Simplified Current Tracking Strategy for LCL-Based Rectifier under Unbalanced Grid Voltage Condition

A new indirect power control strategy based on Model Predictive Current Control (MPCC) for the three-phase Voltage Source Rectifiers (VSR) is proposed in this paper. The proposed strategy regulates the instantaneous AC-side power to ensure stable DC-side voltage under unbalanced gird voltage conditions while no Phase Locked Loop (PLL) or coordinate transformations are needed. This paper analyzes the influence of AC-side inductance on the reference currents, and deduces the direct relationship between the Grid-side current and the AC-side power with a basis of AC-side power mathematical model. To eliminate the impact of AC-side inductance on AC-side power and stabilize the AC-side power under the unbalanced input voltages, MPCC was utilized to regulate the grid-side current. Experiments on a VSR prototype were carried out to validate the effectiveness of the proposed strategy, and the results demonstrated that the ripple of DC-side voltage was reduced and the performance of VSR under unbalanced grid conditions was improved.

An augmented state observer-based sensorless control of grid-connected inverters under grid faults

Under unbalanced grid voltage conditions, negative-sequence will generate the double fundamental frequency in the dq synchronous rotating reference frame. In order to detect the positive-sequence component of the grid voltages fast, this paper proposes a new phase-locked loop method, called double fundamental frequency phase-locked loop (DFF-PLL). The operation principle is analyzed, giving the extract formulas of the positive- and negative-sequence with double fundamental frequency, the process of DFF-PLL and the detection of positive-sequence component of grid voltages. The paper improves the second order generalized integrator quadrature-signals generation (SOGI-QSG), in order to remove the DC bias voltage and guarantee the orthogonality of SOGI-QSG. The performance of DFF-PLL is verified by using simulation and experimental results. Compared with DSOGI-PLL, this method increases the detection speed and accuracy under unbalanced grid voltage conditions.

In this paper, a dual-converter three-phase pulse width modulation (PWM) rectifier based on unbalanced one-cycle control (OCC) strategy is proposed. The proposed rectifier is used to eliminate the second harmonic waves of DC voltage and distortion of line currents under unbalanced input grid voltage conditions. The dual-converter PWM rectifier employs two converters, which are called positive-sequence converter and negative-sequence converter. The unbalanced OCC system compensates feedback currents of positive-sequence converter via grid negative-sequence voltages, as well as compensates feedback currents of negative-sequence converter via grid positive-sequence voltages. The AC currents of positive- and negative-sequence converter are controlled to be symmetrical. Thus, the workload of every switching device of converter is balanced. Only one conventional PI controller is adopted to achieve invariant power control. Then, the parameter tuning is simplified, and the extraction for positive- and negative-sequence currents is not needed anymore. The effectiveness and the viability of the control strategy are demonstrated through detailed experimental verification.

This paper presents a mathematical model for Brushless Doubly-Fed Machine (BDFM) based on Stator Flux Orientation (SFO) in positive and negative reference frames under unbalanced grid voltage conditions. A negative controller is designed to eliminate the effects of unbalanced conditions on electrical torque. The model and controller performance during unbalanced conditions is validated by time domain simulation in MATLAB/Simulink.

This study presents an optimisation control strategy for the power control of voltage source converter‐based high‐voltage direct current transmission under unbalanced grid voltage conditions. To facilitate simple implementation and flexible operation, the current references are derived and synthesised into one generalised equation that introduces two individually adaptable parameters. The two parameters can be adapted to eliminate the oscillations of instantaneous active and reactive power, as well as the currents at the grid side under unbalanced grid voltage conditions. The optimisation method based on particle swarm optimisation algorithm is presented to determine the two parameters. The simulation is conducted with PSCAD/EMTDC. The results show that the proposed strategy is effective and feasible.

Doubly fed induction generator (DFIG) still shares a large part in today's wind power market. It provides the benefits of variable speed operation cost-effectively, and can control its active and reactive power independently. Crowbar protection is often adopted to protect the rotor-side voltage source converter (VSC) from transient overcurrent during grid voltage dip. But under unbalanced grid voltage condition, the severe problems are not the transient overcurrent, but the electric torque pulsation and dc voltage ripple in the back-to-back VSCs. This paper develops dynamic models in MATLAB/Simulink, validates it through experiments, investigates the behavior of DFIG during unbalanced grid voltage condition, and proposes new controllers in separated positive and negative sequence. Methods to separate positive and negative sequence components in real time are also developed, and their responses to unsymmetrical voltage dip are compared. Simulation results prove that the separated positive and negative sequence controllers limit the torque pulsation and dc voltage ripple effectively.

Voltage source converters (VSCs) have been widely used due to their flexibility to control voltages and power independently and bi-directionally. Typically, the control system of VSCs mainly consists of two parts; outer voltage and power controllers and inner current controllers. The vector current control based dq decoupling technique enables to control the active power, reactive power, DC voltage and AC voltage. However, the d- and q-axis of grid voltages and currents comprise AC and DC components under unbalanced grid conditions. The AC components of the d- and q-axis currents make the grid currents unbalanced. In this paper, a novel current control is presented and investigated under unbalanced grid condition for a Back-to-Back Modular Multilevel Converter (B2B-MMC) based HVDC system and validated using the Real Time Digital Simulator (RTDS). Further, the active power oscillation under fault is eliminated by controlling the AC component of grid currents in the dq frame. The RTDS results demonstrate the feasibility of the proposed controllers under unbalanced grid voltage conditions.

This paper presents a new configuration of doubly fed induction generator (DFIG)-based wind farm with multilevel cascaded H-bridge (CHB) converter and its control system, which is suitable not only for unbalanced grid voltage conditions, but also for small voltage sags. The proposed configuration of DFIG can balance the point of common coupling (PCC) voltage without needing additional DC link capacitor or energy storage unlike other configurations and methods. Consequently, the stator voltage and the other loads voltage connected to the PCC are kept constant at the nominal value. The new configuration improves all the DFIG signals under unbalanced grid voltage and small voltage sag conditions unlike other methods. Some of the DFIG units in wind farm require to connect the CHB converter, therefore, it is cost-effective. Matlab/Simulink is used for simulation of a 13.5-MW wind farm consisting of nine 1.5-MW DFIG units and the results demonstrate the good efficiency of the proposed scheme.

Model predictive direct power control for modular multilevel converter under unbalanced conditions with power compensation and circulating current reduction

When a modular multilevel converter (MMC) operates under unbalanced grid voltage conditions, it generates positive-sequence, negative-sequence, and zero-sequence components in the arm currents, which increase system losses and disrupt normal operation. To tackle this problem, this paper introduces a method that employs a second-order generalized integrator (SOGI) to extract the second- and fourth-frequency components from the arm currents. Subsequently, an enhanced quasi-proportional integral resonant (PIR) controller is applied to individually suppress these circulating currents. The approach is validated through MATLAB simulations. The results demonstrate that the proposed suppression method quickly reduces the secondand fourth-frequency circulating currents and significantly minimizes submodule voltage fluctuations, confirming its effectiveness under unbalanced grid conditions.

This paper presents a PIR regulator using direct torque control strategy of grid connected wind turbine driven doubly fed induction generators (DFIGs) when the grid voltage is unbalanced. Under the unbalanced grid voltage condition, the stator voltage and current quality is strongly affected due to the negative and distorted components. It will be reducing the performance of other normal loads connected to the DFIG. That control scheme consisting of indirect matrix converter using DTC. To verify the value of the proposed control strategy, simulation results with 500 MVA DFIG topology are presented and discussed in the paper. Finally,the simulation studies are carried out on a 500 MVA wind-turbine driven DFIG system under unbalanced grid voltage conditions. All the results are validated by using PSCAD simulation.

In order to ride through the unbalanced grid fault, the output power of photovoltaic inverter will fluctuate and its output current will rise and distort under unbalanced grid voltage. This paper proposes an improved formula of reference current and its optimal coefficients method for the photovoltaic inverter under unbalanced grid voltage sag, taking into account the requirements of active and reactive power control. Then the analytic formula of its current harmonic distortion, three-phase current peak, active and reactive power fluctuation, DC voltage fluctuation and the control factor of its output current reference were derived. Considering the constraints of inverter phase current and the dc voltage fluctuation of capacitance in dc side, the optimal model of inverter current reference was established with minimum integrated amplitude of the active and reactive power function as a goal. The correctness of the proposed method has been verified by PSCAD/EMTDC simulation software.

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.

Cascasded delayed signal cancellation (DSC) phase-locked loop (PLL) technique has been attractive for grid synchronization under nonideal grid voltage due to its good harmonics filtering capability. However, it has to face the challenge of slow dynamic response. In this paper, an improved DSC-PLL that features high filtering capability, fast dynamic response and simple structure is presented. This PLL employs only one DSC block and one moving average filter (MAF) block to eliminate all even-order and odd-order harmonics while a second-order phase lead compensator and q-axis feedforward path are introduced to increase the PLL bandwidth. The effect of the phase lead compensator on PLL dynamic performance is analyzed. The feedforward path works only when grid voltage frequency or phase jumps and will not affect the steady state behaviors. Therefore, the PLL can improve the phase estimation accuracy and dynamic speed at the same time even under highly distorted and unbalanced grid voltage. Moreover, linear Lagrange interpolation method is adopted to reduce the discretized errors in the digital implementation of the PLL. The effectiveness of the proposed method is validated by both simulation and experimental results. The comparison results with the existing cascaded DSC-PLL and standard MAF-PLL are also presented.