Enhancing cross-border trade: Implementing 70% rule in SEE CCR

Summary

In this case study, you will discover details regarding:

1. The implementation of the mandated 70% rule for the Day-Ahead cross-border capacities in the South-East Europe Capacity Calculation Region (Greece, Bulgaria, Romania) operated by SEleNe CC,  
2. The complexity and challenges of implementing the 70% rule, which essentially ensures the largest cross-border transfer capacity is provided to the market, while preserving the security of the grid
3. The development and integration of software to support operators at all steps associated with applying the 70% rule.

Introduction

The objective of this project was to support SEleNe CC in adhering to the European-mandated “70% rule”, which requires TSOs to provide to the market at least 70% of the transmission capacity for day-ahead cross-zonal trading in the South East Europe (SEE) Capacity Calculation Region (CCR) operated by SEleNe CC. 

To achieve this objective, the N-SIDE mission was to develop a tool encompassing both an optimization algorithm and a Graphical User Interface (GUI) to support the operators in enforcing the 70% rule and maximising the cross-border capacities at the Northern Greek border with Bulgaria (BG-GR) and the Southern Romanian border with Bulgaria (RO-BG). 

N-SIDE supported SEleNe CC in refining the methodology and facilitating the integration of the tool into the infrastructure and other systems, with the objective of ensuring smooth operation of the process.

Customer

Capacity calculation in South East Europe (Greece, Bulgaria, Romania) is operated by the Regional Coordination Center SEleNe CC, which has been N-SIDE’s direct customer in this project. The TSOs in SEE (namely IPTO, ESO, and Transelectrica) are also heavily involved, both in designing the methodology and in operational steps to enforce the 70% rule. 

Overview of the 70% rule optimization process

The cross-border capacity adjustment to enforce the 70% rule is performed after the initial calculation of cross-border capacities, for each border (RO-BG and BG-GR) and for each direction (import/export) separately. This adjustment process is represented below.  

Figure 1: Overview of the 70% rule optimization process

In the above workflow, the first part of the process (steps 1, 2, and 3) is dedicated to the verification of the potential violation of the 70% rule (formulated within the CC methodology of the SEE CCR) and the consequent adjustment of the cross-border capacities if needed. This adjustment is made without mitigating the risk of overloading in the case of unplanned contingencies. The second part of the process (steps 4, 5, 6) analyses the transmission grid’s security against congestion, optimizes the grid using both topological and redispatch actions, and re-adjusts cross-border capacity until all congestion risks from unplanned contingencies are mitigated. Eventually,  the final cross-border capacity is validated by the TSOs (step 7).  

Contribution of N-SIDE

The software developed by N-SIDE allows the operator to configure and perform all steps in Figure 1 and to monitor each computational component through a comprehensive GUI. Data quality checks on input files are also performed, allowing for the detection early in the process of any issues with the input data. Fallback plans are automatically activated in case the process cannot be properly executed.

Prior to the go-live, N-SIDE supported the intensive testing phase performed by SEleNe CC and helped refine the methodologies to handle all possible corner cases. 

Customer Challenge 

In its role as the Regional Coordination Center (RCC) for the EU TSOs in the South-East Europe (SEE) Capacity Calculation Region (CCR), SEleNe CC must ensure that the calculation of Day-Ahead cross-border capacities for the borders between Romania and Bulgaria, and between Bulgaria and Greece, complies with the EU 70% rule.

The first goal was to verify the 70% rule, which falls outside the scope of the initial Coordinated Capacity Calculation tool. When the cross-border capacity undershoots the target set by the 70% rule, it is adjusted upward to meet the regulation threshold. However, this adjustment may jeopardise grid security.  The second challenge for Selene CC is to optimize the transmission grid to mitigate congestion risks while maximising the cross-border capacity allocated to the Day-Ahead market. This is a complex task, which requires several iterations involving optimization of the transmission grid using both topological and active power redispatch actions.  

Solution/Approach 

The software developed by N-SIDE supports SEleNe CC in all aspects connected to the application of the 70% rule:  

Assessment of compliance with the 70% rule 

The first module supports the operator in verifying the 70% rule. This verification is performed within the CC methodology of the SEE CCR; hence, it entails computing node-to-slack Power Transfer Distribution Factors (PTDFs), evaluating Generation and Load Shifting Keys (GLSKs), and ultimately assessing the impact of cross-border flows on critical network elements via zone-to-zone PTDFs. 

Adjustment of the cross-border capacities, ensuring security and optimization of the grid

A second module adjusts cross-border capacity as required by the 70% rule and assesses the impact on grid security under unplanned contingencies. Overloading risks resulting from the cross-border capacity increase are mitigated by optimizing the grid topology, identifying the optimal redispatch of generator units, and, if needed, further adjusting the cross-border capacity. This optimization is performed iteratively until a secure grid is reached. 

Graphical User interface and reporting

Finally, a GUI has been developed to help the operator monitor all the steps in the cross-border capacity adjustment process. Extensive reporting on the quality of input data as well as on all intermediate steps is made available via multiple channels: SQL database, dedicated GUI screens, and multiple intelligible Excel reports. 

Implementation 

The whole solution is deployed on premises in a secure environment managed by SEleNe CC. The User interface developed by N-SIDE guides the operators through the different steps of the process:

• The RCC operator uploads the multiple input sources: grid models, critical network elements and contingencies, actions, as well as results from the initial capacity calculation tool. 
• A dry run can be executed to verify the quality of input data. In such a case, a set of data quality controls is applied to the different sources, and a report is consolidated with all warnings and errors identified by the dry run. 
• The RCC operator can then trigger the execution of the 70% rule process for different borders, directions, and time units.  
• In case the 70% rule is not satisfied for a given process, the cross-border capacity is automatically adjusted, a security analysis is executed to identify congestion risks, and a Remedial Action Optimization (RAO) attempts to mitigate those risks.  
• For processes with a cross-border capacity increase that lead to congestion risks that cannot be resolved by the RAO, TSOs can provide additional topological actions to mitigate them. 
• A second run is then triggered by the RCC operator to further adjust the cross–border capacity and optimize the grid. 
• Eventually, the TSO can validate (via the user interface) the cross-border capacities or propose an amended value.

An extensive set of reports is also generated, including all intermediate details of the capacity adjustment and optimization process. 

Benefits 

The solution built by N-SIDE allows for assessing compliance with the 70% rule, increasing cross-border capacity when necessary to meet European requirements, and mitigating congestion risks resulting from these adjustments

The benefits of the solution developed by N-SIDE are: 

• Optimization: a sound mathematical formulation of the optimization problem, allowing the gap between DC and AC accuracy to be bridged. 
• User experience: the user interface is built on detailed feedback from operators and has been designed to facilitate their work. 
• Transparency: all steps of the calculation are documented in multiple reports, making it easy for the operator to understand the details of any given step and decision. 
• Support: N-SIDE provides extensive support via the service desk, including the interpretation of the results.
• Maintenance: N-SIDE provides yearly maintenance to the tool for adaptive, evolutive, and corrective measures.

Conclusion

Since the 1st of October 2025, the solution developed by N-SIDE to enforce the 70% rule has been running in production, giving SEleNe CC the ability to perform all the steps required to meet the associated regulatory requirements and to monitor all intermediate and final results.

Extensive validation and testing, supported by SEleNe CC, the TSOs, and N-SIDE, enabled the identification of corner cases, the fine-tuning of the methodology, and the adaptation of the software to achieve high standards of reliability and transparency. This proved essential for achieving a high level of understanding and confidence across all possible cases, resulting in very few issues in Production.