Fang Dazhong

Transmission line fault location for double phase-to-earth fault on non-direct-ground neutral system

An accurate algorithm for locating double phase-to-earth faults on transmission lines of nondirect ground neutral systems is presented. The algorithm employs the faulted phase network and zero-sequence network as the fault location model. It effectively eliminates the effect of load flow and fault resistance on the accuracy of fault location. The technique embodies an accurate location by measuring only one local end data. The algorithm is used in a procedure that provides the automatic determination of faulted line and phase, rather than requires engineer to specify them. Simulation results have shown the effectiveness of the algorithm under the condition of earth faults.

Fast transient stability estimation using a novel dynamic equivalent reduction technique

A new dynamic equivalent power system (DEPS) model for fast first swing transient stability assessment is proposed in this paper. The DEPS model makes use of the phenomenon that during a fault on a large power system, there are many generators that are only lightly disturbed when compared with severely disturbed machines. The method substitutes these less disturbed machines by one equivalent generator but allows some model parameters to become time dependent. The crucial feature is that a novel method of classification of the less disturbed machines based on a Taylor series expansion which achieves a very rapid assessment of first swing critical clearing time (CCT) has been developed. This method is fundamentally different from other published dynamic equivalent techniques, such as the extended equal area criterion method and the identification of coherent generators, and both reliability and efficiency are better. With this DEPS model the efficiency of the traditional first swing transient stability analysis could be fundamentally improved. Extensive tests performed on several IEEE test systems show that the proposed model is suitable for fast transient stability assessment in large systems. >

Fuzzy logic controller for enhancing oscillatory stability of AC/DC interconnected power system

Abstract A novel control strategy is developed for High Voltage DC (HVDC) links to improve oscillatory stability of interconnected power systems. An ‘area’ principle is proposed for controller design to damp AC tie power oscillations by increasing the oscillatory ‘deceleration area’. Exploiting fast control capability of HVDC link, a fuzzy-rule is adopted to smooth the power transition for reducing power ‘shock’ to power system. The strategy also incorporates adaptive control techniques to prevent tie-link power ‘chattering’. Simulation results using the proposed control scheme on two typical power systems are presented. The results demonstrate that significant improvements in both oscillatory and transient stability are obtained.

A fast approach to transient stability estimation using an improved potential energy boundary surface method

In this paper, a new fast algorithm to perform online transient stability assessment (TSA) in a power system is proposed. The TSA is concerned with finding the critical clearing time (CCT), the stability limit, for a specified fault. The method possesses the merits of the fast speed of the potential energy boundary surface method and the accuracy of the conventional time domain simulation technique. To expedite the algorithm, an efficient stopping criterion for postfault trajectory simulation is developed. Examples are given to show the problems of integration stopping criteria published in recent papers and how they are tackled. Extensive test results on several systems, including the 10-generator New England system, the 17-generator Iowa system and the IEEE 50-generator system, are reported.

Evaluation of transient stability limit using a transient time margin sensitivity approach

Abstract This paper addresses the issue of how to evaluate transient stability limit using a transient time margin (TTM) approach and the sensitivity of this margin to key system variables such as generation and load. This allows fast control decisions to be assessed for a power system on-line control purpose. The difference between fault clearing time and critical fault clearing time, called the TTM, is proposed as a transient stability index. The single machine equivalent system (SMES) is employed to represent the TTM as a function of system parameters such as plant generation and load, and the sensitivity of the TTM to these parameters is found. The generation or load changes that can be made while preserving stability, or the changes that are needed to restore stability in dynamically insecure systems, are next derived from this sensitivity. Test results for several real size test power systems show that the proposed approach is feasible and promising for on-line control application.

Inter-area mode transient stability estimation

Inter-area mode transient stability evaluation of large interconnected power systems has proved to be difficult because of the complexity of the phenomena and the long time duration over which instability manifests itself. Very few studies have been published and this paper describes a new method which is reliable for this complex problem, fast and accurate in its evaluation of critical clearing time, provides an estimate of transient energy margin and has some potential for control evaluation. The technique is a hybrid between the detailed multimachine representation, the single-machine equivalent and energy methods. It has been tested on three standard test systems including the IEEE 50-generator system.

Power flow analysis of power system with UPFC using commercial power flow software

This paper proposes a new and efficient approach of load flow analysis for UPFC embedded power systems. The main characteristic of the approach is that an equivalent network without UPFCs, which is obtained easily according to two basic schemes of UPFC control, is used for load flow analysis of the original system including UPFCs. Then control parameters of each UPFC are determined by the load flow solution. The nature of the approach means that commercial software for power system analysis can be employed for load flow solutions of systems with UPFCs, hence it has the advantages of the conventional techniques in load flow analysis. Numerical examples are given to indicate that the approach is reliable and efficient.

Economy and reliability evaluation of generating including wind energy systems

The analysis method used a sequential Monte Carlo simulation technique for wind power modeling and reliability assessment, which is based on an hourly random simulation to mimic the operation of a generating system, taking the wind speeds, the random failure of generating units, and other recognized dependencies into account. An auto-regressive and moving average time series model is used to simulate the hourly wind speeds and thus the available wind power considering chronological characteristics. Considering unit random failures, the analysis model also obtains system available capacity at points in time established sequentially, which provides the probability of existence of each possible outage capacity level. The system reliability index then is formed by observing the probability function of outage capacity over a sufficiently long time period. A number of sensitivity analyses are presented with case studies to illustrate possible applications of the proposed method.

Stability issues and new techniques for on-line transient stability analysis in Asia

With the fast expansion of capacity in electric power systems in recent years, there is a pressing need to include on-line transient security analysis (TSA) capability in the energy management systems in Asian power utilities. Based upon the transient stability issues of todays power systems, the challenge to the implementation of new on-line transient stability assessment tools is first presented. Reviews on TSA methods under development are presented to show their potential for on-line applications. New techniques developed recently and future research issues for this area are introduced.

Dynamic single machine equivalent techniques for on-line transient stability assessment

Abstract According to the post-fault trajectory duration characteristics, power system disturbances, which may cause transient stability problems, are classified into three types: short, long and extremely long duration ones. An improved method using dynamic single machine equivalent system (SMES) model for on-line first swing critical clearing time (CCT) estimation for both short and long duration disturbances on multi-machine power systems is developed in this paper. This dynamic SMES model substitutes for a large power system to provide fast, on-line transient stability assessment (TSA). Techniques such as assessment of energy margin, identification of a group of machines called the ‘dominant critical machines’ and an interpolation formula for CCT evaluation are proposed in this method to achieve high speed and accuracy in TSA. Test results on real size power systems are reported to show that the method is reliable for CCT assessment of both short and long post-fault duration disturbances.