Direct calculation of voltage-stability limit of electric power systems

On-line monitoring of voltage-stability conditions is a new requirement for the energy management systems of the electric power industry. Preferred by the utility engineers and dispatchers is an index which represents explicitly the margins to a voltage-stability limit (collapse point) expressed in a physical unit ordinarily used. For example, such indices are the system load and voltage-voltage-stability-margins (SVVSM) between the present operating point and a voltage collapse point. However, long computer time is required to calculate accurately the voltage collapse point by the conventional method because many multiple load-flow solutions are required. This paper presents a new algorithm to calculate directly, exactly, and rapidly the voltage-stability limit. The algorithm uses the load-flow equations and the explicit expressions of the voltage collapse root conditions are expressed as dR/dV = 0, where R is active or reactive power node injection, and modifying the conventional N-R load-flow program, and is not affected numerically by singular Jacobians. The proposed algorithm is verified through calculating the voltage stability limit of a 176-node power system.     

Approximate linear decoupled solution as the initial value of power system load flow

In this paper an approximate solution of the load flow equations is developed using the same principles employed for developing the fast decoupled method. The approximate decoupled solution takes less time to calculate than one iteration of the fast decoupled method and gives a good approximate picture of the V,D profile of a power system. In the paper, it is used as the initial value of the fast decoupled method, which is applied on standard IEEE test systems, and a solution is achieved to within 0.01 p.u. (1 p.u. = 100 MW/MVAr) of maximum busbar mismatches in less than 50% of the number of iterations that the method takes to converge when a flat initial value (V = 1.0 p.u., D = 0.0 rad) is considered.     

电压稳定分析的潮流算法研究

研究了一种新型的用于电压稳定分析的潮流算法,它可充分考虑电力系统各种类型元件的模型,将常规潮流方程与动态元件的状态方程联立求解,同时解出系统中各个节点的电压、相角以及各种动态元件内部的状态变量。它考虑了系统中各种极限情况,消除了对于PV,PQ及平衡节点等的假设,使计算结果更加接近于系统的实际运行情况。采用上述潮流模型的连续潮流算法,对一个试验系统进行了电压稳定临界点的计算,并与常规潮流算法进行了比较。     

Second-order, 3-phase load flow

The load-flow solution of unbalanced polyphase power systems is obtained using the phase-coordinate technique in which only the 1st-order derivatives of load-flow equations in polar form are used. However, the use of this (1st-order, 3-phase) method is considerably limited because of the long solution time and large core-storage requirement. The present paper outlines an exact method for solving the load-flow problem of unbalanced electrical networks in which full Taylor series expansion of load-flow equations is used. The load-flow equations are expressed in cartesian coordinates. The method is three to five times faster than the 1st-order, 3-phase method but requires slightly more memory due to the cartesian coordiante formulation. The mathematical steps, a solution algorithm and pertinent results are provided for illustration.     

计算最近电压崩溃临界点的实用算法

提出了一种计算最近电压崩溃临界点的实用算法。首先利用直角坐标系下潮流方程的二次性及潮流高低电压解的中点处雅可比矩阵奇异的性质,近似计算最近电压崩溃临界点并将其作为迭代的初值。然后采用牛顿-拉夫逊法直接对最近电压崩溃临界点模型所构成的非线性方程组进行迭代求解。最后通过数字仿真验证了该方法计算精度高,在计算用时上较“主从”迭代算法具有明显的优势。     

某型高炮电源测试系统设计

电源系统是某型高炮电气系统的重要组成部分,电源系统发生故障将导致系统整体瘫痪。电源系统的性能指标包括稳态电压、纹波电压、浪涌电压、尖峰脉冲电压、起动扰动电压、初始啮合扰动电压和启起动电压。为了对电源系统进行测试,开发了一套自动测试系统,采用STM32FRCT6ARM作为核心处理器,设计了必要的外围电路,电源系统输出性能指标经过信号调理和A/D转换后进入单片机,通过CAN总线将数字信号传送到上位机,本测试系统具有友好的人机交互界面,实现了电源系统的性能测试。     

电力系统线性化动态最优潮流模型

电力系统在线经济运行计算亟需可快速获取完整调度信息的线性化动态最优潮流模型。基于系统关联矩阵将节点功率平衡方程解耦为线路功率流和损耗流两部分,对损耗流部分进行等价代换,并消去方程中的三角函数项,采用泰勒级数法对残存的非线性项进行线性化处理,建立起可以同时求解电压幅值和线路无功功率的线性化动态最优潮流模型。基于简化原对偶内点法对所建模型进行求解,在迭代过程中不断更新泰勒级数法所需的基准点信息,以提高模型的计算精度。IEEE 30节点、IEEE 118节点、IEEE 300节点以及某市117节点等值系统的算例测试表明,所建线性化模型能在获取更完备调度信息的同时仍具有较高的计算精度和求解效率。     

一种新的配电网三相潮流算法

提出一种用于不平衡辐射状配电网的三相潮流计算方法。根据配电网的辐射状结构特点,将分支线作为配网结构的基本单元,把馈线分支首节点的电压、电流表示为分支末节点电压、电流的函数．重新建立潮流方程,使方程的数目与分支线数目成正比,大幅度减少方程式和变量的数目。并且根据配电网的特点近似简化雅可比矩阵．以实现解耦。建立了单相系统模型和三相系统模型,给出了潮流的解耦算法。相对于传统计算方法未知数正比于母线数目,可以大幅度减少方程数,缩短计算时间,特别适用于大规模配电系统。     

Probabilistic load-flow computation using point estimate method

A new probabilistic load-flow solution algorithm based on an efficient point estimate method is proposed in this paper. It is assumed that the uncertainties of bus injections and line parameters can be estimated or measured. This paper shows how to estimate the corresponding uncertainty in the load-flow solution. The proposed method can be used directly with any existing deterministic load-flow program. For a system with m uncertain parameters, it uses 2m calculations of load flow to calculate the statistical moments of load-flow solution distributions by weighting the value of the solution evaluated at 2m locations. The moments are then used in the probability distribution fitting. Performance of the proposed method is verified and compared with those obtained from Monte Carlo simulation technique and combined simulation and analytical method using several IEEE test systems.     

A new formulation for load flow solution of power systems with series FACTS devices

This paper addresses the solution of load flow equations for a power system with series flexible AC transmission systems (FACTS) devices. A novel formulation of equations using dual state variables (current magnitude and angle) and dual control variables (series injected real power and series voltage in quadrature with current) for series devices is proposed. These specifications can be related to transmission line loading and device limits. Specifications like power flow through a series device can also be handled using this formulation. The load flow equations are solved using Newton-Raphson technique. A decoupled formulation is also proposed. Case studies are carried out on IEEE test systems with several types of specifications to validate the method.     

Preventive control based on complex-valued load-flow technique to avoid voltage instability in power systems

This paper presents a method for obtaining preventive control strategies based on nonlinear optimization for power systems which will incur voltage instability when load demand increases. An algorithm is proposed to determine an appropriate and effective control action taking into account the operating constraints to widen the margin between the present operating point and a voltage collapse point, thereby improving the system state. The method proposed here is based on a complex-valued load-flow technique using the Newton-Raphson method which has been developed already by the authors. Consequently, the preventive control algorithm also can evaluate the voltage instability in the event the present operating point becomes closer to the critical point and the system state becomes unfeasible at increased loading point. Further, in optimization, two-types of objective functions are introduced so that voltage stabilization and dissolution of constraint violation can be attained simultaneously. In addition, this paper discusses the extension of the proposed preventive control to the successive control method which carries out the control action to avoid voltage collapse while the load demand is increasing. Numerical examples for various model systems demonstrate the effectiveness of the proposed method.     

Linear power flow V-theta

This paper presents a new linear active power flow solution method. The new linear active power flow presents a better performance to calculate MW power flows, as opposed to MW flows, calculated in classical model with a DC power flow. The aforesaid proposed method is based on a decoupling principle. Therefore, the voltage angles and voltage magnitudes are calculate in decoupled forms. A complete demonstration of the proposed method is presented. The algorithm of new method has been tested by extensive numerical studies. This paper gives details of the method's performance on various classical systems. All the results are compared with those from the Newton-Raphson power flow and classical DC power flow.     

Load-flow equivalents for decentralized monitor and control of power systems

Load-flow calculation on electric power systems is one of the most fundamental analyses in system operation and planning. There are many reasons to calculate load-flow solutions, and the contingency evaluation is a representative one. In contingency evaluation, it is necessary to repeat load-flow calculation for tremendous patterns depending on branch or generation outages. Then, computing efforts are generally very heavy. Thus various techniques have been proposed to reduce computing time, especially by adopting equivalencing for external network. On the other hand, the network equivalents techniques also become important at present from the viewpoint of the necessity of decentralized monitor and control on bulk power systems. This paper describes a newly developed method of load-flow equivalents based on the Ward equivalent for both direct (dc) solution and (ac) solution. Numerical results also are shown to clarify the accuracies and practicality of the proposed method through the comparison of the representative REI approach.     

Linear Var Flow Methods for Security Assessment

ABSTRACT

In this paper, an investigation of various approximations for the solution of reactive power flow equations, suitable for security assessment, is presented. When the loads are assumed to be a combination of constant current and constant impedance types, the power flow equations can be decoupled exactly. The reactive power flow equations become linear with parameters which are functions of phase angles. Hence, once the phase angles are known, voltage magnitudes can be computed exactly without iterations But approximations are required to simplify computations for on-line application. Various approximations and the factors that influence the accuracy of the solutions are investigated.lt is shown that iterative linear power flow (ILPF) and fast decoupled load flow (FDLF) methods (for voltage computations) can be derived from one set of approximations.

Case studies of a 30 bus system is presented to compare the accuracy of various approximations. The results indicate that a major factor influencing the accuracy of the solution is the variability of conductance to susceptance ratio in a system. A new method is presented which gives very accurate results with moderate computing effort.     

Real-Time Decoupled Equivalent Models for Static Security Analysis

This paper presents some new decoupled external system equivalent models for use in system security evaluation. Three different models i.e., decoupled Tie-line power flow model, decoupled Boundary bus Impedance model and decoupled Impedance-Admittance model are developed based on P-? and Q-V relations of fast decoupled load-flow formulation. The structure component of the models are then identified by processing real time data using Kalman filtering technique. Then the operating components of the decoupled equivalents are obtained from two new approaches. The performance of the various models with regard to contingency analysis has been presented using simulated data on a 25 bus test system. Persistency analysis of different input-output data is given to determine apriori choice of models. Also the bias conditions are derived to ascertain the accuracy of the models.     

AN EFFICIENT CONTINUATION POWER FLOW METHOD FOR VOLTAGE STABILITY ANALYSIS

ABSTRACT

This paper presents an efficient continuation power flow-method for determining voltage stability limit of interconnected power systems. Decoupled power-flow solutions are extremely popular because of lesser storage and faster solution speed as compared to coupled power flow solution. However, it is generally believed that decoupling cannot be used near critical loading conditions. This paper shows that decoupled power-flow solution is possible even under such adverse operating conditions, it is achieved by judiciously incorporating the effects of non-negligible off diagonal submatrices through a subiteration scheme. The load parameter is included in both sets of equations. Bus angle (other than slack bus) and load parameter are used as continuation parameters for solving real & reactive power equations respectively. Results for 5-bus, 14-bus and a practical 91 -bus test systems have been obtained with the coupled and proposed decoupled versions of continuation power-flow to demonstrate the validity and efficiency of the proposed algorithm.     

计及不确定性的电力系统直流潮流的区间算法

考虑高压输电网络中的不确定性因素,采用区间分析方法来处理直流潮流计算中的不确定问题,提出了一种可应用于求解大规模输电系统的区间直流潮流算法。该算法考虑了节点注入有功功率的不确定性,采用区间高斯消去法求解区间线性方程组,得到母线电压相角和支路有功功率的上下限,即区间潮流方程的区间解。采用IEEE 30算例系统,将该算法的计算结果与蒙特卡罗仿真计算的结果相比较,验证了该算法的有效性和应用价值。     

Static state estimation with DC models and linear programming

A decoupled state estimation algorithm is proposed in which the measurement model is based on linear approximation of the power flow equations and the solution method is based on linear programming. The decoupled least-squares DC-estimator and the undecoupled least-absolute-value estimator are numerically compared. As an overall reference, the classic undecoupled least-squares estimator is used. The error-detecting capabilities and the accuracy of the estimated state of all four types of estimators are treated.     

河源电网电压优化控制应用探讨

针对河源电网夏丰方式下的电压越限问题,分别以网损最小、电压偏移最小和二者结合为目标,采用原-对偶内点法及Matpower潮流优化程序,通过调节主变压器抽头、并联无功补偿装置容量以及电源出力,在考虑负荷裕度的指标下,实现对电网电压及网损的优化控制.该优化方法在河源电网应用后,极大地改善了该区域的电压质量、降低了网损,表明...