Showing papers in "IEEE Transactions on Energy Conversion in 2017"

A Self-Adaptive Inertia and Damping Combination Control of VSG to Support Frequency Stability

Abstract: In the virtual synchronous generator (VSG) field, the traditional methods, such as the constant parameters control method and the self-adaptive inertia control method, always neglect the effect of the damping factor. This letter proposes a self-adaptive inertia and damping combination control method to improve the frequency stability with an interleaving control technique. Tests on the MATLAB/Simulink VSG model demonstrate the effectiveness of the proposed method.

A Grey Wolf-Assisted Perturb & Observe MPPT Algorithm for a PV System

Abstract: This paper proposes a new hybrid maximum power point tracking (MPPT) algorithm combining grey wolf optimization (GWO) and perturb & observe (P&O) technique for efficient extraction of maximum power from a photovoltaic system subjected to rapid variation of solar irradiance and partial shading conditions. GWO handles the initial stages of MPPT followed by application of the P&O algorithm at the final stage in view of achieving faster convergence to the global peak (GP). This MPPT thus overcomes the computational overhead as encountered in the case of a GWO-based MPPT algorithm reported earlier by Mohanty et al. The idea behind using the hybrid technique is to scale down the search space of GWO which helps to speed up for achieving convergence toward the GP. The proposed MPPT algorithm is first implemented using MATLAB/Simulink and subsequently an experimental setup is prepared for its practical implementation. From the obtained results, it is confirmed that the proposed MPPT provides superior tracking performance in any weather conditions compared to both GWO and PSO+PO-based MPPT algorithms.

Characteristic Analysis of Subsynchronous Resonance in Practical Wind Farms Connected to Series-Compensated Transmissions

Abstract: The emerging subsynchronous resonance (SSR) caused by the interaction of wind turbine generators (WTGs) with series compensation has aroused great concerns. For this particular issue, this paper is aimed to fill the gap between theoretical studies and actual observations. By analyzing the field data of 58 SSR events captured in a practical wind power system and examining the observed dynamics with previous theoretical results, the mechanism and characteristics of SSR are revealed in a more explicit and substantial way. The necessary conditions and dominant influential factors are identified and the underlying reasons are discovered. Theoretically derived as well as practically measured impedance models have demonstrated that the converter control of doubly fed induction generator (DFIG) produces negative resistance at the slip frequency and thus causes unstable SSR; while permanent magnet synchronous generators and self-excited induction generators are just passively engaged in those SSR incidents. The distribution of the oscillation frequency has also been examined with field measurements. It is discovered that WTGs at different locations participate into the same SSR mode and their frequencies are not fixed but keep changing with the time, the variation of grid topology, and the number of online generators.

Impedance Modeling of Three-Phase Voltage Source Converters in DQ, Sequence, and Phasor Domains

Abstract: This paper presents a modular approach for the impedance modeling of three-phase voltage source converters (VSC) by representing the VSC dynamics using three-by-three transfer matrix in the dq, sequence, and phasor domains. The transfer matrix form simplifies the modeling process by separately modeling the ac and dc side dynamics, and describing the VSC dynamics independent of the ac and dc side networks. It also explicitly captures coupling among the dominant frequency components of the ac and dc side voltages and currents in the off-diagonal elements. Modeling of the VSC ac and dc side impedances, including the effects of the network on the other side of the VSC, is presented using the transfer matrix models. Transfer matrix based impedance modeling in the three domains and several stability analysis case studies are presented for a VSC-based HVDC station in an offshore wind farm. It is shown that the coupling between the ac and dc networks, and between the positive and negative sequence components of the three-phase quantities, play an important role in the low-frequency stability of the VSC. Impedance models and stability analysis predictions are validated using the wind farm simulations.

Fully Distributed Cooperative Secondary Frequency and Voltage Control of Islanded Microgrids

Abstract: This paper proposes a new distributed cooperative secondary control for both frequency and voltage restoration of an islanded microgrid with droop-controlled, inverter-based distributed generations (DGs). Existing distributed methods commonly design secondary control based on the minimum real part of the nonzero Laplacian matrix eigenvalues related to the microgrid communication graph, which, however, is global information. In contrast to the existing distributed methods, in this paper we design a fully distributed adaptive control based on the dynamic model of DG units and on information from neighboring units. Therefore, the proposed control scheme increases the system reliability, decreases its sensitivity to failures, and eliminates the need for a central processing unit. The fully distributed controllers restore the islanded microgrid frequency and voltage magnitudes to their reference values for all DG units irrespective of parametric uncertainties and disturbances while providing accurate real power sharing. Furthermore, the proposed method considers the coupling between the islanded microgrid frequency and voltages. Finally, we have conducted comprehensive simulation studies in the MATLAB/SimPowerSystems toolbox to verify the proposed control strategy performance.

Coordinated Control Strategies for Offshore Wind Farm Integration via VSC-HVDC for System Frequency Support

Abstract: Coordinated control strategies to provide system inertia support for main grid from offshore wind farm that is integrated through HVdc transmission is the subject matter of this paper. The strategy that seeks to provide inertia support to the main grid through simultaneous utilization of HVdc capacitors energy, and wind turbines (WTs) inertia without installing the remote communication of two HVdc terminals is introduced in details. Consequently, a novel strategy is proposed to improve system inertia through sequentially exerting dc capacitors energy and then WTs inertia via a cascading control scheme. Both strategies can effectively provide inertia support while the second one minimizes the control impacts on harvesting wind energy with the aid of communication between onshore and offshore ac grids. Case studies of a wind farm connecting with a HVdc system considering sudden load variations have been successfully conducted to compare and demonstrate the effectiveness of the control strategies in DIgSILENT/PowerFactory.

Single Sensor-Based MPPT of Partially Shaded PV System for Battery Charging by Using Cauchy and Gaussian Sine Cosine Optimization

Abstract: This paper introduces a battery charging scheme from a solar photovoltaic (SPV) by using a single sensor-based maximum power point tracking (MPPT) strategy. Here, for quick and efficient tracking, a novel hybrid “Cauchy and Gaussian sine cosine optimization” (CGSCO) algorithm is proposed for MPPT, which is based on only a single current sensor. The main objective of the CGSCO algorithm is, maximum extraction of the power from SPV panel and efficiently charging the battery through maximizing the charging current of the battery. Due to the single sensor, the cost of the charging scheme is very low, as well as the algorithm complexity and computational burden are very less, so it can be easily implemented on the low-cost microcontroller. In this paper, a single current sensor-based battery charging scheme by CGSCO algorithm is tested on MATLAB simulator and verified on a developed hardware of the SPV system. The panel condition, with and without shaded as well as dynamic environmental condition (variable temperature and insolation), is considered during simulation as well as on hardware implementation. Moreover, the tracking ability is compared with the most recent state of the art techniques (Grey wolf optimization and Lagrange interpolation particle swarm optimization (LIPSO)) as well as compared with “CGSCO with the conventional dual (voltage and current) sensor-based MPPT scheme.” The efficient battery charging with quick MPPT by CGSCO algorithm w.r.t. all state of the art techniques as well as dual sensor-based MPPT scheme, in steady-state as well as in dynamic conditions meets the motive of the work.

Adjusting Synchronverter Dynamic Response Speed via Damping Correction Loop

Abstract: This paper proposes to augment the conventional synchronverter control scheme with an auxiliary loop to freely adjust the dynamic response speed. This loop is dubbed as a damping correction loop since its form and function are reminiscent of the damping component in the classical synchronous generator model. Central to the proposed auxiliary loop is the creation of an additional tuneable parameter that allows for unrestricted adjustment of the system damping ratio without affecting the steady-state frequency droop characteristic, which is an improvement over the conventional synchronverter design. In the proposed method, relevant parameters are analytically tuned and active- and reactive-power coupling effects are reduced when operating with increased response speed. The dynamic response and robustness of the approach are verified via extensive small-signal analysis. Furthermore, time-domain simulations highlight the advantages of the proposed method over existing ones.

Small Signal Dynamics of DFIG-Based Wind Turbines During Riding Through Symmetrical Faults in Weak AC Grid

Abstract: Instability issues of the grid-connected doubly fed induction generator (DFIG) based wind turbines (WTs) during low-voltage ride-through (LVRT) have got little attention yet. In this paper, the small-signal behavior of DFIG WTs attached to weak ac grid with high impedances during the period of LVRT is investigated, with special attention paid to the rotor-side converter. First, based on the studied LVRT strategy, the influence of the high-impedance grid is summarized as the interaction between phase-looked loop (PLL) and rotor current controller (RCC). As modal analysis result indicates that the underdamped poles are dominated by PLL, complex torque coefficient method, which is conventionally applied in power system to study the interaction between mechanical and electrical subsystems of synchronous generator, is generalized to analyze how the PLL-RCC interaction influence the phase motion of PLL. Then, the concerned small-signal stability of the PLL-synchronized DFIG system can be discerned by the developed complex phase coefficients. Impacts of PLL's and RCC's parameters are highlighted, as well as the system's operating conditions during LVRT. Finally, the analytical result is validated by experiments.

An Efficient Hilbert–Huang Transform-Based Bearing Faults Detection in Induction Machines

Abstract: This paper focuses on rolling elements bearing fault detection in induction machines based on stator currents analysis. Specifically, it proposes to process the stator currents using the Hilbert–Huang transform. This approach relies on two steps: empirical mode decomposition and Hilbert transform. The empirical mode decomposition is used in order to estimate the intrinsic mode functions (IMFs). These IMFs are assumed to be mono-component signals and can be processed using demodulation technique. Afterward, the Hilbert transform is used to compute the instantaneous amplitude (IA) and instantaneous frequency (IF) of these IMFs. The analysis of the IA and IF allows identifying fault signature that can be used for more accurate diagnosis. The proposed approach is used for bearing fault detection in induction machines at several fault degrees. The effectiveness of the proposed approach is verified by a series of simulation and experimental tests corresponding to different bearing fault conditions. The fault severity is assessed based on the IMFs energy and the variance of the IA and IF of each IMF.

Power Loss and Thermal Analysis of a MW High-Speed Permanent Magnet Synchronous Machine

Abstract: High-speed permanent magnet synchronous machines (PMSMs) have attracted much attention due to their high power density, high efficiency, and compact size for direct-drive applications However, the consequent power loss density is high, and hence heat dissipation is a major technical challenge This is particularly the case for high-speed operation In this paper, a MW level high-speed PMSM is designed and its electromagnetic and mechanical power losses comprehensively investigated using finite element analysis The transient machine demagnetization performance is studied, and a composite rotor structure is proposed to improve machine antidemagnetization capability The temperature distribution of the proposed high-speed PMSM is also analyzed using a fluid-thermal coupling method with calculated power loss Experiments conducted on a prototype of the high-speed PMSM demonstrate the effectiveness of the numerical models developed and validate the results obtained

Suppression of Dead-Time Distortion Through Revised Repetitive Controller in PMSM Drives

Abstract: This paper proposes a new dead-time compensation method for voltage-source inverters (VSIs) used in permanent-magnet synchronous motor (PMSM) drives. The proposed technique is developed based on the revised repetitive controller (RRC) to reduce current harmonics and distortion, which is essential to improve efficiency, rotor position observer performance at very low speed, and self-commissioning accuracy. This method significantly suppresses the sixth-order harmonics and its multiples in synchronous reference frame and reduces the current total harmonic distortion (THD) without depending on the precise current sampling especially in the zero-crossing region. Unlike in most average value theory or pulse based compensations, it does not require additional hardware. It is also shown that RRC-based compensation has better robustness against motor parameter variations than disturbance observer based methods and requires very low computational effort. Compared to other repetitive controllers (RC), the proposed RRC is dedicated for dead-time compensation and can easily be integrated because of its simpler structure. In addition, it can be conveniently activated or deactivated on the fly, which adds more flexibility to the control overall algorithm. The effectiveness of the proposed method is validated by theoretical analysis, spectrum analysis, as well as simulation and experimental results.

Novel Cascaded Switched-Diode Multilevel Inverter for Renewable Energy Integration

Abstract: In this paper, a new topology of two-stage cascaded switched-diode (CSD) multilevel inverter is proposed for medium-voltage renewable energy integration. First, it aims to reduce the number of switches along with its gate drivers. Thus, the installation space and cost of a multilevel inverter are reduced. The spike removal switch added in the first stage of the inverter provides a flowing path for the reverse load current, and as a result, high voltage spikes occurring at the base of the stepped output voltage based upon conventional CSD multilevel inverter topologies are removed. Moreover, to resolve the problems related to dc source fluctuations of multilevel inverter used for renewable energy integration, the clock phase-shifting (CPS) one-cycle control (OCC) is developed to control the two-stage CSD multilevel inverter. By shifting the clock pulse phase of every cascaded unit, the staircase-like output voltage waveforms are obtained and a strong suppression ability against fluctuations in dc sources is achieved. Simulation and experimental results are discussed to verify the feasibility and performances of the two-stage CSD multilevel inverter controlled by the CPS OCC method.

Nonlinear Control-Based Modified BFCL for LVRT Capacity Enhancement of DFIG-Based Wind Farm

Abstract: High penetration of wind power into existing grid can be attributed largely to the doubly fed induction generators (DFIG). However, their sensitive nature to the grid faults has created concern for their mass integration into the existing power system. The grid code has been imposed by the regulatory bodies and the transmission system operators to ensure the low-voltage ride through (LVRT) capability of the wind farms for stable operation. DFIGs with their popular topology of partially rated converters are unable to provide the LVRT capability alone. In this work, a nonlinear control-based modified bridge-type fault current limiter (NC-MBFCL) is proposed to enhance the LVRT capability of DFIG-based wind farms. The efficacy of the NC-MBFCL is evaluated through performance comparison with that of the conventionally controlled bridge-type fault current limiter (BFCL) and the modified BFCL (MBFCL). Extensive simulations executed in MATLAB/Simulink environment for both the symmetrical and unsymmetrical faults reveal that the proposed NC-MBFCL is very effective in enhancing the LVRT capability of DFIG-based wind farms and outperforms the conventionally controlled BFCL and MBFCL. Also, it was found that the BFCL demonstrates better performance than the MBFCL.

Fuzzy Logic Based Energy Storage Management System for MVDC Power System of All Electric Ship

Abstract: In this paper, for supporting the medium voltage dc (MVDC) shipboard power system, an energy storage management (ESM) system based on fuzzy logic (FL) has been proposed and its performance with a proportional-integral (PI) control based ESM system is compared. In order to support the peak demand and pulsed load, a hybrid energy storage system incorporating high energy density storage (battery) and high power density storage (supercapacitor) is proposed. For energy transfer among the energy storages and the MVDC system, bidirectional dc–dc converters with dual active bridge (DAB) configuration are used. With the change of the bus voltage and load power demand, the ESM systems provide instantaneous reference powers for charging or discharging of the battery and supercapacitor. The reference powers for the battery and supercapacitor are sent to the respective controllers of the DAB converters. Two power sharing strategies are designed to share power among multiple energy storages. The MVDC shipboard power system with the generators, loads, battery, and supercapacitor with DAB converters are modeled in SimPowerSystems. Simulation results are used to make a comparison of performances of the FL and PI controller based ESM systems. Finally, controller hardware-in-the-loop based experimental results are added to demonstrate the effectiveness of the controller.

Predictive Control for Energy Management in Ship Power Systems Under High-Power Ramp Rate Loads

Abstract: Electrical weapons and combat systems integrated into ships create challenges for their power systems. The main challenge is operation under high-power ramp rate loads, such as rail-guns and radar systems. When operated, these load devices may exceed the ships generators in terms of power ramp rate, which may drive the system to instability. Thus, electric ships require integration of energy storage devices in coordination with the power generators to maintain the power balance between distributed resources and load devices. In order to support the generators by using energy storage systems, an energy management scheme must be deployed to ensure load demand is met. This paper proposes and implements an energy management scheme based on model predictive control to optimize the coordination between the energy storage and the power generators under high-power ramp rate conditions. The simulation and experimental results validate the proposed technique in a reduced scale, notional electric ship power system.

High-Performance Adaptive Torque Control for an IPMSM With Real-Time MTPA Operation

Abstract: In this paper, a high-performance torque control scheme of an interior permanent magnet synchronous machine (IPMSM) is introduced, which focuses on both steady state and transient torque dynamics of the IPMSM under the maximum torque per ampere (MTPA) condition. For the proposed control scheme with model-based torque correction, an accurate and efficient torque control with robust torque response can be achieved for the MPTA operation. Global stability and performance of the proposed torque control scheme are theoretically guaranteed. The current limitation of the IPMSM is easily handled without anti-windup and without degrading the torque dynamics or stability even the torque demand is beyond the maximum reachable torque. Implementation issues of the proposed control scheme to the real IPMSM plant with parameter variation are discussed. With the compensation of the linear and nonlinear inverter voltage drop, a robust and accurate torque response for the real-time MTPA operation can be achieved by an adaptive current control and online parameter estimation. The simulation and experimental results validate the safety and high performance of the proposed torque control scheme.

Analysis of Global and Local Force Harmonics and Their Effects on Vibration in Permanent Magnet Synchronous Machines

Abstract: This paper aims to clarify global and local force harmonics and their effects on vibration in permanent magnet (PM) synchronous machines. The local force includes both the tangential and radial forces distributing on the stator bore while global force refers to cogging torque and torque ripple. First, the expressions of local tangential and radial forces are derived based on the periodicity of magnetic field. The relationship between local tangential force and global force is also established. Then, the work continues with the effects of global and local forces on vibration. It is found global force may induce remarkable lateral motion when the mounting stiffness is low, while the main combined effect of local radial and tangential forces is inducing radial vibration. At last, influence of reducing global force on the vibration is investigated. It is shown even the vibration induced by global force itself does not always decrease, because this sort of vibration depends on both the harmonic components of optimized global force and the specific mounting conditions. The influence on vibration induced by local force is determined by the overall variation of flux density harmonics. Moreover, it is shown that PM arc shaping is a promising candidate for vibration mitigation.

A Decentralized Control Strategy for Economic Operation of Autonomous AC, DC, and Hybrid AC/DC Microgrids

Abstract: Economic operation is a major concern for microgrids (MGs). System operation cost is optimized when the incremental costs (ICs) of all distributed generators (DGs) reach equality. Conventionally, economic dispatch of DGs is solved by centralized control with optimization algorithms or distributed control with consensus algorithms. To improve the reliability, scalability, and economy of MGs, a fully decentralized economic power sharing strategy is proposed in this paper. As frequency is a global state in ac MG and dc bus voltage serves as a natural indicator in dc MG, a frequency-IC droop scheme is proposed for ac MG, a voltage-IC droop scheme is proposed for dc MG, and a normalization scheme is proposed for hybrid ac/dc MG. By using the proposed technique, ICs of DGs reach equality with the convergence of the system global indicator (frequency or dc bus voltage). Then power sharing of each DG is automatically achieved based on its relevant IC function and the total operating cost can be optimized without any communication or central controllers. The proposed approach is implemented in an ac MG, a dc MG, and a hybrid ac/dc MG in MATLAB/Simulink to verify its effectiveness.

Integration of Six-Phase EV Drivetrains Into Battery Charging Process With Direct Grid Connection

Abstract: The paper proposes two novel topologies for integrated battery charging of electric vehicles. The integration is functional and manifests through re-utilization of existing propulsion drivetrain components, primarily a six-phase inverter and a six-phase machine, to serve as components of a fast (three-phase) charging system. An important feature of the proposed charging systems is that they are with direct grid connection, thus nonisolated from the mains. Torque is not produced in machines during the charging process. The paper provides a comprehensive evaluation of the novel systems, together with an existing topology. Various aspects of the considered chargers are detailed and elaborated, including current balancing, interleaving modulation strategy, and influence of rotor field pulsation on control and overall performance. A control strategy is proposed and the theory and control scheme are verified by experiments.

Nonlinear Controller Design for Series-Compensated DFIG-Based Wind Farms to Mitigate Subsynchronous Control Interaction

Abstract: This paper proposes a nonlinear controller to mitigate subsynchronous control interaction (SSCI) in series-compensated doubly fed induction generator (DFIG)-based wind farms. The controller is designed based on partial feedback linearization (PFL) and the proposed design approach involves scrutinizing the partial feedback linearizability of the system. The stability of the internal dynamics, which is not transformed into linear autonomous subsystems by PFL, is also analyzed in the process of deriving the control laws. The frequency scanning method is used to evaluate the performance of the proposed PFL controller, and the performance is compared to that of a finely tuned conventional proportional integral controller. A grid-connected series-compensated 100-MW DFIG-based offshore wind farm is used to demonstrate the performance of the proposed scheme through the identification and mitigation of subsynchronous resonance. An analysis of the power system reveals that the resistance is negative across the entire subsynchronous frequency range, while the reactance becomes negative around 42 Hz. The proposed controller effectively mitigates SSCI, and it can be observed that it results in positive resistance and reactance values across the entire subsynchronous frequency range. Results from the eigenvalue (modal) analysis and electromagnetic transient simulation also confirm the results obtained from frequency scanning.

A Superconducting Magnetic Energy Storage-Emulator/Battery Supported Dynamic Voltage Restorer

Abstract: This study examines the use of superconducting magnetic and battery hybrid energy storage to compensate grid voltage fluctuations. The superconducting magnetic energy storage system (SMES) has been emulated by a high-current inductor to investigate a system employing both SMES and battery energy storage experimentally. The design of the laboratory prototype is described in detail, which consists of a series-connected three phase voltage source inverter used to regulate ac voltage, and two bidirectional dc/dc converters used to control energy storage system charge and discharge. “DC bus level signaling” and “voltage droop control” have been used to automatically control power from the magnetic energy storage system during short-duration, high-power voltage sags, while the battery is used to provide power during longer term, low-power undervoltages. Energy storage system hybridization is shown to be advantageous by reducing battery peak power demand compared with a battery-only system, and by improving long-term voltage support capability compared with an SMES-only system. Consequently, the SMES/battery hybrid dynamic voltage restorer can support both short-term high-power voltage sags and long-term undervoltages with significantly reduced superconducting material cost compared with an SMES-based system.

Stability Analysis of a PMSG-Based Large Offshore Wind Farm Connected to a VSC-HVDC

Abstract: This paper presents modal analysis of a large offshore wind farm using permanent magnet synchronous generator (PMSG)-type wind turbines connected to a voltage source converter HVDC (VSC-HVDC). Multiple resonant frequencies are observed in the ac grid of offshore wind farms. Their control is crucial for the uninterrupted operation of the wind farm system. The characteristics of oscillatory modes are presented using modal analysis and participation factor analysis. Sensitivity of critical modes to wind turbine design parameters and their impact on closed loop stability of the system are discussed. A comparison between a full wind farm model and an aggregated model is presented to show differences in the characteristics of critical modes observed in the models, and implication of using the models for stability studies It is concluded that robust control design is important for reliable operation of the system.

Combined Active and Reactive Power Control of Wind Farms Based on Model Predictive Control

Abstract: This paper proposes a combined wind farm controller based on Model Predictive Control (MPC). Compared with the conventional decoupled active and reactive power controls, the proposed control scheme considers the significant impact of active power on voltage variations due to the low $X/R$ ratio of wind farm collector systems. The voltage control is improved. Besides, by coordination of active and reactive powers, the Var capacity is optimized to prevent potential failures due to Var shortage, especially when the wind farm operates close to its full load. An analytical method is used to calculate the sensitivity coefficients to improve the computation efficiency and overcome the convergence problem. Two control modes are designed for both normal and emergency conditions. A wind farm with 20 wind turbines was used to verify the proposed combined control scheme.

Generic Dynamic Models for Modeling Wind Power Plants and Other Renewable Technologies in Large-Scale Power System Studies

Abstract: With the tremendous growth of wind power worldwide in the past decade, there has been an equally great demand for simplified, standard, and publicly available models for simulating wind power generators in commercially available power system simulation tools for stability analysis. Several efforts have been on the way to meet this need. The Western Electricity Coordinating Council's Renewable Energy Modeling Task Force has successfully achieved this goal and more recently has been working on expanding these models to include the ability to model complex plants, energy storage, and frequency response capabilities. This paper presents an account of these latest developments.

Synchronization and Frequency Regulation of DFIG-Based Wind Turbine Generators With Synchronized Control

Abstract: Synchronized control (SYNC) is widely adopted for doubly fed induction generator (DFIG)-based wind turbine generators (WTGs) in microgrids and weak grids, which applies P-f droop control to achieve grid synchronization instead of phase-locked loop. The DFIG-based WTG with SYNC will reach a new equilibrium of rotor speed under frequency deviation, resulting in the WTG's acceleration or deceleration. The acceleration/deceleration process can utilize the kinetic energy stored in the rotating mass of WTG to provide active power support for the power grid, but the WTG may lose synchronous stability simultaneously. This stability problem occurs when the equilibrium of rotor speed is lost and the rotor speed exceeds the admissible range during the frequency deviations, which will be particularly analyzed in this paper. It is demonstrated that the synchronous stability can be improved by increasing the P-f droop coefficient. However, increasing the P-f droop coefficient will deteriorate the system's small signal stability. To address this contradiction, a modified synchronized control strategy is proposed. Simulation results verify the effectiveness of the analysis and the proposed control strategy.

Self-Synchronization of Wind Farm in an MMC-Based HVDC System: A Stability Investigation

Abstract: The stability of an offshore wind power network connected through a high-voltage dc (HVDC) transmission line can be challenging since a strong ac collection (ACC) bus might not be available, when there is no rotating machine connected in that bus. In addition, the synchronization unit (phase-locked loop, PLL) has shown to have a significant impact in achieving satisfactory performance. To tackle this problem, this paper has proposed a wind energy conversion system (WECS) controller for such an ACC bus based on the synchronverter concept. A synchronverter—an inverter without PLL that mimics the synchronization mechanism inherent to synchronous generator—is introduced in the grid side of WECS voltage-source converter (VSC), where the wind farms are connected to ac network through a modular multilevel converter (MMC)-based HVDC system. In order to determine the stability of the interconnected system, an impedance-based stability method is adopted. The impedances of both the wind power inverter and the MMC-HVDC converter are analytically derived, and the analytical model is verified by comparing the frequency responses obtained from numerical simulation. The detailed analysis and the results presented show the benefits of this controller and its potential for stability. The results highlights the synchronverter's ability in keeping better performance compared to PLL-based dq -domain control in point of stability and control in integrating offshore wind farm through the MMC-based HVDC system, since the impedance of the synchronverter reflects a simple RL characteristic. On the other hand, the impedance of PLL-based dq -domain control impedance is inductive above 2 kHz and reflects composite characteristics below 2 kHz with different resonance points and with higher impedance magnitude at low frequencies, making it more vulnerable to voltage instability. Finally, time-domain simulation results are presented to validate the theoretical analysis and to show how the self-synchronization impacts on the system performance.

An Active Damping Method Based on a Supercapacitor Energy Storage System to Overcome the Destabilizing Effect of Instantaneous Constant Power Loads in DC Microgrids

Abstract: This paper presents a novel active damping method to overcome instability problems of dc microgrids (MGs) caused by constant power loads (CPLs). This method is implemented based on the existing energy storage system (ESS) in the dc MGs. As an indispensable part in the dc MGs, the ESS in this paper is used for more than just compensating the conventional power unbalance in the system, it has taken an additional responsibility and is used for adjusting the system damping to deal with the instability problems induced by the CPLs as well. First, the stability criteria of a simplified dc MG are analyzed. Based on this, an active damping method that aims at virtually reshaping the loads in the dc MG is introduced. With the proposed method, which is realized by a supercapacitor ESS in this paper, the CPLs in the system are virtually reduced and the resistive loads are virtually increased. Therefore, the destabilizing effect of the CPLs in the dc MG is eliminated. Simulations and experiments are conducted and the effectiveness of the proposed method is verified. This method is suitable to be applied in the dc MG with various elements and variable operation modes.

A Novel Online Parameter Estimation Method for Indirect Field Oriented Induction Motor Drives

Abstract: This paper presents a novel online parameter estimation method to overcome the negative influence of parameter mismatch on the field orientation and then on the performance of the indirect field oriented induction motor drives. This method allows for parallel estimation of the rotor resistance and the mutual inductance, which tend to deviate from their nominal values resulting from changes in temperature and flux level. In the case of electrical vehicles or wind power generators, the parameter deviation degrades the drive performance in two ways: calculation of the current reference from the torque command issued by the power control unit and calculation of the slip frequency for the indirect field orientation. Moreover, the mutual inductance estimation is more critical in these wide speed range applications than in general industry field. This proposed estimation method is implemented through a model reference adaptive system about the rotor flux. The reference model comes from two sliding-mode observers (SMOs): a terminal SMO and a linear SMO, implemented in series. The adjustable model is undertaken by the current model of rotor flux. The analysis and design presented in this paper are confirmed both through numerical simulation and experiments.

Efficiency Evaluation of Fully Integrated On-Board EV Battery Chargers With Nine-Phase Machines

Abstract: A fully integrated on-board battery charger for future electric vehicles (EVs) has been recently introduced. It reutilizes all the propulsion components of an EV in charging/vehicle-to-grid (V2G) modes, it does not require any additional components or hardware reconfiguration, and charging/V2G modes are realized with zero electromagnetic torque production. Both fast (three-phase) and slow (single-phase) chargings are possible, with unity power factor operation at the grid side. The solution is based on the use of a triple three-phase machine and a nine-phase inverter/rectifier. This paper reports on the results of efficiency evaluation for the said system. Testing is performed using both a nine-phase induction machine and a nine-phase permanent magnet machine for a range of operating conditions in charging/V2G modes, with both three-phase and single-phase grid connection. Additionally, the impact of converter interleaving on the losses and efficiency is also studied. Losses are separated for different subsystems, thus providing an insight into the importance of optimization of different EV power train components from the efficiency point of view. Promising efficiencies, in the order of 90%, are achieved although none of the system components have been optimized.