• KIEE International Transactions on Power Engineering
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• v.12A no.1
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• pp.31-35
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• 2002

This paper presents a new operation scheme of UPFC to minimize both generation costs and active power losses in a normal operation state of power system. In a normal operation, cost minimization is a matter of primary concern among operating objectives. This paper considers two kinds of costs, generation costs and transmission losses. The total generation cost of active powers can be minimized by optimal power flow, and active power losses in the transmission system can be also minimized by power flow control of UPFC incorporated with minimization of generation costs. In order to determine amounts of active power reference of each UPFC required for the cost minimization, an iterative optimization algorithm based on the power flow calculation using the decoupled UPFC model is proposed. For verification of the proposed method, intensive studies have been performed on a 3-unit 6-bus system equipped with a UPFC.

• The Transactions of the Korean Institute of Electrical Engineers A
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• v.50 no.8
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• pp.388-394
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• 2001

This paper presents how to determine the optimal operating points of Unified Power Flow controllers (UPFC) the line flow control of which can enhance system security level. In order to analyze the effect of these devices on the power system, the decoupled model has been employed as a mathematical model of UPFC for power flow analysis. The security index that indicates the level of congestion of transmission line has been proposed and minimized by iterative method. The sensitivity of objective function for control variables of and UPFC has been derived, and it represents the change in the security index for a given set of changes in real power outputs of UPFC. The proposed algorithm with sensitivity analysis gives the optimal set of operating points of multiple UPECs that reduces the index or increases the security margin and Marquart method has been adopted as an optimization method because of stable convergence. The algorithm is verified by the 10-unit 39-bus New England system that includes multiple FACTS devices. The simulation results show that the power flow congestion can be relieved in normal state and the security margin can be guaranteed even in a fault condition by the cooperative operation of multiple UPECs.

• Journal of Power Electronics
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• v.12 no.5
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• pp.787-799
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• 2012

Real and reactive power flows on a transmission line interact inherently. This situation degrades power flow controller performance when independent real and reactive power flow regulation is required. In this study, a quasi multi-pulse interline power flow controller (IPFC), consisting of eight six-pulse voltage source converters (VSC) switched at the fundamental frequency is proposed to control real and reactive power flows dynamically on a transmission line in response to a sequence of set-point changes formed by unit-step reference values. It is shown that the proposed hybrid fuzzy-PI commanded IPFC shows better decoupling performance than the parameter optimized PI controllers with analytically calculated feed-forward gains for decoupling. Comparative simulation studies are carried out on a 4-machine 4-bus test power system through a number of case studies. While only the fuzzy inference of the proposed control scheme has been modeled in MATLAB, the power system, converter power circuit, control and calculation blocks have been simulated in PSCAD/EMTDC by interfacing these two packages on-line.

• Journal of Power Electronics
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• v.16 no.4
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• pp.1526-1540
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• 2016

This work mainly discusses an accurate and fast islanding detection based on fractional wavelet packet transform (FRWPT)for multibus microgrid systems. The proposed protection scheme uses combined desirable features retrieved from discrete fractional Fourier transform (FRFT) and wavelet packet transform (WPT) techniques, which provides precise time-frequency information on minute perturbation signals introduced in the system. Moreover, this study focuses on the design of decoupling control with a distributed controller based on state feedback for the efficient operation of microgrid systems that are transitioning from the grid-connected mode to the islanded mode. An IEEE 9-bus test system with inverter based distributed generation (DG) units is considered for islanding assessment and smooth operation. Finally, tracking errors are greatly reduced with stability improvement based on the proposed controller. FRWPT based islanding detection is demonstrated via a time domain simulation of the system. Simulated results show an improvement in system stability with the application of the proposed controller and accurate islanding detection based on the FRWPT technique in comparison with the results obtained by applying the wavelet transform (WT) and WPT.

• Journal of Electrical Engineering and Technology
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• v.12 no.5
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• pp.1689-1696
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• 2017

In recent academic and industrial circles of the Republic of Korea, the securement of available reactive power reserve against the line faults is at issue. Thus, simulations have been performed for the securing of effective reactive power reserve (effective Q) to prepare for the line faults and improve reactive power monitoring and control methods. That is, a research has been conducted for the fast-decoupled Newton-Raphson method. In this study, a method that distinguishes source and sink regions to carry out faster provision of information in the event of line fault has been proposed. This method can perform quantification with the formula that calculates voltage variations in the line flow. The line flow and voltage changes can be easily induced by the power flow calculation performed every second in the operation system. It is expected that the proposed method will be able to contribute to securement of power system stability by securing efficient reactive power. Also, the proposed method will be able to contribute to prepare against contingencies effectively. It is not easy to prepare quickly for the situation where voltage drops rapidly due to the exhaustion of reactive power source by observing voltage information only. This paper's simulation was performed on the large scale Korean power system in steady state.

• The Transactions of the Korean Institute of Power Electronics
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• v.23 no.5
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• pp.366-372
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• 2018

Three-port dual-active bridge (DAB) converter in a DC microgrid was studied due to its high power density and cost-effectiveness. The other advantages of DAB include galvanic isolation and bidirectional power conversion capability using simple control modulation. The three-port DAB converter consists of a three winding transformer and three bridges. The transformer has three phases, which means that the ports are coupled. Thus, the three-port DAB converter causes unwanted power flows when the load connected to each port changes. The basic operational principles of the three-port DAB converter are presented in this study. The decoupling control strategy of the independent port power transfer is presented with a mathematical power model to overcome the unexpected power flow problem. The validity of the proposed analysis and control strategy is verified with PSIM simulation and experiments using a 1-kW prototype power converter.

• Proceedings of the KIEE Conference
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• 2002.07a
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• pp.277-279
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• 2002

본 논문은 UPFC(Unified Power Flow Controi)의 유효전력 및 무효전력의 제어를 통해 시스템의 레벨을 향상시키기 위한 운용 알고리즘을 제시한다. 전력 계통에서 UPFC의 효과 분석하기 위해 분리형 모델(decoupled model)을 이용하였다. 본 논문에서는 계통의 조류 및 전압을 이용한 안전도 지수가 제시되었고, 이를 반복계산(iterative method)을 통해 최소화하였다. 이를 위해 각 안전도 지수의 민감도 벡터가 최소화되는 방향으로 UPFC의 운전점을 찾고, 이로써 시스템의 안전도를 증대시킨다. 제안된 알고리즘은 다기의 UPFC를 대상으로 IEEE 30 모선에 적용하여 시스템 안전도 레벨을 검토하였다.

• The Transactions of the Korean Institute of Electrical Engineers A
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• v.48 no.9
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• pp.1081-1087
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• 1999

Load Flow calulation methods can usually be divided into Gauss-Seidel method, Newton-Raphson method and decoupled method. Load flow calculation is a basic on-line or off-line process for power system planning. operation, control and state analysis. These days Newton-Raphson method is mainly used since it shows remarkable convergence characteristics. It, however, needs considerable calculation time in construction and calculation of inverse Jacobian matrix. In addition to that, Newton-Raphson method tends to fail to converge when system loading is heavy and system has a large R/X ratio. In this paper, matrix equation is used to make algebraic expression and then to slove load flow equation and to modify above defects. And it preserve P-Q bus part of Jacobian matrix to shorten computing time. Application of mentioned algorithm to 14 bus, 39 bus, 118 bus systems led to identical results and the same numbers of iteration obtained by Newton-Raphson method. The effect of computing time reduction showed about 28% , 30% , at each case of 39 bus, 118 bus system.

• Proceedings of the KIEE Conference
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• 1998.11a
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• pp.311-315
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• 1998

Load Flow calculation methods can usually be divided into Gauss-Seidel method, Newton-Raphson method and decoupled method. Load flow calculation is a basic on-line or off-line process for power system planning, operation, control and state analysis. These days Newton-Raphson method is mainly used since it shows remarkable convergence characteristics. It, however, needs considerable calculation time in construction and calculation of inverse Jacobian matrix. In addition to that, Newton-Raphson method tends to fail to converge when system loading is heavy and system has a large R/X ratio. In this paper, matrix equation is used to make algebraic expression and then to solve load flow equation and to modify above defects. And it preserve certain part of Jacobian matrix to shorten the time of calculation. Application of mentioned algorithm to 14 bus, 39 bus, 118 bus systems led to identical result and the number of iteration got by Newton-Raphson method. The effect of time reduction showed about 28%, 30%, at each case of 39 bus, 118 bus system.