The OWTS method, developed at Delft University, gently charges the test object using a DC supply and, after a few seconds, the usual service voltage is reached. At this point, a solid-state switch with fast closure time creates a series resonant circuit from the test object and an air-cored inductor. This circuit begins to oscillate at the resonant frequency of ƒ = 1/(2π√LC). The inductance of the air core is selected so the resonant frequency is similar to the power frequency of the service voltage (within the 50- to 100-Hz range). Medium-voltage cable insulation usually has a relatively low dissipation factor. This combines with the low loss factor of the air-core inductor to produce a high Q (30 to 100) resonant circuit.

The result is an oscillating wave at the resonant frequency ƒ with a decay time of 0.3 to 1.0 second. This produces a few tens of cycles to energize the test object and partial discharge (PD) is initiated in a similar fashion to 50 (60) Hz inception conditions.

The specially developed measurement circuit detects all PD pulses that occur during the oscillating wave in compliance with IEC 270 recommendations. Location of PD pulses is performed by the traveling wave method. Values of capacitance C and tan δ (loss factor) can be calculated based on the oscillating wave time and frequency characteristics.

At the heart of the OWTS is an industrial PC that uses the latest Pentium technology. This is the user interface for the system and it provides complete control of the measuring cycle.

Specially designed circuitry measures PD in accordance with IEC 270 using a bandwidth of 150 kHz to 650 kHz. For PD fault location, the upper bandwidth limit is increased to 3 MHz.

Based on three years of development and field research, the OWTS is offered by Haefely Test AF and Hubbell Power Systems.
Circle 178 on Reader Service Card