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This paper presents Space Vector Modulation (SVM) sequences that reduce Common Mode Voltage (CMV) in Dual Three-phase Asymmetrical Induction Motor (DTAIM). At first, two existing SVM techniques such as Reference SVM (RSVM) and CMV1 are studied. Additionally, three new switching sequences are developed as CMV2, CMV3 and DCMV3, which reduce peak magnitude of CMV by 66.8% in comparison with RSVM technique. In SVM techniques, to obtain least CMV, zero vectors are usually replaced by two opposite large Active voltage Vectors (AV) with equal time duration. In such topologies like CMV1, total six AVs are utilized to synthesize the reference vector. The proposed SVM technique CMV2 utilizes only five large AVs to synthesize the reference vector and reduces CMV. Selection of five neighbouring AVs is reflected in reduction of harmonic flux trajectories and stator current ripple in proposed techniques. Additionally, two more switching sequences (CMV3 and DCMV3) are developed related to utilization of specific zero voltage vectors in all sectors (7 and 56) that produce zero CMV. These two switching sequences also provide easy implementation feature during the development of c-code program. In this study, CMV and stator current ripple are considered as performance parameters for different SVM techniques. All SVM techniques are analysed by analytical results, computer simulations, and using prototype test setup of DTAIM fed by Dual Three-phase Voltage Source Inverters (DTVSIs). [ABSTRACT FROM AUTHOR]

The impedance analysis has been proposed and proved to be an effective way to analyze the grid-connected stability of Voltage Source Converter (VSC) based energy conversion systems. Most of the existing impedance analyzes are discussed based on vector oriented control, while there are few discussions about the impedance analysis of Direct Power Control (DPC). In this paper, an impedance modeling approach of VSC using the DPC is firstly proposed and analyzed, which fills the gap of impedance analysis of the DPC based converters and provides a basis for studying the corresponding grid-integration stability problems. Since there is no need for PLL and Park transformation, the impedance matrix is built directly using the $\alpha \beta$ reference frame and features symmetrical characteristics. The proposed impedance matrix can be transformed into a positive-sequence impedance and analyzed by using SISO Nyquist criteria. The stability of a weak-grid connected VSC using DPC is analyzed in this article. The main factors affecting system stability, including the grid and control parameters, are considered in the analysis. The effectiveness of the proposed method is demonstrated by the simulations carried out in the Matlab/Simulink Simscape Power Systems as well as a hardware-in-loop system. [ABSTRACT FROM AUTHOR]

This paper presents a technique for estimating the rotor resistance of an induction machine over the machine's entire speed and torque range. This technique is based on injecting a relatively low-frequency carrier signal into the reference of the rotor flux linkage magnitude and extracting the induction machine's response to the carrier signal, which is then used in a model reference adaptive system. This paper also presents a technique to minimize the torque ripple generated by the carrier signal. This technique utilizes the rotor flux linkage reference in conjunction with a proportional-integral-resonant feedback plus feedforward control to generate references for the direct and quadrature axis stator currents. This paper also improves tracking of the torque and rotor flux linkage in direct field-oriented control of the induction machine through employing electromotive force and resistive compensation methods for tracking q-axis and d-axis reference stator currents, respectively. Simulation and experimental results demonstrate the effectiveness of the technique over the entire operating range. [ABSTRACT FROM AUTHOR]

This paper studies the significant impact of distributed-energy-resource (DER) inverters (which are connected to the same node) on anti-islanding protection of DER-installations. As it concerns the harmonic injection-based schemes, the induced harmonic voltage component that is provoked by each inverter is compromised, as synchronous injection (injection at the same time intervals) among the inverters cannot be reassured. Therefore, harmonic injection-based anti-islanding schemes will fail if the harmonic voltage component falls below a certain level. In this framework, a novel metric (i.e., the upgrade factor, UF) is introduced, indicating the supplementary power (of DERs) that can be installed in the same point of common coupling without compromising the effectiveness of the harmonic injection scheme. As this metric enables the evaluation of harmonic injection schemes prior to the future integration of DERs, it could be a substantial addendum into the relevant standards that evaluates the anti-islanding features of grid-tied DER-inverters. Finally, this paper proposes a technique (implemented in the software level), which maximizes UF and reassures the effectiveness of the harmonic injection schemes under the power variations of installed inverters. Extensive simulations and experimental tests have been conducted that verify the effectiveness of the proposed strategy under various penetration-levels and islanding-network conditions. [ABSTRACT FROM AUTHOR]

Due to the anticipated proliferation of HVDC links up to and beyond 2020 into transmission systems, it is expected that in some locations, two or more HVDC links will be installed with a short electrical distance between them and will form a multiinfeed (MI)-HVDC scenario for the system. The control and operation of MI-HVDC systems are of particular concern for weak grids such as that likely to be found in the North of Scotland network. It is shown that the ac voltage regulation in the VSC-HVDC link may adversely affect the dynamic performance of the host weak ac system under the MI-HVDC scenario. To overcome this identified limitation associated with the ac voltage controller in the VSC-HVDC link, a new controller modification conforming to the grid code is proposed in this paper. Furthermore, this paper presents a robust control design method for ac voltage control using the fractional order (FO) control design theory. The influences of the proposed FO-based ac voltage controller are assessed alongside various voltage controllers for the representative North Scotland system. It is shown that the FO-based ac voltage controller has favorable effects on the dynamic behavior of weak ac systems with MI-HVDC. [ABSTRACT FROM AUTHOR]

This paper presents a quantitative analysis of the losses of the hybrid cascaded modular multilevel converter (HC-MMC). Regulation of full-bridge (FB) submodule-capacitor voltages in an HC-MMC requires dedicated strategies that affect the voltage harmonics generated by the converter's main power stage and hence its switching patterns and losses. This paper uses an electromagnetic transient simulator (PSCAD/EMTDC) to estimate the semiconductor losses using the switches’ turn-on and turn-off characteristics. The paper examines the losses of a 45-MW HC-MMC for two recently introduced voltage-regulation methods based upon the modified sinusoidal pulse width modulation (PWM) and an existing method based upon the trapezoidal PWM. Losses of a conventional FB-MMC that offers dc-fault blocking capability are also investigated and compared with those of HC-MMC. The loss comparison results indicate that: first, the two voltage-regulation methods for HC-MMC based upon the modified sinusoidal PWM result in markedly lower semiconductor losses than the trapezoidal PWM method; second, compared with the original HC-MMC, a mixed submodule HC-MMC may lower the semiconductor losses at certain operating points with proper addition of mixed submodules; and finally, HC-MMC may have up to 9.0% lower semiconductor losses (relative) than the conventional FB-MMC. [ABSTRACT FROM AUTHOR]

Polymer electrolyte membrane fuel cell (PEMFC), because of faster response, portability and the low operating temperature has found wide usage in applications requiring regulated output voltage. Alongwith voltage regulation, optimum stack efficiency is important for reliable operation. Overall, the maximum efficiency point (MEP) operation and voltage regulation need to be achieved while restricting the PEMFC operation till maximum power point (MPP). This demands operational adaptability of PEMFC over a wider range, to meet the desired levels of voltage regulation, maximum efficiency and MPP operation. However, achieving all the objectives simultaneously is challenging since operating points corresponding to the objectives differ. This paper proposes a control strategy that achieves operation dependent adaptability of PEMFC between the objectives as per the demand. The adaptability in controller tuning for MPP and voltage regulation has been formulated as multi-objective optimization (MOO) problem with a constraint on controller parameters, for allowing the PEMFC operation closer to MEP. The solution of MOO problem provides a set of controller configurations, each achieving different operating scenarios/objectives of the PEMFC system. Compared to the existing works aiming at either of the objectives, the effectiveness of the proposed approach is reflected in terms of simultaneous attainment of all the objectives. [ABSTRACT FROM AUTHOR]

This paper presents a modification of the Vienna rectifier (VR) to obtain more than three voltage levels at the DC line connection, while preserving the unity power factor at the AC grid side. The proposed unidirectional multilevel there-phase rectifier, called the “Negev rectifier”, has a reduced device count and utilizes a unique novel Time Division Multiplexed based control scheme, which makes the challenging N-level VR expansion possible. The Negev rectifier can serve as a grid interface for supplying dc voltage to multilevel inverters with 4 up to 7 voltage levels, eliminating the burden of inverter-side voltage balancing control. The Negev hardware can be constructed by two complementary topologies, making it suitable for either high current or high voltage applications. [ABSTRACT FROM AUTHOR]

In wound-field (WF) switched flux (SF) (WFSF) machines, the DC winding induced voltage pulsation causes current ripple in the DC winding and challenges the DC power source, and deteriorates the control performance. In this paper, the induced voltage pulsation in DC winding of five-phase WFSF machines is analyzed and its reduction methods are proposed. The cycles per electric period of the open-circuit and armature reaction induced voltage pulsation in DC winding are derived analytically. Modifying the airgap permeance by optimizing the rotor pole arc or chamfering the rotor pole surface, and axial pairing of rotor segments having rotor pole with different arcs are used to suppress the induced voltage pulsation in DC winding, with >90% average torque maintained. Finite element results show that, by optimizing the rotor pole arc, the peak-to-peak value of the induced voltage pulsation in DC winding can be effectively suppressed to 59.59%, 30.67%, 29.99% and 43.35% for the 10-stator-pole five-phase WFSF machines with 8-, 9-, 11- and 12-rotor-pole rotors, respectively. By applying rotor pole surface shaping, the induced voltage pulsation in DC winding peak-to-peak value can be effectively suppressed to 61.76%, 45.47% and 40.21% for the 8-, 9- and 12-rotor-pole machines, respectively, while by applying axial pairing, it can be suppressed to 46.89%, 7.16%, 15.64% and 12.04%, respectively. The 10-stator-pole/12-rotor-pole WFSF machines having the original rotor, optimized rotor, chamfered rotor and axial paired rotor are prototyped and the experiments validate the analytical and finite element results. [ABSTRACT FROM AUTHOR]

In some power systems, voltage subharmonics and interharmonics of significant value may occur. They exert a harmful effect on various energy receivers, including induction motors. Previous works on an induction machine supplied with voltage containing subharmonics and interharmonics generally concern currents, power losses, rotational speed, electromagnetic torque, and windings temperature. This paper reveals additional detrimental effect of voltage subharmonics and interharmonics: namely, excessive vibration of induction motors. It was found that voltage subharmonics of values a few times less than reported in real power systems may cause unacceptable levels of vibration. [ABSTRACT FROM AUTHOR]

This paper compares transient responses of virtual oscillator control (VOC) to the conventional droop control method, under various inverter terminal voltage amplitude and frequency regulations. The system of interest is an islanded microgrid of three three-phase inverters sharing a resistive load. By letting the two controllers have similar steady-state droop characteristics, the virtual oscillator and droop controlled systems are compared in a small-signal framework with the aid of eigenvalue analyses. The analytical results are verified by simulating the two systems in MATLAB/Simulink. In the simulations, the third inverter in each system is connected to share the load when the other two inverters are already synchronized. Transients are compared by examining synchronization time after the third inverter is connected. The key finding is VOC outperforms droop control in terms of time responses when the terminal voltage frequency regulation range is allowed to be wide; otherwise, the droop control responds faster than the VOC technique. The same conclusions are found for two other load types: constant power load and nonlinear load. In addition, transient energy losses are investigated, which shows positive correlation with the time response. [ABSTRACT FROM AUTHOR]

Conventionally, for a tubular permanent magnet synchronous linear motor (TPMSLM), adopting a modular structure can effectively reduce the detent force; however, the thrust density is sacrificed via the large proportion of the flux barrier to the total primary component volume. To overcome this disadvantage, this paper proposes a novel modular TPMSLM, in which the unit primary component adopts the large slot structure containing several coils. Focusing on this motor, research on the flux linkage characteristic is performed to obtain the design principle, and then the detent force is analyzed and suppressed by reasonable design of end teeth. First, the flux linkage characteristics of the unit motor and the whole motor are investigated. Variation rule of the coil flux linkage affected by the end tooth is discussed, and the principle of realizing three-phase balance for the whole motor flux linkage is elaborated. Moreover, the principle of the detent force suppression is verified and the detent force is further suppressed through the end tooth design. Finally, a prototype is developed. The back electromotive force and the detent force were tested, which verified the effectiveness of the novel modular TPMSLM proposed. [ABSTRACT FROM AUTHOR]

High-frequency oscillations caused by voltage source converters (VSCs) in current control timescale (less than 20 ms) have brought great challenges to the security and stable operation of the VSC-dominated power systems. However, relevant studies on this multi-input multi-output (MIMO) system still lack enough mechanism cognitions of its voltage amplitude/phase dynamics. Thus, this paper will propose a single-input single-output (SISO) equivalent method of this MIMO system to separately study its amplitude/phase dynamics. First, a linearized VSC model based on internal voltage motion equation in current control timescale is derived. This model can explicitly depict VSC's self-characteristic, which is suitable for multi-VSC voltage amplitude/phase dynamic analyses. Then, with reserving and converting the interactions of other outputs into one output, the MIMO system based on this VSC model can be equivalent into the amplitude motion and the phase motion, respectively. These two SISO systems are extremely convenient for engineers to study the mechanisms of the voltage amplitude/phase dynamics. Besides, this SISO equivalent method of the MIMO system can also be used to quantitatively analyze the interactions among the multi-VSC. Finally, the application of this method is illustrated in a single VSC system and the time domain simulations validate the correctness of the analyses. [ABSTRACT FROM AUTHOR]

This paper proposes a novel mesh-based equivalent magnetic network (EMN) model for performance analysis and optimal design of permanent magnet (PM) machines. The key contribution of the EMN model is that a generalized air-gap model solution is proposed by considering the machine motion of PM machines. Meanwhile, a modular modeling method is presented, and then the optimal design of PM machines can be investigated without repetitive programming and modeling. Two kinds of mesh elements are comprehensively utilized in the EMN model, and a hybrid rotor PM (HRPM) machine with interior PM and surface PM configuration is selected as an example to illustrate the development processes of EMN model. Meanwhile, the electromagnetic characteristics of HRPM machine are calculated by the proposed EMN model, such as back electromotive force, air-gap flux density distribution, and average torque. Finally, the accuracy and effectiveness of the proposed EMN model are verified through comparisons with finite element analysis and experimental tests. [ABSTRACT FROM AUTHOR]

In this paper, by solving the Maxwell's equations, an analytical model is developed for the prediction of the air gap flux density in the eccentric permanent magnet (PM)-inset machines at no-load and on-load conditions. Since in this model neither Schwartz–Christoffel transformation nor perturbation analysis is used, it is fast and accurate. In the modeling process, the stator of the eccentric machine is replaced with an equivalent surface current (ESC), which is injected at the stator bore. Distribution of the ESC is unknown and must be selected, such that the resultant tangential field satisfies the boundary condition at the stator bore contour. To apply this condition, the flux density of the PMs, armature current, and the ESC are obtained by applying the subdomain analysis and using a conformal transformation. The armature current and ESC are considered as a surface current sheet at the stator bore. The back electromotive force and inductances of the eccentric machine are obtained in the proposed model. In addition, using the Maxwell stress tensor, the unbalanced magnetic force, and the electromagnetic torque are computed. Finally, the model is verified by means of finite element analysis. [ABSTRACT FROM AUTHOR]

This paper proposes a distributed coordination control strategy for load sharing and energy balancing between heterogeneous energy storages. These control objectives are satisfied through a two-level control structure. At the primary level, the decentralized virtual impedance control, without the requirement of communication links, allocates the low frequency component of the loads to batteries, while the high frequency component is allocated to ultracapacitors. Distributed control strategy, introduced at the secondary level over a sparse communication network, achieves battery state of charge balancing and regulation of the local bus voltages. Two sets of data are exchanged via the communication links, the local bus voltages, and state of charges of batteries. The distributed controller for the restoration of local bus voltages implements an average consensus protocol, while the controller for energy balancing uses a cooperative protocol. In addition, the ultracapacitor voltages are locally restored at a slower time-scale. The proposed control strategy is resilient to communication link failures and features plug-and-play capability. Presented results demonstrate performance of the proposed control strategy for an islanded 380 VDC datacenter with variable loads. Different operating conditions are verified through the RTDS Technologies real-time digital power system simulator using switching converter models and nonlinear battery models. [ABSTRACT FROM AUTHOR]

Terminal voltage distortion in surface-mounted permanent magnet machines caused by local magnetic saturation significantly influences the machine performance. Thus, this paper focuses on the minimization methods for such phenomenon. The traditional harmonics minimization methods, such as rotor skewing and nonuniform airgap (rotor and stator shaping), are investigated. However, neither of them can effectively reduce the terminal voltage distortion since the local magnetic saturation in tooth-tips always exists, especially for designs with small or closed slot openings. This paper proposes that the terminal voltage distortion could be essentially reduced when larger slot opening width is adopted, since the local magnetic saturation by tooth-tip leakage flux decreases as a consequence. A design trade-off on slot opening is recommended to balance the terminal voltage distortion and other performance, e.g., the torque quality and flux weakening capability. The results show that the optimal trade-off should be made according to the machines most frequent operation conditions. Two prototypes with different design tradeoffs have been manufactured and tested as examples to validate the analyses. [ABSTRACT FROM PUBLISHER]

This paper proposes an effective method for suppressing the second harmonic dc bus voltage pulsation accounting for the second harmonic torque ripple in the permanent magnet synchronous generator (PMSG) system with asymmetric impedance. Neither the balanced current control nor the instantaneous power control can cope with the second harmonic dc bus voltage pulsation and the second harmonic torque ripple simultaneously in an asymmetric PMSG system. In this paper, the average power control is employed and only the positive sequence currents appear in the system, thus the second harmonic torque ripple will be avoided with balanced back electromagnetic forces. The second harmonic power and dc link voltage pulsations are then reduced by a compensation unit in parallel with dc bus diverting the second harmonic power. The effectiveness of the proposed method is verified through a small-scale wind turbine PMSG prototype with inherent asymmetry and deliberately introduced asymmetries. [ABSTRACT FROM PUBLISHER]

Variable frequency transformer (VFT) combines the principles of rotary transformer and phase shifting transformer to control active power transfer between two electric grids. The active power transfer control concepts, design, and transient performance of the VFT have been studied in detail and are reported in the literature. However, reactive power flow through the VFT has not been studied so far. In the existing installations, in order to meet the VFT internal reactive current requirements (magnetizing), switched capacitor banks are being used on the two sides of the VFT. These switched capacitor banks are oversized to deal with uncontrolled reactive power exchange that can happen between the two networks through the VFT. This may achieve desired power factor operation, but cannot prevent the undesired reactive power exchange. This paper presents the detailed study on reactive flow through the VFT. The paper also proposes a new VFT configuration (with integrated partially-rated series voltage compensation scheme and corresponding control) to achieve a full control over reactive power flow. Thereby, the proposed configuration can achieve the bidirectional and decoupled active and reactive power flow through VFT. The proposed VFT configuration and control are validated using real-time hardware in-loop evaluations. [ABSTRACT FROM PUBLISHER]

Permanent magnet (PM) synchronous machines (PMSMs) with concentrated windings (CW) exhibit high content of space harmonics leading to non–sinusoidal distribution of PM flux linkage and phase inductances in addition to the non-sinusoidal wave shape of the induced electromotive force. The ideal single reference frame-based PMSM model considers these parameters to be sinusoidal and does not include the effect of harmonics. Hence, computation of electromagnetic torque using ideal model can lead to deviation from the actual waveform. In this paper, a novel analytical model for electromagnetic torque has been derived using multiple reference frames (MRF) incorporating non-sinusoidal aspects of CW PMSM. Initially, the PM flux linkage and inductances in abc frame are extended by incorporating higher order harmonics. Then, the extended model is transformed into dq–axis using MRF rotating at speeds of dominant harmonics in PM flux linkages and inductances, the fundamental being the electrical rotor frequency. Based on this MRF model, a comprehensive electromagnetic torque model for the CW PMSM is derived. Simulation and experiments are conducted on a CW PMSM to validate the proposed model under different operating conditions. [ABSTRACT FROM AUTHOR]

Modern grid codes require wind turbines (WTs), e.g., type-4 WTs, to have low voltage ride-through (LVRT) capability, and recently, great attention has been attracted on the stability issues of WTs during the period of the LVRT. In this paper, we investigate the influences of the LVRT control on the stability of type-4 WTs under severe voltage sags in very weak grids. First, by proposing a novel quasi-static modeling method, we explore the existence of the WT's equilibrium during the LVRT. It is shown that the WT can lose the equilibrium under certain circumstances, leading to instability of the whole WT system. Furthermore, we use the small-signal analysis to illustrate that even there exists an equilibrium, the WT can become linearly unstable due to the dynamics of the current references generated by the LVRT control. We propose an enhanced LVRT control strategy to deal with these instability problems. Simulations verify our analysis and effectiveness of the proposed control strategy. [ABSTRACT FROM AUTHOR]

In AC microgrids, voltage source inverters (VSIs) have become widely employed to interface DC energy sources to the AC network. Apart from power generation, VSI-based energy sources can also perform ancillary services such as demand side management using conservation voltage reduction (CVR). In conventional distribution systems, CVR involves lowering the voltage levels of the system within permissible limits to reduce the amount of power consumed by certain loads. However, in autonomous microgrids, the application of CVR for demand side management has not been fully explored yet. Thus, in this paper, a CVR strategy for a droop-controlled VSI-based distributed generation (DG) operating in an autonomous microgrid is proposed. This strategy employs the deviation in DG output voltage magnitude and frequency, caused by an increase in power demand during peak and overloading conditions, to determine the amount of voltage compensation required to reduce the microgrid bus voltages. In addition, a controller to coordinate between the proposed CVR strategy and load shedding is also developed to operate the DGs during overloading conditions. The proposed strategy is verified with a microgrid test system, and its performance is validated on MATLAB/Simulink software environment. [ABSTRACT FROM AUTHOR]

Consequent-pole permanent magnet (CPM) machines have considerable potential for low-cost applications because they can improve the permanent magnet (PM) utilization ratio. However, the phase back electromotive force (EMF) of CPM machines with Ns/(mt) = odd (t is the greatest common divisor of the stator slot number Ns and the rotor pole-pair number p, and m is the phase number) is asymmetric, i.e., the phase back EMF has even-order harmonics, which will increase torque ripples. This paper utilized the staggered rotor to eliminate the even-order harmonics and reduce the cogging torque of CPM machines, thereby potentially decreasing torque ripples considerably. The axial flux barrier in the middle of the rotor was used and optimized using three-dimensional finite element (FE) analysis to reduce the axial leakage flux of the CPM machine with the proposed staggered rotor. Furthermore, the axial magnetized PM was embedded in the flux barrier, which improved the flux density of the airgap above the iron-pole and, consequently, further increased the output torque. The electromagnetic performance of the proposed machine, including the open-circuit airgap flux density, back EMF, average torque, torque ripples, and PM utilization, was compared with that of the conventional CPM machine. Results showed that the proposed machine obtained a similar torque and PM utilization ratio but a much lower torque ripple compared with the conventional one. [ABSTRACT FROM AUTHOR]

To reduce the current ripples of finite control set model predictive control, this paper proposes a continuous voltage vector model-free predictive current control method for the surface mounted permanent magnet synchronous motor (SMPMSM). In the proposed method, the continuous phase and the amplitude of the voltage vector are optimized in a sequence based on their uncoupling feature, and then the steady-state current fluctuation can be reduced. Only six active voltage vectors are enumerated, and the optimal phase is obtained by establishing the Lagrange interpolation polynomial between the cost function and the phase of voltage vectors. Then, the amplitude of the voltage vector with the optimal phase is optimized based on the principle of minimizing the cost function, and the optimal voltage vector is synthesized by a three-vector method. In addition, the ultra-local model of the SMPMSM drive system from the previous study is used to improve the robustness of the proposed method. The current dynamic and steady-state responses are demonstrated through the simulation and experimental results. [ABSTRACT FROM AUTHOR]

In this paper, existing feasible slot/pole combinations of the fractional slot permanent magnet (FSPM) machines are extended. In comparison with the existing slot/pole combinations, the proposed ones could result in a higher winding factor, lower torque ripple, and lower cogging torque. This diversity in the number of the slot/pole combinations brings the designers more alternative solutions. To reach such slot/pole combinations three-slot pitch coils are applied. The existing slot/pole combinations are limited to the designs with an integer number of slots per phase in one periodicity of the FSPM machine. In the presented work, FSPM machines with fractional numbers of the slots per phase in one periodicity of the machines are introduced. However, the phase back electromotive force signals are balanced. Moreover, to achieve a slot/pole combination with small unbalance magnetic force (UMF) and low THD value of the armature MMF, the multilayer winding technique is applied. In addition, a simple technique is presented to find an arrangement of the multilayer winding with very small UMF. Extensive finite element simulations are used to compare the FSPM machines with the proposed slot/pole combinations with the traditional FSPM machines. [ABSTRACT FROM AUTHOR]

High penetrations of renewable energy sources (RES) in distribution grids lead to new challenges in voltage regulation. These challenges are not just limited to the steady-state voltage rise, but they are extended to rapid voltage changes due to wind speed variations and moving clouds, casting shadows on photovoltaic panels. According to EN50160 in low-voltage (LV) grids, the steady-state voltage should not exceed 1.1 pu (static characteristic), and rapid voltage changes should be kept less than 0.05 pu (dynamic characteristic). These two characteristics may limit the maximum amount of RES that can be installed in LV grids, called, respectively, the static hosting capacity (SHC) and dynamic hosting capacity (DHC). Although existing research just evaluated SHC in distribution grids, high-penetrated RES grids can be faced with such large voltage changes, which cause a smaller DHC than the SHC. This paper studies both SHC and DHC in distribution grids and proposes an adaptive controller for static synchronous compensators to regulate the steady-state and dynamic voltage while avoiding the unnecessary increase in the reactive power. The simulation results in some German distribution grids show considerable effects of the proposed adaptive controller on improving both SHC and DHC. [ABSTRACT FROM AUTHOR]

This paper proposes a new diode-clamped multilevel inverter that can effectively balance the voltages of dc capacitors. For the proposed inverter, the inductors are inserted between the diodes and the main switches. Also, the voltage balance of the dc capacitors can be guaranteed by controlling the opening time of the corresponding main switches in particular modes. Thus, the parallel circuits on the dc side in the conventional inverters are canceled. Consequently, the proposed topology requires fewer switches, diodes, and capacitors, although it adds the compensation currents to the corresponding main switches. Both the simulation and experimental results demonstrate the suitability and applicability of the proposed topology. [ABSTRACT FROM AUTHOR]

DC winding induced voltage pulsation in the wound field synchronous machines (WFSMs) will cause dc winding current ripple, challenge the dc power supply, and deteriorate the control performance, especially at high speed. In this paper, the influence of dc winding configuration including the dc coil number and the parallel branch number on its induced voltage pulsation is investigated for WFSMs. Based on the modeling of both open-circuit and on-load dc winding induced voltages, the preferred dc winding configurations for WFSMs having various stator/rotor pole number combinations with double layer or single layer ac windings are obtained and validated by finite element (FE) analysis on four typical partitioned stator WFSMs (PS-WFSMs). A PS-WFSM prototype is built and tested to validate both analytical and FE analyses. [ABSTRACT FROM AUTHOR]

Resonant switched-capacitor converters (SCCs) are being increasingly used in energy conversion systems due to their low cost, low weight, and high efficiency. The zero-current switching (ZCS) feature of an SCC, which can be achieved at a switching frequency below the resonant frequency, not only results in soft switching and, hence, low switching losses, but also reduces the electromagnetic interference caused by the converter. This paper proposes a cascaded step-down SCC (CSD-SCC) with an arbitrary number of converter stages and an exponential output-to-input voltage ratio. This SCC reduces the number of required converter stages for a low voltage conversion ratio and has a low ripple in the output voltage. A mathematical study is proposed to analyze the operating modes of this generalized CSD-SCC under ZCS operation. This study also enables the calculation of the output voltage, the output voltage ripple, and the efficiency for different switching frequencies and loads. The proposed analysis is validated via several simulation and experimental case studies, including a photovoltaic energy conversion system. The experimental CSD-SCC has two converter stages with an efficiency higher than  $\text{95}\%$. [ABSTRACT FROM AUTHOR]

In a photovoltaic (PV) system, the appropriate voltage regulation is essential to guarantee the maximum power operation. In this paper, a direct pulse width modulation controller based on a single gain (k) is presented. This controller directly estimates the duty cycle (D) through gain k, and simultaneously its mathematical operation contains the properties of integral controller. Initially, a comprehensive study is presented to evaluate the D values according to reference points on I–V curves with contrasted irradiance levels. This study has been carried out for resistive and battery loads. Based on critical observations, separate tuning expressions of k gain have been designed for resistive and battery loads. It is revealed that with regard to varying weather, k gain is dynamic for resistive load, while static for battery load. Moreover, it is also shown that inappropriate setting of load will limit maximum power point tracker controller to reach the MPP point. Therefore, two more expressions have been presented to configure the resistive and battery loads. Comparative analysis between different D-schemes is presented, which indicates that for both loads, the proposed scheme has the superior performance with less complexity. Finally, numerous experimental tests have been conducted to verify the tuning criteria and working principle of the proposed scheme. [ABSTRACT FROM AUTHOR]

Direct power control (DPC) has gained much attention for medium-voltage applications due to its fast power control capability. However, a conventional static switching table based DPC can lead to unsatisfactory response when the converter is connected to an electric grid because it is commonly designed assuming ideal and constant voltages of the grid and the dc link. Therefore, the actual grid voltage and the actual converter dc-link voltage should be considered while designing the switching table. This paper proposes a novel dynamic switching table for the DPC, considering actual grid and dc-link voltages variation. The proposed dynamic switching table incorporates a new definition of power influence crossover angles of each converter space vector. The proposed scheme dynamically adapts the switching table by feeding forward the actual grid and dc-link voltages, and hence the converter's power is precisely controlled. The proposed technique is verified for the three-level neutral point clamped converters connected to a standard IEEE 9-bus test system. Moreover, the real-time simulations are performed in a real-time digital simulator, OPAL-RT. The obtained results verified the effectiveness of the proposed DPC strategy under different grid fault scenarios. [ABSTRACT FROM AUTHOR]

This paper presents a novel decentralized control for offshore wind farms connected to the onshore grid through a high-voltage dc link by means of a diode rectifier. The proposed control system is implemented in each wind turbine generator system (WTGS). The capacitor placed at the filter of the wind turbine front-end converter is used for the proposed control implementation. Frequency control is achieved by aligning the capacitor voltage vector along a reference axis rotating at the reference frequency. Then, a frequency-reactive power droop control allows the synchronization of all the WTGSs. On the other hand, this droop strategy also leads to total reactive power sharing among WTGSs without relying on communications. An additional secondary frequency control is also implemented to compensate the frequency deviation caused by the droop control. The proposed control system has been validated by simulation and results demonstrate the appropriate performance even during start-up and faults. [ABSTRACT FROM AUTHOR]

Dual stator-winding induction generator (DWIG) is one kind of emerging multiport electrical machine. It has become a promising direction in the last two decades, especially for stand-alone power systems and renewable energy power systems. This paper mainly focuses on the DWIG with similar-poles stator windings and cage-type rotor. An overview of DWIG and its system is given. Recent advances of this generator in system topologies, mathematical model, optimal design, control strategies, and performance analysis are presented. The developments and potential applications of the DWIG-based system are also discussed. [ABSTRACT FROM AUTHOR]

Harmonics of back electromotive force (back EMF) always have great effects on torque ripples in flux-reversal permanent magnet (FRPM) machines. In this paper, the back EMF in FRPM machines with different magnetization modes and coil pitches is studied. Then, a simple analytical method is developed to judge whether an FRPM machine with an arbitrary slot–pole combination has compensation to eliminate phase back EMF even-order harmonics or not. On this condition, four different types of machines, i.e., FRPM machine with compensation (magnetization mode 1), FRPM machine with compensation (magnetization mode 2), FRPM machine without compensation (magnetization mode 1), and FRPM machine without compensation (magnetization mode 2), are summarized to derive the winding factors when coil pitches varied from 1 to 3. Thus, the influence of the coil pitch on back EMF and its harmonic components as well as their phase angles can be studied by both analytical expression and 2-D finite element analysis. It is found that the optimal magnetization mode and coil pitch can help to increase the winding factor and hence enlarge the fundamental back EMF. Finally, the prototypes are fabricated and the corresponding experiments are conducted to further verify the effectiveness of derived analytical expressions. [ABSTRACT FROM AUTHOR]

Load-commutated inverters (LCI's) are often used to supply dual-three-phase synchronous motors in high-power variable-speed applications. A pair of LCIs is used in this arrangement to feed the two motor three-phase winding sets. In order to cope with inter-harmonic issues, a drive configuration with an interconnection of the two LCI dc-links has been proposed. In this paper, such a drive design is shown to produce an increased voltage stress on motor windings compared with traditional configurations. The problem is investigated in the paper by proposing an algebraic algorithm capable of predicting the steady-state voltage waveform applied to the motor terminals and arising between the star points of the two winding sets. Unlike conventional dynamic simulations, the proposed approach gives practically instantaneous results, making it possible to quickly investigate a wide number of possible operating conditions. Furthermore, it requires a limited knowledge of system parameters, which are often hardly available. Its reliability and accuracy are assessed by comparison with measurements on a test drive system and examples are given of the method application to the sizing of motor insulation system. [ABSTRACT FROM PUBLISHER]

In this paper, a novel induction motor (IM) stator current controller is proposed based on the stationary frame feed forward controlling structure. The proposed regulator represents an improved variant of the existing stationary and rotational frame IM stator current regulators based on the back EMF compensation. The paper novelty consists of the stationary frame controlling structure based on two feed forward actions that enable asymptotic reference tracking. Namely, the novel stationary frame regulator includes a proportional integral (PI) regulation term combined with a stator resistance voltage drop and back EMF feed forward compensations, with the back EMF compensation based on the stator flux value estimated by means of a programmable low-pass filter. The feedback term is based on a PI controller, with the corresponding parameter tuning procedure outlined in the paper. The simulation runs are included in the paper in order to examine the regulator performance in relation to the factors critical to the IM stator current regulation: variable IM model parameter values, transport delay introduced in the control loop, variable stator frequency, and rotor speed values. Also, the experimental tests are presented and carried out on an experimental setup for different operating conditions typical for an IM drive. [ABSTRACT FROM AUTHOR]

This paper proposes a new simplified model of a doubly fed induction generator (DFIG) that is integrated with a rotor-side converter for studies of large-scale transient stability. This model accurately considers the rotor field dynamics of an induction generator to yield simulation results that are more accurate in terms of voltage stability, as well as angle stability. The purpose of this paper is to provide a simple but effective DFIG model for the study of large-scale transient stability, and to examine the important aspects of rotor voltage and rotor slip power. This new simplified DFIG model with an electric/turbine control system was tested against a six-machine and 23-bus system, and the responses obtained in case studies of transient stability were compared with those of a PSS/E program. Finally, the proposed DFIG model was also validated by comparing the simulation results for a single-machine and infinite-bus system with those of PSCAD/EMTDC. [ABSTRACT FROM AUTHOR]

This paper presents the control strategy of achieving sinusoidal stator current of doubly fed induction generator using improved repetitive control (RC) regulator under generalized harmonic grid voltage. The improved RC regulator, with the larger magnitude response at the higher control frequency due to the magnitude response compensation unit, is proposed to restrain the stator current higher harmonic sequence more effectively. Besides, the appropriate phase leading compensation can also be automatically introduced by the improved RC regulator. The steady-state control performance and the closed-loop operation stability using the improved RC regulator are discussed in this paper. The availability of the proposed strategy under generalized harmonic grid voltage is verified by experiment results. [ABSTRACT FROM AUTHOR]

This paper is the continuation of Part I in which an online reference limitation strategy for shunt-connected converters is presented. This limitation strategy is successfully validated in this Part II of the paper by means of experimental results obtained in a laboratory setup. Two different experiment sets are carried out in order to check the two application scenarios described in Part I. On the one hand, the limitation strategy is implemented in the STATCOM application reducing the reactive power reference. On the other hand, the grid balancer functionality is implemented together with the new limitation strategy reducing the current references. In this last scenario, two different experiments are shown: 1) a switching-on of the converter; and 2) a voltage variation at the point of common coupling. In both functionalities, the corresponding reference limitations are activated due to overcurrent and overvoltage cases and the limitation due to the dc bus voltage oscillations is validated in the grid balancer functionality. [ABSTRACT FROM AUTHOR]

Temperature is one of the key factors when working with lithium-ion battery modules due to its influence in safety, performance, and lifespan concerns in these devices. High working temperatures reduce the available capacity of each cell within the module after several cycles due to aging; nonuniform temperature distributions in the module lead to different aging processes in each cell, and thus it will be impossible to take advantage of all the energy available in the Li-ion storage system, because the most aged cell will limit the available energy. This paper is focused on the minimization of the maximum temperature gradient of a battery module for high depth of discharge applications to avoid different aging processes of the cells within the module. Voltage balancing is proposed as a solution for this purpose, and different strategies and their results are presented in this paper. [ABSTRACT FROM AUTHOR]

Rotor winding interturn short-circuit (RWISC) occurring in turbine generators has been plaguing power producers with respect to production safety and economy. RWISC is characterized by impaired fundamental component of the exciting magnetomotive force and resultantly reduced no-load electromotive force E0. In this paper, an inverse reasoning method is proposed to analyze RWISC. First, it is assumed that the generator's rotor winding is running in good condition and the generator's electromagnetic power (virtual power, VP) is inversely calculated from the obtained operational data on generators. Second, the difference between the virtual power and the actual electromagnetic power is calculated and then used to determine whether an RWISC occurs on the turbine generator. Three real cases of severe rotor winding shorts have demonstrated that this method proposed in this paper not only enables to diagnose RWISC, but also allows determining the RWISC level and its trending accurately. The method is appropriate for RWISC diagnosis, whether offline or online, and can be applied to make the generator sets in the power plant run safer, more economical, and more stable. [ABSTRACT FROM AUTHOR]

Aiming to improve permanent magnet (PM) utilization factor, suppress cogging torque, and reduce back-EMF harmonics in stator-PM machines, a new type of stator-PM doubly complementary machines (DCMs) are proposed in this paper. These machines employ two complementarities in the structure, as one exists in double rotors and the other exists in the units belonging to same phases. The first complementary characteristic supplies a consecutive switching path for PM exciting flux, so that shorted or open magnetic path could be avoided to reduce cogging torque and improve PM utilization. The second one is designed to reduce back electromotive force (EMF) harmonics and improve the torque density. The topology, operation principle, characteristics, and design methodology for DCMs are presented. Simulation and experiments are conducted for a 12/11(stator teeth/rotor teeth) DCM to evaluate its performance and verify this design methodology. It is revealed that this machine could obtain good sinusoidal back-EMF waveform, small torque ripple, and high PM utilization factor due to the doubly complementary characteristic. [ABSTRACT FROM AUTHOR]

Large torque and voltage ripple are inherent drawbacks of doubly salient generators. This paper first introduces torque sharing function (TSF) into the application of a dual-channel three-phase 12/8 poles doubly salient permanent magnet generator (DSPMG) to reduce torque and voltage ripple. Based on the principle of copper losses minimization, a new TSF is proposed through theoretical analysis and numerical computation. The proposed TSF is compared to conventional TSFs according to three evaluation criteria. In order to achieve satisfying and consistent steady state and dynamic performances over the whole operation range, gain scheduling PI controller is adopted to overcome the highly nonlinear characteristic of the DSPMG. Besides, with compensators of back-EMF, cogging torque and losses adopted as feed-forward, a dual closed-loop generation control system is detailed designed. Both simulation and experimental results verify the effectiveness of the proposed control system and the copper losses minimization TSF in reducing torque and voltage ripple. [ABSTRACT FROM AUTHOR]

The definition of standard dynamic wind turbine models for power system stability analysis has been under development in recent years. These standard models, also known as generic or simplified models, have been developed under the framework of a new international standard, IEC 61400-27-1, the first edition of which was published in February 2015. These models are simplified representations of complex systems, such as wind turbines. Hence, the wind power industry urgently needs validation in comparison with real measurements to verify the accuracy and corresponding usability of the models. This paper presents the validation of a generic doubly-fed induction machine wind turbine model, commonly known as Type 3, based on a measurement campaign carried out in a Spanish wind farm. The wind turbine has been subjected to six voltage dips with different characteristics, magnitude, and duration, as well as several wind turbine operating conditions. The real behavior of the wind turbine is compared with the simulated response provided by the corresponding generic dynamic wind turbine model produced by the manufacturer Gamesa . Furthermore, the validation guidelines recently issued by IEC 61400-27-1 have been implemented, and the most important validation errors have been analyzed. [ABSTRACT FROM AUTHOR]

This paper proposes a new approach for separation of saliency signals of the induction machine at very low speed and heavy loads. The change of current responses due to the transient voltage pulse excitation is employed to obtain the inherent impedance saliency information. The proposed separation approach uses the discrete wavelet transform (DWT) in combination with Taylor's expansion and least square error (LSE) methods. The DWT removes the higher and lower harmonic orders to extract the operating frequency band based on the reference speed. Then, Taylor's expansion modeling processed by LSE algorithm is applied to extract the slotting saliency signal from the nearby frequency of the inter-modulation signal. The extracted slotting saliency signal is employed to estimate the rotor/flux position using the phase locked loop observer and current model. Experimental results are provided to prove the effectiveness of the proposed new separation approach at very low speed including zero electrical speed operations. It is found that the proposed new approach is accurate at very low speed and heavy load operations with less noise compared to previous methods. [ABSTRACT FROM AUTHOR]

Because of the current commutation and the double-salient pole structure of single-winding bearingless switched reluctance motors (SWBSRMs), the torque and radial forces have large ripples when present control methods are used. In order to solve those problems, a novel solution is proposed in this paper, which can achieve direct torque control (DTC) and direct force control (DFC) simultaneously. With the hysteresis-loop control of torque and flux linkage, the power switches of converter can be directly controlled based on the proposed space voltage vectors. First, the basic space voltage vectors are transformed correspondingly to realize DTC for BSRMs. After that, considering the generation principle of levitation forces, the basic space voltage vectors are transformed into equivalent space voltage vectors to facilitate the implementation of DFC. As a result, the torque and levitation forces can be controlled at the same time only with the developed equivalent space voltage vectors. The complicated current control algorithm is not needed, which is necessary for conventional methods. Moreover, the levitation performance is improved with the proposed method. Experimental results show that the torque ripple and radial displacements of SWBSRMs can be reduced approximately by 80% and 29%, respectively. [ABSTRACT FROM AUTHOR]

This paper proposes a selective voltage harmonic compensation strategy for stand-alone inverters. The use of LC output filters makes it possible to reduce switching harmonics at the output terminals of the inverter; however, it also produces voltage distortion when feeding nonlinear loads due to the harmonic voltage drop across the filter inductances. To compensate the voltage harmonic distortion and produce sinusoidal voltage, it is proposed to add virtual impedances to the fundamental voltage control. The whole process and considerations for the impedance design are described and a stability analysis of the system is presented. Finally, simulations and experimental results are shown to confirm the good performance of the proposed strategy under both steady state and load steps. [ABSTRACT FROM AUTHOR]

Loss-of-field (LOF) phenomenon in synchronous generators may cause equipment damage and voltage drop in the power system, which can be terminated to a blackout. Conventional schemes, which detect LOF on the basis of the measured impedance from the generator terminal viewpoint, have some major drawbacks such as low speed, complicated setting procedure, and unsecure performance in the face of similar phenomena, e.g., stable power swing. In this paper, different electrical quantities of the generator including terminal voltage (V), current (I ), active (P) and reactive power (Q), and power angle (δ) variations are studied to achieve a secure technique for LOF protection in synchronous generators. Comprehensive studies show that a simple criterion based on polarities of V, Q, and δ variations is accessible to achieve this goal, which does not need a complicated procedure to set it. To evaluate the performance of the proposed algorithm, some cases are simulated on sample power systems under various operating conditions. Also, superiority of the proposed algorithm in comparison with the other schemes is proven by the obtained results, as well. Moreover, the proposed technique is experimentally tested by using some 2-kVA laboratory generators to detect LOF. [ABSTRACT FROM AUTHOR]

A brushless doubly fed machine (BDFM) usually contains two sets of independent three-phase symmetrical stator windings. Judgment of the direct electromagnetic coupling phenomenon between the two sets of stator windings is one of the most important prerequisites when safeguarding the excellent operation and control performances of the BDFM. However, because of the diversity among the stator winding structures, the generation mechanism and the existing criterion of direct coupling between the two sets of stator windings have not been fully revealed yet. This paper analyzes the mutually induced electromotive force (EMF) in the coils per pole per phase of a single stator winding that is induced by the rotating magnetic field of the other stator winding. Detailed studies of the effects of the winding structures on the mutually induced EMF and the circulating current are performed and the results are verified by finite element analysis and experimental studies of an existing BDFM prototype. The results show that the direct coupling between the two sets of stator windings is related to the combination of poles number, the number of parallel branches, the winding type, and the winding connection. [ABSTRACT FROM PUBLISHER]

In this paper, a novel configuration of a three-level neutral-point-clamped (NPC) inverter that can integrate solar photovoltaic (PV) with battery storage in a grid-connected system is proposed. The strength of the proposed topology lies in a novel, extended unbalance three-level vector modulation technique that can generate the correct ac voltage under unbalanced dc voltage conditions. This paper presents the design philosophy of the proposed configuration and the theoretical framework of the proposed modulation technique. A new control algorithm for the proposed system is also presented in order to control the power delivery between the solar PV, battery, and grid, which simultaneously provides maximum power point tracking (MPPT) operation for the solar PV. The effectiveness of the proposed methodology is investigated by the simulation of several scenarios, including battery charging and discharging with different levels of solar irradiation. The proposed methodology and topology is further validated using an experimental setup in the laboratory. [ABSTRACT FROM PUBLISHER]