MV/LV transformer protection with circuit breaker

From Electrical Installation Guide

MV/LV transformer protection with circuit-breaker is usually used in large Commercial, Industrial and Building applications and especially when the transformer power exceeds 800 kVA. In these applications, switchboards made of modular units provide high flexibility.

The protection chain of each unit may include self powered relays (see Fig. B23) bringing a high level of safety and optimized CTs (See Fig. B24).

This solution provides interesting benefits concerning:

Fig. B24 – Schneider Electric SM6 and Premset switchboards including MV/LV transformer protection with circuit breaker associated to self powered relay


Modern protective relays are now almost maintenance free, as they include self testing features. However it remains necessary to check the protection chain at commissioning stage and periodically (every 5 or 10 years).

Protection performance

Circuit breakers combined with electronic protection relays bring many protection selectivity benefits, including:

  • coordination with upstream and downstream devices;
  • selectivity of inrush currents;
  • detection of low level of phase to phase and phase to earth fault currents.

Selectivity with LV installation

In cases where the LV installation includes an incoming LV Air circuit breaker, selectivity with the MV circuit-breaker is easy, as it is possible to choose the right curve in the electronic relay to ensure selectivity between MV and LV protection.

Inrush current

Transformer energizing produces very high transient inrush current that can reach peak values, up to about ten times the peak rated current for step-down transformer, and 25 times for step-up transformer. This is a natural phenomenon and the protection should not operate. The circuit breaker allows high flexibility to avoid tripping current while still maintaining a good level of protection due to the electronic relay time/current characteristic.

Low magnitude phase fault current

A MV/LV transformer has usually a very low failure rate. Most of the faults are interturn faults or phase to earth faults. Phase-to-phase faults between MV bushing are of more seldom occurrences (see Fig. B25).

Fig. B25 – Localization of a fault

Most common faults are short-circuit inside a turn of the MV winding where the fault level is of low magnitude (1 to 6 times the rated current) (see Fig. B25).

In case of circuit breaker, as soon as the fault reaches the setting, the relay will detect it and trip safely the circuit breaker, disconnecting the MV/LV transformer circuit.

High magnitude fault currents

In the rare event of a short-circuit between MV bushings, the protection must act quickly. In that case the circuit breaker is slower than the MV fuse that has current limiting capabilities. However, the circuit breaker will clear the fault in less than 100 ms, and this is effective enough to avoid any serious damages.

Low level MV earth-faults

In case of either high impedance earth fault on MV winding or solid earth-faults in impedance earthed neutral system, the earth fault magnitude is below the rated current of the transformer. Modern self powered relays integrate sensitive earth fault protection and then provide effective coverage on these conditions.

Case of public distribution

In public distribution applications, such as MV ring network configurations, utilities look for the simplest repetitive MV/LV substations that are dispersed in a large geographical area. The power of MV/LV transformer is generally limited to 630 kVA or less. Compact and often non extensible 3 function switchgear are specified by the utilities. In these cases, protection of MV/LV transformers by MV fuses offers an optimized solution (see Fig. B26).

Fig. B26 – Compact 3 function switchgear