Low Frequency Inter-area Oscillations Research Articles

STABILISATEUR DE SYSTÈME D'ALIMENTATION ROBUSTE BASÉ SUR UN CONTRÔLE H∞ STRUCTURÉ POUR LA STABILITÉ D'UN SYSTÈME MULTI-MACHINE

A robust design of power system stabilizers (PSSs) using H∞ output feedback control has been introduced in this work. To facilitate the implementation of the designed PSSs, the proposed technique employs the H∞ to tune the fixed-structure conventional lead-lag PSS parameters of the multi-machine system. These PSSs are used to improve the damping of the local and inter-area low-frequency oscillations in power systems under different operating conditions. The proposed control is tested on a multi-machine system, which is a three-machine nine-bus system. A comparative simulation study shows a significant enhancement and good performance of the proposed design compared to an IEEE conventional power system stabilizer.

Wide area damping of electromechanical oscillations based on implicit reference model adaptive control

Assessing the stability of low-frequency inter-area oscillations remains an important issue in modern electrical power systems. The paper proposes a novel method for optimal tuning of power system stabilizers for wide area damping of electromechanical oscillations based on implicit reference model adaptive control. An implicit reference model of the system is formed on the basis of choosing the desired values of the roots of the characteristic polynomial of the system by computing the determinant of its characteristic matrix. An adaptive tuning procedure seeks to preserve the “reference” values of the coefficients in the characteristic polynomial of the swing equation model of the system identified at the current operating point. The proposed approach can be also considered as an adaptive pole placement method applied to the linearized swing equation model. Numerical experiments with the IEEE 30 bus test model confirm that the proposed method can be effective in maintaining dynamic and transient stability.

Optimal fractional SSSC auxiliary controller for power system low frequency oscillations damping improvement

This work presents a new controller design to improve inter-area low frequency electromechanical oscillations (LFEOs) damping and enhance the overall stability of the electrical network. The control strategy used the static synchronous series compensator (SSSC) based flexible AC transmission systems (FACTS) series-type device mainly performed for power flow and voltage regulation. Then, a fractional order proportional integral derivative (FOPID) is introduced as an auxiliary controller for the SSSC using the difference of rotor speed deviations of generators as input signal to improve the oscillations damping. Moreover, genetic algorithm (GA) is applied to seek for optimum controller gains. The proposed control approach is examined on two-area four-machine (2A4M) test system. The FOPID performance is compared with the integer order proportional integral derivative (PID). Obtained results show that the proposed SSSC-based FOPID controller achieves high performance for enhancement of inter-area low frequency oscillations damping.

Design and simplified stability analysis of wide-area damping control with variable feedback delay

Poorly damped low-frequency inter-area oscillations are a major concern in large power systems. Wide-area damping controllers which modulate power flow using power electronic systems like HVDC links and FACTS can be used to improve the damping of the critical modes in a targeted manner. These controllers use wide-area measurements, which are communicated from several remote locations. The communication time delay is generally variable in nature, and the data from different locations arrive at the controller with different delays. The overall delay in synthesizing the controller input signal affects the closed-loop damping performance. This paper proposes a delay-handling strategy and derives a discrete-time model which is used to characterize its performance. The relative insensitivity of the mode shape of swing modes to the damping controller is used to facilitate a simplified analysis. A strategy for the adaptation of controller gain when the delays are excessive and persistent is also presented. A case study is presented to illustrate the delay-handling and gain adaptation strategies.

Identification and damping of low-frequency oscillations based on WAMS data and the revisited residue method – part I

The results of low-frequency oscillations identification in the Republic of Kazakhstan power grid by using a Wide Area Measurement System are presented and an algorithm for damping low-frequency oscillations is proposed in this paper. Analysis of weakly damped inter-area low-frequency oscillations revealed a constant mode with a frequency range of 0.3‒0.4 Hz. It was determined that at these low-frequency oscillations, the amplitude of active power fluctuations along the transmission line was 150 MW with a duration of 9 minutes. The modal analysis calculation of the Republic of Kazakhstan power system model in the «DigSilent Power Factory» software shows the dangerous low-frequency oscillation modes having a damping ratio is 2.2 % and an eigenfrequency 0.328 Hz. These oscillation modes identified by the real data and in the developed model indicate the incorrect tuning of power system stabilizer parameters at power plants. It is necessary to retune the power system stabilizer parameters whenever changing the system’s and mode’s configurations. An analysis of existing power system stabilizer tuning methods was performed, and revisited residue method was determined as sufficiently effective. Thus, the developed algorithm for identification and damping of low-frequency oscillation consists of three tasks. The first task is data collection from the Wide Area Measurement System and Supervisory Control and Data Acquisition system and updating the calculation model based on the current status of equipment (generators, transformers, transmission lines, etc.). The second task is the identification of dangerous electromechanical oscillations and modal analysis based on information obtained in real-time. The third task is tuning the power system stabilizer parameters for damping dangerous low-frequency oscillation modes based on the revisited residue method

Influence evaluation of signal transmission delay on supplementary damping controller for SVC in power system

Supplementary damping controller (SDC) for static var compensator (SVC) can be used to damp interarea low-frequency oscillations, but the influence of signal transmission delay on damping performance is generally neglected in the previous research. Because the delay may deteriorate damping performance or even cause instability for power system, this work intensively investigates the influence of signal transmission delay on input signal selection and parameter setting in SDC design. A novel evaluation method based on delay-dependent stability criterion and model order reduction is proposed to obtain the tradeoff between damping performance and delay margin for the SDC to be designed. Delay margin, determined by the proposed method, is taken as a critical index to evaluate the delay influence on SDC design. Case studies are carried out on two-area four-generator power system. Results show that for input signals with higher mode observability by residue method, such as current magnitude and active power of transmission line, delay margin decreases with the increase of SDC gain or delay variation ratio, which indicates that appropriately setting the gain can ensure SDC with better damping performance and larger delay margin.

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.

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.

Artificial electric field algorithm based superconducting magnetic energy storage system for damping inter area oscillations

This paper presents an artificial electric field algorithm (AEFA) based optimal control of superconducting magnetic energy storage (SMES) system for damping inter area oscillations. The artificial electric field algorithm is based upon Coulomb’s law of electrostatics and Newton’s law of motion. The proposed technique is used to optimize the SMES controller parameters for effective damping of inter area oscillations. Because of the potential for system damage and the subsequent constraints on power transmission along select lines, low frequency inter-area oscillations have been identified as a serious problem in utility systems. With the advancement in power electronics technologies, a new option for damping of power system oscillations has been created by integrating energy storage devices to it. The proposed scheme is tested on a two area interconnected power system which is suitably modelled and it’s modal analysis is done. The participation of each area towards the inter area mode is analyzed by using participation matrix and the suitable placement of SMES units is decided accordingly. MATLAB Simulations are carried out to prove the efficacy of proposed scheme. The simulation results show that proposed SMES controller along with the incorporation of SMES units to each generator bus effectively damps out the inter area oscillations.

Selective LQ wide area damping of power networks based on the spectral decomposition of Gramians

Inter-area oscillations in large power network are poorly controlled via local decentralized control. One promising strategy for wide-area damping of these oscillations is to minimize the quadratic state and control cost function for a linearized system model. In this paper, we improve this linear-quadratic method by introducing novel selective approach. Namely, instead of using the full state we minimize in the cost function only the selected spectral part of the state. The proposed selective LQ optimal problem is reformulated via the selected spectral part of the closed-loop observability Gramian. This spectral part is a solution to the modal Lyapunov equation specially designed for a chosen spectral range. Novel approach allows to take into account in global optimization only the selected set of oscillations and therefore to improve flexibility and accuracy of controllers setting. Numerical experiments with the IEEE New England test system confirm that the selective approach improves the performance of the conventional LQ method for suppressing low frequency inter-area oscillations.

An analytical approach for LQR design for improving damping performance of multi-machine power system

In a multi-machine environment, the inter-area low-frequency oscillations induced due to small perturbation(s) has a significant adverse effect on the maximum limit of power transfer capacity of power system. Conventionally, to address this issue, power systems were equipped with lead-lag power system stabilizers (CPSS) for damping oscillations of low-frequency. In recent years the research was directed towards optimal control theory to design an optimal linear-quadratic-regultor (LQR) for stabilizing power system against the small perturbation(s). The optimal control theory provides a systematic way to design an optimal LQR with sufficient stability margins. Hence, LQR provides an improved level of performance than CPSS over broad-range of operating conditions. The process of designing of optimal LQR involves optimization of associated state (Q) and control (R) weights. This paper presents an analytical approach (AA) to design an optimal LQR by deriving algebraic equations for evaluating optimal elements for weight matrix ‘Q’. The performance of the proposed LQR is studied on an IEEE test system comprising 4-generators and 10-busbars.

The Delay-Dependent DOFC for Damping Inter-Area Low-Frequency Oscillations in an Interconnected Power System Considering Packet Loss of Wide-Area Signals

In this paper, the inter-area low-frequency oscillations are restrained in the interconnected power system by making use of the delay-dependent wide-area dynamic output feedback controller (DOFC). Modal analysis is adopted to obtain the modes of inter-area oscillation to be controlled and the Schur truncation model reduction technique is represented to reduce the order of the power system. The augmented closed-loop system model, where the transmission delay and packet loss of wide-area signals are considered, is established. The sufficient conditions of exponentially mean-square stable are obtained according to Lyapunov’s stability theory. Finally, case studies are carried out on a two-area four-machine power system, where our proposed controller, a conventional controller, and the wide-area damping controller in the existing references are installed, respectively. The simulation results under different external disturbances, packet loss rates, and delays are presented to show the effectiveness and advantages of our proposed controller.

Controller design for output constrained thyristor controlled series capacitor (TCSC) of time-delayed power system

Abstract Thyristor Controlled Series Capacitor (TCSC) is employed as actuator in an interconnected power system using Supplementary Damping Controller (SDC) to enhance the power transfer capabilities between areas as well as damp out the Inter-Area Low-Frequency (IALF) oscillations by varying the impedance of transmission lines. The time-varying delays in the feedback loop and asymmetric saturation limits at the output of TCSC may reduce the performance of the designed controller and lead to closed-loop instability if these are not considered in the process of controller design. Usually, a Minimum Absolute Saturation Bound technique (MASB) is used to convert asymmetric saturation limits into symmetric one. However, such consideration provides an apparent conservative and degrades the performance of the control system. To reduce the conservative in the control design process, a Normalized Saturation Bound (NSB) technique is used in this paper to convert asymmetric saturation limits of TCSC of time-delayed power system. The L-K functional and generalized sector conditions are utilized in the proposed controller design process to compensate the effect of time-varying delay and converted symmetric saturation limits. Sufficient conditions required for controller design are formulated in LMI form. 2-area 4-machine power model is used to verify the performance of the designed controller. From the simulation results, it is observed that the designed controller is giving superior performance in the present of time-varying delays and asymmetric saturation as compared to the designed controller using MASB technique.

Research on improving low frequency oscillation characteristics of wind power system with DFIG-PSS-VI

The new energy represented by strong random wind power connecting to the power system may make the problem of inter-area low-frequency oscillation more serious. In this paper, a DFIG-PSS controller based on virtual impedance is constructed to solve the low-frequency oscillation problem in the wind power system. The step response of PSS-VI was carried out to test the effect of the controller to verify the advantages of PSS-VI than traditional PSS. The input signal of PSS-VI which is a controller based on PSS installed virtual impedance is the active power of DFIG. The output signal of PSS-VI is added to the reactive power control loop of rotor side controller of DFIG. DFIG-PSS-VI was built in Digsilent/Powerfactory software, and the simulation was carried out on the system of 4 machines and 2 regions. It is verified that PSS-VI can improve the low-frequency oscillation of wind power system.

Unified Smith predictor‐based loop‐shaping H ∞ damping controller for mitigating inter‐area oscillations in power system

Low-frequency inter-area oscillation is one of the major problems to transfer the bulk power from one area to another area in an interconnected large power system. A wide-area signal-based controller is more effective to improve the damping of the inter-area oscillation than a local area signal-based controller. A wide-area measurement system makes it easier to transmit the wide-area signal through the communication system from the remote location to the controller site. However, the involvement of the unavoidable time delay in the wide-area signal transmission from a remote area to a controller site is to possess a great deal of challenge to design a damping controller. In this study, a unified Smith predictor (USP)-based loop shaping controller is designed to handle the negative effect of the delay using the wide-area signal. The robust stabilization normalised co-prime factor problem is converted into a generalised H ∞ optimisation problem for additional pole placement constraints. The performances of the USP-based loop-shaping H ∞ controller are compared with the USP-based H ∞ controller. From the obtained results, it is verified that the proposed controller gives an excellent damping performance and handles the effect of time delay.

Impact of HVDC dynamic modelling on power system small signal stability assessment

This paper presents a new measure that can be used to identify the locations where high levels of HVDC modelling can be simplified while maintaining the accuracy of the small-signal stability results. Low-frequency inter-area oscillations have posed a major issue to power system operations and stability, for example by reducing the maximum power transfer on tie lines connected between different regions or causing cascading failure due to ever-growing oscillations. In modern power systems, a significant number of High Voltage Direct Current (HVDC) systems are connected to power networks. An appropriate level of HVDC modelling is required in order to obtain accurate small-signal stability results in multi-infeed HVDC systems. Although accurate results can be obtained by using complex modelling details, this can be prohibitively computationally expensive due to increasing simulation time. In this paper, different levels of modelling fidelity for HVDC systems are investigated in order to establish the impact of the included HVDC system dynamics on small-signal stability. It is shown that some dynamic HVDC modelling, particularly LCC-HVDC and VSC-HVDC with P-Q control, can be replaced by simplified models in order to reduce the simulation time and model complexity while maintaining the accuracy of small-signal stability results. Furthermore, a method to identify which HVDC systems require detailed modelling based on the quantification of changes to critical mode shapes is developed.

Distributed inter-area oscillation damping control for power systems by using wind generators and load aggregators

This paper investigates the potential of wind turbine generators (WTGs) and load aggregators (LAs) to provide supplementary damping control services for low frequency inter-area oscillations (LFOs) through the additional distributed damping control units (DCUs) proposed in their controllers. In order to provide a scalable methodology for the increasing number of WTGs and LAs, a novel distributed control framework is proposed to coordinate damping controllers. Firstly, a distributed algorithm is designed to reconstruct the system Jacobian matrix for each damping bus (buses with damping controllers). Thus, the critical LFO can be identified locally at each damping bus by applying eigen-analysis to the obtained system Jacobian matrix. Then, if the damping ratio of the critical LFO is less than a preset threshold, the control parameters of DCUs will be tuned in a distributed and coordinated manner to improve the damping ratio and minimize the total control cost at the same time. The proposed control framework is tested in a modified 10-machine New England system and a modified 14-generator Australian power system. The simulation results with and without the proposed control framework are compared to demonstrate the effectiveness of the proposed framework.

Unified Smith predictor based H wide-area damping controller to improve the control resiliency to communication failure

Inter-area low frequency oscillation in power system is one of the major problems for bulk power transmission through weak tie lines. Use of wide-area signal is more effective than the local area signal in damping out the inter-area oscillations. Wide area measurement system &#x0028 WAMS &#x0029 is convenient to transmit the wide area signal through the communication channel to the remote location. Communication failure is one of the disastrous phenomena in a communication channel. In this paper, a dual input single output &#x0028 DISO &#x0029 H&#x221E controller is designed to build the control resiliency by employing two highest observability ranking wide area signals with respect to the critical damping inter-area mode. The proposed controller can provide sufficient damping to the system and also the system remains stabilized if one of the wide-area signals is lost. The time delay is an unwanted phenomenon that degrades the performance of the controllers. The unified Smith predictor approach is used to design a H&#x221E controller to handle the time delay. Kundur&#x02BC s two-area and IEEE-39 bus test systems are considered to verify the effectiveness of the proposed controller. From the simulation results, it is verified that, the proposed controller provides excellent damping performance at normal communication and improves the controller resiliency to counteract the communication failure.

Unified Residue Method for Design of Compact Wide-area Damping Controller Based on Power System Stabilizer

A wide-area damping controller (WADC) is effective in damping inter-area low-frequency oscillation (LFO), if the time delay in a wide-area control loop can be properly handled. In order to simplify the WADC design and enlarge the delay adaptation range, the classic power system stabilizer (PSS) is adopted, and a new unified residue (UR) method is proposed for compact WADC design. The strategy of control loop selection is also improved by modifying the relative residue index based on a few dominant oscillation modes. The designed PSS-based compact WADC is as simple as classic PSS with no more than two lead-lag phase compensation units. Case studies are carried out on an IEEE 16-machine 68-bus power system. Simulation results demonstrate that the control loop selection before the WADC design is necessary and that the proposed selection strategy can easily pick out the suitable candidate control loops. In addition, it is feasible for the UR method to design WADCs with different time delays in the selected control loops. All the designed WADCs are effective in damping inter-area LFO and robust to time delay variations under operation conditions. Comparisons among five design methods for PSS-based WADC show that the proposed UR method is superior in delay adaptation, the conciseness of WADC structure and computation speed of parameters.

Self Tuning Wide Area Damping Control for Distributed Power Systems

Damping of low-frequency inter-area oscillations in a power system has been the main concern for ascertaining the small-signal stability. Much of these efforts had been paid on designing damping controller with controller parameters obtained by offline analysis using a linearized model of the power system around a most probable operating point. A modern power system is however subjected to frequent changes in operating points due to load, renewable penetration and topology variations. An adaptive controller has the advantage of flexibility in getting the controller parameters auto-tunned online thus can be used for different operating conditions. This paper presents a WADC controller using a self tunning control (STC) strategy. The evaluation of the designed controller under various conditions has been carried out with renewable integrated IEEE 11 bus and IEEE 39 bus test systems. From the results, it is found that the proposed WADC can provide improved damping on inter-area oscillations under variable operating conditions.