Croatian Transmission Research Articles

On the practical aspects of machine learning based active power loss forecasting in transmission networks

AbstractThe cost for covering active power losses makes a significant item in transmission system operators (TSO) annual budgets, and still it received limited attention in the existing literature. The focus of accurate power loss forecasting and procurement is of high increase during the past 2 years due to spikes in electricity prices, making the cost of covering the active power losses a dominant factor of TSO operational costs. This paper presents practical aspects of the highly accurate models for transmission loss forecast in the day ahead time frame for the Croatian transmission system. The contributions are two‐fold: 1) Practical insights into usable TSO data are provided, filling a critical research gap and a foundational literature review is established on transmission loss forecasting. 2) A novel method utilizing only electricity transit data as input which outperforms existing practices is presented. For this, several algorithms such as gradient boosted decision tree model (XGB), support vector regressors, multiple linear regression and fully connected feedforward artificial neural networks are developed, and implemented and validated on data obtained from the Croatian TSO. The results show that the XGB model outperforms current TSO model by 32% for 4 months of comparison and TSCNET's commercial solution by 25% during a year‐long testing period. The developed XGB model is also implemented as a software tool and put into everyday operation with the Croatian TSO.

Alignment of aFRR and mFRR prequalification process in Croatia with the target market design

Commission Regulation (EU) 2017/1485 of 2 August 2017 establishing a guideline on electricity transmission system operation defines a prequalification process as a process to verify the compliance of a reserve providing unit or a reserve providing group with the requirements set by the transmission system operator. Croatian Transmission System Operator Plc. (HOPS) carries out a prequalification process in order for (potential) ancillary service providers to prove their capability of providing ancillary services, which means that the prequalification process is a prerequisite for the provision of ancillary services. As part of the European Network of Transmission System Operators for Electricity, HOPS participates in the European implementation projects PICASSO and MARI, which aim is to establish a single European platform for aFRR and mFRR balancing services. Within the PICASSO and MARI projects, the technical characteristics of the products traded on unique European platforms were defined. This paper analyzes the difference between the requirements from the existing prequalification process rules and the technical requirements for products within the PICASSO and MARI projects, explores practice in Slovenia and Germany and suggests the required adoptions of aFRR and mFRR prequalification processes in Croatia.

Application of fully automated centralized voltage regulation in transmission system operation management

The influence of wind farms, inclusion of distributed energy and the energy market has a large impact on the voltage profile in a transmission system. The Croatian Transmission System Operator HOPS has implemented VVC (Volt Var Control) system which was one of the subprojects included in the EU co-founded Sincro.Grid smart grid project that was financed by the EU CEF fund. The main goal of the project is to raise voltage quality in the transmission network. VVC system is an optimal power flow-based application which calculates the optimal solution regarding the desired objective function, available control variables and a defined set of constraints. To achieve the calculated optimal power system state, control of field devices is included in the process of optimization by shifting tap changer positions or changing setpoint values of reactive power injection. Voltage and reactive power constraints are set accordingly to available regulating devices included in the optimization.
 The lack of automatic coordination of reactive power resources motivated this work for implementing an advanced VVC for real time control of reactive power in partially automatic and fully automatic (closed loop) mode using the optimal power flow (OPF) algorithm. Due to its complexity, the closed loop approach of reactive power regulation is rarely used in the TSO community and the HOPS is one of the first TSOs that implemented regulation in such manner. Except fully automatic mode of operation, VVC can be used in semi-auto or manual mode. For a successful implementation several safety algorithms are implement to avoid many unwanted situations in the power system such as voltage breakdown, overloading of regulating equipment and similar

APPLICATION OF PHASE SHIFTING TRANSFORMERS IN THE CROATIAN POWER SUPPLY SYSTEM

The phase shifting transformers with on load tap changer are increasingly found in modern power supply systems, especially under the present conditions of the full opening of the electricity market. The construction of the Žerjavinec TS 400/220/110 kV for the first time introduces into the Croatian transmission system a 400/220/(10,5) kV 400 MVA phase shifting network transformer with on load tap changer. The present article highlights some specific aspects of phase shifting transformers in the light of their application in the Croatian system. For a more efficient use of the advantages of phase shifting transformers, the transformer of this type at Žerjavinec would need to be provided with a complementary transformer (pair), either within the Croatian transmission network or within the neighbouring systems.

Calculation and analysis of the steady-state line ampacity weather sensitivity coefficients

The continuous increase in electrical power demand entails the need for larger ratings of overhead lines. For spatial, economic and ecological reasons, contemporary solutions are focused on incrementing the ratings of existing lines. One of these solutions is the real-time determination of line ampacity according to the measured or predicted weather and/or line parameters. The paper is focused on a novel calculation procedure and analysis of the steady-state line ampacity (SSLA) weather sensitivity coefficients (WSCs). The calculation of WSCs is based on the sensitivity analysis of the methodology for the SSLA given in Cigre N° 601 Technical Brochure. On the one hand these WSCs allow a fast estimate of the potential in the introduction of real-time ampacity calculation concept. On the other hand they give a good basis on the requirements for the measurement equipment installed in contemporary dynamical line rating systems (DLRSs). The interrelations between the WSCs and the SSLA are investigated using correlation and regression analysis. Finally, according to the typical design parameters of overhead lines the WSCs and their interrelations with the SSLA are calculated and analysed for typical aluminium steel-reinforced conductors (ACSRs) present in the Croatian transmission system.

Energy correction procedure on cross-border energy exchange using a virtual measuring point

Measurement procedures in cross-border energy exchanges usually neglect the associated measurement uncertainties, which is likely to result in financial damage to one, or both, transmission system operators. In this paper, measurement uncertainty analysis at the measurement point is extensively described and a correction procedure for its systematic contributions is presented. Aiming to develop a fair procedure for the exchanged energy distribution among transmission system operators, the concept of the virtual measuring point is introduced, taking into consideration measurement uncertainties and generated transmission losses. After performing the correction procedure, the virtual measuring point is converted into the corrected virtual measuring point, whose energy measurement is used for the final billing procedure. The presented method was tested in a practical example with data from a Croatian transmission system operator Ltd. and it was confirmed that the correction procedure can be of great interest for transmission system operators.

Reliability and Adequacy Assessment of a Transmission Network in the Eastern Croatia

The paper analyzes network reliability and generator adequacy indicators using Monte Carlo simulation. The reliability indicators are analyzed for the eastern part of the Croatian transmission system and the influence of new combined cogeneration plants on the network reliability. All simulations were conducted with DIgSILENT PowerFactory software tool which uses two types of algorithms - state enumeration method and Monte Carlo simulation method. Firstly, two conditions of the transmission system were analyzed - the condition with the maximum and the condition with the minimum time varying load along with the comparison of the network reliability indicators. Then, new constructed combined-cogeneration plants connected at 400 kV busbars were compared to the combined-cogeneration plants of smaller size connected to the different point in the network at 110 kV busbars. Results were compared and analyses show that the option with three CCGT is better than option of one huge CCGT plant placed in Ernestinovo or Slavonski Brod.

Cross‐border transmission line losses calculation using adaptive Monte–Carlo method

This study deals with calculation of transmission line losses. Having in mind that this type of losses, besides the line load and the other external input quantities such as atmospheric conditions, strongly depends on measurement uncertainty as well, the issue can be considered through finding the most convenient approach in calculation of measurement uncertainty. In this respect, two approaches (methods) are the most common: traditional method based on the guide to the expression of uncertainty in measurement (GUM) and the adaptive Monte–Carlo method (AMC). The study reveals the main disadvantages of the GUM, which is so far considered as a most accurate method. The observed drawbacks of the GUM can be successfully overcome by using of the AMC method. The comparison between the methods is performed on 110 kV cross-border transmission line from Croatian Transmission System Operator Ltd. The results show the great difference in the estimated variances, i.e. GUM variances are significantly overestimated what results in incorrect transmission losses allocation procedure with respect to its final financial effect. The study confirms that the AMC method is likely to be more practically suitable method for transmission line resistance and losses calculation.

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.