Dynamic security constrained optimal power flow/VAr planning

Traditionally security constrained optimal power flow and VAr planning methods consider static security observing voltage profile and flow constraints under normal and post contingency conditions. Ideally, these formulations should be extended to consider dynamic security. This paper reports on a BC Hydro/CEPEL joint effort establishing a dynamic security constrained OPF/VAr planning tool which considers simultaneously static constraints as well as voltage stability constraints. This paper covers the details of formulation and implementation of the tool together with the test results on a large scale North American utility system and a reduced Brazilian system     

Contingency constrained optimal reactive power flow procedures forvoltage control in planning and operation

The paper presents a preventive secure contingency constrained approach to power system voltage profile optimization suitable both for VAr planning and for short term reactive scheduling. The solution of the problems is based upon the implementation of two optimal reactive power flow (ORPF) programs: the first relevant to determining a workable state (security aspect); and the second relevant to attaining the optimal and secure point (global target). The first ORPF is solved by recursively employing a linear programming algorithm, whilst the second is based on the Han-Powell algorithm. In the paper, emphasis is given to the introduction of the contingency constraints in the ORPF models. The N-1 security constraints are explicitly introduced in the programs in order to obtain an operating point preventive-secure with respect to a selected contingency set. The performances of the procedures are shown by presenting the numerical results obtained from their applications to a small test network and to the large ENEL transmission system     

Artificial bee colony algorithm for solving multi-objective optimal power flow problem

This paper presents a new and efficient method for solving optimal power flow (OPF) problem in electric power systems. In the proposed approach, artificial bee colony (ABC) algorithm is employed as the main optimizer for optimal adjustments of the power system control variables of the OPF problem. The control variables involve both continuous and discrete variables. Different objective functions such as convex and non-convex fuel costs, total active power loss, voltage profile improvement, voltage stability enhancement and total emission cost are chosen for this highly constrained nonlinear non-convex optimization problem. The validity and effectiveness of the proposed method is tested with the IEEE 9-bus system, IEEE 30-bus system and IEEE 57-bus system, and the test results are compared with the results found by other heuristic methods reported in the literature recently. The simulation results obtained show that the proposed ABC algorithm provides accurate solutions for any type of the objective functions.     

Dynamic performance and control of a static VAr generator usingcascade multilevel inverters

A cascade multilevel inverter is proposed for static VAr compensation/generation applications. The new cascade M-level inverter consists of (M-1)/2 single-phase full bridges in which each bridge has its own separate DC source. This inverter can generate almost sinusoidal waveform voltage with only one time switching per cycle. It can eliminate the need for transformers in multipulse inverters. A prototype static VAr generator (SVG) system using 11-level cascade inverter (21-level line-to-line voltage waveform) has been built. The output voltage waveform is equivalent to that of a 60-pulse inverter. This paper focuses on dynamic performance of the cascade-inverter-based SVG system. Control schemes are proposed to achieve a fast response which is impossible for a conventional static VAr compensator (SVC). Analytical, simulated, and experimental results show the superiority of the proposed SVG system     

Evaluation of wheeling and nonutility generation (NUG) optionsusing optimal power flows

The application of optimal power flow for the evaluation of wheeling and nonutility generation (NUG) related options is presented. Both wheeling and NUG options affect vital attributes such as system security, voltage profile, losses, and VAr reserves. These options put a strain on the existing transmission system and may restrict economic utility dispatching. Optimal power flows may be used to determine the best control settings to accommodate wheeling or NUG options so as to maintain system security while minimizing losses or production cost. Short-term marginal wheeling costs may be derived using optimal power flows. Case studies involving the Northeast Utilities and IEEE test systems are presented. These cases show that optimal power flows can be used effectively to address a broad range of wheeling or NUG related planning issues     

Coordinated preventive control of transient stability with multi-contingency in power systems using trajectory sensitivities

In this paper, the challenging multi-contingency transient stability constrained optimal power flow problem is partitioned into two sub-problems, namely optimal power flow (OPF) and transient stability control, solved in turn with conventional well trusted power system analysis tools instead of tackling it directly using a complicated integrated approach. Preventive multi-contingency transient stability control is carried out with generation rescheduling based on trajectory sensitivities using results obtained from a conventional transient stability simulation. A new iterative approach is proposed to optimally redistribute the generation from the critical machines to noncritical machines with the help of conventional OPF. Results on the New England 10-machine 39-bus systems demonstrate that the proposed method is capable of handling multiple contingencies and complex power system models effectively with solution quantity and time comparable with conventional integrated approaches.     

一种基于EEAC和轨迹灵敏度的暂态稳定约束 最优潮流模型与方法

暂态稳定约束最优潮流问题是当前电力系统的研究热点之一。将该问题分解为暂态稳定评估、灵敏度分析和最优潮流等三个子问题交替迭代求解。采用EEAC法进行多预想故障的暂态稳定评估,并给出各不同稳定情况下的裕度表达式;通过轨迹灵敏度法得到稳定裕度关于控制变量的灵敏度,由此构造线性不等式约束;采用非线性原对偶内点法求解含简单稳定约束的OPF问题,并通过增加校正因子的方法避免过度调整问题,通过迭代求解使系统能够满足稳定裕度要求。该方法不仅能考虑多摆失稳问题,也能处理不同失稳模式的多故障问题。通过IEEE-39算例的仿真计算,验证了该方法的有效性。     

Synchronizing and damping torque coefficients and power systemsteady-state stability as affected by static VAr compensators

The utilization of static VAr compensators (SVC) for supplying reactive power at certain points of an electric power system is an efficient way for fast control of transient and steady-state voltage changes following short-circuits, load rejection, opening of severely loaded transmission circuits, etc. Other SVC applications include the increase of power transmission capacity through interconnections between areas of a power system and the damping enhancement of local or inter-area electromechanical oscillation modes. This paper fundamentally deals with these last two issues by analyzing the results of synchronizing and damping torque coefficient calculations for a generation station connected radially to a much larger power system. The results presented pertain to a double-circuit, 800 km, 500 kV transmission system during a one-circuit out contingency     

AI approach to optimal VAr control with fuzzy reactive loads

This paper presents an artificial intelligence (AI) approach to the optimal reactive power (VAr) control problem. The method incorporates the reactive load uncertainty in optimizing the overall system performance. The artificial neural network (ANN) enhanced by fuzzy sets is used to determine the memberships of control variables corresponding to the given load values. A power flow solution determines the corresponding state of the system. Since the resulting system state may not be feasible in real-time, a heuristic method based on the application of sensitivities in an expert system is employed to refine the solution with minimum adjustments of control variables. Test cases and numerical results demonstrate the applicability of the proposed approach. Simplicity, processing speed and ability to model load uncertainties make this approach a viable option for online VAr control     

基于L指标的电压稳定约束下的最优潮流

为追求更大的经济利益,电力系统的运行日益接近其稳定临界,增加了系统出现电压失稳的可能性,故在传统的最优潮流中考虑电压稳定约束十分必要。本文利用电压稳定￡指标,建立了包含电压稳定约束的四种最优潮流模型（VSCOPF）：最大化稳定裕度模型、固定稳定裕度模型、线性组合模型和混合模型。通过多个测试系统对上述模型进行了分析和计算,基于计算结果对各个模型解的存在性、能否科学地兼顾电压稳定性和经济性等问题进行了较为详细的讨论。该项工作对于寻求更为科学实用的、能够合理计及电压稳定约束的最优潮流模型,具有一定的帮助。     

A solution of optimal power flow with multicontingency transient stability constraints

This paper presents a formulation of the multicontingency transient stability constrained optimal power flow (MC-TSCOPF) problem and proposes a method to solve it. In the MC-TSCOPF formulation, this paper introduces a modified formulation for integrating transient stability model into conventional OPF, which reduces the calculation load considerably. In our MC-TSCOPF solution, the primal-dual Newton interior point method (IPM) for nonlinear programming (NLP) is adopted. Computation results on the IEEJ WEST10 model system demonstrate the effectiveness of the presented MC-TSCOPF formulation and the efficiency of the proposed solution approach. Moreover, based on quite convincing simulation results, some phenomena occurred when considering multicontingency are elaborated.     

考虑电压稳定约束的最优潮流

采用指数负荷模型研究了有载调压变压器对系统最优潮流的影响。在阐述电压稳定性L指标的基础上,将L指标引入传统最优潮流中形成电压稳定约束的最优潮流,运用原-对偶内点法求解该模型。算例结果表明,该模型鲁棒性较好,能更加真实地反映系统实际运行情况。不同静态负荷模型对系统电压稳定性和经济性有一定的影响,系统电压稳定性的提高是以牺牲经济性为代价的,在考虑电压稳定性时,有载调压变压器的加入可以有效降低系统燃料费用,平抑系统有功出力和燃料费用的波动,将波动维持在一个较小的范围内。     

电压稳定约束的最优潮流研究

针对电力系统将运行在更加接近电压稳定临界点的情况,建立了一种包含电压稳定约束的最优潮流(OPF)数学模型,在常规OPF模型基础上增加了一组电压稳定裕度约束作为不等式约束。引入能正确反映系统当前电压稳定程度的L指标函数作为电压稳定约束,通过调整该约束的上限值,可以获得系统所要求的电压稳定裕度。利用基于扰动KKT(Karush-Kuhn-Tucker)条件的原始-对偶内点算法对问题进行求解。多个IEEE测试系统上的仿真结果表明,该模型能较好地兼顾系统对电压安全性与经济性的要求;在电压稳定约束起作用时,发电总费用会随着系统对电压稳定要求的提高而增大;L指标约束的加入对系统的调节能力有一定影响;所用算法收敛性好、鲁棒性强,能有效地处理大量等式和不等式约束。     

Real-time VAr control by SCADA

The operation and advantages of the SCADA (supervisory control and data acquisition) capacitor control called CAPCON (capacitor control) are presented. CAPCON is used to control VArs systemwide by switching substation capacitor banks based on real-time VAr and voltage data instead of the traditional time clock method. CAPCON has many benefits, which should increase as it is expanded to more capacitor banks on the system. There are cost savings in line losses, generator fuel, and better planning and utilization of substation equipment. Control is based upon real-time conditions, providing much more efficient and precise operation of capacitors and a resulting improvement in reactive power flow. By improving the control of capacitors, CAPCON has given the company the ability to serve its customers better     

Static VAr compensators-a solution to the big motor/weak systemproblem?

Static VAr compensators (SVCs) with reactive power capabilities of hundreds of both positive and negative MVAr have been used for years by utilities to control reactive power flow in transmission and distribution systems. In this paper, the authors describe how static VAr compensators provide an economical way to control reactive power flow and maintain stable utility and plant voltages     

Optimal multi-objective VAr planning using an interactivesatisfying method

In this paper, an interactive satisfying method, a two-level structure, is presented for solving a multiobjective power system VAr planning problem. In this structure, the analysis level is concerned with the calculation of a possible or it set of possible solutions to the multiobjective problem, and the decision level is concentrated on the task of inducing the analysis level to generate noninferior solutions that meet the decisionmakers' preferences. On the analysis level, the ϵ-constraint method based on the simulated annealing approach is developed to find a global noninferior solution. Through interaction with the decisionmaker, the proposed method guarantees the solution to be a desirable, global noninferior solution for a general multiobjective VAr planning problem. The presented method was applied to the AEP-14 bus system as well as to a large, actual-size system with satisfactory results     

Optimal power flow using differential evolution algorithm

This paper presents an efficient and reliable evolutionary-based approach to solve the optimal power flow (OPF) problem. The proposed approach employs differential evolution algorithm for optimal settings of OPF problem control variables. The proposed approach is examined and tested on the standard IEEE 30-bus test system with different objectives that reflect fuel cost minimization, voltage profile improvement, and voltage stability enhancement. The proposed approach results are compared with the results reported in the literature. The results show the effectiveness and robustness of the proposed approach.     

A security-constrained decomposition approach to optimal reactivepower planning

A two-level decomposition technique for VAr (voltampere reactive) sources planning in electric power systems is presented. Several operating conditions of the system (different base cases with associated contingencies) can be considered simultaneously. Total flexibility in the corrective or preventive treatment for control variables is provided. New VAr sources are modeled by discrete variables, and the operating and investment costs are discussed in detail. Examples of applications to actual cases of reactive compensation planning in a large power system are reported to demonstrate the efficiency and capabilities of the approach     

Further developments in LP-based optimal power flow

The authors describe developments that have transformed the LP (linear programming) approach into a truly general-purpose OPF (optimal power flow) solver, with computational and other advantages over even recent nonlinear programming (NLP) methods. it is pointed out that the nonseparable loss-minimization problem can now be solved, giving the same results as NLP on power systems of any size and type. Coupled formulations, where for instance voltages and VAr become constraints on MW scheduling, are handled. Former limitations on the modeling of generator cost curves have been eliminated. In addition, the approach accommodates a large variety of power system operating limits, including the very important category of contingency constraints. All of the reported enhancements are fully implemented in the production OPF software described here, and most have already been utilized within the industry     

Load and voltage balancing in harmonic power flows by means of static VAr compensators

In special applications, static VAr compensators (SVCs) with thyristor-controlled reactors (TCRs) are used to balance unbalanced loads or to eliminate voltage unbalances at the terminals of the SVC. In both cases, load balancing and voltage balancing, the TCR can present a significant unbalanced behavior which produces important quantities of noncharacteristic harmonics. In this paper, the formulation and solution of the load and voltage balancing problems are developed for harmonic power flows obtained from a combination of two blocks: (1) a conventional load flow (CLF) and (2) an iterative harmonic analysis (IHA). In both blocks, the treatment of load and voltage balancing is described in detail. Load flow (LF) calculations and harmonic analysis show the presence of noncharacteristic harmonics in these two situations.