Resonant-Based Backstepping Direct Power Control Strategy for DFIG Under Both Balanced and Unbalanced Grid Conditions

This paper presents an improved direct power control (DPC) strategy for grid-connected voltage source inverter (VSI) under unbalanced and harmonic grid voltage conditions. Based on the mathematic model of VSI with the negative sequence, 5th and 7th harmonic voltage components consideration, a PI controller is used in the proposed DPC strategy to achieve the average output power regulation. Furthermore, vector PI controller with the resonant frequency tuned at the two times and six times grid fundamental frequency is adopted to regulate both negative and harmonic components, and then two alternative targets of the balanced/sinusoidal current and smooth active/reactive output power can be achieved. Finally, simulation results based on Matlab validate the availability of the proposed DPC strategy.

This paper presents an optimized control strategy based on a Doubly Fed Induction Generator (DFIG) and Gravity Energy Storage System (GESS) for AC voltage support in unbalanced grid conditions. The presented control aims to achieve precise rotational speed control, voltage stabilization, and harmonic component suppression. The optimization strategy responds to voltage and frequency fluctuations in an unbalanced grid. Based on Grid-Forming (GFM) control, it adjusts the DFIG’s operating state in real time. This ensures stable voltage support and mitigates harmonic distortion caused by the unbalanced grid. Simulation results, under a weak grid (SCR = 3) and unbalanced (0.9 p.u. voltage sag) conditions, validate the strategy, which reduces rotor current THD from 12.57% to 1.71% and maintains precise speed tracking during a 0.8 p.u. to 0.7 p.u. load change. The results demonstrate that the presented control method effectively improves grid power quality. It also enhances system stability and reliability. This approach provides strong support for integrating renewable energy into unbalanced grids.

A novel control scheme for DFIG-based wind energy systems under unbalanced grid conditions

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

The modular multilevel converter (MMC) is an emerging and attractive topology for the high-voltage direct-current (HVDC) transmission system. This paper presents a generalized mathematical model for MMC in HVDC applications under balanced and unbalanced grid conditions. The dynamics of the positive-, negative-, and zero-sequence components are derived from the model. Then, a dual current control scheme with positive- and negative-sequence current controllers is applied to MMC. The power controller to eliminate negative-sequence current components and the other one to eliminate double-line-frequency voltage ripple are compared. Moreover, a zero-sequence current controller is proposed in addition to the positive- and negative-sequence current controllers. Time-domain simulations on a 61-level MMC-HVDC test system are performed in the PSCAD/EMTDC software environment. The results demonstrate that the MMC-HVDC system with or without converter transformer is able to operate under unbalanced conditions by the use of the proposed control scheme.

The modular multilevel converter-based unified power flow controller (MMC-UPFC) is able to operate under unbalanced grid conditions with symmetric component decoupling. However, the constraint of the voltage limit of UPFC is not considered and no protection schemes are investigated to protect the UPFC from overmodulation under unbalanced grid conditions. To solve this problem, this paper proposes the cascaded control scheme for MMC-UPFC based on voltage limit control and symmetric component decoupling to balance the ac current of the transmission line. With appropriate transformer connections for MMC-UPFC, the negative- and zero-sequence currents are suppressed by the corresponding inner current loops. Considering the voltage limit of MMC, the operating ranges of UPFC-MMC under balanced and unbalanced grid conditions are investigated in both mathematical derivation and point scanning methods. The results are depicted and analyzed in 3-D vision. Based on the analysis of operating regions, the voltage limit control is proposed to protect the MMC from overmodulation and to maximize the controllable region under unbalanced grid conditions. Finally, the final cascaded control structure is constructed. The simulation results obtained in PSCAD/EMTDC are provided to validate the effectiveness and robust performance of the proposed control strategies.

In the DC power distribution systems, One-cycle control (OCC) based three-phase VIENNA rectifiers achieves high power factor (PF) easily. However, under unbalanced grid conditions, the fundamental phase shift between the input current and corresponding phase voltage obviously deteriorates PF, especially when carrier-fundamental frequency ratio varies. In this paper, based on the mathematical model of the rectifier with OCC control under unbalanced grid conditions, the fundamental phase shift problem is analyzed in detail, including variable carrier-fundamental frequency ratio. Moreover, aiming at solving this problem, a fundamental phase shift compensation scheme is proposed. By introducing a second order generalized integrator (SOGI) to generate the complex vector grid currents feedback, which combines with unbalanced grid voltages, three-phase modulation signals are dynamically modified to eliminate the fundamental phase shift. To facilitate the digital control for the rectifier, the digital implementation of the proposed compensation scheme is described in this paper. At last, a digital-controlled VIENNA rectifier experimental test bench is built, and the effectiveness of the proposed scheme is verified by experimental results.

For the power control problem of a bidirectional voltage-source converter, because only one voltage vector is applied at each control period, the traditional model predictive direct power control strategy brings in high harmonic content. In this paper, a hybrid vector-modulation model predictive direct power control scheme is developed to address the high harmonic distortion issue under unbalanced grid conditions. Based on the instantaneous power theory, the expression for the power fluctuation is derived under unbalanced grid voltages. The new reference power under unbalanced grid voltages is represented by the α β components of the grid voltages and their orthogonal signals lagging by 9 0 ∘ . By means of the proposed strategy, neither the phase-locked loops nor the sequence extraction of the grid voltages are required. In addition, the optimal voltage vector is selected from eighteen alternative hybrid voltage vectors to further improve control performance. The effectiveness of the proposed strategy is verified through real-time simulations conducted on an RT-LAB hardware-in-the-loop platform.

Many wind turbine generators (WTGs) are installed in remote, rural areas, where the power grids are usually weak, characterized by unbalanced voltage conditions. If the voltage unbalance is not taken into account in the control system, it will cause poor power quality and poor operating performance of the WTG systems. This paper proposes a novel control scheme to improve the operating performance of a wind turbine equipped with a doubly fed induction generator (DFIG) under unbalanced network voltage conditions. The rotor-side converter (RSC) and grid-side converter (GSC) of the DFIG are controlled in a positive (dq)+ reference frame as well as in a negative (dq)- reference frame. Control of the RSC and GSC in the (dq)+ reference frame is the same as the regular control under balanced network conditions. The supplementary control of the RSC in the (dq)- reference frame minimizes the electromagnetic torque pulsations of the DFIG caused by the unbalanced network voltage; the supplementary control of the GSC in the (dq)- reference frame contributes to balance the total output currents of the DFIG. The proposed control scheme is implemented in PSCAD/EMTDC on a 3.6 MW DFIG wind turbine connected to a power network with unbalanced voltage. Results show that it improves the operating performance of the DFIG wind turbine system.

Compared with 3-phase parallel-connected modular multilevel converter (PC-MMC), the 3-phase series-connected MMC (SC-MMC) is more beneficial due to possible reduction of construction costs and footprints, which is more suitable for HVDC tapping station. To determine the power safe transmission range of SC-MMC under both balanced and unbalanced conditions, this paper introduced the analytical and point-scanning methods. Firstly, the equivalent mathematical model of SC-MMC was derived. Next, constraints of the operation boundary are discussed and decided. Then, the corresponding boundary control strategy is designed for the possible over power operation of SC-MMC under unbalanced ac grid conditions. The simulation model of SC-MMC and its control systems are built in PSCAD/EMTDC. Simulation results show the correctness of the theoretical analysis and the effectiveness of the proposed power safe transmission range boundary control method.

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 steady-state model of MMC for the second-order phase voltage ripple prediction under unbalanced conditions, taking the impact of negative-sequence current control into account. From the steady-state model, a circular relationship is found among current and voltage quantities, which can be used to evaluate the magnitudes and initial phase angles of different circulating current components. Moreover, in order to calculate the circulating current in a point-to-point MMC-based HVdc system under unbalanced grid conditions, the derivation of equivalent dc impedance of an MMC is discussed as well. According to the dc impedance model, an MMC inverter can be represented as a series connected R–L–C branch, with its equivalent resistance and capacitance directly related to the circulating current control parameters. Experimental results from a scaled-down three-phase MMC system under an emulated single-line-to-ground fault are provided to support the theoretical analysis and derived model. This new models provides an insight into the impact of different control schemes on the fault characteristics and improves the understanding of the operation of MMC under unbalanced conditions.

An improved finite control-set model predictive control for nested neutral point-clamped converters under both balanced and unbalanced grid conditions

When a grid voltage is unbalanced, the unbalanced grid voltages act as an external disturbance for the grid-connected PWM inverter systems, resulting in an unbalanced grid current. Firstly, the mathematical model of the grid-connected converter, in view of unbalanced grid voltages, is proposed and analyzed in detail. In order to achieve the decomposition of positive and negative sequence voltage, this paper adopts the software phase-locked loop to decouple based on double synchronous reference frame. This method has good dynamic performance. In order to suppress negative sequence current on the AC side, the positive and negative sequence components of the grid current are controlled in the double synchronous reference frame respectively. Finally, Simulation results verify the correctness of the control strategy.

To solve the neutral-point (NP) voltage unbalancing problem, a conventional proportional integral (PI) controller is used for NP clamped (NPC) three-level grid-connected inverters under balanced grid conditions. However, when the three-phase grid voltage is unbalanced, the NP voltage unbalancing problem becomes more serious and complex, which cannot be solved by a conventional PI controller with the variation of the unbalanced grid. Based on the transfer function of the injected zero-sequence component to the NP voltage, the NP voltage module has different characteristic performances in frequency-domain, which is a leading or lagging system with the variation of the unbalanced grid and output power factor. Besides, being appropriate for this NP voltage module in two cases, a PI controller with hybrid parameters (HP-PI controller) is proposed to balance the NP voltage in this paper. According to varied unbalanced grid and output power factor, hybrid parameters are designed for this controller in two cases by the Lagrange multiplier method appropriately. Furthermore, frequency-domain characteristics and the balancing time of the proposed HP-PI control method are analyzed. Experimental and simulation results show that, compared with conventional PI and quasi-proportional resonant (PR) control methods, the proposed HP-PI controller has a strong ability to balance NP voltage with a good dynamic and steady-state performance under varied unbalanced grid conditions, which also improves the quality of grid currents.