Andreas I. Chrysochos

Application of Narrowband Power-Line Communication in Medium-Voltage Smart Distribution Grids

Narrowband power-line communication (PLC) over distribution networks is gaining the attention of researchers, as a communication medium in future power grids. This paper investigates the application of narrowband PLC in smart distribution grids, starting from a historical overview on the accomplished technological progress and continuing with a comparison of the advantages and drawbacks of PLC technology to other smart-grid communication solutions. The analysis shows that narrowband PLC applications are best suitable for medium-voltage (MV) networks, due to the vast and complex geographical extent. The channel and topological characteristics of MV distribution networks are examined for different operational states and configurations, since they are important for the optimal design and implementation of the PLC infrastructure. The analysis is also extended to an existing overhead MV distribution network with distributed-generation units. The obtained results and the proposed methodology are useful and comprehensive tools for the efficient implementation of PLC technology in future smart grids.

Promoting PV energy through net metering optimization: The PV-NET project

As the Feed-in-Tariff (FiT) scheme that was used widely in the past years start to fade away, new schemes and policies are needed to revitalize the Photovoltaic (PV) market, which in danger of stagnation. FiTs have rapidly reduced in most countries mainly due to the sharp decline in PV system costs and the fact that targets set in terms of Wp installed by 2020 have already been reached. One of the schemes proposed is net-metering. This scheme implies that an algebraic deduction is performed between the electrical energy produced by the PV system and the energy consumed. The net result if the energy produced is higher, is fed back to the grid preferably at a certain premium or retail prices. Although net-metering is not widely adopting currently, the combination of PV systems cost decline and increase in electricity prices observed worldwide, will probably make such a scheme attractive to both investors and policy makers. In this context, PV-NET Metering is a project under Europe in the Mediterranean (MED) programme addressing the design of energy policies and strategies in the Mediterranean for cost-optimized utilization of Renewable Energy Sources (RES). It involves involves optimizing smart energy management schemes, in particular net metering, to provide an economically sustainable alternative to government FiT subsidies. This paper presents the initial steps taken in this project and presents an analysis concerning the Levelized Cost of Electricity (LCOE) in four regions - project partners and a grid parity calculation for Greece. Also, the methodology and technical specifications for the pilot installations foreseen are outlined.

Fair power curtailment of distributed renewable energy sources to mitigate overvoltages in low-voltage networks

Penetration of distributed renewable energy sources in low-voltage (LV) networks is increasing steadily. This trend helps the vision of sustainability, but it is hindered by various economic and technical constraints. Some of the existing solutions put some customers in a disadvantageous position, which is often overlooked while developing technical solutions. In this paper, the most common solutions to cope with overvoltage are analyzed, including inverters with ON/OFF switching and droop-based control features. These two techniques place customers connected to the end of a radial feeder in a disadvantageous position, by curtailing their power generation more than their counterparts who are connected closer to the MV/LV transformer; a situation generally known as unfair power curtailment. In this paper, an efficient solution for voltage rise is proposed, which mitigates voltage rise using conventional droop control techniques, but also provides fair power curtailment among all customers by exploiting the sensitivity matrix of the examined radial feeder. The proposed control strategy is implemented in a flexible simulation platform, based on MATLAB and OpenDSS. Results on different LV networks are compared with the corresponding obtained by conventional droop control methods, highlighting the superior performance of the proposed control strategy.

Robust Calculation of Frequency-Dependent Transmission-Line Transformation Matrices Using the Levenberg–Marquardt Method

This paper presents a new method for the calculation of smooth frequency-dependent transmission-line (TL) transformation matrices. The proposed method, based on the Levenberg-Marquardt algorithm, solves an equivalent real-valued approach of the generalized complex eigenproblem. The implemented formulation incorporates a robust convergence criterion and is applicable to all TL configurations, due to the included numerically well-defined computational scheme. Smooth modal transformation matrices are calculated for overhead and underground TL configurations under different representations of the imperfect earth. Results are compared and validated with the corresponding results obtained from the Newton-Raphson and the sequential quadratic programming methods, revealing the accuracy, efficiency, and robustness of the proposed formulation, even in cases where the other methods fail.

Damping-Based Droop Control Strategy Allowing an Increased Penetration of Renewable Energy Resources in Low-Voltage Grids

The increased penetration of renewable distributed energy sources increases the challenges for distribution systems operators to keep the voltage variations within the prescribed limits. One of the voltage problems is caused by single-phase systems leading to voltage unbalance. Another problem is overvoltages caused by local injection of distributed energy sources. An effective solution for this problem is by active power curtailment. In this paper, a control strategy is proposed that mitigates voltage unbalance by using active power and it is equipped with a linear active power curtailment solution. The effect of the proposed control strategy and three state-of-the-art control strategies on the voltage profile and the curtailed power are studied. It can be concluded that the proposed control strategy has a beneficial effect on the voltage profile along a feeder and leads to less power curtailment compared to the state-of-the-art solutions. The proposed control strategy thus allows an increased penetration of distributed energy sources in the low-voltage grid.

A simulation tool for extended distribution grids with controlled distributed generation

In this paper, a new software tool is presented for the simulation of electrical networks under steady-state conditions. Its distinct advantage is the robust integration of distributed generation droop controls, while offering the ability to simulate extended networks fast and reliably. The proposed simulation tool is based on the combination of two well-known software products, namely MATLAB and OpenDSS. The latter is employed as an unbalanced power flow solver, whereas the former implements the droop control of DG units. Simulation results for a simple and extended low-voltage network show the effectiveness of the proposed tool and mainly the reduction in the execution times over other conventional time-domain-based software products.

Measurement-Based Hybrid Approach for Ringdown Analysis of Power Systems

System identification methods have been widely used for the study of low frequency electromechanical oscillations and the development of low order dynamic models. This paper introduces a hybrid frequency/time-domain approach to estimate the dominant modes contained in ringdown responses of power systems. Practical issues and solutions encountered in the application of the hybrid method are discussed. The performance of the proposed technique is evaluated by applying the Monte Carlo method to synthetic signals and simulated responses from a large-scale power system, as well as to measurements recorded in a microgrid laboratory test facility. Results in all cases proved to be very accurate, verifying the robustness of the proposed method.

Development of measurement-based generic load models for dynamic simulations

In this paper the development of robust measurement-based load models for dynamic simulations is discussed. Load model parameters vary significantly, due to different loading conditions, thus, load models obtained from measurements are valid only for a specific operating condition and cannot be easily generalized. Scope of the paper is to develop a generic load model, suitable for dynamic simulations over a wide range of operating and loading conditions. In order to derive the proposed generic model, three methodologies are thoroughly investigated. Several simulation scenarios of different operational conditions are examined with NEPLAN software and are used to validate the accuracy of the proposed models.

A wide band black-box model of power transformers in ATP/MODELS

The high-frequency power transformer modeling is of significant importance for most power systems applications. This paper presents a simple black-box modeling methodology for power transformers using transfer functions, defined by the recorded voltage ratios at the transformer open-circuited terminals. The model parameters are estimated using an optimization method, minimizing the error between the measured and the calculated data. The proposed model is implemented in the ATP/EMTP software, using the ATP/MODELS language in order to simulate the transmission of power-line communication signals through the transformer.

Analysis of the propagation characteristics of single-core cables from experimental results using modal decomposition

In this paper various field test transient responses from a medium-voltage single-core cable lying on the ground surface are presented. The results from six different excitation cases are compared, applying the modal decomposition theory with the use of a complex-frequency-dependent modal transformation matrix. The resulting modal voltages present unique modal travel time and attenuation, leading to significant remarks about the wave propagation along the cable in each examined case. The decoupled voltages are also compared to the corresponding obtained from the assumption of a real-constant transformation matrix. Results are in good agreement, validating the use of the simplified modal transformation matrix for the calculation of high-frequency transient phenomena in single-phase cables with short lengths. The calculated propagation modes are eventually related with the voltage differences and potentials of the examined cable, revealing their significance in the study of various engineering applications.