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 = NTC - AAC
- 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:
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.