June-Hee Lee

Current Sensorless MPPT Control Method for Dual-Mode PV Module-Type Interleaved Flyback Inverters

This paper presents a current sensorless maximum power point tracking (MPPT) control method for dual-mode photovoltaic (PV) module-type interleaved flyback inverters (ILFIs). This system, called the MIC (Module Integrated Converter), has been recently studied in small PV power generation systems. Because the MIC is an inverter connected to one or two PV arrays, the power system is not affected by problems with other inverters. However, since the each PV array requires an inverter, there is a disadvantage that the initial installation cost is increased. To overcome this disadvantage, this paper uses a flyback inverter topology. A flyback inverter topology has an advantage in terms of cost because it uses fewer parts than the other transformer inverter topologies. The MPPT control method is essential in PV power generation systems. For the MPPT control method, expensive dc voltage and current sensors are used in the MIC system. In this paper, a MPPT control method without current sensor where the input current is calculated by a simple equation is proposed. This paper also deals with dual-mode control. Simulations and experiments are carried out to verify the performance and effectiveness of the proposed current sensorless MPPT control method on a 110 [W] prototype.

Modulation Technique for Single-Phase Transformerless Photovoltaic Inverters With Reactive Power Capability

This paper underpins the principles for generating reactive power in single-phase transformerless photovoltaic (PV) inverters. Two mainstream and widely adopted PV inverters are explored, i.e., H5 and HERIC. With conventional modulation techniques, reactive power cannot be realized in H5 and HERIC due to the absence of freewheeling path in negative power region. Based on the study, modulation techniques are proposed to provide bidirectional current path during freewheeling period. With proposed modulation technique, reactive power control is achieved in H5 and HERIC inverters, without any modification on the converter structures. The performances of the proposed modulation techniques are studied via MATLAB simulation and further validated with experimental results.

Active Frequency Drift Method for Islanding Detection Applied to Micro-inverter with Uncontrollable Reactive Power

This paper proposes active frequency drift (AFD) as an anti-islanding method applied to micro-inverters with uncontrollable reactive power. When using ordinary inverter topologies, such as full bridge inverters in photovoltaic systems, the islanding phenomenon can be detected with reactive power-based methods, such as reactive power variation. However, when the inverter topology cannot control the reactive power, conventional anti-islanding methods with reactive power cannot be utilized. In this work, the topology used in this paper cannot control the reactive power. Thus, an anti-islanding method that can be used in topologies that cannot control the reactive power is proposed. The conventional anti-islanding method of the topology that cannot control reactive power is introduced and analyzed. Unlike the conventional AFD method, the proposed method extends a zero current interval every predetermined cycle. The proposed method offers certain advantages over conventional AFD methods, such as total harmonic distortion. The proposed method is validated through simulation and experiment.

Current sensorless MPPT method for a PV flyback microinverters using a dual-mode

This paper proposes a current sensorless maximum power point tracking (MPPT) control strategy for a dual-mode photovoltaic (PV) module-type interleaved flyback inverter (ILFI). A flyback inverter, which is one of many transformer topologies, is generally used because of its simplicity and low cost. The topology used in this paper is the interleaved topology; therefore, the secondary ripple current of the transformer can be reduced. The flyback inverter is typically operated in three modes: continuous conduction mode (CCM), discontinuous conduction mode (DCM) and boundary conduction mode (BCM). This paper uses a dual-mode control that includes both the DCM and BCM operations. In addition, the simulations are conducted to verify the performance and effectiveness of the proposed current sensorless MPPT control strategy.

CLC filter design of a flyback-inverter for photovoltaic systems

This paper proposes a CLC filter design of a flyback-inverter for photovoltaic systems. This applied system is small power system; therefore, the capacity and size of the entire system are critical factor. The proposed CLC filter consists of a conventional CL filter and an additional capacitor. The CLC filter, which guarantees the same performance about the current ripple reduction as that of the CL filter, can reduce the size of capacitor and inductor, comparing with components of CL filter. It can help the applied flyback-inverter to be smaller size and cost less. In this paper, the CL filter with the passive damping method is analyzed. Next, the one passive damping method with low power consumption is applied the effect of this method is analyzed. In order to verify the performance of the proposed CLC filter and the validity of passive damping method, a simulation is conducted.

Model predictive control of a grid-connected inverter to reduce current ripples and computation loads

This paper proposes the model predictive control with the space vector modulation (MPC-SVM) method for the current control of the voltage source inverter (VSI). Unlike the conventional MPC method using the limited number of voltage vectors by switching states, the various voltage vectors can be taken into account to find the optimized voltage vector in the proposed method. The various voltage vectors are obtained by subdividing existing voltage vectors more. Among them, some voltage vectors, which are determined by the reference voltage angle (Oref) and magnitude dVrefi), are used to calculate the cost function to mitigate high computational load during the prediction. The various voltage vectors and SVM reduce the current ripples in the output AC side of the inverter compared to the conventional MPC method. The effectiveness and performance of the proposed method are verified by PSIM simulation and experimental results with a three-phase two-level voltage source grid-connected inverter.

A Dead-Beat Control for Bridgeless Inverter Systems to Reduce the Distortion of Grid Current

This paper proposes a method to reduce the distortion of the grid current using a dead-beat control (DBC) scheme for bridgeless inverter systems. In the case of the general proportional integral (PI) current controller for power factor correction (PFC) scheme, the grid current is generally distorted due to the slow dynamic response of the PI controller. If the bandwidth of the PI controller increases to overcome this characteristic, the system shows a fast dynamic response. However, it becomes unstable as wide bandwidth leads to the control of unnecessary values such as a grid current noise. On the contrary, the proposed PFC-DBC method has a fast response for the reference current and accuracy compared to the conventional PFC-PI method owing to the optimal duty cycle for the next state current. In addition, the proposed PFC-DBC method can operate robust control by using the estimation method for actual filter inductance. Therefore, the proposed method can reduce the grid current distortion. The proposed method is verified through powersim simulations and experiments.

Optimal phase shifted method to reduce current ripples for parallel grid-connected voltage source inverter under unequal DC-link voltages

This paper proposes optimal switching period phase-shifted method for slave inverter to reduce grid current ripples in synchronized two parallel three-phase grid-connected voltage-source inverters (VSI). In parallel operation, a 180 degree phase shifted method generally takes of advantage of reducing grid current ripples in same DC-link voltage for each grid-connected VSI. However, in case of unequal DC-link voltage, the grid current ripples have increased using a 180 degree phase shifted method because a dwelling time between two inverters is changed by the different modulation index (MI) of each inverter. To overcome these drawbacks, the proposed method applies the phase angle for reducing grid current ripples to the slave inverter through the calculation of dwelling time. Using the proposed method, it is possible to increase the quality of grid current without any additional device or re-designing of the filter. The performances of the proposed phase-shifted method are studied via PSIM simulation.

MPC-SVM method with subdivision strategy for current ripples reduction and neutral-point voltage balance in three-level inverter

This paper proposes a model predictive control-space vector modulation (MPC-SVM) method to achieve current ripple reduction and neutral-point voltage balance in three level inverter. The proposed MPC-SVM method is based on subdivision strategy for improving the performance of the conventional model predictive control (MPC) method. The proposed subdivision strategy consists of two parts: current controller and neutral-point voltage balance controller. In the current controller, various voltage vectors are obtained by subdividing space vector diagram. In the neutral-point voltage balance controller, various offset voltages are obtained by subdividing the offset voltage range. The optimal voltage vector and offset voltage value are independently determined from the cost function of each controller and they are applied to the inverter through the space vector modulation (SVM) method. Since both controllers are based on the finite set model predictive control (FS-MPC) method, the fast dynamic response can be achieved. The simulation and experiment are conducted to validate the effectiveness and performance of the proposed method.

Reduction of DC-link current ripple in unbalanced three-parallel PV module condition

This paper proposes reduction of dc-link current ripple in unbalanced input voltage of three-parallel interleaved boost converter. The non-overlapped interval of diode currents of each boost converter can be occurred in case of the unbalanced input voltage. Therefore, the ripple of the dc-link current is increased. The non-overlapped interval is removed by adding the calculated phase to the carriers. Therefore, the current ripple is reduced by applying proposed method. The simulation results proved that dc-link current ripple is reduced about 36% by applying proposed switching method. The proposed method is verified through PSIM simulations.