Learnings

Outcomes

Models for the computation of transients in GIS environments have been developed using a combination of Physical simulations using finite element techniques (COMSOL) and transient circuits electromagnetic analysis (EMTP/ATP). The simulations have highlighted key characteristics of VFTs in GIS and their propagation effects. It is now realised that it is important to undertake key experimental work to measure these VFTs.

The results obtained from the modelling have highlighted that, in contrast to previous understanding, the role of VFTs and their propagation is very important in determining the transients during GIS disconnector operations. The initial measurements have indicated high frequency transient of relatively low magnitude on the GIL length and at the surge arrester which might explain the high count seen on the surge arrester counter.

Good progress in developing new approaches to measure very fast GIS transients. The techniques are being improved to overcome challenges due to electromagnetic interference and high bandwidth.

Aside from the modelling and measurement techniques developed, one of the key outcomes of the project, related particularly to equipment studied, is that from measurement of the magnitudes of transients within the bus, has provided some quantification of the risk, particularly as limited switching operations are carried out at present due to this risk. The magnitudes calculated for the disconnector should be well below insulation limits for a clean defect-free system on the line side bus of the circuit being switched.

Recommendations for further work

The results from this project have helped to understand the VFT phenomenon in GIS and will contribute to understanding the root cause of the problem.  NGET will continue to work with the GIS manufacturer at Swansea North to understand the type of discharging that is occurring.  Optical windows will be installed to enable any discharge activity to be observed, suitable windows for the purpose were specified during this project.  Combined with the research carried out during this project it may be possible to engineer a modification to the asset to reduce the risk of insulation failure under normal operational switching.

Wider than any specific asset issue, this work has shown that measurements of VFTs are challenging but can help improve our understanding and will point to future solutions for the better measurements.  Photomultiplier and optical techniques are already being investigated for more reliable discharge analysis.

Lessons Learnt

High voltage measurements at the VFT range of frequencies is an extremely complex process. Access to switching of live assets is necessarily restricted to ensure such activities are conducted safely.  Development of sensors is therefore almost exclusively laboratory and simulation based. Reproducing realistic VFTs in the laboratory environment is difficult and will unlikely yield results or methods that apply generically to all GIS. 

Due to the complexity of VFTs and TEVs, many variables must be considered, layouts, heights, materials and even environmental factors. It was therefore essential to further develop the understanding of VFTs in this project, bringing in existing concepts from RF and EMC disciplines and elucidating the phenomenon through numerical modelling. Obtaining component characteristics from datasheets for the creation of high frequency models can be challenging even when it is possible, as equipment manufacturers are in most instances unlikely to have investigated the high frequency behaviour. The current practice is to make assumptions about the high frequency parameters; or utilise analytic calculations where available. This can lead to significant errors in the modelling process and make the model very difficult to validate. Overly optimistic models can lead to insulation failures; overly pessimistic models can lead to over-engineering and increased cost. Where equipment drawings are available and material types are easily deduced, the modelling methods explored as part of this project for extraction of equivalent circuits will greatly enhance EMT simulations.

A D-dot probe was designed, built and tested both in the HV laboratory and at a GIS substation. This will help to explore the shapes and magnitudes of VFTs in GIS. The probe was specially designed for a particular substation. By doing so we have learnt how to design and carry out measurements using the probe.

It is desirable, in future projects, to build a laboratory GIS model to replicate the VFT/TEV seen on in-service GIS. This will allow high performance measurement and mitigation techniques to be designed.

The measurements showed that D-dot principle can be used to measure the transient voltage. Due to the high EMI environment, sensor shielding, and acquisition system components require a greater degree of immunity and ideally fibre-optic transmission.

Throughout the suite of measurements carried out at Swansea North, a great deal of experience was gained and an extensive knowledge of the transients characteristics has been realised. Threshold magnitudes and durations have been identified, providing future measurement activities with a greater chance of success. More importantly, measurements have provided a degree of validation of the modelling concepts developed. Due to the magnitudes involved and nanosecond timescales, validated models and concepts are key to understanding the potential implications of switching in GIS.

Dissemination

A paper (Application of multiple modelling techniques for analysis of very fast transient overvoltages in GIS) was published at the International Conference on High Voltage Engineering (ICHVE) which was held in Athens, Greece in September 2018. The paper has been selected for publication in a future special issue of the MDPI Energies Journal (ongoing).

The paper can be found at http://orca.cf.ac.uk/121186/

Work to date has been presented at Cardiff University, at the National Grid headquarters and at the 2018 UHVnet conference in Winchester on 16th January 2018.

Posters were also presented at the UHVnet 2019 at the University of Manchester and at UHVnet 2020 at the University of Strathclyde in Glasgow.

A journal paper, entitled ‘Analysis of Very Fast Transients Using Black Box Macromodels in ATP-EMTP’ was published in MDPI Energies Special Issue ‘Overvoltage Protection of Electrical Networks’. The manuscript is available at https://www.mdpi.com/1996-1073/13/3/698.