• The Transactions of the Korean Institute of Power Electronics
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• v.21 no.1
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• pp.88-93
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• 2016

LCL filters are widely used in high-order harmonics attenuation of output currents in grid-connected inverters. However, output currents of grid-connected inverters with LCL filters can become unstable because of the resonance of the filters. Given that the characteristics of output currents in inverters mostly depend on filter performance, the exact analysis of filters by considering parasitic components is necessary for both harmonics attenuation and current control. LCL filters have three or four parasitic components: the series and/or parallel resistance of the filter capacitor and the series resistance of the two filter inductors. Most studies on LCL filters have focused on the parasitic components of the filter capacitor. Although several studies have addressed the parasitic components of the filter inductor at the inverter side, no study has yet investigated the concurrent effects of series resistance in both filter inductors in detail. This paper analyzes LCL filters by considering series resistance in both filter inductors; it proposes an active damping method based on the virtual series resistance of LCL filters. The performance of the proposed active damping is then verified through both simulation and experiment using Hardware-in-the-Loop Simulator(HILS).

• Journal of Power Electronics
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• v.11 no.3
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• pp.311-318
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• 2011

LCL filters installed at converter outputs offer a higher harmonic attenuation than L filters. However, as a three order resonant circuit, it is difficult to stabilize and has a risk of oscillating with the power grid. Therefore, careful design is required to damp LCL resonance. Compared to a passive damping method, an active damping method is a more attractive solution for this problem, since it avoids extra power losses. In this paper, the damping capabilities of capacitor current, capacitor voltage, and grid-side current feedback methods, are analyzed under the discrete-time state-space model. Theoretical analysis shows that the grid-side current feedback method is more suitable for use in active power filters, because it can damp LCL resonance more effectively than the other two methods when the ratio of the resonance and the control frequency is between 0.225 and 0.325. Furthermore, since there is no need for extra sensors for additional states measurements, this method provides a cost-efficient solution. To support the theoretical analysis, the proposed method is tested on a 7-kVA single-phase shunt active power filter.

• Journal of Power Electronics
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• v.9 no.2
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• pp.288-299
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• 2009

This paper proposes a feedback linearization control scheme of AC/DC PWM converters with LCL input filters using no damping resisters. Feedback linearization techniques use a transformation from nonlinear system models into equivalent linear models in a simpler form. The feedback linearization scheme in this work has cascade structures unlike usual feedback linearization, therefore it has an advantage that it is possible to limit the capacitor current to a certain level. The performance of the proposed controller is validated with simulation and experimental results.

• Journal of Power Electronics
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• v.13 no.6
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• pp.1058-1069
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• 2013

This paper proposes and designs a new output filter called an LCFL filter for application to three phase three wire shunt active power filters (SAPF). This LCFL filter is derived from a traditional LCL filter by replacing its capacitor with a C-type filter, and then constructing an L-C-type Filter-L (LCFL) topology. The LCFL filter can provide better switching ripple attenuation capability than traditional passive damped LCL filters. The LC branch series resonant frequency of the LCFL filter is set at the switching frequency, which can bypass most of the switching harmonic current generated by a SAPF converter. As a result, the power losses in the damping resistor of the LCFL filter can be reduced when compared to traditional passive damped LCL filters. The principle and parameter design of the LCFL filter are presented in this paper, as well as a comparison to traditional passive damped LCL filters. Simulation and experimental results are presented to validate the theoretical analyses and effectiveness of the LCFL filter.

• Journal of Power Electronics
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• v.11 no.6
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• pp.904-913
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• 2011

Previous publications regarding the design and specifications of input filters for STATCOMs usually deal with the input filter only, and seldom pay any attention to the influence of the input filters on the performance of the STATCOM systems. A detailed analysis of the influences of input filters on the stability of STATCOM systems and the corresponding design considerations are presented in this paper. Three types of input filters, L filters, LC filters, and LCL filters, are examined separately. The influences of the parameters of input filters on system stability are investigated through frequency domain methods. With direct current control taken as the major control strategy for the STATCOMs, the different situations when adopting different current detection points are covered in this analysis. A comparison between LC filters and LCL filters is also presented with optimized filter parameters. Based on the analysis, the phase margin, as one of the design considerations for the different types of input filters under different current detection schemes, is discussed. This leads to filter parameters that are different than those of the traditional design. Hardware experimental results verify the validity of the above analysis and design.

• Journal of Power Electronics
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• v.11 no.4
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• pp.456-463
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• 2011

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 grid-connected 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.

• Journal of Power Electronics
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• v.13 no.2
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• pp.287-295
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• 2013

The use of LCL filters in pulse width modulation voltage source converters is a standard solution for providing proper attenuation of high-order grid-current harmonics. However, these filters can cause the undesired effect of resonance. This paper proposes an active damping strategy with harmonics compensation. It can alleviate the harmonics around the resonance frequency caused by the LCL filters. The proposed strategy is attractive since it is simple, does not depend on grid parameters and does not increase the number of sensors. Simulation and experimental results verify the effectiveness of the proposed active damping strategy.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.21 no.12
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• pp.16-22
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• 2020

The characteristics of the LCL filter for grid-connected inverters have been discussed in academia and industry. An online simulation tool was applied to compare and analyze the difference between the LCL filter and L filter. LCL filters were modeled and simulated using a range of professional simulation simulators, and the LCL filters were found to have good filtering effects for high-frequency harmonics. First, this paper summarizes the transfer functions of the LCL filter and provides the Bode plot diagram. The accuracy and validity of the filter attenuation characteristics were confirmed by a fast Fourier transform based on off-line simulation tools, such as PSIM and MATLAB, depending on the given parameters of the LCL filter. Finally, the Typhoon HIL402 real-time simulation was performed for hardware in the loop simulation to verify the actual filtering characteristics of the LCL filter.

• The Transactions of the Korean Institute of Power Electronics
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• v.18 no.1
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• pp.103-109
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• 2013

The conventional LCL filter design method of the utility interactive inverter considers only harmonics attenuation of the current injected to the grid. However, in case of utility-interactive inverter with critical load the voltage quality of the critical load should also be considered for LCL filter design. Also, considering cost and volume of LCL filters. it is important to have minimum values of inductance and capacitance as far as the harmonic standards are satisfied. In this paper a LCL filter design method is proposed to satisfy not only the harmonic standards of the grid current during the grid-connected mode but the voltage quality of the critical load during grid-connected mode and stand-alone mode. With the proposed method optimized values of LCL filters could be obtained by applying weighting factor to voltage ripple across the critical load, inductor volume, amount of reactive current and system bandwidth.

• The Transactions of the Korean Institute of Power Electronics
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• v.13 no.1
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• pp.55-62
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• 2008

This paper proposes a feedback linearization control scheme of AC/DC PWM converters with LCL input filters using no damping resisters. This feedback linearization scheme can eliminate the non-linearity of the system. So, the controller of the system can be designed by using linear control theory, which gives a good transient response. The cascade structure of the controller makes the converter current be controlled within a certain limit. To reduce the number of sensors, the source voltage and current is estimated. The validity of the proposed control algorithm is verified by simulation and experimental results.