Frequency Droop Research Articles

An Adaptive Droop Control for Power Sharing Between Three Phase Parallel Inverter in Autonomous Microgrid

The development of an adaptive droop controller is for power sharing between three-phase parallel inverters. Traditionally, droop controllers have been employed in conventional power systems to allocate the load among generators with varying capacities. Our approach involves the utilization of an adaptive control technique. This method is designed for a setup involving three-phase parallel inverters linked to an AC microgrid, to enhance the proportional distribution of active power despite changes in feeder network characteristics and loads. The proposed control mechanism automatically tunes the frequency droop parameter to address mismatch frequencies. The system includes two sets of three-phase inverters regulated by a combination of P-F droop control, an enhanced droop control loop, and PI controllers. The adaptive droop coefficient control is capable of addressing problems where power distribution inaccuracies arise due to different line loads as well as different inverter capacities.

Droop Frequency Limit Control and Its Parameter Optimization in VSC-HVDC Interconnected Power Grids

With the gradual emergence of trends such as the asynchronous interconnection of power grids and the increasing penetration of renewable energy, the issues of ultra-low-frequency oscillations and low-frequency stability in power grids have become more prominent, posing serious challenges to the safety and stability of systems. The voltage-source converter-based HVDC (VSC-HVDC) interconnection is an effective solution to the frequency stability problems faced by regional power grids. VSC-HVDC can participate in system frequency stability control through a frequency limit controller (FLC). This paper first analyses the basic principles of how VSC-HVDC participates in system frequency stability control. Then, in response to the frequency stability control requirements of the sending and receiving power systems, a droop FLC strategy is designed. Furthermore, a multi-objective optimization method for the parameters of the droop FLC is proposed. Finally, a large-scale electromagnetic transient simulation model of the VSC-HVDC interconnected power system is constructed to verify the effectiveness of the proposed droop FLC method.

Inertia–Active Power Filter Design Based on Repetitive Control

The advent of distributed generation has brought with it a plethora of challenges for the nascent power systems that are being deployed on a large scale. Firstly, the majority of power electronic converters connected to the grid are in current source mode, which results in a lack of inertia and an inability to provide effective inertia or achieve damping support during fluctuations in grid frequency. Secondly, the issue of power quality, caused by the presence of harmonics, is becoming increasingly severe. This is particularly problematic in microgrids or systems with high line impedance, where harmonics can be amplified, thereby further compromising the stability of the power system. To address the deficiency in system inertia, numerous scholars are currently utilizing grid-forming (GFM) technology to achieve virtual inertia. In order to address the issue of system harmonics, it is possible to install active power filter (APF) devices at the point of common coupling (PCC), which serve to mitigate the effects of harmonics. This paper puts forth a proposal for the implementation of an APF with virtual inertia, based on PR + RC composite control. This composite control mechanism serves to enhance the harmonic suppression capabilities of the APF. The introduction of a frequency droop enables the capacitor voltage amplitude to be adjusted during fluctuations in system frequency, thereby achieving virtual inertia and providing active support for system frequency. The experimental results demonstrate that this strategy not only reduces the total harmonic distortion (THD) by 13% in comparison to PI control, indicating excellent harmonic suppression performance, but also allows the system to be inert, achieving positive results in suppressing frequency fluctuations during transients.

Photovoltaic Power Injection Control Based on a Virtual Synchronous Machine Strategy.

The increasing participation of photovoltaic sources in power grids presents the challenge of enhancing power quality, which is affected by the intrinsic characteristics of these sources, such as variability and lack of inertia. This power quality degradation mainly generates variations in both voltage magnitude and frequency, which are more pronounced in microgrids. In fact, the magnitude problem is particularly present in the distribution systems, where photovoltaic sources are spread along the grid. Due to the power converter's lack of inertia, frequency problems can be seen throughout the network. Grid-forming control strategies in photovoltaic systems have been proposed to address these problems, although most proposed solutions involve either a direct voltage source or energy storage systems, thereby increasing costs. In this paper, a photovoltaic injection system is designed with a virtual synchronous machine control strategy to provide voltage and frequency support to the grid. The maximum power point tracking algorithm is adapted to provide the direct voltage reference and inject active power according to the droop frequency control. The control strategy is validated through simulations and key experimental setup tests. The results demonstrate that it is possible to inject photovoltaic power and provide voltage and frequency support.

A Resilient Control for Distributed Energy Storage Units in an Islanded AC Microgrid

A high number of charge/discharge cycles results in premature aging of Distributed Energy Storage Units (DESUs). Since classical droop control does not consider this issue and leads to MG voltage and frequency fluctuations in steady state, controls such as State of Charge (SoC) synchronization control are used to minimize DESUs number of charge/discharge cycles. However, few methods in the literature investigate frequency and voltage (f & V) restoration while ensuring a functional SoC equalization control in both charge and discharge modes. In this paper, an Adaptive Frequency Droop based on Virtual Power (AFDVP) method is proposed to achieve DESUs SoC equalization to prolong their lifespan while ensuring MG voltage and frequency regulation. In addition, AFDVP strategy ensures optimal use of second life batteries, since they are naturally less solicited due to the decrease in their capacity. DESUs SoC equalization is achieved thanks to virtual active powers using a PI controller. A complete stability analysis is provided to choose optimal parameters for the proposed distributed control. Experimental tests are performed to check the performance of the AFDVP technique in several study cases. A comparison is also performed with a SoC equalization strategy from the literature.

Variable Power Tracking Control Strategy of Doubly Fed Induction Generators for Fast Frequency Responses

Frequency regulation and droop control of doubly fed induction generators (DFIGs) can quickly respond to frequency changes and reduce the maximum rate of frequency (MROFF) in power systems. However, due to real-time dynamic changes in the MPPT control loop, the ability to improve the lowest frequency point is limited. Therefore, this article first describes an in-depth analysis of the dynamic characteristics of the incremental power of frequency regulation with droop control using an equivalent linear model. The limitations of improving the lowest frequency point under the influence of dynamic changes in the MPPT control loop are revealed. Secondly, to address the impact of these dynamics, an improved decoupling frequency regulation (IDFR) strategy based on power tracking is proposed, aiming to increase the maximum frequency deviation (MFD) and MROCOF. Then, in order to overcome the difficulty of adjusting control coefficients in the IDFR strategy, an adaptive control coefficient tuning fuzzy control method based on frequency deviation and ROCOF was proposed to flexibly adjust control requirements under various working conditions, thereby improving the control stability and performance of the system and effectively solving the problem of control coefficient allocation. Finally, to verify the frequency regulation performance of the proposed IDFR strategy under various operating conditions, simulations were conducted based on different disturbances and wind conditions. The results show that the proposed IDFR strategy significantly improves the system MFD and MROCOF improvement ability under various conditions.

Adaptive Frequency Droop Feedback Control-Based Power Tracking Operation of a DFIG for Temporary Frequency Regulation

Frequency regulation scheme of doubly-fed induction generators (DFIGs) are implemented to improve the frequency stability achieving by rotor speed control. However, the coupling dynamics between the maximum power point tracking operation control loop and frequency regulation control loop would impair the frequency regulation capability. To eliminate the adverse influences, firstly, analytical derivation for coupling output power dynamics is represented when adopting frequency regulation scheme of the DFIG. Then, an adaptive frequency droop feedback control based on power tracking operation strategy, which is a decoupling control scheme for frequency regulation, is proposed to respond the dynamic grid frequency without reserving power. The frequency deviation-based droop control with dynamics gains is used to calculate the coefficient of power tracking operation of the DFIG. The system stability with various power tracking operation of the DFIG is analyzed, in addition, the stable boundary is used to limit the setting of coefficient of power tracking operation. Simulations are carried based on an EMTP-RV simulator. It is found that the suggested adaptive frequency droop feedback control based on power tracking strategy can provide dual frequency regulation capability and reduce the maximum frequency excursion under various scenarios.

Frequency response of microgrids with PV power generation and energy storage system (battery and supercapacitor)

Since renewable energy sources (RES) have almost little inertia, an increase in their electricity might harm the power system's ability to run steadily and dependably. Numerous solutions to the issue mentioned above are offered. This paper aims to assess the technological possibility of using energy storage system (ESS) devices built from batteries and supercapacitors to enhance the interia response of sources in microgrids with a large amount of PV power penetration. The microgrid's inertia was altered by varying the penetration level of RES. To obtain a rigid microgrid, batteries and supercapacitors are suggested in this study to enhance frequency stability and droop control is utilized to complete this assessment. The model of the on-grid power network was designed using Simulink in MATLAB to evaluate the high level of RES penetration impact on the frequency stability of the system. Results verify that the microgrid stiffness is significantly enhanced when the suggested storage elements are incorporated. The findings show that the rate of change of frequency (RoCoF) is reduced when the size of the ESS increases and vice versa. The supercapacitor energy storage system (SCESS) can increase the stability of the system's frequency more effectively than the battery energy storage systems (BESS) with a slower time response.

Droop parameter-based economic dispatch for islanded microgrid considering demand response

The droop parameter setting of the distributed generator (DG) has an essential role in islanded microgrid (MG) systems. This paper presents droop-based economic load dispatch (ELD) in an islanded MG. In this approach, the Power–frequency (P–f) droop characteristic of DG is integrated into its incremental cost, which is the derivative of the cost function of DG. The optimisation problem is formulated by considering the standard deviations of the incremental costs of DGs. Under steady-state conditions, DGs operate at a single frequency and the P–f droop-based ELD ensures that the incremental costs of DGs are equal, minimising the overall cost of active power generation, following the equal incremental cost principle. Furthermore, an incentive-based demand response (DR) programme is integrated into the system to maximise the utility benefit function by incentivising customers for power curtailment. The results exhibit significant power curtailment during peak load hours, indicating the effectiveness of the DR programme. The proposed combined mathematical model is applied to the modified 33-bus islanded MG and solved using basic particle swarm optimisation. It calculates optimal droop parameters for a day-ahead scenario and emphasises the efficiency benefits of optimised droop settings and the integration of DR in islanded MGs.

Variable speed turbine based wind farm including storage system connected to a power grid or islanded.

In this paper, the possibility of a generating system, which associates a variable speed wind turbine and a flywheel energy storage system, to participate to the ancillary services is investigated. The connection to a power grid of a farm including three of them is considered. A solution is proposed to allow wind generators to work as voltage sources with the help of a frequency droop. It is shown that the power generated by these systems can be controlled by fixing the power set point when they are connected to the grid, and that, in case of islanding, they can share the global load of the isolated network

Fast-Frequency-Response Control Method for Electrode Boilers Supporting New Energy Accommodation

With the large-scale integration of new energy generation, represented by wind and photovoltaic power, into the power grid, the intermittency, randomness, and fluctuations of their output pose significant challenges to the safe and stable operation of the power system. Therefore, this paper proposes a control method for electrode boiler systems participating in rapid grid frequency response based on a fuzzy control strategy. This method improves the traditional electrode boiler control strategy, giving it characteristics similar to those of synchronous generators in terms of active power–frequency droop, allowing it to actively adjust active power based on system frequency disturbances. Furthermore, it optimizes its control performance indicators using fuzzy algorithms. The simulation results on the Matlab/Simulink platform demonstrate that the modified electrode boiler control system, when applying this method, can effectively address power disturbances in the system, reduce system frequency deviations, and contribute to enhancing the grid frequency regulation capability and system stability.

Investigation on Tuning Power–Frequency Droop for Improved Grid–Forming Inverter and Synchronous Generator Transient Load Sharing

Poor transient load sharing has been observed during the parallel operation of synchronous generators (SGs) and droop-controlled grid-forming (GFM) inverter-based resources (IBRs) in islanded microgrids. This can result in overcurrent protections, causing the inverters to trip, which leads to the collapse of voltage and frequency. In this article, small-signal analysis supported by electromagnetic transients (EMTs) simulation is performed on a detailed model of a microgrid containing IBRs and an SG to identify underdamped modes and their sensitivities to the power–frequency (P-F) droop parameter. Further EMT simulation of a more complex microgrid model with multiple GFM IBRs and SGs is performed to determine whether trends identified from the modal analysis extend to cases where the number of SGs in operation may vary. The objective is to investigate the effectiveness of tuning this parameter to reduce the likelihood of overcurrents in response to load and generation disturbances. The work uncovers both fast and slow modes of concern. Primarily, the findings show that increasing the P-F droop value improves the damping of the slower mode while degrading the damping of the faster mode, leading to an upper limit on the damping improvement of the GFM IBR output current. In EMT simulations of the more complex microgrid, the greatest damping improvement still exhibits significant maximum overshoot. Furthermore, the optimal value of the P-F droop parameter is sensitive to the number of SGs in operation. The conclusion is that tuning the P-F droop parameter is not an effective means to avoid the overcurrent tripping of GFM IBRs during response to large load or generation disturbances when operating in parallel to SGs in islanded microgrids.

Fast Frequency Control Strategy for Direct Drive Wind Turbines Considering the Suppression of Secondary Frequency Drop in the Power Grid

Inertia and damping in the power system are important parameters in frequency regulation and oscillation suppression. Inertia will reduce the rate and amplitude of frequency changes after active power sudden changes. Damping in the system can effectively suppress low-frequency oscillations after grid disturbances. As the penetration rate of wind turbines increases, it will significantly reduce the effective inertia in the power grid and reduce the friendliness of wind power grid connection. To provide better frequency support for the power grid in the event of large disturbances in the power system, this paper proposes a fast frequency control strategy for wind farms that considers the suppression of secondary frequency dips in the power grid. The strategy combines frequency droop control with grid frequency bias control and evaluates the kinetic energy reserve online based on wind power prediction and hourly speed, thus balancing the frequency regulation and suppression of secondary frequency dips in the power grid. A 2.5 MW direct drive wind turbine simulation model was built based on the PSCAD simulation platform, verifying the effectiveness of the proposed comprehensive control strategy.

Power Angle–Frequency Droop Control to Enhance Transient Stability of Grid-Forming Inverters Under Voltage Dips

Due to the replacement of synchronous generators, grid operators are currently demanding to control grid-connected inverters in grid&#x2013;forming mode to make them participate in the maintenance of the grid. To carry this out, the traditional droop controls based on the active and reactive powers are usually adopted, achieving a satisfactory performance in normal operation. Nevertheless, the power-frequency (<i>P</i>-&#x03C9;) droop may become transiently unstable under voltage dips. This is because of the modification of the active power response caused by the inverter current limitation together with the voltage reduction. To enhance this, the power angle-frequency (&#x03B4;_inv-&#x03C9;) droop is proposed, consisting in employing an estimation of the inverter power angle as input to obtain the inverter frequency. The proposed method provides the inverter with the same performance as the <i>P</i>-&#x03C9; droop in normal operation, while enhancing considerably the transient stability margins under current limitation. This is thanks to the higher variation of the inverter power angle with the phase difference between the inverter and the grid. Simulation results show the transient stability problems of the <i>P</i>-&#x03C9; droop as well as the superior performance of the proposed &#x03B4;_inv-&#x03C9; droop control, which has also been verified by means of HIL results.

Frequency droop characteristic for grid forming battery inverters - operation in islanded grids with the infeed of distributed generation systems

Frequency droop characteristic for grid forming battery inverters - operation in islanded grids with the infeed of distributed generation systems

Improving resilience of islanding protection - implementation of a Q-f droop for inverter-based generation

As the prevalence of distributed generation (DG) continues to increase in the GB network, anti-islanding protection has been the subject of industry focus, with work done to alter the existing Rate of Change of Frequency (ROCOF) settings. However, there are still concerns regarding the resilience and security of this method based upon non-detection zones (NDZ), and the potential of system-wide events and network faults leading to nuisance tripping. This paper provides a review of the theory and literature, exploring the issues with power system islanding and the current anti-islanding protection methods. A novel protection method, based on a reactive power - frequency (Q-f) droop, is developed and implemented into a Simulink inverter-connected DG model, to test under various distribution network scenarios including islanding and system-wide frequency excursions. This new method was compared to the existing ROCOF method and showed improved performance in all network scenarios studies. The Q-f droop method had no NDZ and was developed to be customisable for a range of network regulation requirements. Finally, aspects of the method such as multi-DG operation, financial and practical implications have been explored.

Accurate Reactive Power Sharing Strategy for Droop-Based Islanded AC Microgrids

In islanded ac microgrids, the mismatched impedances of the interfacing feeders between the inverters and the load bus cause poor reactive power sharing when the conventional frequency and voltage droop control technique is employed. Such operation endangers the whole microgrid reliability as it may lead to overloading certain inverters and, consequently, triggering protection relays and causing cascaded failure. Thus, this article proposes an accurate reactive power sharing strategy that considers the mismatched feeder impedances in islanded ac microgrids. It is based on the optimal tuning of the virtual complex impedance for each inverter. The proposed strategy has several advantages. First, it has a physical meaning as it establishes an explicit relationship between the mismatched values of the actual resistive–inductive feeders and the assigned values for the proposed virtual complex impedance to each inverter. Hence, further degradations in the microgrid voltages are prevented. Second, there is no need for prior knowledge about the actual feeder impedances in the design stage as they are estimated online from the available measurements. Finally, the proposed control is reliable and fault-tolerant as it copes with unexpected failures such as failure of (or sudden switching <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">off</small> ) some inverters and communication disruptions/delays. It ensures accurate power sharing even under the primary controllers after losing communication links with the secondary controller. The simulation and experimental verification results are presented to validate the performance of the proposed technique.

Inter-connected AC/DC HMGS power management with 3-phase and 1-phase ILC

In AC/DC hybrid micro grid system (HMGS) power converters are always tested for is performance in distribution, its ability to provide accurate power sharing, transient stability and load dynamics. The existing control methods either complex or limited to achieve optimal power flow. This paper proposes a modified decentralized droop control scheme for interlinking converter (ILC) connected to interconnected AC and DC grids. A three coordinated model is proposed where AC frequency, ILC power and DC voltage are corresponding axis. The power sharing through the ILC is dependent on the AC frequency droop and DC voltage droop which occurs due to overloading. The control scheme is designed for single and three phases ILC. The obtained results are compared with double loop control method which shows less frequency deviations, accurate power sharing. The ILC performs autonomously and transfer power bidirectional under islanded mode. The simulation is carried in MATLAB/Simulink.

Adaptive Control Strategy for Frequency Support From MTDC Connected Offshore Wind Power Plants

Grid integration of large-scale power electronic interfaced renewable energy sources leads to a low inertia system, thus resulting in a high rate of change of frequency (RoCoF) and frequency deviations in the power system following a credible contingency. In this article, a realistic adaptive communication-independent control strategy for effective inertial and primary frequency support from multiterminal HVDC (MTDC) connected offshore wind farms (OWFs) has been proposed. The proposed strategy can identify and distinguish the grid under disturbance among the other healthy grids in the MTDC system by an adaptive droop frequency support (ADFS) scheme at each receiving end converter (REC). On the OWF side, the control system of wind turbine generators has been designed to coordinate with the ADFS scheme to comply with the grid code regulations (GCRs) for inertial and primary frequency support. The proposed ADFS control scheme ensures that while providing frequency support to the grid under disturbance, frequency deviation limits of the healthy neighboring grids are not breached. Moreover, the proposed strategy complies adequately with the maximum permissible limits of HVDC voltage deviation. Furthermore, the proposed strategy is dependent on only local measurements and can be easily customized to comply with any applicable GCR. The reliability and effectiveness of the proposed robust control strategy have been demonstrated on three power systems interconnected through the MTDC system to two OWFs.

Active Power Control Integrated With Reactive Power Compensation of Battery Energy Stored Quasi-Z Source Inverter PV Power System Operating in VSG Mode

The battery energy stored quasi-Z source inverter (BES-qZSI)-based photovoltaic (PV) power system combines the advantages of the qZSI and energy storage system. However, as the BES-qZSI is a fast-response power converter without any inertia, when applied as a grid-connected system, it leads to decreased power system inertia. The low inertia problem will degrade the system’s performance and affect the system’s stability. Moreover, the BES-qZSI PV power system usually employs a phase-locked loop and PR regulator to control the grid current to track the desired reference in phase with the grid voltage. These do not reflect the characteristics of frequency modulation and voltage regulation. To enhance the system’s frequency and voltage self-adjustment ability, a control strategy based on the virtual synchronous generator (VSG) technique for the BES-qZSI PV power system is proposed in this article. By introducing the concept of synchronous generator, the BES-qZSI PV power system has the characteristics of the heavy moment of inertia, droop frequency active, droop characteristic of voltage–reactive power, and so on. Then, an active power control strategy integrated with reactive power compensation is proposed to implement PV generation and reactive power compensation at the same time. The simulation and experimental results verify the proposed control strategy.