Optimal Topology Design for Disturbance Minimization in Power Grids

The transient response of power grids to external disturbances influences their stable operation. This paper studies the effect of topology in linear time-invariant dynamics of different power grids. For a variety of objective functions, a unified framework based on $H_2$ norm is presented to analyze the robustness to ambient fluctuations. Such objectives include loss reduction, weighted consensus of phase angle deviations, oscillations in nodal frequency, and other graphical metrics. The framework is then used to study the problem of optimal topology design for robust control goals of different grids. For radial grids, the problem is shown as equivalent to the hard "optimum communication spanning tree" problem in graph theory and a combinatorial topology construction is presented with bounded approximation gap. Extended to loopy (meshed) grids, a greedy topology design algorithm is discussed. The performance of the topology design algorithms under multiple control objectives are presented on both loopy and radial test grids. Overall, this paper analyzes topology design algorithms on a broad class of control problems in power grid by exploring their combinatorial and graphical properties.Comment: 6 pages, 3 figures, a version of this work will appear in ACC 201

Revisiting the Merit-Order Effect of Renewable Energy Sources

An on-going debate in the energy economics and power market community has raised the question if energy-only power markets are increasingly failing due to growing feed-in shares from subsidized renewable energy sources (RES). The short answer to this is: No, they are not failing. Energy-based power markets are, however, facing several market distortions, namely from the gap between the electricity volume traded at day-ahead markets versus the overall electricity consumption as well as the (wrong) regulatory assumption that variable RES generation, i.e., wind and photovoltaic (PV), truly have zero marginal operation costs. In this paper we show that both effects over-amplify the well-known merit-order effect of RES power feed-in beyond a level that is explainable by underlying physical realities, i.e., thermal power plants being willing to accept negative electricity prices to be able to stay online due to considerations of wear & tear and start-stop constraints. We analyze the impacts of wind and PV power feed-in on the day-ahead market for a region that is already today experiencing significant feed-in tariff (FIT)-subsidized RES power feed-in, the EPEX German-Austrian market zone ($\approx\,$20% FIT share). Our analysis shows that, if the necessary regulatory adaptations are taken, i.e., increasing the day-ahead market's share of overall load demand and using the true marginal costs of RES units in the merit-order, energy-based power markets can remain functional despite high RES power feed-in.Comment: Working Paper (9 pages, 11 figures, 5 tables) - Some revisions since last version (10 February 2014). (Under 2nd review for IEEE Transactions on Power Systems

Impact of Low Rotational Inertia on Power System Stability and Operation

Large-scale deployment of RES has led to significant generation shares of variable RES in power systems worldwide. RES units, notably inverter-connected wind turbines and PV that as such do not provide rotational inertia, are effectively displacing conventional generators and their rotating machinery. The traditional assumption that grid inertia is sufficiently high with only small variations over time is thus not valid for power systems with high RES shares. This has implications for frequency dynamics and power system stability and operation. Frequency dynamics are faster in power systems with low rotational inertia, making frequency control and power system operation more challenging. This paper investigates the impact of low rotational inertia on power system stability and operation, contributes new analysis insights and offers mitigation options for low inertia impacts.Comment: Presented at IFAC World Congress 2014, Capetown, South Africa (Flaws in Table I corrected.

Grid Integration Costs of Fluctuating Renewable Energy Sources

The grid integration of intermittent Renewable Energy Sources (RES) causes costs for grid operators due to forecast uncertainty and the resulting production schedule mismatches. These so-called profile service costs are marginal cost components and can be understood as an insurance fee against RES production schedule uncertainty that the system operator incurs due to the obligation to always provide sufficient control reserve capacity for power imbalance mitigation. This paper studies the situation for the German power system and the existing German RES support schemes. The profile service costs incurred by German Transmission System Operators (TSOs) are quantified and means for cost reduction are discussed. In general, profile service costs are dependent on the RES prediction error and the specific workings of the power markets via which the prediction error is balanced. This paper shows both how the prediction error can be reduced in daily operation as well as how profile service costs can be reduced via optimization against power markets and/or active curtailment of RES generation.Comment: Accepted for SUSTECH 2014, Portland, Oregon, USA, July 201

Intracameral Injection of Bevacizumab for the Treatment of Neovascular Glaucoma

Purpose: To assess the duration of the effect of intracameral bevacizumab in patients presenting with rubeosis iridis and neovascular glaucoma (NVG). Methods: Retrospective analysis of 24 consecutive eyes of 24 patients with decompensated NVG (> 21 mm Hg) treated with a single intracameral injection of bevacizumab over a minimum follow-up of 6 months. The endpoint of the study was the need for retreatment due to recurrence of raised intraocular pressure (IOP). Secondary outcome was the course of visual acuity (VA) and IOP over 6 months. Results: A Kaplan-Meier calculation revealed a mean duration of the treatment effect of 23 +/- 4.4 days. Compared to mean IOP before treatment (26.3 mm Hg), decreases to 17.5 mm Hg at 1 week after treatment (p < 0.002) and to 17.1 mm Hg (p < 0.005) at 6 months following a single injection were seen. At 6 months, additional treatment was performed in 87.5% (n = 21) of eyes. VA remained stable or improved in 75% (n = 18) of all cases. Conclusion: The IOP-lowering effect of intracameral bevacizumab can be seen 1 week after the injection, but is limited to a period of approximately 3 weeks. However, the fast and effective response to intracameral bevacizumab injection opens a time window for additional treatments, which are often necessary. Copyright (C) 2011 S. Karger AG, Base

Alpha-Lipoic Acid for the Prevention of Diabetic Macular Edema

Introduction: To evaluate the effect of alpha-lipoic acid (ALA) on the occurrence of diabetic macular edema. Methods: Randomized, double-blind, placebo-controlled, multicenter, multinational study. Patients were randomized to the treatment group with 600 mg ALA per day or the placebo group. Every 6 months stereo fundus photographs, HbA1c levels, and an ophthalmological examination were documented. The primary endpoint was the occurrence of clinically significant macular edema (CSME) within a follow-up period of 2 years. Results: We randomized 235 patients with type II diabetes mellitus into the treatment group (mean age 58.0 years) and 232 into the placebo group (mean age 57.9 years). Mean HbA1c level was 8.1, with no significant differences between the treatment (mean 8.2, SD +/- 1.35) and placebo groups (mean 8.1, SD +/- 1.29). HbA1c values remained constant over time. In the treatment and placebo groups, 84 and 86 patients (35.7 and 37.1%) had insulin-dependent diabetes mellitus (IDDM) with a median duration of diabetes of 9.3 versus 9.0 years in the placebo group. Visual acuity remained unchanged during the entire trial. Concerning the primary endpoint, the study provided a negative result, i.e. 26/235 patients in the treatment group and 30/232 patients in the placebo group developed CSME. Confirmatory intention-to-treat analysis of the primary endpoint revealed no statistically significant difference between groups (log-rank test, p = 0.7108, HR = 0.9057 with CI = 0.5355-1.5317). Median follow-up was identical (2.00 years). Conclusions: A daily dosage of 600 mg ALA does not prevent the occurrence of CSME in IDDM patients. Copyright (C) 2011 S. Karger AG, Base

Central serous chorioretinopathy

The pathogenesis of central serous chorioretinopathy (CSC) is still not fully understood. The involvement of corticosteroids is undisputed, although their exact role has not been clarified; other parts of the underlying mechanism of CSC have been mainly elucidated by imaging techniques such as fluorescein and indocyanine green angiography. Even though most cases of CSC are self-limiting, severe as well as recurrent courses exist, and for these patients only a limited number of treatment options are available: laser photocoagulation, with a risk of scotoma and choroidal neovascularization, and photodynamic therapy. In this review article, we give an overview of its epidemiology, the current understanding of its pathogenesis as well as systemic and ocular risk factors. We illuminate modern diagnostic tools as well as current treatment options in the context of CSC, particularly in the light of a better understanding of corticosteroids and their receptors involved in its pathogenesis