S. Krishna

Transient stability assessment using artificial neural networks

Online transient stability assessment (TSA) of a power system is not yet feasible due to the intensive computation involved. Artificial neural networks (ANN) have been proposed as one of the approaches to this problem because of their ability to quickly map nonlinear relationships between the input data and the output. In this paper a review of the previously published papers on TSA using ANN is presented. The paper also reports the results of the application of ANN to the problem of TSA of a 10 machine 39 bus system.

On-line detection of loss of synchronism using locally measurable quantities

Maintaining dynamic security of a power system subjected to large disturbances is of utmost importance. Fast and accurate on-line detection of instability is essential in initiating certain emergency control measures. The techniques reported in the literature involve mainly application of global phasor measurements and heuristic algorithms. In this paper, an accurate technique for the on-line detection of loss of synchronism based on voltage and current measurements in a line is presented. The technique makes use of the concept of potential energy in a line. The conditions for the system instability are derived from energy function analysis. However no assumptions are made regarding the power-angle relationship in a line, nor any data on the system equivalents are necessary in implementing the detection scheme which is simple and requires only local measurements.

On-line Dynamic Security Assessment: Determination of Critical Transmission Lines

Abstract The main objective of on-line dynamic security assessment is to take preventive action if required or decide remedial action if a contingency actually occurs. Stability limits are obtained for different contingencies. The mode of instability is one of the outputs of dynamic security analysis. When a power system becomes unstable, it splits initially into two groups of generators, and there is a unique cutset in the transmission network known as critical cutset across which the angles become unbounded. The knowledge of critical cutset is additional information obtained from dynamic security assessment, which can be used for initiating preventive control actions, deciding emergency control actions, and adaptive out-of-step relaying. In this article, an analytical technique for the fast prediction of the critical cutset by system simulation for a short duration is presented. Case studies on the New England ten-generator system are presented. The article also suggests the applications of the identification of critical cutsets.

Teaching Calculation of Inductance of Power Transmission Lines

The conventional derivation of the expression for inductance of power transmission lines given in books is more a recipe than a derivation starting from a law of physics. The serious shortcomings of the traditional approach are that the definition of flux linkage lacks strong connection to Faradays law, and unnecessary assumptions are made. This paper gives a complete rigorous derivation of the expression for inductance of power transmission lines. This paper will help instructors as well as students, since it addresses the deficiencies present in the derivation given in textbooks.

Optimal parameters of UPFC for power flow

Unified power flow controller (UPFC) is a versatile flexible AC transmission system (FACTS) controller which can be used for voltage and power flow control. In some situations such as transient stability control, it is required to maximize and minimize the power flow on certain transmission lines. Since UPFC has three independent variables, the values of these for optimal power flow are not obvious. In this paper, Kuhn-Tucker conditions are used to obtain global optimal values of the variables of UPFC, which is located at the midpoint of a lossless symmetrical line.

Bifurcation analysis of Static Synchronous Compensator with reactive current controller

This paper presents bifurcation analysis of static synchronous compensator (STATCOM) (with detailed model) with reactive current controller. The discrete time model of the system is derived. The stability region in the parameter space is obtained for different values of reactive current. The analysis is done for type II controller where the parameters are the proportional and integral gains. It is found that increase in integral gain results in period doubling bifurcation and increase in proportional gain results in border collision bifurcation. The stability analysis is compared with that for simplified STATCOM model neglecting harmonics.

A Novel Load Shedding Scheme for Voltage Stability

Abstract Undervoltage load shedding serves to maintain voltage stability when a majority of loads are fast acting. An undervoltage load shedding scheme should address two tasks: the detection of voltage instability following a large disturbance and the determination of the amount of load to be shed. Additionally, in case of short-term voltage instability, the scheme should be fast. This paper proposes a method to predict voltage instability arising due to a large disturbance. The amount of load to be shed to maintain voltage stability is then determined from the Thevenin equivalent of the network as seen from the local bus. The proposed method uses local measurements of bus voltage and power, and does not require knowledge of the network. The method is validated by simulation of three test systems subjected to a large disturbance. The proposed scheme is fairly accurate in estimating the minimum amount of load to be shed to maintain stability. The method is also successful in maintaining stability in cases where voltage collapse is detected at multiple buses.

A new measure of local error in the numerical solution for online dynamic security assessment

Dynamic security assessment (DSA) examines whether the power system is stable under large disturbances. There is a time constraint for online analysis since it has to be performed for a large number of contingencies in every few tens of minutes. In order to reduce the amount of computation, screening is performed to identify those contingencies which are sure to not cause system instability and hence can be eliminated; the other contingencies have to be scrutinized further. The computation required is in the numerical solution of differential equations. The amount of computation depends on step size; but increase in step size results in larger errors. This paper proposes a measure of local error in the numerical solution. Case studies are conducted on 10 generator New England system and 17 generator IEEE test system. Runge-Kutta explicit and implicit methods with different step sizes and tolerances are compared based on the proposed measure of error. Hence the most accurate numerical method satisfying the time constraint can be chosen for online DSA.