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.

Wide-Area Damping Controller of FACTS Devices for Inter-Area Oscillations Considering Communication Time Delays

In this paper the design of power system stabilizers for small-signal stability using phasor measurements is considered. An approach based on optimal control with structural constraints is proposed to design a two-level control structure. Time delays are included in the design. The method is combined with order reduction to ensure faster convergence of the design algorithm and to facilitate the choice of the weighting matrices for damping interarea modes. The control scheme is discussed and modal analysis and time-domain simulations of two Brazilian equivalent systems are performed to assess the control performance. The robustness to loss of communication links, topological changes, and variations of time delay is evaluated.

Wide-Area Measurements-Based Two-Level Control Design Considering Signal Transmission Delay

The vision of a smart grid is predicated upon pervasive use of modern digital communication techniques to today's power system. As wide area measurements and control techniques are being developed and deployed for a more resilient power system, the role of communication network is becoming prominent. Power system dynamics gets influenced by the communication delays in the network. Therefore, extensive integration of power system and communication infrastructure mandates that the two systems be studied as a single distributed cyber-physical system. This paper proposes a power system and communication network co-simulation framework (GECO) using a global event-driven mechanism. The accuracy is tunable based on the time-scale requirements of the phenomena being studied. This co-simulation can improve the practical investigation of smart grid and evaluate wide area measurement and control schemes. As a case study, a communication-based backup distance relay protection scheme is co-simulated and validated on this co-simulation framework.

GECO: Global Event-Driven Co-Simulation Framework for Interconnected Power System and Communication Network

This paper investigates the delay-dependent stability of a power system equipped with a wide-area damping controller (WADC) using a new method which consists of a delay-dependent criterion based on Lyapunov theory and model reduction technique. The delay margin, the maximal delay which allows the closed-loop power system to retain stable, is calculated based on the reduced order model and linear matrix inequality (LMI) technique. Both constant and time-varying delays introduced by the transmission of the remote signals are investigated. Cases studies are carried out based on a two-area four-machine power system controlled by a conventional lead-lag WADC and a robust WADC. The relationship between the parameters of WADC and the delay margin has been investigated and is used to guide the design of WADC and achieve a trade-off between the damping performance and delay margin. The effectiveness of the proposed method is verified by simulation studies.

Delay-Dependent Stability Analysis of the Power System With a Wide-Area Damping Controller Embedded

Wide-area damping controller (WADC) requires communication networks to transmit remote signals. The usage of communication networks will introduce time delays into the control loop of the WADC. Ignoring this time delay would deteriorate the damping performance provided by the WADC or even cause the whole system instability. This paper employs networked predictive control (NPC) to design a WADC for the generator exciter to enhance the damping of interarea oscillations in a large-scale power system. The NPC incorporates with a generalized predictive control (GPC) to generate optimal control predictions, and a network delay compensator to detect and compensate both constant and random delays. Moreover, model identification is used to obtain an equivalent reduced-order model of the large-scale power system and deal with the model uncertainties and variation of operating conditions. Case studies are based on the New England 10-machine 39-bus system. Effectiveness of the proposed WADC is verified by simulation studies and compared with a conventional WADC and a GPC-based WADC without delay compensation.

Wide-Area Damping Controller for Power System Interarea Oscillations: A Networked Predictive Control Approach

A method of calculating the communication delay (latency) for measurements and control signals in a power system is shown. The basis of the calculation is a dedicated communication channel for control signals. The time delay calculation is examined using a dynamic equivalent of the Western Electricity Coordinating Council transmission system. The impact on control system response is discussed. The application given is a wide area control system for interarea mode damping. Results demonstrate that control signal latency can degrade the performance of controls in a wide area control system.

Latency Viewed as a Stochastic Process and its Impact on Wide Area Power System Control Signals

Power systems are stochastic in nature due to uncertainty in bus loading and transmission system asset status. The available transfer capability (ATC) is a measure of "available capacity" in a power transmission system beyond already committed uses, i.e., beyond base case loading. To render the ATC as a more realistic measure of transmission availability, a stochastic calculation of ATC is proposed. A stochastic power flow algorithm can be used to help quantify and evaluate the uncertainty of the ATC. The limitations considered in the stochastic ATC calculation algorithm are: transmission thermal rating limits; bus voltage magnitude limits; and transmission circuit outages. The stochastic-algebraic calculation of ATC is described and tested using a reduced order WECC 179 bus system

J.W. Stahlhut

Papers

Latency Viewed as a Stochastic Process and its Impact on Wide Area Power System Control Signals

Stochastic-Algebraic Calculation of Available Transfer Capability