• Journal of Power Electronics
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• 제16권4호
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• pp.1483-1493
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• 2016

Multiple parallel inverters have multiple resonant frequencies that are influenced by many factors. This often results in stability and power quality problems. This paper develops a multiple input multiple output model of grid-connected inverter systems using a closed-loop transfer function. The influence factors of the resonant characteristics are analyzed with the developed model. The analysis results show that the resonant frequency is closely related to the number, type and composition ratio of the parallel inverters. To suppress resonance, a scheme based on virtual impedance is presented, where the virtual impedance is emulated in the vicinity of the resonance frequency. The proposed scheme needs one inverter with virtual impedance control, which reduces the design complexity of the other inverter controllers. Simulation and experimental tests are carried out on two single phase converter-based setups. The results validate the correctness of the model, the analytical results and the resonant suppressing scheme.

• Journal of Power Electronics
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• 제13권4호
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• pp.701-711
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• 2013

This paper takes into account the influence of the different impedances of distribution lines on power distribution among inverters when the inverters are paralleled with the droop control method. The impact of distribution lines on the power distribution of inverters can be divided into two aspects. Firstly, since the distributed generators are in low voltage grids, there is resistive impedance in the distribution lines, which will cause control coupling and reduce system stability. The virtual negative resistive impedance of inverters is adopted in this paper to neutralize the resistive element of distribution lines and thus make the distribution line impedance purely inductive. Secondly, after solving the resistive impedance problem, the difference in the inductive impedance value of distribution lines due to the low density of distributed generators will cause an unequal share of reactive power. With regards to this problem, modification is put forward for the droop control strategy to share the reactive power equally. The feasibility of the design is validated by simulation and experimental results.

• 전력전자학회:학술대회논문집
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• 전력전자학회 2003년도 춘계전력전자학술대회 논문집(1)
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• pp.430-433
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• 2003

IGBTS are widely used for the industrial inverters in the mid power range at low voltage (440V$\~$660V) application. Advantageous features of the device are simple gate drive and high speed switching capability. Due to these advantages the application of IGBTS is enlarging into the high power application. However, to increase the power handling capacity at lower input voltage level, the current rating in each bridge arm must be enlarged. Therefore the parallel operation of IGBT devices is essentially needed. This paper describes the feasible parallel structures of the power circuit for the mid & the high power inverters and introduces the important design condition for the parallel operation of IGBT devices. To verify feasibility of the IGBT parallel operation, the feature of several IGBT devices (EUPEC, SEMIKRON's IGBT) are investigated and the power stacks are implemented and tested with these devices. The experimental results show the good characteristics for the parallel operation of IGBTS.

• 전력전자학회논문지
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• 제18권4호
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• pp.387-396
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• 2013

This paper investigates the droop control of parallel inverters for an islanded mode of microgrid. Frequency and voltage droop control is one of power control and load demand sharing methods. However, although the active power is properly shared, the reactive power sharing is inaccurate with conventional method due to the unequal line impedances and the power coupling of active - reactive power. In order to solve this problem, an improved droop method with virtual inductor concept and a voltage and current controller properly designed have been considered and analyzed through the PSiM simulation. The performance of improved droop method is analyzed in not only low-voltage line but also medium voltage line.

• Journal of Electrical Engineering and Technology
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• 제13권1호
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• pp.280-286
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• 2018

A sensorless active damping scheme for LCL filters in grid-connected parallel inverters for battery energy storage systems is proposed. This damping method is superior to the conventional notch filter and virtual damping methods with respect to robustness against the variation of the resonance of the filter and unnecessary additional current sensors. The theoretical analysis of the proposed damping method is explained in detail, along with the characteristic comparison to the conventional active damping methods. The performance verification of the proposed sensorless active damping method shows that its performance is comparable to that of the conventional virtual damping method, even without additional current sensors. Finally, simulation and experimental results are provided to examine the overall characteristics of the proposed method.

• 전력전자학회논문지
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• 제20권1호
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• pp.81-90
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• 2015

This paper investigates the droop control-based real and reactive power load sharing with a virtual inductor when the line impedance between inverter and Point of Common Coupling (PCC) is partly and unequally resistive in high-capacity systems. In this paper, the virtual inductor method is applied to parallel inverter systems with resistive and inductive line impedance. Reactive power sharing error has been improved by applying droop control after considering each line impedance voltage drop. However, in high capacity parallel systems with large output current, the reference output voltage, which is the output of droop controller, becomes lower than the rated value because of the high voltage drop from virtual inductance. Hence, line impedance voltage drop has been added to the droop equation so that parallel inverters operate within the range of rated output voltage. Additionally, the virtual inductor value has been selected via small signal modeling to analyze stability in transient conditions. Finally, the proposed droop method has been verified by MATLAB and PSIM simulation.

• Journal of Power Electronics
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• 제16권3호
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• pp.1152-1162
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• 2016

The unequal impedances of the interconnecting cables between paralleled inverters in the island mode of microgrids cause inaccurate reactive power sharing when the traditional droop control is used. Many studies in the literature adopt low speed communications between the inverters and the central control unit to overcome this problem. However, the losses of this communication link can be very detrimental to the performance of the controller. This paper proposes an improved reactive power-sharing control method. It employs infrequent measurements of the voltage at the point of common coupling (PCC) to estimate the output impedance between the inverters and the PCC and then readjust the voltage droop controller gains accordingly. The controller then reverts to being a traditional droop controller using the newly calculated gains. This increases the immunity of the controller against any losses in the communication links between the central control unit and the inverters. The capability of the proposed control method has been demonstrated by simulation and experimental results using a laboratory scale microgrid.

• Journal of Power Electronics
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• 제18권1호
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• pp.70-80
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• 2018

A novel strategy based on a zero common mode voltage pulse-width modulation (ZCMV-PWM) technique and zero-sequence circulating current (ZSCC) feedback control is proposed in this study to eliminate ZSCCs between three-level neutral point clamped (NPC) voltage source inverters, with common AC and DC buses, that are operating in parallel. First, an equivalent model of ZSCC in a three-phase three-level NPC inverter paralleled system is developed. Second, on the basis of the analysis of the excitation source of ZSCCs, i.e., the difference in common mode voltages (CMVs) between paralleled inverters, the ZCMV-PWM method is presented to reduce CMVs, and a simple electric circuit is adopted to control ZSCCs and neutral point potential. Finally, simulation and experiment are conducted to illustrate effectiveness of the proposed strategy. Results show that ZSCCs between paralleled inverters can be eliminated effectively under steady and dynamic states. Moreover, the proposed strategy exhibits the advantage of not requiring carrier synchronization. It can be utilized in inverters with different types of filter.

• 전력전자학회논문지
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• 제24권2호
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• pp.71-77
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• 2019

This study proposes a circulating current reduction method that uses high-frequency voltage compensation when carrier phase difference occurs between two inverters in MSIS. In MSIS, inverters are configured in parallel to increase power capacity and to increase efficiency by using inverters only as needed. However, in the parallel inverter structure, circulating current is inevitably generated. Circulating current increases the stress on the switch, adversely affects the current control performance, and renders load sharing difficult. The proposed method compensates for the output voltage reference of the slave module by using the high-frequency voltage so that the switching pattern of each module is matched even in asynchronous carriers. The validity of the proposed method is verified by simulations and experiments with 600 W IPMSM.

• 대한전기학회:학술대회논문집
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• 대한전기학회 1996년도 하계학술대회 논문집 A
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• pp.309-311
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• 1996

In this paper, we propose two types of novel discrete time current control methods of modified fixed band hysteresis control and optimal control for Parallel Resonant DC Link Inverters(PRDCLI). Because zero bus voltage intervals are generated on the DC link of PRDCLI, we can obtain the information of counter electromotive force(emf) by a simple estimation strategy. The proposed current controllers predict the currents of the next resonant cycle using the obstained information of counter emf and the average values of DC link voltages. The computer simulation results for a simple equivalent circuit of induction motor show that the proposed control methods are more effective than conventional methods.