See-Young Choi

Instantaneous control of average power for grid tie inverter using single phase D-Q rotating frame with all pass filter

This paper proposes a new instantaneous power control method using the rotating frame with an all pass filter for a single-phase grid tie inverter. The recently proposed single-phase rotating frame is reanalyzed based on the phasor notation that has been widely used to understand linear electrical circuits and then the relationship between them is defined. A grid tie inverter circuit model with a LCL filter is derived in a single-phase rotating frame. A novel average power control method with an all pass filter suitable for digital implementation is introduced. Various results from simulations are shown to verify the proposed scheme and actual experiments taken from a 3 kW grid tie inverter will be carried out in a near future.

The control system of the active power filter considering power factor in unbalanced load

This paper presents the performance of a parallel active power filter system in unbalanced load. The unbalanced load leads to negative sequence of current, and this makes 120 Hz ripple in the DC-link voltage and increases the rating of the APF. Thus, the separation of the negative sequence was performed in the synchronous reference frame and was controlled to flow into the supply network. The validity of this scheme is investigated through experimental results for a prototype active power filter system rated at 10 kVA.

Design consideration of CC-CV controller of LLC resonant converter for Li-ion battery charger

In this paper, the design consideration of the dual control scheme of a LLC resonant converter for a Li-ion battery charger is proposed to stabilize operation during whole charging process. The theoretical approach of constant-current and constant-voltage controller is presented when considering the nonlinear property of a Li-ion battery under wide ranged output voltage. By applying the extended describing function method, the small-signal ac model of the resonant converter is obtained. The feedback controller is designed to guarantee stable and robust, while providing a desired crossover frequency and sufficient phase margin. Several simulation results are provided in order to verify the effectiveness of the proposed design consideration.

A novel battery cell balancing circuit using an auxiliary circuit for fast equalization

This paper proposes an improved cell balancing circuit for fast equalization among the lithium ion (Li-ion) batteries. Usually, the battery cell balancing circuit uses the sensor to estimate the accurate SOC (state of charge) of each battery cell and to operate the switch selectively (Manenti et al., 2011), but it makes the system bulky and expensive. A simple voltage sensorless charge balancing circuit has been proposed in (Lim et al., 2014). This cell balancing circuit automatically transfers the energy from the high-voltage battery cell to the low-voltage battery cell. But it needs the switch which has low on-resistance, since the balancing speed is limited by the on-resistance of the switch. Also, as the voltage difference among the battery cells is decreased, the balancing speed is decreased. To ensure that the battery can operate in the safety state, the fast balancing speed must be obtained. In this paper, the balancing speed of the cell balancing circuit is enhanced by using the auxiliary circuit. Switching operation of the proposed circuit is controlled by the comparator and the flip-flop, gate devices. So there is no need for the complicated algorithm to control the switching operation. The simulation results are provided to verify the validity of the proposed cell balancing circuit.

A novel droop control method for distribution loss minimization in DC microgrids with decentralized communication

Dc microgrids including grid-tied inverter, energy storage system, and distributed generations have the advantage of high efficiency as compared to the ac system. Droop control has been realized to suppress the current mismatching among distributed sources. However, the conventional droop control involves distribution loss due to the line impedance. In this paper, a novel droop control is proposed to minimize the distribution loss by adjusting controllable droop coefficient. The control objective of the novel droop control is that energy storage system supplies the most adjacent load with their energy within rated power. Hence, the distribution loss can be minimized significantly by tracking the new reference voltage in order to maintain same output voltage of each converter. Simulation results based on PSCAD/EMTDC are provided to verify proposed droop control method.

A novel adaptive magnetizing inductance control scheme for high-efficiency LLC resonant converter for PV applications

In this paper, A novel control scheme of LLC resonant converter is proposed to maximize energy harvest of photovoltaic resources by adapting an integrated transformer with variable magnetizing inductance adaptively. The proposed resonant converter changes its magnetizing inductance according to the PV panel operating condition, and thus it is possible to achieve a minimum circulating energy under wide input voltage range by the appropriate DC gain of the resonant tank. In addition, the proposed method introduces the feedback loop based on the switching frequency to regulate the magnetizing inductance, and as a result, those advantages can be easily obtained in even wide output power levels. The operating principles of the proposed converter and its control structure are explained in detail, and the design example of a 250-W converter is also presented by using the fundamental harmonic analysis. Several simulation results under various conditions of PV source and output power levels are provided in order to verify its effectiveness.

A novel charge equalizer with auxiliary circuit to control the allowable charging and discharging current of the Lithium-ion battery

This paper proposes a novel type of active cell balancing circuit using the multi-winding transformer. This scheme is applied auxiliary circuit for controlling the charging and discharging current of the Lithium-ion battery. Conventional schemes using a multi-winding transformer require a large magnetizing inductance to satisfy the allowable charging and discharging current of the battery cell. In order to obtain a large magnetizing inductance, the cross-sectional area of the core and the number of turns must be increased. For this reason, there are drawbacks that the system becomes bulky and difficult to design. To overcome this problem, the proposed circuit limits the current flow through the diode of the auxiliary circuit to the magnetizing inductance of the multi-winding transformer and controls the slope of the current with the inductor of the auxiliary circuit. Therefore, since the magnetizing inductor does not participate in the slope of the discharge current during the balancing operation, it can be designed with a small magnetizing inductance and has an advantage of easily designing the transformer. In order to verify the validity of the proposed circuit, we analyzed the configuration, operation principle and operation mode of the proposed circuit. Also, the waveforms of the balancing performance according to various conditions, the charging and discharging current control performance of the conventional circuit and the proposed circuit are presented.

An integrated voltage-current compensator of LLC resonant converter for Li-ion battery charger applications

The typical conventional battery charger requires the two separated voltage and current compensators to achieve both the constant current and the constant current (CC-CV) charging profile. This paper proposes a single integrated voltage-current compensator of the LLC resonant converter for lithium-ion battery charger applications. The proposed compensator is designed to provide a smooth and reliable performance during entire charging process, while providing the reduced design efforts and seamless mode transient response due to its inherent integration. The theoretical analysis and the design consideration of the proposed compensator is illustrated based on a frequency response domain, especially with the considerations of the nonlinearly varied equivalent resistor and/or effective capacitance based on the proposed battery model, and as a result, the integrated compensator guarantees a stable and robust operation. Finally several experimental results verify the effectiveness of the proposed posed compensator and its theoretical analysis.

A low cost lithium-ion battery tester with a zero voltage discharge capability

A battery tester is an important equipment for battery manufacturers to understand several characteristics of a lithium-ion battery including dc and/or over-discharge properties. This paper proposes a low cost battery tester that can discharge the battery to zero voltage under a minimized current ripple to evaluate the characteristics of a lithium-ion battery. The circuit of the proposed tester consists of two basic bidirectional buck converters with no additional switch or auxiliary circuitry structure. In addition, to ensure the minimization of the current ripple components, a link voltage variation scheme is adapted. Due to these properties, the proposed circuit enables to provide a low cost and a good DC characteristics with a low current ripple. The performance of the proposed battery tester is verified by several simulation and experimental results with a lithium-ion battery as a load.

A Novel Cell Balancing Circuit for Fast Charge Equalization

This study proposes an improved cell balancing circuit for fast equalization among lithium-ion (Li-ion) batteries. A simple voltage sensorless charge balancing circuit has been proposed in the past. This cell balancing circuit automatically transfers energy from high-to low-voltage battery cells. However, the circuit requires a switch with low on-resistance because the balancing speed is limited by the on-resistance of the switch. Balancing speed decreases as the voltage difference among the battery cells decrease. In this study, the balancing speed of the cell balancing circuit is enhanced by using the auxiliary circuit, which boosts the balancing current. The charging current is determined by the nominal battery cell voltage and thus, the balancing speed is almost constant despite the very small voltage differences among the batteries. Simulation results are provided to verify the validity of the proposed cell balancing circuit.