Central Electricity Generating Board

Expansion of power network has led to the construction of multiple transmission and distribution lines within constrained spaces inevitably resulting in them being mutually coupled. This introduces a new set of challenges to protection engineers to accurately determine the reach of conventional protection schemes like differential, distance and directional earth fault with carrier aided schemes, making accurate protection grading a complex task. Protection settings without meticulous investigation have led to maloperation of the relays which degrades the overall power system reliability. Zero-sequence impedance-based tripping logics have shown operational issues under such mutual coupling of power lines, whereas secondary tripping logic, such as negative sequence protection has warranted an accurate fault recognition. In this paper, the usability of negative sequence protection of such instances is extensively analysed to mitigate the nuisance tripping of such mutually coupled transmission lines. A realworld case study of relay maloperation is analysed using detailed PSCAD modelling of the network under different fault conditions with protection scheme applied. The results of the case study have revealed that the proposed scheme successfully eliminates the instances of incorrect tripping, ensuring precise fault identification across various fault scenarios, while maintaining compatibility with the existing protection coordination of mutually coupled lines.

The performance of a new approach for multiobjective voltage stability constrained-optimal power flow (VSC-OPF) formulation is compared with a conventional VSC-OPF approach for improving the steady-state power systems security in this paper. The compared OPF problems involve the minimization of the fuel cost and minimization of novel line stability index (NLSI) for case 1 and minimization of fuel cost and maximization of line static stability margin (CBI) for the proposed case 2. Clerc's constricted PSO algorithm modified with the nondominated sorting algorithm is used for obtaining the best solution point for each of the decision variables. Network security constraints and bus voltage limits are considered along with the constraint on permissible line stability margin limit adopting the quadratic penalty model for constraints violations. The approach is tested on a load-modified IEEE 30-bus and actual Nigerian 28-bus systems. From the results obtained, the proposed OPF formulation performed better for simultaneous consideration of voltage stability improvement and line loss reduction.

This work proposes an Integer-coded, multiobjective Genetic Algorithm (IGA) applied to the full Reactive-power Compensation Planning (RCP) problem considering both intact and contingent operating states. The IGA is used to simultaneously solve both the siting problem-optimization of the installation of new devices-and the operational problem-optimization of preventive transformer taps and the controller characteristics of dynamic compensation devices. The aim is to produce an optimal siting plan that does not violate any system or operational constraint and is optimal in terms of the voltage deviation from the ideal and the cost incurred through the installation and use of reactive power compensation devices. This multiobjective problem is solved through the use of Pareto optimality. The developed algorithm is tested on the IEEE 30-bus system and on a reduced practical system that was developed with the cooperation of the National Grid. The algorithm is validated via the comparison with the SCORPION software package, which is a Linear Programming-based (LP) planning tool developed and used by the National Grid for the England and Wales transmission system. This work demonstrates that the IGA is superior to the LP-based method, both in terms of system conditions and installation and utilization cost when fixed and dynamic compensation devices are being sited; the system performance is optimized via the adjustment of tap settings and controller characteristic across multiple operating states.

Abstract This paper investigates the switching impulse and temporary overvoltage (TOV) duty for 400 kV underground power cables using ATP. This work together with the work reported in [1], investigates the full range of transient overvoltages due to lightning, switching and other temporary overvoltages (TOVs) using ATP. [1] re‐assesses the basic insulation level (BIL) for 400 kV oil‐filled cables. The objective of this work was to check the impact of switching surges on the choice of BIL and to re‐assess the switching impulse insulation level (SIL) and TOV withstand level for 400 kV cables. A comprehensive study of the transient overvoltages experienced by cables within National Grid's transmission system has been performed. This paper provides an argument, for reducing the BIL and SIL for 400 kV cables.

An Fe-1200ppmP-30ppmC alloy has been examined after aging and irradiation exposure at 240°C. Thermal aging for 97 days reduced the yield stress by 10% but increased the 40J Charpy transition temperature by 50°C. Irradiation for a similar duration, in a highly thermalised neutron spectrum at 5.8 ± 1.7×10-11dpa/sec, increased both the yield stress by 15% and the transition temperature by 70°C. Longer irradiation, in spectra with or without the high thermal fraction, produced additional hardening but little extra embrittlement. Irradiation and aging gave a fivefold increase in the fraction of brittle intergranular fracture. Auger Spectroscopy showed that the P grain boundary monolayer coverage was virtually unchanged for all start-of-life, aging and irradiation conditions. In contrast, the C coverage reduced by 50% during the initial aging or irradiation, but changed little after longer irradiations. Grain boundary N increased initially but then stabilised. No grain boundary S occurred. Transmission microscopy revealed some dislocation loops and small regions of strain contrast in the grains after irradiation but no P-rich precipitates were seen. From this, and other published work, it is concluded that irradiation hardening arises from dislocation loops and possibly from P-rich cluster formation. Embrittlement developed due to loss of C from the grain boundaries, rather than from changes to the boundary P content. Loss of C reduced the boundary fracture stress but embrittlement was independent of the matrix strength. For the irradiation conditions examined, there was no evidence of irradiation-induced grain boundary P segregation.

SUMMARYGrain boundaries in metals provide preferential sites for impurity atoms to segregate, forming an area of the metal with different chemical and physical properties compared to the bulk. This also means that the electron beam damage characteristics of the grain boundary might be different from that of the bulk. Segregation can be expected to make grain boundaries particularly vulnerable to electron‐beaminduced hole drilling and beam damage, if the segregation produces a near two‐dimensional compound with increased ionicity. Here we report preferential hole drilling at grain boundaries in an Fe‐0–4 wt%P alloy. Such a phenomenon confirms evidence from energy‐loss spectroscopy that charge transfer occurs between P and Fe at the grain boundary. It also explains why it is easier to detect phosphorus using EDX compared to PEELS, and suggests that the electron beam damage may be a major limiting factor in grain‐boundary studies.

SUMMARYIncreasingly sophisticated demands placed on metals in the modern world have stimulated the use of tailor‐designed multi‐component alloys in many applications. Solute segregation, whether due to thermodynamic or non‐equilibrium processes, can dramatically alter the alloy composition near the grain boundary leading to drastic (and often detrimental) change in the properties of the alloy. The solute segregation profile is often a nearly singular function centred at the boundary, leading to great difficulty in analysis. The traditional method of depth profiling using Auger electron spectroscopy can only examine fractured surfaces and, because of limited spatial resolution, yields laterally averaged information. Recent improvement in the efficiency of electron‐microscope‐based electron energy‐loss spectroscopy has allowed a new high‐resolution approach in analysing the chemical and electronic properties of buried interfaces on a nanometer scale, i.e. we can study the complicated details of the alloy boundary itself. As an example, we will present the result of a study on stainless‐steel alloys used in the U.K. nuclear industry. The effect of irradiation on the structure of the alloy boundary is investigated. The nanoscale change in the chemical state of the alloying element at the grain boundary is observed for the first time in situ. The result can be potentially utilized to predict the corrosion‐resistance of the stainless steels.

Significant improvements have been taking place in the power and extent of the monitoring and control systems employed by electrical power utilities, with the result that strategic decisions are having to be made concerning the use of the increased data now available. The evolutionary changes being made to power systems are introducing increased expectations of the performance of system management and control. However, more effective system control can only be exercised if the increased volume of data is matched by an increased capacity to interpret system conditions from the control room. The decision support system described in this paper makes use of expert system techniques in order to provide intelligent reasoning based on the incoming alarm data. Both the readily available benefits, and the advance towards the next generation of power systems which this type of system will afford shall be presented along with a description of the decision support system.