• Journal of IKEEE
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• v.24 no.3
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• pp.877-882
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• 2020

This paper presents performance comparison study of current control methods for grid connected inverter (GCI) system. Different current control methods have been developed for GCI systems and each controller has its own advantages and limitations. Steady state and transient dynamic performance of the GCI current controllers are compared in this paper. The performance of the proposed command feedforward control (CFFC) and disturbance rejection control (DRC) is analyzed before and after application to all GCI current controllers. The proposed CFFC and DRC control algorithms is analyzed in a frequency domain and the simulation and experiment models of each GCI current control methods are developed for verification of the performance.

• The Transactions of the Korean Institute of Power Electronics
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• v.21 no.2
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• pp.168-174
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• 2016

In this paper, current control using PI controller in the synchronous reference frame is analyzed through the relationship among bandwidth, resonance frequency, and sampling frequency in the grid-connected inverter with LCL filter. Stability is investigated by using bode plot in frequency domain and root locus in discrete domain. The feedback variable is the grid current, which is regulated by the PI controller in the synchronous reference frame. System delay is modeled as 1.5Ts, which contains computational and PWM modulator delay. Two resonance frequencies are given at 815 Hz and 3.16 kHz from LCL filter parameters. Sufficient phase and gain margins can be obtained to guarantee stable current control, in case that resonance frequency is above one-sixth of the sampling frequency. Unstable current control is performed when resonance frequency is below one-sixth of the sampling frequency. Analysis results of stability from frequency response and discrete response is the same regardless of resonance frequency. Finally, stability of current control based on theoretical analysis is clearly verified through simulation and experiment in grid-connected inverters with LCL filter.

• Journal of Electrical Engineering and Technology
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• v.13 no.5
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• pp.2050-2058
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• 2018

Impulse characteristic of transmission tower grounding grid is needed for lightning protection of transmission line. This paper describes an outdoor experimental test facility established for large-scale grounding grid of transmission tower, made up of four impulse current generators and a circle current return electrode. The amplitude of impulse current is up to 100 kA. The results of the CDEGS simulation and GPR measurement reveal the uniform current distribution in the test arrangement. An impulse test for a square electrode with extended conductors is carried out in condition of three current waveforms with different amplitude. Based on the electrical network model and iterative algorithm method, a calculation model is proposed to simulate the impulse characteristic of large-scale grounding grid considering soil ionization. The curve of impulse resistance against the current amplitude shows the soil ionization both from the simulation and test. Deviation between the simulation and test result is less than 15%.

• Proceedings of the KIPE Conference
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• 2011.07a
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• pp.83-84
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• 2011

This paper presents an analysis and a novel strategy for a doubly fed induction generator (DFIG) based wind energy conversion system under unbalanced grid voltage and non-linear load conditions. A proportional-resonant (PR) current controller is applied in both grid side converter (GSC) and rotor side converter (RSC). The RSC is controlled to mitigate the stator active power and the rotor current oscillations at double supply frequency under unbalanced grid voltage while the GSC is controlled to mitigate ripples in the dc-link voltage and compensate harmonic components of the network current. Simulation results using Psim simulation program are presented for a 2 MW DFIG to confirm the effectiveness of the proposed control strategy.

• The Transactions of the Korean Institute of Power Electronics
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• v.22 no.5
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• pp.440-448
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• 2017

This paper proposes a controller design of a distributed source inverter in stand-alone mode or grid-connected mode to compensate the current or voltage harmonics caused by local nonlinear load. The PR-based multi loop controller has been used to improve the dynamic performance of the system and to compensate the output voltage or grid current harmonics. The multi-loop controller consists of an outer current controller and an inner voltage controller for the output voltage control in stand-alone mode. In grid-connected mode, an outer current controller is added to the output voltage controller for the grid current control. The design performance of each controller is described through the Root locus and Bode plot of the transfer functions. The validity of the proposed control algorithm and design parameters has been verified through the PSiM simulation and experimental results.

• The Transactions of the Korean Institute of Power Electronics
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• v.23 no.6
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• pp.373-380
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• 2018

This study proposes a new islanding detection method that uses the phase shift of feed-forward voltage and evaluates the performance of an existing method and the proposed method when the grid frequency changes within the allowable range under steady-state conditions. The investigated existing method, which is slip mode frequency shift (SMS), uses current phase shift to detect islanding. The SMS method supplies reactive current to the grid under this condition, but the proposed method does not generate additional reactive power because it does not depend on the current control loop. The performance in steady-state grid condition is evaluated through simulations and experiments.

• The Transactions of the Korean Institute of Power Electronics
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• v.28 no.1
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• pp.1-7
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• 2023

A grid-connected 3-level NPC inverter is a power conversion device that connects renewable energy generators, such as photovoltaic or wind turbines to the grid. Although many studies have focused on this inverter, commercializing it requires strictly satisfying various safety and power quality-related standards. Among many standards, leakage current and grid current total harmonic distortion(THD) can be affected by external factors such as installation environment, aging, and grid conditions. Hence, inverter operations that can satisfy these standards need to be explored. In this study a 3-level NPC inverter operation algorithm using the Phase Opposition Disposition-PWM method that can effectively reduce leakage current and switching frequency adjustment to reduce THD effectively has been proposed.

• The Transactions of the Korean Institute of Power Electronics
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• v.20 no.3
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• pp.201-208
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• 2015

The effects of dead-time and offset error, which cause output current distortion in single-phase grid-connected inverters are investigated this paper. Offset error is typically generated by measuring phase current, including the voltage unbalance of analog devices and non-ideal characteristics in current measurement paths. Dead-time inevitably occurs during generation of the gate signal for controlling power semiconductor switches. Hence, the performance of the grid-connected inverter is significantly degraded because of the current ripples. The current and voltage, including ripple components on the synchronous reference frame and stationary reference frame, are analyzed in detail. An algorithm, which has the proportional resonant controller, is also proposed to reduce current ripple components in the synchronous PI current regulator. As a result, computational complexity of the proposed algorithm is greatly simplified, and the magnitude of the current ripples is significantly decreased. The simulation and experimental results are presented to verify the usefulness of the proposed current ripple reduction algorithm.

• The Transactions of The Korean Institute of Electrical Engineers
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• v.65 no.10
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• pp.1648-1654
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• 2016

This paper analyzes complex vector current control for the enhanced cross-coupling compensation in accordance with parameter variation in grid-connected inverter system, and verifies through simulation and experiment. Complex vector current control is performed in the synchronous reference frame through d-q transformation. It generates cross-coupling components with rotating nominal angular frequency. In general, cross-coupling elements are compensated by decoupling terms added to output of conventional decoupling PI controller. But, it is impossible to compensate them perfectly which transient response is especially deteriorated such as large overshoot and slow tracking, when variation of grid impedance or measurement error occurs. However, complex vector current control can improve stability and response characteristic of current control regardless of the situation as before. Decoupling controller and complex vector controller are represented through complex forms, and these controllers are analyzed by using frequency response in s-domain, respectively. It is verified that complex vector controller has more superior response characteristic than decoupling controller through MATALB, PSIM and experimental in 5kW grid-connected inverter when L filter parameter is varied from 1.1mH to increase double, 2.2mH.

• Proceedings of the KIEE Conference
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• 2002.04a
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• pp.213-215
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• 2002

This paper presents grid connected photovoltaic inverter system using a new Zero-Current-Transition(ZCT) technique. The main switches of the proposed grid connected inverter are turned off under the zero current condition by operating the auxiliary circuit and also all semiconductor devices, switches and diodes, are applied to low rated voltage regardless of the load condition. In additionally, the proposed ZCT scheme has advantages, which are without the additional current stresses and the conduction losses on the main switches during the resonance period of the auxiliary circuit. The simulation was performed to verify the validity of the proposed grid connected photovoltaic ZCT inverter system.