Rade M. Ciric

Power flow in four-wire distribution networks-general approach

The neutral wire in most power flow software is usually merged into phase wires using Krons reduction. Since the neutral wire and the ground are not explicitly represented, neutral wire and ground currents and voltages remain unknown. In some applications, like power quality and safety analyses, loss analysis, etc., knowing the neutral wire and ground currents and voltages could be of special interest. In this paper, a general power flow algorithm for three-phase four-wire radial distribution networks, considering neutral grounding, based on backward-forward technique, is proposed. In this novel use of the technique, both the neutral wire and ground are explicitly represented. A problem of three-phase distribution system with earth return, as a special case of a four-wire network, is also elucidated. Results obtained from several case studies using medium- and low-voltage test feeders with unbalanced load, are presented and discussed.

Integrated fuzzy state estimation and load flow analysis in distribution networks

In this paper, a fuzzy algorithm for integrated state estimation and load flow analysis in the distribution networks is presented. The algorithm uses an available set of historical data and real-time measurements. The influence of historical data and participation of consumer categories in the node load, as well as influence of real-time measurements on the load estimation, are linguistically described using fuzzy set notation. The corrective fuzzy estimation for minimization of uncertainty fuzzy variables is based on available real-time measurements. Fuzzy solutions can be used directly, as input data for other distribution management system (DMS) fuzzy applications, or indirectly, in the classical algorithms (after the defuzzification is performed). The results and practical aspects of the proposed methodology application on two characteristic real-life distribution networks are presented.

Intermittent Fault Location in Distribution Feeders

Due to the utilization of fundamental frequency, current impedance-based fault-location methods are able to locate only permanent and linear faults. The duration of the arc in low- and medium-voltage systems can be as short as a quarter of a cycle. This period, which is normal for intermittent faults, is insufficient for fundamental frequency-based fault-location algorithms. Therefore, available methods are not applicable for an intermittent arcing fault location. In this paper, a novel method is proposed for arcing fault location in radial feeders, utilizing time-based formulation considering the short duration of the faults. The advantage of the proposed method over available methods is its capability for locating faults using fewer samples, which is suitable for arcing faults as well as normal faults in the network. Also, different types of faults are taken into account in the proposed algorithm. The validity of the devised algorithm is studied within the PSCAD-EMTDC environment and the results obtained show good accuracy for arcing faults. The application of the proposed algorithm in real systems is based on the availability of measured voltage and current waveforms at one end of the network and knowledge of cable/line parameters (self and mutual impedances).

Improved Fault Analysis Method Based on a New Arc Resistance Formula

To calculate fault currents in distribution systems accurately, the fault model must include the electrical arc existing at the fault point, which plays an important role in the fault currents calculation. In existing approaches, the value of the arc resistance at the fault location must be known in advance. Since the fault current depends on the arc resistance, which itself is a nonlinear function of the fault current, the key question here is how to calculate the arc resistance and the fault current at the fault location consecutively and accurately. In this paper, a new solution for the aforementioned defined problem using an iterative procedure is proposed and validated. The solution is based on an improved method for calculating short-circuit currents in distribution networks, in which the iterative hybrid compensation short-circuit method and a new formula for arc resistance are used. A significant achievement is that the improved method calculates short-circuit currents and arc resistance consecutively, improving the accuracy of the computation. The results of fault analysis and arc resistance calculation in the IEEE-34 distribution network with/without a distributed generator are presented and discussed. The practical importance of the improved method is addressed as well.

Integration of the dispersed generators in the distribution management system

Since the 80s huge efforts have been made to utilize renewable energy sources to generate electric power. An important issue about using renewable energy sources is a distribution management system (DMS) in presence of dispersed generators. This paper reports some aspects of integration of the dispersed generators in the DMS. Besides, an investigation of impact of the dispersed generators on the overall performances of the distribution systems in steady state is performed. In order to observe losses in the distribution networks with dispersed generators, several loss allocation methods are applied. Results obtained from case study using IEEE test network, are presented and discussed.

Multi-objective integration of real-time functions in distribution management system

Abstract In this paper, a new iterative algorithm for unique optimal deterministic solution, selected from fuzzy environment, in the integrated distribution management system real-time functions, is presented. The integrated fuzzy algorithm simultaneously finds solution of the consumer node load estimation, Var/Volt coordination and load flow problems. As fuzzy variables, the effective current and power factor of consumer nodes and source-node voltage are considered. The topology and parameters of the distribution system, as well as, on/off switching of shunt capacitors and the adjustment of off-nominal turn ratios of transformers, are considered as crisp variables. For determination of the most credible defuzzified solution, the minimum operator procedure applied on active loss minimization, subject to ‘soft’ voltage constraints in the different type of consumer nodes (commercial, industrial and residential), is used. Proposed integrated fuzzy model is tested on a real-life distribution system.

Power flow in four-wire distribution networks - general approach

Summary form only given. The neutral wire in most power flow software is usually merged into phase wires using Krons reduction. Since the neutral wire and the ground are not explicitly represented, neutral wire and ground currents and voltages remain unknown. In some applications, like power quality and safety analyses, loss analysis, etc., knowing the neutral wire and ground currents and voltages could be of special interest. In this paper, a general power flow algorithm for three-phase four-wire radial distribution networks, considering neutral grounding, based on backward-forward technique, is proposed. In this novel use of the technique, both the neutral wire and ground are explicitly represented. A problem of three-phase distribution system with earth return, as a special case of a four-wire network, is also elucidated. Results obtained from several case studies using medium and low voltage test feeders with unbalanced load, are presented and discussed.

Overview of the development, simplification and numerical analysis of synchronous machine models for stability studies

Synchronous machines, that is, practically all generators together with synchronous motors and synchronous compensators, are the most important power system components in the analysis of electromechanical and electromagnetic dynamics in power systems. Power system modelling (PSM) in general is an area of ongoing interest in the transmission management and control systems community. Continual development is driven by the traditional task of model maintenance and management to achieve increased accuracy with the expenditure of fewer resources [3]. Derived higher order models are simplified because they are very difficult to use for simulations, analysis and control. Simplification is therefore necessary and must be done methodically; this makes an understanding of the underlying assumptions imperative. Traditional methods of simplification include linearization for small signal stability analyses and reduced order procedures used in transient stability analyses [4-5]. This paper presents three of the existing synchronous machine mathematical models and the assumptions made in the process of their simplification, and introduces methods which are used to solve these models numerically for stability studies assessment.

Investigation into the impact of embedded generators on distribution system performance using theorem of superposition

Since the 1980s, huge efforts have been made to utilise renewable energy sources to generate electric power. One of the interesting issues about embedded generators is the question of optimal placement and sizing of the embedded generators. This paper reports an investigation of impact of the integration of embedded generators on the overall performances of the distribution networks in the steady state, using theorem of superposition. Set of distribution system indices is proposed to observe performances of the distribution networks with embedded generators. Results obtained from the case study using IEEE test network are presented and discussed.

Investigating effects of neutral wire and grounding in distribution systems with faults

In some applications, like fault analysis, fault location, power quality studies, safety analysis, loss analysis, etc., knowing the neutral wire and ground currents and voltages could be of particular interest. In order to investigate the effects of neutrals and system grounding on the operation of distribution feeders with faults, a hybrid short circuit algorithm is generalized. In this novel use of the technique, the neutral wire and assumed ground conductor are explicitly represented. Results obtained from several case studies using the IEEE 34-node test network are presented and discussed.