Brushless Exciters Research Articles

Measurement and Analysis of Rotating Magnetic Field in Multiphase Annular Brushless Exciters With Armature Winding Short-Circuit Faults

Measurement and Analysis of Rotating Magnetic Field in Multiphase Annular Brushless Exciters With Armature Winding Short-Circuit Faults

Multi-Parametric Condition Monitoring of Medium-Power Generators with Brushless Exciters under Mechanical Faults

Multi-Parametric Condition Monitoring of Medium-Power Generators with Brushless Exciters under Mechanical Faults

Modeling, Analysis, and Identification of Armature Winding Interturn Fault in Multiphase Brushless Exciters

As the core of large-capacity nuclear power generation system, the multiphase annular brushless exciter (MPABE) may cause a serious catastrophe to the nuclear power plant if it fails. In order to effectively protect the armature winding of MPABE, a rapid simulation model and a sensitive diagnosis method of armature winding short-circuit (AWSC) fault are studied systematically in this article. First, this article proposes a general and rapid simulation model of MPABE with AWSC fault. To verify the correctness of this model, corresponding simulations and experiments of AWSC faults are carried out, respectively, through which the fault characteristics are obtained. Then, a detailed mechanism analysis is presented, which explains the induction process of the related harmonics in the stator and rotor currents. The variation law of the characteristics value when the number of short-circuited coils changes is also analyzed. Finally, a new method to diagnose AWSC fault of the MPABE is proposed and then optimized. The test results on the prototype show that this method can sensitively diagnose even the slightest AWSC fault. Therefore, it can provide a reliable basis for the monitoring and protection of MPABE in nuclear power plants.

Double Layer Capacitive Power and Heat Transfer in Rotating Machinery

Doubly fed induction machines, wound field synchronous machines and dc homopolar machines all require a brushed slip ring connection to the rotor conductors. This connection inherently limits the performance and/or lifespan of these machines. Noncontact methods, such as brushless exciters and wireless power transfer, have been used to support some of these machine types, but they can be physically large and/or overly reliant on power electronics. Additionally, these methods do not support the cooling of the rotor conductors since their function is purely electrical. This article introduces a rotating electric double layer capacitor concept, or supercapacitor, to form a noncontact, slip-ring-like, ac electrical connection between a stator and rotor that simultaneously provides a cooling platform. An experimental proof-of-concept consists of concentric stainless-steel sleeves separated by a ceramic bearing to facilitate relative rotary motion but maintain constant facing area and gap distance. Electrolyte floods the space between the sleeves to form double layer capacitances and can also serve as a heat transfer mechanism. The sleeves were tested as bare metal and with an activated carbon coating to enhance capacitance further. Measurements demonstrate coupling capacitances up to Farad levels with three to six orders of magnitude improvement in A/Hz over rotary air capacitors, enabling low frequency operation.

Balancing of Common DC-Bus Parallel-Connected Modular Inductive Power Transfer Systems

The aim of this article is to design a modular, fault-tolerant multi-transmitter (Tx)/multi-receiver (Rx) parallel-connected common dc-bus inductive power transfer (IPT) system to replace slip rings in wind turbines or brushless exciters. In parallel-connected common dc-bus systems, current unbalance is a major issue that results in thermal stresses and over current or voltages. In this article, two different new current balancing methods are proposed: cross-coupled Rx modules and intentional detuning of Rx-side resonant frequency. These methods are investigated both analytically and experimentally for a single Tx and two Rx system for a 500-W prototype. The proposed methods are tested independently, and then, the combined current balancing method is also investigated. For the same misalignment case, cross coupled/detuned has a 55.8% current balancing improvement compared with decoupled/fully tuned topology.

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.

On the Electromagnetic Performance Prediction of Turbo Synchronous Condensers Based on Wound-Field Flux Switching Machine Design

The revival of the old synchronous condenser (SC) technology is presently occurring. Hence, this study is proposed on the electromagnetic design and evaluation of turbo wound-field flux switching machines (WF-FSMs) for SC operation, for the first time. Unlike classical wound-field rotating machines, WF-FSMs do not need brushless exciters while possessing a robust topology. A design process based on the frozen permeability technique in static 2-D finite element analysis script-based package, which enhances both iterative and analytic adjudications, is proposed to accurately predict the SC operation. To attain the specific design requirements (~300 MVAR, 16 kV), a global-optimized model (GOM) of the SC machine is obtained through a stochastic optimization process involving multiple objectives. Based on the GOM, simple torque ripple minimization techniques are implemented, from which a final benchmark design is selected. The proposed SC analytical design procedure is then tested and the expected V-curves clearly obtained. The whole design approach is further verified by experimenting on a small-scale prototype, which when synchronized to the grid, confirms the novelty and feasibility of the proposed SC design and operation.

Online Measurement Based Joint Parameter Estimation of Synchronous Generator and Exciter

In this article, an online approach for joint parameter estimation of the exciter and Synchronous Generator (SG) is proposed. It only utilizes the Phasor Measurement Unit (PMU) based measurements, which can be obtained at the SG's terminal and does not require field measurements (as in brushless exciters). Compared to existing joint estimation approaches, the proposed work includes multiple aspects to put forward a pragmatic estimation framework. Firstly, the unobservable erroneous initial states are duly considered by utilizing the proposed Augmented Parameter Vector (APV). It is an essential feature in practical scenarios as an accurate representation of a dynamic system entails knowledge of true initial states and parameters. Secondly, suitable models of SG and exciter necessitate the inclusion of saturation characteristics, which is also considered in present work. Further, the estimation procedure is independent of multiple pre-tuned parameters, as such dependence may lead to tedious implementation and inaccurate estimates. Additionally, Wavelet denoising is used to improve the parameter estimation accuracy by pre-processing the noisy measurements prior to estimation. Real-Time Digital Simulator (RTDS) based simulation results, considering noise and measurement rate analogous to commercial synchrophasors, demonstrate the proposed approach's improved accuracy.

Estimation Algorithm for Current and Temperature of Field Winding in Electrically Excited Synchronous Machines With High-Frequency Brushless Exciters

Electrically excited synchronous machines have become one of the potential alternatives to permanent magnet synchronous machines in electric vehicles to avoid rare-earth materials. Utilizing high-frequency brushless exciters for rotor excitation is a promising choice to reduce friction losses and maintenance effort and cost. However, with the usage of brushless exciters, the field current and temperature cannot be measured directly. In this article, an algorithm is proposed to dynamically estimate the field current as well as the field winding temperature. The dc-link current is utilized as a feedback to correct the estimation. The performance of the estimation algorithm is initially evaluated in simulations and then verified by experimental measurements in a prototype. Based on the estimation algorithm, closed-loop control of the field current is developed and verified experimentally.

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 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.

An Online Diagnostic Method for Rotary Diode Open-Circuit Faults in Brushless Exciters

A rotating diode failure is the most common electrical fault in brushless exciters. It brings a safety hazard to the generator excitation system so that the whole unit is forced to be shut down. It is necessary to detect and deal with this fault accurately. According to the electrical structure characteristics of brushless exciters, this paper analyzes the influence of an open circuit rotating diode on the armature current and armature magnetic field. It is proposed here that we install the magnetic field detection coil on the stator yoke and use the rotational frequency component in the induced voltage to diagnose the open circuit fault in the rotating diode online. In this paper, a two-dimensional model of a 22-pole 39-phase brushless exciter is established, and the new detection method is verified using a finite element simulation. This provides a new solution for rotating diode fault diagnosis in brushless exciters. The detection method exhibits good performance in terms of practicability, real-time usage, and versatility.

Synchronous Generator Brushless Field Excitation and Voltage Regulation via Capacitive Coupling Through Journal Bearings

Wound field synchronous generators (WFSG) are the standard electromechanical converter for back-up and utility scale power generation. Maintenance costs may be minimized by adopting noncontact or “brushless” technologies to replace sliding slip ring connections for rotor field excitation. This paper presents a brushless excitation approach using ceramic insulated sleeve (journal) bearings with oil lubrication to form capacitively coupled slip rings, in contrast to more traditional inductive brushless exciters and rotary transformers. This capacitive power transfer (CPT) approach exhibits advantages including low weight, low volume, and has a relatively simple construction using off-the-shelf components. Analysis, design, and prototype construction of the CPT system are presented. Experimental results demonstrate that 1.7 nF of capacitive coupling transfers 340 W to the rotor field winding of a 10 kW 208 V WFSG. Voltage regulation of a WFSG is demonstrated during steady state and 1 per unit load step changes yielding a NEMA-MG1 class G2 rating.

Design and Characterization of a Rotating Brushless Outer Pole PM Exciter for a Synchronous Generator

Generally, PM machines are used as PMG pre-exciters in 3-stage brushless excitations systems. This paper presents the design, characterization, and prototyping of a rotating brushless PM exciter used in a proposed 2-stage excitation system for a synchronous generator. The proposed design reduces the number of components compared with conventional systems. A comparison with the state-of-the-art conventional excitation systems is given. The design of a fast-response, or high initial response, brushless exciter requires active rectification on the rotating frame, replacing the noncontrollable diode bridge. The objective was to construct an exciter with the capability of a 50 A output field current, as well as a high value of the available ceiling voltage and ceiling current. The final exciter was constructed to be fitted into an in-house synchronous generator test setup. A finite element model of the exciter was validated with experimental measurements. The exciter prototype is also compared with an alternative armature design with nonoverlapping single-layer concentrated windings, but with the same main dimensions. The paper includes a general design procedure suitable for optimization of PM brushless exciters that fulfill the requirements of their synchronous generators and the grid.

Testing of Active Rectification Topologies on a Six-Phase Rotating Brushless Outer Pole PM Exciter

The static exciter is dominating among large grid-connected generators due to the weak dynamic performance of conventional brushless exciters. In this contribution, a six-phase double-star outer pole permanent magnet rotating brushless exciter is evaluated with different active rectification topologies. Both thyristor-based and chopper-based topologies are considered. A high speed response brushless excitation system is obtained by replacing the conventional rotating diode bridge rectifier with the proposed active rectification topologies on the shaft. The given two-stage system generates its own excitation power directly from the shaft, contrary to static exciters. The selection of an appropriate rectification topology could minimize the rotor armature phase currents for a given generator field current. The objective is a high-power factor and a high utilization of the exciter machine. An optimal rectification topology makes higher ceiling currents was possible, improving the transient behavior of the synchronous generator. In this paper, we show that six-phase topologies add complexity, but improve exciter redundancy, increase the available ceiling voltage, and reduce the steady-state torque ripple. Experimental results are given for validating the models implemented for the analysis.

Failure Modes Demonstration and Redundant Postfault Operation of Rotating Thyristor Rectifiers on Brushless Dual-Star Exciters

The excitation system plays a critical role in the operation of synchronous generators. An equipment failure could impact the voltage quality for smaller grids. Furthermore, it can lead to cost penalties and reduced production for the power plant owner. Recently, a new high-speed-response rotating brushless exciter was developed that employs remote control of the rotating thyristors on the generator shaft. This has led to new possibilities for improving the performance of brushless exciters. This contribution investigates the failure modes of a dual-star outer pole exciter that feeds two separate thyristor bridges connected in parallel during a normal operation. The possibility of a redundant postfault operation due to open-thyristor or open-phase faults is demonstrated using experimental testing. The system is compared with the fault performance of a conventional three-phase system. This paper includes the implementation and validation of a fault-predicting double d–q exciter model. In addition, the dangerous effects of a shorted-thyristor fault are investigated. A “skip firing” protection technique is briefly demonstrated for the fast isolation of such faults, yielding nondestructive postfault recovery and redundant failure-mode operation. The evidence shows that the dual-star exciter is a competitive choice for the future development of fault-tolerant brushless exciters.

Comparison of Thyristor-Controlled Rectification Topologies for a Six-Phase Rotating Brushless Permanent Magnet Exciter

The thyristor bridge rectifier has proven to be a reliable solution regarding control of excitation equipment for synchronous generators. However, in rotating brushless exciters, the diode rectifier is the dominant topology on the shaft. In order to improve the step response of rotating exciters, one could put a thyristor bridge rectifier on the rotating part and control the firing angle remotely from a stationary controller. This paper compares different multiphase configurations of permanent magnet synchronous machines as a rotating exciter and discusses the possibility to reduce the torque ripple by selecting the appropriate rectification topology. The paper also explains the implications of the self and mutual inductances of the armature windings for the performance of the exciter.

Simulation of Magnetically Triggered ${\rm MgB}_{2}$ Switches

Superconducting synchronous machines require Direct Current (DC) excitation for their field windings which can be achieved via brushless exciters or by direct connection to current leads. However, the resulting heat loads limit their application. An excitation system consisting of a High-temperature Superconducting (HTS) full wave rectified flux pump eliminates these issues and enables operation at high currents. Rectification is achieved through superconducting magnetic switches, which also allow persistent current operation. Initial experimental results demonstrated that magnetic switching in MgB2 can be achieved without any significant temperature increase, enabling a fast recovery of the switches to the superconducting state. An empirical model has been developed which allows estimating MgB2 switch performance over the operational range needed for rotating machine excitation. The model uses measured critical current densities and n-values of commercially available MgB2 conductor and enables the design of optimized switches for field winding excitation and other applications.

Current flow and losses in brushless exciters with polygon-connected windings and dc rectifiers

The current flow within a salient pole brushless exciter with polygon-connected winding is calculated. The winding is connected to a rectifier, which causes transients in the winding during switching. Brushless exciters are over-dimensioned for their rated operation. During ceiling conditions the required voltage may be two times higher and the exciter is strongly saturated. Therefore the normal asynchronous circuit theory is no longer applicable for a detailed study within the exciter itself. A nonlinear finite difference time stepping method with rotating rotor and diode circuits is applied instead. Basic theories for the combination between the circuit theory and field theory are summarized in one mathematical model. Calculation results are compared to measurements on a real power plant exciter for different operation conditions.

Transfer Function Model of a Brushless Exciter

Brushless exciters are widely used in power stations to furnish the excitation current for the main alternators. These exciters are maintenance free and possess high gain and forcing margin. Transfer function models available for such exciters are somewhat complicated and deal with few variables. The objective of this paper is to develop a detailed transfer function model relating all the important exciter variables and covering the operation in a wide range of frequency. Starting with a state space mathematical model based upon a direct-phase circuit model for the exciter, various frequency responses are obtained. These are used to derive the transfer function model.