAC values in different periods:

Gibanje vrednosti TTC, TRM, NTC, ATC in AAC

The entire ENTSO-E methodology for the determination of NTC/ATC is further described in »NTC and ATC in the IEM, information for user, ETSO, March 2000«.


All system operators on the northern Italian border calculate the total NTC. The methodology for the calculation of the total NTC is described in the document »Methodology for the joint assessment of the net transfer capacity at the northern italian interconnection«.

The method for the total NTC distribution among individual system operators or the NTC value for individual operators is based on some ad hoc distribution factors resulting from past bilateral agreements and relations.

NTC in ATC for Allocation

The NTC values mean the values of the total cross-border transmission capacities on a certain border dedicated to commercial allocations under the consideration of all the criteria for the reliable and safe operation of both neighbouring grids. The system operator determines the NTC values for each direction and each border by publishing the indicative NTC value, which is determined for the entire year by neighbouring system operators, as well as the NTC value on day D-1, which means the actual NTC value that can differ from the indicative value in the case of special operating conditions or unexpected events.

The ATC value means the value of available cross-border transmission capacities that are still available for allocation for commercial purposes.

When an auction participant who was allocated CTC at an annual or monthly auction returns the capacities to the system operator for resale, the ATC value for re-allocation is increased by the returned CTC. Also the ATC for the daily auction is increased by the nominated long-term CTC in the opposite direction due to the netting performed after the nomination of long-term CTC.

CTC Auctions

ELES allocates CTC at explicit and implicit auctions. Explicit auctions are auctions at which participants give their offers in order to obtain access to CTC. The marginal CTC price is defined by the last accepted offer. If the demand for CTC is less than the offered quantity, the CTC price is zero. Implicit auctions mean an upgrade of the explicit CTC allocation since it is a simultaneous obtainment of CTC and electricity. Such auctions are executed by energy exchanges and the system operators announce the quantities of available CTC representing the trading limitation between two neighbouring exchanges. In practice, the order books of two or more exchanges are combined, whereas the exchanges calculate the new pricing optimum using an algorithm as a consequence of the more efficient electricity exchange.

The table shows the CTC allocation method and the entity responsible for the execution of the auctions for both directions and each of the three borders:

Border Auction type Auction performer
SI-AT Explicit coordinated; annual, monthly
Market Coupling – implicit; daily
SI-IT Explicit coordinated; annual, monthly
Market Coupling – implicit; daily
Implicit intraday auctions
SI-HR Explicit coordinated; annual, monthly, daily JAO

CTC are allocated at annual auctions that are executed once a year, monthly auctions once a month, and daily auctions.

Daily auctions on the Slovenian-Italian, Slovenian-Austrian and Slovenian-Croatian borders are executed every day of the week.

The intraday CTC allocation is performed on the Slovenian-Austrian, Slovenian-Croatian and Slovenian-Italian borders. This is possible upon the coordination of schedules for a day in advance.

On the Slovenian-Austrian and Slovenian-Croatian borders, the intraday allocation begins at 6:00pm on D-1 and is performed for every full hour until H-1 following the first come first serve principle.

Electricity Market Coupling

Market coupling refers to energy market coupling. It is one of the methods of managing cross-border electricity trading blockages. It represents an implicit allocation of cross-border transmission capacities where these are the input parameters of the optimisation algorithm executed during the purchase or sale of electricity at at least two neighbouring electricity exchanges among which the blockages occur.

Reliability of Allocated CTC

The reliability rate of the allocated CTC is defined in the auction rules on the allocation of cross-border transmission capacities. Further on, there is a summary of the information about the reliability of the allocated CTC by individual allocation border.

The reliability rate of the allocated CTC is defined based on statistical data. It represents the ratio between the total amount of allocated CTC in MWh for a certain period and the total amount of non-reduced CTC for the same period, whereas the non-reduced values mean the difference between the allocated CTC and the reduced CTC for this period. The reliability rate of the allocated CTC is calculated in the following way:



  • SD is the reliability rate of allocated CTC,
  • CTCallocated is the total value of the allocated CTC for hour h,
  • CTCreduced is the total value of the reduced CTC for hour h,
  • h are the hours in the period for which the statistical analysis was performed.

The reliability rate of the allocated CTC on Slovenian borders for 2015:

SI-IT 99,9
IT-SI 99,3
SI-HR 100,0
HR-SI 100,0
SI-AT 100,0
AT-SI 100,0

CTC Usage

The auction participants to whom the CTC was allocated announced their usage in the form of so-called schedules. The schedules are used to announce how much CTC at a certain border and in a certain direction will be used for the cross-border transmission of electrical power.

ELES accepts the usage announcement for the long-term CTC (allocated at the annual and monthly auction) until the deadline for the morning scheduling, and the short-term CTC (allocated at the daily auction) until the deadline for the afternoon scheduling.

The procedure regarding the schedule announcements and coordination is defined in the Operation Handbook, Policy 2 on the European level.

After the scheduling deadline, ELES combines all the received schedules for each border and direction, and harmonises them with the neighbouring system operator. After the harmonisation of the schedule, ELES will approve an individual electricity transfer to the user by confirming the schedule and in accordance with the System Operating Instructions (SOITN).

Managing the Congestion in the Slovenian Transmission Network

One of the main tasks of the system operator is the management of congestion in the transmission network. The congestion of the transmission network is a condition in which the sum of the working powers of the concluded purchase and sale agreements in a certain period on an individual transmission path exceeds its technical or operational limitations.

With regard to the reason for the congestion, we know congestion in the internal network or a consequence of links between the neighbouring electric power systems.

Congestion in Connection with Neighbouring EES

Congestion in connection with the neighbouring EES means a state where the demand for the cross-border transfer of electricity exceeds the technical capacities of the EES. In such cases, the system operator limits the electricity transfer in regulation areas in order to prevent the overloading of individual elements of the electric power system or to be able to ensure operational reliability within the standards (standard N-1).

With regard to the location of the Slovenian electrical power system and the conditions in the regional electricity markets, the electricity flows and as a consequence, the demand for cross-border transmission capacities on Slovenian borders mostly follow two patterns:

  • electric power transmission from mid-Eastern and Southeast Europe towards Italy – this occurrence is quite frequent and occurs under normal conditions on the electricity markets. As a consequence, there is a lack of cross-border transmission capacities on the Austrian-Slovenian border and predominantly on the Slovenian-Italian border;
  • electric power transmission from mid-Eastern Europe and Italy towards Southeast Europe – this coincides with dry periods in Southeast Europe. As a consequence, there is a lack of cross-border transmission capacities on the Austrian-Slovenian border and periodically on the Slovenian-Croatian border.

Congestion in the Internal Network

Under normal operational conditions, there is usually no congestion in the Slovenian internal network.

In the case of irregular operating conditions leading to the overloading of the transmission lines and transformers, the system operator can, in accordance with the statutory authorizations, temporarily limit the supply of electricity to consumers or generation for producers in order to prevent the outages of grid elements that could cause a total malfunction.

Congestion-Relief Measures

We distinguish between long- and short-term congestion relief measures.

Long-term congestion-relief measures are additional investments and upgrades of the existing transmission network with which we can relieve the existing grid elements. Also the definition of the scope of the permitted cross-border transmission capacities for commercial use that still allows safe EES operation belongs among the long-term and mid-term congestion relief measures. For this purpose, the system operator calculates and publishes the amount of annual, monthly and daily available cross-border transmission capacities that are distributed among market participants in a marketable way.

Despite long- and mid-terms congestion relief measures, congestion can still occur in real time. This is caused by unexpected events in the electric power system that deviate from the foreseen network states and based on which the calculation of the available cross-border transmission capacities was performed.

In general and in order to relieve congestion in real time (short-term measures), system operators first use technical measures to reduce the flows through congested elements (changes in the network’s topology, use of phase shifting transformers, etc.). If these measures do not suffice or are not available, the system operator executes additional measures that are used depending on availability:

  • Generation redispatching – a system operator can resolve congestion in their area with local generation redispatching by intervening in the market and thus changing the generation location.
  • In the case of congestion in certain areas, countertrading among different system operators is necessary, whereas the system operators shall harmonise the time frames and agreements.
  • Curtailment – if none of the aforementioned congestion-relief measures achieve the desired effect, the system operator shall curtail the already allocated cross-border transmission capacities at the congested borders in order to ensure operational safety.

Consumption and Generation in the Transmission Network

ELES aims to implement a balance between the generation and consumption of electrical energy from the transmission network by using leased system services and buying/selling energy to compensate for deviations, thus preventing the deviations of the electric power consumption from the plan posing a hazard to the safe operation of the transmission network, as well as the supply of electricity to end users.

Consumption of Electrical Energy from the Transmission Network

The consumption of electric power from the transmission network refers to the energy that is taken from the transmission network by the connected consumers, i.e. the distribution operator who transmits the energy to end users and direct consumers who are connected directly to the transmission network.

Based on the data obtained from distribution companies and large consumers connected to the transmission network, ELES prepares and published the annual, monthly and weekly consumption forecasts.

The daily consumption forecast is prepared by ELES themselves. The forecasts and realised consumption values include consumption by distribution companies and large consumers from the transmission network, however they do not include the losses in the transmission network and consumption by pumping power plants.

Electricity Generation in Slovenia

The significant producers feeding electricity into the transmission network are the nuclear power plant, thermal power plants and hydroelectric power plants. There are a growing number of producers of renewable power sources who are mostly connected to the distribution network, however a smaller share is connected directly to the transmission network.

Based on the data obtained from electricity producers connected to the transmission network, ELES prepares and published the generation forecasts. The company published annual, monthly, weekly and daily generation forecasts, as well as the realised generation.

The electricity generation share in hydroelectric power plants and power plants running on other renewable sources changes annually with regard to the hydrological conditions, as well as the scope of investment into the construction of generation units for the usage of renewable sources.

Larger Electricity Producers (more than 100 MW power output)

  • Krško Nuclear Power Plant (KNPP) with a rated output of 696 MW (the Slovenian share is 348 MW);
  • Šoštanj Thermal Power Plant (ŠTPP) with a total rated output of 1,183 MW;
  • Dravske elektrarne Maribor (DEM): Dravograd HPP, Vuzenica HPP, Vuhred HPP, Ožbalt HPP, Fala HPP, Mariborski otok HPP, Zlatoličje HPP, and Formin HPP with a total rated output of 587 MW;
  • Brestanica Thermal Power Plant (BTPP) with a total rated output of 297 MW;
  • Trbovlje Thermal Power Plant (TTPP) with a total rated output of 168 MW;
  • Savske elektrarne Ljubljana (SEL): Moste HPP, Medvode HPP, Mavčiče HPP, and Vrhovo HPP with a total rated output of 118 MW;
  • Hidroelektrarne na spodnji Savi (HESS); Boštanj HPP, Arto-Blanca HPP, and Krško HPP with a total rated output of 116 MW;
  • Soške elektrarne Nova Gorica (SENG): Doblar HPP, Plave HPP, Solkan HPP, and Avče PHPP with a total rated output of 316 MW;
  • Javno podjetje energetika Ljubljana enota TE-TOL (TE-TOL) with a total rated output of 134 MW.

Transmission Network Access

Transmission Network Access

(1) Electricity supply is performed according to the principle of regulated access by a third party to the system. Network users pay the network costs based on the tariff that the Energy Agency published in the Official Journal of the Republic of Slovenia at least five days before they enter into force.

(2) All persons who wish to become users of the network and power system operators have the right to a connection to the network in accordance with the criteria in the System Operating Instructions.

(3) The power system operator defines the scope of the rights to use the network for each user of the network or power system operator in the consent for the connection to the grid as stipulated in Article 147 of EZ-1 by defining the highest connection power or any other operational limitation.

(4) Before the start of power transmission into the network or power consumption from the network, or before the connection to the network, the power system operator and the user of the network conclude an agreement for the connection to the network. After the payment of the grid fee for the connection power, they conclude an agreement on the usage of the network within the limitations from the previous paragraph.

(5) The conditions for the connection to the network are defined by the signed power purchase agreement.

Access requirements provided to the system operator by the users of the network shall include the forecast amount of electric power and the final power requirement for the consumption or provision of electric power, as well as the duration of the power sale and/or purchase agreement. The user of the network who is buying the electric power shall also state the information about the supplier.

Transmission System Usage Calculation in 2015

Every user is classified into a customer group in accordance with the characteristics of their consumption/transmission based on the Act determining the methodology for setting the network charge, the criteria for establishing the eligible costs for electricity networks, and the methodology for charging the network charge (Official Journal of the RS, No. 81/2012), (the Act) with its amendments.

The received/transmitted quantities of electrical power are measured by the system operator at measuring points defined in the agreement on the usage of the transmission system in 15-minute intervals. The power is distributed among tariffs according to the applicable tariff periods, i.e. to the higher daily tariff periods (HT) and the lower daily tariff period (LT) defined in the Act. Within the higher daily tariff, the time of the peak tariff (PT) is also defined. This is stipulated by ELES by 15 October for the subsequent year and published on its website (

The price for the usage of the transmission system (network charge, fees, supplements) paid by the user of the transmission system for 2015 is defined in:

  • Decision of the Energy Agency number 111-12/2012-01/452 (the tariff for the network charges are defined and published on the ELES website);
  • The Document determining the contribution for ensuring support for the production of electricity from high-efficiency cogeneration and renewable energy sources (Official Journal of the RS, No. 38/2014);
  • notifications of the Slovenian Ministry of Infrastructure (on the amounts of supplements to the price for the usage of the network),;
  • Decree on Energy Savings Requirements (Official Journal of the RS, No. 96/2014).

System Services

System Services

In accordance with the national energy legislation, the system operator shall always ensure that the electric power transfer is seamless, as well as ensure the safe and reliable operation of the network. In order to ensure the seamless operation of the electric power system under free market conditions, various system services shall be provided via market providers, whereas their technical characteristics and provision methods are defined in more detail in SOITN, the ENTSO-E operating instructions, and the provisions issued by the Energy Agency. Together with the consideration of the actual needs, the system operator prepares various methods for generation- and load-side capacity lease, usually executed via public auctions or in negotiation with providers.

The system operator executes the leases of the following types of system services:

  • frequency and power regulation;
  • voltage regulation;
  • start-up of the generator without an external power source.

The lease of the reserve for primary frequency regulation is currently not being executed in Slovenia since the participation is stipulated by the law for all producers connected to the transmission network. The minimum recommended amount of the provided reserve is stipulated as the peak power of the regulation range, which is ±14 MW for Slovenia, of which every producer shall contribute a proportional amount with regard to the rated output of the individual generators.

The reserve lease for the provision of automatic frequency regulation (secondary regulation) is mostly executed by means of negotiation due to the limited competition on the offer side. Only the providers owning regulation units equipped with suitable facilities for the provision of such a service and meeting all the requirements of the contracting authority, as well as meeting the technical specifications defined in SOITN, can apply to the call for tenders. The minimum recommended amount of the reserve lease is defined with the deterministic method, which takes into consideration the peak power of the regulation range, or the probability method, which is based on the statistical processing of the regulation range deviations from the previous period. For Slovenia, this is ±60 MW.

The reserve lease process for manual frequency regulation (tertiary regulation) in Slovenia is executed via public calls for tenders, which are executed in two parts. The first part refers to the process for establishing the technical capacity of the tenderers, and the second part refers to the selection of the most favourable tenders at an auction. The reserve lease differs in terms of the product type and the service duration period. Generation units and loads with demand-side management (DSM) can participate in the lease. The dimensioning of the leased quantities of the reserve is stipulated by SOITN, which is based on the ENTSO-E rules.

Umbrella Contract

r the purchase/sale of electricity for the needs of the system operator of a transmission network executed on day D or in real time, ELES will conclude an umbrella contract on the sale and purchase of electricity for the needs of the transmission network system operator with the interested providers..

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