Search

To transmit constant powers particularly at light load, the isolated dual active bridge (DAB) with conventional dual-phase-shift (DPS) control exhibits large variations of currents, losses, and efficiencies if there is a small change of outer phase shift ratio, which is caused by the change of command to adjust power transmission. This is mainly because of the narrow operation region of outer phase shift ratio if the conventional DPS modulation is used. To solve this problem, a novel DPS control with bidirectional inner phase shifts for the DAB is developed to reduce the current surge in the high-frequency transformer, which can stabilize the output power with high efficiency. From the analysis of operation modes and power characteristics of the proposed DPS control, it is shown that due to wider operation region of outer phase shift ratios compared with the conventional DPS control in which the inner phase shifts of both the primary and secondary H-bridges are in the same direction, lower surge current can be obtained in the change of phase shift command, which results in stable power transmission particularly at light load without scarifying the efficiency. Moreover, the DAB of using the proposed DPS control can transmit bidirectional energy with the inner and outer shift ratios of 0–1. For experimental verifications, the test results are also presented in this paper. [ABSTRACT FROM AUTHOR]

This paper proposes a novel sensorless method for open-winding (OW)-permanent magnet synchronous machine (PMSM) (OW-PMSM) with circulating current suppression. First, a zero-sequence steerable space vector pulse width modulation together with zero-sequence controller (ZSC) is implemented to suppress the circulating current existing in the OW-PMSM drive system with common dc bus. Afterward, the zero-sequence back EMF reconstructed from the output signal of the ZSC is utilized to extract the position information via synchronous phase-locked loop-based position observer. Hence, the voltage transducer and resistance network used in the conventional zero-sequence back-electromotive force-based sensorless methods can be consequently eliminated. Moreover, the proposed method requires no machine parameters, such as winding resistance, d- and q-axis inductances which are indispensable for the conventional fundamental back-EMF-based methods. The voltage disturbance components due to the resistance, inductance, and rotating cross coupling voltage drops can be avoid. Therefore, stronger robustness against the variation of machine parameters due to load or temperature variation can also be obtained. Finally, the proposed method is validated via experiments on a 3-kW OW-PMSM drive system. [ABSTRACT FROM AUTHOR]

Due to asymmetric three-phase system and influence of inverter nonlinearity, large torque, and stator flux harmonics would exist in the space vector modulation-based direct torque controlled (DTC-SVM) three-phase permanent magnet synchronous machines (PMSMs). A general expression for electromagnetic torque is derived for DTC of asymmetric three-phase PMSMs considering the influence of inverter nonlinearity. To mitigate the influence of the undesired harmonics, a compensation method is developed by enhancing the standard proportional-integral (PI)-regulator in the torque and flux loops with appropriate resonant controllers. Since the 2nd- and 6th-order harmonics are the most dominant components, two resonant controllers tuning at the 2nd and 6th of stator angular frequency, respectively, are utilized. The proposed method can achieve good steady-state performance in terms of low torque and flux ripples, while keeping the merits of the conventional DTC-SVM strategy such as good dynamic torque response and machine parameter independence. The experimental results validate the effectiveness of the proposed strategy. [ABSTRACT FROM AUTHOR]

A novel variable reactor based on transformer is proposed in the paper. In the variable reactor, a transformer with air gap is chosen, and the primary voltage is detected and used as the reference signal. A voltage-sourced inverter is applied to track the reference signal in order to yield a controllable voltage, which is applied across the secondary winding of the transformer. When the secondary-controlled voltage and the primary voltage satisfy a proportional condition, the equivalent impedance of primary winding of the transformer will vary continuously. According to the novel variable reactor, a novel shunt power quality controller is proposed and its principle is analyzed in detail. The shunt power quality controller is characterized by parallel topology and single frequency control. The approaches of decreasing the transformer leakage inductance are addressed. The experimental results verify the validity of the shunt power quality controller. [ABSTRACT FROM AUTHOR]

Aposition offset-based multiparameter estimation for permanent magnet synchronous machines (PMSMs) under variable speed control is proposed in this paper, which does not need the aid from nominal parameter values of the PMSM and could estimate the rotor permanent magnet (PM) flux linkage and winding resistance separately. For the estimation of rotor PM flux linkage, two sets of PMSM data corresponding to two position offsets are measured while the dq-axis reference currents are set to constants so as to ensure that the estimated machine parameters will not vary during the data measurement. Afterwards, the winding resistance will be estimated from measured PMSM data without addition of position offsets, in which the estimation and compensation of distorted voltage due to inverter nonlinearity are also taken into account. The conventional quantum genetic algorithm is used for aiding the calculation of proposed estimation,which is finally tested on an interior PMSM and shows very good performance in the estimation of rotor PM flux linkage and winding resistance. Thus, it could be used for the condition monitoring of stator winding and rotor PMs of PMSMs. [ABSTRACT FROM AUTHOR]

This paper proposes a method for online estimating the rotor flux linkage and voltage-source inverter (VSI) nonlinearity of permanent magnet synchronous machine (PMSM) drives. Thermocouples are employed for measuring the temperature variation of the stator winding in order to obtain the actual value of stator winding resistance. An Adaline estimator is used for online estimation of distorted voltage Vdead due to VSI nonlinearity. Both are subsequently used for the estimation of the rotor flux linkage. The proposed method is experimentally validated on a PMSM drive system and shows good performance in tracking the variation of the rotor flux linkage and compensating the VSI nonlinearity. [ABSTRACT FROM AUTHOR]

This paper presents a dead-beat predictive direct power control (DPC) strategy and its improved voltage-vector sequences for reversible three-phase grid-connected voltage-source converters (VSCs). The instantaneous variation rates of active and reactive powers, by applying each converter voltage vector in 12 different sectors, are deduced and analyzed. Based on the power variation rates, it is found that the values of the predicted duration times for the two conventional active converter voltage vectors are less than zero when the grid-connected VSC operates as either a rectifier or an inverter. In order to solve this issue, two new alternative vector sequences are proposed and compared. Experimental results on a 1.5 kW reversible grid-connected VSC system are presented to validate the feasibility of the proposed voltage-vector sequences on the dead-beat predictive DPC strategy. [ABSTRACT FROM AUTHOR]

This paper proposes an improved flux-weakening control method to improve the drive performance of nonsalient pole permanent magnet synchronous motors having large winding inductance and consequently an infinite constant power speed ratio. The proposed method consists of two parts. The first part utilizes the voltage differences between the input and output of a minimum-magnitude-error over-modulation block to modify the d-axis current references. The second part uses the compensated q-axis voltage, in which the resistive voltage drop and the distorted voltage due to inverter nonlinearities are taken into account, to detect the maximum torque per voltage (MTPV) line. Under the MTPV condition, a PI controller with a low-pass filter is presented to modify the q-axis current reference. With these two parts, the bus voltage utilization in the proposed method is close to the six-step operation and the actual current trajectory approaches the ideal MTPV line. As a result, the drive performance in the whole flux-weakening region is optimized in terms of maximizing the output torque/power. The method also shows a good performance in transient response. Effectiveness and superiorities of the proposed method are verified by experimental results. [ABSTRACT FROM AUTHOR]

This paper investigates the switching patterns of direct power control (DPC) strategies for three-phase grid-connected voltage-sourced dc-ac converters. The instantaneous variation rates of active and reactive powers by applying each converter voltage vector in 12 different sectors are deduced. In terms of hysteresis look-up-table DPC, based on the power variation rates, modified switching tables are presented to alleviate the active power pulsations, and an asymmetrical hysteresis controller is proposed for active power control to eliminate its steady-state errors. As to predictive DPC, according to the power variation rates, the cause is identified to that the values of the predicted duration times for the auxiliary active voltage vector become negative. Consequently, to solve this problem, two new alternative vector sequences are proposed and compared. A simple compensation method is further added to the basic control scheme of predictive DPC to deal with grid voltage unbalances. Experimental results on a 1.5-kVA grid-connected dc-ac converter are presented to validate the feasibility of the improved switching patterns and the unbalanced compensation method on the DPC strategies for dc-ac converters. [ABSTRACT FROM AUTHOR]

This paper presents direct torque control (DTC) methodology for a four-switch three-phase (FSTP) inverter-fed permanent magnet brushless ac (PM BLAC) machine, with reference to a conventional six-switch three-phase (SSTP) inverter. It has been found that when derived from conventional voltage model flux estimation scheme, the predicted stator flux imbalance may be caused by unbalanced inverter voltage drop in the FSTP inverter, in which one phase winding is directly connected to dc-link midpoint. While this imbalanced problem does not adversely affect the performance of current-model-based DTC, it causes significantly nonsinusoidal current waveforms and considerably unbalanced current magnitudes in voltage-model-based DTC. A new compensation scheme taking into account the different forward voltage-drop values in the switching device and the freewheeling diode is proposed for the voltage-model-based DTC to correct for stator flux imbalance via the addition of corrective voltages to flux equations. The proposed scheme has significantly improved the shape of current waveforms with satisfactory balanced magnitudes, total harmonic distortion, and torque ripple factor, as verified by both simulation and experimental results. It has been shown that it is possible for an FSTP inverter to provide similar performance to an SSTP inverter when driving a PM BLAC machine. [ABSTRACT FROM AUTHOR]

For dual three-phase machines, vector space decomposition allows the control of fundamental and main harmonic components in αβ and xy subspaces, respectively. However, the modulation of voltage references in the two subspaces using the space vector pulsewidth modulation (SVPWM) is not independent but coupled. In that, voltage references of two subspaces cannot be modulated simultaneously beyond the modulation restraints, thereby distorting the voltage regulation and causing harmonics or even system misbehavior. Meanwhile, the fundamental voltage in αβ subspace responsible for electromagnetic torque generation should be prioritized and secured. Thus, this article proposes a systematic analysis of modulation restraints of three representative SVPWM techniques, i.e., the linear modulation range of xy subspace under an assured modulation index of the fundamental voltage. Voltage references of xy subspace within this range can be modulated successfully without affecting the assured modulation index and the margin left for harmonic control in xy subspace is then acknowledged. This can be used to limit the output of current controllers not to exceed the voltage modulation restraint, thereby avoiding the modulation failure. Finally, the experimental results validate the linear modulation range and demonstrate the modulation behaviors within and out of the linear modulation range. [ABSTRACT FROM AUTHOR]

This paper presents a flexible compensation strategy for voltage source converter (VSC) connected to an unbalanced and harmonic distorted grid, which can achieve flexible tradeoff between the two common control targets, i.e., compensating the voltage at the point of common coupling (PCC), and compensating the output current of the VSC. To achieve the control target, a hybrid virtual impedance method is proposed, which uses both the feedforward of the PCC voltage and the feedback of the output current to develop the harmonic (including the negative sequence) voltage references. Compared with the conventional virtual impedance methods using only feedforward or only feedback, the proposed hybrid method can control the harmonic output impedance of the VSC in a wider range, without the potential overmodulation or instability problems caused by the conventional methods when the desired virtual impedance varies in a wide range. The effectiveness and superiority of the proposed strategy is theoretically analyzed and experimentally verified. [ABSTRACT FROM AUTHOR]

This paper proposes a coordinated direct power control (DPC) scheme for the rotor-side converter (RSC) and the grid-side converter (GSC) of the doubly fed induction generator (DFIG) under unbalanced grid voltage conditions. In order to eliminate the coupling interactions between the phase-locked loop (PLL) and the local unbalanced network, a virtual phase angle is used to replace the actual one acquired by the PLL. Thus, the PLL is removed out of RSC and GSC in the proposed DPC scheme. During network unbalance, the RSC is controlled to reduce torque ripples, while three selectable control targets are identified for the GSC, i.e., constant total active power, constant total reactive power, and balanced currents. A single-side resonant controller with the frequency discrimination between the positive- and negative-sequence signals of the same frequency is employed in the coordinated DPC scheme to avoid the complex calculations of the power compensating components. Meanwhile, the sequential separations of the voltages and currents are also eliminated. Then, the control performance, including the limits of the dc-link voltage, the dc-capacitor power oscillations, the impacts of the frequency deviations, and the grid synchronization of the proposed DPC strategy, is discussed. Finally, the experimental results of DFIG system verify the effectiveness of the proposed DPC strategy under unbalanced grid voltage conditions. [ABSTRACT FROM PUBLISHER]

This article develops a new closed-form analytical solution to the harmonic spectrum of the full-bridge (FB) modular multilevel converter (MMC) with phase shifted carrier pulse width modulation. It considers FB-MMC operation in boost mode where the negative-voltage states of the FB submodules (SMs) are utilized to extend the ac operating voltage range and/or reduce the dc link voltage which is known to reduce the energy storage requirement. The solution is based on double Fourier series expansion in two variables. The harmonic distribution of the output voltage is analyzed and the effects of the modulation index and the number of SMs on the output voltage harmonics are revealed. The influence of the upper/lower arm carrier phase shift angle is also investigated. Using the analysis, selection of the number of SMs, the modulation index and the upper/lower arm carrier phase shift angle are specified for output voltage harmonic minimization. The findings are verified by simulation and experimental results. [ABSTRACT FROM AUTHOR]

This paper presents an independent operation of the rotor-side converter (RSC) and grid-side converter (GSC) for a doubly fed induction generator (DFIG)-based wind energy conversion system under unbalanced grid voltage conditions. In this paper, the RSC is controlled to achieve four different control targets, including balanced stator current, sinusoidal rotor current, smooth stator active and reactive powers, and constant DFIG electromagnetic torque. The GSC is commanded to keep the dc voltage at a constant value. Additional feedback compensators using resonant regulators for the RSC are employed, and the decompositions of the positive and negative sequence components and calculations of the rotor negative current references can be avoided. Another similar compensator is used in the GSC to suppress the dc voltage fluctuates and remove the GSC reactive power oscillations without the stator or rotor power information. The proposed method can make the RSC and GSC available to an independent operation with a simple implementation for higher reliability. The experimental results demonstrate the effectiveness of the proposed control strategy for both the RSC and GSC under unbalanced grid voltage conditions. [ABSTRACT FROM AUTHOR]

This article develops a new closed-form analytical solution to the harmonic spectrum of the pulsewidth modulated (PWM) output voltage of the single-phase inverter connected to the dc bus with considerable voltage ripple. The solution is based on a double Fourier series expansion in two variables. As the single-phase inverter forms the basic building block of most converters, the developed expressions can be applied to various topologies. This article first identifies the interactions of a single-frequency sinusoidal modulation signal and the carrier signal with the dc bus voltage harmonics. The dc bus voltage harmonics are constrained in this article to multiples of the inverter output voltage fundamental frequency. The analysis is then extended to include a multifrequency modulation signal. Starting from the general solution, new expressions for the output voltage harmonics of the modular multilevel converter (MMC) are developed. The low-capacitance MMC is chosen in this analysis due to inherent low-frequency voltage oscillations in the converter internal capacitors. The MMC analytical harmonic spectrum can incorporate the effects of the circulating current control and third harmonic injection PWM, by including the second- and third-order harmonic components in the modulation signal, respectively. The MMC analytical harmonic spectrum is benchmarked against the experimental results. [ABSTRACT FROM AUTHOR]

This paper proposes a coordinated control strategy for the rotor-side converter (RSC) and the grid-side converter (GSC) of an LC-filtered doubly fed induction generator (DFIG) under distorted grid voltage condition, to eliminate the distortion of the total output current (including the stator current, the GSC current, and the filter capacitor current). For an LC-filtered DFIG connected to a distorted grid, the low-order current harmonics can freely flow into the filter capacitor thereby polluting the total output current, even when the stator current and the GSC current are both controlled as sinusoidal. Although this problem can be addressed by measuring the output current or the capacitor current, the extra current sensors are undesired in industrial applications. In this paper, a novel hybrid virtual impedance method is implemented in both the RSC and the GSC, to reshape the parallel impedance of the stator, the GSC and the filter capacitor to nearly infinite for low-order harmonics, thus, the freely flowing output current harmonics can be mitigated. Meanwhile, the low-order current harmonics required by the output current compensation can be flexibly distributed between the stator and the GSC according to their operating conditions, to disperse the side-effects on the drive train and the GSC, thereby better protecting the whole system. The effectiveness of the proposed strategy is theoretically analyzed and verified with both simulation and experiment. [ABSTRACT FROM AUTHOR]

This paper proposes a sliding-mode direct power control (SMDPC) strategy for doubly fed induction generator (DFIG) under both balanced and unbalanced grid conditions using extended active power. When the traditional power theory is used under unbalanced grid condition, the control strategies usually need to be modified and become more complicated. Therefore, an extended active power is proposed in this paper, which is effective under both balanced and unbalanced grid conditions with a simple control strategy. Based on the extended active power, elaborated analysis of the mathematical model of DFIG is obtained. Furthermore, an SMDPC strategy using the extended active power is proposed, which can obtain sinusoidal stator currents and restrain electromagnetic torque ripples under unbalanced grid condition without the need of decomposition process and phase-locked loop (PLL). Comparative experimental studies of the SMDPC using the extended and traditional active powers for DFIG are conducted to validate the effectiveness of the proposed strategy under both balanced and unbalanced grid conditions. In addition, the dynamic performance and robustness of the proposed SMDPC are also proved to be satisfying by the experimental results. [ABSTRACT FROM AUTHOR]

This paper proposes a direct stator current vector control strategy of a doubly fed induction generator (DFIG) without phase-locked loop (PLL) during network unbalance. In the proposed control strategy, a virtual phase angle produced by the nominal frequency is used for coordinate transformations instead of the actual voltage one estimated by the PLL. Since the calculation of the commanded stator currents is based on the instantaneous power theory regardless of the generator parameters, it can greatly reduce the parameter dependence of the control system. Then, for a satisfactory grid synchronization, an improved synchronization method is proposed. It can smoothly produce the stator voltage with no overshoot voltage and no parameter dependence. During network unbalance, a reduced-order vector integrator is used to directly regulate the pulsating components without the complex calculations of the commanded current values and the sequential separations of the voltages and currents. Meanwhile, four alternative control targets can be achieved for the DFIG during network unbalance. Furthermore, the control performance, including the control system stability, the robustness against frequency disturbances and parameter deviations, and the rejection of voltage unbalance, are fully analyzed. Finally, the experimental results are given to validate the effectiveness of the proposed control strategy. [ABSTRACT FROM AUTHOR]

This paper proposes a new direct active and reactive power control (DPC) for the three-phase grid connected dc/ac converters. The proposed DPC strategy employs a nonlinear sliding mode control (SMC) scheme to directly calculate the required converter's control voltage so as to eliminate the instantaneous errors of active and reactive powers without involving any rotating coordinate transformations. Meanwhile, there are no extra current control loops involved, which simplifies the system design and enhances the transient performance. Constant converter switching frequency is achieved by using space vector modulation, which eases the design of the ac harmonic filter. Simulation and experimental results are provided and compared with those of the classic voltage-oriented vector control (VC) and conventional lookup table (LUT) DPC strategies. The proposed SMC-DPC is capable of providing enhanced transient performance similar to that of the LUT-DPC, and keeps the steady-state harmonic spectra at the same level as those of the VC scheme. The robustness of the proposed DPC to line inductance variations is also inspected during active and reactive power changes. [ABSTRACT FROM PUBLISHER]