The Tunisian relay protection experiment demonstrates the setup, coordination, and testing of overcurrent and distance relays in medium voltage distribution networks to ensure selectivity and system reliability.

Overview of Tunisian MV Protection Systems

In Tunisia, medium voltage (MV) distribution networks, such as those in Gabes, are protected using a combination of overcurrent relays, reclosers, sectionalisers, and fuses to maintain service continuity and safeguard equipment . The MV network is connected to high voltage (HV) buses via transformers, typically 150/33 kV, with grounding systems influencing relay coordination and fault detection . Transformers are equipped with internal protections like Buchholz relays, differential protection, and tank-ground protection to prevent thermal overloads .

Experimental Setup

A practical experiment for relay protection involves a radial feeder system where students can observe the operation of electromechanical relays . The setup includes:

• Generators, transformers, circuit breakers, and transmission lines to simulate a real power system.
• Overcurrent and earth fault relays with adjustable Time Setting Multiplier (TSM) and Plug Setting Multiplier (PSM).
• Fault simulation to test relay response to short circuits, overloads, and ground faults.
• Observation of relay coordination and backup protection to ensure selectivity and minimize outage impact .

Relay Protection Settings

Relay settings in Tunisian substations are based on short-circuit calculations and time-current coordination:

• Definite time overcurrent relays trip after a preset current threshold and time delay.
• Inverse time overcurrent relays operate faster for higher fault currents, following CEI standards .
• Time grading ensures upstream and downstream relays coordinate to isolate only the faulted section, maintaining service continuity .
• Backup protection is provided by upstream relays in case downstream devices fail .

Distance Protection

For transmission lines, distance relays are used to detect faults based on impedance measurement, dividing the line into zones:

• Zone 1 trips instantaneously for faults within 80% of the line.
• Zone 2 trips with a short delay for faults extending into adjacent lines.
• Zone 3 provides backup protection for remote faults .

Educational and Practical Benefits

The experiment allows students and engineers to:

• Understand relay wiring and operation in real power systems.
• Analyze fault detection, relay coordination, and selectivity.
• Learn the impact of earthing methods on protection schemes.
• Gain hands-on experience with overcurrent and distance relays, preparing for real-world MV network operations .

Conclusion

The Tunisian relay protection experiment provides a comprehensive understanding of MV network protection, combining theoretical calculations with practical testing. It emphasizes relay coordination, fault detection, and system reliability, ensuring that protective devices operate efficiently to minimize outages and equipment damage in Tunisian distribution networks .