Online coordinated voltage control in distribution systems subjected to structural changes and DG availability

The responses of multiple DG units and voltage regulating devices such as tap changers and capacitor banks for correcting the voltage may lead to operational conflicts and oscillatory transients, where distribution systems are subjected to network reconfiguration and changes in availability of the DG units. Therefore, coordinated voltage control is required to minimize control interactions while accounting for the impact of structural changes associated with the network. This paper proposes a strategy for coordinating the operation of multiple voltage regulating devices and DG units in medium voltage (MV) distribution systems, under structural changes and DG availability, for effective voltage control. The proposed strategy aids to minimize the operational conflicts by allowing the farthest voltage regulating device to operate first on a priority scheme designed based on the electrical-distance between voltage regulating devices and DG units, while maximizing the voltage support by the DG units. The proposed coordination scheme is designed to enact with an aid of a substation centered distribution management system (DMS) for online voltage control. The control actions of proposed coordination strategy are tested on a MV distribution system, derived from the state of New South Wales, Australia, through simulations, and results are reported

Coordinated Voltage Control in Modern Distribution Systems

Modern distribution systems, especially with the presence of distributed generation (DG) and distribution automation are evolving as smart distribution systems. Distribution management systems (DMSs) with communication infrastructure and associated software and hardware developments are integral parts of the smart distribution systems. With such advancement in distribution systems, distribution system voltage and reactive power control are dominant by Volt/VAr (voltage and reactive power) optimisation and utilisation of DG for system Volt/VAr support. It is to be noted that the respective controls and optimisation formulations are typically adopted from primary, secondary and tertiary voltage and reactive power controls at upstream system level. However, the characteristics of modern distribution systems embedded with high penetration of DG are different from transmission systems and the former distribution systems with uni-directional power flow. Also, coordinated control of multiple Volt/VAr support DG units with other voltage control devices such as on-load tap changer (OLTC), line voltage regulators (VRs) and capacitor banks (CBs) is one of the challenging tasks. It is mainly because reverse power flow, caused predominantly by DG units, can influence the operation of conventional voltage control devices. Some of the adverse effects include control interactions, operational conflicts, voltage drop and rise cases at different buses in a network, and oscillatory transients. This research project aimed to carry out in-depth study on coordinated voltage control in modern MV distribution systems utilising DG for system Volt/VAr support. In the initial phase of the research project, an in-depth literature review is conducted and the specific research gaps are identified. The design considerations of the proposed coordinated voltage control, which also uses the concept of virtual time delay, are identified through comprehensive investigations. It emphasises on examining and analysing both steady-state and dynamic phenomena associated with the control interactions among multiple Volt/VAr support DG units and voltage control devices. It would be essential for ensuring effective coordinated voltage control in modern distribution systems. In this thesis, the interactions among multiple DG units and voltage control devices are identified using their simultaneous and non-simultaneous responses for voltage control through time domain simulations. For this task, an analytical technique is proposed and small signal modelling studies have also been conducted. The proposed methodology could be beneficial to distribution network planners and operators to ensure seamless network operation from voltage control perspective with increasing penetration of DG units. Notably, it has been found that the significant interactions among multiple DG units and voltage control devices are possible under conventional standalone, rule-based, and analytics based control strategies as well as with real-time optimal control under certain system conditions. In the second phase of the research project, the proposed coordinated voltage control strategy is elaborated. The control design considerations are fundamentally based on eliminating the adverse effects, which can distinctly be caused by the simultaneous and non-simultaneous responses of multiple Volt/VAr support DG units and voltage control devices. First, the concept of virtual time delay is applied for dynamically managing the control variables of Volt/VAr support DG units and voltage control devices through the proposed control parameter tuning algorithm. Because it has been found that the conventional time-graded operation cannot eliminate the adverse effects of DG-voltage control device interactions under certain system conditions. Secondly, the distinct control strategies are designed and tested for effectively and efficiently coordinating the operation of multiple Volt/VAr support DG units and voltage control devices in real-time. The test results have demonstrated that the proposed coordinated voltage control strategy for modern MV distribution systems can effectively be implemented in real-time using advanced substation centred DMS. The proposed coordinated voltage control strategy presented in this thesis may trigger paradigm shift in the context of voltage control in smart distribution systems. In the final phase of the research project, short-term and/or long-term oscillations which can be possible for a MV distribution system operation embedded with Volt/VAr support DG are discussed. Typically, the short-term oscillations are occurred due to interactions among different DG units and their controllers (i.e., inter-unit electro-mechanical oscillations in synchronous machine based DG units) while the long-term oscillations occurred due to DG-voltage control device interactions. Also, sustained oscillations may occur due to tap changer limit cycle phenomenon. The concept of alert-state voltage control is introduced for mitigating the sustained oscillations subjected to OLTC limit cycles in the presence of high penetration of DG. The investigative studies in this thesis further emphasise the requirements of supplementary control and other mitigating strategies for damping the oscillations in modern active MV distribution systems. The proposed research will pave the way for managing increasing penetration of DG units, with different types, technologies and operational modes, from distribution system voltage control perspective

Multi-objective optimisation method for coordinating battery storage systems, photovoltaic inverters and tap changers

The many well-established advantages of distributed generation (DG) make their usage in active distribution networks prevalent. However, uncontrolled operation of DG units can negatively interfere with the performance of other equipment, such as tap-changers, in addition to resulting in sub-optimal usage of their potential. Thus, adequate scheduling/control of DG units is critical for operators of the distribution system to avoid those adverse effects. A linearised model of a multi-objective method for coordinating the operation of photovoltaics, battery storage systems, and tap-changers is proposed. Three objective functions are defined for simultaneously enhancing voltage profile, minimising power losses, and reducing peak load power. The formulated multi-objective problem is solved by means of the epsilon-constraint technique. A novel decision-making methodology is offered to find the Pareto optimality and select the preferred solution. To assess to proposed model's performance, it is tested using 33-bus IEEE test system. Consequently, tap-changers suffer lessened stress, the batteries state-of-charge is kept within adequate limits, and the DG units operation is at higher efficiency. The obtained results verify the effectiveness of this approach.fi=vertaisarvioitu|en=peerReviewed

A Comprehensive Method For Coordinating Distributed Energy Resources In A Power Distribution System

Utilities, faced with increasingly limited resources, strive to maintain high levels of reliability in energy delivery by adopting improved methodologies in planning, operation, construction and maintenance. On the other hand, driven by steady research and development and increase in sales volume, the cost of deploying PV systems has been in constant decline since their first introduction to the market. The increased level of penetration of distributed energy resources in power distribution infrastructure presents various benefits such as loss reduction, resilience against cascading failures and access to more diversified resources. However, serious challenges and risks must be addressed to ensure continuity and reliability of service. By integrating necessary communication and control infrastructure into the distribution system, to develop a practically coordinated system of distributed resources, controllable load/generation centers will be developed which provide substantial flexibility for the operation of the distribution system. On the other hand, such a complex distributed system is prone to instability and black outs due to lack of a major infinite supply and other unpredicted variations in load and generation, which must be addressed. To devise a comprehensive method for coordination between Distributed Energy Resources in order to achieve a collective goal, is the key point to provide a fully functional and reliable power distribution system incorporating distributed energy resources. A road map to develop such comprehensive coordination system is explained and supporting scenarios and their associated simulation results are then elaborated. The proposed road map describes necessary steps to build a comprehensive solution for coordination between multiple agents in a microgrid or distribution feeder.\u2

Accurate Battery Modelling for Control Design and Economic Analysis of Lithium-ion Battery Energy Storage Systems in Smart Grid

Adoption of lithium-ion battery energy storage systems (Li-ion BESSs) as a flexible energy source (FES) has been rapid, particularly for active network management (ANM) schemes to facilitate better utilisation of inverter based renewable energy sources (RES) in power systems. However, Li-ion BESSs display highly nonlinear performance characteristics, which are based on parameters such as state of charge (SOC), temperature, depth of discharge (DOD), charge/discharge rate (C-rate), and battery-aging conditions. Therefore, it is important to include the dynamic nature of battery characteristics in the process of the design and development of battery system controllers for grid applications and for techno-economic studies analyzing the BESS economic profitability. This thesis focuses on improving the design and development of Li-ion BESS controllers for ANM applications by utilizing accurate battery performance models based on the second-order equivalent-circuit dynamic battery modelling technique, which considers the SOC, C-rate, temperature, and aging as its performance affecting parameters. The proposed ANM scheme has been designed to control and manage the power system parameters within the limits defined by grid codes by managing the transients introduced due to the intermittence of RESs and increasing the RES penetration at the same time. The validation of the ANM scheme and the effectiveness of controllers that manage the flexibilities in the power system, which are a part of the energy management system (EMS) of ANM, has been validated with the help of simulation studies based on an existing real-life smart grid pilot in Finland, Sundom Smart Grid (SSG). The studies were performed with offline (short-term transient-stability analysis) and real-time (long-term transient analysis) simulations. In long-term simulation studies, the effect of battery aging has also been considered as part of the Li-ion BESS controller design; thus, its impact on the overall power system operation can be analyzed. For this purpose, aging models that can determine the evolving peak power characteristics associated with aging have been established. Such aging models are included in the control loop of the Li-ion BESS controller design, which can help analyse battery aging impacts on the power system control and stability. These analyses have been validated using various use cases. Finally, the impact of battery aging on economic profitability has been studied by including battery-aging models in techno-economic studies.Aurinkosähköjärjestelmien ja tuulivoiman laajamittainen integrointi sähkövoimajärjestelmän eri jännitetasoille on lisääntynyt nopeasti. Uusiutuva energia on kuitenkin luonteeltaan vaihtelevaa, joka voi aiheuttaa nopeita muutoksia taajuudessa ja jännitteessä. Näiden vaihteluiden hallintaan tarvitaan erilaisia joustavia energiaresursseja, kuten energiavarastoja, sekä niiden tehokkaan hyödyntämisen mahdollistaviea älykkäitä ja aktiivisia hallinta- ja ohjausjärjestelmiä. Litiumioniakkuihin pohjautuvien invertteriliitäntäisten energian varastointijärjestelmien käyttö joustoresursseina aktiiviseen verkonhallintaan niiden pätö- ja loistehon ohjauksen avulla on lisääntynyt nopeasti johtuen niiden kustannusten laskusta, modulaarisuudesta ja teknisistä ominaisuuksista. Litiumioniakuilla on erittäin epälineaariset ominaisuudet joita kuvaavat parametrit ovat esimerkiksi lataustila, lämpötila, purkaussyvyys, lataus/ purkausnopeus ja akun ikääntyminen. Akkujen ominaisuuksien dynaaminen luonne onkin tärkeää huomioida myös akkujen sähköverkkoratkaisuihin liittyvien säätöjärjestelmien kehittämisessä sekä teknis-taloudellisissa kannattavuusanalyyseissa. Tämä väitöstutkimus keskittyy ensisijaisesti aktiiviseen verkonhallintaan käytettävien litiumioniakkujen säätöratkaisuiden parantamiseen hyödyntämällä tarkkoja, dynaamisia akun suorituskykymalleja, jotka perustuvat toisen asteen ekvivalenttipiirien akkumallinnustekniikkaan, jossa otetaan huomioon lataustila, lataus/purkausnopeus ja lämpötila. Työssä kehitetyn aktiivisen verkonhallintajärjestelmän avulla tehtävät akun pätö- ja loistehon ohjausperiaatteet on validoitu laajamittaisten simulointien avulla, esimerkiksi paikallista älyverkkopilottia Sundom Smart Gridiä simuloimalla. Simuloinnit tehtiin sekä lyhyen aikavälin offline-simulaatio-ohjelmistoilla että pitkän aikavälin simulaatioilla hyödyntäen reaaliaikasimulointilaitteistoa. Pitkän aikavälin simulaatioissa akun ikääntymisen vaikutus otettiin huomioon litiumioniakun ohjauksen suunnittelussa jotta sen vaikutusta sähköjärjestelmän kokonaistoimintaan voitiin analysoida. Tätä tarkoitusta varten luotiin akun ikääntymismalleja, joilla on mahdollista määrittää akun huipputehon muutos sen ikääntyessä. Akun huipputehon muutos taas vaikuttaa sen hyödynnettävyyteen erilaisten pätötehon ohjaukseen perustuvien joustopalveluiden tarjoamiseen liittyen. Lisäksi väitöstutkimuksessa tarkasteltiin akkujen ikääntymisen vaikutusta niiden taloudelliseen kannattavuuteen sisällyttämällä akkujen ikääntymismalleja teknis-taloudellisiin tarkasteluihin.fi=vertaisarvioitu|en=peerReviewed

Voltage Control in Active Distribution Grids: A Review and a New Set-Up Procedure for Local Control Laws

Planning, operation and control of active distribution grids by increasing the number of dispersed generators is becoming more important but also more complex. Hence, the importance of controlling the voltage is highlighted in many research papers. Traditionally, in passive distribution networks the voltage rise has been mitigated by network reinforcement. Nowadays, local voltage control, coordinated voltage control and centralized voltage control have been discussed for active networks in research papers. Although all the approaches have been proven to solve the problem of voltage rise in distribution grids, using plenty of sensors to gather huge number of measurement could cause complexity. This paper represents a literature review of different voltage control approaches in active distribution grids and proposes a new procedure to set up a local voltage control law devoted to properly manage the voltage profile (e.g. minimizing losses on MV feeders)

Control and Energy Management of Standalone Interconnected AC Microgrids

This thesis considered microgrids as local area distribution mini-power grids formed by distributed generation sources, energy storage systems and loads. They are reliable and can operate at different voltages and frequencies to meet the requirements of the load. Microgrids have limited renewable energy source (RES) capacity, which can only supply a limited load and increasing the load beyond a specifically designed limit can lead to stability issues. Irrespective of its limited capacity, there has been an increased widespread deployment of renewable energy-based microgrids worldwide orchestrated by the 2015 Paris Agreement and the war in Ukraine and as a solution to meet the global demand for energy in electricity deficit zones aimed to achieve universal access to affordable, reliable, and sustainable energy. Fast forward to the future, flooded singly operated microgrids face the problem of more curtailing of RES and load shedding. Multiple microgrids can be interconnected to mitigate the limitations of single microgrids and improve supply reliability, enhance power supply availability, stability, reserve capacity, reduce investment in new generating capacity and control flexibility. As a result, this thesis proposes a new structure and control technique for interconnecting multiple standalone AC microgrids to a common alternating current (AC) bus using a back-to-back power electronic converter and a traditional transformer. Each microgrid considered in this thesis comprises a renewable energy source (RES), battery, auxiliary unit, and load. The battery maintains the AC bus voltage and frequency and balances the difference in power generated by the RES and that consumed by the load. Each microgrid battery’s charge/discharge is maintained within the safest operating limit to maximise the RES power utilisation. The back-to-back converters are used to decouple the connecting standalone microgrid frequencies and facilitate power exchange between microgrids. The transformer is used to transmit electric power over long distances efficiently. The control technique for all the connecting bidirectional back-to-back converters is developed to manage the bidirectional power flow between each microgrid and other microgrids in the network and to balance the energy in the global bus of the interconnected microgrid with no communication. The control strategy uses a frequency signalling mechanism to limit the power demand of individual global converters and adjusts its droop coefficients accordingly and in proportion to deviation in frequency. The global droop controllers of the global connecting converters receive information about the status of the frequencies of individual microgrids using a low bandwidth communication link to enhance network power flow. MATLAB/Simulink results validate the performance of the proposed structure and control strategy. A decentralised control scheme is further proposed for the standalone interconnected AC microgrid structure. This thesis presented a high-level global droop controller that exchanges power between the interconnected microgrids. Renewable power curtailment and auxiliary power supplement mechanisms are designed based on the bus frequency signalling technique to achieve balance and continuity of supply. In case of power shortage in one microgrid, priority will first be given to power import from other microgrids. A power supplement is used if the power imported is insufficient to control the battery state of charge (SOC). Similarly, in case of a power surplus, priority will be given to power export, and if this is not enough, power from RES will be curtailed. Performance evaluation shows that the proposed controller maximises renewable power utilisation and minimises auxiliary power usage while providing better load support. The performance validation of the proposed structure and control strategy has been tested using MATLAB/Simulink. Furthermore, this thesis investigated a centralised control and energy management of multiple interconnected standalone AC microgrids using the Nelder-Mead simplex algorithm (Fminsearch optimisation toolbox in MATLAB) based on the new proposed model. The main objective is to minimise the total cost of energy from the auxiliary unit produced from gas. The results obtained are compared with those obtained from an unoptimised system. The performance evaluation investigation results are compared with the unoptimised results to determine the percentage optimal performance of the system. The comparison outcome shows that the proposed optimisation method minimises the total auxiliary energy cost by about 9% compared with the results of the unoptimised benchmark