A line outage study for prediction of static voltage collapse

A simple but useful technique is proposed to study steady-state line outage and hence voltage collapse of the system. Voltage collapse prediction can be performed through the study of line outages, though techniques for nodal-based prediction or critical-node identification are frequently proposed. Experimentally, it was proven that both line outage and system voltage collapse take place simultaneously. In fact, line outages can be treated as the secondary cause of voltage collapse. A scalar index called line-stability index for each line is calculated based on the power flow through the line. The line-stability index may have a value that varies from zero (no power flows through the line) to one (maximum power flows through the line). Any line exceeding the maximum limit of stability index (1.00) can cause system-wide voltage collapse. The proposed technique is tested on a six-bus standard test system, and encouraging results are observed. The results obtained indicate that the technique has the potential to be used as a tool for system monitoring and future load planning     

A novel method to investigate voltage stability of IEEE-14 bus wind integrated system using PSAT

The maximum demand of power utilization is increasing exponentially from base load to peak load in day to day life. This power demand may be either industrial usage or household applications. To meet this high maximum power demand by the consumer, one of the options is the integration of renewable energy resources with conventional power generation methods. In the present scenario, wind energy system is one of the methods to generate power in connection with the conventional power systems. When the load on the conventional grid system increases, various bus voltages of the system tend to decrease, causing serious voltage drop or voltage instability within the system. In view of this, identification of weak buses within the system has become necessary. This paper presents the line indices method to identify these weak buses, so that some corrective action may be taken to compensate for this drop in voltage. An attempt has been made to compensate these drops in voltages by integration of renewable energy systems. The wind energy system at one of the bus in the test system is integrated and the performance of the system is verified by calculating the power flow (PF) using the power system analysis tool box (PSAT) and line indices of the integrated test system. The PF and load flow results are used to calculate line indices for the IEEE-14 bus test system which is simulated on PSAT.     

Overview and investigation of the planned dynamic voltage restorer for autonomous electrical grid: Case of Adrar,south of Algeria

This paper provides an overview and study of diesel power plant (DPP) map, configuration and design in Ain-Belbel Adrar (Situated in south of Algeria) which was constructed as basis to feed the costumers by electricity in this isolated village. Its capacity of electrical production is 5 MW; this power covers the energy demand of customers connected to the electrical distribution network. First, a brief introduction of the evolution of Algeria's electrical energy consumption and power plants installed in the south is presented, followed by studies of theoretical and experimental data. Several experimental works based on simulation and primary data from field measurement are presented to show the performance of the interconnection of Diesel power plant through 15 kV distribution line to improve electrical power supply. In power system voltage sag is considered to be the most common and serious power quality issue, Due to electrical disturbances occurred through autonomous networks such as voltage sags, swells and outages in 15 kV Ain-Belbel city feeder distribution system a dynamic voltage restorer is proposed to diminish the impact of voltage perturbation in this area. DVR should inject the equivalent dropped voltage as a reflection of active power and energy into the distribution system. However, the capability of energy storage is considered how the injection energy can be minimized and the load voltage can be close to the pre-fault voltage.     

Medium-term electric load forecasting using singular value decomposition

Medium-term load forecasting is an important stage in electric power system planning and operation. It is used in maintenance scheduling, and to plan for outages and major works in the power system. A new technique is proposed which uses hourly loads of successive years to predict hourly loads and peak load for the next selected time span. The proposed method implements a new combination of some existing and well established techniques. This is done by first filtering out the load trend, then applying the SVD (singular value decomposition) technique to de-noise the resulting signal. Hourly load is thus divided to three main components: a) a load trend-following component, b) a random component, and c) a de-noised component. Results of applying the technique to the Jordanian power system showed that good forecasting accuracies are attained. In addition, the proposed method outperforms the traditional exponential curve fitting method. The peak load error was found to be less than 5% using the proposed methodology. It was also found that a lag period of 4 years suits the load forecasting purposes of the Jordanian power system. The proposed method is generic and can be implemented to the hourly loads of any power system.     

Non-sequential Monte Carlo simulation tool in order to minimize gaseous pollutants emissions in presence of fluctuating wind power

In this paper, an original non-sequential Monte Carlo simulation tool is developed. This tool permits to compute the optimal dispatch of classical (coal, oil, etc.) thermal generation in order to minimize polluting gases (NO_x, CO₂, etc.) emissions in presence of wind power and under constraints. These constraints include, e.g., the maximal generation cost or the ability of the electrical system to cover the load, … In comparison with existing analytical tools that are based on restrictive hypotheses when it comes to wind power modelling (generally represented by a single entirely correlated global wind park), unexpected outages of conventional parks or fluctuating representation of the load, the use of Monte Carlo simulation allows to remove all those limitations. Indeed, thanks to the developed tool, the optimal dispatch of classical thermal generation can be reached under several load conditions. Well-known reliability indices can also be computed and, moreover, following the wind speed sampling that is used, entirely correlated, independent or more accurate correlation level between wind parks can be considered. Finally, it is thought that the proposed solution can be a useful tool for electrical system operators in order to dispatch the polluting thermal units under cost, reliability, emissions, fluctuating wind power and unexpected outages constraints.     

Portable PEM fuel cell‐ultracapacitor system: Model and experimental verification

This paper presents a comparative study between a model of portable direct hydrogen‐fed proton exchange membrane fuel cell‐ultracapacitor (PEMFC‐UC) power source and experimental results obtained from an actual PEMFC‐UC system in the authors' laboratory. In the proposed system the UC is directly connected to the PEMFC output terminals. The UC is used to supply the power mismatch during the sudden load variations when the load is higher than the PEMFC maximum capacity. The model is then used to estimate the output voltage and study the transient response of the PEMFC‐UC system when subjected to rapid changes in the load. To validate the model, laboratory experiments are carried out using a 100 W commercially available PEMFC and an UC. The model results are verified against the experimental data using three statistical indices to measure the variations, unbiasedness, and accuracy. The indices indicate a maximum difference of 1.06%, which shows a close agreement between the voltage and power responses of the proposed model and the actual PEMFC‐UC system. Copyright © 2009 John Wiley & Sons, Ltd.     

Single-stage sine-wave inverter for an autonomous operation of solar photovoltaic energy conversion system

This paper proposes a high performance single-stage inverter topology for the autonomous operation of a solar photovoltaic system. The proposed configuration which can boost the low voltage of photovoltaic (PV) array, can also convert the solar dc power into high quality ac power for driving autonomous loads without any filter. An MPPT circuit with parallel connection is implemented so that the part of the energy generated is processed by the dc–dc converter to supply dc loads. The line current total harmonic distortion (THD) obtained using this configuration is quite reasonable. The proposed topology has several desirable features such as low cost and compact size as number of switches used, are limited to four as against six switches used in classical two-stage inverters. In this paper analysis, simulation and experimental results are presented.     

Performance improvement of a slip energy recovery drive system by a voltage-controlled technique

This paper introduces the performance improvement of a slip energy recovery drive system for the speed control of a wound rotor induction motor by a voltage-controlled technique. The slip energy occurred in the rotor circuit is transferred back to ac mains supply through a reactor instead of a step up transformer. The objective of the voltage-controlled technique is to increase power factor of the system and to reduce low order harmonics of the input line current. The drive system is designed and implemented using a voltage source inverter in conjunction with a boost chopper for DC link voltage, instead of a conventional drive using a 6 pulse converter or a Scherbius system. The slip power is recovered by the help of a voltage source inverter (VSI) based on a space vector pulse width modulation (SVPWM) technique. In order to keep the speed of the wound rotor induction motor constant over a certain range of operating conditions, the servo state feedback controller designed by a linear quadratic regulator (LQR) is also introduced in this paper. The overall control system is implemented on DSP, DS1104’TMS320F240 controller board. The performance improvement of the proposed system is tested in comparison with the conventional Scherbius system and the modified conventional Scherbius system by a 12 pulse converter in conjunction with a chopper at steady state and at dynamic conditions. A 220 W wound motor is employed for testing. It is found that the motor speed can be controlled to be constant in the operating range of 450–1200 rpm at no load and full load. It is also found that the efficiency of the proposed system is remarkably increased since the harmonics of the input ac line current is reduced while the ac line input power factor is increased.     

Optimal placement and sizing of PV systems and electric parking lots considering reactive power capability and load variation

Power systems should operate in reliable, stable, and efficient conditions. Addition of new generation units or loads to the power systems may change their performance. Therefore, appropriate decisions should be made to manage these elements to improve the power system performance. In this study, optimal placement and sizing of photovoltaic systems and electric parking lots (EPLs) are studied considering the reactive power capability of the inverters and load variation in a 24-h period. For the EPL, a proper charge/discharge scheme (CDS) is initially proposed to flatten the daily load profile; then the EPL with the associated CDS is considered to find its optimal location. Voltage profile, energy losses, bus, and line voltage stability are considered as the objectives of the problem. Genetic algorithm and backward–forward power flow method are utilised to solve the problem considering the IEEE 33-bus system. The results show that all objectives are improved utilising the proposed method.     

Real-time AC voltage control and power-following of a combined proton exchange membrane fuel cell,and ultracapacitor bank with nonlinear loads

The nonlinear loads create a wide range of current harmonics in the system. Such loads can make distortions on the output voltage profile, influence on the fuel cell (FC) performance, and endanger safe operation of the FC unit. In this paper, new strategies for power-following and AC voltage control have been developed. The proposed system consists of the ultracapacitor (UC) bank and proton exchange membrane fuel cell (PEMFC) supplying nonlinear AC loads. The power tracking strategy is based on the Fourier analysis of total load demand. The Fourier analysis is used as an effective tool to eliminate destructive effect of current harmonics on the PEMFC output current. To supply the nonlinear AC loads under sinusoidal voltage with the fast response, a dynamic model for the inverter control loop is also presented. This model is used to enhance the input reference tracking and reject input/output disturbances. The simulation outcomes confirm the desirable PEMFC performance against nonlinear load disturbances. In addition, the output AC voltage is kept sinusoidal and has low deviations under nonlinear load variations.     

Experimental investigation on the mechanism of variable fan speed control in Open cathode PEM fuel cell

This study aims to reveal the mechanism of variable fan speed control in Open cathode PEM fuel cell (OC-PEMFC) by the experiments, which analyze the effects of variable fan speed on the operating parameters under different fuel cell loads. The operating parameters are cell temperature, stack voltage, voltage uniformity, and parasitic power. The results reveal that the fan speed has strong effects on the operating parameters: (1) it affects the temperature distribution and overheats the middle cells of fuel cell stack, especially, when it operates at high power; (2) it affects the variation trend of the voltage under different loads; (3) it affects the voltage uniformity, which can be improved via forced convection; (4) it affects the system efficiency due to the parasitic load consumed by the fan. Considering the varying effects of these operating parameters on performance, durability and system efficiency under different fuel cell loads, the variable fan speed control should adapt the variation of fuel cell loads and weigh multiple operating parameters.     

Reliability and economic evaluation of small autonomous power systems containing only renewable energy sources

Evaluation of reliability performance in every power system has to be done within a cost–benefit framework. This approach, however, is a very time consuming task, especially for systems that contain a large number of possible configurations, so simpler techniques referred to the calculation of reliability indices are used. In small autonomous power systems (SAPSs), such an evaluation uses mainly deterministic criteria. This approach, however, cannot be applied in SAPS that contain only renewable energy sources, due to the intermittent nature of the provided energy. In this paper, a complete reliability cost and worth analysis is implemented for these systems, combined with the calculation of some basic probabilistic indices, in order to discover their performance and propose the appropriate of them as a criterion of optimal system configuration. This paper proposes that normalized energy reliability indices as system minutes and energy index of unavailability can be used as adequate criteria of system's optimal performance. This conclusion is validated through a large number of sensitivity analysis studies that are based on different maximum annual loads and different mix of load types.     

Fuzzy dispatching of solar energy in distribution system

This paper presents a novel approach for solar energy using in distribution system as distributed generation (DG) unit. A nonlinear fuzzy controller tunes the modulation index of PWM inverter to feed the load in the grid via photovoltaic arrays. The controller also dispatches two dc sources to control input of inverter. The proposed system controls the voltage even during changing sunlight voltage condition or unbalanced load. A low pass LC filter is linked to the output of voltage source converter to bypass switching harmonics. The evolutionary method based on fuzzy theory is used to determine the value of modulation index and disperse the sources from a fuzzy rule-based defined on load voltage error of the point of common coupling. This system gives a full flexibility to the grid to obtain power from the solar photovoltaic units depending on its cost and load requirement at any given time. Simulation results illustrate the effectiveness of performance of proposed method.     

Starting and Steady-State Characteristics of DC Motors Powered by Solar Cell Generators

The performance of dc motors (series, separately-excited, and shunt motors) powered by a solar cell generator and loaded by two different types of loads, one a constant load and one a ventilator load, were analyzed with respect to the transient (starting) and steady state operation. Direct current motors are employed in photovoltaic water pumping systems; therefore, the understanding of the system operation and the matching of the system components (solar cells, dc motor type, and load type) are important factors of the system design. Since the solar cell generator in a nonlinear and time-dependent power supply with an output that varies with the insolation (hourly and daily), the performance characteristics of the dc motor are different when supplied by a solar cell generator than when supplied by a conventional constant voltage source. The transient solution was obtained by using an available computer program - SUPER SCEPTRE. The separately - excited (or permanent magnet) motor with a ventilator load was found to be the most suitable for the solar cell generator. The series motor is quite acceptable, but the shunt motor gives poor performance. In all cases the ventilator load is more compatible with the solar cell generator than with the constant load.     

Experimental analysis of the performance of the air supply system in a 120 kW polymer electrolyte membrane fuel cell system

For analyzing the performance of 120 kW polymer electrolyte membrane fuel cell (PEMFC) system and its air supply system, an air system test bench was built, then applied on a 120 kW PEMFC system test bench composed of air supply subsystem, hydrogen supply subsystem, stack, cooling subsystem and electronic control subsystem. The strategy composed of feedforward table and Piecewise proportional integral (PI) feedback control strategy is employed to regulate the flow rate and pressure of air supply system. Firstly, the air compressor map and the mapping relationship between the speed of air compressor, opening of back-pressure valve and stack current are obtained by carrying out experiments on the PEMFC air system bench. Then, the max output performance, steady-state performance, the startup performance, the dynamic response abilities of PEMFC system are tested, respectively. During the experiments, performances under different test conditions were analyzed by comparing parameters such as voltage inconsistency, average voltage, minimum voltage, voltage range, net power of the PEMFC system, and stack power. The test results show that the air supply system can provide qualified flow rate and pressure for the PEMFC stack. The peak power of the stack is 120 kW and net power of the system is 97 kW when the current is 538 A. The response time from rated net power to idle net power is 12 s and from idle net power to rated net power is 23 s. The overshoot of average voltage and minimum voltage in the process of increasing load is both 0.01 V, which are 0.015 V and 0.02 V lower than that when the load is decreased, respectively. The dynamic response speed and stability of the PEMFC system in the process of decreasing the load are better than those in the process of increasing the load.     

Restructuring Choices for the Indian Power Sector

This letter demonstrates the use of line stability index termed as fast voltage stability index (FVSI) in order to determine the maximum loadability in a power system. The bus that is ranked highest is identified as the weakest bus since it can withstand a small amount of load before causing voltage collapse. It involves the experimental process of voltage stability analysis and evaluation of line index based on the load variation. The point at which FVSI close to unity indicates the maximum possible connected load termed as maximum loadability at the point of bifurcation. This technique is tested on the IEEE system and results proved that the proposed technique is able to estimate the maximum loadability in a system.     

Design and laboratory test of black‐start control mode for wind turbines

In this article, an operational strategy and control concept for wind turbines (WTs) are described, which would allow them to actively contribute to black‐start events after major power system outages. The approach is based on (a) a new generator/converter control strategy implementing a so called virtual synchronous machine (VSM) and (b) a number of modifications to the superimposed WT controller allowing for operation in black‐start conditions. In order to operate stably even at very low active power levels and to cope with sudden changes in active power, due to switching of loads in the recovered grid, the rotor speed/pitch controller had to be redesigned. The extension of the operational range of the WT towards negative power, ie, power consumption, is discussed, which would allow the turbine to temporarily provide a controllable minimum load to conventional power plants until a sufficient number of consumers has been reconnected. The control system has been implemented and verified using two experimental power converters, each linked to a hardware‐in‐the‐loop (HiL) simulator of a WT and connected to a real medium‐voltage laboratory grid.     

Wide range load controllable MCFC cycle with pressure swing operation

Partial load efficiencies of a natural gas fuelled MCFC/GT system are calculated; the efficiencies of four systems are compared. A constant pressure air compressor is applied in system cases 1 and 2, whereas a pressure swing air compressor is provided in system cases 3 and 4. A gas cooler is integrated in the cathode gas recycling line of cases 2–4, and an anode recycling with sub-reformer is provided in case 4. The cathode pressure loss in the MCFC stack is kept below 3 kPa during the calculation procedure to avoid a leakage of cathode gas. The range of the power load is limited to 50–100% in the constant operating pressure system (cases 1 and 2), mainly because of the limited cathode gas pressure loss of 3 kPa. The range of the power load is enlarged to 20–100% in cases 3 and 4 by combining the pressure swing operation with gas cooling in the cathode recycling line. In system cases 3 and 4, the efficiency at the lowest load operation (approx. 20–30% load) remains over 35% HHV-CH₄, whereas the maximum efficiency is calculated to be 53% HHV-CH₄ in middle load operation; the efficiency of case 4 at 100% load is estimated to be 50% HHV-CH₄. The combination of the pressure swing operation and gas cooling in the cathode recycling line offers a high efficiency of the MCFC system in a wide range of loads.     

Incorporation of a new wind turbine generating system model into distribution systems load flow analysis

This paper describes a new model for wind turbine generating systems (WTGSs) that is widely used as distributed generation sources. The model is developed by using the bi‐quadratic equation, which is generally used for the calculation of the line voltages in distribution systems' load flow analysis, and facilitates computation of real and reactive power outputs of the WTGSs for a specified wind speed and terminal voltage. The developed model is validated with an experimental setup composed by an induction generator coupled with an induction motor as a prime mover. In addition to that, measured values are also compared with the calculated values, obtained by using the turbine models found in the literature. The incorporation of the developed model into some well‐known distribution systems' load flow algorithms is detailed. The effect of WTGSs on the power losses, voltage profile of radial distribution systems are evaluated for the sample test systems. Additionally, the performance of the load flow algorithms with the new model are examined and found to be robust and reliable. Copyright © 2008 John Wiley & Sons, Ltd.     

Load control of a wind-hydrogen stand-alone power system

A new generation of load controllers enable stand-alone power systems (SAPS) to use one or many standard (grid connected) wind turbines. The controllers use fuzzy logic software algorithms. The strategy is to use the control loads to balance the flow of active power in the system and hence control system frequency. The dynamic supply of reactive power by a synchronous compensator maintains the system voltage within the limits specified in EN50160. The resistive controller loads produce a certain amount of heat that is exchanged down to the end user (hot water). It was decided to investigate the implementation of a hydrogen subsystem into the SAPS that can work in parallel with the Distributed Intelligent Load Controller (DILC). The hydrogen subsystem can then function as energy storage on long-term basis and an active load controller on short-term basis.