266 / 2018-09-25 00:33:41

HFCT-based Detection of Partial Discharge Currents on GIS Enclosures

Partial Discharge (PD) location,GIS,VHF/UHF,high frequency current transformer

The growing trend towards renewable energies is bringing new challenges to the conventional power networks. To accommodate the increasing power from wind offshore farms in deep waters, gas insulated HVDC systems are arising as the most suitable technical option over AC networks. Gas insulated systems (GIS) provide advantages such as significant space reduction over Air insulated systems, which becomes a cornerstone for offshore platforms where space comes to a high premium. However, HVDC GIS technology is still under development and as such nowadays there is not yet the availability of international standards for specification requirements and test procedures. The European Union-funded “PROMOTioN” project was established as the combined effort of several top manufacturers and Universities aiming to increase the level of readability of the HVDC GIS technology. PROMOTioN is researching a plurality of topics involved in the realization of offshore transmission HVDC networks, including guidelines for specification of requirements, procedures for testing, testing of new insulation gases and development of new diagnostics and monitoring (M&D) techniques with special attention to measurements of partial discharges (PD), being this last subject the purpose of this paper.
Difficulty in monitoring and interpretation of partial discharge under DC due to the lack of records as compared to the case with AC GIS has led to the application of the same principles and test methods for PD detection in AC systems on DC systems.
Under this approach, PD detection predominantly has been carried out by RF techniques in the VHF/UHF range. This technique has been preferred because its sensitivity is high, comparable to that of the IEC measuring systems and because a high signal-to-noise ratio can be achieved by tuning filters. One of the limitations of VHF/UHF methods is that the electromagnetic waves produced by a PD event are strongly attenuated as they propagate along the GIS. Therefore, the GIS has to be provided with antennas every few meters, otherwise, PD signals away from the antenna are not picked up. A novel and an alternative method, subject of this paper, proposes to pick up the conductive current of the PD event that is induced on the GIS compartments. In our measurement approach, HFCT sensors are installed in one or several of the bolts that fix mechanically two adjacent compartments with the spacer in between. When properly installed, the HFCT sensors can pick up a portion of the PD current that flows along the GIS and that unlike electromagnetic waves is less attenuated as it propagates. Measurements with an actual size, SF6, 245kV GIS at TU Delft allowed to validate the proof of concept of this new measuring system. Tests with calibration pulse and PD test-cells showed that the measuring system can pick up PD signals tens of meters away from its source, meaning an increased coverage of the GIS with a small amount of sensor. Moreover, since the HFCT sensors were designed with a lower cut-off frequency of 50kHz and sufficient bandwidth of roughly 110MHz, the system may have a potential for estimation of apparent charge of the PD under certain constraints. Some of the challenges that this HFCT-based system faces will be reported in this paper such as: the relation of the measured current to the PD current, the effect of distortion and attenuation as the PD pulse travels alone the different GIS components seen as changes in characteristic impedances, the effect of noise in the sensitivity and its performance compared to established VHF/UHF systems.