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Title: Contact erosion and nozzle ablation in gas blast circuit breakers
Author: Wang, W.
Awarding Body: University of Liverpool
Current Institution: University of Liverpool
Date of Award: 2018
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Circuit breaker is key protection equipment in the conventional and smart grid to isolate a faulty part of the network and maintain system stability. The physical characteristics of arc plasma in the presence of contact erosion and nozzle ablation are studied in the present work with the aim to produce optimised design of circuit breakers with extended service time, minimised maintenance cost, and adequate intelligence for using in smart grid. It is achieved by using a differential arc model in two-dimensional axisymmetric coordinate system and the model is implemented in a commercial Computational Fluid Dynamics (CFD) software package, PHOENICS. The thermodynamic properties and transport coefficients of pure copper vapour under local thermodynamic equilibrium (LTE) and non-equilibrium conditions are calculated first in Chapter 2, which can be used in the near electrode layer to determine the energy flux towards the cathode bulk. The non-equilibrium thermophysical properties of pure copper vapour have not been calculated before. The composition and thermophysical properties are obtained according to fundamental theory from 300 K to 30,000 K at 4 pressure levels of 0.1 MPa, 0.4 MPa, 0.8 MPa and 1.0 MPa. Non-equilibrium degree ranging from 1 to 4 is considered to cover potential plasma states. The LTE results from present work are compared with existing data. A near cathode non-LTE layer exists between LTE arc column and cathode surface. It is divided into a pre-sheath layer and a sheath layer. The energy flux for contact erosion depends on the physical processes inside the layers. Up to now, there has been no model that can correctly predict the erosion rate of the cathodic contact. A mathematical model that consists of the two-temperature (2T) pre-sheath and the collisionless space charge sheath is established in the present work in Chapter 3 to describe the physical processes inside the non-LTE layer. Copper vapour is the arcing gas because it has a much lower boiling temperature than tungsten when their mixture is used as the contact material in the real applications. A novel approach has been developed to bridge the solutions from the pre-sheath layer and the sheath layer, thus allowing a self-consistent determination of the near cathode layer potential drop for the non-refractory materials (copper). Our prediction of this overall potential drop (16.44 V) is reasonably in view of measurement results (15 V-17.5 V). Total mass evaporated from the cathode surface due to contact erosion is predicted at three current levels (i.e. 22 kA, 27 kA and 32 kA). The predicted mass loss is only 30% of the measured mass loss because the latter includes the droplets of the cathode material that are spitted by melting and mechanical stress. A computational study of nozzle ablation and its influences on the flow environment in a 145 kV/40 kA auto-expansion circuit breaker is carried out in Chapter 4. When the increase in nozzle diameter due to nozzle ablation is considered in the simulation, less subsequent nozzle ablation is normally obtained. The decreased ablated mass reduces the enthalpy and mass fluxes flowed into the expansion volume which results in a lower pressure rise. Results show that ablation leads to an excessive enlargement of the nozzle flat throat at high current and long arc duration, presenting as a limited factor that affects the lifetime of auto-expansion circuit breakers. The predicted specific ablation falls well within the experimental results obtained by ABB. Results obtained in the present work strongly suggest that, to achieve more realistic arc modelling, the dimensional variation of the nozzle due to ablation must be considered. The influence of nozzle ablation on the interruption process of an auto-expansion circuit breaker is studied in Chapter 5. The turbulence model is first calibrated using results from two tests (similar cases but differing in arc duration). It is found that the turbulence parameter c=0.32 for Prandtl mixing length model is applicable to predict interruption performance of 145 kV/40 kA auto-expansion circuit breaker. The critical rate of rise of recovery voltage (RRRV), which represents the interruption capability of the circuit breaker, are predicted with results well matching the test results. The arc model is then used to study the deterioration of breaker's interruption capability by computationally repeating the same test duty on the breaker with the consideration of nozzle enlargement. For a typical case (Test 98), it has been shown that the reduction of approximately 20% in the predicted RRRV of the breaker is mainly caused by the dimensional variation of the main and auxiliary nozzles, especially the change of nozzle diameter from 19 mm (auxiliary nozzle) and 21 mm (main nozzle) to 21.4 mm/24 mm (auxiliary nozzle) and 23 mm/24.6 mm (main nozzle), giving an increase in the flow area of averagely 20% (auxiliary nozzle) and 14% (main nozzle).
Supervisor: Yan, J. D. Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral