Using Broken Delta Protection for Earth Faults

The Broken Delta configuration is used to protect against earth faults. It is a configuration that works by monitoring the vector sum of the phase voltages.

It consists of a delta connection on the secondary of a potential transformer that is open at one point as in the figure. In such a construction, when a balanced three phase voltage is present in the star connected primary, the voltage across the broken point of the delta connection would be zero, as it would be a vector sum of all three voltages.

In the event of a ground fault in one phase, the phase-to-ground voltage in the remaining two phases is now equal to the phase-to-phase voltage with a displacement of 60 degrees. The voltage at the broken delta becomes 3Vo or three times the phase-to-ground voltage.

This voltage can be measured by a relay and can be used to trip the power system. A resistor is usually connected across the broken delta connection to prevent Ferro resonance, a condition that occurs when the line capacitance and the inductance in the potential transformers reach a state of resonance.

Broken delta transformers are usually marked as 11kV/√3:110 volts. It means that the transformer is designed that in the event of a ground fault in the primary side, the secondary output will be 110 V.

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