联营电力市场中需求侧的直接参与

在现有的允许需求侧竞标的日前联营市场中,每当某个时段的市场清算电价高于投标报价时,需求侧的兆瓦级购电竞标就被拒绝。由于用户不参与通过削减供电量来牟利的商业运作,因此未得到满足的用户需求只能从平衡市场中在更接近于预期用电的时段中获得。这就使用户遭受更大的不能以所希望的价格来满足其对电能的需要的风险。文章介绍了一种新的市场概念,它能在日前市场对需求侧的竞标者关闭后,以合并拍卖规则中需求移动机制的方法来降低其需求不平衡的风险。     

Value of security: modeling time-dependent phenomena and weather conditions

Deterministic security criteria provide a degree of security that may be insufficient under some operating conditions and excessive for others. To determine an appropriate level of security, one should perform a probabilistic cost/benefit analysis that balances the cost of the security margin against its benefits, i.e., the expected societal cost of the avoided outages. This paper shows how a previously published method based on Monte Carlo simulation can be enhanced to take into account time-dependent phenomena (TDP) such as cascade tripping of elements due to overloads, malfunction of the protection system, and potential power system instabilities. In addition, the importance of using failure rates that reflect the weather conditions is discussed. Studies based on the South-Western part of the transmission network of England and Wales demonstrate the validity of the models that have been developed.     

Data fitting with geometric-programming-compatible softmax functions

Motivated by practical applications in engineering, this article considers the problem of approximating a set of data with a function that is compatible with geometric programming (GP). Starting with well-established methods for fitting max-affine functions, it is shown that improved fits can be obtained using an extended function class based on the softmax of a set of affine functions. The softmax is generalized in two steps, with the most expressive function class using an implicit representation that allows fitting algorithms to locally tune softness. Each of the proposed function classes is directly compatible with the posynomial constraint forms in GP. Max-monomial fitting and posynomial fitting are shown to correspond to fitting special cases of the proposed implicit softmax function class. The fitting problem is formulated as a nonlinear least squares regression, solved locally using a Levenberg–Marquardt algorithm. Practical implementation considerations are discussed. The article concludes with numerical examples from aerospace engineering and electrical engineering.     

Improved sensitivities in MW dispatch for control of voltage

This paper describes the derivation of linear sensitivities used in the dispatch of MW controls for alleviation of voltage violations. Such a set of sensitivities is important for the selection and co-ordination of necessary changes to MW generation, phase shifter settings and, as a last resort, load shedding where reactive power controls are insufficient. The sensitivity analysis has been embedded in a qualitative reasoning based decision support tool in which operators' considerations such as cost, control effectiveness, preservation of control margin and ease of implementation are modeled in such a way that priorities can be adjusted and results achieved quickly. Results are presented demonstrating the accuracy and effectiveness of the proposed approach both for a standard test system and a system representing a real network     

Assessing the competitiveness of demand-side bidding

Demand-side bidding (DSB) provides a very flexible way of meeting the peak load in a production schedule. This paper demonstrates that, in a competitive electricity pool, DSB and other forms of flexible generation can cause sharp and unwarranted price increases if the production schedule is established on the basis of a minimization of the total production cost. Furthermore, the competitiveness of DSB is artificially inflated if the load recovery periods which accompany these load reductions are not taken into consideration by the scheduling program. It is argued that the actual value of DSB should be re-examined in the light of these findings     

Experimental determination and computation of the liquid miscibility gap in the system CaO-MgO-SiO2-TiO2

Coexisting liquids in the CaO-MgO-SiO₂-TiO₂ system were synthesized and quenched at ambient pressure in air from 1600 °C using a Rh-Pt resistance furnace and from 1800 to 2000 °C using a laser heated air levitation setup. Compositions of quenched glasses determined by electron microprobe analysis are reproduced in detail by weighted extrapolations of binary Margules-type excess functions. Using a generalized Kohler method, the authors illustrate the correlation between the degree of binary excess polynomials and their extrapolation behavior to higher-order systems. The presented extrapolation approach avoids additional ternary parameters in the CaO-SiO₂-TiO₂ and MgO-SiO₂-TiO₂ systems and affords a reasonable extrapolation outside of the compositional region of stable liquid immiscibility. In contrast, ternary excess parameters had to be added to binary interaction terms in the CaO-MgO-TiO₂ and CaO-MgO-SiO₂ systems to reproduce the solvus and liquidus phase relations reported in the literature. The excess entropy terms of the liquid were minimized in order to avoid unjustified stable miscibility gaps of the melt up to at least 3000 °C. The reliability of the authors’ polynomial approximation for the excess Gibbs energy of the melt is reduced only when ternary interaction terms are added to binary terms in the melt. However, the proposed method permits a valuable approximation of the highly complex excess Gibbs energy at solvus compositions in the system CaO-MgO-SiO₂-TiO₂ with a minimum number of excess terms.     

A model of PV generation suitable for stability analysis

This paper describes a model of photovoltaic (PV) generation suitable for studying its interactions with the power system. Experimental results suggest that the maximum power point tracking part of the control system of the PV generator dominates the dynamic behavior of the system. These experimental results are used to develop and validate the proposed model. It is shown that the model accurately reflects the behavior of the generator following both small and fast changes in irradiance and AC grid voltage. The proposed model is designed to be integrated in a dynamic simulation program.     

Comparison of risk-based and deterministic security assessments

The traditional N - 1 security criterion provides only a limited perspective on the actual level of security of a power system and a risk-based approach to security assessment provides considerably more information on which to base operating decisions. It further argues that this risk should be measured in terms of expected outage costs to the consumers. Furthermore, the risk calculation should not be limited to the consideration of a predefined set of contingencies but should factor in the actual probabilities of outages leading to load disconnections. A case study based on the IEEE-Reliability Test System shows that different operating points on the system's deterministic security boundary have very different levels of risk. For a given operating point, the risk level changes considerably between fair, average and bad weather conditions. Finally, the paper shows how, using adaptive deterministic security boundaries, it is possible to compare the cost and benefit of relaxing operating limits.     

Application of an Off‐Gas Analysing System to Control Oxidation during Stainless Steelmaking in an EAF

During stainless steelmaking in the electric arc furnace at Deutsche Edelstahlwerke GmbH, oxygen is injected to oxidize unwanted tramp elements mainly carbon and silicon. Unfortunately the injected oxygen also oxidizes valuable elements such as chromium and iron, which causes an economical loss and has a negative environmental effect. Since the temperature and dilution techniques to minimise chromium oxidation are seldom applied in the electric arc furnace, a new strategy to minimise chromium oxidation has to be developed. This paper proposes a new strategy which involves the use of a continuous off‐gas analysing system to minimise chromium oxidation by monitoring the oxidation products in the off‐gas, i.e. CO and CO₂. During stainless steelmaking in the electric arc furnace, for which initial carbon and silicon input cannot be precisely known due to imprecise scrap analysis, the installed off‐gas analysing system should provide precise information concerning an efficient oxygen injection. This would then directly prevents excess chromium and iron oxidation. A continuous off‐gas analysing system installed at Deutsche Edelstahlwerke GmbH delivers a promising result for future applications. During the plant trial, the efficiency of oxygen injection as well as the chromium and iron yields were increased.     

Centralised and distributed electricity systems

Because of their high level of integration, centralised energy supply systems are vulnerable to disturbances in the supply chain. In the case of electricity especially, this supply paradigm is losing some of its appeal. Apart from vulnerability, a number of further aggravating factors are reducing its attractiveness. They include the depletion of fossil fuels and their climate change impact, the insecurities affecting energy transportation infrastructure, and the desire of investors to minimise risks through the deployment of smaller-scale, modular generation and transmission systems.