71 results found
Perez-Olvera J, Green TC, Junyent-Ferre A, 2021, Self-learning Control for Active Network Management
Active network management (ANM) using power electronic devices will become an essential tool for distribution network operators to deal with the variability of a large number of low-carbon technologies. To enable ANM, this paper proposes a control scheme based on deep reinforcement learning, as an alternative to traditional optimisation. The algorithm uses only a small number of network measurements and can learn approximations of optimal control actions, identified in offline simulations, via a neural network. Once trained, the control scheme chooses power converter set-points that can, for instance, even out loadings on different substations in real-time without the computational burden of high-level optimisation. The performance of the proposed control algorithm is validated against the optimal power flow (OPF) using data from real low-voltage networks. The results show that the solution and benefits are comparable to those obtained by the OPF.
Tosatto A, Misyris G, Junyent-Ferre A, et al., 2021, Towards optimal coordination between regional groups: HVDC supplementary power control, IEEE Transactions on Power Systems, Pages: 1-1, ISSN: 0885-8950
With Europe dedicated to limiting climate change and greenhouse gas emissions, large shares of Renewable Energy Sources (RES) are being integrated in the national grids, phasing out conventional generation. The new challenges arising from the energy transition will require a better coordination between neighboring system operators to maintain system security. To this end, this paper studies the benefit of exchanging primary frequency reserves between asynchronous areas using the Supplementary Power Control (SPC) functionality of High-Voltage Direct-Current (HVDC) lines. First, we focus on the derivation of frequency metrics for asynchronous AC systems coupled by HVDC interconnectors. We compare two different control schemes for HVDC converters, which allow for unilateral or bilateral exchanges of reserves between neighboring systems. Second, we formulate frequency constraints and include them in a unit commitment problem to ensure the N-1 security criterion. A data-driven approach is proposed to better represent the frequency nadir constraint by means of cutting hyperplanes. Our results suggest that the exchange of primary reserves through HVDC can reduce up to 10% the cost of reserve procurement while maintaining the system N-1 secure.
Rodriguez Bernuz JM, McInerney I, Junyent Ferre A, et al., 2021, Design of a linear time-varying Model Predictive Control energy regulator for grid-tied VSCs, IEEE Transactions on Energy Conversion, Vol: 36, Pages: 1425-1434, ISSN: 0885-8969
This paper presents an energy regulator based on a Model Predictive Control (MPC) algorithm for a Voltage Source Converter (VSC). The MPC is formulated to optimise the converter performance according to the weights defined in an objective function that trades off additional features, such as current harmonic distortion, reactive power tracking and DC bus voltage oscillation. Differently from most approaches found in the research literature, the MPC proposed here considers the coupling dynamics between the AC and DC sides of the VSC. This study is focused on the example case of a single-phase VSC, which presents a nonlinear relationship between its AC and DC sides and a sustained double-line frequency power disturbance in its DC bus. To reduce the burden of the MPC, the controller is formulated to benefit from the slow energy dynamics of the system. Thus, the cascaded structure typically used in the control of VSCs is kept and the MPC is set as an energy regulator at a reduced sampling frequency while the current control relies on a fast inner controller. The computational burden of the algorithm is further reduced by using a linear time-varying approximation. The controller is presented in detail and experimental validation showing the performance of the algorithm is provided.
Tu G, Junyent Ferre A, Xiang J, et al., 2021, Optimal power sharing of wind farms for frequency response, IET Renewable Power Generation, Vol: 15, Pages: 1005-1018, ISSN: 1752-1416
This paper presents a uniform optimal power sharing strategy to coordinate the wind turbines (WTs) in a wind farm (WF)to provide occasional and continuous frequency response (FR). The coordination of WTs is formulated as an optimisation problemthat takes into account the WT dynamics and tries to reduce the long term loss of energy yield caused by the provision of FR. Thisis achieved by maximising the total kinetic energy of the WF over time while reducing wear and tear of WTs. The proposed optimalpower sharing strategy relies on periodic communication between each WT and a WF controller. Local linear approximations areemployed to predict the system behaviour and the solution of the optimisation problem is obtained using the proposed centralisedand/or distributed algorithm. The distributed algorithm only requires one-way communication between the WF controller and localWTs, reducing the communication overheads. Simulation studies are carried out on a WF model to demonstrate the effectivenessof the proposed strategy. The results show the strategy enables reduction of yield loss over previous methods while avoiding overtorque operation during FR provision.
Hunter L, Booth CD, Egea-Alvarez A, et al., 2021, A new fast-acting backup protection strategy for embedded MVDC links in future distribution networks, IEEE Transactions on Power Delivery, Vol: 36, Pages: 861-869, ISSN: 0885-8977
This paper presents a new fast-acting backup protection strategy for future hybrid ac-dc distribution networks. By examining the impedance measured by an ac-connected distance protection relay, a unique characteristic is established for faults occurring on the dc-side of an embedded medium-voltage dc (MVDC) link, interconnecting two 33 kV distribution network sections. Based on the identified impedance characteristic, appropriate settings are developed and deployed on a verified software model of a commercially available distance protection relay. To remain stable for ac-side faults, it is found that the tripping logic of the device must be altered to provide correct time grading between standard, ac, protection zones and the fast-acting dc region, which can identify faults on the dc system within 40 ms. An additional confirmatory check is also employed to reduce the likelihood of mal-operation. The proposed solution is trialled on a test system derived from an actual distribution network, which employs distance protection, and is shown to provide stable operation for both standard ac-side faults and dc-side pole-pole-ground and pole-pole events.
Rodriguez Bernuz JM, Junyent Ferre A, Xiang X, 2021, Optimal droop offset adjustments for accurate energy trading in rural DC mini-grid clusters, 2020 International Conference on Smart Grids and Energy Systems (SGES 2020), Publisher: IEEE, Pages: 453-458
Off-grid micro-grids and solar home systems have become an enabler for the electrification of rural areas in developing countries. Their future integration into full-scale grids poses multiple technical challenges. This paper presents a concept of local DC mini-grid formed by the interconnection of nearby micro-grids. This is envisioned as a step towards the bottom-up development of a larger grid. The paper places special emphasis on the analysis of the controller structure, which is based on a variation of the common droop controller. Generally, decentralised droop schemes present inherent limitations to achieve, simultaneously, DC voltage regulation and accurate energy tracking. The decentralised controller proposed here improves conventional approaches by optimally adjusting the droop controller setpoints based on an Optimal Power Flow (OPF) algorithm that takes into account the characteristics of individual converters and the energy trading commands. This approach can benefit from the communication layer required by the energy trading system. The control design of the mini-grid is customised for its deployment on a variation of the dual active bridge (DAB) power electronic converter. Despite the use of a centralised OPF, the system stability is guaranteed by the decentralised low-level controller. The performance of the control approach and its sensitivity to unknown system parameters is measured and validated through detailed simulation studies.
Li Y, Yunjie G, Yue Z, et al., 2021, Impedance circuit model of grid-forming inverter: visualizing control algorithms as circuit elements, IEEE Transactions on Power Electronics, Vol: 36, Pages: 3377-3395, ISSN: 0885-8993
The impedance model is widely used for analyzing power converters. However, the output impedance is an external representation of a converter system, i.e., it compresses the entire dynamics into a single transfer function with internal details of the interaction between states hidden. As a result, there are no programmatic routines to link each control parameter to the system dynamic modes and to show the interactions among them, which makes the designers rely on their experience and heuristic to interpret the impedance model and its implications. To overcome these obstacles, this paper proposes a new modeling tool named as impedance circuit model, visualizing the closed-loop power converter as an impedance circuit with discrete circuit elements rather than an all-in-one impedance transfer function. It can reveal the virtual impedance essence of all control parameters at different impedance locations and/or within different frequency bandwidths, and show their interactions and coupling effects. A grid-forming voltage-source inverter (VSI) is investigated as an example, with considering its voltage controller, current controller, control delay, voltage/current dq-frame cross-decoupling terms, output-voltage/current feedforward control, droop controllers, and three typical virtual impedances. The proposed modeling tool is validated by frequency-domain spectrum measurement and time-domain step response in simulations and experiments.
Mian S, Judge P, Junyent Ferre A, et al., 2020, A delta-connected Modular Multilevel STATCOM with partially-rated energy storage for provision of ancillary services, IEEE Transactions on Power Delivery, ISSN: 0885-8977
This paper proposes a delta-connected Modular Multilevel STATCOM with partially rated storage (PRS-STATCOM), capable of providing both reactive and active power support. The purpose is to provide short-term energy storage enabled grid support services such as inertial and frequency response, either alongside or temporarily instead of standard STATCOM voltage support. The topology proposed here contains two types of sub-modules (SM) in each phase-leg: standard sub-modules (STD-SMs) and energy storage element sub-modules (ESE-SMs) with a dc-dc interface converter between the SM capacitor and the ESE. A control structure has been developed that allows energy transfer between the SM capacitor and the ESE resulting in active power exchange between the converter and the grid. Injecting 3rd harmonic current into the converter waveforms can be used to increase the amount of power that can be extracted from the ESE-SMs and so reduce the required ESE-SMs fraction in each phase-leg. Simulation results demonstrate that for three selected active power ratings, 1 up, 2/3 pu, & 1/3 pu, the fraction of SMs that need be converted to ESE-SMs are only 69%, 59% & 38%. Thus, the proposed topology is effective in adding real power capability to a STATCOM without a large increase in equipment cost.
Hategekimana P, Junyent Ferre A, Ntagwirumugara E, et al., 2020, Assessment of feasible DC microgrid network topologies for rural electrification in Rwanda: studying the Kagoma Village, 2020 International Conference on Smart Grids and Energy Systems (SGES 2020), Publisher: IEEE
This paper investigates the network topology fordistributing electricity in the remote village using solar PVwith DC microgrid. The expansion of the national gridelectricity into rural villages remains a challenge due to theincreased costs of distribution and transmission systems.The optimum selection of the network topology isimportant, relative to the density of the population, villagesize, and the group arrangement of a particular village.The modified Newton–Raphson’s algorithm (N-R) hasbeen programmed in MATLAB to evaluate the voltagedrop and system efficiency, obtained with different typesof the distribution system and the nominal voltages to beused. In this paper, the methodology to decide the bestdistribution topology, nominal operating voltage level, andthe conductor size (based on voltage drop and total cost)was addressed for a specific remote village in Rwanda.
Seok H, Junyent-Ferre A, Cha H, et al., 2020, Reactive Power Elimination for High Conversion-Ratio Bidirectional Resonant Converter, IEEE TRANSACTIONS ON POWER ELECTRONICS, Vol: 35, Pages: 3740-3753, ISSN: 0885-8993
Rodriguez-Bernuz JM, Junyent Ferre A, 2020, Operating region extension of a modular multilevel converter using model predictive control: a single phase analysis, IEEE Transactions on Power Delivery, Vol: 35, Pages: 171-182, ISSN: 0885-8977
The modular multilevel converter is the state-of-the-art topology for voltage source converter HVDC. Despite its advantages, this converter handles large internal low-frequency energy ripples, and the capacitance that supports these dynamics is a key design parameter that affects the operating region of the converter. Different strategies can be found in the literature to increase the feasible region of operation of the converter. Nevertheless, they are typically open loop in nature and use precalculated control references. This paper presents an alternative based on model predictive control that steers the system through optimal control trajectories that are calculated online. This provides feedback and corrective control action in real time. The predictive controller used for this purpose is presented and a linear time-varying approximation is used to reduce the computational burden of the algorithm. The feasible boundaries of the converter are sought and the final performance of the control algorithm is evaluated through detailed simulations using a switching model of the converter.
Sang Y, Junyent Ferre A, Green TC, 2020, Operational principles of three-phase single active bridge DC/DC converters under duty cycle control, IEEE Transactions on Power Electronics, Vol: 35, Pages: 8737-8750, ISSN: 0885-8993
Single Active Bridge (SAB) DC/DC converters are attractive options for unidirectional DC/DC conversion in future medium power generation applications, offering galvanic isolation and a diode-based output side. SAB DC/DC converters must be controlled by adjusting the active bridge switching duty cycle, unlike Dual Active Bridge (DAB) converters where the phase-shift angle control is normally used. This paper presents a comprehensive analysis of the operational principles of three-phase SAB (3p-SAB) DC/DC converters that arise from different duty cycle operation ranges. Moreover, the converter performances such as transformer harmonic currents are analyzed and are compared between different input and output DC voltage conditions. Finally, an experimental validation using a small scale prototype is presented.
Sanchez-Sanchez E, Junyent-Ferre A, Prieto-Araujo E, et al., 2020, Modelling and experimental validation of a laboratory-scaled HVDC cable emulator tested in an MMC-based platform, 22nd European Conference on Power Electronics and Applications (EPE ECCE Europe), Publisher: IEEE, ISSN: 2325-0313
Prieto-Araujo E, Junyent-Ferre A, Clariana-Colet G, et al., 2020, Control of Modular Multilevel Converters Under Singular Unbalanced Voltage Conditions With Equal Positive and Negative Sequence Components, IEEE-Power-and-Energy-Society General Meeting (PESGM), Publisher: IEEE, ISSN: 1944-9925
Batzelis E, Junyent-Ferre A, Pal BC, 2020, MPP Estimation of PV Systems keeping Power Reserves under Fast Irradiance Changes, IEEE-Power-and-Energy-Society General Meeting (PESGM), Publisher: IEEE, ISSN: 1944-9925
Perez-Olvera J, Green TC, Junyent-Ferre A, 2020, Active network management in LV networks: a case study in the UK, IEEE-Power-and-Energy-Society General Meeting (PESGM), Publisher: IEEE, ISSN: 1944-9925
Gu Y, Li Y, Yoo H-J, et al., 2019, Transfverter: imbuing transformer-like properties in an interlink converter for robust control of a hybrid ac-dc microgrid, IEEE Transactions on Power Electronics, Vol: 34, Pages: 11332-11341, ISSN: 0885-8993
In a hybrid ac-dc microgrid, stiff voltage sources may appear in either the dc or ac subgrids which gives rise to multiple operation modes as power dispatch changes. This creates a challenge for designing the interlink converter between the ac and dc subgrids since the different modes require different interlink controls. To solve this problem, this paper proposes the concept of a transfverter inspired by how transformers link ac grids. Like a transformer, a transfverter can react to the presence of stiff voltage sources on either the dc or ac side and reflect the “stiffness” and voltage stabilizing capability to the other side. A back-to-back converter with droop control is used as the underlying technology to implement this concept. A novel optimization method called model bank synthesis is proposed to find control parameters for the interlink converter that offer the best controller performance across the different microgrid modes without requiring mode-changing of the controller. The effectiveness of the proposed solution is validated through both simulation and experiments.
Wang Y, Li Y, Junyent Ferre A, et al., 2019, H5+ converter: A bidirectional AC-DC converter with DC-fault-blocking and self-pre-charge capabilities, IEEE Transactions on Power Electronics, Vol: 34, Pages: 10619-10634, ISSN: 0885-8993
A pulse-width-modulation (PWM) bidirectional ac-dc converter (i.e., active rectifier) with dc-faultblocking and self-pre-charge capabilities is proposed in this paper for low-voltage dc (LVDC) applications. The proposed converter, which is named as ”H5 + converter”, consists of an H4 bridge, a bidirectional switch, and a transient-voltage-surge (TVS) diode. The bidirectional switch and the TVS diode enable the dc-fault blocking and dc-bus self-pre-charge, while preserving the low common mode voltage noise and low leakage current of the converter. Additionally, the proposed H5+ converter has advanced features under a dc-side short circuit fault, such as fault diagnosis and fault recovery. Operation principles of the proposed converter are presented and analyzed. A down scaled prototype is built. Experiment results are shown and analyzed, including steady-state waveforms, common-mode performance, start-up dynamics, as well as dc fault blocking, fault diagnosis, and fault recovery. Moreover, the proposed converter is compared with other two dc-fault blocking converters for LVDC applications, in terms of converter capabilities, required devices, and power switch losses.
Makkieh A, Emhemed A, Wang D, et al., 2019, Investigation of different system earthing schemes for protection of low-voltage DC microgrids, JOURNAL OF ENGINEERING-JOE, Pages: 5129-5133
Li Y, Junyent Ferre A, Judge P, 2019, A boost-full-bridge-type single-active-bridge isolated AC-DC converter, 34th Annual IEEE Applied Power Electronics Conference & Exposition, Publisher: IEEE
A boost-full-bridge (BFB) single-active-bridge (SAB) ac-dc converter is proposed in this paper. The topology has high power density andgalvanic isolation, thanks to its high-frequency (HF) transformer link. In addition, it can be easily controlled by using the proposed modulation and control scheme. Furthermore, it is able to block dc-side short-circuit fault current contribution. The converter operation principles are analyzed. Experiment results are also given to show the startup dynamics, steady-state waveforms, and the dc fault blocking capability.
Son S, Montes OA, Junyent Ferre A, et al., 2019, High step-up resonant DC/DC converter with balanced capacitor voltage for distributed generation systems, IEEE Transactions on Power Electronics, Vol: 34, Pages: 4375-4387, ISSN: 0885-8993
We propose a high step-up resonant dc-dc converter that can achieve voltage balance of the resonant capacitors in distributed generation systems. By incorporating a switching mechanism on the secondary side, we achieve high step-up voltage gain with a minimum number of devices and without reverse-recovery problem. An active-clamp circuit installed on the primary side suppresses the surge voltage that occurs at switch components, recycles the energy stored in the leakage inductance, and provides an alternate resonant-current path formed by the leakage inductance and the output resonant capacitors. A dual resonance that occurs at the secondary side of the converter is exploited to reduce the turn-off current and switching loss significantly, and to achieve high power conversion efficiency. The resonant capacitor voltages remain in balance because the duty cycle of the primary-side switches is always set to 0.5 regardless of the input voltages and load variations. Design and analysis of the proposed converter are presented, and tests using a 400 W experimental prototype verify its superior performance.
Lachichi A, Junyent Ferre A, Green T, 2019, Comparative optimization design of a modular multilevel converter tapping cells and a 2L-VSC for hybrid LV ac/dc microgrids, IEEE Transactions on Industry Applications, Vol: 55, Pages: 3228-3240, ISSN: 0093-9994
The paper presents the performance of the modular multilevel converter tapping cells associated with an ac filter in term of efficiency and power density in a hybrid LV ac/dc microgrid application and compares it to the performance of the conventional topology used in LV application, i.e., the Two-Level voltage source converter (2L VSC). A bi-objective optimization based on the Genetic Algorithm is hence developed, providing details on designing the components of the LCL filter, the MMC and the 2L VSC. The MMC reaches an efficiency of 99.4% when the main dcgrid is left floating. However, due to its modularity and scalability, offering multiport connections option, the MMC tapping cells has the disadvantage of low power density. Exploring the filtering capability of the equivalent arm inductance of the MMC seen from the ac grid side, optimization design results show that higher switching frequencies allow a significant volume reduction of the inductive components of the MMC/LCL filter while higher switching frequencies have little impact on the switching losses of the MMC. This has the benefit of reducing the overall footprint of the converter and encouraging the use of the MMC in LV application.
Seok H, Jeong S-G, Kim K-S, et al., 2019, Instantaneous Reactive Power Reduction of Ripple-Free Resonant Buck Converter Using Bidirectional Switch, 34th Annual IEEE Applied Power Electronics Conference and Exposition (APEC), Publisher: IEEE, Pages: 3174-3179, ISSN: 1048-2334
Mandic DP, Kanna S, Xia Y, et al., 2019, A Data Analytics Perspective of Power Grid Analysis-Part 1: The Clarke and Related Transforms, IEEE SIGNAL PROCESSING MAGAZINE, Vol: 36, Pages: 110-116, ISSN: 1053-5888
Lachichi A, Junyent-Ferre A, Green T, 2019, LCL Filter Design Optimization for LV Modular Multilevel Converters in Hybrid ac/dc Microgrids Application, 3rd International Conference on Electrical Sciences and Technologies in Maghreb (CISTEM), Publisher: IEEE, Pages: 945-949
Olvera JP, Green T, Junyent-Ferre A, 2018, Using Multi-terminal DC Networks to Improve the Hosting Capacity of Distribution Networks, IEEE PES Innovative Smart Grid Technologies Conference Europe (ISGT-Europe), Publisher: IEEE, ISSN: 2165-4816
Rodriguez-Bernuz J-M, Junyent-Ferre A, 2018, Model predictive circulating current regulator for single-phase modular multilevel converter, 10th IEEE Annual Energy Conversion Congress and Exposition (ECCE), Publisher: IEEE, Pages: 4824-4830, ISSN: 2329-3721
This paper describes a model predictive strategy to reduce the sub-module voltage oscillation in a single-phase modular multi-level converter. This task is accomplished by a predictive controller which solves an optimal control problem sequentially. By choosing the objective function weights appropriately, this controller can naturally trade-off sub-modules voltage ripple and recirculating current. It is shown that the sub-module voltage oscillation can be reduced without degrading the efficiency excessively, enhancing the performance of the overall converter. Additionally, it is guaranteed that the recirculating current can be regulated without exceeding the physical limitations of the device.
Makkieh A, Emhemed A, Wang D, et al., 2018, Fault Characterisation of a DC Microgrid with Multiple Earthing under Grid Connected and Islanded Operations, 53rd International Universities Power Engineering Conference (UPEC), Publisher: IEEE
Sánchez-Sánchez E, Prieto-Araujo E, Gomis-Bellmunt O, et al., 2018, Analysis of MMC energy-based control structures for VSC- HVDC links, IEEE Journal of Emerging and Selected Topics in Power Electronics, Vol: 6, Pages: 1065-1076, ISSN: 2168-6777
An analysis of control structures for modular multilevel converters (MMCs) used in high-voltage direct current (HVDC) applications is addressed. In particular, this paper focuses on the case of a point-to-point link with master-slave control, considering an energy-based scheme (also known as closed loop or energy controlled) for the MMC, meaning that the internal energy of the converter is explicitly controlled. With such an approach, the MMC internal energy can be controlled independently of the energy of the HVDC link, and whereas the internal capacitance of the MMC depends on the converter's rating, the capacitance at the dc terminal depends on the cable length. Therefore, several possibilities regarding the outer control structure (internal energy and dc voltage) arise, affecting the overall dynamics differently. Whereas for a long link, the classic control structures should perform well, for shorter links the transient performance might not be acceptable and other alternatives shall be used instead. Different control structures are presented and evaluated in this paper through small-signal and frequency-domain analysis, and validated through time-domain simulation with MATLAB Simscape Power Systems.
Sang Y, Junyent Ferre A, Xiang X, et al., 2018, Analysis and Control of a Parallel DC Collection System for Wind Turbines with Single Active Bridge Converters, IEEE ENERGY CONVERSION CONGRESS & EXPO
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