Showing papers on "Voltage droop published in 2023"

Distributed Power Sharing Control Based on Adaptive Virtual Impedance in Seaport Microgrids with Cold Ironing

Abstract: In a seaport microgrid (SMG), power sharing among distributed generation (DG) units is hindered by power coupling, line changes as well as frequent power fluctuations caused by ship charging and discharging through cold ironing, which in turn threatens system stability and inverter security. This paper proposes a delay-tolerant distributed adaptive virtual impedance control strategy for assigned active and reactive power sharing, so as to suppress the circulating current among DGs. Firstly, the nonlinear impedance-power droop equations (IPDEs) are derived to actively update the resistive and inductive components of virtual impedance, which can accommodate changes in line structure and output power. Secondly, by means of low-bandwidth communication with quantized data and time delay, the desired power terms derived from practical consensus protocols are fed into the IPDEs, for which the proposed scheme gives a quantitative maximum tolerable communication delay with respect to power sharing accuracy. Thirdly, considering the voltage drop caused by virtual impedance, inspired by traditional synchronous generators, we design a virtual impedance loop based on virtual damping and inertia to preserve voltage dynamics. Finally, an SMG containing four DG units is simulated to verify the effectiveness of the proposed strategy on both active and reactive power sharing.

State-Space Modeling and Control of Grid-Tied Power Converters With Capacitive/Battery Energy Storage and Grid-Supportive Services

Abstract: Advances in the fields of renewable generation, electric vehicles, and energy storage systems push forward the research on ac–dc and dc–ac grid-tied power converters. However, the variabilities of power converters create new challenges in modeling and control. Existing state-space models fail to accurately describe various types of grid-tied converters (GTCs), particularly those with grid-supportive services, which are increasingly required by upcoming grid codes. As such, this article first proposes to classify GTCs into four basic types according to their ac and dc characteristics. Subsequently, corresponding detailed state-space models of GTCs are introduced, which serve as a useful tool for stability analysis. On top of that, this article further proposes control implementations of grid-supportive services related to active and reactive power control, including droop, inertia, and oscillation damping. Finally, simulation and experimental results demonstrate the effectiveness of the proposed models and grid-supportive services.

Distributed Networked Microgrids Power Flow

Abstract: An integrative power flow approach is established for networked microgrids. Our new contributions include: 1) A distributed augmented power flow (APF) algorithm for networked microgrids is devised to incorporate hierarchical control effects in/among microgrids; 2) Based upon APF, an enhanced distributed continuation power flow (CPF $^+$ ) algorithm is established to explore operating regions of droop coefficients and power interchanges for static voltage stability assessment; and 3) A programmable distributed platform is designed to coordinate power interchanges and support plug-and-play while protecting local customers’ privacy. RTDS experiments validate the high fidelity of APF, while its scalability and convergence performance are verified on medium and large networked microgrids. Extensive tests demonstrate the effectiveness of CPF $^+$ in quantifying secure operation regions of networked microgrids.

Frequency Restoration of Grid-Forming Inverters in Pulse Load and Plug-in Events

Abstract: This article presents a restoration technique using grid-forming inverters in an islanded microgrid during pulse load and plug-in events. In a microgrid powered by droop-controlled inverters, frequency is the variable that is accessible to all inverters for adjusting their power contributions. The problem is that the microgrid frequency deviates from its nominal value after a load change. The presented method enables the inverters to restore the frequency to the nominal value and regulate their bus voltage amplitudes. The frequency and voltage restorations are performed without communication while achieving the desired power-sharing between grid-forming inverters. The restoration is activated after detecting any active or reactive power changes. In this article, the dynamic model of an inverter equipped with enabled restoration paths is developed to verify the stability of the inverter controller under various conditions. The frequency and voltage restorations are examined during pulse load and plug-in events. This study is performed using a laboratory-scale islanded microgrid powered by two 208 V, 60 Hz, 5 kVA inverters.

A Generic Voltage Control for Grid-Forming Converters With Improved Power Loop Dynamics

Abstract: Grid-forming converters (GFMCs) provide voltage support and other grid services, such as inertia and droop, in more-electronics power systems. The GFMCs with small droop coefficients or connected to the grid through a small impedance may have a fast power loop, which leads to serious conflicts with the inner ac voltage control loop, resulting in power oscillations or even instability. To address this problem, an extremely fast ac voltage loop should be expected for GFMCs. On top of that, GFMCs must operate well in both stand-alone and grid-tied conditions. Through detailed analysis, this article first reveals that the grid-tied operation of a dual-loop controlled GFMC with an LCL output filter features a much slower voltage control loop than that of the stand-alone mode. To improve the dynamics of GFMCs, we propose a generic voltage control scheme with a high-pass filter in the current feedback loop. The generic voltage controller has fast voltage tracking performances under both grid-tied and stand-alone operations, thereby enabling GFMCs to achieve better power regulation and grid service. The improvements in both voltage tracking and power control are verified via experiments.

Distributed Optimal Power Sharing Strategy in an Islanded Hybrid AC/DC Microgrid to Improve Efficiency

Abstract: In an islanded hybrid ac/dc microgrid (IHMG), ac and dc subgrids are connected via interlinking converters (ICs). For reliable operation of an IHMG, achieving distributed generators’ power sharing (DGPS) throughout the entire IHMG and ICs’ power sharing (IPS) is the main objective in an IHMG. To improve efficiency of an IHMG, minimizing operating costs of DGs is required and thus cost-minimized DGPS needs to be achieved. Similarly, when multiple ICs operate in an IHMG, efficiency-maximized IPS can further improve the efficiency of an IHMG, as total loss of ICs can vary by the IPS ratios. Thus, this paper proposes a distributed power sharing strategy to achieve cost-minimized DGPS and efficiency-maximized IPS simultaneously, without any communication lines for DGs. Incremental cost-based droop controlled DGs and distributed controlled ICs are coordinated for cost-minimized DGPS and a distributed optimal power sharing method of ICs is proposed to achieve efficiency-maximized IPS. The proposed strategy secures both cost-minimized DGPS and efficiency-maximized IPS, with only sparse communication lines among ICs, thus resulting in a significant improvement in IHMG efficiency and reliability.

A Novel State-of-Charge-Based Droop Control for Battery Energy Storage Systems to Support Coordinated Operation of DC Microgrids

Abstract: A modern dc microgrid often comprises renewable energy sources (RESs), such as photovoltaic (PV) generation units, battery energy storage systems (BESSs), and local load, and it is also connected to the utility grid through a point of common coupling (PCC). While most existing approaches have to rely on communication links to achieve the desired control performance, this article proposes a novel control strategy without resorting to the communication links. This is achieved by assigning BESSs as master units and regulating the dc bus voltage with a novel state-of-charge (SoC)-based droop control, where the BESSs coordinate the slave units (e.g., RES and utility grid) with the aid of the dc bus signaling (DBS) technique to avoid overcharging and overdischarging of these BESSs. In the proposed droop control, the reference voltage for these BESSs is designed for coordinated operation between BESSs and utility grid, and it is maintained constant in the normal SoC range, which can reduce dc voltage variation. Droop coefficients designed for SoC balance of BESSs are dynamically adjusted based on their own SoC values. Furthermore, the preset maximum deviation between the reference voltage and the dc bus voltage ensures the reliable coordinated operation. Real-time hardware-in-loop (HIL) tests considering three different scenarios are conducted to validate the effectiveness of the proposed method.

A New Communication-Free Grid Frequency and AC Voltage Control of Hybrid LCC-VSC-HVDC Systems for Offshore Wind Farm Integration

Abstract: We propose frequency and voltage control method for a hybrid high-voltage direct current (HVDC) system for integrating an offshore wind farm into transmission networks. Conventionally, DC voltage level is maintained as constant regardless of the wind velocity and the grid frequency, so AC voltage fluctuates with active power variation in a hybrid HVDC system due to the innate reactive power absorption of inverter side line-commutated converter (LCC). However, in the proposed method, the DC voltage changes according to the wind velocity. Furthermore, DC voltage is simultaneously regulated to participate in frequency control in a communication-free manner. These two characteristics can be achieved by using five droop control methods proposed here. Additionally, by determining droop constants using the proposed determination methods, AC voltage can be maintained as constant during active power fluctuations. Therefore, using the proposed method, an offshore wind farm can be stably connected to mainland transmission networks and successfully used as frequency supporting resources, because voltage characteristics of the AC network are not destabilized. A small-signal state-space (SS) model of the target system was also developed to investigate stability of the proposed control. The utility of the proposed method is demonstrated by case studies using PSCAD and SS models.

Distributed Cooperative Control of Offshore Wind Farms Integrated via MTDC System for Fast Frequency Support

Abstract: This article proposes a distributed cooperative control (DCC) scheme of offshore wind farms integrated via the multiterminal direct current system for fast frequency support. The DCC scheme employs the consensus algorithm to share the frequency support burden among offshore wind turbines (WTs) suitably. The proposed DCC scheme can exploit the kinetic energy of all WTs adequately, while ensuring the security by employing the consensus state index and changing droop control coefficients adaptively. After the frequency support, the asymptotic recovery scheme is proposed at the leader WT for smooth rotor speed restorage, and other WTs will follow the leader to recover and reduce the second frequency drop. Besides, to realize the fast frequency support, the communication-free estimator is employed to estimate the onshore dc voltage using offshore local measured signals. Case studies are carried out on MATLAB and OPAL-RT real-time simulation platforms, respectively. Different control schemes are compared to elaborate the performance of the proposed DCC scheme considering the parameter uncertainty and noise disturbance.

Adaptive Dual Droop Control of MTDC Integrated Offshore Wind Farms for Fast Frequency Support

Abstract: This paper proposes an adaptive dual droop control (ADDC) scheme, it can provide the disturbed onshore system with fast frequency support from both other undisturbed onshore systems, and voltage source converter-based multi-terminal direct current (VSC-MTDC) integrated offshore wind farms (OWFs). With conventional droop control at onshore converters, the DC voltage and power flow will change once frequency events occur, it will lead to frequency variations in other undisturbed systems. The proposed ADDC scheme firstly detects the disturbed and undisturbed systems, and then makes the undisturbed onshore system provide more frequency support power, while ensuring safe operation by settling the support power limitation and regulating the droop coefficients. Moreover, the offshore stations will estimate the onshore DC voltage as the control signal for fast frequency support. After that, the OWFs will recover their rotor speed with an asymptotic control scheme to reduce the second frequency drop. Case studies are carried out on 3-terminal and 5-terminal test systems, and the Opal-RT real-time simulation platform, respectively. Different control schemes are compared, and the parameter uncertainty and noise disturbance are considered to illustrate the performance and effectiveness of the proposed ADDC scheme.

Intelligent Secondary Control of Islanded AC Microgrids: A Brain Emotional Learning-Based Approach

Abstract: This article proposes a distributed intelligent secondary control approach based on brain emotional learning-based intelligent controller (BELBIC) for power electronic-based ac microgrid (MG). The BELBIC controller is able to learn quick-auto and handle model complexity, nonlinearity, and uncertainty of the MG. The proposed controller is fully model-free, indicating that the voltage amplitude and frequency deviations are regulated without previous knowledge of the system model and parameters. This approach ensures low steady-state variations with higher bandwidth and maintains accurate power-sharing of the droop mechanism. Furthermore, primary control is realized with a robust finite control set-model predictive control in the inner level to increase the system frequency bandwidth and a droop control in the outer level to regulate the power-sharing among the distributed generations. Finally, experimental tests obtained from a hardware-in-the-loop testbed validate the proposed control strategy for different cases.

Stability Constrained Optimal Operation of Standalone DC Microgrids Considering Load and Solar PV Uncertainties

Abstract: Operation and control dynamics of islanded DC microgrids (DC mGs) have received enormous interest recently in distribution system management. In this regard, research concern on droop-controlled distributed generations (DGs) integrated within the standalone DC mG is becoming more profound. This paper proposes an optimal operation of droop-controlled standalone DC mG with combined small-signal stability and economic-environment-related objectives. The proposed approach optimizes the principal eigenvalue that affects the system stability and two other objectives. An eigen value-based stability analysis for network configurations, with the possibility of its buses solely connected to either dispatchable DG or constant power load (CPL), is proposed. Moreover, uncertainties involved in mG network variables that originate due to the integration of solar PV generation with load consumption were considered and modelled with relevant probability characterization. A recent swarm-based intelligent technique (dragonfly algorithm) for optimization of droop parameters is adopted in the proposed approach. A unique bi/tri-objective combinations are solved, and obtained results are discussed. To show the adaptability of the proposed approach for any given DC mG network, a standalone 6-bus mG was adopted to verify the applicability. Furthermore, the time-domain simulations were carried out on a sample 3-bus DC mG to validate the proposed strategy.

Distributed Model-Based Predictive Secondary Control for Hybrid AC/DC Microgrids

Abstract: This article presents a novel scheme based on distributed model-based predictive control for the secondary level control of hybrid ac/dc microgrids (MGs). Prediction models based on droop control and power-transfer equations are proposed to characterize the generators in both the ac and dc sub-MGs, whereas power balance constraints are used to predict the behavior of interlinking converters. The operational constraints (such as powers and control action limits) are included in all the formulations. Experimental results validate the proposed scheme for the following cases: 1) load changes, working within operating constraints; 2) managing frequency regulation in the ac sub-MG, voltage regulation in the dc sub-MG, and global power consensus in the whole hybrid MG; and 3) maintaining the MG performance in the presence of communication malfunction while ensuring that plug-and-play capability is preserved.

Enhanced Frequency-Constrained Unit Commitment Considering Variable-Droop Frequency Control From Converter-Based Generator

Abstract: To improve the frequency stability of power systems with a high penetration of converter-based generators, this paper proposes an Enhanced Frequency-Constrained Unit Commitment (E-FCUC) considering variable-droop-controlled Frequency Control (FC) from converter-based generators. Unlike previous studies emphasizing conventional and fix-droop converter-based generators participating in primary frequency response, the impact of converter-based generators with variable-droop FC (VD-FC) is investigated, modeled, and then incorporated into the UC problems. Firstly, the transfer function of variable-droop-controlled converter-based generators is modeled; the available primary power reserves bounding provided by converter-based generators is analyzed. Secondly, the frequency dynamic of power systems considering joint frequency control from conventional generators and variable-droop-controlled converter-based generators is derived based on an analytical aggregated System Frequency Response (SFR) model. Since the nonlinear non-smooth feature of obtained frequency dynamic function, the “max-affine” Piece-Wise-Linearization method (PWL) is adopted here to fit the calculated frequency function. The fitting function is subsequently reformulated into mix-integer linearized constraints participating in UC studies. Case studies based on a modified IEEE 6 bus test system and a modified IEEE 118 bus test system are carried out to verify the effectiveness of the proposed E-FCUC through comparisons of results with existing empirical models.

Inverse Application of Artificial Intelligence for the Control of Power Converters

Abstract: This article proposes a novel application method, inverse application of artificial intelligence (IAAI) for the control of power electronic converter systems. The proposed method can give the desired control coefficients/references in a simple way because, compared to conventional methods, IAAI only relies on a data-driven process with no need for an optimization process or substantial derivations. Noting that the IAAI approach uses artificial intelligence to provide feasible coefficients/references for the power converter control, rather than building a new controller. After illustrating the IAAI concept, a conventional application method of artificial neural network is discussed, an optimization-based design. Then, a two-source-converter microgrid case is studied to choose the best droop coefficients via the optimization-based approach. After that, the proposed IAAI method is employed for the same microgrid case to quickly find good droop coefficients. Furthermore, the IAAI method is applied to a modular multilevel converter (MMC) case, extending the MMC operation region under unbalanced grid faults. In the MMC case, both simulation and experimental online tests validate the operation, feasibility, and practicality of IAAI.

Energy Optimal Management of Microgrid With High Photovoltaic Penetration

Abstract: In order to further reduce carbon emissions, a large number of distributed photovoltaics (PVs) are connected to customer sider, which can form microgrids (MGs) with high PV penetration combined with energy storage system (ESS) adopting droop control. Due to the uncontrollability of PV output and frequent charging and discharging of ESS, the economic optimization of MG with high PV penetration is full of challenges, especially island state. Aiming at the lowest daily operating cost, the multi-factor collaborative energy optimization models are established for the grid-connected and islanded MG respectively. Then using particle swarm optimization (PSO) with inertial weight factor to find the optimal solutions of the models under stable operating constraints, the day-ahead energy optimal management strategy (EOMS) for the MG is obtained. In order to reduce the influence of PV and load prediction errors on the energy management accuracy, model predictive control (MPC) is applied to improve the day-ahead EOMS, and intraday rolling horizon energy optimal management strategy (RHEOMS) is obtained. The RHEOMS corrects the forecast errors by feeding back the PV and load current operating value continuously and rolling updating the EOMS control value. The economy and effectiveness of the proposed strategies are verified on a typical MG with high PV penetration.

Modeling of Frequency Security Constraints and Quantification of Frequency Control Reserve Capacities for Unit Commitment

Abstract: The high penetration of converter-based renewable energy sources has brought challenges to the power system frequency control. It is essential to consider the frequency security constraints and frequency control reserve requirements in unit commitment (UC). First, a novel extreme learning machine-based method is proposed to approximate the highly nonlinear frequency nadir constraint (FNC) by a set of linear constraints. Second, considering the variation of the frequency insecurity risk under the changing operational condition, we propose to optimize the primary frequency control (PFC) droop gains and reserve capacities in the UC model to provide diverse control efforts in different risk levels adaptively. Third, a secondary frequency control (SFC) reserve capacity quantification method is proposed by combining the Copula theory and distributionally robust optimization technique. The UC simulation is conducted on the IEEE 118-bus system to test the proposed optimal PFC droop gain strategy and SFC reserve capacity quantification method. Simulation results show that the proposed optimal PFC droop gain strategy is better than the traditional fixed PFC droop gain setting on economic efficiency and operational flexibility. Besides, the SFC reserve capacity calculated by the proposed method is more appropriate than the actual SFC reserve capacity in the historical operation.

A Modified Droop Control Technique for Accurate Power-Sharing of a Resilient Stand-Alone Micro-grid

Abstract: This paper proposes a modified droop control technique for accurate power-sharing of a resilient stand-alone microgrid. Two cases were presented in this study; In case 1, a stand-alone microgrid system with a single inverter operating as a voltage source and a local load is presented. A conventional droop-based control strategy was used to modify the voltage magnitude and frequency in relation to reactive and active power signals in order to employ an inner current loop and an outer voltage loop. Although the control scheme accomplishes high flexibility and reliability, the only drawback is that the sharing is realized via frequency and voltage variations in the system, which has motivated the case-2 study to solve these problems. The islanded micro-grid system considered in case 2 consists of two inverters operating as voltage sources (also known as network-forming converters) and a shared load. The load is shared between the first and second converter after a specified period. Therefore, since case- 2 aims to solve the problems of frequency and voltage variations induced by the primary control in case-L, centralized secondary control (modified droop control technique) is implemented in the micro-grid in case-2 to reinstate the nominal frequency and voltage amplitude value in the micro-grid. Additionally, case 2 considered the integration of virtual impedance in the converter's output using an additional closed-loop control to attenuate distortion, minimize the influence of circulating current, and ensure the sharing of harmonic current under unbalanced and non-linear loads. The system model was simulated using the MATLAB/Simulink environment.

Voltage and frequency stabilization by fuzzy integrated droop control of a multi renewable source micro grid

Abstract: A multi renewable source micro-grid system has many power quality issues when it is operated in grid islanding condition. During grid connected mode the voltage and frequency of the system will remain stable for small disturbances like load changes or environmental changes. During these sudden variations in the system, the grid supports the load maintaining the system stable. In this paper a droop control is introduced for the renewable source module 3-ph inverter during standalone mode supporting the load. For detection of grid islanding, an active islanding detection control is adopted which makes the 3-ph inverter to shift from synchronous reference frame (SRF) control to droop control. The performance of the droop control is improvised with fuzzy interference structure (FIS) module replacing the conventional proportional integral and derivative (PID) control. A comparative analysis is done in this paper with different controllers in droop control scheme using matrix laboratory (MATLAB) Simulink software. Stability analysis is carried out on the system determining the voltage and frequency fluctuations during grid disturbances. There is an improvement in load compensation, voltage ripple, voltage magnitude, frequency ripple and settling time with the introduced fuzzy droop controller.

A Novel Control Technique for Enhancing the Operation of MTDC Grids

Abstract: This paper develops a novel control approach for the droop-controlled Voltage Source Converters (VSC) of Multi-Terminal High Voltage Direct Current (MTDC) systems. The frequency consensus controller is shown to assist in damping the inter-area oscillations and providing enhanced mutual frequency support. Such features, however, might be achieved at the expense of overloading some of the VSCs interfaced to the synchronously connected ac grids. Thus, a new power-sharing control loop, based on the proposed power deviation ratio (PDR) index, is developed to enhance the distribution of active power mismatches between those VSCs. The developed PDR loop, which regulates the ratios of the mismatched power-sharing by considering both the scheduled power injections and the available capacities of the VSCs, enhances the mutual frequency support capability between ac areas of the MTDC system. Furthermore, a newly proposed equidistant voltage control (EVC) loop of the proposed controller regulates the dc system’s voltages such that they are equally far from upper and lower voltage limits. This technique increases the safety margin in voltage regulation during events that cause dc system’s voltage profile variation. The comparative advantage of the proposed controller is verified through modal and participation factor analysis and through comprehensive time-domain simulations.

An Optimized Nonlinear Droop Control Method Using Load Profile for DC Microgrids

Abstract: Paralleling of power electronic converters to achieve current sharing is a critical aspect of a dc microgrid. Droop control is a popular technique to parallel converters. This article presents a novel perspective of different practically realizable nonlinear droop characteristics. Using the probability distribution function of the load current, a methodology is proposed to optimize the characteristics of nonlinear droop control. In a stand-alone dc microgrid system, the load current information used in the proposed control method is typically known using historical data or can be estimated using load forecasting methods. The advantages of the proposed nonlinear droop control method over several state-of-the art nonlinear droop control methods are discussed and shown. The effectiveness of the proposed nonlinear droop control method is validated using circuit simulations and hardware-based experiments on a multiconverter system for different load profiles.