No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
An Adaptive BESS Controller for Stability Enhancement of Islanded Low Voltage Microgrids
Abstract
Battery energy storage system (BESS), as grid forming unit, can quickly regulate voltage and frequency for a 100% inverter-based islanded low voltage microgrid. However, due to some inherent characteristics of this network, such as: (a) coupling among voltage and frequency dynamics, (b) dynamics of dc source, and (c) timescale coupling among converter and network, small-signal stability is a major concern. This paper proposes an adaptive feed-forward compensation scheme for each BESS unit to reduce dynamic interactions among converters and network/load parameters. Additionally, the proposed scheme can enhance system damping capability for a wide range of operating conditions without the need for any prior/ continuous generation/network information or additional sensors. This technique can preserve the voltage/frequency regulation capability of the traditional (ω − P/V − Q) droop control scheme for any low voltage networks. The existing small-signal model is modified to include dc-source, dc link, and proposed feed-forward dynamics, which assists in analyzing the impact of dc-side, acside, and network parameters on system small-signal stability. The system performance is analyzed with extensive case studies conducted on a CIGRE TF C6 : 04 : 02 benchmark system. The proposed model is validated using a real-time digital simulator with hardware-in-loop setup
Due to the overlap in the time scales of controllers, interaction relationships exist between converters. These characteristics are related to the AC grid and controller parameters. To quantitatively analyze the dynamic characteristics of the system, amplitude-phase models of LCC, VSC converters and the AC grid have been established. The self-stability and en-stability damping between converters, which quantifying the interaction relationships, have been proposed. The impact of AC grid, converter capacity and PLL bandwidth on converter interaction has been investigated. Comparative analysis is conducted to examine the impact of the PLL bandwidth on converter stability under the extreme operating capacity. The time domain responses are solved for variations in converter capacity and control parameters, which indirectly validating the accuracy of the frequency domain analysis results. The results indicate that the interaction between converters is primarily influenced by the bandwidth of the PLL controller. The interaction is weakest when the LCC operates at high capacity and the VSC operates at low capacity.
With the recent developments in renewable energy generation and addition of power electronic devices, power system dynamics have become extremely complex. One of the challenges faced due to this transition is the sub synchronous oscillations caused by the interaction of renewable energy sources and various components of the power grid. Recently reported incidents due to sub synchronous oscillations highlight the need of monitoring and suppression of these harmful oscillations in real time. This paper gives an overview of the phenomena of sub synchronous oscillations and discusses the existing monitoring and damping techniques along with their limitations. Further, it highlights the research trends along this path.
The increasing penetration of behind-the-meter distributed energy resources opens up opportunities to improve distribution grids’ resilience and reliability. In this context, the paper studies islanded operation of radial low-voltage (LV) distribution feeders with a high penetration of prosumers equipped with grid-forming inverters. In contrast to the existing studies that focused on droop control as the main source of instability, we extend the analysis to the microgrid topology, in particular the number of grid-forming inverters. The eigenvalue analysis shows that with the increasing number of grid-forming inverters, the system becomes practically unstable with its critical eigenvalue approaching zero as progressively more inverters are added. Furthermore, we extend the analysis by explicitly modelling the DC side dynamics, including the DC/DC converter with its controller and inherent dynamic response of the DC energy source. Eigenvalue analysis shows that the DC voltage controller impacts the system stability by introducing new dominant eigenvalues. The results suggest that the DC energy source should have a fast response to ensure a stable operation. The small-signal stability results, benchmarked against time-domain simulations, indicate that careful parameter tuning is required to ensure stable islanded operation of LV feeders.
To facilitate the transformation from conventional power systems towards smart grids, the concept of microgrids has been widely applied in practice, serving as the medium to accommodate renewable generators. One crucial problem is the stability associated with microgrids. This paper proposes a converter-based power system stabilizer (CBPSS), acting as a supplementary control loop, to enhance the stability of the islanded microgrids. The goal of the proposed CBPSS is to stabilize the critical microgrid mode with the generation of a damping torque, and it is achieved with the identification of the forward loop from the CBPSS to the microgrid. Then, the parameters of the proposed CBPSS can be designed accordingly to compensate the phase lag of the identified forward loop. Besides, an eigenvalue-mobility-based method is presented to identify the optimal installation location of the CBPSS in microgrids. As a consequence, the maximum stabilizing effect can be realized with the least control effort. Finally, modal analysis and time-domain simulations as well as hardware experimental results confirm the effectiveness of the proposed method.
Large-scale integration of renewable generation, usually interfaced to the network through power electronics, has led to drastic changes in power system dynamics. This paper presents novel insights into stability properties of such systems. For that purpose, a high-fidelity dynamic model of a generic low-inertia power system has been developed. The full-order, state-of-the-art control schemes of both synchronous and converter-based generators are included, with the latter differentiating between grid-forming and grid-following mode of operation. Furthermore, the dynamics of transmission lines and loads are captured in the model. Using modal analysis techniques such as participation factors and parameter sensitivity, the most vulnerable segments of the system are determined and the adverse effects of timescale coupling and control interference are investigated. More precisely, this work characterizes the maximum permissible penetration levels of inverter-based generation as well as the nature of the associated unstable modes and the underlying dynamics. Finally, potential directions for improving the system stability margin under different generation portfolios are proposed for several benchmark systems.
A battery energy storage system (BESS) can play a critical role in regulating system frequency and voltage in an islanded microgrid. A
-synthesis-based robust control has been proposed for dc link voltage regulation of BESS for achieving frequency regulation and voltage quality enhancement of islanded microgrid. Variation in the operating condition of ac microgrid affects the operating condition of the BESS converter. This controller synthesis accounts for such uncertain variations as parametric uncertainty. The stability and performance of the proposed controller can be guaranteed for bounded parametric variations. The bounds on parameters are selected based on practical limitations of BESS. The proposed controller's performance is tested on an islanded CIGRE TF C6:04:02 benchmark low voltage ac microgrid system. The importance of dc link voltage regulation is analyzed based on performance comparison with a benchmark controller. The controller performance is also validated using a real-time Typhoon HIL emulator.
This paper presents a simulation study of standalone hybrid Distributed Generation Systems (DGS) with Battery Energy Storage System (BESS). The DGS consists of Photovoltaic (PV) panels as Renewable Power Source (RPS), a Diesel Generator (DG) for power buck-up and a BESS to accommodate the surplus of energy, which may be employed in times of poor PV generation. While off-grid DGS represent an efficient and cost-effective energy supply solution particularly to rural and remote areas, fluctuations in voltage and frequency due to load variations, weather conditions (temperature, irradiation) and transmission line short-circuits are major challenges. The paper suggests a hierarchical Power Management (PM) and controller structure to improve the reliability and efficiency of the hybrid DGS. The first layer of the overall control scheme includes a Fuzzy Logic Controller (FLC) to adjust the voltage and frequency at the Point of Common Coupling (PCC) and a Clamping Bridge Circuit (CBC) which regulates the DC bus voltage. A maximum power point tracking (MPPT) controller based on FLC is designed to extract the optimum power from the PV. The second control layer coordinates among PV, DG and BESS to ensure reliable and efficient power supply to the load. MATLAB Simulink is used to implement the overall model of the off-grid DGS and to test the performance of the proposed control scheme which is evaluated in a series of simulations scenarios. The results demonstrated the good performance of the proposed control scheme and effective coordination between the DGS for all the simulation scenarios considered.
Jumping disturbances and large noises in input voltage sources to a power converter can cause substantial excursion of its output voltage even under a well-designed feedback controller. Predictive compensation can achieve improved disturbance rejection and tracking performance in such scenarios, resulting in a two-degree-of-freedom design. While the feedback controller has embedded robustness, designing feedforward controllers, which are open-loop compensators, is challenging due to the fact that converter internal parameters change from aging and variations in operating conditions, and loads themselves are part of the converter dynamics. When converter dynamics change, system performance deteriorates significantly, making adaptation mandatory. By integrating system identification with the feedforward compensator, an adaptive feedforward compensation design is proposed in this brief. Working on a boost dc-dc converter as a typical platform, combined feedback and adaptive feedforward design is explored. The results show that the two-degree-of-freedom adaptive design results in much improved performance in rejecting disturbances from input power sources.
The analysis of the small-signal stability of conventional power systems is well established, but for inverter based microgrids there is a need to establish how circuit and control features give rise to particular oscillatory modes and which of these have poor damping. This paper develops the modeling and analysis of autonomous operation of inverter-based microgrids. Each sub-module is modeled in state-space form and all are combined together on a common reference frame. The model captures the detail of the control loops of the inverter but not the switching action. Some inverter modes are found at relatively high frequency and so a full dynamic model of the network (rather than an algebraic impedance model) is used. The complete model is linearized around an operating point and the resulting system matrix is used to derive the eigenvalues. The eigenvalues (termed "modes") indicate the frequency and damping of oscillatory components in the transient response. A sensitivity analysis is also presented which helps identifying the origin of each of the modes and identify possible feedback signals for design of controllers to improve the system stability. With experience it is possible to simplify the model (reduce the order) if particular modes are not of interest as is the case with synchronous machine models. Experimental results from a microgrid of three 10-kW inverters are used to verify the results obtained from the model
The full utilization of solar energy is of great significance in reducing carbon emissions and alleviating environmental problems. Fast frequency regulation plays an important role in the power system with grid-connected two-stage photovoltaic (PV) plants. The presented fast frequency regulation method is composed of droop control, virtual inertia control and de-loading control. This work focuses on improving droop control and virtual inertia control for PV plants connected to grid. Due to non-Gaussian disturbance induced by solar irradiance, an adaptive control strategy is proposed by minimizing an improved performance index which combines the mean square error (MSE) and the minimum error entropy (MEE) criteria. Based on the collected frequency deviation series data, the entropy of the frequency deviation is estimated with the aid of a quantizer. The droop control coefficient and inertia gain are then solved by an improved gravitational search algorithm (IGSA). Finally, the effectiveness of the proposed strategy is testified by an illustrative power system with a two-stage grid-connected PV plant.
In this paper, an adaptive virtual impedance-based universal voltage source converter (VSC) control scheme has been proposed to improve the stability, and power-sharing performance of VSC interfaced distributed energy resources in hybrid AC/DC microgrids with different feeder characteristics. The proposed control scheme modulates the VSC output impedance by tracking the active and reactive power transfer difference between the inverter terminal and the point of connection to the microgrid. Firstly, the stability limits of microgrid with resistive and inductive microgrid feeders are investigated by theoretical analysis. Subsequently, influence of the feeder parameters on the stability of microgrid is evaluated. From the theoretical analysis, the proposed control scheme shows resistive inverter output impedance at low frequency, while it shows more inductive behavior at system frequency for both the low and medium voltage microgrid feeders. The effectiveness of the proposed control strategy is demonstrated through a range of scenarios for two different microgrid types. The results show that the proposed controller can autonomously vary the output impedance of the VSC and provides significantly improved damping and power-sharing performance of the microgrid.
Stability of grid-forming converters (GFCs) inter-facing renewable resources in a power system with multiple synchronous generators (SGs) are studied in the context of primary frequency response. The GFCs are divided in two classes based on the control methods class-A: droop control, dispatch-able virtual oscillator control (dVOC), and virtual synchronous machine (VSM); and class-B: matching control. First, averaged phasor models of these GFC classes are developed, which can be seamlessly integrated with positive sequence fundamental frequency planning models. Next, simplified averaged models are derived to study stability of the dc-link voltage of the GFCs under dc-side current limitation in a generic multimachine system during primary frequency response. To that end, sufficiency conditions for stability of both classes and that of instability for class-A GFCs are established. Finally, the proposed stability conditions are verified using detailed switched models of two small systems and phasor models of a4-machine and a16-machine IEEE benchmark systems.
This research addresses the following question: For a weak grid with multiple-IBRs, what are the nature of oscillation modes The analysis of a system consisting of two identical IBRs offers a glimpse of the picture. It shows that the system has a weak-grid mode and an inter-IBR mode. For the weak-grid mode, two IBRs work as an aggregated one to interact with the grid. This mode is sensitive to IBR exporting power level and grid strength. For the inter-IBR mode, two IBRs oscillate against each other. This mode is less sensitive to grid strength and power level.
The balanced, unbalanced and harmonic current sharing inaccuracy caused by the mismatched wire impedances of distributed generation (DG) units influences the normal operation of islanded microgrids. To deal with this issue, an iterative virtual impedance regulation (IVIR) strategy is proposed in this paper, both the resistive and reactive parts in the output impedances of DG units are regulated. The whole process is decomposed into several successive iterations. In each iteration, the fixed virtual impedance and adaptive virtual impedance regulation are combined to generate a virtual resistance increment and uniform virtual reactance increment according to the uniform output current magnitude and phase angle respectively. Based on these increments, the virtual impedance at fundamental-negative and harmonic frequencies also are modified. With the proposed strategy, all the balanced, unbalanced and harmonic current sharing is accurate with a relatively small virtual impedance. Only the local current information is required for the IVIR process, and the communication is merely used to exchange the VIR mode status flags at the beginning and the end of the IVIR process, the dependency on communication is minimized. The feasibility of the proposed strategy is verified with the simulation and experimental results.
Several drawbacks of the conventional and inverse droop control based decentralized techniques for islanded microgrids are being addressed actively, over the years by various researchers. One of the prominent issues is the inaccurate power sharing (reactive power in the conventional droop and real power in the inverse droop) among the distributed generators (DGs) due to the feeder/line impedances. Virtual impedance (VI) is a popular technique to overcome impedance mismatches but the challenge is in the setting of the appropriate value. This paper presents an adaptive decentralized technique for adjusting the virtual impedance in the controller of a DG, based on its output current, without the need of communication, extra sensors or network parameter/load estimations. The methodology is tested through simulations for a wide variety of cases including unbalanced, harmonic, constant power and induction motor loads, DG plug and play functionality and performance in larger networks (modified 13 and 33 bus systems) under meshed configurations. The range for parameter stability is verified through modelling and small signal eigenvalue analyses. Further, improved power sharing performances using the proposed scheme is validated through implementation on a laboratory experimental setup.
Most of the existing reactive power sharing schemes that assume parallel architecture are known to be less effective for multi-bus radial microgrids. This paper proposes an improved reactive power sharing scheme that exploits the novel concept of droop equivalent impedance into designing a consensus virtual-output-impedance-based droop control scheme. The control scheme leads to two notable improvements: (a) it proves that only either virtual resistance or virtual reactance is sufficient to restore equal reactive power sharing; (b) only a global coupling gain needs to be tuned and no proportional-integral controller is required. A systematic guideline that establishes the approximate range of stable coupling gain is developed. This simplifies the tuning process of the coupling gain. The power correction performance, the resulting bus voltage behavior, consensus control stability, and the robustness to time delay have been investigated in conjunction with an islanded microgrid modified from the IEEE 34 Node Test Feeder. It is shown that the consensus control scheme is capable to demonstrate accurate power sharing regardless of the changes in the network topology, network impedances, loading conditions, and communication delay.
Traditional droop control strategy is lack of the decoupling ability, so the efficiency and stability of the microgrid based on the traditional droop control strategy is easily affected by the uncontrollable coupling of the distributed generation (DG) units. In this paper, an improved droop control method incorporating the coupling compensation and virtual impedance is proposed to improve the efficiency and stability of the microgrid. Its coupling compensation comprises the angular frequency deviation compensation and voltage deviation compensation, which decreases the influence of the uncontrollable coupling. And the virtual impedance can change the characteristics of the line impedance of the DG units and reduce their coupling. Therefore, the combination of the coupling compensation and virtual impedance can greatly reduce the power circulating-current of the microgrid and improve its dynamic performance. Then, an improved particle swarm optimization (IPSO) algorithm incorporating the multi-swarm and multi-velocity is taken to optimize the parameters of the coupling compensation with high accuracy and high reliability. Simulation and experimental results validate the high performance of the improved droop control strategy.
Load sharing and stability are two main issues for parallel operation of an inverter based distributed generators in microgrids. To address these issues, many novel control techniques like conventional droop control, generalized droop control, inverse droop control, transient droop control and virtual synchronous machines etc. are proposed in the literature. Majority of the work presented in the literature assumes a homogeneous mixture of these inverters i.e., same type of control in all the generators. However, integration of inverter based DGs manufactured by different vendors into the system operating on different droop laws makes the system more heterogeneous. Such systems are prone to have stability and transient issues. In this present work, stability and transient response of distributed generators characterized by various droop control techniques in an islanded microgrid with homogeneous and heterogeneous mix are evaluated through mathematical analysis, simulation and experiments.
Microgrids with increased penetration of renewables experience serious challenges due to large frequency excursions under power system disturbances. An energy storage system (ESS) can provide frequency regulation, but the effectiveness depends on whether it is configured for grid-forming or grid-following mode of operation. For this purpose, two critical parameters are studied for frequency regulation in distribution systems: (i) the initial rate-of-change-of-frequency (ROCOF), and (ii) the minimum value of frequency, also known as frequency nadir, following a load change. The aim of this work is to identify analytical methods for accurately calculating the frequency parameters like ROCOF and frequency nadir. Reduced-order models are developed to determine the frequency deviation against power system disturbances. The results are verified against simulation models validated by testing at the CERTS Microgrid test bed.
Proper modeling of inverter-based microgrids is crucial for accurate assessment of stability boundaries. It has been recently realized that the stability conditions for such microgrids are significantly different from those known for large- scale power systems. While detailed models are available, they are both computationally expensive and can not provide the insight into the instability mechanisms and factors. In this paper, a computationally efficient and accurate reduced-order model is proposed for modeling the inverter-based microgrids. The main factors affecting microgrid stability are analyzed using the developed reduced-order model and are shown to be unique for the microgrid-based network, which has no direct analogy to large-scale power systems. Particularly, it has been discovered that the stability limits for the conventional droop-based system (omega - P/V - Q) are determined by the ratio of inverter rating to network capacity, leading to a smaller stability region for microgrids with shorter lines. The theoretical derivation has been provided to verify the above investigation based on both the simplified and generalized network configurations. More impor- tantly, the proposed reduced-order model not only maintains the modeling accuracy but also enhances the computation efficiency. Finally, the results are verified with the detailed model via both frequency and time domain analyses.
Traditional droop-controlled system has assumed that generators can always generate the powers demanded from them. This is true with conventional sources, where fuel supplies are usually planned in advance. For renewable sources, it may also be possible if energy storage is available. Energy storage, usually as batteries, may however be expensive, depending on its planned capacity. Renewable sources are therefore sometimes installed as non-dispatch-able sources without storage. This may not be viable for remote grids, where renewable sources may be the only or major type of sources. In those cases, traditional droop scheme may not work well when its demanded power cannot be met by some renewable sources due to intermittency. When that happens, the system may become unstable with some sources progressively brought out of generation. To avoid such occurrence, an enhanced dual droop scheme is proposed for general two-stage converters with front rectifiers or dc-dc converters for conditioning powers from renewable sources and rear inverters for channeling powers to remote grids. Unlike the traditional droop scheme, the proposed dual droop scheme uses both dc-link voltage and generated powers for determining the required control actions, which have subsequently been proven stable by small-signal analysis. Experimental results have also verified the effectiveness of the dual droop scheme.
This paper analyzes the stability of a battery energy storage system (BESS) connected to the grid using a power-electronic interface. It is shown that the internal resistance and internal voltage of the battery affect system stability. Variations in these parameters may occur due to aging and changes in the state-of-charge (SoC). Using average-value modeling, this problem is framed into a nonlinear system formulation and the region of stability as a function of the internal resistance and the internal voltage of the battery is determined. This paper also extends its results in determining the configuration of a battery pack in terms of the number of battery cells in series and parallel to prevent instability while meeting demand power requirements. The findings are useful both in the design and operation stages of large-scale battery storage systems in the grid.
Accurate estimation of the state of charge in battery systems is of essential importance for battery system management. Due to nonlinearity, high sensitivity of the inverse mapping from external measurements, and measurement errors, SOC estimation has remained a challenging task. This is further compounded by the fact that battery characteristic model parameters change with time and operating conditions. This paper introduces an adaptive nonlinear observer design that compensates nonlinearity and achieves better estimation accuracy. A two-time-scale signal processing method is employed to attenuate the effects of measurement noises on SOC estimates. The results are further expanded to derive an integrated algorithm to identify model parameters and initial SOC jointly. Simulations were performed to illustrate the capability and utility of the algorithms. Experimental verifications are conducted on Li-ion battery packs of different capacities under different load profiles.
The enabling of ac microgrids in distribution networks allows delivering distributed power and providing grid support services during regular operation of the grid, as well as powering isolated islands in case of faults and contingencies, thus increasing the performance and reliability of the electrical system. The high penetration of distributed generators, linked to the grid through highly controllable power processors based on power electronics, together with the incorporation of electrical energy storage systems, communication technologies, and controllable loads, opens new horizons to the effective expansion of microgrid applications integrated into electrical power systems. This paper carries out an overview about microgrid structures and control techniques at different hierarchical levels. At the power converter level, a detailed analysis of the main operation modes and control structures for power converters belonging to microgrids is carried out, focusing mainly on grid-forming, grid-feeding, and grid-supporting configurations. This analysis is extended as well toward the hierarchical control scheme of microgrids, which, based on the primary, secondary, and tertiary control layer division, is devoted to minimize the operation cost, coordinating support services, meanwhile maximizing the reliability and the controllability of microgrids. Finally, the main grid services that microgrids can offer to the main network, as well as the future trends in the development of their operation and control for the next future, are presented and discussed.
Research roadmap on grid-forming inverters
- Y Lin
- J H Eto
- B B Johnson
- J D Flicker
- R H Lasseter
- H N Villegas Pico
- G.-S Seo
- B J Pierre
- A Ellis
Y. Lin, J. H. Eto, B. B. Johnson, J. D. Flicker, R. H. Lasseter, H. N.
Villegas Pico, G.-S. Seo, B. J. Pierre, and A. Ellis, "Research roadmap
on grid-forming inverters," National Renewable Energy Lab.(NREL),
Golden, CO (United States), Tech. Rep., 2020.
Ieee standard for interconnection and interoperability of distributed energy resources with associated electric power systems interfaces
- D G Photovoltaics
- E Storage
D. G. Photovoltaics and E. Storage, "Ieee standard for interconnection
and interoperability of distributed energy resources with associated
electric power systems interfaces," IEEE Std, pp. 1547-2018, 2018.
Power system stability
- P Kundur
P. Kundur, "Power system stability," Power system stability and control,
pp. 7-1, 2007.
Ieee standard for the specification of microgrid controllers
"Ieee standard for the specification of microgrid controllers," IEEE Std
2030.7-2017, pp. 1-43, 2018.
Real-time simulation and hardware-in-the-loop testing
- R Technologies
R. Technologies, "Real-time simulation and hardware-in-the-loop testing," https://www.rtds.com, [Online; accessed 06-February-2023].
Hardware-in-the-loop testing for microgrids
- Technologies