Traditional Synchronous Generators Research Articles

Stability assessment of inverter‐based renewable energy sources integrated to weak grids

AbstractThe worldwide electricity network is undergoing a crucial transformation, shifting from traditional synchronous generators to inverter‐based renewable energy sources (IRESs). This shift is expected to reduce the grid's available fault level (AFL), potentially impacting grid functionality, particularly during the integration of IRESs into weak grids. This paper examines the challenges associated with weak grids, focusing on the steady‐state and dynamic stability of IRES when integrated into these systems. In the steady‐state analysis, the effects of AFL, injected power volume, and grid characteristics on the steady‐state voltage at the point of interconnection were explored. For dynamic stability, eigenvalue and H2 norm analyses are used for evaluation. Sensitivity analysis is conducted to assess the impact of these factors on the stability of IRESs connected to weak grids. A detailed case study using the IEEE 39 bus test power system is included to demonstrate our findings, where the steady‐state and dynamic stability of IRES connected to the test system are assessed using the proposed methods. The accuracy of these analyses is confirmed by extensive simulation studies on the OPAL‐RT real‐time digital simulator platform.

A methodological assessment to determine the frequency response of utility-scale deloaded PV under variability

Replacing the traditional synchronous generators with photovoltaic systems (PV) causes an overall reduction in grid inertia. Further, as the newly installed PV plants do not require any rotational mechanism to generate active power, they are not also able to ramp the generation during contingency situations leading to frequency collapse of the grid. To deal with this issue, a portion of peak solar generation can be kept as deloaded reserve which can be released during emergency conditions emulating the conventional system. However, the allocation of deloading as a percentage of PV power can introduce randomness on the expected support as the dispatched PV power varies depending on the irradiance and temperature values. Therefore, it is essential to accurately estimate the frequency support of a given deloading percentage to maintain frequency stability considering PV variability. In this regard, this study presents a generalized approach to determine the frequency response of deloaded support under utility-scale PV integration. To achieve this, linear and quadratic regression are employed to establish a mathematical correlation using the data derived from dynamic simulations conducted on a modified IEEE 39 bus grid at various PV penetration levels. The obtained results indicate a minimum generation of PV power is required to prevent under-frequency load shedding for a fixed deloading percentage. These findings are later verified by comparing them with two other techniques - battery energy storage system and synchronous condenser based on the simulated results achieved from DIgSILENT PowerFactory software. Moreover, this paper provides insights into how this required minimum PV generation changes with the variation in sanctioned deloading percentage. This can aid the grid designers to allow proper deloaded operation depending on the environmental condition.

A metaheuristic algorithm for regulating virtual inertia of a standalone microgrid incorporating electric vehicles

AbstractModern electrical networks, particularly microgrids, have seen a sharp rise in acquiring non‐conventional sources. The inertia of the microgrid decreases drastically because RESs have been used in place of traditional synchronous generators. The reduced inertia negatively impacts the dynamics and performance of the microgrid with RESs, which decreases the microgrid's stability, especially in operation on an island. The primary purpose of this research study is to enhance the dynamic security of an island microgrid by merging an electric vehicle with a frequency control technique based on virtual inertia control. A Proportional Integral Derivative Filter Constant (PIDFN) controller optimally created using the Modified Differential Evolution (MDE) method served as the foundation for control in the virtual inertia control loop. The effectiveness of the MDE‐based PIDFN controller was examined considering the diverse operational scenarios are compared and contrasted with those of traditional methods Differential Evolution (DE) and Teaching Learning Based Optimization (TLBO)‐based PIDFN controllers. Real‐time wind and solar power statistics and random load fluctuations were incorporated to provide realistic simulation settings. The outcomes demonstrate that the MDE‐based PIDFN controller performs better in reference frequency tracking and reducing frequency disturbances than the other optimization strategies.

Assessment and management of frequency stability in low inertia renewable energy rich power grids

AbstractThe integration of the renewable energy sources (RESs) into the power grid, drives a significant transformation in the conventional power generation landscape. This transition from traditional synchronous generators to inverter based RESs introduces unique challenges in maintaining the grid frequency stability due to the reduced system inertia. The inherent stochastic nature of the RES power generation, load demand, and grid inertia includes further complexity in the assessment of frequency stability. Existing studies have limitations, including neglecting the stochastic nature of RES generation and load demand fluctuations, relying on limited metrics, and lacking a comprehensive day‐to‐day assessment. To address these shortcomings of the existing approaches, this paper introduces a novel methodology for assessing frequency stability in power grids with high RES penetration. It proposes three indices for evaluating grid frequency sensitivity, resiliency, and permissibility amidst varying RES integration. Utilizing a stochastic approach, the study incorporates uncertainties in RES generation and load demand, offering a comprehensive framework for day‐to‐day frequency stability analysis. Additionally, it presents a systematic method to ascertain the necessary inertial support for maintaining desired frequency reliability in RES‐dominated grids. The effectiveness of these methodologies is validated through a case study on a modified IEEE 39‐bus test system, demonstrating their applicability in ensuring reliable grid operation under high RES scenarios.

Grid Strength Impedance Metric: An Alternative to SCR for Evaluating System Strength in Converter Dominated Systems

Short circuit ratio is an outdated measure of system strength in converter dominated systems as the useful behaviour of inverter-based resources is not represented. While still useful for determining fault current provision, converter connected generation contributes to short circuit level and system strength in a different way to traditional synchronous generation. Additionally, traditional measures only account for system strength at the fundamental frequency while converters possess complex interactions across a frequency range. A novel method of determining system strength across this frequency range from the network impedances is proposed in this paper. The approach known as the Grid Strength Impedance Metric, calculates the system strength independently from short circuit level. The unique contribution of grid-following and grid-forming converters is represented by including the multiple-input multiple-output converter output impedances in the calculation. Small-signal impedance models are employed, and the metric is largely concerned with short-term voltage stability, but some inferences can be made for longer timescales. The approach is validated using time domain simulations to check for any identified areas of weakness in the frequency range. The Grid Strength Impedance Metric successfully represents the useful behaviour of grid-forming converters, indicating an enhancement in voltage stiffness in locations of deployment.

Control of grid-following converter-interfaced resources for improved transient stability of power systems

Power-electronic converters are increasingly utilized to integrate renewable and distributed energy resources (DERs) with conventional power grids. The increasing penetration of converter-interfaced resources (CIRs) could, in turn, influence the transient stability of traditional synchronous generators (SGs) in power systems due to their fast response and low inertia nature. This paper evaluates the transient stability of power systems containing SGs and grid-following CIRs under various control schemes and penetration of CIRs. The critical clearing time (CCT) of the SGs is used as the criterion for assessing transient stability following a fault. It is demonstrated that stability margins can be improved by actively controlling the grid-following CIRs during fault and post-fault periods. The proposed method is shown to increase the CCT compared to the conventional/alternative approaches, particularly the common practice that requires CIRs to inject only reactive power to support/regulate voltage during the fault period. It is also shown that with increased penetration of CIRs, if controlled appropriately, the transient stability of SGs can be improved, which is not commonly expected in low-inertia systems.

A Multivariable, Adaptive, Robust, Primary Control Enforcing Predetermined Dynamics of Interest in Islanded Microgrids Based on Grid-Forming Inverter-Based Resources

This paper proposes a multivariable, adaptive, robust (MAR) control strategy for islanded inverter-based resources (IBRs) operating as grid-forming inverters. The proposed method is employed in the inner control loop of the primary layer in the hierarchical or decentralized structures for the islanded operation of microgrids. The MAR control scheme is responsible for stabilizing IBRs’ output voltage in autonomous operations of microgrids, considering mismatched input voltage disturbances from the grid side and a large amount of system uncertainty. The control methodology introduced in this paper does not rely on the system’s physical parameters, such as microgrid topology, load dynamics, LCL filters, and output connectors. As a result, there is no need to know the nominal values or the bounds of uncertainties in system dynamics. The MAR control method uses online adaptation rules first to identify and then adjust the control parameters of the closed-loop system based on an arbitrary dynamic model. In other words, the MAR method replaces the actual dynamics of IBRs with predetermined dynamics of interest. Simulation results in the MATLAB/Simulink environment confirm the capability of the scheme introduced for the closed-loop stabilization and voltage regulation in the presence of disturbances and a significant amount of uncertainty under various case studies; moreover, comparative simulations by comparing the presented method with other studies using sliding mode control are provided. Finally, experiments verify the effectiveness and practicality of the proposed MAR control scheme. <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Note to Practitioners</i> —Inverter-based resources are integral parts of current and especially future power and energy systems; with increasing concerns about carbon footprints, the tendency to substitute traditional synchronous generators with inverter-based resources increases. This transition towards the widespread use of power electronics devices needs careful studies regarding the stability and control of power converters. Although existing studies are addressing potential control system challenges, they suffer from complex mathematical computations and the need for the system’s preliminary information. With this in mind, this study proposes a multivariable, adaptive, robust control strategy for the inner voltage control loop of grid-forming inverters. This method utilizes online estimation algorithms to identify inverter-based resources’ parameters and tune control system parameters simultaneously, making it applicable even to cases with slow parameter variations caused by aging or environmental changes. It can compensate for potential voltage disturbances from the grid side and enable the designer to replace undesirable dynamics of inverter-based resource units with arbitrary and stable dynamics of interest. In fact, unlike traditional methods that need control parameters and gains to be tuned to achieve a proper dynamic response, the control system designer can choose reference dynamics and enforce the closed-loop system to imitate the dynamical model selected. This model is usually chosen based on established priorities, such as response time and other transient behaviors. Moreover, this method does not require complex mathematical and algebraic calculations to design and implement. It can be easily applied to inverter-based resource units after selecting the desired reference dynamics, as shown through this study’s experimental result. The above points give this method a competitive edge over the existing algorithms, especially in practical applications.

Analysis of fast frequency control using battery energy storage systems in mitigating impact of photovoltaic penetration in Ethiopia–Kenya HVDC link

The high penetration of photovoltaic (PV) in power grids typically leads to the displacement of traditional synchronous generators (SGs). However, with a high penetration of PV, fewer SGs are running, and the sharing of responsibility to control the system frequency is reduced and easily exacerbates the problem of reduced inertia response in the power system. This can increase frequency deviation when there is a load-generation imbalance or unexpected disturbances. The limited amount of inertial response from the PV generation means that it cannot provide the same frequency support as SGs. Therefore, this paper suggests a fast frequency control (FFC) technique for the battery energy storage system (BESS) to reduce the instantaneous frequency deviation (IFD) in the Ethiopian grid. The authors specifically provide knowledge of the modeling of droop-type controlled BESS, which can provide additional damping, enhance the inertial ability of the system, and reduce IFD. The simulation results indicate the effectiveness of BESS in mitigating the adverse inertial impact of PV and meeting the necessary grid requirement. The proposed method was tested on the Ethiopian power system model through simulations by using DIgSILENT simulation software. To validate the model, transient frequency simulation studies are performed for various fault conditions, and comparisons are made with and without BESS for different levels of PV penetration.

Automated Data–Driven Model Extraction and Validation of Inverter Dynamics with Grid Support Function

This research focuses on the evolving dynamics of the power grid, where traditional synchronous generators are being replaced by non-synchronous power electronic converter (PEC)-interfaced renewable energy sources. The non-linear dynamics must be accurately modeled to ensure the stability of future converter-dominated power systems (CDPS). However, obtaining comprehensive dynamic models becomes more complex and computationally intensive as the system grows. This study proposes a scalable and automated data-driven partitioned modeling framework for CDPS dynamics. The method constructs reduced-ordered dynamic linear transfer function models using input-output measurements from a PEC switching model. Validation experiments were conducted on single-house and multi-house scenarios, demonstrating high accuracy (over 97%) and significant computational speed improvements (6.5 times faster) compared to comprehensive models. This framework and modeling approach offer valuable insights for efficient analysis of power system dynamics, aiding in planning, operation, and dispatch.

Influence mechanism and virtual power system stabiliser method of virtual synchronous generator for low‐frequency oscillation of power system

AbstractThe virtual synchronous generator (SG) (VSG) can not only enhance the inertia of the grid, but also introduce the oscillation characteristics of SG, which is easy to interact with the power angle of SG in the grid, and even produce low‐frequency oscillation (LFO). The authors first construct a two‐machine interconnected power system model containing VSG and traditional SG. The model is linearised to construct the state space equations to obtain the Phillips–Heffron model with VSG. The LFO path of action between VSG and SG is analysed. To reduce the negative damping torque provided by VSG to SG through this path, a virtual power system stabiliser (VPSS) is proposed and the controller parameters are adjusted according to the phase compensation method. Finally, the effectiveness of VPSS is verified by modal analysis and simulation comparison.

Open Source High Fidelity Modeling of a Type-5 Wind Turbine Drivetrain

The increasing integration of renewable energy resources in evolving bulk power system (BPS) is impacting the system inertia. Type-5 wind turbine generation has the potential to behave like a traditional synchronous generator and can help mitigate the impact on system inertia. A hydraulic torque converter (TC) and gearbox with torque limiting feature are integral parts of a Type-5 wind turbine unit. A high fidelity model of Type-5 wind turbine drivetrain is not openly and widely available for grid integration and transient stability studies. This hinders appropriate assessment of Type-5 wind power plant’s contribution to bulk grid resilience. This work develops and validates a TC model based on those generally used in automobile’s transmission system. Moreover, the concept of torsional coupling is leveraged to integrate the TC and gearbox system dynamics. The entire integrated model will be open sourced and publicly available for grid integration studies.

A recursive system identification inertia estimator for traditional and converter-interfaced generators

This paper proposes a method to estimate the inertia constant of synchronous generators (SGs) and the virtual inertia provided by converter-interfaced generators (CIGs). It can be applied when the system is under normal operating conditions or after power imbalance events. The algorithm is based on a recursive system identification approach and uses the active power and bus frequency measurements proceeding from phasor measurement units. The measurements are preprocessed to improve the estimation accuracy. Subsequently, a model-independent rotor speed estimator is used to compute rotor speed variations (or internal frequency variations, in the case of CIGs). Afterwards, a recursive least-square equation error formulation is implemented to compute the SG and CIG parameters in the z-domain, which are later converted into their respective counterparts in the s-domain. The method is applied to several generation devices in the IEEE 39-bus and IEEE 118-bus test systems, including traditional SGs and CIGs providing virtual inertia. Numerical results show the accuracy and the low computational burden of the proposed method.

Synchronization stability and multi-timescale analysis of renewable-dominated power systems.

Synchronization is one of the key issues in three-phase AC power systems. Its characteristics have been dramatically changed with the large-scale integration of power-electronic-based renewable energy, mainly including a permanent magnetic synchronous generator (PMSG) and a double-fed induction generator (DFIG) for wind energy and a photovoltaic (PV) generator for solar energy. In this paper, we review recent progresses on the synchronization stability and multi-timescale properties of the renewable-dominated power system (RDPS), from nodes and network perspectives. All PMSG, DFIG, and PV are studied. In the traditional synchronous generator (SG) dominated power system, its dynamics can be described by the differential-algebraic equations (DAEs), where the dynamic apparatuses are modeled by differential equations and the stationary networks are described by algebraic equations. Unlike the single electromechanical timescale and DAE description for the SG-dominated power system, the RDPS dynamics should be described by the multiscale dynamics of both nodes and networks. For three different timescales, including the AC current control, DC voltage control, and rotor electromechanical timescales, their corresponding models are well established. In addition, for the multiscale network dynamics, the dynamical network within the AC current control timescale, which should be described by differential equations, can also be simplified as algebraic equations. Thus, the RDPS dynamics can be put into a similar DAE diagram for each timescale to the traditional power system dynamics, with which most of power electrical engineers are familiar. It is also found that the phase-locked loop for synchronization plays a crucial role in the whole system dynamics. The differences in the synchronization and multiscale characteristics between the traditional power system and the RDPS are well uncovered and summarized. Therefore, the merit of this paper is to establish a basic physical picture for the stability mechanism in the RDPS, which still lacks systematic studies and is controversial in the field of electrical power engineering.

Assessment of traveling wave-based functions in inverter-based resource interconnecting lines

In this paper, the behavior of traveling waves (TWs) on transmission lines which interconnect inverter-based resources (IBRs) is investigated, assessing the performance of well-known TW functions applied in protection and fault location schemes. To do so, traditional synchronous generators and wind turbine-based IBRs of types III and IV are investigated, allowing comparative studies regarding the shape of fault-induced transients measured at the monitored line terminals. Then, the impacts of typical terminations of IBR-interconnecting lines on the classical double-ended TW-based fault location method, as well as on directional, overcurrent and differential TW-based protections are analyzed, highlighting the effects of busbar and transformer stray capacitances on the performance of these functions. The obtained results reveal that TW solutions are promising for IBR-interconnecting lines. However, a significant influence of busbar and transformer stray capacitances on the performance of TW functions is also identified, revealing the need for taking these capacitances into account during the definition of settings used in TW protection and fault location schemes.

Active Power Oscillation Suppression Based on Decentralized Transient Damping Control for Parallel Virtual Synchronous Generators

Similar to the traditional synchronous generator (SG), a drawback of the virtual synchronous generator (VSG) is the active power oscillation, especially in the system with parallel VSGs. In this paper, a decentralized transient damping (DTD) control strategy is proposed to suppress active power oscillation during load fluctuations in the parallel VSGs system, which is based on embedding the transient electromagnetic power compensation and transient damping power compensation (TDPCP) control into the original VSG active power controller, providing two new degrees of freedom for the flexible regulation of the system damping ratio without frequency measurement. In this way, the damping ratio of the parallel VSGs system can be adjusted more freely without affecting the steady-state characteristics of the system. Therefore, this flexible transient damping characteristic can facilitate the suppression of active power oscillation under load fluctuations. Moreover, the proposed method is immune to the parameter mismatch of the parallel VSGs. The design of key parameters and stability analysis are presented. Finally, the feasibility of the proposed control strategy is verified through the simulation and experimental results.

Distributed Power Sharing Control Based on Adaptive Virtual Impedance in Seaport Microgrids With Cold Ironing

In a seaport microgrid (SMG), power sharing among distributed generation (DG) units is hindered by power coupling, line changes as well as frequent power fluctuations caused by ship charging and discharging through cold ironing, which in turn threatens system stability and inverter security. This paper proposes a delay-tolerant distributed adaptive virtual impedance control strategy for assigned active and reactive power sharing, so as to suppress the circulating current among DGs. Firstly, the nonlinear impedance-power droop equations (IPDEs) are derived to actively update the resistive and inductive components of virtual impedance, which can accommodate changes in line structure and output power. Secondly, by means of low-bandwidth communication with quantized data and time delay, the desired power terms derived from practical consensus protocols are fed into the IPDEs, for which the proposed scheme gives a quantitative maximum tolerable communication delay with respect to power sharing accuracy. Thirdly, considering the voltage drop caused by virtual impedance, inspired by traditional synchronous generators, we design a virtual impedance loop based on virtual damping and inertia to preserve voltage dynamics. Finally, an SMG containing four DG units is simulated to verify the effectiveness of the proposed strategy on both active and reactive power sharing.

Two-Stage Grid-Connected Frequency Regulation Control Strategy Based on Photovoltaic Power Prediction

The large number of photovoltaics connected to the distribution network via power electronic converters squeezes the functional space of traditional synchronous generators in the power system and reduces the inertia of the network itself. However, due to the random and fluctuating nature of PV power generation, different types of meteorological conditions can also affect the inertia support capability of PV output power. Therefore, this paper proposes a frequency regulation control strategy based on the dynamic characteristics of the grid-side DC capacitor. Firstly, the control strategy of the grid-side inverter is improved and the mechanism of the frequency dynamic response model under PV penetration is analysed. Secondly, data from different weather types are correlated and analysed to predict PV power, and wireless sensor technology is used to introduce the data signals into the control link. Finally, a simulation model of a two-stage PV power generation system is developed and the fast-response capability of the system under different control parameters when the load is increased or decreased is analysed. The results show that under the proposed frequency regulation strategy, the larger the regulation coefficient and virtual inertia time constant, the greater the virtual inertia provided by the PV power generation, which improves the stability of the distribution network.

Inertia estimation of power system with new energy considering with high renewable penetrations

The emerging energy technologies, such as wind energy and photovoltaic (PV), will gradually replace the traditional synchronous generator in wide-area power system. As the wind, PV and energy storage equipment are all controlled by power electronic inverters, which are decoupled from the system and cannot provide effective inertial support to the power system with new energy (PS-NE), resulting in stability problems caused by the low inertia of the PS-NE. In particular, this paper investigates the inertia response of synchronous generator and PS-NE, and the inertia model of PS-NE considering with high renewable penetrations. Then the relation between inertia and frequency of PS-NE with high renewable penetrations are explored in this paper. Besides, the real-time inertia of the PS-NE is estimated by using the statistical algorithm based on the historical data of the PS-NE, and the range of the inertia and synchronous generator start-up capacity of the PS-NE can be estimated by using the statistical algorithm based on the renewable penetrations when we cannot obtain the historical data of the PS-NE. In addition, IEEE39 system of PS-NE is used to verify the relation between inertia and frequency of PS-NE with high renewable penetrations, and the inertia estimation of PS-NE with high renewable penetrations is analyzed in this paper.

A Rotor Ground Fault Protection Method Based on Injection Principle for Variable Speed Pumped Storage Generator-Motor

Three-phase AC excitation is adopted in the variable speed pumped storage generator-motors, which is different from the DC excitation in traditional synchronous generators. The special rotor winding structure makes the existing rotor ground fault protection methods no longer applicable. Therefore, a rotor ground fault protection method for variable speed pumped storage generator-motor is proposed in this paper. Considering that there is no precondition for additional installation of the injection device at the rotor winding neutral point, the rotor ground fault protection scheme is proposed by connecting the injection device through current limiting resistors outside the rotor winding terminals. The key parameters of injection source, band-pass filter, and current limiting resistance are analyzed and calculated. Then, the online calculation methods of grounding resistance and grounding current are proposed based on the injection equivalent circuit. On this basis, the composite protection criterion considering grounding resistance and grounding current is constructed, and the protection outlet mode is clarified. Simulation and experiments verify the effectiveness of the proposed method.

Voltage and frequency stabilization control strategy of virtual synchronous generator based on small signal model

Power system with large scale Inverter-based Resources (IBR) penetration is challenged with insufficient voltage and frequency support capacity. This paper restructures the control blocks of virtual synchronous generator (VSG) from the perspective of traditional synchronous generators’ control, the control path of the system over its active and reactive power output is clarified and simplified, which has facilitated the tuning of virtual inertia and damping parameters. Firstly, the small signal model of VSG control IBR is established, including instantaneous power calculation module, LC filter and power loop, voltage and current loops. In the meantime, the improved VSG voltage and frequency regulation control strategy is proposed based on the stability analysis of the small signal model. The stable operation range of the VSG control grid-connected inverter system is studied with the objective to improve the stability and robustness of the VSG-controlled grid-connected inverter system. Finally, MATLAB/Simulink simulation verifies the correctness of the proposed VSG’s dynamic model of power grid voltage and frequency regulation. The effectiveness of the restructures of VSG is verified as well. The results show that the control strategy not only improves the inertia of the system, but also enables the IBR to support the frequency and voltage of the system.