Telephone interference problem caused by generator triplen harmonic earth currents on an island system

The island community and systems are supplied from eight 1.35 MW and 1.5 MW diesel generators feeding directly into the island´s 11 kV transmission distribution system. When these generators are directly connected to earth at their star points a loud noise is heard on the island´s telephone system. This noise varies in magnitude and at times is loud enough to make communications impossible. To overcome this problem the generators have for the last 12 years or so been earthed through a spark gap in series with a nonlinear resistor. However there are disadvantages to this system. The author describes the problem and possible solutions including filtering the 9th harmonic and earthing the generators through zig-zag earthing transformers connected to the 11 kV busbars