Damir Šljivac

Prediction of the time-dependent failure rate for normally operating components taking into account the operational history

Abstract The prediction of the time-dependent failure rate has been studied, taking into account the operational history of a component used in applications such as system modeling in a probabilistic safety analysis in order to evaluate the impact of equipment aging and maintenance strategies on the risk measures considered. We have selected a time-dependent model for the failure rate which is based on the Weibull distribution and the principles of proportional age reduction by equipment overhauls. Estimation of the parameters that determine the failure rate is considered, including the definition of the operational history model and likelihood function for the Bayesian analysis of parameters for normally operating repairable components. The operational history is provided as a time axis with defined times of overhauls and failures. An example for demonstration is described with prediction of the future behavior for seven different operational histories.

Small wind turbine power curve comparison

This paper focus is on the small wind turbines resource potential estimation. Assessment is done for seven selected small wind turbines and one measured set of wind speed data with the micropower optimization modeling tool HOMER. Goal was to investigate how estimated energy production and economical parameters are sensitive to the selection of small wind turbine. Selected turbines have similar rated power, but different blades diameter and aerodynamic characteristics. Energy production was quantified for one year with hourly resolution. Results from all different wind turbines were compared on the power production base, and on the economical base. Two sensitivity cases related to the wind speed and installation lifetime were also simulated. Results are showing significant importance of the small wind turbine selection for the both total energy production and economical feasibility. This makes small wind turbine characteristics such as reliability and power curves testing very important.

PROBABILITY DENSITY FUNCTIONS OF VOLTAGE SAGS MEASURED INDICES

Probability Density Functions of Voltage Sags Measured Indices Voltage sags can cause interruptions of industrial processes, which could result as a malfunction of equipment and considerable economic losses. Thus, it is very useful to see certain rules of voltage sags occurrence due to duration and depth. This paper presents statistical analyses of voltage sags in several domestic and industrial transformer stations. Voltage sag probability functions are calculated from actual measurement data, by means of a hill climbing algorithm. Lognormal and Weibull frequency distribution functions are used to describe distribution of measured voltage dips.

State enumeration approach in reliability assessment of the Eastern Croatia bulk power system

As an introduction to reliability assessment of a power system, the paper deals with the state enumeration approach in computation of the reliability indices. The Markov state-space model of power system reliability components. is highlighted. The possibilities of COMREL (composite reliability) computer program used for all computations are presented. The conditional probability evaluation technique as a concept for calculation of reliability indices is explained. In addition, the results of the performed reliability assessment of the bulk power system of Eastern Croatia are discussed.

Exploitation of public lighting infrastructural possibilities

Paper deals with the possibility for exploitation of public lighting infrastructure for multiple purposes. Public lighting infrastructure is not fully utilized. Therefore there is a great potential to reduce electricity and operational costs by establishing smart public lighting. The first step in that intention is to replace existing lamps with LED-based technology that will result with reduction of energy and operational costs. By connecting those LED lamps municipalities can additionally cut energy costs which is result of real time intelligent management and faster response. Smart public lighting includes many functional units such as remote management system, sensors for traffic monitoring, traffic counter, digital traffic signs, integrated electric vehicle charging stations and much more. The combination of all these features makes public lighting system smarter and more exploited. At the same time this is a good way to establish the smart lighting system and an intelligent traffic system. The paper presents public lighting operational profiles with analysis of dimming possibilities of LED lighting lamps.

Upravljanje potražnjom u distribucijskom sustavu s fotonaponskim elektranama

In the last years, there was a great effort on inclusion electricity generation from the renewable energy sources in power system. Random renewable generation creates the imbalance between electricity production and consumption, which requires power plants with fast response and also storage systems. Generally accepted solution for load balancing is concept of smart grids and one of the elements of smart grid efficiency is the ability of real-time demand-supply balancing. In this paper, the model of the part of power distribution network of the city of Osijek has been created based on results of the power measurements of total electricity consumption in a family house in Osijek, air conditioning system consumption and PV power plant production. Also, algorithm for real-time load management is proposed. It assumes coordinated control of air conditioning system units depending on the production of PV power plants and electricity consumption of distribution network, in order to reduce peak demand in the distribution network.

Coordination of protective relays in MV transformer stations using EasyPower protector software

The analysis of digital protection relays setting in a MV transformer station grounded via a low-ohmic resistor is presented. Theoretical background on low-ohmic resistor grounding in a transformer station is provided. EasyPower PowerProtector software is used for simulation and relay settings. Protective relay coordination after the simulation of a 3-phase fault and a line-to-ground fault on different selected buses in transformer station 35/10 kV is performed. Digital ABB relays series REF 541 were used for low-ohmic resistor thermal, short-circuit and overcurrent protection and overload protection of the low voltage transformer side. The results of analysis indicate correct selectivity of overcurrent protection devices in all cases.

Influence of distributed generation from renewable energy sources on distribution network hosting capacity

The influence of distributed generation (DG) from renewable energy sources on the distribution network (DN) hosting capacity was analyzed in this paper. The analysis was conducted on the simulation model which represents a real DN with the DG consisting of medium size biogas combined heat & power (CHP) and small size household photovoltaic (PV) systems. Main goal of this paper was to analyze how the different voltage regulation methods and network topology manipulations like the integration of a new substation near the DG and adding new power lines affects the DN hosting capacity for the DG. Power flow simulations of different cases in this paper were analyzed by monitoring the voltage or current limitation levels of the elements into the network. Effect of the amount of power injected by the DG into the network on power losses was also studied.

Influence of load management in distribution network on voltages and active power losses: Case study

Production of photovoltaic (PV) power plants depends on weather conditions which can cause sudden and often unpredictable variation in their output power. Balancing the production from renewable sources with consumption requires utilization of power plants with fast response or energy storage systems what causes considerable costs. In recent years, in terms of balancing, the emphasis is on the higher role of the consumers. In this paper computer model of the part of distribution network is used to simulate and analyze demand side load management (using air conditioner installed in households) in the distribution feeder considering PV power plant expansion scenarios. Three case studies are analyzed: first with low penetration of PV plants in distribution system, second with moderate penetration of PV power plants and third with high penetration of PV power plants. Influence of demand side load management on network voltages and active power losses is investigated.

Microgrid energy management system in a public building

Penetration of renewable energy sources, as distributed sources, and market liberalization have introduced a new concept in the power system. This globally new concept is to some extent based on the stochastic production of renewable energy sources which creates imbalance between production and consumption. One possible solution for that imbalance is a generally accepted concept of microgrids. Microgrids as an integral part of the power system, combine distributed energy sources, energy storage and load. However, in order to develop and deploy microgrids it is necessary to ensure active management of distributed sources and distribution networks. The aim of this paper is to present implementation of a microgrid in a public building by describing the concept of microgrids and offering a variety of models. Apart from indicating the possibilities and advantages of microgrids, the models identify main technical drawbacks and limitations.