Reactive Power Limits Research Articles

Application of Reactive Power Optimization in Power Grid in AVC

Automatic voltage control(AVC) is the highest form of current power grid voltage and reactive power control,during the implementation of AVC, the whole network reactive power optimization isthe core and foundation. Thispaper researches and discuses the application of reactive power optimization inpower grid AVC. In the traditional reactive power optimization, the reactivepower limits of synchronous generators are fixed. In this paper, thesynchronous generator PQ operating limits change with external conditions,thus establishes reactive power optimization model in accordance with therequirements of AVC. Thispaper presents reactive power optimization method based on the principle ofpartition. The method decomposes the system to several partitions. Eachpartition separately optimized, thus reduces the system scale.And the convergence of the algorithm, the calculation speed and the discretevariable processing etc. improve. At the same time, this method reflects theclassification, hierarchical, partition, characteristics of coordinated controlof AVC.

Comparative study of STATCOM and SVC performance on Dynamic Voltage Collapse of an Electric Power System with Wind Generation

One of the major problems of voltage stability is the reactive power limit of the system. Improving the system's reactive power handling capacity via Flexible AC transmission System (FACTS) devices is a solution for prevention of voltage instability. In this paper it is studied the influence of Static Synchronous Compensators (STATCOM) and Static Var Compensators (SVC) in dynamic Voltage Stability during Low Voltage Ride Through (LVRT), in wind farms. The wind turbines are equipped with pitch control coupled with a Fixed Speed Induction Generator (FSIG). Due to the nature of asynchronous operation, system voltage instability of wind farms based on FSIG is largely caused by the excessive reactive power absorption by FSIG after a fault due to the large rotor slip gained during fault. Wind farm models based on FSIG and equipped with either STATCOM or SVC are developed in EUROSTAG. The Automatic Voltage Regulators (AVR) of the generating units and the turbine speed governors were modeled in detail. Different load models were used and the Under Load Tap Changers (ULTC) were also taken into account. Finally, some conclusions that provide a better understanding of the dynamic voltage stability of a system with FSIG model during LVRT, using various capacities of STATCOM and SVC are pointed out.

Voltage Stability Improvement using STATCOM and SVC

stability of a system is affected by reactive power limit of the system. FACTs devices improve the reactive power flow in system thereby improving voltage stability. This paper explores the effect of SVC and STATCOM on static voltage stability. IEEE- 14 bus system has been used to demonstrate the ability of SVC and STATCOM in improving the voltage stability margin. These FACTs controllers help to increase the load ability margin of the power network. KeywordsSTATCOM, Voltage stability

A Static Voltage Security Region for Centralized Wind Power Integration—Part II: Applications

In Part I of this work, a static voltage security region was introduced to guarantee the safety of wind farm reactive power outputs under both base conditions and N-1 contingency. In this paper, a mathematical representation of the approximate N-1 security region has further studied to provide better coordination among wind farms and help prevent cascading tripping following a single wind farm trip. Besides, the influence of active power on the security region is studied. The proposed methods are demonstrated for N-1 contingency cases in a nine-bus system. The simulations verify that the N-1 security region is a small subset of the security region under base conditions. They also illustrate the fact that if the system is simply operated below the reactive power limits, without coordination among the wind farms, the static voltage is likely to exceed its limit. A two-step optimal adjustment strategy is introduced to shift insecure operating points into the security region under N-1 contingency. Through extensive numerical studies, the effectiveness of the proposed technique is confirmed.

A modified Newton–Raphson load flow scheme for directly including generator reactive power limits using complementarity framework

A modified Newton–Raphson (NR) load flow scheme which automatically incorporates generator reactive power limits is proposed. Traditionally, handling generator reactive power limits is considered as an adjusted load flow solution and this adjustment is carried out through bus type switching method. Incorporating bus type switching in a NR algorithm is not straight forward, as it involves several subjective decisions and hence, there is no commonly used implementation. The modified NR scheme proposed here combines this adjustment feature in the load flow problem formulation directly. This is achieved by augmenting the set of complementary reactive power relations at voltage controlled buses with the conventional set of active power balance equations at all buses and reactive power balance equations at load buses in a Mixed Complementarity Problem (MCP) framework. Effectiveness of the scheme has been demonstrated through extensive simulations considering a number of test systems.

INTEGRACIÓN DE RECURSOS RENOVABLES Y REQUERIMIENTOS DE CONEXIÓN EN EL SISTEMA ELÉCTRICO ESPAÑOL: ANÁLISIS DE DATOS EN INSTALACIONES FOTOVOLTAICAS

The integration of renewable energy resources into power systems have increased considerably along the last decade, mainly encouraged by both development technical solutions and policy proposals as a way to reduce energy dependence from third-countries. For the Spanish case, the remarkable penetration of renewable energy sources has been mainly driven by wind and photovoltaic power plants. As a result of these relevant modifications in the mix of generation, a set of changes in the grid-code requirements have been proposed by most governments, mainly those characteristics related with grid connections under disturbances of renewable energy resources. Under this framework, the present paper discusses the evolution of wind farm and PV power plant integration in the Spanish power system, describing the requirements and electrical behaviors of these sources under disturbances. Both active and reactive power limits have also described in the paper, including disconnection constraints. In addition, the last draft issued by the Spanish Transmission Operator System (REE) provides modifications and greater requirements aiming to support disturbances and voltage dips even more critical. Real field-test measurements carried out along several years in different Spanish PV power plants gives a comparison between collected data and grid-code requirements. This study provides a preliminary analysis regarding the severity of the current grid-code requirements compared with real disturbances. Moreover, these data gives additional information in reference to the inverter performance under the presence of disturbances, and their electrical behavior in current LV PV plant installations.

Real Power Loss Minimization and Voltage Stability Limit Enhancement by Using Shuffled Frog Leap Algorithm

This paper deals with the Optimal location and control of a unified power flow controller (UPFC) along with transformer taps are tuned to simultaneously optimize the voltage stability limit (VSL) and real power losses of a mesh power system network. This problem is dealt as a nonlinear equality and inequality constrained optimization problem with an objective function incorporating both the real power loss and VSL. An evolutionary algorithm known as shuffled leap is applied for solving the UPFC location, its injected series voltage, and also the tap positions of transformers as the variables. The then obtained results of SFC algorithm are compared with the results of Bacteria Foraging-Based Algorithm using IEEE 14 bus. Index Terms: Shuffled leap, continuation power flow, linearprogramming, optimal power flow (OPF). I. Introduction: The OPF methods are conventional and intelligent and solved by varieties of methods such as successive linear programming, the Newton-based nonlinear programming method, and with varieties of recently proposed interior point methods. The Optimal Power Flow solution is used to optimize a selected objective function such as fuel cost with optimal adjustment of the power system control variables, at the same time satisfying various equity and inequality constraints. The drawback of the OPF is solved from different perspectives, like analyzing the effects of load on voltage stability/power flow solvability, generation rescheduling for cost minimization of power generation, controls such as taps, shunts, and other modern VAR sources adjustments to minimize real power losses in the system. The advent of Flexible ac transmission systems (FACTS) system made the possibility for optimizing the power flow without the restoration of generation rescheduling or changes to topology. Unified power flow controller (UPFC) is the advanced in the controllers family and can provide the OPF with significant flexibility by injecting compensation in series and shunt in controlled manner. The UPFC can provide simultaneous control of all basic power system parameters (transmission voltage, impedance and phase angle). The controller can fulfil functions of reactive shunt compensation, series compensation and phase shifting meeting multiple control objectives. From a functional perspective, the objectives are met by applying a boosting transformer injected voltage and a exciting transformer reactive current. The injected voltage is inserted by a series transformer. The continuation power flow (CPF) method is robust; however has some weakness in large electric power system considering generators reactive power limits and gives information regarding how much percentage overloading the system can withstand before a possible voltage collapse. The CPF problem is incorporated into an OPF problem so that both the issues can be addressed simultaneously. In this paper, the voltage stability limit is defined as the maximum percentage overloading (λmax) the system can withstand and incorporated along with the objective of real power loss minimization. The classical techniques of OPF solution has the disadvantage that they are sensitive to starting points and leading to non-monotonic solution. To eliminate this problem evolutionary techniques have been applied in solving the OPF problem (10), (11) like particle swarm optimization (PSO) to the problem of OPF. In this paper a new evolutionary algorithm known as Shuffled Frog Leap algorithm (SFLA) is used to solve the combined problem of CPF-OPF for real power loss minimization and VSL maximization of the system. The algorithm has been inspired from memetic evolution of a group of frogs when seeking for food. In this method, a solution to a given problem is presented in the form of a string, called frog which has been considered as a control vector in this paper. The main objective is transformer taps optimization, location of UPFC and its series injected voltage for the single objective of real power loss minimization and then for the multiple objectives of loss minimization and VSL maximization. Finally, a cost analysis for installation of UPFC is carried out to establish the investment in putting a UPFC for the cause. Test results indicate that SFLA method can obtain better results than the simple heuristic search method on the 14-bus radial distribution systems.

Firm and Non-Firm Wind Generation Planning Considering Distribution Network Sterilization

Conventional generation is no longer the only source of electricity, as medium and small-scale units, mostly referred to as distributed generation, are increasingly being connected to the power system. This paper introduces a planning methodology, based on AC optimal power flow, which takes a detailed account of the network sterilization effect and maximizes the net power export from the distribution network to the transmission network. It also introduces a mathematical formulation of the reactive power capabilities that are consistent with the reactive power limitations of the wind turbine generators. A comparison is made between the generation planning results, in terms of sterilization characteristics, when using the nameplate and extended reactive power capability, for both firm and non-firm (i.e., active power curtailment applies) generation. Finally, the transition from a firm to non-firm connection policy is examined.

Revisiting the power flow problem based on a mixed complementarity formulation approach

A novel optimisation-based model of the power flow (PF) problem is proposed using complementarity conditions to properly represent generator bus voltage controls, including reactive power limits and voltage recovery processes. This model is then used to prove that the Newton-Raphson (NR) solution method for solving the PF problem is basically a step of the generalised reduced gradient algorithm applied to the proposed optimisation problem. To test the accuracy, flexibility and the numerical robustness of the proposed model, the IEEE 14-bus, 30-bus, 57-bus, 118-bus and 300-bus test systems and large real 1211-bus and 2975-bus systems are used, benchmarking the results of the proposed PF model against the standard NR method. It is shown that the proposed model yields adequate solutions, even in the case when the NR method fails to converge.

Probabilistic load flow incorporating generator reactive power limit violations with spline based reconstruction method

The increase in the penetration of intermittent generation and uncertainty in load patterns calls for the need to include these uncertainties in the conventional power flow programs. Consideration of the generator reactive power limit violation is an essential aspect in load flow studies for planning. In this paper, a probabilistic load flow method is proposed in which the violation of the reactive power limits of the generators are adequately represented. The seven point estimate method along with the spline based reconstruction technique is proposed for constructing the probability density function of the resulting multimodal distributions. The proposed load flow method has been tested on the IEEE-118 bus and IEEE-300 bus test systems for unimodal and multimodal load distributions, with and without correlation, and the accuracy of the results has been validated by comparing these results with those obtained by Monte Carlo simulation studies. Further, the effect of slack bus power limits has also been studied in this work.

The Research of Reactive Power Output of Doubly-Fed Induction Generator when Large-Scale Wind Power Integration

The technology of large-scale wind power connected to grid is a key technology in the application of wind power. However, the large-scale wind power will cause the negative effect on the voltage of grid. So, the regulation of reactive power is necessary, which can stabilize the voltage level of the grid. The doubly-fed induction generators (DFIG) themselves own the ability of providing reactive power. Considering the reactive power demand when the power grid is operating, and then rationally allocating static and dynamic reactive power compensation equipment, the operational efficiency can be improved. The particle analyzes the reactive power limit of DFIG, considering the limit of stator and rotor. In this way, the reactive power compensation capacity of DFIG is used first, the use of reactive power compensation equipments are greatly reduced, so the economy of the wind farm is obviously improved.

A Vector-Controlled Distributed Generator Model for a Power Flow Based on a Three-Phase Current Injection Method

This paper proposes a vector-controlled distributed generator (DG) model for a power flow based on a three-phase current injection method (TCIM). In order to represent the DG models in the power flow, steady-state phase current output equations are formulated. Using these equations, the TCIM power flow formulation is modified to include the DG models. In the proposed power flow, a DG-connected bus is modeled as either a load bus (PQ bus) or a voltage-controlled bus (PV bus), depending on the control mode of the reactive power. However, unlike conventional bus models, the values of the DG-connected bus models are represented by three-phase quantities: three-phase active and reactive power output for a PQ bus, and three-phase active power and positive-sequence voltage for a PV bus. In addition, a method is proposed for representing the reactive power limit of a voltage-control-mode DG by using the q-axis current limit. Utilizing a modified IEEE 13-bus test system, the accuracy of the proposed method is verified by comparison to the power systems computer aided design (PSCAD) model. Furthermore, the effect of the number of DGs on the convergence rate is analyzed, using the IEEE 123-bus test system.

Particle swarm optimization with constriction factor for optimal reactive power dispatch

This paper proposes a simple particle swarm optimization with constriction factor (PSO-CF) method for solving optimal reactive power dispatch (ORPD) problem. The proposed PSO-CF is the conventional particle swarm optimization based on constriction factor which can deal with different objectives of the problem such as minimizing the real power losses, improving the voltage profile, and enhancing the voltage stability and properly handle various constraints for reactive power limits of generators and switchable capacitor banks, bus voltage limits, tap changer limits for transformers, and transmission line limits. The proposed method has been tested on the IEEE 30-bus and IEEE 118-bus systems and the obtained results are compared to those from other PSO variants and other methods in the literature. The result comparison has shown that the proposed method can obtain total power loss, voltage deviation or voltage stability index less than the others for the considered cases. Therefore, the proposed PSO-CF can be favorable solving the ORPD problem.

Reactive-Power Optimization Research Based on Load Fluctuation and Distributed Control

This paper proposes a voltage-reactive power optimization, which is an integrated system based on centralized optimization and distributed control, to confine the action numbers of control devices. The system includes VQC (voltage and reactive power control system) and the optimal power flow calculation in the master station. By using the fractal theory, the original data in the range of nonperiodic cycles are divided into several time-intervals. After this processing, the optimal points are obtained by optimal power flow in every segment. Based on the load fluctuation rate, in every period, the voltage / reactive power limits of nine area figure are set. As the load fluctuation rates changed in every segment, voltage / reactive power limits are different. And the tap changers and switchable capacitor banks are regulated according to each upper/lower limits and the optimal points. Furthermore, correctness and effectiveness of the proposed method have been confirmed. And the simulation results show that it not only reduces power losses but also restricts the action numbers of control device.

A novel multi-stage fuel cost minimization in a VSC-based microgrid considering stability, frequency, and voltage constraints

This paper presents a multi-stage fuel consumption minimization method in an autonomous voltage source converter (VSC)-based microgrids considering microgrid small signal stability (SSS) margin, frequency and voltage regulation, and the minimum and maximum allowable range of active and reactive output power of VSCs. In VSC-based microgrid, the parameters of each VSC power sharing controller have significant influence on microgrid SSS margin, frequency and voltage regulation, and the ratio of active and reactive power shared between VSCs. Considering all of these effects in fuel consumption minimization method with straightforward technique leads to having a complicated constrained optimization problem, which cannot be solved in a short time. The computation time of solving this optimization problem is very important in practical applications. In order to eliminate this problem, a multi-stage method is developed in this paper. This method can optimize the fuel consumption and consider the output active and reactive power limit, SSS margin, and frequency and voltage regulation in different stages without significant interactions on each other. Hence, the computation time is strongly reduced. The simulation results in a microgrid, which has a wind turbine, two gas turbines, and a micro-turbine with heat load, demonstrate the effectiveness of the proposed method.

A Range Arithmetic-Based Optimization Model for Power Flow Analysis Under Interval Uncertainty

This paper presents a novel framework based on range arithmetic for solving power flow problems whose input data are specified within real compact intervals. Reliable interval bounds are computed for the power flow problem, which is represented as an optimization model with complementary constraints to properly represent generator bus voltage controls, including reactive power limits and voltage recovery processes. It is demonstrated that the lower and upper bounds of the power flow solutions can be obtained by solving two determinate optimization problems. Several numerical results are presented and discussed, demonstrating the effectiveness of the proposed methodology and comparing it to a previously proposed affine arithmetic based solution approach.

Reactive Power Compensation in Single-Phase Operation of Microgrid

A coordinated control of distributed generators (DG) and distribution static compensator (DSTATCOM) in a microgrid is proposed in this paper. With high penetration of distributed sources and single-phase operation of the system, voltage unbalance can often go beyond the acceptable limit. With the feeders geographically spread out, it is not always possible to achieve reactive compensation at optimum location with the three-phase devices. In this paper, a simple control strategy for DSTATCOM with communication in loop is proposed. The proposed reactive compensation technique is based on the voltage sag and the power flow in the line. The power flow and the voltage at different locations of the feeders are communicated to the DSTATCOM to modulate the reactive compensation. The single-phase DSTATCOM compensates for the reactive power deficiency in the phase while the DGs supply “maximum available active power.” During reactive power limit of the DG, the “maximum available active power” is fixed to a value lower than maximum active power to increase reactive power injection capability of the DGs. A primary control loop based on local measurement in the DSTATCOM always ensures a part of reactive compensation in case of communication failure. It is shown that the proposed method can always ensure to achieve acceptable voltage regulation. The data traffic analysis of the communication scheme and closed-loop simulation of power network and communication network are presented to validate the proposed method.

Study of SVC for Voltage Stability Enhancement Using MATLAB

Severe and increasing strain has been observed in the power system in recent years due to incongruence between the generation and transmission infrastructure. The kind of stress developed in the system has caused concerns for voltage instability. One of the major causes of voltage instability is the reactive power limit of the system. The Modal Analysis method is used to investigate the stability of the power system and is performed for IEEE 14 Bus system. Then, the weakest bus is identified using the participation factor. Improving the system's reactive power handling capacity via Flexible AC transmission System (FACTS) devices is a remedy for prevention of voltage instability and hence voltage collapse. Here, SVC is used to improve the voltage profile of the system. The simulations are done by using PSAT and continuation power flow technique. The continuation power flow (CPF) is an accurate method for estimating the maximum loading margin.

Emission Constraint Optimal Power Flow using Differential Evolution

Objective of the power systems firm is to achieve the maximum profit and customer goodwill by providing reliable and quality power supply. This power system operation problem is solved by Optimal Power Flow (OPF). It gives requirement of operating states which satisfy the objective of the firm. Thermal power plants are the main source of power generation. Fuel cost of these power plants has to minimize for better profit at the same time it should satisfy system load demand, real, reactive power limit, voltage limit, power transmission limit and other limitations. For generation cost minimization Economic Load Dispatch (ELD) and Optimal Power Flow (OPF) was developed. When cost is the single objective, the power generation may pollute the environment. Thermal electric power could not be generated without pollution but this pollution can be reduced for the sake of good and healthy atmospheric condition. Differential Evolution (DE) algorithm is used in this paper to solve Emission Constraint Optimal Power Flow problem. Standard IEEE 30 bus, power system having 6 thermal power plants, is considered to validate the simulation.

Preventive reactive power management for improving voltage stability margin

Voltage stability imposes important limitations on the power systems operation. Adequate voltage stability margin needs to be obtained through the appropriate scheduling of the reactive power resources. The main countermeasures against voltage instability could be distinctly classified into preventive and corrective control actions. This paper proposes a preventive countermeasure to improve the voltage stability margin through the management of the reactive power and its reserve. The voltage and reactive power management is studied from the generator's point of view to maximize effective generator reactive power reserve (EGRPR). Detailed model of the generators including the armature and field current limits, as well as the switch mode between the voltage control and the reactive power limitations are considered to maximize the reactive power capability of the generators in emergency states. One-stage and two-stage optimization approaches are utilized to find the optimum solution. The proposed optimization procedure is applied on a 6-bus system and the New England 39-bus system to illustrate the effectiveness of the method.