Generator Terminal Voltage Research Articles

연계선로의 조건 변화에 따른 DFIG와 FSIG 풍력발전시스템의 운전특성 비교

This paper analyzes the steady-state output characteristics of variable-speed wind turbine systems using doubly-fed induction generators(DFIG) compared with fixed-speed induction generator(FSIG) wind turbine systems. It also presents simulations of a grid-connected wind turbine generation system for dynamics analysis on MATLAB/Simulink and compares the responses between DFIG and FSIG wind turbine systems with respect to wind speed variation, impedance changes and X/R ratio changes of interconnecting circuits. Simulation results show the variation of generator's active output, terminal voltage and fault currents at the interconnecting point. Case studies demonstrate that DFIG wind turbine systems illustrate better performance to 3-phase fault than FSIG's.

Robust transient stabilisation problem for a synchronous generator in a power network

The robust transient stabilisation problem (with stability proof) of a synchronous generator in an uncertain power network with transfer conductances is rigorously formulated and solved. The generator angular speed and electrical power are required to be kept close, when mechanical and electrical perturbations occur, to the synchronous speed and mechanical input power, respectively, while the generator terminal voltage is to be regulated, when perturbations are removed, to its pre-fault reference constant value. A robust adaptive nonlinear feedback control algorithm is designed on the basis of a third-order model of the synchronous machine: only two system parameters (synchronous machine damping and inertia constants) along with upper and lower bounds on the remaining uncertain ones are supposed to be known. The conditions to be satisfied by the remote network dynamics for guaranteeing ℒ2 and ℒ∞ robustness and asymptotic relative speed and voltage regulation to zero are weaker than those required by the single machine-infinite bus approximation: dynamic interactions between the local deviations of the generator states from the corresponding equilibrium values and the remote generators states are allowed.

An improved direct feedback linearization technique for transient stability enhancement and voltage regulation of power generators

In this paper, a simple improved direct feedback linearization design method for transient stability and voltage regulation of power systems is discussed. Starting with the classical direct feedback linearization technique currently applied to power systems, an adaptive nonlinear excitation control of synchronous generators is proposed, which is new and effective for engineering. The power angle and mechanical power input are not assumed to be available. The proposed method is based on a standard third-order model of a synchronous generator which requires only information about the physical available measurements of angular speed, active electric power and generator terminal voltage. Experimental results of a practical power system show that fast response, robustness, damping, steady-state and transient stability as well as voltage regulation are all achieved satisfactorily.

Extending the Life of Gear Box in Wind Generators by Smoothing Transient Torque With STATCOM

Gearboxes for wind turbines must ensure high reliability over a period of 20 years, withstanding cumulative and transient loads. One main challenge to this is represented by electromagnetic torque transients caused by grid faults and disturbances, which will result in significant stresses and fatigue of the gearbox. Possibilities for limiting the torque transients in fixed-speed wind generators have not been previously reported. This paper presents a technique by which the transient torques during recovery after a grid fault can be smoothed in a wind farm with induction generators directly connected to the grid. A model-based control technique using the quasi-stationary equivalent circuit of the system is suggested for controlling the torque with a static synchronous compensator (STATCOM). The basis of the approach consists of controlling the induction generator terminal voltage by the injection/absorption of reactive current using the STATCOM. By controlling the terminal voltage as a function of the generator speed during the recovery process, the electromagnetic torque of the generator is indirectly controlled, in order to reduce the drive train mechanical stresses caused by the characteristics of the induction machine when decelerating through the maximum torque region. The control concept is shown by time-domain simulations, where the smoothing effect of the proposed technique on a wind turbine is seen during the recovery after a three-phase-to-ground-fault condition. The influence of the shaft stiffness in a multimass drive train model is discussed, and the performance of the control concept in the case of parallel connection of several turbines is investigated to discuss the applicability in a wind farm.

Optimal Reactive Power Dispatch for Voltage Stability Enhancement Using Real Coded Genetic Algorithm

This paper presents an algorithm for solving the multi-objective reactive power dispatch problem in a power system. Modal analysis of the system is used for static voltage stability assessment. Loss minimization and maximization of voltage stability margin are taken as the objectives. Generator terminal voltages, reactive power generation of the capacitor banks and tap changing transformer setting are taken as the optimization variables. An improved genetic algorithm which permits the control variables to be represented in their natural form is proposed to solve this combinatorial optimization problem. For effective genetic operation, crossover and mutation operators which can directly operate on floating point number and integers are used. The proposed method has been tested on IEEE 30 bus system and has resulted in loss which is less than the value reported earlier and is well suited for solving the mixed integer optimization problem.

Real-time transient stabilization and voltage regulation of power generators with unknown mechanical power input

A nonlinear adaptive excitation controller is proposed to enhance the transient stability and voltage regulation of synchronous generators with unknown power angle and mechanical power input. The proposed method is based on a standard third-order model of a synchronous generator which requires only information about the physical available measurements of relative angular speed, active electric power, infinite bus and generator terminal voltages. The operating conditions are computed online using the above physical available measurements, the terminal voltage reference value and the estimate of the mechanical power input. The proposed design is therefore capable of providing satisfactory voltage in the presence of unknown variations of the power system operating conditions. Using the concept of sliding mode equivalent control techniques, a robust decentralized adaptive controller which insures the exponential convergence of the outputs to the desired ones, is obtained. Real-time experimental results are reported, comparing the performance of the proposed adaptive nonlinear control scheme to one of the conventional AVR/PSS controller. The high simplicity of the overall adaptive control scheme and its robustness with respect to line impedance variation including critical unbalanced operating condition and temporary turbine fault, constitute the main positive features of the proposed approach.

発電機を電源とするマトリックスコンバータの制御法

This paper describes a strategy for control of the input current of a matrix converter with a generator as the input. The line impedance is larger than the power grid when the generator is used as the input of the matrix converter. The resonance between the synchronous reactance of the generator and the input capacitance results in unstable behavior of the matrix converter. For a stable operation, stability control is provided to the rotating frame on the generator side. With the proposed method, both generator current and output voltage can be controlled, regardless of the generator speed. In addition, optimal input power factor control is proposed for achieving the maximum modulation index of the matrix converter.This study confirms the validity of the proposed control strategy on the basis of the results of simulations and experiments. Oscillations in the generator terminal voltage and current can be suppressed by using the proposed stability control. Moreover, the proposed control helps achieve stable operations at a motor load of 1.5kW, and the THD of the input current is reduced to 3.7%.

Dynamic optimal reactive power dispatch based on parallel particle swarm optimization algorithm

In this paper, Message Passing Interface (MPI) based parallel computation and particle swarm optimization (PSO) algorithm are combined to form the parallel particle swarm optimization (PPSO) method for solving the dynamic optimal reactive power dispatch (DORPD) problem in power systems. In the proposed algorithm, the DORPD problem is divided into smaller ones, which can be carried out concurrently by multi-processors. This method is evaluated on a group of IEEE power systems test cases with time-varying loads in which the control of the generator terminal voltages, tap position of transformers and reactive power sources are involved to minimize the transmission power loss and the costs of adjusting the control devices. The simulation results demonstrate the accuracy of the PPSO algorithm and its capability of greatly reducing the runtimes of the DORPD programs.

経済性とセキュリティを考慮したタブサーチによる電圧・無効電力制御の最適化

Due to deregulation of electric power market, the several power transactions and the distributed generators will increase year by year. The power flow in power system will vary increasingly more complex and more uncertain by them. So it is difficult for Voltage and reactive power(Q) Control (VQC) to sustain the adequate voltages in the power system. On the other hand, the economy and security are requested for power system operation and control under the competitive environment. The optimization algorithm is effective to satisfy these trade-off problem. The optimization of VQC traces the combinational optimization problem with shunt devices, transformer tap and generator terminal voltage. Tabu Search (TS), which is one of meta-heuristic approaches, is suitable for the optimization problem of VQC. Because it is able to deal with the discrete variables such as shunt devices easily and it needs few tuning parameters.In this paper, in order to realize flexible VQC which responds to the uncertain power flow with competitive environment, we propose the optimization of VQC with TS algorithm taking into account of both economy and security. The security is considered as the minimization of generator reactive outputs and transformer tap positions difference from the neutral. Moreover, the novel neighborhood condition setting focused on the control interference between voltage control equipments is applied to minimizing the objective function. It is confirmed by several numerical simulations using IEEE 14 and 57 bus system that the proposed scheme is able to ensure the security and to find the useful optimal solution efficiently.

Differential evolution approach for optimal reactive power dispatch

Differential evolution based optimal reactive power dispatch for real power loss minimization in power system is presented in this paper. The proposed methodology determines control variable settings such as generator terminal voltages, tap positions and the number of shunts to be switched, for real power loss minimization in the transmission system. The problem is formulated as a mixed integer nonlinear optimization problem. A generic penalty function method, which does not require any penalty coefficient, is employed for constraint handling. The formulation also checks for the feasibility of the optimal control variable setting from a voltage security point of view by using a voltage collapse proximity indicator. The algorithm is tested on standard IEEE 14, IEEE 30, and IEEE 118-Bus test systems. To show the effectiveness of proposed method the results are compared with Particle Swarm Optimization and a conventional optimization technique – Sequential Quadratic Programming.

Steady-State Analysis of a Self-Excited Single-Phase Reluctance Generator

This paper presents an analytical method for predicting the steady-state performance of a self-excited single-phase reluctance generator (SESPRG), which supplies <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R</i> -- <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">L</i> load. The proposed analysis is based on the <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">d</i> -- <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">q</i> axis model and phasor diagram of such a generator in the steady-state condition. Excitation capacitors are connected across both the main and auxiliary windings. Magnetic saturation is taken into account and is assumed to be confined to the direct axis, and is accounted for a variable direct-axis magnetizing reactance. Conditions of self-excitation and the minimum value of the capacitance required to achieve self-excitation are also given. Special attention is focused on the machine performance when it operates as a pure single-phase reluctance generator (PSPRG). A fixed-capacitor (FC) thyristor-controlled reactor (TCR) scheme is used to regulate the generator terminal voltage by controlling the thyristor conduction angle. Further stability limits are investigated by developing the active-reactive ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">P-Q</i> ) power diagram. Reasonably close agreement between the measured and predicted results is observed confirming the validity of the proposed analysis.

Improved genetic algorithm for multi‐objective reactive power dispatch problem

AbstractThis paper presents an improved genetic algorithm (GA) approach for solving the multi‐objective reactive power dispatch problem. Loss minimization and maximization of voltage stability margin are taken as the objectives. Maximum L‐index of the system is used to specify the voltage stability level. Generator terminal voltages, reactive power generation of capacitor banks and tap changing transformer setting are taken as the optimization variables. In the proposed GA, voltage magnitudes are represented as floating point numbers and transformer tap‐setting and reactive power generation of capacitor bank are represented as integers. This alleviates the problems associated with conventional binary‐coded GAs to deal with real variables and integer variables with total number of permissible choices not equal to 25. Crossover and mutation operators which can deal with mixed variables are proposed. The proposed method has been tested on IEEE 30‐bus system and is compared with conventional methods and binary‐coded GA. The proposed method has produced the loss which is less than the value reported earlier and is well suitable for solving the mixed integer optimization problem. Copyright © 2007 John Wiley &amp; Sons, Ltd.

Rotor flux magnitude and angle control strategy for doubly fed induction generators

AbstractA new control strategy is investigated for Doubly Fed Induction Generators (DFIGs). It exercises control over the generator terminal voltage and output power by adjusting the magnitude and angle of the rotor flux vector. It is shown that this control strategy leads to low interaction between the power and voltage control loop, better system damping and voltage recovery following faults, and it also provides enhanced frequency regulation capability compared with that achieved with existing DFIG controllers described in the open literature. The dynamic performance of the proposed DFIG control is tested for small and large disturbances using a generic network that combines wind and conventional synchronous generation. Simulation results are presented and discussed that demonstrate the capabilities of the new strategy to enhance DFIG performance and its contribution to network operation.

STABILITY ANALYSIS OF AN ISLANDED GENERATOR

The authors investigate the stability of an islanded generator. They examine the effect of a predominantly resistive load on small-signal (frequency) stability of an islanded generator. The authors derive a new linear generator model for the islanded power station, and perform small-signal analyses using this model. The analyses are also performed using two industrial–grade software packages, and the results are compared. The authors show that predominantly resistive loads lead to poor damping of the speed response, and that an increase in the L/R ratio further decreases the damping. The use of a power system stabilizer is proposed to increase the damping of the speed response by varying the generator terminal voltage in response to speed changes. In this sense the PSS is also contributing to frequency control. Finally, the authors study the effect of the turbine and its control system on stability.

Coherency and Aggregation Techniques Incorporating Rotor and Voltage Dynamics

Dynamic equivalents have been widely used to reduce computational efforts when a large number of scenarios have to be studied to evaluate the stability of interconnected power systems. Most coherency methods developed for model reduction do not consider the effect of voltage variations on the coherency of generators due to weak coupling between real power and voltage. In reality, however, the coherency of generators can be influenced by voltage related factors such as location and speed of AVR controls, generator terminal voltage variation, and load bus voltage variation. The reduced dynamic equivalent is not accurate if the voltage characteristics of the original system are not included in the model. This paper presents coherency identification techniques incorporating rotor and voltage dynamics in order to improve the accuracy of dynamic equivalents. This paper also presents a new method to match the power flow conditions for generator aggregation. This new technique is shown to be critical for the accuracy of aggregated dynamic models. Numerical results based on the model of an actual power system in Asia are presented to demonstrate the performance of the proposed method.

A self-tuning power system stabilizer based on artificial neural network

This paper presents a systematic approach for designing a self-tuning power system stabilizer (PSS) based on artificial neural network (ANN). An ANN is used for self-tuning the parameters of PSS in real-time. The nodes in the input layer of the ANN receive generator terminal active power ( P), reactive power ( Q), and voltage ( V t), while the nodes in the output layer provide the optimum PSS parameters, e.g. stabilizing gain ( K STAB), time constants ( T 1 and T 2). A new approach for the selection of number of neurons in the hidden layer has been proposed. Investigations reveal that the dynamic performance of the system with self-tuning PSS based on ANN (ST-ANNPSS) is quite robust over a wide range of loading conditions and equivalent reactance, X e.

Intelligent dual input power system stabilizer

This paper presents a new approach for real-time tuning of a dual input power system stabilizer using neuro-fuzzy system (NFS). Intelligent dual input power system stabilizer (IDIPSS) comprises of NFS and conventional dual input power system stabilizer. The NFS is a fuzzy inference system implemented in the framework of multi-layered feed forward adaptive network. NFS network is trained using hybrid training algorithm for real-time tuning of the dual input power system stabilizer. The generator real power ( P e), reactive power ( Q e), and terminal voltage ( V t) characterizing the operating condition are input signals to the network while optimum DIPSS parameters K S1 and T 1 are the outputs. Investigations have been carried out considering three, five and seven membership functions (MFs) of Triangular, Trapezoidal, and Gaussian shapes. Studies reveal that for real-time tuning of the dual input PSS, the NFS network with three MFs of any shape is adequate. The proposed IDIPSS exhibits quite a robust performance to wide variations in loading condition, system parameters and large perturbations.

A novel modelling approach for decentralized voltage and speed control of multi-machine power systems

This paper proposes a novel modelling approach of multi-machine power systems for the design of decentralized multi-variable voltage and speed regulators. In this approach, each generator views the rest of the network as effective instantaneous impedance. The new model contains time-varying parameters representing the interactions among the system generators. The time-varying parameters depend only on the operating conditions of the power system and are independent of the structure of the latter. The advantages of this model are its simplicity, the fact that each generator's terminal voltage and rotor speed are expressed only in terms of local measurable variables and its capability to model inter-machines interactions. A decentralized multi-input multi-output (MIMO) non-linear voltage and speed regulator based on the new model is then used to dampen oscillations in a four-machine power system and to illustrate the advantages of the proposed modelling approach. Simulation results show that voltage, rotor and inter-area oscillations are well damped.

Linear‐programming‐based method for optimum schedule of reactive power sources in integrated AC‐DC power systems

AbstractThis paper is aimed at obtaining the optimal flow of reactive power which corresponds to minimum real power losses in integrated AC‐DC power systems including one DC link. The DC power or DC current, drawn by the link at its rectifier side is introduced as a new control variable added to the normal control variables, i.e. transformer tap‐settings, generator terminal voltages and reactive‐power outputs, and switchable reactive power sources. The constraints include the reactive power limits of the generators, limits on the load bus voltages and the operating limits of the control variables. Dual linear programming is applied to minimize an objective function for system losses. Application of the proposed method on different test AC‐DC systems confirmed that less system losses is achieved with the introduction of the DC control variable.

Transient stability preventive control for stable operating condition with desired CCT

In recent years, power systems have become larger and more complicated. Deregulation of electric power industries has begun in some countries, and power system operations have become more difficult. However, the stability of the power supply is the most important consideration for customers. Therefore, power system operators should consider not only economic load dispatch but also on-line stability aspects. In this paper, the authors pay attention to one of the most important transient stability indices [critical clearing time (CCT)] and propose a new transient stability preventive control method using linear relationships between CCTs and generator rotor angles. At first, CCT calculations, as contingency screenings, are carried out to find contingencies that have smaller CCTs than predefined target values. The target values are taken as larger values than the actual circuit breaker operating times in the power system. A preventive control to achieve a more stable power system operating point is carried out by generator output rescheduling and generator terminal voltage control, which are determined using the relationships between CCTs and generator rotor angles. This proposed method is demonstrated using a Japanese standard power system called the IEE of Japan (IEEJ) EAST 10 machine system.