Use of the Traditional Wattmetric Function in Networks with Zig-Zag Transformer Grounding
I. San Sebastian(1,2), Felipe Uriondo(2), Endika Garcia(1), Juan María Garcia(1) 1. INGETEAM.Power Technology, Parque Tecnológico de Bizkaia, Zamudio, Spain 2. Department of Electrical Engineering, UPV/EHU University of the Basque Country, School of Engineering Bilbao, Bilbao, Spain 2026-06-27

Abstract

The earliest electrical protection systems consisted of simple overcurrent relays (functions 50/51) that measured only the magnitude of the current.

Later, European engineers—particularly in Germany and the Nordic countries, where compensated-neutral networks were common—began to adapt the directional principle to zero-sequence currents (function 67N). The purpose was to determine whether the ground fault was located within the protected section, thus avoiding unnecessary tripping in other parts of the system [1–4].

In systems with isolated or Petersen-coil grounded neutrals, the 67N function provides excellent selectivity but when a system is grounded through a zig-zag transformer, selectivity becomes even more challenging if the protected line has significant capacitive contribution [5].

This paper presents the existing difficulties in achieving selectivity in networks grounded through zig-zag transformers combined with lines that carry high capacitive current. Once the problem is described, a simple and cost-effective solution is proposed based on the wattmetric protection function to obtain an appropriate selectivity. The solution presented in this paper is already implemented in the field.

Key words: Zig-Zag transformer, Wattmetric protection, Forward fault, Reverse fault, Zero-sequence.

References

[1] Schneider Electric. (2023). Protección contra sobrecorriente direccional (ANSI 67) – MicroLogic X User Guide.

[2] Electro-Genesis. (s.f.). Aplicaciones de las protecciones direccionales ANSI 67 y 67N.

[3] IMSE Ingeniería. (2020, abril 16). Protecciones direccionales (ANSI 67 y ANSI 67N) – Parte 2ª. Blog IMSE Ingeniería

[4] Roberts, J. B., Altuve, H. J., & Hou, D. (2001). Review of ground fault protection methods for grounded, ungrounded, and compensated distribution systems. Schweitzer Engineering Laboratories, Inc. Pullman, WA, USA.

[5] Cimadevilla, R., García, A., Viosca, J. Á. G., & Domínguez, C. A. (2025). Protection of MV Networks Using Zig-Zag Grounding Transformers. In IET Conference Proceedings. The Institution of Engineering and Technology.