BPF-based Grid Voltage Feedforward Control of Grid-connected Converters for Improving Robust Stability

Yang, Shude,Tong, Xiangqian,Yin, Jun,Wang, Haiyan,Deng, Yaping,Liu, Le The Korean Institute of Power Electronics 2017 JOURNAL OF POWER ELECTRONICS Vol.17 No.2

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.

Influence of Filter Inductances and Number of Paralleled Grid-Connected Converters on the System’s Stable Region Under Weak Grid Condition

Yang Shude,Sun Yipeng,Jiang Wei,Zhang Jiyong,Wang Qianlong,Tong Xiangqian 대한전기학회 2021 Journal of Electrical Engineering & Technology Vol.16 No.2

It has been found that both the number of paralleled grid-connected converters and the fi lter inductance value will aff ect the system’s stability in weak grid condition. However, a quantitative study on this eff ect is missing in the current literature. The main contribution of this paper is to quantitatively evaluates the infl uence of the number of paralleled grid-connected converters as well as the inductance value of the L fi lters on the system’s stable region. It is fi rst revealed that, when the same controller design principles are applied for L -type grid-connected converters, the stable region of the paralleled grid-connected converter system is dependent on the ratio between the fi lter inductances and the number of paralleled converters. Specifi - cally, the stable region is proportional to the ratio. Compared to the qualitative analysis in previous work, this fi nding can provide quantitative guidelines on the fi lter inductors or control system design for grid-connected converter applications in weak grid. Finally, experimental tests are carried out using two paralleled grid-connected converters to confi rm the fi ndings.

BPF-based Grid Voltage Feedforward Control of Grid-connected Converters for Improving Robust Stability

Shude Yang,Xiangqian Tong,Jun Yin,Haiyan Wang,Yaping Deng,Le Liu 전력전자학회 2017 JOURNAL OF POWER ELECTRONICS Vol.17 No.2

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.

Complex Vector Analysis and Suppression of Parallel Vibration of Distributed Power Grid Connected Converter

Rui Guo,Cunxu Wang,Shu Liu,Xin Guan,Wei Gao,Shuaijie Wang,Chuan Chen 보안공학연구지원센터 2016 International Journal of Grid and Distributed Comp Vol.9 No.10

With the continuous expansion of the scale of distributed power generation,multi grid connected converters often prone to oscillation problems when parallel operation.In this paper,we study the equivalent circuit of parallel connection of multi grid connected converters , system response characteristics of voltage source and in-depth analysis of the causes of the parallel oscillation of the converters.This paper analyzes converter’s parallel complex vector model by complex vector and comes to a conclusion that power grid voltage feed-forward will produce strong coupling between converter so that cause oscillation parallel converters. In view of the above problems and the flow of the converter starting or the voltage fluctuation of the grid, we design a low feed forward method, that is to make the fundamental and the main sub components of the grid voltage forward without delay and for the relative high frequency components, the filter technology is used to attenuate, so as to achieve the maximum elimination of the feed-forward coupling between the converter, to prevent the occurrence of system oscillation. In order to verify the use of this paper, the theory of the parallel converter is applied in the field of MW level wind field. The experimental results prove the correctness of the theory.

계통 연계형 그리드 포밍 컨버터의 Soft Start-up 기법

맹준열,최성휘 전력전자학회 2024 전력전자학회 논문지 Vol.29 No.2

Inverter-based resources connected to the power system have increased rapidly in recent years. However, this trend weakens grid strength, which is maintained by the inertia of synchronous generators. Therefore, grid-forming converters have attracted much attention owing to their ability to maintain stable connections with weak grids compared with conventional grid-following converters. Various control methods have been proposed for configuring virtual synchronous generators to control grid-forming converters. However, the stable grid integration and initial operation for each of these methods require further research. In this paper, the start-up characteristics of grid-forming converters were examined. The proposed method implements a soft start-up method in a synchronous power control-based grid-forming converter. The validity of the proposed method was verified with 250 kW simulation and hardware-in-the-loop simulation.

Stabilizing dc‐link voltage and neutral point current fluctuation in grid‐connected three‐level NPC converter under generalized grid fault conditions

Jaehoon Choi,Yongsug Suh 전력전자학회 2023 JOURNAL OF POWER ELECTRONICS Vol.23 No.4

This study introduces a stabilizing scheme that decreases the fluctuations in dc-link voltage and neutral point current in a grid-connected three-level NPC converter under generalized grid fault conditions. The fundamental component of the neutral point current is generated under grid-unbalanced conditions. Regardless of the type of unbalance in the grid, the behavior and pattern of the fundamental components of the neutral current are determined by the imbalance factor. The proposed control scheme, which involves regulating the negative sequence component of the ac input current, effectively reduces the size of the dc-link voltage ripple and the fundamental component of neutral point current under diverse types of grid unbalanced conditions. This is verified by simulations and experiments under various conditions. The proposed control method will improve the stability and reliability of the grid-connected converter under various types of imbalance conditions.

State-Space Modeling of Grid-Connected Power Converters Considering Power-Internal Voltage Characteristics

Qiao Peng,Yongheng Yang,Frede Blaabjerg 전력전자학회 2019 ICPE(ISPE)논문집 Vol.2019 No.5

To analyze the stability of power electronicsdominated systems, this paper develops a state-space model of power converters including the DC-link voltage control, where the power-internal voltage (PIV) characteristics of the power converters are considered. The PIV concept comes from the inertia characteristic and swing equation of synchronous generators. It reflects the relationship between the output power and converter internal voltage, the converter response to grid disturbances, and finally, the impact of power converters on the grid stability. In the proposed model, the inputs are the outer controller references and the converter output power, i.e., the power at the point of common coupling (PCC). The output of the model is the internal voltage that determines the grid power distribution, including the power at the PCC. The power will be fed back to the converters as an input, and thinks the closed-loop model of the converterbased system is obtained. Importantly, the parameters of the proposed model are independent of the power grid, but related to the converter operation points and parameters. This makes the model more general and can be employed to analyze multiple converter-based systems. Furthermore, a case study is conducted in this paper to demonstrate the model and the stability analysis.

Voltage Feedforward Control with Time-Delay Compensation for Grid-Connected Converters

Shude Yang,Xiangqian Tong 전력전자학회 2016 JOURNAL OF POWER ELECTRONICS Vol.16 No.5

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.

Dual-Loop Power Control for Single-Phase Grid-Connected Converters with LCL Filter

Shuangjian Peng,An Luo,Yandong Chen,Zhipeng Lv 전력전자학회 2011 JOURNAL OF POWER ELECTRONICS Vol.11 No.4

Grid-connected converters have widely adopted LCL filters to acquire high harmonic suppression. However, the LCL filter increases the system order so that the design of the system stability would be complicated. Recently, sole-loop control strategies have been used for grid-connected converters with L or LC filters. But if the sole-loop control is directly transplanted to gridconnected converters with LCL filters, the systems may be unstable. This paper presents a novel dual-loop power control strategy composed of a power outer loop and a current inner loop. The outer loop regulates the grid-connected power. The inner loop improves the system stability margin and suppresses the resonant peak caused by the LCL filter. To obtain the control variables, a single-phase current detection is proposed based on PQ theory. The system transfer function is derived in detail and the influence of control gains on the system stability is analyzed with the root locus. Simulation and experimental results demonstrate the feasibility of the proposed control.

An Active Damping Scheme Based on a Second Order Resonant Integrator for LCL-Type Grid-Connected Converters

Chen Chen,Jian Xiong,Kai Zhang 전력전자학회 2017 JOURNAL OF POWER ELECTRONICS Vol.17 No.4

This paper proposes a novel active damping scheme to suppress LCL-filter resonance with only grid-current feedback control in grid-connected voltage-source converters. The idea comes from the concept of the model reference adaptive control (MRAC). A detailed theoretical derivation is given, and the effectiveness of this method is explained based on its physical nature. According to the control structure of this method, the active damping compensator, which is essentially a second order resonant integrator (SORI) filter, provides an effective solution to damp LCL resonance and to eliminate the need for additional sensors. Compared with extra feedback methods, the cost and complexity are reduced. A straightforward tuning procedure for the active damping method has been presented. A stability analysis is illustrated in the discrete domain while considering a one-step delay. Finally, experimental results are presented to validate the analysis and to demonstrate the good performance of the proposed method.