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This is the presentation of the paper: "Effect of non-ideal power take-off on the electric output power of a wave energy converter under suboptimal control", presented at the RENEW 2020 - 4th International Conference on Renewable Energies Offshore, 12-15 October 2020, Lisbon, Portugal.
This is the presentation of the paper "Stability Analysis of a Virtual Synchronous Machine-based HVDC Link by Gear's Method" for the ENERGYCON 2020 - 6th IEEE International Energy Conference held at the Ramada Plaza Hotel, Gammarth, Tunisia (28. Sept - 1. Oct, 2020)
Declining equivalent inertia caused by the massive integration of converter-based technologies can increase the risk of stability problems in the future power system. Control of HVDC systems as Virtual Synchronous Machines (VSMs) for providing emulated inertia is a promising countermeasure against this development. However, low inertia converter-dominated power systems are raising new challenges for stability analysis. Thus, new numerical techniques and tools are needed for efficient and accurate analysis of small-and large-signal stability issues in complex power systems with different kinds of generation units, converter systems and control loops that can experience potential interactions, internally or with the network. Gear's method with a variable time step, offers the possibility to obtain fast and flexible procedures for large-signal stability analysis. Additionally, it can make stability analysis efficient by combining the small-signal and large-signal analysis into the same process. This paper will show how the stability of a power system with a VSM-based HVDC link can be evaluated by the method and presents an approach for simultaneous small-and large-signal stability assessment.
The paper describes an implementation of a set of object-oriented Python library functions for distribution system analysis. To illustrate its functionalities, an example of load flow calculation is implemented with the core method based on the Forward-Backward Sweep. The functionality of the method is extended with several alternatives for voltage and power flow control. The voltage control may either be based on a specified voltage or on a droop based approach. The performance of the method is demonstrated on a voltage compensation case. It is also demonstrated how the sensitivities may be used to develop a close to optimal voltage profile. The sensitivities applied are calculated during the forward and the backward sweeps. The recursive method is fast and flexible and makes it possible to solve many different topologies quickly. The common data structure with the object-oriented design is a key contributor to flexibility and makes it possible to work on local copies and compare alternatives. The intended purpose of the implementation is to offer a fully open and transparent platform that can be a basis for further development and combined with other tools, to enable more complex studies. The code of this software is available from a reference of the paper. The tool has been validated on the IEEE69 Bus system.
This paper describes a library for power system analysis. It is based on an object-oriented representation of the transmission system components and the design of the library is also object-oriented. The data description is connected to a CSV-file format where either Microsoft Excel or Open Office Spreadsheet may be used. Currently, the tools available for load flows are Newton-Rhapson, Decoupled Power Flow, DC Power Flow, Continuation Power Flow, Contingency Analysis, Security- Constrained DC Power Flow. The program frame work is written in Python 3.6 and it is tested on Windows 10 and Ubuntu 18.04.
The discussions on the development of an electricity market model for accommodating cross-border cooperation remains active in Europe. The main interest is the establishment of market couplings without curtailing the fair use of the scarce transmission capacity. However, it is difficult to gain mutual consensus on this subject because of the absence of convincing simulation results for the entire region. To achieve that, researchers need a common grid model (CGM) which is a simplified representation of the detailed transmission model which comprises aggregated buses and transmission lines. A CGM should sufficiently represent the inter-area power flow characteristics. Generally, it is difficult to establish a standard CGM that represents the actual transmission network with a sufficient degree of exactness because it requires knowledge on the details of the transmission network, which are undisclosed. This paper addresses the issue and reviews the existing approaches in transmission network approximation, and their shortcomings. Then, it proposes a new approach called the adaptive CGM approximation (ACA) for serving the purpose. The ACA is a data-driven approach, developed based on the direct current power flow theory. It is able to construct a CGM based on the published power flow data between the interconnected market areas. This is done by solving the issue as a non-linear model fitting problem. The method is validated using three case studies. Index Terms-Common grid model, flow-based market mechanism, genetic algorithms, linear optimal power flow problems, transmission network approximation.
The presentation covers the use of Instantaneous Frequency (IF) based tools and techniques for studying dynamic performance of power systems. The focus is on the concept of instantaneous frequency where the principles for the IF calculation are illustrated. The sifting process of the Empirical Mode Decomposition (EMD) is also covered. Presented in project granted under: "EU-CHINA Research and Innovation Partnership Ref: EuropeAid/135-587/DD/ACT/Multi"
This paper investigates a combination of methods to extract modal information from synchrophasor measurements. Ringdown measurements have been preprocessed with Empirical Mode Decomposition (EMD), analyzed with Prony's method and the results postprocessed with clustering techniques. Ambient measurements before and after a disturbance have been analyzed with Robust Recursive Least Squares, with the same clustering technique applied for continuous evaluation of the estimated modes. Together, ambient and ringdown analysis provide accurate, near real-time awareness of electromechanical oscillations. Postprocessing logic is an issue for parametric methods, as higher model orders often are chosen to improve the estimation process. This paper demonstrates that close inspection of true and trivial modes can reveal important properties of the estimation, and that they can be used to evaluate if the choice of parametric model is justified for the analyzed signal.
Phasor estimation is crucial for monitoring and control of smart power systems. The classical signal processing method named Prony has been used for estimating the parameters of measured signals such as frequency, damping factor and phasor. To reduce the impact of noise on the parameters estimated by Prony, multi-channel Prony has been previously explored and presented in the literature. The basic approach for multi-channel Prony is a generalized solution, in which new rows are added to matrices for every channel. Since the generalized multi-channel Prony is time-consuming, a new method based on recursive solution is proposed in this paper to make it suitable for real-time application. Here, several channels of one Phasor Measurement Unit (PMU) are used to estimate the phasor of current/voltage in a recursive pattern, in which the phasor is computed recursively over time based on previously calculated estimates and new measurements. The proposed method is compared with three other solutions for multi-channel Prony: a) data fusion which is based on the Kalman filter concept, b) an alternating direction method of multipliers (ADMM), and c) a consensus update approach which is based on an iterative procedure. Simulation results demonstrate the ability of the proposed method for real-time phasor estimation, both in terms of maintaining the accuracy and reducing computation time.
Phasor estimation has many application areas and has therefore attracted significant research focus. Classical pha-sor estimation proposed many years ago, considers the phasor to be time independent which means constant amplitude and phase. However, the dynamic phasor concept introduced recently, improves the accuracy of the phasor estimation under a non-stationary signal as is typically the case of low frequency oscillations (LFO). However, more accurate estimates lead to higher computation time. To achieve both low computation time and more accurate estimates, an adaptive phasor estimation concept based on both static and dynamic phasors is proposed in this paper. A new method based on the Adaptive Prony algorithm is presented, in which the Static Prony is employed under steady state conditions and the Dynamic Prony under dynamic conditions. To switch between these two algorithms, a Cumulative Summation of the Phasor Estimation Error (CSPEE) is used. Simulation results show the applicability of the proposed method to achieve the most accurate estimates at the lowest computation time. Total Vector Error (TVE) and Floating Point Operation (FLOP) are used to evaluate the proposed method.
This is the presentation of the paper "EMD-Prony for Phasor Estimation in Harmonic and Noisy Condition" presented at the Conference: 24th IEEE International Symposium on Power Electronics, Electrical Drives, Automation and Motion - SPEEDAM 2018 at Amalfi Coast, Italy.
Synchronized Measurement Technology represented by Phasor Measurement Units (PMUs) is a sophisticated tool for power system monitoring and contributes to a more reliable and secure operation of the power systems. Phasor estimation is the key of PMUs, providing amplitude and phase of voltage/current of the power network buses. In this paper, the performance of a recursive Prony algorithm for the phasor estimation is examined under steady state and dynamic conditions. It is shown that the properties of the recursive algorithm have opposite impacts on the accuracy and the speed of the estimation, under these two conditions. To reach a good compromise between these two opposing properties, a modified recursive Prony, based on a time varying λ, is proposed in this paper.
This paper investigates the theory, intuition and performance of two known implementations of Prony's method. Such methods are useful for identifying the individual modes of a system without constructing a component-based model. In the Smart Grid, Prony Analysis has been widely used on post-disturbance ring-down measurements, which have been increasingly available with the extensive deployment of PMU's. Both methods decompose the signal into decaying sinusoidals, and estimate the frequency, damping, amplitude and phase of each modal component. The first method is based on the original Prony's method, whilst the second method is based on the thought that the system can be viewed as a digital synthesis problem where the system has the properties of an infinite impulse response filter. Both methods employ EMD-based pre-filtering. Additionally, a cluster based approach is proposed for circumventing the issue of determining model order, so that the true modes of the estimation can be distinguished from the trivial modes.
Wide area monitoring systems (WAMSs) enables the real-time monitoring of power system dynamics by bringing together new developments in the field of measurement, communication and computing. Measurements of voltage and current phasors are recorded by phasor measurement units (PMUs) installed across a wide area power system and time tagged at the point of measurement using a common time reference. Estimates of Phasor, Frequency and rate of change of frequency (ROCOF) are duties of PMU in every installed bus. For this purpose, the Prony algorithm is one of the promising method since its phasor estimates are calculated adaptively based on estimated frequency. For fundamental phasor estimation, the Prony algorithm with the order of one is suitable but its performance in terms of accuracy is diminished when non-fundamental components interfere in the measured signal. These components can be eliminated from fundamental phasor estimates by increasing the Prony's model order. In order to specify adaptively the order of the Prony algorithm, the Empirical Mode Decomposition (EMD) method is proposed in this paper to be combined with the Prony algorithm as EMD-Prony. The EMD decomposes a signal into finite Intrinsic Mode Functions (IMF) based on the number of modes in the measured signal. The number of IMFs is utilized by Prony to extend its model to higher order and so purifies the fundamental phasor estimates. In addition, EMD is also used as a pre-processor to filter the noise from the input signal of Prony. Finally, the proposed method can estimate phasors accurately under noisy and harmonic conditions.
Dynamical phenomena like oscillations and instability in railway power systems have come into concern in the experts' community the recent years. In several occasions, modern advanced electrical rail vehicles have been a source for low-frequency power oscillations leading to an unstable power system due to lack of damping, and as a consequence operational problems. Traditionally the dynamical behaviour of such a power-electronic based vehicle is studied by instantaneous-value time-domain simulations. In this paper, the entire single-phase railway power supply including the vehicle is modelled together in the time-invariant rotating reference frame. Such a reference-frame transformation is common for power-system analysis and allows utilization of linearization tools as eigenvalue analysis including participation factors and parameter sensitivity analysis. Such tools are used here for study and analysis of possible improvement of the stability of the complex railway power system. The results from the linear analysis are compared to time simulations.
Use of power electronic equipment has increased and introduced new dynamical phenomena in power systems. For example, new electric rail vehicles (locomotives) equipped with modern power electronic traction chains have caused situations of low-frequency power oscillations and instability in single-phase railway power supply systems. This paper presents the development and implementation of an instantaneous value model and simplified fundamental frequency (RMS) models of such an advanced electric rail vehicle in order to investigate their representation of low-frequency dynamics. The dynamical behaviour is studied by use of both time-domain simulations and linear analysis (eigenvalues) and the degree of simplifications regarding controller dynamics and power system dynamics are presented and discussed. An enhanced RMS model is tested in order to account for the impact of fast current dynamics on the low-frequency behaviour. The results show that this enhanced model is corresponding more closely to the instantaneous value model than what can be obtained by the traditional RMS simplifications and indicate that current dynamics should be included in stability studies involving power electronic inverters.
The future electric grid development plan in Europe foresees the North Sea Super Grid (NSSG). This grid will need to integrate individually and independently planned projects, leading to a rather grown than optimised structure. This will offer a lot of technical challenges, since it is a clear pioneer project. Offshore cluster grids could be either realised in AC or DC, and significant advantages and drawbacks come along with both options. Power balancing in offshore cluster grids is challenging, due to the characteristics of modern wind turbines, which will play an important role. These clusters can be connected to each other and to shore via HVDC links. VSC technology is heavily in focus nowadays, but CSC technology might also offer good solutions in the future and could either be applied as a regular CSC link or as a hybrid link. Series connected HVDC will gain importance with increasing number of offshore clusters. Parallel HVDC connections can offer the needed capacities and the important redun-dancy. A meshed HVDC grid structure can offer both, but additional challenges come along with it. In this article an overview over the relevant technologies is given and the challenges of realising the NSSG are discussed.
The large remote offshore wind clusters that are planned in the North Sea will most likely be connected with a meshed HVDC grid. Power will mostly flow from the offshore wind clusters to shore, creating asymmetrical requirements for the HVDC links that will consist of several parallel HVDC systems. To realise an asymmetrical link, some of those systems could be designed unidirectional, resulting in possible changes and simplifications (especially to the protection system). Assessment of a future scenario has shown that 42% of the HVDC systems can only be operated unidirectional. The remaining systems could in theory be used both directions, but power flow optimisation has shown, that this will in many cases not happen. A first cost calculation has shown that almost 6% of the investment cost can be saved when asymmetric design is implemented. This indicates the need to consider asymmetric design and to develop unidirectional HVDC systems.
The earlier developed control method using a piecewise linear droop curve, with different droop values for the different segments, has now been optimised for dynamic performance. Non-linearities at the junctions of two linear droop sections have been adressed. Also non-linearity of power based DC voltage control has been adressed. Dynamic instability due to high control gains has been treated and a new improved control structure has been proposed. The concepts have been validated with RMS simulation with the DIgSILENT PowerFactory software on the CIGRE B4 DC grid test system.
PV-micro-grids are becoming an affordable alternative to provide electricity access to isolated or remote regions due to both a reduction in prices and a strong focus on the free availability of renewable energy sources. When designing these microgrids, the use of optimization methods has been rather limited. Flexible systematic design tools that can adapt to different project sizes, the lack of data, and the different systems constraints, are of limited availability. This paper presents a methodology for a cost-effective design of microgrids based on composable tools using optimization. The optimization is aimed at identifying the optimum size of components for a required performance with the minimum possible cost due to given budget constraints. The developed methodology for optimizing micro-grids is then applied to an off-grid PV microgrid installation in Bhutan.
This paper attempts to identify some basic relations between damping of the oscillation modes and voltage stability through the analysis of a simplified yet realistic two-area test system. The system consists of four equivalent generators, one of which represents a sizeable wind farm connected to the transmission system through a weak radial. The approach chosen has been to perform different types of stability analyses on different power flows and system configurations. Both static and dynamic simulations have been performed.
In this paper we have analyzed the integration of large scale wind power in a regional network. The purpose of this study was to maximize wind power sharing in a regional network given a specific thermal capacity, voltage level and voltage stability constraints for the existing grid interface. The approach used is based on a combined assessment of sensitivity matrices and QV curves for different loading scenarios. The sensitivity analysis has given information about where the reactive power injection is the most effective, whereas the QV curves have given information about how much reactive power is needed with varying voltage levels. The lowest rating of reactive power support and the best placement for controlling voltages in the regional network has been decided.