Grid-connected Conditions Research Articles

Grid-following and grid-forming control modes of the rotor and grid sides converters for seamless and universal operation of the hybrid DFIG-wind/battery energy storage system in grid-connected and stand-alone conditions

The system examined in this paper is a hybrid doubly-fed induction generator wind-turbine (DFIG-WT) combined with a battery energy storage system (BESS). It operates in both stand-alone and grid-connected modes, with the capability for seamless transitions between these states, and manages the state of charge (SOC) of the battery under all conditions. In this paper, by modifying control structures of the rotor-side and grid-side converters (RSC and GSC), the RSC is controlled in the grid-following (GFL) mode and the GSC in the grid-forming mode using the virtual synchronous generator technique. The proposed control scheme with detailed considerations for both the RSC and GSC simultaneously addresses the following aspects: (a) seamless operation in both grid-connected and stand-alone conditions, (b) synchronization and smooth connection of the DFIG during initial moments of grid-connection without using a phase-locked loop (PLL), (c) enabling the converters control systems to allow the grid-connected DFIG to operate not only in MPPT mode but also in power dispatch mode based on the power grid requirements, and (d) SOC adjustment of the battery in both grid-connected and islanded modes. In the grid-connected state, the battery SOC is managed by the GSC, and for SOC < 90 %, the battery is charged by the GSC according to the SOC level, and for SOC > 95 %, the GSC limits the SOC below the upper limit. In the stand-alone state, the SOC upper limits are mainly managed by the RSC, and if the SOC exceeds the value of 85 %, the RSC works in the power curtailment mode and reduces the stator power.

Takagi-Sugeno fuzzy gain controller for Vehicle-to-Grid (V2G) load frequency control

Load Frequency Control (LFC) has gained more importance with the introduction of deregulated Renewable Energy Sources (RES) connectivity with the grid. Electrical Vehicles (EVs) can feed electricity back into the grid in Vehicle-to-Grid (V2G) mode to maintain stability. However, the increasing number of EVs penetrating the grid causes frequency instability in the power system. If required, EVs may utilize bi-directional chargers to transfer power back to the grid in the V2G mode while they are charging or in a grid-connected state, restoring the frequency instability of the grid. The frequency restoration response time is important to reset the grid frequency fluctuations in the shortest time possible to avoid shutting down the power system. This paper presents a Takagi-Sugeno (T-S) fuzzy linear output controller for LFC in two-area systems with tie-line control. This work models EV batteries as a single lump of large-capacity battery energy storage systems. The EV's battery system provides ancillary power to the two-area power system to reset it to a steady state after a load disturbance. The T-S fuzzy controller's linear output dependency on its inputs enables it to respond efficiently to load variations in the nonlinear two-area power systems. The proposed controller parameters are evaluated from stability analyses and its robustness is tested with sensitivity analysis. It is compared with other fuzzy controllers, and it demonstrates a fast-settling time and reduced frequency deviation response.

Impact of Multi-Energy System and Different Control Strategies on a Generic Low-Voltage Distribution Grid

The rising electricity costs, cost of space heating, and domestic hot water end up driving consumers toward reducing expenses by generating their electricity through devices like photovoltaic systems and efficient combined heat and power plants. When coupled with thermal systems via an energy management system (EMS) in a Multi-Energy System (MES), this self-produced electricity can effectively lower electricity and heating bills. However, MESs with EMSs can serve various purposes beyond cost reduction via self-consumption, such as reacting to variable electricity prices, meeting special grid connection conditions, or minimizing CO2 emissions. These diverse strategies create unique prosumer profiles, deviating significantly from standard load profiles. The potential threat to the power grid arises as grid operators lack visibility into which consumers employ which control strategies. This paper investigates the impact of controlled MESs on the power grid compared to average households and answers whether new control strategies affect the planning strategies of low voltage grids. It proposes a comprehensive four-step toolchain for the detailed simulation of thermal–electrical load profiles, MES control strategies, and grid dynamics. It includes a new method for the grid impact analysis of extreme and average bulk values. As a result, this study identifies three primary factors influencing distribution power grids by MESs. Firstly, the presence and scale of photovoltaic (PV) systems significantly affect extreme values in the grid. Secondly, MESs incorporating combined heat and power (CHP) and heat pump (HP) units impact the overall grid performance, mainly reflected in bulk values. Thirdly, the placement of an MES with heating systems, especially when concentrated in one feeder, plays a crucial role in grid dynamics. Despite the three distinct factors identified as impactful on the power grid, this study reveals that the various control strategies, despite leading to vastly different grid profiles, do not exhibit divergent impacts on buses, lines, or transformers. Remarkably, the impact of MESs remains consistently similar across the range of control strategies studied. Therefore, different control strategies do not pose an additional challenge to the grid integration of MESs.

Microgrid Pre-Synchronization Scheme for Suppressing Voltage Fluctuations and Avoiding Frequency Out-of-Limits

Pre-synchronization control is needed when the microgrid changes from an off-grid state to a grid-connected state. Aiming to resolve the problems of frequency overstep and voltage fluctuation in traditional pre-synchronous grid-connection schemes, a micro-grid pre-synchronous grid-connection scheme is proposed that can solve both frequency overstep and voltage fluctuation at the same time. Firstly, the feedforward disturbance compensation V/f control strategy of the load voltage and current is introduced to the energy storage system to suppress voltage fluctuations caused by power changes. Then, the V/f control strategy of the energy storage system is improved to suppress voltage fluctuations caused by frequency fluctuations when the starting phase of pre-synchronization is enacted. Finally, PI parameters are optimized based on the golden jackal optimization algorithm to meet the requirements of frequency fluctuation and grid-connection speed in the pre-synchronization process. A large number of simulation experiments demonstrate the effectiveness of the proposed method.

The Frequency Regulation Strategy for Grid‐Forming Wind Turbine Generator and Energy Storage System Hybrid System in Grid‐Connected and Stand‐Alone Modes

This paper proposes a coordinated frequency regulation strategy for grid‐forming (GFM) type‐4 wind turbine (WT) and energy storage system (ESS) controlled by DC voltage synchronous control (DVSC), where the ESS consists of a battery array, enabling the power balance of WT and ESS hybrid system in both grid‐connected (GC) and stand‐alone (SA) modes. The newly developed GFM framework, i.e., DVSC, is adopted in both WT and ESS, which utilizes DC voltage dynamics for synchronization purposes. In this paper, the GC mode and SA mode are transferred by changing the status of the series‐connected switch, and it is necessary to meet the grid connection conditions when the system is transferred to the GC mode, namely, voltage, frequency, and phase sequence. For the GC mode, the inertia response from WT can be realized using the reserved energy of the DC capacitor, while the ESS serves to eliminate the steady‐state frequency deviation and reduce the DC voltage fluctuation of WT using the designed secondary frequency regulation scheme. For the SA mode, the proposed strategy can keep the power balance without external power sources. The small‐signal model of the WT and ESS hybrid system is derived. The stability analysis in both GC and SA modes is fully conducted utilizing the modal analysis method, where the impacts of control parameters on stability are assessed. The performance of the proposed strategy in a weak system is evaluated. Simulation studies are carried out under various power system contingencies to verify the effectiveness of the proposed strategy and validate the correctness of the theoretical analysis.

Advantage of battery energy storage systems for assisting hydropower units to suppress the frequency fluctuations caused by wind power variations

The integration of renewable energy sources into power grids has led to new challenges for maintaining the frequency stability of power systems. Hydropower has traditionally played a key role in frequency regulation due to its flexibility in output power. However, the water hammer effect can lead to the phenomenon of inverse regulation, which can degrade the transient characteristics of power systems, particularly in hydropower-dominant systems, such as the Southwest China Power Grid. On the other hand, battery energy storage systems (BESSs) are well-suited for frequency regulation due to their fast response speed, high response accuracy, and flexible control capabilities. Hence, it is a meaningful topic to evaluate the advantage of integrated battery energy storage systems for assisting hydropower units (HPUs) in frequency regulation. First, the frequency dynamic response model of power system with BESSs assisting HPUs to participate in frequency regulation is established. Then, a two-part strategy is presented. The primary frequency regulation strategy for BESSs uses droop control with a variable regulation coefficient and the secondary frequency regulation strategy considers the costs associated with both HPUs and BESSs. Last, a performance assessment is carried out based on the management rules for auxiliary services of power plants in China and the USA. The results of this study demonstrate that the incorporation of the BESS into the HPU can significantly improve the regulating performance, and can provide important economic and technical benefits for the power system. Specifically, the maximum ratio of HPU with BESS to conventional HPU is 33.13 %, 33.85 %, 7.60 % and 41.52 % for the different indicators (difference, delay, distance, and cost). Notably, the implementation of BESS in the HPU not only contributes to the stability of the power system, but it also provides considerable economic benefits by achieving higher evaluations for auxiliary services in the power market. These findings are critical in supporting the transformation and development of conventional HPUs under large-scale new energy grid connection conditions.

Research on the Hybrid Wind–Solar–Energy Storage AC/DC Microgrid System and Its Stability during Smooth State Transitions

The hybrid AC/DC microgrid is an independent and controllable energy system that connects various types of distributed power sources, energy storage, and loads. It offers advantages such as a high power quality, flexibility, and cost effectiveness. The operation states of the microgrid primarily include grid-connected and islanded modes. The smooth switching between these two states is a key technology for ensuring the flexible and efficient operation of the microgrid. In this paper, the typical structure of an AC–DC hybrid microgrid and its coordination control strategy are introduced, and an improved microgrid model is proposed. Secondly, an adaptive current–voltage–frequency integrated control method based on signal compensation is proposed to solve the impulse current and voltage generated during the switching between a grid-connected state and an off-grid state. Finally, in response to unplanned grid-connected scenarios, an improved pre-synchronization control strategy based on BP neural networks is introduced to rapidly restore stable operation. The proposed control strategies enhanced the steady-state and transient stability of the hybrid wind–solar–energy storage AC/DC microgrid, achieving seamless grid-connected and islanded transitions without disturbances. The simulation and experimental results validated the correctness and effectiveness of the proposed theories.

Control Strategy of Seamless Switching Between Grid-connected and Islanding Operation for Weak-connected Microgrid

A control strategy of seamless switching is proposed for the high-capacity microgrid, which is at the end of long-distance transmission. Firstly, the control modes of energy storage and diesel generator set under grid-connected conditions and islanding conditions are analyzed. Meanwhile, the seamless switching strategy for the microgrid, in which energy storage or diesel generator sets can be taken as the main power source adaptively, is proposed. During the switching from the grid-connected condition to the islanding condition, the connection line power is optimized according to the respective operational characteristics of the energy storage and diesel generator sets. And the interaction of the power step control with voltage blocking and the regulation characteristics of the line voltage regulator is utilized to effectively suppress the fluctuation of system voltage and frequency. Finally, the combination of auto-synchronization control and multiple synchronization control improves the success rate of switching from the islanding condition to the grid-connected condition. The on-site test results of an actual microgrid project verify the effectiveness of the proposed control strategy.

A Nonlinear Multimode Controller for Seamless off-Grid of Energy Storage Inverter Under Unintentional Islanding

A nonlinear multimode controller (NMC) is proposed to achieve the whole process seamless off-grid of energy storage inverter (ESI) from the grid-connected state of current control mode (CCM) to the islanded state of voltage control mode (VCM) under unintentional islanding. Based on the presented design idea of intelligent equivalent substitution, NMC is composed of multimode voltage controller (MVC), multimode power compensator (MPC) and multimode current controller (MCC). MVC can control load voltage in islanding and maintain manipulated variables of load current in other modes. MPC can compensate the power flowing into grid and help inner loop reference value achieve the smooth drop, and also compensate for load current variation during grid-connection. MCC can track the given current during grid-connection and make MPC quickly obtain the compensation of load power variation. The reference values of inner loop are logically split into the output of MVC, MPC and MCC, so that the manipulated variables can be combined nonlinearly to achieve seamless off-grid of ESI under unintentional islanding. This paper also analyzes the ESI stability with NMC during the off-grid process, and analyzes the relationship between manipulated variables by simulations. Finally, the correctness and effectiveness of proposed NMC are verified by experiments.

Distributed Power Grid Connected Panoramic Perception Technology Based on Digital Twin Model

Abstract To address the issue that large-scale distributed Proton-exchange membrane fuel cell grid-connected operation status is difficult to effectively monitor, this study carried out clustering screening of Proton-exchange membrane fuel cell grid connected status variables. The digital twin model of the Proton-exchange membrane fuel cell group grid connection is established by using key important variables, and the digital model accurately reflects the real-time operation status of the Proton-exchange membrane fuel cell. Then, it predicts the operation status of the Proton-exchange membrane fuel cell. The experiment showcases that the optimal accuracy of fuzzy C-means clustering algorithm, K-means clustering algorithm and fuzzy C-means clustering algorithm in view of the objective function for clustering screening of grid-connected state quantity of Proton-exchange membrane fuel cell is 0.74, 0.65 and 0.59 respectively; The optimal recall rates are 0.84, 0.76, and 0.68, respectively; The optimal F-measure values are 0.78, 0.70, and 0.64, respectively. Among the three clustering analysis algorithms, the fuzzy C-means clustering algorithm, in view of the objective function, has the shortest running time and the least memory consumption. The monitoring results are basically consistent with the actual situation, indicating that this design method can complete the task of monitoring the operating status of distributed power grid-connected equipment. This study’s proposed method is more accurate than conventional methods and is better suited for monitoring the grid connection status of distributed power systems.

A Non-Intrusive Identification Approach for Residential Photovoltaic Systems Using Transient Features and TCN with Attention Mechanisms

In order to reduce the negative impact of the large-scale grid connection of residential photovoltaic (PV) equipment on the distribution network, it is of great significance to realize the real-time accurate identification of the grid connection state and its switching of residential PV equipment from the distribution network side. This paper introduces a non-intrusive method for identifying residential PV systems using transient features, leveraging the temporal convolutional network (TCN) model with attention mechanisms. Firstly, the discrimination and redundancy of transient features for residential PV devices are measured using a feature selection method based on the semi-Fisher score and maximal information coefficient (MIC). This enables the construction of a subset of identification features that best characterize the PV devices. Subsequently, a sliding window two-sided cumulative sum (CUSUM) event detection algorithm, incorporating a time threshold, is proposed for the real-time capturing of PV state switching and grid connection behavioral events. This algorithm effectively filters out disturbances caused by the on/off cycles of low-power residential devices and captures the transient time windows of PV behaviors accurately. On this basis, a TCN model with attention mechanisms is proposed to match the discerned event features by assigning varying weights to different types of characteristics, thereby facilitating the precise recognition of a PV grid connection and state-switching events. Finally, the proposed method is validated on a custom-designed non-intrusive experimental platform, demonstrating its precision and real-time efficiency in practical applications.

A Novel Islanding Detection Method for Distributed PV System Based on μPMUs

With the rapid development of distributed PV systems in distribution grids, unintentional islanding is one major concern which maybe cause safety hazards. The existing passive islanding detection methods are mainly based on the voltage and frequency deviations during islanding conditions. Passive methods have relatively large non-detection zone (NDZ) and susceptible to power system faults. Active islanding detection methods applied in inverters have relatively small non-detection zone, but they are usually harmful to power quality. This paper presents a new islanding detection method based on equivalent inter-connection line impedance, which is normally a small value. A measured impedance is calculated with the synchronous phasors from two lPMUs which are installed in a utility substation and a PV system. Under the grid-connected condition, the measured impedance is no more than the equivalent line impedance. While under the islanding condition, the measured impedance is much larger than the equivalent line impedance. So, an islanding condition can be effectively detected by the comparison of the measured impedance with the equivalent line impedance. The non-detection zone of this proposed method is nearly zero. The detection time is normally less than 250ms. This method is applicable for PV systems connected to the power system with a distribution transformer. It is also applicable for a distribution grid with multiple distributed generation systems.

A Hybrid Islanding Detection Method Based on Lissajous Pattern Having Robust Performance Under Various Power Quality Scenarios

A fast, accurate, robust, and two-staged islanding detection technique (IDT) is proposed. It is the hybridization of second-order general integrator-frequency locked loop (SOGI-FLL), Lissajous pattern (LP), and active power absorption-cum-reactive power injection via dc-bus connected battery unit-based inverter. The LP is used to identify islanding condition on the basis of frequency variation of fundamental voltage. Here, the measured ac voltage signal is preprocessed by SOGI-FLL to obtain fundamental bus voltage under various power quality scenarios. This assures robust islanding detection by removing uncertainty, and thus, reliability is improved. The uncertainty effects due to the partial shading condition of the PV module (source-side disturbance) and weak grid-connected condition are avoided by dc-bus connected battery storage unit. Thus, it removes above-said limitations of the existing scheme. The LP-based detection within the nondetection zone is expedited due to active and reactive powers variation via controlling of the inverter and charging operation of the dc-bus connected battery unit. During the reactive power injection by solar inverter, active power is absorbed by dc-bus connected battery unit. In effect, an active power output of the inverter is reduced. Here, the reactive power injection at the reduced active power output condition of the inverter accelerates frequency variation allowing rapid islanding detection. The real-time experiments are carried out using Typhoon-HIL tools to compare the proposed method with other works reported in the literature. Improved performances under diversified scenarios are found in the proposed case.

Analysis and Optimization of the Impact of Distributed Generation Integration on Distribution Network Loss

The distribution loss of the distribution network has changed greatly because of the access of distributed generation, which has a certain impact on the power quality of the distribution network. In order to solve the above problems, this paper constructs a 10 kV grid model containing distributed generation, studies the impact of distributed generation on the distribution of network loss, and grasps the characteristics of network loss changes under different scales of distributed generation and different grid-connected conditions. Finally, this paper constructs a mathematical model containing distributed generation and proposes control measures to ensure the safe and stable operation of the grid.

Detecting Speed Improvement and System Stability Enhancement for DC Microgrids Islanding Detection Based on Impedance Characteristic Analysis

Islanding detection is the essential basis for the safety and protection of dc microgrids. Due to the advantages of rapid detection speed and small nondetection zone, the positive feedback islanding detection technique has attracted much interest in recent years. The implementation of a positive feedback loop facilitates the islanding detection for dc microgrids, but it may reduce the system stability margin at the grid-connected state. In this article, the dc output impedance model of voltage source converter and distributed generators are developed to reveal the dynamic interactions and instability mechanism accurately between the two subsystems considering the impact of islanding detection with voltage positive feedback of selected frequency (VPFOSF). Then, the stability region of the interconnected system based on the output admittance model is derived to guide the design method of the damping compensator. Moreover, a grid-current-based active-damping control scheme is further proposed to mitigate instability caused by the VPFOSF loop. Thus, the conflict among islanding detection rapidity, power quality, and grid-connected system stability is effectively addressed. In the end, both the simulations and experimental results are provided to validate the effectiveness of the proposed control method.

Design of A Grid-connected Control System for Distributed Photovoltaic Power Generation Based on PLC

Distributed photovoltaics interfere with continuous power generation after grid connection. In the face of the failure of a single module, the current grid-connected control system needs to readjust system parameters to flexibly adapt to equipment operating conditions. Therefore, this paper is researching a photovoltaic power generation grid-connected control system based on PLC. In the hardware part, PLC is used to complete power generation control, monitoring MCU, data acquisition, control, and other modules. In the software part, the grid-connected state is optimized and controlled according to the distributed photovoltaic output power and the remaining energy storage capacity. The PLC is the execution center, which will control the on-off state of the equipment switch after the data operation. The experimental results show that the designed system can optimally control the power generation and energy storage units according to the power change, reduce the cost of electricity consumption through the adjustment of charge and discharge, reduce the IO usage rate, and improve the concurrent response ability of users. The design system not only guarantees the quality and efficiency of photovoltaic grid connection but also realizes rapid networking and independent tracking of power points to meet the requirements of flexible communication.

Research on grid-connected photovoltaic energy storage to stabilize power fluctuations

Due to the instability of photovoltaic power generation, device aging and other factors, the PI parameters of the original PI controller were no longer applicable. So a grid-connected control strategy of photovoltaic energy storage with PI controller parameters optimized by an algorithm was proposed to realize the smooth suppression of photovoltaic power fluctuations. Firstly, the PI controller parameters of the energy storage control part were optimized by using the algorithm, and the optimization process was designed. Then, the second-order low-pass filter was used to distribute the photovoltaic power, and the energy control mode of the energy storage part was designed. Finally, the PI parameter effects of the optimized energy storage control part were displayed by comparing the DC bus voltage in the isolated state, and the power suppression effects under grid-connected condition were displayed by the mixed energy storage power. The rationality of the control mode of the energy storage part was verified by the SOC state diagram of the mixed energy storage. The results showed that the control strategy could effectively suppress pv power fluctuation.

Energy management scheme of single‐phase electric energy router with two‐layer intelligent control combined with optimal scheduling of battery

As a key equipment in energy internet, the Electric Energy Router (EER) which takes the converter as the core device solved the problems of power diversity and multi-directional flow of power. This paper designs a household single-phase electric energy router (SPEER), and proposes an energy management scheme with two-layer intelligent control combined with optimal scheduling of battery under isolated grid and grid-connected conditions. The upper instruction layer mainly uses holistic intelligent decision-making and the lower execution layer controls each converter to complete the energy management of the entire system according to the upper instructions. In the isolated grid, The SPEER operates in three modes based on AC/DC distributed generation, AC/DC load and energy storage status to maintain the stability of the system. When connected to the grid, according to the actual situation of a certain place and the peak-valley price policy, the charging/discharging time and power flow direction of the battery can be optimized, which can improve the operating economy. Finally, the correctness of the proposed energy management scheme was verified through experiments.

Quantitative Influence of Wind Farm Reactive Power and Voltage Control Capability on Hunan Power Grid Thermal Power Spinning Reserve

Affected by factors such as the inverse distribution of power sources and loads in the province, and the mismatch between the construction of load center power sources and load growth, the problem of voltage stability in central and southern Hunan is prominent, which mainly restricts the thermal power maximum operating capacity of the Hunan Power Grid. Under the heavy load mode, the hydro power units and thermal power units operating in the province need to reserve a certain amount of spinning reserve to response the voltage stability problems in central and southern Hunan. Thus, it leads to insufficient power supply during peak load periods. In recent years, the grid load has maintained rapid growth, and the increase in power sources is mainly the wind power. Affected by the anti-peak-shaving characteristics of wind power, the lack of wind power has aggravated the problem of insufficient power supply. Actually, the wind turbine is in a hot standby state during standby/backup, and its grid-connected converter is still in a grid-connected state within four hours. Besides, the continuous running SVG in the wind farm booster station can still be used temporarily. The above devices can provide reactive power support to the power grid during the transient process. Therefore, this paper takes the southern Hunan region as an example, and quantified the reduced thermal power requirement reserve by analyzing the reactive power support effect of the grid-connected wind farms. In this way, the power supply capacity can be improved.

Microgrid Operation Optimization Using Hybrid System Modeling and Switched Model Predictive Control

Optimization of economic aspects of microgrid operation in both grid-connected and islanded mode leads to contradictive definitions of optimality for both modes. There is no general agreement on how to cope with this duality. To address this issue, as well as modern energy market requirements and a better renewable energy utilization necessity in the case of large facilities, a comprehensive control solution utilizing the appropriate model is needed. In response, the authors propose a hybrid microgrid model covering fundamental features and designed to work in conjunction with two switched receding horizon control laws. A relevant controller is chosen according to the current microgrid operation mode and its cost function tailored to specific demands of the islanded or grid-connected operation. Performed research led to a new switched hybrid model predictive control approach focused on microgrid economic optimization. This approach utilizes an appropriate hybrid microgrid model also contributed by the authors. The introduced solution turned out to be effective in overall energy cost reduction in the case of large commercial facilities, regardless of grid-connection and renewable generation scenarios. Furthermore, it also provides satisfactory renewable energy and storage capabilities utilization in changing grid connection conditions.