Droop Gain Research Articles

On-Line Inertia Estimation for Synchronous and Non-Synchronous Devices

This paper proposes an on-line estimation method able to track the inertia of synchronous machines as well as the equivalent, possibly time-varying inertia from the converter-interfaced generators. For power electronics devices, the droop gain of the Fast Frequency Response (FFR) is also determined as a byproduct of the inertia estimation. The proposed method is shown to be robust against noise and to track accurately the inertia of synchronous generators, virtual synchronous generators with constant and adaptive inertia, and wind power plants with inclusion of energy storage-based frequency control.

A DC Droop-Based Optimal Dispatch Control and Power Management of Hybrid Photovoltaic-Battery and Diesel Generator Standalone AC/DC System

This article deals with the parallel operation of photovoltaic (PV), battery energy storage (BES), and diesel generator (DG) for standalone ac/dc system. It is known that the maximum penetration of PV systems in DG-based power system is limited in the range of 40%-60%. When the PV capacity becomes comparable to DG capacity, the system suffers from stability and synchronization issues. Hence, this article proposes a dc droop-based hierarchical control of the PV-BES-DG system based on dc integration at a common dc bus. The dc integration alleviates the scalability limit on PV and facilitates better control of the hybrid system than ac counterpart. Moreover, an optimal regulator-based secondary control is proposed to ensure optimal load sharing among DG and BES. Further, a power management scheme (PMS) is designed to ensure reliable operation of the standalone system during source and load power imbalances and operation under critical state of charge limit condition of BES. Also, the PMS constraints the operation of DG below a defined low-level loading. Further, small signal stability analysis is presented to analyze the effects of droop and optimal regulator gain on the system's stability. The laboratory-scale prototype is developed to evaluate the performance of proposed PMS and dc droop-based hierarchical control of the PV-BES-DG system.

Voltage Restoration and Adjustable Current Sharing for DC Microgrid With Time Delay via Distributed Secondary Control

As a key part of modern power systems, DC microgrid is becoming increasingly important. Among different control methods for DC microgrid, secondary control has been widely investigated since it can guarantee both current sharing and DC bus voltage restoration. However, the existing secondary control results only consider fixed current sharing ratio among DC converters, and thus they cannot be applied to the case where an adjustable current sharing ratio is desired. Motivated by this observation, this paper presents a new distributed secondary control strategy. By imposing a time-varying droop gain and specifying the “virtual voltage drop,” this strategy is able to ensure adjustable current sharing ratio among DC converters. Moreover, the effects of time delay on the control performance is also analyzed. Three case studies and two hardware-in-the-loop (HIL) tests are provided to verify the efficacy of the presented strategy.

Frequency Regulation in Isolated Microgrids Through Optimal Droop Gain and Voltage Control

This article presents an adaptive active power droop controller and voltage setpoint control in isolated microgrids for optimal frequency response and stability after disturbances. The control scheme involves an optimal and model predictive control approach that continuously adjusts the active power droop gains and the voltage setpoints of Distributed Energy Resources (DERs) to maintain the frequency of the system within acceptable limits and enhance the primary frequency response of the system, while taking into account the active power sensitivity of the microgrid loads to the system’s operating voltage. The proposed control scheme is tested, validated, and compared with previously proposed techniques using time-domain simulations for a test system based on a CIGRE medium voltage benchmark microgrid under different realistic conditions, demonstrating the advantages of the proposed approach.

DC Voltage Droop Control Design for MMC-Based Multiterminal HVDC Grids

This article addresses the design of the DC voltage droop control in modular multilevel converter (MMC)-based multiterminal HVDC grids. First, two energy-based control approaches, namely classic and cross control, are explored for the implementation of the voltage-power droop controller. The cross control, as the better solution for droop implementation, is further improved, making it more robust against disturbances. Then, a methodology is derived to select the droop gain combinations considering the AC grid, DC grid and MMC dynamics and their limitations. The methodology is based on a linear analysis to identify the valid droop gains which comply with the limits imposed on: the transient power sharing among MMCs, the DC grid voltage, the MMC AC and DC currents, the total MMC stored energy, and the stability margin of the complete multiterminal HVDC grid. Finally, time-domain simulations are conducted using the nonlinear model to validate the dynamic performance of the selected droop combinations obtained from the suggested methodology.

Stability of Isolated Microgrids With Renewable Generation and Smart Loads

This paper investigates the low frequency (<; 30 Hz) oscillations in isolated microgrids (IMGs) with smart loads (SLs) alongside converter-interfaced distributed generators (CDGs) fuelled by renewable energy resources (e.g. wind, solar) with battery energy storage. In an IMG with normal loads (active or passive), such oscillations are typically associated with the droop control of the CDGs operating in grid forming mode. This paper shows that SLs have marginal influence on these low frequency oscillations but introduce a new oscillatory mode at a slightly higher frequency (> 20 Hz). First, the stability analysis model (linearized state-space model) of an IMG is extended to include the dynamics of a smart load with a series-shunt converter arrangement in its voltage compensator. It is shown that the dynamics of the phase-locked loop (PLL), DC link along with the control loops of the series and shunt converters of the smart load dictates the lower limit of its droop gain for stable operation. This is not apparent from the simplified SL models (i.e. neglecting the dynamics of the shunt converter and DC link) reported previously. Impact of smart loads on low frequency oscillations in IMGs is demonstrated in this paper through stability analysis and time domain simulation.

Optimal power transmission in multi‐terminal HVDC systems for large offshore wind farms: a matheuristic approach

Nowadays power transmission losses in multi-terminal high-voltage DC (MT-HVDC) systems of large offshore wind farms are the main concerns to wind farm operators. Maintaining the DC voltages and currents within pre-specified limits and simultaneously optimising DC power flows of MT-HVDC systems using optimum droop control settings becomes a complex optimisation problem. This droop control scheme was less explored with power loss minimisation applications due to the difficulties in the optimisation of these droop gain settings. This study presents a new mathematical programming plus meta-heuristic-based optimisation approach to solve this optimisation problem. This hybrid optimisation methodology is also termed as 'matheuristic approach'. The proposed matheuristic method optimises the droop control settings for different operating conditions of offshore wind farms and simultaneously minimises the DC power losses in the MT-HVDC system. The proposed optimisation method has been tested upon a six-terminal HVDC system connected with large offshore wind farms. The superiority of the proposed optimisation method is demonstrated using the steady state as well as dynamic simulation studies. The simulation results show that DC power losses could be reduced significantly by adopting the proposed method to facilitate optimal power flows in the MT-HVDC system of large offshore wind farms.

A New Robust Control Strategy for Parallel Operated Inverters in Green Energy Applications

This research work puts forward a hybrid AC/DC microgrid with renewable energy sources pertaining to consumer’s residential area for meeting the demand. Currently, the power generation and consumption have experienced key transformations. One such tendency would be integration of microgrids into the distribution network that is characterized by high penetration of renewable energy resources as well as operations in parallel. Traditional droop control can be employed in order to get an accurate steady state averaged active power sharing amongst parallel inverters pertaining to hybrid AC/DC microgrid. It is presumed that there would be similar transient average power responses, and there would be no circulating current flowing between the units for identical inverters possessing the same droop gain. However, the instantaneous power could be affected by different line impedances considerably and thus resulting in variation in circulating power that flows amongst inverters, especially during unexpected disturbances like load changes. This power, if absorbed by the inverter, could result in sudden DC-link voltage rise and trip the inverter, which in turn causes performance degradation of the entire hybrid microgrid. When the hybrid generators act as unidirectional power source, the issue worsens further. In this research work, we have put forward a new distributed coordinated control pertaining to hybrid microgrid, which can be applied for both grid connected and islanded modes that include variable loads and hybrid energy resources. Also, in order to choose the most effective controller scheme, a participation factor analysis has been designed for binding the DC-link voltage as well as reducing the circulating power. Moreover, to both photovoltaic stations and wind turbines, maximum power point tracking (MPPT) techniques have been used in order to extract the maximum power from hybrid power system when there is discrepancy in environmental circumstances. Lastly, the feasibility and effectiveness pertaining to the introduced strategy for hybrid microgrid in various modes are confirmed via simulation results.

State of Charge Balancing for Distributed Battery Units Based on Adaptive Virtual Power Rating in a DC Microgrid

This paper proposes an adaptive virtual power rating method for state of charge (SoC) balancing among distributed battery units (BUs) in a DC microgrid. The virtual power rating is flexibly determined according to the SoC to obtain the droop gain of BU, and the balanced SoC is achieved by means of the modified droop controller. Because an accurate power sharing among BUs is satisfied by using only virtual power rating, SoC balancing performance is consistently ensured regardless of the line resistance difference. Moreover, the voltage restoration to keep the grid voltage at a desired value is easily realized without PI controller, and the proposed control strategy is implemented based on the distributed control method with simple low-bandwidth communication. The system stability is investigated, and the performance of the control method is demonstrated through both simulations and experiments.

The Influence of Network Factors on Frequency Stability

Retaining frequency stability is becoming increasingly challenging as the power system incorporates more non-synchronous generation. Assessing the frequency stability in a system has been predominantly completed by focusing on the quantity of connected synchronous kinetic energy in the system, or inertia . This traditionally was considered a function of generator construction – network factors typically were not considered. The research in this paper investigates how network topology, power flow, droop gain distribution, and inertia distribution all impact frequency stability. A generic four-area model has been created that allows discrete system setups. This research has shown that certain topologies lead to a more severe rate of change of frequency. A key finding is that the frequency drop is further increased when there is greater power flow into the area that experiences the disturbance. The extent to which the rate of change of frequency and frequency drop are influenced differently is emphasized, highlighting the need to procure different services depending on which metric is of primary significance at a specific location.

Cost-Based Adaptive Droop Control Strategy for VSC-MTDC system

This study proposes a cost-based adaptive (CBA) droop control strategy for use in a voltage source converter (VSC)-based multi-terminal high voltage direct current (MTDC) system. Rather than using a fixed droop gain, we suggest the CBA droop control scheme, which reduces the total incremental generation cost of ac systems, while sharing the burden based on the available capacity of VSCs at the post-contingency-steady-state operating point. Following a certain VSC outage in the MTDC system, unbalanced power is allocated based on the equal incremental cost principle to reduce the total active power generation cost. The results were verified through a transient simulation on an MTDC system with four monopole VSCs, and outage contingency scenarios were presented with three groups of generation cost curves. The results indicate that the CBA droop control strategy can provide greater contributions to the economic operation of the MTDC system, while achieving robust control.

Adaptive Droop Gain-Based Event-Triggered Consensus Reactive Power Sharing in Microgrids

This paper proposes an adaptive droop gain-based consensus approach for reactive power sharing in microgrids (MGs) with the event triggered communication protocol (ETCP). A multi-agent system-based network is constructed to establish the communication with distributed generators (DGs) in MGs. An ETCP is proposed to reduce the communication among agents to save resources and improve system reliability, as the communication is only needed when the event triggered condition is fulfilled. A stability analysis is conducted to guarantee the existence of the equilibrium point and the freeness of the Zeno solution. Moreover, an adaptive droop gain is designed to reduce the impact of imbalanced feeder impedances. Four case studies are conducted to verify the effectiveness and performance of the proposed method. The simulation results show that the ETCP-based approach is capable of achieving power sharing consensus, communication reduction and shifting the information exchange mode based on the operation scenarios.

Combined Control of DFIG-Based Wind Turbine and Battery Energy Storage System for Frequency Response in Microgrids

This paper presents a novel methodology for frequency control of a microgrid through doubly fed induction generator (DFIG) employing battery energy storage system (BESS) and droop control. The proposed microgrid frequency control is the result of the active power injection from the droop control implemented in the grid side converter (GSC) of the DFIG, and the BESS implemented in the DC link of the back-to-back converter also in the DFIG. This methodology guarantees the battery system charge during operation of the connected DFIG in the network, and the frequency control in microgrid operation after an intentional disturbance. In order for the DFIG to provide frequency support to the microgrid, the best-performing droop gain value is selected. Afterwards its performance is evaluated individually and together with the power injected by the battery. The power used for both battery charging and frequency support is managed and processed by the GSC without affecting the normal operation of the wind system. The simulation tests are performed using Matlab/Simulink toolbox.

Data-Driven Online System Equivalent for Self-Adaptive Droop Voltage Control of Wind Power Plants

This letter considers the closed-loop stability issue of droop-based voltage control for wind power plants (WPPs) under varying grid strength. We first propose a data-driven online system equivalent/identification method (DOnSE) for WPPs using the weighted least-squares estimation (WLSE). Then, upon the analysis of link between stability boundary of droop gain and external grid strength, a self-adaptive gain-scheduling droop voltage control scheme for WPPs based on the DOnSE is developed to enhance the closed-loop stability and reactive power utilization of wind turbines (WTs).

Control strategy to improve power transmission capacity from wind farms to weak AC grids

—With the increasing connection of large wind farms based on direct driven Permanent Magnet Synchronous Generators (PMSGs) to weak AC grids, the steady state characteristics of such wind farms, particularly those connected to very weak AC systems with a Short Circuit Ratio (SCR) of 1, are important factors for analysis with a view to improvement. In this paper, the issue is thus analysed in order to identify the factors limiting transfer capability from wind farm output. This paper thus presents a simulation model of a wind farm connected to a very weak AC grid, and a vector control strategy is used with respect to the full power converters of the wind farm in order to track current order injected from the converters into the AC system. A supplementary outer loop control is proposed to support grid voltage and to maximise transferred power into the very weak AC grid, based on a droop gain which is allowed to update the reactive power reference for reactive power control for the grid side converter based on changes to the output of the wind turbine, within grid code requirements. The simulation results prove that the proposed control system offers a promising method for addressing challenges arising in active power transmission, and thus increasing transferred active power from such wind farms to very weak grids.

Primary Frequency Response of Microgrid Using Doubly Fed Induction Generator With Finite Control Set Model Predictive Control Plus Droop Control and Storage System

This paper presents a new methodology for primary frequency response (PFR) in a microgrid through the finite control set-model predictive control (FCS-MPC) plus droop control applied to the grid side converter (GSC) of a doubly fed induction generator (DFIG). In this configuration, the rotor side converter (RSC) is responsible for maintaining wind turbine operation at the maximum power point (MPP) extraction, even at the time of a disturbance, while the GSC is responsible for processing the power required to reestablish the microgrid frequency at its rated value. The power required for frequency control comes from a battery energy storage system (BESS) connected to the DC-link, and its value is selected via the FSC-MPC by continuously adjusting the droop gain value. This control configuration has considerable benefits such as continuous operation at the MPP extraction, injection of power proportional to the frequency imbalance, the capability to impose restrictions through the control and it does not use any type of communication between the storage system and the control. Through the FCS-MPC, the gain of the droop controller is selected, which maximizes the power needed to control the frequency of the microgrid. To verify the performance of the proposed control strategy, simulations are performed for an unexpected islanding of the microgrid under different wind speed scenarios. The results show that the DFIG equipped with the proposed control strategy is able to provide ancillary services such as PFR in all DFIG operating modes.

Innovated inertia control of DFIG with dynamic rotor speed recovery

High wind power penetration (WPP) is challenging system frequency stability. As a countermeasure, virtual inertia controls are complemented to utilize kinetic energy (KE) stored in wind turbine generator (WTG) for frequency regulation. While without restoration, generation efficiency of WTG will be degraded after inertia contribution. To counter this issue, we propose an inertia control scheme of doubly fed induction generator (DFIG), aiming at achieving dynamic inertia recovery regarding both of KE and DC link energy. An asymmetrical droop control referred to rate of change of frequency (RoCoF) is proposed for KE management. The upper boundary of droop gain is extended to give full play of converter and revised considering system frequency state to counter positive feedback issue induced by reversible gain regulation, which is restricted by KE in store to ensure stable operation as well. The inertial DC energy to cooperate with KE control regarding countering small fluctuations, is improved with an orderly recovery behavior. Case studies are conducted under dynamic wind condition and the results indicate that with proposed scheme, the ability of dynamic inertia recovery is obtained, bringing DFIG higher generation efficiency and more adequate operation margin for sustained regulation. Essentially, the inertial frequency response and fluctuation suppression ability is well maintained.

Distributed Secondary Control for Current Sharing and Voltage Restoration in DC Microgrid

Direct current (DC) microgrid is being increasingly investigated in modern power grid. An important issue in DC microgrid operation is to ensure proper current sharing among converters. While this has been addressed through droop control, the resulting voltage deviation in DC bus has to be compensated. To solve this problem, a new distributed secondary control scheme is presented in this paper for both current sharing and voltage restoration. A key part of the presented scheme is the integration of a new parameter ‘virtual voltage drop’ defined from droop gain and line resistance. Since the DC bus voltage is not required as a feedback signal, the proposed secondary control is simple and easy to design and implement. In addition, as the proposed scheme has no requirement for the loads, it can handle both resistance loads and constant power loads (CPLs). Simulations as well as experimental studies are carried out to demonstrate the effectiveness of the proposed scheme.

Novel Piecewise Linear Formation of Droop Strategy for DC Microgrid

This paper proposes a new load current sharing control strategy for dc microgrid (MG). It adopts the piecewise linear formation to adaptively assign droop gains to balance the load sharing accuracy with the deviation of bus voltage. Communication among the distributed units is unnecessary for the proposed operation. The approach is considered as a comprehensive solution to deal with the unbalanced performance in dc MG for different load levels. The balancing performance is shown by comparing with the traditional linear droop control and the latest nonlinear droop gain solution. The study is proceeded by the in-depth analysis of both steady state and system dynamics with simulation support. The claimed advantage is finally verified by both simulation and experimental evaluations.

A Coordinated Droop Control Method Using a Virtual Voltage Axis for Power Management and Voltage Restoration of DC Microgrids

In this paper, a coordinated droop control method through virtual voltage axis is proposed for voltage restoration and energy management of dc microgrids (MGs). Droop control is a control method that operates a dc MG using only local information. However, there are several drawbacks in that output voltage deviations occur, and the utilization priorities of various sources are not considered. To solve these problems, the voltage compensation term is defined as the virtual axis voltage value. It can minimize the voltage deviation of the dc MG and enable energy management by determining the operation band. However, the value of the virtual voltage axis is affected by communication delays. Therefore, its influence on the values of the virtual voltage axis was analyzed. The reference voltage and the droop gain of the droop control were designed, and the validity of the proposed method was verified using a lab-scale dc MG.