Chun-Lien Su

Probabilistic load-flow computation using point estimate method

A new probabilistic load-flow solution algorithm based on an efficient point estimate method is proposed in this paper. It is assumed that the uncertainties of bus injections and line parameters can be estimated or measured. This paper shows how to estimate the corresponding uncertainty in the load-flow solution. The proposed method can be used directly with any existing deterministic load-flow program. For a system with m uncertain parameters, it uses 2m calculations of load flow to calculate the statistical moments of load-flow solution distributions by weighting the value of the solution evaluated at 2m locations. The moments are then used in the probability distribution fitting. Performance of the proposed method is verified and compared with those obtained from Monte Carlo simulation technique and combined simulation and analytical method using several IEEE test systems.

Stochastic Analyses of Electric Vehicle Charging Impacts on Distribution Network

A stochastic modeling and simulation technique for analyzing impacts of electric vehicles charging demands on distribution network is proposed in this paper. Different from the previous deterministic approaches, the feeder daily load models, electric vehicle start charging time, and battery state of charge used in the impact study are derived from actual measurements and survey data. Distribution operation security risk information, such as over-current and under-voltage, is obtained from three-phase distribution load flow studies that use stochastic parameters drawn from Roulette wheel selection. Voltage and congestion impact indicators are defined and a comparison of the deterministic and stochastic analytical approaches in providing information required in distribution network reinforcement planning is presented. Numerical results illustrate the capability of the proposed stochastic models in reflecting system losses and security impacts due to electric vehicle integrations. The effectiveness of a controlled charging algorithm aimed at relieving the system operation problem is also presented.

Interconnected Network State Estimation Using Randomly Delayed Measurements

In an interconnected power system, the network security and power transfer capability analyses require a complete network base case solution. With the advent of communications among operations control center computers, real-time telemetered data can be exchanged for complete network modeling. Measurement time skew is a consideration when combining large area data received via a data communication network. In order to accommodate the effects of randomly varying arrival of measurement data, this paper presents an implementation of a stochastic extended Kalman filter (EKF) algorithm, which provides optimal estimates of interconnected network states for systems in which some or all measurements are delayed. The method relies on the delay statistics of exchanged data arrival at the control center and the delay is assumed to have binary statistics, i.e., either the measurements arrive in time or they are delayed by one time sample. Performance of the proposed method is compared with that of the standard weighted least square (WLS) technique.

Two-point estimate method for quantifying transfer capability uncertainty

A two-point estimate method is proposed in this paper to assess the power transfer capability uncertainty. This paper assumes that the uncertainty of the line parameters and bus injections involved in transfer capability calculations can be estimated or measured and shows how to estimate the corresponding uncertainty in the transfer capability. Instead of using a large number of simulations as required in the Monte Carlo approach, for a system with n uncertain parameters, the two-point estimate method uses 2n calculations of transfer capability to quantify the uncertainty. The proposed method uses a numerical method to calculate the moments of the transfer capability. The moments are then used in the probability distribution fitting. Using the obtained transfer capability uncertainty information and a desired level of reliability, an adequate transmission reliability margin can be determined for each transmission service. The proposed method can be used directly with a deterministic computer program and it does not require derivatives of the transfer capability. Test results of the proposed method are compared with those obtained from the Monte Carlo simulations and a truncated Taylor series expansion method.

Stochastic Evaluation of Voltages in Distribution Networks With Distributed Generation Using Detailed Distribution Operation Models

Distributed generation (DG) connected to distribution networks affects the currents or power flows in the networks; thus, node voltages that are strongly related to power flows also are changed. As the voltage must be within permitted limits to comply with utility and customer requirements, this effect should be assessed prior to DG connection. At the distribution level there are a number of attributes that can influence the voltage profiles; therefore, this assessment requires detailed distribution operation models. To provide reliable voltage evaluation results, a new probabilistic methodology is proposed in this paper. The distribution system operation uncertainties including daily time varying load, stochastic DG power production, network configuration, and voltage control devices operation are all taken into account. A probabilistic load flow method based on an efficient algorithm is used to handle uncertainties in loads and DG output power and voltage control devices operation. Accordingly, a probabilistic network reconfiguration model is employed to take stochastic network configurations for service restoration or load balancing into consideration. The proposed probabilistic method provides a better knowledge of the voltage effects and can be used for evaluating the level of DG that might be accepted on a distribution network. The models developed can also be used for assessing impact of voltage mitigation equipment on distribution operating performance. Test results of a modified IEEE feeder test system demonstrate the performance of the proposed method.

Economic Evaluation of a Distribution Automation Project

To deal with the deregulated challenges in future power system operations, Taiwan Power Company (Taipower) has taken a strategic approach to implement an extensive Distribution Automation (DA) project including feeder automation (FA), trouble call management, load management, and remote metering functions in its Tai-Chung District. The project was completed in December 2002. For future extension purposes, Taipower tries to develop a comprehensive generalized step-by-step methodology for the economic evaluation of the DA project. This paper presents the methodology and results for the economic evaluation of the Tai-Chung DA project. The potential benefits of the implemented DA functions are quantified using standard mathematic formulas. An economic evaluation method using value-based analysis and present-worth analysis is performed to identify the most beneficial functions in the Taipowers implemented DA functions. Those DA functions that have great benefits should have high priority to be implemented in other distribution districts. Moreover, to find the candidate feeders for further FA extension, the sensitivity analysis of the benefit/cost ratio with respect to different customer types, feeder loads, feeder lengths, numbers of switch, and failure rates is also conducted. Accordingly, the optimum number of switch can be determined under the expected service reliability. The results and formulas obtained in this paper can also be used for return-of-investment or possible performance measure of the DA systems.

An Energy-Savings Evaluation Method for Variable-Frequency-Drive Applications on Ship Central Cooling Systems

Variable-frequency-drive (VFD) applications are gradually being recognized by maritime industries as one of the most effective tools for energy savings. Significant energy benefit can be realized from VFD-driven central seawater cooling systems because the ambient seawater temperature varies greatly as ships travel through different sea areas. Before making a decision about which drive to choose, information about the actual energy and cost savings is needed. A simple and fast method that is able to effectively determine the energy savings for closely estimating the economics for their application is essential. This paper aims at presenting a fast method for calculating energy savings that would result from installing VFDs on the seawater cooling pumps on ships. The ship main engine cooling system and navigation environment characteristics, including related marine equipment, pipe allocation, ambient seawater temperature, etc., are taken into account. The process system that involves mechanical, instrumental, and pump hydraulics, as well as the electrical aspects of the drive, is mathematically modeled so that the pump hydraulic formulas can be properly applied. The performance of the proposed method was validated through the energy-savings analyses of an actual ship seawater cooling system. The effect of shipping routes on the energy savings was fully described. The analysis model developed in this paper can assist in performing the energy-savings calculations to permit the checking of the economics of adjustable-speed flow control for similar systems in industrial applications.

Simulation study of Internet based inter control center data exchange for complete network modeling

The model used for real-time network security and transmission capability analyses requires the use of a large area network and has to be brought up to real-time conditions. To build a complete network model, SCADA data must be made available to many external bodies. An Internet interface to SCADA systems is proposed for intercontrol-center data exchange. A discrete even technique is utilized to simulate the interarea data exchange. The performance modeling of data exchange on the Internet is established, and a quantitative analysis of the data exchange delay is presented. Simulation and experimental results have indicated that the Intemet provides a convenient way for inter-control-center data exchange, and, by using state estimation (SE), accurate complete network models can be obtained for practical security and made available for transfer capability analyses.

A Method for Evaluating Energy Efficiency to Justify Power Factor Correction in Ship Power Systems

A poor power factor is often overlooked in ship power systems where electric motors are widely used to drive a variety of machinery and operate numerous processes. By reducing the reactive power produced by power generators through power factor correction (PFC), not only have system operating efficiency and economic targets been enhanced but greenhouse gas emissions have been also reduced. An evaluation of the economics of power factor improvement should take into account diesel electric generator loading, fuel oil consumption rate, running time, operating efficiency, and the production of emissions. This will help achieve energy-saving evaluation and ensure that the additional capital costs of installing a PFC system will provide an adequate rate of return on investment. This paper presents the methodology and results for the economic evaluation of PFC in marine vessels. A mathematical model, used to determine the relationships among fuel oil consumption, emission, and power factor for the diesel electric generator, is derived to quantify the possible benefits with PFC. An economic evaluation method using present worth analysis is performed to evaluate the PFC system. Numerical analysis and field test of practical ship power systems confirm the validity of the proposed approach and the critical parameters for PFC economics are determined.

Design of passive harmonic filters to enhance power quality and energy efficiency in ship power systems

Variable frequency drives (VFDs) driven motors for pumps, blowers and fans, and heating, ventilation and air conditioning (HVAC) equipment on ships are rapidly growing in number. Due to their nonlinear nature, they become the major sources of harmonics in electrical power systems on board. To mitigate the harmonics, passive harmonic filters that are an LC circuit tuned to each harmonic order to be filtered have been used. In compliance with the new rule in the emission regulation of the maritime industry, the installation of passive filters has attracting more attention in managing service power energy efficiency. This paper proposed a design of passive filters to reduce the voltage and current distortions in ship power systems while increasing energy efficiency with power factor corrected by passive filters. The inductance and capacitance in the filter are designed according to harmonic currents and reactive power required. The on-board power system of a practical ship with a great number of VFDs is analyzed to illustrate the filter design. The field testing results verified the performance of system with the filter designed by the proposed method.