Croatian Power System Research Articles

Impact of the new electric arc furnace on the level of flicker in surrounding transmission and distribution power system

Point of connection of Ironworks is 110 kV bus in 110/35(33) kV "Željezara 1" substation in Sisak in Croatian transmission power system. Installation of new electric arc furnace, requires increased agreed power for Ironworks from TSO. New electric arc furnace is connected on low voltage side of T3 transformer with ratio 110/33 kV. As a part of the test operation of the new electric arc furnace in Ironworks in Sisak, detailed measurements of Voltage Quality, in different operating modes of the arc furnace were performed. The purpose of the measurements was to determine Voltage Quality on point of connection of Ironworks during the test operation of new arc furnace and to determine negative influence of Ironworks on Voltage Quality according to the Grid Code of Croatian Power System. Measurements were performed simultaneously at point of connection of Ironworks and surrounding transmission and distribution power network. Measurement results show that in all measuring points flicker short-term and long-term flicker severity exceeds prescribed limits according to the Grid Code of Croatian Power System. From the measurement flicker transfer coefficients between different voltage levels were calculated and conclusion about flicker attenuation was given.

Provision of Virtual Inertia Support Using Battery Energy Storage System

The paper presents the importance of the grid inertia constant for the frequency stability of the future high-res low-inertia power systems. Since more and more renewable energy sources (RES) are being connected to the power system via frequency converters, the grid inertia constant is reduced. This issue can be mitigated by applying appropriate control mechanisms through which RES can provide virtual inertia and provide rotating reserves for primary frequency control. The concept of a virtual synchronous generator for providing virtual inertia is elaborated, as a solution to the presented problems. By applying virtual synchronous generators, RES can provide support for frequency control during disturbances almost like conventional synchronous generators. The influence of virtual inertia on the stability of the Croatian power system was analyzed using a battery energy storage systems (BESS) with a control mechanism that enables its participation in frequency control.

The real-time coordination of a wind-hydro power generation

This paper introduces the real-time coordination of the wind and hydro power plants in the case of a part of the Croatian Power System where hydro–wind coordination is represented by Vrataruša wind farm and Senj and Vinodol hydropower plants. The model uses real data which represent generation units that make the power system. Also, the paper describes the general problem which is specific for these type of energy sources. For modeling hydro-wind coordinated generation, the MATLAB/Simulink model is developed. Obtained results, rotor speed, an active and reactive power of wind power plant, voltages, rotor speed, rotor speed deviation, active output power, and stator current of a hydroelectric generator are presented and analyzed

Market Power of Coordinated Hydro-wind Joint Bidding: Croatian Power System Case Study

The paper analyses the coordinated hydro-wind power generation considering joint bidding in the electricity market. The impact of mutual bidding strategies on market prices, traded volumes, and revenues has been quantified. The coordination assumes that hydro power generation is scheduled mainly to compensate the differences between actual and planned wind power outputs. The potential of this coordination in achieving and utilizing of market power is explored. The market equilibrium of asymmetric generation companies is analyzed using a game theory approach. The assumed market situation is imperfect competition and non-cooperative game. A numerical approximation of the asymmetric supply function equilibrium is used to model this game. An introduced novelty is the application of an asymmetric supply function equilibrium approximation for coordinated hydro-wind power generation. The model is tested using real input data from the Croatian power system.

Challenges of High Renewable Energy Sources Integration in Power Systems—The Case of Croatia

This paper presents a high-level overview of the integration of renewable energy sources (RES), primarily wind and solar, into the electric power system (EPS) in Croatia. It presents transmission system integration aspects for the particular case of this country. It explains the current situation and technical characteristics of the current conventional generation units and currently installed wind energy capacities. Based on the current situation future development scenario is determined and used to evaluate the impacts of the wide-scale integration of renewables. Grid connections aspects, power balancing, market participation, and inertia reduction aspects are considered. Furthermore, some specifics of both solar and wind integration are discussed identifying problems and potential solutions. Primarily through the provision of the inertial response of both solar and wind and through better forecasting of wind production. Finally, the outlook for the Croatian power system is given, that will most probably double its RES capacity in the coming 3-year period and a certain level of investments and changes of current operational practices will need to be provided.

Role of Electric Vehicles in Transition to Low Carbon Power System—Case Study Croatia

One of the major tools for the implementation of low carbon strategy goals is increasing the penetration of renewable sources, which are mostly intermittent in nature, into the power system that also increases the needs for additional storage and flexibility capacity in the system. Among other possible solutions, one very most promising tool is the significant electrification of the transport sector. A slightly modified and already verified power system model used for Croatian low-carbon strategy was used here. The PLEXOS software was used to model the Croatian power system by simulating different scenarios. Two scenarios were examined: with and without electric vehicles. This research aimed to evaluate the total decrease in CO2 emissions from both the transport and power sectors due to the increased number of electrical vehicles. The analysis of the Croatian power system was used to assess the flexibility potential of such a large number of electrical vehicles on power system flexibility while considering the volatile nature of wind and solar. Additionally, a question regarding solar availability and simultaneous low-availability of parked electrical vehicles was also examined.

Impact of wind capacity share, allocation of inertia and grid configuration on transient RoCoF: The case of the Croatian power system

Increased wind energy penetration influences the power system dynamic response to transient disturbances. Replacement of conventional production units with converter-connected wind turbines reduces natural power system inertia contained in rotational masses of synchronously connected turbine-generator units, therefore creating low-inertia power systems. Such a transition has an adverse effect on system resilience to disturbances and on the capability to maintain stable operation. This research examines the impact of high regional wind power production on system transient stability in the case of island operation of the Croatian power system. The system is divided into four geographical areas modeled as four centers of inertia with aggregated parameters. The study investigated initial transient RoCoF values in different areas for current and future wind capacity share scenarios, loading data, and primary frequency regulation settings. The modeling and scenario analysis have been performed on a detailed phasor power system model in the MATLAB/Simulink environment.

Role of energy storage in ensuring transmission system adequacy and security

The main purpose of a Transmission System Operator is to ensure stabile, reliable and efficient operation of its power system. Large-scale integration of renewable energy sources has introduced additional challenges to active control of transmission power systems. Traditionally, generation adequacy has been achieved through investments in generating units and transmission adequacy through investments in transmission lines. However, energy storage can be regarded as both the generation asset, as it reduces peak load and acts as a generator when injecting electricity into the network, and transmission asset, as it can move electricity in time thus reducing congestion and curtailment of renewable energy sources. This paper examines the role of energy storage in increasing power system adequacy and security. A method is proposed to define the charging/discharging schedule of energy storage after a contingency in order to preserve the system within the operating limits and to provide the system operator enough time to redispatch the system and relieve the overloaded lines. The method is applied to an actual part of the Croatian power system using scenarios that describe representative network states. The simulations are performed in a transmission operations and planning software using actual operating data. The results are analysed in details and conclusions on the role of the energy storage in providing transmission system adequacy and security is assessed.

The impact of wind-power generation on the planning of regulating reserve

This doctoral thesis focuses on the efficient management of a power system with a large share of wind-power generation and on the methods for planning the operating reserve. The approach proposed in this research involves the use of data directly from the network’s dispatch center and the forecasting systems, as well as already existing day-ahead plans for wind and load power to determine the regulating reserve for the load frequency control (LFC). The same principle is used to find the optimal regulating reserve distribution (RRD) between the available generation units in the system for automatic generation control. It is developed using iterative power-flow computation with a reactive power correction and a stochastic search algorithm for transmission-loss minimization. Moreover, variations in the generation of individual wind-power plants (WPPs) from the plan are considered in the optimal RRD. A special focus of this research has been put on the Croatian power system due to the accessibility of relevant information and operating data. The obtained results of the testing with actual data from the Croatian power system indicate substantial savings in ancillary service costs for the LFC, while ensuring safe system operation. The considerable impact of variations from the plan for each individual WPP generation on the optimal RRD was also identified. Comparing the results for the daily sum of regulating reserves obtained with the proposed and the currently used approaches, the corresponding costs indicate a total saving of 21.2% for all 12 selected days in 2015 when using the proposed approach. In practice, a part of the reserve can be slow, i.e., with a lower unit price, thus the savings would be even higher. Furthermore, the obtained optimal RRD computed according to the proposed approach was compared with four commonly used RRDs in Croatia. The results obtained using the proposed approach indicate a decrease in the total transmission losses of between 1% and 2%. With a larger share of the generation from the WPPs and with more dispersed locations of the WPPs and regulating power plants a larger reduction in the transmission losses is expected.

Impact of high penetration of wind and solar PV generation on the country power system load: The case study of Croatia

Even though the Republic of Croatia is on track of achieving goals set in the Europe 2020 strategy, to achieve the goals set in the 2030 European framework for climate and energy policies will require more effort. The new 2030 framework calls for a reduction in greenhouse gas emissions by at least 40% compared to the 1990 level, yet the Republic of Croatia does not have such an ambitious plan. In times when wind plants and photovoltaic systems have reached grid parity in the majority of European countries, this paper analysed the influence of construction of wind and photovoltaic power plants in order to present the optimal constructing ratio of such systems on the Croatian power system load. Simulations have been conducted in the EnergyPLAN model for the year 2012. After the simulation presented promising scenarios, applying the Pareto analysis showed the optimal scenario for generating electricity from renewables, scenario with the lowest import of electricity, scenario with the lowest CO2 emissions and with the lowest critical excess electricity production. In addition, all of the scenarios were subjected to a multiple criteria decision analysis in order to find the best overall scenario. After showing that the best overall scenario was 1.65GW of wind power plants and 1.6GW of installed PV capacity, a multi-criteria analysis was performed in order to observe the behaviour of grading the scenarios. Indeed, all of the simulations proved that PV will have a bigger role in the Republic of Croatia than expected.

Long-term energy planning of Croatian power system using multi-objective optimization with focus on renewable energy and integration of electric vehicles

Due to the stochastic nature and variability of renewable energy sources (RES), it is necessary to integrate still expensive storage capacities into an energy system with a high share of RES and to model appropriate energy market. The study presented here considers all energy carriers, however, only the electricity carrier is modeled in detail, with notion taken for the heating demand that is covered but without proper modeling of storage.A proposed two-level approach with multi-objective optimization on the global level, was used to design a Croatian Energy System (CES), where electric vehicles (EVs) are integrated to serve as battery storage in Vehicle-to-Grid (V2G) mode, for a scenario between 2015 and 2050. In addition, case study includes nine aggregated hydro power plants, one for each river basin in Croatia. Also, case study includes solar and wind power plants modeled for six locations in Croatia: Osijek, Zagreb, Rijeka, Sibenik, Split and Dubrovnik. The resulting Pareto front suggests that with assumed future costs of fuels and technology certain level of conventional energy sources will have to remain in the energy system to take into the account unfavourable weather conditions and to cover heating demand, which also results in significantly lower load factors for those power plants. Also, variants with more RES share have lower total energy system load factor and significantly higher installed capacity.

Hybrid State Estimation Model Based on PMU and SCADA Measurements

Abstract: In this paper, a new method of hybrid non-linear state estimation with PMU and SCADA measurements will be proposed. PMU measurements, the voltage and current phasors, are used for real time transmission line parameters (impedance and admittance) calculation. Transmission line parameters are input data for the conventional state estimator. Calculated parameters of the transmission lines based on the real time PMU measurements are compared with the measured impedance and admittance. The simulation results were carried out on 400 kV Croatian power system. An index of accuracy is set and used to quantify the impact of PMU measurements on power system state estimation by using proposed hybrid model. Software for the proposed hybrid model was developed and it is compatible with PMUs form different vendors. This research was made possible by implementation of 5 PMUs and restoration of wide area monitoring (WAM) system in Croatian power system. Those PMUs were implemented during the reconnection process - reintegration of the 1 st and 2 nd UCTE synchronous zone.

Impact of Wind Power on the ATC Values in Hydro- Dominated Power System

The wind power generation is increasing in many countries as a result of decreasing technology costs, active government policies for renewable energy sources, environmental concerns, etc.. This paper investigates the impact of wind power generation on ATC (available transmission capacity) calculation. In order to determine the maximum incremental MW transfer possible between two parts of a power system without violating any specified limits, ATCs are calculated. When calculating ATC values, it is necessary to assume production and consumption pattern in power system. Production of wind power depends on the wind speed, which is a random variable and it is impossible to forecast exactly the production of wind power that is needed for the ATCs calculation. In order to investigate influence of the stochastic wind power production on the ATCs value, computer model of Croatian electric power system is made in Power World Simulator. ATCs are calculated for southern part of Croatian power system in which besides wind power, hydro power plants are only type of power generation. Available wind speed measurements are used as input data for wind power production. The results of the ATC calculation for different scenario of wind power production and location in the Southern Croatian power system are presented and discussed in the paper.

Staged-Fault Testing of Distance Protection Relay Settings

In order to analyze the operation of the protection system during induced fault testing in the Croatian power system, a simulation using the CAPE software has been performed. The CAPE software (Computer-Aided Protection Engineering) is expert software intended primarily for relay protection engineers, which calculates current and voltage values during faults in the power system, so that relay protection devices can be properly set up. Once the accuracy of the simulation model had been confirmed, a series of simulations were performed in order to obtain the optimal fault location to test the protection system. The simulation results were used to specify the test sequence definitions for the end-to-end relay testing using advanced testing equipment with GPS synchronization for secondary injection in protection schemes based on communication. The objective of the end-to-end testing was to perform field validation of the protection settings, including verification of the circuit breaker operation, telecommunication channel time and the effectiveness of the relay algorithms. Once the end-to-end secondary injection testing had been completed, the induced fault testing was performed with three-end lines loaded and in service. This paper describes and analyses the test procedure, consisting of CAPE simulations, end-to-end test with advanced secondary equipment and staged-fault test of a three-end power line in the Croatian transmission system.

Modelling Sustainable Development Scenarios of Croatian Power System

Modelling Sustainable Development Scenarios of Croatian Power SystemThe main objective of power system sustainable development is to provide the security of electricity supply required to underpin economic growth and increase the quality of living while minimizing adverse environmental impacts. New challenges such as deregulation, liberalization of energy markets, increased competition on energy markets, growing demands on security of supply, price insecurities and demand to cut CO2 emissions, are calling for better understanding of electrical systems modelling. Existing models are not sufficient anymore and planners will need to think differently in order to face these challenges. Such a model, on the basis on performed simulations, should enable planner to distinguish between different options and to analyze sustainability of these options. PLEXOS is an electricity market simulation model, used for modeling electrical system in Croatia since 2005. Within this paper, generation expansion scenarios until 2020 developed for Croatian Energy Strategy and modeled in PLEXOS. Development of sustainable Croatian energy scenario was analyzed in the paper - impacts of CO2 emission price and wind generation. Energy Strategy sets goal for 1200 MW from wind power plants in 2020. In order to fully understand its impacts, intermittent nature of electricity generation from wind power plant was modeled. We conclude that electrical system modelling using everyday growing models has proved to be inevitable for sustainable electrical system planning in complex environment in which power plants operate today.

Modelling CO2 emissions impacts on Croatian power system

Today?s electrical energy landscape is characterized by new challenges such as deregulation, liberalization of energy markets, increased competition, growing demands on security of supply, price insecurities, and demand to cut CO2 emissions. All mentioned challenges are calling for consideration of various options (like nuclear, coal, gas or renewable scenarios) and for better understanding of energy systems modelling in order to optimize proper energy mix. Existing models are not sufficient any more and planners will need to think differently in order to face these challenges. European emission trading scheme (EU ETS) started in 2005 and it has great influence on power system short term and long term planning. Croatia is obliged to establish a national scheme for trading of greenhouse gas emission allowances from the year 2010, which will be focused on monitoring and reporting only until accession to EU when it will be linked with EU ETS. Thus, for Croatian power system it is very important to analyze possible impacts of CO2 emissions. Analysis presented in this paper was done by two different models: mathematical model, based on short run marginal costs (SRMC, relevant for fuel switch in existing power plant and merit order change) and long run marginal costs (LRMC, relevant for new investment decisions); and electricity market simulation model PLEXOS, which was used for modelling Croatian power system during development of the Croatian energy strategy in 2008. Results of the analysis show important impacts that emission trading has on Croatian power system, such as influence of emission price rise on price of electricity and on emission quantity, and changes in power plants output that appear with emission price rise. Breakeven point after which gas power plant becomes more competitive than coal is 62 ?/tCO2 for SRMC and 40 ?/tCO2 for LRMC. With CO2 prices above 31 ?/tCO2 wind is more competitive than gas or coal, which emphasizes importance that emission price has on competitiveness of renewables.

Potential and use of renewable energy sources in Croatia

The current share of renewable energy sources in electricity production in Croatia is very high, around 50%. Nevertheless it is expected that the share of renewables will have to rise and efficient strategies must be examined and adopted. The Croatian government has recognised the important role renewable energy sources could play in Croatian energy and electricity supply. The most important barrier for a wider deployment of renewables in energy production is their cost which is still above those of conventional energy sources. As the energy market is currently undergoing the process of liberalisation, support mechanisms that are compatible with an open market philosophy must be adopted. The characteristics of the Croatian power system, the expected consumption, growth and possible future role of renewables in energy and electricity production is presented. The current legislative framework relevant for renewables is analysed and discussed.

Analysis of future nuclear power plants competitive investment costs with stochastic methods

Abstract One of main issues in planning future power system expansion is forecast of expected electricity generating costs. Due to the fact that the forecast should be extended to plants lifetime it is best to consider and compare lifetime levelized cost of generated electricity for each of the candidate plants. The analysis is a part of studies for the future expansion of the Croatian power system, but is valid generally. The main competitors to nuclear power plants are combined cycle natural gas fired plants whose economy strongly depends upon gas cost. The expected increase of gas cost in following decades can substantially influence the competitiveness of nuclear plants versus gas fired plants. In addition, the costs of electricity produced by coal fired plants and wind electricity generators were also considered. Due to uncertainties of input parameters it is advantageous to use probabilistic instead of deterministic method of analysis. The paper considers the problem of the probabilistic distribution of nuclear power plant investment costs for which such plants could be competitive to gas fired plants for an estimated probable range of gas cost increase rates.

External costs: an attempt to make power generation a fair game (case study Croatia)

External costs of electricity represent the monetary value of the environmental damage caused by electricity generation. They are here calculated applying the impact pathway methodology on Croatian specific conditions. This paper estimates the external costs of coal and gas fired power plants determined as main candidates for Croatian power system expansion till 2030. It is analyzed how the estimated external costs, when incorporated into total production costs, would affect the competitiveness of fossil–fired plants compared to other electricity generation options, i.e. how they influence the optimal expansion strategy of the Croatian power system.