Inter-area Oscillations Research Articles

Influence of Photovoltaic Power Plants Based on VSG Technology on Low Frequency Oscillation of Multi-Machine Power Systems

With the increase of photovoltaic penetration, the power-electronic converter (PEC)-interfaced generators with traditional control strategy may cause severe power oscillations after being disturbed due to lack of inertia and damping. The photovoltaic power plant controlled by virtual synchronous generator (VSG) technology, while introducing virtual inertia and virtual damping to improve the stability of the system, will also inherit its oscillation characteristics and cause a new oscillation mode. Aiming at this problem, this paper analyzes the difference between the system low-frequency oscillation (LFO) modes before and after the VSG is connected to the two-area system by the modal analysis. It is pointed out that the PEC-interfaced renewable energy generators based on VSG control technology could change the oscillation mode of the original system. The influence of VSG's main parameters and capacity on system dynamics is analyzed. Finally, a synchrophasor-based inter-area oscillation suppressed comprehensive control strategy is proposed to suppress the LFO of active power on the tie line, and the superiority of the strategy is verified by DIgSILENT /PowerFactory.

Wide-area damping control system for large wind generation with multiple operational uncertainty

• Illustrates the contribution of wind generation, and investigates the influence of the grid-forming mode-based VSWT-DFIGs. • Coordinated design of wide-area damping controller for utility-scale wind generation power plants considering time delays uncertainty. • Optimization is used to optimize the parameters of damping controller. • Contains case studies on a large power system, including FACTS, and wind generation. • MATLAB/Simulink-based nonlinear simulations are performed, for variable wind speed, 3-phase faults, and dynamic load model. The large integration of wind generation resources into power systems may causes low-frequency oscillations and deterioration in power system stability. This paper presents a novel coordinated approach-based Wide-Area Damping Control System (WADCS) for large wind generation integrated power system, while taking into consideration of the operational uncertainties. The coordinated WADCS approach is used to enhance the damping of inter-area oscillations while maintaining the voltage and frequency in the prescribed range. In a coordinated WADCS, a wide-area damping controller is coordinated with the rotor side converter model of wind generation resources (WGRs). The Static compensator (STATCOM) is used to enhance voltage and reactive power support for WGRs. To provide the additional damping, a static synchronous series compensator (SSSC) is also employed with the WADCS. The geometric measures of controllability/observability and residues method is utilized to select the appropriate input control signals and optimal location of wide-area damping controller. A multi-objective grey wolf optimizer (MOGWO) algorithm is used to optimize the parameters of the wide-area damping controller to enhance the damping. MATLAB/Simulink-based nonlinear simulations are performed considering operational uncertainties such as variable wind speed, 3-phase faults, variable load, and time delays to demonstrate the efficacy of the proposed coordinated control for modified WGRs integrated test power system.

Damping Torque Analysis of the PMSG-Based WT with Supplementary Damping Control for Mitigating Interarea Oscillations

This study presents a comprehensive analysis of the impact that a supplementary damping controller of the permanent magnet synchronous generator (PMSG)-based wind turbine (WT) has on low-frequency oscillation (LFO) damping. A reduced mathematical model of an interarea system is first established. Then, an auxiliary damping controller is designed using the PMSG active power control loop, enabling the WT to actively support the interarea LFO mitigation. Based on this, the damping torque analysis (DTA) method is applied to explore the contribution of the PMSG-based WT with an auxiliary damping controller to LFO damping enhancement, whose analytical expressions of the damping torque coefficients reveal the impacts of the control parameters and the operation conditions on the system damping characteristics. It is evident that the installation location of the wind power plant (WPP) plays a leading role in determining whether the damping provided by the PMSG controller is positive or negative. Also, the contribution of damping from the PMSG can be improved by tuning the droop coefficient of the PMSG controller properly. The results indicate that the proposed damping control is always helpful in improving the system damping when the wind power penetration increases. Accordingly, analytical conclusions can serve as guidelines for the control design. Case studies of a two-area test system integrated with a PMSG-based wind farm have been conducted to verify the theoretical analysis.

Additional Compound Damping Control to Suppress Low-Frequency Oscillations in a Photovoltaic Plant with a Hybrid Energy Storage System

The use of the conventional dual closed-loop control strategy by photovoltaic (PV) plants with grid-connected inverters may weaken the damping of a power system, which may aggravate low-frequency oscillations (LFOs). This influence will become more severe as the penetration of PV plants increases. Therefore, it is necessary to incorporate damping controls into PV plants to suppress LFOs. This paper proposed an additional compound damping control (ACDC) system that combines additional damping control (ADC) for the inverter with ADC-based dynamic power compensation control (DPCC), allowing hybrid energy storage systems (HESSs) to suppress LFOs. First, the feasibility of suppressing low-frequency oscillations in PV plants is demonstrated by the torque method and a small signal model. Then, an additional damping controller is added to the active power control link of the PV inverter to enhance the damping abilities of the system. However, given that the damping performance of PV plants with only ADC is limited by the compensated power, PV plants require devices that can rapidly compensate for the damping power. Therefore, we added the HESS to the DC bus and proposed DPCC. Finally, a three-machine nine-node system for a PV plant was modeled and simulated in the PSCAD platform. The simulation results showed that the proposed control strategy could provide effective damping for interarea oscillation.

Power System Stability Enhancement Using Robust FACTS-Based Stabilizer Designed by a Hybrid Optimization Algorithm

Improving the stability of power systems using FACT devices is an important and effective method. This paper uses a static synchronous series compensator (SSSC) installed in a power system to smooth out inter-area oscillations. A meta-heuristic optimization method is proposed to design the supplementary damping controller and its installation control channel within the SSSC. In this method, two control channels, phase and magnitude have been investigated for installing a damping controller to improve maximum stability and resistance in different operating conditions. An effective control channel has been selected. The objective function considered in this optimization method is multi-objective, using the sum of weighted coefficients method. The first function aims to minimize the control gain of the damping controller to the reduction of control cost, and the second objective function moves the critical modes to improve stability. It is defined as the minimum phase within the design constraints of the controller. A hybrid of two well-known meta-heuristic methods, the genetic algorithm (GA) and grey wolf optimizer (GWO) algorithm have been used to design this controller. The proposed method in this paper has been applied to develop a robust damping controller with an optimal control channel based on SSSC for two standard test systems of 4 and 50 IEEE machines. The results obtained from the analysis of eigenvalues and nonlinear simulation of the power system study show the improvement in the stability of the power system as well as the robust performance of the damping in the phase control channel.

Decentralized Model-Reference Adaptive Control Based Algorithm for Power Systems Inter-Area Oscillation Damping

Being the primary cause of inter-area oscillations and due to the fact that they limit the generation’s output, Low-Frequency Electromechanical Oscillations (LFEOs) represent a real threat to power system networks. Mitigating their effects is therefore crucial as it may lead to system collapse if not properly damped. As rotor angle instability is the primary cause of LFEOs, this paper presents a novel Model-Reference Adaptive Control (MRAC) scheme that enhances its stability. The proposed scheme is tested using the Single-Machine Infinite Bus (SMIB) network. The results obtained validate the proposed decentralized control architecture. The robustness of this oscillation damping controller is verified through simulations in MATLAB/SIMULINK. With Gaussian noise added to the structure of the generator to emulate small load variations responsible for the rotor angle instability, the results of the simulations show that the rotor angle remains stable. Furthermore, when subjected to faults, the recovery time is less than 500 ms.

Power System Rotor Angle Stability Enhancement Using Lyapunov-Based Trajectory Tracking Controller and Model Reference Adaptive Control

It is vital that the synchronous generator rotor angle be kept stable to avoid disastrous consequences such as the loss of synchronism amongst generators within a power system network. Once it is unstable, the time that is taken to remedy this is advised to be 5–10 s for smaller power systems and 15–20 s for larger ones. The instability that is caused by poorly damped Low-Frequency Electromechanical Oscillations (LFEOs) may result in inter-area oscillations, where a group of generators in one area oscillates against those in another area, thus affecting the stability of the entire network. This paper explores two control architectures, namely, a nonlinear Lyapunov-based trajectory tracking controller and a Model Reference Adaptive Controller (MRAC) as options to enhance the stability of the rotor angle. The performance of each of these controllers was assessed under steady-state conditions, and then, the synchronous generator was subjected to Gaussian noise and an impulse. While the first one is aimed at emulating small variations in the system loads that are responsible for inter-area oscillations, the latter one is an attempt to explore their performance for transient stability.

Wind power plant grid-forming control and influences on the power train and power system oscillations

In future power systems, wind power plants are supposed to contribute in the voltage and frequency stability of the grid. For this purpose, the control strategy of the wind power-frequency converter system needs to change from grid-following to grid-forming control. In this paper, the influence of future wind power plants equipped with the grid-forming control strategy on the wind turbine itself and the power system oscillations is studied. For this purpose, the coupled model of wind power plant and a relatively realistic model of future power system is employed, which is able to capture the internal dynamics of wind turbine power train system and its dynamic interactions with the rest of power system. It is shown how wind power plant local- and inter-area oscillations can influence/be influenced by the power system if insufficient damping is presented by wind turbines' power train system. It is shown how the variation of demand in the power grid can consistently excite wind turbine local- and inter-area modes which can cause undamped oscillations in both wind turbine power train system and the rest of power system if insufficient damping is provided by the wind turbine power train system. A test case, using 1.5 MW NREL wind turbine drivetrain, is demonstrated.

Small-Signal Stability Improvement of Grid Integrated DFIG and SOFC System with SSSC-POD Using Coordinated Optimization Approach

Background: Focusing on the stability problems brought by integrated wind power and fuel cell, the objective of this paper is to analyze small-signal stability and improvement of a hybrid renewable energy system connected with doubly-fed induction generators (DFIGs) and solid oxide fuel cells (SOFCs) energy with the static series synchronous compensator (SSSC) and power-oscillation damper (POD). Methods: For this purpose, a SSSC-POD controller is designed and the state-space representation of such a hybrid power system for modal analysis is developed. Then an approach of coordination and optimization of SSSC-POD parameters based on genetic algorithm (GA) and modal analysis is proposed to further improve the power system small-signal stability. Results: Several designed scenarios, including changing the tie-line power, POD input signal and SOFC power output, are considered in IEEE two-area system to test the proposed method through small-disturbance eigenvalue analysis and time-domain simulations. Conclusion: Simulation results demonstrate that the proposed approach can effectively suppress the local and inter-area oscillations and improve power system stability.

A Novel Hardware-in-the-Loop Approach to Investigate the Impact of Low System Inertia on RoCoF Relay Settings

This paper presents a novel hardware-in-the-loop (HIL) approach as used to investigate the impact of the reduction in inertia on the Great Britain (GB) electrical power system with regard to rate of change of frequency (RoCoF) settings for Loss-of-Mains (LoM) protection. Furthermore, the research as presented in this paper updates, enhances, and validates a reduced model of the Great Britain transmission system, as originally developed in DIgSILENT PowerFactory by the National Grid Electricity System Operator. The enhanced model has been developed for integrated use with the OPAL-RT real-time HIL simulation toolkit and is validated against phasor measurement unit (PMU) data from actual disturbance events using novel automated interfacing between both integrated simulation platforms, PowerFactory from DIgSILENT and ePHASORSIM from OPAL-RT. The corresponding simulations show that the updated reduced model is capable of capturing the dynamic behaviour of the GB transmission system, including both local and inter-area oscillations, with satisfactory accuracy. Finally, the paper presents HIL study results with the reduced model to investigate the influence of decreasing system inertia on the response of LoM protection relays. The studies show that decreasing system inertia may have a significant impact on LoM relays using RoCoF detection, particularly relays using the legacy G59 setting of 0.125 Hz/s. Initial studies have also demonstrated the potential for a previously unrecognised interaction between system oscillations and the 500 ms operating delay, as specified in G59 and G99 Engineering Recommendations. Consequently, faster local oscillations (>1 Hz) reset the relay and decrease the sensitivity, whereas slower inter-area oscillations (<1 Hz) appear to cause the relay to overestimate the average RoCoF.

Coordinated Charging and Discharging of Electric Vehicles: A New Class of Switching Attacks

In this work, we investigate that the abundance of Electric Vehicles (EVs) can be exploited to target the stability of the power grid. Through a cyber attack that compromises a lot of available EVs and their charging infrastructure, we present a realistic coordinated switching attack that initiates inter-area oscillations between different areas of the power grid. The threat model as well as linearized state-space representation of the grid are formulated to illustrate possible consequences of the attack. Two variations of switching attack are considered, namely, switching of EV charging and discharging power into the grid. Moreover, two possible attack strategies are also considered (i) using an insider to reveal the accurate system parameters and (ii) using reconnaissance activities in the absence of the grid parameters. In the former strategy, the system equations are used to compute the required knowledge to launch the attack. However, a stealthy system identification technique, which is tailored based on Eigenvalue Realization Algorithm (ERA), is proposed in latter strategy to calculate the required data for attack execution. The two-area Kundur, 39-Bus New England, and the Australian 5-area power grids are used to demonstrate the attack strategies and their consequences. The collected results demonstrate that by manipulation of EV charging stations and launching a coordinated switching attack to those portions of load, inter-area oscillations can be initiated. Finally, to protect the grid from this anticipated attack, a Support Vector Machine (SVM) based framework is proposed to detect and eliminate this attack even before being executed.

The Unified Supplementary Damping Method Based on Voltage-Soured Converter High-Voltage Direct Current Transmission Through Robust Control

The VSC-HVDC system can damp the inter-area low-frequency oscillation effectively. To enhance the ability of the VSC-HVDC supplementary control, this article proposes a unified low-frequency oscillation controller design using the robustness theory. The unified control strategy uses both the active power control and reactive power control loop of the VSC to expand the supplementary control dimensions. To design the controller, the TLS-ESPRIT identification method is used to obtain the system small signal model and the oscillation characteristic. Based on the model identified out, the linear matrix inequality method based robust control theory is applied for the controller design, and the robust controllers for active power control and reactive power control loop are both designed to improve the control effect. Finally, the simulation results show that the controller can reach better control effect and the robustness can also be guaranteed.

Reduce Frequency Oscillation Using Energy Storage System and Phasor Measurement Units Technology

Inter-Area oscillations result from system events coupled with a poorly damped electric power system. The oscillations are observed in the large system with groups of generators, or generating plants connected by relatively weak tie lines. The frequency oscillation is the major problem for interconnected power systems; hence, the stability of these oscillations is an important condition. The inter-area oscillation, equipment such as Static Var Compensator and various Flexible AC Transmission System devices, are being increasingly used. Although Power System Stabilizers exist on many generators, there effect is only on the local area and do not effectively damp out inter-area oscillations. The injection of a stable electrical energy into disturbed power system decreases the oscillations; in our research we proposed the use of an electric energy storage system to reduce the frequency oscillations by using phasor measurement unit. This technique is applied in the Algerian interconnection power.

Optimal power flow schedules with reduced low-frequency oscillations

The dynamic response of power grids to small events or persistent stochastic disturbances influences their stable operation. Low-frequency inter-area oscillations are of particular concern due to insufficient damping. This paper studies the effect of the operating point on the linear time-invariant dynamics of power networks. A pertinent metric based on the frequency response of grid dynamics is proposed to quantify power system’s stability against inter-area oscillations. We further put forth an optimal power flow formulation to yield a grid dispatch that optimizes this novel stability metric. A semidefinite program (SDP) relaxation is employed to yield a computationally tractable convex problem. Numerical tests on the IEEE-39 bus system demonstrate that the SDP relaxation is exact yielding a rank-1 solution. The relative trade-off of the proposed small-signal stability metric versus the generation cost is also studied.

A special protection scheme for damping power system oscillations by controlling wind farms

This paper proposes a special protection scheme (SPS) as an additional tool for system operators to damp power oscillations and thereby improve the overall power system stability. This SPS is activated by an online power oscillation detection algorithm and utilizes a power system stabilizer (PSS) controlling a wind farm to damp power oscillations. Moreover, an adaptive time delay compensator (ATDC) compensates for the time delays in the control system. Different alternatives for the PSS input signals are compared by time-domain simulations and by small-signal analysis. The most significant improvement in the damping of the inter-area oscillations is achieved when the PSS uses the active tie-line power as input and controls the active power of the wind farm. The results also prove that the ATDC operates correctly if the total time delay remains below 300 ms. This SPS can improve the system stability when the conventional PSSs may be insufficient.

Robust Nonlinear Control of DFIG-Based Wind Farms for Damping Inter-Area Oscillations of Power Systems

This article presents a perturbation observer–based robust nonlinear damping control (RNDC) scheme for a doubly fed induction generator (DFIG)–based wind farm to damp the inter-area oscillations of multi-area power systems. In the RNDC scheme, the perturbation term is introduced to describe the combined effects of nonlinearities, uncertainties, and disturbances of multi-area power systems with wind farms. The feedback linearization control is realized for reactive power control loops of the DFIG based on state estimation and perturbation estimation derived from a perturbation observer, thus achieving the damping control of the DFIG. The proposed RNDC scheme only needs one measurement signal, and it does not require any model parameters of the power grid and wind farm. Simulation studies are carried out on a two-area power system model connected with the wind farm to validate the control performance of the RNDC scheme under the conditions of three-phase-to-ground faults, parameter variations, and time delays.

A wide-area nodal frequency estimation and tracking approach

Synchronization of the nodal frequencies at a nominal frequency value throughout the network is essential for ensuring the reliable operation of the entire power system. This synchronization is achieved by maintaining a near real-time generation load balance. A large disturbance in this balance results in frequency deviations in time and space. Since significant frequency deviations can lead to load shedding, operating instability, and even blackouts, system operators use a combination of control actions to restore the system frequency very close to its nominal level. The effectiveness of these control actions, however, will depend on the accuracy with which the system-wide frequency is measured. A novel approach for directly tracking the frequency at each node of an electric power system is proposed in this paper. This approach is based on a tracking state estimation and considers two different methodologies, according to the existing time frame between estimations. Lastly, case studies are presented to evaluate the proposal's effectiveness in estimating nodal frequencies and directly observing interarea oscillations from the state estimation results.

Digital Twin Development of a Dynamic Hardware Emulator

The increasing deployment of new technologies to contribute to the decarbonization of power systems is imposing new challenges in terms of system dynamics and stability. To deal with different operating and control issues in this sense, and support actual needs, advanced tools and solutions are required. Therefore, this paper presents a digital twin of a dynamic hardware emulator that can be used for controller hardware in the loop (CHIL) testing and is based on a small-scale laboratory system. To build the simulation model, the parameters of involved synchronous machines, excitation systems, prime movers and transmission lines have been identified and then compared to laboratory measurements to assess the accuracy of the digital twin. Static and dynamic accuracy have been investigated and an overall good accuracy can be shown with the help of quantification of errors. Furthermore, a case study is presented where the digital twin was used to design a controller to damp inter-area oscillations with the help of wide area measurements. This controller was then implemented and tested within the dynamic hardware emulator in the laboratory.

Implementation of a Multiterminal Line Commutated Converter HVDC Scheme with Auxiliary Controller on South Africa’s 765 kV Corridor

The deployment of a 765-kV transmission line on Eskom’s South African Grid marks the beginning of a new era in power industries. The integration of renewable energies by independent power producers (IPPs) leads to an infrastructural change in the stability performance of the entire grid. These developments are expected to bring about a multiterminal direct current (MTDC) system for practical implementation on this grid. Therefore, this study focuses on the dynamic response of the South African transmission grid during a system disturbance. In the carrying out of this study, the South African grid was modeled on PSCAD, and its performance was evaluated. The impact of the MTDC link on the grid’s interarea oscillation was also investigated. An additional current order controller for the MTDC link was developed, and its impact on the MTDC power transfer was analyzed. The results show a better system performance and reduced interarea power swings with the inclusion of the MTDC link.

Coordinated Wide-Area Damping Control in Modern Power Systems Embedded with Utility-Scale Wind-Solar Plants

This paper presents a coordinated wide-area damping control of power system stabilizers (PSSs), static synchronous compensator (STATCOM), and static synchronous series compensator (SSSC) for grid-forming based wind and solar PV generation resources. The main objective of this work is to enhance the damping for inter-area oscillations while maintaining the voltage and frequency in the prescribed range, taking into consideration the operational uncertainties. Better voltage and reactive power support are provided by the use of STATCOM and for additional damping, an SSSC is employed with the coordinated wide-area damping controller (CWADC). Variable-speed wind turbine control methodology strategically combined with inertia control, de-loading control, pitch angle control, and electronic rotor speed control is employed to obtain an optimum response for discontinuous generation mixes and variable wind speed. The geometric measures of controllability/observability and residues method have been utilized to choose the appropriate input control signals and optimal location of CWADC. A multi-objective salp swarm algorithm has been used to optimize the parameters of the proposed CWADC. To demonstrate the efficacy of the proposed CWADC, nonlinear time-domain simulations are performed on a modified IEEE 68-bus test system embedded with hybrid wind-solar power plants considered multiple time delays.