Starting a transformer

[chouakis]

Iwas told that in certain case we have to inhibit the 2nd harmonic protection during a transformer start.
Can you help me with that

 

[GTstartup]

Harmonic restraint is a function of the differential relay. Check the instruction bulletin for the relay settings and test procedures.

 

[oldfieldguy]

We don't inhibit the second harmonic. Some protective relays use this characteristic of inrush current to desensitize differential elements during the energization of the transformer.

You understand that a differential relay looks at the current going into the transformer and compares it with the current leaving the transformer, and operates when the two do not match within certain limits.

When the transformer is first energized, current rushes in, which is why we cleverly call it "inrush current" but no corresponding current flows out.

An uninhibited differential element would operate under such circumstance. In order to prevent that from happening while still allowing the settings of the element to be sensitive enough, some relay manufacturers use the presence of harmonic energy in the inrush current to inhibit the element.

old field guy