Intermittent renewable generation and maintaining power system reliability

There have been attempts, using various approaches, to assess the additional cost of running an electricity system when intermittent renewable generation is used to provide a significant proportion of the energy. The key issues are the difference, in statistical terms, between the resource availability of the intermittent source and conventional generation and the contribution the intermittent source can make to meet the system peak demand while maintaining system reliability. There is considerable agreement over the capacity credits that can be attributed to renewable energy sources, that is the amount of conventional capacity that renewables can reliably displace, yet the implications for costs have proved more controversial. Approaches to calculate changes in overall system cost are examined and an expression for the additional cost that intermittent generation imposes on a system that is attributable to its intermittent nature is identified. Further, it is shown that this expression can be reconciled with approaches that look at intermittent renewables on a stand-alone basis and factor in the additional costs of 'standby' capacity. It is shown that the main source of divergence between estimates of the cost of intermittency is the load factor implicitly assumed for the conventional plant used as a reference. There is only one consistent way to impute the costs of intermittency when the unit cost of intermittent plant is being compared with that of baseload generation plant.     

Probabilistic load flow in systems with wind generation

A method for solving a probabilistic power flow that deals with the uncertainties of (i) wind generation, (ii) load and (iii) generation availability in power systems is proposed. Dependence between random variables has been considered. The method is based on the properties of cumulants of random variables. Cornish-Fisher expansion series are used to obtain the cumulative distribution function (CDF) of the output variables. Multimodal CDF are obtained by convolutions, whose number has been minimised in order to decrease the computation requirements.     

新疆风力发电中的计量问题研究

在对新疆达坂城风力发电场进行调研的基础上,通过与企业科技工作人员的座谈,总结了新疆地区风力发电中的计量测试需求等计量科学问题.列举了近几年来国内发生的重大风电事故,引出了风电系统中的监管问题,指出由于监管部门对风电场质量的监管空白,造成了制造企业、施工单位对质量控制的忽视.从风电项目设计、安装、运行三个环节分析了所涉及到的风电设备质量问题,在每一个环节中计量检测技术都是风电行业质量与安全的重要保障;由于风电场的位置和设备复杂性,企业和计量监管部门都十分重视风电系统的在线监测、检定和校准的功能.这也是"十二五"规划中智能电网的重要组成部分.     

Solving multi-objective optimal power flow using differential evolution

A differential evolution approach to solve optimal power flow problem with multiple and competing objectives is presented. Two sub-problems of optimal power flow namely active power dispatch and reactive power dispatch are considered. The problem is formulated as a nonlinear constrained true multi-objective optimisation problem with competing objectives. Constrain-domination approach have been used to handle inequality constraints, which eliminates the use of penalty factors. The performance of the proposed approach was tested on standard IEEE 30-bus system and is compared with a conventional method. The result demonstrates the capability of the proposed approach to generate diverse and well-distributed Pareto-optimal solutions.     

Study of differential evolution for optimal reactive power flow

Differential evolution (DE) is studied in detail for optimal reactive power flow (ORPF) problems. The concept, mechanism, and parameter setting of DE are discussed. Based on the IEEE 14-, 30- and 57-bus system test cases, DE is compared with some basic or improved evolutionary algorithms that have been applied to ORPF. It is found that DE is generally a good algorithm for ORPF and worthy of more attention. However, it is also found that DE requires relatively large populations to avoid premature convergence. The impact of this shortcoming is made clear in the IEEE 118-bus system test case. The effectiveness of parallel computing technology for speeding up the computation of DE-based ORPF is also analysed     

Power-current hybrid rectangular formulation for interior-point optimal power flow

A non-linear interior-point optimal power flow algorithm based on power?current hybrid mismatch formulation in rectangular coordinates is proposed. In the proposed algorithm, all buses are divided into two types: the buses with non-zero injections and the buses with zero injections. For the buses with non-zero injections, power flow equations are presented by power mismatch formulation. For the buses with zero injections, power flow equations are presented by current mismatch formulation. The proposed power?current hybrid formulation combines the advantages of both power mismatch and current mismatch formulations. For the buses with zero injections, the proposed hybrid formulation shares the advantages of current mismatch formulation: first-order derivatives of power flow equations become constants, second-order derivatives of power flow equations become zeros so that Jacobian and Hessian matrices of power flow equations are easier to compute. The hybrid mismatch formulation is also easier to handle the buses with non-zero injections than current mismatch formulation. Numerical studies on various testing systems indicate that the proposed hybrid formulation has better convergence performance and computational efficiency, especially for large-scale optical power flow problems with a large percentage of zero-injection buses.     

Transient stability constrained optimal power flow using particle swarm optimisation

A novel approach based on the particle swarm optimisation (PSO) technique is proposed for the transient-stability constrained optimal power flow (TSCOPF) problem. Optimal power flow (OPF) with transient-stability constraints considered is formulated as an extended OPF with additional rotor angle inequality constraints. For this nonlinear optimisation problem, the objective function is defined as minimising the total fuel cost of the system. The proposed PSO-based approach is demonstrated and compared with conventional OPF as well as a genetic algorithm based counterpart on the IEEE 30-bus system. Furthermore, the effectiveness of the PSO-based TSCOPF in handling multiple contingencies is illustrated using the New England 39-bus system. Test results show that the proposed approach is capable of obtaining higher quality solutions efficiently in the TSCOPF problem     

穿孔管式消声单元气流再生噪声总功率模型

采用正交试验设计方法确定了穿孔管式消声单元试验样件的具体结构参数,在自行研制的试验台上进行了气流再生噪声测量。根据量纲分析建立了气流再生噪声总功率与结构参数及气流速度的关系模型,利用超静定最小二乘法求解了模型参数,并检验了模型的显著性,利用该模型对气流再生噪声的影响因素进行探讨。结果表明,在显著性水平0.01下,所建模型是高度显著的;穿孔直径和穿孔部分长度的减小均有利于气流再生噪声的降低,而穿孔率和腔体直径对气流再生噪声的影响较小。频谱分析发现,随气流速度增大,气流再生噪声突出峰值频率有向中高频移动的趋势,强度也有所增大,在斯德鲁哈尔数为0.2～0.35的范围内,存在一突出峰值频率,该处的声能量约占总能量的60%以上。     

N -21 security in optimal power flow control applied to limited areas

Blackouts in recent years have demonstrated that a reliable and secure power system is a key component of an efficient economy. Therefore control devices such as flexible AC transmission system devices (FACTS) are placed in the system and utilised to improve the security of the system. A method to determine appropriate settings for these devices is optimal power flow control. As the area of influence of a FACTS device is usually limited, it is sufficient to include only a reduced area in the optimisation problem. Here, such an optimal power flow problem is formulated where the considered area is defined using sensitivity analysis. To include N 2 1 security as an objective, a current injection method is applied, which facilitates the determination of the system state in the case of a line outage, without having to carry out a full-load flow simulation.     

柔性隔振系统功率流特性及最优化控制策略

针对实际工程中力和力矩复杂激励下柔性耦全系统的隔振问题，综合考虑弹簧的纵向和横向刚度，运用导纳矩阵法推导了主动控制力下系统的动态传递方程。分析了力的对称性及弹簧刚度对传递到基础的功率流的影响，探讨了最优化控制的效果。     

Hybrid algorithm of differential evolution and evolutionary programming for optimal reactive power flow

Differential evolution (DE) is a promising evolutionary algorithm for solving the optimal reactive power flow (ORPF) problem, but it requires relatively large population size to avoid premature convergence, which will increase the computational time. On the other hand, evolutionary programming (EP) has been proved to have good global search ability. Exploiting this complementary feature, a hybrid algorithm of DE and EP, denoted as DEEP, is proposed in this study to reduce the required population size. The hybridisation is designed as a novel primary-auxiliary model to minimise the additional computational cost. The effectiveness of DEEP is verified by the serial simulations on the IEEE 14-, 30-, 57-bus system test cases and the parallel simulations on the IEEE 118-bus system test case.     

Cryogenic power generation

The ac superconducting generator may well be the first large-scale commercial application of superconductivity. In part, this is because the anticipated advantages should occur at power generation levels of today (～ 1000 MVA), rather than at future power levels. Recent definitive feasibility studies indicate that among the advantages to be accrued will be reduced size and weight, higher efficiency, lower capital cost, and greater system stability. This machine, unlike the conventional one, has the potential for generation at transmission line voltages, eliminating the expensive step-up transformer. Consideration is also given to a cryoresistive generator, and to a superconducting stored field generator.     

Semi-definite programming-based method for security-constrained unit commitment with operational and optimal power flow constraints

Considering the economics and securities for the operation of a power system, a semi-definite programming (SDP) model for the security-constrained unit commitment (SCUC) problem is described here, which is directly solved by the interior-point method for SDP within the polynomial times. The proposed method is promising for the SCUC problems because of its excellent convergence and the ability of handling the non-covex integer variables. No model decomposition and initial relaxation are needed when applying the SDP-based method. When the solution contains minor mismatches in the integer variables, a simple rounding strategy is used to correct the non-integer into integer efficiently. Different test cases from 6 to 118 buses over a 24 h horizon are presented. Extensive numerical simulations have shown that the proposed method is capable of obtaining the optimal UC schedules without any network and bus voltage violations, and minimising the operation cost as well.     

Dynamic optimal power flow approach to account for consumer response in short term hydrothermal coordination studies

This paper deals with the representation of price-responsive loads in the short term coordination of predominantly hydroelectric hydrothermal systems. The proposed approach allows the representation of cross-relationships between demand and price during different dispatch intervals of a given time horizon. Also, consumer energy constraints which behave as dynamic restrictions can be taken into account. By that means, a load shifting program in which the consumers' overall energy consumption remains constant throughout the time horizon can be simulated. The short-term hydrothermal coordination is solved as a single optimisation problem employing the primal-dual interior point method. Realistic subsystems, based on the Brazil interconnected power system, are used to illustrate the effect of price-responsive loads on reservoir storages     

Transmit beamforming for optimal second-harmonic generation

A simulation study of transmit ultrasound beams from several transducer configurations is conducted to compare second-harmonic imaging at 3.5 MHz and 11 MHz. Second- harmonic generation and the ability to suppress near field echoes are compared. Each transducer configuration is defined by a chosen f-number and focal depth, and the transmit pressure is estimated to not exceed a mechanical index of 1.2. The medium resembles homogeneous muscle tissue with nonlinear elasticity and power-law attenuation. To improve computational efficiency, the KZK equation is utilized, and all transducers are circular-symmetric. Previous literature shows that second-harmonic generation is proportional to the square of the transmit pressure, and that transducer configurations with different transmit frequencies, but equal aperture and focal depth in terms of wavelengths, generate identical second-harmonic fields in terms of shape. Results verify this for a medium with attenuation f1. For attenuation f1.1, deviations are found, and the high frequency subsequently performs worse than the low frequency. The results suggest that high frequencies are less able to suppress near-field echoes in the presence of a heterogeneous body wall than low frequencies.     

Hydroelectric power in emissions-constrained electricity generation

The positioning of generating plant in rank (merit) order has been based traditionally on minimised system economic cost. If regulations were introduced to place a financial penalty on gaseous emissions from fossil-fuelled power stations, the result would be an increased economic burden on such generating plant. This paper examines the effect on merit order of the imposition of a tax on carbon dioxide emissions, and looks in particular at the potential role to be played by hydroelectric power     

Titanium in the power generation industry

Abstract

The role and construction of steam condensers are reviewed and the factors affecting materials selection are outlined. Comparisons are made between the behaviour of copper-base alloys and commercially pure titanium tubing under test and in service. The implications of a change in tube material on condenser design are reviewed. The potential for using the Ti–6Al–4V alloy for last stage low-pressure turbine blading is also reviewed.

MST/598     

Integral methodology for distribution systems reconfiguration based on optimal power flow using Benders decomposition technique

A new and efficient methodology for distribution network reconfiguration integrated with optimal power flow (OPF) based on a Benders decomposition approach is presented. The objective minimises power losses, balancing load among feeders and subject to constraints such as capacity limit of branches, minimum and maximum power limits of substations or distributed generators, minimum deviation of bus voltages and radial optimal operation of networks. A variant of the generalised Benders decomposition algorithm is applied to solve the problem. The formulation can be embedded under two stages; the first one is the master problem and is formulated as a mixed integer non-linear programming problem. This stage determines the radial topology of the distribution network. The second stage is the slave problem and is formulated as a non-linear programming problem. This stage is used to determine the feasibility of the master problem solution by means of an OPF and provides information to formulate the linear Benders cuts that connect both problems. The model is programmed in GAMS mathematical modeling language. The effectiveness of the proposal is demonstrated through two examples extracted from the specialised literature.     

Digital generation of multivariate wind field processes

A very efficient procedure for the generation of multivariate wind velocity stochastic processes by wave superposition as well as autoregressive time series is proposed in this paper. The procedure starts by decomposing the wind velocity field into a summation of fully coherent independent vector processes using the frequency dependent eigenvectors of the Power Spectral Density matrix. It is shown that the application of the method allows to show some very interesting physical properties that allow to reduce drastically the computational effort.Moreover, using a standard finite element procedure for approximating the frequency dependent eigenvectors, the generation procedure requires the generation of a limited number of univariate fully coherent processes for describing the entire multivariate velocity processes independently of the number of components of the process.     

Emission-constrained optimal generation scheduling incorporating equity principles

An annual emission-constrained generation scheduling model that combines equity principles with cost optimisation is developed. By employing this annual generation scheduling model, an estimation of the daily emission allowance of each generation unit is obtained. A daily emission-constrained generation scheduling model, based on cost optimisation, is proposed. The equity-related issues in the daily scheduling model are discussed. The Shapley value is employed in the daily scheduling problem to allocate the operating cost reduction among the units. Characteristics of the allocation game of operating cost reduction in daily scheduling are discovered, and several methods are proposed to overcome the combinatorial explosion problem in the calculation of the Shapley value. The effectiveness of the proposed models is shown with simulation results on a test power system.