Many steady-state power-flow-based models for cascading outage simulation have not considered frequency, which, however, is an important indicator of generation-load imbalance. This paper proposes a novel steady-state approach for simulating cascading outages. The approach employs a power-flow-based model that considers static power-frequency characteristics of both generators and loads. Thus, the frequency deviation due to active power imbalance can be calculated under cascading outages. Furthermore, a new ac optimal power-flow model considering frequency deviation is proposed to simulate the remedial control when system collapse happens as indicated by the divergence of power flows. Case studies first benchmark the steady-state frequency calculated by the power-flow-based model with time-domain simulation results on a two-area power system, and then test the proposed approach for simulation of cascading outages on the IEEE 39-bus system and an NPCC 48-machine 140-bus power system. The test results are compared with a traditional frequency-independent approach using the conventional power flow and ac optimal power-flow models, and verify that by capturing frequency variations under cascading outages, the proposed approach can more accurately simulate the mechanism of outage propagation.

Simulation of Cascading Outages Using a Power-Flow Model Considering Frequency

The natural gas system and electricity system are coupled tightly by gas turbines in an integrated energy system. The uncertainties of one system will not only threaten its own safe operation but also be likely to have a significant impact on the other. Therefore, it is necessary to study the variation of state variables when random fluctuations emerge in the coupled system. In this paper, a multi-slack-bus model is proposed to calculate the power and gas flow in the coupled system. A unified probabilistic power and gas flow calculation, in which the cumulant method and Gram–Charlier expansion are applied, is first presented to obtain the distribution of state variables after considering the effects of uncertain factors. When the variation range of random factors is too large, a new method of piecewise linearization is put forward to achieve a better fitting precision of probability distribution. Compared to the Monte Carlo method, the proposed method can reduce computation time greatly while reaching a satisfactory accuracy. The validity of the proposed methods is verified in a coupled system that consists of a 15-node natural gas system and the IEEE case24 power system.

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Unified probabilistic gas and power flow

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

From the study of dispatcher power flow in a distributed energy management system (DEMS) among multiple control centers, a fixed-point iteration scheme for boundary bus states is constructed to modify the exterior network equivalent through data exchange. Implementation details such as determination of combination parameters, independent choice of reference bus, and distribution of unbalanced power are analyzed. Using this method, each subsystem involved in distributed computing can maintain its method of modeling the exterior network. Adverse effects of communication interference on computing can also be eliminated. Results consistent with integrated power flow are obtained with little loss of accuracy. Thus, the proposed algorithm is especially suitable for application in DEMS, in which control centers can only communicate by WAN. Tests are conducted on the IEEE 118-bus system partitioned into five subsystems, the IEEE 300-bus system, and an actual power system, which verifies the favorable convergence and effectiveness of the method.

A Distributed Multi-Control-Center Dynamic Power Flow Algorithm Based on Asynchronous Iteration Scheme

Le probleme de reprise de service des reseaux electriques a la suite d'un incident partiel ou generalise est un probleme bien connu depuis tres longtemps. Cependant, ces dernieres annees, a cause de la concurrence economique et de la deregulation, les reseaux electriques sont appeles a fonctionner de plus en plus pres de leurs limites. Parallelement a cela, les reseaux electriques sont devenus de plus en plus complexes et leurs dimensions ne cessent de croitre, ce qui a augmente le risque de pannes generalises. La maitrise de la reprise de service revet alors une importance nouvelle. La reprise de service d'un reseau, apres un incident quelconque, doit etre a la fois rapide et fiable. Une telle operation exige la disponibilite de plans detailles. Cette these porte plus particulierement sur la phase initiale de la reprise de service d'un reseau electrique, phase pendant laquelle quatre problemes sont identifies. Afin de pouvoir apporter des solutions a ces problemes, quatre approches ont ete developpees tout au long de ce travail: La premiere consiste a determiner la sequence optimale de la mise en marche des groupes pendant la reprise de service des centrales electriques. L'evaluation du comportement frequentiel du systeme lors de la re-alimentation des charges fait l'objet de la deuxieme approche. Dans la troisieme approche, nous expliquerons comment reduire l'angle de la phase stationnaire. La quatrieme permet l'estimation et le controle des surtensions harmoniques pendant le renvoi de la tension ver les transformateurs. Ainsi, un blockset dans l'outil de simulation MATLAB/SIMULINK a ete developpe pour la simulation dynamique a long terme afin de pouvoir etudier la reponse frequentielle du systeme, a la suite d'un desequilibre entre la charge et la production lors d'un scenario de reprise de service.

Nouvelle approches pour la restauration d'un reseau electrique apres un incident generalise

This paper presents a dynamic load flow technique for power system simulators. The approach presented can be applied to a system when there is a net accelerating or decelerating power in the system. It possesses all the attractive features and basic structure of the normal fast decoupled method, and it does not depend on the selected reference bus. The modifications to the standard fast decoupled power flow algorithm are derived, and power system simulator implementation considerations are discussed. The algorithm has been tested on standard IEEE test cases and on a metropolitan utility 559 bus model. A numerical example of the above algorithm on the 559 test case is presented which demonstrates convergence characteristics and computation requirements of the proposed algorithm. These results are compared with those of normal fast decoupled load flow.

S. A. Sackett

Papers

Dynamic Load Flow Technique for Power System Simulators