Tomislav Tomiša

Nonlinear digital simulation model of hydroelectric power unit with Kaplan turbine

Mathematical and simulation models enable hydroelectric power unit dynamic behavior analysis using computers. To approximate the power unit dynamics as accurate as possible, nonlinear high-order model is required. This paper describes a hydroelectric power unit with double regulated Kaplan turbine. The runners of such turbines permit the blades angle to be varied on the run depending on operating conditions. This ensures maximum turbine efficiency under all operating conditions. Detailed mathematical and digital simulation model of the hydroelectric power unit with double regulated turbine was presented for turbine control system analysis. Simulation model developed for Varazdin HPP (power unit 1) is verified by several comparisons with real measurement results.

Load forecast of a university building for application in microgrid power flow optimization

Microgrid is defined as a cluster of distributed generation sources, storages and loads that cooperate together in order to improve power supply reliability and overall power system stability. Short-term power production and load profile prediction is very important for power flow optimization in a microgrid, thus enhancing the management of distributed generation sources and storages in order to improve the microgrid reliability, as well as the economics of energy trade with electricity markets. However, short-term load prediction is a complex procedure, mainly because of the highly nonsmooth and nonlinear behaviour of the load time series. In this paper we develop and verify a neural-network-based short-term load profile prediction model. Neural network inputs are lagged load data, as well as meteorological and time data, while neural network output is load at the particular moment. Neural network training and validation is performed on load data recorded at University of Zagreb Faculty of Electrical Engineering and Computing, and on meteorological data obtained from Meteorological and Hydrological Service of Croatia, in period 2011-2013.

Simulation of undervoltage load shedding to prevent voltage collapse

Recent blackouts in USA, Italy and London discovered problems with systems in operation near to their limits. After deregulation of power systems in industrialized countries, large problems with voltage stability incurred due to the larger and larger amounts of power transmitted to larger and larger distances. Systems often work on their limits and single contingency is sometimes enough to cause system collapsing. Best way for voltage instability prevention is fast and enough local reactive power reserve. Also, building of new power plants and transmission lines helps in voltage collapse prevention. These projects often take several years to complete, and in a lot of cases there are big problems with acquiring suitable locations and corridors. One of the most economic methods to prevent larger scale voltage collapse is undervoltage load shedding (UVLS). In this article, most important characteristics of UVLS are shown and their implementation in New England test system. Loads are modeled with different amounts of motor load. Simulation of system behavior is presented as well.

Comparison of various methods for determining direction of harmonic distortion by measuring in point of common coupling

Various methods dealing with determining of harmonic distortion direction are summarized in two groups, as multi point and single point measurement. This paper presents comparison of various methods for determining direction of harmonic distortion by measurements taken in single point of common coupling (PCC). Advance methods require complete system parameters for its calculation and often unavailable tests (harmonic current injection, switching tests) for determining the values of harmonic impedances. In this paper we are dealing with methods that require only measurement of current and voltage value in PCC. Some methods are based on utility and customer impedances modeling, some on calculation of harmonic reactive power and some only on harmonic current and voltage parameters. In this paper five methods will be described and processed through measurement on the simple test system.

A mathematical model for studying power system frequency changes

Mathematical model for studying electric power system (EPS) frequency changes is developed. The model gives user an estimation of the weighted average frequency of the entire electric power system during time range of primary control. In paper are presented effects of changing some model parameters on system frequency behavior.

Dynamics analysis of 220 V DC auxiliary system in power plant using different mathematical models

The calculation of short-circuit currents in DC auxiliary systems of power plants and substations is essential for design and application of distribution and protective devices used in these systems. Because of differences between DC and AC auxiliary systems, DC system protection designers should be take into account that important fact in calculation and dimensioning of DC auxiliary system. There are several mathematical models that are usually used for calculation of short-circuit currents in DC auxiliary system. In this paper, the authors performed short-circuit currents calculation for one real DC auxiliary system of power plant using three different, well-known, mathematical models. The results of calculations were compared with results given by testing of selectivity of DC auxiliary system on the site. Based on comparison results of different calculations and testing on the site conclusion related to choice one of three the most often used mathematical models as the most suitable has deduced.

Inter-area oscillations damping using dynamic braking and phasor measurements

In this paper, the use of dynamic braking as a cost-effective measure for damping inter-area oscillations is considered. Proposed control scheme assumes possibility of multiple insertions of dynamic braking resistor triggered whenever the speed of equivalent one-machine infinite bus (OMIB) system exceeds predefined value or controlled based on commanded electrical power calculated from OMIB transformation. OMIB is formed based on the measurements provided by phasor measurement units (PMUs) and data collected from energy management system. Results obtained using simple four machine and 68-bus test systems, are included to illustrate capabilities of investigated control scheme.

Sun trajectory modeling for specific location using PV measured data

In order to research the PV module characterization it was necessary to develop the Prototype PV characterization station. Among other possibilities derived from the measured results of Prototype PV characterization station it is also possible to model the Sun trajectory at installation location. Such measured results can be used to model Sun trajectory. Also, the Sun trajectory generated using presented model is verified with conventional analytical model Sun trajectory. Some conclusions for new approach are presented and discussed.

Specific features of excitation system of different distributed power plants

The paper describes through the examples the excitation system in small hydro and wind units with a synchronous generator. The specificities depending on its application are emphasized. The ability to minimize differences in hardware and software in order to achieve the universal product in terms of production, without losing the quality of a product is described.

Digital simulator for transient condition analyses in hydroelectric power plants

This paper presents a basic non-linear mathematical model for investigation of normal and transient operations in high pressure hydroelectric power plants. On the basis of this model an original digital simulating model has been developed by using software-hardware XANALOG system and the MATRIXx software package because they are suitable as a training simulator for the operators in hydroelectric power plants as well as in real operating conditions. Theoretical results have been verified in hydroelectric power plants in Croatia (75 MW and 480 MW).