Understanding the Implications of Partial Discharge Performance of Non-SF6 Gases
These days we are all aware of the need to reduce greenhouse gas emissions to address the climate emergency. Driven by this increasing awareness, the electricity sector is responding to public, political and regulatory pressures on many fronts including renewable power generation, storage technologies, R&D on carbon capture and, strengthening grids to facilitate e-mobility and replacement of fossil heating: of particular interest to D1, are materials with a lower CO2 footprint. The recently published 2030 Strategic Plan explains how CIGRE’s expanded scope will support many elements of the energy transition.
By Michael Walter, Convenor CIGRE WG D1.78 and Simon Sutton, Chairperson of Study Committee D1
The contribution of D1 on this path
Most of the green technologies rely on innovations in new materials that are applied in primary equipment, to make it less harmful for the environment. The introduction of such new technologies must be accompanied by extensive development tests and systematic type-, routine- and onsite tests to ensure the same reliability as we have today. D1 is at the heart of both, working to define the necessary test procedures and supporting the better understanding of the physical mechanisms associated with these new materials.
Green technologies at substation level
At the substation level, one of the key initiatives for GIS-, AIS- and GIL-equipment is the elimination of sulphur hexafluoride (SF6) gas, used for switching and for insulating purpose, being one of the most potent greenhouse gases known.
For switching, medium voltage vacuum circuit breakers (VCB) have been used for more than 30 years. In recent years, manufacturers have expanded the VCBs range towards progressively higher voltages: today 145 kV equipment is commercially available, with an outlook towards higher voltages in the near future. As an alternative for switching, as well as for insulation purposes, the application of non-SF6 insulating gases has been the focus of considerable research. These solutions are based on either:
- Mixtures of natural-origin gases (nitrogen, oxygen and carbon dioxide) or,
- Halogenated gases (with lower global warming potential compared to SF6) mixed with natural-origin gases.
The addition of a low quantity of a halogenated gas such as C4-Fluoronitrile (C4-FN) brings the advantage of maintaining insulation gaps distances, comparable as in SF6 products, but at the price of not reducing the GWP to <1. Mixtures based on natural-origin gases do reach GWP<1 but at the price of increased equipment size and with that an increase of grey energy of the material used. Both technologies, however, are suitable to reduce the ecological footprint of gas insulated equipment massively, especially when compared to existing SF6 technology.
Green substation of a Swiss distribution grid operator combining an air-insulated HV GIS with VCB technology and an air-insulated MV-AIS in a fully wooden environmental friendly building
In recent years Study Committee D1, and in particular the Advisory Group on Gases (D1.04), has studied the technical properties of these gas mixtures (work groups D1.51 and D1.67). At the same time, SC A3 and B3 have addressed aspects of these gas mixtures in MV and HV switchgear, including the arc quenching performance and their application (e.g. ageing, gas handling procedures and, health, safety and environmental aspects) respectively.
The importance of understanding the partial discharge properties of non-SF6 gas mixtures
To understand whether the test methods developed to ensure the reliability of SF6-filled primary equipment are suitable for the alternative insulation gas-mixtures, we must understand the risks associated with defects in such products. This includes potential ageing mechanisms and the detectability of critical defects during quality checks and in-service monitoring. Partial discharge (PD) measurements play a key role in addressing these questions. Conventional PD-measurement (IEC 60270) is today widely used for quality check at production sites, whereas UHF measurements are a sensitive tool for onsite quality checks before equipment goes into operation or during service.
PD measurement techniques are essential for detecting typical imperfections in GIS like protrusions, particles in the gas and at insulator surfaces, which may affect the integrity of gas-insulated systems during its life cycle. However, PD activity is dependent on many factors including the defect type, the gas or gas-mixture, and the gas pressure. To make use of the accumulated knowledge regarding PD behaviour in SF6-filled equipment, we must firstly understand the influence of changing the gas-mixture and pressure on the breakdown strengths in the presence of defects, and secondly, the PD activity before a potential breakdown. The first question was addressed in D1.51 and D1.67, while the latter is the focus of D1.78, “Partial discharge properties of non-SF6 insulating gases and gas mixtures”, launched in 2023 by SC D1.
This new Working Group will collect and summarise the current knowledge on the PD properties of the major natural-origin and fluorinated non-SF6 insulating gases and gas mixtures, which are currently proposed by the equipment manufacturers. If needed the group will also carry out further tests. Some of the topics in the scope of the group include:
- Impact of gas mixture on PD impulse waveforms, frequency spectra and phase synchronous patterns in the typical pressure range of HV and MV equipment in comparison to SF6
- Applicability of current PD measurement techniques for non-SF6 insulating gases
- Estimation of the detectability of critical imperfections
- Gas decomposition by PD, its measurement and potential impact on electric properties
These items will help asset managers in the future understand the performance of their non-SF6 insulating gas filled equipment; giving them the knowledge to interpret PD data and risk manage developing defects.
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