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High performance reactive control for unbalanced three‐phase load
Abstract
In this paper we consider the problem of balancing a three-phase load and how to optimize the TCR operation. For an unbalanced load change a VAT technique is developed by the authors to determine the three compensating susceptance values and then the unsymmetrical firing angles of TCR, which are necessary for a balanced load operation. An objective function (THD) is determined to measure the discontinuity of the TCR operation. For an unbalanced load change, the availability of TSC and the control of TCR produce different values of reactive volt–amperes, in which all produce the balanced operation but with different amounts of harmonics. The paper introduces and develops an iterative algorithm to obtain the optimum firing angle values of TCR, and this is based on minimum generation of harmonics. The results show that a modulated THD is achieved, and this approach guarantees the high performance reactive control for unbalanced three-phase load. Copyright © 2009 John Wiley & Sons, Ltd.
... Te authors in [19] have proposed an algorithm for voltage unbalance detection based on Clark transformation, which converts the three-phase voltages into complex zplane. Te static volt-ampere-reactor compensative (SVC) has the ability to balance the loads and stabilize the electrical power system [20][21][22]. Te authors in [23] used the SVC to balance the load voltages and improve the power factor. ...
In this paper, an intelligent integrated approach is proposed to control the reactive power and restore the voltage balance in a three-phase power system using particle swarm optimization (PSO), Gaussian process regression (GPR), and support vector machine (SVM). The PSO algorithm is used in offline mode to determine the optimal set of firing angles for the thyristor-controlled-reactor (TCR) compensator according to the smallest fitness value required for voltage balancing. The optimum firing angles are then used to train the GPR and SVM regression models. The GPR and SVM models are finally used as a real-time controller to retrieve the voltage balance in online mode. A simulation model and experimental setup of the electrical power system are built. The modeled system consists of a 500 km long transmission line. The line is divided into three-pi sections to guarantee a real system response. Several simulation and practical case studies have been conducted to test and validate the capability of the proposed integrated approach in solving the voltage unbalance problem. The results have revealed the supreme ability of the proposed integrated approach to restore the voltage balance quickly (within 20 ms) and for a wide range of voltage unbalance factors (VUFs) (3.90–8.42%).
... Employing the static volt-ampere-reactor compensation (SVC) in the electric power system provides many advantages such as voltage regulation and load balancing, and also enhances the system stability [22]- [24]. To balance three-phase load currents, the SVC needs to absorb a particular amount of positive or negative reactive power to produce zero resultant reactive power at the SVC load common point [25]. ...
In this paper, the problem of voltage unbalance in the three-phase power systems is examined. A fast detection technique (FDT) is proposed to detect the voltage unbalance precisely and speedily. The well-known detection methods require more than one cycle time to detect the unbalanced voltages, whereas the proposed technique detects the unbalanced situations speedily in a discrete manner. Reducing the time duration required to detect the unbalanced voltages will enhance the dynamic response of the control system used to balance these voltages. The FDT acquires the instantaneous values of the three load voltages, calculates the sum and the space vector for these voltages at each sample, and utilizes these parameters to detect the voltage unbalance accurately within a quarter of the cycle time. A proof-of-concept simulation model for a real power system has been built. The parameters of the aqaba-qatrana-south amman (AQSA) Jordanian power system are considered in the simulation model. Also, several test cases have been conducted to test and validate the capabilities of the proposed technique.
... To ensure a stable operation of the electrical power system and conforming to the allowable voltage deviation standards, an external control on reactive power flow is required, based on Static VAR Compensation (SVC) [7][8][9]. A dynamic voltage restorer technique was used in [10] to compensate the voltage variations due to electrical load changes. ...
In this work, a Particle Swarm Optimization (PSO) technique is proposed to determine the optimal firing angles of the Thyristor-Controlled Reactor (TCR) to regulate the voltage of the electrical power system. A 500 km-length electrical power system is considered. The transmission line is modeled by three pi-section networks each represents a distance of 500/3 km. The mathematical model of the electrical power system is derived and used by the PSO algorithm to find the required TCR firing angles for voltage regulation. Different test cases have been conducted to assess and validate the proposed PSO technique capabilities. The results have revealed the ability of the proposed PSO technique to regulate the load voltage efficiently to average load voltage change equals 0.271%.
... Another, technique that provides a satisfactory performance in load balancing is the employment of static Volt-Ampere Reactive (VAR) compensators and Static Synchronous Compensators (STATCOM) which are widely used for unbalance mitigation [13]- [15]. Enhancement of Static-VAR Compensator (SVC) performance based on minimizing harmonics generated by the compensator was discussed in [16]. It is proposed to reduce the Third Harmonic Distortion (THD) by introducing a new technique called Iterative Volt Ampere Technique (IVAT). ...
... Many research efforts have been directed toward estimating, detecting and correcting the voltage unbalance conditions [12,13]. The authors in [14] assigned the location of individual lines causing voltage unbalance by determining the line coupling impedance between the positive and negative sequence networks. ...
A three-phase unbalance compensation system based on magnetic control transformer is proposed, changes the three-phase system from unbalance to balance. Firstly, the working principle of the magnetic control transformer and the balance compensation principle based on the magnetic control transformer are analyzed. Secondly, the situation of three-phase unbalanced load is analyzed. The three-phase unbalanced system at the low voltage side becomes balanced at the grid side, and the power factor of the system is compensated to 1. Finally, a three-phase unbalance simulation model was built in Matlab, the simulation results show that the combination of the magnetic control transformer and the fixed capacitor bank can effectively eliminate the negative sequence current, compensate the reactive power of the system, improve the power factor, and balance the three-phase current.KeywordsMagnetic control transformerThree-phase imbalanceNegative-sequence current
In this paper, genetic algorithm and commutation switch based three-phase imbalanced distribution network are studied. A genetic algorithm-based commutation switch three-phase imbalance management strategy was developed, and the three-phase current distribution station area was established. A mathematical model was developed for the multi-objective optimal commutation based on the minimum imbalance degree and the minimum switching times of the low-voltage load online automatic commutation device during the commutation process. Additionally, the real-time online governance control strategies were balanced. Finally, the effectiveness of the proposed method was verified using simulation studies.
The current management of three-phase imbalance of distribution transformers mainly relies on distribution operation and inspection personnel to determine the phase sequence switching of load according to their own experience, which is a large workload and low precision of phase sequence adjustment, and cannot meet the increasingly expanding scale of power grid. There is an urgent need for a research solution that can accurately judge the phase of station users and timely adjust the station users with heavier phase sequence to another phase sequence with lighter load for power supply, so that the three-phase load can be balanced. To this end, a solution is proposed to manage the three-phase imbalance of distribution transformers by intelligently identifying the phase sequence of station users through genetic algorithm. The example shows that the method is easy to use and can effectively guide the distribution operation and inspection personnel to accurately determine the phase sequence of station users and reduce the three-phase imbalance of distribution transformers.
In this work, a wide range of reactive power compensation is achieved for voltage unbalance mitigation in 500 km electrical power systems. An Optimal Control Technique (OCT) is proposed to encompass the unbalance load changes at a wide range of Voltage Unbalance Factor (VUF), between 3.33% and 12.4601%, and to minimize it to an acceptable value (at average less than 2%). The technique uses a combination of Particle Swarm Optimization (PSO) and Artificial Neural Networks (ANN) in three stages. In the first stage, the PSO finds the optimal firing angles of the Thyristor Controlled Reactor (TCR) and the optimal number of bank capacitors for the Thyristor Switched Capacitor (TSC) to restore the voltage balance. In the second stage, the voltage unbalance evaluations obtained by the PSO algorithm are used to train the ANN. In the third stage, the ANN is connected to the system to control and overcome the voltage unbalance problem accurately and quickly. Results are compared with other techniques available in the literature to confirm the superiority of the OCT performance. Furthermore, a laboratory model for the electrical power system is built and the proposed OCT for real voltage unbalance mitigation is validated.
In order to solve the problem of unbalanced three-phase load in low-voltage distribution network, the method of using three-phase load automatic adjustment device which is the type of the change-phase switch and the intelligent change-phase terminal to govern the three-phase load imbalance in the distribution station area is studied. First of all, make a load balancing real-time control strategy, choose suitable objective function and optimal algorithm. In order to verify the effectiveness of the algorithm, establishing a distribution station area model and use genetic algorithm to calculate the optimal change-phase instruction which can reduce the three-phase load imbalance based on the simulated distribution network load data. Afterwards make use of the change-phase control switch and the intelligent change-phase terminal to form a three-phase real-time load balance control system, make a real-time continuous governance of three-phase load imbalance, the results of experiments verify the effectiveness of the control system.KeywordsThree-phase load imbalanceChange-phase switchGenetic algorithmIntelligent change-phase terminal
With the wide application of doubly-fed induction generator (DFIG), accurate calculation and analysis of the short circuit currents of DFIG with crowbar protection has become very important key to realizing the low voltage ride through (LVRT) capability of DFIG and establishing the corresponding grid protection scheme. In view of this, a method to calculate the short circuit currents of DFIG with crowbar protection when different types of fault occur in the grid is proposed in this paper. First, the DFIG stator and rotor 2nd-order differential equations in the cases of grid symmetrical fault and asymmetrical fault are established respectively. And the short circuit current calculation model when crowbar resistance remains unchanged in the fault duration is obtained. On this basis, the short circuit current calculation model when crowbar resistance varies in the fault duration is derived. And then, the influence of crowbar resistance and DFIG rotor speed on the short circuit current calculation model is analyzed, including the variation pattern of flux eigenvalues with the crowbar resistance, and the variation feature of flux amplitude and phase angle with the rotor speed. Besides, the transient variation characteristics of different flux components are studied. Finally, time-domain simulation tests verify that the proposed calculation method is correct and applicable to different types of crowbar circuits in the market. Besides, the influence of crowbar resistance and DFIG rotor speed on the stator short circuit current peak, peak time and current frequency in different fault cases is revealed. 2017
Voltage regulation at weak distribution networks and feeders imposes several challenges. The degree of complexities of the overall design and implementation is network and site dependent. In this paper, design and implementation of a thyristor controlled reactor–based static VAr compensator (SVC) system to enhance voltage regulation and avoid voltage instability are presented and discussed. The SVC system is installed for an extremely weak distribution network with long feeders in a remote area in the Kingdom of Saudi Arabia. This work is the first implementation of an SVC system in the utility's distribution network in Kingdom of Saudi Arabia. Principles of operation, overall design assessment, and protection modifications in the installation area are described. Power stage and digital control system are custom designed according to the specific network and operation requirements. The overall system is examined under steady-state and dynamic conditions. Power system at the corresponding area with/without the compensator is simulated using PSCAD/EMTDC software. Field test results are also obtained, and the performance of the system is discussed along with the design considerations required for such installations. The various technical discussions presented in this paper highlight several practical considerations and practices that are valid in similar installations for extremely weak distribution networks around the world.
This paper presents a new method named Sampled, Data and Space Vector Technique (SDSV) to control the threephase, voltages subjected to load changes. The proposed SDSV, uses the on record and measured three-phase voltages to obtain, the reference space vector (SVr) and measured one (SVm), respectively at different samples. The two space vectors SVr and, SVm are compared at regular samples of the system sinusoidal, cycle, to generate an error. This error is employed to produce the, new reactive power of compensator in a small period of time, rather than waiting for a complete system sinusoidal cycle, thereby allowing a quick and an efficient Volt-Ampere Reactive, (VAR) control for the three-phase load voltages.
An improved tracking technique of the reference current waveform in a shunt type active power filter is proposed in this paper using sliding mode control and feedback linearization. Feedback linearization approach helps to reduce the complexity of the controller. Excellent tracking is achieved in both for steady state and dynamic performance. A three-phase four-wire system is considered. The proposed algorithm is simulated first in MATLAB/SIMULINK. An experimental prototype using a dspace1104 based controller is also produced in the laboratory. Results from simulation match well with the corresponding results from experimental prototype confirming the usefulness of the proposed technique.
In order to make a better compensation for the three-phase unbalanced load, the unbalanced load was analyzed and a method of reactive compensation admittance calculation based on balance component method was proposed. Any neutral non-grounded three-phase load can be decomposed into a group of balanced load and single line load, and the load compensation admittance can be obtained by the decomposition of the three-phase unbalance load. The load compensation admittance calculation can be divided into three parts, the power factor compensation for the balanced load and the single line load together with the balanced compensation for the single line resistive load. Then, the ideal Steinmetz compensation network algorithm was amended under the condition of asymmetric voltage. A freedom degree existed in this case, and the asymmetry voltage compensation admittance can be accurately calculated according to the control objectives and the revised ideal compensation network calculation method. The rationality and validity of the method was verified using example and simulation. Compared with existing methods, the proposed method has easy calculation and convenient realization.
A method for real-time online control of three-phase unbalanced load in distribution area was proposed in this paper. The general idea was presented firstly, and then the implementations of online automatic commutation device and control terminal for low-voltage load were described in details. In order to pursue the lowest unbalance radio of three-phase current and the least switching frequency of online automatic commutation device during the commutation process, a multi-objective mathematical model for optimal commutation was established. Then the control strategies for real-time online management can be obtained based on vector-genetic optimization algorithm. The simulation results prove the validity and feasibility of the proposed method.
This paper explores and compares the performance of alternative voltage control strategies applied to doubly fed induction generator (DFIG). Different combinations of reactive power control of rotor- and grid-side converters are investigated for voltage-control purposes. Simulations are performed using detailed models built in DIgSILENT PowerFactory in order to illustrate the influence of controllers on transient stability and steady-state operation of the DFIG-based wind plant. This paper also proposes appropriate control strategies for different sets of network operating conditions and topologies. Operational limits, such as current margins and pulse-width modulation limits, are also taken into account.
In this paper, the practical impedance approach steady-state analysis in the frequency domain for the three-phase self-excited induction generator (SEIG) with squirrel-cage rotor is presented along with its operating performance evaluations. The three-phase SEIG is driven by a variable-speed prime mover(VSPM) in addition to a constant-speed prime mover (CSPM) such as a wind turbine and a micro gas turbine for clean alternative renewable energy in rural areas. The basic steady-state characteristics of the VSPM are considered in the three-phase SEIG approximate equivalent circuit and the operating performance of the three-phase SEIG coupled with a VSPM and/or a CSPM are evaluated and discussed online under the conditions related to the speed changes of the prime mover and the electrical inductive load power variations with simple computation processing procedures. A three-phase SEIG prototype setup with a VSPM is implemented for small-scale clean renewable and alternative energy utilizations. The experimental performance results give good agreement with those obtained from the simulation results. Furthermore, a proportional-integral (PI) closed-loop feedback voltage regulation of the three-phase SEIG driven by the VSPM on the basis of the static var compensator (SVC) composed of the thyristor phase-controlled reactor in parallel with the thyristor switched capacitor and the fixed-excitation capacitor bank is designed and considered for the wind generation as a renewable power conditioner. The simulation analysis and experimental results obtained from the three-phase SEIG with SVC for its voltage regulation prove the practical effectiveness of the additional SVC with the PI-controller-based feedback loop in steady-state operation in terms of high performance with low cost.
The purpose of this book is to describe the theory of Digital Power Electronics and its applications. The authors apply digital control theory to power electronics in a manner thoroughly different from the traditional, analog control scheme. In order to apply digital control theory to power electronics, the authors define a number of new parameters, including the energy factor, pumping energy, stored energy, time constant, and damping time constant. These parameters differ from traditional parameters such as the power factor, power transfer efficiency, ripple factor, and total harmonic distortion. These new parameters result in the definition of new mathematical modeling: A zero-order-hold (ZOH) is used to simulate all AC/DC rectifiers. A first-order-hold (FOH) is used to simulate all DC/AC inverters. A second-order-hold (SOH) is used to simulate all DC/DC converters. A first-order-hold (FOH) is used to simulate all AC/AC (AC/DC/AC) converters.
Various static devices for raising the stability limit of a power system are examined with respect to their employment at or near the electrical center of a two-machine system. Among these devices are series capacitors, shunt capacitors, combined series and shunt capacitors, and shunt static reactive compensators consisting of a fixed capacitor and a variable reactor. New concepts of current gain, voltage gain, and power gain are defined and illustrated. The natural frequencies of each circuit are examined, and it is shown that with series capacitors these frequencies are subsynchronous and hence produce a risk of subsynchronous resonance, whereas all shunt-connected devices give supersynchronous natural frequencies and therefore do not involve such a risk.
In this paper, a modified shunt active filter is used under unbalanced three-wire supply voltages with non-linear loads. The main objective of the proposed shunt active filter is to control the three-phase supply currents both in its waveform, magnitude and phase to follow three-phase reference signals. The reference signals are derived from one of the phase voltages. When this is achieved, ideally the supply current will then be always sinusoidal, with robust control over its magnitude and phase, irrespective of the harmonics and unbalance of the load demand or the supply voltage system. This confirms nearly unity power factor on supply side and reactive compensation by the active filter in static-compensator mode. The proposed shunt active filter control strategy is based on hysteresis control, which is very simple to implement and does not even require any measurement of the load demand. The controller performance is improved by incorporating an additional loop that can respond to the variation in the DC bus voltage due to changes in the real and reactive power demand. This paper presents the detailed design analysis of the proposed active filter with simulation and test results obtained from the laboratory implementation.
The operation of thyristor-controlled static VAR compensators (SVCs) at various conduction angles can be used advantageously to meet the unablanced reactive power demands in a system. However, such operation introduces harmonic currents into the AC system. This paper presents an algorithm to evaluate an optimum combination of the phase-wise reactive power generations from SVC and balanced reactive power supply from the AC system, based on the defined performance indices, namely, the telephone influence factor (TIF), the total harmonic current factor (IT) and the distortion factor (D). Results of the studies conducted on a typical distribution system are presented and discussed.
This paper characterizes the operation of a thyristor controlled reactor (TCR) which consists of an inductance and a bi-directional thyristor switch connected in parallel. Firing angle control of the thyristor switches regulates the time for which the inductance is included in the circuit. This controls the average value of the inductance. The continuously regulated inductance can be used for line flow regulation and short circuit current reduction. It can also be used for series compensation when the TCR is connected in series with a fixed capacitor and when the combination is inserted in series with transmission lines. This paper analysis the operation of the TCR using equations and EMTP simulations.The analysis shows that a series compensator using TCR regulates the level of compensation from inductive to capacitive and the system response is faster than the recently commercialized ‘Advanced Series Compensation’ (ASC) scheme. Also, the problems of resonance and harmonic instability associated with the ASC are eliminated. The TCR can control short circuit current within half cycle.
Series compensation can substantially increase the transmission capability of EHV (extra-high-voltage) systems, permitting a higher loading per right of way. Several of the possible circuit arrangements that can be used for insertion of series capacitors into a transmission system are cited in this paper. Certain technical problems of using series capacitors are analyzed. The effects of fault switching time on stability limit for series-compensated circuits and of delaying reinsertion of series capacitors after fault clearing are indicated for particular configurations. The relative economics of various degrees of series compensation and number of intermediate switching stations for 500 kv and distances up to 400 miles are also shown.
The paper presents a comparison of the admittance models for static reactive-power compensators along with a microcomputer-based digital model for power-system dynamic-performance improvement. Emphasis is placed on stability, and it is shown that static reactive-power (VAr) controllers (SVCs) can provide significant benefits in terms of increased transient stability limits and improved damping in terms of synchronising power-flow oscillations. Further introduction of a simplified self tuner for the SVC can dramatically increase the power-system performance under a variety of abnormal operating conditions, such as 3-phase short circuits, load shedding, reactance switching, pole slipping and resynchronisation conditions. The self-tuning digital control for the SVC can be easily implemented on a microcomputer. Simulation results for a sample power system using an LSI-11/23 microcomputer are presented in the paper.
Two basic schemes for thyristor-controlled static compensators are described, namely, thyristorswitched capacitors (TSC) and thyristor-controlled reactors (TCR). A more advanced scheme using a combination of TSC and TCR is presented. It is shown that this combination gives a greater degree of flexibility in the designing of a compensator. The paper also briefly describes the control system in a compensator comprising both thyristor.controlled reactors, thyristor-switched capacitors and reactors (TSR). One method to damp power oscillations, using TSC with a certain control strategy, is presented. The problem of unbalanced loads and load balancing methods using TSC and TCR aie discussed. It is shown that the combined system (TSC/TCR) generates very low harmonics, because the TSC does not generate any harmonics. Finally, some pictures from the first installations comprising both TSC and TCR are shown
A fast calculation method to calculate the firing angle of a thyristor-controlled reactor (TCR) and/or the number of thyristor-switched reactor (TSR) ‘switch-ons’ and/or the number of thyristor-switched capacitor (TSC) ‘switch-ons’ for four types of static VAR compensator (SVC) is proposed in this paper. The calculation is an optimal compensation problem that is written in terms of the minimization of power line loss by minimizing the line currents. The three-phase unbalanced line currents after installation of an optimal reactive power compensator will become three-phase balanced line currents.In this paper, theoretical derivations and the computation procedures are formulated. A numerical example for real-time simulation is discussed to demonstrate the performance of the proposed method and to support the application capability.
In this paper, the steady-state analysis of the three-phase self-excited induction generator (SEIG) is presented. The three-phase SEIG with a squirrel cage rotor is driven by a variable-speed prime mover (VSPM) or a constant-speed prime mover (CSPM) such as a wind turbine or a micro gas turbine. Furthermore, a PI closed-loop feedback voltage regulation scheme of the three-phase SEIG driven by a VSPM on the basis of the static VAR compensator (SVC) is designed and evaluated for the stand-alone AC and DC power applications. The simulation and experimental results prove the practical effectiveness of the additional SVC with the PI controller-based feedback loop in terms of the fast responses and the high performances.
In this paper, the steady-state operating performances of single-phase squirrel-cage self-excited induction generator (SEIG) are evaluated by using the per-unit frequency in addition to a simple steady-state analysis based on the per-unit slip frequency. The single-phase SEIG operating performances, estimated by the per-unit slip frequency steady-state analysis at the prime move speed and the inductive load power variations, are discussed with those obtained by using the per-unit frequency. The results provide close agreements with a simple analysis and an efficient computation processing procedures. Moreover, the single-phase static VAr compensator (SVC) composed of the thyristor controlled reactor, the thyristor switched capacitor and the fixed excitation capacitor is applied to regulate smoothly the generated output voltage of the stand-alone single-phase induction generator with a variable inductive load. The fixed gain PI controller is employed to adjust the equivalent capacitance of the single-phase SVC. The experimental and simulation results are illustrated the practical effectiveness of the additional SVC with the PI controller-based feedback loop to regulate smoothly the output voltage of the single-phase SEIG.
In recent years, there has been a rapid increase in the number of thyristor-controlled shunt compensators used in industrial and utility systems for dynamic power factor correction and terminal voltage stabilization. These thyristor-controlled shunt compensators function as variable reactances operated in both the inductive and capacitive domains.
The development of new static VAr compensator (SVC) models and their implementation in a power flow program is presented. The modelling is carried out in the phase domain considering the SVC delta-connected physical structure. The SVC is assumed to be a continuous, variable susceptance, which is adjusted in order to achieve a specified nodal voltage magnitude in an adaptive fashion. A polyphase Newton-Raphson power flow program is modified in order to implement the proposed models. Validation of these models is carried out in a benchmark network operating under balanced conditions. SVC performance is assessed in both balanced and unbalanced power network operating conditions. The power flow implementation exhibits a very strong convergence characteristic regardless of the network size.
The RMS value of the unbalanced and reactive currents in
three-phase unbalanced power systems can be reduced with a balancing
compensator that has an adaptive property. Circuits that provide a
controllable susceptance are the main components of these compensators.
They can be built from thyristors, switched inductors and reactive
elements. The susceptance of such circuits is referred to as `thyristor
controlled susceptance' (TCS) in the paper. The harmonic currents
generated by switched thyristors and injected into the supply cause a
waveform distortion, however. They have to be often operated, moreover,
at distorted supply voltage, and a resonance with the system inductance
may occur for harmonic frequencies. As a consequence, the load balancing
and power factor improvement are obtained at the expense of an increase
in waveform distortion. This distortion can be reduced if the structure
of the circuit that provides the TCS is properly selected. In this
paper, various circuits that can provide the TCS are analysed and
compared with respect to generated harmonic currents and frequency
properties
Static VAr compensators with thyristor-controlled reactors
generate harmonics. Due to the voltage regulation characteristics of the
compensator, the generated harmonics depend on unknown thyristor firing
angles and the network load flow conditions. The inclusion of load flow
dependent firing angle determination in the author's computer-based
multiphase harmonic load flow (MHLF) technique (see ibid., vol.6, no.1,
p.174-82, 1991) is described. It uses a novel conduction angle
adjustment scheme. This general and simple scheme can easily be used for
the analysis of other nonlinear elements with control characteristics.
Various case studies are presented to demonstrate the performance and
application of the technique. Field test comparisons are included
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