Protections based by higher harmonics absolute measurements the zero sequence currents of the protected object connections against single-phase earth faults in resonant grounded cable networks of medium voltage industrial and urban energy supply systems have been widely applied in Russia since the late 60s of the 20th century. However, some operational problems connected with sufficient selectivity and sensitivity of these protection devices appeared with time. Sensitivity and selectivity of this protection are considerably determined by the instability degree of the higher harmonics total level in single-phase earth fault current of the protected network. Well-known Russian expert Kiskachi V.M. gave approximate estimate of the higher harmonics instability degree at the end of the 60s. Nowadays due to load changes in the main substations load in resonant grounded cable networks of medium voltage higher harmonics fluctuations in single-phase to earth fault current. The simulation models of this networks application and the accumulated experimental data about real networks allow to specify the existing estimates of higher harmonics instability in single-phase to earth fault current and their applicability conditions.
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