Differential Protection and Selectivity in MV Networks

Fundamental Principles of MV Protection

In a Medium Voltage network (1 kV to 36 kV), protection's mission is to quickly isolate any fault (short circuit, ground fault, overload) while keeping the rest of the network energized. This principle is called selectivity.

Without selectivity, the slightest fault trips the entire network. With well-designed selectivity, only the faulty section is isolated: other customers keep receiving power.

Types of Selectivity

Four approaches coexist in MV networks:

• Current-based selectivity: tripping current thresholds increase from downstream to upstream.
• Time-graded selectivity: time delays increase from downstream to upstream (0.3 s → 0.6 s → 1.0 s).
• Logic selectivity (LS): relays communicate with each other so that only the relay closest to the fault is allowed to trip. Fast clearing times everywhere in the network.
• Differential selectivity: compares the currents entering and leaving a piece of equipment. The most selective and fastest, used for HV/MV transformers and major cables.

Transformer Differential Protection

Longitudinal differential protection of an HV/MV transformer measures the vector sum of the currents on the HV side and MV side. Under normal conditions, this sum is zero. In the event of an internal short circuit, a differential current appears and instantly trips the HV-side breaker.

Settings challenges include:

• Magnetizing inrush current: a transient current that can reach 10 times In appears at energization. The relay distinguishes it from a real fault via second-harmonic detection.
• Phase shift: the Dy11 vector group introduces a 30° shift between HV and MV, digitally compensated by the relay.
• Ratio mismatch: the HV-side and MV-side instrument transformers have different ratios. The relay normalizes the currents by calculation.

Selectivity Study with ETAP

ERPT uses ETAP for its protection coordination studies. The Star-View module plots the time-current curves (TCC) of all relays, fuses and breakers in an MV network on a single logarithmic graph.

The ERPT procedure:

• Calculating maximum short-circuit currents (3-phase, phase-phase, phase-ground) at every node of the network.
• Positioning TCC curves to guarantee selectivity margins of at least 0.3 s between two relays in series.
• Verifying overload protection.
• Simulating energization transients to validate inrush settings.

Standards Applicable in Morocco

MV projects in Morocco must comply with ONEE requirements (MV Specifications), the IEC 60255 standard (protection relays) and IEC 60909 (short-circuit current calculation). IEC 61850-8-1 applies to communicating relays in digital substations.

Application Example: OCP Industrial MV Network

On the OCP Khouribga MV network project, ERPT implemented logic selectivity (LS) on a 60 kV/22 kV loop network feeding 8 MV substations. Clearing times achieved: under 80 ms for any fault, compared to 600 ms to 1 s with conventional time-graded selectivity. Network availability rose from 99.5% to 99.92%.