Low Frequency Power Oscillations Research Articles

Active Damping for Dynamic Improvement of Multiple Grid-Tied Virtual Synchronous Generators

To eliminate low-frequency oscillations, this paper proposes an active-damping method for multiple grid-tied virtual synchronous generators (VSGs) in a power plant. Firstly, using the Lyapunov's indirect method, the damping ratio of multiple VSGs in parallel is analyzed. The average damping ratio reveals that this multi-VSG power plant can be poorly damped in a wide range of inertia and damping settings. Then, self- and mutual-damping controllers are developed to suppress self- and mutually induced low-frequency power oscillations, respectively. For the practical implementation, an adaptive tuning algorithm that enables automatic realization is proposed. Through a reassessment, a remarkable damping-ratio improvement is validated. Moreover, the inertial response improvement is validated by the frequency response analysis. Finally, simulations in Digsilent/PowerFactory and experiments are performed to demonstrate the accuracy of the analyses and the effectiveness of the proposed method.

Low-Frequency and Sub-Synchronous Power Oscillation Suppression and Analysis of Hybrid Line Commutated Converter for HVDC Grid

Building high-proportion renewable energy grid requires economical and reliable High Voltage Direct Current (HVDC) converters. Line Commutated Converter (LCC) is widely used because of its long transmission distance, low power loss, low manufacturing cost, and large transmission capacity, which modular multilevel converter (MMC) cannot achieve, but has commutation failure (CF) problems. Low-frequency and sub-synchronous power oscillation caused by CF will affect the safety of bulk power systems. This paper theoretically analyzes the power oscillation during CF and CF recovery, and then proposes a novel hybrid line commutated converter (H-LCC) based on integrated gate commutated thyristor (IGCT), which can effectively mitigate CF and suppress power oscillation. The principle of H-LCC suppressing power oscillation is analyzed. On this basis, the performance of LCC, MMC and H-LCC under the same fault conditions is compared by a simulation study, which proves that H-LCC can improve the security and stability of high-proportion renewable energy power systems.

A Grey Wolf Optimizer algorithm based fuzzy logic power system stabilizer for single machine infinite bus system

Low-frequency power oscillations in power systems have always been a serious risk for a stable power system. A fuzzy logic power system stabilizer (FLPSS) is proposed to mitigate system oscillation problems. Aimed at parameter configuration and rule setting of FLPSS, Grey Wolf Optimizer (GWO) based FLPSS is utilized to achieve quicker settling time and improve the damping oscillation effectiveness. Comparative studies between the proposed GWO-based FLPSS and conventional stabilizers are performed on a single-machine infinite bus system(SMIBS). The simulation results demonstrate the GWO-based FLPSS can better dampen out low-frequency oscillations and maintain the electricity network stability.

Power flow control and power oscillation damping in a 2-machine system using SSSC during faults

Abstract With the rapidly growing population, energy demand is increasing. The power supply to consumers must be free from distortions. By injecting voltage in quadrature with line current and varying the magnitude, the SSSC offers series compensation to the line. The injected voltage, which offers the effect of inserting an inductive or else capacitive reactance in series with the transmission line, is in quadrature with the line current. Using MATLAB/Simulink software, a phasor model of a 2-machine device with SSSC integration and POD as a subsidiary controller is simulated in this paper to evaluate efficient power flow regulation. The simulation has been used to study the time domain behavior of SSSC under normal and faulty conditions. The SSSC is implemented for correcting the voltage and analyzing power responses during a low voltage fault in the power system, whereas in normal conditions, the power system’s voltage stability for maintaining steady acceptable voltages at every bus is analyzed. It has been revealed that POD controller assists SSSC by supplying the reference voltage signal to damp out the low frequency power oscillations. The objective of this study is to reduce the crest outreach and clearing time during the fault thus improving the transient stability.

A Reference-Feedforward-Based Damping Method for Virtual Synchronous Generator Control

The virtual synchronous generator (VSG) control has been extensively applied for realizing grid-friendly interconnections of power converters. Nevertheless, due to the direct shaft dynamics emulation, low-frequency power oscillations may be introduced in the case of limited damping effects. To solve this issue, power control law modifications have been proposed based on the feedback control theory. Although power oscillations can be suppressed to an acceptable level, the original inertial response of the VSG will be significantly degraded. This letter proposes a new damping method based on the reference feedforward control. The proposed method is able to effectively attenuate poorly damped power oscillations in VSG-controlled converters without affecting the original inertial response. Theoretical analysis and effectiveness of the proposed damping method have been validated by both electromagnetic transient simulations and experiments.

Small‐disturbance stability of a concentrating solar power collecting system with multiple synchronous generators

This paper examines the small-disturbance stability in planning a concentrating solar power collecting system (CSPCS) with multiple concentrating solar power (CSP) generation plants, which are modelled as synchronous generators (SGs). The examination indicates that an oscillation mode of the CSPCS gradually moves to the right half of complex plane when the planned number of the CSP generation plants increases. Such oscillatory instability is different with the low-frequency electromechanical power oscillations (LEPOs) in a conventional multi-machine power system, because the unstable oscillation mode is associated with the excitation systems, rather than the electromechanical dynamics of the SGs. Analysis is carried out in the paper to explain why the instability risk found in the planning is associated with the excitation systems instead of electromechanical dynamics of the SGs. The analysis helps to understand the finding made in the planning. Furthermore, rules are proposed to adjust the gain of feedback loop of the excitation systems to eliminate the risk of oscillatory instability. This suggests that the parameters’ setting of excitation systems recommended by the IEEE standard may need to be modified to accommodate the planning of CSPCS. Tests are presented to demonstrate and evaluate the analysis and conclusions made.

A Hybrid Power Sharing Control to Enhance the Small Signal Stability in DC Microgrids

Low-frequency power oscillations pose serious problems to the operation of Distributed Generations (DGs) equipped with droop control. This paper presents a new hybrid power-sharing control for damping low-frequency power and current oscillations in DC microgrids. The proposed control consists of a decentralized droop control together with a centralized average current sharing control. A small-signal equivalent model is analytically derived for the proposed hybrid control and is incorporated into the DC microgrid model. The parameters of the PI controllers involved in the average current sharing control units are tuned individually to achieve a satisfactory performance in a stand-alone operation of DGs with a local load. The contributions of control signals contained in the hybrid control signal to the damping characteristic are analyzed. The dynamic response of the DC microgrid under a variety of disturbance scenarios is investigated. The power-sharing accuracy of the proposed control is also examined under the investigated scenarios. The method performance is also investigated for the oscillations caused by constant-power loads. Sensitivity analyses are conducted to evaluate the impact of the loading level and line parameters on the damping ratio associated with the dominant low-frequency modes. The effectiveness of the control method is examined in case of losing a number of communication links. The impact of the delay in the communication links on the method performance is also examined. The simulations are conducted on a multi-source DC microgrid, and the results are verified in the OPAL-RT real-time simulator.

Coordinated design of PSS and STATCOM-POD based on the GA-PSO algorithm to improve the stability of wind-PV-thermal-bundled power system

The application of renewable energy has a significant impact on the stability of power systems. This article jointly uses static synchronous compensator (STATCOM), additional power oscillation damping (POD) controller and power system stabilizers (PSSs) to improve the stability of power systems in transmission network. An enhanced intelligent optimization algorithm is utilized to optimize the controller parameters. The proposed algorithm introduces the idea of combined genetic algorithm (GA) and particle swarm algorithm (PSO) to overcome the local convergence problem of the PSO. The voltage control and damping control of STATCOM are comprehensively considered in the objective function to suppress low-frequency power oscillation (LFO) while ensuring the effectiveness of voltage control. Numerical results from the IEEE 4 machine 2 area system and the IEEE 16-machine five-areas system demonstrate the effectiveness of the proposed method and the superiority on restraining the LFO of the wind-PV-thermal-bundled (WPTB) system.

Simulation control model of synchronous motor based on PSO algorithm optimization in power system

Under normal operation, the power angle, voltage and frequency of a power system should be stable. The periodic instability of power angle occurs after small disturbance, which is the low-frequency power oscillation of power system. In this paper, the problems and methods regarding optimization of low frequency power oscillation in power system are introduced. The intelligent algorithm is used to solve the problem. The modern intelligent algorithm is analyzed. The particle swarm optimization algorithm is mainly introduced. The basic principle of program design and parameter selection of the particle swarm optimization algorithm are described in detail. The fitness function of the damping characteristic of the speed regulation system is listed, and the damping characteristic parameters of the speed regulation system are optimized. The optimization results show that the damping characteristic of the speed regulation system has been significantly improved after optimization.

DC‐link low‐frequency current and voltage ripple analysis in multiphase voltage source inverters with unbalanced load

Inverter's performance and operating mode may be negatively affected by inverter input (dc-link) current and voltage ripple. It is a common experience that even theoretically balanced loads with perfectly balanced supply voltages, such as multiphase ac motors supplied by pulse-width modulation (PWM) inverters, in practice show a certain degree of current unbalance, in the range of a few percent, which introduces a low-frequency instantaneous power oscillation. This reflects in current and voltage low-frequency ripple on the dc-link inverter side (i.e. at the double-fundamental frequency). A possible method to analyse this matter is through the symmetric sequence components. In particular, based on the first negative current sequence component and by considering the equivalent dc-link impedance calculated at the dominant double-fundamental frequency, the amplitude of the corresponding dc-link voltage ripple component is calculated in this work for a general multiphase load. Finally, the design of the dc-link capacitor in multiphase inverters is proposed considering requirements referred to the double-fundamental dc voltage ripple. The feasibility of the proposed developments has been verified for three-, five- and seven-phase inverters by both numerical simulations and comprehensive experimental tests, always showing a good matching.

Electromechanical interactions of full scale converter wind turbine with power oscillation damping and inertia control

In this paper, the impacts of power oscillation damping (POD) and inertia controller on the structural system of wind turbine (WT) are evaluated. Synchronous generators (SGs) fitted with well-established power system stabilisers (PSS) are declining leading to reduced power system inertia and damping capability which are critical to system stability performance. The WTs will become more dominant and should therefore be capable of providing inertia response and POD to maintain system performance. However, there has been some concern amongst the industry regarding effects of inertia and POD controls on WT performance when the active power modulation is utilised by both controllers particularly during frequency event that leads to low-frequency power oscillations. A detailed grid-connected full-scale converter (FSC)-WT system which combines aerodynamic, electrical, and mechanical characteristics was developed to assess its interactions with the POD and inertia controllers and also the resulting impact on the WT structural system. The obtained results show that the implementation of the POD/inertia controllers on the FSC-WT can improve the inertia response and oscillation damping but will affect the dynamics of its drivetrain, blades, and tower. However, these effects are shown to be smaller than the effects caused by the credible electricity system faults.

The quantum‐mode regulated power point tracking in a photovoltaic array for application under the quantised converter duty ratio

The objective of this work is to develop a realistic control technique for stably regulating the power output of a photovoltaic array. The associated process is referred to as the regulated power point tracking (RPPT). Ideally, the converter duty ratio is expected to be a continuous variable so that the photovoltaic power output can be freely adjusted. However, such a continuous-mode RPPT is possible only with the help of an analogue pulse-width-modulation (PWM) control. In contrast, the modern digital PWM control causes a duty ratio quantisation. As a result, the photovoltaic power output can, practically, be adjusted only at some discrete power levels. Therefore, existing RPPT control techniques, which optimistically assume a continuous power production range, may lead to persistent low-frequency power oscillations because of the digital PWM. To overcome the particular problem, a quantum-mode RPPT control technique is proposed in this paper. The quantum-mode RPPT basically limits its search only to the practically attainable discrete power levels by suitably modifying the given power reference command. Thus, the power tracking can always reach a converged state. For the faster RPPT, the perturbation step size is dynamically adapted. The proposed RPPT control technique is verified via both simulations and hardware experiments.

Analysis of Small-Signal Power Oscillations in MTDC Power Transmission System

This paper investigates the small-signal power oscillations in a VSC-based multi-terminal DC (MTDC) power transmission system. Regarding the DC power oscillation modes (DPOMs), this paper demonstrates that when the master-slave control is adopted for the MTDC power system, there is only one DPOM. In contrast, when the DC voltage droop control is adopted, the DC electric power oscillations (DEPOs) may be multi-modal oscillations as there are multiple DPOMs. Furthermore, a better understanding of DEPOs, which is analogous to low-frequency electric power oscillations (LEPOs) in conventional AC power systems, is obtained, unveiling how the DPOMs determine the power oscillations in AC and MTDC power systems. Moreover, this paper theoretically verifies that the DPOMs determine the behaviors of both DEPOs in MTDC systems and power oscillations between the VSCs (which adopt the DC voltage control or DC voltage droop control) and external AC systems. By analogy to LEPOs, damping torque analysis is applied to examine the stability risk of the MTDC power system. The results indicate that even if VSCs are self-stable, there is a potential risk of growing DEPOs occurring in MTDC power systems under heavy DC loads.

A Method to Design Power System Stabilizers in a Multi-Machine Power System Based on Single-Machine Infinite-Bus System Model

This paper proposes a method to design power system stabilizers (PSSs) based on single-machine infinite-bus system models to mitigate the risk of low-frequency electro- mechanical power oscillations in an N-machine power system. First, models of N fabricated identical-machine power systems are established for the N-machine power system. Analysis in the paper indicates that the electromechanical oscillation mode of fabricated identical-machine power systems with the least damping is of less damping than the electromechanical oscillation modes of N-machine power system. Second, it is proved that models of fabricated identical-machine power systems are dynamically equivalent to the single-machine infinite-bus system models. Finally, it is suggested that the PSSs are designed using single-machine infinite-bus system models to enhance the least damped electromechanical oscillation mode of fabricated identical-machine power systems. This eventually improves the damping of electromechanical oscillation modes of N-machine power system, thus mitigating the risk of low-frequency electromechanical power oscillations. In the paper, the proposed method is demonstrated and evaluated by using three well-known example multi-machine power systems. Effectiveness of PSSs is confirmed by the results of modal computation and simulation.

An Oscillation Energy Calculation Method Suitable for the Disturbance Source Location of Generator Control Systems

Low frequency power oscillation threatens the secure and stable operation of power systems. Since the appearance of low frequency oscillation there are lots of approaches to analyze this problem. The oscillation source location method based on oscillation energy is adopted to find the cause of the oscillation, and to locate the oscillation source. Forced power oscillation is a kind of low frequency oscillation based on resonance mechanism, and the key is to quickly locate and remove the disturbance source. Aiming at the shortcomings of the existing disturbance source location methods, an oscillation energy calculation method suitable for the generator control systems level location is proposed. With the rich acquisition of generator data by means of phasor measurement unit (PMU), this method has the advantage of not relying on generator parameters. Taking the forced power oscillation caused by the abnormality of the governor system as an example, the superiority of the method is verified.

An Alternative Way to Locate Disturbance Source for Low Frequency Oscillation Considering Control Devices of Generator

Fast and accurate location of the power oscillation disturbance source is of great significance for the implementation of power system safety and stability control measures. This paper focuses on the applicability study of the oscillation energy flow. Aiming at the evaluation of the oscillation energy flow in disturbance source location of low frequency power oscillation, the effects of different methods are compared. After that, a fast Fourier transform (FFT)-based oscillation energy flow method considering control devices of generator is proposed. Taking a low frequency oscillation case caused by the abnormal governor system as an example, the effectiveness and accuracy of the proposed method is verified.

Dynamic Instability of a Power System Caused by Aggregation of Induction Motor Loads

Aggregated loads of induction motors are often used in the study of power system dynamic stability. This paper reports the finding that aggregation of the same or similar induction motors may likely cause the dynamic instability. Theoretical analysis is presented in the paper to show that a group of N same induction motors in parallel connection is equivalent to N dynamic independent subsystems. Oscillation modes of one of the equivalent subsystems are affected by the number of induction motors. Hence, when the number of induction motors increases, it is possible that the oscillation modes of the group of induction motors may move toward the right on the complex plane, leading to the dynamic instability in the worst case. The analysis explains why aggregation of induction motors may possibly bring about the risk of power system dynamic instability. In this paper, case studies of an example power system with a cluster of induction motors are presented. Results of modal computation and simulation demonstrate that when the number of induction motors increases, aggregation of the same or similar induction motors leads to the dynamic voltage instability and growing low-frequency electromechanical power oscillations in the test case.

Describing Function Method Based Power Oscillation Analysis of LCL-Filtered Single-Stage PV Generators Connected to Weak Grid

For the LCL -Filtered single-stage photovoltaic (PV) generators, the low-frequency power oscillation ( LCL -Filtered single-stage PV generators. First, the complete model of the PV generators is established including the perturbation and observation (P&O) based PV power loop and dc voltage loop, which are rarely considered by the existing stability analysis methods. Especially, the P&O based PV power loop is nonlinear and discontinuous, and the conventional small-signal modeling is inapplicable. Hence, its influence is completely ignored in the existing stability analysis methods. Second, considering the nonlinear discontinuous link, the describing function method is adopted to analyze the whole system stability based on the established complete model. In this way, the accuracy and completeness of the stability analysis are enhanced. Furthermore, for the critically stable state, both the oscillation amplitude and frequency can be calculated accurately. At the same time, different influence factors are analyzed quantitatively including the operation points, grid strength, and the interaction of multiple control loops. It is first revealed that the grid impedance has different influence on the system high-frequency and low-frequency stability. Also, the proposed analysis method and the conventional method are compared, which presents the advantages of the proposed analysis method. Finally, all the theoretical analyses are verified by the real-time hardware-in-loop tests.

A Case Study of Low Frequency Oscillation Caused by PSS Field Test

In this paper, the event of low-frequency power oscillation caused by power system stabilizer (PSS) field test in a power plant is introduced, and the mechanism of oscillation is analyzed. In view of the existence of several dominant components in this low frequency oscillation accident, an improved measure is proposed to achieve the disturbance source localization. Based on fast Fourier transform (FFT) decomposition of active power and frequency in steady-state oscillation, several dominant components of the oscillation are obtained. By superimposing the non-periodic components of the oscillation energy for several dominant modes, the disturbance source of the low frequency oscillation accident is determined.

Active Power Decoupling for Current Source Converters: An Overview Scenario

For single-phase current source converters, there is an inherent limitation in DC-side low-frequency power oscillation, which is twice the grid fundamental frequency. In practice, it transfers to the DC side and results in the low-frequency DC-link ripple. One possible solution is to install excessively large DC-link inductance for attenuating the ripple. However, it is of bulky size and not cost-effective. Another method is to use the passive LC branch for bypassing the power decoupling, but this is still not cost-effective due to the low-frequency LC circuit. Recently, active power decoupling techniques for the current source converters have been sparsely reported in literature. However, there has been no attempt to classify and understand them in a systematic way so far. In order to fill this gap, an overview of the active power decoupling for single-phase current source converters is presented in this paper. Systematic classification and comparison are provided for researchers and engineers to select the appropriate solutions for their specific applications.