Kyeon Hur

Design and Control of a Modular Multilevel HVDC Converter With Redundant Power Modules for Noninterruptible Energy Transfer

This paper presents design and control methods for fault-tolerant operations with redundant converter modules, one of the most prominent features in modular multilevel converter (MMC) topology. In fully implementing MMC functionalities, a nearest level control is applied as a low-switching modulation method. A dual sorting algorithm is newly proposed for effectively reducing the switching commutations of each power module as well as for voltage balancing control. Built upon these primary MMC topological and control features, its redundant operation is comprehensively investigated for fail-safe energy transfer. In particular, a novel spare process is proposed to handle an emergency situation when the number of faulty power modules exceeds the module redundancy. Since topological redundancy may cause the switching commutations of power modules in an arm to be unevenly distributed, a practical and effective mitigation measure is incorporated to keep the energy balance while avoiding the undesired switching stresses. Rigorous simulation studies for MMC and its application for high-voltage direct current are performed to demonstrate the validity and effectiveness of the proposed spare process under normal and emergency conditions.

Synergistic Control of SMES and Battery Energy Storage for Enabling Dispatchability of Renewable Energy Sources

The use of renewable energy source can reduce greenhouse gas emission and fossil fuel pollution. Compared with fossil fuel energy, renewable energy is not stable and cannot supply firm electrical output (i.e., it is nondispatchable). Fluctuating power from renewables may result in grid power oscillation. To reduce grid swing, energy storage is necessary to smooth output from renewable energy. Energy storage with high energy density and fast response time or high power capacity is desired for compensation of fluctuating output. Generally, superconducting magnetic energy storage (SMES) has higher power capacity than battery energy storage, while battery provides higher energy density. Thus, this research proposes a hybrid energy storage system (HESS) composed of an SMES and battery. Novel and practical synergistic control is presented for firming power fluctuation by exploiting the strong power and energy capabilities of the SMES and the battery while within the efficient operating range of (i.e., state of charges of) HESS. Comprehensive case studies demonstrate the efficacy of the proposed HESS topology and control algorithm using PSCAD/EMTDC.

A New Unified Scheme for Controlled Power System Separation Using Synchronized Phasor Measurements

Controlled power system separation, which separates the transmission system into islands in a controlled manner, is considered the final resort against a blackout under severe disturbances, e.g., cascading events. Three critical problems of controlled separation are where and when to separate and what to do after separation, which are rarely studied together. They are addressed in this paper by a proposed unified controlled separation scheme based on synchrophasors. The scheme decouples the three problems by partitioning them into sub-problems handled strategically in three time stages: the Offline Analysis stage determines elementary generator groups, optimizes potential separation points in between, and designs post-separation control strategies; the Online Monitoring stage predicts separation boundaries by modal analysis on synchrophasor data; the Real-time Control stage calculates a synchrophasor-based separation risk index for each boundary to predict the time to perform separation. The proposed scheme is demonstrated on a 179-bus power system by case studies.

High-wire act

This paper discusses how Energy Reliability Council of Texas (ERCOT) balances transmission flows for Texas-size savings using its dynamic thermal ratings application. Competition in a deregulated environment requires companies to take advantage of all their transmission resources to the fullest extent possible to maximize profits.

On Two Fundamental Signatures for Determining the Relative Location of Switched Capacitor Banks

This paper describes two fundamental signatures of shunt capacitor bank switching transient phenomena from which one can accurately determine the relative location of an energized capacitor bank whether it is upstream or downstream from the monitoring location. Mathematical analysis of a capacitor bank energizing proves that: 1) the energized capacitor bank affects only the upstream reactive power flow and 2) at the energizing instant, the gradients (time derivatives) of voltage and current waveforms measured upstream from the capacitor location will have opposite signs. The reverse is true in that at the energizing instant, gradients of voltage and current waveforms measured downstream from the same capacitor location will have equal signs. Thus, we can precisely determine the relative location of the switched capacitor bank by simply evaluating power factor changes and the signs of voltage and current waveform gradients at the switching instant. The efficacy of our practical direction-finding technique is demonstrated analytically and by way of time-domain simulation models and actual data.

Improvement of Composite Load Modeling Based on Parameter Sensitivity and Dependency Analyses

This paper presents an effective optimization scheme for the measurement-based load modeling based on the sensitivity analysis of composite load model parameters. Each parameter of load model has different effects on its dynamic response. Moreover, some parameters are insensitive to the change of others. To estimate the dynamic interactions between parameters, their sensitivity is analyzed by using the eigenvalues of Hessian matrix used in the optimization algorithm. Also, the linear dependence between two load model parameters is then identified by examining the condition number of Jacobian matrix. With this parameter analysis, the performance of optimization process for measurement-based composite load modeling is improved by reducing the number of necessary parameters to consider. The performance of proposed method is verified with the practical data measured at a feeder in a real substation.

Estimation of System Damping Parameters Using Analytic Wavelet Transforms

This paper presents an efficient methodology for estimating the damping of a power system in which multiple resonant frequency components exist. The proposed method was developed using the analytic wavelet transforms and requires only the intrinsic transient portion of the voltage waveform (i.e., the free response due to capacitor bank energizing). The method can estimate the damping ratios of the selected modes and is free from the unimodal restriction of the Hilbert damping analysis. Thus, the method can provide more accurate system modal information.

Distance Estimation of Switched Capacitor Banks in Utility Distribution Feeders

This paper proposes an efficient and accurate technique for estimating the location of an energized capacitor bank downline from a monitoring equipment. The proposed technique is based on fundamental circuit theory and works with existing capacitor switching transient data of radial power distribution systems. Once the direction of a switched capacitor bank is found to be downstream from a power quality monitoring point, the feeder line reactance is estimated using the initial voltage change. The efficacy of the proposed technique is demonstrated with system data from IEEE test feeders modeled in a commercial time-domain modeling software package.

Capacitor bank predictive maintenance and problem identification using conventional power quality monitoring systems

Functional specifications for an automated capacitor bank predictive maintenance system that works with a conventional power quality monitoring system, are described in This work. This system uses raw power quality monitoring data to automatically evaluate and characterize transient disturbances and system conditions associated with the operation of capacitor banks, i.e., during energization, deenergization, and when the capacitor bank is in service. The system is composed of six rule based modules. The main characteristics and requirements of each module are described. An example output of this predictive maintenance system is provided in order to illustrate the utility and adaptability of the system. A brief discussion of the future system development plans concludes the paper.

Dynamic interactions among multiple FACTS controllers — A survey

Increasing number of Flexible AC Transmission System (FACTS) devices have been be installed to reinforce the existing grid and build the envisioned “Smartness” into the grid through controls and optimization. However, it has been noticed that adverse interactions among multiple FACTS controllers may occur when they are not properly coordinated with each other and other slowly acting system equipment. These interactions can amplify oscillations and even destabilize the system by influencing the damping properties of individual FACTS controllers or increasing voltage deviations. This paper presents an extensive survey on the existing cases, system studies and assessment techniques to help system planners understand the underlying mechanism of diverse interactions among multiple FACTS controllers and develop coordinated control schemes for preventing or mitigating any harmful interactions. Control interactions are categorized and discussed in terms of their root causes and resulting frequency ranges. Unfriendly interactions involving shunt FACTS devices are detailed in which Korean Electric Power Corporation (KEPCO) is particularly interested.