Investigation of transient and safety issues in gas insulated systems

This thesis investigates the occurrence, characteristics and effects of Very Fast Transients (VFTs) associated with disconnector switching operations in Gas Insulated Substations. VFTs are analysed and efforts are made to elucidate their behaviour through advanced simulation techniques. The initial motivation for this work was the occurrence of a surface flashover at a spacer, leading to a prolonged outage of the circuit in question and a significant repair effort. While post failure investigations were carried out by the manufacturer and yielded no significant observations, through modelling and measurements efforts while working towards this thesis, a phenomenon that could have led or contributed to the failure. VFTs at a live, operational 400kV Substation (un-named for confidentiality but termed throughout as Substation ‘A’) are quantified through both modelling and measurements. Significant progress in the modelling of VFTs and TEVs is demonstrated. Numerical Electromagnetic Analysis is shown to be most effective method in studying the behaviour of the GIS and earthing systems. Multiple NEA techniques are utilised, all solving a full-Maxwell’s equations through a Wave equation. The behaviour of the system (both internally and externally) is captured with great accuracy and lucidity, without the need to use analytic approximations or assumed parameters, which has traditionally been the case. Detailed models were built using equipment drawings from Substation ‘A’ for the GIB, spacer-flange assembly, double-elbow assembly, disconnector, gas to air bushing. Frequency and time domain behaviour is analysed and a potential contributor to the failure at Substation ‘A’ is identified. Furthermore, elements of the earthing system were evaluated for effectiveness in mitigating TEVs. The methods highlight some of flaws and inaccuracies that are present with existing ‘standard practice’ modelling efforts. The need for circuit-based modelling for VFT studies is apparent, as NEA techniques at very high frequencies are limited in their interaction with the wider system. Efforts are therefore made to enhance circuit-based models; utilising NEA methods and Vector Fitting to produce accurate, large bandwidth equivalent circuits, which demonstrate the computed frequency responses of the various GIS equipment types studied. Vector Fit models at lower orders of approximation are prone to unstable time domain responses, leading to numerical oscillations or even a complete divergence from a solution. A method was developed to identify model orders that demonstrate stability in the time domain, allowing the lowest model order of approximation to be selected, thereby reducing the additional computational requirements of very high orders of approximation, while retaining accuracy and stability in the time and frequency domains. The conversion process is augmented with a new method for identifying model orders that will be stable in the time domain. Several measurement techniques and sensors were developed to capture the entire cycle of transients associated with disconnector operations. Device prototypes were designed and optimised through NEA/circuit-based modelling, prior to undergoing laboratory-based measurements. Laboratory based testing was conducted using a custom built, half scale GIB, with impedance matching cones at each end to allow measurement and signal generating equipment to be connected with minimal interference. While, essential, laboratory-based measurements will never replicate the transient and high EMI environmental conditions seen at a live GIS, therefore, the bulk of the measurement efforts were focused on live measurements at Substation ‘A’. Throughout the course of this project several opportunities to undertake measurements were presented and a significant amount of data was recorded. Each measurement also identified areas for improvement of the measurement system