Frequency Stability Of Power System Research Articles

Optimising power system frequency stability using virtual inertia from inverter‐based renewable energy generation

The objective of this study is to optimise the use of virtual inertia given a virtual inertia budget and a network topology. Following a brief investigation into inertia distribution, it was observed that distributed inertia is more effective than centralised inertia for a large and sparse power system network. A subsection of the South African transmission network, the Western Transmission network, is expected to host large-scale inverter-based generation. The total power system inertia is in decline due to the large-scale integration of inverter-based renewable energy generation. In the literature, mitigation methods such as virtual inertia and ‘noise-cancelling’ network topology design has been proposed to improve frequency stability. In this study, the Western Transmission network is modelled as a network of coupled oscillators. A state-space model for the power system network is developed with virtual inertia as a feedback control loop. The H 2 -norm is used to design optimal feedback control through virtual inertia allocation/distribution, to increase power system frequency stability. The results show that the optimal distribution significantly improves system frequency stability by decreasing the rate of change of frequency and delaying the time to reach similar frequency nadir values.

Adaptive Robust SMC-Based AGC Auxiliary Service Control for ESS-Integrated PV/Wind Station

Power source structure has developed significantly because of the increasing share of renewable energy sources (RESs) in the power system. RESs bring inevitable impacts on power system frequency, voltage regulation, and power system stability. The conventional automatic generation control (AGC) loops which relay only on the synchronous generating units cannot meet the requirements of these new circumstances. This paper presents an ESS-integrated PV/wind station topology and its control structure for AGC auxiliary service in order to provide existing RESs the additional functionality of AGC auxiliary service without changing their control strategies conceived for MPPT mode. The shifting operation modes and external disturbances make ESSs in an ESS-integrated PV/wind station inherently nonlinear and time variable. Therefore, an adaptive robust sliding-mode control (ARSMC) system is proposed. The ARSMC colligates the advantages of adaptive control and SMC contains state feedback term, robust control term, and adaptive compensation term. The strictly logical and rigorous proof using Lyapunov stability analysis indicates the ARSMC system is insensitive to parametric uncertainties and external disturbances; meanwhile, it guarantees fast response speed and high control precision. The case studies on NI-PXI platform validate the effectiveness of the proposed approach.

Adaptive virtual-inertia control and chicken swarm optimizer for frequency stability in power-grids penetrated by renewable energy sources

In this article, a control scheme based on chicken swarm optimizer (CSO) in cooperation with adaptive virtual-inertia control (AVIC) is investigated. The proposed control scheme aims at improving the frequency stability of an interconnected power system which is penetrated by renewable energy sources. The CSO is applied to produce the best values of the gains of the adapted standard proportional-integral-derivative (PID) controllers and required parameters of AVICs. Various scenarios are addressed in this study such as applications of sudden step load disturbances and severe variations in the inertia of the system. In addition, realistic conditions such as uncertainties of tidal power source and random load disturbances are demonstrated. Compulsory assessments with subsequent discussions to evaluate the results of the CSO are made. The proposed CSO–AVIC based control method is verified by comparisons with well-matured interesting algorithms such as differential evolution and particle swarm optimizers. Various quality specifications of the dynamic responses and the demonstrated results indicate clearly the viability of the proposed CSO–AVIC based on control scheme. It can be emphasized that the utilization of AVIC along with PID controllers are significantly improved the system dynamic performances and their dynamic response specifications meet the terms of standard acceptable criteria’s.

Research on interconnection power system Construction of traditional energy Control Structure

With the increasing proportion of new energy in the power system energy. It has become an inevitable trend for new energy grid connected to participate in power system frequency modulation. As the representative energy of new energy, wind power has become the research focus of frequency modulation with traditional energy in recent years. In order to realize the frequency stability of power system, the load frequency control technology is used for power system frequency modulation at this stage. That is to say, in the case of any load disturbance, the power system can realize the load disturbance following the random change of power generation, so as to maintain the frequency deviation within a allowable range (±0.2Hz). Based on this purpose, this design starts from the load frequency control model of conventional units, and first does not consider the role of wind power to build the control structure of traditional energy. In order to facilitate the study of the design of traditional energy units are selected thermal power units. Starting from the establishment of power system frequency response model, we can understand the composition of traditional energy power system frequency response model and the structural characteristics of multi regional interconnected power system. The characteristics of load frequency control module of governor, turbine, generator, load and other basic conventional units are analyzed, and the related transfer function is listed. After the completion of the basic module, the mathematical model of traditional energy single area and traditional energy two areas is built. At the same time, when considering the construction of two regional interconnected thermal power units, we should also pay attention to the characteristic analysis of tie line power and regional control deviation.

Impact of Renewable Energy Sources on Inertia and Frequency Response of Power Systems

With its extensive and growing use in power systems, renewable energy sources (RES) affect power system dynamics and stability. This paper presents the impact of renewable energy sources on power system dynamics, especially through their effect on reducing the system inertia. Thus, affecting power system frequency response and stability. An analysis of the effect of inertia on system frequency dynamics is presented. Also, the possibility of determining the system inertia through online measurements is discussed and illustrated with a double-bus case study. Finally, some suggested solutions for enhancing power system dynamics with high penetration levels of renewables are presented

Grid Inertia Support Enabled by Smart Loads

The increasing penetration of renewable energy resources (RESs) leads to a continuous displacement of synchronous generators. Since the grid integration of RESs is usually achieved through inertialess power converters, the system inertia reduces and the power system frequency stability is compromised. Consequently, unexpected generator tripping, load shedding, and even blackouts may occur and pose a great threat to the system normal operation. In view of these challenges, this article explores the possibility of providing the inertia support from power-electronics interfaced smart loads. Specifically, a transformerless electric spring topology and the associated control strategy have been developed to provide a substantial inertia support from the noncritical load. As compared with existing inertia enhancement techniques, the proposed solution is cost-effective without the need of additional energy storage units. The smart load inertia provision capability is quantitatively analyzed and experimentally verified on a laboratory-scale test. In the testing results, 40% and 18% reductions of the rate of change of frequency and the maximum frequency deviation are, respectively, observed with a 12% smart load penetration ratio.

Application of Bang–Bang Controller to Emulate Primary Frequency Response in DFIGs

This article proposes a bang–bang auxiliary frequency control scheme for doubly-fed induction generator (DFIG)-based wind turbines. It results in the DFIGs emulate the primary frequency control of conventional units and, thus, increases the power system frequency stability. Indeed, the suggested hysteresis controller causes to deliver the maximum extractable kinetic energy of DFIGs to the grid and consequently further enhance the power system frequency. This is accomplished using different frequency deviation indices, which determine the size of power mismatch and lead to an appropriate frequency response by the DFIGs. The operational constraints of the DFIGs are taken into account in the design procedure of the controller. The proposed controller implies negligible implementation cost and its parameters must be chosen considering a compromise between the amount of DFIGs’ provided power during the frequency support period and the subsequent wear and tear due to the frequency response. Extensive time domain simulations on an IEEE 9-bus test system verify the efficiency of proposed controller.

Frequency stability and under frequency load shedding of the Southern Sulawesi power system with integration of wind power plants

The Government of Indonesia is encouraging investments in renewable energy based power plants in Indonesia, including wind power plants (WPPs). Two large WPPs in the Southern Sulawesi interconnected power system are Sidrap WPPs and Jeneponto WPPs. Both Sidrap WPP and Jeneponto WPP are the largest WPPs in Indonesia and they account for significant contribution for the Southern Sulawesi power generation mix. Considering the intermittency characteristics of WPPs and system’s failure probability, therefore it is essential to assess the system’s stability after their integration. This work evaluates the frequency stability of the Southern Sulawesi interconnected power system with the integration of both Sidrap WPPs and Jeneponto WPPs.

Implementation of Grasshopper optimization algorithm based cascaded fuzzy PD-PI controller for frequency stability in a multi-area power system

This article proposed a Grasshopper optimization algorithm (GOA) based cascade controller i.e. fuzzy-PD with PI controller (FPD-PI) for the study of load frequency control. A three unequal-area interconnected power system employed with thermal units has been considered for the present study. Initially, the performance of the GOA technique is verified over the Ant-lion optimization technique with integral square error (ISE) as an objective function. Finally, the supremacy of the proposed FPD-PI controller is verified over other benchmark controllers namely PID and fuzzy-PID with GOA, after ensuring the superiority of integral time multiplied absolute error (ITAE) over ISE.

Performance Tuning for Power Electronic Interfaces Under VSG Control

Renewable generation, such as solar PV and wind power, is commonly integrated into the power grid through inertialess power electronic interfaces (PEIs). Due to the increasing penetration of renewable generation, the frequency stability of the current power system deteriorates. In order to sustain the desired level of the overall inertia, the virtual synchronous generator (VSG) algorithm has been proposed. The concept of VSG is to enable the PEIs to emulate the external properties of traditional synchronous generators (SGs), such as inertia and primary frequency responses. By exploitation of the well-established knowledge system of conventional SG-based power grids, the VSG can also be implemented with the capabilities of primary, secondary, and tertiary frequency control in multiple temporal stages. This paper focuses on parameter tuning for VSG-PEIs by performance indices. The emulation strategies are completed with the capability of secondary and tertiary frequency regulation. The transfer functions of the dynamic systems of PEIs are simplified and referred to the control theory. The composite influences of different parameters on performance indices are analyzed. The methods of the parameter tuning are proposed according to the temporal sequences of the control stages. By typical performance standards, the proposed method is verified through simulation.

Frequency Stability of Power System with Large Share of Wind Power Under Storm Conditions

The operation of transmission systems with large share of wind power is specially challenging under storm conditions. Under the stormy wind speed conditions, wind turbine protection system is designed to shut down the turbine to avoid excessive mechanical load. The sudden loss of wind power from large offshore plants is difficult to forecast accurately, which results in a large amount of power imbalance. The severity of such a wind power imbalance towards frequency stability needs to be studied for the future power systems. In addition, the overhead transmission lines can also be affected during storms, thereby increasing their probability of failure in the operation of power system under the islanded conditions. This paper investigates how the stormy weather can threaten the frequency stability of future Danish power system with large share of wind power and how to avoid the frequency instability through proper control and defence strategies such as high-voltage direct current (HVDC) control and load shedding. Sensitivity studies are performed for ramp rates of HVDC control, load shedding strategies, inertia of the system with different volumes of disturbances to understand their impact on frequency stability.

An Efficient Control Strategy for Enhancing Frequency Stability of Multi-Area Power System Considering High Wind Energy Penetration

This paper proposes an efficient control strategy to enhance frequency stability of three-area power system considering a high penetration level of wind energy. The proposed strategy is based on a combination of a Proportional Integral Derivative (PID) controller with a Linear Quadratic Gaussian (LQG) approach. The parameters of the proposed controller (i.e., PID-LQG) are optimally designed by a novel natural physical based-algorithm called Lightning Attachment Procedure Optimization (LAPO). The main objective is to keep the frequency fluctuation at its acceptable value in the presence of high penetration of wind energy, high load disturbance and system uncertainties. The superiority of the proposed PID-LQG controller is validated by comparing its performance with optimal Coefficient Diagram Method (CDM) controller, conventional CDM controller, optimal PID controller-based LAPO, and integral controller. Moreover, the exhaustive results completely demonstrate that the proposed controller gives better performance in terms of overshoot, undershoot, and settling time as well as provides reliable frequency stability for interconnected power systems considering high wind penetration and system uncertainties.

Coordinated False Data Injection Attacks in AGC System and Its Countermeasure

The Automatic Generation Control (AGC) system is vital for power system frequency stability. The frequency and tie-line power flow are measured and transmitted to the control room to form Area Control Error (ACE), which is then sent to each generator for power generation adjustment. Due to the vulnerability of the Inter-Control Center Communication (ICCP) protocol, which is used for data transmission, many attacks such as Denial of Service, timing de-synchronization, and False Data Injection (FDI) attack can be inflicted upon the compromised system. In this paper, we investigated the attacking mechanism and the impact of the coordinated FDI attack. Compared with the single attack model, the coordinated FDI attack has a smaller Time to Emergency (TTE) value and wider parameter ranges. Therefore, it is stealthier and more harmful to the AGC system. However, it is found that the pattern of the corrupted ACEs (attacked by a specific coordination FDI attack) follows a specific fashion. Therefore, we proposed a self-learning and evolving approach to detect this stealthy attack. The real data from an electric company helps to train and test the pattern recognition model. The coordinated attack is simulated and compared in a 3-area AGC system, while the proposed detection method is verified via the IEEE 39-bus test system.

Frequency stability improvement in wind‐thermal dominated power grids

With the proliferation of intermittent renewable energy sources, power systems need to withstand an increasing number of disturbances that may affect system frequency. In this paper, we focus on the effects of high penetration level of wind turbine-generators (WTG) on the power system operational planning from the frequency point of view. Specifically, an exact formulation for the minimum frequency calculation is presented to increase accuracy and speed of analyses. Then, the effect of WTG's frequency response on the power system frequency stability is investigated to compute the maximum acceptable generation outage as a function of penetration level. Finally, as an application to power system operational planning, the obtained frequency dynamics of the power system and the wind turbine frequency response are integrated into the unit commitment problem. Simulations are performed on the IEEE 39-bus test system to study the effects of the WTGs on the frequency stability of the system. Results illustrate a more secure and reliable operation of the power system when the wind turbines participate in the frequency regulation task. Moreover, including the frequency response of the wind turbines in the unit commitment problem can reduce the ancillary cost, meanwhile increase the frequency stability of the system.

Dynamic load frequency control for high-penetration wind power considering wind turbine fatigue load

The frequency stability of power systems can be improved by the participation of high-penetration wind power in grid frequency regulation. However, the wind turbine (WT) fatigue load will be increased after the wind power participates in the load frequency control (LFC). This paper proposes a control method that takes into account the WT fatigue load and the frequency response of the power system. First, wind turbines are added to the two-areas LFC model including thermal power and hydropower models to participate in frequency regulation. And considering the fatigue load experienced by the tower bending moment and the low-speed shaft torque of the WT, a WT model capable of reflecting the main dynamics is established. Then, the model identification adaptive Proportional-Integral-Differential (PID) controller is designed to be applied to LFC of wind power participating in the grid considering both frequency response and fatigue load. In order to achieve better control effects, the differential evolution (DE) algorithm is used to optimize the PID parameters based on multi-cost functions. The controller parameters are optimized by considering both the fatigue load and frequency deviation. To verify the effectiveness of the proposed method, the controller is applied to the established model and the simulation results are compared.

Integrating Model-Driven and Data-Driven Methods for Power System Frequency Stability Assessment and Control

With increase of practical power system complexity, power system online stability assessment and control is more and more important. Application of the traditional model-driven methods is always limited by contradiction between accuracy and efficiency, while data-driven methods demonstrate strong abilities for the online decision-making support with advancement of various data mining techniques. Instead of direct application of data-driven methods in the power system, this paper first discusses feasible integration approaches for the model-driven and data-driven methods based on the existing achievements, and then, proposes to integrate both methods for the power system online frequency stability assessment and control. The integrated method consists of frequency dynamics prediction and load shedding procedure. In frequency dynamics prediction procedure, integration of system frequency response (SFR) model and the extreme learning machine (ELM)-based learning model is applied, where basic physical causality is kept in the SFR model and ELM is used to fit and correct error of the SFR. The ELM also plays a part in load shedding prediction model construction by digging out mapping relationship from samples. Finally, the proposed prediction and control scheme for the frequency stability is verified by simulations on WSCC 9-bus, New England 39-bus, and NPCC 140-bus system. Results show that the reliability, time efficiency, and accuracy are enhanced with the proposed method.

Continuous Under-Frequency Load Shedding Scheme for Power System Adaptive Frequency Control

Frequency drop due to loss of massive generation is a threat to power system frequency stability. Under-frequency load shedding (UFLS) is the principal measure to prevent successive frequency declination and blackouts. Based on traditional stage-by-stage UFLS scheme, a new continuous UFLS scheme is proposed in this paper to shed loads proportional to frequency deviation. The characteristic of the proposed scheme is analyzed with a closed-form solution of frequency dynamics. Frequency threshold and time delay are added to make the proposed scheme practical. A line-by-line scheme based on precise load control is introduced to implement the continuous scheme for systems without enough continuously controllable loads. The load shedding scale factor of the proposed scheme is tuned with an analytical method to achieve adaptability to different operating conditions. The adaptability of the proposed scheme is validated with 39-bus New England model and simplified Shandong Power Grid of China.

Frequency control reserve with multiple micro grid participation for power system frequency stability

The introduction of this micro grids into the conventional distribution network system forces a new challenge to the system operation. The failure factor of the power system performance essentially due to the limitation of electrical power generation in which could not meet the load demand. In order to maintain the frequency stability of the system, the power sources must be matched instantaneously among all generators and constantly supply to the load demand. A deviation of system frequency from the set-point value will affect the entire stability of power system network. This paper investigates the impact of utilizing multiple micro grids in supporting and facilitating on grid’s frequency. A method called Frequency Control Reserve (FCR) is introduced, with intention to share the excessive power from all available micro grids. These power will be controlled effectively before being injected into the main grid to stabilize the power frequency. Simulation using MATLAB Simulink have been used to simulate the result and shows great potential to be integrated with distributed generation i.e. solar photovoltaic (PV) for Malaysia power system vicinity.

Adaptive frequency control with variable speed wind turbines using data-driven method

The increasing penetration of variable speed wind turbines which use the traditional controllers reduces the inertia of power systems. Consequently, the maintenance of the frequency stability of power systems would be an important challenge. Currently, droop and inertial control methods have been used in variable speed wind turbines to control the frequency of the power network. Adjusting the gains on the droop and inertia control loops is very influential on the performance of wind turbines. However, due to the variable wind speed and power system conditions, the adjustment of the control coefficients which produces the best response in all situations seems to be impossible. In this paper, a new method for the adaptive adjustment of droop and inertia control loop gains in the doubly fed induction generator is presented. In order to eliminate the defects and problems of the power system and wind turbine modeling, it is also proposed to use the data-driven method, which only operates based on the input and output of the system. The second derivative of the error can be used to provide the adaptive adjustment of the droop and inertia control loop gains. This is done to enable faster control and to prevent frequency drooping. In the proposed control method, through implementing the K-vector nearest neighborhood, the output of the next moment is estimated. Then, the coefficients of the frequency control loops are adjusted adaptively, using the Hessian matrix. Simulation results demonstrated the proper performance of a data-driven based adaptive approach to increase the frequency nadir and to decrease the frequency variations of the power system in a steady state and a transient state.

Market Scheduling and Pricing for Primary and Secondary Frequency Reserve

The high penetration of renewable energy (RE) significantly impacts the power system frequency stability for the following reasons: first, the replacement of conventional thermal generator by RE such as wind and solar degrades the frequency control capability; second, the variability of wind and solar imposes more challenges on the frequency stability. The current unit commitment (UC) program in the industrial practice cannot fully provide frequency reserve requirement to maintain the satisfactory frequency performance and the frequency reserves are not fully compensated through the existing market scheme. In this paper, a security constrained unit commitment (SCUC) model with primary and secondary frequency reserve is proposed to co-optimize the energy and frequency reserve to provide satisfactory frequency performance. Post-contingency transmission constraints are enforced to account for the deliverability of the frequency reserve. The locational based reserve pricing is proposed to differentiate the compensating prices for reserves procured at various locations. A five-bus system and a 118-bus system are used to test the proposed model. Frequency dynamic simulation is also implemented to illustrate the frequency performance improvement based on the proposed method in terms of various frequency metrics.