Inertia Frequency Research Articles

Coordinated frequency control strategy for modern power system considering engagement willingness

The large scale integration of renewable energy sources (RES) is posing unprecedented challenges to the frequency stability of modern power systems, with system inertia being the primary concern. Under such a landscape, this paper introduces a coordinated control strategy aiming at enhancing frequency stability by swiftly adjusting the power output of flexible resources to mitigate power imbalances and further dampen frequency fluctuations. Firstly, a communication-free center of inertia (COI) frequency estimation method is proposed. An equivalent inertia estimation method tailored for modern power systems is then developed by exploiting the correlation between synchronous inertia and active power fluctuations. By integrating the COI frequency variation rate with the estimated equivalent inertia, the imbalance power is calculated and compensated through the proposed coordinated control strategy. In this work, two typical flexible resources including photovoltaic (PV) generation and electric vehicles (EVs) are modeled and integrated into the testing system in PSCAD/EMTDC. Considering the contradiction between maximizing generation profit and reducing power output to mitigate frequency variation, the Corporate Social Responsibility (CSR) of renewable Generation Companies (GENCOs) is modeled to sketch their willingness of engaging into frequency support. Moreover, the frequency modulation potential of EVs in both commercial and residential areas are quantified for practical implementation. The simulations results successfully validate the effectiveness of the proposed control strategy.

Oscillations-aware frequency security assessment via efficient worst-case frequency nadir computation

Frequency security assessment following major disturbances has long been one of the central tasks in power system operations. The standard approach is to study the center of inertia frequency, an aggregate signal for an entire system, to avoid analyzing the frequency signal at individual buses. However, as the amount of low-inertia renewable resources in a grid increases, the center of inertia frequency is becoming too coarse to provide reliable frequency security assessment. In this paper, we propose an efficient algorithm to determine the worst-case frequency nadir across all buses for bounded power disturbances, as well as identify the power disturbances leading to that severest scenario. The proposed algorithm allows oscillations-aware frequency security assessment without conducting exhaustive simulations and intractable analysis.

Sizing method of a novel hybrid energy storage considering adaptive inertia control

This paper introduces a novel hybrid energy storage system (HESS) with a focus on adaptive inertia control and its sizing methodology. The HESS is built upon the modular multilevel converter (MMC) topology, incorporating hybrid submodules, batteries, and supercapacitors (SC). The sizing method for the hybrid energy storage system in the proposed HESS is discussed, with a specific emphasis on its application in frequency response scenarios. The center of inertia frequency (COIF) is selected to represent the overall system's frequency characteristic. By examining COIF deviation and the Rate of Change of Frequency (ROCOF) during contingency events, the inertia and damping parameters of the HESS are designed. Additionally, an adaptive inertia approach is incorporated to further reduce the SC capacity. The analysis of COIF dynamics under contingency with adaptive inertia is performed, and an analytical expression of COIF dynamics is derived. Subsequently, a sizing method for the SC and battery in the HESS, considering adaptive inertia, is proposed. The effectiveness of the proposed sizing method is verified through simulation results, demonstrating its suitability for optimizing the design of the hybrid energy storage system in the novel HESS configuration.

Recovery time and strategy of DFIG speed based on a high-proportional hydropower grid

Large-scale wind power access reduces the equivalent inertia of a power system. The wind turbine has a considerable reserve of rotational inertia, which is important for reducing the risk of low-inertia operation in a grid and for improving frequency reliability. In this work, a doubly fed induction generator (DFIG) inertia frequency regulation strategy is studied. The secondary dip in frequency caused by speed recovery after the wind turbine participates in frequency regulation is an important factor that limits regulation capability on the DFIG. To address such problem, the power dip and recovery generated by the DFIG after active speed recovery is analyzed and modeled. Then, a linearized frequency response model of a hydropower single machine is established to investigate the secondary disturbance effect of wind turbine active speed recovery moment on the frequency of the high-ratio hydropower system.A conclusion is drawn that the moment that corresponds to the first recovery of system frequency to the quasi-steady-state frequency is the optimal recovery moment of the turbine in the linear SFR model. Combined with the frequency control characteristics of the frequency limit control(FLC)’s “large dead zone, ” an active speed recovery strategy of the turbine based on frequency deviation judgment is designed.

Optimal Model Predictive Control for Virtual Inertia Control of Autonomous Microgrids

For the time being, renewable energy source (RES) penetration has significantly increased in power networks, particularly in microgrids. The overall system inertia is dramatically decreased by replacing traditional synchronous machines with RES. This negatively affects the microgrid dynamics under uncertainties, lowering the microgrid frequency stability, specifically in the islanded mode of operation. Therefore, this work aims to enhance the islanded microgrid frequency resilience using the virtual inertia frequency control concept. Additionally, optimal model predictive control (MPC) is employed in the virtual inertial control model. The optimum design of the MPC is attained using an optimization algorithm, the African Vultures Optimization Algorithm (AVOA). To certify the efficacy of the proposed controller, the AVOA-based MPC is compared with a conventional proportional–integral (PI) controller that is optimally designed using various optimization techniques. The actual data of RES is utilized, and a random load power pattern is applied to achieve practical simulation outcomes. Additionally, the microgrid paradigm contains battery energy storage (BES) units for enhancing the islanded microgrid transient stability. The simulation findings show the effectiveness of AVOA-based MPC in improving the microgrid frequency resilience. Furthermore, the results secure the role of BES in improving transient responses in the time domain simulations. The simulation outcomes are obtained using MATLAB software.

A Stability Result for Network Reduced Power Systems Using Virtual Friction and Inertia

We prove that virtual friction can stabilize a power grid containing several virtual synchronous machines (VSMs), connecting line impedances and loads. Virtual friction is a torque added to the swing equation of each VSM, proportional to the deviation of its frequency from the overall center of inertia (COI) frequency. Our analysis is based on the network reduced power system (NRPS) model. We support our results with simulations for a two-area network of four VSMs, looking at the transients induced by a change of tie-line impedance and an asymmetric load change. We compare the results for the NRPS model with the corresponding results using detailed models of synchronverters and line impedances. We find that virtual friction has a strong stabilizing effect both for the NRPS model and for the detailed model. Furthermore we show simulation results on the influence of time delays in the communication between the inverters. This communication is used to compute the COI frequency. It turns out that communication delays of less than 100ms have practically no effect on the overall system.

Study on Improving Fault Stability of Doubly Fed Induction Wind Turbine by Using Active-Power Transient Frequency Characteristics

With the continuous increase in wind power penetration, doubly fed wind turbines can quickly respond to changes in grid frequency, and have particularly important inertia-response characteristics. This article starts with the excitation control principle of a doubly fed induction generator, compares the transient frequency characteristics of the synchronous generator under fault, and proposes that the doubly fed induction generator can control the speed or active power of the generator through excitation. It proves the unique “active power transient frequency characteristics” of doubly fed induction generators. Under different wind speeds, the inertia response capability of the wind turbine is quantified, and the degrees of influence of the inertia time constant and frequency characteristic slope of the doubly fed induction generator on the transient change process are analyzed. Finally, simulation and experiments verify the correctness of the above theory, which provides a basis for significantly improving the transient stability of the power system and realizing the controllability of the transient stability of the power system.

Review of Grid Codes and Implementation for Wind Farm Frequency Regulation

With the increase of wind power and the construction of UHVDC transmission, the wind turbines must actively participate in the system frequency regulation through inertia response and primary frequency regulation to ensure the safe and stable operation of the power system. However, the wind power penetration rate and the grid structure of each regional power grid are different, so that the corresponding power grid codes are also slightly different. Firstly, this paper introduces the specific requirements of wind power grid guidelines in typical countries and regions, and with reference to the experience of international frequency regulation codes, it puts forward the proposed implementation mode of active support new energy frequency regulation suitable for domestic power grid, and the recommended parameter for wind farm frequency regulation. Secondly, this paper summarizes the frequency modulation implementation schemes of the main wind turbine manufacturers in the world, and demonstrates the potential frequency modulation capability of the wind turbine. Finally, the challenges and realizable solutions of virtual inertia and primary frequency regulation of wind farms are prospected, and some suggestions are given.

Power system inertia estimation method based on maximum frequency deviation

In order to solve the problem that the inertia estimation method based on the rate of change of frequency (RoCoF) is only applicable to the centre of inertia (COI) frequency, a novel inertia estimation method is proposed based on the maximum frequency deviation. This method utilizes the property that the maximum frequency deviation of each generator is basically consistent and easy to obtain. Firstly, the inertia support power of the system is quantified according to the disturbance power and frequency, and then the inertia support energy of the disturbance process is obtained. Finally, the system inertia is estimated by the ratio of the inertia support energy to the maximum frequency deviation. The proposed estimation method is suitable for non-COI frequency, which makes it unnecessary to obtain the COI frequency and RoCoF. On this basis, a simplified estimation method is proposed based on the linearization of regulation power, which improves the estimation accuracy and simplifies the estimation process. According to the proposed method, the governor test data is used to estimate, which increases the estimation times and provides more information for system stable operation. The effectiveness and accuracy of the proposed method are validated by some cases.

Center of Inertia Frequency Estimation Using Deep Learning Algorithm

Abstract Increasing the number of generation units connected to the grid via power electronic devices potentially implies negative impacts on the power system frequency stability and, depending on the power system inertia value, implies the necessary contribution of wind power plants to inertial response of the system. An alternative approach to the active power control of wind power plants, without the impact of local frequency deviation on the output power, is the application of a control strategies based on the center of inertia frequency. Since control schemes based on the input variable of the center of inertia frequency require a satisfactory level of signal transmission capacity in real time and the advanced telecommunication infrastructure of the power system, the paper considers an alternative approach to estimate the input signal value. According to the developed long short-term memory recurrent neural network, paper presents the idea of center of inertia frequency estimation by monitoring the speed of several generators in the system and passing the sequence of input data for a certain time interval, after the occurrence of imbalance, to the artificial intelligence module.

Inertia and primary frequency modulation strategy for a doubly fed induction generator based on supercapacitor energy storage control

ABSTRACT Doubly fed wind turbines cannot respond to changes in grid frequency under maximum power point tracking control. The traditional deloading frequency control suffers from problems, such as low power generation efficiency, small speed adjustment range, and frequent starting of pitch angle control. An inertia and primary frequency modulation (FM) strategy for a doubly fed wind turbine based on supercapacitor energy storage control is proposed in this study. Virtual inertia and primary frequency adjustments are realized by supercapacitor control. Changing or increasing the additional control of the wind turbine is unnecessary. The proposed strategy can improve the disturbance resistance and self-stabilization of a single wind turbine. The capacity of the energy storage unit is optimally configured in accordance with the cost and charging/discharging efficiency of the actual supercapacitor module. Comparing and evaluating the economics of the primary FM scheme of an active power reserve provide a powerful economic advantage. Simulations and experiments show that inertial support, primary frequency adjustment capability, and power generation benefits are significantly improved compared with those in the conventional primary frequency control. This study provides new ideas for the modified inertia and primary FM function of a doubly fed wind turbine.

A new approach for system‐wide power system frequency model validation via measurement data

AbstractModels are commonly used for various power system studies. This article presents a new framework to estimate and validate the parameters of the turbine‐governor and also the load model in a multi‐machine power system environment. The proposed method utilizes the center of inertia frequency and generating unit signals (especially active power) recorded by measurement devices to validate the parameters. The objective function is to minimize the total squared error between the simulated and the measured responses for some events. Particle swarm optimization is used to estimate the parameters. The accuracy of the estimated parameters is investigated through sensitivity analysis. Moreover, in order to improve the efficiency of the identification procedure for large‐scale networks, a new scheme is proposed, which is based on the impact of each estimated parameter during the simulation time frames. The simulation results on the IEEE 9‐bus and IEEE 118‐bus systems show the validity and benefit of the proposed method.

Operation and Control of a Hybrid Power Plant with the Capability of Grid Services Provision

The integration of distributed power plants that rely on renewable energy sources (RESs) is a major challenge for system operators (SOs) due to the variable nature of the input energy (e.g., wind and solar irradiation) to these power sources. A key solution to such a challenge is to coordinate and combine the power generation of these sources such that their behavior is closer to a conventional and dispatchable power station, taking into account the limitations imposed by the battery storage system (BESS), so it is seen as a hybrid power plant (HPP) from the SOs’ viewpoint. This paper develops a model of HPP that encompasses two generation technologies, wind and photovoltaic farms, which are assisted by a BESS. The paper proposes a comprehensive control method that can smooth the HPP output with minimized energy rejection whilst enabling the HPP to provide synthetic inertia and primary frequency response, which are grid-code compliant. The proposed control method is validated through various scenarios, which are implemented on a detailed electromechanical test system modeled in MATLAB/Simulink. The results show and quantify the achieved improvement on stabilizing the HPP capacity factor under variable wind speed. The HPP also enhances the system response to frequency events.

Wind Inertial Response Based on the Center of Inertia Frequency of a Control Area

As a result of the increased integration of power converter-connected variable speed wind generators (VSWG), which do not provide rotational inertia, concerns about the frequency stability of interconnected power systems permanently arise. If the inertia of a power system is insufficient, wind power plants’ participation in the inertial response should be required. A trendy solution for the frequency stability improvement in low inertia systems is based on utilizing so-called “synthetic” or “virtual” inertia from modern VSWG. This paper presents a control scheme for the virtual inertia response of wind power plants based on the center of inertia (COI) frequency of a control area. The PSS/E user written wind inertial controller based on COI frequency is developed using FORTRAN. The efficiency of the controller is tested and applied to the real interconnected power system of Southeast Europe. The performed simulations show certain conceptual advantages of the proposed controller in comparison to traditional schemes that use the local frequency to trigger the wind inertial response. The frequency response metrics, COI frequency calculation and graphical plots are obtained using Python.

Inertia Emulation Uncorrelated With Electromechanical Dynamics to Improve Frequency Transients Using Center of Inertia (COI) Frequency Signal

This paper proposes an inertia emulation method that uses the frequency of the center of inertia (COI) as the feedback signal, aiming to slow the frequency change rate and raise the frequency nadir during frequency-dropping events. An approximated model depicting the COI frequency dynamics is first introduced, which shows the COI frequency signal has low observabilities of electromechanical modes. Thus, this feature enables the proposed inertia emulation to react quickly to steep frequency changes without suffering any adverse impacts caused by electromechanical dynamics, such as excessively saturating the controlled objects. Moreover, the electromechanical dynamics associated with small-signal stability properties will not be interrupted by the proposed inertia emulation. In addition, a simple yet effective computing procedure is proposed to configure the critical parameters, as the inertia emulation is implemented with multiple flywheel-based energy storage systems. Simulation results obtained based on two large interconnected systems verify the effectiveness of the proposed inertia emulation in terms of improving the frequency transients as well as its limited correlation with the electromechanical dynamics of the system. The signal transmission latency's impacts on the proposed inertia emulation are also investigated and discussed.

Adjustable inertia implemented by bidirectional power converter in hybrid AC/DC microgrid

Hybrid AC/DC microgrid is regarded as a low inertia system due to the extensive integration of renewable energy sources. Thus, the AC bus frequency and DC bus voltage are easily damaged when a step change or random fluctuation appears, which threatens the system power quality. Focusing on this problem, this study proposes an adjustable inertia implementing method for the two-stage bidirectional power converter (BPC) to enhance the dynamic performance of the hybrid power system. Concretely, the first stage of the BPC is controlled as a virtual synchronous generator to support the AC subgrid and plays the role of imitating the inertia frequency response in the AC bus. The second stage controls the built-in capacitor of the BPC to implement adjustable inertia for the DC subgrid and improves the DC bus voltage response. Besides, the terminal voltage of the built-in capacitor will be restricted to ensure the normal operation of the BPC. The small-signal analysis of the hybrid system is provided to illustrate the stability of the proposed control method. Finally, the experimental results based on the real-time hardware-in-loop platform demonstrate the effectiveness of the proposed method.

KPI-based real-time situational awareness for power systems with high proportion of renewable energy sources

With the increasing complexity of power systems and the widespread penetration of renewable energy sources (RES), real-time situational awareness for power systems is of great significance for operation scheduling. With the impact of RES on power system operation considered, a situational awareness key performance index (KPI) system for power systems with high proportion of RES is proposed in this work, which consists of reserve capacity abundance, ramp resource abundance, center of inertia (COI) frequency deviation, interface power flow margin, synthesized voltage stability, and angle stability margin. Then, the KPIs are synthesized and visualized by decision tree method and radar chart method, respectively, for monitoring the operation states (i.e., normal, alert, and emergency states) of power systems with high proportion of RES. Numerical simulations are conducted in the revised New England 16-machine 68-bus power system and the actual CEPRI-RE power system in the northwest region of China with high proportion of RES. The results show that the proposed KPI-based situational awareness method is able to monitor the real-time state of power systems with high proportion of RES accurately, and can assist power dispatchers to make effective decisions.

Control Methods for Standalone and Grid Connected Micro-Hydro Power Plants With Synthetic Inertia Frequency Support: A Comprehensive Review

The penetration of renewable energy sources on a large scale in conventional electric networks increases exponentially for environmental preservation. However, the increase in RESs leads to some technical problems that include (a) fluctuations in power production, (b) less or no generation units for power balancing, (c) reduced inertia due to RESs decoupling from the conventional grid using converters. Micro-hydro power plants (MHPPs) are emerging as a mature balancing technology and a great alternative to large hydropower plants as they encounter population displacement and many environmental problems. Modern pump storage MHPPs uses a power converter between the grid and machine to control the consumption of power during pumping mode. This power electronic interface loses the ability of the rotating mass of the machine to contribute to the grid inertia by making it independent of the grid frequency. This problem is solved through the control of power converters in such a way that the inertia effect is synthesized termed as synthetic inertia. MHPPs can also be operated in a standalone mode where they are not connected to the grid. This paper reviews control schemes applied in literature for the frequency, voltage, and inertia control, in both the grid-connected and standalone modes. This study starts with the standalone MHPPs, covering the literature review and control structure for voltage and frequency control of standalone MHPPs. Then it presents a detailed operation, control principle, synthetic inertia concepts, and architecture of grid-connected MHPPs. The mathematical formulation of the grid, permanent magnet synchronous generator, synthetic inertia inclusion, and control structures, including direct torque control, virtual synchronous machine, and model predictive control, is also presented. Finally, the paper presents the concluding remarks with the comparative analysis of various control structures to include synthetic inertia, its suitability, and future scope for MHPPs.

Design of nonlinear coordinate damping controller for HVDC and SVC based on synergetic control

This article presents a novel nonlinear coordinate damping controller (NCDC) for high‐voltage direct current (HVDC) and static var compensator (SVC) based on synergetic control theory, which can improve the stability of interarea power system and multi‐infeed HVDC system by quickly changing the transmission power of HVDC and reactive power of SVC. The traditional control design of HVDC and SVC can improve the stability, but there may be a negative interaction between the controllers of HVDC and HVDC or HVDC and SVC. Furthermore, the power system is a strong nonlinear system, and the controller designed by a linearized model may reduce its performance. To solve these problems, this article first constructs a unified macro variable and manifold, which contains the model of HVDC and SVC. The manifold sets the deviation value of the center of inertia (COI) frequency of the two systems connected by HVDC to zero. Then, a nonlinear HVDC and SVC coordinated controller is derived based on the synergetic control theory. The global stability of the controller is proven by the Lyapunov theorem. Finally, the simulation results in a two‐area power system and a multi‐infeed system using the power systems computer aided design (PSCAD) show the effectiveness and superiority of the proposed controller. © 2019 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

New Transient Stability and LVRT Improvement of Multi-VSG Grids Using the Frequency of the Center of Inertia

This paper examines a completely novel method for improving the transient stability of multi virtual synchronous generator (VSG) grids. In this method, it has been tried to control the oscillation of VSGs with respect to the center of inertia frequency (COI-frequency) of the grid, during short-circuit. It is also well demonstrated here that the proposed method is able to prevent the increase in irreversible distance angle of VSGs relative to each other, which is the main factor in preventing VSGs reaching to instability point. The method can enhance both the synchronizing and damping terms efficiently. Detailed analyzing of the direct Lyapunov method illustrates the non-linear proof of the method. Also, non-linear algebraic equations for a two-VSG system have been derived to prove the possibility of corroborating simulation results. Equal area criterion is also used to clarify this method further. One interesting feature of the method is that there is no conflict between the requirements of the low voltage ride-through and transient stability improvement.