Congestion Management using Local Flexibility Markets: Recent Development and Challenges

Increasing amount of renewable based distributed generation at distribution systems, leads to an increased need for active distribution network management dealing with local network congestion and voltage issues. Development of local flexibility markets aims to provide a market-based solution to these issues. This paper presents a comprehensive review of proposed approaches towards markets exploiting the flexibilities from the demand-side. Efforts have been made on presenting a systematic overview of market design, including e.g. framework, participation, bidding and clearing mechanisms, of local flexibility market proposals developed in recent years. The implementation and regulatory issues and challenges are also discussed. The paper also presents the conceptual framework of the local flexibility service market which is currently being developed within UNITED-GRID project. This proposal aims to provide a holistic approach on local service markets, so that Distribution System Operators (DSOs) are provided with a market-based instrument to manage their networks efficiently

Assessment indexes for converter p-Q control coupling

This work presents a concise methodology for the calculation of assessment indexes regarding the coupling between active and reactive power control observed on distribution level converters. First, the reader is introduced to the concept of power coupling; when, where and how it appears in power control of converters. A brief summary of the theory and formulation behind it is also included, together with relevant literature. Then, the methodology for the assessment of active and reactive power control performance of any grid-connected converter is presented. The impact of small control disturbances during a testing procedure is monitored, analyzed and converted to meaningful indexes, so that the type and level of coupling is quantified without putting the converter or the grid at risk. The efficiency of the methodology to assess the type and level of coupling is verified experimentally. This is done by assessing several power control approaches with different level of decoupling efficiency on the same power converter connected to a distribution grid. While the assessment is performed with safe, minimal disturbances, its exceptional accuracy is later confirmed by the level and type of coupling observed during significant power step changes

Local flexibility market framework for grid support services to distribution networks

The increasing volume of distributed resources and user-dependent loads in local networks has increased the concern for congestion and voltage management in distribution networks. To mitigate these issues, the implementation of local flexibility markets has been proposed to assist distribution system operators (DSOs) to manage their networks efficiently. This paper presents the framework of a local flexibility market, including the market participants and their roles. This framework aims to empower DSOs with a market-based instrument for the alleviation of congestion incidents by exploiting the flexibility of local resources. The proposed market aims to provide a tool for the holistic management of distribution networks by trading both reservation and activation of flexibility services, indifferent of the type and the timeline of the needed service. Three market modes are proposed, i.e., long-term, short-term and real-time market, and the interactions among those modes are shown. The operation of the market is explained in detail, including the identification of the needed services, the activation of the market as well as the proposed bidding, clearing and settlement mechanisms. The modelling of the long-term and real-time markets is also presented, along with some indicative simulation results for long-term and real-time services. Finally, the future developments as well as the major conclusions are discussed

Development of a DSO support tool for congestion forecast

This paper presents a novel DSO support tool with visualisation capability for forecasting network congestion in distribution systems with a high level of renewables. To incorporate the uncertainties in the distribution systems, the probabilistic power flow framework has been utilised. An advanced photovoltaic production forecast based on sky images and a load forecast using an artificial neural network is used as the input to the tool. In addition, advanced load models and operating modes of photovoltaic inverters have been incorporated into the tool. The tool has been applied in case studies to perform congestion forecasts for two real distribution systems to validate its usability and scalability. The results from case studies demonstrated that the tool performs satisfactorily for both small and large networks and is able to visualise the cumulative probabilities of nodes voltage deviation and network components (branches and transformers) congestion for a variety of forecast horizons as desired by the DSO. The results have also shown that explicit inclusion of load-voltage dependency models would improve the accuracy of the congestion forecast. For demonstrating the applicability of the tool, it has been integrated into an existing distribution management system via the IoT platform of a DMS vendor, Atos Worldgrid

Current Harmonics Compensation in Microgrids Exploiting the Power Electronics Interfaces of Renewable Energy Sources

This work presents a method of current harmonic reduction in a distorted distribution system. In order to evaluate the proposed method a grid with high-order current harmonics is assumed. The reduction of current distortion is feasible due to the pulse modulation of an active filter, which consists of a buck-boost converter connected back-to-back to a polarity swapping inverter. For a practical application, this system would be the power electronic interface of a Renewable Energy Source (RES) and therefore it changes a source of harmonics to a damping harmonics system. Using the proposed method, the current Total Harmonic Distortion (THD) of the grid is reduced below the acceptable limits and thus the general power quality of the system is improved. Simulations in the MATLAB/SIMULINK platform and experiments have been performed in order to verify the effectiveness of the proposed method

Study of a laboratory voltage source converter for the interconnection of microsource units to microgrids, with the “plug and play” capability and the ability of harmonics compensation

This PhD thesis regards the study of a laboratory voltage source converter for the interconnection of generation units to microgrids, the “plug and play” capability and the ability of power quality enhancement through higher harmonics compensation. The targets of this PhD thesis are:1.Study and analysis of the used voltage source converter.2.Application of the proper control that will induce power quality enhancement to converter’s output.3.Development of a voltage and current higher harmonics compensation method and its application to converter’s modulation.4.Exploitation of the power produced by Renewable Energy Sources, such as photovoltaic arrays.5.Investigation of the safe microsources interconnection to microgrids, along with “plug and play” operation.6.Development of a methodology that guarantees the steady state stability of the studied system, irrelevant of the inductive or resistive nature of the grid.7.Development of an evaluation method regarding the imposed control scheme, through decoupling indexes for active and reactive power.Initially, the most important characteristics of microgrids and smart grids will be presented, along with the basic electronic converters technologies used in them. Special emphasis will be given to the operation of the used converter, which consists of a buck-boost DC/DC converter connected in series with a polarity swapping inverter.Subsequently, the capability of higher harmonics compensation through the used converter control is studied. For this reason, it has been developed a method of higher harmonics compensation through mirror harmonics integration to the modulation stage of the buck-boost converter. This method is explained in detail and simulation and experimental results are presented, verifying its suitability for microgrid applications. Initially, simulations of a weak harmonically distorted microgrid are performed, where through the proposed method voltage higher harmonics are mitigated. Subsequently, simulation and experimental results regarding current harmonics compensation in a stiff distribution grid are presented. Afterwards, it has been studied the ability of the used converter regarding voltage harmonics compensation in an isolated system, when the energy source is a photovoltaic array. It is demonstrated that the used method is effective in all these cases, as it provides the ability of both voltage and current harmonics compensation, independently from whether the studied system is isolated or interconnected. The power quality study ends with the investigation of the interconnected operation of the used converter supplied by a photovoltaic array. In this case, it is studied the effect of the injection angle of voltage higher harmonics included in converter modulation regarding the higher harmonics of the current flowing to the grid. It is demonstrated that in every case there is a voltage harmonic injection angle that minimizes the harmonic distortion of the current flowing to the grid.Subsequently, the interconnection of the converter to the distribution grid is studied, so that converter’s ability regarding microsources interconnection to microgrids, along with its “plug and play” capability is investigated. To verify the feasibility of the connection and the proper converter control in a grid with non-trivial line resistance, a modified virtual impedance method has been developed. Afterwards, it is presented a method of virtual impedance determination that guarantees the safe connection and operation of the converter to any grid and its dynamic stability to the continuous disturbances, occurring due to converter’s control. Finally, proper assessment indexes of the imposed control regarding active and reactive power decoupling have been developed. This way the evaluation of the imposed control method is feasible, so that its suitability in every application is determinated.Η παρούσα διατριβή αφορά τη μελέτη ενός εργαστηριακού μετατροπέα πηγής τάσης για τη διασύνδεση μονάδων παραγωγής σε μικροδίκτυα, τη δυνατότητα της επίτευξης «σύνδεσης και λειτουργίας» και τη βελτίωση της ποιότητας ισχύος μέσω της απαλοιφής ανώτερων αρμονικών. Οι στόχοι της παρούσας διατριβής είναι:1.Η μελέτη και η ανάλυση της λειτουργίας του χρησιμοποιούμενου μετατροπέα πηγής τάσης.2.Η εφαρμογή του κατάλληλου ελέγχου οδήγησής του που θα προκαλέσει τη βελτίωση της ποιότητας ισχύος στην έξοδό του.3.Η ανάπτυξη μίας μεθοδολογίας απόσβεσης ανώτερων αρμονικών τάσης και ρεύματος και η επιβολή της στην παλμοδότηση του μετατροπέα.4.Η αξιοποίηση της ισχύος που παράγεται από Ανανεώσιμες Πηγές Ενέργειας, όπως φωτοβολταϊκές συστοιχίες.5.Η διερεύνηση της ασφαλούς διασύνδεσης μικροπαραγωγών σε μικροδίκτυα και η επίτευξη δυνατότητας «σύνδεσης και λειτουργίας».6.Η ανάπτυξη μεθοδολογίας για την εξασφάλιση ευστάθειας μόνιμης κατάστασης του μελετώμενου συστήματος, ασχέτως της επαγωγικής ή ωμικής φύσης του δικτύου.7.Η ανάπτυξη μεθοδολογίας αξιολόγησης της εκάστοτε επιβαλλόμενης μεθόδου ελέγχου, μέσω δεικτών αξιολόγησης της απόζευξης ενεργού και άεργου ισχύος.Για την επίτευξη των παραπάνω στόχων αναλύεται αρχικά η λειτουργία του χρησιμοποιούμενου μετατροπέα που αποτελείται από έναν μετατροπέα ανύψωσης/υποβιβασμού συνεχούς τάσης συνδεδεμένου εν σειρά με έναν αντιστροφέα εναλλαγής πολικότητας.Στη συνέχεια μελετάται η δυνατότητα απαλοιφής ανώτερων αρμονικών μέσω της οδήγησης του χρησιμοποιούμενου μετατροπέα. Για αυτόν τον σκοπό αναπτύχθηκε μία μέθοδος απαλοιφής ανώτερων αρμονικών μέσω ενσωμάτωσης καθρεπτικών αρμονικών στην παλμοδότηση του μετατροπέα ανύψωσης/υποβιβασμού συνεχούς τάσης. Η μέθοδος αυτή εξηγείται αναλυτικά και παρουσιάζονται εξομοιωτικά και πειραματικά αποτελέσματα που αποδεικνύουν την καταλληλότητά της για εφαρμογές μικροδίκτυων. Αρχικά, γίνεται προσομοίωση ενός αρμονικά διαταραγμένου ασθενούς μικροδίκτυου, όπου μέσω της προτεινόμενης μεθόδου απαλείφονται ανώτερες αρμονικές τάσης. Εν συνεχεία παρουσιάζονται εξομοιωτικά και πειραματικά αποτελέσματα για την απαλοιφή αρμονικών ρεύματος σε ένα ισχυρό δίκτυο διανομής. Έπειτα, μελετήθηκε πειραματικά η δυνατότητα απαλοιφής αρμονικών τάσης σε απομονωμένο σύστημα με τροφοδοσία φωτοβολταϊκής συστοιχίας. Αποδεικνύεται ότι η χρησιμοποιούμενη μέθοδος είναι αποτελεσματική σε όλες τις περιπτώσεις αφού παρέχει τη δυνατότητα απαλοιφής τόσο αρμονικών τάσης όσο και αρμονικών ρεύματος, ανεξάρτητα από το αν μελετάται απομονωμένο ή διασυνδεδεμένο σύστημα. Η μελέτη της βελτίωσης της ποιότητας ισχύος τελειώνει με τη μελέτη της διασυνδεδεμένης λειτουργίας του μετατροπέα τροφοδοτούμενου από φωτοβολταϊκές συστοιχίες. Σε αυτήν την περίπτωση μελετάται η επίδραση της γωνίας έγχυσης ανώτερων αρμονικών τάσης στην παλμοδότηση του μετατροπέα ως προς την απόσβεση των ανώτερων αρμονικών του ρεύματος που ρέει στο δίκτυο. Αποδεικνύεται ότι σε κάθε περίπτωση υπάρχει μία γωνία έγχυσης ανώτερης αρμονικής τάσης που ελαχιστοποιεί την αρμονική παραμόρφωση του ρεύματος που ρέει στο δίκτυο.Στη συνέχεια μελετήθηκε η σύνδεση του μετατροπέα στο δίκτυο διανομής για να διαπιστωθεί η δυνατότητα του για τη διασύνδεση μικροπαραγωγών σε μικροδίκτυο και για την επίτευξη «σύνδεσης και λειτουργίας». Για να είναι δυνατή η σύνδεση και να διασφαλιστεί ο κατάλληλος έλεγχος του μετατροπέα σε δίκτυο με μη αμελητέα αντίσταση γραμμής αναπτύχθηκε μία τροποποιημένη μέθοδος εικονικής εμπέδησης. Έπειτα, παρουσιάζεται μία μεθοδολογία εύρεσης της εικονικής εμπέδησης που διασφαλίζει την ασφαλή σύνδεση και λειτουργία του μετατροπέα σε οποιοδήποτε δίκτυο και τη δυναμική ευστάθεια του συστήματος κατά τις συνεχείς μεταβολές που προκύπτουν λόγω του ελέγχου του μετατροπέα. Τέλος, προσδιορίστηκαν κατάλληλοι δείκτες αξιολόγησης του ελέγχου ως προς την απόζευξη ενεργού και άεργου ισχύος. Με αυτόν τον τρόπο είναι δυνατή η αξιολόγηση του εκάστοτε επιβαλλόμενου ελέγχου, ώστε να προσδιορίζεται επαρκώς η καταλληλότητα του για την κάθε εφαρμογή

Current Harmonics Compensation in Microgrids Exploiting the Power Electronics Interfaces of Renewable Energy Sources

This work presents a method of current harmonic reduction in a distorted distribution system. In order to evaluate the proposed method a grid with high-order current harmonics is assumed. The reduction of current distortion is feasible due to the pulse modulation of an active filter, which consists of a buck-boost converter connected back-to-back to a polarity swapping inverter. For a practical application, this system would be the power electronic interface of a Renewable Energy Source (RES) and therefore it changes a source of harmonics to a damping harmonics system. Using the proposed method, the current Total Harmonic Distortion (THD) of the grid is reduced below the acceptable limits and thus the general power quality of the system is improved. Simulations in the MATLAB/SIMULINK platform and experiments have been performed in order to verify the effectiveness of the proposed method