Contingency Screening Research Articles

Assessment of oscillatory stability constrained available transfer capability

This paper utilizes a bifurcation approach to compute oscillatory stability constrained available transfer capability (ATC) in an electricity market having bilateral as well as multilateral transactions. Oscillatory instability in non-linear systems can be related to Hopf bifurcation. At the Hopf bifurcation, one pair of the critical eigenvalues of the system Jacobian reaches imaginary axis. A new optimization formulation, including Hopf bifurcation conditions, has been developed in this paper to obtain the dynamic ATC. An oscillatory stability based contingency screening index, which takes into account the impact of transactions on severity of contingency, has been utilized to identify critical contingencies to be considered in determining ATC. The proposed method has been applied for dynamic ATC determination on a 39-bus New England system and a practical 75-bus Indian system considering composite static load as well as dynamic load models.

Fetal nasal bone in screening for trisomies 21, 18 and 13 and Turner syndrome at 11–13 weeks of gestation

To investigate the performance of first-trimester screening for aneuploidies by including assessment of the fetal nasal bone in the combined test of maternal age, fetal nuchal translucency (NT) thickness, fetal heart rate (FHR) and serum free beta-human chorionic gonadotropin (beta-hCG) and pregnancy-associated plasma protein-A (PAPP-A). Screening by the combined test was performed in singleton pregnancies, including 19,614 with euploid fetuses, 122 with trisomy 21, 36 with trisomy 18, 20 with trisomy 13 and eight with Turner syndrome. In all cases the fetal nasal bone was assessed and classified as present or absent. We examined the performance of two screening strategies: firstly, assessment of the nasal bone in all patients and secondly, first-stage screening using the combined test in all patients followed by second-stage assessment of the nasal bone only in those with an intermediate risk of 1 in 51 to 1 in 1000 after the first stage. To validate the new risk algorithm we used a second independent dataset of 19 651 fetuses, including 139 with trisomy 21. The nasal bone was absent in 2.6% of the euploid fetuses, 59.8% with trisomy 21, 52.8% with trisomy 18, 45.0% with trisomy 13 and in none of the fetuses with Turner syndrome. Respective figures for an absent nasal bone in the validation population, which contained fewer Black women, were 0.6%, 62.6%, 55.3%, 35.3% and 41.7%. In a screening policy based on maternal age, fetal NT, FHR, serum free beta-hCG and PAPP-A, for a fixed risk cut-off of 1 : 100, the false-positive rate was 3.0%. The standardized detection rates were 91% for trisomy 21 and 100% for trisomy 18, trisomy 13 and Turner syndrome, respectively. Assessment of the nasal bone in all pregnancies reduced the false-positive rate to 2.5% without changing the detection rate. A detection rate of 93% was achieved with the two-stage strategy at a false-positive rate of 2.4% in which it was necessary to assess the nasal bone in only 15% of the total population. In the validation dataset, screening by the combined test and using a risk cut-off of 1 : 100 detected 90% of the cases with trisomy 21 for a false-positive rate of 4%. Inclusion of the nasal bone increased the detection rate to 92% for a false-positive rate of 2.9%. Contingent screening detected 92% of cases for a false-positive rate of 2.9%. Assessment of the fetal nasal bone improves the performance of first-trimester screening for trisomy 21.

Comparison of different strategies in prenatal screening for Down’s syndrome: cost effectiveness analysis of computer simulation

Objectives To assess and compare the cost effectiveness of three different strategies for prenatal screening for Down’s syndrome (integrated test, sequential screening, and contingent screenings) and to determine the most...

First‐trimester contingent screening for Down syndrome can reduce the number of nuchal translucency measurements required

To assess the performance of a two-stage screening protocol for Down syndrome based on initial serum marker analysis for all women and nuchal translucency (NT) measurement only in women with intermediate risks. Biochemical marker and NT data in 10 189 women who had had combined ultrasound and biochemical (CUB) screening, were re-analysed using the contingent model. A risk was calculated from the results of the pregnancy-associated plasma protein A (PAPP-A) and free beta human chorionic gonadotrophin (FbetahCG) measurements and maternal age. For risks between 1 in 42 and 1 in 1000, the likelihood ratio from the NT measurement was incorporated and assessed against a final cut-off risk of 1 in 250. A total of 3.1% unaffected and 61.4% Down syndrome pregnancies had risks>or=1:42. In women with risks<1 in 42 and >1 in 1000 (29%), a further 2.7% unaffected pregnancies and 27.3% Down syndrome pregnancies had risks above 1 in 250 when NT was incorporated. Overall detection rate was 88.6%, and false positive rate 5.8% (compared with 90.9% and 6.4% for CUB screening). NT measurements were required in 29% of women. Within first-trimester, contingent screening provides good sensitivity and specificity with the potential for considerable saving in ultrasound resources.

Combining first and second trimester markers for Down syndrome screening: Think twice

This study compares different screening strategies for the detection of Down syndrome and considers practical implications of using multiple screening protocols. The performance characteristics of each screening strategy were assessed based on datasets of Down syndrome (n=11) and unaffected pregnancies (n=1006) tested in both first and second trimester, as well as data from first trimester (n=18,901) and second trimester (n=40,748) pregnancies. For a detection rate of 91%, the false positive rates for integrated and serum integrated screening were 2.5% and 6.3%, respectively, compared with combined first trimester (4.6%) and second trimester (12.6%) screening. Contingent and sequential screening protocols achieved detection rates of 82 to 91% with false positive rates between 2.6 and 2.9%. Contingent protocols require retesting of 15 to 20% of cases in the second trimester. Sequential and integrated protocols require retesting of 98 to 100% of cases in the second trimester. The various screening strategies did not always detect the same Down syndrome pregnancies. Combining first and second trimester markers for Down syndrome screening better defines the at-risk population. However, integrated protocols complicate management of screening programs and may not be suitable as primary screening strategies. It may be a better use of resources to refine current first and second trimester programs through improved access and new markers. We therefore suggest thinking twice before embracing integrated population screening programs.

Contingent screening for Down syndrome

Prenatal DiagnosisVolume 28, Issue 8 p. 781-781 Correspondence Contingent screening for Down syndrome Nicholas J. Wald, Corresponding Author Nicholas J. Wald Wolfson Institute of Preventive Medicine, Barts and the London School of Medicine and Dentistry, Charterhouse Square, London EC1M 6BQ, UKWolfson Institute of Preventive Medicine, Barts and the London School of Medicine and Dentistry, Charterhouse Square, London EC1M 6BQ, UKSearch for more papers by this authorJonathan P. Bestwick, Jonathan P. Bestwick Wolfson Institute of Preventive Medicine, Barts and the London School of Medicine and Dentistry, Charterhouse Square, London EC1M 6BQ, UKSearch for more papers by this authorJacob A. Canick, Jacob A. Canick Division of Prenatal and Special Testing, Department of Pathology and Laboratory Medicine, Women and Infants Hospital of Rhode Island, Providence, Rhode Island, USASearch for more papers by this author Nicholas J. Wald, Corresponding Author Nicholas J. Wald Wolfson Institute of Preventive Medicine, Barts and the London School of Medicine and Dentistry, Charterhouse Square, London EC1M 6BQ, UKWolfson Institute of Preventive Medicine, Barts and the London School of Medicine and Dentistry, Charterhouse Square, London EC1M 6BQ, UKSearch for more papers by this authorJonathan P. Bestwick, Jonathan P. Bestwick Wolfson Institute of Preventive Medicine, Barts and the London School of Medicine and Dentistry, Charterhouse Square, London EC1M 6BQ, UKSearch for more papers by this authorJacob A. Canick, Jacob A. Canick Division of Prenatal and Special Testing, Department of Pathology and Laboratory Medicine, Women and Infants Hospital of Rhode Island, Providence, Rhode Island, USASearch for more papers by this author First published: 25 July 2008 https://doi.org/10.1002/pd.2032Citations: 1AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article.Citing Literature Volume28, Issue8August 2008Pages 781-781 RelatedInformation

Contingent screening for Down syndrome

Contingent screening for Down syndrome

Correlation between nuchal translucency and nuchal skin‐fold measurements in Down syndrome and unaffected fetuses

To assess whether there is a correlation between nuchal translucency (NT) and nuchal skin-fold (NF) measurements, in Down syndrome and in normal pregnancies. Nineteen Down syndrome and 224 normal fetuses underwent ultrasound sequential examinations at 11-13 weeks and 14-28 weeks' gestation. NT was measured at the earlier examination and NF at the later one. Both markers were expressed in multiples of the normal gestation-specific median (MoM). The affected cases had been referred to us for termination of pregnancy; NT had been measured locally and NF was measured at our center prior to the procedure. All unaffected pregnancies were scanned routinely at our center. There was no statistically significant correlation between NT and NF, in either the Down syndrome (r = 0.076, P = 0.76) or the unaffected (r = - 0.021, P = 0.76) pregnancies. The median NF value in Down syndrome fetuses was 1.538 MoM, compared with 0.990 in unaffected fetuses (P < 0.0001). Gaussian modeling with parameters from a published meta-analysis, updated to include the current series, predicted a 91% detection rate of Down syndrome for a 5% false-positive rate when NF replaced second-trimester biochemistry in a sequential contingent screening strategy. While this study cannot exclude a small correlation between NT and NF, and the Down syndrome karyotype was known at the time of the NF scan, the markers can be considered as independent determinants of Down syndrome risk. Modeling suggests that sequential contingent screening incorporating NF is an effective screening strategy, although this needs to be confirmed in a prospective study.

A new transient stability margin based on dynamic security region and its applications

A new transient stability margin is proposed based on a new expression of dynamic security region (DSR) which is developed from the existing expression of DSR. Applications of the DSR based transient stability margin to contingency ranking and screening are discussed. Simulations in the 10-machine 39-bus New England system are performed to show the effectiveness of the proposed DSR based transient stability margin.

Severe Multiple Contingency Screening in Electric Power Systems

We propose a computationally efficient approach to detect severe multiple contingencies. We pose a contingency analysis problem using a nonlinear optimization framework, which enables us to detect the fewest possible transmission line outages resulting in a system failure of specified severity, and to identify the most severe system failure caused by removing a specified number of transmission lines from service. Illustrations using a three-bus system and the IEEE 30-bus system aim to exhibit the effectiveness of the proposed approach.

에너지함수법을 이용한 가용송전용량(ATC) 계산에 관한 연구

가용송전용량(ATC)은 계통내의 한 지역에서 다른 지역까지 실제 전력을 증가시키는 것이다. 지금까지 ATC 계산은 대부분 정상상태에서 실행가능성을 주로 고려하여 계산되어 왔다. 하지만 ATC 평가시 과도안정도로 제약된 ATC 계산은 매우 중요한 부분이다.BR ATC 평가시에는 제약조건으로 열적용량, 전압 및 과도안정도로 제약된 상정사고(n-1)시 안전도 명가가 요구된다.BR 본 논문은 자코비안 행렬의 고유치를 이용하여 상정사고 우선순위를 선정하였고, 에너지 함수법을 이용하여 선로의 열적용량, 전압안정도 및 과도안정도를 고려한 ATC를 계산하였다.

Contingent screening for Down syndrome—results from the FaSTER trial

Comparison of contingent, step-wise and integrated screening policies. Mid-trimester Down syndrome risks were retrospectively calculated from FaSTER trial data. For contingent screening, initial risk was calculated from ultrasound measurement of nuchal translucency (NT), maternal serum pregnancy-associated plasma protein (PAPP)-A and free beta-human chorionic gonadotrophin (hCG) at 11-13 weeks, and classified positive (>1 in 30), borderline (1 in 30-1500) or negative. Borderline risks were recalculated using alpha-fetoprotein, hCG, unconjugated estriol (uE3) and inhibin at 15-18 weeks, and reclassified as positive (>1 in 270) or negative. For step-wise screening, initial negative risks were also recalculated. For integrated screening, a single risk was calculated from NT, PAPP-A and the second trimester markers. There were 86 Down syndrome and 32,269 unaffected pregancies. The detection rate for contingent screening was 91% and false-positive rate was 4.5%; initial detection rate was 60%, initial false-positive rate was 1.2% and borderline risk was 23%. Step-wise screening had 92% detection rate and 5.1% false-positive rate; integrated screening had 88% and 4.9% respectively. As predicted by modelling, the contingent screening detection rate for a fixed false-positive rate is comparable with step-wise and integrated screening, but substantially reduces the number needing to return for second trimester testing.

Fast Computation and Assessment Methods in Power System Analysis-Fast Power System Analysis Techniques for Online Dynamic Security Assessment-

Power system analysis is essential for efficient and reliable power system operation and control. Recently, online security assessment system has become of importance, as more efficient use of power networks is eagerly required. In this article, fast power system analysis techniques such as contingency screening, parallel processing and intelligent systems application are briefly surveyed from the view point of their application to online dynamic security assessment.

588: Down syndrome screening strategies utilized by United States Maternal-Fetal Medicine specialists in 2007

MATERNAL-FETAL MEDICINE SPECIALISTS IN 2007 YU MING VICTOR FANG, PETER BENN, KATHERINE TRIBULATO, CONNOR FITZPATRICK, JAY BOLNICK, JAMES EGAN, University of Connecticut, Obstetrics and Gynecology, Farmington, Connecticut, University of Connecticut, Department of Pediatrics, Farmington, Connecticut, University of Nebraska, Omaha, Nebraska, Boston College, Boston, Massachusetts OBJECTIVE: Ball and colleagues suggested a cohesive national strategy regarding prenatal diagnosis and screening for Down syndrome (DS). (Obstet Gynecol 2007;110: 10-17) We determined the most common DS screening strategies currently utilized by Maternal-Fetal Medicine specialists (SMFM) in the United States (US). STUDY DESIGN: SMFM members in the US were surveyed in April 2007 regarding screening patterns for fetal aneuploidy. They were asked which of the following first trimester (trim) DS screening strategies they offered: combined screening (1st trim NT, PAPP-A, B-hCG. No 2nd trim screen), integrated screening (single risk given after 1st and 2nd trim tests are completed), stepwise sequential screening (results given after each trim, final risk is calculated from 1st and 2nd trim tests), contingency screening (use specific cut-offs to offer either invasive testing, no further testing, or 2nd trim screen), independent screening (1st and 2nd trim results are evaluated independently and not combined), or other. They could choose more than one strategy. Descriptive statistics are reported. RESULTS: There were 448/1756 (26%) responses of which 30.4% offered more than one screening strategy. The frequencies of the strategies offered by respondents are presented in the graph.

A Hybrid Decision Tree Model for Fast Voltage Contingency Screening and Ranking

This paper presents a new decision tree (DT) based approach for fast voltage contingency screening and ranking for on-line applications in energy management systems. The hybrid decision tree model is developed to learn all the selected contingencies simultaneously, therefore fewer DTs are required. To reduce the size and improve the accuracy of the decision tree, the K-class problem is converted into the set of K two-class problems, and separate decision tree modules are trained for each of the two class problems. All the selected contingencies are presented to the filter module, which is trained to separate them in critical and non-critical contingency classes, which reduces the burden on ranking modular DT. The critical contingencies screened out by the filter module are presented to the ranking modular decision tree for their further ranking. To measure the severity of contingencies, bus voltage violation based scalar performance index is used. Full AC load flow is performed to generate the training and testing patterns for the proposed hybrid decision tree, under each contingency. The effectiveness of the proposed approach is tested on IEEE test systems. Once trained, a hybrid decision tree method gives fast and accurate screening and ranking of contingencies for unknown load patterns.

A new ultra‐fast contingency screening algorithm without time‐domain simulation for online transient security assessment

AbstractThis paper proposes a new ultra‐fast contingency screening (CS) method for the transient security assessment (TSA). The method is based on the Single Machine Equivalent (SIME), but computes the critical clearing times (CCT) for given disturbances without the use of time‐domain simulations. All generators and loads are represented by a classical model and constant impedance model, respectively. The critical clustering is done using two stability indices, which are defined as the ratio of the accelerating power to the synchronizing power coefficient and the accelerating area to the decelerating area, respectively. Finally, the CCT is calculated using the equal‐area criterion to the equivalent one‐machine infinite‐bus system with the assumption that the rotor angles of the critical generators increase uniformly and the rotor angles of the non‐critical ones remain constant. The proposed CS method is tested on the 950‐bus, 230‐machine, 60‐Hz KEPCO system. Copyright © 2007 John Wiley &amp; Sons, Ltd.

Genetic Sonography After First-trimester Down Syndrome Screening

Although second-trimester Down syndrome screening using maternal analytes was long accepted as the standard of care, first-trimester screening using both maternal analytes and fetal nuchal translucency measurement has become a viable alternative and may even be preferable, since it identifies approximately 90% of Down syndrome cases. This simulation study used published statistical parameters to determine whether following a first-trimester screening test with second-trimester sonography is an effective sequential screening protocol. Both stepwise screening (all patients have both a first-trimester screening and a second-trimester ultrasound exam) and contingent screening (all patients have a first-trimester screening; only those with a Down syndrome risk between 1/300 and 1/2500 have a second-trimester ultrasound exam) were evaluated. Serum analytes assessed in the first trimester included free β-human chorionic gonadotropin and pregnancy-associated plasma protein A. Ultrasound markers included nuchal translucency thickness and nasal bone in the first trimester, and nuchal fold, hyperechoic bowel, short humerus, short femur, echogenic intracardiac focus, pyelectasis, or a major abnormality in the second trimester. Combined first-trimester screening with maternal serum analytes and nuchal thickness detected 88.5% of Down syndrome cases at a false-positive rate of 4.2%. If women who were screen negative underwent a second-trimester genetic ultrasound and everyone whose fetus had at least one ultrasound marker was considered screen positive, another 8% of Down syndrome cases were identified at an additional false-positive rate of 13.2%. This protocol thus identified 96.5% of all Down syndrome cases at a total screen positive rate of 17.5%. In an alternative approach, the second-trimester ultrasound markers were converted into likelihood ratios that were used to modify the Down syndrome risk determined by the first-trimester screening test. This protocol identified a total of 94.1% cases at a total screen positive rate of 5.4%. In the contingent protocol, only women whose first-trimester risk of Down syndrome was between 1/300 and 1/2500 underwent second-trimester ultrasound, and sonographic markers were converted into likelihood ratios that were used to modify the Down syndrome risk determined by the first-trimester screening test. This yielded an overall Down syndrome detection rate of 93.3% and a total false-positive rate of 4.9%. Regardless of whether a stepwise sequential or contingent protocol is used, second-trimester genetic sonography may be considered as an alternative to second-trimester serum screening for maximizing the detection of Down syndrome at reasonable rates of false-positive findings.

Power system static security assessment and evaluation using external system equivalents

Static security assessment and analysis using external system (Ward network) equivalent is presented. Contingency analysis is performed on the reduced network obtained using extended Ward equivalent. The Ward equivalent provides reasonably good results for contingency studies involving active power flows. Distribution factors method is used to perform contingency analysis. Fast line flow contingency screening is implemented for both the original unreduced network and the reduced network. The real power performance index ( PI MW) that reflects the degree of severity of contingencies is calculated using the screening method and full AC load flow method. The effectiveness of the proposed method is demonstrated by performing contingency analysis on a standard 6-bus system, IEEE 14-bus, IEEE 30-bus and IEEE 57-bus systems, respectively. The performances of the reduced network and the screening method are compared with the traditional Newton–Raphson (NR) method and the results are discussed. The external network equivalent approach to security analysis was found to be suitable for fast line flow contingency analysis.

First- and Second-Trimester Evaluation of Risk for Down Syndrome

To investigate the differences in costs and outcomes of Down syndrome screening using data from the First and Second Trimester Evaluation of Risk (FASTER) Trial. Seven possible screening options for Down syndrome were compared: 1) Triple Screen-maternal serum alpha fetoprotein, estriol, and hCG; 2) Quad-maternal serum alpha fetoprotein, estriol, hCG, and Inhibin A; 3) Combined First-nuchal translucency, pregnancy-associated plasma protein A (PAPP-A), free beta-hCG; 4) Integrated-nuchal translucency, PAPP-A, plus Quad; 5) Serum Integrated-PAPP-A, plus Quad; 6) Stepwise Sequential-Combined First plus Quad with results given after each test; and 7) Contingent Sequential-Combined First and only those with risk between 1:30 and 1:1,500 have Quad screen. The detection rates for each option were used given a 5% false-positive rate except for Contingent Sequential with a 4.3% false-positive rate. Outcomes included societal costs of each screening regimen (screening tests, amniocentesis, management of complications, and cost of care of Down syndrome live births), Down syndrome fetuses identified and born, the associated quality-adjusted life years, and the incremental cost-utility ratio. Based on the screening results derived from the 38,033 women evaluated in the FASTER trial, the Contingent Sequential screen dominated (lower costs with better outcomes) all other screens. For example, the Contingent Sequential cost 32.3 million dollars whereas the other screens ranged from 32.8 to 37.5 million dollars. The Sequential strategy led to the identification of the most Down syndrome fetuses of all of the screens, but at a higher cost per Down syndrome case diagnosed ($719,675 compared with $690,427) as compared with the Contingent Sequential. Because of the lower overall false-positive rate leading to fewer procedure-related miscarriages, the Contingent Sequential resulted in the highest quality-adjusted life years as well. The Contingent Sequential remained the most cost-effective option throughout sensitivity analysis of inputs, including amniocentesis rate after positive screen, rate of therapeutic abortion after Down syndrome diagnosis, and rate of procedure-related miscarriages. Analysis of this actual data from the FASTER Trial demonstrates that the Contingent Sequential test is the most cost-effective. This information can help shape future policy regarding Down syndrome screening.

Line flow contingency selection and ranking using cascade neural network

Line flow or real-power contingency selection and ranking is performed to choose the contingencies that cause the worst overloading problems. In this paper, a cascade neural network-based approach is proposed for fast line flow contingency selection and ranking. The developed cascade neural network is a combination of a filter module and a ranking module. All the contingency cases are applied to the filter module, which is trained to classify them either in critical contingency class or in non-critical contingency class using a modified BP algorithm. The screened critical contingencies are passed to the ranking module (four-layered feed-forward artificial neural network (ANN)) for their further ranking. Effectiveness of the proposed ANN-based method is demonstrated by applying it for contingency screening and ranking at different loading conditions for IEEE 14-bus system. Once trained, the cascade neural network gives fast and accurate screening and ranking for unknown patterns and is found to be suitable for on-line applications at energy management centre.