Showing papers on "Voltage regulation published in 2021"
TL;DR: This survey shows that MPC is at the beginning of the application in microgrids and that it emerges as a competitive alternative to conventional methods in voltage regulation, frequency control, power flow management and economic operation optimization.
Abstract: The development of microgrids is an advantageous option for integrating rapidly growing renewable energies. However, the stochastic nature of renewable energies and variable power demand have created many challenges like unstable voltage/frequency and complicated power management and interaction with the utility grid. Recently, predictive control with its fast transient response and flexibility to accommodate different constraints has presented huge potentials in microgrid applications. This paper provides a comprehensive review of model predictive control (MPC) in individual and interconnected microgrids, including both converter-level and grid-level control strategies applied to three layers of the hierarchical control architecture. This survey shows that MPC is at the beginning of the application in microgrids and that it emerges as a competitive alternative to conventional methods in voltage regulation, frequency control, power flow management and economic operation optimization. Also, some of the most important trends in MPC development have been highlighted and discussed as future perspectives.
155 citations
TL;DR: This paper develops a model-free volt-VAR optimization (VVO) algorithm via multi-agent deep reinforcement learning (DRL) in unbalanced distribution systems and realizes the dual goals for VVO.
Abstract: This paper develops a model-free volt-VAR optimization (VVO) algorithm via multi-agent deep reinforcement learning (DRL) in unbalanced distribution systems. This method is novel since we cast the VVO problem in distribution networks to an intelligent deep Q-network (DQN) framework, which avoids solving a specific optimization model directly when facing time-varying operating conditions in the systems. We consider statuses/ratios of switchable capacitors, voltage regulators, and smart inverters installed at distributed generators as the action variables of the agents. A delicately designed reward function guides these agents to interact with the distribution system, in the direction of reinforcing voltage regulation and power loss reduction simultaneously. The forward-backward sweep method for radial three-phase distribution systems provides accurate power flow results within a few iterations to the DRL environment. The proposed method realizes the dual goals for VVO. We test this algorithm on the unbalanced IEEE 13-bus and 123-bus systems. Numerical simulations validate the excellent performance of this method in voltage regulation and power loss reduction.
113 citations
TL;DR: A new artificial intelligence (AI)-based method for the detection of cyber-attacks in direct current (dc) microgrids and also the identification of the attacked distributed energy resource (DER) unit is proposed.
Abstract: Cyber-physical systems (CPSs) are vulnerable to cyber-attacks. Nowadays, the detection of cyber-attacks in microgrids as examples of CPS has become an important topic due to their wide use in various practical applications from renewable energy plants to power distribution and electric transportation. In this article, we propose a new artificial intelligence (AI)-based method for the detection of cyber-attacks in direct current (dc) microgrids and also the identification of the attacked distributed energy resource (DER) unit. The proposed method works based on the time-series analysis and a nonlinear auto-regressive exogenous model (NARX) neural network, which is a special type of recurrent neural network for estimating dc voltages and currents. In the proposed method, we consider the effect of cyber-attacks named false data injection attacks (FDIAs), which try to affect the accurate voltage regulation and current sharing by affecting voltage and current sensors. In the presented strategy, first, a dc microgrid is operated and controlled without any FDIAs to gather enough data during the normal operation required for the training of NARX neural networks. It is worth mentioning that in the data generation process, load changing is also considered to have distinguishing data sets for load changing and cyber-attack scenarios. Trained and fine-tuned NARX neural networks are exploited in an online manner to estimate the output dc voltages and currents of DER units in dc microgrid. Then, based on the error of estimation, the cyber-attack is detected. To show the effectiveness of the proposed method, offline digital time-domain simulation studies are performed on a test dc microgrid system in the MATLAB/Simulink environment, and the results are verified using real-time simulations using the OPAL-RT real-time digital simulator (RTDS).
110 citations
TL;DR: In this paper, the authors proposed a soft-switching solid-state transformer (S4T), which has full-range zero-voltage switching (ZVS), electrolytic capacitor-less dc link, and controlled dv/dt, which reduces EMI.
Abstract: Solid-state transformers (SSTs) are a promising solution photovoltaic (PV), wind, traction, data center, battery energy storage system (BESS), and fast charging electric vehicle (EV) applications. The traditional SSTs are typically three-stage, i.e., hard-switching cascaded multilevel rectifiers and inverters with dual active bridge (DAB) converters, which leads to bulky passives, low efficiency, and high electromagnetic interference (EMI). This article proposes a new soft-switching solid-state transformer (S4T). The S4T has full-range zero-voltage switching (ZVS), electrolytic capacitor-less dc link, and controlled dv/dt , which reduces EMI. The S4T comprises two reverse-blocking current-source inverter (CSI) bridges, auxiliary branches for ZVS, and transformer magnetizing inductor as a reduced dc link with 60% ripple. Compared with the prior S4T, an effective change on the leakage inductance diode is made to reduce the number of the devices on the main power path by 20% for significant conduction loss saving and retain the same functionality of damping the resonance between the leakage and resonant capacitors and recycling trapped leakage energy. The conduction loss saving is crucial, being the dominating loss mechanism in SSTs. Importantly, the proposed single-stage SST not only holds the potential for high power density and high efficiency but also has full functionality, e.g., multiport dc loads integration, voltage regulation, and reactive power compensation, unlike the traditional single-stage matrix SST. The S4T can achieve single-stage isolated bidirectional dc–dc, ac–dc, dc–ac, or ac–ac conversion. It can also be configured input-series output-parallel (ISOP) in a modular way for medium-voltage (MV) grids. Hence, the S4T is a promising candidate for the SST. The full functionality, e.g., voltage buck–boost, multiport, etc., and the universality of the S4T for the dc–dc, dc–ac, and ac–ac conversion are verified through the simulations and experiments of two-port and three-port MV prototypes based on 3.3 kV SiC mosfet s in dc–dc, dc–ac, and ac–ac modes at 2 kV.
65 citations
TL;DR: In this paper, a detailed asymmetric parameters methodology (APM) is proposed to design gain curves for charging and discharging modes separately, which enables overlapping of the switching frequency range for both modes, thereby reducing the bidirectional frequency range variation.
Abstract: The CLLC bidirectional resonant converter has significant potential in battery chargers and dc microgrids, due to its bidirectional power transfer capability. To ensure uniform characteristics for bidirectional operation, secondary LC resonant tank components are usually designed to equal the primary LC components after reflection. This conventional design method is regarded as the symmetric design. It is used in applications where wide voltage regulation is not required, such as CLLC dc transformers. However, for bidirectional battery charger applications, the battery has a wide voltage range variation, and gain requirements during charging and discharging are different. These asymmetric characteristics could lead to undesirable large switching frequency range if a conventional symmetric CLLC design is employed. To address this issue, a detailed asymmetric parameters methodology (APM) is proposed in this article. It can design gain curves for charging and discharging modes separately. This enables overlapping of the switching frequency range for both modes, thereby reducing the bidirectional frequency range variation. It brings lower switching loss caused by excessive high frequency, and relieve the extra conduction loss and current stress of power switches as well. Finally, a detailed analysis of the proposed APM is provided and validated with simulations and experiments.
64 citations
TL;DR: A delay-tolerant predictive PCC is presented for voltage regulation in distribution feeders by estimating the maximum tolerable communication delay (MTCD) based on voltage and power mutation, and uses normal PCC for effective operation when communication delay is within MTCD, or switches to predictive P CC under ACD conditions.
Abstract: Voltage regulation is imperative for the successful operation of electricity distribution networks, especially with a high penetration level of photovoltaic (PV) systems. Power compensation control (PCC) that uses both reactive power compensation and active power curtailment has shown promising results in alleviating voltage rise problems. It crucially relies on real-time communications among distributed PV systems. However, the transmission of state measurements and control signals in PCC is hampered by inevitable communication delays. Therefore, it is important to not only estimate the maximum tolerable communication delay (MTCD) but also develop an alternative technique for PCC under abnormal communication delay (ACD) conditions. This article presents a delay-tolerant predictive PCC for voltage regulation in distribution feeders. After estimating the MTCD based on voltage and power mutation, it uses normal PCC for effective operation when communication delay is within MTCD, or switches to predictive PCC under ACD conditions. An accurate prediction is achieved using a double neural network with online adjustment of weights and samples. Simulations on a sample distribution network demonstrate the effectiveness of our presented approach.
63 citations
TL;DR: In this article, the power system dynamics with non-incremental local voltage control can be seen as a distributed algorithm for solving a well-defined optimization problem (reverse engineering), and two incremental voltage control schemes based on the subgradient and pseudo-gradient algorithms are designed for solving the same optimization problem.
Abstract: The increasing penetration of renewable and distributed energy resources in distribution networks calls for real-time and distributed voltage control. In this article, we investigate local Volt/VAR control with a general class of control functions, and show that the power system dynamics with nonincremental local voltage control can be seen as a distributed algorithm for solving a well-defined optimization problem (reverse engineering). The reverse engineering further reveals a fundamental limitation of the nonincremental voltage control: the convergence condition is restrictive and prevents better voltage regulation at equilibrium. This motivates us to design two incremental local voltage control schemes based on the subgradient and pseudo-gradient algorithms, respectively, for solving the same optimization problem (forward engineering). The new control schemes decouple the dynamical property from the equilibrium property, and have much less restrictive convergence conditions. This article presents another step toward developing a new foundation—network dynamics as optimization algorithms—for distributed real-time control and optimization of future power networks.
62 citations
TL;DR: In the proposed hybrid control strategy, LCC-S compensated WPT system can realize wide output voltage regulation with zero voltage switching (ZVS), and the reactive current in a resonant tank can be minimized in the whole voltage range.
Abstract: To regulate the output voltage of inductor–capacitor–capacitor-series ( LCC -S) compensated wireless power transfer (WPT) system, a hybrid control strategy of phase shift modulation (PSM) and a switch-controlled capacitor is presented in this article. In the proposed hybrid control strategy, LCC -S compensated WPT system can realize wide output voltage regulation with zero voltage switching (ZVS), and the reactive current in a resonant tank can be minimized in the whole voltage range. Besides, the current in the primary coil of LCC -S compensated WPT system can be regulated with PSM, which helps to reduce the power loss of the primary coil at light load. Compared with conventional dual-loop hybrid control, the proposed hybrid control can regulate two control variables with one feedback loop simultaneously. To verify the theoretical analysis, a 500-W prototype with 400-V input voltage, 100−250-V output voltage is built. The experimental results show that the inverter can realize ZVS within the whole voltage range and a peak efficiency of 94.7% can be obtained.
60 citations
TL;DR: An improved finite-set MPC is first proposed for the inner loop of the VSG, achieving simplified control structure, faster dynamic response, enhanced bandwidth and stability, as well as improved current limitation.
Abstract: Conventional primary control employs outer-loop droop and inner-loop cascaded linear control to realize local voltage regulation and power-sharing of an islanded ac microgrid. However, it has a complex structure, limited dynamic response, and a rapid rate of change of frequency when disturbances occur. This article resolves these issues by proposing a model predictive control-based virtual synchronous generator (VSG-MPC). An improved finite-set MPC is first proposed for the inner loop, achieving simplified control structure, faster dynamic response, enhanced bandwidth and stability, as well as improved current limitation. In the outer control loop, a simplified VSG without a phase-locked loop is employed to realize active power-sharing and inertia emulation. The merits above are verified by a description function of MPC and the frequency-domain response of the overall VSG. Simulation and experimental results verify the feasibility of the proposed method.
58 citations
TL;DR: A novel distributed coordinated control framework is proposed to handle the uncertain voltage violations in active distribution networks and is verified on the modified IEEE 34-bus and 123-bus test feeders.
Abstract: In this article, a novel distributed coordinated control framework is proposed to handle the uncertain voltage violations in active distribution networks. It addresses the problem of coordination of different types of devices in a distributed manner. In our control design, on-load tap changers (OLTCs) are firstly employed to handle the potential voltage violations based on the prediction of renewable outputs and load variations. During real-time operation, once an unmanageable voltage violation is detected, the reactive power of distributed energy resources (DERs) will be coordinated immediately to provide fast corrective control. The control schedules of OLTCs are calculated by solving a multitime-step constrained optimization problem via the alternating direction method of multipliers, whereas the reactive power injections of DERs are determined by a novel online distributed algorithm. The effectiveness of the proposed control framework is verified on the modified IEEE 34-bus and 123-bus test feeders.
53 citations
TL;DR: The test results demonstrate that the PSR GDPS is more preferable because of lower interwinding capacitance, lower load voltage regulation rate, higher conversion efficiency, and simpler control circuit.
Abstract: High-performance gate drive power supply (GDPS) plays a crucial role in ensuring the reliability and safety of the gate driver for power semiconductor devices. This article focuses on the design of a high-voltage-insulated GDPS for the 10-kV silicon carbide MOSFET in medium-voltage (MV) application. Design considerations, including insulation scheme, high-voltage-insulated transformer design, and load voltage regulation scheme, are proposed. In addition, the performance of the secondary-side-regulated (SSR) GDPS and that of the primary-side-regulated (PSR) GDPS are compared for several aspects, including interwinding capacitance, load voltage regulation rate, conversion efficiency, and hardware complexity. Finally, an SSR GDPS and a PSR GDPS, with an insulation voltage of 20 kV, are built in the lab. The test results demonstrate that the PSR GDPS is more preferable because of lower interwinding capacitance, lower load voltage regulation rate, higher conversion efficiency, and simpler control circuit.
TL;DR: The case studies indicate that the proposed DMPC is robust to communication latency (CML) and works effectively in both balanced and unbalanced DNs without any control center, which is a significant advantage for the promotion of real-time reactive power V2G in DNs with irregular user integration and relatively poor communication infrastructure.
Abstract: It has been demonstrated theoretically and experimentally that the Vehicle-to-Grid (V2G) enabled electric vehicle (EV) charger is of a reactive power compensation ability with a battery or capacitor connected. To exploit the aggregated reactive power V2G abilities of massively dispersed EV chargers, a distributed model predictive control (DMPC) strategy applying to both balanced and unbalanced distribution networks (DNs) is proposed to integrate them into real-time DN voltage regulation. Firstly, based on the instantaneous power theory and voltage sensitivity matrices, a voltage regulation model considering the reactive response of EV chargers is established without violating the normal EV active charging demands. Then, a completely distributed framework is achieved by DMPC, in which prediction information and calculation results are shared in a Peer-to-Peer (P2P) way to realize the asynchronous broadcast. The proposed model and techniques are validated by numerical results obtained from the IEEE European low voltage test feeder system. The case studies indicate that the proposed DMPC is robust to communication latency (CML) and works effectively in both balanced and unbalanced DNs without any control center, which is a significant advantage for the promotion of real-time reactive power V2G in DNs with irregular user integration and relatively poor communication infrastructure.
TL;DR: An improved dc-link voltage regulation strategy is proposed for grid-connected converters applied in dc microgrids and has robustness properties to handle parameter mismatches compared with the input–output linearization method.
Abstract: In this article, an improved dc-link voltage regulation strategy is proposed for grid-connected converters applied in dc microgrids. For the inner loop of the grid-connected converter, a voltage modulated direct power control is employed to obtain two second-order linear time-invariant systems, which guarantees that the closed-loop system is globally exponentially stable. For the outer loop, a sliding mode control strategy with a load current sensor is employed to maintain a constant dc-link voltage even in the presence of constant power loads at the dc-side, which adversely affect the system stability. Furthermore, an observer for the dc-link current is designed to remove the dc current sensor at the same time improving the reliability and decreasing the cost. From both simulation and experimental results obtained from a 15-kVA prototype setup, the proposed method is demonstrated to improve the transient performance of the system and has robustness properties to handle parameter mismatches compared with the input–output linearization method.
TL;DR: Improvement in voltage regulation and optimal utilization of resources by using the proposed coordinated scheme over the traditional uncoordinated scheme is demonstrated for the IEEE 13 bus and 33 bus distribution systems in MATLAB/ Simulink.
Abstract: Accommodating increased penetration of renewable energy resources like solar Photo-Voltaics (PV) imposes severe challenges on the voltage regulation of the traditionally designed distribution system. Battery Energy Storage Systems (BESS) can mitigate voltage regulation issues, as they can act quickly in response to the uncertainties introduced due to solar PV. However, if there is no coordination between existing devices such as On Load Tap Changing Transformers (OLTC) and BESS, then BESS takes all the burden and is generally over-utilized. The uncoordinated control schematic shall also lead to underutilization of OLTC. Hence, in this paper, a coordinated control strategy to control BESS along with OLTC is proposed to warrant acceptable voltage magnitudes across the distribution feeder. The formulated optimization problem aims to mitigate the voltage deviation from its required values while reducing the number of changes in tap positions and also enhancing the battery life. The improvement in voltage regulation and optimal utilization of resources by using the proposed coordinated scheme over the traditional uncoordinated scheme is demonstrated for the IEEE 13 bus and 33 bus distribution systems in MATLAB/ Simulink.
02 Feb 2021
TL;DR: In this paper, the authors provide an overview of the power system support functions from renewable DER inverters, which are categorized as voltage regulation by active/reactive power control, frequency regulation, active power control and voltage ride-through.
Abstract: Penetration of renewable energy in power systems has been increasing in the past decades in response to increased global electricity demand and concerns for the environment Distributed energy resources (DERs) based on renewables have experienced rapid growth thanks to the incentive programs and broad-based participation With the growing prevalence of DERs, the risk of grid instability and vulnerability increases due to the intermittent nature of renewable energy At the same time, the voltage and frequency deviation problems emerge more often when the reverse power flow occurs under supply-demand imbalance in distributed power systems Standards and grid codes have been issued for DER inverters to interconnect with the distribution grid The updated standard and grid codes expect DERs to provide a variety of power system support functions in order to incorporate higher DER penetration and to maximize DER value to the grid This paper provides an overview of the power system support functions from renewable DER inverters, which are categorized as: voltage regulation by active/reactive power control, frequency regulation by active power control, voltage ride-through, and frequency ride-through The benefits and drawbacks of each algorithm are presented and compared with its predecessor, manifesting the logic in the evolution of the algorithms
TL;DR: A novel IT2FNN-based dynamic SMC law is derived, by applying simplified ellipsoidal-type membership functions in the type-2 fuzzy neural network, to improve the capacity to manage the uncertainties.
Abstract: This paper proposes a dynamic sliding mode control (SMC) approach to the robust voltage regulation of dc–dc boost converters by using interval type-2 fuzzy neural networks (IT2FNNs). First, uncertainties caused by the perturbation of the input inductor and the output capacitor are represented with some bounded approximation errors, by the utilization of a Takagi–Sugeno (T–S) fuzzy modeling approach. Based on the represented model of the boost converter, a new type of sliding surface is designed depending on the duty cycle and reference inputs of the converter. Then, a dynamic SMC law is designed, by considering that the perturbation of the uncertain parameters, including input inductor, output capacitor, load resistor, and input voltage, is bounded. Meanwhile, we adopt an exponential plus power approaching law in the sliding mode controller for fast reachability of the sliding surface and a small chattering in the duty cycle input. Moreover, in terms of the considered uncertainties, a novel IT2FNN-based dynamic SMC law is derived, by applying simplified ellipsoidal-type membership functions in the type-2 fuzzy neural network. To improve the capacity to manage the uncertainties, some online learning algorithms for the updating of the IT2FNN are designed by a gradient descent method (GDM), without the requirement of the boundedness of the uncertainties. The resulting tracking error system is synthesized to be bounded stable based on the designed IT2FNN-based dynamic SMC. Finally, the effectiveness of the proposed adaptive IT2FNN-based dynamic SMC method is verified by some comparative simulation results.
TL;DR: Compared with existing droop control methodologies, the proposed controller ensures accurate power sharing, tight voltage regulation, and closed-loop stability with a current limitation when connected to Z, I, or P load, for multiple paralleled boost converters, which introduce nonlinear dynamics.
Abstract: In this brief, a novel current-limiting droop controller for paralleled dc–dc boost converters loaded by constant impedance Z, constant current I, or constant power P loads in a dc microgrid is proposed to guarantee closed-loop stability and power sharing. Using an improved version of the recently proposed nonlinear current-limiting controller, an inherent current-limiting property is guaranteed for each converter independently of the load type or magnitude variations. Then, sufficient conditions to ensure closed-loop stability for the entire dc microgrid system with a constant Z, I, or P load are analytically obtained. Hence, compared with existing droop control methodologies, the proposed controller ensures accurate power sharing, tight voltage regulation, and closed-loop stability with a current limitation when connected to Z, I, or P load, for multiple paralleled boost converters, which introduce nonlinear dynamics. To verify the effectiveness of the proposed controller and the stability analysis, simulation results for the three parallel operated dc–dc boost converters with Z, I, and P loads and experimental results for two parallel operated dc–dc boost converters with a P load are performed under several changes of the load power demand.
TL;DR: This brief addresses the problem of instability occurring in the voltage control mode of a non-minimum phase (NMP) DC-DC boost converter with quantitative feedback theory (QFT) adapted to systematically design a robust proportional integral derivative (PID) controller.
Abstract: This brief addresses the problem of instability occurring in the voltage control mode of a non-minimum phase (NMP) DC-DC boost converter. To solve this instability issue in the presence of uncertainties and the external disturbances, quantitative feedback theory (QFT) is adapted to systematically design a robust proportional integral derivative (PID) controller, which is realized using only sensed output voltage as feedback. The advantages of the proposed PID design using the QFT are: (i) it eliminates the burden of tedious and ad-hoc tuning of PID gains using the conventional PID design approaches, (ii) current measurement is not required, (iii) disturbance dynamics (input voltage and load current variations) are included in the design stage itself, which further enhances the disturbance rejection performance of the output voltage, and (iv) it allows direct design for the non-minimum phase boost converter despite the bandwidth limitations. Extensive simulations and experiments are carried out to validate the efficacy of the proposed PID controller in the presence of the external disturbances and compared its superiority over a conventional PID controller.
TL;DR: This article proposes an ST-enabled meshed hybrid distribution grid to achieve voltage and power flow control simultaneously with a centralized controller to achieve improved voltage regulation as compared to conventional microgrid.
Abstract: Renewable energy sources (RES) induce problems such as voltage and current limit violations, absence of inertia and consequent stability problems, and poor power quality. Smart transformer (ST) is a promising solution for avoiding such a changing grid scenario leading to strong grid reinforcements. This article proposes an ST-enabled meshed hybrid distribution grid to achieve voltage and power flow control simultaneously with a centralized controller. In this configuration, a low-voltage dc (LVdc) line is proposed which connects the ST LVdc link with the dc bus of distributed generation (DG) converters. This introduces various active power flow paths to support the loads. The DG converters supply active power near the load points which ensures that line losses are reduced significantly while achieving improved voltage regulation as compared to conventional microgrid. Moreover, the DG converters can draw active power from ST LVdc link during absence of RES to support the load, resulting in improved utilization of these converters. The control complexity of the DG converters is reduced as the ST controls both the LVac and LVdc line voltages. Further, the newly developed power flow path allows reverse power flow from DG plants more efficiently. Performance of the proposed system is verified with simulation and experimental results.
TL;DR: A distributed hierarchical control framework for energy storage systems (ESSs) in DC microgrid clusters, which achieves voltage regulation and current sharing for ESSs in each microgrid as well as the whole microgrid cluster.
Abstract: This paper proposes a distributed hierarchical control framework for energy storage systems (ESSs) in DC microgrid clusters, which achieves voltage regulation and current sharing for ESSs in each microgrid as well as the whole microgrid cluster. The primary control stage adopts a droop controller which only requires local information while the secondary control stage provides correction terms for ESSs within microgrids. The tertiary control stage samples the pinned ESSs in different microgrids with low sampling rate to provide the voltage setpoint, which ensures global current sharing among microgrid cluster. The corresponding multilayered event-triggered consensus algorithm for clusters is proposed to reduce the communication cost generated by operation of the distributed controller. Both the control framework and the consensus algorithm can be extended for satisfying higher dimensional regulation needs. The controller is validated in a DC microgrid cluster through simulation under different scenarios, and the results illustrate the effectiveness of the proposed controller.
TL;DR: A nonlinear high-gain observer (NHGO)-based second-order sliding mode (SOSM) control strategy is proposed for the three-phase three-level neutral-point-clamped (NPC) converter, which provides a fast dynamic for the dc-link voltage, and also assures a good steady-state behavior for the NPC converter.
Abstract: In this paper, a nonlinear high-gain observer (NHGO) based second-order sliding mode (SOSM) control strategy is proposed for the three-phase three-level neutral-point-clamped (NPC) converter. This controller applies the advanced SOSM algorithm both in the voltage regulation loop and in power tracking loop, which provides a fast dynamic for the dc-link voltage, and also assures a good steady state behavior for the NPC converter. Additionally, a NHGO technique is implemented in the voltage regulator combining with the SOSM algorithm. The conventional observer-based controllers suffer from the destructive effects of measurement noise, and it can only be addressed by diminishing the observer gain, which sacrifices the observer property. The NHGO technique adopts a time varying gain, that is, high gain in transient while low gain in steady state, which minimizes the adverse influence of measurement noise. The tuning method of the proposed NHGO-based SOSM controller is given to simplify the implementation process. Finally, the simulation and experimental results of the proposed control scheme for the NPC converter are given and compared with the conventional PI controller as well as the well-known linear extended state observer-based control method, which validates the feasibility and superiority of the proposed controller.
TL;DR: A single-stage dual active bridge (DAB) LLC hybrid bidirectional converter based on the sigma converter structure is proposed, and DAB is used for the power flow direction control and output voltage regulation.
Abstract: Working as a dc–dc transformer (DCX), which operates at the resonant frequency, is a suitable solution to achieve the maximum conversion efficiency of the LLC converter. However, because the fixed-frequency modulation is applied, the voltage regulation performance is limited. To achieve tight voltage regulation and fully utilize the advantages of LLC -DCX operation under bidirectional isolated power application, in this article, a single-stage dual active bridge (DAB) LLC hybrid bidirectional converter based on the sigma converter structure is proposed. In the proposed converter, the main power flows through the LLC converter, and DAB is used for the power flow direction control and output voltage regulation. In addition, a mathematical model of the proposed converter is derived for the controller design, which is verified by circuit simulation. Based on the model, the design of the controller and the circuit parameters are discussed with the goal of fast dynamics. Finally, the experimental results of a 1200-W prototype verify the effectiveness and advantages of the proposed topology and controller design.
TL;DR: Comparisons have shown the proposed hybrid AEONM algorithm to be superior in terms of enhancing the buck converter’s transient and frequency responses.
Abstract: Over the last decade, there has been a constant development in control techniques for DC-DC power converters which can be classified as linear and nonlinear. Researchers focus on obtaining maximum efficiency using linear control techniques to avoid complexity although nonlinear control techniques may achieve full dynamic capabilities of the converter. This paper has a similar purpose in which a novel hybrid metaheuristic optimization algorithm (AEONM) is proposed to design an optimal PID controller for DC-DC buck converter’s output voltage regulation. The AEONM employs artificial ecosystem-based optimization (AEO) algorithm with Nelder-Mead (NM) simplex method to ensure optimal PID controller parameters are efficiently tuned to control output voltage of the buck converter. Initial evaluations are performed on benchmark functions. Then, the performance of AEONM-based PID is validated through comparative results of statistical boxplot, non-parametric test, transient response, frequency response, time-domain integral-error-performance indices, disturbance rejection and robustness using AEO, particle swarm optimization and differential evolution. A comparative performance analysis of transient and frequency responses is also performed against simulated annealing, whale optimization and genetic algorithms for further performance assessment. The comparisons have shown the proposed hybrid AEONM algorithm to be superior in terms of enhancing the buck converter’s transient and frequency responses.
TL;DR: A novel design method based on the usage of the Equilibrium Optimizer (EO) algorithm for the determination of the optimal values of the Proportional – Integral – Derivative controller parameters of an Automatic Voltage Regulation (AVR) system is proposed.
TL;DR: The output voltage regulation problem of a buck–boost converter feeding a CPL is addressed and an immersion and invariance parameter estimator is proposed to compute online the extracted load power, which is difficult to measure in practical applications.
Abstract: Rapid developments in power distribution systems and renewable energy have widened the applications of dc–dc buck–boost converters in dc voltage regulation. Applications include vehicular power systems, renewable energy sources that generate power at a low voltage, and dc microgrids. It is noted that the cascade connection of converters in these applications may cause instability due to the fact that converters acting as loads have a constant power load (CPL) behavior. In this brief, the output voltage regulation problem of a buck–boost converter feeding a CPL is addressed. The construction of the feedback controller is based on the interconnection and damping assignment control technique. In addition, an immersion and invariance parameter estimator is proposed to compute online the extracted load power, which is difficult to measure in practical applications. It is ensured through the design that the desired operating point is (locally) asymptotically stable with a guaranteed domain of attraction. The approach is validated via computer simulations and experimental prototyping.
TL;DR: This paper addresses the problem of distributed secondary control for islanded AC microgrids with external disturbances by using a full-order sliding-mode (FOSM) approach, voltage regulation and frequency restoration are achieved in finite time.
Abstract: This paper addresses the problem of distributed secondary control for islanded AC microgrids with external disturbances. By using a full-order sliding-mode (FOSM) approach, voltage regulation and frequency restoration are achieved in finite time. For voltage regulation, a distributed observer is proposed for each distributed generator (DG) to estimate a reference voltage level. Different from some conventional observers, the reference voltage level in this paper is accurately estimated under directed communication topologies. Based on the observer, a new nonlinear controller is designed in a backstepping manner such that an FOSM surface is reached in finite time. On the surface, the voltages of DGs are regulated to the reference level in finite time. For frequency restoration, a distributed controller is further proposed such that a constructed FOSM surface is reached in finite time, on which the frequencies of DGs are restored to a reference level in finite time under directed communication topologies. Finally, case studies on a modified IEEE 37-bus test system are conducted to demonstrate the effectiveness, the robustness against load changes, and the plug-and-play capability of the proposed controllers.
TL;DR: Control structure along with power sharing scheme to operate the system under various operating modes, such as: 1) grid-connected mode; 2) islanded mode; 3) state of charge of battery less than or greater than specified limits; and 4) operating renewable sources (PV and wind) at maximum power point are presented.
Abstract: In this article, a new dc–dc multisource converter configuration-based grid-interactive microgrid consisting of photovoltaic (PV), wind, and hybrid energy storage (HES) is proposed. Control structure along with power sharing scheme to operate the system under various operating modes, such as: 1) grid-connected mode; 2) islanded mode; 3) state of charge of battery ( $SoC_{b}$ ) less than or greater than specified limits; and 4) operating renewable sources (PV and wind) at maximum power point, is presented. The detailed analysis, modeling, and design of the proposed configuration and control structure are presented. The key highlights of the proposed configuration are: 1) low component count; 2) voltage boosting, voltage regulation of supercapacitor and power-sharing among the battery and supercapacitor are inherent; and 3) simple control structure with a reduced number of sensors. Supercapacitor-battery-based HES is interfaced which effectively handle the power fluctuations due to the wind, PV, and sudden load disturbances. Integration of supercapacitor to respond to high-frequency fluctuations increases the lifetime of battery storage and reduces the sizing of the storage unit. The proposed system is verified through digital simulation and experimental results.
TL;DR: A computationally-efficient algorithm is proposed to solve the formulated optimal control problem and show the effectiveness of the employed control framework on a commercial utility-scale 720 kVA/560 kWh BESS.
TL;DR: In this article, the impact of communication time delays (CTDs) on combined load frequency control (LFC) and automatic voltage regulation (AVR) of a multi-area system with hybrid generation units is addressed.
Abstract: In this paper, the impact of communication time delays (CTDs) on combined load frequency control (LFC) and automatic voltage regulation (AVR) of a multi-area system with hybrid generation units is addressed. Investigation reveals that CTDs have significant effect on system performance. A classical PID controller is employed as a secondary regulator and its parametric gains are optimized with a differential evolution - artificial electric field algorithm (DE-AEFA). The superior performance of the presented algorithm is established by comparing with various optimization algorithms reported in the literature. The investigation is further extended to integration of redox flow batteries (RFBs) and interline power flow controller (IPFC) with tie-lines. Analysis reveals that IPFC and RFBs coordinated control enhances system dynamic performance. Finally, the robustness of the proposed control methodology is validated by sensitivity analysis during wide variations of system parameters and load.
TL;DR: A supercapacitor-battery based HES is interfaced which effectively handle the power fluctuations due to the wind, photovoltaic and sudden load disturbances and less number of switches is proposed.
Abstract: In this paper, a new multi-source and Hybrid Energy Storage (HES) integrated converter configuration for DC microgrid applications is proposed. Unlike most of the multi-input converter configurations, a supercapacitor-battery based HES is interfaced which effectively handle the power fluctuations due to the wind, photovoltaic and sudden load disturbances. Integration of supercapacitor to respond for high-frequency fluctuations increase the lifetime of battery storage and reduce the sizing of the storage unit. The control structure is framed to achieve power balance in the system along with basic functionality such as operating renewable sources (PV and wind) at maximum power point and charging and discharging of energy storage based on power availability. The key highlights of the proposed configuration are: (i) Less number of switches, (ii) Voltage boosting, voltage regulation of supercapacitor and power-sharing among battery and supercapacitor are inherent, (iii) Simple control structure with a reduced number of sensors. The detailed analysis, modeling, and design of the proposed configuration and control structure are presented along with MATLAB simulations and experimental validations.