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The pole drop test is the most common test for detecting inter-turn insulation failure in the rotor field winding of salient pole synchronous machines. Shorted field turns are detected by comparing the voltage drop across each rotor pole with ac voltage applied to the field winding terminals. It is a simple and effective test that is usually performed with the rotor inserted for convenience and high sensitivity. In this work, the influence of rotor eccentricity on the pole drop test is analyzed. It is shown that anomalies in the airgap can interfere with the pole drop test to cause false positive indications. Alternative test options immune to false indications are also suggested in this letter. The analysis and test options for screening false indications due to eccentricity are verified on 4 and 6 pole, 5, 5.5, and 30 kVA synchronous machines under emulated fault conditions. It is shown that eccentricity interferes with the pole drop test and that testing under dc excitation is immune to eccentricity although the sensitivity is lower, which are not reported elsewhere. [ABSTRACT FROM AUTHOR]

The air-cooled turbo-generator often works in the more frequent start-up and shut-down operation. Either the misoperation or the failure of synchronizing controllers can cause the out of phase synchronization fault. The fault can disturb the power angle and cause a strong impact electromagnetic force for the generator. With the traditional numerical calculation method, it is difficult to calculate the variations of key physical parameters and the electromagnetic field in the end region of the generator under complex working conditions, especially the out of phase synchronization fault. In order to solve the problem, a calculation method of the nonlinear transient electromagnetic field with multi-domain field-circuit-grid coupled is proposed. The correctness of the model and method is validated by comparing the calculated and test values of the magnetic density flux at the key points of the generator under the no-load condition. Based on this method, the power angle variation, the magnetic flux leakage distribution in the end region, and the electromagnetic force on the end winding in the transient process of the fault are calculated and analyzed. [ABSTRACT FROM AUTHOR]

The energy transition is aimed to take advantage of the operational flexibility of hydropower to extend the integration of intermittent renewable sources. Consequently, the hydrogenerators will have to operate in regimes far away from their designed best-point operation. In order to accurately assess the impact, this paper presents a useful approach to determine the overall operating efficiency of synchronous generators under intermittent operation. An accumulated average efficiency (AAE) model is proposed and compared against an alternative approach; the weighted average efficiency (WAE) model. It is found that the WAE approach produces unrealistic low efficiencies when the generator operates in synchronous condenser mode (SCM) for long periods. In general, the AAE supersedes the WAE for all the different load distributions that were investigated. This was further illustrated by a worked example and by constructing more complex load distributions. A load distribution dominated by SCM yields a difference as high as $33.18 \,\%$ , while an even distribution deviates $1.43 \,\%$ in their respective efficiencies. Finally, a yearly on-site measurement of our studied $103 \,\mathrm{MVA}$ generator’s concentrated load distribution revealed a discrepancy of $0.67 \,\%$ , which is a significant deviation considering what the operating regime would mean in terms of economic implications. [ABSTRACT FROM AUTHOR]

Virtual synchronous generator (VSG)-based control approaches for AC machine drivers have attracted much attention in recent years. The contradiction between the strong motor loading capacity and the small DC voltage fluctuation is disregarded in previous studies. A larger virtual inertia is required to raise the torque gain and drive the motor with heavier loads, thus resulting in greater DC voltage variations. A systematic method is proposed for the optimal design of the virtual inertia for VSG-based motor starting systems to achieve the maximal motor loading capacity and the low DC voltage fluctuation in this paper. Firstly, the relationship between the virtual inertia and the loading capacity is revealed on the basis of the closed-loop analysis. Considering the DC voltage variation, the constraint on the maximal value of the virtual inertia is proposed from the energy-balance perspective despite the unknown contribution of the DC capacitors to the virtual inertia. To fully exploit the energy storage capacity of the DC capacitors and raise the upper limit of the virtual inertia under the same permitted DC voltage variation, a modified DC voltage reference is introduced. Extensive simulation and experimental results are presented to validate the effectiveness of the proposed methods. [ABSTRACT FROM AUTHOR]

The conventional model predictive power control suffers from heavy computational burden and large power ripple. To address the problem, this paper proposes a simplified three-vector-based model predictive direct power control, which can greatly optimize the control set. Firstly, a switching table is designed to exclude the redundant virtual voltage vectors (VVs). Only half of the candidate VVs are retained. Then, the effect of each VV on power is analyzed in detail. The deviation of reactive power is used as the evidence to further simplify the control set. In this way, the prediction behavior is reduced from 12 to 2, which effectively reduces the computational burden. In addition, the concept of power deadbeat is employed to determine the duty cycles of the two virtual VVs and one zero VV. Thus, the power ripples and current harmonics can be considerably suppressed. With this proposed method, the calculation complexity is reduced, and the steady-state performance is improved. The simulation and experimental results are given to verify the proposed method. [ABSTRACT FROM AUTHOR]

This paper presents the variable speed applications of a recently developed generator named Brushless Induction Excited Synchronous Generator (BINSYG) for offshore wind power applications. The machine is basically a Synchronous generator (SG) with its excitation fed from a three-phase Induction Machine (IM). Both the machines are embedded in the same machine frame and share the same magnetic core. The SG feeds power to the dc grid via a five-level Unidirectional Dual Flying Capacitor Active Neutral Point Converter. The converter is controlled such that the Total Harmonic Distortion (THD) of the current flowing through the stator of SG is below 5% and at unity power factor at SG terminal. The excitation developed by IM-type exciter is adjusted using a new control strategy that is proposed for Voltage Source Inverter, such that BINSYG can operate at wide speed range necessary for Wind Energy Systems (WES). The complete system is controlled without using ac voltage sensors and speed encoders. The proposed WES with its control algorithm is first modelled in MATLAB/Simulink. Further, rapid control prototyping verification in the form of real-time results using OPAL-RT is presented for a small scale (5.5 kVA) BINSYG feeding power to a 750 V dc grid. [ABSTRACT FROM AUTHOR]

In this paper, the transient stability for dc-link voltage of doubly-fed induction generator (DFIG)-based wind turbine (WT) is studied in detail during low voltage ride-through (LVRT). Firstly, referring to the rotor swing equation of synchronous generator (SG), the nonlinear large-signal model of dc-link voltage is established. In addition, the damping power, static slip power, and dynamic slip power are derived. Consequently, the transient performance of dc-link voltage under the different operation conditions of DFIG is revealed during LVRT. Furthermore, the instability form of dc-link voltage with the moving process of equivalent power angle is analyzed by the energy function of dc-link voltage. In addition, the impacts of dc voltage control loop's parameter, the active current of the stator, and the slip on the stabilization process of dc-link voltage are studied. Analysis result indicates that the insufficient damping and large unbalanced power would deteriorate the transient behavior and steady-state level of dc-link voltage. Therefore, an additional damping and slip power feedforward control strategy is proposed, which can make the transition process of dc-link voltage smoother and significantly improve the voltage steady-state level. Finally, simulation and experimental results validate the effectiveness of theoretical analysis. [ABSTRACT FROM AUTHOR]

This paper investigates the transient stability of power systems co-dominated by different types of grid-forming (GFM) devices. Synchronous generators (SGs and VSGs) and droop-controlled inverters are typical GFM devices in modern power systems. SGs/VSGs are able to provide inertia while droop-controlled inverters are generally inertialess. The transient stability of power systems dominated by homogeneous GFM devices has been extensively studied. Regarding the hybrid system jointly dominated by heterogeneous GFM devices, the transient stability is rarely reported. This paper aims to fill this gap. It is found that the synchronization behavior of the hybrid system can be described by a second-order motion equation, resembling the swing equation of SGs. Moreover, two significant differences from conventional power systems are discovered. The first is that the droop control dramatically enhances the damping effect, greatly affecting the transient stability region. The second is that the frequency state variable exhibits a jump at the moment of fault disturbances, thus impacting the post-fault initial-state location and stability assessment. The underlying mechanism behind the two new characteristics is clarified and the impact on the transient stability performance is analyzed and verified. The findings provide new insights into transient stability of power systems hosting heterogeneous devices. [ABSTRACT FROM AUTHOR]

The virtual synchronous generator (VSG) control has been widely used in grid-connected inverter to effectively increase the equivalent inertia of power system. However, the active power oscillation similar to electromechanical transient characteristics of conventional synchronous generator (SG) is introduced to power grid, especially under the condition of multi-virtual synchronizer. In this paper, the dynamic stability of the multiple grid-connected voltage source converters (VSC) controlled by VSG is studied. Firstly, the system linearization model is established to analyze the oscillation risk. In order to improve the damping characteristics of multi-VSG grid-connected systems, an additional power system stabilizer (PSS) method is proposed. Then considering the influence of controller installation location and control parameters on the dynamic stability of the system, the design methods of multi-VSG damping controller based on residue index and hybrid particle swarm optimization algorithm are proposed respectively. Finally, the effectiveness of the damping controller design method is verified through the simulation and test cases of the multi-VSG grid-connected system. [ABSTRACT FROM AUTHOR]

Frequency response analysis (FRA) is a popular tool to detect transformer winding deformation faults. It has been recently used to diagnose the winding inter-turn short circuit fault of the electric machine. The application of FRA is based on the fact that the winding can be equivalent to an electrical circuit consisting of resistance, inductance, and capacitance. Therefore, to obtain an accurate broadband equivalent circuit model of synchronous machine and perform the fault analysis and diagnosis, this paper introduces the gray box model technique based on the state space and genetic algorithm. The electrical circuit of a form-wound coil of a 4 kW machine with known electrical parameters is used to verify the proposed method primarily. Besides, the equivalent circuit modeling is carried out and experimentally verified on an actual 5 kW synchronous machine. Finally, the fault simulation analyses of winding inter-turn short circuits are performed based on the proposed equivalent electrical circuit. The verification result shows that the circuit parameters identified by the proposed method are accurate. Moreover, the fault simulation results indicate that the inter-turn short circuit faults can cause apparent variations in the frequency response curves. It is feasible to detect the winding fault of the synchronous machine by the FRA method. [ABSTRACT FROM AUTHOR]

Islanded microgrids have low inertia due to large penetration of non-inertial inverter based power sources. Such systems are prone to instability due to a higher rate of change of frequency (RoCoF) and large peak frequency deviation in case of a sudden load change or generation loss. This paper proposes a novel fast frequency control method for an islanded microgrid comprised of both inverter based distributed generators (DGs) and synchronous generators (SGs). The frequency controller estimates the power imbalance during the transient and compensates for it through a battery energy storage system (BESS). For the estimation of the power imbalance, an approximated linear model of the microgrid is developed considering multiple SGs operating in parallel. To transfer the compensated power from BESS to SGs, and also to maintain the state of charge (SoC) of the BESS, a SoC balancing control method is presented which is implemented by controlling the biased frequency of the SGs. The performance of the proposed frequency control method is verified using Typhoon real-time hardware-in-the-loop simulator. [ABSTRACT FROM AUTHOR]

Inverter based distributed generators (DGs) are increasingly being employed for interfacing renewable energy sources into the grid. Droop control is a very popular technique for ensuring parallel operation of these sources based on local measurements alone, in which the inverters are made to emulate the behaviour of synchronous generators of the power system. However, several drawbacks of the droop control technique are being addressed actively, over the years by various researchers. One of the prominent issues is the inaccurate power sharing among the distributed generators (DGs) due to the feeder/line impedances. The performance further deteriorates in the presence of unbalanced loads. This work presents an adaptive technique for adjusting the negative sequence (NS) virtual impedance of the DG, based on its output current, without the need of communication or extra sensors, to achieve enhanced unbalanced power sharing. The proposed control is shown to have the capability of seamless integration with the existing conventional and inverse droop techniques. The methodology is validated through simulations for different DG ratings, controls and on a modified 13 bus system. Small signal eigenvalue analysis about the operating point is used to determine the stable range of control parameters and the strategy is validated on an experimental setup in the laboratory. [ABSTRACT FROM AUTHOR]

For a marine electrical machine, its high power density and large electromagnetic (EM) load tend to bring high saturation to the magnetic circuit. When the non-linearity of ferromagnetic material is not taken into consideration, large errors will appear in the calculation of air-gap flux density and EM force wave. Considering the radial and tangential EM force waves that cause the stator core to vibrate and deform are the primary sources of EM vibration of a non-salient pole synchronous generator (NSPSG), this paper introduces a semi-analytical method of calculating the radial and tangential EM force waves, with the non-linearity of ferromagnetic material taken into account. The method includes the distributed magnetic circuit method (DMCM), the separation variable method (SVM) and the complex variable conformal transformation method (CVCTM). First of all, the radial air-gap flux density is obtained by using the DMCM to calculate the no-load magnetic circuit of the NSPSG and dealing with the non-linearity of ferromagnetic material by B-H curve. Then, the tangential air-gap flux density is calculated by the SVM and the complex relative air-gap permeance is calculated by the CVCTM to determine the effect of slotting. Finally, the radial and tangential EM force waves are obtained by Maxwell stress tensor method. The proposed semi-analytical method is verified through measuring vibration acceleration of the NSPSG. [ABSTRACT FROM AUTHOR]

This paper derives in analytical closed form the feasible pole-placement region of the synchronverter active-power loop (APL) so as to eliminate all trial-and-error effort in parameter tuning. We consider the well-established setting of a reduced third-order APL model with two controller parameters that can be tuned freely. Thus, only two of the three APL poles can be specified independently in hopes of achieving desired system dynamics. Central to the presented derivation is the realization that the two specified poles must represent the dominant mode of the system. Otherwise the actual system dynamics may be dictated by the third unspecified pole, leading to unexpected or undesired dynamic behaviour. Numerical simulations involving the full-order synchronverter dynamical model and a modified New England 39-bus test system validate the analysis and the resulting region within which poles must be placed for actual system dynamics to match desired ones. [ABSTRACT FROM AUTHOR]

A feasible online test procedure is proposed in this paper to estimate the full set parameters of synchronous generators (SG). The recommended test procedure involves applying step perturbations to one of the power system stabilizer (PSS) inputs like active power or rotor speed, such that the stator and field winding signals change considerably, which makes them suitable for parameter estimation purposes. Furthermore, an accurate approach is suggested to estimate SG's parameters by utilizing the recorded data from the proposed test procedure. The effect of magnetic saturation, which can influence the estimation results, is also taken into account continuously during the parameter estimation. Finally, the proposed test is applied to a 200 MVA gas unit SG, and the parameters are estimated using a Genetic algorithm (GA) from the measurement data. The results show the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

The power quality of virtual synchronous generator (VSG) inevitably deteriorates in the presence of local nonlinear loads and distorted grid. In this paper, the conflict involved in the simultaneous elimination of distortion for both the inverter local load voltage and the grid exchanged current is first described. A unified control structure is presented that enables a tunable tradeoff between the two constrained harmonic sources. Then, a hybrid harmonic suppression scheme is proposed to enable the further improvement of the adaptability of VSG, which mainly consists of a local voltage harmonic control loop and an adaptive grid current-controlled loop. The local voltage harmonic control loop aims to scale down the inverter output impedance via a negative feedback loop, while the grid current-controlled compensator is intended to counteract the adverse effects from a weak grid via an additional voltage, which leads to substantially lower total harmonic distortion for both the local load voltage and the grid current at the same time. Small-signal modelling is performed to investigate the system stability and its robustness to parameter perturbations. The effectiveness of the proposed methodology is verified using hardware-in-the-loop simulations. [ABSTRACT FROM AUTHOR]

In order to realize the grid-friendly wind turbine system (WTS), applying of the virtual synchronous generator (VSG) control strategy to WTS has become a trend of research, and it is also required to analyze the grid-connected operation characteristics of the VSG controlled WTS. In this paper, the impedance modeling and impedance-based stability analysis of the VSG controlled type-IV WTS are carried out. Different from existing impedance modeling method that the WTS is modeled by a high dimensional impedance model with complex expressions, a model to describe the shaping effects of dc-link dynamics is proposed, and combined with the space vector diagram to analyze the system impedance characteristics. It is found that the impedance of the VSG controlled type-IV can be capacitive below the fundamental frequency, which is inherited from its GSC impedance and the capacitive characteristics has been weakened by the influence of dc-link dynamics. Instable risk has also been found between the VSG controlled type-IV WTS and the connected weak grid, and the instability can be avoided if the capacitive characteristics of the VSG controlled type-IV WTS are weakened by reshaping its GSC impedance or by enhancing the dc-link dynamics. Experiments are also carried out as verifications. [ABSTRACT FROM AUTHOR]

The brushless excitation structure characteristics of the aviation wound-rotor synchronous starter/generator (WRSSG) put forward the requirement on the brushless exciter (BE) model that both the stator and rotor windings can be interfaced with the external circuit directly. In this paper, a double-side variable-parameter voltage-behind-reactance (DS-VP-VBR) model of the BE considering magnetic saturation was established firstly to realize the direct connection between the stator and rotor windings with external circuits. Then, a double-side constant-parameter VBR (DS-CP-VBR) model with completely decoupled RL circuit was developed through the algebraic transformation and numerical approximation from the DS-VP-VBR model. In order to further improve the model accuracy, the leakage inductance variation of the stator and rotor windings were also considered. Computer studies under steady-state and transient operating conditions verified the validity and accuracy of the proposed BE DS-VBR models. [ABSTRACT FROM AUTHOR]

The article presents a calculation of a turbogenerator rotor air-cooling system with self-ventilation of the rotor winding from a sub-slot duct. The mathematical model proposed in the article reflects all the significant geometric parameters of the physical model as well as the gas dynamics of the cooling process. Research has been done into the impact of the profile of a sub-slot duct along the length of the rotor on air velocity distribution in radial ducts and on the efficiency of heat transfer in a 320 MW turbogenerator. The research has also shown that the maximum temperature of the rotor winding may be lowered by means of a sub-slot duct smoothly tapering to the middle of the axial rotor length (17% of the inlet section area of the sub-slot duct). Such configuration compensates air heat up along the rotor sub-slot duct due to the forced unevenness of air distribution in the radial ducts. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

This article presents a potential wind energy conversion system (WECS) using the recently proposed brushless induction excited synchronous generator (BINSYG), which can operate as a self-excited synchronous-machine. In BINSYG, an induction-machine (IM) and a synchronous-machine (SM) are wound for different number of poles and are embedded in the same machine structure. By operating the SM in self-excited mode, dc-link voltage of grid-side inverter can be controlled smoothly from the stator side of the IM. A set of conditions has to be maintained for stable operation in self-excited mode. This is particularly critical when the machine needs to be operated in wide variation in speed. So far, no stability-study is available to explore the stable-operating-zone, when the machine operates in self-excited mode. Therefore, in this paper stability of self-excited BINSYG is investigated to check suitability of the proposed system for wind energy applications. The study clearly indicates the presence of a critical speed (i.e. a minimum speed below which self-excitation is not possible). The movement of eigen values are presented to show the influence of several parameters on stable behavior of the system. Simulations using PLECS and supporting experimental-results using a laboratory-prototype (configured for 4/6 poles, 5.5 kVA) have been presented. [ABSTRACT FROM AUTHOR]

This article presents a comprehensive analysis on the end winding vibration characteristics due to different stator inter-turn short circuit (SISC) positions in synchronous generators. Different from other studies, this article not only pays attention to the effect of SISC on the end winding electromagnetic forces, but also considers the impact of the short circuit position, which is related to the static air-gap eccentricity, on the end winding vibrations at the meantime. The whole work is based on the qualitatively theoretical analysis, the finite element analysis (FEA), and the experimental validation on a 6-pole synchronous generator. The study shows that SISC will considerably enlarge the electromagnetic force and intensify the radial vibration of the end winding, especially at 100 Hz, 200 Hz and 300 Hz. The closer the short circuit position is to the minimum air-gap point, the more severe the end winding vibration will be. [ABSTRACT FROM AUTHOR]

Emulation mechanisms of the physical synchronous generators obtained through convert-interfaced units are becoming extremely popular in literature. Still the impact of these strategies on networks frequency transients often does not take into account the effect of the converter DC-bus regulation, that plays a significant role in the dynamics definition. Thus two of the main synthetic inertia architectures are analysed both mathematically and experimentally in this article: differently from the available literature, the goal is not just to compare their macroscopic effects on network behaviour, but rather to identify the impact of the multiple internal control feedbacks on the inertia provision itself. [ABSTRACT FROM AUTHOR]

This article investigates the total demagnetization fault in Axial Flux Permanent Magnet (AFPM) synchronous generator, suitable for wind power applications, using the three-dimensional Finite Element Method (3D-FEM). The machine, in which the demagnetization diagnosis has been investigated, consists of one coreless stator with concentrating winding, embed in resin and two rotors. In the current study, faulty cases of different number and relative position of total demagnetized magnets are studied. Finally, simulation results are compared to experimental results for validation. The stator current spectrum is studied as fault identification mean and it is shown that the fault signatures depend on the number of total demagnetized magnets, the relative position of the magnets on the rotor and the stator winding topology. In addition, there is an investigation of the partial demagnetization fault for comparison purposes. Moreover, analytical expressions for the magnetic flux density in the healthy case and in every faulty case are exported. These novel expressions, related to every demagnetized case, are presented for the first time in the literature. [ABSTRACT FROM AUTHOR]

This article presents a comprehensive small signal model for a virtual synchronous generator (VSG) integrated into a microgrid. In the developed state-space, active and reactive power control loops of the VSG as well as resistances, and reactances of the lines are considered. Based on this model, the robust D-stable region in terms of different values of inertia constant (M), and damping coefficient (D) values of the VSG is obtained. To enhance both transient performance as well as guarantee the robust D-stable operation of VSG, an optimization problem is also proposed. Moreover, a new control method is suggested to damp frequency oscillations of the microgrid. The proposed controller is a state feedback which uses the pole placement method and can be used in both of the grid-connected, and isolating modes of operation. In this controller, poles can be placed in the desired locations in order for the microgrid to achieve the desired transient response. In order to access all state variables, a state observer is used to avoid measurement of state variables. Accordingly, one of the features of the proposed method is its ability to move poles without requiring additional sensors. The performance of the proposed method is evaluated in MATLAB/SIMULINK, which shows enhanced dynamic behavior of the microgrid. [ABSTRACT FROM AUTHOR]

This paper proposes a novel simulation method to analyze the characteristics of the electromagnetic noise of claw pole alternators under stator winding interturn short circuit faults. For the two types of faults: short circuit between two wires of the same phase and short circuit between turns of the same wire, a general circuit model built in Matlab/Simulink was introduced to obtained the phase current and output current signals. The phase current was then used as current source and the claw pole alternator was simulated as an electric motor rather than a generator to analyze the magnetic force and electromagnetic noise by using 3-D finite element method and boundary-element method, respectively. A noise bench test was conducted to verify the simulation results. It is shown that under the stator winding interturn short circuit faults, the output current harmonics and electromagnetic noise of the three-phase 12-pole/36-slot claw pole alternator would increase greatly. The 12th order noise replaces the 36th order noise as the most dominant order noise at low speed. The method proposed in this paper can also be used to diagnose the stator winding interturn short circuit faults of claw pole alternators. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

In virtual synchronous generator (VSG)-related discussions, the inner voltage and current controls are always omitted due to their fast dynamics. However, converters require a lower switching frequency as their power rating increases, which constrains the cascaded controls’ bandwidths. Hence, the interactions between the controls at different time-scales should be analyzed. Specifically, the aim is to reveal the effects of inner control loops on the VSG system from the torque perspective. First, this paper derives a linearized model of the internal voltage phase and amplitude, which accurately depicts the VSG system characteristics. Subsequently, based on this linearized model, we derive an equivalent single-input single-output motion-equation- based model. The proposed model can be used to investigate the change in damping and synchronizing torque components under low control bandwidths and weak grid conditions. Finally, simulation results are presented to validate the proposed modeling and effects analysis. Our model is general and can be used for any power ratings, and the study also provides a guide to designing control parameters for VSG systems. [ABSTRACT FROM AUTHOR]

In this article, the transient synchronization process of the grid-connected voltage source converters (VSC) is studied detailly. Firstly, the phase-locked loop (PLL)-synchronized VSC is modeled according to the rotor motion equation of synchronous generator (SG). Furthermore, the VSC's damping ratio is derived, and the effects of the VSC's control parameters and the operation status on the equivalent power angle (EPA) is investigated. Moreover, the influence of the PLL's parameters and the voltage dips degree on the VSC's transient frequency behavior are analyzed. Analysis result reveals that the VSC's damping ratio decreases once the grid voltage drops, so that the EPA and frequency of the VSC may appear large overshoot and oscillation, which may trigger the frequency protection and deteriorate the VSC's transient stability. In order to ensure that the VSC can smoothly operate to new equilibrium points, an improved PLL is proposed, which can adaptively adjust the VSC's damping ratio for different voltage sags. The proposed method can not only reduce the overshoot of the EPA but also restrain the VSC's frequency dips degree. Finally, the simulations and experimental tests validate the effectiveness of the theoretical analysis. [ABSTRACT FROM AUTHOR]

Low speed, high torque machines are used in wind turbines where the turbine rotor is directly connected to the generator. A permanent magnet synchronous generator using high-energy rare-earth permanent magnets (PMs) is one common choice for this application, but rare-earth PMs have supply insecurities and cost risks. A rare-earth free 32-pole PM generator, using ferrite PMs in a spoke-type rotor, for use in a 12 kW experimental wind turbine is built and tested. Voltages and currents at load and no load are measured, as well as the magnetic field in the end regions of the machine. The generator can deliver the required power at nominal speed and has low harmonic content in the output. The electrical measurements are compared to two-dimensional finite element design calculations and the experimental results are lower than expected from simulations. The measured voltage is lower than expected, requiring a higher current than calculated for the rated power. Three-dimensional magnetic field simulations show that there are leakage flux paths in the end-regions that the two-dimensional design calculations overlook, explaining the discrepancy between simulations and measurements. [ABSTRACT FROM AUTHOR]

This article deals with the stator cooling of an air-cooled, synchronous generator with a power rating of 400 kVA, which has been improved by adding two radial vents to the stator. To ensure an optimal vent design, a novel combined thermal and electromagnetic modelling approach is developed. A parametric 3D Conjugate Heat Transfer Computational Fluid Dynamics (CFD) model is used for the thermal modelling. An electromagnetic 2D Finite Element Analysis determined the impact that venting the stator has on the loss distribution. The models are coupled by deriving analytical correlations between the combined vent width and rotor copper, rotor iron and stator iron losses. These correlations are implemented into the optimisation procedure of the parametric CFD model. Five design parameters are optimised simultaneously with the aim of minimising the peak stator winding temperature. The modeling approach was validated experimentally by thermal, torque and mass flow measurements on the benchmark machine, as well as the newly designed prototype. [ABSTRACT FROM AUTHOR]

To make the distributed generation provide better support for microgrid, the idea of virtual synchronous generator (VSG) is proposed. Typically, the VSG consists of the outer power loop and the inner cascaded voltage and current control loops. Due to the three-loop cascaded structure, a voltage loop with higher bandwidth is significant for ensuring good power control performance. Moreover, most loads in the microgrid are variable and nonlinear. So the voltage control scheme must possess strong robustness against load disturbance. A linear active disturbance rejection control (LADRC) scheme based on the improved error feedback control law (IEFCL) is proposed for VSG voltage control. Compared with the traditional EFCL, the proposed IEFCL can effectively improve the gain and compensate phase lag in the mid-frequency, thereby increasing the bandwidth of the LADRC based voltage loop. Quantitative analysis demonstrates the proposed scheme can achieve highly decoupled voltage control in $ dq$ frame. In comparison with the traditional strategies, the proposed scheme has higher bandwidth and can significantly improve the power control dynamics for VSG in parallel operation. Additionally, it possesses a good capability of harmonic mitigation and strong robustness against load disturbance. Simulations and experiments verify the effectiveness of the proposed scheme. [ABSTRACT FROM AUTHOR]

In this article, we present a new approach to the detection of Loss-Of-Excitation (LOE), a typical failure of synchronous generators. Unlike most of the algorithms proposed in the literature, which only use the information available at the point of connection, we also take advantage of prior knowledge of the generator model. To track the field voltage and the other state variables, we have chosen the Constrained Unscented Kalman Filter (CUKF) as the core estimation technique, with phasor measurements as the input for this filtering algorithm. Detection of LOE is then performed by using the Faulty Modes Detection and Diagnosis (FMDD) algorithm, which combines the normal operation and a LOE-based model to decide in real-time whether an LOE has occurred or not. Results of simulations using a small two-area power system and the IEEE 39-bus system show that LOE detection times can be significantly reduced as compared to conventional and state-of-the-art approaches. Moreover, we observe that the new fault detection signal used to trip the generator can avoid short-term voltage stability problems. [ABSTRACT FROM AUTHOR]

In this article, a comprehensive mathematical model for synchronous generators under stator winding faults near the star point is described. The model is developed using the Modified Winding Function. The model includes space harmonics and their effects on windings inductances. The model is verified using electromagnetic analysis through 2D Finite Element Analysis simulation and experimental testing. The results prove the accuracy of the proposed mathematical model in reflecting the features of synchronous generators under stator windings ground faults in different parts of the winding and specifically near the star point. The developed modeling technique can be used for designing protections schemes for the very challenging 100% stator winding ground fault protection. The model enables protection designers to obtain rich set of data for protection relays designing and testing. [ABSTRACT FROM AUTHOR]

This paper studies the acceleration of numerical simulations executed on Field-Programmable Gate Arrays (FPGAs) for electric machines presented by voltage-behind-reactance (VBR) models. In VBR models, the stator dynamics are modeled in abc coordinates, while the rotor dynamics are formulated in qd reference frame. Both induction motors and synchronous generators, operating without and with magnetic saturation, are considered. Once VBR models of these machine types are reviewed, their dynamic models are discretized using Runge-Kutta numerical routines. The detailed mapping of such discrete models to FPGA is provided using High-Level Synthesis, which directly converts untimed descriptions into VHDL or Verilog. An automated method finds the fastest FPGA architecture by finding the best set of synthesis options. Experimental results show that our FPGA-based acceleration flow leads to about 92-168 times average simulation speed-up for various machine types compared to the MATLAB simulation. [ABSTRACT FROM AUTHOR]

Oscillation phenomena of offshore wind power plant (OWPP) in a wide frequency range can be caused due to impedance interactions between grid-connected inverters (GCIs) and transmission cables. In this article, impedance model of GCI with outer power control loop, inner current control loop and phase-locked loop is first established in dq reference frame. The correctness is validated by frequency scanning method. Then, the effects of active and reactive power/current references on dq impedance characteristics of GCI with/without consideration of power control loop are investigated using complex space vectors and complex transfer functions. Furthermore, RLC circuit model of transmission cable considering frequency-dependent characteristics is also established for dq-domain IBSC. On the basis of them, it's found that low-frequency oscillation phenomena of OWPP under power control mode may occur if active power reference exceeds a certain threshold value, which can be mitigated by injecting a certain amount of negative reactive power. Impacts of PLL parameters, length of transmission cable and number of paralleled GCIs on required negative reactive power for low-frequency stabilization are further investigated. Both Matlab/Simulink-based simulation and OPAL-RT-based real-time verification are implemented in an OWPP with four permanent magnet synchronous generators to validate the correctness of the reactive power characteristic analysis results and the feasibility of mitigating low-frequency oscillation phenomena by negative reactive power injection. [ABSTRACT FROM AUTHOR]

Field-oriented control (FOC) is standardly used in high-performance induction machine (IM) applications. However, the performance of FOC algorithms strongly depends on the level of agreement between the actual IM and the respective model used for the algorithm design. IM parameters’ values are known to vary with the operating conditions, so their online identification is preferable. In addition, it is often preferred to acquire the rotor speed by estimation because speed sensors increase the cost and size of the system while reducing its reliability. Model-reference-adaptive-system (MRAS) estimators are popular due to their simple implementation, but they are sensitive to IM parameters and, hence, usually include a parameter tuning mechanism. This article proposes an indirect rotor-field-oriented controller with the parallel MRAS speed estimation and IM parameter tuning – implemented in an induction generator system. Both the control algorithm and the MRAS include the stray load and iron losses, magnetic saturation, and rotor resistance variation, whereas the MRAS additionally accounts for variations of the stator and stray-load resistances. The system performance is experimentally evaluated over wide ranges of the IM flux, speed, torque, and temperature. [ABSTRACT FROM AUTHOR]

This article presents an improved virtual synchronous generator (VSG) control strategy of doubly-fed induction generator (DFIG) during symmetrical voltage fault. Based on VSG control, DFIG can behave like a conventional synchronous generator (SG), which can support frequency and voltage in grid-connected mode. Moreover, when microgrid disconnects from grid, DFIG can provide voltage and power for local loads. However, during symmetrical voltage fault, the transient components in DFIG cannot be controlled properly by VSG. To solve this problem, the air-gap flux feedback is designed in VSG control to accelerate the decay of transient components, and rotor current limiting method is also introduced in the article. In the proposed method, there is no need to switch the control structure or control targets. DFIG always works in the VSG control. Therefore, DFIG can operate in the standalone mode when the connection to grid is cut off. The decay time constant, the voltage control, current limiting control and parameter variation of the improved VSG control are analyzed in detail. Finally, the simulation and experimental results in both normal and fault grid condition are given to validate the effectiveness of proposed VSG control strategy. [ABSTRACT FROM AUTHOR]

The paper describes a synchronous generator model developed based on the multiple reference frame theory. The main physical phenomena included in the model are the machine armature non-sinusoidal voltage waveform and the influence of armature current in load conditions on the armature voltage waveform higher harmonic components. The modified multiple reference frame theory model is proposed. In this modified theory model the field and armature currents are the cause of the non-sinusoidal airgap spatial field distribution. The influence of this airgap spatial field distribution is also investigated. The resulting nonuniform saturation of the pole shoe affects the machine voltage waveform in load conditions. Simulation and measurements of the machine performance in steady and transient states are conducted in order to validate the model. [ABSTRACT FROM AUTHOR]

A flux-based technique is proposed for the on-line detection, localization and discrimination of stator and rotor turn-to-turn faults in synchronous generators, using the measured air-gap rotational magnetic field. Simple magnetic field sensors (MFSs) are installed in the stator slots at different regions, and the corresponding induced voltages are measured. Several differential voltages are then obtained to give a picture of the rotational magnetic field symmetricity, whose distortions indicate the presence of the stator or rotor faults. The proposed diagnostic technique is presented in detail and validated initially by simulation results obtained with a finite element simulation package and afterwards through the experimental tests. The obtained results demonstrate that the proposed technique can be used to detect and discriminate stator and rotor turn-to-turn faults with appropriate sensitivity. In addition, the region of the stator turn-to-turn faults can be identified as well. [ABSTRACT FROM AUTHOR]

In recent years, the increasing penetration of distributed generation in microgrids challenges the control and coordination of energy resources. Especially in microgrids with virtual synchronous generator (VSG)-controlled converters and conventional synchronous generators (SG), the inherent inertia difference (i.e., the VSG and SG) results in a poor transient performance when the VSG and/or loads are cut in/out. Thus, this paper explores the transient performance of microgrids with parallel VSG and SG systems. More importantly, a novel pre-synchronization control method is proposed to eliminate the phase jump while meeting the requirements in case of closures or re-closures of generation units. A small-signal dynamic model is presented, and accordingly, the VSG inertia and its damping can be designed considering the capacity ratio of VSG and SG units. In addition, with the power angle stability analysis, an active power provision strategy is introduced to suppress the transient power oscillation due to the inertia difference. Finally, the feasibility of the proposed methods is verified by simulations on a microgrid consisting of parallel VSG and SG units. [ABSTRACT FROM AUTHOR]

Benchmark models provide a common platform that facilitates comparison of different methods and strategies that can be further validated in a real representation of the power system. Nevertheless, when using RMS test cases, transients present in modern power systems may be neglected. Modern energy conversion systems require an enhanced level of detail in order to properly represent their behavior. Moreover, due to the steady increase of non-synchronous generation, new operation and management strategies need to be tested to ensure power system security. One way to overcome these constraints is to use EMT real-time digital simulations which allows to obtain high precision results in a timeframe suited to a specific study. This work presents an EMT real-time model for integration studies of advanced energy conversion systems that can be flexibly expanded to higher renewable penetration. The model, based on the Simplified Australian 14-Generator test system, has been expanded to represent in an improved way the dynamics of power system components as well as the high penetration of non-synchronous power electronics generation. Transient simulations are undertaken to illustrate system's response due to non-synchronous generation and when faults are applied. This work also addresses the current gap in benchmark models for real-time simulations. [ABSTRACT FROM AUTHOR]

As internal combustion engine in automobile operates much quieter than before, the noise emitted by claw-pole alternator becomes more non-negligible. To address this problem, the magnetic force, which caused acoustic noise, was analyzed. Force harmonic relevant to the noise peak was identified and set as an optimization object to weaken. The influence of each rotor structural parameters on the force harmonic and output current was analyzed, and the four most influential parameters were chosen as the optimization variables. With the genetic algorithm, an optimum rotor shape generating minimum force harmonic, without severely deteriorate the output performance of the alternator, was obtained. Calculated by the noise predicting model which was verified by experiment, acoustic noise produced by the optimized claw-pole alternator was 14.3 dB (A) less than the initial one, with the output current decreased within 5 percent. [ABSTRACT FROM AUTHOR]

The term Virtual Synchronous Machine (VSM) commonly refers to the emulation of synchronous machines operation by using power electronics converters. However, a wider application of this control technique is still pending on further refinement of its design aspects and digital implementation. In this paper, a systematic approach to design the VSM parameters is introduced. First, a simplified third-order small-signal model of a VSM based on the conventional model of synchronous generators is developed. Unlike the existing works published in literature, the proposed model takes into account the effects of inductive and resistive weak grids. The model is then used to design the controller parameters by appropriately placing the poles of the closed-loop system. VSM robustness is studied for weak grids dominated by either inductance or resistance, and the aspects of digital control implementation are addressed. All the control system features and improvements were tested on a 15 kW prototype of a battery-supported VSM connected to a weak grid. [ABSTRACT FROM AUTHOR]

Failures in the field winding of synchronous generators cause degradation in performance and accelerated wear of components. A number of offline and online test methods has been developed over the years, and airgap flux monitoring has been proven to be the most effective solution for reliable detection of the fault. However, there are concerns on reduced sensitivity for cases where the number of turns per pole is large. In this letter, the fault indicators used in commercial applications for airgap flux based fault detection are analyzed for salient pole synchronous generators, and a new simplified method with improved sensitivity is proposed. Experimental testing on a 330-MW generator under controlled fault conditions is given to verify the claims. A comparative study shows that the new method can provide a simplified means of improving the sensitivity and reliability of the technology presently employed in the field. [ABSTRACT FROM AUTHOR]

Standard vector controllers for induction machines (IMs) are based on the conventional IM model. Consequently, they are unaware of the existence of stray load and iron losses. However, these losses may induce a considerable mismatch between the actual and reference flux vectors in vector controllers. This mismatch is greatly affected by the operating flux and load. In this paper, two different approaches are considered for compensation of this mismatch—the first involves investment in IMs with low stray load and iron losses, whereas the other involves development of a control algorithm in which both these losses are compensated. The performance of the developed vector controller is compared to the standard controller and tested on three small IMs of different efficiency class and rotor cage material (aluminum versus copper). Simulation and experimental testing is carried over wide ranges of loads, speeds, and magnetization levels. The estimated rotor speed is used as a measure of the controller detuning. A standard estimator that utilizes rotor flux-linkage estimates within the model reference adaptive system is here for the first time modified to include stray load and iron losses. Finally, problems regarding numerical integration, sampling frequency, and temperature-induced variations of IM parameters are addressed. [ABSTRACT FROM AUTHOR]

This paper presents characterization and operational envelope of a hybrid permanent magnet (HPM) generator in a series hybrid electric vehicle (SHEV) powertrain. A proposed control strategy is applied to characterize the system during vehicle operation within new european driving cycle (NEDC). The HPM generator utilizes a permanent magnet topology assisted with a wound field (WF) rotor while the machine 9-phase output is rectified and connected to the SHEV main dc-link. A brushless exciter with dc stator and multiphase rotor is designed to provide an adjustable dc current and voltage to the HPM generator WF. The brushless exciter dc stator is fed from the vehicle dc-link using a dc/dc converter while its rotor output connects to an integrated 32-leg rotating rectifier the output of which provides a high quality dc to the HPM generator WF. A control strategy incorporating a dynamic model of the HPM generator, brushless exciter, and dc/dc converter is proposed under which the system operational envelope is obtained. The HPM generator efficiency map, torque speed, and WF current behavior is analyzed to realize an optimum operating region while the SHEV operates over the NEDC driving cycle. The analytically predicted results are validated against measurements from a laboratory prototype HPM generator and exciter system. The comparison shows the effectiveness of the control method and accuracy of the developed models. [ABSTRACT FROM AUTHOR]

This paper presents an approach for systematically modeling a class of microgrid (MG) systems. The derived model accommodates grid-connected and islanded operation of the MG simultaneously, and allows modeling of converter-based as well as directly interfaced resources. Full modeling of virtual synchronous machines and actual synchronous generators is addressed. The originally nonlinear model is then converted to a linear model whose eigenvalues determine local stability of the MG. The model is used to analyze impacts of cross-coupling droop terms on the MG stability. The conclusions of this analysis are as follows. Addition of cross-coupling droop terms reduces the losses, compromises the stability in the grid-connected mode, and improves the stability in standalone mode. Therefore, for an MG that is intended to continuously operate at grid-connected and standalone mode, only a modest amount of cross-coupling terms is recommended to increase the efficiency without compromising the grid-connected stability. Simulation and experimental results are presented to verify the derivations. [ABSTRACT FROM AUTHOR]