The overvoltage relay (device 59 in Figure 18.3) used for ground faults should be sensitive
to fundamental frequency currents and insensitive to third harmonic currents and voltages,
which may be 15% or so of the fundamental due to unbalanced loading on the generator.
Some relays that are used for this type of ground protection have harmonic restraint, which
filters out the third harmonic and allows the relay to respond only to the fundamental [5].