Droop Control Research Articles

A novel adaptive droop control strategy for SoC balance in PV-based DC microgrids

Battery energy storage systems (BESSs) are generally used as a buffer stage for photovoltaic (PV) power generation to tolerate the output power unpredictability in DC microgrids, in which the State-of-Charge (SoC) balance is a necessary and urgent issue to be solved. To this end, an integral feedforward sliding mode controller (SMC) is adopted to replace the traditionally proportional integral (PI) controller such that the voltage response speed of the converter in each BESS can be significantly enhanced. Further, a novel adaptive droop control strategy for SoC balance with three different working modes is proposed, in which all batteries can be cooperated through three different stages corresponding to their different SoC degrees. Compared with most existing SoC balancing approaches, the proposed SoC strategy can improve the transient performance for the BESSs and their robustness against the volatility of PV output powers. The effectiveness of the proposed SoC balancing strategy is verified through a simulation in a DC microgrid network consisting of several PV generators, batteries, and loads utilizing Simulink/SimPower Systems.

Accurate Active and Reactive Power Sharing Based on a Modified Droop Control Method for Islanded Microgrids.

When multiple paralleled distributed generation (DG) units operate in an islanded microgrid, accurate power sharing of each DG unit cannot be achieved with a conventional droop control strategy due to mismatched feeder impedance. In this paper, a small AC signal (SACS)-injection-based modified droop control method is presented for accurate active and reactive power sharing among DG units. The proposed control method adjusts the voltage amplitude of each DG unit by injecting small AC signals to form a reactive power control loop. This strategy does not need communication links or to specifically obtain the physical parameter of the feeder impedance and only requires the local information. Moreover, the parameter design procedure and stability analysis are given full consideration. Finally, simulation and experimental results verify the effectiveness of the proposed control scheme, and accurate active and reactive power sharing can be achieved.

HVDC grids stability enhancement through the integration of battery energy storage systems

AbstractIn high voltage direct current (HVDC) grids, the reference values of power controllers are normally adjusted based on the DC‐link voltage as the DC voltage is regulated by the droop controllers. As a matter of fact, except for the DC slack bus, there is a trade‐off between power‐sharing and voltage regulation of all the remaining HVDC buses. The main issue with this control strategy is that in case of contingencies, significant voltage and power oscillations throughout HVDC grids can be expected. To address such issues, the integration of a battery energy storage system into HVDC grids through a multi‐port DC/DC power converter is investigated in this paper. The DC/DC converter used in this paper consists of three ports: (1) two ports are connected in cascade with the intended DC transmission line, formerly named cascaded power flow controller, and (2) the third port is for the integration of the battery energy storage system to form a multi‐functional device. Therefore, the proposed framework can potentially achieve small‐signal stability enhancement and regulate/adjust the power throughout the HVDC grids.

On the stability of distributed secondary control for DC microgrids with grid-forming and grid-feeding converters

This paper investigates the stability problem of a single-bus DC microgrid with mixed grid-forming/grid-feeding converters and the constant impedance, constant current, and constant power (ZIP) loads. Considering the different output properties of grid-forming and grid-feeding converters, distributed secondary controllers with mixed V–I droop and I–V droop controllers are designed to achieve accurate bidirectional current sharing and voltage regulation. As few converters have access to the information of the common bus, an observer is designed for each local controller to estimate the bus voltage. Then, the observer state is used to generate voltage correction term in the feedback loop, which gets potential faster regulation compared with the existing approaches that use pinning gains. Based on the rank-one perturbation analysis, a stability criterion of DC microgrid is given for the case of ZI load, where global voltage regulation and current sharing are guaranteed. Then, the stability criterion is extended to the case of ZIP load, where local stability of the DC microgrid is guaranteed. Both numerical examples and experimental tests are given to verify the effectiveness of the theoretical results.

Improving Current Sharing and Voltage Regulation for DC Microgrids: A Decentralized Demand Response Approach

Droop control is a widely applied method to improve the current sharing and voltage regulation of each distributed generator in a DC microgrid. However, the droop coefficients are usually fixed and assigned based on the designed capacity of the distributed generators, leading to fluctuations in the voltage and current at the DC buses. This paper introduces a novel droop control based on a decentralized demand response approach with DC boost converter, to derive the optimal droop coefficients. In the proposed model, the switch states (i.e., closed, and open) of the DC boost converter at each distributed generator are utilized to employ the decentralized demand response for dealing with the uncertainties in the output power of distributed generators and the resistive loads. A multi-objective optimization approach based on reinforcement learning and interactive fuzzy programming is proposed to regulate the system voltage, minimize the total power loss, and improve the current sharing error among the distributed generators. First, the Pareto solutions are obtained using the deep-Q network-based reinforcement learning method to handle the uncertainties. Then, the best compromise solution is selected based on the interactive fuzzy method with a modified aggregation membership function to consider both dominant and compromise values for an optimal solution. The proposed approach is tested with a parallel-connected DC modified 30-bus system. Further, its performance is also validated by comparing it to the centralized demand response-based droop control and the classical stochastic method.

A Decentralized Power Allocation Strategy for Dynamically Forming Multiple Hybrid Energy Storage Systems Aided With Power Buffer

Multiple hybrid energy storage systems (HESSs) consisting of batteries and super-capacitors (SCs) are widely used in DC microgrids to compensate for the power mismatch. According to their specific energy and power characteristics, batteries and SCs are used to compensate low-frequency and high-frequency power mismatches, respectively. This paper proposes a decentralized power allocation strategy for dynamically forming multiple HESSs aided with a novel power buffer. The power buffer is a device combing a capacitor and a bidirectional DC-DC converter, it is used as an interface between the batteries and DC bus, allowing easy Plug-and-Play of different energy storage units and effective and efficient power allocation. First, the power buffer and SCs split the power mismatch into a low-frequency and high-frequency part with a modified I-V droop control. Then the power buffer transfers the low-frequency mismatch to the batteries for compensation based on their respective state-of-charges (SoCs), while the high-frequency part is dealt by the SCs directly. This new scheme further allows elimination of the DC bus voltage deviations. Finally, the real-time hardware-in-loop (HIL) tests of three case studies confirm the effectiveness of the proposed control strategy.

Optimal Allocation of Virtual Inertia and Droop Control for Renewable Energy in Stochastic Look-Ahead Power Dispatch

To stabilize the frequency of the renewable energy sources (RESs) dominated power system, frequency supports are required by RESs through virtual inertia emulation or droop control in the newly published grid codes. Since the long-term RES prediction involves significant errors, we need online configure the frequency control parameters of RESs in a rolling manner to improve the operation economics under the premise of stabilizing system frequency. To address this concern, this paper proposes a frequency constrained stochastic look-ahead power dispatch (FCS-LAPD) model to formulate the frequency control parameters of RESs and Energy Storage Systems (ESSs) as scheduling variables, which can optimally allocate the virtual inertia and droop coefficient of RESs and ESSs. In this FCS-LAPD model, the uncertainties of RESs are characterized using Gaussian Mixture Model (GMM). The required reserves are determined by frequency control parameters, and the reserve cost coefficients are adjusted properly to allocate the reserves according to the predicted power generation. Due to the nonlinearity of the frequency nadir constraint, a convex hull approximation method is proposed to linearize it with guaranteed feasibility. The proposed FCS-LAPD is ultimately cast as an instance of quadratic programming and can be efficiently solved. Case studies on modified IEEE 24-bus system and a provincial power system in China are conducted to show the effectiveness of the proposed model.

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

This paper proposes a distributed intelligent secondary control (SC) 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, non-linearity, 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 (FCS-MPC) 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.

Sensitivity Region-Based Optimization for Maximizing Renewable Generation Hosting Capacity of an Islanded Microgrid

Renewable energy based distributed generators are key components in islanded microgrids. However, their power intermittency and uncertainty may impair power quality and cause system operating constraint violations. It is imperative to evaluate and maximize the hosting capacity of an islanded mi-crogrid for renewable generation. Besides, conventional optimi-zation methods focus on improving the solution robustness on constraints under uncertainties but ignoring that on the optimi-zation objective. To address these unsolved issues, a sensitivity region (SR) based optimization method for maximizing renewa-ble generation hosting capacity of an islanded microgrid is pro-posed. This paper firstly proposes an optimization model consid-ering microgrid frequency variation and microturbine droop control functionality. Secondly, SR and feasibility-SR are adopt-ed to quantify solution robustness against uncertainties of renew-able generation and load. It is expected to enlarge these two re-gions to cover all the possible uncertainty realizations, thus providing robust solutions. Last, this paper develops an SR based optimization method with a new solution algorithm. Through comprehensive numerical simulations, the proposed SR based hosting capacity maximization method is verified with high solu-tion robustness on both objective and operating constraints.

Distributed Unified Control for Global Economic Operation and Resilience Reinforcement of Hybrid AC–DC Microgrids

The paralleled bidirectional interlinking converters (BILCs) of the hybrid AC/DC microgrid (HMG) provide a flexible and reliable power interaction way between AC and DC subgrids with high power density. A distributed unified control (DUC) is proposed for BILCs to achieve both resilience reinforcement and global economic operation of the HMG. For distributed generators (DGs), the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$f_{ac}-\lambda _{ac}$</tex-math></inline-formula> and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$v_{dc}-\lambda _{dc}$</tex-math></inline-formula> economic droop controls are employed for AC DGs and DC DGs to decrease the individual subgrid's generation expense by equaling DGs' incremental expenses (IEs). For BILCs, the normalized AC subgrid's frequency and DC subgrid's voltage are coordinated to achieve the economic power interaction (EPI), which further decreases the total generation expense (TGE). Besides economic operation, the proposed DUC is capable of adopting different MG operation modes into a unified control structure without any mode switching. Paralleled BILCs are managed to reinforce the system resilience by supporting the DC voltage or AC voltage once all DC or AC DGs fail. In addition, the proposed DUC enables BILCs with plug-and-play characteristics, resistance to the maximum acceptable communication delay, and the communication disruption robustness. The corresponding hardware-in-the-loop (HIL) results verify the effectiveness of the proposed control strategy.

Improved active and reactive power sharing on distributed generator using auto-correction droop control

Improved active and reactive power sharing on distributed generator using auto-correction droop control

Circulating current minimization based adaptive droop control for grid-connected DC microgrid

Circulating current minimization based adaptive droop control for grid-connected DC microgrid

Transformer and line energisation via grid forming converter based on multi-loop droop control

Transformer and line energisation via grid forming converter based on multi-loop droop control

A Droop-Controlled Interlink Converter for a Dual DC Bus Nanogrid with Decentralized Control

This paper proposed a dual DC bus nanogrid with 380 V and 48 V buses and allows the integration of distributed energy resources on two buses. The proposed system employs an interlink converter to enable power sharing between the buses. The integration of distributed energy resources has been found to enhance the reliability of the low-voltage bus in comparison to those that lack such integration. The integration process requires the introduction of a new V-I curve for the interlink converter within a DC nanogrid controlled by DC bus signaling and droop control. Furthermore, selecting a power electronics converter for the interlink converter is essential. This paper employs a dual active bridge with galvanic isolation as an interlink converter and proposes a control strategy for the converter that relies on DC bus signaling and droop control. Moreover, this control methodology serves the purpose of preventing any detrimental impact of the interlink converter on the DC buses through the reprogramming of the V-I curve. Subsequently, the suggested control methodology underwent simulation testing via MATLAB/Simulink, which included two different test categories. Initially, the DAB was evaluated as an interlink converter, followed by a comprehensive assessment of the interlink converter in a complete dual DC bus nanogrid. The results indicate that the DAB has the potential to function as an interlink converter while the suggested control approach effectively manages the power sharing between the two buses.

Deviation of Frequency and Inertia with penetration of Renewable

Inertia response is a function of bulky synchronous generators with large rotating rotor masses that cover the instantaneous difference between supply and grid demand, typically in power grids. The rotor rotation frequency helps speed up or slow down the rotor rotation due to the lack of adequate gap between the power supply and the electrical power demand. When demand increases or a fault occurs, or load fluctuations exceed the balance of supply and demand, grid inertia is always induced at this stage. This helps the automatic grid operator rebalance the system to avoid speed drifts, resulting in a relatively small change in AC frequency over the system's frequency range. But the system consists of a system with low or zero inertia, like the wind, it lacks the effect of inertia and could not gain equilibrium. So, to balance and to reach the target frequency; the frequency deviation of the system due to the lack of inertia of the wind turbine is improved by the droop control technique in this research because the wind system reduced the total inertia of the system. Droop rate control mode for AC generator techniques was used to control the frequency fluctuations due to the lack of inertia of the renewable energy system. A synchronous generator connected to the mains was used as the governor control mode. It works by controlling power generation using the mains frequency of the prime mover. This mode allows synchronous generators to run in parallel and share the loads with the same drooping technique in proportion to their rated output. Injected grid inertia increased generator inertia on large power grids.

Load-Frequency and Power Sharing Control in a Multi-Area Power System Using Intelligent Sliding Surface Approach

With the penetration of DGs in power systems, some restrictions such as economic dispatch in non-dispatchable sources, their control technology and voltage/frequency stability may be challenging. In conventional power systems, the transient stability has been obtained by high-inertia of upstream network in all conditions; however, in modern power grids, the micro grids (MGs) which could operate in both islanded mode and grid-connected mode are taken into account to overcome this issue. AC or DC types of DGs usually connect to the loads through power electronic converters, in which for researchers, this issue has been interesting and applicable to continue. When loads have been changed, the frequency has influenced correspondingly and must not exceed the permissible range. Therefore in this paper, this intention has been studied using conventional droop controller and intelligent sliding surface approach (ISSA) to evaluate the DGs effects on load-frequency control. While two or more areas (as some individual MGs) connect to each other via a transmission line, the voltage/frequency stability and power balancing constrained may be changed, which the ISSA is hired to track the frequency changes. To examine this problem, in this paper there are three MGs operate separately to show if the load may not supply, they should connect to adjacent block. If this important concern has not considered, it may cause global black-out. The simulation results obtained by ISSA compared with droop controller method represent the superiority of proposed technique to achieve best power sharing.

Power Control Strategy for a Ferry’s DC Power System Using Supercapacitors

Integrated power systems are gaining popularity in the field of power systems and DC integrated power systems are considered promising for electric propulsion ships due to their simple grid topology, low fuel consumption, and easy access to new energy sources. However, the dynamic response characteristics of the power plant can be compromised when a variable speed generator is used in a DC power system, despite achieving energy savings. In this research, we investigate the power control strategy of a specific type of a ferry’s DC power plant. We establish a mathematical model and a Matlab/Simulink-based simulation model to analyze the performance of the proposed strategy. The research utilizes the fast charging and discharging advantages of supercapacitor storage devices to compensate for the dynamic impact delay of the power output when using the variable speed generator set. Additionally, an improved DC bus voltage droop control method that incorporates voltage compensation is proposed to mitigate problems related to large bus voltage fluctuations under sudden load change conditions, enabling better load distribution between different power sources. The simulation results confirm the effectiveness of the proposed strategy in optimizing the speed-seeking method of the variable speed diesel engine sets matching with the supercapacitor, and its positive impact on the dynamic performance of the propulsion system is demonstrated under variable load conditions resulting from ferry operations.

Adaptive Dynamic Droop Control for Photovoltaic-Rich Distribution Networks Considering Photovoltaic Uncertainties

ABSTRACT The integration of photovoltaic (PV) units into distribution networks is becoming crucial as renewable energy sources gain more prominence. However, the intermittent nature of solar energy causes voltage fluctuations and power losses that need to be addressed. To address these challenges associated with integrating PV units into distribution networks, a novel approach is proposed that provides reactive power ancillary support using a decentralized local dynamic droop control (DDC) strategy. The proposed approach, combined with a meta-heuristic reptile search algorithm (RSA) technique that identifies the appropriate sizing and placement of multiple PV arrays in distribution networks, will substantially improve the voltage profile and full utilization of the PV inverter’s capacity to provide reactive power support over a 24-hour period. The PV systems are modeled using the Lambert-W function, which simplifies the estimation of PV at its maximum power point (MPP) without resorting to computationally intensive MPP tracking techniques. The study leverages 24 hours of realistic data, including ambient temperature and solar irradiance, together with the inverter’s terminal voltage, to estimate the amount of reactive power an inverter can handle. The proposed method substantially improves the voltage profile with full utilization of the PV inverter’s capacity to provide reactive power support over a 24-hour period. The study demonstrates that this approach prioritizes a decentralized network and PV inverters over a centralized management system, making it a promising option for future PV systems that require improved grid compatibility and integration without the installation of additional reactive power devices. In addition, a customized performance index based on the integral square of the voltages at various buses where PVs are located in IEEE-33 and IEEE-69 test bus systems, summed over a 24-hour time window, was calculated for comparative analysis. The proposed method results in an improvement of 93.9% and 95.6% over the base case test systems, whereas conventional droop control results in an improvement of 33.47% and 30.9% only over the base case, respectively, for the test systems.

Voltage Drop Estimation during Shore Connection with the Use of Motor Drives Modified as Static Frequency Converters

Ship-to-shore connection is an important technological element that reduces air pollution in ports. Therefore, ports install facilities that allow mooring ships to connect to the port distribution network. By 2025, this will be mandatory for all ports in Europe. This can be a challenging task in most ports due to the different frequency of the network and ship frequency. This problem can be solved by the use of grid-forming static frequency converters. This solution also brings some other advantages: The ship is not threatened by high shore short-circuit currents, and the port distribution network is not affected by the character of the ship load. However, frequency converter software must include a droop control algorithm to ensure that voltage deviations do not exceed the allowed limits during transients. Typical frequency converters used for shore connection are those developed as static frequency converters (SFCs). However, those converters were not developed for large power outputs, which are needed to power large vessels, such as ferries or cruise ships. This paper proposes motor drives that were modified to operate as SFCs. This approach has quite a lot of advantages which are described in this article. This paper describes both a standard shore connection system without a frequency converter and a solution that includes static frequency converters. The paper then focusses on voltage deviation estimations during connection/disconnection of large load (ferry or cruise ship) to static frequency converters. In this work, a high-voltage shore connection (HVSC) simulation model is developed, including a frequency converter, a shoreside transformer, medium-voltage (MV) connection cables, and a power system of the ship, to analyze in detail the behavior of the system in the case of connection or disconnection of the ship load. The model was made in DIgSILENT PowerFactory for the case of a commercial port in southern France. The model gives credible estimations of voltage drops/surges during transient and steady states.

Dynamic and Model Predictive Controllers for Frequency Regulation of an Isolated Micro—Grid with Electrical Vehicles and the ESS Integration

Electric vehicles (EVs) improve the performance of Energy Storage Systems (ESS). The inclusion of electric vehicles in the microgrid system has attracted considerable attention due to the emerging potential of vehicle-to-grid options. The proper coordination among ESS & EVs has a high capacity to tolerate frequency discrepancies and the functioning of the MG (Micro-Grid). In this research, two innovative control techniques-To address the issue of frequency regulation in the standalone MG, Model Predictive Control (MPC) and Dynamical Droop Control (D2C) are linked to affect the structure of ESS and EVs following extensive RES inclusion or a significant change in load demand. The D2C protects EV energy in line with system demands by preserving the least amount of power for anticipated EV demand. Combined MPC and D2C settings are tuned using a complex evolutionary technique to further improve performance. In MATLAB/Simulink, a single MG (micro-Grid) is modeled and evaluated using the controlling strategies indicated above. As well, a range of case papers is taken into account to confirm the integration of the D2C and MPC for a single MG's modulation scheme. Moreover, the MPC outperforms both fuzzy-based proportional-integral (FPI) and proportional-integral (PI) controllers in terms of performance results.