Excitation system protective limiters and their effect on volt/VArcontrol-design, computer modeling, and field testing

The action of excitation system protective limiters and VAr regulation are often not taken into account in planning studies, and actual units may be quite different than simulations. This may lead to incorrect predictions of actual stability limitations, or even voltage collapse phenomena. The design of the excitation system protective limiters such as over excitation limiters (OEL), under excitation limiters (UEL) and volts/hertz (V/Hz) is explained, and models presented. For system events where these controls and limiters may become active, modeling in transient stability studies is important to correctly predict volt/VAr performance. Specific models applicable to the GE EX2000 digital-based control systems are shown for reference, although the general concepts discussed here are applicable to any other excitation equipment. An important aspect of the new digital-based excitation controls is that they allow for more intelligent limiter functions and protective features not convenient with older equipment. Test procedures and results are shown to illustrate the limiter performance     

汽轮发电机静止励磁的轴电压问题分析

轴电压是大型汽轮发电机运行中备受关注的问题,随着静止励磁系统在汽轮发电机中的广泛应用,励磁系统产生了一个高频轴电压。它是在基本轴电压上叠加幅值为60v的低频电压和峰值为30V的高频电压彤成的。轴电压会引起旋转机械的严重损坏,传统的接地装置不能消除高频轴电压峰值。介绍了四种产生轴电压的情况,重点是由静止励磁系统引起的轴电压的产生、传递及危害。提出在发电机的励磁端通过接地电刷对地接入一个新型无源RC电路的防范措施,将轴电压降到了无害的水平。     

Thyristor-controlled voltage regulators for critical inductionmotor loads during voltage disturbances

The transient stability of a critical induction machine load group during momentary as well as continuing voltage sags can be enhanced by using a fast-response voltage regulator in the form of a static VAr compensator (SVC) or a thyristor controlled tap changer (CTC). A procedure is presented for determining the minimum SVC rating or CTC off-nominal turn ratio including control of the SVC admittances or CTC taps to meet dynamic and steady-state specifications. The limitations of their effectiveness to preserve operation continuity are also shown     

IEEE-PES Transmission and Distribution Conference and Exposition Call for Papers

Static network-related system voltage stability margin (VSM) depends on the availability of reactive power to support the transport of real power from sources to sinks. Based on this premise, the total VAr loss is minimized in the unified OPF framework considering real and reactive power controllers, and its effect on VSM is studied. Studies are conducted on a three-bus system, the IEEE 30-bus system, and a 191-bus Indian electric power system, and their results are reported.     

Voltage Distribution among the Coils and Turns of a Form Wound AC Rotating Machine Exposed to Impulse Voltage

This paper describes a method of calculating voltage distribution in a stator winding exposed to impulse voltage. In this method, the voltage distribution in an equivalent-circuit representation of the stator winding is calculated. The winding is treated as an infinite number of identical coils connected in series, with each coil represented by an equivalent circuit including inductance, turn-to-ground capacitance and conductance, and with mutual inductance, capacitance, and conductance between turns. The impulse voltage is approximated by a Fourier series of components. The voltage distribution for each component is calculated, and the complete voltage distribution is obtained by summing the voltages due to each component. The complete calculating procedure, including the calculation of the equivalent circuit parameters has bee programmed in Basic language for computer calculation. Examples of the coil and turn voltages distributions calculated by this program and a comparison of calculated data with test data for a particular stator winding are included.     

直接转矩控制中基于模糊推理的相位补偿

基于数字信号处理器ＴＭＳ３２０Ｆ２４０给出了一个动态的模糊预测电流补偿的方法,针对感应电机提高了直接转矩控制的低速性能,引入了一种模糊控制方法来补偿ＰＷＭ信号中由死区和芯片延时引起的相位误差,根据电流的幅值与方向,结合瞬时电压,采用模糊推理对相位误差进行补偿,可以在一定程度上提高控制的性能,仿真与实验结果表明的这种方法是可行的。     

Fuzzy logic based MPPT controller for high conversion ratio quadratic boost converter

In this study, a maximum power point tracking DC–DC quadratic boost converter for high conversion ratio required applications is proposed. The proposed system consists of a quadratic boost converter with high step-up ratio and fuzzy logic based maximum power point tracking controller. The fuzzy logic based maximum power point tracking algorithm is used to generate the converter reference signal, and the change in PV power and the change in PV voltage are selected as fuzzy variables. Determined membership functions and fuzzy rules which are design to track the maximum power point of the PV system generates the output signal of the fuzzy logic controller's output. It is seen from MATLAB/Simulink simulation and experimental results that the quadratic boost converter provides high step-up function with robustness and stability. In addition, this process is achieved with low duty cycle ratio when compared to the traditional boost converter. Furthermore, simulation and experimental results have validated that the proposed system has fast response, and it is suitable for rapidly changing atmospheric conditions. The steady state maximum power point tracking efficiency of the proposed system is obtained as 99.10%. Besides, the output power oscillation of the converter, which is a major problem of the maximum power point trackers, is also reduced.     

Voltage and capacity stability of the Hubble telescope nickel-hydrogen battery

The power system of the Hubble Space Telescope includes two orbital replacement units, each containing three nickel-hydrogen (NiH₂) batteries of 88 Ah capacity. Since launch in April 1990, the batteries have completed 23 000 charge and discharge cycles and continue to meet the power demands of the satellite. The voltage, capacity, and pressure characteristics of all six batteries were analyzed to determine the state of health of the battery and to identify any signs of performance degradation. The battery pressures have changed to varying degrees. The end-of-charge pressure for battery 4 increased by 96 psi, while that for battery 3 decreased by 37 psi. The voltages of the individual cells show a decay rate of 0.69 mV per 1000 cycles, and the capacity of the batteries has apparently decreased, possibly due to the system being operated at a lower stage of charge. Autonomous battery operation involving charge termination at a preselected voltage continues to restore the energy dissipated during each orbit. The accumulated data on voltages and recharge ratios can be used to design new temperature-compensated voltage levels for similar missions that employ NiH₂ batteries.     

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.     

Control design and dynamic performance analysis of a windturbine-induction generator unit

This paper presents the modeling and control design for a wind energy conversion scheme using induction generators. The scheme consists of a three-phase induction generator driven by a horizontal axis wind turbine and interfaced to the utility through a double overhead transmission line. A static VAr compensator was connected at the induction generator terminals to regulate its voltage. The mechanical power input was controlled using the blade pitch-angle. Both state and output feedback controllers are designed using MATLAB software to regulate the generator output. From the simulation results, the response of closed loop system exhibited a good damping and fast recovery under different type of large disturbances     

基于模糊划分理论的山西省多层次水资源系统评价

针对山西省水资源系统存在的水资源短缺、节水潜力挖掘不足、河川基流不能保障和地下水超采严重等问题,构建多层次水资源系统矩阵,以最严格水资源管理、节水评价技术要求和地下水压采计划等为依据选取各个指标的模糊评判标准,采用改进的模糊划分隶属函数模型,逐层计算山西省水资源系统3个子系统14个指标154个因子对应于劣、中、优模糊等级的隶属度,得出3个子系统16个指标11个地市的排序结果及综合系统的3个子系统11个地市的模糊级别和排序结果。结果表明,太原、晋中和运城水资源综合评价模糊级别较差,其余地市相对较好。     

Integrated SVC and step-down transformer model for Newton-Raphsonload flow studies

This letter presents an integrated static VAr compensator (SVC) and step-down transformer model suitable for power flow studies using the Newton-Raphson method. The SVC is taken to be a continuous, variable-shunt susceptance that is adjusted automatically to satisfy a specified voltage magnitude. The step-down transformer is represented by an admittance that combines easily with the variable susceptance, representing the SVC plant. The resulting equivalent model allows for a direct SVC voltage regulation at the high-voltage side of the transformer, which does not compromise the quadratic convergence characteristics of the Newton-Raphson method     

Analysis of grid connected induction generators driven byhydro/wind turbines under realistic system constraints

Results of an investigation dealing with the behaviour of grid-connected induction generators (GCIGs) driven by typical prime movers such as mini-hydro/wind turbines are presented. Certain practical operational problems of such systems are identified. Analytical techniques are developed to study the behavior of such systems. The system consists of the induction generator (IG) feeding a 11 kV grid through a step-up transformer and a transmission line. Terminal capacitors to compensate for the lagging VAr are included in the study. Computer simulation was carried out to predict the system performance at the given input power from the turbine. Effects of variations in grid voltage, frequency, input power, and terminal capacitance on the machine and system performance are studied. An analysis of self-excitation conditions on disconnection of supply was carried out. The behavior of a 220 kW hydel system and 55/11 kW and 22 kW wind driven system corresponding to actual field conditions is discussed     

Novel dynamic semiempirical proton exchange membrane fuel cell model incorporating component voltages

This paper introduces a novel dynamic semiempirical model for the proton exchange membrane fuel cell (PEMFC). The proposed model not only considers the stack output voltage but also provides valid waveforms of component voltages, such as the no‐load, activation, ohmic, and concentration voltages of the PEMFC stack system. Experiments under no‐load, ramping load, and dynamic load conditions are performed to obtain various voltage components. According to experimental results, model parameters are optimised using the lightning search algorithm by providing valid theoretical ranges of parameters to the lightning search algorithm code. In addition, the correlation between the vapour and water pressures of the PEMFC is obtained to model the component voltages. Finally, all component voltages and the stack output voltage are validated by using the experimental/theoretical waveforms mentioned in previous research. The proposed model is also compared with a recently developed semiempirical model of PEMFC through particle swarm optimisation. The proposed dynamic model may be used in future in‐depth studies on PEMFC behaviour and in dynamic applications for health monitoring and fault diagnosis.     

Application of magnetic energy storage unit as continuous VArcontroller

It is shown that magnetic energy storage units can simultaneously operate as continuous VAr (volt-ampere reactive) controllers while performing the role of load-frequency stabilizers in electrical power systems. This is achieved by operating the converter in the buck-boost mode with a switched capacitor bank placed across its terminals. The P versus Q modulation ranges of the 12-pulse converter depend on the source inductance, secondary voltage of the input transformers, and output current. Once the input transformer is chosen, the Q modulation range depends on the active power transfer and the current through the inductor at any instant of time. The actual reactive power consumption of the converter is varied continuously, depending on the requirements of the power system, while keeping within the Q-modulation range. Switching of the capacitor bank keeps the required Q consumption of the converter within the available range. It is shown that this mode of control improves the overall performance of the power system in P-f and Q-V loops and obviates the use of any additional VAr compensator in the power area where the SMES (superconducting magnetic energy storage) unit is located     

Prestriking voltages associated with motor breaker closing

Steep-fronted surges arriving at motor terminals due to breaker closing result from the prestrike voltages present across each breaker pole as an arc is established. This prestrike voltage has been shown to depend on the instantaneous 60 Hz source voltage, and transients transferred through the motor from poles already closed, and the dielectric withstand of the closing contact gap. The worst-case prestrike voltage reaches a higher value than previously reported. Statistical studies show that a motor is likely to be stressed within a few percent of the maximum surge level, if switched daily over 30 years     

Influence of generator load conditions on third-harmonic voltages in generator stator winding

The influence of changes in load conditions of the generator-transformer unit on the operating conditions of the 100% ground-fault protection of the generator stator winding based on the third-harmonic voltages was analyzed. The analyzed stator ground-fault protection system is by ratio of the voltages third harmonic in generator neutral and at the generator terminals. The third-harmonic voltages were determined on the base of the experimental tests (measurements) run in real conditions at changes in active and reactive load conditions of the generator. It was found that mainly the generator active load influences the voltage feeding the measuring element of ground-fault protection. Therefore, determining operating conditions of ground-fault protection the influence of the generator active and reactive load should be deeply considered. This can ensure the proper operation of the ground-fault protection based on the voltage third harmonics in the whole range of the changes in generator load. Considering the influence of changes in generator load on the third-harmonic voltages (feeding measuring element of the protection system) will allow eliminating irregular generator tripping as a result of unselective operation of these protections.     

A self-tuning automatic voltage regulator designed for anindustrial environment

Examination of the performance of fixed parameter controllers has resulted in the development of self-tuning strategies for excitation control of turbogenerator systems. In conjunction with the advanced control algorithms, sophisticated measurement techniques have previously been adopted on micromachine systems to provide generator terminal quantities. In power stations, however, a minimalist hardware arrangement would be selected leading to relatively simple measurement techniques. The performance of a range of self-tuning schemes is investigated on an industrial test-bed, employing a typical industrial hardware measurement system. Individual controllers are implemented on a standard digital automatic voltage regulator (AVR), as installed in power stations. This employs a VME platform, and the self-tuning algorithms are introduced by linking to a transputer network. The AVR includes all normal features, such as field forcing, VAr limiting and overflux protection. Self-tuning controller performance is compared with that of a fined gain digital AVR     

Enhancement of micro-grid performance during islanding mode using storage batteries and new fuzzy logic pitch angle controller

Power system deregulation, shortage of transmission capacities and needing to reduce green house gas have led to increase interesting in distributed generations (DGs) especially renewable sources. This study developed a complete model able to analysis and simulates in details the transient dynamic performance of the Micro-Grid (MG) during and subsequent islanding process. Wind speed fluctuations cause high fluctuations in output power of wind turbine which lead to fluctuations of frequency and voltages of the MG during the islanding mode. In this paper a new fuzzy logic pitch angle controller is proposed to smooth the output power of wind turbine to reduce MG frequency and voltage fluctuations during the islanding mode. The proposed fuzzy logic pitch controller is compared with the conventional PI pitch angle controller which usually used for wind turbine power control. Results proved the effectiveness of the proposed fuzzy controller in improvement of the MG performance. Also, this paper proposed using storage batteries technique to reduce the frequency deviation and fluctuations originated from wind power solar power fluctuations. Results indicate that the storage batteries technique is superior than fuzzy logic pitch controller in reducing frequency deviation, but with more expensive than the fuzzy controller. All models and controllers are built using Matlab® Simulink® environment.     

Fuzzy stochastic long-term model with consideration of uncertainties for deployment of distributed energy resources using interactive honey bee mating optimization

This paper presents a novel modified interactive honey bee mating optimization (IHBMO) base fuzzy stochastic long-term approach for determining optimum location and size of distributed energy resources (DERs). The Monte Carlo simulation method is used to model the uncertainties associated with long-term load forecasting. A proper combination of several objectives is considered in the objective function. Reduction of loss and power purchased from the electricity market, loss reduction in peak load level and reduction in voltage deviation are considered simultaneously as the objective functions. First, these objectives are fuzzified and designed to be comparable with each other. Then, they are introduced into an IHBMO algorithm in order to obtain the solution which maximizes the value of integrated objective function. The output power of DERs is scheduled for each load level. An enhanced economic model is also proposed to justify investment on DER. An IEEE 30-bus radial distribution test system is used to illustrate the effectiveness of the proposed method.