Frequency Stability Enhancement Research Articles

Grid–Following DVI–Based Converter Operating in Weak Grids for Enhancing Frequency Stability

Inertia requirement is the paramount challenge in future power systems with a significant share of renewable energy generators. A promising solution to this issue is applying Distributed Virtual Inertia (DVI) concept, i.e. releasing energy preserved in the DC–link capacitors of employed power converters in the grid following a frequency disturbance. Nonetheless, small–signal stability analyses affirm that a local mode associated with the control system is prone to become unstable when the grid–interactive converter augmented with the DVI operates in weak grids. To overcome this problem, an efficient compensator is proposed in this paper. This compensator introduces one degree–of–freedom to the direct–axis current controller in synchronous reference frame, which eliminates the adverse impact of DVI function on converter stability. Finally, the efficacy of the proposed control framework is depicted by simulations in MATLAB. The results illustrate that the grid frequency rate of change following a step–up load change is improved by 30% compared to the case in which the DVI loop is nullified.

A robust PID controller based on linear quadratic gaussian approach for improving frequency stability of power systems considering renewables

This paper proposes a novel robust controller for frequency stabilization of electrical systems taken into consideration a high renewable energy sources (RESs) penetration. The suggested controller, robust PID (RPID) controller, is combination of a proportional–integral–derivative (PID) controller and a linear quadratic gaussian (LQG) controller. Furthermore, the Improved lightning attachment procedure optimization (ILAPO) technique is applied for determining the optimal setting of the parameters of the introduced RPID controller. A studied power system with RESs is used as a test system to justify the performance of the RPID controller. The superiority of the introduced robust controller is verified through a comparison of its performance under system uncertainties with those of other robust controllers presented in the literature. The results elucidate that the RPID controller can effectually enhance frequency stability and guarantee reliable performance for power grids supplied with high share of renewable energy for all studied scenarios. Consequently, the proposed RPID controller purveys creditable for modern power systems considering RESs.

Small‐signal analysis of multiple virtual synchronous machines to enhance frequency stability of grid‐connected high renewables

The virtual synchronous machine technology is considered as an important technique to effectively control the shortcomings of renewable energy-based power electronics interfaces, providing backup inertia and regulating grid stability. Conventionally, the virtual synchronous machine with a large capacity is responsible for controlling the entire grid stability against renewable penetration. It is usually operated as a centralised control system. But what if virtual synchronous machines with small capacities are independently operated by their additional droop control schemes, and will they present better performance than the single virtual synchronous machine? This study proposes the multiple virtual synchronous machine system with different active power-frequency (P-f) droop characteristics to improve inertia support regarding frequency stability improvement. The comprehensive small-signal modelling of the multiple virtual synchronous machine unit is designed to include the additional P-f droop characteristics. Then, the dynamic characteristics (steady-state and transient responses) and static stability of the multiple virtual synchronous machines are compared with the single virtual synchronous machine at the same rated capacity in both eigenvalue/sensitivity-domain and time-domain analysis. The obtained results reveal that the system with the presence of several virtual synchronous machines is more stable than the system with single virtual synchronous machine, maintaining stable and secure system operation during the contingency.

Višestepena brza regulacija frekvencije u elektroenergetskim sistemima sa neravnomernom distribucijom inercije

Integration of renewable energy sources (RES) is one of the key factors in the fight against climate change and they are becoming to take a larger share in electricity production. The systems with a high penetration of RES have small rotational inertia and are more vulnerable in terms of frequency stability. This paper proposes strategy for multistage fast frequency control (FFC) provided by converter-connected resources. They can quickly change the output active power and provide frequency support immediately after the disturbance during the period before that frequency reaches its nadir. The proposed control strategy uses only local measurements of the rate of change of frequency (RoCoF) and there is no need for complex telecommunications infrastructure. The multistage approach enables dispatched reserve to be proportional to the size of disturbance. RoCoF based FFC provides that more reserves would be dispatched in low-inertia areas that are more sensitive to disturbance and therefore enhance frequency stability. The proposed control strategy is validated on a test system of 3 coherent areas and the simulation results confirm that more reserve is dispatched in low-inertia areas that are more affected by disturbance.

Optimized Tilt Fractional Order Cooperative Controllers for Preserving Frequency Stability in Renewable Energy-Based Power Systems

The low system inertia and the high sensitivity to load and generation fluctuations represent the main challenges for future ambitious plans of modern power systems accompanied by high penetrations levels of the renewable energy sources (RESs). Therefore, this article presents a new approach for solving the load frequency control (LFC) in addition to the virtual inertia control (VIC) in interconnected RESs penetrated power systems using cooperative tilt-based controllers and a hybrid modified particle swarm optimization with genetic algorithm (MPSOGA). The VIC system is adopted using superconducting magnetic energy storage (SMES) to provide sufficient inertial energy for system stability. Two tilt-based controllers are employed in each area using the tilt-integral-derivative (TID) controller for the SMES and TID with filter (TIDF) for the LFC function. The cooperative optimum design of the TID/TIDF controllers leads to the enhancement of frequency stability in studied two-area power systems. The formulated optimization process aims to minimize the frequency nadir settling time during abrupt changes of RESs and/or load changes, considering the cooperative control of LFC and VIC. The proposed approach has been applied to a case study consisting of two-area power systems, connected via hybrid high voltage DC/AC (hybrid HVAC/HVDC) tie-line, integrated with distributed conventional generations, photovoltaic (PV), and wind generation systems. Performance analysis has been conducted to demonstrate the effectiveness of the proposed method is compared to the genetic algorithm (GA) and particle-swarm optimization (PSO) using high fluctuations of renewable generations under extreme changes in loading conditions and physical parameters variation. The obtained results show the superiority of MPSOGA approach on the other competitive optimization techniques.

Artificial Intelligence-Based Power System Stabilizers for Frequency Stability Enhancement in Multi-Machine Power Systems

Artificial Intelligence-Based Power System Stabilizers for Frequency Stability Enhancement in Multi-Machine Power Systems

Enhanced dielectric properties of Ca0.9Sr0.1Cu3-yZnyTi4-xBxO12 ceramics by modifying grain boundary response

Ca0.9Sr0.1Cu3-yZnyTi4-xBxO12 ceramics (0.01 ≤ x ≤ 0.07, y = 0 and 0.05) were prepared using the citrate precursor process and sintered at 1100 °C for 3 h. X-ray diffraction (XRD) has confirmed that CaCu3Ti4O12 ceramics with single phase can be prepared for a short-term sintering. The results of XRD and Raman reveals the existence of secondary phase as x = 0.07. The energy dispersive spectrometer confirms the presence of Cu-rich phase at grain boundary, which leads to a weakened dielectric properties. B3+ doping can speed the grain growth, whereas Zn2+ doping decreases grain size slightly and high dense microstructures were observed. The introduction of Zn2+ leads to an increase of grain boundary resistance, a decrease of dielectric loss and an enhancement of frequency stability of dielectric properties. Enhanced dielectric properties with dielectric constant of ~13410 and dielectric loss of ~0.038 were observed for the sample at x = 0.04 and y = 0.05. Moreover, the nonlinear electrical characteristics are enhanced with proper concentration of Sr, B and Zn additions. The present results indicate that the improved dielectric properties of doped CCTO ceramics are mainly attributed to the improved microstructure and the modified grain boundary response.

Effect of the Ba/K ratio on structural, dielectric and energy storage properties of BaO–K2O–TiO2–P2O5 glass-ceramics

xBaO–(20−x)K2O–30TiO2–50P2O5 with (0≤x≤20 mol%) glasses were successfully elaborated by the melt quenching while their related glass-ceramics were developed by controlled crystallisation. Density and molar volume measurements, differential thermal analysis and Raman spectroscopy were carried out to examine the glassy structure, the results revealed that the addition of BaO increases the reticulation and reinforces the glass network by the creation of strengthened linkages. X-ray diffraction has identified the formation of MTi2(PO4)3 with M=(K, Ba0·5) in all the glass-ceramics (GC) and the precipitation of a secondary BaTiP2O8 phase when x increase beyond 10 mol%. The dielectric properties of the glass-ceramics were studied by impedance spectroscopy, it showed that the addition of BaO induces an enhancement of both thermal and frequency stability of the dielectric parameters (εr and tanδ). The glass-ceramic with 5 mol% of BaO GC-(x=5) presents the highest dielectric constant and the lowest dielectric loss. The P-E hysteresis loops were recorded at room temperature and the energy storage parameters of the glass-ceramics were determined. These parameters were significantly improved by the increase of the BaO content and the optimum parameters were obtained for GC-(x=5). The dielectric and energy storage parameters were discussed according to the structure data.

Coordinated multiple HVDC modulation emergency control for enhancing power system frequency stability

High-voltage direct current (HVDC) power modulation can effectively improve the system frequency stability after high power shortage fault disturbance. This study presents an emergency control method by coordinating the active powers of multiple HVDC lines for enhancing the frequency stability. Firstly, the system frequency response model with HVDC modulation is built, and then a simplified method for calculating the sensitivity of maximum frequency to HVDC modulation is proposed based on the first-order differential method. Next, based on a functional relationship of the AC bus voltage and the actual HVDC modulation capacity, the methodology for calculating the HVDC real-time modulation capacity is proposed. Finally, the coordinated multiple HVDC modulation emergency control model is established which aims to minimise the modulation cost considering the maximum frequency threshold. The proposed methodology has been tested in a real-power system. The simulation results of different scenarios have verified its effectiveness in suppressing the rise of maximum frequency and enhancing frequency stability.

Adaptive Control of HVDC Links for Frequency Stability Enhancement in Low-Inertia Systems

Decarbonization of power systems has put Renewable Energy Sources (RES) at the forefront when it comes to electric power generation. The increasing shares of converter-connected renewable generation cause a decrease of the rotational inertia of the Electric Power System (EPS), and consequently deteriorate the system capability to withstand large load-generation imbalances. Low-inertia systems are subjected to fast and large frequency changes in case of in-feed loss, where the traditional primary frequency control is not sufficient to preserve the frequency stability and to maintain the frequency above the critical value. One possible solution to this rising problem is seen in Fast Frequency Response (FFR) provided by the High-Voltage Direct-Current (HVDC)-based systems. This paper presents the adaptive FFR control of HVDC-based systems for frequency stability enhancement in the low-inertia system. The EPS is considered as a “black box” and the HVDC response is determined only using the locally measured frequency change. Sliding Mode Control (SMC) of the Modular Multilevel Converter (MMC) was developed and demonstrated to provide faster and more appropriate frequency response compared to the PI controller. The described adaptive HVDC control considers the size of disturbance and the inertia of the power system, and it is verified by simulations on the IEEE 39 bus test system implemented in MATLAB/Simulink for different system configurations and different sizes of disturbance.

Microgrid Frequency Stability Enhancement Through Controlling Electric Vehicles Batteries Based on Fuzzy Logic Controller

The large-scale renewable energy contribution tends to expand vastly; this is due to the numerous benefits of the renewable energy, but renewable resources are intermittent and changeable in nature, which would affect the microgrid frequency stability. In this paper, a renewable and green microgrid is proposed aiming to emission power generation; the microgrid consists of biogas plant, biodiesel plant, photovoltaic system, wind turbines, battery energy storage system, load, and electric vehicles. The main contribution of this paper is providing the frequency stability enhancement of the proposed microgrid through controlling the electric vehicles batteries using fuzzy logic controller based on three main factors, the state of charge of the electric vehicles batteries, the microgrid frequency deviation signal, and the departure time of the vehicle that is set in advance by the owner. This work is discussing three scenarios, in the first one, the batteries of the electric vehicles are going to be charged directly once they get plugged into the charging point, in the second one, the electric vehicles batteries are going to be charged based on fuzzy charging control technique, in the third scenario, the vehicle to grid concept is applied, so the charging and discharging of the electric vehicles batteries are going to be controlled through fuzzy logic controller.

A Simulation-Based Procedure to Determine Optimum Range of Settings of Fast Valving Action of Steam Turbines to Enhance Frequency Stability

Fast Valving (FV) mechanism of steam turbines is considered to be one of the most effective actions for enhancing the security of a power system subsequent to disturbances. It plays a significant role in mitigating the impact of severe disturbances by instantly decreasing steam turbine power, thus ensuring the power system stability. There is a direct relationship between the settings of Over-Speed Protection Control (OPC) unit of steam turbines and the control functions of FV. This paper proposes a generalized simulation-based procedure to determine the optimum intermediate valve actuation timings of the FV control action. A coal-fired thermal power plant has been used as the case study and the accuracy of the settings made using the proposed approach has been demonstrated using actually happened contingencies in a real power system. Finally, it is concluded that the proper selection of actuation timings of intermediate valves for FV scheme has a significant effect on enhancing frequency stability under transient conditions.

Control strategy resilient to unbalanced faults for interlinking converters in hybrid microgrids

Hybrid microgrids (HMGs) consist of AC and DC microgrids (MGs) that are connected through bidirectional interlinking converters (ICs). In order to optimally utilize renewable generations and address their intermittency, efficient control strategies are needed for power management of ICs. The performance of available control strategies for HMGs in case of faults may be limited since the converters are usually set to trip in the event of transient stability related problems to protect converters from being damaged. Also, HMGs may face frequency instability problems due to the lower damping capability of converter-based generations compared with classical synchronous generators. In this paper, an enhanced control strategy based on the concept of 3-D droops is proposed for ICs. The proposed method makes HMGs more resilient against faults at the utility side. It also improves stability of HMGs by mitigating quasi-static active and reactive power oscillations that may be generated by renewable intermittent generations. Also, it ensures a proper transient power sharing without deteriorating the transfer capability of ICs when unbalanced faults, as the most prevalent ones, occur in HMGs. In addition, the synchronverter technology with an adaptive virtual inertia is applied to the control strategy of ICs to enhance frequency stability of HMGs in case of occurring faults or volatile renewable generations. The performance of the proposed features is evaluated by their testing on a typical HMG.

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 control of conventional power sources and PHEVs using jaya algorithm optimized PID controller for frequency control of a renewable penetrated power system

In renewable penetrated power systems, frequency instability arises due to the volatile nature of renewable energy sources (RES) and load disturbances. The traditional load frequency control (LFC) strategy from conventional power sources (CPS) alone unable to control the frequency deviations caused by the aforementioned disturbances. Therefore, it is essential to modify the structure of LFC, to handle the disturbances caused by the RES and load. With regards to the above problem, this work proposes a novel coordinated LFC strategy with modified control signal to have Plug-in Hybrid Electric Vehicles (PHEVs) for frequency stability enhancement of the Japanese power system. Where, the coordinated control strategy is based on the PID controller, which is optimally tuned by the recently developed JAYA Algorithm (JA). Numerous simulations are performed with the proposed methodology and, the results have confirmed the effectiveness of a proposed approach over some recent and well-known techniques in literature. Furthermore, simulation results reveal that the proposed coordinated approach significantly minimizing the frequency deviations compared to the JAYA optimized LFC without PHEVs & with PHEVs but no coordination.

A Fast Load Control System Based on Mobile Distribution-Level Phasor Measurement Unit

The increase of renewable penetration in power grids calls for loads to participate in frequency regulation to avoid under-frequency load shedding after large resource contingencies. The motivation of this paper is to fulfill a fast residential load control for frequency stability enhancement. For this purpose, a novel mobile distribution-level phasor measurement unit (MDPMU) is developed for disturbance event detection, power mismatch estimation and fast load control. First, the systematic design of the MDPMU introduced. The proposed MDPMU has the major advantages of low cost and high frequency measurement accuracy. Second, the distribution-level measurement is utilized for the rate of change of frequency (ROCOF) calculation. Upon detection of a power system frequency event, power mismatch is estimated at the early stage of the event. Details of a robust approach to calculate ROCOF and determine the event starting point are presented. Practical issues including the measurement reporting rate and estimation time are also considered. Last, with the proposed control system, residential load responses can be controlled coordinately based on load availability and compensation prices offered by customers, providing adaptive frequency regulation service after large resource contingencies. The accuracy of frequency measurement and power mismatch estimation are evaluated via experimental analysis. The frequency stability improvement by load control is validated via PSS/E simulation.

A Novel Adaptive Supervisory Controller for Optimized Voltage Controlled Demand Response

Recently, it has been shown that voltage-controlled demand response (VCDR), which is based on the principle of the dependence of the active power of demand on the system voltage, can provide ancillary services to future power systems. Voltage control devices used for VCDR can improve system frequency stability to various extents, depending on their locations, load size and load-voltage dependency in the grid. Thus, designing a supervisory controller to guarantee that such devices are optimally utilized for VCDR is necessary, as this can contribute in enhancing system frequency stability. In this paper, a novel adaptive supervisory controller (ASC) is proposed to optimally use VCDR resources in large power systems for such purposes. To ensure the effective operation of the ASC, an assessment of the impacts of on-load tap changer (OLTC) transformers on the system frequency is essential. In this regard, clustering techniques and principal component regression are used as offline tools to evaluate the influences of OLTCs transformers on VCDR in large-scale power systems using the IEEE 39-bus test system. Also, to estimate the effects of the OLTC transformer clusters on VCDR, a comprehensive sensitivity analysis with respect to the gain of the modified OLTC controllers’ frequency input is conducted.

Dynamic event-triggered robust secondary frequency control for islanded AC microgrid

The permeability of renewable energy sources rapidly increases in the microgrid for economic and environmental concerns. However, the uncertainty of renewable energy may lead to frequency fluctuation or even instability in the microgrid. This paper proposes a dynamic event-triggered robust secondary frequency control scheme for the islanded microgrid to handle the uncertainty of the renewable energy source. In addition to the enhancement of frequency stability, the communication burden is also considered in this work. The proposed dynamic event-triggered communication scheme provides a good balance between dynamic performance and communication burden. Based on the discontinuous Lyapunov function, the stability analysis helps the optimal gain selecting. Given the designed controller, the derived theorems in this paper can help to examine whether the communication system fulfills the requirements from the controllers. To validate the proposed method, case studies are carried out based on two typical microgrids. The simulation results are compared with conventional frequency controllers. It shows that the proposed controller outperforms other types of controllers in term of the frequency response performance and communication burden.

Optimized coordinated control of LFC and SMES to enhance frequency stability of a real multi-source power system considering high renewable energy penetration

With rapidly growing of Renewable Energy Sources (RESs) in renewable power systems, several disturbances influence on the power systems such as; lack of system inertia that results from replacing the synchronous generators with RESs and frequency/voltage fluctuations that resulting from the intermittent nature of the RESs. Hence, the modern power systems become more susceptible to the system instability than conventional power systems. Therefore, in this study, a new application of Superconducting Magnetic Energy Storage (SMES) (i.e., auxiliary Load Frequency Control (LFC)) has been integrated with the secondary frequency control (i.e., LFC) for frequency stability enhancement of the Egyptian Power System (EPS) due to high RESs penetration. Where, the coordinated control strategy is based on the PI controller that is optimally designed by the Particle Swarm Optimization (PSO) algorithm to minimize the frequency deviations of the EPS. The EPS includes both conventional generation units (i.e., non-reheat, reheat and hydraulic power plants) with inherent nonlinearities, and RESs (i.e., wind and solar energy). System modelling and simulation results are carried out using Matlab/Simulink® software. The simulation results reveal the robustness of the proposed coordinated control strategy to preserve the system stability of the EPS with high penetration of RESs for different contingencies.

Frequency Stability Enhancement of Integrated AC/VSC-MTDC Systems With Massive Infeed of Offshore Wind Generation

This paper investigates how the operation of the onshore AC grid and the offshore voltage source converter-based multiterminal dc (VSC-MTDC) grid can be cooptimized within the unit commitment (UC) framework to enhance system frequency stability following various contingencies. Frequency dynamic constraints corresponding to three typical types of outages (i.e., loss of a synchronous unit, loss of a grid-side VSC, and loss of a wind farm-side VSC) are established. A novel frequency dynamics constrained UC for integrated AC/VSC-MTDC systems is then proposed. Case studies on three integrated AC/VSC-MTDC systems demonstrate that major frequency disturbances caused by credible contingencies occurring in either the AC grid or VSC-MTDC grid can be effectively stabilized by the proposed approach.