Droop Loop Research Articles

Adaptive Power Control Strategy for Smart Droop-Based Grid-Connected Inverters

Grid-connected inverters play an important role in the integration of renewable energy sources such as solar and wind. However, due to the unneglectable grid impedance value seen by the inverters at the point of common coupling (PCC), especially in the weaks and resistive low voltage distribution networks, there is an inherent strong coupling between active and reactive power flow. This power coupling causes significant power quality problems including 1) voltage fluctuation of the common AC bus resulted from high penetration of intermittent renewable generation systems, 2) non-optimal control of neither power flow nor power factor of the delivered power by inverters to the common AC bus, and 3) unintended/uncompensated transmission losses, where the flow of active power through the transmission/distribution lines will cause unintended reverse reactive power flow from the grid-side and vice versa with the reactive power flow. To solve these issues, this paper proposes an adaptive mechanism for droop-based grid-connected inverters to decouple the power flow by compensating the associated unintended active and reactive power losses flowing through the transmission line (or any desired segment of it). This control strategy relies on modifying the power command provided to the frequency and voltage droop loops by considering the effects of both the transmission line resistance and inductance components on the power flow between the inverter and the grid. It uses only the local current and voltage measurements to first perform an online estimation of the transmission line resistance and inductance and then to calculate the proposed adaptive power terms. The performance of the proposed control is validated in MATLAB/Simulink and HIL experiment for a 350 kW droop-based grid-connected inverter system. The proposed control strategy can be utilized to provide ancillary services to the grid such as accurate frequency and voltage support at the location of interest.

A Decentralized Impedance Reshaping Strategy for Balanced, Unbalanced and Harmonic Power Sharing in Islanded Resistive Microgrids

In an islanded microgrid, the wire impedance mismatches among distributed generation (DG) units lead to inaccurate power sharing results, which influences the system’s normal operation. The output impedances of DG units can be reshaped to deal with this issue. In this paper, an impedance reshaping strategy for the resistive microgrid is proposed based on the combination of droop control and adaptive virtual impedance regulation (AVIR). Firstly, DG units operate in droop mode and the steady-state reactive power from <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$Q-f$</tex-math></inline-formula> droop is restored as the reference. Then the droop loops are removed and an AVIR based on the error between instantaneous reactive power and its reference is implemented. Since reactive power sharing with <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$Q-f$</tex-math></inline-formula> is relatively accurate, the impedance mismatches could be eliminated by the virtual impedances introduced with the proposed strategy, all the accurate active, reactive, unbalanced and harmonic power sharing could be realized simultaneously. In this strategy, communication among DG units or between DG unit and central control is not required, neither the wire impedances nor bus voltage or load current is measured, which reveals the high system reliability. Finally, the simulation and experimental results are presented to verify the effectiveness of the proposed control method.

Inrush Current Management During Medium Voltage Microgrid Black Start With Battery Energy Storage System

This paper addresses the black start of medium voltage distribution networks (MV-DNs) by a battery energy storage system (BESS). The BESS consists of a two-level voltage source inverter interfacing MV-DN which has limited overcurrent capability. On the other hand, MV-DN normally includes several step-up and step-down transformers that are drawing sympathetic inrush current in energizing stage. Therefore, the major difficulty to perform black start by BESS during MV-DN island operation lies in the fact that the inverter has to simultaneous control the network voltage and its output current. This paper presents two control approaches for a BESS in order to control the inrush current during MV-DN black start. The proposed control schemes consist of droop, voltage and current loops in the stationary reference frame. The droop loop is used to generate the voltage reference. An intermediate voltage and inner current loops are designed for output voltage regulation, current reference generation as well as current tracking, respectively. New reference modifiers are included into droop and voltage loops to limit inrush current. The proposed control schemes are experimentally tested for energizing Ingå MV-DN in Finland by 1 MVA BESS, and their performance is compared in terms of inrush current value and voltage quality. The obtained results prove that both methods are able to feed the load with a fixed voltage in the steady-state as well as to limit the transformer inrush current considering the inverter overcurrent limit.

A Novel Unified Controller for Grid-Connected and Islanded Operation of PV-Fed Single-Stage Inverter

This paper presents a novel robust current droop controller (RCDC) using a single droop loop. This scheme is unified supporting dual mode of operation for micro-grids (MGs), including grid connected mode (GCM) and islanded mode (ISM) while ensuring seamless transition between the two modes with proportional power sharing maintained. The proposed controller is further incorporated with an improved maximum power point tracking (MPPT) technique presented for the parallel operation of single-stage inverters fed by multi-string PV array topology. In addition, an improved phase-locked-loop-less (PLL-less) method is presented supporting self-synchronization strategy of the parallel operation of PV-inverters with the main grid while maintaining the full capabilities of the unified control architecture. This obviates the usage of conventional PLLs, which are widely used with active synchronization techniques. The performance of the proposed control scheme is validated using real time simulations (RTS) developed by dSPACE MicroLabBox.

A Full State-Variable Direct Predictive Control for Islanded Microgrids With Parallel Converters

In this work, we propose a high-quality control solution for islanded microgrids with multiparallel power converters; it uses a full state-variable direct model predictive control (FSV-DMPC) and has a simple structure. Unlike the conventional cascaded control loops, the proposed FSV-DMPC solution tracks the optimal reference generated by a robust droop loop using a unified cost function. This proposal enables the FSV-DMPC to be inserted into the entire control framework with plug-and-play capability; it is robust to parameter variations while also guaranteeing dynamics and stability. We conduct a deep analysis of the proposed approach, taking into account both the characteristics of the solution and the bounded stability of the system. Through comprehensive comparative studies with a classical double-loop linear controller, we validate that our solution achieves superior output voltage regulation during the load transients in terms of voltage error and settling time. Meanwhile, similar steady-state performances are accomplished for both methods. Finally, we verify our approach experimentally in different scenarios through a lab-constructed microgrid test bench. Experimental data confirm that the proposed approach achieves excellent steady-state and transient performances and obtains accurate load sharing.

Control and synchronization of a fixed-frequency control method in microgrid based on global positioning system

Control methods based on global positioning systems (GPS) are reported in multiple literatures recently, which achieve a fixed frequency operation of the microgrid and therefore has the tremendous benefit that any problems related to frequency instability are eliminated. However, the possible interruption of GPS timing signals may cause current circulating and finally leads to instability of the microgrid, yet it is largely neglected in literatures. This paper presents an angle synchronizing mechanism which utilizes the timing signal from GPS satellites and an auxiliary frequency droop loop to ensure synchronization during GPS offline events. Smooth transfer is guaranteed between primary and auxiliary control loops with discrete control architecture. Also, to allow microgrid using GPS-based control to work in tandem with the bulk power system or another frequency droop microgrid, an extra synchronization algorithm is proposed. The viability and performance of the proposed control structure is validated by case studies.

Reverse droop control-based smooth transfer strategy for interface converters in hybrid AC/DC distribution networks

Hybrid AC/DC distribution networks are promising candidates for future applications due to rapid advancement in power electronics technology. They use interface converters (IFCs) to link DC and AC distribution networks. However, they possess drawbacks of the AC voltage and frequency offsets when transferring from grid-tied to islanding modes. To tackle this problem, this paper proposes a simple but effective strategy based on reverse droop method. Initially, the power balance equation of the distribution system is derived, it reveals that the cause of voltage and frequency offsets is the mismatch between IFC output power and the rated load power. Then, the reverse droop control is introduced into the IFC controller. By using a voltage-active power / frequency-reactive power (U-P /f-Q) reverse droop loop, the IFC output power enables adaptive tracking of the rated load power. Therefore, the AC voltage offset and frequency offset are suppressed during the transfer process of operational modes. In addition, the universal parameter design method is discussed based on the stability limitations of the control system and the voltage quality requirements of AC critical loads. Finally, simulation and experimental results clearly validate the proposed control strategy and parameter design method.

Adaptive fractional‐order control of power system frequency in the presence of wind turbine

Inertial and droop control are common methods for improving grid frequency control. Using the first-order derivative of the frequency in the inertial control loop only investigates the rate of frequency variation, while other useful information is not taken into consideration. Accordingly, in this study, the application of a fractional-order derivative (between 1 and 2) in the inertial control loop is proposed to have more useful frequency-based information. To achieve this goal, firstly continued fraction approximations and secondly, the discretization is adopted by using the Tustin Method with Prewarping. Inertia loop and droop loop gain have a significant impact on the operation of the wind turbines and the power system frequency control. Therefore, in this paper, a novel method for the adaptive adjusting of droop and inertia control loops gains in DFIG is proposed. To utilize more frequency error information, fractional-order adaptive law is proposed to update the loop gains and the inertia control loop. In order to have a robust system, which is permanently stable for every fractional order (FO), is used parameter space method. Simulation results demonstrate the proper performance of the FO-based adaptive approach to increase the frequency nadir (FN) and decrease the frequency variations.

A Control System for Stable Operation of Autonomous Networked Microgrids

The interaction of droop controllers through power network is high in networked microgrids (NMGs) due to the low X/R ratio of the power lines impedance and lack of inertia in converter-based NMGs, which has raised stability concerns. On the other hand, inaccurate reactive power sharing and poor power quality due to the voltage and frequency deviations still remain as noticeable issues in NMGs. In this paper, a novel fuzzy consensus protocol is proposed to improve the droop controller performance in power sharing by incorporating the X/R ratio of the power lines impedance into droop loops. Power quality is also enhanced by restoring the average voltage profile based on a new consensus protocol, which is designed to be in coordination with reactive power sharing. In order to guarantee stability of the closed-loop system, linear matrix inequality method is adopted to determine the consensus signal coefficients as structured static output feedback gains. To this end, a novel small-signal model is proposed for NMGs to be adopted in the design process, by which the cross-coupling as well as interaction of droop controllers through the power network is properly realized. The numerical results in MATLAB/SIMULINK prove the effectiveness and accuracy of the proposed method.

Reinforced Droop for Active Current Sharing in Parallel NPC Inverter for Islanded AC Microgrid Application

The paper investigates the current sharing in parallel 3-level Neutral point clamped inverter for islanded AC microgrid application. In Distributed generation, parallel power electronic interface based microgrid suffers from power quality issues due to inaccurate output current sharing. To address the current sharing problem, the paper proposes an improved droop technique that reinforces the droop loop by infusing the incurred voltage drop accountable for inaccurate current sharing at the inverter output. The control based on droop reference is bounded with regard to output current since the output impedance influences the output current of the inverter largely. Besides that, the parallel NPC inverter also suffers from dc link voltage imbalance due to neutral currents. To address these issues a control strategy is proposed in the research work in which the processed DC offset is incorporated into the reinforced droop loop and the generated reference is utilized with feedback control to accurately share the currents under linear and nonlinear load conditions. The effectiveness of devised reinforced droop strategy is realized in MATLAB/Simulink environment and experimental validation is carried out with the field-programmable gate array as hardware interface to the hardware laboratory prototype of parallel NPC inverter.

Active Power Sharing and Frequency Restoration in an Autonomous Networked Microgrid

Microgrid (MG) concept is considered as the best solution for future power systems, which are expected to receive a considerable amount of power through renewable energy resources and distributed generation units. Droop control systems are widely adopted in conventional power systems and recently in MGs for power sharing among generation units. However, droop control causes frequency fluctuations, which leads to poor power quality. This paper deals with frequency fluctuation and stability concerns of f-P droop control loop in MGs. Inspired from conventional synchronous generators, virtual damping is proposed to diminish frequency fluctuation in MGs. Then, it is demonstrated that the conventional frequency restoration method inserts an offset to the phase angle, which is in conflict with accurate power sharing. To this end, a novel control method, based on phase angle feedback, is proposed for frequency restoration in conjunction with a novel method for adaptively tuning the feedback gains to preserve precise active power sharing. Nonlinear stability analysis is conducted by drawing the phase variations of the nonlinear second-order equation of the δ-P droop loop and it is proved that the proposed method improves the stability margin of f-P control loop. Simulation results demonstrate the effectiveness of the proposed method.

A Modified Droop Control for Grid-Connected Inverters With Improved Stability in the Fluctuation of Grid Frequency and Voltage Magnitude

The droop controlled grid-connecting inverter (DC-GCI) has been widely used in microgrid (MG). However, the power flow of the droop control is very sensitive to the fluctuation of grid frequency and voltage magnitude, which will result in a very fast inrush current of DC-GCI. Owing to the linear relationship of P-F, current limiters are required to prevent DC-GCI output power from exceeding maximum power. However, the impact of grid frequency fluctuation on DC-GCI cannot be mitigated by the current limiter for the reason that the fluctuation will lead to the current limiter saturation, which will cause the error between the angular frequency of the P-F droop output and the angular frequency of the grid voltage, and the system becomes unstable. It is a challenge to limit the output current, meanwhile ensuring the stable operation of the DC-GCI. To solve these problems, a modified droop control strategy is proposed to mitigate the impacts of the fluctuation of grid frequency and voltage magnitude on the system stability. In addition, the proposed control strategy in this paper realizes the stable operation of the DC-GCI through the cooperation between the modified droop control and the current limiter, and the current limiter saturation is avoided. The current limiter is used to restrain the transient current to compensate for the slow bandwidth of the droop loop, and the modified droop loop is used to stabilize output power and restrain the steady-state output current. The simulation and experimental results from a scaled-down laboratory prototype have been presented to verify the major contribution of this paper.

Almost global asymptotic stability of a grid-connected synchronous generator

We study the global asymptotic behavior of a grid-connected constant field current synchronous generator (SG). The grid is regarded as an “infinite bus,” i.e., a three-phase AC voltage source. The generator does not include any controller other than the frequency droop loop. This means that the mechanical torque applied to this generator is an affine function of its angular velocity. The negative slope of this function is the frequency droop constant. We derive sufficient conditions on the SG parameters under which there exist exactly two periodic state trajectories for the SG, one stable and another unstable, and for almost all initial states, the state trajectory of the SG converges to the stable periodic trajectory (all the angles are measured modulo $$2\pi $$ ). Along both periodic state trajectories, the angular velocity of the SG is equal to the grid frequency. Our sufficient conditions are easy to check computationally. An important tool in our analysis is an integro-differential equation called the exact swing equation, which resembles a forced pendulum equation and is equivalent to our fourth-order model of the grid-connected SG. Apart from our objective of providing an analytical proof for a global asymptotic behavior observed in a classical dynamical system, a key motivation for this work is the development of synchronverters which are inverters that mimic the behavior of SGs. Understanding the global dynamics of SGs can guide the choice of synchronverter parameters and operation. As an application we find a set of stable nominal parameters for a 500-kW synchronverter.

Robust Droop and DC-Bus Voltage Control for Effective Stabilization and Power Sharing in VSC Multiterminal DC Grids

Voltage-source converter (VSC)-based multiterminal dc grids are receiving widespread acceptance as an enabling technology to integrate renewable energy sources, energy storage units, and modern dc-type loads into existing ac grids. Droop control is a common power-sharing strategy to facilitate autonomous power sharing among different terminals in dc grids. However, the dynamics and stability of a gird-connected VSC with dc power sharing droop control can be affected by several important factors that are not addressed in the current literature. Important among these are: 1) ignoring the effect of the outer droop loop on the dc-link voltage dynamics when the dc-link voltage controller is designed, which induces destabilizing dynamics particularly under variable droop gain needed for optimum economic operation, energy management, and successful network operation under converter outages and contingencies; 2) uncertainties in the dc grid parameters (e.g., passive load resistance and equivalent capacitance as viewed from the dc side the VSC); and 3) disturbances in the dc grid (i.e., power absorbed or injected from/to the dc grid), which change the operating point and the converter dynamics by acting as a state-dependent disturbance. To overcome these difficulties, this paper presents a robust power sharing and dc-link voltage regulation controller for grid-connected VSCs in multiterminal dc grids applications. A detailed dynamic model that considers the droop controller dynamics and the impact of the effective dc-side load parameters and disturbances imposed on the VSC is developed. Then, a robust controller that preserves system stability and robust performance is developed. Theoretical analyses, including the MIMO dynamic analysis of a multiterminal dc grid with the proposed controller, as well as simulation and experimental results are provided to show the effectiveness of the proposed controller.

Model predictive control of distributed generation micro-grids in island and grid connected operation under balanced and unbalanced conditions

Model Predictive Control (MPC) uses the model of system at current time to predict the system behavior at the future sampling interval in the prediction horizon and sets a number of variables to their references due to their controllability from input variables. This paper concentrates on the design and analysis of a controller for Distributed Generation (DG) microgrids in islanding and grid connected operation modes using a Receding Horizon MPC scheme. In this contribution, active and reactive powers are used in a cost function as an inner control loop to produce switching states. Two outer voltage and frequency droop loops share active and reactive powers between DGs in the microgrid. The design concept of the proposed control system is evaluated through simulation studies and experiment under different test scenarios. The impact of the simulation and the experimental results shows that the operations of the DG units within the microgrid can be coordinated effectively under the proposed control system to ensure stable operation of the overall microgrid and power quality improvement.

Analytical determination of stable droop loop gain for a DFIG participating in frequency regulation

Analytical determination of stable droop loop gain for a DFIG participating in frequency regulation

Dynamic Droop–Based Inertial Control of a Doubly-Fed Induction Generator

If a large disturbance occurs in a power grid, two auxiliary loops for the inertial control of a wind turbine generator have been used: droop loop and rate of change of frequency (ROCOF) loop. Because their gains are fixed, difficulties arise in determining them suitable for all grid and wind conditions. This paper proposes a dynamic droop-based inertial control scheme of a doubly-fed induction generator (DFIG). The scheme aims to improve the frequency nadir (FN) and ensure stable operation of a DFIG. To achieve the first goal, the scheme uses a droop loop, but it dynamically changes its gain based on the ROCOF to release a large amount of kinetic energy during the initial stage of a disturbance. To do this, a shaping function that relates the droop to the ROCOF is used. To achieve the second goal, different shaping functions, which depend on rotor speeds, are used to give a large contribution in high wind conditions and prevent over-deceleration in low wind conditions during inertial control. The performance of the proposed scheme was investigated under various wind conditions using an EMTP-RV simulator. The results indicate that the scheme improves the FN and ensures stable operation of a DFIG.

Releasable Kinetic Energy-Based Inertial Control of a DFIG Wind Power Plant

Wind turbine generators (WTGs) in a wind power plant (WPP) contain different levels of releasable kinetic energy (KE) because of the wake effects. This paper proposes a releasable KE-based inertial control scheme for a doubly fed induction generator (DFIG) WPP that differentiates the contributions of the WTGs depending on their stored KE. The proposed KE-based gain scheme aims to make use of the releasable KE in a WPP to raise the frequency nadir. To achieve this, two additional loops for the inertial control are implemented in each DFIG controller: the rate of change of frequency and droop loops. The proposed scheme adjusts the two loop gains in a DFIG controller depending on its rotor speed so that a DFIG operating at a higher rotor speed releases more KE. The performance of the proposed scheme was investigated under various wind conditions. The results clearly indicate that the proposed scheme successfully improves the frequency nadir more than the conventional same gain scheme by releasing more KE stored in a WPP, and it helps all WTGs to ensure stable operation during inertial control by avoiding the rotor speed reaching the minimum speed limit.

Intelligent Distributed Generation and Storage Units for DC Microgrids—A New Concept on Cooperative Control Without Communications Beyond Droop Control

Low voltage dc microgrids have been widely used for supplying critical loads, such as data centers and remote communication stations. Consequently, it is important to ensure redundancy and enough energy capacity in order to support possible increments in load consumption. This is achieved by means of expansion of the energy storage system by adding extra distributed energy storage units. However, using distributed energy storage units adds more challenges in microgrids control, since stored energy should be balanced in order to avoid deep discharge or over-charge in one of the energy storage units. Typically, voltage droop loops are used for interconnecting several different units in parallel to a microgrid. This paper proposes a new decentralized strategy based on fuzzy logic that ensures stored energy balance for a low voltage dc microgrid with distributed battery energy storage systems by modifying the virtual resistances of the droop controllers in accordance with the state of charge of each energy storage unit. Additionally, the virtual resistance is adjusted in order to reduce the voltage deviation at the common dc bus. The units are self-controlled by using local variables only, hence, the microgrid can operate without relying on communication systems. Hardware in the loop results show the feasibility of the proposed method.