Integrating Market Efficiency and Grid Security: The 70% Rule in the SEE Capacity Calculation Region

Transmission System Operators (TSOs) offer at least 70% of cross-zonal transmission capacity to the market while ensuring operational security. By increasing cross-border transmission capacity, the rule enhances market liquidity, fosters competition, and facilitates price convergence across bidding zones. It also incentivizes TSOs to coordinate more effectively and utilize Remedial Actions (RAs) to maximize capacity without compromising grid security. This paper presents the methodology and practical implementation of the 70% rule in the South-East

Europe Capacity Calculation Region (SEE CCR), focusing on the day-ahead market time horizon and covering the Bulgaria–Greece and Romania–Bulgaria bidding zone borders. SEE

CCR applies a coordinated Net Transfer Capacity (NTC) approach to determine secure crosszonal transfer limits across bidding zone borders. The methodology focuses on evaluating the margin available on the Critical Network Element (CNE) and its associated Contingency

(CNEC) limiting the NTC, dubbed the limiting CNEC. The margin must meet or exceed the minimum threshold, which is typically 70% of the thermal capacity of the CNE. If this threshold is not satisfied, a series of corrective actions is triggered. These actions include adjustment of the Total Transfer Capacity (TTC) via Common Grid Model (CGM) modifications using

Generation Shift Keys. If overloads persist, a Remedial Action Optimization (RAO) process is executed, applying non-costly RAs such as topological changes or phase-shifting transformer adjustments, and, if deemed necessary, costly re-dispatching actions. If overloads are not resolved, an iterative procedure is performed to reduce TTC until system security is fully restored under all possible contingencies. Once a secure TTC is achieved, final NTC values for

BG-GR and RO-BG for Import and Export directions are validated by each TSO to confirm operational security. The paper presents the solution algorithm, the mathematical modeling of the RAO and includes quantitative results derived from the application of the process in real operational context within the SEE CCR.