Generator Earth Fault Protection

1. 95% protection - only 95% of generator stator winding is protected (5% at "star point" end of winding is not protected).
2. 100% - the whole of the winding is protected.
3. Harmonics of fundamental and unbalance at fundamental cause star point not to be at zero volts. Measurement of voltage at star point gives some % of nominal on healthy system, limiting setting of relay to 5% or greater. Note that the usual construction of power transformers is not totally symmetrical and capacitance coupling from HV windings can cause an LV potential off-set.
   
4. A short to earth less than 5% of turns from nominal earth potential at star point is not detectable. The displacement of the star point is less than 5% for a short to earth less than 5% of turns from earth end of winding.
5. The solution, where generator is costly enough to warrant protection, is to bias the star point above ground at a frequency not found in normal service (and not a sub-harmonic) - Using a power source of about 20 Hz is common - and use a frequency selective relay. If the generator stator actually has a short to ground in the bottom 5%, a 20 Hz current flows and is detected.
6. Note it is usual to earth the generator star point through a high resistance, which limits the current to say 10 amps at 100% of turns, to minimise damage by an earth fault arc - this earthing is not a problem with a big generator [I mean 100s of MVA] which is connected through its own transformer to the Grid. Note that big generators may use "graded" insulation, which is thinner at the star end, but saves space for more copper or cooling - smaller generators with full voltage insulation on all the winding have little risk of a fault where the the working voltage is 5% or less (near star end).

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