Modern high-voltage cable-testing plant

Describes the equipment put down by Standard Telephones and Cables, Limited, at their North Woolwich works for cable testing up to 87,000-volt single-core and 66,000-volt three-core cables. The plant for the high-pressure tests consists of three 100 kVA, 150,000-volt transformers which can be connected in series to give 450,000 volts, or in three-phase star to give 260,000 volts between lines. Voltage control is obtained by induction regulators and by exciter field variation. Both controls are interlocked so that it is not possible to close the main switch except when the voltage is at a minimum. The pressure is measured by a four-range Abraham-Villard type of electrostatic voltmeter reading up to 300,000 volts, or by the flashover distance between 10-in. spheres. Frequencies from 25 to 50 cycles are obtainable, but the maximum voltage can only be obtained at 50 cycles. For load tests three 25-kVA transformers giving 400 A are provided, the secondaries being insulated to withstand 150,000 volts, so that the load test can be applied while the cables are under pressure. The temperature-rise of the cable is measured by superposing a small direct current from a 2-volt traction battery on to the load current and comparing the drop due to it in the cable with that due to it in a shunt in series with the cable. This method has not previously been applied to high-pressure circuits. Dielectric losses are measured by a Dudell-Mather astatic wattmeter insulated to enable it to be used direct in the high-pressure circuit. A Dudell oscillograph is used for wave-shape measurement, also insulated for use in the high-pressure circuit. The testing tanks can be filled with water and steam heated as an alternative to heating by circulating currents in the cable. The whole apparatus is controlled from a single-control desk, and power is automatically cut off by the opening of the testing enclosure doors; power is also cut off from the overhead crane if it approaches the enclosure during a test.