A Low-frequency Oscillation Suppression Method for Power Systems Based on Self-impedance Control

The problem of low-frequency power swings is a matter of concern for power systems engineers in the operation and planning of power systems. Therefore, this paper is an investigation of the use of unified power flow controller (UPFC) that is suitably tuned to improve the damping of low frequency power oscillations present in Nigerian grid system. A large disturbance is initiated, during simulation in PSAT, in the network to induce nonlinear oscillations of the power system in order to ascertain the damping effect of UPFC. The dynamic responses of the system simulated show that the system is well damped and is stabilized at minimal time which confirms the robustness of the UPFC. Finally, a comparative study in a typical transient stability problem on IEEE 14-bus system using a UPFC model and the Nigerian high voltage interconnected system is used to illustrate the concepts presented in this paper. Key words: Power oscillation damping, power systems, unified power flow controller (UPFC), flexible AC transmission system (FACTS) devices, low frequency oscillation

The problem of low-frequency power swings is a matter of concern for power systems engineers in the operation and planning of power systems. Therefore, this paper is an investigation of the use of unified power flow controller (UPFC) that is suitably tuned to improve the damping of low frequency power oscillations present in Nigerian grid system. A large disturbance is initiated, during simulation in PSAT, in the network to induce nonlinear oscillations of the power system in order to ascertain the damping effect of UPFC. The dynamic responses of the system simulated show that the system is well damped and is stabilized at minimal time which confirms the robustness of the UPFC. Finally, a comparative study in a typical transient stability problem on IEEE 14-bus system using a UPFC model and the Nigerian high voltage interconnected system is used to illustrate the concepts presented in this paper. Key words: Power oscillation damping, power systems, unified power flow controller (UPFC), flexible AC transmission system (FACTS) devices, low frequency oscillation

Guest Editorial: Situational awareness of integrated energy systems

A walking assistance apparatus which can promote gait training for motor palsy patients and outdoor walking for the elderly was developed. Our prototype uses a novel spatial parallel link mechanism and restraint free type of equipment. The apparatus supports complete leg alignment, including the soles of the feet from under the soles with flat steps, and assists walking behavior at the ankle, knee, and hip joints simultaneously without the use of Velcro tape. We proposed the control method according to the purpose of the user. For example, to promote gait training, this apparatus can select the impedance control method. User's legs are assisted along the trajectory with the ideal gait as well as walking. To promote outdoor walking for the elderly, this apparatus can select the torque control method. The apparatus calculates the necessary torques of each joint according to the posture and motion of the apparatus and user. In the case of the impedance control method, by adjusting the natural frequency of the ideal dynamic equation for the flat steps under the soles, the user felt it was easier to walk by frequency entrainment. On the other hand, to promote outdoor walking of the user, this apparatus has to be used not only on the flat ground but also on stairs or slopes with freedom behaviors, and it has to make the user feel comfortable. To address this problem, the torque control method proposed to use real-time acceleration data instead of using the template trajectory. By using this method, the subject could walk on the slope and stairs continuously. Finally, %MVC of the user's EMG while walking equipped with this apparatus was measured, and the influence of the difference using between the impedance and torque control method was confirmed by interviewing the subjects. From the result, the impedance control method assisted the motion of kicking the ground, and the torque control method assisted the motion of raising-up the leg. Therefore, the impedance control method is good for gait training, and the torque control method is good for walk promotion.

Due to power system oscillations induced by complex interconnections, power system stability has been a major concern and will continue to be a major concern in power system operations. For improving power system stability, efforts have been directed toward damping power system oscillations. The small signal stability is the instability that has been influenced by complex interconnections. Small signal stability can be dampened with power system stabilisers (PSS). PSS, on the other hand, have certain disadvantages, such as significant variations in voltage profiles and the potential for leading power factor operations, which may reduce system stability in the face of severe disturbances. In wide multi machine power systems, using only traditional PSS can not be enough to dampen inter-area oscillations and the effect of voltage profile variation. Power electronic devices known as Flexible Alternating Current Transmission System (FACTS) devices, on the other hand, are good at damping inter-area oscillations and can handle voltage variations. As a result, based on computational intelligence techniques, this paper provides a thorough analysis and evaluation of PSS and FACTS power oscillation damping (POD) controllers in reducing low frequency oscillations. The importance of technical paper publications on PSS and FACTS-based controller design, optimum location, and parameter tuning is highlighted. Finally, the potential future research directions for novel PSS and FACTS devices is proposed as a reference for interested researchers.

With rapid proliferation of wind generation in current generation mix, the issue of low frequency oscillations (LFOs) may get escalated in the modern power grids. The eigenvalue and dynamic sensitivity analysis have been employed to examine the effect of wind integration on system damping. Further, a wide area based robust damping improvement control is suggested. It involves the coordinated control of power system stabilizers (PSSs) of synchronous generators (SGs) and power oscillation dampers (PODs) of doubly fed induction generators (DFIGs). The robust control is attained by employing a new fitness function based on eigenvalue and damping ratio and optimized by Whale Optimization Algorithm (WOA). The wide area POD inputs are selected using modal observability criterion, obtained using phasor measurement units (PMUs) located optimally in the system. The results are verified on IEEE benchmark 68 bus NY-NE (New York- New England) test system. The simulation results highlights the robustness of proposed control to changing system conditions and shows its effectiveness in augmenting system damping and thus small signal stability with high level of wind penetration.

Synchrophasor has evolved as an effective tool for detection of low frequency oscillation (LFO) inherent to a power system. This paper presents case studies to illustrate utilization of Synchrophasors for post-mortem analysis of oscillations to identify the root cause and evaluate the effectiveness of the remedial measures like Power System Stabilizer (PSS) and System Protection Schemes (SPS) for improving damping in the system. Authors suggest installation of Phasor Measurement Unit (PMU) at suitable locations for performance evaluation of other controllers installed in the system for power oscillation damping (POD).

This paper proposes an intelligent massage control system by using multi-fingered robot hand with hybrid impedance and force control, which is able to create the movement and force of robot to behave as similar as the human’s massage. Therefore, the various massage points, such as the changing of hardness and softness of human skin muscle condition, can be appropriately controlled by using hybrid impedance and force control method. The impedance control is used to control the lateral position of massage on the human skin muscle. The perpendicular direction one, while force control is carried out for the vertical direction on human skin muscle. This paper also gives a modeling of human skin muscle through robot perception of impedance to decide the parameter of impedance controller. The control strategy using a hybrid impedance and force control to implement an adaptive control system is presented, when human condition is changed with soft and hard skin muscle. For the robot fingertip force, PI servo compensation is used together with impedance control. Based on using both impedance and PI servo compensation, the effectiveness of the massage position and force control is shown through actual massage motion experiments.

Electro-mechanical oscillations between interconnected synchronous generators and oscillations in system frequency are an inherent part of the operation of large power systems. Very Low Frequency (VLF) oscillations are usually classified as oscillations in the 0.01-0.1 Hz range. With the move towards variable renewable energy sources and low-inertia power systems, VLF oscillations are being observed with increasing regularity in many small and island grids. If left undamped, these can present a threat to system stability. However, finding the root cause and source(s) of VLF oscillations is an extremely challenging task for network operators. Recent work has identified a need for improved tools for identifying and characterising VLF oscillations, in order to determine the combination of system conditions that can be used as predictors for VLF events. A suitable small signal model is also required in order to enable verification of the root cause of VLF events and study of mitigation measures. Accordingly, this paper presents a new approach for detrending and characterizing system frequency oscillations using an adapted Zhou algorithm. The paper also describes a method for applying this algorithm for the detection/location of oscillations, and for their detrending and characterization. Finally, an approach for relating detected oscillation events to power system operating conditions for diagnostic purposes is described. The effectiveness of the proposed approach is demonstrated using a single frequency power system model, and using system frequency oscillations recorded from the Irish power system.

With the increasing of wind power, the influence of wind farm accessing in power system is becoming more and more significant. The randomness and volatility of wind power can cause frequency stability, voltage stability, low frequency oscillation and other problems. It will more challenges to the regulation and operation of power system. The power system which is composed of controllable and schedulable synchronous power generators couples with a large number of wind power with different behavior characteristics. The integration of large-scale wind power into the synchronous power grid may lead to a large random move of power system slack bus, and change the intrinsic structure of the system transient stability. The mathematical model and control architecture is discussed and analyzed in the paper. The influences of transient stability, including voltage stability, angle stability and frequency stability, with wind power accessing in regional grid are analyzed by simulation under different sences. The research results have important guiding meaning to stable operation of power grid.

The PT low frequency nonlinear oscillation is easily excited by external disturbances in the distribution network with ungrounded neutral point, which can cause the PT fuse fuse, burning loss and even the explosion accident frequently. The mechanism of PT low frequency oscillation is analyzed theoretically, and the phenomenon and key factors are simulated based on the electromagnetic transient simulation program PSCAD. A new low frequency oscillation suppression method for PT is proposed, that is, the PT neutral point is grounded by low frequency surge suppressor. The effectiveness of the suppression method is verified by simulation, and the low frequency inrush current can be controlled within the safe range. The theoretical research and the new suppression method proposed in this paper can provide guidance for PT fault analysis and the formulation of countermeasures in distribution network.

The integration of ICT into power systems has increased the interdependencies between the two systems. The operation of power system depends on several ICT-enabled grid services which manifest the interdependencies. ENTSO-E system state classification is a tool that is widely used by operators to determine the current operational state of the power system. However, it does not adequately describe the impact of ICT disturbances on the operation of the power system. Despite their interconnections, the operational states of both systems have been described separately so far. This paper bridges the well-established ENTSO-E systems state classification with an ICT system state classification, forming a new model considering the state classification of the ICT-enabled grid services. The model is developed by first identifying the ICT-enabled services, remedial actions and the respective performance requirements that are required by the power system. Then the states of these services are specified based on the supporting ICT system. The resulting joint state description shows how performance degradation of ICT-enabled services (introduced by disturbances) can affect the operation of the interconnected power system. Two case studies of such ICT-enabled services, namely state estimation and on-load tap changer control, are investigated in terms of how their operational states affect the states of the power system. A third case study highlights the interdependencies that exist between the services. These case studies demonstrate the interdependencies that exist between power and ICT systems in modern cyber-physical energy systems, thus highlighting the usage of a unified system state description.

Low frequency oscillation (LFO) is dangerous for power systems secure operation. Over the past decades, installations of power system stabilizer (PSS) on synchronous generator (SG) exciter or flexible AC transmission systems (FACTS) are widely-used methods in electrical power industry to suppress power system oscillation and improve the system oscillation stability. But with the rapid increase of renewable energy sources, more and more power electronic converters with seriously different dynamic characteristics from that of SGs’ are connected and operated in the power system. Thus, the future power system is very likely to generate more complex oscillation phenomenon and system stability problems are maybe more serious than ever before. Under these circumstances, adding supplementary control on these existing power electronic converters is becoming an attractive choice. In this paper, inspired by working principle of the PSS, an improved virtual synchronous generator (VSG) control strategy is presented to damp LFO actively via adaptively adjusting its parameter values of virtual inertia and damping coefficient, according to deviation value and changing rate of its output active power. This idea takes full advantage of flexible adjustability of VSG’s parameters. The effectiveness of the proposed VSG strategy is proved by simulation results, which indicates remarkable performance in fast damping of LFO.

In the face of challenges posed by increasing wind power integration, penetrations, SG replacement with Wind Farm (WF), and significant disturbances and their impact on small signal and transient stability. This paper presents a novel solution employing coordinated control of Power System Stabilisers (PSS) and Power Oscillation Dampers (POD). The stochastic nature of wind power and nonlinear wind speed behaviour creates complexities and is exacerbated by severe disturbances. Traditional PSS and POD designs struggle with these non-linearities. An intelligent coordinated control strategy based on a Fuzzy Logic Controller (FLC) is proposed to address this issue. The FLC synchronises and coordinates PSS and POD, enhancing stability by dampening Low-Frequency Oscillations (LFOs) and mitigating undamped fluctuations even under extreme conditions. Sensitivity analysis identifies key LFOs affecting stability, enabling optimal control gain selection. RT-LAB experimental platform on the IEEE-9 bus system validates the approach, showcasing superior performance compared to individual methods. The results underscore the robustness and reliability of FLC coordination, ensuring enhanced system stability in practical applications.

In this paper, a robust method for the simultaneous coordinated design of the interline power flow controller (IPFC) and power system stabilizer (PSS) based on output feedback controllers in a single-machine infinite-bus power system for the enhancement of power systems low-frequency oscillations damping is presented. The Honey Bee Algorithm, which has a strong ability to find the most optimistic results, is employed to search for optimal IPFC and PSS output feedback controller parameters. A singular value decomposition method is utilized to select the most effective damping control signal of the IPFC output feedback controller. To assess the effectiveness and robustness of the proposed method, simulation studies are carried out for three operating conditions. Analysis of the results shows that the combined design has an excellent capability for damping a power system’s low-frequency oscillations. Moreover, system performance analysis under different operating conditions and performance indices shows the effectiveness of the proposed method even when a severe fault is applied.