Brushless Excitation System Research Articles

Analysis and Monitoring of the Fuse Conditions in Nuclear Power Multiphase Brushless Excitation System

Analysis and Monitoring of the Fuse Conditions in Nuclear Power Multiphase Brushless Excitation System

Mutual Inductance Identification and Bilateral Cooperation Control Strategy for MCR-BE System

Considering that the excitation method of an electric excitation synchronous motor has the disadvantages of the brush and slip ring, this article proposes a new brushless excitation system, which includes two parts: a wireless charging system and a motor. To meet the requirements of maximum transmission efficiency and constant voltage output of the system, a bilateral cooperation control strategy is proposed. For the strategy, the buck converter in the receiving side of the system can maintain maximum transmission efficiency through impedance matching, while the inverter in the transmitting side can keep the output voltage constant through phase shift modulation. In the control process, considering that the offset of coupling coils will affect the control results, a grey wolf optimization–particle swarm optimization algorithm is proposed to identify mutual inductance. Simulation and experimental results show that this identification algorithm can improve the identification accuracy and maximize the avoidance of falling into local optima. The final experimental result shows that the bilateral cooperation control strategy can maintain the output voltage around 48 V and the transmission efficiency around 84.5%, which meets the expected requirements.

Synchronous Generator Feeding DC Grid Having Brushless Excitation System With Induction Machine Operating in Deep Plugging Mode

This paper proposes a novel excitation control for a recently proposed synchronous generator (SG) entitled Brushless Induction Excited Synchronous Generator (BINSYG) for wind energy applications. In BINSYG, one SG and another Induction machine (IM) are embedded in the same machine frame to make the overall system brushless. Here, IM acts as the excitation machine and regulates the dc excitation for SG by rectifying the rotor-induced currents through a rotating rectifier. SG feeds power to dc grid via a two-level six-switch Voltage Source Rectifier that is operated at unity power factor at the SG terminal. The excitation developed by IM is regulated using a new frequency modulation control strategy for a two-level Voltage Source Inverter connected to the stator of IM. The excitation machine helps operate SG at variable wind speeds with reduced flux distortions. The proposed system is speed/position sensorless and controlled without using any voltage or current sensors on the IM side. The system with its proposed control algorithm is modeled in MATLAB/Simulink and experimental verification for a small-scale laboratory prototype BINSYG feeding power to a 220 V dc grid are presented to validate the effectiveness of the controller.

Rotating Rectifier Fault Diagnosis of Nuclear Multiphase Brushless Excitation System Based on DTW Metric and kNN Classifier

Multi-phase annular brushless excitation system is widely used in large-capacity nuclear power units. Accurate fault diagnosis of the rotating rectifier is of great significance to improve the reliability of the excitation system. However, the traditional diagnosis method based on harmonic analysis of stator field current has some deficiencies. In this paper, a novel diagnosis method using field current waveforms and artificial intelligence is presented. Firstly, the various shape features in field current waveforms of the different rotating rectifier faults are analyzed. Then, the field current waveforms are used as the input of a hybrid algorithm based on the dynamic time warping (DTW) metric and the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k</i> -Nearest Neighbors ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k</i> NN) classifier (DTW- <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k</i> NN). That is, a DTW metric is used to calculate the distance among the shape features in field current waveforms and <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k</i> NN classifier is used to diagnose the specific rotating rectifier fault. Finally, experiments on an 11-phase prototype prove the effectiveness of the hybrid method DTW- <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k</i> NN. It is worth mentioning that an improved training set including all trends of field current waveforms should be selected to avoid the asymmetry between each pair of field poles. The learning method provides a new idea for fault diagnosis of the rotating rectifier.

Condition Monitoring of Rotating Diodes in Synchronous Machines Through the Exciter Stray Flux Analysis

Brushless excitation systems have many advantages mainly related to the low maintenance. A failure in the rotating diodes leads to an increase in the exciter field current and an unbalance in the currents supplied by the exciter armature. This produces additional losses in the exciter and, in some cases, severe damage in the machine if operated for a long time under fault conditions. The supervision of the condition of the rotating diodes is, therefore, essential for the protection of the exciter and the machine. In this paper, monitoring of the diodes is carried out via the analysis of the exciter´s stray flux. Numerous experimental tests have been performed on a synchronous machine with axial and radial flux measurements under healthy condition, under one open diode fault and finally under a shorted diode. The experimental results prove that a failure in the diodes can be reliably detected due to the axial and radial stray flux signatures which significantly increase in comparison to the healthy condition.

A Simplified Analytical Approach for Hybrid Exciters of Wound-Field Generators

Midpower wound-field generators are predominantly used for grid applications, marine naval ships, industrial buildings, and critical standby power applications. They are often equipped with brushless excitation systems (ESs), where an exciter supplies the generator through a diode rectifier. Here, the uncontrolled nature of the rectification process can determine a significant reduction of the mean value of the output voltage. Hence, the root causes must be identified and properly modeled for an accurate evaluation of the system performance. In this article, a simplified analytical model is proposed for such purposes, taking as case study a consequent-pole, hybrid exciter of a wound-field generator for naval applications. The approach is based on the calculation of the diodes’ commutation angle, which permits to estimate the exciter’s armature current waveforms. Special focus is given to the voltage waveforms, which are determined using the estimated current waveforms as an input, thus removing the need for a numerical resolution. In addition, simplified equivalent functions calculated without relying upon finite-element (FE) analyses are employed to determine the machine inductances. Being fully released from any numerical computation, the proposed model permits a fast but accurate evaluation of the exciter performance. For validation purposes, FE and experimental investigations are finally performed.

A Novel Field Current Estimation Method for Brushless Wound-Field Synchronous Machine

Compared with other electric machines, wound-field synchronous machines (WFSMs) have the advantage of having three control variables (stator <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$d$ </tex-math></inline-formula> - and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$q$ </tex-math></inline-formula> -axis currents and rotor current). Determining the optimum combination of field and armature currents is necessary for ensuring the high-performance control of the WFSM. Two methods are usually employed to supply the field winding of the WFSM. The first one uses brushes and slip rings, while the second one uses a brushless excitation system. Concerning the WFSM with a brushless excitation system, a dc field current is provided by a rotating diode bridge rectifier fed by an exciter machine. Therefore, direct measurement of the field current is not possible. As a solution, field current needs to be accurately estimated. Previously, field current estimation methods proposed in the literature have been based on modeling the brushless exciter machine and the rotating rectifier. Using the brushless exciter machine model, these methods estimate electric quantities of the exciter rotor (currents and voltages); then, through the rotating rectifier model, the rectified field current is estimated. Accordingly, any inaccuracy in the models leads to estimation errors. The effectiveness of the techniques relying on the brushless excitation system model is not clearly justified, especially because the nonlinearity of the brushless system has been intensified by the existence of the rotating uncontrolled diode bridge rectifier. This article proposes an accurate online field current estimation method for WFSMs without the need neither to estimate exciter rotor voltages or currents nor to use a rotating rectifier model. The accuracy and effectiveness of the proposed field current estimation method are demonstrated by experimental validation tests.

Research on the Brushless Excitation System with Linear Current Amplifier Characteristics

In terms of improving the dynamic response characteristics of brushless AC synchronous generators, the concept of integrated design of AC exciter and rotating rectifier and their electromagnetic characteristics are studied, and the conditions and judgment methods for determining their operating modes are obtained in theoretical design and simulation design, pointing out that when the brushless excitation system operates in or is close to mode III, i.e., the rectifier always has three or four diodes in each operating cycle. The linear current amplifier characteristics can be basically achieved when the brushless excitation system is operated in mode III or is close to mode III, i.e., the rectifier always has three or four diodes alternating in each operating cycle. The impact of the key parameters of the AC exciter at different rotational speeds and temperatures on the characteristics of the linear current amplifier was explored, and an engineering prototype was manufactured. The related test verified the correctness of the concept of integrated electromagnetic design and simulation calculation. This study provides an engineered design method for enhancing the dynamic response properties of the brushless AC synchronous generators.

A new approach to parameter identification of generation unit equipped with brushless exciter using estimated field voltage

Parameter estimation of power plants is one of the main challenges of power system studies. Among different components of a power plant, excitation system (EXS) has great importance because of its effect on dynamic stability of power systems. Thus, it is vital to have accurate models of EXSs for power system dynamic studies. Since the field voltage and current are not accessible in brushless EXSs, parameter estimation of them is more difficult and challenging. Therefore, a new method is proposed in this paper to estimate field voltage signal using other measurements of the synchronous generator (SG). The proposed method is carried out through three stages; 1) parameter estimation of the SG using load rejection tests, 2) field voltage estimation, and 3) parameter estimation of EXS. The proposed method is applied to a 147 MVA industrial gas unit. The estimated model outputs are compared to the experimental results to show the accuracy and effectiveness of the proposed method.

Alternative Test Methods for Monitoring the Condition of Brushless Exciters in Synchronous Machines

Although brushless excitation systems in synchronous machines have reduced the maintenance requirements and brush-related failures, they have increased the operating stresses on the excitation system components. Since the rotating excitation system is not accessible, it is difficult to detect excitation system failures such as open circuit diodes or open armature windings to prevent degradation in machine performance and reliability. Detection of such failures currently relies on monitoring the magnitude and harmonic content of the exciter field current. However, it is desirable to improve the reliability of fault detection with multiple test methods, considering the high cost of brushless exciter testing and repair due to the requirement of machine outage/disassembly. In this work, alternative off-line testing and on-line monitoring options are proposed for detection of excitation system failures. An off-line test based on ac excitation of the exciter field and on-line monitoring based on stator current and airgap flux analysis, are investigated, and fault indicators sensitive to excitation system failures are proposed for reliable fault monitoring. An experimental study on 3.8, 5.5, and 30 kW synchronous machines is given to validate the claims on the sensitivity and reliability of the proposed test methods.

Estimation of Damage Rotor Shaft of Turbogenerators with a Diode Brushless Excitation System

The damage to the rotor shafts of high-power turbo generators is estimated as a result of the resonant in-teraction between the generator and the exciter caused by the operation of the automatic excitation regulator (AER) by mathematical modeling. The analysis covers turbo generators with a capacity of 300 to 1000 MW with a diode brushless excitation system (DBES). The simulated circuit includes a turbo generator with a block transformer, a power transmission line, an exciter in the form of a reversed synchronous generator with a rotat-ing rectifier diode converter unit, a static thyristor converter, and a sub-exciter in the form of a synchronous machine with permanent magnets. When modeling the electrical part, an approach is used from the positions of its own coordinates, which ensures maximum methodological consistency of the models of the listed devices and allows directly reproducing resonant phenomena at torsional vibration frequencies with the determination of instantaneous values of currents, voltages and electromagnetic moments of the turbo generator and exciter. The mechanical system is presented taking into account the exciter and sub-exciter as a seven-mass system. AER is introduced into the mathematical model by means of transfer functions with corresponding coefficients and time constants. The control system of the thyristor converter is represented in the model by a generator of line-arly increasing signals, a body for comparing these signals with the signal from the AER and a control pulse generator. To assess the damage rate, the deformation criterion for soft and hard loads in the zone of low-cycle fatigue and the force criterion in the zone of multi-cycle fatigue were used. A comparative quantitative assess-ment of the damage in the neck of the G-E shaft line in the resonant interaction between the exciter and the generator with different automatic excitation control systems is given. The influence of attenuation of electro-magnetic transients and damping of torsional vibrations on the damage values is analyzed. The results ob-tained can be used to analyze the functioning and determine the settings of the AER.

Methods and Means of Improving the Dynamic Characteristics of Brushless Excitation Systems

Possibilities of improving the dynamic characteristics of brushless excitation systems are considered. A technique for choosing the value of the additional resistance in the excitation winding of a reverse synchronous generator, the gain of the hard negative feedback and the excitation-forcing ratio of the exciter by voltage has been developed. The reasons for the appearance of high-frequency pulsations in the excitation voltage of a reverse synchronous generator are considered. The technique was applied when choosing the means of improving the dynamic characteristics of the BVD-4600-1500 brushless exciter, operating with the TVV-1000 – 4 generator.

A Consequent-Pole Hybrid Exciter for Synchronous Generators

In low-to-medium power generating sets, a self-powered brushless excitation system is typically employed. This solution is cost-effective, simple and compact, but it suffers from an unreliable voltage build-up at start-up, a slow dynamic response and a relatively low efficiency for the exciter. The push towards more effective, reliable and efficient products has recently led to consider excitation systems equipped with permanent magnet exciters and controlled rotating converters, but their diffusion is limited by their higher complexity and cost. This article investigates the utilization of a hybrid excitation for the exciter, aiming to join the benefits of field windings and permanent magnets. As a case study, this concept is applied to a commercial mid-size generating set adopting an industrial perspective, aiming to maximize the benefits while minimizing the required modifications in the system design. After a preliminary analysis, a consequent-pole layout with surface-mounted bonded magnets is then selected as the most effective solution. Theoretical considerations, numerical analysis and experimental validation are reported to show that the hybrid excitation concept can actually lead to a significant reduction of the exciter field losses as well as to other appreciable side benefits with a very limited impact on the present design of the generating set.

Analysis of the Interturn Short Circuits of Stator Field Windings in Multiphase Angular Brushless Exciter at Nuclear Power Plant

An excitation system with a multiphase angular brushless exciter is the preferred nuclear power excitation scheme because of its low output voltage ripple. However, interturn short circuits of the stator field windings (ISCFW) often occur in the brushless exciter, affecting the normal operation of the system. To study this type of fault, this paper presents research on theoretical analysis, simulation calculation, and model machine experiments. First, through the analysis of the magnetomotive force produced by windings and their interaction in the air gap under fault, the frequency domain characteristics of the postfault steady-state current are obtained. A general multiloop-method-based mathematical model of multiphase angular brushless excitation system with ISCFW is established. Correlated simulations and experiments with ISCFW are carried out, verifying the result of the theoretical analysis: m / P times harmonics are in the field current, while all harmonics are in the armature current. The general theoretical analysis method and mathematical modeling method proposed in this paper provide a basis for the protection scheme to rapidly remove ISCFW that occurs in the multiphase angular brushless exciter.

Unified Reduced Model for a Dual-Control Scheme of the High-Speed Response Brushless Excitation System of Synchronous Generators

Developments of the high-speed response brushless excitation systems (HSRBESs) are ongoing in the power industry. This is because the transient response of the excitation system (ES) is a key performance indicator for the grid owner. In dealing with this problem, accurate prediction and control of the ES ceiling voltage are desirable. However, the brushless exciters’ nonlinear armature reaction causes the ceiling voltage to be unknown under varying operating conditions. This article proposes a numerical average-value model (AVM) that captures all the main dynamics of the HSRBES. It is shown that the AVM relationships can be utilized for online prediction of the ceiling voltage and employed in a dual-control scheme. The proposed model is validated against a dynamic voltage build-up test. Moreover, it is derived from a detailed model, which is verified from instantaneous field measurements and further from finite-element analysis. Finally, the accuracy and effectiveness of the AVMs’ transient relationships prove the feasibility of the proposed dual-control scheme and shows that it can be easily implemented in existing systems.

Modeling and Simulation of Excitation System for Third Harmonic Brushless Synchronous Generator

The excitation system of third harmonic excitation brushless synchronous generator (HESG) can improve the voltage stability and is widely used in small and medium generator set. In order to establish the model of harmonic brushless excitation system, the formula of third harmonic EMF is derived. Then, based on the traditional synchronous generator model, the influence of harmonic windings is counted in. Finally, the simulation model of closed-loop excitation system is analyzed with the automatic voltage regulator (AVR) as the core. On the basis of theoretical analysis, simulation and comparative analysis are carried out according to parameters from actual system. The results show that compared with normal synchronous generator, HESG has the advantage of excellent dynamic characteristics and automatic voltage adjustment.

Controlled Brushless De-Excitation Structure for Synchronous Generators

The main weakness of the brushless excitation system in a synchronous generator (SG) is the slow de-excitation response obtained during a load rejection. That is why voltage overshoots may be observed on the generator terminals. This behavior is mainly due to the exciter machine response time and the rotating diode bridge which is not able to quickly de-excite the generator by negative excitation voltages. This paper presents a new brushless de-excitation structure able to perform a quick de-excitation of the generator by providing controlled negative excitation voltage to the generator main field winding. The proposed structure is based on a new brushless de-excitation machine, called a control machine, and mounted on the same shaft of the generator and the brushless exciter. The brushless control machine is a low power one and used to transfer the orders from the voltage regulator to the discharge system located on the rotor side of the main generator. The dynamic performance of the proposed de-excitation system is evaluated in terms of system stability, voltage regulation response times and voltage overshoots during different load rejection tests. The proposed system is compared to the conventional brushless excitation system without the proposed de-excitation structure. In addition, a comparison is done with the static excitation system. The simulation tests are realized on an experimentally validated model of 11kVA synchronous generator developed in Matlab/Simulink.

Modeling and Experimental Verification of High-Frequency Inductive Brushless Exciter for Electrically Excited Synchronous Machines

Electrically excited synchronous machines have shown potential to be an alternative to permanent magnet synchronous machines in electromobility and wind power applications. High-frequency wireless power transferring technology enables a compact design of brushless exciters for the machine. In this paper, a dynamic model of high-frequency brushless exciters is proposed for the purposes of operating condition monitoring and excitation control. The modeling is done by using arithmetic and differential equations as well as considering different operation modes of the system. The operation modes are defined based on the physical behaviors of the excitation circuit. Experiments are performed to verify the model with variations of different circuit parameters. Furthermore, parameter sensitivity study, component parameter selection, and loss analysis are conducted to demonstrate the effectiveness of the model. The model is therefore proposed as an effective tool to assist the design and optimization of the brushless excitation system.

Detection of Rotating Diode Failure Condition &amp; its Protection in Brushless Alternator

Brushless excitation system is widely used in large capacity synchronous generators since it removes the usage of commutator, brushes and slip rings, hence reduces the losses, maintenance and increases reliability. Rotating Rectifier Assembly (RRA) is the main part of brushless alternator. Due to ageing phenomenon and continuous process regime, diodes in rotating rectifier can fail either due to Open Circuit (OC) or Short Circuit (SC), which overloads the exciter and hence the alternator can no longer run securely. If such condition is prolonged, the Automatic Voltage Regulator (AVR) as well as the exciter windings can be damaged.This paper presents two different methods of diode failure detection in brushless alternators. First method uses an algorithm based on output voltage and the second uses the ripple factor of the exciter field current. Diode failure condition is detected for different type of loads connected to 4 kVA, 380 V, 50 Hz, 4 poles generator with 14 pole exciter (brushless alternator) and the results are verified using MATLAB/Simulink. Also, the protection schemes for rotating diode assembly as well as exciter field windings are presented using Metal Oxide Varistor (MOV) and Discharge Resistor.

Research on an online diagnosis for rotating diode faults in three‐phase brushless exciter with two coils

The rotating rectifier is prone to be damaged in the exciter of the brushless excitation system, wherein a diode open-circuit fault which is common can affect the normal and safe operations of a generator. On the basis of the structure of the brushless exciter, this study, analysed the effect that an open-circuit in one and two diodes had on armature current and armature field, and put forward a new online detection method for diode open-circuit fault. In this method, two magnetic field detection coils are installed on the stator iron yoke of the brushless exciter, and the open-circuit fault of the diodes can be reflected by the induced voltage difference of the two detection coils. Using a 16-pole, three-phase brushless exciter as an example, the performance of this new diagnosis method was verified by using a finite element simulation.