Wide-area damping controller of FACTS devices for inter-area oscillations considering communication time delays

The usage of remote signals obtained from a wide-area measurement system (WAMS) introduces time delays to a wide-area damping controller (WADC), which would degrade system damping and even cause system instability. The time-delay margin is defined as the maximum time delay under which a closed-loop system can remain stable. In this paper, the delay margin is introduced as an additional performance index for the synthesis of classical WADCs for flexible ac transmission systems (FACTS) devices to damp inter-area oscillations. The proposed approach includes three parts: a geometric measure approach for selecting feedback remote signals, a residue method for designing phase-compensation parameters, and a Lyapunov stability criterion and linear matrix inequalities (LMI) for calculating the delay margin and determining the gain of the WADC based on a tradeoff between damping performance and delay margin. Three case studies are undertaken based on a four-machine two-area power system for demonstrating the design principle of the proposed approach, a New England ten-machine 39-bus power system and a 16-machine 68-bus power system for verifying the feasibility on larger and more complex power systems. The simulation results verify the effectiveness of the proposed approach on providing a balance between the delay margin and the damping performance.

Robust design and best control channel selection of FACTs-based WADC for improving power system stability using Grey Wolf Optimizer

With the development of the Wide-Area Measurement System (WAMS), researchers has proposed different kinds of Wide-Area Damping Controller (WADC) to suppress inter-area power oscillations. However time delays in WAMS communication networks have made it less effective to improve dynamic stability of large-scale power system. Traditional methods such as modal analysis and simulation cannot study the time delay impacts on the dynamic characters theoretically and obtain boundary conditions of stability for the optimization of damping controllers. To solve these problems, this paper studies the influence of time delays on the inter-area dynamic characters using two-machine Electric Torque Analysis (ETA) method. The mathematical relationships between the time delay, electric torque coefficients, and damping ratio of a power system installed with a WADC are explored. The stability criteria of inter-area mode and the delay margin are studied. An optimization scheme of WADC is proposed to implement fast online damping control considering time-varying delays. This method is verified in a four-generator-two-machine benchmark installed with a SVC-based WADC. Simulation results show the proposed scheme can improve the robustness of WADC against time delays and has advantages over traditional methods.

A new wide‐area damping control strategy is investigated for flexible AC transmission systems (FACTS) device using wide‐area measurement system (WAMS) signals. The purpose is to design a dynamic output wide‐area damping controller (WADC) for improving the stability of interconnected power systems. The time‐varying delay of wide‐area signal is incorporated into the design process, which can effectively reduce the delay effect on the damping performance. First, a discrete‐time plant model with time‐varying delay is established for power systems; then by using the proposed improved free‐weighting matrices (IFWMs) approach and a convex optimization algorithm, a new and less conservative delay‐dependent stability criterion, expressed in the terms of linear matrix inequalities (LMIs), is obtained without ignoring any useful terms on the difference of a Lyapunov function. Detailed case studies on a 4‐machine two‐area benchmark test system and 16‐machine five‐area NETS‐NYPS interconnected system show that the designed WADC can not only maintain effective damping performance under the condition of time‐varying delay but also get the maximum wide‐area time delay. © 2015 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

Synchronous and non synchronous delay-dependent robust wide-area controllers for power system

Increasing introduction of PVs and expanding operation areas, inter-area low frequency oscillations become a serious problem and are often caused from poor system damping. According to rapid development of wide-area measurement systems (WAMS), wide-area damping controllers (WADC) designed using wide-area signals obtained from WAMS have been proposed to enhance damping of inter-area low frequency oscillations. As wide-area signals are remote signals, they need to be transmitted through a communication network. Communication time delays caused by transmission of remote signals are one of the key factors influencing system stability and damping performance of WADCs. In this paper, we propose a WADC accommodating a state feedback controller. The proposed WADC's feedback gain K is updated for each time communication delay that occurs in the wide-area signals sent by the PMU. Therefore, the eigenvalues of delay using the Pade approximation method are given desirable eigenvalues using the pole placement method to compensate for communication delays. To evaluate system stability and damping performance of the proposed WADC, we perform transient analysis with three-phase faults using the IEEJ EAST 30-machine System Model. We verify the effectiveness of the proposed WADC through simulation studies with different time delays. Simulations showed that the proposed WADC could achieve a damping performance similar to the damping performance when there are no communication time delays, even if communication delays occur in wide-area signals.

Large amount of power is required for long-distance transmission with the increase of cross -transmission of electricity demand, thus it is difficult to inhibit the power oscillation between the regions to guarantee the stable operation of the whole power system which only relies on local feedback signal designed controller. Therefore, it is one of the hot spots for the analysis and design with time delay influence of wide-area controller in recent study of interconnected grid. Based on Hamilton system theory, a nonlinear time-delay generalized Hamiltonian model of corresponding wide area measurement power system model was constructed. Based on the proposed model, the corresponding Lyapunov-Krasovskii functional was given to derive a delay-dependent steady stability criterion in term of matrix inequalities. It was noticed that control input contains the influence of time-delay signal; the corresponding wide-area damping controller (WADC) was designed. The WAN feedback control with time-delay was realized; Time delay margin delay can be calculated which makes closed loop power system is stable. The relationship between the damping performance of WADC and the time delay margin was investigated. Thus corresponding control parameters of WADC were given. Finally, taking a 16-machine 68-bus power system as an example, the results of simulation in time-domain show that performance of the proposed controller is effective.

Mitigating inter-area oscillation of an interconnected power system considering time-varying delay and actuator saturation

The wide-area damping controller (WADC), using remote signals obtained from a wide-area measurement system (WAMS) as the input, has been employed to enhance the stability of the large-scale inter-connected power system. The time delay caused by transmission of remote signals is one of the key factors influencing the whole system stability and damping performance. This paper presents an adaptive WADC based on generalized predictive control and model identification. The proposed WADC is implemented by adding its output signal to the excitation system of a selected generator. A recursive least-squares algorithm (RLSA) with a varying forgetting factor is used to identify the model of power system. Based on this model, the generalized predictive control considering control output constraints is employed. Simulations studies undertaken on a two-area four-machine power system show that the proposed adaptive WADC not only can damp the inter-area mode oscillations effectively under varying operation conditions and different disturbances, but also has better robustness against to the time delay existing in the remote signals. The comparison studies with the conventional WADC are also provided.

Low frequency oscillation occurs frequently in power systems. Wide-area damping controller (WADC) based on Wide-Area Measurement System (WAMS) can effectively suppress inter-area low frequency oscillations. However, with the introduction of global signals, time delays are inevitable. Electromagnetic torque analysis method(ETA) can establish the analytic relation between electromagnetic torque coefficients and control parameters of power component, which is facilitative to analyze small signal stability. Considering that WADC has inherent characteristics of multiple time delays, based on the ETA for equivalent two-machine system, this paper deduces the general mathematics expression among electromagnetic torque coefficients, WADC parameters and time delays. The influence mechanism of time delay on electromagnetic torque coefficients and small signal stability is further analyzed. Based on these analysis, performance of damping control of WADC can be improved, on one hand, time-delay small signal stability region is established under fixed WADC parameters, on the other hand, if small signal stability is not achieved under the given WADC parameters due to set of the time delays, optimization scheme is proposed to carry out parameters coordination according to the measured time delays. Finally, taking two time delays as an example, the paper verifies the effectiveness of the proposed method by comparing the system dynamic performance considering two time delays with the system only considering single time delay and the system without time delay in a two-area four-machine power system.

Remote signal transmission time delays are the major factors affecting damping performance of wide-area damping controller (WADC), and it even deteriorates small signal stability of large-scale power system. Traditional time delay damping controller based on linear matrix inequalities (LMI) has relatively large conservatism and complexity. Conventional time delay compensation controller suffers large amount of simulations and there is no general formulation to determine control parameters. In this paper, the design method of wide-area time-delay damping controller based on parametric Lyapunov theory is derived. The explicit expression of the controller parameter value associated with the time delay is determined. The controller is determined according to the system stability original definition (The system energy function is finite and positive and its first derivative is negative), which avoids complex matrix inequalities cyclic solution and reduces the conservatism of the controller. Moreover, it can directly give simple and explicit control law and specific control parameters, which enable this method to adjust its parameters online according to the exact time delay obtained by GPS. Case study is undertaken based on the New England ten-machine 39-bus power system. Compared with the free weight matrix controller, which is a kind of LMI-based design strategy with excellent performance, the effectiveness and superiority of the proposed design method for WADC are verified.

The aim of this paper is to investigate the design method of robust thyristor controlled series capacitor (TCSC) wide-area damping controller (WADC) to enhance the stability of power system with considering time delay of wide-area signals. Although wide-area measurement system (WMAS) is being widely applied to monitor and control the interconnected power systems, which can enable the use of measured signals from remote locations for centralized control purpose, the impact of time delays introduced by remote signal transmission and processing in WAMS cannot be ignored. So this paper describes the TCSC wide-area damping controller design problem of power system as a public time-delay feedback control problem. Based on the established time-delay power system model, the TCSC wide-area damping controller is proposed by the free-weighting matrices method, which is expressed with a set of linear matrix inequality (LMI) constraints, and the pole placement method. Finally, the simulation results on a two-area four-machine test system are illustrated by comparing with various time delays of wide-area signals. It demonstrates that the designed TCSC damping controller not only inhibits inter-area oscillation evidently, but also is robust to various time delays.

This paper investigates coordinated design of multiple wide‐area damping controllers (WADCs) for damping multiple inter‐area oscillations considering communication delays in multiple control loop. Two approaches have been investigated: the sequential approach which uses the stability criteria for single delay and design the WADC independently, and the coordinated approach which employs a less conservative and fast Lyapunov stability criterion for linear system with multiple delays to calculate delay margins simultaneously and coordinately design the multiple WADCs. The sequential approach cannot reveal the interaction among several WADCs loops with multiple delays and may lead to inaccurate delay margins, while the coordinated approach takes into account interactions of delays between different WADC control loops and can result in accurate delay stable region and relationship between delay margins and the WADC gains. Gains of WADCs are determined based on a trade‐off between damping performance and delay margin. Case studies are based on the modified IEEE 10‐machine 39‐bus power system equipped with two FACTS devices and two WADCs. The feasibility and effectiveness of the proposed approach is verified by simulation studies.

Wide area damping controller (WADC) is usually utilized to damp interarea low frequency oscillation in power system. However, conventional WADC design method neglects the influence of signal transmission delay and damping performance of WADC designed by the conventional method may deteriorate or even has no effect when signal transmission delay is beyond delay margin, an index that denotes delay endurance degree of power system. Therefore, a new design method for WADC under the condition of expected damping factor and required signal transmission delay is presented in this work. An improved delay margin with less conservatism is derived by adopting a new Lyapunov-Krasovskii function and more compact bounding technique on the derivative of Lyapunov-Krasovskii functional. The improved delay margin, which constructs the correlation of damping factor and signal transmission delay, can be used to design WADC. WADC designed by the proposed method can ensure that power system satisfies expected damping factor when WADC input signal is delayed within delay margin. Satisfactory test results demonstrate the effectiveness of the proposed method.

<span>With increasing implementation of Wide Area Measurement Systems (WAMS) in power grids, application of wide area damping controllers (WADCs) to damp power system oscillations is of interest. On the other hand it is well known that rapidly increasing integration of renewable energy sources into the grid can dangerously reduce the inertia of the system and degrade the stability of power systems. This paper aimed to design a novel WADC for a utility-scale PV solar farm to damp out inter area oscillations while the main focus of the work is to eliminate the impact of communication delays of wide-area signals from the WAMS. Moreover the PV farm impact on inter area oscillation mitigation is investigated in various case studies, namely, with WADC on the active power control loop and with WADC on the reactive power control loop. The Quantum Particle Swarm Optimization (QPSO) technique is applied to normalize and optimize the parameters of WADC for inter-area oscillations damping and continuous compensation of time-varying latencies. The proposed method is prosperously applied in a 16-bus six-machine test system and various case studies are conducted to demonstrate the potential of the proposed structure.</span>