Screening Method for EMT Studies of Border Transformers in the Brazilian Interconnected Power System

(EMT) studies are required for transformer energization in the Brazilian Interconnected Power

System. Currently, all transformers connected at or above 230 kV undergo detailed EMT analysis, regardless of systemic relevance. This creates a significant workload for the Brazilian

System Operator (ONS).

The proposed screening approach introduces a pre-evaluation stage combining two key indices:

i) the Transformer Short-Circuit Ratio (TSCR), and ii) the harmonic impedance Z(ω) observed at the transformer’s point of connection. Using both operational experience and a newly developed parametric simulation framework, these indices identify the conditions under which simplified analysis is sufficient or full EMT studies are indispensable.

The method builds upon years of experience and incorporates international benchmarking. The system is represented by an equivalent RLC parallel filter combined with a reference transformer model. A series of parametric simulations determines critical harmonic impedance thresholds across the 2nd-10th harmonics, correlating them with TSCR values. Results are condensed in an abacus that delineates safe, cautionary, and critical regions for transformer energization screening. This applies to transformers connected to the 230 kV trunk. Otherwise, a general empirical harmonic impedance envelope is utilized.

Validation with a wide database of historical EMT cases demonstrates a strong correlation between predicted screening outcomes and actual study results. The proposed approach enables significant workflow optimization, reducing study demand while maintaining system security.

This method does not replace equipment-level design verifications (e.g., inrush withstand, arrester energy capability) but focuses solely on systemic impacts. Since 2023, it has been progressively applied and validated in the Brazilian transformer integration processes, confirming its robustness and alignment with international practices.