Coordinated Capacity Calculation (CCC) is one of core RSC services established with a goal to allocate available cross-border electricity transfer capacity for different time horizons, in order to maximise transfer capacity offered to the market. Capacity Calculation Region (CCRs) represents the geographic area in which unique CCC methodology is applied. CCRs are established by Agency for the Cooperation of Energy Regulators (ACER) and currently there are 10 CCRs.
In general terms, the CCC procedure takes place in two different time frames:
1. Long-term capacity calculation for the year- and month-ahead market time frames is considered to be coordinated by the TSOs at regional level to ensure that capacity calculation is reliable, and that optimal capacity is made available to the market. Harmonized long-term cross-the-region capacity allocation rules require the establishment of a single allocation mechanism at that regional level. It is up to the TSOs to commonly agree what percentage of the capacity should be allocated on the long term and what percentage on the short term. Further on, after long term allocations, the rest of available capacity is subject of allocation on a short-term basis.
The Network Code on Forward Capacity Allocation (FCA) deals with rules for long term markets, the forward markets. These have an important role in allowing market participants to secure capacity on cross border lines a long time in advance and therefore have a sort of trade insurance.
2. Short-term capacity calculation refers to the combination of day-ahead, intraday and balancing time frames, where the available capacity needs to be calculated in a coordinated manner by the involved TSOs and regional RSC. For this purpose, they should also use a CGM including estimates of generation, load and network status for each hour. The Generation and Load Data Provision (GLDP) methodology clarify which generation units and loads are required to provide information to their respective TSOs for the purposes of capacity calculation.
Network code on Capacity Allocation & Congestion Management (CACM) sets out rules for allocation of available cross-border capacity on the electricity transmission infrastructure in day-ahead and intraday timescales, and outlines the way in which capacity is calculated across the different bidding zones. The goal of the CACM guideline is to provide the coordination and harmonization of capacity calculation and allocation in the day-ahead and intraday cross-border markets. CACM guideline specify that there are two permissible approaches when calculating cross-zonal capacity:
- Flow-Based (FB) approach or
- coordinated net transmission capacity (CNTC) approach.
SCC performs CNTC approach as a part of dry-run day-ahead CCC process for its TSO service users. This dry-run process means that each Monday SCC collect two days-ahead individual grid models (D-2 IGMs) from SEE TSOs and performs day-ahead CCC process for Wednesday. Prior to the CCC process, SCC creates D-2 CGMs for SEE region.
In contrast to the FB approach, CNTC approach determines Available Transfer Capacity (ATC) based on bilateral detection of congestions that are related with cross-border power flows. Basically, in CNTC approach ATC values on each border are determined independently by simulating power flow transaction between two areas – export area has stepwise increase of generation (or decrease of load), while import area has stepwise decrease of generation (or increase of load). This simulation process is performed until relevant congestion was detected, thus determining Total Transfer Capacity (TTC) – the maximum cross-border transmission capacity between two areas compatible with operational security standards applicable at each system if the future network conditions, generation and load patterns were perfectly known in advance.
However, uncertainties associated to the forecast of the power system state, for a given time period in the future, may decrease according to the selected time frame. The Transmission Reliability Margin (TRM) is a security margin that copes with uncertainties on the computed TTC values arising from:
- unintended deviations of physical flows during operation due to the physical functioning of load-frequency regulation,
- emergency exchanges between TSOs to cope with unexpected unbalanced situations in real time,
- inaccuracies, e.g. data collection and measurements”.
Net Transmission Capacity (NTC) is the maximum exchange between two areas compatible with security standards applicable in both areas (N-1 criteria) and taking into account the technical uncertainties on the future network conditions.
Finally, Available Transfer Capacity (ATC) is the transfer capacity remaining available between two interconnected areas for further commercial activity over and above already committed utilization of the transmission networks. ATC is calculated by subtracting Already Allocated Capacity (AAC), determined in month-ahead or year-ahead CCC process, from the NTC values obtained in D-1 CCC process:
The available cross-border capacity is one of the key inputs into the further market calculation process, in which all region bids and offers collected and matched, taking into account available cross-border capacity in an economically optimal manner.