Computer-Aided Analysis of Unbalanced Operating Conditions in Three-Phase Circuits Containing a Dynamic Load

A new approach to analyzing three-phase circuits in the phase coordinates under unbalanced normal and emergency operating conditions is proposed, in which the information about the three-phase circuit to be analyzed by means of software is entered in a simplified manner. The equivalent circuits of three-phase generators, power lines, and static and dynamic loads are aggregated and considered in a generalized form. With such presentation, the work with a three-phase circuit diagram is significantly simplified even if it contains unbalanced loads, a few faulty sections, and control links in the equivalent circuits of electrical machines. The labeling of three-phase circuit nodes is proposed that allows three-phase and single-phase parts of the circuit to be distinguished. The topologic list of branches intended for computer-aided calculations of currents and voltages and currents is compiled for three-phase branches in a generalized form. The obtained list is compact and retains a clear representation of the three-phase circuit. The analogy between the basic electrical equations written for electrical circuit three-phase and single-phase branches is shown. Thus, the voltages and currents in a three-phase element are interrelated by equations similar to the generalized Ohm’s law, while Kirchhoff's current law is written for three-phase nodes and has the same form as for single-phase circuits. The analogy of drawing up the incidence matrix and the matrix of nodal equations is shown. Submatrices of dimensions 3 × 3, 1 × 3, or 1 × 1 depending on the node label appear as entries in the incidence matrices and nodal admittance matrices of a three-phase circuit. The nodal equations used for carrying out the subsequent analysis of the circuit in the phase coordinates are written in a standard way as in single-phase circuits. In analyzing emergency operating conditions, it is proposed to keep the simplicity and clarity of the approach by representing the circuit faulty section of as a corresponding branch embedded into the three-phase circuit. The developed approach is illustrated by calculation of unbalanced and emergency operating conditions in a complex three-phase unbalanced circuit containing two synchronous generators, one dynamic load, and one static load. The calculation has been carried for four- and three-wire three-phase circuits.

Technical and economic comparison of power supply systems providing various degrees of reliability

The article presents a method of technical and economic comparison of power supply systems with different degrees of reliability, taking into account the potential economic damage from power supply interruptions. At the same time, the economic damage is determined based on the probability of emergency operating conditions and planned repair of a power supply system. In the vast majority of cases, it is inappropriate to achieve an excessive increase in the reliability of a power supply system, as this may lead to an economically unprofitable solution.

Robustness analysis of technological units for drinking water clarification: Normal and emergency operating conditions

The primary goal of a water supply system is the protection of human health by providing microbiologically and chemically safe drinking water. Significant changes in water quality require sufficiently robust systems for water preparation, performances of which are unaffected by present variations and changing operational conditions. Water turbidity is an important parameter for the water filtration control and efficiency of disinfection. The efficiency of turbidity removal in the drinking water treatment plant ?Vodovod? in Banjaluka under normal and emergency operating conditions was examined in this paper. At normal conditions the maximal detected value was 25 NTU while at emergency operating conditions it was above 240 NTU. Robustness evaluation of the water clarification system was performed separately for periods of normal and emergency operating conditions (during and after emptying the accumulation). The robustness index was calculated based on a more stringent target turbidity value (0.5 NTU) than that specified by the current legislation, which represents a new criterion in the risk analysis in the existing practice. Data processing results indicate high operational stability of technological units under normal conditions. The filtered water quality was below the target value during most of the time of filter operation in all cycles. The recorded turbidity value was ? 0.3 NTU for 92.9 % of filtered water samples. Analysis of the water turbidity data has shown that 17% of all taken measurements under emergency operating conditions (336 samples) had higher turbidity than the target value (0.5 NTU). Large variations in raw water turbidity over short periods of times during the emergency operating conditions, present a problem for prompt response in the drinking water plant. Calculated robustness index values point to inadequate efficiency of the water clarification process in a certain number of filter operating cycles. We have found a significant impact of the plant operating conditions on the filtered water turbidity under emergency conditions, such as suboptimal coagulation and flocculation conditions as well as the nature of suspended and colloid particles inducing turbidity and insufficient particle interactions with the coagulant. Along with the negative influence on water turbidity, excessive coagulant dosage leads to increased concentrations of residual aluminum in filtered water. Optimization of emergency working conditions could be performed based on adequate monitoring of water sources, which would further decrease potential risks of pathogen appearance in drinking water.

Applicability of (K, T-Stress) Methodology to Analyze RPV Under Thermal-Hydraulic Transients

The (K, T-stress) methodology developed by Gao and Dodds [1] is being utilized to introduce crack front plasticity with constraint effects when plastic deformation occurs in structures, for example, when the Reactor Pressure Vessels (RPVs) are subjected to thermal-hydraulic loadings. One crucial step in this procedure is to quantify combinations of flaw geometries and loading conditions (transient sequences) that illustrate the limits of applicability of the two-parameter (K, T-stress) advanced fracture methodology relevant to integrity analyses of RPVs subjected to normal and emergency operating conditions. Numerical analyses were conducted to determine the limits of applicability of (K, T-stress) advanced fracture technology for RPV under thermal-hydraulic loadings. The numerical results indicate that the (K, T-stress) methodology captures the constraint condition of the RPV with typical embedded flaws under a postulated dominant thermal-hydraulic transient.