• Journal of Power Electronics
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• 제16권5호
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• pp.1833-1842
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• 2016

In grid-connected converter control, grid voltage feedforward is usually introduced to suppress the influence of grid voltage distortion on the converter's grid-side AC current. However, owing to the time-delay in control systems, the suppression effect of the grid voltage distortion is seriously affected. In this paper, the positive effects of the grid voltage feedforward control are analyzed in detail, and the time-delay caused by the low-pass filter (LPF) in the voltage filtering circuits and digital control are summarized. In order to reduce the time-delay effect on the performance of the feedforward control, a voltage feedforward control strategy with time-delay compensation is proposed, in which, a leading correction of the feedforward voltage is used. The optimal leading step used in this strategy is derived from analyzing the phase-frequency characteristics of a LPF and the implementation of digital control. By using the optimal leading step, the delay in the feedforward path can be further counteracted so that the performance of the feedforward control in terms of suppressing the influence of grid voltage distortion on the converter output current can be improved. The validity of the proposed method is verified through simulation and experiment results.

• 전력전자학회논문지
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• 제22권1호
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• pp.89-93
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• 2017

This study proposes a grid voltage estimation scheme without a phase delay in the voltage-sensorless control of a grid-connected inverter to enhance its economic feasibility, such as manufacturing cost and system complexity. The proposed scheme estimates grid voltages using a disturbance observer (DOB)-based current controller to control the grid-connected inverter without grid-side voltage sensors. The proposed voltage-sensorless control scheme can be applied successfully to grid-connected inverters, which should be operated with synchronization to the grid, considering the phase angle of the grid can be effectively detected through estimating the grid voltages by DOB. However, a problem associated with the phase delay in estimated grid voltages remains because the DOB has dynamic behavior similar to low-pass filter. Hence, the estimated grid voltages are compensated by a phase lead compensator to overcome the limitation. The effectiveness of the proposed control and estimation schemes is proven through simulations and experiments using a 2 kVA prototype inverter.

• Journal of Electrical Engineering and Technology
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• 제7권6호
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• pp.898-904
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• 2012

This paper presents a ride-through skill of PMSG wind turbine system under the distorted and unbalanced grid voltage dips. When voltage dips occur in the grid, pitch control and generator speed control as well as a parallel resistor of DC-link help to keep the turbine's safety. Modern grid code requires a wind turbine to supply reactive currents to help voltage recovery after grid faults clearance. In order to supply reactive currents to the grid in case of the distortedly unbalanced grid voltage dips, a special PLL is needed to control the grid side converter and to regulate the grid voltages symmetrically. The proposed method is applied to 2MW multi-pole PMSG wind turbine system, and verified by simulation.

• Journal of Power Electronics
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• 제15권1호
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• pp.235-245
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• 2015

This paper proposes a reactive current assignment and control strategy for a doubly-fed induction generator (DFIG) based wind-turbine generation system under generic grid voltage sag or swell conditions. The system's active and reactive power constrains during grid faults are investigated with both the grid- and rotor-side convertors (GSC and RSC) maximum ampere limits considered. To meet the latest grid codes, especially the low- and high-voltage ride-through (LVRT and HVRT) requirements, an adaptive reactive current control scheme is investigated. In addition, a torque-oscillation suppression technique is designed to reduce the mechanism stress on turbine systems caused by intensive voltage variations. Simulation and experiment studies demonstrate the feasibility and effectiveness of the proposed control scheme to enhance the fault ride-through (FRT) capability of DFIG-based wind turbines during violent changes in grid voltage.

• 전기학회논문지
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• 제62권11호
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• pp.1560-1565
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• 2013

This paper proposes the control method for compensating for unbalanced grid current and reducing a total harmonic distortion (THD) of the grid current at the three-phase grid-connected inverter systems under unbalancd and distorted grid voltage conditions. The THD of the grid current caused by grid voltage harmonics is derived by considering the phase delay and magnitude attenuation due to the hardware low-pass filter (LPF). The Cauchy-Schwarz inequality theory is used in order to search more easily for a minimum point of THD. Both the gain and angle of a compensation voltage at the minimum point of THD of the grid current are derived. The negative-sequence components in the three-phase unbalanced grid voltage are cancelled in order to achieve the balanced grid current. The simulation and experimental results show the validity of the proposed control methods.

• Journal of Power Electronics
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• 제12권3호
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• pp.495-502
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• 2012

This paper presents an improved grid voltage control strategy for wind farms with doubly-fed induction generators (DFIGs) connected to distribution networks based on an analysis of the operation limits of DFIG systems. A modified reactive power limit calculation method in different operation states is proposed and a reactive power control strategy during grid voltage dips/rises is further discussed. A control strategy for compensating unbalanced grid voltage, based on DFIG systems, by injecting negative sequence current into the grid through the grid side converter (GSC) is proposed. In addition, the negative current limit of the GSC is discussed. The distribution principle of the negative sequence current among the different DFIG systems in a wind farm is also introduced. The validity of the proposed voltage control strategy is demonstrated by Matlab/Simulink simulations. It is shown that the stability of a wind farm and the power grid can be improved with the proposed strategy.

• Journal of Power Electronics
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• 제17권2호
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• pp.432-441
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• 2017

Grid voltage feedforward is extensively used for controlling grid-connected converters. However, the conventional voltage feedforward control reduces the stability margins of the converter connected to a high-impedance grid. The effect mechanism of voltage feedforward on the grid-connected converter control under high-inductive conditions of the grid impedance is clearly explained in this study using the equivalent transformations of control block diagrams. Results show that the delay produced by the digital control is the root cause of this effect. An improved voltage feedforward strategy, in which a bandpass filter (BPF) is introduced into the feedforward path, is proposed to strengthen the converter's robust stability against grid impedance variations. The selection method of the BPF's bandwidth is also provided considering the tradeoff between the response speed to the grid voltage sag and the system's robust stability. The converter can work stably over a wide range of the grid impedance through the proposed approach. Simulation and experimental results fully verify the effectiveness of the BPF-based voltage feedforward strategy.

• Journal of Power Electronics
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• 제16권4호
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• pp.1551-1564
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• 2016

The high penetration level of inverter-based distributed generation (DG) power plants is challenging the low-voltage ride-through requirements, especially under unbalanced voltage sags. Recently, a flexible injection of both positive- (PS) and negative-sequence (NS) reactive currents has been suggested for the next generation of grid codes. This can enhance the ancillary services for voltage support at the point of common coupling (PCC). In light of this, considering distant grid faults that occur in a mainly inductive grid, this paper proposes a complete voltage support control scheme for the interface inverters of medium or high-rated DG power plants. The first contribution is the development of a reactive current reference generator combining PS and NS, with a feature to increase the PS voltage and simultaneously decrease the NS voltage, to mitigate voltage imbalance. The second contribution is the design of a voltage support control loop with two flexible PCC voltage set points, which can ensure continuous operation within the limits required in grid codes. In addition, a current saturation strategy is also considered for deep voltage sags to avoid overcurrent protection. Finally, simulation and experimental results are presented to validate the effectiveness of the proposed control scheme.

• 전력전자학회논문지
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• 제16권2호
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• pp.182-190
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• 2011

본 논문은 계통 전압 사고 상황에서 계통 연계형 풍력 발전 시스템이 만족시켜줘야 할 LVRT(Low Voltage Ride Through) 제어 전략을 제안한다. LVRT 규정은 계통 전압 사고 시 풍력 발전 시스템이 지켜야 할 부분들을 전압 감소율과 사고 시간에 대해 나타내고 있다. 특히 전압 감소율이 10% 이상일 경우에는 풍력 발전 시스템은 규정된 무효 전류를 전력 계통에 제공하여 계통 전압 확보에 이바지해야 한다. 본 논문에서의 LVRT 규정은 세계적으로 가장 엄격한 규정인, 독일 계통 연계 규정(German Grid Code)을 기준으로 하고 풀 스케일(Full-scale) 가변 속도 전력 변환 시스템을 고려하여 제어 전략을 수립한다. 본 LVRT 제어 전략은 계통 사고 시 LVRT 규정을 모두 만족시킴과 동시에 직류단 전압 제어의 추가적인 알고리즘으로 직류단 전압의 제어를 통하여 전체 풍력 발전 시스템의 전력 균형을 기할 수 있다. 3상 전압 지락 사고의 경우 계통으로의 전력 제어가 불가능하여 계통 측 컨버터가직류단 전압을 제어할 수 없으므로, 전력 제어의 기능을 발전기 측 인버터로 이행 시켜 상황에 따라 유연한 직류단 전압 제어가 가능함을 보였다. 시뮬레이션과 실험을 통해 LVRT 제어 전략의 타당성을 검증하였다.

• Journal of Power Electronics
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• 제8권1호
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• pp.10-22
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• 2008

The wind-driven doubly fed induction generator (DFIG) is currently under pressure to be more grid-compatible. The main concern is the fault ride-through (FRT) requirement to keep the generator connected to the grid during faults. In response to this, the paper introduces a novel model and new control scheme for the DFIG. The model provides a means of direct stator power control and considers the stator transients. On the basis of the derived model, a robust linear quadratic (LQ) controller is synthesized. The control law has proportional and integral actions and takes account of one sample delay in the input owing to the microprocessor's execution time. Further, the influence of the grid voltage imperfection is mitigated using frequency shaped cost functional method. Compensation of the rotor current pulsations is proposed to improve the FRT capability as well as the generator performance under grid voltage unbalance. As a consequence, the control system can achieve i) fast direct power control without instability risk, ii) alleviation of the problems associated with the DFIG operation under unbalanced grid voltage, and iii) high probability of successful grid FRT. The effectiveness of the proposed solution is confirmed through simulation studies on 2MW DFIG.