Byoung-Hoon Cho

Modified PID load-frequency control with the consideration of valve position limits

This paper proposes a new load frequency control (LFC) scheme with a modified PID controller which guarantees the stability of the LFC loop for the wide control ranges of PID feedback gains. By observing the fact that the ratios between PID gains have more information than the individual gains, we proposed a modified PID controller by implementing feedback circuits by differentiating and integrating the output of the flyball speed governor. The proposed control scheme guarantees wide-range stability boundary of the PID gain controls, which can be easily checked by observing the root locus diagram. This paper shows that the valve position limits can be modeled in several ways, and that the control performance significantly depends on steam valve modeling in the case of large disturbances. The proposed LFC scheme has been tested for a sample 4-machine 10-bus system.

The development of equivalent system technique for deriving an energy function reflecting transfer conductances

This paper shows that a well-defined energy function can be developed to reflect the transfer conductances for multi-machine power systems under an assumption that all transmission lines have uniform R/X ratios. The energy function is derived by introducing a pure reactive equivalent system for the given system. In this study, a static energy function reflecting transfer conductances is also derived as well as the transient energy function. The proposed static energy function is applied to voltage stability analysis and tested for various sample systems. The test results show that the accuracy of voltage stability analysis can be considerably improved by reflecting transfer conductances into the energy function.

Fault restoration algorithm using fast tracing technique based on the tree-structured database for the distribution automation system

This paper presents a fast tracing algorithm by adopting the LC/RS (left child/right sibling) tree structure for the database. In the distribution automation system (DAS), fast tracing of network connectivity is a vital issue for the application to large distribution systems. In this study, a tree-structured database has been adopted with the use of nondirectional data rather than the directional. The tree-structured database can afford to speed up the tracing algorithm with the use of the systematic search engine. The features of the proposed algorithm are (i) fast network tracing, (ii) convenience in the system data management, and (iii) convenient and fast modification of system data due to network changes.

Fast and reliable distribution system load flow algorithm based on the Y/sub BUS/ formulation

This paper presents a new and efficient method for solving the load flow problem in distribution systems. A simple algebraic matrix equation to solve the load flow problem is derived by using the complex power balance equations. By adopting the rectangular coordinate, which requires the neglect of only second order terms in the linearization procedure, the proposed method gives better convergence characteristics than the conventional Newton Raphson (NR) or the fast decoupled load flow (FDLF) methods. Only diagonal elements of the Jacobian matrix are subjected to be updated while other elements remain constant. This enhances the proposed algorithm considerably in computation speed. The proposed method can be applied to both radial and meshed networks. The proposed algorithm is tested with 33, 302, 700, and 1004-bus sample systems. The test results show that the proposed method can be well applied to the distribution system with wide ranged R/X values. The computation speed is remarkably improved compared with the conventional NR and FDLF methods.

Energy conservation law and its application for the direct energy method of power system stability

Many papers have contributed to the development of energy functions for power systems. However, there exists limitation in that the conventional energy functions cannot reflect the control effects of the speed governor and exciter efficiently. In this paper, two useful theorems are developed regarding the relationships of the energy integral of generator power versus its phasor current and voltage. With the use of the two theorems, the energy conservation law can be derived to reflect the resistance, flux-decaying effects, saliency with wide adaptability to various detailed generator models including the governor and exciter control loops. By the rigorous mathematical analysis and numerical tests with its application to sample systems, the proposed energy conservation law is proven to be very exact and valid for any kind of generator models including the speed governor and exciter controls.

Derivation of energy conservation law by complex line integral for the direct energy method of power system stability

This paper presents a complex line integral approach to derive an energy conservation law which reflects the resistance, flux-decaying effects, saliency with wide adaptability to various detailed generator models including the governor and exciter control loops. Especially, the previous work has shown that an exact energy conservation law can be derived under the assumption of stator network transients being negligible. In this paper, it is shown that an exact energy conservation law can be derived by using the complex line integral approach without any assumptions. Consequently, the proposed energy conservation law is applicable to power systems adopting any kind of detailed generator model. The stator resistance of the generator can be taken into account in a simple manner. The proposed energy conservation law is tested for several sample systems and the results show its validity.

Estimation of maximum loadability in power systems by using elliptic properties of P-e curve

This paper presents an efficient algorithm to estimate the maximum load level for heavily loaded power systems with the load-generation variation vector obtained by ELD (economic load dispatch) and/or short term load forecasting while utilizing the elliptic pattern of the P-e curve. It is well known the power flow equation in the rectangular coordinate is fully quadratic. However, the coupling between e and f makes it difficult to take advantage of this quadratic characteristic. In this paper, the elliptic characteristics of P-e curve are illustrated and a simple technique is proposed to reflect the e-f coupling effects on the estimation of maximum loadability with theoretical analysis. An efficient estimation algorithm has been developed by utilizing the elliptic properties of the P-e curve. The proposed algorithm is tested on IEEE 14 bus system, New England 39 bus system and IEEE 118 bus system, which shows that the maximum load level can be efficiently estimated with remarkable improvement in accuracy.

Improved method of maximum loadability estimation in power systems by transforming the distorted P-e curve

This paper presents an improved method to estimate the maximum load level for heavily loaded power systems with the load-generation variation vector obtained by ELD (economic load dispatch) and/or short term load forecasting by using the elliptic pattern of the P-e curve. The previous study suggested a simple technique of removing e-f coupling, where only high voltage load flow solutions to calculate transforming angle of system reference is needed. The proposed algorithm is improved to require only one load flow solution at a specific load level in addition to the operating point at the beginning stage, which reduces the computation time and the iteration number of estimation. The proposed method can be efficiently applied to heavily loaded systems with the combination of CPFlow when the reactive power limit and ULTC (under load tap changer) are considered. In this paper, the effect of ULTC on the estimation of maximum loadability index is also investigated. The proposed algorithm is tested on IEEE 14 bus system, New England 39 bus system and IEEE 118 bus system.

Second-kind energy function of power system and its applications

This paper presents a second-kind energy functions for power systems. The previously introduced complex integral approach yields two kinds of energy integral expressions for its real and imaginary parts. The former provides a second-kind energy function while the latter the conventional energy function. It has been shown that a series of energy functions can be generated by taking the weighted sum of the conventional energy function and the second-kind one. By taking appropriate weights, a well-defined energy function can be derived to reflect the transfer conductances when the system has uniform R/X ratios for all transmission lines with separation of generator reactances.

Analysis of polarized linear electric network with two-reactance components

This paper introduces the concept of polarized linear electric network and proposes a new approach to analyze it. It is well-known that the internal impedance of salient-pole generator has two reactance X/sub d/, X/sub q/ corresponding to the direct and quadratic current components. Two different reactance possessed by one circuit element provide the polarization characteristics in the current transfer. This study proposes a new analysis method which shows that the linearity of the polarized linear network makes it possible to calculate generator currents in one step for the given internal voltages and rotor angles of the generators. Tests have been performed for the sample systems, which shows remarkable improvement in computation time.