A new methodology is presented for estimating the frequency behaviour of power systems necessary for an indication of under-frequency load shedding in steady-state security assessment. It is well known that large structural disturbances such as generator tripping or load outages can initiate cascading outages, system separation into islands, and even the complete breakup. The underfrequency load shedding takes place during the beginning phase of a dynamic change of the system frequency initiated by these distrubances. In this context the authors examine the ability of neural nets to properly interpolate among training data sets and to accurately predict the system frequency variations. The neutral-net approach provides a fairly accurate method of estimating the system average frequency response without making simplifications or neglecting non-linearities and small time constants in the equations of generating units, voltage regulators and turbines. The understanding and selection of the input features comes from the developed, simple low-order system frequency response model. Additional features are defined in terms of the centre of inertia acceleration and admittance distances. The efficiency of the new procedure is demonstrated using the New England power system model for a series of characteristic perturbations. The validity of the proposed approach is verified by comparison with the simulation of short-term dynamics including effects of control and automatic devices.

Prediction of power system frequency response after generator outages using neural nets

A modified load flow algorithm for microgrids operating in islanded mode is developed in this paper. The proposed algorithm is able to perform load flow calculations in networks without making use of the slack bus concept, and is suitable for applications to islanded microgrids. The developed algorithm includes the explicit representation of the steady-state characteristics of microsource controllers, making it possible to perform the load sharing between all generating units. Through this approach, the mathematical concept of a slack bus is not anymore necessary, since the load and the network losses are distributed among all generating units according to the steady-state characteristics of their respective controllers. The results obtained by applying the proposed algorithm to a microgrid test-system are validated through comparisons with steady-state values obtained through nonlinear time-domain simulations of the same test-system, performed with the PSCAD/EMTDC software package.

A modified load flow algorithm for microgrids operating in islanded mode

The long term dynamic system behavior under severe frequency and voltage changes, and the logics of automatic control actions (e.g., underfrequency load shedding and underfrequency generator separation) are discussed. The long term power system dynamic simulation is devised as a tool for studying the coordination of emergency control techniques in order to prevent significant deviations in power systems variables. A modification of the fast, decoupled power flow technique is considered for the quasi-stationary power flow solution. This approach is enhanced by the trapezoidal rule integration algorithm and the Runge-Kutta fourth order method for the solution of power system dynamic equations. Typical scenarios for the system recovery after power outages are discussed, and recommendations for the implementation of these techniques in large-scale power systems are provided. These topics are illustrated by two practical-examples for modern power systems. >

A comprehensive long term dynamic simulation for power system recovery

Dragan P. Popović

Papers

An approach to the evaluation of electromechanical transient processes in power systems

An efficient unified methodology for steady-state and dynamic security assessment of power systems

Fast solution method for evaluation of long-term dynamic processes in power systems with provision for island operation