다중혈관 관상동맥 환자에서 y-문합을 이용하여 양쪽 내흉동맥만을 사용한 우회술의 조기 성적

High-voltage direct current (HVDC) has received considerable attention due to several advantageous features such as minimum transmission losses, enhanced stability, and control operation. An appropriate model of HVDC is necessary to assess the operating conditions as well as to analyze the transient and steady-state stabilities integrated with the AC networks. Nevertheless, the construction of an HVDC model is challenging due to the high computational cost, which needs huge ranges of modeling experience. Therefore, advanced dynamic modeling of HVDC is necessary to improve stability with minimum power loss. This paper presents a comprehensive review of the various dynamic modeling of the HVDC transmission system. In line with this matter, an in-depth investigation of various HVDC mathematical models is carried out including average-value modeling (AVM), voltage source converter (VSC), and line-commutated converter (LCC). Moreover, numerous stability assessment models of HVDC are outlined with regard to stability improvement models, current-source system stability, HVDC link stability, and steady-state rotor angle stability. In addition, the various control schemes of LCC-HVDC systems and modular multilevel converter- multi-terminal direct current (MMC-MTDC) are highlighted. This paper also identifies the key issues, the problems of the existing HVDC models as well as providing some selective suggestions for future improvement. All the highlighted insights in this review will hopefully lead to increased efforts toward the enhancement of the modeling for the HVDC system.

Dynamic Modeling of HVDC for Power System Stability Assessment: A Review, Issues, and Recommendations

This study investigates the performance of FACTS device based on different methods of placements for available transfer capability enhancement (ATC). Total real power loss, real power performance index, real power loss sensitivity with respect to line reactance, power transfer distribution factor (PTDF), line thermal limitation, available transfer capability (ATC) value, and least bus voltage magnitude are the placement methods considered. Each of these methods is used independently for location of Thyristor Controlled Series Compensator (TCSC) device for ATC enhancement. The study has been conducted on standard IEEE 30 bus test systems for comparison. The enhanced ATC values, bus voltage magnitudes and real power loss are then used in assessment of performance of TCSC with respect to locations. Disparities in enhancement ranges between 2% and 85% were achieved while real power loss minimization of up to 25% was obtained based on different placement methods.

Performance Evaluation of FACTS Placement Methods for Available Transfer Capability Enhancement in a Deregulated Power Networks

Power network operations are increasingly becoming complex due to upsurge in power transactions, following the ongoing power system deregulation and restructuring. The alteration in dynamics of power operations based on this upsurge, usually results into contingency because power facilities are made to operate in region of their limits. In this regards, high voltage direct current (HVDC) based voltage source converter (VSC) and other flexible alternating current transmission systems (FACTS) imbedded with VSC have established competence for noticeable diverse power network characteristics improvements. Therefore, this study applies optimized VSC-HVDC transmission system for mitigation of bus voltage and line thermal limit violation as a result of n-1-line outage contingency during bilateral, simultaneous and multilateral transactions. Line contingency evaluation through real power performance index (RPPI) is adopted for severe contingency identification purposes. Optimization of VSC-HVDC system is achieved through statically developed sensitivity-based analysis of VSC control parameters, while its performance for both steady state and contingency conditions are verified with IEEE 6 and 30 bus networks. An improvement in power flow, voltage deviation minimization and automatic alleviation of violated thermal and voltage limits during contingency present optimized VSC-HVDC system as a solution for network performance optimization especially during various transactions occasioned by unbundling power processes. Besides, the superiority of the proposed optimization techniques is established through performance comparison with other sensitivity methods involving reactive power (RPSM), active power (APSM) and performance index (PISM), where the proposed sensitivity method (PSM) outperformed these other methods.

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Contingency Control Capability of an Optimized HVDC-Based VSC Transmission System

This paper incorporates Flexible AC Transmission System (FACTS) device loss with the general loss sensitivity equation for the determination of optimum location for its placement in deregulated power networks, with objectives of Available Transfer Capability (ATC) enhancement, bus voltage improvement and loss reduction. A detailed mathematical model in terms of circuit system parameters is presented based on FACTS loss amalgamation approach. Thyristor Controlled Series Capacitor (TCSC) FACTS device is considered for simulation and analysis because of its capability to control active power among other parameters. The TCSC location is established based on analysis of sensitivity factors obtained from partial derivatives of the resultant loss equations (including FACTS) with respect to control parameters. ATC values are obtained using ACPRDF method and with TCSC in place, these values are enhanced for different bilateral and multilateral power transactions. IEEE 5 Bus system is used for the demonstration of the effectiveness of this approach. Placement with this method resulted into ATC enhancement of more than 60% well above the values obtained when TCSC was placed with thermal limit method. In addition, a substantial bus voltage improvement of up to 3% deviation minimization as well as up to 10% active power loss reduction was recorded with this placement.

FACTS Devices Loss Consideration in Placement Approach for Available Transfer Capability Enhancement

In a deregulated environment, Available Transfer Capability (ATC) values accurately depict the transferable quantum of power over the transmission network with due consideration for limits violations. This paper applies repeated alternating current power flow (RACPF) approach for the calculation of ATC. In this work, RACPF based on Newton Raphson (NR) power flow equation was developed in a MATLAB software environment to obtain various values of ATC using IEEE 30 bus reliability system as a test case. ATC values for inter-area line outage and generator outage contingency conditions are obtained and analyzed. ATC for bilateral and multilateral transactions as well as outage of inter-area tie lines during these conditions are also presented. It is observed from the results that RACPF approach is more accurate when compared with the results obtained using ac power flow in previous work. Numerically to mention but few, 11.574 MW was obtained above the recorded ATC value for transmission line 1–2 while 29.014 MW values was obtained below the recorded value for line 6–8. It is also established from the results that ATC values present an understanding of transmission network conditions and limits needed for operational purposes especially in a deregulated power sector.

Determination and Analyses of Available Transfer Capability: Deregulated and Restructured Power Systems Perspective

Static Var Compensator (SVC) and Thyristor Controlled Series Compensator (TCSC) models were incorporated into Newton Raphson power flow equations for enhancement of available transfer capability (ATC) of a deregulated power network in this study. Bus voltage profile improvement and real power loss minimization were considered as part of objectives and were investigated during ATC enhancement. The performance of the duo based on these objectives were analytically compared. IEEE 30 bus network system was used for these comparative analyses and the implementation was done in Matlab environment. Real power loss minimization of up to 0.06 MW and voltage improvement of bus 21 to 30 were achieved with SVC, while ATC enhancement of up to 14% were recorded for both devices. ATC values, bus voltage magnitude profile and transmission line real power losses suggested an advantage of SVC over TCSC for ATC enhancement deployment.

Babatunde. O. Adewolu

Papers

Performance Evaluation of FACTS Placement Methods for Available Transfer Capability Enhancement in a Deregulated Power Networks

Determination and Analyses of Available Transfer Capability: Deregulated and Restructured Power Systems Perspective

Contingency Control Capability of an Optimized HVDC-Based VSC Transmission System

FACTS Devices Loss Consideration in Placement Approach for Available Transfer Capability Enhancement

Available Transfer Capability Enhancement with FACTS: Perspective of Performance Comparison