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This study proposes a phase-locked loop (PLL) algorithm employed for phase-angle detection of single-phase utility grid voltage. A detailed stability analysis is performed, as well as its performance is evaluated under several power quality problems. The proposed PLL structure is based on the instantaneous active power theory for three-phase power systems (pPLL), which is investigated into the fictitious two-phase stationary reference frame (αβ-pPLL). A non-autonomous adaptive filter (AF) operates in conjunction with the PLL, and its main function is to extract the fundamental component of the utility grid voltage allowing the rejection of voltage harmonics. The stability analysis of the proposed AF-αβ-pPLL scheme is carried out in order to provide adequate tuning procedures for choosing the parameters used in the proposed algorithm. In addition, the dynamic response and robustness of the AF-αβ-pPLL algorithm are evaluated by means of simulation and experimental tests, under utility grid disturbances, such as voltage harmonics, voltage sag, phase jumps and frequency variations. To emphasise the effectiveness of the algorithm, a comparative analysis with three other single-phase PLL schemes is carried out, such as the conventional power-based PLL (pPLL), the two-phase stationary reference frame pPLL (αβ-pPLL) and the well-known enhanced PLL (EPLL).

Electronic power converters play an essential role in power grids, aiming to improve the electrical energy quality and also enabling bidirectional energy transfer between DC lines and circuits. They facilitate the achievement of a sinusoidal current waveform and effective power transfer control with a high power factor. This paper introduces a stationary reference frame based control strategy for grid-connected three phase modular multilevel converters (MMC). This strategy employs conventional PI controllers to track the instantaneous power components that include intentional oscillations at double grid frequency. By employing this method, the MMC converter can maintain an output sinusoidal waveform even under unbalanced grid voltage conditions. Also, there is no need for a transformation from a stationary frame to a synchronous frame, eliminating the requirement for a PLL to estimate the grid voltage phase angle. Furthermore, the use of MMC converter over common two-level and three-level VSC converters is proposed since MMC converters offer merits such as low harmonic components, no need for filters at the DC terminals, no need for filters at the AC side, and low losses, despite some drawbacks such as a large number of IGBT switches or a higher amount of stored energy in the sub-module capacitors. Therefore, the voltage THD and consequently active and reactive powers of the converter have been impressively mitigated by using MMC. To confirm the capability and effectiveness of the proposed method, various simulations are performed in MATLAB/Simulink software. Finally, the results are compared with common methods.

Electronic power converters play an essential role in power grids, aiming to improve the electrical energy quality and also enabling bidirectional energy transfer between DC lines and circuits. They facilitate the achievement of a sinusoidal current waveform and effective power transfer control with a high power factor. This paper introduces a stationary reference frame based control strategy for grid-connected three phase modular multilevel converters (MMC). This strategy employs conventional PI controllers to track the instantaneous power components that include intentional oscillations at double grid frequency. By employing this method, the MMC converter can maintain an output sinusoidal waveform even under unbalanced grid voltage conditions. Also, there is no need for a transformation from a stationary frame to a synchronous frame, eliminating the requirement for a PLL to estimate the grid voltage phase angle. Furthermore, the use of MMC converter over common two-level and three-level VSC converters is proposed since MMC converters offer merits such as low harmonic components, no need for filters at the DC terminals, no need for filters at the AC side, and low losses, despite some drawbacks such as a large number of IGBT switches or a higher amount of stored energy in the sub-module capacitors. Therefore, the voltage THD and consequently active and reactive powers of the converter have been impressively mitigated by using MMC. To confirm the capability and effectiveness of the proposed method, various simulations are performed in MATLAB/Simulink software. Finally, the results are compared with common methods.

High penetration of wind power into the grid necessitates the coordinated action of wind energy conversion systems and the grid. A suitable generation control is required to fulfill the grid integration requirements, especially during faults. A system using a pair of voltage source converters with a squirrel cage induction generator coupled to a wind turbine is proposed to provide fault ride-through during grid faults. A threefold action is used for providing the effective fault ride-through via coordinated action of the machine side and the grid side converter. The entire wind energy conversion system is controlled such that the wind turbine remains connected even during the faults. To implement the threefold action: (i) A decoupled current controller is placed in the grid side converter, which separately controls the positive and negative sequence currents arising during faults. The grid side converter controller is capable of eliminating the double frequency oscillations at the dc-link voltage and, hence, real power, which arises during the unsymmetrical faults; (ii) Reactive power injection is additionally provided by the grid side converter for better grid support; and (iii) The vector control technique is used in machine side converter along with the droop control to adjust the generator speed and the torque resulting in actuation of the pitch control mechanism to limit power generation without shutdown of the turbine.

This paper proposes the grid synchronization method of inverter using a quasi Proportional+Multi Resonant (P+MR) control-based voltage controller a stationary reference frame. The inverter supplies a non-linear load under the islanded mode. In islanded mode, the inverter is defined as a voltage source to supply the full local load demand without a connection to the grid. On the other hand, if the grid is restored from a previous fault or the strategic islanding is unnecessary, the inverter needs to be synchronized with the phase of the grid before the transfer from islanded mode to grid-connected mode. When the system is modeled and controlled based on the stationary reference frame control, the AC reference voltage, which has a constant voltage and frequency in islanded mode, is substituted to the AC grid voltage. Significant error can occur due to the large phase differences between the phase of reference and that of the measured value. This error also can cause severe voltage dynamic problems. In addition, if any nonlinear local load is connected to the output of the inverter, it becomes more serious due to the harmonics generated from the loads. In this paper, the PR control under a stationary reference frame is used for voltage control under islanded mode considering the harmonic effects from the nonlinear load. The seamless grid synchronization method based on this PR control is proposed to solve the aforementioned problems. The validity of the proposed seamless grid synchronization method is verified through PSiM simulations and experimental results.

Sensorless schemes are mainly used in fault-tolerant control of interior permanent magnet synchronous motor (IPMSM) drives. Traditional sensorless methods with injection of high-frequency (HF) carrier signals into the stationary reference frame have a stable performance. However, phase shifts caused by conventional high-pass filters (HPFs) decrease the estimation accuracy. Besides, the demodulation process is usually complicated, which increases the difficulty of application. To solve these problems, this study presents an HF-induced current direct demodulation method. Rotor position angle can be solved from the sampled currents at adjacent moments without demodulation signals. Therefore, the errors related to filters in the rotor position estimation are eliminated. Besides, the efficiency of the demodulation algorithm is improved. Furthermore, an online compensation method for cross-saturation effect is proposed. Experimental results verify that the proposed strategy can obtain an accurate rotor position with good steady-state and dynamic performance.

In this study, a direct torque and flux control is described for a six-phase asymmetrical speed and voltage sensorless induction machine (IM) drive, based on non-linear backstepping control approach. First, the decoupled torque and flux controllers are developed based on Lyapunov theory, using the machine two axis equations in the stationary reference frame. In this control scheme, the actual stator voltages are determined from dc-link voltage using the switching pattern of the space vector pulse-width modulation inverter. Then, for a given motor load torque and rotor speed, a so-called fast search method is used to maximise the motor efficiency. According to this method, the rotor reference flux is decreased in the small steps, until the average of real input power to the motor reaches to a minimum value. In addition, a model reference adaptive system-based observer is employed for online estimating of the rotor speed. Finally, the feasibility of the proposed control scheme is verified by simulation and experimental results.

The study discusses the problem of speed and flux estimation for the induction motor (IM) drive and presents the design of two sliding mode observers (SMO) with compound manifolds. Both observers are developed using the IM model in the stationary reference frame. The first observer is a single-manifold SMO – it estimates the motor fluxes and yields an approximate value of the speed; however, it is not a converging observer. The single-manifold design is transformed into a double-manifold observer by adding extra feedback terms – this leads to a fully convergent observer that also yields an accurate estimate of the speed. The observers are designed using compound manifolds, which are chosen as a combination of the estimated fluxes and current mismatches. Observers with compound manifolds have been rarely investigated because they cannot be designed using a standard procedure; however, they are shown to have interesting properties. Observer uniqueness is also discussed. The methods proposed are suited to a sensorless IM drive control algorithm where the speed, the flux magnitude and the rotor flux angle are needed. The theoretical developments are supported with simulations and experiments.